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Spinach-Derived Porous Carbon Nanosheets as High-Performance Catalysts for Oxygen Reduction Reaction

  • Xiaojun Liu
    Xiaojun Liu
    Department of Chemistry, American University, Washington, District of Columbia 20016, United States
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    • , 
  • Casey Culhane
    Casey Culhane
    Department of Chemistry, American University, Washington, District of Columbia 20016, United States
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    • , 
  • Wenyue Li
    Wenyue Li
    Department of Chemistry, American University, Washington, District of Columbia 20016, United States
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    • , and 
  • Shouzhong Zou*
    Shouzhong Zou
    Department of Chemistry, American University, Washington, District of Columbia 20016, United States
    *Email: [email protected]
    More by Shouzhong Zou
    Orcidhttp://orcid.org/0000-0003-1952-8799
Cite this: ACS Omega 2020, 5, 38, 24367–24378
Publication Date (Web):September 15, 2020
https://doi.org/10.1021/acsomega.0c02673
Copyright © 2020 American Chemical Society
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Abstract

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Biomass-derived porous carbon materials are effective electrocatalysts for oxygen reduction reaction (ORR), with promising applications in low-temperature fuel cells and metal–air batteries. Herein, we developed a synthesis procedure that used spinach as a source of carbon, iron, and nitrogen for preparing porous carbon nanosheets and studied their ORR catalytic performance. These carbon sheets showed a very high ORR activity with a half-wave potential of +0.88 V in 0.1 M KOH, which is 20 mV more positive than that of commercial Pt/C catalysts. In addition, they showed a much better long-term stability than Pt/C and were insensitive to methanol. The remarkable ORR performance was attributed to the accessible high-density active sites that are primarily from Fe–Nx moieties. This work paves the way toward the use of metal-enriching plants as a source for preparing porous carbon materials for electrochemical energy conversion and storage applications.

1. Introduction

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Electrochemical energy conversion and storage play a key role in sustainable and clean energy production. Among different electrochemical devices, fuel cells and metal–air batteries show great promises. Oxygen reduction reaction (ORR) is the cathode reaction of these devices and is one of the bottlenecks of their practical applications because of its sluggish kinetics that limits their power output. Platinum-based carbon-supported catalysts have long been recognized as the most effective electrocatalysts for ORR. However, the scarcity and high cost of Pt, together with the lack of long-term stability and the vulnerability to surface poisoning by various chemicals such as methanol and carbon monoxide, call for the development of non-Pt group metal (NPGM) catalysts. To address these issues, carbon-based catalysts have attracted increasing attention because of their excellent stability, resistance to surface poisoning, and use of low-cost and earth-abundant elements. A variety of NPGM catalysts have been developed by doping carbon with various heteroatoms such as N, S, P, B, or co-doping with N and Fe, or N and Co.(1−4) Among them, iron- and nitrogen-co-doped carbon materials exhibited the best performance for ORR, and some of these catalysts showed key performance descriptors that are superior to Pt catalysts.(1,2)
Given the great promise these catalysts hold, it is of importance to explore approaches for obtaining them in a sustainable fashion. A reasonable starting point would be to use inexpensive and readily available materials as sources for carbon, nitrogen, and iron, as opposed to sometimes expensive and/or toxic chemicals. In this regard, biomass is an attractive renewable natural resource because it is typically rich in carbon and nitrogen, and some forms contain metal elements such as iron or cobalt. These metal elements are key to the formation of highly active catalytic centers for ORR.(5−7) Progress has been made in this direction. Doped carbon ORR catalysts have been made from plants,(8−14) plant products,(15−18) biomass wastes of animal or human origin,(19,20) fungi,(21,22) and microorganisms.(23,24) Of particular interest among these biomass resources are plants because some plants are able to selectively store metals.(25) Therefore, they can be used to remove toxic metals from contaminated soil or water (phytoremediation)(26,27) or enrich valuable metals from mineral wastes (phytomining).(28) It is envisioned that doped carbons with high catalytic activity can be obtained from these metal-rich plants.
As a first step toward this direction, we chose spinach to conduct proof-of-principle studies because spinach contains appreciable amounts of N and Fe, among other elements such as S and P,(29,30) which are important elements for carbon-based ORR catalysts. We found that by using spinach as the precursor, melamine as a nitrogen promoter, and NaCl/KCl as the pore producer, multielement-doped carbon with high ORR activity can be obtained through pyrolysis at 900 °C under Ar. The resultant carbon materials have a sheet-like structure with a specific area of 289.6 m2 g–1 and a high density of active sites for ORR, as demonstrated by the high oxygen reduction current peak in the cyclic voltammograms. The polarization measurements further show that the obtained carbon sheets have ORR performance superior to Pt/C in alkaline solutions in terms of half-wave potential, methanol tolerance, and long-term stability. They are also active ORR catalysts in acidic media. The Fe–Nx moiety is the main active center, as supported by the results from the X-ray photoelectron spectroscopy (XPS) measurements and the acid etching studies, as well as from the observations of poisoning effects of cyanide and fluoride. Control experiments revealed that the presence of micro- and mesopores and the sufficiently high density of Fe–Nx active sites are the main contributors to the remarkable ORR performance. This study paves an avenue to preparing high-performing NPGM ORR catalysts by using metal-enriching plants as the precursor.

2. Results and Discussion

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2.1. Synthesis and Characterization of Porous Carbon Nanosheets

A schematic illustration of the preparation procedure of spinach-derived carbon nanosheets is shown in Scheme 1. Fresh spinach leaves were first cleaned and blended into juice, which was then freeze-dried and ground into a fine powder. Afterward, the spinach powder, melamine, NaCl, and KCl were dissolved and mixed in water and heated to 120 °C to form a uniform mixture. An elevated temperature is needed to dissolve melamine because of its relatively low solubility at room temperature.(31) The mixture was then cooled down quickly in liquid nitrogen and freeze-dried to obtain spinach–salt–melamine composites. During this process, the uniformity of the mixture was preserved. The composites were then pyrolyzed at high temperatures, and the metal particles were leached out by soaking in a heated sulfuric acid solution. Finally, the obtained carbon powder was pyrolyzed again at 900 °C for 1 h and denoted as M+S+C900-900.

Scheme 1

Scheme 1. Schematic of the Procedure for Preparing Spinach-Derived Carbon Nanosheets
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Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) micrographs of the spinach-derived carbon prepared by pyrolyzing at 900 °C (M+S+C900-900) are shown in Figure 1. M+S+C900-900 consists of agglomerates of graphene-like carbon sheets with wrinkles and defects. There are many voids created by the dissolution of NaCl/KCl salt crystals. These macropores provide passages for transporting the reactants to active sites, which are beneficial for improving the catalytic activity (vide infra). The high-magnification TEM image in Figure 1e demonstrates that M+S+C900-900 has crystalline domains with a lattice distance of about 0.36 nm, corresponding to the (002) direction of the graphitic carbon lattice. The corresponding selected-area electron diffraction pattern (the inset in Figure 1e) exhibits a sixfold symmetry, confirming that M+S+C900-900 has a graphite-like structure.

Figure 1

Figure 1. Representative SEM (a–c) and TEM (d,e) images of M+S+C900-900. The inset in (e) is an electron diffraction pattern of M+S+C900-900.

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Figure 2a shows the X-ray diffraction (XRD) patterns of M+S+C900-900. The two main peaks at around 25.0 and 44.1° can be assigned to the (002) and (101) planes of graphite (JCPDS card no. 41-1487), respectively,(15,22) which indicate that graphite-like structures were formed after the high-temperature pyrolysis. This result is in good agreement with the TEM observations. To further characterize the structure of M+S+C900-900, a Raman spectrum was recorded. As shown in Figure 2b, two relatively strong peaks were observed at 1332 and 1590 cm–1, which can be assigned to the D and G bands, respectively.(32) The G band arises from the Raman-active in-plane bond-stretching of the sp2 carbon atoms and has an E2g symmetry. The D band is attributed to the ring breathing mode of A1g symmetry. This mode is Raman-inactive in perfect graphite and graphene but becomes active in defective graphite and graphene.(32−34) The observation of these Raman peaks suggests the formation of defective graphite structure after pyrolysis, which is further confirmed by the appearance of a weak broad band centered at 2725 cm–1, assignable to the overtone of the D band.(32) The position of the G and D bands and the lower ID/IG ratio (0.92) suggest the carbon nanosheets are more crystalline than the Vulcan XC (ID/IG = 2.1) used in commercial Pt/C.(34,35) Surface area and pore distribution are the critical parameters for carbon-based catalysts. Figure 2c displays the nitrogen adsorption–desorption isotherm of M+S+C900-900, which exhibits a type IV sorption isotherm with a type H4 hysteresis loop. The corresponding Brunauer–Emmett–Teller (BET) surface area is 289.6 m2 g–1. At low relative pressures (<0.1 P/P0), a sharp increase indicates the presence of micropores. The observation of the type H4 hysteresis loop extending from P/P0 = 0.45 to 1.0 signals the existence of narrow slit-like pores, large mesopores in a matrix with much smaller pores, and pores of irregular shape and broad size distribution.(36) In addition, an obvious tail appeared at P/P0 = ∼1.0, implying the formation of macropores as well.(37−39) This conjecture is corroborated with the pore size distribution (PSD) calculated using the density functional theory (DFT) method shown in Figure 2d, which shows the presence of micropores, mesopores, and macropores. The slit-like pores may arise from the sheet-like structure of M+S+C900-900, as shown in the TEM images in Figure 1. The presence of mesoporous and macroporous structures has been shown to facilitate the diffusion and transport of reactants to the active sites for ORR.(40−42)

Figure 2

Figure 2. Structural characterizations of M+S+C900-900. (a) XRD. (b) Raman spectrum. (c) N2 adsorption–desorption isotherm. (d) PSD. Inset: zoom-in of the PSD. (e) XPS survey spectrum. (f–i) High-resolution XPS spectrum of: (f) C 1s, (g) N 1s, (h) S 2p, and (i) Fe 2p.

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XPS measurements were employed to characterize the chemical states of different elements in M+S+C900-900. As shown in Figure 2e, the full survey XPS spectrum reveals that M+S+C900-900 is mainly composed of carbon, oxygen, and nitrogen and trace amounts of iron, sulfur, and phosphorus. The atomic percentage of each element is summarized in Table S1. The high-resolution C 1s, N 1s, S 2p, and Fe 2p XPS spectra are shown in Figure 2f–i. The C 1s spectrum can be resolved into four peaks at the binding energies of 284.5, 285.6, 286.5, and 289.0 eV (Figure 2f), representing graphitic carbon, C–N, C–O, and O–C═O species, respectively.(43) The N 1s spectrum can be fitted by multicomponent peaks. The peaks at 397.6 and 398.2 eV (Figure 2g) can be assigned to imine and pyridinic N.(44,45) The peak at 399.3 is attributed to Fe-coordinated N (Fe–Nx),(46,47) which is highly ORR-active.(45,48,49) The other peaks at 399.9, 400.9, and 402.6 eV arise from pyrrolic N, graphitic N, and oxidized N, respectively.(10,47,50) As shown in Figure 2h, the high-resolution S 2p XPS spectrum can be deconvoluted into several peaks. The two peaks at binding energies of around 162.0 and 163.0 eV can be attributed to the spin–orbit coupling of S 2p3/2 and S 2p1/2 for thiol sulfurs.(51,52) The peaks at binding energies of 163.9 and 165.1 eV can be assigned to the spin–orbit coupling positions of 2p3/2 and 2p1/2 for thiophene S that has been reported active for ORR.(53,54) A strong peak at the binding energy of 164.7 eV is from sulfur in conjugated C═S bonds.(55) The peaks at 165.8, 166.7, 167.5, and 168.0 eV are assigned to the oxidized sulfur groups (−C–SOx–C–, x = 2–4, 165.0–171.5 eV).(56,57) The Fe 2p spectrum (Figure 2i) displays two broad features centered around 711 and 725 eV, and each can be deconvoluted into two peaks at 710.6, 713.6 and at 723.0, 725.3 eV, respectively. These binding energies are very close to those observed on various iron oxides(58) and an iron-coordinated nitrogen-doped carbon catalyst.(45) Following these previous reports, we assign the 710.6 and 723.0 eV peaks to Fe 2p3/2 and Fe 2p1/2 from an Fe(II) species and the 713.6 and 725.3 eV peaks to Fe 2p3/2 and Fe 2p1/2 of an Fe(III) species. The peak at 716.8 eV is identified as a satellite peak corresponding to Fe 2p3/2.(53) The P2p signal is too weak for a meaningful peak fitting.

2.2. ORR Activity and Stability

To test the ORR performance of M+S+C900-900, cyclic voltammograms were first recorded in 0.1 M KOH saturated with N2 and O2, respectively (Figure 3a). The current was normalized to the geometric area of the glassy carbon supporting electrode. A quasi-rectangular voltammogram was observed in N2-saturated 0.1 M KOH, which suggests that only capacitive charging current was present. In contrast, when the solution was saturated with O2, a very strong reduction peak appeared at +0.88 V, indicating the effective electrochemical reduction of oxygen. The high magnitude of the O2 reduction peak current density at a relatively low scan rate (10 mV s–1) suggests the existence of abundant accessible active sites on the catalyst. Rotating ring-disk electrode (RRDE) measurements were carried out to gain further insights into the ORR performances of M+S+C900-900. A representative result is shown in Figure 3b along with that from a commercial Pt/C (20 wt % Pt) of the same mass loading as a benchmark. For clarity, only the disk current density is shown. Clearly, M+S+C900-900 exhibited a much higher ORR activity than Pt/C. Thus, M+S+C900-900 has a half-wave potential (E1/2) of +0.88 V, which is 20 mV more positive than that of Pt/C, whereas the two catalysts have a similar ORR onset potential (Eo). Similar ORR activities were also obtained on carbon materials derived from spinach purchased from different supermarkets (Figure S1), suggesting the high ORR performance is insensitive to the source of spinach. Compared with other NPGM ORR catalysts, including those derived from biomass, M+S+C900-900 is the most active, especially after the differences in surface area and catalyst loading are considered (Table S2). For example, mesoporous carbon derived from okara showed an ORR E1/2 of +0.86 V, and the same was observed on lysine-derived nitrogen-doped carbon hollow cubes.(17,59) These values are 20 mV more negative than that of M+S+C900-900. As mentioned in the outset, a variety of biomass have been used as sources for preparing carbon-based ORR catalysts.(60) One advantage of spinach is that it naturally contains a high amount of Fe, which has been extensively demonstrated to be an important component of ORR active sites.(1,2) In the present case, no additional Fe source was used in preparing the carbon nanosheets, unlike okara- and catkin-derived carbons where FeCl3 was used to increase the Fe content.(11,17) This notion suggests that high ORR-performing plant-derived carbon catalysts containing active metal centers could be obtained from selected metal-enriching plants, such as those proposed for phytoremediations(26,27) or phytomining.(28)

Figure 3

Figure 3. Oxygen reduction studies of M+S+C900-900 in 0.1 M KOH. (a) Cyclic voltammograms recorded in N2- and O2-saturated solutions; scan rate: 10 mV s–1. (b) Linear sweep voltammetry (LSV) curves obtained in O2-saturated solutions; electrode rotation rate: 1600 rpm and potential scan rate: 10 mV s–1. (c) Plot of the number of electron transfer (n) vs the electrode potential. (d) LSV curves recorded in an O2-saturated solution at various rotation rates. (e) Koutecky–Levich (K–L) plots. (f) Tafel plots. Results from Pt/C are included for comparison.

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The high ORR activity of M+S+C900-900 was further demonstrated by the number of electron transfer (n) and the HO2– percent yield involved in ORR. The n value and HO2– percent yield at different potentials were calculated by using eqs 3 and 4, respectively. As shown in Figure 3c, for M+S+C900-900, the average n was ca. 3.91, comparable to that of Pt/C (n = 3.97), and the HO2– yield was <5% in the potential range from +0.20 to +0.80 V (see Figure S2). To further examine the ORR activity, we also performed RRDE measurements at various rotation speeds from 400 to 2500 rpm. As expected from the K–L equation (eq 1), the limiting current density increased with the increasing rotation speed (Figure 3d). The corresponding K–L plots (j–1 vs ω–1/2) derived from the RRDE voltammograms are presented in Figure 3e. The K–L plots from +0.61 to +0.81 V display good linearity with a similar slope, indicating the first-order reaction kinetics for ORR with respect to the dissolved oxygen concentration. To shed light on the mechanism of the ORR on M+S+C900-900, Tafel analysis was performed. As shown in Figure 3f, the Tafel slope for M+S+C900-900 is 61.5 mV dec–1, which is lower than that for Pt/C of 63.1 mV dec–1. These values are close to the theoretical value of 60 mV dec–1 when the transfer of the first electron is the rate-determining step in the ORR process.(47) M+S+C900-900 also showed a strong ORR activity in acidic media. As shown in Figure S3, the LSV curve of M+S+C900-900 has an E1/2 of +0.75 V, which is only 98 mV more negative than that from Pt/C in O2-saturated 0.1 M HClO4.
Selectivity and durability of the catalysts are important performance metrics for their practical applications. To investigate the methanol crossover effects, chronoamperometric responses (i–t curves) were recorded in O2-saturated 0.1 M KOH at +0.88 V with the addition of methanol into the electrolyte solution to make up a 1 M methanol solution. As shown in Figure 4a, M+S+C900-900 is insensitive to methanol, indicating the excellent selectivity to ORR over methanol oxidation, whereas Pt/C shows a high methanol oxidation current, as indicated by the appearance of a large oxidation current (opposite to the ORR current) after the addition of methanol. The durability of M+S+C900-900 and Pt/C was also evaluated and compared by chronoamperometric measurements, with an electrode rotation speed of 900 rpm in O2-saturated 0.1 M KOH. As depicted in Figure 4b, the resultant chronoamperometric profile for M+S+C900-900 exhibited a nearly invariant current response after the initial 15% drop within the first hour. The current remained at 83% of the initial value even after 10 h of continuous operation. In stark contrast, the activity of Pt/C kept decreasing, and only 57% of the initial current remained after 10 h under the same experimental conditions. The result indicated a markedly higher stability of M+S+C900-900 than the commercial Pt/C catalyst in the alkaline solution. M+S+C900-900 also exhibited a remarkable long-term stability. The RDE measurements showed almost no change of limiting current and half-wave potential after the M+S+C900-900 modified electrode was immersed in the electrolyte for 2 months (Figure 4c). In contrast, the commercial Pt/C displayed a 44 mV negative shift of the half-wave potential in the same period (Figure 4d).

Figure 4

Figure 4. Methanol tolerance and stability comparison between M+S+C900-900 and Pt/C in O2-saturated 0.1 M KOH. (a) Current–time (i–t) responses at +0.88 V, with the addition of 1 M methanol at 1200 s. (b) Normalized current–time profiles at +0.88 V, at a rotation rate of 900 rpm. (c,d) LSV curves obtained with freshly prepared catalysts (solid traces) and after storage in the electrolyte solution for 60 days (dashed traces): (c) M+S+C900-900 and (d) Pt/C. Electrode rotation rate: 1600 rpm and potential scan rate: 10 mV s–1.

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2.3. ORR Active Sites

To probe the nature of the ORR catalytic active sites, we measured the ORR activity of M+S+C900-900 in 0.1 M KOH containing 10 mM KCN. As can be seen in Figure 5a, the addition of KCN negatively shifts the onset and half-wave potentials by ∼73 and ∼79 mV, respectively. In addition, the diffusion-limited current decreased 6% at +0.4 V. These observations indicate that the iron centers are the active sites, and the decreased activity is attributed to the strong binding of CN– ions to the iron centers.(53,61) This assertion is further confirmed by the 41 mV negative shift of E1/2 after the addition of 5 mM of NaF to O2-saturated 0.1 M KOH (Figure 5b). The decrease of ORR activity by these anions has been attributed to their high affinity to the iron centers that compete with O2 binding to these sites.(62−64) This hypothesis cannot explain the observation of the minute decrease of diffusion-limited current accompanied with the tens of millivolts negative shift of E1/2 in the present and previous studies by others.(62,63) Recent Mössbauer spectroscopic and in situ X-ray absorption spectroscopic studies by Mukerjee and others revealed that the active site for Fe–N–C catalysts is an Fe2+–N4 moiety that is formed by the reduction of Fe3+–N4.(65,66) Therefore, the ORR activity in terms of E1/2 is dictated by the Fe2+/3+ redox potential. This active site model is consistent with the earlier studies of Fe and Co porphyrins by Anson and co-workers(67,68) and can reconcile the large negative shift of E1/2 and the small decrease of the limiting current. Thus, the negative E1/2 arises from the much larger binding constant of these anions to Fe3+ than to Fe2+, as has been shown extensively in iron porphyrins and hemes,(69,70) and thereby the formation of the ORR active center Fe2+–N4 is shifted to more negative potentials. The small decrease of the limiting current can be accounted for by the competitive binding of these anions and O2 to the Fe2+–N4 center. The critical role of Fe species in the high ORR activity was further demonstrated by the chemical etching of M+S+C900-900 with 12 M HCl. The ORR activity of M+S+C900-900 diminished markedly after the etching, with an 81 mV negative shift of the onset potential and a 17% decrease of the limiting current (Figure S4). These results clearly indicate that the Fe species play an indispensable role in the high ORR activity of M+S+C900-900 and provide additional support for the hypothesis of the Fe2+–N4 active center discussed above.

Figure 5

Figure 5. (a) LSV curves of M+S+C900-900 recorded before (red) and after (black) the addition of (a) 10 mM KCN and (b) 5 mM NaF in O2-saturated 0.1 M KOH. Electrode rotation rate: 1600 rpm and potential scan rate: 10 mV s–1.

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2.4. Factors Affecting ORR Activity

To aid in dissecting how different factors in the synthesis procedure contribute to the high performance of the obtained carbon-based catalysts, various controlled experiments were performed. The influence of the pyrolysis temperature on the electrocatalytic properties was first explored. As shown in Figure 6a, the ORR activity for the catalysts prepared under the same protocol (first heat treatment followed by an acid wash and second heat treatment) but with different pyrolysis temperatures (800 °C for M+S+C800-800 and 1000 °C for M+S+C1000-1000) is significantly inferior to that of M+S+C900-900, which displays more negative half-wave potentials and lower limiting current densities. The higher activity of M+S+C900-900 can at least partly be attributed to its higher surface area and pore volume (see Figure S5 and Table S3), which could expose more active sites for ORR, and facilitate mass transport through the porous structures. In addition, the variation of chemical speciation at different temperatures can strongly influence the catalyst activity. As disclosed by the XPS measurements, the percentage of the overall N and O contents decreased with the increasing pyrolysis temperature, whereas the Fe and C contents increased (Table S1). These observations agree well with that reported on catalysts prepared by pyrolyzing polyaniline.(71) Further inspection of the high-resolution N 1s XPS spectrum revealed that catalysts prepared at 900 °C have the highest percentage of graphitic and Fe–N species (Table S4). As discussed above, the Fe–N species is the most active site in NPGM catalysts. This result adds an explanation for the optimal pyrolysis temperature at 900 °C.(71,72)

Figure 6

Figure 6. Results from controlled experiments. (a) Effects of pyrolysis temperature. (b,c) Effects of second heat treatment at 900 °C: (b) LSV and (c) EIS. (d) Effects of different reagents on the starting materials. The LSV curves were recorded in O2-saturated 0.1 M KOH with a rotation speed of 1600 rpm and a potential scan rate of 10 mV s–1.

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We also attempted to explore the effects of the second heat treatment in the catalyst preparation on the catalytic activity by comparing the ORR activities of the samples before and after the second heat treatment. As shown in Figure 6b, the sample subjected to the second heat treatment (M+S+C900-900) exhibited dramatically improved activity in both kinetic-controlled and mass transport-controlled potential regions. The onset potential shifted from +0.91 V for M+S+C900 to +0.98 V for M+S+C900-900, and the limiting current density also increased significantly. We would like to point out that the sample without the second heat treatment was acid-washed; therefore, the observed difference was from the second heat treatment alone. It has been shown by others that a second heat treatment could remove oxygen-containing functional groups and corrosive carbon species from the material surface and improve the degree of graphitization, thereby further enhancing the ORR activity.(72,73) The increased degree of graphitization can increase the electric conductivity of the catalyst and therefore improve the catalytic activity.(10,72,74) The electrochemical impedance spectroscopy (EIS) results in Figure 6c show that the serial resistance (Rs) of M+S+C900-900 (47.6 Ω) is lower than that of M+S+C900 (62.9 Ω), indicating the conductivity of the catalysts improved after the second heat treatment. We also varied the temperature of the first heat treatment from 800 to 1000 °C but kept the second heat treatment at 900 °C (Figure S6). Interestingly, the ORR onset potentials for these samples are largely the same, unlike those in Figure 6a where the onset potentials for the samples prepared at 800 and 1000 °C are significantly more negative than the one prepared at 900 °C, suggesting that the second heat treatment at 900 °C is critical for obtaining the high ORR activity. It has been postulated that the second heat treatment may repair the disrupted active sites.(71,73) We speculate that the lower temperature is unable to repair the disrupted active sites, and the higher temperature may destroy the Fe–N active sites. Detailed surface characterizations are needed to further digest these results.
Control experiments were also performed to reveal the roles of NaCl/KCl and melamine in forming the high-performance spinach-derived carbon materials. From the ORR polarization curves shown in Figure 6d, on the sample prepared under identical conditions as that for M+S+C900-900, but in the absence of NaCl/KCl and melamine (M–S–C900-900), the onset potential was significantly shifted to a negative potential at +0.84 V, and the limiting current was reduced to half of that observed on M+S+C900-900. The sample prepared with the addition of melamine, but no NaCl/KCl, to spinach showed slightly improved ORR onset potential and limiting current (M+S–C900-900). Adding NaCl/KCl mixture to spinach in the absence of melamine (M–S+C900-900) further increased the ORR limiting current. These performance differences can be understood in terms of surface area, pore formation, and nitrogen content in the samples. Nitrogen adsorption–desorption isotherms clearly reveal that NaCl/KCl crystals play a major role in forming mesopores and increasing the surface area (Table S3). The BET surface area for M–S–C900-900 is only 40.3 m2 g–1, with a pore volume of 0.33 cm3 g–1. Most pores are micropores. With the addition of the NaCl and KCl salts, the surface area increased to 175.5 m2 g–1 with a pore volume of 0.70 cm3 g–1 (M–S+C900-900). Most pores are mesopores. Chen and co-workers have shown that NaCl can facilitate the formation of mesopores and act as a template to form macropores.(75,76) Our results agree well with these previous studies. A close inspection of Table S3 further reveals that melamine also plays a role in increasing the surface area and pore volume. The surface area of M+S–C900-900 is more than 60% higher than that of M–S–C900-900. Similarly, the surface area of M+S+C900-900 is about 65% higher than that of M–S+C900-900. The only difference in the two pairs is the absence of melamine in M–S–C900-900 and M–S+C900-900. Accompanied with the increasing surface area is the pore volume. The micropore and mesopore areas increased significantly after the addition of melamine. These structural changes most likely arise from the formation of volatile compounds in melamine decomposition.(77,78) It has been shown that a combination of micropores and mesopores in Fe, N-doped carbon material can boost the ORR activity.(79) The micropores are where the active sites reside,(79−81) and the mesopores facilitate the mass transport of the reactants. Melamine has also been extensively used as a source of nitrogen in the formation of high-performance Fe, N-doped ORR catalysts(82−85) because its decomposition at elevated temperatures releases NH3 gas, which is commonly used for doping nitrogen to carbon materials.(1) As revealed by the XPS measurements (Figure S7 and Tables S1 and S4), samples prepared with the addition of melamine (M+S–C900-900 and M+S+C900-900) have significantly higher nitrogen contents than the one without melamine (M–S–C900-900 and M–S+C900-900), which increase the density of the active sites.

3. Conclusions

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We have developed an effective synthetic strategy to fabricate spinach-derived heteroatom-doped porous carbon sheets. These materials exhibit ORR performance superior to Pt/C in alkaline media in terms of half-wave and onset potentials, longtime, stability and resistance to methanol poisoning. They also show ORR activity close to that of Pt/C in acidic media. The control experiments reveal the active centers as Fe–Nx moieties and the important roles of micro- and mesopores. Given that many plants have the ability to accumulate metals, we envision that they can be used not only for metal remediation from contaminated environments but also as a natural precursor for the preparation of heteroatom-doped carbon materials as catalysts for various reactions and electrode materials for electrochemical energy storage.(86)

4. Experimental Section

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4.1. Chemicals

Spinach was purchased from various local supermarkets. Sodium chloride (NaCl, ≥99.5%), potassium chloride (KCl, ≥99%), potassium cyanide (KCN, ≥98.0%), sodium fluoride (NaF, ≥99%), Nafion solution (5%), and potassium hydroxide (KOH, 85%), were purchased from Sigma-Aldrich. Double-distilled perchloric acid (HClO4, 70%) and sulfuric acid (H2SO4, 95–98%) were procured from GFS Chemicals. Melamine (99%) was purchased from Alfa Aesar. Commercial 20 wt % Pt/C was obtained from Fuel Cell Store. Water was supplied from a Milli-Q water purification system (18.2 MΩ cm).

4.2. Catalyst Preparation

The spinach leaves were washed with water and ground into juice using a kitchen blender. The juice was freeze-dried for 48 h with a Labconco FreeZone freeze-dryer and ground into powder with a mortar and pestle. A 0.50 g of the obtained spinach powder, 0.50 g melamine, 3.0 g KCl, 3.0 g NaCl, and 20.0 mL deionized water were added into a beaker and mixed with magnetic stirring. The beaker was covered with a parafilm, and the mixture was stirred and heated to 120 °C until a uniform mixture was formed. The resulting mixture was quickly transferred to centrifuge tubes and frozen with liquid nitrogen and then freeze-dried for 48 h. The samples were then collected and ground into fine powders. Subsequently, the composite powders were pyrolyzed in an Ar atmosphere according to the following heating program: (1) at 500 °C for 2 h at a ramp rate of 5 °C min–1 to dehydrate and carbonize the composite powders; (2) at 900 °C for 2 h to further carbonize and graphitize the material. Finally, the obtained sample was soaked in 0.5 M H2SO4 at 85 °C for 24 h to remove metallic particles, followed by filtration with water and freeze-drying for 24 h. The freeze-dried sample was calcined at 900 °C for 1 h in Ar flow. The obtained sample is denoted as M+S+C900-900, where M and S represent melamine and salt, respectively, and the presence or absence of melamine and salt is presented with “+” and “–”. The first “900” indicates the first pyrolysis temperature and the second “900” denotes the second pyrolysis temperature. Similar notation method is used for control samples, as noted in the text.

4.3. Characterization

SEM observation was performed on a field emission scanning electron microscope (Hitachi S-4800). High-resolution TEM images were obtained using a JEOL-2100F microscope operated under an accelerating voltage of 200 kV. XRD data were collected at room temperature using a Bruker D8 diffractometer with Cu Kα radiation (λ = 0.1541 nm). Raman spectra were recorded on a confocal LabRAM HR800 spectrometer (Horiba) with an excitation wavelength of 514.5 nm from an argon ion laser. N2 adsorption–desorption isotherms and PSD were obtained at 77 K using a Micromeritics ASAP 2020 Plus instrument. The specific surface areas of the samples were calculated using the BET method. The PSD was calculated via the DFT model. XPS measurements were conducted on a PHI X-tool instrument with Al Kα radiation.

4.4. Electrochemical Measurements

The electrochemical measurements were carried out on a CHI700C electrochemical workstation (CH instruments Inc.) with a three-electrode two-compartment glass cell at room temperature. A platinum wire and an Ag/AgCl (1 M KCl) electrode were used as the counter and reference electrodes, respectively. A RRDE with a glassy carbon disk (5.0 mm in diameter) surrounded by a platinum ring (with a collection efficiency of 20%) served as the working electrode. The working electrode was placed in the main compartment of the electrochemical cell that is separated from the compartment housing the counter and reference electrodes by a fine glass frit to minimize contaminations. To prepare the catalyst ink, 2 mg of the carbon sample was ultrasonically dispersed in 1.0 mL of a 7:3 (v/v) ethanol/water mixture solvent along with 20 μL of Nafion solution (5%) for 30 min. Then, 15 μL of the catalyst ink was dropped onto the glassy carbon disk and dried at room temperature for 30 min at a catalyst loading of 0.15 mg cm–2. For comparison, commercial Pt/C was loaded onto the glassy carbon electrode surface using the same procedure and catalyst loading. The RRDE tests were carried out in N2- or O2-saturated 0.1 M KOH or 0.1 M HClO4, and the scan rate was 10 mV s–1. In all of the measurements, the Ag/AgCl reference electrode was calibrated with respect to a homemade reversible hydrogen electrode (RHE), using E(RHE) = E(Ag/AgCl) + 0.982 V in 0.1 M KOH and E(RHE) = E(Ag/AgCl) + 0.288 V in 0.1 M HClO4. The disk current density was normalized to the geometric surface area of the glassy carbon electrode.
The LSV curves were recorded in O2-saturated 0.1 M KOH, with varying electrode rotation rates from 400 to 2500 rpm. The K–L plots (j–1 vs ω–1/2) were analyzed at various potentials according to eqs 1 and 2(1)(2)where j, jK, and jD are the measured, kinetic, and diffusion-limited current densities, respectively. ω is the angular velocity of the rotating electrode, n is the number of electron transfer involved in the ORR, F is the Faraday constant (96485 C mol–1), D is the oxygen diffusion coefficient (1.9 × 10–5 cm2 s–1), ν is the kinematic viscosity of the electrolyte solution (0.01 cm2 s–1), and CO2 is the bulk concentration of oxygen (1.2 × 10–3 mol L–1).
The number of electron transfer (n) and HO2– percent yield at different potentials were calculated by eqs 3 and 4(3)(4)where ID represents the disk current, IR is the ring current, and N is the RRDE collection efficiency (20%).

