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Noble-Metal-Free Iron Nitride/Nitrogen-Doped Graphene Composite for the Oxygen Reduction Reaction
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  • Tamás Varga
    Tamás Varga
    Department of Applied and Environmental Chemistry, University of Szeged, Rerrich Bela tér 1, H-6720 Szeged, Hungary
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  • Lívia Vásárhelyi
    Lívia Vásárhelyi
    Department of Applied and Environmental Chemistry, University of Szeged, Rerrich Bela tér 1, H-6720 Szeged, Hungary
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  • Gergő Ballai
    Gergő Ballai
    Department of Applied and Environmental Chemistry, University of Szeged, Rerrich Bela tér 1, H-6720 Szeged, Hungary
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  • Henrik Haspel
    Henrik Haspel
    Department of Applied and Environmental Chemistry, University of Szeged, Rerrich Bela tér 1, H-6720 Szeged, Hungary
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  • Albert Oszkó
    Albert Oszkó
    Department of Physical Chemistry and Materials Science, University of Szeged, Aradi Vértanúk tere 1, H-6720 Szeged, Hungary
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  • Ákos Kukovecz
    Ákos Kukovecz
    Department of Applied and Environmental Chemistry, University of Szeged, Rerrich Bela tér 1, H-6720 Szeged, Hungary
    SZTE “Lendület” Porous Nanocomposites Research Group, Rerrich Béla tér 1, H-6720 Szeged, Hungary
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    Orcidhttp://orcid.org/0000-0003-0716-9557
  • Zoltán Kónya*
    Zoltán Kónya
    Department of Applied and Environmental Chemistry, University of Szeged, Rerrich Bela tér 1, H-6720 Szeged, Hungary
    MTA-SZTE Reaction Kinetics and Surface Chemistry Research Group, Rerrich Béla tér 1, H-6720 Szeged, Hungary
    *E-mail: [email protected]
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    Orcidhttp://orcid.org/0000-0002-9406-8596
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Cite this: ACS Omega 2019, 4, 1, 130–139
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https://doi.org/10.1021/acsomega.8b02646
Published January 3, 2019

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Abstract

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Considerable effort has been devoted recently to replace platinum-based catalysts with their non-noble-metal counterparts in the oxygen reduction reaction (ORR) in fuel cells. Nitrogen-doped carbon structures emerged as possible candidates for this role, and their earth-abundant metal-decorated composites showed great promise. Here, we report on the simultaneous formation of nitrogen-doped graphene and iron nitride from the lyophilized mixture of graphene oxide and iron salt by high-temperature annealing in ammonia atmosphere. A mixture of FeN and Fe2N particles was formed with average particle size increasing from 23.4 to 127.0 nm and iron content ranging from 5 to 50 wt %. The electrocatalytic oxygen reduction activity was investigated via the rotating disk electrode method in alkaline media. The highest current density of 3.65 mA cm–2 at 1500 rpm rotation rate was achieved in the 20 wt % catalyst via the four-electrode reduction pathway, exceeding the activity of both the pristine iron nitride and the undecorated nitrogen-doped graphene. Since our catalysts showed improved methanol tolerance compared to the platinum-based ones, the formed non-noble-metal system offers a viable alternative to the platinum-decorated carbon black (Pt/CB) ORR catalysts in direct methanol fuel cells.