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.0c02673.

  • Elemental contents in different carbon catalysts determined by XPS measurements; comparison of the catalytic performances of M+S+C900-900 and noble metal-free carbon materials in the literature; results from nitrogen adsorption−desorption analysis; comparison of nitrogen species in different carbon catalysts; LSV curves of M+S+C900-900 from different spinach; hydrogen peroxide yield of M+S+C900-900 and Pt/C during the ORR process; RDE voltammograms for ORR of M+S+C900-900 and 20 wt % Pt/C in 0.1 M HClO4; LSV curves of M+S+C900-900 before and after acid leaching in O2-saturated 0.1 M KOH; N2 adsorption–desorption isotherms at 77 K for different samples; LSV curves of different catalysts prepared under different first heat treatments and the same second heat treatment at 900 °C; and XPS survey spectra and deconvolutions of N1s spectra of M+S+C800-800, M-S+C900-900, M-S-C900-900, M+S-C900-900, and M+S+C1000-1000 (PDF)

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Author Information

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  • Corresponding Author
  • Authors
    • Xiaojun Liu - Department of Chemistry, American University, Washington, District of Columbia 20016, United States;  Present Address: Department of Chemistry, Oakland University, Rochester, MI, 48309, USA
    • Casey Culhane - Department of Chemistry, American University, Washington, District of Columbia 20016, United States
    • Wenyue Li - Department of Chemistry, American University, Washington, District of Columbia 20016, United States;  Present Address: Department of Electrical & Computer Engineering, Texas Tech University, Lubbock, TX 79409, USA
  • Author Contributions

    X.L.: investigation, data curation, formal analysis, writing—original draft. C.C.: data curation, validation, review and editing. W.L.: data curation, validation, review and editing. S.Z.: conceptualization, resources, supervision, funding acquisition, writing—review and editing.

  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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This work was partially supported by the American University.

References

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    Biomass-derived nitrogen self-doped porous carbon as effective metal-free catalysts for oxygen reduction reaction
    Liu, Xiaojun; Zhou, Yucheng; Zhou, Weijia; Li, Ligui; Huang, Shaobin; Chen, Shaowei
    Nanoscale (2015), 7 (14), 6136-6142CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
    Biomass-derived nitrogen self-doped porous carbon was synthesized by a facile procedure based on simple pyrolysis of water hyacinth (eichhornia crassipes) at controlled temps. (600-800 °C) with ZnCl2 as an activation reagent. The obtained porous carbon exhibited a BET surface area up to 950.6 m2 g-1, and various forms of nitrogen (pyridinic, pyrrolic and graphitic) were found to be incorporated into the carbon mol. skeleton. Electrochem. measurements showed that the nitrogen self-doped carbons possessed a high electrocatalytic activity for ORR in alk. media that was highly comparable to that of com. 20% Pt/C catalysts. Exptl., the best performance was identified with the sample prepd. at 700 °C, with the onset potential at ca. +0.98 V vs. RHE, that possessed the highest concns. of pyridinic and graphitic nitrogens among the series. Moreover, the porous carbon catalysts showed excellent long-term stability and much enhanced methanol tolerance, as compared to com. Pt/C. The performance was also markedly better than or at least comparable to the leading results in the literature based on biomass-derived carbon catalysts for ORR. The results suggested a promising route based on economical and sustainable biomass towards the development and engineering of value-added carbon materials as effective metal-free cathode catalysts for alk. fuel cells.
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  13. 13
    Pan, F.; Cao, Z.; Zhao, Q.; Liang, H.; Zhang, J. Nitrogen-doped porous carbon nanosheets made from biomass as highly active electrocatalyst for oxygen reduction reaction. J. Power Sources 2014, 272, 8– 15,  DOI: 10.1016/j.jpowsour.2014.07.180
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    13
    Nitrogen-doped porous carbon nanosheets made from biomass as highly active electrocatalyst for oxygen reduction reaction
    Pan, Fuping; Cao, Zhongyue; Zhao, Qiuping; Liang, Hongyu; Zhang, Junyan
    Journal of Power Sources (2014), 272 (), 8-15CODEN: JPSODZ; ISSN:0378-7753. (Elsevier B.V.)
    The successful commercialization of fuel cells requires the efficient electrocatalyst to make the O redn. reaction (ORR) fast because of the sluggish nature of ORR and the high cost of the Pt catalysts. The authors report the excellent performance of metal-free N-doped porous C nanosheets (NPCN) with hierarchical porous structure and a high surface area of 1436.02 m2 g-1 for catalyzing ORR. The active NPCN is synthesized via facile high-temp. carbonization of natural ginkgo leaves followed by purifn. and NH3 post-treatment without using addnl. supporting templates and activation processes. In O2-satd. 0.1 M KOH soln., the resultant NPCN exhibits a high kinetic-limiting c.d. of 13.57 mA cm-2 at -0.25 V (vs. Ag/AgCl) approaching that of the com. Pt/C catalyst (14 mA cm-2) and long-term electrochem. stability. Notably, the NPCN shows a slightly neg. ORR half-wave potential in comparison with Pt/C (ΔE1/2 = 19 mV). The excellent electrocatalytic properties of NPCN originate from the combined effect of optimal N doping, high surface area, and porous architecture, which induce the high-d. distribution of highly active and stable catalytic sites.
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  14. 14
    Zhang, P.; Gong, Y.; Wei, Z.; Wang, J.; Zhang, Z.; Li, H.; Dai, S.; Wang, Y. Updating Biomass into Functional Carbon Material in Ionothermal Manner. ACS Appl. Mater. Interfaces 2014, 6, 12515– 12522,  DOI: 10.1021/am5023682
    [ACS Full Text ACS Full Text], [CAS], Google Scholar
    14
    Updating Biomass into Functional Carbon Material in Ionothermal Manner
    Zhang, Pengfei; Gong, Yutong; Wei, Zhongzhe; Wang, Jing; Zhang, Zhiyong; Li, Haoran; Dai, Sheng; Wang, Yong
    ACS Applied Materials & Interfaces (2014), 6 (15), 12515-12522CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
    The development of meaningful ways to transfer biomass into useful materials, more efficient energy carriers, and/or carbon storage deposits is a profound challenge of our days. Herein, an ionothermal carbonization (ITC) method, via treating natural resources (glucose, cellulose, and sugar cane bagasse) in nonmetal ionic liqs. (ILs) at ∼200 °C, is established for the fabrication of porous heteroatom-doped carbon materials with high yield. Com. ILs with bulky bis(trifluoromethylsulfonyl)imide anion or cross-linkable nitrile group were found to be efficient and recyclable templates for porosity control, leading to exciting nanoarchitectures with promising performance in oxygen redn. reaction. The optimized ILs (12 mL) can dissolve and directly convert up to 15 g of glucose into porous carbon materials (SBET: 272 m2/g) one time. This ITC method relies on the synergistic use of structure-directing effect, good biomass soly., and excellent thermal stability of ILs, and provides a sustainable strategy for exploiting biomass.
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  15. 15
    Gao, S.; Chen, Y.; Fan, H.; Wei, X.; Hu, C.; Luo, H.; Qu, L. Large scale production of biomass-derived N-doped porous carbon spheres for oxygen reduction and supercapacitors. J. Mater. Chem. A 2014, 2, 3317– 3324,  DOI: 10.1039/c3ta14281g
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    15
    Large scale production of biomass-derived N-doped porous carbon spheres for oxygen reduction and supercapacitors
    Gao, Shuyan; Chen, Yanli; Fan, Hao; Wei, Xianjun; Hu, Chuangang; Luo, Hongxia; Qu, Liangti
    Journal of Materials Chemistry A: Materials for Energy and Sustainability (2014), 2 (10), 3317-3324CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
    The urgent need for sustainable energy development depends on the progress of green technologies, which have steered hot research areas into environmentally benign approaches via inexpensive precursors and abundant resources obtained directly from nature for energy devices such as fuel cells and supercapacitors. Using fermented rice as a starting material, a facile, green and scalable approach to synthesize porous N-doped carbon spheres was demonstrated, which was characterized by high sp. surface areas (2105.9 m2 g-1) and high porosity (1.14 cm3 g-1), which exhibit not only excellent electrocatalytic activity toward the four-electron oxygen redn. reaction with long-term stability for fuel cells, but also have excellent resistance to crossover effects and CO poisoning superior to that of the com. Pt/C catalyst. Furthermore, the naturally derived porous N-doped carbon spheres, used as the active electrode materials, present superior performance for capacitors with a capacitance of 219 F g-1 at a high discharge c.d. of 15 A g-1 and good cycling stability for over 4400 cycles. This work shows a good example for taking advantage of the abundant resources provided by nature, and opening the door for the creation of functional materials with promising applications in high-performance renewable devices related to energy conversion and storage.
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  16. 16
    Li, J.-C.; Hou, P.-X.; Zhao, S.-Y.; Liu, C.; Tang, D.-M.; Cheng, M.; Zhang, F.; Cheng, H.-M. A 3D bi-functional porous N-doped carbon microtube sponge electrocatalyst for oxygen reduction and oxygen evolution reactions. Energy Environ. Sci. 2016, 9, 3079– 3084,  DOI: 10.1039/c6ee02169g
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    16
    A 3D bi-functional porous N-doped carbon microtube sponge electrocatalyst for oxygen reduction and oxygen evolution reactions
    Li, Jin-Cheng; Hou, Peng-Xiang; Zhao, Shi-Yong; Liu, Chang; Tang, Dai-Ming; Cheng, Min; Zhang, Feng; Cheng, Hui-Ming
    Energy & Environmental Science (2016), 9 (10), 3079-3084CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)
    A flexible, large-area three-dimensional porous N-doped carbon microtube (NCMT) sponge was prepd. via a simple and low-cost process of pyrolyzing facial cotton. Due to its unique structure with a micron-scale hollow core and well-graphitized and interconnected porous walls, the NCMT sponge exhibits incomparable electrocatalytic activity for the oxygen redn. reaction (ORR) and the oxygen evolution reaction (OER) with a small p.d. of 0.63 V between the OER c.d. at 10 mA cm-2 and the ORR c.d. at -3 mA cm-2, which is the best to date.
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  17. 17
    Wang, R.; Wang, H.; Zhou, T.; Key, J.; Ma, Y.; Zhang, Z.; Wang, Q.; Ji, S. The enhanced electrocatalytic activity of okara-derived N-doped mesoporous carbon for oxygen reduction reaction. J. Power Sources 2015, 274, 741– 747,  DOI: 10.1016/j.jpowsour.2014.10.049
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    17
    The enhanced electrocatalytic activity of okara-derived N-doped mesoporous carbon for oxygen reduction reaction
    Wang, Rongfang; Wang, Hui; Zhou, Tianbao; Key, Julian; Ma, Yanjiao; Zhang, Zheng; Wang, Qizhao; Ji, Shan
    Journal of Power Sources (2015), 274 (), 741-747CODEN: JPSODZ; ISSN:0378-7753. (Elsevier B.V.)
    Nitrogen-doped carbon (N-C) catalysts can potentially offer high ORR (oxygen redn. reaction) electrocatalytic activity comparable to Pt/C catalysts. Here, we establish a correlation between N-species (pyridinic-N and graphitic-N) with high ORR activity and a key role for Fe in their prepn. N-C catalysts are prepd. from okara (a cheap, nitrogen-rich, biomass precursor) using a facile synthesis method with inclusion of FeCl3 at different steps of synthesis. Mesoporous N-C catalyst is produced that had ORR activity comparable to that of com. Pt/C catalyst. High ORR-activity N-C results from the presence of FeCl3 at a specific step during synthesis. Detailed investigation by XPS reveals that increased levels of pyridinic-N and graphitic-N arose from pyridinic-N-oxide conversion in the presence of Fe. We conclude that transforming inert N species to active N species underlies the increase in active catalytic sites on the carbon surface and offers a means to improve N-C catalyst performance.
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  18. 18
    Borghei, M.; Laocharoen, N.; Kibena-Põldsepp, E.; Johansson, L.-S.; Campbell, J.; Kauppinen, E.; Tammeveski, K.; Rojas, O. J. Porous N,P-doped carbon from coconut shells with high electrocatalytic activity for oxygen reduction: Alternative to Pt-C for alkaline fuel cells. Appl. Catal., B 2017, 204, 394– 402,  DOI: 10.1016/j.apcatb.2016.11.029
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    18
    Porous N,P-doped carbon from coconut shells with high electrocatalytic activity for oxygen reduction: Alternative to Pt-C for alkaline fuel cells
    Borghei, Maryam; Laocharoen, Nikorn; Kibena-Poldsepp, Elo; Johansson, Leena-Sisko; Campbell, Joseph; Kauppinen, Esko; Tammeveski, Kaido; Rojas, Orlando J.
    Applied Catalysis, B: Environmental (2017), 204 (), 394-402CODEN: ACBEE3; ISSN:0926-3373. (Elsevier B.V.)
    This study introduces a new, environmentally-friendly method to synthesize N,P-doped porous carbon by high conversion (46% yield) of coconut shell residues for the redn. of oxygen in alk. media. The obtained materials display an excellent electrocatalytic activity, making them suitable as cathode catalyst for alk. fuel cells. The synthesis procedure included an efficient single-step activation with phosphoric acid to achieve high surface area (1216 m2 g-1) and pore vol. (1.15 cm3 g-1 with 72% mesopores). Urea was used as a low-cost and ecol.-sound source for nitrogen doping of the as-synthesized porous carbon. Remarkably, the biomass-derived electroactive carbon demonstrates a superior performance compared to a ref. material, the state-of-the-art com. Pt-C catalyst: (a) comparable electrocatalytic activity; (b) better tolerance to methanol crossover effects and, (c) improved long-term durability towards oxygen redn. reaction in alk. media.
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  19. 19
    Chaudhari, K. N.; Song, M. Y.; Yu, J.-S. Transforming Hair into Heteroatom-Doped Carbon with High Surface Area. Small 2014, 10, 2625– 2636,  DOI: 10.1002/smll.201303831
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    19
    Transforming Hair into Heteroatom-Doped Carbon with High Surface Area
    Chaudhari, Kiran N.; Song, Min Young; Yu, Jong-Sung
    Small (2014), 10 (13), 2625-2636CODEN: SMALBC; ISSN:1613-6810. (Wiley-VCH Verlag GmbH & Co. KGaA)
    We present a unique approach to dispose of human hair by pyrolyzing it in a regulated environment, yielding highly porous, conductive hair carbons with heteroatoms and high surface area. α-Keratin in the protein network of hair serves as a precursor for the heteroatoms and C. The C framework is ingrained with heteroatoms such as N and S, which otherwise are incorporated externally through energy-intensive, hazardous, chem. reactions using proper org. precursors. This judicious transformation of org.-rich waste not only addresses the disposal issue, but also generates valuable functional C materials from the discard. Our unique synthesis strategy involving moderate activation and further graphitization enhances the elec. cond., while still maintaining the precious heteroatoms. The effect of temp. on the structural and functional properties is studied, and all the as-obtained carbons are applied as metal-free catalysts for the O redn. reaction (ORR). C graphitized at 900° emerges as a superior ORR electrocatalyst with excellent electrocatalytic performance, high selectivity, and long durability, demonstrating that hair C can be a promising alternative for costly Pt-based electrocatalysts in fuel cells. The ORR performance can be discussed in terms of heteroatom doping, surface properties, and elec. cond. of the resulting porous hair C materials.
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  20. 20
    Amiinu, I. S.; Zhang, J.; Kou, Z.; Liu, X.; Asare, O. K.; Zhou, H.; Cheng, K.; Zhang, H.; Mai, L.; Pan, M.; Mu, S. Self-organized 3D porous graphene dual-doped with biomass-sponsored nitrogen and sulfur for oxygen reduction and evolution. ACS Appl. Mater. Interfaces 2016, 8, 29408– 29418,  DOI: 10.1021/acsami.6b08719
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    20
    Self-Organized 3D Porous Graphene Dual-Doped with Biomass-Sponsored Nitrogen and Sulfur for Oxygen Reduction and Evolution
    Amiinu, Ibrahim Saana; Zhang, Jian; Kou, Zongkui; Liu, Xiaobo; Asare, Owusu Kwadwo; Zhou, Huang; Cheng, Kun; Zhang, Haining; Mai, Liqiang; Pan, Mu; Mu, Shichun
    ACS Applied Materials & Interfaces (2016), 8 (43), 29408-29418CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
    3D graphene-based materials offer immense potentials to overcome the challenges related to the functionality, performance, cost, and stability of fuel cell electrocatalysts. Herein, a nitrogen (N) and sulfur (S) dual-doped 3D porous graphene catalyst is synthesized via a single-row pyrolysis using biomass as solitary source for both N and S, and structure directing agent. The thermochem. reaction of biomass functional groups with graphene oxide facilitates in situ generation of reactive N and S species, stimulating the graphene layers to reorganize into a trimodal 3D porous assembly. The resultant catalyst exhibits high ORR and OER performance superior to similar materials obtained through toxic chems. and multistep routes. Its stability and tolerance to CO and methanol oxidn. mols. are far superior to com. Pt/C. The dynamics governing the structural transformation and the enhanced catalytic activity in both alk. and acidic media are discussed. This work offers a unique approach for rapid synthesis of a dual-heteroatom doped 3D porous-graphene-architecture for wider applications.
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  21. 21
    Guo, C.; Liao, W.; Li, Z.; Sun, L.; Chen, C. Easy conversion of protein-rich enoki mushroom biomass to a nitrogen-doped carbon nanomaterial as a promising metal-free catalyst for oxygen reduction reaction. Nanoscale 2015, 7, 15990– 15998,  DOI: 10.1039/c5nr03828f
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    21
    Easy conversion of protein-rich enoki mushroom biomass to a nitrogen-doped carbon nanomaterial as a promising metal-free catalyst for oxygen reduction reaction
    Guo, Chaozhong; Liao, Wenli; Li, Zhongbin; Sun, Lingtao; Chen, Changguo
    Nanoscale (2015), 7 (38), 15990-15998CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
    The search for low-cost, highly active, and stable catalysts to replace the Pt-based catalysts for oxygen redn. reaction (ORR) has recently become a topic of interest. Herein, we report a new strategy to design a nitrogen-doped carbon nanomaterial for use as a metal-free ORR catalyst based on facile pyrolysis of protein-rich enoki mushroom (Flammulina velutipes) biomass at 900 °C with carbon nanotubes as a conductive agent and inserting matrix. We found that various forms of nitrogen (nitrile, pyrrolic and graphitic) were incorporated into the carbon mol. skeleton of the product, which exhibited more excellent ORR electrocatalytic activity and better durability in alk. medium than those in acidic medium. Remarkably, the ORR half-wave potential measured on our material was around 0.81 V in alk. medium, slightly lower than that on the com. 20 wt% Pt/C catalyst (0.86 V). Meanwhile, the ORR followed the desired 4-electron transfer mechanism involving the direct redn. pathway. The ORR performance was also markedly better than or at least comparable to the leading results in the literature based on biomass-derived carbon-based catalysts. Besides, we significantly proposed that the graphitic-nitrogen species that is most responsible for the ORR activity can function as the electrocatalytically active center for ORR, and the pyrrolic-nitrogen species can act as an effective promoter for ORR only. The results suggested a promising route based on economical and sustainable fungi biomass towards the large-scale prodn. of valuable carbon nanomaterials as highly active and stable metal-free catalysts for ORR under alk. conditions.
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  22. 22
    Gao, S.; Fan, H.; Zhang, S. Nitrogen-enriched carbon from bamboo fungus with superior oxygen reduction reaction activity. J. Mater. Chem. A 2014, 2, 18263– 18270,  DOI: 10.1039/c4ta03558e
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    22
    Nitrogen-enriched carbon from bamboo fungus with superior oxygen reduction reaction activity
    Gao, Shuyan; Fan, Hao; Zhang, Shuxia
    Journal of Materials Chemistry A: Materials for Energy and Sustainability (2014), 2 (43), 18263-18270CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
    Fuel cells are promising candidates for clean and high-efficient energy conversion in the future. The development of C-based inexpensive metal-free ORR catalysts has become one of the most attractive topics in the fuel cell field. Herein, the authors report a N-doped C catalyst with a surface area of up to 1895.5 m2 g-1 using a natural product (bamboo fungus) as the starting material. In 0.1M KOH electrolyte, the ORR onset potential for the catalyst is up to 0.089 V vs. Ag/AgCl. Also, it shows superior stability, fuel crossover resistance, and selective activity to a com. Pt/C catalyst. The sample displays excellent stability, i.e. no obvious decrease in current was obsd. after 1000 continuous cycles between -0.7 and 0.3 V in O2-satd. 0.1M KOH. Structural characterizations and electrochem. tests verify that the treatment of biomass have an important impact on the materials.
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  23. 23
    Zhu, H.; Yin, J.; Wang, X.; Wang, H.; Yang, X. Microorganism-derived heteroatom-doped carbon materials for oxygen reduction and supercapacitors. Adv. Funct. Mater. 2013, 23, 1305– 1312,  DOI: 10.1002/adfm.201201643
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    23
    Microorganism-Derived Heteroatom-Doped Carbon Materials for Oxygen Reduction and Supercapacitors
    Zhu, Hui; Yin, Jiao; Wang, Xiaolei; Wang, Hongyu; Yang, Xiurong
    Advanced Functional Materials (2013), 23 (10), 1305-1312CODEN: AFMDC6; ISSN:1616-301X. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Heteroatom-doped C (HDC) has attracted attention due to its application in energy conversion and storage. Herein, due to its abundance high rate of reprodn., the microorganism, Bacillus subtilis, is selected as a precursor. An effective ionothermal process is adopted to produce the HDCs. Using acid activation, the obtained sample exhibits excellent electrocatalytic activity, long-term stability, and excellent resistance to crossover effects in O redn. Addnl., the base-treated sample exhibits superior performance in capacitors to most com. available C materials. Even at a high c.d., a relatively high capacitance is retained, indicating a great potential for direct application in energy storage.
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    Gong, X.; Liu, S.; Ouyang, C.; Strasser, P.; Yang, R. Nitrogen- and Phosphorus-Doped Biocarbon with Enhanced Electrocatalytic Activity for Oxygen Reduction. ACS Catal. 2015, 5, 920– 927,  DOI: 10.1021/cs501632y
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    24
    Nitrogen- and Phosphorus-Doped Biocarbon with Enhanced Electrocatalytic Activity for Oxygen Reduction
    Gong, Xin; Liu, Shanshan; Ouyang, Chuying; Strasser, Peter; Yang, Ruizhi
    ACS Catalysis (2015), 5 (2), 920-927CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
    The oxygen redn. reaction (ORR) at the cathode of fuel cells and metal-air batteries requires efficient electrocatalysts to accelerate its reaction rate due to its sluggish kinetics. Nitrogen- and phosphorus-doped biocarbon was fabricated via a simple and low-cost biosynthesis method using yeast cells as a precursor. The as-prepd. biocarbon exhibits excellent electrocatalytic activity for the ORR. An onset potential of -0.076 V (vs. Ag/AgCl) and a neg. shift of only ∼29 mV in the half-wave potential of the biocarbon as compared to com. Pt/C (20 wt. % Pt on Vulcan XC-72, Johnson Matthey) is achieved. The biocarbon possesses enhanced electron poverty in carbon atoms and a decreasing amt. of less electroactive nitrogen and phosphorus dopants due to the biomineralization during the synthesis. The surface gap layer along with the mesopores in the biocarbon increases accessible active sites and facilitates the mass transfer during the ORR. These factors correlate with the high ORR activity of the biocarbon. The results demonstrate that biomineralization plays a crit. role in tailoring the structure and the electrocatalytic activity of the biocarbon for ORR.
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    van der Ent, A.; Baker, A. J. M.; Reeves, R. D.; Pollard, A. J.; Schat, H. Hyperaccumulators of metal and metalloid trace elements: Facts and fiction. Plant Soil 2013, 362, 319– 334,  DOI: 10.1007/s11104-012-1287-3
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    25
    Hyperaccumulators of metal and metalloid trace elements: Facts and fiction
    van der Ent, Antony; Baker, Alan J. M.; Reeves, Roger D.; Pollard, A. Joseph; Schat, Henk
    Plant and Soil (2013), 362 (1-2), 319-334CODEN: PLSOA2; ISSN:0032-079X. (Springer)
    A review. Background: Plants that accumulate metal and metalloid trace elements to extraordinarily high concns. in their living biomass have inspired much research worldwide during the last decades. Hyperaccumulators have been recorded and exptl. confirmed for elements such as nickel, zinc, cadmium, manganese, arsenic and selenium. However, to date, hyperaccumulation of lead, copper, cobalt, chromium and thallium remain largely unconfirmed. Recent uses of the term in relation to rare-earth elements require crit. evaluation. Scope Since the mid-1970s the term hyperaccumulator' has been used millions of times by thousands of people, with varying degrees of precision, aptness and understanding that have not always corresponded with the views of the originators of the terminol. and of the present authors. There is therefore a need to clarify the circumstances in which the term 'hyperaccumulator' is appropriate and to set out the conditions that should be met when the terms are used. We outline here the main considerations for establishing metal or metalloid hyperaccumulation status of plants, (re)define some of the terminol. and note potential pitfalls. Conclusions: Unambiguous communication will require the international scientific community to adopt std. terminol. and methods for confirming the reliability of anal. data in relation to metal and metalloid hyperaccumulators.
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    Rezania, S.; Ponraj, M.; Talaiekhozani, A.; Mohamad, S. E.; Md Din, M. F.; Taib, S. M.; Sabbagh, F.; Sairan, F. M. Perspectives of phytoremediation using water hyacinth for removal of heavy metals, organic and inorganic pollutants in wastewater. J. Environ. Manage. 2015, 163, 125– 133,  DOI: 10.1016/j.jenvman.2015.08.018
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    26
    Perspectives of phytoremediation using water hyacinth for removal of heavy metals, organic and inorganic pollutants in wastewater
    Rezania, Shahabaldin; Ponraj, Mohanadoss; Talaiekhozani, Amirreza; Mohamad, Shaza Eva; Md. Din, Mohd. Fadhil; Taib, Shazwin Mat; Sabbagh, Farzaneh; Sairan, Fadzlin Md.
    Journal of Environmental Management (2015), 163 (), 125-133CODEN: JEVMAW; ISSN:0301-4797. (Elsevier Ltd.)
    The development of eco-friendly and efficient technologies for treating wastewater is one of the attractive research area. Phytoremediation is considered to be a possible method for the removal of pollutants present in wastewater and recognized as a better green remediation technol. Nowadays the focus is to look for a sustainable approach in developing wastewater treatment capability. Water hyacinth is one of the ancient technol. that has been still used in the modern era. Although, many papers in relation to wastewater treatment using water hyacinth have been published, recently removal of org., inorg. and heavy metal have not been reviewed extensively. The main objective of this paper is to review the possibility of using water hyacinth for the removal of pollutants present in different types of wastewater. Water hyacinth is although reported to be as one of the most problematic plants worldwide due to its uncontrollable growth in water bodies but its quest for nutrient absorption has provided way for its usage in phytoremediation, along with the combination of herbicidal control, integratated biol. control and watershed management controlling nutrient supply to control its growth. Moreover as a part of solving wastewater treatment problems in urban or industrial areas using this plant, a large no. of useful byproducts can be developed like animal and fish feed, power plant energy (briquette), ethanol, biogas, composting and fiber board making. In focus to the future aspects of phytoremediation, the utilization of invasive plants in pollution abatement phytotechnologies can certainly assist for their sustainable management in treating waste water.
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    Raskin, I.; Ensley, B. D. Phytoremediation of Toxic Metals: Using Plants to Clean Up the Environment; John Wiley & Sons, Inc.: New York, 2000.
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    van der Ent, A.; Baker, A. J. M.; Reeves, R. D.; Chaney, R. L.; Anderson, C. W. N.; Meech, J. A.; Erskine, P. D.; Simonnot, M.-O.; Vaughan, J.; Morel, J. L.; Echevarria, G.; Fogliani, B.; Rongliang, Q.; Mulligan, D. R. Agromining: Farming for Metals in the Future?. Environ. Sci. Technol. 2015, 49, 4773– 4780,  DOI: 10.1021/es506031u
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    Agromining: Farming for Metals in the Future?
    van der Ent, Antony; Baker, Alan J. M.; Reeves, Roger D.; Chaney, Rufus L.; Anderson, Christopher W. N.; Meech, John A.; Erskine, Peter D.; Simonnot, Marie-Odile; Vaughan, James; Morel, Jean Louis; Echevarria, Guillaume; Fogliani, Bruno; Rongliang, Qiu; Mulligan, David R.
    Environmental Science & Technology (2015), 49 (8), 4773-4780CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
    A review. Phytomining technol. employs hyperaccumulator plants to take up metal in harvestable plant biomass. Harvesting, drying and incineration of the biomass generates a high-grade bio-ore. We propose that "agromining" (a variant of phytomining) could provide local communities with an alternative type of agriculture on degraded lands; farming not for food crops, but for metals such as nickel (Ni). However, two decades after its inception and numerous successful expts., com. phytomining has not yet become a reality. To build the case for the minerals industry, a large-scale demonstration is needed to identify operational risks and provide "real-life" evidence for profitability.
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    Bhattacharjee, S.; Dasgupta, P.; Paul, A. R.; Ghosal, S.; Padhi, K. K.; Pandey, L. P. Mineral element composition of spinach. J. Sci. Food Agric. 1998, 77, 456– 458,  DOI: 10.1002/(sici)1097-0010(199808)77:4<456::aid-jsfa55>3.0.co;2-m
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    Mineral element composition of spinach
    Bhattacharjee, Santanu; Dasgupta, Pranab; Paul, Adhir R.; Ghosal, Sunanda; Padhi, Karuna K.; Pandey, Lallan P.
    Journal of the Science of Food and Agriculture (1998), 77 (4), 456-458CODEN: JSFAAE; ISSN:0022-5142. (John Wiley & Sons Ltd.)
    An extensive study has been made on the mineral element compns. of spinach leaves and stems. Twenty two locally grown different spinach samples have been analyzed for 16 elements using ICP and at. absorption spectrophotometric techniques. Both spinach leaves and stems were analyzed sep. A detailed elucidation of the inorg. matrix in spinach leaves and stems has been provided.
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    Citak, S.; Sonmez, S. Mineral Contents of Organically and Conventionally Grown Spinach (Spinacea oleracea L.) during Two Successive Seasons. J. Agric. Food Chem. 2009, 57, 7892– 7898,  DOI: 10.1021/jf900660k
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    Mineral Contents of Organically and Conventionally Grown Spinach (Spinacea oleracea L.) during Two Successive Seasons
    Citak, Sedat; Sonmez, Sahriye
    Journal of Agricultural and Food Chemistry (2009), 57 (17), 7892-7898CODEN: JAFCAU; ISSN:0021-8561. (American Chemical Society)
    Spinach (Spinacea oleracea L.) plants were grown organically and conventionally during two successive seasons (late autumn and early winter) in order to examine the nutrient content of the plants. In a series of 17 org. applications including chicken manure (CM), farmyard manure (FM), blood meal (BM), and one mineral fertilizer treatment and one control, collectively 19 treatments, were used at each season. The optimum doses to be recommended should be divided into groups depending on the mineral nutrients and also the seasons as follows: 1.7 CM+7.5 FM in the late autumn season and 2.5 CM + 4.0 FM in the early winter season for N, P, K content; 5.0 FM + 1.2 CM + 0.4 BM in the late autumn; and 2.5 CM + 4.0 FM in the early winter season for Ca and Mg. Regarding the micro nutrients, the group divisions should be as follows: 10.0 FM + 0.4 BM in the late autumn season and 5.0 FM + 2.5 CM in the early winter season for Fe and Cu, and 3.5 CM in the late autumn season and 10.0 FM + 1.2 CM in the early winter season for Mn and Zn content. High rates of farmyard manure (FM) and chicken manure (CM) can be successfully used in org. prodn., and high rates of these manures may substitute for mineral fertilizer, esp. in the late autumn season.
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    Solubility of melamine in water
    Chapman, R. P.; Averell, P. R.; Harris, R. R.
    Industrial and Engineering Chemistry (1943), 35 (), 137-8CODEN: IECHAD; ISSN:0019-7866.
    The soly. of melamine in g. per 100 g. H2O is given by the equation, log (soly.) = - 1642/T + 5.101, in the range 0-100°.
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    Ferrari, A. C.; Meyer, J.; Scardaci, V.; Casiraghi, C.; Lazzeri, M.; Mauri, F.; Piscanec, S.; Jiang, D.; Novoselov, K.; Roth, S. Raman spectrum of graphene and graphene layers. Phys. Rev. Lett. 2006, 97, 187401,  DOI: 10.1103/physrevlett.97.187401
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    Physical Review Letters (2006), 97 (18), 187401/1-187401/4CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
    Graphene is the 2-dimensional building block for C allotropes of every other dimensionality. Its electronic structure is captured in its Raman spectrum that clearly evolves with the no. of layers. The D peak 2nd order changes in shape, width, and position for an increasing no. of layers, reflecting the change in the electron bands via a double resonant Raman process. The G peak slightly down-shifts. This allows unambiguous, high-throughput, nondestructive identification of graphene layers, which is critically lacking in this emerging research area.
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    Influence of the Atomic Structure on the Raman Spectra of Graphite Edges
    Cancado, L. G.; Pimenta, M. A.; Neves, B. R. A.; Dantas, M. S. S.; Jorio, A.
    Physical Review Letters (2004), 93 (24), 247401/1-247401/4CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
    A study of step edges in graphite with different at. structures combining Raman spectroscopy and scanning probe microscopy is presented. The orientation of the C hexagons with respect to the edge axis, in the so-called armchair or zigzag arrangements, is distinguished spectroscopically by the intensity of a disorder-induced Raman peak. This effect is explained by applying the double resonance theory to a semi-infinite graphite crystal and by considering the 1-dimensional character of the defect.
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    Ferrari, A. C.; Robertson, J. Interpretation of Raman spectra of disordered and amorphous carbon. Phys. Rev. B: Condens. Matter Mater. Phys. 2000, 61, 14095– 14107,  DOI: 10.1103/physrevb.61.14095
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    Interpretation of Raman spectra of disordered and amorphous carbon
    Ferrari, A. C.; Robertson, J.
    Physical Review B: Condensed Matter and Materials Physics (2000), 61 (20), 14095-14107CODEN: PRBMDO; ISSN:0163-1829. (American Physical Society)
    The model and theor. understanding of the Raman spectra in disordered and amorphous C are given. The nature of the G and D vibration modes in graphite is analyzed in terms of the resonant excitation of π states and the long-range polarizability of π bonding. Visible Raman data on disordered, amorphous, and diamondlike C are classified in a 3-stage model to show the factors that control the position, intensity, and widths of the G and D peaks. The visible Raman spectra depend formally on the configuration of the sp2 sites in sp2-bonded clusters. In cases where the sp2 clustering is controlled by the sp3 fraction, such as in as-deposited tetrahedral amorphous C (ta-C) or hydrogenated amorphous C (a-C:H) films, the visible Raman parameters can be used to derive the sp3 fraction.
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    Ma, J.; Habrioux, A.; Guignard, N.; Alonso-Vante, N. Functionalizing Effect of Increasingly Graphitic Carbon Supports on Carbon-Supported and TiO2–Carbon Composite-Supported Pt Nanoparticles. J. Phys. Chem. C 2012, 116, 21788– 21794,  DOI: 10.1021/jp304947y
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    Functionalizing Effect of Increasingly Graphitic Carbon Supports on Carbon-Supported and TiO2-Carbon Composite-Supported Pt Nanoparticles
    Ma, J.; Habrioux, A.; Guignard, N.; Alonso-Vante, N.
    Journal of Physical Chemistry C (2012), 116 (41), 21788-21794CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    Strong interaction between Pt nanoparticles and graphitic C in C-based and oxide-C composite substrates is demonstrated using electrochem. CO stripping expts. A correlation between the in-plane crystallite size of the C supports and the oxidn. charge of CO stripping wave was made. It appears that π-system of graphitized C anchors Pt particles in a way that strongly modifies the electronic properties of the Pt valence band. The effects of graphitized C on Pt are even observable on TiO2-C composite-supported Pt, where a known strong metal-support interaction between Pt and TiO2 is already present, demonstrating the significant extent of the interaction between Pt and graphite. Finally, a preliminary proof of the role played by the interfacial Pt-Ti nanoalloy on oxide-C composite is given.
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    Kruk, M.; Jaroniec, M. Gas adsorption characterization of ordered organic– inorganic nanocomposite materials. Chem. Mater. 2001, 13, 3169– 3183,  DOI: 10.1021/cm0101069
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    Gas adsorption characterization of ordered organic-inorganic nanocomposite materials
    Kruk, Michal; Jaroniec, Mietek
    Chemistry of Materials (2001), 13 (10), 3169-3183CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
    A crit. review, with 133 refs. on adsorption methods that are currently used in the characterization of ordered org.-inorg. nanocomposite materials is presented, and the adsorption methodol. that is potentially useful for this characterization, but has not yet been applied, is discussed. The ordered org.-inorg. nanocomposites include surface-functionalized ordered mesoporous materials (OMMs) with siliceous frameworks (synthesized either via postsynthesis surface modification or via direct co-condensation method), periodic mesoporous organosilicas, and surfactant-contg. OMMs. This review covers the methods for detn. of the sp. surface area and pore vol. The available methods for mesopore size anal. are critically compared and evaluated, with special emphasis on the recent developments related to the application of advanced computational methods for studying adsorption in porous media and to the direct modeling of adsorption using highly ordered surface-functionalized OMMs as model adsorbents. The review also covers adsorption methods for studying the surface properties of org.-inorg. nanocomposites, including those based on adsorption of mols. of different polarities. An emphasis is placed on the emerging opportunity for studying the surface properties of nanocomposites using low-pressure adsorption of nonpolar mols., such as N2 and Ar. The opportunities and challenges in adsorption characterization of sp. surface sites, uniformity of coated or bonded layers, bonding d. of groups on the surface, type of surface species, and so forth, are presented. Thus, this review provides an overview of adsorption studies dealing with org.-inorg. nanocomposites, a crit. discussion of adsorption methods available for such studies, and some recommendations for thorough characterization of these materials using gas adsorption.
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    Wang, H.; Li, X.-d.; Yu, J.-s.; Kim, D.-p. Fabrication and characterization of ordered macroporous PMS-derived SiC from a sacrificial template method. J. Mater. Chem. 2004, 14, 1383– 1386,  DOI: 10.1039/b313405a
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    Fabrication and characterization of ordered macroporous PMS-derived SiC from a sacrificial template method
    Wang, Hao; Li, Xiao-dong; Yu, Jong-sung; Kim, Dong-pyo
    Journal of Materials Chemistry (2004), 14 (9), 1383-1386CODEN: JMACEP; ISSN:0959-9428. (Royal Society of Chemistry)
    Three-dimensionally long-range ordered macroporous SiC ceramics were prepd. using a low mol. wt. polymer precursor, polymethylsilane (PMS), by utilizing sacrificial colloidal silica cryst. arrays as templates which were subsequently etched off with HF (45%) after pyrolysis in an argon atm. SEM, TEM, HRTEM and BET were used to specify the morphologies and BET surface areas of the porous materials. The pore sizes of about 84-658 nm and BET surface areas (pore vols.) of 584.64-299.44 m2.g-1 (0.64-0.25 cm3.g-1) of the achieved macroporous SiC ceramic were approx. proportional to the sizes 112-700 nm of the sacrificial silica sphere templates used. The unexpectedly high surfaces area and pore vols. of the achieved porous SiC ceramic were related to the microchannels on the walls that were caused by the interfacial diffusion of oxygen or silica from the silica sphere template into the walls of the SiC framework structures.
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    Liu, X.; Zhou, W.; Yang, L.; Li, L.; Zhang, Z.; Ke, Y.; Chen, S. Nitrogen and sulfur co-doped porous carbon derived from human hair as highly efficient metal-free electrocatalysts for hydrogen evolution reactions. J. Mater. Chem. A 2015, 3, 8840– 8846,  DOI: 10.1039/c5ta01209k
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    Nitrogen and sulfur co-doped porous carbon derived from human hair as highly efficient metal-free electrocatalysts for hydrogen evolution reactions
    Liu, Xiaojun; Zhou, Weijia; Yang, Linjing; Li, Ligui; Zhang, Zhenyuan; Ke, Yunting; Chen, Shaowei
    Journal of Materials Chemistry A: Materials for Energy and Sustainability (2015), 3 (16), 8840-8846CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
    Design and engineering of low-cost and high-efficiency electrocatalysts for hydrogen evolution reactions (HER) has attracted increasing interest in renewable energy research. Herein, a highly active and stable metal-free electrocatalyst, N + S-codoped porous carbon derived from human hair, was developed for HER for the first time, with an electrocatalytic performance comparable to that of state-of-the-art com. 20 wt% Pt/C catalysts. SEM, TEM and nitrogen adsorption-desorption measurements showed that the resultant carbon exhibited a porous structure with a high sp. surface area (up to 830.0 m2 g-1) and rich porosity. XPS measurements showed that N and S were codoped into the carbon mol. skeletons. Importantly, electrochem. measurements showed high activity for hydrogen evolution with a low overpotential of only -12 mV, a Tafel slope of 57.4 mV dec-1, a c.d. of 10 mA cm-2 at -0.1 V vs. RHE, and remarkable durability. The results highlight a unique paradigm for the prepn. of highly efficient electrocatalysts for HER based on abundant biowastes.
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    Li, Y.; Zhang, H.; Wang, Y.; Liu, P.; Yang, H.; Yao, X.; Wang, D.; Tang, Z.; Zhao, H. A self-sponsored doping approach for controllable synthesis of S and N co-doped trimodal-porous structured graphitic carbon electrocatalysts. Energy Environ. Sci. 2014, 7, 3720– 3726,  DOI: 10.1039/c4ee01779j
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    A self-sponsored doping approach for controllable synthesis of S and N co-doped trimodal-porous structured graphitic carbon electrocatalysts
    Li, Yibing; Zhang, Haimin; Wang, Yun; Liu, Porun; Yang, Huagui; Yao, Xiangdong; Wang, Dan; Tang, Zhiyong; Zhao, Huijun
    Energy & Environmental Science (2014), 7 (11), 3720-3726CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)
    A facile self-sponsored doping approach is developed to synthesize S and N co-doped trimodal-porous structured graphitic carbon network electrocatalysts. It utilizes a sole precursor (1-allyl-2-thiourea) to realize a precisely controlled co-doping of S and N during a concurrent graphitic carbon growth process by simple control of the pyrolysis temp. The results reveal that the doping effect is heavily dependent on the doping d. and a maximal catalytic activity could only be achieved with an optimal doping level. The presence of a macro-pore structure in the trimodal-porous network enhances the mass transport, enabling the full utilization of large surface areas created by micro- and meso-pores. The resultant electrocatalyst possesses high ORR catalytic activity with excellent durability and high resistance to the inhibition effect of fuel mols. The findings of this work would be valuable for design and fabrication of high performance carbon-based electrocatalysts.
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    He, W.; Jiang, C.; Wang, J.; Lu, L. High-Rate Oxygen Electroreduction over Graphitic-N Species Exposed on 3D Hierarchically Porous Nitrogen-Doped Carbons. Angew. Chem., Int. Ed. 2014, 53, 9503– 9507,  DOI: 10.1002/anie.201404333
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    High-Rate Oxygen Electroreduction over Graphitic-N Species Exposed on 3D Hierarchically Porous Nitrogen-Doped Carbons
    He, Wenhui; Jiang, Chunhuan; Wang, Jiabo; Lu, Lehui
    Angewandte Chemie, International Edition (2014), 53 (36), 9503-9507CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Nitrogen-doped species (NDs) are theor. accepted as a determinant of the catalytic activity of metal-free N-doped carbon (NC) catalysts for oxygen redn. reaction (ORR). However, direct relationships between ND type and ORR activity have been difficult to ext. because the complexity of carbon matrix impairs efforts to expose specific NDs. Herein, we demonstrate the fabrication of a 3D hierarchically porous NC catalyst with micro-, meso-, and macroporosity in one structure, in which sufficient exposure and availability of inner-pore catalytic sites can be achieved due to its super-high surface area (2191 cm2 g-1) and interconnected pore system. More importantly, in-situ formation of graphitic-N species (GNs) on the surface of NC stimulated by KOH activation enables us to exptl. reveal the catalytic nature of GNs for ORR, which is of great significance for the design and development of advanced metal-free NC electrocatalysts.
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    Liang, J.; Du, X.; Gibson, C.; Du, X. W.; Qiao, S. Z. N-Doped Graphene Natively Grown on Hierarchical Ordered Porous Carbon for Enhanced Oxygen Reduction. Adv. Mater. 2013, 25, 6226– 6231,  DOI: 10.1002/adma.201302569
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    N-Doped Graphene Natively Grown on Hierarchical Ordered Porous Carbon for Enhanced Oxygen Reduction
    Liang, Ji; Du, Xin; Gibson, Christopher; Du, Xi Wen; Qiao, Shi Zhang
    Advanced Materials (Weinheim, Germany) (2013), 25 (43), 6226-6231CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
    This novel hybrid material consists of natively grown graphene on hierarchical porous carbon. The material is prepd. through a convenient bottom-up route, which combines the advantages of both hierarchical porous carbon and graphene such as high surface area, large porous channel size, and high electron cond. Due to its excellent structural properties and inherent hetero-atom dopants, this material can serve as a metal-free catalyst for ORR. The efficient electron transfer between carbon and the in situ formed graphene makes its catalytic performance synergistically enhanced. Moreover, this carbon-based material could also potentially be used in other applications which require high surface area, high accessibly, and high cond.
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    Hu, Q.; Li, G.; Liu, X.; Zhu, B.; Chai, X.; Zhang, Q.; Liu, J.; He, C. Superhydrophilic Phytic-Acid-Doped Conductive Hydrogels as Metal-Free and Binder-Free Electrocatalysts for Efficient Water Oxidation. Angew. Chem. 2019, 131, 4362– 4366,  DOI: 10.1002/ange.201900109
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    Wang, Z.; Dong, Y.; Li, H.; Zhao, Z.; Wu, H. B.; Hao, C.; Liu, S.; Qiu, J.; Lou, X. W. D. Enhancing lithium–sulphur battery performance by strongly binding the discharge products on amino-functionalized reduced graphene oxide. Nat. Commun. 2014, 5, 5002,  DOI: 10.1038/ncomms6002
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    Enhancing lithium-sulphur battery performance by strongly binding the discharge products on amino-functionalized reduced graphene oxide
    Wang, Zhiyu; Dong, Yanfeng; Li, Hongjiang; Zhao, Zongbin; Wu, Hao Bin; Hao, Ce; Liu, Shaohong; Qiu, Jieshan; Lou, Xiong Wen
    Nature Communications (2014), 5 (), 5002CODEN: NCAOBW; ISSN:2041-1723. (Nature Publishing Group)
    Lithium-sulfur batteries are one very appealing power source with high energy d. But their practical use is still hindered by several issues including short lifespan, low efficiency and safety concern from the lithium anode. Polysulfide dissoln. and insulating nature of sulfur are generally considered responsible for the capacity degrdn. However, the detachment of discharge products, i.e., highly polar lithium sulfides, from nonpolar carbon matrix (for example, graphene) has been rarely studied as one crit. factor. Here we report the strongly covalent stabilization of sulfur and its discharge products on amino-functionalized reduced graphene oxide that enables stable capacity retention of 80% for 350 cycles with high capacities and excellent high-rate response up to 4 C. The present study demonstrates a feasible and effective strategy to solve the long-term cycling difficulty for lithium-sulfur batteries and also helps to understand the capacity decay mechanism involved.
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    Artyushkova, K.; Matanovic, I.; Halevi, B.; Atanassov, P. Oxygen binding to active sites of Fe–N–C ORR electrocatalysts observed by ambient-pressure XPS. J. Phys. Chem. C 2017, 121, 2836– 2843,  DOI: 10.1021/acs.jpcc.6b11721
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    Oxygen Binding to Active Sites of Fe-N-C ORR Electrocatalysts Observed by Ambient-Pressure XPS
    Artyushkova, Kateryna; Matanovic, Ivana; Halevi, Barr; Atanassov, Plamen
    Journal of Physical Chemistry C (2017), 121 (5), 2836-2843CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    The authors report the 1st in situ ambient pressure XPS (APXPS) study of the binding of oxygenated species to the active sites of Fe-N-C O redn. reaction (ORR) electrocatalysts. To better interpret the results, DFT calcns. were used to calc. absorption energies of reactants and intermediates on potential active sites and calc. the core level shifts for those. The obsd. O binding to N coordinated to Fe centers correlates with the enhanced measured ORR fuel cell activity of these materials with respect to metal-free analogs and sheds light on the ORR mechanism on PGM-free electrocatalysts.
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    Lin, L.; Zhu, Q.; Xu, A.-W. Noble-metal-free Fe–N/C catalyst for highly efficient oxygen reduction reaction under both alkaline and acidic conditions. J. Am. Chem. Soc. 2014, 136, 11027– 11033,  DOI: 10.1021/ja504696r
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    Noble-Metal-Free Fe-N/C Catalyst for Highly Efficient Oxygen Reduction Reaction under Both Alkaline and Acidic Conditions
    Lin, Ling; Zhu, Qing; Xu, An-Wu
    Journal of the American Chemical Society (2014), 136 (31), 11027-11033CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    In this work, the synthesis and assessment are reported of a new non-precious-metal oxygen redn. reaction (ORR) catalyst from pyrolysis of an iron-coordinated complex which manifests superior activity in both alk. and acidic media. 11,11'-bis(dipyrido[3,2-a:2',3'-c]phenazinyl) (bidppz) was selected as a ligand for the formation of a nitrogen-rich iron-coordinated coordination polymer (Fe-bidppz) which forms a self-supporting catalyst contg. high densities of nitrogen and iron doping by pyrolysis. The catalyst pyrolyzed at 800° (Fe-N/C-800) shows the highest ORR activity with onset and half-wave potentials of 923 and 809 mV in 0.1 M KOH, resp., which are comparable to those of Pt/C (half-wave potential 818 mV vs. RHE) at the same catalyst loading. Besides, the Fe-N/C-800 catalyst has an excellent ORR activity with onset and half-wave potentials only 38 and 59 mV less than those of the Pt/C catalyst in 0.1 M HClO4. The optimal Fe-N/C-800 catalyst displays much greater durability and tolerance of methanol than Pt/C. It is proposed that the Fe-N/C-800 catalyst has a considerably high d. of surface active sites because Fe-N/C-800 possesses excellent ORR activity while its sp. surface area is not so high. Electrochem. measurements show that the Fe-N/C-800 catalyst in KOH and HClO4 follows the effective four-electron-transfer pathway.
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    Shah, S. S. A.; Najam, T.; Cheng, C.; Peng, L.; Xiang, R.; Zhang, L.; Deng, J.; Ding, W.; Wei, Z. Exploring Fe-Nx for Peroxide Reduction: Template-Free Synthesis of Fe-Nx Traumatized Mesoporous Carbon Nanotubes as an ORR Catalyst in Acidic and Alkaline Solutions. Chem.—Eur. J. 2018, 24, 10630– 10635,  DOI: 10.1002/chem.201802453
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    Exploring Fe-Nx for Peroxide Reduction: Template-Free Synthesis of Fe-Nx Traumatized Mesoporous Carbon Nanotubes as an ORR Catalyst in Acidic and Alkaline Solutions
    Shah, Syed Shoaib Ahmad; Najam, Tayyaba; Cheng, Chao; Peng, Lishan; Xiang, Rui; Zhang, Ling; Deng, Jianghai; Ding, Wei; Wei, Zidong
    Chemistry - A European Journal (2018), 24 (42), 10630-10635CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Fe-based electrocatalysts are elegant due to their better performance towards the O redn. reaction. Nevertheless, they commonly contain different moieties, for example Fe-Nx, Fe, Fe3C and N-doped C, primarily the debatable assistance of these components towards ORR electrocatalysis, specifically for intermediate peroxide redn. reactions (PRR). To explore the role of Fe-Nx centers for PRR, a Fe-N-C electrocatalyst rooted in N-doped C nanotubes with mesoporous structures was synthesized from a Fe/Zn-dicyanoimidazolate framework. The use of dicyanoimidazole coordinated with Fe can introduce the Fe-Nx active sites as well as directional N-doped C nanotubes, which is good for enhancing electronic conductance of the catalyst. The attained electrocatalyst shows tremendous enactment to ORR, being comparable to the activity of Pt/C in acidic and better in alk. electrolytes. This study also reveals that Fe-Nx active centers are responsible for less H2O2 prodn. Though the Fe-Nx moieties and Fe3C/Fe particles encapsulated N-doped C, both are active centers for ORR, however, Fe-Nx sites are more active than others for peroxide redn. reaction. These perceptions suggest rational methodologies for more active and consequently further durable Fe-N-C catalysts.
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    Jiang, W.-J.; Gu, L.; Li, L.; Zhang, Y.; Zhang, X.; Zhang, L.-J.; Wang, J.-Q.; Hu, J.-S.; Wei, Z.; Wan, L.-J. Understanding the High Activity of Fe–N–C Electrocatalysts in Oxygen Reduction: Fe/Fe3C Nanoparticles Boost the Activity of Fe–Nx. J. Am. Chem. Soc. 2016, 138, 3570– 3578,  DOI: 10.1021/jacs.6b00757
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    Understanding the High Activity of Fe-N-C Electrocatalysts in Oxygen Reduction: Fe/Fe3C Nanoparticles Boost the Activity of Fe-Nx
    Jiang, Wen-Jie; Gu, Lin; Li, Li; Zhang, Yun; Zhang, Xing; Zhang, Lin-Juan; Wang, Jian-Qiang; Hu, Jin-Song; Wei, Zidong; Wan, Li-Jun
    Journal of the American Chemical Society (2016), 138 (10), 3570-3578CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Understanding the origin of high activity of Fe-N-C electrocatalysts in O redn. reaction (ORR) is crit. but still challenging for developing efficient sustainable nonprecious metal catalysts in fuel cells and metal-air batteries. We developed a new highly active Fe-N-C ORR catalyst contg. Fe-Nx coordination sites and Fe/Fe3C nanocrystals ([email protected]), and revealed the origin of its activity by intensively studying the compn. and the structure of the catalyst and their correlations with the electrochem. performance. The detailed analyses unambiguously confirmed the coexistence of Fe/Fe3C nanocrystals and Fe-Nx in the best catalyst. A series of designed expts. disclosed that (1) N-doped C substrate, Fe/Fe3C nanocrystals or Fe-Nx themselves did not deliver the high activity; (2) the catalysts with both Fe/Fe3C nanocrystals and Fe-Nx exhibited the high activity; (3) the higher content of Fe-Nx gave the higher activity; (4) the removal of Fe/Fe3C nanocrystals severely degraded the activity; (5) the blocking of Fe-Nx downgraded the activity and the recovery of the blocked Fe-Nx recovered the activity. These facts supported that the high ORR activity of the [email protected] electrocatalysts should be ascribed to that Fe/Fe3C nanocrystals boost the activity of Fe-Nx. The coexistence of high content of Fe-Nx and sufficient metallic Fe nanoparticles is essential for the high ORR activity. DFT calcn. corroborated this conclusion by indicating that the interaction between metallic Fe and Fe-N4 coordination structure favored the adsorption of O mol. These new findings open an avenue for the rational design and bottom-up synthesis of low-cost highly active ORR electrocatalysts.
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    Ahn, S. H.; Yu, X.; Manthiram, A. “Wiring” Fe-Nx-Embedded Porous Carbon Framework onto 1D Nanotubes for Efficient Oxygen Reduction Reaction in Alkaline and Acidic Media. Adv. Mater. 2017, 29, 1606534,  DOI: 10.1002/adma.201606534