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Introduction

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Anthropogenic global warming is one of the greatest challenges of humankind today. Its effect could be attenuated by replacing fossil fuels with more sustainable resources. To this end, fuel cells can be of utmost importance as they can convert green fuels like hydrogen, methanol, ethanol, formic acid, etc. into electricity in a carbon neutral manner. (1) The fuel is oxidized on the anode side separated by a thin polymeric membrane from the cathode side, where the oxygen reduction reaction (ORR) takes place. ORR also plays a crucial role at metal–air battery cathodes.
Porous carbon-supported platinum and platinum-based alloys were the traditional catalysts of choice for ORR in fuel cells. There are some practical issues with using platinum-based catalysts in fuel cells, though. Since oxygen reduction kinetics is sluggish, high amounts of costly precious-metal catalysts are needed. A further issue in direct methanol fuel cells (DMFCs) is the so-called methanol crossover through the ion-exchange membrane. Platinum catalyzes both ORR and methanol oxidation (MOR), and the CO intermediate of the latter reaction poisons the catalyst surface. This reduces the durability and performance of the catalyst and therefore the whole cell. (2)
Finding efficient ORR catalysts is challenging. Several non-noble-metal catalyst systems have been proposed as platinum alternatives, like transition-metal carbides, (3) oxides, (4) chalcogenides, (5) oxynitrides, (2) or nitrides. (6) In group 4–6 transition-metal nitrides, the high ORR activity is caused by D-band contraction: the higher electron density near the Fermi level makes electron donation to the adsorbed oxygen easier. (7) According to previous reports, the most active transition-metal nitrides are cobalt and iron nitrides. (8−10) The fact that these catalysts usually work better in alkaline than in acidic media results in a higher interest from the alkaline membrane fuel cell field. (11)
A further aspect to be considered regarding sustainability in green energy research is geopolitical availability. As it is not directly connected to materials science, it is often overlooked by scientists in this field, justifying the usage of cobalt instead of precious metals in their studies. It has to be emphasized that more than half of the world’s cobalt mine production in 2017 (∼58.2%) came from one single country, while the next largest producers supplied at an order of magnitude lower annual rates (∼4–5%). Both the unstable political situation of the largest producer and the highly unbalanced nature of the cobalt market can threaten the global supply chain. Along with the rising demand for Li-ion battery cathodes, cobalt prices increased by almost a factor of 3 between November 2015 and January 2018 on the London Metal Exchange (LME) “owing to strong demand from consumers, limited availability of cobalt on the spot market, and an increase in metal purchases by investors” as concluded by the U.S. Geological Survey, Mineral Commodity Summaries, January 2018. (12) Moreover, “growth in world refined cobalt supply was forecast to increase at a lower rate than that of world cobalt consumption, which was driven mainly by strong growth in the rechargeable battery and aerospace industries. As a result, the global cobalt supply was expected to remain limited in the near term”. (12) Therefore, iron seems to be a safer choice for producing non-noble-metal catalysts for the industry in the long run.
Several iron nitride synthesis methods have been reported in the literature so far. They are often based on the pyrolysis of transition-metal salts with nitrogen-containing molecules or polymeric organic precursors. Zhang et al. made porous polyaniline-derived FeNxC/C catalysts by using iron chloride as the precursor of iron nitride and carbide. (13) Wang et al. mixed an iron oxide/graphene aerogel composite with urea and thermally treated it in argon atmosphere to obtain an iron nitride/nitrogen-doped graphene (NG) composite. (14) Chen et al. treated the mixture of iron chloride, glucose, and melamine first in nitrogen and then in ammonia/nitrogen atmosphere to get an iron nitride/nitrogen-doped carbon structure. (15) Park et al. tested several precursors to synthesize iron nitride by ammonia treatment and found that the as-prepared nitride from iron(III) acetylacetonate (Fe(acac)3) showed the best oxygen reduction reaction activity in contrast to that obtained from ferrocene (C10H10Fe), iron chloride (FeCl3), iron sulfate (FeSO4), and iron(II) acetylacetonate (Fe(acac)2). (16)
Another type of non-precious metal ORR catalyst is the so-called metal-free catalysts. Typical representatives are carbon nitride, (17,18) boron nitride, (19,20) or heteroatom-doped (nitrogen or sulfur) carbon structures, like carbon nanotubes or graphene. (21−26) Nitrogen-doped graphene (NG) is one of the most widely investigated heteroatom-doped carbon structures due to its advantageous physical–chemical properties and high ORR activity in both acidic and alkaline media. (25,27,28) Density functional theory calculations by Zhang et al. revealed the role of carbon atoms with incorporated nitrogen atom neighbors in facilitating oxygen adsorption and hence promoting the ORR reaction. Pyridinic and pyrrolic nitrogen moieties were identified as key elements for the latter effects. (29,30)
Nitrogen-doped graphene composites can be synthesized using various approaches. Ma et al. and Tao et al. synthesized NG composites hydrothermally from either ammonia or urea mixed with graphene oxide, (31,32) whereas Zhu et al. co-annealed graphene oxide with polypyrrole. (33) Nitrogen-doped graphene can also be produced by chemical vapor deposition using methane as the carbon source and ammonia as the nitrogen source in the reaction. The resulting materials showed good ORR activity, accompanied with high stability and methanol tolerance. (25) Zheng et al. used a hydrothermal approach to treat the mixture of urea and graphene oxide, and the as-prepared NG exhibited a higher ORR activity than nondoped graphene. Even so, the activity of the widely used Pt/carbon black (CB) catalysts is difficult to surpass using non-noble-metal catalysts. (34) A further possible preparation pathway toward nitrogen-doped graphene is the reaction of ammonia with pristine graphene oxide at high temperatures. (35)
In this report, we present a simple, one-step synthesis method to prepare nitrogen-doped graphene decorated by iron nitride particles. To this end, the mixture of graphene oxide and iron(III) acetylacetonate with different ratios was annealed under ammonia atmosphere, resulting in the simultaneous formation of iron nitride and nitrogen-doped graphene.
Structural and morphological differences in the composites were investigated by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and transmission electron microscopy (TEM). The electrocatalytic activity of the as-prepared composites was studied using the rotating disk electrode (RDE) method in alkaline medium, and linear sweep voltammetry (LSV) was utilized to obtain electron-transfer numbers for the reaction. The methanol tolerance of the composite with the highest ORR activity was tested along with a noble-metal Pt/CB catalyst.

Results and Discussion

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Morphology

Transmission electron microscopy was used to characterize the morphology of the as-prepared composites. Figure 1a–d shows that well-dispersed particles were grown onto the surface of the two-dimensional graphene surface after thermal treatment. The diameter of these particles increased with increasing iron content. According to the inset graphs in Figure 1a–d, average particle sizes of 23.4 ± 9.2, 78.2 ± 33.8, 105.1 ± 56.4, and 127.0 ± 41.8 nm were found for 5, 10, 20, and 50 wt % iron contents, respectively. In the absence of graphene, 309.6 ± 109.1 nm iron nitride particles were formed (Figure 1e).

Figure 1

Figure 1. TEM images of the FeNx/NG samples with various weight percents of iron (a–d), and the pristine iron nitride particles synthesized without the presence of graphene oxide (e).

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Larger particles in Figure 1d were formed by the merging of primary grains. This is also discernible in Figure 1e, but with increased particle size and wider size distribution. This happens because the precursor salt can disperse well over the graphene oxide surface, which in turn yields more dispersed iron nitride nanoparticles during the salt decomposition under ammonia atmosphere.

Structure

The crystal structure of the composites, stand-alone iron nitride, and nitrogen-doped graphene was investigated with X-ray diffractometry. As Figure 2a shows, a mixture of FeN and Fe2N was formed during the ammonia treatment. A reflection at 35.7° can be assigned to the (111) plane of FeN, (36) while the additional reflections at 43.1, 57.1, and 63.1° correspond to the (011), (012), and (003) planes of Fe2N (JCPDS 01-072-2126). (37) A similar result was reported in a previous study by Yin et al. (38) The intensities of these reflections are lower in the case of low iron nitride content. The reflection at 26.6° can be assigned to the layered structure of nitrogen-doped graphene. (39)

Figure 2

Figure 2. (a) X-ray diffractograms and (b) Raman spectra of the FeNx/NG samples with varying iron contents.