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    Sa, Y. J.; Seo, D.-J.; Woo, J.; Lim, J. T.; Cheon, J. Y.; Yang, S. Y.; Lee, J. M.; Kang, D.; Shin, T. J.; Shin, H. S.; Jeong, H. Y.; Kim, C. S.; Kim, M. G.; Kim, T.-Y.; Joo, S. H. A general approach to preferential formation of active Fe–N x sites in Fe–N/C electrocatalysts for efficient oxygen reduction reaction. J. Am. Chem. Soc. 2016, 138, 15046– 15056,  DOI: 10.1021/jacs.6b09470
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    A General Approach to Preferential Formation of Active Fe-Nx Sites in Fe-N/C Electrocatalysts for Efficient Oxygen Reduction Reaction
    Sa, Young Jin; Seo, Dong-Jun; Woo, Jinwoo; Lim, Jung Tae; Cheon, Jae Yeong; Yang, Seung Yong; Lee, Jae Myeong; Kang, Dongwoo; Shin, Tae Joo; Shin, Hyeon Suk; Jeong, Hu Young; Kim, Chul Sung; Kim, Min Gyu; Kim, Tae-Young; Joo, Sang Hoon
    Journal of the American Chemical Society (2016), 138 (45), 15046-15056CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Iron-nitrogen on carbon (Fe-N/C) catalysts have emerged as promising nonprecious metal catalysts (NPMCs) for oxygen redn. reaction (ORR) in energy conversion and storage devices. It was widely suggested that an active site structure for Fe-N/C catalysts contains Fe-Nx coordination. However, the prepn. of high-performance Fe-N/C catalysts mostly involves a high-temp. pyrolysis step, which generates not only catalytically active Fe-Nx sites, but also less active large iron-based particles. Herein, we report a general "silica-protective-layer-assisted" approach that can preferentially generate the catalytically active Fe-Nx sites in Fe-N/C catalysts while suppressing the formation of large Fe-based particles. The catalyst prepn. consisted of an adsorption of iron porphyrin precursor on carbon nanotube (CNT), silica layer overcoating, high-temp. pyrolysis, and silica layer etching, which yielded CNTs coated with thin layer of porphyrinic carbon (CNT/PC) catalysts. Temp.-controlled in situ x-ray absorption spectroscopy during the prepn. of CNT/PC catalyst revealed the coordination of silica layer to stabilize the Fe-N4 sites. The CNT/PC catalyst contained higher d. of active Fe-Nx sites compared to the CNT/PC prepd. without silica coating. The CNT/PC showed high ORR activity and excellent stability in alk. media. Importantly, an alk. anion exchange membrane fuel cell (AEMFC) with a CNT/PC-based cathode exhibited record high current and power densities among NPMC-based AEMFCs. In addn., a CNT/PC-based cathode exhibited a high volumetric c.d. of 320 A cm-3 in acidic proton exchange membrane fuel cell. We further demonstrated the generality of this synthetic strategy to other carbon supports.
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    Wu, Z.-S.; Chen, L.; Liu, J.; Parvez, K.; Liang, H.; Shu, J.; Sachdev, H.; Graf, R.; Feng, X.; Müllen, K. High-Performance Electrocatalysts for Oxygen Reduction Derived from Cobalt Porphyrin-Based Conjugated Mesoporous Polymers. Adv. Mater. 2014, 26, 1450– 1455,  DOI: 10.1002/adma.201304147
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    Wu, Zhong-Shuai; Chen, Long; Liu, Junzhi; Parvez, Khaled; Liang, Haiwei; Shu, Jie; Sachdev, Hermann; Graf, Robert; Feng, Xinliang; Muellen, Klaus
    Advanced Materials (Weinheim, Germany) (2014), 26 (9), 1450-1455CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Cobalt-nanoparticle-integrated nitrogen-enriched porous carbons derived from cobalt porphyrin-based conjugated mesoporous polymers (CoP-CMP) by template-free pyrolysis have proven an outstanding value for high-performance ORR catalysts.
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    Influence of aromatic groups incorporated in long-chain alkanethiol self-assembled monolayers on gold
    Buckel, Frank; Effenberger, Franz; Yan, Chun; Golzhauser, Armin; Grunze, Michael
    Advanced Materials (Weinheim, Germany) (2000), 12 (12), 901-905CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH)
    Self-assembled monolayers of I (R = 2-thienyl, Ph), II, and III are bound via the thiolate function to the Au surface, as demonstrated by XPS. The contact angle of the thiophenol II (111 ± 1°) deviates from those of the aryl functionalized thiols (between 90 ± 1° and 95 ± 1°). RAIR spectroscopy shows densely packed, highly ordered SAMs in all cases. the incorporation of aryl groups, independent of their position, influences the chain mol. orientation to the surface, but it does not prevent dense packing. Probably due to the lower cross section and the better intermol. interactions of the aryl groups, a perpendicular orientation of the arom. ring with respect to the surface is preferred. Therefore, the incorporation of aroms. into alkyl and the kind of chem. linkages between the arom. and the aliph. moieties in these mols. play a decisive role in the orientation of the mols. on the surface. Compds. I and II, with directly connected aliph. and arom. moieties, possess larger tilt angles than the normal alkanethiols, whereas smaller tilt angles are obsd. for III where the arom. and the aliph. groups are connected via an O bridge. Taking these results into consideration, it is possible to influence the tilt angle of SAMs on Au a desired manner. In this way the authors were able to prep. a novel tightly packed oligo(arylalkyl) SAM with an upright alignment of mols. III on a Au surface.
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    Turchanin, A.; Käfer, D.; El-Desawy, M.; Wöll, C.; Witte, G.; Gölzhäuser, A. Molecular mechanisms of electron-induced cross-linking in aromatic SAMs. Langmuir 2009, 25, 7342– 7352,  DOI: 10.1021/la803538z
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    Molecular Mechanisms of Electron-Induced Cross-Linking in Aromatic SAMs
    Turchanin, Andrey; Kaefer, Daniel; El-Desawy, Mohamed; Woell, Christof; Witte, Gregor; Goelzhaeuser, Armin
    Langmuir (2009), 25 (13), 7342-7352CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
    When arom. self-assembled monolayers (SAMs) are electron-irradiated, intermol. cross-links are formed and the SAMs transform into carbon nanosheets with mol. thickness. These nanosheets have a very high mech. stability and can withstand temps. >1000 K. The authors study the electron induced crosslinking of 1,1'-biphenyl-4-thiol (BPT) SAMs on gold by combining XPS, x-ray absorption spectroscopy (NEXAFS), thermal desorption spectroscopy (TDS), and UPS. The exptl. data were acquired as a function of electron dose and temp. and compared with quantum chem. calcns. Details of the intermol. crosslinking, the microstructure of cross-linked films, and their structural transformations upon heating were obtained to derive a view of the mechanisms involved. The anal. shows that room-temp. electron irradn. causes a lateral crosslinking via the formation of C-C linked Ph species as well as a new sulfur species. The thermal stability of the BPT films increases with the electron dose and sats. at ∼50 mC/cm2. Nevertheless, nonlinked fragments in the thermal desorption spectra indicate an incomplete crosslinking even at high doses, which can be attributed to steric reasons and quenching due to the reduced band gap of partially linked mols. At temps. >800 K, all sulfur species are thermally desorbed, while the remaining film reveals an onset of carbonization.
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    Ferrero, G. A.; Preuss, K.; Marinovic, A.; Jorge, A. B.; Mansor, N.; Brett, D. J. L.; Fuertes, A. B.; Sevilla, M.; Titirici, M.-M. Fe–N-Doped Carbon Capsules with Outstanding Electrochemical Performance and Stability for the Oxygen Reduction Reaction in Both Acid and Alkaline Conditions. ACS Nano 2016, 10, 5922– 5932,  DOI: 10.1021/acsnano.6b01247
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    Fe-N-Doped Carbon Capsules with Outstanding Electrochemical Performance and Stability for the Oxygen Reduction Reaction in Both Acid and Alkaline Conditions
    Ferrero, Guillermo A.; Preuss, Kathrin; Marinovic, Adam; Jorge, Ana Belen; Mansor, Noramalina; Brett, Dan J. L.; Fuertes, Antonio B.; Sevilla, Marta; Titirici, Maria-Magdalena
    ACS Nano (2016), 10 (6), 5922-5932CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    High surface area N-doped mesoporous carbon capsules with iron traces exhibit outstanding electrocatalytic activity for the oxygen redn. reaction in both alk. and acidic media. In alk. conditions, they exhibit more pos. onset (0.94 V vs RHE) and half-wave potentials (0.83 V vs RHE) than com. Pt/C, while in acidic media the onset potential is comparable to that of com. Pt/C with a peroxide yield lower than 10%. The Fe-N-doped carbon catalyst combines high catalytic activity with remarkable performance stability (3500 cycles between 0.6 and 1.0 V vs RHE), which stems from the fact that iron is coordinated to nitrogen. Addnl., the newly developed electrocatalyst is unaffected by the methanol crossover effect in both acid and basic media, contrary to com. Pt/C. The excellent catalytic behavior of the Fe-N-doped carbon, even in the more relevant acid medium, is attributable to the combination of chem. functions (N-pyridinic, N-quaternary, and Fe-N coordination sites) and structural properties (large surface area, open mesoporous structure, and short diffusion paths), which guarantees a large no. of highly active and fully accessible catalytic sites and rapid mass-transfer kinetics. Thus, this catalyst represents an important step forward toward replacing Pt catalysts with cheaper alternatives. In this regard, an alk. anion exchange membrane fuel cell was assembled with Fe-N-doped mesoporous carbon capsules as the cathode catalyst to provide current and power densities matching those of a com. Pt/C, which indicates the practical applicability of the Fe-N-carbon catalyst.
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    Li, Y.; Li, M.; Jiang, L.; Lin, L.; Cui, L.; He, X. Advanced oxygen reduction reaction catalyst based on nitrogen and sulfur co-doped graphene in alkaline medium. Phys. Chem. Chem. Phys. 2014, 16, 23196– 23205,  DOI: 10.1039/c4cp02528h
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    Advanced oxygen reduction reaction catalyst based on nitrogen and sulfur co-doped graphene in alkaline medium
    Li, Yongfeng; Li, Meng; Jiang, Liqing; Lin, Lin; Cui, Lili; He, Xingquan
    Physical Chemistry Chemical Physics (2014), 16 (42), 23196-23205CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    A novel nitrogen and sulfur co-doped graphene (N-S-G) catalyst for oxygen redn. reaction (ORR) has been prepd. by pyrolyzing graphite oxide and poly[3-amino-5-mercapto-1,2,4-triazole] composite (PAMTa). The at. percentage of nitrogen and sulfur for the prepd. N-S-G can be adjusted by controlling the pyrolysis temp. Furthermore, the catalyst pyrolyzed at 1000 °C, denoted N-S-G 1000, exhibits the highest catalytic activity for ORR, which displays the highest content of graphitic-N and thiophene-S among all the pyrolyzed samples. The electrocatalytic performance of N-S-G 1000 is significantly better than that of PAMTa and reduced graphite oxide composite. Remarkably, the N-S-G 1000 catalyst is comparable with Pt/C in terms of the onset and half-wave potentials, and displays larger kinetic limiting c.d. and better methanol tolerance and stability than Pt/C for ORR in an alk. medium.
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    Wang, X.; Wang, J.; Wang, D.; Dou, S.; Ma, Z.; Wu, J.; Tao, L.; Shen, A.; Ouyang, C.; Liu, Q.; Wang, S. One-pot synthesis of nitrogen and sulfur co-doped graphene as efficient metal-free electrocatalysts for the oxygen reduction reaction. Chem. Commun. 2014, 50, 4839– 4842,  DOI: 10.1039/c4cc00440j
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    One-pot synthesis of nitrogen and sulfur co-doped graphene as efficient metal-free electrocatalysts for the oxygen reduction reaction
    Wang, Xin; Wang, Jie; Wang, Deli; Dou, Shuo; Ma, Zhaoling; Wu, Jianghong; Tao, Li; Shen, Anli; Ouyang, Canbin; Liu, Qiuhong; Wang, Shuangyin
    Chemical Communications (Cambridge, United Kingdom) (2014), 50 (37), 4839-4842CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
    Novel N, S co-doped graphene (NSG) was prepd. by annealing graphene oxide with thiourea as the single N and S precursor. The NSG electrodes, as efficient metal-free electrocatalysts, show a direct four-electron reaction pathway, high onset potential, high c.d. and high stability for the oxygen redn. reaction.
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    Yuan, S.; Guo, Z.; Wang, L.; Hu, S.; Wang, Y.; Xia, Y. Leaf-Like Graphene-Oxide-Wrapped Sulfur for High-Performance Lithium–Sulfur Battery. Adv. Sci. 2015, 2, 1500071,  DOI: 10.1002/advs.201500071
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    Leaf-Like Graphene-Oxide-Wrapped Sulfur for High-Performance Lithium-Sulfur Battery
    Yuan Shouyi; Guo Ziyang; Wang Lina; Hu Shuang; Wang Yonggang; Xia Yongyao
    Advanced science (Weinheim, Baden-Wurttemberg, Germany) (2015), 2 (8), 1500071 ISSN:2198-3844.
    Carbon/sulfur composites are attracting extensive attention because of their improved performances for Li-S batteries. However, the achievements are generally based on the low S-content in the composites and the low S-loading on the electrode. Herein, a leaf-like graphene oxide (GO), which includes an inherent carbon nanotube midrib in the GO plane, is synthesized for preparing GO/S composites. Owing to the inherent high conductivity of carbon nanotube midribs and the abundant surface groups of GO for S-immobilization, the composite with an S-content of 60 wt% exhibits ultralong cycling stability over 1000 times with a low capacity decay of 0.033% per cycle and a high rate up to 4C. When the S-content is increased to 75 wt%, the composite still shows a perfect cycling performance over 1000 cycles. Even with the high S-loading of 2.7 mg cm(-2) on the electrode and the high S-content of 85 wt%, it still shows a promising cycling performance over 600 cycles.
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    Liang, X.; Hart, C.; Pang, Q.; Garsuch, A.; Weiss, T.; Nazar, L. F. A highly efficient polysulfide mediator for lithium–sulfur batteries. Nat. Commun. 2015, 6, 5682,  DOI: 10.1038/ncomms6682
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    A highly efficient polysulfide mediator for lithium-sulfur batteries
    Liang Xiao; Hart Connor; Pang Quan; Nazar Linda F; Garsuch Arnd; Weiss Thomas
    Nature communications (2015), 6 (), 5682 ISSN:.
    The lithium-sulfur battery is receiving intense interest because its theoretical energy density exceeds that of lithium-ion batteries at much lower cost, but practical applications are still hindered by capacity decay caused by the polysulfide shuttle. Here we report a strategy to entrap polysulfides in the cathode that relies on a chemical process, whereby a host--manganese dioxide nanosheets serve as the prototype--reacts with initially formed lithium polysulfides to form surface-bound intermediates. These function as a redox shuttle to catenate and bind 'higher' polysulfides, and convert them on reduction to insoluble lithium sulfide via disproportionation. The sulfur/manganese dioxide nanosheet composite with 75 wt% sulfur exhibits a reversible capacity of 1,300 mA h g(-1) at moderate rates and a fade rate over 2,000 cycles of 0.036%/cycle, among the best reported to date. We furthermore show that this mechanism extends to graphene oxide and suggest it can be employed more widely.
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    A study of the core level electrons in iron and its three oxides by means of x-ray photoelectron spectroscopy
    Mills, P.; Sullivan, J. L.
    Journal of Physics D: Applied Physics (1983), 16 (5), 723-32CODEN: JPAPBE; ISSN:0022-3727.
    The core level electrons in Fe, FeO, Fe3O4, and α-Fe2O3 were studied by XPS. The main features of the spectra were identified and electron binding energies measured. The Fe 2p peaks of the oxide spectra were broadened (full width at half max. ∼4.5 eV) due to unresolved multiplet splitting and shake-up satellites were obsd. for both Fe(II) and Fe(III) species. For all the samples studied the O 1s spectra consisted of 2 overlapping peaks, 1 due to the oxide and a higher binding energy peak due to an adsorbed species. The higher binding energy peak was frequently dominant indicating a high degree of adsorption on the oxide surfaces. The effect of Ar-ion etching on the surfaces was also investigated and the results indicate that the anal. of surfaces by ion etching in conjunction with XPS can give misleading results.
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  59. 59
    Zheng, X.; Cao, X.; Li, X.; Tian, J.; Jin, C.; Yang, R. Biomass lysine-derived nitrogen-doped carbon hollow cubes via a NaCl crystal template: an efficient bifunctional electrocatalyst for oxygen reduction and evolution reactions. Nanoscale 2017, 9, 1059– 1067,  DOI: 10.1039/c6nr07380h
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    59
    Biomass lysine-derived nitrogen-doped carbon hollow cubes via a NaCl crystal template: an efficient bifunctional electrocatalyst for oxygen reduction and evolution reactions
    Zheng, Xiangjun; Cao, Xuecheng; Li, Xiaowei; Tian, Jinghua; Jin, Chao; Yang, Ruizhi
    Nanoscale (2017), 9 (3), 1059-1067CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
    Nitrogen-doped carbon hollow cubes (NCHCs) are fabricated from biomass L-lysine monohydrochloride via a facile and low-cost NaCl template process, showing efficient bifunctional electrocatalytic activities towards the oxygen redn. reaction (ORR) and oxygen evolution reaction (OER). The resultant lysine-derived carbon hollow cubes with hierarchical pores on the wall are conducive to mass transport and high utilization of nitrogen dopant-induced active sites during the electrocatalytic process. When used as electrocatalysts for the ORR, an onset potential of 0.92 V vs. RHE has been achieved for NCHCs. A neg. shift of only 61 mV exists in the half-wave potential of NCHCs compared to that of the com. Pt/C (20 wt%). Moreover, the NCHCs show high activity for the OER comparable to that of com. RuO2/C (20 wt%). The sustainable conversion of biomass lysine to heteroatom-doped carbon hollow cubes and the recyclability of the NaCl template allow a scalable prodn. and practical application of carbon materials for energy storage and conversion.
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  60. 60
    Wang, M.; Wang, S.; Yang, H.; Ku, W.; Yang, S.; Liu, Z.; Lu, G. Carbon-Based Electrocatalysts Derived From Biomass for Oxygen Reduction Reaction: A Minireview. Front. Chem. 2020, 8, 116,  DOI: 10.3389/fchem.2020.00116
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    60
    Carbon-based electrocatalysts derived from biomass for oxygen reduction reaction: a minireview
    Wang, Mi; Wang, Shiyu; Yang, Haoqi; Wen, Ku; Yang, Shuchen; Liu, Zhenning; Lu, Guolong
    Frontiers in Chemistry (Lausanne, Switzerland) (2020), 8 (), 116CODEN: FCLSAA; ISSN:2296-2646. (Frontiers Media S.A.)
    Oxygen redn. reaction (ORR) electrocatalysts derived from biomass have become one of the research focuses in hetero-catalysis due to their low cost, high performance, and reproducibility properties. Related researches are of great significance for the development of next-generation fuel cells and metal-air batteries. Herein, the prepn. methods of various biomass-derived catalysts and their performance in alk., neutral, and acidic media are summarized. This review clarifies the research progress of biomass carbon-based electrocatalysts for ORR in acidic, alk. and neutral media, and discusses the future development trends. This minireview can give us an important enlightenment to practical application in the future.
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    Thorum, M. S.; Hankett, J. M.; Gewirth, A. A. Poisoning the Oxygen Reduction Reaction on Carbon-Supported Fe and Cu Electrocatalysts: Evidence for Metal-Centered Activity. J. Phys. Chem. Lett. 2011, 2, 295– 298,  DOI: 10.1021/jz1016284
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    61
    Poisoning the Oxygen Reduction Reaction on Carbon-Supported Fe and Cu Electrocatalysts: Evidence for Metal-Centered Activity
    Thorum, Matthew S.; Hankett, Jeanne M.; Gewirth, Andrew A.
    Journal of Physical Chemistry Letters (2011), 2 (4), 295-298CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
    Whether or not the active sites for the oxygen redn. reaction (ORR) in electrocatalysts based on carbon-supported transition-metal complexes are metal-centered has become controversial, esp. for heat-treated materials. Some have proposed that the transition metal only serves to form highly active sites based on nitrogen and carbon. Here, we examine the oxygen redn. activity of carbon-supported iron(II) phthalocyanine (FePc) before and after pyrolysis at 800 °C and a carbon-supported copper(II) complex with 3,5-diamino-1,2,4-triazole (CuDAT) in the presence of several anions and small-mol. poisons, including fluoride, azide, thiocyanate, ethanethiol, and cyanide. CuDAT is poisoned in a manner consistent with a Cu-based active site. Although FePc and pyrolyzed FePc are remarkably resilient to most poisons, they are poisoned by cyanide, indicative of Fe-based active sites.
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  62. 62
    Wang, Q.; Zhou, Z.-Y.; Lai, Y.-J.; You, Y.; Liu, J.-G.; Wu, X.-L.; Terefe, E.; Chen, C.; Song, L.; Rauf, M.; Tian, N.; Sun, S.-G. Phenylenediamine-Based FeNx/C Catalyst with High Activity for Oxygen Reduction in Acid Medium and Its Active-Site Probing. J. Am. Chem. Soc. 2014, 136, 10882– 10885,  DOI: 10.1021/ja505777v
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    62
    Phenylenediamine-Based FeNx/C Catalyst with High Activity for Oxygen Reduction in Acid Medium and Its Active-Site Probing
    Wang, Qiang; Zhou, Zhi-You; Lai, Yu-Jiao; You, Yong; Liu, Jian-Guo; Wu, Xia-Ling; Terefe, Ephrem; Chen, Chi; Song, Lin; Rauf, Muhammad; Tian, Na; Sun, Shi-Gang
    Journal of the American Chemical Society (2014), 136 (31), 10882-10885CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    High-temp. pyrolyzed FeNx/C catalyst is one of the most promising nonprecious metal electrocatalysts for O redn. reaction (ORR). However, it suffers from 2 challenging problems: insufficient ORR activity and unclear active site structure. Herein, the authors report a FeNx/C catalyst derived from poly-m-phenylenediamine (PmPDA-FeNx/C) that possesses high ORR activity (11.5 A g-1 at 0.80 V vs. RHE) and low H2O2 yield (<1%) in acid medium. The PmPDA-FeNx/C also exhibits high catalytic activity for both redn. and oxidn. of H2O2. Further the ORR activity of PmPDA-FeNx/C is not sensitive to CO and NOx but can be suppressed significantly by halide ions (e.g., Cl-, F-, and Br-) and low valence state S-contg. species (e.g., SCN-, SO2, and H2S). This result reveals that the active sites of the FeNx/C catalyst contains Fe element (mainly as FeIII at high potentials) in acid medium.
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  63. 63
    Thorum, M. S.; Hankett, J. M.; Gewirth, A. A. Poisoning the Oxygen Reduction Reaction on Carbon-Supported Fe and Cu Electrocatalysts: Evidence for Metal-Centered Activity. J. Phys. Chem. Lett. 2011, 2, 295– 298,  DOI: 10.1021/jz1016284
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    63
    Poisoning the Oxygen Reduction Reaction on Carbon-Supported Fe and Cu Electrocatalysts: Evidence for Metal-Centered Activity
    Thorum, Matthew S.; Hankett, Jeanne M.; Gewirth, Andrew A.
    Journal of Physical Chemistry Letters (2011), 2 (4), 295-298CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
    Whether or not the active sites for the oxygen redn. reaction (ORR) in electrocatalysts based on carbon-supported transition-metal complexes are metal-centered has become controversial, esp. for heat-treated materials. Some have proposed that the transition metal only serves to form highly active sites based on nitrogen and carbon. Here, we examine the oxygen redn. activity of carbon-supported iron(II) phthalocyanine (FePc) before and after pyrolysis at 800 °C and a carbon-supported copper(II) complex with 3,5-diamino-1,2,4-triazole (CuDAT) in the presence of several anions and small-mol. poisons, including fluoride, azide, thiocyanate, ethanethiol, and cyanide. CuDAT is poisoned in a manner consistent with a Cu-based active site. Although FePc and pyrolyzed FePc are remarkably resilient to most poisons, they are poisoned by cyanide, indicative of Fe-based active sites.
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    Li, W.; Wu, J.; Higgins, D. C.; Choi, J.-Y.; Chen, Z. Determination of Iron Active Sites in Pyrolyzed Iron-Based Catalysts for the Oxygen Reduction Reaction. ACS Catal. 2012, 2, 2761– 2768,  DOI: 10.1021/cs300579b
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    64
    Determination of Iron Active Sites in Pyrolyzed Iron-Based Catalysts for the Oxygen Reduction Reaction
    Li, Wenmu; Wu, Jason; Higgins, Drew C.; Choi, Ja-Yeon; Chen, Zhongwei
    ACS Catalysis (2012), 2 (12), 2761-2768CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
    Fe-based oxygen redn. reaction (ORR) catalyst materials are considered promising nonprecious alternatives to traditional platinum-based catalysts. These catalyst materials are generally produced by high-temp. pyrolysis treatments of readily available carbon, nitrogen, and iron sources. Adequate control of the structure and active site formation during pyrolysis methods is nearly impossible. Thus, the chem. nature, structure, and ORR mechanism of catalytically active sites in these materials is a subject of significant debate. A method is proposed, utilizing CN- ions as ORR inhibitors on Fe-based catalysts, to provide insight into the exact nature and chem. of the catalytically active sites. Moreover, two possible catalytically active site formation mechanisms occurring during high-temp. pyrolysis treatments, dependent on the specific type of precursor and synthesis methods utilized are proposed. Further direct evidence is provided of the proposed active site formations using ToF-SIMS neg. and pos. ion imaging. This knowledge will be beneficial to future work directed at the development of Fe-based catalysts with improved ORR activity and operational stabilities for fuel cell and battery applications.
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    Xiao, M.; Zhu, J.; Ma, L.; Jin, Z.; Ge, J.; Deng, X.; Hou, Y.; He, Q.; Li, J.; Jia, Q.; Mukerjee, S.; Yang, R.; Jiang, Z.; Su, D.; Liu, C.; Xing, W. Microporous framework induced synthesis of single-atom dispersed Fe-NC acidic ORR catalyst and its in situ reduced Fe-N4 active site identification revealed by X-ray absorption spectroscopy. ACS Catal. 2018, 8, 2824– 2832,  DOI: 10.1021/acscatal.8b00138
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    65
    Microporous Framework Induced Synthesis of Single-Atom Dispersed Fe-N-C Acidic ORR Catalyst and Its in Situ Reduced Fe-N4 Active Site Identification Revealed by X-ray Absorption Spectroscopy
    Xiao, Meiling; Zhu, Jianbing; Ma, Liang; Jin, Zhao; Ge, Junjie; Deng, Xin; Hou, Yang; He, Qinggang; Li, Jingkun; Jia, Qingying; Mukerjee, Sanjeev; Yang, Ruoou; Jiang, Zheng; Su, Dangsheng; Liu, Changpeng; Xing, Wei
    ACS Catalysis (2018), 8 (4), 2824-2832CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
    Developing highly efficient, low-cost O redn. catalysts, esp. in acidic medium, is of significance toward fuel cell commercialization. Although pyrolyzed Fe-N-C catalysts have been regarded as alternatives to platinum-based catalytic materials, further improvement requires precise control of the Fe-Nx structure at the mol. level and a comprehensive understanding of catalytic site structure and the ORR mechanism on these materials. Here, we present a microporous metal-org.-framework-confined strategy toward the preferable formation of single-atom dispersed catalysts. The onset potential for Fe-N-C is 0.92 V, comparable to that of Pt/C and outperforming most noble-metal-free catalysts ever reported. A high-spin Fe3+-N4 configuration is revealed by the 57Fe Moddossbauer spectrum and X-ray absorption spectroscopy for Fe L-edge, which will convert to Fe2+-N4 at low potential. The in situ reduced Fe2+-N4 moiety from high-spin Ox-Fe3+-N4 contributes to most of the ORR activity due to its high turnover frequency (TOF) of ∼1.71 e/s-sites.
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    Li, J.; Ghoshal, S.; Liang, W.; Sougrati, M.-T.; Jaouen, F.; Halevi, B.; McKinney, S.; McCool, G.; Ma, C.; Yuan, X.; Ma, Z.-F.; Mukerjee, S.; Jia, Q. Structural and mechanistic basis for the high activity of Fe–N–C catalysts toward oxygen reduction. Energy Environ. Sci. 2016, 9, 2418– 2432,  DOI: 10.1039/c6ee01160h
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    66
    Structural and mechanistic basis for the high activity of Fe-N-C catalysts toward oxygen reduction
    Li, Jingkun; Ghoshal, Shraboni; Liang, Wentao; Sougrati, Moulay-Tahar; Jaouen, Frederic; Halevi, Barr; McKinney, Samuel; McCool, Geoff; Ma, Chunrong; Yuan, Xianxia; Ma, Zi-Feng; Mukerjee, Sanjeev; Jia, Qingying
    Energy & Environmental Science (2016), 9 (7), 2418-2432CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)
    The development of efficient non-platinum group metal (non-PGM) catalysts for oxygen redn. reaction (ORR) is of paramount importance for clean and sustainable energy storage and conversion devices. The major bottleneck in developing Fe-N-C materials as the leading non-PGM catalysts lies in the poor understanding of the nature of active sites and reaction mechanisms. Herein, we report a scalable metal org. framework-derived Fe-N-C catalyst with high ORR activity demonstrated in practical H2/air fuel cells, and an unprecedented turnover frequency (TOF) in acid in rotating disk electrode. By characterizing the catalyst under both ex situ and operando conditions using combined microscopic and spectroscopic techniques, we show that the structures of active sites under ex situ and working conditions are drastically different. Resultantly, the active site proposed here, a non-planar ferrous Fe-N4 moiety embedded in distorted carbon matrix characterized by a high Fe2+/3+ redox potential, is in contrast with those proposed hitherto derived from ex situ characterizations. This site reversibly switches to an in-plane ferric Fe-N4 moiety poisoned by oxygen adsorbates during the redox transition, with the population of active sites controlled by the Fe2+/3+ redox potential. The unprecedented TOF of the active site is correlated to its near-optimal Fe2+/3+ redox potential, and essentially originated from its favorable biomimetic dynamic nature that balances the site-blocking effect and O2 dissocn. The porous and disordered carbon matrix of the catalyst plays pivotal roles for its measured high ORR activity by hosting high population of reactant-accessible active sites.
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    Shi, C.; Anson, F. C. Catalytic pathways for the electroreduction of oxygen by iron tetrakis (4-N-methylpyridyl) porphyrin or iron tetraphenylporphyrin adsorbed on edge plane pyrolytic graphite electrodes. Inorg. Chem. 1990, 29, 4298– 4305,  DOI: 10.1021/ic00346a027
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    67
    Catalytic pathways for the electroreduction of oxygen by iron tetrakis(4-N-methylpyridyl)porphyrin or iron tetraphenylporphyrin adsorbed on edge plane pyrolytic graphite electrodes
    Shi, Chunnian; Anson, Fred C.
    Inorganic Chemistry (1990), 29 (21), 4298-305CODEN: INOCAJ; ISSN:0020-1669.
    Spontaneous adsorption of Fe tetrakis(4-N-methylpyridyl)porphyrin on edge plane pyrolytic graphite electrodes (EPG) from aq. solns.of the sol. porphyrin produces an adsorbed species that catalyzes the electroredn. of O2 at potentials notably more pos. than can be obtained with electrodes on which the porphyrin does not adsorb (e.g., glassy C). Adsorption of insol. Fe tetraphenylporphyrin (FeTPP) on EPG electrodes produces two surface waves for the adsorbed porphyrin, one prominent and one partially hidden by the response from the EPG surface itself. Only the form of the FeTPP responsible for the latter wave participates in the catalytic redn. of O2. Possible explanations for the obsd. behavior are offered that allow the apparently disparate catalytic reactivity exhibited by iron and cobalt porphyrins to be understood as examples of a single mechanistic pattern.
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    Ni, C. L.; Anson, F. C. Relation between the potentials where adsorbed and unadsorbed cobalt (III) tetrakis (N-methylpyridinium-4-yl) porphyrin is reduced and those where it catalyzes the electroreduction of dioxygen. Inorg. Chem. 1985, 24, 4754– 4756,  DOI: 10.1021/ic00220a067
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    Relation between the potentials where adsorbed and unadsorbed cobalt(III) tetrakis(N-methylpyridinium-4-yl)porphyrin is reduced and those where it catalyzes the electroreduction of dioxygen
    Ni, Ching Long; Anson, Fred C.
    Inorganic Chemistry (1985), 24 (26), 4754-6CODEN: INOCAJ; ISSN:0020-1669.
    The large difference in the formal potentials of CoIII/II between the adsorbed and unadsorbed Co-tetrakis(N-methylpyridinium-4-yl)porphyrin complex (CoTMPyP) as measured with a graphite electrode indicates unusually favorable interactions between the Co center and the electrode surface when the Co is in the reduced state. The adsorbed CoTMPyP, with a formal potential (CoIII/II) near 0.5V vs. SCE catalyzes the redn. of O at potential near 0.3V vs. SCE. The fact that the catalyzed redn. of O proceeds near the formal potential of the CoIII/II couple for the unadsorbed porphyrin is coincidental. Cycles voltammetry of CoTMPyP was studied at edge-plane pyrolytic graphite electrodes and at stationary Au electrodes in 0.5M CF3CO2H. Adsorption of CoTMPyP is much weaker by Au.
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    Bottomley, L. A.; Kadish, K. M. Counterion and solvent effects on the electrode reactions of iron porphyrins. Inorg. Chem. 1981, 20, 1348– 1357,  DOI: 10.1021/ic50219a003
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    Counterion and solvent effects on the electrode reactions of iron porphyrins
    Bottomley, L. A.; Kadish, K. M.
    Inorganic Chemistry (1981), 20 (5), 1348-57CODEN: INOCAJ; ISSN:0020-1669.
    The 1st systematic study of the interacting effects of solvent and axially coordinated monovalent anions on the electroredn. mechanisms and redox potentials of Fe porphyrins is presented. Five different anions were coordinated to (5,10,15,20-tetraphenylporphinato)iron(III), and their resp. redox reactions were investigated in 12 nonaq. solvents. Potential shifts with changes in solvent were directly related to solvent donicity. Potential shifts with changes in counterion were related to the degree of Fe(III)-counterion interaction and were also solvent dependent. The counterion is the most predominant factor governing the type of electron-transfer mechanism obsd. for the electrode reactions of Fe porphyrins. The apparent independence of redox potentials on the Fe-center spin state is established.
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    Milani, M.; Ouellet, Y.; Ouellet, H.; Guertin, M.; Boffi, A.; Antonini, G.; Bocedi, A.; Mattu, M.; Bolognesi, M.; Ascenzi, P. Cyanide Binding to Truncated Hemoglobins:  A Crystallographic and Kinetic Study. Biochemistry 2004, 43, 5213– 5221,  DOI: 10.1021/bi049870+
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    Cyanide Binding to Truncated Hemoglobins: A Crystallographic and Kinetic Study
    Milani, Mario; Ouellet, Yannick; Ouellet, Hugues; Guertin, Michel; Boffi, Alberto; Antonini, Giovanni; Bocedi, Alessio; Mattu, Marco; Bolognesi, Martino; Ascenzi, Paolo
    Biochemistry (2004), 43 (18), 5213-5221CODEN: BICHAW; ISSN:0006-2960. (American Chemical Society)
    Cyanide is one of the few diat. ligands able to interact with the ferric and ferrous heme-Fe atom. Here, the X-ray crystal structure of the cyanide deriv. of ferric Mycobacterium tuberculosis truncated Hb-N (M. tuberculosis trHbN) has been detd. at 2.0 Å (R-general = 17.8% and R-free = 23.5%), and analyzed in parallel with those of M. tuberculosis truncated Hb-O (M. tuberculosis trHbO), Chlamydomonas eugametos truncated Hb (C. eugametos trHb), and sperm whale myoglobin, generally taken as a mol. model. Cyanide binding to M. tuberculosis trHbN is stabilized directly by residue TyrB10(33), which may assist the deprotonation of the incoming ligand and the protonation of the outcoming cyanide. In M. tuberculosis trHbO and in C. eugametos trHb the ligand is stabilized by the distal pocket residues TyrCD1(36) and TrpG8(88), and by the TyrB10(20) - GlnE7(41) - GlnE11(45) triad, resp. Moreover, kinetics for cyanide binding to ferric M. tuberculosis trHbN and trHbO and C. eugametos trHb, for ligand dissocn. from the ferrous trHbs, and for the redn. of the heme-Fe(III)-cyanide complex have been detd., at pH 7.0 and 20.0°. Despite the different heme distal site structures and ligand interactions, values of the rate const. for cyanide binding to ferric (non)vertebrate heme proteins are similar, being influenced mainly by the presence in the heme pocket of proton acceptor group(s), whose function is to assist the deprotonation of the incoming ligand (i.e., HCN). On the other hand, values of the rate const. for the redn. of the heme-Fe(III)-cyanide (non)vertebrate globins span over several orders of magnitude, reflecting the different ability of the heme proteins considered to give productive complex(es) with dithionite or its reducing species SO2-. Furthermore, values of the rate const. for ligand dissocn. from heme-Fe(II)-cyanide (non)vertebrate heme proteins are very different, reflecting the different nature and geometry of the heme distal residue(s) hydrogen-bonded to the heme-bound cyanide.
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    Wu, G.; Johnston, C. M.; Mack, N. H.; Artyushkova, K.; Ferrandon, M.; Nelson, M.; Lezama-Pacheco, J. S.; Conradson, S. D.; More, K. L.; Myers, D. J.; Zelenay, P. Synthesis–structure–performance correlation for polyaniline–Me–C non-precious metal cathode catalysts for oxygen reduction in fuel cells. J. Mater. Chem. 2011, 21, 11392– 11405,  DOI: 10.1039/c0jm03613g
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    71
    Synthesis-structure-performance correlation for polyaniline-Me-C non-precious metal cathode catalysts for oxygen reduction in fuel cells
    Wu, Gang; Johnston, Christina M.; Mack, Nathan H.; Artyushkova, Kateryna; Ferrandon, Magali; Nelson, Mark; Lezama-Pacheco, Juan S.; Conradson, Steven D.; More, Karren L.; Myers, Deborah J.; Zelenay, Piotr
    Journal of Materials Chemistry (2011), 21 (30), 11392-11405CODEN: JMACEP; ISSN:0959-9428. (Royal Society of Chemistry)
    The authors present the systematic prepn. of active and durable non-precious metal catalysts (NPMCs) for the oxygen redn. reaction in polymer electrolyte fuel cells (PEFCs) based on the heat treatment of polyaniline/metal/carbon precursors. Variation of the synthesis steps, heat-treatment temp., metal loading, and the metal type in the synthesis leads to markedly different catalyst activity, speciation, and morphol. Microscopy studies demonstrate notable differences in the carbon structure as a function of these variables. Balancing the need to increase the catalyst's degree of graphitization through heat treatment vs. the excessive loss of surface area that occurs at higher temps. is a key to prepg. an active catalyst. XPS and XAFS spectra are consistent with the presence of Me-Nx structures in both the Co and Fe versions of the catalyst, which are often proposed to be active sites. The av. speciation and coordination environment of nitrogen and metal, however, depends greatly on the choice of Co or Fe. Taken together, the data indicate that better control of the metal-catalyzed transformations of the polymer into new graphitized carbon forms in the heat-treatment step will allow for even further improvement of this class of catalysts.
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    Gadipelli, S.; Zhao, T.; Shevlin, S. A.; Guo, Z. Switching effective oxygen reduction and evolution performance by controlled graphitization of a cobalt–nitrogen–carbon framework system. Energy Environ. Sci. 2016, 9, 1661– 1667,  DOI: 10.1039/c6ee00551a
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    Switching effective oxygen reduction and evolution performance by controlled graphitization of a cobalt-nitrogen-carbon framework system
    Gadipelli, Srinivas; Zhao, Tingting; Shevlin, Stephen A.; Guo, Zhengxiao
    Energy & Environmental Science (2016), 9 (5), 1661-1667CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)
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    We demonstrate a new strategy for tuning the size of large-diam. and few-walled nitrogen-doped carbon nanotubes (N-CNTs) from 50 to 150 nm by varying the transition metal (TM = Fe, Co, Ni or Mn) used to catalyze graphitization of dicyandiamide. Fe yielded the largest tubes, followed by Co and Ni, while Mn produced a clot-like carbon morphol. We show that morphol. is correlated with electrocatalytic activity for the oxygen redn. reaction (ORR). A clear trend of Fe > Co > Ni > Mn for the ORR catalytic activity was obsd., in both alk. media and more demanding acidic media. The Fe-derived N-CNTs exhibited the highest BET (∼870 m2 g-1) and electrochem. accessible (∼450 m2 g-1) surface areas and, more importantly, the highest concn. of nitrogen incorporated into the carbon planes. Thus, in addn. to the intrinsic high activity of Fe-derived catalysts, the high surface area and nitrogen doping contribute to high ORR activity. This work, for the first time, demonstrates size-controlled synthesis of large-diam. N-doped carbon tube electrocatalysts by varying the metal used in N-CNT generation. Electrocatalytic activity of the Fe-derived catalyst is already the best among studied metals, due to the high intrinsic activity of possible Fe-N coordination. This work further provides a promising route to advanced Fe-N-C nonprecious metal catalysts by generating favorable morphol. with more active sites and improved mass transfer.
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    We firstly propose the application of CNTs as novel catalysts and mol. oxygen as the oxidant for the oxidative desulfurization (ODS) of a model fuel contg. benzothiophene (BT), dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (4,6-DMDBT) at atm. pressure and low temp. Results showed that when three CNTs including CNT-SZ, CNT-TS, and CNT-CD were sep. used as catalysts with mol. oxygen as the oxidant, the conversion of DBT to its corresponding sulfone reached 100% at 150°C sep. within 40, 120 and 180 min. The CNT-SZ exhibited a superior catalytic activity even at a high fuel-to-catalyst wt. ratio of 7.5:1. The oxidn. reactivity of these benzothiophenic compds. followed the order: 4,6-DMDBT > DBT > BT. The deactivated CNT can be effectively regenerated by heat treatment under an argon atm. at 900°C. Raman spectroscopy anal. revealed that the graphitization degree of the CNT played a decisive role in its catalytic activity for DBT oxidn. The CNT with the higher degree of graphitization had higher catalytic activity for DBT oxidn. since its higher elec. cond. benefited the transfer of electrons involved in the redox reaction.
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    ACS Catalysis (2017), 7 (9), 6144-6149CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
    Three-dimensional macroporous carbon materials with hierarchical pore structures (3D MPC) have wide applications, but the scale-up synthesis is limited by the cumbersome procedures of template formation and removal. Herein, we show that NaCl crystallites, which form in situ in a lyophilizing process of a NaCl soln. contg. a carbon precursor and are removable simply through water washing, can act as templates to grow 3D MPC materials with graphene-like ultrathin and mesoporous walls through pyrolitic carbonization. Further, by use of a nitrogen (N)-rich polymer (polyvinylpyrrolidone, PVP) as the carbon precursor and introduction of Fe salt in the precursor, an MPC catalyst with high Fe/N doping content is achieved due to the NaCl crystallites serving as confining agents to simultaneously prevent the large wt. loss and N evapn., a severe problem in usual pyrolytic syntheses of Fe-N-C catalysts. Benefiting from the mass transport convenience of the macropores as indicated by the impedance spectroscopy results, the Fe/N-doped 3D MPC exhibits high catalytic performance toward the oxygen redn. reaction. The dual functionality, facile formation and removal, and reusability of NaCl make the present method a promising way to gain cost-effective porous Fe-N-C catalysts.
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    ACS Applied Materials & Interfaces (2018), 10 (10), 8721-8729CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
    Heteroatoms doping is able to produce catalytic sites in C materials for O redn. reaction (ORR); while hierarchically porous structure is necessary for efficient exposure and accessibility of the usually limited catalytic sites in such activated C catalysts. This work reports an in situ generated dual-template method to synthesize the Fe/N/S co-doped hierarchically porous C (FeNS/HPC), with NaCl crystallites formed during the precursor lyophilization process as the primary template to generate ∼500 nm macropores with ultrathin graphene-like C-layer walls, and Fe3O4 nanoparticles formed during the high-temp. carbonization process as the secondary template to produce mesopores on the walls of macropores. As well as the coexistence of graphitic-N, pyridinic-N, and thiophene-S which are beneficial to ORR, the as prepd. FeNS/HPC possesses a highly graphitized and interconnected hierarchical porous structure, giving a sp. surface area as high as 938 m2/g. As a consequence, it exhibits excellent 4-electron O redn. performance in both alk. and acid electrolytes. The in situ generation and facile soln. removal make the present template method a promising way for scale-up prepn. of active porous C materials for various applications.
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    Utilization of heat released from the classical aluminothermic reaction (Fe2O3 + Al) for thermal condensation of melamine allows prodn. of a large amt. of C/N/H graphitic ternary polymers within a matter of a few minutes. This approach could allow instant, large-scale synthesis of g-C3N4. A detailed, comparative study of melamine condensation to hydrogen-rich melon-related CxNyHz graphitic ternary polymers (which can be considered as a pre-stage to carbon(iv) nitride) initiated by resistance furnace or by thermite reaction is presented. The products obtained by both heating routes at two different temps. (560 and 640 °C) are compared and analyzed. It is shown that the photocatalytic activity of the CxNyHz polymers correlates with the hydrogen content in the materials. The superiority of the thermite-heated process over elec. resistance furnace heating is elucidated. The exceptional characteristics of the thermite reaction, that is the use of cheap, ubiquitous substrates, rapid increase of temp. and peerless exothermicity, make it suitable for efficient, large-scale transformation of a variety of primary org. compds. (melamine and beyond) into new advanced functional materials.
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    Carbonaceous porous structures have instigated global research interest as promising low-cost electrocatalysts for numerous energy technologies. However, the rational design principle of pore structures for activity maximization is still unclear. In this work, a series of N-doped carbon (N-C) catalysts with exclusively different micro-mesoporosity are investigated for the oxygen redn. reaction (ORR). By combining the expt. results and a pioneering math. model, it was obsd. that the best catalytic activity can only be attained by balancing the micro-mesoporosity. These findings offer a definite criterion for pore structure optimization in carbon-based ORR catalysts, which is of great importance for various energy technologies.
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    Limited availability of Pt is a threat to fuel cell commercialization. Alternative catalysts for the electrochem. redn. of O were obtained by heat treatment at >600° of C with non-noble metals and a source of N atoms. However, the process by which heat treatment activates these materials is uncertain. The activation of C black and Fe acetate heat-treated in NH3 comprises 3 consecutive steps: (a) incorporation of N atoms in the C, (b) micropore formation through reaction between C and NH3, and (c) completion of active sites in the micropores by reaction of Fe with NH3. Step (b) is the slowest. The microporous surface per mass of catalyst controls the macroscopic activity when enough N atoms are incorporated in the structure of the C support. These facts should help in detg. the structure of the active sites and in identifying methods to increase the site d. of such catalysts.
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    Fe-based catalysts for the O-redn. reaction in polymer electrolyte fuel cells are not as good as Pt catalysts, in part because they have a comparatively low no. of active sites per unit vol. Microporous C-supported Fe-based catalysts, with active sites contg. Fe cations coordinated by pyridinic N functionalities in the interstices of graphitic sheets within the micropores, were prepd. The greatest increase in site d. was obtained when a mixt. of C support, phenanthroline, and ferrous acetate was ball-milled and then pyrolyzed in Ar and then in NH3. The c.d. of a cathode made with the best Fe-based electrocatalyst equals that of a Pt-based cathode with a loading of 0.4 mg Pt/cm2 at a cell voltage of ≥0.9 V.
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    Effect of Transition Metals on the Structure and Performance of the Doped Carbon Catalysts Derived From Polyaniline and Melamine for ORR Application
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    The effects of the addn. of transition metals (Mn, Fe, Co, Ni, Cu) on the structure and performance of the doped C catalysts M-PANI/C-Mela are studied. The doping of various transition metals affected structures and performances of the catalysts significantly. Doping with Fe and Mn leads to a catalyst with a graphene-like structure, and doping with Co, Ni, and Cu leads to a disordered or nanosheet structure. The doping of transition metals can enhance the performance of the catalysts, and their ORR activity follows the order of Fe > Co > Cu > Mn > Ni, which is consistent with the order of their active N contents. Probably the various performance enhancements of the transition metals may be the result of the joint effect of the following 3 aspects: the N content/active N content, metal residue, and the surface area and pore structure, but not the effect of any single factor.
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    Catalyst-Free Synthesis of Nitrogen-Doped Graphene via Thermal Annealing Graphite Oxide with Melamine and Its Excellent Electrocatalysis
    Sheng, Zhen-Huan; Shao, Lin; Chen, Jing-Jing; Bao, Wen-Jing; Wang, Feng-Bin; Xia, Xing-Hua
    ACS Nano (2011), 5 (6), 4350-4358CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    The electronic and chem. properties of graphene can be modulated by chem. doping foreign atoms and functional moieties. The general approach to the synthesis of N-doped graphene (NG), such as CVD performed in gas phases, requires transitional metal catalysts which could contaminate the resultant products and thus affect their properties. The authors propose a facile, catalyst-free thermal annealing approach for large-scale synthesis of NG using low-cost industrial material melamine as the N source. This approach can completely avoid the contamination of transition metal catalysts, and thus the intrinsic catalytic performance of pure NGs can be studied. Detailed x-ray photoelectron spectrum anal. of the resultant products shows that the at. percentage of N in doped graphene samples can be adjusted up to 10.1%. Such a high doping level was not reported previously. High-resoln. N1s spectra reveal that the as-made NG mainly contains pyridine-like N atoms. Electrochem. characterizations clearly demonstrate excellent electrocatalytic activity of NG toward the O redn. reaction (ORR) in alk. electrolytes, which is independent of N doping level. The present catalyst-free approach opens up the possibility for the synthesis of NG in gram-scale for electronic devices and cathodic materials for fuel cells and biosensors.
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    A Highly Efficient Electrocatalyst for the Oxygen Reduction Reaction: N-Doped Ketjenblack Incorporated into Fe/Fe3C-Functionalized Melamine Foam
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    Angewandte Chemie, International Edition (2013), 52 (3), 1026-1030CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    To dramatically enhance mass transfer through porous O-breathing electrodes, we created unique electrode architectures that are similar to breakwaters composed of highly porous tetrapod structures. To imitate this structure, we started with a com. available melamine foam. After pyrolysis, the open-cell network structure of the carbonized melamine foam becomes very fragile, and it is easily broken to pieces by grinding with a mortar. Interestingly, the broken pieces appear similar to the tetrapod architecture. our unique electrodes consist of carbonized melamine foam and nano-sized ketjenblack, thus offering high sp. surface area, large no. of active sites, and large amt. of pore vol. for fast mass transport. Although Lin et al. (2012) used melamine resin to prep. metal-free N-doped electrocatalysts for ORR, the unique architecture of carbonized melamine foam has not yet been used in synthesizing electrocatalysts for ORR.
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    The high cost and limited reserves of noble metals such as Pt have hampered their large-scale com. applications in the oxygen redn. reaction (ORR) and hydrogen evolution reaction (HER). Herein, we developed a simple silica template approach to form porous carbons codoped with nitrogen and cobalt as highly efficient bifunctional electrocatalysts for the ORR and HER. The resulting porous carbons exhibited excellent electrocatalytic activity for the ORR in alk. media, which compares favorably with that of com. Pt/C (20 wt%), and had superior durability and excellent methanol tolerance. The porous carbons also showed superior performance for the HER, with a low onset potential of -0.04 V, a Tafel slope of 49 mV dec-1, an overpotential of -0.106 V at a c.d. of 10 mA cm-2, and remarkable durability. These results demonstrate that the Co, N-doped carbons are promising bifunctional catalysts for fuel cell applications.
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    Energy & Environmental Science (2019), 12 (6), 1751-1779CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)
    A review. Global warming, environmental pollution, and an energy shortage in the current fossil fuel society may cause a severe ecol. crisis. Storage and conversion of renewable, dispersive and non-perennial energy from the sun, wind, geothermal sources, water, or biomass could be a promising option to relieve this crisis. Carbon materials could be the most versatile platform materials applied in the field of modern energy storage and conversion. Conventional carbon materials produced from coal and petrochem. products are usually energy intensive or involve harsh synthetic conditions. It is highly desired to develop effective methods to produce carbon materials from renewable resources that have high performance and limited environmental impacts. In this regard, biochar, a bio-carbon with abundant surface functional groups and easily tuned porosity produced from biomass, may be a promising candidate as a sustainable carbon material. Recent studies have demonstrated that biochar-based materials show great application potential in energy storage and conversion because of their easily tuned surface chem. and porosity. In this review, recent advances in the applications of biochar-based materials in various energy storage and conversion fields, including hydrogen storage and prodn., oxygen electrocatalysts, emerging fuel cell technol., supercapacitors, and lithium/sodium ion batteries, are summarized, highlighting the mechanisms and open questions in current energy applications. Finally, contemporary challenges and perspectives on how biochar-based materials will develop and, in particular, the fields in which the use of biochar-based materials could be expanded are discussed throughout the review. This review demonstrates significant potential for energy applications of biochar-based materials, and it is expected to inspire new discoveries to promote practical applications of biochar-based materials in more energy storage and conversion fields.
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  • Abstract