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Raman spectra in Figure 2b show Stokes G- and D-bands at 1586 and 1347 cm–1, respectively. Both are characteristic features of carbonaceous materials. The D-band correlates with the degree of structural defects in the carbon structure, while the G-band is associated with the Raman active vibrations of sp2 carbon atoms, i.e., with the graphitic domains of the graphene structure. (40) The intensity ratio of the D- and G-bands was found to be 1.02 in the case of NG. Somewhat higher, but constant values of 1.05, 1.06, 1.05, and 1.06 occurred in the case of composites with 5, 10, 20, and 50 wt % iron contents, respectively. These values are in good agreement with the literature data on nitrogen-doped graphene composites. (31) The observed ratios imply that the amount of defects and/or the incorporated nitrogen atoms in the graphene structure could be higher in the composites than in the pristine nitrogen-doped graphene. This result agrees well with the literature data. Choi et al. showed that the presence of iron could promote the incorporation of nitrogen into the carbon structure. (41) Further peaks were observed at 217.9, 280.8, and 391.2 cm–1. These peaks correspond to the iron nitride phase. (42)
The initial condition of the catalysts was characterized by ex situ X-ray photoelectron spectroscopy. Some of the measured spectra are depicted in Figure 3. The high-resolution Fe 2p spectrum of the 20% composite in Figure 3a shows peaks at 711.1 and 724.6 eV. These correspond to the 2p1/2 and 2p3/2 photoelectron lines of Fe(III), while their corresponding satellite peaks can also be observed at 719.7 and 733.2 eV. (37) These Fe(III) species can interact with nitrogen or oxygen to form FeNx or iron oxides. (43) The same was obtained in other iron nitride composites, as it can be seen in Figure S1a,c,e,g. Although it refers to an iron oxide phase, it was not detected in the XRD patterns (Figure 2a), as most likely, only a thin layer was formed due to surface oxidation. Similar findings were reported in other systems too. (37,44) The proof of nitride formation can be seen in Figure 3b, though. Since the N 1s peak of iron nitride was recorded in the absence of graphene, it was deconvoluted into three features at binding energies of 398.6, 400.6, and 404.1 eV. The first peak corresponds to the bonding between the nitrogen and iron atoms, while the second and third peaks indicate the presence of adsorbed ammonia and nitrogen oxide species, respectively. (44) The iron/nitrogen ratio in iron nitride was calculated to be 1.7:1. This agrees well with the XRD analysis above and confirms the formation of the FeN–Fe2N mixture.

Figure 3

Figure 3. Fe 2p (a) spectrum of the 20 wt % iron-containing FeNx/NG sample. N 1s (b) spectrum of the pure iron nitride particles synthesized without the presence of graphene oxide. O 1s (c) and (d) N 1s spectra of the FeNx/NG composite with 20 wt % iron content.

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O 1s spectra show a hydroxyl feature at 532.3 and the sign of carboxyl groups at 534.8 eV binding energies in Figure 3c. The latter peaks could appear in the spectra because of water and carbon dioxide molecules adsorbed on the surface, and/or they could also be due to various functional groups of graphene oxide that remained intact during the ammonia treatment. (45,46) A further peak at 530.5 eV corresponds to the iron–oxygen interaction (37) and thus serves as an additional proof for oxide layer formation on the particle surface. Azuma et al. investigated the surface of a sputtered transition-metal nitride and found that there was a thin surface layer containing oxygenated groups. When the surface was etched with Ar+ ions, the O 1s peak was weakened. (47) Similar differences in the O 1s spectra for the nitrogen-doped graphene, iron nitride-decorated composites, and iron nitride particles can be seen in Figure S2.
Various nitrogen moieties can be formed by the incorporation of nitrogen into the carbon structure during the ammonia treatment of graphene oxide. Figure 3d shows the deconvoluted N 1s spectrum of iron nitride/nitrogen-doped graphene composite with 20 wt % iron content. Features at 398.7 and 400.7 eV can be assigned to the pyrrolic- and pyridinic-type nitrogen moieties, respectively. Several theoretical and experimental studies revealed that the incorporation of pyrrolic- and pyridinic-type nitrogen species improves the ORR activity of carbon structures. (25,29,30,48,49) According to previous reports and the small difference between the binding energies of pyridinic-type and iron-bonded nitrogen, we concluded that the peak at 400.7 eV should also include the contribution of nitrogen, which was bonded to iron. (50) The additional peaks at higher binding energy evidence the presence of oxidized pyridinic-type nitrogen moieties. (51) Similar results were obtained from the N 1s spectra of nitrogen-doped graphene and composites with different iron contents in Figure S1b,d,f,h. The atomic ratio of nitrogen and carbon in the graphene structure was calculated from the XPS images and is depicted in Figure S3. N/C increased from 0.098 to 0.122 as the iron content of the composite increased from 5 to 20%. However, the N/C ratio dropped upon any further increase in the iron content. It was also documented in the literature in, e.g., iron oxide/carbon nanotube (41) or iron oxide/nitrogen-doped graphene composites. (52)

Electrocatalytic Activity

Alkaline electrocatalytic oxygen reduction activity was tested in 0.1 M KOH using cyclic voltammetry (CV) (Figures S4 and S7a) and linear sweep voltammetry (LSV). Figure 4a shows that the current density was increasing with increasing rotation rate in the 20 wt % iron-containing sample, and Figure S5 demonstrates the same trend for the other working electrodes. Linear sweep voltammograms at 1500 rpm rotation rate are compared in Figure 4b. The onset potentials of the 5, 10, and 20 wt % iron content samples were similar and around 0.90 V (vs reversible hydrogen electrode (RHE))—slightly more positive than that of the pristine nitrogen-doped graphene (0.87 V (vs RHE)). Further increase in the iron content, however, decreased the onset potential: 0.84 and 0.69 V (vs RHE) were found for the 50 wt% iron containing composite and for pristine, graphene oxide-free iron nitride, respectively. The obtained reduction current densities at 1500 rpm rotation rate (recorded at 0 V (vs RHE)) are summarized in Figure 4c. The composites with 5, 10, and 20 wt % iron contents have higher current densities than the stand-alone nitrogen-doped graphene (2.70 mA cm–2) and iron nitride (2.21 mA cm–2). The highest reduction current density (3.65 mA cm–2) was reached at 10 and 20 wt % iron contents. However, even higher current densities were achieved in the 20 wt % iron-containing composite at more positive potentials between 0.1 and 0.8 V (vs RHE). The performance of the 5 wt % iron-containing composite (3.32 mA cm–2) falls between that of the iron-free and the best performing catalysts. A similar trend manifests in the onset potentials: the 50 wt % iron composite featured a lower current density than the other composites with lower iron contents. These results imply that the ORR activity of iron nitride can be improved by decreasing its particle size. Nitrogen-doped graphene as a support material can prevent particle aggregation besides exhibiting an inherent ORR activity as well. These findings are consistent with the X-ray photoelectron spectroscopy results. The N/C atomic ratio of nitrogen-doped graphene was higher at 20 wt % iron content, which in turn implied that there may be more incorporated nitrogen atoms in the graphene structure. This resulted in a higher number of catalytically active sites in the graphene surface.

Figure 4

Figure 4. (a) LSV curves of the 20% iron-containing FeNx/NG using rotating disk electrode at various rotation rates. (b) Voltammograms recorded at fixed 1500 rpm for the FeNx/NG and Pt/CB composites on glassy carbon electrode (GCE). (c) Reduction current densities (at 0 V (vs RHE) and 1500 rpm) and (d) electron-transfer numbers were determined from LSV measurements. Voltammograms were recorded in alkaline conditions (0.1 M KOH) applying a scan rate of 10 mV s–1.

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However, the reduction current density of our most promising composite (recorded at 0 V (vs RHE) at 1500 rpm rotation rate) was still lower than that of the most commonly used Pt/CB catalyst (4.45 mA cm–2) (seen in Figure 4c). Furthermore, the onset potential was slightly more positive in the case of Pt/CB-modified GCE (0.96 V (vs RHE)), but considering the price difference between Pt/CB and FeNx/NG, our composites certainly represent a competitive value proposition.
Further information about the ORR mechanism was obtained via the Koutecký–Levich analysis of the linear sweep voltammograms recorded at different rotation rates (Figures 4a and S5 and S7b). Electron-transfer numbers were determined by using the Koutecký–Levich equation
(1)
where j is the apparent current density, jk and jd are the kinetic and diffusion-limited current densities, respectively, n is the number of transferred electron of one oxygen molecule, F is the Faraday constant, k is the rate constant, the cO2b and DO2 values are 1.2 × 10–6 mol cm–3 and 1.9 × 10–5 cm2 s–1, the oxygen concentration and diffusion coefficient in 0.1 M KOH at 23 °C, respectively, (53) ν is the solution’s kinetic viscosity (8.9 × 10–3 cm2 s–1), (54) and ω denotes the electrode’s angular velocity in rad s–1. (55)
The Koutecký–Levich plots of the sample series were determined from LSV measurements on glassy carbon RDE electrodes and are depicted in Figure 5 at different potentials. The fitted linear lines of the plots refer to the first-order kinetics of the reaction (Koutecký–Levich plots of the other samples are depicted in Figures S6 and S7c), (56) while off-zero intercepts indicate a mixed kinetic diffusion control. (57)Figure 4d summarizes the corresponding electron-transfer numbers. The pristine iron nitride and the nitrogen-doped graphene yielded 3.2, while higher values were obtained in the case of composites. The onset potential for FeNx in the linear voltammograms can be seen at more negative potentials; therefore, measurements were done only in the lower potential regime. By changing the potential from 4.5 to 1.5 V (vs RHE), the electron-transfer number increased from 3.3 to 3.8 and from 3.5 to 4.0 for 5 and 10 wt % iron contents, respectively. This indicates that increasing the overpotential favors the four-electron process over the two-electron pathway. The four-electron pathway was observed at higher overpotentials in the 10 wt % sample and in almost the whole investigated potential range in the 20 wt % composite. This result is comparable with the behavior of our as-prepared Pt/CB composite and other commercially used platinum-based catalysts. (58,59)

Figure 5

Figure 5. Koutecký–Levich plots determined from linear sweep voltammetry. Composites with (a) 5%, (b) 10%, (c) 20%, and (d) 50% iron contents were deposited onto a glassy carbon electrode.

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To investigate the role of iron nitride in the composites, the iron nitride was removed from the surface of the nitrogen-doped graphene by etching the 20 wt % iron-containing sample in a 4 M HCl solution. Figure S8a shows the TEM image of the composite after etching (curve “20% Fe_e”). The successful removal of the iron nitride particles was confirmed by XRD, as only the reflections corresponding to nitrogen-doped graphene can be seen in the diffractogram in Figure S8b. The ORR activity of the etched sample was compared to that of the 20 wt % composite and the pristine nitrogen-doped graphene in Figure S9. LSV curves of the modified GCE electrodes recorded at 1500 rpm rotation rate (Figure S9a) show that although the current density decreased slightly due to the etching process, it was still higher than in the case of nitrogen-doped graphene prepared in the absence of iron nitride. Furthermore, the onset potential showed a negative shift from 0.9 to 0.85 V (vs RHE) after removing the iron nitride particles. A similar trend was observed for the electron-transfer number insofar as it decreased after the removal of the metal nitride phase (3.6). However, the latter value is still higher than in the case of nitrogen-doped graphene prepared in the absence of the metal precursor (3.2) (Figure S9b). The deteriorating performance, i.e., the drop in current density and electron-transfer number and the shift in the onset potential, clearly exposes the importance of iron nitride in the ORR activity of the composite. A stronger Fe–N–C interaction between the metal nitride and the support was formed during the synthesis, and this synergistic effect further improved the electrocatalytic ORR activity. (60)
There is an ongoing debate on the location and chemical nature of the active sites in M–N–C ORR catalysts, and thus, several theories allow in recent literature. It was suggested that the iron species only participate in the active C/N site formation during catalyst preparation, but they do not work as ORR active sites. (9) According to other studies, including the present work, the activity of the nonprecious metal-containing nitrogen-doped carbon catalysts decreased after the removal of the metallic part in an acidic treatment. This clearly underlines the importance of the metal nitride in the electrocatalytic reduction of oxygen. (9,61) The higher ORR activity of the etched sample compared to that of the pristine nitrogen-doped graphene can be explained with the increased amount of nitrogen in the graphene structure. Lai et al. showed that the increased pyridinic nitrogen content in the carbon structure converts the mechanism from the two-electron process to the four-electron pathway. (62)
A possible oxygen reduction reaction mechanism with a four-electron process is summarized in Figure 6. After the adsorption of an oxygen molecule onto the surface of the nitride nanoparticle, the oxygen–oxygen bond is weakened due to the strong interaction between the transition metal and oxygen. The first electron-transfer step results in the formation of an adsorbed O2– species. In the next step, peroxide is formed on the surface in the reaction of adsorbed O2– with a water molecule. In the third reaction step, adsorbed OH is formed, which then desorbs as hydroxide ion in the final step, closing the reaction cycle. It is worth mentioning that this mechanism can take place on the nitrogen-doped graphene surface as well. Since the incorporation of nitrogen atoms can substantially modify the charge and spin distribution at the neighboring carbon atoms, the latter become more prone to the interaction with oxygen molecules. This in turn results in higher ORR activity, as it is well known from the literature. (63,64)