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    Scheme 1

    Scheme 1. Schematic of the Procedure for Preparing Spinach-Derived Carbon Nanosheets
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    Figure 1

    Figure 1. Representative SEM (a–c) and TEM (d,e) images of M+S+C900-900. The inset in (e) is an electron diffraction pattern of M+S+C900-900.

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    Figure 2

    Figure 2. Structural characterizations of M+S+C900-900. (a) XRD. (b) Raman spectrum. (c) N2 adsorption–desorption isotherm. (d) PSD. Inset: zoom-in of the PSD. (e) XPS survey spectrum. (f–i) High-resolution XPS spectrum of: (f) C 1s, (g) N 1s, (h) S 2p, and (i) Fe 2p.

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    Figure 3

    Figure 3. Oxygen reduction studies of M+S+C900-900 in 0.1 M KOH. (a) Cyclic voltammograms recorded in N2- and O2-saturated solutions; scan rate: 10 mV s–1. (b) Linear sweep voltammetry (LSV) curves obtained in O2-saturated solutions; electrode rotation rate: 1600 rpm and potential scan rate: 10 mV s–1. (c) Plot of the number of electron transfer (n) vs the electrode potential. (d) LSV curves recorded in an O2-saturated solution at various rotation rates. (e) Koutecky–Levich (K–L) plots. (f) Tafel plots. Results from Pt/C are included for comparison.

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    Figure 4

    Figure 4. Methanol tolerance and stability comparison between M+S+C900-900 and Pt/C in O2-saturated 0.1 M KOH. (a) Current–time (i–t) responses at +0.88 V, with the addition of 1 M methanol at 1200 s. (b) Normalized current–time profiles at +0.88 V, at a rotation rate of 900 rpm. (c,d) LSV curves obtained with freshly prepared catalysts (solid traces) and after storage in the electrolyte solution for 60 days (dashed traces): (c) M+S+C900-900 and (d) Pt/C. Electrode rotation rate: 1600 rpm and potential scan rate: 10 mV s–1.

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    Figure 5

    Figure 5. (a) LSV curves of M+S+C900-900 recorded before (red) and after (black) the addition of (a) 10 mM KCN and (b) 5 mM NaF in O2-saturated 0.1 M KOH. Electrode rotation rate: 1600 rpm and potential scan rate: 10 mV s–1.

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    Figure 6

    Figure 6. Results from controlled experiments. (a) Effects of pyrolysis temperature. (b,c) Effects of second heat treatment at 900 °C: (b) LSV and (c) EIS. (d) Effects of different reagents on the starting materials. The LSV curves were recorded in O2-saturated 0.1 M KOH with a rotation speed of 1600 rpm and a potential scan rate of 10 mV s–1.

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  • References

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    This article references 86 other publications.