Figure 6

Figure 6. Schematic of the possible ORR mechanism in FeNx/NG composite via the four-electron pathway.

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Another reaction pathway could be the so-called outer-sphere mechanism, which is also possible in alkaline media. In this case, the solvated oxygen molecules are in the outer Helmholtz plane, and a noncovalent hydrogen bond is formed between the oxygen and the adsorbed OH, which acts as an outer-sphere bridge in this case. However, this pathway promotes the two-electron pathway. (65)
A further important technological aspect of ORR catalysts is their resistance against impurities. In direct methanol fuel cells, methanol is used as fuel, which diffuses through the polymeric membrane separating the anode and cathode compartments. This so-called methanol crossover reduces the ORR activity and therefore reduces the efficiency of the whole fuel cell. The methanol resistance of our most active 20 wt % iron-containing composite was tested in a chronoamperometric test. In Figure 7, the relative current (I/I0), i.e., the ratio of the current measured at a given time and the initial current of the composite, is compared to that of commercial Pt/CB catalyst-modified GCE electrode.

Figure 7

Figure 7. Alkaline (0.1 M KOH) chronoamperometric methanol tolerance ORR response at 0.51 V (vs RHE) of commercial Pt/CB catalyst and the 20 wt % Fe-containing composite deposited onto a glassy carbon electrode.

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After the addition of methanol to the electrolyte solution at 360 s, a sharp drop in the relative current was observed in the case of the Pt/CB-modified electrode. The electrocatalytic activity decreased steeply in the presence of methanol due to the activity of Pt/CB toward the methanol oxidation reaction (MOR). (66) On the other hand, there was no significant change in the relative current of our non-noble-metal composite, thus evidencing considerably higher methanol tolerance compared to the widely used precious-metal Pt/CB catalyst.

Conclusions

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A single-step synthesis was developed to obtain iron nitride nanoparticle-decorated nitrogen-doped graphene catalysts for the electrochemical oxygen reduction reaction (ORR). The composites were synthesized via the high-temperature annealing of a lyophilized mixture of graphene oxide and iron(III) acetylacetonate under ammonia atmosphere. A mixture of FeN and Fe2N was formed during the thermal treatment, with an increasing particle size from 23.4 to 127.0 nm by increasing the iron content in the 5–50 wt % range. Increasing amounts of pyridinic- and pyrrolic-type nitrogen moieties were successfully incorporated into the graphene structure along with the increase in the iron content. The electrocatalytic activity of the catalyst was tested in a three-electrode system in alkaline media using the rotating disk electrode technique at different rotation rates. The highest current density (3.65 mA cm–2 at 1500 rpm) was reached in the 20 wt % iron-containing composite, outperforming both the pristine iron nitride and the nitrogen-doped graphene. The composite catalyzes the oxygen reduction via the four-electron pathway, as evidenced from the determination of the electron-transfer number. Furthermore, the catalyst exhibited high methanol tolerance compared to a commercial platinum-based system. The latter benefit is particularly relevant for handling cathode-side methanol oxidation, one of the major drawbacks of precious-metal ORR fuel cell catalysts in direct methanol fuel cells (DMFCs). Additional advantages of the suggested system include refraining from both the high price of platina group metals and the limited availability of cobalt, a widely studied metal in non-precious-metal ORR catalyst.

Experimental Section

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Synthesis of Graphene Oxide

The modified Hummers method was used in the graphene oxide synthesis. (67) Typically, 4.5 g of graphite powder and sodium nitrate (NaNO3) were mixed with 210 mL of c.c. H2SO4 under vigorous stirring for 30 min. Then, 27 g of KMnO4 was added to the solution with continuous cooling in an ice bath. After 24 h stirring, 500 mL of H2O and 10 mL of 50% H2O2 were poured into the solution and stirred for another 2 h. The product was washed with deionized (DI) water to obtain sulfate-free supernatant and then dried at 80 °C overnight.

Synthesis of Iron Nitride/Nitrogen-Doped Graphene Oxide

In a typical synthesis, graphene oxide and iron(III) acetylacetonate were mixed in DI water. To obtain composites with 5, 10, 20, and 50 wt % iron loadings, 33.3, 70.2, 158.2, and 632.1 mg of iron salts were weighed in, respectively. The mixture was stirred overnight, followed by lyophilization with a Labconco freeze dryer at 0.014 mbar and −55 °C. Ammonia (120 sccm) heat treatment at 600 °C was then applied for 1 h, purging the furnace with ammonia to prevent oxygen contamination in the product. Bare iron nitride and nitrogen-doped graphene were also prepared for comparison. Pristine iron nitride was prepared via annealing of iron(III) acetylacetonate powder, while nitrogen-doped graphene was prepared according to the previous recipe of the composites, but without adding the iron precursor. Untreated graphene oxide was subjected to the same lyophilization and annealing steps. To investigate the role of iron nitride in the catalytic activity, the as-prepared composite with 20% iron content was annealed in a round-bottom flask under continuous stirring in a 4 M HCl solution for 48 h to remove the iron nitride from the graphene surface.

Characterization Methods

Sample morphology was studied by an FEI Tecnai G2 20 X Twin transmission electron microscope (TEM) at a 200 kV accelerating voltage. Samples were drop-cast onto a copper mounted holey carbon film from isopropyl alcohol suspension. The diameter of 50–50 iron nitride particles was measured in 10 representative TEM images to determine the corresponding particle size distribution. The powder X-ray diffractograms were recorded by a Rigaku Miniflex II diffractometer using Cu Kα radiation at a scan speed of 4° min–1. Raman spectra were measured using 532 nm laser excitation at 5 mW power (Thermo Scientific DXR Raman Microscope), averaging 10 scans from 200 to 3500 cm–1 Raman shift at 1 cm–1 resolution. X-ray photoelectron spectroscopy (XPS) was done using the Al anode’s Kα radiation (SPECS instrument with a PHOIBOS 150 MCD 9 hemispherical analyzer), where the analyzer was used in the fixed analyzer transmission mode with a 20 eV pass energy and the X-ray gun at 210 W (14 kV, 15 mA).

Electrode Preparation

For all electrochemical measurements, a 3 mm glassy carbon electrode (purchased from BASi) was used. The carbon surface was thoroughly cleaned before any subsequent surface modification. After carefully polishing it with a 0.05 μm diameter alumina slurry, it was rinsed and sonicated in ethanol and ion-exchanged water, followed by drying in air at room temperature. For the RDE samples, a homogeneous suspension of 1.0 mg of sample in a mixture of 250 μL of ethanol/water (1:1 volume ratio) and 10 μL of 5 wt % Nafion solution was prepared via ultrasonication. A 15.0 μL aliquot from the resulted suspension was then drop-cast onto the pretreated glassy carbon electrode and dried at room temperature. The same method was used to prepare the 10 wt % Pt-containing Pt/CB-modified CGE electrode, which served as an ORR benchmark catalyst.

Electrochemical Measurements

The ORR activity of the composites was investigated in a three-electrode configuration, where the surface-modified glassy carbon electrode and a platinum wire served as the working and counter electrodes, respectively. The working electrode potential was determined against a Ag/AgCl (3 M NaCl, BASi) reference electrode. The potential was varied during cyclic and linear sweep voltammetry (CV and LSV) measurements (ACM Instruments GILL AC electrochemical workstation) from 0 to −1000 mV against Ag/AgCl (3 M NaCl) at a scan rate of 10 mV s–1. During the rotating disk electrode (RDE) LSV measurements, the rotation rate was varied between 500 and 3000 rpm. Background correction was done to all voltammograms, and the resulted corrected data were used for further calculations. All measurements were carried out in nitrogen- and oxygen-saturated 0.1 M KOH solutions at room temperature (23 °C).
Potentials against the reversible hydrogen electrode (RHE) were used throughout the study and calculated according to the well-known equation
(2)
where E is the measured potential against the reference electrode and EAg/AgCl (vs SHE) is its standard electrode potential (0.209 V) at 23 °C. SHE denotes the standard hydrogen electrode.
Methanol tolerance was investigated in a 0.1 M KOH solution with continuous oxygen bubbling via chronoamperometry at 0.51 V (vs RHE). After continuously recording the current for 6 min, 1 mL of 3 M methanol solution was quickly added to the system and the corresponding change was recorded.

Supporting Information

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.8b02646.

  • XPS, CV, and LSV results of the composites, and TEM, XRD, and LSV data for the etched FeNx/NG composites (PDF)

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Author Information

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  • Corresponding Author
    • Zoltán Kónya - Department of Applied and Environmental Chemistry, University of Szeged, Rerrich Bela tér 1, H-6720 Szeged, Hungary;  MTA-SZTE Reaction Kinetics and Surface Chemistry Research Group, Rerrich Béla tér 1, H-6720 Szeged, Hungary;  Orcidhttp://orcid.org/0000-0002-9406-8596;  Email: [email protected]
  • Authors
    • Tamás Varga - Department of Applied and Environmental Chemistry, University of Szeged, Rerrich Bela tér 1, H-6720 Szeged, Hungary
    • Lívia Vásárhelyi - Department of Applied and Environmental Chemistry, University of Szeged, Rerrich Bela tér 1, H-6720 Szeged, Hungary
    • Gergő Ballai - Department of Applied and Environmental Chemistry, University of Szeged, Rerrich Bela tér 1, H-6720 Szeged, Hungary
    • Henrik Haspel - Department of Applied and Environmental Chemistry, University of Szeged, Rerrich Bela tér 1, H-6720 Szeged, Hungary;  Present Address: Division of Physical Sciences and Engineering, KAUST Catalysis Center (KCC), King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900 Saudi Arabia (H.H.)
    • Albert Oszkó - Department of Physical Chemistry and Materials Science, University of Szeged, Aradi Vértanúk tere 1, H-6720 Szeged, Hungary
    • Ákos Kukovecz - Department of Applied and Environmental Chemistry, University of Szeged, Rerrich Bela tér 1, H-6720 Szeged, Hungary;  SZTE “Lendület” Porous Nanocomposites Research Group, Rerrich Béla tér 1, H-6720 Szeged, Hungary;  Orcidhttp://orcid.org/0000-0003-0716-9557
  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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Financial support from the Hungarian National Research Development and Innovation Office through grants GINOP-2.3.2-15-2016-00013, NKFIH OTKA K120115 (Z.K.), and K126065 (Á.K.) is gratefully acknowledged.