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      Pan, Fuping; Cao, Zhongyue; Zhao, Qiuping; Liang, Hongyu; Zhang, Junyan
      Journal of Power Sources (2014), 272 (), 8-15CODEN: JPSODZ; ISSN:0378-7753. (Elsevier B.V.)
      The successful commercialization of fuel cells requires the efficient electrocatalyst to make the O redn. reaction (ORR) fast because of the sluggish nature of ORR and the high cost of the Pt catalysts. The authors report the excellent performance of metal-free N-doped porous C nanosheets (NPCN) with hierarchical porous structure and a high surface area of 1436.02 m2 g-1 for catalyzing ORR. The active NPCN is synthesized via facile high-temp. carbonization of natural ginkgo leaves followed by purifn. and NH3 post-treatment without using addnl. supporting templates and activation processes. In O2-satd. 0.1 M KOH soln., the resultant NPCN exhibits a high kinetic-limiting c.d. of 13.57 mA cm-2 at -0.25 V (vs. Ag/AgCl) approaching that of the com. Pt/C catalyst (14 mA cm-2) and long-term electrochem. stability. Notably, the NPCN shows a slightly neg. ORR half-wave potential in comparison with Pt/C (ΔE1/2 = 19 mV). The excellent electrocatalytic properties of NPCN originate from the combined effect of optimal N doping, high surface area, and porous architecture, which induce the high-d. distribution of highly active and stable catalytic sites.
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    14. 14
      Zhang, P.; Gong, Y.; Wei, Z.; Wang, J.; Zhang, Z.; Li, H.; Dai, S.; Wang, Y. Updating Biomass into Functional Carbon Material in Ionothermal Manner. ACS Appl. Mater. Interfaces 2014, 6, 12515– 12522,  DOI: 10.1021/am5023682
      [ACS Full Text ACS Full Text], [CAS], Google Scholar
      14
      Updating Biomass into Functional Carbon Material in Ionothermal Manner
      Zhang, Pengfei; Gong, Yutong; Wei, Zhongzhe; Wang, Jing; Zhang, Zhiyong; Li, Haoran; Dai, Sheng; Wang, Yong
      ACS Applied Materials & Interfaces (2014), 6 (15), 12515-12522CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
      The development of meaningful ways to transfer biomass into useful materials, more efficient energy carriers, and/or carbon storage deposits is a profound challenge of our days. Herein, an ionothermal carbonization (ITC) method, via treating natural resources (glucose, cellulose, and sugar cane bagasse) in nonmetal ionic liqs. (ILs) at ∼200 °C, is established for the fabrication of porous heteroatom-doped carbon materials with high yield. Com. ILs with bulky bis(trifluoromethylsulfonyl)imide anion or cross-linkable nitrile group were found to be efficient and recyclable templates for porosity control, leading to exciting nanoarchitectures with promising performance in oxygen redn. reaction. The optimized ILs (12 mL) can dissolve and directly convert up to 15 g of glucose into porous carbon materials (SBET: 272 m2/g) one time. This ITC method relies on the synergistic use of structure-directing effect, good biomass soly., and excellent thermal stability of ILs, and provides a sustainable strategy for exploiting biomass.
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    15. 15
      Gao, S.; Chen, Y.; Fan, H.; Wei, X.; Hu, C.; Luo, H.; Qu, L. Large scale production of biomass-derived N-doped porous carbon spheres for oxygen reduction and supercapacitors. J. Mater. Chem. A 2014, 2, 3317– 3324,  DOI: 10.1039/c3ta14281g
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      15
      Large scale production of biomass-derived N-doped porous carbon spheres for oxygen reduction and supercapacitors
      Gao, Shuyan; Chen, Yanli; Fan, Hao; Wei, Xianjun; Hu, Chuangang; Luo, Hongxia; Qu, Liangti
      Journal of Materials Chemistry A: Materials for Energy and Sustainability (2014), 2 (10), 3317-3324CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
      The urgent need for sustainable energy development depends on the progress of green technologies, which have steered hot research areas into environmentally benign approaches via inexpensive precursors and abundant resources obtained directly from nature for energy devices such as fuel cells and supercapacitors. Using fermented rice as a starting material, a facile, green and scalable approach to synthesize porous N-doped carbon spheres was demonstrated, which was characterized by high sp. surface areas (2105.9 m2 g-1) and high porosity (1.14 cm3 g-1), which exhibit not only excellent electrocatalytic activity toward the four-electron oxygen redn. reaction with long-term stability for fuel cells, but also have excellent resistance to crossover effects and CO poisoning superior to that of the com. Pt/C catalyst. Furthermore, the naturally derived porous N-doped carbon spheres, used as the active electrode materials, present superior performance for capacitors with a capacitance of 219 F g-1 at a high discharge c.d. of 15 A g-1 and good cycling stability for over 4400 cycles. This work shows a good example for taking advantage of the abundant resources provided by nature, and opening the door for the creation of functional materials with promising applications in high-performance renewable devices related to energy conversion and storage.
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    16. 16
      Li, J.-C.; Hou, P.-X.; Zhao, S.-Y.; Liu, C.; Tang, D.-M.; Cheng, M.; Zhang, F.; Cheng, H.-M. A 3D bi-functional porous N-doped carbon microtube sponge electrocatalyst for oxygen reduction and oxygen evolution reactions. Energy Environ. Sci. 2016, 9, 3079– 3084,  DOI: 10.1039/c6ee02169g
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      16
      A 3D bi-functional porous N-doped carbon microtube sponge electrocatalyst for oxygen reduction and oxygen evolution reactions
      Li, Jin-Cheng; Hou, Peng-Xiang; Zhao, Shi-Yong; Liu, Chang; Tang, Dai-Ming; Cheng, Min; Zhang, Feng; Cheng, Hui-Ming
      Energy & Environmental Science (2016), 9 (10), 3079-3084CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)
      A flexible, large-area three-dimensional porous N-doped carbon microtube (NCMT) sponge was prepd. via a simple and low-cost process of pyrolyzing facial cotton. Due to its unique structure with a micron-scale hollow core and well-graphitized and interconnected porous walls, the NCMT sponge exhibits incomparable electrocatalytic activity for the oxygen redn. reaction (ORR) and the oxygen evolution reaction (OER) with a small p.d. of 0.63 V between the OER c.d. at 10 mA cm-2 and the ORR c.d. at -3 mA cm-2, which is the best to date.
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    17. 17
      Wang, R.; Wang, H.; Zhou, T.; Key, J.; Ma, Y.; Zhang, Z.; Wang, Q.; Ji, S. The enhanced electrocatalytic activity of okara-derived N-doped mesoporous carbon for oxygen reduction reaction. J. Power Sources 2015, 274, 741– 747,  DOI: 10.1016/j.jpowsour.2014.10.049
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      17
      The enhanced electrocatalytic activity of okara-derived N-doped mesoporous carbon for oxygen reduction reaction
      Wang, Rongfang; Wang, Hui; Zhou, Tianbao; Key, Julian; Ma, Yanjiao; Zhang, Zheng; Wang, Qizhao; Ji, Shan
      Journal of Power Sources (2015), 274 (), 741-747CODEN: JPSODZ; ISSN:0378-7753. (Elsevier B.V.)
      Nitrogen-doped carbon (N-C) catalysts can potentially offer high ORR (oxygen redn. reaction) electrocatalytic activity comparable to Pt/C catalysts. Here, we establish a correlation between N-species (pyridinic-N and graphitic-N) with high ORR activity and a key role for Fe in their prepn. N-C catalysts are prepd. from okara (a cheap, nitrogen-rich, biomass precursor) using a facile synthesis method with inclusion of FeCl3 at different steps of synthesis. Mesoporous N-C catalyst is produced that had ORR activity comparable to that of com. Pt/C catalyst. High ORR-activity N-C results from the presence of FeCl3 at a specific step during synthesis. Detailed investigation by XPS reveals that increased levels of pyridinic-N and graphitic-N arose from pyridinic-N-oxide conversion in the presence of Fe. We conclude that transforming inert N species to active N species underlies the increase in active catalytic sites on the carbon surface and offers a means to improve N-C catalyst performance.
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    18. 18
      Borghei, M.; Laocharoen, N.; Kibena-Põldsepp, E.; Johansson, L.-S.; Campbell, J.; Kauppinen, E.; Tammeveski, K.; Rojas, O. J. Porous N,P-doped carbon from coconut shells with high electrocatalytic activity for oxygen reduction: Alternative to Pt-C for alkaline fuel cells. Appl. Catal., B 2017, 204, 394– 402,  DOI: 10.1016/j.apcatb.2016.11.029
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      18
      Porous N,P-doped carbon from coconut shells with high electrocatalytic activity for oxygen reduction: Alternative to Pt-C for alkaline fuel cells
      Borghei, Maryam; Laocharoen, Nikorn; Kibena-Poldsepp, Elo; Johansson, Leena-Sisko; Campbell, Joseph; Kauppinen, Esko; Tammeveski, Kaido; Rojas, Orlando J.
      Applied Catalysis, B: Environmental (2017), 204 (), 394-402CODEN: ACBEE3; ISSN:0926-3373. (Elsevier B.V.)
      This study introduces a new, environmentally-friendly method to synthesize N,P-doped porous carbon by high conversion (46% yield) of coconut shell residues for the redn. of oxygen in alk. media. The obtained materials display an excellent electrocatalytic activity, making them suitable as cathode catalyst for alk. fuel cells. The synthesis procedure included an efficient single-step activation with phosphoric acid to achieve high surface area (1216 m2 g-1) and pore vol. (1.15 cm3 g-1 with 72% mesopores). Urea was used as a low-cost and ecol.-sound source for nitrogen doping of the as-synthesized porous carbon. Remarkably, the biomass-derived electroactive carbon demonstrates a superior performance compared to a ref. material, the state-of-the-art com. Pt-C catalyst: (a) comparable electrocatalytic activity; (b) better tolerance to methanol crossover effects and, (c) improved long-term durability towards oxygen redn. reaction in alk. media.
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    19. 19
      Chaudhari, K. N.; Song, M. Y.; Yu, J.-S. Transforming Hair into Heteroatom-Doped Carbon with High Surface Area. Small 2014, 10, 2625– 2636,  DOI: 10.1002/smll.201303831
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      19
      Transforming Hair into Heteroatom-Doped Carbon with High Surface Area
      Chaudhari, Kiran N.; Song, Min Young; Yu, Jong-Sung
      Small (2014), 10 (13), 2625-2636CODEN: SMALBC; ISSN:1613-6810. (Wiley-VCH Verlag GmbH & Co. KGaA)
      We present a unique approach to dispose of human hair by pyrolyzing it in a regulated environment, yielding highly porous, conductive hair carbons with heteroatoms and high surface area. α-Keratin in the protein network of hair serves as a precursor for the heteroatoms and C. The C framework is ingrained with heteroatoms such as N and S, which otherwise are incorporated externally through energy-intensive, hazardous, chem. reactions using proper org. precursors. This judicious transformation of org.-rich waste not only addresses the disposal issue, but also generates valuable functional C materials from the discard. Our unique synthesis strategy involving moderate activation and further graphitization enhances the elec. cond., while still maintaining the precious heteroatoms. The effect of temp. on the structural and functional properties is studied, and all the as-obtained carbons are applied as metal-free catalysts for the O redn. reaction (ORR). C graphitized at 900° emerges as a superior ORR electrocatalyst with excellent electrocatalytic performance, high selectivity, and long durability, demonstrating that hair C can be a promising alternative for costly Pt-based electrocatalysts in fuel cells. The ORR performance can be discussed in terms of heteroatom doping, surface properties, and elec. cond. of the resulting porous hair C materials.
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    20. 20
      Amiinu, I. S.; Zhang, J.; Kou, Z.; Liu, X.; Asare, O. K.; Zhou, H.; Cheng, K.; Zhang, H.; Mai, L.; Pan, M.; Mu, S. Self-organized 3D porous graphene dual-doped with biomass-sponsored nitrogen and sulfur for oxygen reduction and evolution. ACS Appl. Mater. Interfaces 2016, 8, 29408– 29418,  DOI: 10.1021/acsami.6b08719
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      20
      Self-Organized 3D Porous Graphene Dual-Doped with Biomass-Sponsored Nitrogen and Sulfur for Oxygen Reduction and Evolution
      Amiinu, Ibrahim Saana; Zhang, Jian; Kou, Zongkui; Liu, Xiaobo; Asare, Owusu Kwadwo; Zhou, Huang; Cheng, Kun; Zhang, Haining; Mai, Liqiang; Pan, Mu; Mu, Shichun
      ACS Applied Materials & Interfaces (2016), 8 (43), 29408-29418CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
      3D graphene-based materials offer immense potentials to overcome the challenges related to the functionality, performance, cost, and stability of fuel cell electrocatalysts. Herein, a nitrogen (N) and sulfur (S) dual-doped 3D porous graphene catalyst is synthesized via a single-row pyrolysis using biomass as solitary source for both N and S, and structure directing agent. The thermochem. reaction of biomass functional groups with graphene oxide facilitates in situ generation of reactive N and S species, stimulating the graphene layers to reorganize into a trimodal 3D porous assembly. The resultant catalyst exhibits high ORR and OER performance superior to similar materials obtained through toxic chems. and multistep routes. Its stability and tolerance to CO and methanol oxidn. mols. are far superior to com. Pt/C. The dynamics governing the structural transformation and the enhanced catalytic activity in both alk. and acidic media are discussed. This work offers a unique approach for rapid synthesis of a dual-heteroatom doped 3D porous-graphene-architecture for wider applications.
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    21. 21
      Guo, C.; Liao, W.; Li, Z.; Sun, L.; Chen, C. Easy conversion of protein-rich enoki mushroom biomass to a nitrogen-doped carbon nanomaterial as a promising metal-free catalyst for oxygen reduction reaction. Nanoscale 2015, 7, 15990– 15998,  DOI: 10.1039/c5nr03828f
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      21
      Easy conversion of protein-rich enoki mushroom biomass to a nitrogen-doped carbon nanomaterial as a promising metal-free catalyst for oxygen reduction reaction
      Guo, Chaozhong; Liao, Wenli; Li, Zhongbin; Sun, Lingtao; Chen, Changguo
      Nanoscale (2015), 7 (38), 15990-15998CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
      The search for low-cost, highly active, and stable catalysts to replace the Pt-based catalysts for oxygen redn. reaction (ORR) has recently become a topic of interest. Herein, we report a new strategy to design a nitrogen-doped carbon nanomaterial for use as a metal-free ORR catalyst based on facile pyrolysis of protein-rich enoki mushroom (Flammulina velutipes) biomass at 900 °C with carbon nanotubes as a conductive agent and inserting matrix. We found that various forms of nitrogen (nitrile, pyrrolic and graphitic) were incorporated into the carbon mol. skeleton of the product, which exhibited more excellent ORR electrocatalytic activity and better durability in alk. medium than those in acidic medium. Remarkably, the ORR half-wave potential measured on our material was around 0.81 V in alk. medium, slightly lower than that on the com. 20 wt% Pt/C catalyst (0.86 V). Meanwhile, the ORR followed the desired 4-electron transfer mechanism involving the direct redn. pathway. The ORR performance was also markedly better than or at least comparable to the leading results in the literature based on biomass-derived carbon-based catalysts. Besides, we significantly proposed that the graphitic-nitrogen species that is most responsible for the ORR activity can function as the electrocatalytically active center for ORR, and the pyrrolic-nitrogen species can act as an effective promoter for ORR only. The results suggested a promising route based on economical and sustainable fungi biomass towards the large-scale prodn. of valuable carbon nanomaterials as highly active and stable metal-free catalysts for ORR under alk. conditions.
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    22. 22
      Gao, S.; Fan, H.; Zhang, S. Nitrogen-enriched carbon from bamboo fungus with superior oxygen reduction reaction activity. J. Mater. Chem. A 2014, 2, 18263– 18270,  DOI: 10.1039/c4ta03558e
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      22
      Nitrogen-enriched carbon from bamboo fungus with superior oxygen reduction reaction activity
      Gao, Shuyan; Fan, Hao; Zhang, Shuxia
      Journal of Materials Chemistry A: Materials for Energy and Sustainability (2014), 2 (43), 18263-18270CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
      Fuel cells are promising candidates for clean and high-efficient energy conversion in the future. The development of C-based inexpensive metal-free ORR catalysts has become one of the most attractive topics in the fuel cell field. Herein, the authors report a N-doped C catalyst with a surface area of up to 1895.5 m2 g-1 using a natural product (bamboo fungus) as the starting material. In 0.1M KOH electrolyte, the ORR onset potential for the catalyst is up to 0.089 V vs. Ag/AgCl. Also, it shows superior stability, fuel crossover resistance, and selective activity to a com. Pt/C catalyst. The sample displays excellent stability, i.e. no obvious decrease in current was obsd. after 1000 continuous cycles between -0.7 and 0.3 V in O2-satd. 0.1M KOH. Structural characterizations and electrochem. tests verify that the treatment of biomass have an important impact on the materials.
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    23. 23
      Zhu, H.; Yin, J.; Wang, X.; Wang, H.; Yang, X. Microorganism-derived heteroatom-doped carbon materials for oxygen reduction and supercapacitors. Adv. Funct. Mater. 2013, 23, 1305– 1312,  DOI: 10.1002/adfm.201201643
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      23
      Microorganism-Derived Heteroatom-Doped Carbon Materials for Oxygen Reduction and Supercapacitors
      Zhu, Hui; Yin, Jiao; Wang, Xiaolei; Wang, Hongyu; Yang, Xiurong
      Advanced Functional Materials (2013), 23 (10), 1305-1312CODEN: AFMDC6; ISSN:1616-301X. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Heteroatom-doped C (HDC) has attracted attention due to its application in energy conversion and storage. Herein, due to its abundance high rate of reprodn., the microorganism, Bacillus subtilis, is selected as a precursor. An effective ionothermal process is adopted to produce the HDCs. Using acid activation, the obtained sample exhibits excellent electrocatalytic activity, long-term stability, and excellent resistance to crossover effects in O redn. Addnl., the base-treated sample exhibits superior performance in capacitors to most com. available C materials. Even at a high c.d., a relatively high capacitance is retained, indicating a great potential for direct application in energy storage.
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      Gong, X.; Liu, S.; Ouyang, C.; Strasser, P.; Yang, R. Nitrogen- and Phosphorus-Doped Biocarbon with Enhanced Electrocatalytic Activity for Oxygen Reduction. ACS Catal. 2015, 5, 920– 927,  DOI: 10.1021/cs501632y
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      24
      Nitrogen- and Phosphorus-Doped Biocarbon with Enhanced Electrocatalytic Activity for Oxygen Reduction
      Gong, Xin; Liu, Shanshan; Ouyang, Chuying; Strasser, Peter; Yang, Ruizhi
      ACS Catalysis (2015), 5 (2), 920-927CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
      The oxygen redn. reaction (ORR) at the cathode of fuel cells and metal-air batteries requires efficient electrocatalysts to accelerate its reaction rate due to its sluggish kinetics. Nitrogen- and phosphorus-doped biocarbon was fabricated via a simple and low-cost biosynthesis method using yeast cells as a precursor. The as-prepd. biocarbon exhibits excellent electrocatalytic activity for the ORR. An onset potential of -0.076 V (vs. Ag/AgCl) and a neg. shift of only ∼29 mV in the half-wave potential of the biocarbon as compared to com. Pt/C (20 wt. % Pt on Vulcan XC-72, Johnson Matthey) is achieved. The biocarbon possesses enhanced electron poverty in carbon atoms and a decreasing amt. of less electroactive nitrogen and phosphorus dopants due to the biomineralization during the synthesis. The surface gap layer along with the mesopores in the biocarbon increases accessible active sites and facilitates the mass transfer during the ORR. These factors correlate with the high ORR activity of the biocarbon. The results demonstrate that biomineralization plays a crit. role in tailoring the structure and the electrocatalytic activity of the biocarbon for ORR.
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      van der Ent, A.; Baker, A. J. M.; Reeves, R. D.; Pollard, A. J.; Schat, H. Hyperaccumulators of metal and metalloid trace elements: Facts and fiction. Plant Soil 2013, 362, 319– 334,  DOI: 10.1007/s11104-012-1287-3
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      Hyperaccumulators of metal and metalloid trace elements: Facts and fiction
      van der Ent, Antony; Baker, Alan J. M.; Reeves, Roger D.; Pollard, A. Joseph; Schat, Henk
      Plant and Soil (2013), 362 (1-2), 319-334CODEN: PLSOA2; ISSN:0032-079X. (Springer)
      A review. Background: Plants that accumulate metal and metalloid trace elements to extraordinarily high concns. in their living biomass have inspired much research worldwide during the last decades. Hyperaccumulators have been recorded and exptl. confirmed for elements such as nickel, zinc, cadmium, manganese, arsenic and selenium. However, to date, hyperaccumulation of lead, copper, cobalt, chromium and thallium remain largely unconfirmed. Recent uses of the term in relation to rare-earth elements require crit. evaluation. Scope Since the mid-1970s the term hyperaccumulator' has been used millions of times by thousands of people, with varying degrees of precision, aptness and understanding that have not always corresponded with the views of the originators of the terminol. and of the present authors. There is therefore a need to clarify the circumstances in which the term 'hyperaccumulator' is appropriate and to set out the conditions that should be met when the terms are used. We outline here the main considerations for establishing metal or metalloid hyperaccumulation status of plants, (re)define some of the terminol. and note potential pitfalls. Conclusions: Unambiguous communication will require the international scientific community to adopt std. terminol. and methods for confirming the reliability of anal. data in relation to metal and metalloid hyperaccumulators.
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      Rezania, S.; Ponraj, M.; Talaiekhozani, A.; Mohamad, S. E.; Md Din, M. F.; Taib, S. M.; Sabbagh, F.; Sairan, F. M. Perspectives of phytoremediation using water hyacinth for removal of heavy metals, organic and inorganic pollutants in wastewater. J. Environ. Manage. 2015, 163, 125– 133,  DOI: 10.1016/j.jenvman.2015.08.018
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      26
      Perspectives of phytoremediation using water hyacinth for removal of heavy metals, organic and inorganic pollutants in wastewater
      Rezania, Shahabaldin; Ponraj, Mohanadoss; Talaiekhozani, Amirreza; Mohamad, Shaza Eva; Md. Din, Mohd. Fadhil; Taib, Shazwin Mat; Sabbagh, Farzaneh; Sairan, Fadzlin Md.
      Journal of Environmental Management (2015), 163 (), 125-133CODEN: JEVMAW; ISSN:0301-4797. (Elsevier Ltd.)
      The development of eco-friendly and efficient technologies for treating wastewater is one of the attractive research area. Phytoremediation is considered to be a possible method for the removal of pollutants present in wastewater and recognized as a better green remediation technol. Nowadays the focus is to look for a sustainable approach in developing wastewater treatment capability. Water hyacinth is one of the ancient technol. that has been still used in the modern era. Although, many papers in relation to wastewater treatment using water hyacinth have been published, recently removal of org., inorg. and heavy metal have not been reviewed extensively. The main objective of this paper is to review the possibility of using water hyacinth for the removal of pollutants present in different types of wastewater. Water hyacinth is although reported to be as one of the most problematic plants worldwide due to its uncontrollable growth in water bodies but its quest for nutrient absorption has provided way for its usage in phytoremediation, along with the combination of herbicidal control, integratated biol. control and watershed management controlling nutrient supply to control its growth. Moreover as a part of solving wastewater treatment problems in urban or industrial areas using this plant, a large no. of useful byproducts can be developed like animal and fish feed, power plant energy (briquette), ethanol, biogas, composting and fiber board making. In focus to the future aspects of phytoremediation, the utilization of invasive plants in pollution abatement phytotechnologies can certainly assist for their sustainable management in treating waste water.
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      Raskin, I.; Ensley, B. D. Phytoremediation of Toxic Metals: Using Plants to Clean Up the Environment; John Wiley & Sons, Inc.: New York, 2000.
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      van der Ent, A.; Baker, A. J. M.; Reeves, R. D.; Chaney, R. L.; Anderson, C. W. N.; Meech, J. A.; Erskine, P. D.; Simonnot, M.-O.; Vaughan, J.; Morel, J. L.; Echevarria, G.; Fogliani, B.; Rongliang, Q.; Mulligan, D. R. Agromining: Farming for Metals in the Future?. Environ. Sci. Technol. 2015, 49, 4773– 4780,  DOI: 10.1021/es506031u
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      28
      Agromining: Farming for Metals in the Future?
      van der Ent, Antony; Baker, Alan J. M.; Reeves, Roger D.; Chaney, Rufus L.; Anderson, Christopher W. N.; Meech, John A.; Erskine, Peter D.; Simonnot, Marie-Odile; Vaughan, James; Morel, Jean Louis; Echevarria, Guillaume; Fogliani, Bruno; Rongliang, Qiu; Mulligan, David R.
      Environmental Science & Technology (2015), 49 (8), 4773-4780CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
      A review. Phytomining technol. employs hyperaccumulator plants to take up metal in harvestable plant biomass. Harvesting, drying and incineration of the biomass generates a high-grade bio-ore. We propose that "agromining" (a variant of phytomining) could provide local communities with an alternative type of agriculture on degraded lands; farming not for food crops, but for metals such as nickel (Ni). However, two decades after its inception and numerous successful expts., com. phytomining has not yet become a reality. To build the case for the minerals industry, a large-scale demonstration is needed to identify operational risks and provide "real-life" evidence for profitability.
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      Bhattacharjee, S.; Dasgupta, P.; Paul, A. R.; Ghosal, S.; Padhi, K. K.; Pandey, L. P. Mineral element composition of spinach. J. Sci. Food Agric. 1998, 77, 456– 458,  DOI: 10.1002/(sici)1097-0010(199808)77:4<456::aid-jsfa55>3.0.co;2-m
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      Mineral element composition of spinach
      Bhattacharjee, Santanu; Dasgupta, Pranab; Paul, Adhir R.; Ghosal, Sunanda; Padhi, Karuna K.; Pandey, Lallan P.
      Journal of the Science of Food and Agriculture (1998), 77 (4), 456-458CODEN: JSFAAE; ISSN:0022-5142. (John Wiley & Sons Ltd.)
      An extensive study has been made on the mineral element compns. of spinach leaves and stems. Twenty two locally grown different spinach samples have been analyzed for 16 elements using ICP and at. absorption spectrophotometric techniques. Both spinach leaves and stems were analyzed sep. A detailed elucidation of the inorg. matrix in spinach leaves and stems has been provided.
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      Citak, S.; Sonmez, S. Mineral Contents of Organically and Conventionally Grown Spinach (Spinacea oleracea L.) during Two Successive Seasons. J. Agric. Food Chem. 2009, 57, 7892– 7898,  DOI: 10.1021/jf900660k
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      Mineral Contents of Organically and Conventionally Grown Spinach (Spinacea oleracea L.) during Two Successive Seasons
      Citak, Sedat; Sonmez, Sahriye
      Journal of Agricultural and Food Chemistry (2009), 57 (17), 7892-7898CODEN: JAFCAU; ISSN:0021-8561. (American Chemical Society)
      Spinach (Spinacea oleracea L.) plants were grown organically and conventionally during two successive seasons (late autumn and early winter) in order to examine the nutrient content of the plants. In a series of 17 org. applications including chicken manure (CM), farmyard manure (FM), blood meal (BM), and one mineral fertilizer treatment and one control, collectively 19 treatments, were used at each season. The optimum doses to be recommended should be divided into groups depending on the mineral nutrients and also the seasons as follows: 1.7 CM+7.5 FM in the late autumn season and 2.5 CM + 4.0 FM in the early winter season for N, P, K content; 5.0 FM + 1.2 CM + 0.4 BM in the late autumn; and 2.5 CM + 4.0 FM in the early winter season for Ca and Mg. Regarding the micro nutrients, the group divisions should be as follows: 10.0 FM + 0.4 BM in the late autumn season and 5.0 FM + 2.5 CM in the early winter season for Fe and Cu, and 3.5 CM in the late autumn season and 10.0 FM + 1.2 CM in the early winter season for Mn and Zn content. High rates of farmyard manure (FM) and chicken manure (CM) can be successfully used in org. prodn., and high rates of these manures may substitute for mineral fertilizer, esp. in the late autumn season.
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      Chapman, R. P.; Averell, P. R.; Harris, R. R. Solubility of melamine in water. Ind. Eng. Chem. 1943, 35, 137– 138,  DOI: 10.1021/ie50398a003
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      Solubility of melamine in water
      Chapman, R. P.; Averell, P. R.; Harris, R. R.
      Industrial and Engineering Chemistry (1943), 35 (), 137-8CODEN: IECHAD; ISSN:0019-7866.
      The soly. of melamine in g. per 100 g. H2O is given by the equation, log (soly.) = - 1642/T + 5.101, in the range 0-100°.
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      Ferrari, A. C.; Meyer, J.; Scardaci, V.; Casiraghi, C.; Lazzeri, M.; Mauri, F.; Piscanec, S.; Jiang, D.; Novoselov, K.; Roth, S. Raman spectrum of graphene and graphene layers. Phys. Rev. Lett. 2006, 97, 187401,  DOI: 10.1103/physrevlett.97.187401
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      Physical Review Letters (2006), 97 (18), 187401/1-187401/4CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
      Graphene is the 2-dimensional building block for C allotropes of every other dimensionality. Its electronic structure is captured in its Raman spectrum that clearly evolves with the no. of layers. The D peak 2nd order changes in shape, width, and position for an increasing no. of layers, reflecting the change in the electron bands via a double resonant Raman process. The G peak slightly down-shifts. This allows unambiguous, high-throughput, nondestructive identification of graphene layers, which is critically lacking in this emerging research area.
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      Cançado, L. G.; Pimenta, M.; Neves, B.; Dantas, M.; Jorio, A. Influence of the atomic structure on the Raman spectra of graphite edges. Phys. Rev. Lett. 2004, 93, 247401,  DOI: 10.1103/physrevlett.93.247401
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      Influence of the Atomic Structure on the Raman Spectra of Graphite Edges
      Cancado, L. G.; Pimenta, M. A.; Neves, B. R. A.; Dantas, M. S. S.; Jorio, A.
      Physical Review Letters (2004), 93 (24), 247401/1-247401/4CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
      A study of step edges in graphite with different at. structures combining Raman spectroscopy and scanning probe microscopy is presented. The orientation of the C hexagons with respect to the edge axis, in the so-called armchair or zigzag arrangements, is distinguished spectroscopically by the intensity of a disorder-induced Raman peak. This effect is explained by applying the double resonance theory to a semi-infinite graphite crystal and by considering the 1-dimensional character of the defect.
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      Ferrari, A. C.; Robertson, J. Interpretation of Raman spectra of disordered and amorphous carbon. Phys. Rev. B: Condens. Matter Mater. Phys. 2000, 61, 14095– 14107,  DOI: 10.1103/physrevb.61.14095
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      Interpretation of Raman spectra of disordered and amorphous carbon
      Ferrari, A. C.; Robertson, J.
      Physical Review B: Condensed Matter and Materials Physics (2000), 61 (20), 14095-14107CODEN: PRBMDO; ISSN:0163-1829. (American Physical Society)
      The model and theor. understanding of the Raman spectra in disordered and amorphous C are given. The nature of the G and D vibration modes in graphite is analyzed in terms of the resonant excitation of π states and the long-range polarizability of π bonding. Visible Raman data on disordered, amorphous, and diamondlike C are classified in a 3-stage model to show the factors that control the position, intensity, and widths of the G and D peaks. The visible Raman spectra depend formally on the configuration of the sp2 sites in sp2-bonded clusters. In cases where the sp2 clustering is controlled by the sp3 fraction, such as in as-deposited tetrahedral amorphous C (ta-C) or hydrogenated amorphous C (a-C:H) films, the visible Raman parameters can be used to derive the sp3 fraction.
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      Ma, J.; Habrioux, A.; Guignard, N.; Alonso-Vante, N. Functionalizing Effect of Increasingly Graphitic Carbon Supports on Carbon-Supported and TiO2–Carbon Composite-Supported Pt Nanoparticles. J. Phys. Chem. C 2012, 116, 21788– 21794,  DOI: 10.1021/jp304947y
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      Functionalizing Effect of Increasingly Graphitic Carbon Supports on Carbon-Supported and TiO2-Carbon Composite-Supported Pt Nanoparticles
      Ma, J.; Habrioux, A.; Guignard, N.; Alonso-Vante, N.
      Journal of Physical Chemistry C (2012), 116 (41), 21788-21794CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      Strong interaction between Pt nanoparticles and graphitic C in C-based and oxide-C composite substrates is demonstrated using electrochem. CO stripping expts. A correlation between the in-plane crystallite size of the C supports and the oxidn. charge of CO stripping wave was made. It appears that π-system of graphitized C anchors Pt particles in a way that strongly modifies the electronic properties of the Pt valence band. The effects of graphitized C on Pt are even observable on TiO2-C composite-supported Pt, where a known strong metal-support interaction between Pt and TiO2 is already present, demonstrating the significant extent of the interaction between Pt and graphite. Finally, a preliminary proof of the role played by the interfacial Pt-Ti nanoalloy on oxide-C composite is given.
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      Kruk, M.; Jaroniec, M. Gas adsorption characterization of ordered organic– inorganic nanocomposite materials. Chem. Mater. 2001, 13, 3169– 3183,  DOI: 10.1021/cm0101069
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      Gas adsorption characterization of ordered organic-inorganic nanocomposite materials
      Kruk, Michal; Jaroniec, Mietek
      Chemistry of Materials (2001), 13 (10), 3169-3183CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
      A crit. review, with 133 refs. on adsorption methods that are currently used in the characterization of ordered org.-inorg. nanocomposite materials is presented, and the adsorption methodol. that is potentially useful for this characterization, but has not yet been applied, is discussed. The ordered org.-inorg. nanocomposites include surface-functionalized ordered mesoporous materials (OMMs) with siliceous frameworks (synthesized either via postsynthesis surface modification or via direct co-condensation method), periodic mesoporous organosilicas, and surfactant-contg. OMMs. This review covers the methods for detn. of the sp. surface area and pore vol. The available methods for mesopore size anal. are critically compared and evaluated, with special emphasis on the recent developments related to the application of advanced computational methods for studying adsorption in porous media and to the direct modeling of adsorption using highly ordered surface-functionalized OMMs as model adsorbents. The review also covers adsorption methods for studying the surface properties of org.-inorg. nanocomposites, including those based on adsorption of mols. of different polarities. An emphasis is placed on the emerging opportunity for studying the surface properties of nanocomposites using low-pressure adsorption of nonpolar mols., such as N2 and Ar. The opportunities and challenges in adsorption characterization of sp. surface sites, uniformity of coated or bonded layers, bonding d. of groups on the surface, type of surface species, and so forth, are presented. Thus, this review provides an overview of adsorption studies dealing with org.-inorg. nanocomposites, a crit. discussion of adsorption methods available for such studies, and some recommendations for thorough characterization of these materials using gas adsorption.
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      Wang, H.; Li, X.-d.; Yu, J.-s.; Kim, D.-p. Fabrication and characterization of ordered macroporous PMS-derived SiC from a sacrificial template method. J. Mater. Chem. 2004, 14, 1383– 1386,  DOI: 10.1039/b313405a
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      Fabrication and characterization of ordered macroporous PMS-derived SiC from a sacrificial template method
      Wang, Hao; Li, Xiao-dong; Yu, Jong-sung; Kim, Dong-pyo
      Journal of Materials Chemistry (2004), 14 (9), 1383-1386CODEN: JMACEP; ISSN:0959-9428. (Royal Society of Chemistry)
      Three-dimensionally long-range ordered macroporous SiC ceramics were prepd. using a low mol. wt. polymer precursor, polymethylsilane (PMS), by utilizing sacrificial colloidal silica cryst. arrays as templates which were subsequently etched off with HF (45%) after pyrolysis in an argon atm. SEM, TEM, HRTEM and BET were used to specify the morphologies and BET surface areas of the porous materials. The pore sizes of about 84-658 nm and BET surface areas (pore vols.) of 584.64-299.44 m2.g-1 (0.64-0.25 cm3.g-1) of the achieved macroporous SiC ceramic were approx. proportional to the sizes 112-700 nm of the sacrificial silica sphere templates used. The unexpectedly high surfaces area and pore vols. of the achieved porous SiC ceramic were related to the microchannels on the walls that were caused by the interfacial diffusion of oxygen or silica from the silica sphere template into the walls of the SiC framework structures.
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      Liu, X.; Zhou, W.; Yang, L.; Li, L.; Zhang, Z.; Ke, Y.; Chen, S. Nitrogen and sulfur co-doped porous carbon derived from human hair as highly efficient metal-free electrocatalysts for hydrogen evolution reactions. J. Mater. Chem. A 2015, 3, 8840– 8846,  DOI: 10.1039/c5ta01209k
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      Nitrogen and sulfur co-doped porous carbon derived from human hair as highly efficient metal-free electrocatalysts for hydrogen evolution reactions
      Liu, Xiaojun; Zhou, Weijia; Yang, Linjing; Li, Ligui; Zhang, Zhenyuan; Ke, Yunting; Chen, Shaowei
      Journal of Materials Chemistry A: Materials for Energy and Sustainability (2015), 3 (16), 8840-8846CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
      Design and engineering of low-cost and high-efficiency electrocatalysts for hydrogen evolution reactions (HER) has attracted increasing interest in renewable energy research. Herein, a highly active and stable metal-free electrocatalyst, N + S-codoped porous carbon derived from human hair, was developed for HER for the first time, with an electrocatalytic performance comparable to that of state-of-the-art com. 20 wt% Pt/C catalysts. SEM, TEM and nitrogen adsorption-desorption measurements showed that the resultant carbon exhibited a porous structure with a high sp. surface area (up to 830.0 m2 g-1) and rich porosity. XPS measurements showed that N and S were codoped into the carbon mol. skeletons. Importantly, electrochem. measurements showed high activity for hydrogen evolution with a low overpotential of only -12 mV, a Tafel slope of 57.4 mV dec-1, a c.d. of 10 mA cm-2 at -0.1 V vs. RHE, and remarkable durability. The results highlight a unique paradigm for the prepn. of highly efficient electrocatalysts for HER based on abundant biowastes.
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      Li, Y.; Zhang, H.; Wang, Y.; Liu, P.; Yang, H.; Yao, X.; Wang, D.; Tang, Z.; Zhao, H. A self-sponsored doping approach for controllable synthesis of S and N co-doped trimodal-porous structured graphitic carbon electrocatalysts. Energy Environ. Sci. 2014, 7, 3720– 3726,  DOI: 10.1039/c4ee01779j
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      A self-sponsored doping approach for controllable synthesis of S and N co-doped trimodal-porous structured graphitic carbon electrocatalysts
      Li, Yibing; Zhang, Haimin; Wang, Yun; Liu, Porun; Yang, Huagui; Yao, Xiangdong; Wang, Dan; Tang, Zhiyong; Zhao, Huijun
      Energy & Environmental Science (2014), 7 (11), 3720-3726CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)
      A facile self-sponsored doping approach is developed to synthesize S and N co-doped trimodal-porous structured graphitic carbon network electrocatalysts. It utilizes a sole precursor (1-allyl-2-thiourea) to realize a precisely controlled co-doping of S and N during a concurrent graphitic carbon growth process by simple control of the pyrolysis temp. The results reveal that the doping effect is heavily dependent on the doping d. and a maximal catalytic activity could only be achieved with an optimal doping level. The presence of a macro-pore structure in the trimodal-porous network enhances the mass transport, enabling the full utilization of large surface areas created by micro- and meso-pores. The resultant electrocatalyst possesses high ORR catalytic activity with excellent durability and high resistance to the inhibition effect of fuel mols. The findings of this work would be valuable for design and fabrication of high performance carbon-based electrocatalysts.
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      He, W.; Jiang, C.; Wang, J.; Lu, L. High-Rate Oxygen Electroreduction over Graphitic-N Species Exposed on 3D Hierarchically Porous Nitrogen-Doped Carbons. Angew. Chem., Int. Ed. 2014, 53, 9503– 9507,  DOI: 10.1002/anie.201404333
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      High-Rate Oxygen Electroreduction over Graphitic-N Species Exposed on 3D Hierarchically Porous Nitrogen-Doped Carbons
      He, Wenhui; Jiang, Chunhuan; Wang, Jiabo; Lu, Lehui
      Angewandte Chemie, International Edition (2014), 53 (36), 9503-9507CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Nitrogen-doped species (NDs) are theor. accepted as a determinant of the catalytic activity of metal-free N-doped carbon (NC) catalysts for oxygen redn. reaction (ORR). However, direct relationships between ND type and ORR activity have been difficult to ext. because the complexity of carbon matrix impairs efforts to expose specific NDs. Herein, we demonstrate the fabrication of a 3D hierarchically porous NC catalyst with micro-, meso-, and macroporosity in one structure, in which sufficient exposure and availability of inner-pore catalytic sites can be achieved due to its super-high surface area (2191 cm2 g-1) and interconnected pore system. More importantly, in-situ formation of graphitic-N species (GNs) on the surface of NC stimulated by KOH activation enables us to exptl. reveal the catalytic nature of GNs for ORR, which is of great significance for the design and development of advanced metal-free NC electrocatalysts.
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      Liang, J.; Du, X.; Gibson, C.; Du, X. W.; Qiao, S. Z. N-Doped Graphene Natively Grown on Hierarchical Ordered Porous Carbon for Enhanced Oxygen Reduction. Adv. Mater. 2013, 25, 6226– 6231,  DOI: 10.1002/adma.201302569
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      N-Doped Graphene Natively Grown on Hierarchical Ordered Porous Carbon for Enhanced Oxygen Reduction
      Liang, Ji; Du, Xin; Gibson, Christopher; Du, Xi Wen; Qiao, Shi Zhang
      Advanced Materials (Weinheim, Germany) (2013), 25 (43), 6226-6231CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
      This novel hybrid material consists of natively grown graphene on hierarchical porous carbon. The material is prepd. through a convenient bottom-up route, which combines the advantages of both hierarchical porous carbon and graphene such as high surface area, large porous channel size, and high electron cond. Due to its excellent structural properties and inherent hetero-atom dopants, this material can serve as a metal-free catalyst for ORR. The efficient electron transfer between carbon and the in situ formed graphene makes its catalytic performance synergistically enhanced. Moreover, this carbon-based material could also potentially be used in other applications which require high surface area, high accessibly, and high cond.
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      Hu, Q.; Li, G.; Liu, X.; Zhu, B.; Chai, X.; Zhang, Q.; Liu, J.; He, C. Superhydrophilic Phytic-Acid-Doped Conductive Hydrogels as Metal-Free and Binder-Free Electrocatalysts for Efficient Water Oxidation. Angew. Chem. 2019, 131, 4362– 4366,  DOI: 10.1002/ange.201900109
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      Wang, Z.; Dong, Y.; Li, H.; Zhao, Z.; Wu, H. B.; Hao, C.; Liu, S.; Qiu, J.; Lou, X. W. D. Enhancing lithium–sulphur battery performance by strongly binding the discharge products on amino-functionalized reduced graphene oxide. Nat. Commun. 2014, 5, 5002,  DOI: 10.1038/ncomms6002
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      Enhancing lithium-sulphur battery performance by strongly binding the discharge products on amino-functionalized reduced graphene oxide
      Wang, Zhiyu; Dong, Yanfeng; Li, Hongjiang; Zhao, Zongbin; Wu, Hao Bin; Hao, Ce; Liu, Shaohong; Qiu, Jieshan; Lou, Xiong Wen
      Nature Communications (2014), 5 (), 5002CODEN: NCAOBW; ISSN:2041-1723. (Nature Publishing Group)
      Lithium-sulfur batteries are one very appealing power source with high energy d. But their practical use is still hindered by several issues including short lifespan, low efficiency and safety concern from the lithium anode. Polysulfide dissoln. and insulating nature of sulfur are generally considered responsible for the capacity degrdn. However, the detachment of discharge products, i.e., highly polar lithium sulfides, from nonpolar carbon matrix (for example, graphene) has been rarely studied as one crit. factor. Here we report the strongly covalent stabilization of sulfur and its discharge products on amino-functionalized reduced graphene oxide that enables stable capacity retention of 80% for 350 cycles with high capacities and excellent high-rate response up to 4 C. The present study demonstrates a feasible and effective strategy to solve the long-term cycling difficulty for lithium-sulfur batteries and also helps to understand the capacity decay mechanism involved.
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      Artyushkova, K.; Matanovic, I.; Halevi, B.; Atanassov, P. Oxygen binding to active sites of Fe–N–C ORR electrocatalysts observed by ambient-pressure XPS. J. Phys. Chem. C 2017, 121, 2836– 2843,  DOI: 10.1021/acs.jpcc.6b11721
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      Oxygen Binding to Active Sites of Fe-N-C ORR Electrocatalysts Observed by Ambient-Pressure XPS
      Artyushkova, Kateryna; Matanovic, Ivana; Halevi, Barr; Atanassov, Plamen
      Journal of Physical Chemistry C (2017), 121 (5), 2836-2843CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      The authors report the 1st in situ ambient pressure XPS (APXPS) study of the binding of oxygenated species to the active sites of Fe-N-C O redn. reaction (ORR) electrocatalysts. To better interpret the results, DFT calcns. were used to calc. absorption energies of reactants and intermediates on potential active sites and calc. the core level shifts for those. The obsd. O binding to N coordinated to Fe centers correlates with the enhanced measured ORR fuel cell activity of these materials with respect to metal-free analogs and sheds light on the ORR mechanism on PGM-free electrocatalysts.
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      Lin, L.; Zhu, Q.; Xu, A.-W. Noble-metal-free Fe–N/C catalyst for highly efficient oxygen reduction reaction under both alkaline and acidic conditions. J. Am. Chem. Soc. 2014, 136, 11027– 11033,  DOI: 10.1021/ja504696r
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      Noble-Metal-Free Fe-N/C Catalyst for Highly Efficient Oxygen Reduction Reaction under Both Alkaline and Acidic Conditions
      Lin, Ling; Zhu, Qing; Xu, An-Wu
      Journal of the American Chemical Society (2014), 136 (31), 11027-11033CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      In this work, the synthesis and assessment are reported of a new non-precious-metal oxygen redn. reaction (ORR) catalyst from pyrolysis of an iron-coordinated complex which manifests superior activity in both alk. and acidic media. 11,11'-bis(dipyrido[3,2-a:2',3'-c]phenazinyl) (bidppz) was selected as a ligand for the formation of a nitrogen-rich iron-coordinated coordination polymer (Fe-bidppz) which forms a self-supporting catalyst contg. high densities of nitrogen and iron doping by pyrolysis. The catalyst pyrolyzed at 800° (Fe-N/C-800) shows the highest ORR activity with onset and half-wave potentials of 923 and 809 mV in 0.1 M KOH, resp., which are comparable to those of Pt/C (half-wave potential 818 mV vs. RHE) at the same catalyst loading. Besides, the Fe-N/C-800 catalyst has an excellent ORR activity with onset and half-wave potentials only 38 and 59 mV less than those of the Pt/C catalyst in 0.1 M HClO4. The optimal Fe-N/C-800 catalyst displays much greater durability and tolerance of methanol than Pt/C. It is proposed that the Fe-N/C-800 catalyst has a considerably high d. of surface active sites because Fe-N/C-800 possesses excellent ORR activity while its sp. surface area is not so high. Electrochem. measurements show that the Fe-N/C-800 catalyst in KOH and HClO4 follows the effective four-electron-transfer pathway.
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    46. 46
      Shah, S. S. A.; Najam, T.; Cheng, C.; Peng, L.; Xiang, R.; Zhang, L.; Deng, J.; Ding, W.; Wei, Z. Exploring Fe-Nx for Peroxide Reduction: Template-Free Synthesis of Fe-Nx Traumatized Mesoporous Carbon Nanotubes as an ORR Catalyst in Acidic and Alkaline Solutions. Chem.—Eur. J. 2018, 24, 10630– 10635,  DOI: 10.1002/chem.201802453
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      46
      Exploring Fe-Nx for Peroxide Reduction: Template-Free Synthesis of Fe-Nx Traumatized Mesoporous Carbon Nanotubes as an ORR Catalyst in Acidic and Alkaline Solutions
      Shah, Syed Shoaib Ahmad; Najam, Tayyaba; Cheng, Chao; Peng, Lishan; Xiang, Rui; Zhang, Ling; Deng, Jianghai; Ding, Wei; Wei, Zidong
      Chemistry - A European Journal (2018), 24 (42), 10630-10635CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Fe-based electrocatalysts are elegant due to their better performance towards the O redn. reaction. Nevertheless, they commonly contain different moieties, for example Fe-Nx, Fe, Fe3C and N-doped C, primarily the debatable assistance of these components towards ORR electrocatalysis, specifically for intermediate peroxide redn. reactions (PRR). To explore the role of Fe-Nx centers for PRR, a Fe-N-C electrocatalyst rooted in N-doped C nanotubes with mesoporous structures was synthesized from a Fe/Zn-dicyanoimidazolate framework. The use of dicyanoimidazole coordinated with Fe can introduce the Fe-Nx active sites as well as directional N-doped C nanotubes, which is good for enhancing electronic conductance of the catalyst. The attained electrocatalyst shows tremendous enactment to ORR, being comparable to the activity of Pt/C in acidic and better in alk. electrolytes. This study also reveals that Fe-Nx active centers are responsible for less H2O2 prodn. Though the Fe-Nx moieties and Fe3C/Fe particles encapsulated N-doped C, both are active centers for ORR, however, Fe-Nx sites are more active than others for peroxide redn. reaction. These perceptions suggest rational methodologies for more active and consequently further durable Fe-N-C catalysts.
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    47. 47
      Jiang, W.-J.; Gu, L.; Li, L.; Zhang, Y.; Zhang, X.; Zhang, L.-J.; Wang, J.-Q.; Hu, J.-S.; Wei, Z.; Wan, L.-J. Understanding the High Activity of Fe–N–C Electrocatalysts in Oxygen Reduction: Fe/Fe3C Nanoparticles Boost the Activity of Fe–Nx. J. Am. Chem. Soc. 2016, 138, 3570– 3578,  DOI: 10.1021/jacs.6b00757
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      Understanding the High Activity of Fe-N-C Electrocatalysts in Oxygen Reduction: Fe/Fe3C Nanoparticles Boost the Activity of Fe-Nx
      Jiang, Wen-Jie; Gu, Lin; Li, Li; Zhang, Yun; Zhang, Xing; Zhang, Lin-Juan; Wang, Jian-Qiang; Hu, Jin-Song; Wei, Zidong; Wan, Li-Jun
      Journal of the American Chemical Society (2016), 138 (10), 3570-3578CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Understanding the origin of high activity of Fe-N-C electrocatalysts in O redn. reaction (ORR) is crit. but still challenging for developing efficient sustainable nonprecious metal catalysts in fuel cells and metal-air batteries. We developed a new highly active Fe-N-C ORR catalyst contg. Fe-Nx coordination sites and Fe/Fe3C nanocrystals ([email protected]), and revealed the origin of its activity by intensively studying the compn. and the structure of the catalyst and their correlations with the electrochem. performance. The detailed analyses unambiguously confirmed the coexistence of Fe/Fe3C nanocrystals and Fe-Nx in the best catalyst. A series of designed expts. disclosed that (1) N-doped C substrate, Fe/Fe3C nanocrystals or Fe-Nx themselves did not deliver the high activity; (2) the catalysts with both Fe/Fe3C nanocrystals and Fe-Nx exhibited the high activity; (3) the higher content of Fe-Nx gave the higher activity; (4) the removal of Fe/Fe3C nanocrystals severely degraded the activity; (5) the blocking of Fe-Nx downgraded the activity and the recovery of the blocked Fe-Nx recovered the activity. These facts supported that the high ORR activity of the [email protected] electrocatalysts should be ascribed to that Fe/Fe3C nanocrystals boost the activity of Fe-Nx. The coexistence of high content of Fe-Nx and sufficient metallic Fe nanoparticles is essential for the high ORR activity. DFT calcn. corroborated this conclusion by indicating that the interaction between metallic Fe and Fe-N4 coordination structure favored the adsorption of O mol. These new findings open an avenue for the rational design and bottom-up synthesis of low-cost highly active ORR electrocatalysts.
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      Ahn, S. H.; Yu, X.; Manthiram, A. “Wiring” Fe-Nx-Embedded Porous Carbon Framework onto 1D Nanotubes for Efficient Oxygen Reduction Reaction in Alkaline and Acidic Media. Adv. Mater. 2017, 29, 1606534,  DOI: 10.1002/adma.201606534
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      Sa, Y. J.; Seo, D.-J.; Woo, J.; Lim, J. T.; Cheon, J. Y.; Yang, S. Y.; Lee, J. M.; Kang, D.; Shin, T. J.; Shin, H. S.; Jeong, H. Y.; Kim, C. S.; Kim, M. G.; Kim, T.-Y.; Joo, S. H. A general approach to preferential formation of active Fe–N x sites in Fe–N/C electrocatalysts for efficient oxygen reduction reaction. J. Am. Chem. Soc. 2016, 138, 15046– 15056,  DOI: 10.1021/jacs.6b09470
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      A General Approach to Preferential Formation of Active Fe-Nx Sites in Fe-N/C Electrocatalysts for Efficient Oxygen Reduction Reaction
      Sa, Young Jin; Seo, Dong-Jun; Woo, Jinwoo; Lim, Jung Tae; Cheon, Jae Yeong; Yang, Seung Yong; Lee, Jae Myeong; Kang, Dongwoo; Shin, Tae Joo; Shin, Hyeon Suk; Jeong, Hu Young; Kim, Chul Sung; Kim, Min Gyu; Kim, Tae-Young; Joo, Sang Hoon
      Journal of the American Chemical Society (2016), 138 (45), 15046-15056CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Iron-nitrogen on carbon (Fe-N/C) catalysts have emerged as promising nonprecious metal catalysts (NPMCs) for oxygen redn. reaction (ORR) in energy conversion and storage devices. It was widely suggested that an active site structure for Fe-N/C catalysts contains Fe-Nx coordination. However, the prepn. of high-performance Fe-N/C catalysts mostly involves a high-temp. pyrolysis step, which generates not only catalytically active Fe-Nx sites, but also less active large iron-based particles. Herein, we report a general "silica-protective-layer-assisted" approach that can preferentially generate the catalytically active Fe-Nx sites in Fe-N/C catalysts while suppressing the formation of large Fe-based particles. The catalyst prepn. consisted of an adsorption of iron porphyrin precursor on carbon nanotube (CNT), silica layer overcoating, high-temp. pyrolysis, and silica layer etching, which yielded CNTs coated with thin layer of porphyrinic carbon (CNT/PC) catalysts. Temp.-controlled in situ x-ray absorption spectroscopy during the prepn. of CNT/PC catalyst revealed the coordination of silica layer to stabilize the Fe-N4 sites. The CNT/PC catalyst contained higher d. of active Fe-Nx sites compared to the CNT/PC prepd. without silica coating. The CNT/PC showed high ORR activity and excellent stability in alk. media. Importantly, an alk. anion exchange membrane fuel cell (AEMFC) with a CNT/PC-based cathode exhibited record high current and power densities among NPMC-based AEMFCs. In addn., a CNT/PC-based cathode exhibited a high volumetric c.d. of 320 A cm-3 in acidic proton exchange membrane fuel cell. We further demonstrated the generality of this synthetic strategy to other carbon supports.
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      Wu, Z.-S.; Chen, L.; Liu, J.; Parvez, K.; Liang, H.; Shu, J.; Sachdev, H.; Graf, R.; Feng, X.; Müllen, K. High-Performance Electrocatalysts for Oxygen Reduction Derived from Cobalt Porphyrin-Based Conjugated Mesoporous Polymers. Adv. Mater. 2014, 26, 1450– 1455,  DOI: 10.1002/adma.201304147
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      High-performance electrocatalysts for oxygen reduction derived from cobalt porphyrin-based conjugated mesoporous polymers
      Wu, Zhong-Shuai; Chen, Long; Liu, Junzhi; Parvez, Khaled; Liang, Haiwei; Shu, Jie; Sachdev, Hermann; Graf, Robert; Feng, Xinliang; Muellen, Klaus
      Advanced Materials (Weinheim, Germany) (2014), 26 (9), 1450-1455CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Cobalt-nanoparticle-integrated nitrogen-enriched porous carbons derived from cobalt porphyrin-based conjugated mesoporous polymers (CoP-CMP) by template-free pyrolysis have proven an outstanding value for high-performance ORR catalysts.
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      Buckel, F.; Effenberger, F.; Yan, C.; Gölzhäuser, A.; Grunze, M. Influence of Aromatic Groups Incorporated in Long-Chain Alkanethiol Self-Assembled Monolayers on Gold. Adv. Mater. 2000, 12, 901– 905,  DOI: 10.1002/1521-4095(200006)12:12<901::aid-adma901>3.0.co;2-b
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      Influence of aromatic groups incorporated in long-chain alkanethiol self-assembled monolayers on gold
      Buckel, Frank; Effenberger, Franz; Yan, Chun; Golzhauser, Armin; Grunze, Michael
      Advanced Materials (Weinheim, Germany) (2000), 12 (12), 901-905CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH)
      Self-assembled monolayers of I (R = 2-thienyl, Ph), II, and III are bound via the thiolate function to the Au surface, as demonstrated by XPS. The contact angle of the thiophenol II (111 ± 1°) deviates from those of the aryl functionalized thiols (between 90 ± 1° and 95 ± 1°). RAIR spectroscopy shows densely packed, highly ordered SAMs in all cases. the incorporation of aryl groups, independent of their position, influences the chain mol. orientation to the surface, but it does not prevent dense packing. Probably due to the lower cross section and the better intermol. interactions of the aryl groups, a perpendicular orientation of the arom. ring with respect to the surface is preferred. Therefore, the incorporation of aroms. into alkyl and the kind of chem. linkages between the arom. and the aliph. moieties in these mols. play a decisive role in the orientation of the mols. on the surface. Compds. I and II, with directly connected aliph. and arom. moieties, possess larger tilt angles than the normal alkanethiols, whereas smaller tilt angles are obsd. for III where the arom. and the aliph. groups are connected via an O bridge. Taking these results into consideration, it is possible to influence the tilt angle of SAMs on Au a desired manner. In this way the authors were able to prep. a novel tightly packed oligo(arylalkyl) SAM with an upright alignment of mols. III on a Au surface.
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      Turchanin, A.; Käfer, D.; El-Desawy, M.; Wöll, C.; Witte, G.; Gölzhäuser, A. Molecular mechanisms of electron-induced cross-linking in aromatic SAMs. Langmuir 2009, 25, 7342– 7352,  DOI: 10.1021/la803538z
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      Molecular Mechanisms of Electron-Induced Cross-Linking in Aromatic SAMs
      Turchanin, Andrey; Kaefer, Daniel; El-Desawy, Mohamed; Woell, Christof; Witte, Gregor; Goelzhaeuser, Armin
      Langmuir (2009), 25 (13), 7342-7352CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
      When arom. self-assembled monolayers (SAMs) are electron-irradiated, intermol. cross-links are formed and the SAMs transform into carbon nanosheets with mol. thickness. These nanosheets have a very high mech. stability and can withstand temps. >1000 K. The authors study the electron induced crosslinking of 1,1'-biphenyl-4-thiol (BPT) SAMs on gold by combining XPS, x-ray absorption spectroscopy (NEXAFS), thermal desorption spectroscopy (TDS), and UPS. The exptl. data were acquired as a function of electron dose and temp. and compared with quantum chem. calcns. Details of the intermol. crosslinking, the microstructure of cross-linked films, and their structural transformations upon heating were obtained to derive a view of the mechanisms involved. The anal. shows that room-temp. electron irradn. causes a lateral crosslinking via the formation of C-C linked Ph species as well as a new sulfur species. The thermal stability of the BPT films increases with the electron dose and sats. at ∼50 mC/cm2. Nevertheless, nonlinked fragments in the thermal desorption spectra indicate an incomplete crosslinking even at high doses, which can be attributed to steric reasons and quenching due to the reduced band gap of partially linked mols. At temps. >800 K, all sulfur species are thermally desorbed, while the remaining film reveals an onset of carbonization.
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    53. 53
      Ferrero, G. A.; Preuss, K.; Marinovic, A.; Jorge, A. B.; Mansor, N.; Brett, D. J. L.; Fuertes, A. B.; Sevilla, M.; Titirici, M.-M. Fe–N-Doped Carbon Capsules with Outstanding Electrochemical Performance and Stability for the Oxygen Reduction Reaction in Both Acid and Alkaline Conditions. ACS Nano 2016, 10, 5922– 5932,  DOI: 10.1021/acsnano.6b01247
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      53
      Fe-N-Doped Carbon Capsules with Outstanding Electrochemical Performance and Stability for the Oxygen Reduction Reaction in Both Acid and Alkaline Conditions
      Ferrero, Guillermo A.; Preuss, Kathrin; Marinovic, Adam; Jorge, Ana Belen; Mansor, Noramalina; Brett, Dan J. L.; Fuertes, Antonio B.; Sevilla, Marta; Titirici, Maria-Magdalena
      ACS Nano (2016), 10 (6), 5922-5932CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      High surface area N-doped mesoporous carbon capsules with iron traces exhibit outstanding electrocatalytic activity for the oxygen redn. reaction in both alk. and acidic media. In alk. conditions, they exhibit more pos. onset (0.94 V vs RHE) and half-wave potentials (0.83 V vs RHE) than com. Pt/C, while in acidic media the onset potential is comparable to that of com. Pt/C with a peroxide yield lower than 10%. The Fe-N-doped carbon catalyst combines high catalytic activity with remarkable performance stability (3500 cycles between 0.6 and 1.0 V vs RHE), which stems from the fact that iron is coordinated to nitrogen. Addnl., the newly developed electrocatalyst is unaffected by the methanol crossover effect in both acid and basic media, contrary to com. Pt/C. The excellent catalytic behavior of the Fe-N-doped carbon, even in the more relevant acid medium, is attributable to the combination of chem. functions (N-pyridinic, N-quaternary, and Fe-N coordination sites) and structural properties (large surface area, open mesoporous structure, and short diffusion paths), which guarantees a large no. of highly active and fully accessible catalytic sites and rapid mass-transfer kinetics. Thus, this catalyst represents an important step forward toward replacing Pt catalysts with cheaper alternatives. In this regard, an alk. anion exchange membrane fuel cell was assembled with Fe-N-doped mesoporous carbon capsules as the cathode catalyst to provide current and power densities matching those of a com. Pt/C, which indicates the practical applicability of the Fe-N-carbon catalyst.
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      Li, Y.; Li, M.; Jiang, L.; Lin, L.; Cui, L.; He, X. Advanced oxygen reduction reaction catalyst based on nitrogen and sulfur co-doped graphene in alkaline medium. Phys. Chem. Chem. Phys. 2014, 16, 23196– 23205,  DOI: 10.1039/c4cp02528h
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      Advanced oxygen reduction reaction catalyst based on nitrogen and sulfur co-doped graphene in alkaline medium
      Li, Yongfeng; Li, Meng; Jiang, Liqing; Lin, Lin; Cui, Lili; He, Xingquan
      Physical Chemistry Chemical Physics (2014), 16 (42), 23196-23205CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
      A novel nitrogen and sulfur co-doped graphene (N-S-G) catalyst for oxygen redn. reaction (ORR) has been prepd. by pyrolyzing graphite oxide and poly[3-amino-5-mercapto-1,2,4-triazole] composite (PAMTa). The at. percentage of nitrogen and sulfur for the prepd. N-S-G can be adjusted by controlling the pyrolysis temp. Furthermore, the catalyst pyrolyzed at 1000 °C, denoted N-S-G 1000, exhibits the highest catalytic activity for ORR, which displays the highest content of graphitic-N and thiophene-S among all the pyrolyzed samples. The electrocatalytic performance of N-S-G 1000 is significantly better than that of PAMTa and reduced graphite oxide composite. Remarkably, the N-S-G 1000 catalyst is comparable with Pt/C in terms of the onset and half-wave potentials, and displays larger kinetic limiting c.d. and better methanol tolerance and stability than Pt/C for ORR in an alk. medium.
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      Wang, X.; Wang, J.; Wang, D.; Dou, S.; Ma, Z.; Wu, J.; Tao, L.; Shen, A.; Ouyang, C.; Liu, Q.; Wang, S. One-pot synthesis of nitrogen and sulfur co-doped graphene as efficient metal-free electrocatalysts for the oxygen reduction reaction. Chem. Commun. 2014, 50, 4839– 4842,  DOI: 10.1039/c4cc00440j
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      One-pot synthesis of nitrogen and sulfur co-doped graphene as efficient metal-free electrocatalysts for the oxygen reduction reaction
      Wang, Xin; Wang, Jie; Wang, Deli; Dou, Shuo; Ma, Zhaoling; Wu, Jianghong; Tao, Li; Shen, Anli; Ouyang, Canbin; Liu, Qiuhong; Wang, Shuangyin
      Chemical Communications (Cambridge, United Kingdom) (2014), 50 (37), 4839-4842CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
      Novel N, S co-doped graphene (NSG) was prepd. by annealing graphene oxide with thiourea as the single N and S precursor. The NSG electrodes, as efficient metal-free electrocatalysts, show a direct four-electron reaction pathway, high onset potential, high c.d. and high stability for the oxygen redn. reaction.
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      Yuan, S.; Guo, Z.; Wang, L.; Hu, S.; Wang, Y.; Xia, Y. Leaf-Like Graphene-Oxide-Wrapped Sulfur for High-Performance Lithium–Sulfur Battery. Adv. Sci. 2015, 2, 1500071,  DOI: 10.1002/advs.201500071
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      Leaf-Like Graphene-Oxide-Wrapped Sulfur for High-Performance Lithium-Sulfur Battery
      Yuan Shouyi; Guo Ziyang; Wang Lina; Hu Shuang; Wang Yonggang; Xia Yongyao
      Advanced science (Weinheim, Baden-Wurttemberg, Germany) (2015), 2 (8), 1500071 ISSN:2198-3844.
      Carbon/sulfur composites are attracting extensive attention because of their improved performances for Li-S batteries. However, the achievements are generally based on the low S-content in the composites and the low S-loading on the electrode. Herein, a leaf-like graphene oxide (GO), which includes an inherent carbon nanotube midrib in the GO plane, is synthesized for preparing GO/S composites. Owing to the inherent high conductivity of carbon nanotube midribs and the abundant surface groups of GO for S-immobilization, the composite with an S-content of 60 wt% exhibits ultralong cycling stability over 1000 times with a low capacity decay of 0.033% per cycle and a high rate up to 4C. When the S-content is increased to 75 wt%, the composite still shows a perfect cycling performance over 1000 cycles. Even with the high S-loading of 2.7 mg cm(-2) on the electrode and the high S-content of 85 wt%, it still shows a promising cycling performance over 600 cycles.
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      Liang, X.; Hart, C.; Pang, Q.; Garsuch, A.; Weiss, T.; Nazar, L. F. A highly efficient polysulfide mediator for lithium–sulfur batteries. Nat. Commun. 2015, 6, 5682,  DOI: 10.1038/ncomms6682
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      A highly efficient polysulfide mediator for lithium-sulfur batteries
      Liang Xiao; Hart Connor; Pang Quan; Nazar Linda F; Garsuch Arnd; Weiss Thomas
      Nature communications (2015), 6 (), 5682 ISSN:.
      The lithium-sulfur battery is receiving intense interest because its theoretical energy density exceeds that of lithium-ion batteries at much lower cost, but practical applications are still hindered by capacity decay caused by the polysulfide shuttle. Here we report a strategy to entrap polysulfides in the cathode that relies on a chemical process, whereby a host--manganese dioxide nanosheets serve as the prototype--reacts with initially formed lithium polysulfides to form surface-bound intermediates. These function as a redox shuttle to catenate and bind 'higher' polysulfides, and convert them on reduction to insoluble lithium sulfide via disproportionation. The sulfur/manganese dioxide nanosheet composite with 75 wt% sulfur exhibits a reversible capacity of 1,300 mA h g(-1) at moderate rates and a fade rate over 2,000 cycles of 0.036%/cycle, among the best reported to date. We furthermore show that this mechanism extends to graphene oxide and suggest it can be employed more widely.
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      Mills, P.; Sullivan, J. L. A study of the core level electrons in iron and its three oxides by means of X-ray photoelectron spectroscopy. J. Phys. D: Appl. Phys. 1983, 16, 723– 732,  DOI: 10.1088/0022-3727/16/5/005
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      A study of the core level electrons in iron and its three oxides by means of x-ray photoelectron spectroscopy
      Mills, P.; Sullivan, J. L.
      Journal of Physics D: Applied Physics (1983), 16 (5), 723-32CODEN: JPAPBE; ISSN:0022-3727.
      The core level electrons in Fe, FeO, Fe3O4, and α-Fe2O3 were studied by XPS. The main features of the spectra were identified and electron binding energies measured. The Fe 2p peaks of the oxide spectra were broadened (full width at half max. ∼4.5 eV) due to unresolved multiplet splitting and shake-up satellites were obsd. for both Fe(II) and Fe(III) species. For all the samples studied the O 1s spectra consisted of 2 overlapping peaks, 1 due to the oxide and a higher binding energy peak due to an adsorbed species. The higher binding energy peak was frequently dominant indicating a high degree of adsorption on the oxide surfaces. The effect of Ar-ion etching on the surfaces was also investigated and the results indicate that the anal. of surfaces by ion etching in conjunction with XPS can give misleading results.
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      Zheng, X.; Cao, X.; Li, X.; Tian, J.; Jin, C.; Yang, R. Biomass lysine-derived nitrogen-doped carbon hollow cubes via a NaCl crystal template: an efficient bifunctional electrocatalyst for oxygen reduction and evolution reactions. Nanoscale 2017, 9, 1059– 1067,  DOI: 10.1039/c6nr07380h
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      Biomass lysine-derived nitrogen-doped carbon hollow cubes via a NaCl crystal template: an efficient bifunctional electrocatalyst for oxygen reduction and evolution reactions
      Zheng, Xiangjun; Cao, Xuecheng; Li, Xiaowei; Tian, Jinghua; Jin, Chao; Yang, Ruizhi
      Nanoscale (2017), 9 (3), 1059-1067CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
      Nitrogen-doped carbon hollow cubes (NCHCs) are fabricated from biomass L-lysine monohydrochloride via a facile and low-cost NaCl template process, showing efficient bifunctional electrocatalytic activities towards the oxygen redn. reaction (ORR) and oxygen evolution reaction (OER). The resultant lysine-derived carbon hollow cubes with hierarchical pores on the wall are conducive to mass transport and high utilization of nitrogen dopant-induced active sites during the electrocatalytic process. When used as electrocatalysts for the ORR, an onset potential of 0.92 V vs. RHE has been achieved for NCHCs. A neg. shift of only 61 mV exists in the half-wave potential of NCHCs compared to that of the com. Pt/C (20 wt%). Moreover, the NCHCs show high activity for the OER comparable to that of com. RuO2/C (20 wt%). The sustainable conversion of biomass lysine to heteroatom-doped carbon hollow cubes and the recyclability of the NaCl template allow a scalable prodn. and practical application of carbon materials for energy storage and conversion.
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    60. 60
      Wang, M.; Wang, S.; Yang, H.; Ku, W.; Yang, S.; Liu, Z.; Lu, G. Carbon-Based Electrocatalysts Derived From Biomass for Oxygen Reduction Reaction: A Minireview. Front. Chem. 2020, 8, 116,  DOI: 10.3389/fchem.2020.00116
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      60
      Carbon-based electrocatalysts derived from biomass for oxygen reduction reaction: a minireview
      Wang, Mi; Wang, Shiyu; Yang, Haoqi; Wen, Ku; Yang, Shuchen; Liu, Zhenning; Lu, Guolong
      Frontiers in Chemistry (Lausanne, Switzerland) (2020), 8 (), 116CODEN: FCLSAA; ISSN:2296-2646. (Frontiers Media S.A.)
      Oxygen redn. reaction (ORR) electrocatalysts derived from biomass have become one of the research focuses in hetero-catalysis due to their low cost, high performance, and reproducibility properties. Related researches are of great significance for the development of next-generation fuel cells and metal-air batteries. Herein, the prepn. methods of various biomass-derived catalysts and their performance in alk., neutral, and acidic media are summarized. This review clarifies the research progress of biomass carbon-based electrocatalysts for ORR in acidic, alk. and neutral media, and discusses the future development trends. This minireview can give us an important enlightenment to practical application in the future.
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    61. 61
      Thorum, M. S.; Hankett, J. M.; Gewirth, A. A. Poisoning the Oxygen Reduction Reaction on Carbon-Supported Fe and Cu Electrocatalysts: Evidence for Metal-Centered Activity. J. Phys. Chem. Lett. 2011, 2, 295– 298,  DOI: 10.1021/jz1016284
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      61
      Poisoning the Oxygen Reduction Reaction on Carbon-Supported Fe and Cu Electrocatalysts: Evidence for Metal-Centered Activity
      Thorum, Matthew S.; Hankett, Jeanne M.; Gewirth, Andrew A.
      Journal of Physical Chemistry Letters (2011), 2 (4), 295-298CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
      Whether or not the active sites for the oxygen redn. reaction (ORR) in electrocatalysts based on carbon-supported transition-metal complexes are metal-centered has become controversial, esp. for heat-treated materials. Some have proposed that the transition metal only serves to form highly active sites based on nitrogen and carbon. Here, we examine the oxygen redn. activity of carbon-supported iron(II) phthalocyanine (FePc) before and after pyrolysis at 800 °C and a carbon-supported copper(II) complex with 3,5-diamino-1,2,4-triazole (CuDAT) in the presence of several anions and small-mol. poisons, including fluoride, azide, thiocyanate, ethanethiol, and cyanide. CuDAT is poisoned in a manner consistent with a Cu-based active site. Although FePc and pyrolyzed FePc are remarkably resilient to most poisons, they are poisoned by cyanide, indicative of Fe-based active sites.
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    62. 62
      Wang, Q.; Zhou, Z.-Y.; Lai, Y.-J.; You, Y.; Liu, J.-G.; Wu, X.-L.; Terefe, E.; Chen, C.; Song, L.; Rauf, M.; Tian, N.; Sun, S.-G. Phenylenediamine-Based FeNx/C Catalyst with High Activity for Oxygen Reduction in Acid Medium and Its Active-Site Probing. J. Am. Chem. Soc. 2014, 136, 10882– 10885,  DOI: 10.1021/ja505777v
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      62
      Phenylenediamine-Based FeNx/C Catalyst with High Activity for Oxygen Reduction in Acid Medium and Its Active-Site Probing
      Wang, Qiang; Zhou, Zhi-You; Lai, Yu-Jiao; You, Yong; Liu, Jian-Guo; Wu, Xia-Ling; Terefe, Ephrem; Chen, Chi; Song, Lin; Rauf, Muhammad; Tian, Na; Sun, Shi-Gang
      Journal of the American Chemical Society (2014), 136 (31), 10882-10885CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      High-temp. pyrolyzed FeNx/C catalyst is one of the most promising nonprecious metal electrocatalysts for O redn. reaction (ORR). However, it suffers from 2 challenging problems: insufficient ORR activity and unclear active site structure. Herein, the authors report a FeNx/C catalyst derived from poly-m-phenylenediamine (PmPDA-FeNx/C) that possesses high ORR activity (11.5 A g-1 at 0.80 V vs. RHE) and low H2O2 yield (<1%) in acid medium. The PmPDA-FeNx/C also exhibits high catalytic activity for both redn. and oxidn. of H2O2. Further the ORR activity of PmPDA-FeNx/C is not sensitive to CO and NOx but can be suppressed significantly by halide ions (e.g., Cl-, F-, and Br-) and low valence state S-contg. species (e.g., SCN-, SO2, and H2S). This result reveals that the active sites of the FeNx/C catalyst contains Fe element (mainly as FeIII at high potentials) in acid medium.
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    63. 63
      Thorum, M. S.; Hankett, J. M.; Gewirth, A. A. Poisoning the Oxygen Reduction Reaction on Carbon-Supported Fe and Cu Electrocatalysts: Evidence for Metal-Centered Activity. J. Phys. Chem. Lett. 2011, 2, 295– 298,  DOI: 10.1021/jz1016284
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      63
      Poisoning the Oxygen Reduction Reaction on Carbon-Supported Fe and Cu Electrocatalysts: Evidence for Metal-Centered Activity
      Thorum, Matthew S.; Hankett, Jeanne M.; Gewirth, Andrew A.
      Journal of Physical Chemistry Letters (2011), 2 (4), 295-298CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
      Whether or not the active sites for the oxygen redn. reaction (ORR) in electrocatalysts based on carbon-supported transition-metal complexes are metal-centered has become controversial, esp. for heat-treated materials. Some have proposed that the transition metal only serves to form highly active sites based on nitrogen and carbon. Here, we examine the oxygen redn. activity of carbon-supported iron(II) phthalocyanine (FePc) before and after pyrolysis at 800 °C and a carbon-supported copper(II) complex with 3,5-diamino-1,2,4-triazole (CuDAT) in the presence of several anions and small-mol. poisons, including fluoride, azide, thiocyanate, ethanethiol, and cyanide. CuDAT is poisoned in a manner consistent with a Cu-based active site. Although FePc and pyrolyzed FePc are remarkably resilient to most poisons, they are poisoned by cyanide, indicative of Fe-based active sites.
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    64. 64
      Li, W.; Wu, J.; Higgins, D. C.; Choi, J.-Y.; Chen, Z. Determination of Iron Active Sites in Pyrolyzed Iron-Based Catalysts for the Oxygen Reduction Reaction. ACS Catal. 2012, 2, 2761– 2768,  DOI: 10.1021/cs300579b
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      64
      Determination of Iron Active Sites in Pyrolyzed Iron-Based Catalysts for the Oxygen Reduction Reaction
      Li, Wenmu; Wu, Jason; Higgins, Drew C.; Choi, Ja-Yeon; Chen, Zhongwei
      ACS Catalysis (2012), 2 (12), 2761-2768CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
      Fe-based oxygen redn. reaction (ORR) catalyst materials are considered promising nonprecious alternatives to traditional platinum-based catalysts. These catalyst materials are generally produced by high-temp. pyrolysis treatments of readily available carbon, nitrogen, and iron sources. Adequate control of the structure and active site formation during pyrolysis methods is nearly impossible. Thus, the chem. nature, structure, and ORR mechanism of catalytically active sites in these materials is a subject of significant debate. A method is proposed, utilizing CN- ions as ORR inhibitors on Fe-based catalysts, to provide insight into the exact nature and chem. of the catalytically active sites. Moreover, two possible catalytically active site formation mechanisms occurring during high-temp. pyrolysis treatments, dependent on the specific type of precursor and synthesis methods utilized are proposed. Further direct evidence is provided of the proposed active site formations using ToF-SIMS neg. and pos. ion imaging. This knowledge will be beneficial to future work directed at the development of Fe-based catalysts with improved ORR activity and operational stabilities for fuel cell and battery applications.