References

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    Zheng, Yao; Jiao, Yan; Jaroniec, Mietek; Jin, Yonggang; Qiao, Shi Zhang
    Small (2012), 8 (23), 3550-3566CODEN: SMALBC; ISSN:1613-6810. (Wiley-VCH Verlag GmbH & Co. KGaA)
    A review. Replacing precious and nondurable Pt catalysts with cheap and com. available materials to facilitate sluggish cathodic oxygen redn. reaction (ORR) is a key issue in the development of fuel cell technol. The recently developed cost effective and highly stable metal-free catalysts reveal comparable catalytic activity and significantly better fuel tolerance than that of current Pt-based catalysts; therefore, they can serve as feasible Pt alternatives for the next generation of ORR electrocatalysts. Their promising electrocatalytic properties and acceptable costs greatly promote the R and D of fuel cell technol. This review provides an overview of recent advances in state-of-the-art nanostructured metal-free electrocatalysts including nitrogen-doped carbons, graphitic-carbon nitride (g-C3N4)-based hybrids, and 2-dimensional graphene-based materials. A special emphasis is placed on the mol. design of these electrocatalysts, origin of their electrochem. reactivity, and ORR pathways. Finally, some perspectives are highlighted on the development of more efficient ORR electrocatalysts featuring high stability, low cost, and enhanced performance, which are the key factors to accelerate the commercialization of fuel cell technol.
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    Chen, X.; Chang, J.; Yan, H.; Xia, D. Boron Nitride Nanocages as High Activity Electrocatalysts for Oxygen Reduction Reaction: Synergistic Catalysis by Dual Active Sites. J. Phys. Chem. C 2016, 120, 28912– 28916,  DOI: 10.1021/acs.jpcc.6b08560
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    Boron Nitride Nanocages as High Activity Electrocatalysts for Oxygen Reduction Reaction: Synergistic Catalysis by Dual Active Sites
    Chen, Xin; Chang, Junbo; Yan, Huijun; Xia, Dingguo
    Journal of Physical Chemistry C (2016), 120 (51), 28912-28916CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    The O redn. reaction (ORR) catalytic mechanism and activity on B12N12 and B60N60 nanocages were studied in detail by d. functional theory methods. The calcd. results indicate that all the adsorption energies of ORR intermediates on B12N12 are close to those known for the Pt(111) catalyst, implying that it can be an effective catalyst for the ORR, with catalytic properties similar to Pt. A relative energy profile suggests that the ORR process could spontaneously take place on the studied two BN nanocages, with a four-electron redn. mechanism. More importantly, during the entire redn. process, the BN nanocages can provide dual-catalytic sites, esp. in the 2nd and 3rd H transfer step, further accelerating the ORR pathways. Thus, the synergistic catalytic effect between B and N atoms is considerable in BN nanocages.
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    Uosaki, K.; Elumalai, G.; Noguchi, H.; Masuda, T.; Lyalin, A.; Nakayama, A.; Taketsugu, T. Boron nitride nanosheet on gold as an electrocatalyst for oxygen reduction reaction: theoretical suggestion and experimental proof. J. Am. Chem. Soc. 2014, 136, 6542– 6525,  DOI: 10.1021/ja500393g
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    Boron Nitride Nanosheet on Gold as an Electrocatalyst for Oxygen Reduction Reaction: Theoretical Suggestion and Experimental Proof
    Uosaki, Kohei; Elumalai, Ganesan; Noguchi, Hidenori; Masuda, Takuya; Lyalin, Andrey; Nakayama, Akira; Taketsugu, Tetsuya
    Journal of the American Chemical Society (2014), 136 (18), 6542-6545CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Boron nitride (BN), which is an insulator with a wide band gap, supported on Au is theor. suggested and exptl. proved to act as an electrocatalyst for oxygen redn. reaction (ORR). D.-functional theory calcns. show that the band gap of a free h-BN monolayer is 4.6 eV but a slight protrusion of the unoccupied BN states toward the Fermi level is obsd. if BN is supported on Au(111) due to the BN-Au interaction. A theor. predicted metastable configuration of O2 on h-BN/Au(111), which can serve as precursors for ORR, and free energy diagrams for ORR on h-BN/Au(111) via two- and four-electron pathways show that ORR to H2O2 is possible at this electrode. It is exptl. proved that overpotential for ORR at the gold electrode is significantly reduced by depositing BN nanosheets. No such effect is obsd. at the glassy carbon electrode, demonstrating the importance of BN-substrate interaction for h-BN to act as the ORR electrocatalyst. A possible role of the edge of the BN islands for ORR is also discussed.
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    Alexeyeva, N.; Shulga, E.; Kisand, V.; Kink, I.; Tammeveski, K. Electroreduction of oxygen on nitrogen-doped carbon nanotube modified glassy carbon electrodes in acid and alkaline solutions. J. Electroanal. Chem. 2010, 648, 169– 175,  DOI: 10.1016/j.jelechem.2010.07.014
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    Electroreduction of oxygen on nitrogen-doped carbon nanotube modified glassy carbon electrodes in acid and alkaline solutions
    Alexeyeva, N.; Shulga, E.; Kisand, V.; Kink, I.; Tammeveski, K.
    Journal of Electroanalytical Chemistry (2010), 648 (2), 169-175CODEN: JECHES; ISSN:1572-6657. (Elsevier B.V.)
    The electrochem. redn. of oxygen was studied on vertically aligned N-doped carbon nanotube (NCNT) modified glassy carbon (GC) electrodes in 0.5 M H2SO4 and in 0.1 M KOH solns. using the rotating disk electrode (RDE) method. For comparison purposes, the oxygen redn. behavior of undoped carbon nanotube material has been also investigated. Both catalysts were prepd. by chem. vapor deposition (CVD). Acetonitrile was used as the precursor in the synthesis of the NCNT material. The surface morphol. and chem. compn. of the NCNT catalysts were studied by SEM and XPS. The NCNT/GC electrodes showed a significant enhancement of the kinetics of oxygen redn. in both solns. In acid media the half-wave potential of O2 redn. on NCNT-modified electrodes shifted by 250 mV to more pos. potentials as compared to that of vertically aligned undoped CNT materials. The factors that det. the high electrocatalytic activity of nitrogen-doped carbon nanotubes towards oxygen redn. are discussed.