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    65. 65
      Xiao, M.; Zhu, J.; Ma, L.; Jin, Z.; Ge, J.; Deng, X.; Hou, Y.; He, Q.; Li, J.; Jia, Q.; Mukerjee, S.; Yang, R.; Jiang, Z.; Su, D.; Liu, C.; Xing, W. Microporous framework induced synthesis of single-atom dispersed Fe-NC acidic ORR catalyst and its in situ reduced Fe-N4 active site identification revealed by X-ray absorption spectroscopy. ACS Catal. 2018, 8, 2824– 2832,  DOI: 10.1021/acscatal.8b00138
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      65
      Microporous Framework Induced Synthesis of Single-Atom Dispersed Fe-N-C Acidic ORR Catalyst and Its in Situ Reduced Fe-N4 Active Site Identification Revealed by X-ray Absorption Spectroscopy
      Xiao, Meiling; Zhu, Jianbing; Ma, Liang; Jin, Zhao; Ge, Junjie; Deng, Xin; Hou, Yang; He, Qinggang; Li, Jingkun; Jia, Qingying; Mukerjee, Sanjeev; Yang, Ruoou; Jiang, Zheng; Su, Dangsheng; Liu, Changpeng; Xing, Wei
      ACS Catalysis (2018), 8 (4), 2824-2832CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
      Developing highly efficient, low-cost O redn. catalysts, esp. in acidic medium, is of significance toward fuel cell commercialization. Although pyrolyzed Fe-N-C catalysts have been regarded as alternatives to platinum-based catalytic materials, further improvement requires precise control of the Fe-Nx structure at the mol. level and a comprehensive understanding of catalytic site structure and the ORR mechanism on these materials. Here, we present a microporous metal-org.-framework-confined strategy toward the preferable formation of single-atom dispersed catalysts. The onset potential for Fe-N-C is 0.92 V, comparable to that of Pt/C and outperforming most noble-metal-free catalysts ever reported. A high-spin Fe3+-N4 configuration is revealed by the 57Fe Moddossbauer spectrum and X-ray absorption spectroscopy for Fe L-edge, which will convert to Fe2+-N4 at low potential. The in situ reduced Fe2+-N4 moiety from high-spin Ox-Fe3+-N4 contributes to most of the ORR activity due to its high turnover frequency (TOF) of ∼1.71 e/s-sites.
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    66. 66
      Li, J.; Ghoshal, S.; Liang, W.; Sougrati, M.-T.; Jaouen, F.; Halevi, B.; McKinney, S.; McCool, G.; Ma, C.; Yuan, X.; Ma, Z.-F.; Mukerjee, S.; Jia, Q. Structural and mechanistic basis for the high activity of Fe–N–C catalysts toward oxygen reduction. Energy Environ. Sci. 2016, 9, 2418– 2432,  DOI: 10.1039/c6ee01160h
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      66
      Structural and mechanistic basis for the high activity of Fe-N-C catalysts toward oxygen reduction
      Li, Jingkun; Ghoshal, Shraboni; Liang, Wentao; Sougrati, Moulay-Tahar; Jaouen, Frederic; Halevi, Barr; McKinney, Samuel; McCool, Geoff; Ma, Chunrong; Yuan, Xianxia; Ma, Zi-Feng; Mukerjee, Sanjeev; Jia, Qingying
      Energy & Environmental Science (2016), 9 (7), 2418-2432CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)
      The development of efficient non-platinum group metal (non-PGM) catalysts for oxygen redn. reaction (ORR) is of paramount importance for clean and sustainable energy storage and conversion devices. The major bottleneck in developing Fe-N-C materials as the leading non-PGM catalysts lies in the poor understanding of the nature of active sites and reaction mechanisms. Herein, we report a scalable metal org. framework-derived Fe-N-C catalyst with high ORR activity demonstrated in practical H2/air fuel cells, and an unprecedented turnover frequency (TOF) in acid in rotating disk electrode. By characterizing the catalyst under both ex situ and operando conditions using combined microscopic and spectroscopic techniques, we show that the structures of active sites under ex situ and working conditions are drastically different. Resultantly, the active site proposed here, a non-planar ferrous Fe-N4 moiety embedded in distorted carbon matrix characterized by a high Fe2+/3+ redox potential, is in contrast with those proposed hitherto derived from ex situ characterizations. This site reversibly switches to an in-plane ferric Fe-N4 moiety poisoned by oxygen adsorbates during the redox transition, with the population of active sites controlled by the Fe2+/3+ redox potential. The unprecedented TOF of the active site is correlated to its near-optimal Fe2+/3+ redox potential, and essentially originated from its favorable biomimetic dynamic nature that balances the site-blocking effect and O2 dissocn. The porous and disordered carbon matrix of the catalyst plays pivotal roles for its measured high ORR activity by hosting high population of reactant-accessible active sites.
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      Shi, C.; Anson, F. C. Catalytic pathways for the electroreduction of oxygen by iron tetrakis (4-N-methylpyridyl) porphyrin or iron tetraphenylporphyrin adsorbed on edge plane pyrolytic graphite electrodes. Inorg. Chem. 1990, 29, 4298– 4305,  DOI: 10.1021/ic00346a027
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      67
      Catalytic pathways for the electroreduction of oxygen by iron tetrakis(4-N-methylpyridyl)porphyrin or iron tetraphenylporphyrin adsorbed on edge plane pyrolytic graphite electrodes
      Shi, Chunnian; Anson, Fred C.
      Inorganic Chemistry (1990), 29 (21), 4298-305CODEN: INOCAJ; ISSN:0020-1669.
      Spontaneous adsorption of Fe tetrakis(4-N-methylpyridyl)porphyrin on edge plane pyrolytic graphite electrodes (EPG) from aq. solns.of the sol. porphyrin produces an adsorbed species that catalyzes the electroredn. of O2 at potentials notably more pos. than can be obtained with electrodes on which the porphyrin does not adsorb (e.g., glassy C). Adsorption of insol. Fe tetraphenylporphyrin (FeTPP) on EPG electrodes produces two surface waves for the adsorbed porphyrin, one prominent and one partially hidden by the response from the EPG surface itself. Only the form of the FeTPP responsible for the latter wave participates in the catalytic redn. of O2. Possible explanations for the obsd. behavior are offered that allow the apparently disparate catalytic reactivity exhibited by iron and cobalt porphyrins to be understood as examples of a single mechanistic pattern.
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      Ni, C. L.; Anson, F. C. Relation between the potentials where adsorbed and unadsorbed cobalt (III) tetrakis (N-methylpyridinium-4-yl) porphyrin is reduced and those where it catalyzes the electroreduction of dioxygen. Inorg. Chem. 1985, 24, 4754– 4756,  DOI: 10.1021/ic00220a067
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      Relation between the potentials where adsorbed and unadsorbed cobalt(III) tetrakis(N-methylpyridinium-4-yl)porphyrin is reduced and those where it catalyzes the electroreduction of dioxygen
      Ni, Ching Long; Anson, Fred C.
      Inorganic Chemistry (1985), 24 (26), 4754-6CODEN: INOCAJ; ISSN:0020-1669.
      The large difference in the formal potentials of CoIII/II between the adsorbed and unadsorbed Co-tetrakis(N-methylpyridinium-4-yl)porphyrin complex (CoTMPyP) as measured with a graphite electrode indicates unusually favorable interactions between the Co center and the electrode surface when the Co is in the reduced state. The adsorbed CoTMPyP, with a formal potential (CoIII/II) near 0.5V vs. SCE catalyzes the redn. of O at potential near 0.3V vs. SCE. The fact that the catalyzed redn. of O proceeds near the formal potential of the CoIII/II couple for the unadsorbed porphyrin is coincidental. Cycles voltammetry of CoTMPyP was studied at edge-plane pyrolytic graphite electrodes and at stationary Au electrodes in 0.5M CF3CO2H. Adsorption of CoTMPyP is much weaker by Au.
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      Bottomley, L. A.; Kadish, K. M. Counterion and solvent effects on the electrode reactions of iron porphyrins. Inorg. Chem. 1981, 20, 1348– 1357,  DOI: 10.1021/ic50219a003
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      Counterion and solvent effects on the electrode reactions of iron porphyrins
      Bottomley, L. A.; Kadish, K. M.
      Inorganic Chemistry (1981), 20 (5), 1348-57CODEN: INOCAJ; ISSN:0020-1669.
      The 1st systematic study of the interacting effects of solvent and axially coordinated monovalent anions on the electroredn. mechanisms and redox potentials of Fe porphyrins is presented. Five different anions were coordinated to (5,10,15,20-tetraphenylporphinato)iron(III), and their resp. redox reactions were investigated in 12 nonaq. solvents. Potential shifts with changes in solvent were directly related to solvent donicity. Potential shifts with changes in counterion were related to the degree of Fe(III)-counterion interaction and were also solvent dependent. The counterion is the most predominant factor governing the type of electron-transfer mechanism obsd. for the electrode reactions of Fe porphyrins. The apparent independence of redox potentials on the Fe-center spin state is established.
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    70. 70
      Milani, M.; Ouellet, Y.; Ouellet, H.; Guertin, M.; Boffi, A.; Antonini, G.; Bocedi, A.; Mattu, M.; Bolognesi, M.; Ascenzi, P. Cyanide Binding to Truncated Hemoglobins:  A Crystallographic and Kinetic Study. Biochemistry 2004, 43, 5213– 5221,  DOI: 10.1021/bi049870+
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      Cyanide Binding to Truncated Hemoglobins: A Crystallographic and Kinetic Study
      Milani, Mario; Ouellet, Yannick; Ouellet, Hugues; Guertin, Michel; Boffi, Alberto; Antonini, Giovanni; Bocedi, Alessio; Mattu, Marco; Bolognesi, Martino; Ascenzi, Paolo
      Biochemistry (2004), 43 (18), 5213-5221CODEN: BICHAW; ISSN:0006-2960. (American Chemical Society)
      Cyanide is one of the few diat. ligands able to interact with the ferric and ferrous heme-Fe atom. Here, the X-ray crystal structure of the cyanide deriv. of ferric Mycobacterium tuberculosis truncated Hb-N (M. tuberculosis trHbN) has been detd. at 2.0 Å (R-general = 17.8% and R-free = 23.5%), and analyzed in parallel with those of M. tuberculosis truncated Hb-O (M. tuberculosis trHbO), Chlamydomonas eugametos truncated Hb (C. eugametos trHb), and sperm whale myoglobin, generally taken as a mol. model. Cyanide binding to M. tuberculosis trHbN is stabilized directly by residue TyrB10(33), which may assist the deprotonation of the incoming ligand and the protonation of the outcoming cyanide. In M. tuberculosis trHbO and in C. eugametos trHb the ligand is stabilized by the distal pocket residues TyrCD1(36) and TrpG8(88), and by the TyrB10(20) - GlnE7(41) - GlnE11(45) triad, resp. Moreover, kinetics for cyanide binding to ferric M. tuberculosis trHbN and trHbO and C. eugametos trHb, for ligand dissocn. from the ferrous trHbs, and for the redn. of the heme-Fe(III)-cyanide complex have been detd., at pH 7.0 and 20.0°. Despite the different heme distal site structures and ligand interactions, values of the rate const. for cyanide binding to ferric (non)vertebrate heme proteins are similar, being influenced mainly by the presence in the heme pocket of proton acceptor group(s), whose function is to assist the deprotonation of the incoming ligand (i.e., HCN). On the other hand, values of the rate const. for the redn. of the heme-Fe(III)-cyanide (non)vertebrate globins span over several orders of magnitude, reflecting the different ability of the heme proteins considered to give productive complex(es) with dithionite or its reducing species SO2-. Furthermore, values of the rate const. for ligand dissocn. from heme-Fe(II)-cyanide (non)vertebrate heme proteins are very different, reflecting the different nature and geometry of the heme distal residue(s) hydrogen-bonded to the heme-bound cyanide.
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      Wu, G.; Johnston, C. M.; Mack, N. H.; Artyushkova, K.; Ferrandon, M.; Nelson, M.; Lezama-Pacheco, J. S.; Conradson, S. D.; More, K. L.; Myers, D. J.; Zelenay, P. Synthesis–structure–performance correlation for polyaniline–Me–C non-precious metal cathode catalysts for oxygen reduction in fuel cells. J. Mater. Chem. 2011, 21, 11392– 11405,  DOI: 10.1039/c0jm03613g
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      71
      Synthesis-structure-performance correlation for polyaniline-Me-C non-precious metal cathode catalysts for oxygen reduction in fuel cells
      Wu, Gang; Johnston, Christina M.; Mack, Nathan H.; Artyushkova, Kateryna; Ferrandon, Magali; Nelson, Mark; Lezama-Pacheco, Juan S.; Conradson, Steven D.; More, Karren L.; Myers, Deborah J.; Zelenay, Piotr
      Journal of Materials Chemistry (2011), 21 (30), 11392-11405CODEN: JMACEP; ISSN:0959-9428. (Royal Society of Chemistry)
      The authors present the systematic prepn. of active and durable non-precious metal catalysts (NPMCs) for the oxygen redn. reaction in polymer electrolyte fuel cells (PEFCs) based on the heat treatment of polyaniline/metal/carbon precursors. Variation of the synthesis steps, heat-treatment temp., metal loading, and the metal type in the synthesis leads to markedly different catalyst activity, speciation, and morphol. Microscopy studies demonstrate notable differences in the carbon structure as a function of these variables. Balancing the need to increase the catalyst's degree of graphitization through heat treatment vs. the excessive loss of surface area that occurs at higher temps. is a key to prepg. an active catalyst. XPS and XAFS spectra are consistent with the presence of Me-Nx structures in both the Co and Fe versions of the catalyst, which are often proposed to be active sites. The av. speciation and coordination environment of nitrogen and metal, however, depends greatly on the choice of Co or Fe. Taken together, the data indicate that better control of the metal-catalyzed transformations of the polymer into new graphitized carbon forms in the heat-treatment step will allow for even further improvement of this class of catalysts.
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      Gadipelli, S.; Zhao, T.; Shevlin, S. A.; Guo, Z. Switching effective oxygen reduction and evolution performance by controlled graphitization of a cobalt–nitrogen–carbon framework system. Energy Environ. Sci. 2016, 9, 1661– 1667,  DOI: 10.1039/c6ee00551a
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      Switching effective oxygen reduction and evolution performance by controlled graphitization of a cobalt-nitrogen-carbon framework system
      Gadipelli, Srinivas; Zhao, Tingting; Shevlin, Stephen A.; Guo, Zhengxiao
      Energy & Environmental Science (2016), 9 (5), 1661-1667CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)
      We report a purposely designed route for the synthesis of a promising carbon-based electrocatalyst for both ORR (oxygen redn. reaction) and OER (oxygen evolution reaction) from zeolitic imidazolate frameworks (ZIFs). Firstly, precursor ZIFs are rationally designed with a blend of volatile zinc to induce porosity and stable cobalt to induce graphitic domains. Secondly, the self-modulated cobalt-nitrogen-carbon system (SCNCS) is shown to be an effective ORR catalyst after graphitization at mild temps. Finally, the best OER catalyst is developed by enhancing graphitization of the SCNCS. For the first time, solely by switching the graphitization conditions of SCNCS, excellent ORR or OER performance is realized. This approach not only opens up a simple protocol for simultaneous optimization of nitrogen doping and graphitization at controlled cobalt concns., but also provide a facile method of developing such active catalysts without the use of extensive synthesis procedures.
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      Wang, X.; Li, Q.; Pan, H.; Lin, Y.; Ke, Y.; Sheng, H.; Swihart, M. T.; Wu, G. Size-controlled large-diameter and few-walled carbon nanotube catalysts for oxygen reduction. Nanoscale 2015, 7, 20290– 20298,  DOI: 10.1039/c5nr05864c
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      Size-controlled large-diameter and few-walled carbon nanotube catalysts for oxygen reduction
      Wang, Xianliang; Li, Qing; Pan, Hengyu; Lin, Ye; Ke, Yujie; Sheng, Haiyang; Swihart, Mark T.; Wu, Gang
      Nanoscale (2015), 7 (47), 20290-20298CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
      We demonstrate a new strategy for tuning the size of large-diam. and few-walled nitrogen-doped carbon nanotubes (N-CNTs) from 50 to 150 nm by varying the transition metal (TM = Fe, Co, Ni or Mn) used to catalyze graphitization of dicyandiamide. Fe yielded the largest tubes, followed by Co and Ni, while Mn produced a clot-like carbon morphol. We show that morphol. is correlated with electrocatalytic activity for the oxygen redn. reaction (ORR). A clear trend of Fe > Co > Ni > Mn for the ORR catalytic activity was obsd., in both alk. media and more demanding acidic media. The Fe-derived N-CNTs exhibited the highest BET (∼870 m2 g-1) and electrochem. accessible (∼450 m2 g-1) surface areas and, more importantly, the highest concn. of nitrogen incorporated into the carbon planes. Thus, in addn. to the intrinsic high activity of Fe-derived catalysts, the high surface area and nitrogen doping contribute to high ORR activity. This work, for the first time, demonstrates size-controlled synthesis of large-diam. N-doped carbon tube electrocatalysts by varying the metal used in N-CNT generation. Electrocatalytic activity of the Fe-derived catalyst is already the best among studied metals, due to the high intrinsic activity of possible Fe-N coordination. This work further provides a promising route to advanced Fe-N-C nonprecious metal catalysts by generating favorable morphol. with more active sites and improved mass transfer.
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      Zhang, W.; Zhang, H.; Xiao, J.; Zhao, Z.; Yu, M.; Li, Z. Carbon nanotube catalysts for oxidative desulfurization of a model diesel fuel using molecular oxygen. Green Chem. 2014, 16, 211– 220,  DOI: 10.1039/c3gc41106k
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      Carbon nanotube catalysts for oxidative desulfurization of a model diesel fuel using molecular oxygen
      Zhang, Wei; Zhang, Hong; Xiao, Jing; Zhao, Zhenxia; Yu, Moxin; Li, Zhong
      Green Chemistry (2014), 16 (1), 211-220CODEN: GRCHFJ; ISSN:1463-9262. (Royal Society of Chemistry)
      We firstly propose the application of CNTs as novel catalysts and mol. oxygen as the oxidant for the oxidative desulfurization (ODS) of a model fuel contg. benzothiophene (BT), dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (4,6-DMDBT) at atm. pressure and low temp. Results showed that when three CNTs including CNT-SZ, CNT-TS, and CNT-CD were sep. used as catalysts with mol. oxygen as the oxidant, the conversion of DBT to its corresponding sulfone reached 100% at 150°C sep. within 40, 120 and 180 min. The CNT-SZ exhibited a superior catalytic activity even at a high fuel-to-catalyst wt. ratio of 7.5:1. The oxidn. reactivity of these benzothiophenic compds. followed the order: 4,6-DMDBT > DBT > BT. The deactivated CNT can be effectively regenerated by heat treatment under an argon atm. at 900°C. Raman spectroscopy anal. revealed that the graphitization degree of the CNT played a decisive role in its catalytic activity for DBT oxidn. The CNT with the higher degree of graphitization had higher catalytic activity for DBT oxidn. since its higher elec. cond. benefited the transfer of electrons involved in the redox reaction.
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      Wang, W.; Chen, W.; Miao, P.; Luo, J.; Wei, Z.; Chen, S. NaCl crystallites as dual-functional and water-removable templates to synthesize a three-dimensional graphene-like macroporous Fe-NC catalyst. ACS Catal. 2017, 7, 6144– 6149,  DOI: 10.1021/acscatal.7b01695
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      NaCl Crystallites as Dual-Functional and Water-Removable Templates To Synthesize a Three-Dimensional Graphene-like Macroporous Fe-N-C Catalyst
      Wang, Wang; Chen, Wenhui; Miao, Peiyu; Luo, Jin; Wei, Zidong; Chen, Shengli
      ACS Catalysis (2017), 7 (9), 6144-6149CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
      Three-dimensional macroporous carbon materials with hierarchical pore structures (3D MPC) have wide applications, but the scale-up synthesis is limited by the cumbersome procedures of template formation and removal. Herein, we show that NaCl crystallites, which form in situ in a lyophilizing process of a NaCl soln. contg. a carbon precursor and are removable simply through water washing, can act as templates to grow 3D MPC materials with graphene-like ultrathin and mesoporous walls through pyrolitic carbonization. Further, by use of a nitrogen (N)-rich polymer (polyvinylpyrrolidone, PVP) as the carbon precursor and introduction of Fe salt in the precursor, an MPC catalyst with high Fe/N doping content is achieved due to the NaCl crystallites serving as confining agents to simultaneously prevent the large wt. loss and N evapn., a severe problem in usual pyrolytic syntheses of Fe-N-C catalysts. Benefiting from the mass transport convenience of the macropores as indicated by the impedance spectroscopy results, the Fe/N-doped 3D MPC exhibits high catalytic performance toward the oxygen redn. reaction. The dual functionality, facile formation and removal, and reusability of NaCl make the present method a promising way to gain cost-effective porous Fe-N-C catalysts.
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      Zeng, H.; Wang, W.; Li, J.; Luo, J.; Chen, S. In Situ Generated Dual-Template Method for Fe/N/S Co-Doped Hierarchically Porous Honeycomb Carbon for High-Performance Oxygen Reduction. ACS Appl. Mater. Interfaces 2018, 10, 8721– 8729,  DOI: 10.1021/acsami.7b19645
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      In Situ Generated Dual-Template Method for Fe/N/S Co-Doped Hierarchically Porous Honeycomb Carbon for High-Performance Oxygen Reduction
      Zeng, Hongju; Wang, Wang; Li, Jun; Luo, Jin; Chen, Shengli
      ACS Applied Materials & Interfaces (2018), 10 (10), 8721-8729CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
      Heteroatoms doping is able to produce catalytic sites in C materials for O redn. reaction (ORR); while hierarchically porous structure is necessary for efficient exposure and accessibility of the usually limited catalytic sites in such activated C catalysts. This work reports an in situ generated dual-template method to synthesize the Fe/N/S co-doped hierarchically porous C (FeNS/HPC), with NaCl crystallites formed during the precursor lyophilization process as the primary template to generate ∼500 nm macropores with ultrathin graphene-like C-layer walls, and Fe3O4 nanoparticles formed during the high-temp. carbonization process as the secondary template to produce mesopores on the walls of macropores. As well as the coexistence of graphitic-N, pyridinic-N, and thiophene-S which are beneficial to ORR, the as prepd. FeNS/HPC possesses a highly graphitized and interconnected hierarchical porous structure, giving a sp. surface area as high as 938 m2/g. As a consequence, it exhibits excellent 4-electron O redn. performance in both alk. and acid electrolytes. The in situ generation and facile soln. removal make the present template method a promising way for scale-up prepn. of active porous C materials for various applications.
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      Costa, L.; Camino, G. Thermal behaviour of melamine. J. Therm. Anal. 1988, 34, 423– 429,  DOI: 10.1007/bf01913181
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      Thermal behavior of melamine
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      Journal of Thermal Analysis (1988), 34 (2), 423-9CODEN: JTHEA9; ISSN:0368-4466.
      Melamine undergoes condensation on heating with elimination of ammonia and formation of insol. products. Thermogravimetry and IR characterization show that two products of successive condensation can be reproducibly obtained upon heating at T<500°, and then at 600°. Above 620°, the melamine condensate undergoes thermal degrdn. with quant. formation of volatile products.
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      Dyjak, S.; Kiciński, W.; Huczko, A. Thermite-driven melamine condensation to C x N y H z graphitic ternary polymers: towards an instant, large-scale synthesis of gC3N4. J. Mater. Chem. A 2015, 3, 9621– 9631,  DOI: 10.1039/c5ta00201j
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      Thermite-driven melamine condensation to CxNyHz graphitic ternary polymers: towards an instant, large-scale synthesis of g-C3N4
      Dyjak, S.; Kicinski, W.; Huczko, A.
      Journal of Materials Chemistry A: Materials for Energy and Sustainability (2015), 3 (18), 9621-9631CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
      Utilization of heat released from the classical aluminothermic reaction (Fe2O3 + Al) for thermal condensation of melamine allows prodn. of a large amt. of C/N/H graphitic ternary polymers within a matter of a few minutes. This approach could allow instant, large-scale synthesis of g-C3N4. A detailed, comparative study of melamine condensation to hydrogen-rich melon-related CxNyHz graphitic ternary polymers (which can be considered as a pre-stage to carbon(iv) nitride) initiated by resistance furnace or by thermite reaction is presented. The products obtained by both heating routes at two different temps. (560 and 640 °C) are compared and analyzed. It is shown that the photocatalytic activity of the CxNyHz polymers correlates with the hydrogen content in the materials. The superiority of the thermite-heated process over elec. resistance furnace heating is elucidated. The exceptional characteristics of the thermite reaction, that is the use of cheap, ubiquitous substrates, rapid increase of temp. and peerless exothermicity, make it suitable for efficient, large-scale transformation of a variety of primary org. compds. (melamine and beyond) into new advanced functional materials.
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      Du, C.; Liu, X.; Ye, G.; Gao, X.; Zhuang, Z.; Li, P.; Xiang, D.; Li, X.; Clayborne, A. Z.; Zhou, X.; Chen, W. Balancing the Micro-Mesoporosity for Activity Maximization of N-Doped Carbonaceous Electrocatalysts for the Oxygen Reduction Reaction. ChemSusChem 2019, 12, 1017– 1025,  DOI: 10.1002/cssc.201802960
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      Balancing the Micro-Mesoporosity for Activity Maximization of N-Doped Carbonaceous Electrocatalysts for the Oxygen Reduction Reaction
      Du, Cheng; Liu, Xinlei; Ye, Guanghua; Gao, Xiaohui; Zhuang, Zhihua; Li, Ping; Xiang, Dong; Li, Xiaokun; Clayborne, Andre Z.; Zhou, Xinggui; Chen, Wei
      ChemSusChem (2019), 12 (5), 1017-1025CODEN: CHEMIZ; ISSN:1864-5631. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Carbonaceous porous structures have instigated global research interest as promising low-cost electrocatalysts for numerous energy technologies. However, the rational design principle of pore structures for activity maximization is still unclear. In this work, a series of N-doped carbon (N-C) catalysts with exclusively different micro-mesoporosity are investigated for the oxygen redn. reaction (ORR). By combining the expt. results and a pioneering math. model, it was obsd. that the best catalytic activity can only be attained by balancing the micro-mesoporosity. These findings offer a definite criterion for pore structure optimization in carbon-based ORR catalysts, which is of great importance for various energy technologies.
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      Jaouen, F.; Lefèvre, M.; Dodelet, J.-P.; Cai, M. Heat-treated Fe/N/C catalysts for O2 electroreduction: are active sites hosted in micropores?. J. Phys. Chem. B 2006, 110, 5553– 5558,  DOI: 10.1021/jp057135h
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      Heat-Treated Fe/N/C Catalysts for O2 Electroreduction: Are Active Sites Hosted in Micropores?
      Jaouen, Frederic; Lefevre, Michel; Dodelet, Jean-Pol; Cai, Mei
      Journal of Physical Chemistry B (2006), 110 (11), 5553-5558CODEN: JPCBFK; ISSN:1520-6106. (American Chemical Society)
      Limited availability of Pt is a threat to fuel cell commercialization. Alternative catalysts for the electrochem. redn. of O were obtained by heat treatment at >600° of C with non-noble metals and a source of N atoms. However, the process by which heat treatment activates these materials is uncertain. The activation of C black and Fe acetate heat-treated in NH3 comprises 3 consecutive steps: (a) incorporation of N atoms in the C, (b) micropore formation through reaction between C and NH3, and (c) completion of active sites in the micropores by reaction of Fe with NH3. Step (b) is the slowest. The microporous surface per mass of catalyst controls the macroscopic activity when enough N atoms are incorporated in the structure of the C support. These facts should help in detg. the structure of the active sites and in identifying methods to increase the site d. of such catalysts.
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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD28Xhs12ksrg%253D&md5=ace3633286f0724cbc64f337fe200549
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      Lefèvre, M.; Proietti, E.; Jaouen, F.; Dodelet, J.-P. Iron-based catalysts with improved oxygen reduction activity in polymer electrolyte fuel cells. Science 2009, 324, 71– 74,  DOI: 10.1126/science.1170051
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      Iron-Based Catalysts with Improved Oxygen Reduction Activity in Polymer Electrolyte Fuel Cells
      Lefevre, Michel; Proietti, Eric; Jaouen, Frederic; Dodelet, Jean-Pol
      Science (Washington, DC, United States) (2009), 324 (5923), 71-74CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
      Fe-based catalysts for the O-redn. reaction in polymer electrolyte fuel cells are not as good as Pt catalysts, in part because they have a comparatively low no. of active sites per unit vol. Microporous C-supported Fe-based catalysts, with active sites contg. Fe cations coordinated by pyridinic N functionalities in the interstices of graphitic sheets within the micropores, were prepd. The greatest increase in site d. was obtained when a mixt. of C support, phenanthroline, and ferrous acetate was ball-milled and then pyrolyzed in Ar and then in NH3. The c.d. of a cathode made with the best Fe-based electrocatalyst equals that of a Pt-based cathode with a loading of 0.4 mg Pt/cm2 at a cell voltage of ≥0.9 V.
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      Peng, H.; Liu, F.; Liu, X.; Liao, S.; You, C.; Tian, X.; Nan, H.; Luo, F.; Song, H.; Fu, Z.; Huang, P. Effect of transition metals on the structure and performance of the doped carbon catalysts derived from polyaniline and melamine for ORR application. ACS Catal. 2014, 4, 3797– 3805,  DOI: 10.1021/cs500744x
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      Effect of Transition Metals on the Structure and Performance of the Doped Carbon Catalysts Derived From Polyaniline and Melamine for ORR Application
      Peng, Hongliang; Liu, Fangfang; Liu, Xiaojun; Liao, Shijun; You, Chenghang; Tian, Xinlong; Nan, Haoxiong; Luo, Fan; Song, Huiyu; Fu, Zhiyong; Huang, Peiyan
      ACS Catalysis (2014), 4 (10), 3797-3805CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
      The effects of the addn. of transition metals (Mn, Fe, Co, Ni, Cu) on the structure and performance of the doped C catalysts M-PANI/C-Mela are studied. The doping of various transition metals affected structures and performances of the catalysts significantly. Doping with Fe and Mn leads to a catalyst with a graphene-like structure, and doping with Co, Ni, and Cu leads to a disordered or nanosheet structure. The doping of transition metals can enhance the performance of the catalysts, and their ORR activity follows the order of Fe > Co > Cu > Mn > Ni, which is consistent with the order of their active N contents. Probably the various performance enhancements of the transition metals may be the result of the joint effect of the following 3 aspects: the N content/active N content, metal residue, and the surface area and pore structure, but not the effect of any single factor.
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    83. 83
      Sheng, Z.-H.; Shao, L.; Chen, J.-J.; Bao, W.-J.; Wang, F.-B.; Xia, X.-H. Catalyst-Free Synthesis of Nitrogen-Doped Graphene via Thermal Annealing Graphite Oxide with Melamine and Its Excellent Electrocatalysis. ACS Nano 2011, 5, 4350– 4358,  DOI: 10.1021/nn103584t
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      Catalyst-Free Synthesis of Nitrogen-Doped Graphene via Thermal Annealing Graphite Oxide with Melamine and Its Excellent Electrocatalysis
      Sheng, Zhen-Huan; Shao, Lin; Chen, Jing-Jing; Bao, Wen-Jing; Wang, Feng-Bin; Xia, Xing-Hua
      ACS Nano (2011), 5 (6), 4350-4358CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      The electronic and chem. properties of graphene can be modulated by chem. doping foreign atoms and functional moieties. The general approach to the synthesis of N-doped graphene (NG), such as CVD performed in gas phases, requires transitional metal catalysts which could contaminate the resultant products and thus affect their properties. The authors propose a facile, catalyst-free thermal annealing approach for large-scale synthesis of NG using low-cost industrial material melamine as the N source. This approach can completely avoid the contamination of transition metal catalysts, and thus the intrinsic catalytic performance of pure NGs can be studied. Detailed x-ray photoelectron spectrum anal. of the resultant products shows that the at. percentage of N in doped graphene samples can be adjusted up to 10.1%. Such a high doping level was not reported previously. High-resoln. N1s spectra reveal that the as-made NG mainly contains pyridine-like N atoms. Electrochem. characterizations clearly demonstrate excellent electrocatalytic activity of NG toward the O redn. reaction (ORR) in alk. electrolytes, which is independent of N doping level. The present catalyst-free approach opens up the possibility for the synthesis of NG in gram-scale for electronic devices and cathodic materials for fuel cells and biosensors.
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    84. 84
      Lee, J.-S.; Park, G. S.; Kim, S. T.; Liu, M.; Cho, J. A highly efficient electrocatalyst for the oxygen reduction reaction: N-doped ketjenblack incorporated into Fe/Fe3C-functionalized melamine foam. Angew. Chem., Int. Ed. 2013, 52, 1026– 1030,  DOI: 10.1002/anie.201207193
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      A Highly Efficient Electrocatalyst for the Oxygen Reduction Reaction: N-Doped Ketjenblack Incorporated into Fe/Fe3C-Functionalized Melamine Foam
      Lee, Jang-Soo; Park, Gi Su; Kim, Sun Tai; Liu, Meilin; Cho, Jaephil
      Angewandte Chemie, International Edition (2013), 52 (3), 1026-1030CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      To dramatically enhance mass transfer through porous O-breathing electrodes, we created unique electrode architectures that are similar to breakwaters composed of highly porous tetrapod structures. To imitate this structure, we started with a com. available melamine foam. After pyrolysis, the open-cell network structure of the carbonized melamine foam becomes very fragile, and it is easily broken to pieces by grinding with a mortar. Interestingly, the broken pieces appear similar to the tetrapod architecture. our unique electrodes consist of carbonized melamine foam and nano-sized ketjenblack, thus offering high sp. surface area, large no. of active sites, and large amt. of pore vol. for fast mass transport. Although Lin et al. (2012) used melamine resin to prep. metal-free N-doped electrocatalysts for ORR, the unique architecture of carbonized melamine foam has not yet been used in synthesizing electrocatalysts for ORR.
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      Liu, X.; Li, W.; Zou, S. Cobalt and nitrogen-codoped ordered mesoporous carbon as highly efficient bifunctional catalysts for oxygen reduction and hydrogen evolution reactions. J. Mater. Chem. A 2018, 6, 17067– 17074,  DOI: 10.1039/c8ta06864j
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      Cobalt and nitrogen-codoped ordered mesoporous carbon as highly efficient bifunctional catalysts for oxygen reduction and hydrogen evolution reactions
      Liu, Xiaojun; Li, Wenyue; Zou, Shouzhong
      Journal of Materials Chemistry A: Materials for Energy and Sustainability (2018), 6 (35), 17067-17074CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
      The high cost and limited reserves of noble metals such as Pt have hampered their large-scale com. applications in the oxygen redn. reaction (ORR) and hydrogen evolution reaction (HER). Herein, we developed a simple silica template approach to form porous carbons codoped with nitrogen and cobalt as highly efficient bifunctional electrocatalysts for the ORR and HER. The resulting porous carbons exhibited excellent electrocatalytic activity for the ORR in alk. media, which compares favorably with that of com. Pt/C (20 wt%), and had superior durability and excellent methanol tolerance. The porous carbons also showed superior performance for the HER, with a low onset potential of -0.04 V, a Tafel slope of 49 mV dec-1, an overpotential of -0.106 V at a c.d. of 10 mA cm-2, and remarkable durability. These results demonstrate that the Co, N-doped carbons are promising bifunctional catalysts for fuel cell applications.
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      Liu, W.-J.; Jiang, H.; Yu, H.-Q. Emerging applications of biochar-based materials for energy storage and conversion. Energy Environ. Sci. 2019, 12, 1751– 1779,  DOI: 10.1039/c9ee00206e
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      Emerging applications of biochar-based materials for energy storage and conversion
      Liu, Wu-Jun; Jiang, Hong; Yu, Han-Qing
      Energy & Environmental Science (2019), 12 (6), 1751-1779CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)
      A review. Global warming, environmental pollution, and an energy shortage in the current fossil fuel society may cause a severe ecol. crisis. Storage and conversion of renewable, dispersive and non-perennial energy from the sun, wind, geothermal sources, water, or biomass could be a promising option to relieve this crisis. Carbon materials could be the most versatile platform materials applied in the field of modern energy storage and conversion. Conventional carbon materials produced from coal and petrochem. products are usually energy intensive or involve harsh synthetic conditions. It is highly desired to develop effective methods to produce carbon materials from renewable resources that have high performance and limited environmental impacts. In this regard, biochar, a bio-carbon with abundant surface functional groups and easily tuned porosity produced from biomass, may be a promising candidate as a sustainable carbon material. Recent studies have demonstrated that biochar-based materials show great application potential in energy storage and conversion because of their easily tuned surface chem. and porosity. In this review, recent advances in the applications of biochar-based materials in various energy storage and conversion fields, including hydrogen storage and prodn., oxygen electrocatalysts, emerging fuel cell technol., supercapacitors, and lithium/sodium ion batteries, are summarized, highlighting the mechanisms and open questions in current energy applications. Finally, contemporary challenges and perspectives on how biochar-based materials will develop and, in particular, the fields in which the use of biochar-based materials could be expanded are discussed throughout the review. This review demonstrates significant potential for energy applications of biochar-based materials, and it is expected to inspire new discoveries to promote practical applications of biochar-based materials in more energy storage and conversion fields.
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    • Elemental contents in different carbon catalysts determined by XPS measurements; comparison of the catalytic performances of M+S+C900-900 and noble metal-free carbon materials in the literature; results from nitrogen adsorption−desorption analysis; comparison of nitrogen species in different carbon catalysts; LSV curves of M+S+C900-900 from different spinach; hydrogen peroxide yield of M+S+C900-900 and Pt/C during the ORR process; RDE voltammograms for ORR of M+S+C900-900 and 20 wt % Pt/C in 0.1 M HClO4; LSV curves of M+S+C900-900 before and after acid leaching in O2-saturated 0.1 M KOH; N2 adsorption–desorption isotherms at 77 K for different samples; LSV curves of different catalysts prepared under different first heat treatments and the same second heat treatment at 900 °C; and XPS survey spectra and deconvolutions of N1s spectra of M+S+C800-800, M-S+C900-900, M-S-C900-900, M+S-C900-900, and M+S+C1000-1000 (PDF)


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