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    Chen, L.; Cui, X.; Wang, Y.; Wang, M.; Qiu, R.; Shu, Z.; Zhang, L.; Hua, Z.; Cui, F.; Wei, C.; Shi, J. One-step synthesis of sulfur doped graphene foam for oxygen reduction reactions. Dalton Trans. 2014, 43, 3420– 3423,  DOI: 10.1039/c3dt52253a
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    Jiang, S.; Ma, Y.; Jian, G.; Tao, H.; Wang, X.; Fan, Y.; Lu, Y.; Hu, Z.; Chen, Y. Facile Construction of Pt-Co/CNx Nanotube Electrocatalysts and Their Application to the Oxygen Reduction Reaction. Adv. Mater. 2009, 21, 4953– 4956,  DOI: 10.1002/adma.200900677
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    Facile Construction of Pt-Co/CNx Nanotube Electrocatalysts and Their Application to the Oxygen Reduction Reaction
    Jiang, Shujuan; Ma, Yanwen; Jian, Guoqiang; Tao, Haisheng; Wang, Xizhang; Fan, Yining; Lu, Yinong; Hu, Zheng; Chen, Yi
    Advanced Materials (Weinheim, Germany) (2009), 21 (48), 4953-4956CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Taking advantage of the inherent chem. activity of CNx nanotubes that arise from nitrogen incorporation, a facile strategy was developed for the construction of Pt-Co/CNx electrocatalysts. Pt-based alloyed nanoparticles were highly and homogeneously dispersed on CNx nanotubes with a size about 3 nm. Compared with the com. Pt/C and the monometallic Pt/CNx catalysts, the binary Pt-Co/CNx catalysts show much higher electrocatalytic activities and similar stabilities for oxygen redn. in acidic electrolyte but with much less consumption of precious Pt. The good performance of the catalysts mainly arise from the high dispersion of Pt-based species, the alloying effect of Pt-Co and the intrinsic catalytic capacity of CNx nanotubes with high cond.
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    Li, M.; Zhang, L.; Xu, Q.; Niu, J.; Xia, Z. N-doped graphene as catalysts for oxygen reduction and oxygen evolution reactions: Theoretical considerations. J. Catal. 2014, 314, 66– 72,  DOI: 10.1016/j.jcat.2014.03.011
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    N-doped graphene as catalysts for oxygen reduction and oxygen evolution reactions: Theoretical considerations
    Li, Mingtao; Zhang, Lipeng; Xu, Quan; Niu, Jianbing; Xia, Zhenhai
    Journal of Catalysis (2014), 314 (), 66-72CODEN: JCTLA5; ISSN:0021-9517. (Elsevier Inc.)
    Electrocatalysts are essential to two key electrochem. reactions, oxygen evolution reaction (OER) and oxygen redn. reaction (ORR) in renewable energy conversion and storage technologies such as regenerative fuel cells and rechargeable metal-air batteries. Here, we explored N-doped graphene as cost-effective electrocatalysts for these key reactions by employing d. functional theory (DFT). The results show that the substitution of carbon at graphene edge by nitrogen results in the best performance in terms of overpotentials. For armchair nanoribbons, the lowest OER and ORR overpotentials were estd. to be 0.405 V and 0.445 V, resp., which are comparable to those for Pt-contg. catalysts. OER and ORR with the min. overpotentials can occur near the edge on the same structure but different sites. These calcns. suggest that engineering the edge structures of the graphene can increase the efficiency of the N-doped graphene as efficient OER/ORR electrocatalysts for metal-air batteries, water splitting, and regenerative fuel cells.
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    Qu, L.; Liu, Y.; Baek, J.-B.; Dai, L. Nitrogen-Doped Graphene as Efficient Metal-Free Electrocatalyst for Oxygen Reduction in Fuel Cells. ACS Nano 2010, 4, 1321– 1326,  DOI: 10.1021/nn901850u
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    Nitrogen-Doped Graphene as Efficient Metal-Free Electrocatalyst for Oxygen Reduction in Fuel Cells
    Qu, Liangti; Liu, Yong; Baek, Jong-Beom; Dai, Liming
    ACS Nano (2010), 4 (3), 1321-1326CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    Nitrogen-doped graphene (N-graphene) was synthesized by CVD of methane in the presence of ammonia. The resultant N-graphene was demonstrated to act as a metal-free electrode with a much better electrocatalytic activity, long-term operation stability, and tolerance to crossover effect than platinum for oxygen redn. via a four-electron pathway in alk. fuel cells. This is the 1st report on the use of graphene and its derivs. as metal-free catalysts for oxygen redn. The important role of N-doping to oxygen redn. reaction (ORR) can be applied to various carbon materials for the development of other metal-free efficient ORR catalysts for fuel cell applications, even new catalytic materials for applications beyond fuel cells.
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    Yang, Z.; Yao, Z.; Li, G.; Fang, G.; Nie, H.; Liu, Z.; Zhou, X.; Chen, X.; Huang, S. Sulfur-Doped Graphene as an Efficient Metal-free Cathode Catalyst for Oxygen Reduction. ACS Nano 2012, 6, 205– 211,  DOI: 10.1021/nn203393d
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    Sulfur-Doped Graphene as an Efficient Metal-free Cathode Catalyst for Oxygen Reduction
    Yang, Zhi; Yao, Zhen; Li, Guifa; Fang, Guoyong; Nie, Huagui; Liu, Zheng; Zhou, Xuemei; Chen, Xi'an; Huang, Shaoming
    ACS Nano (2012), 6 (1), 205-211CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    Tailoring the electronic arrangement of graphene by doping is a practical strategy for producing significantly improved materials for the O-redn. reaction (ORR) in fuel cells (FCs). Recent studies proved that the C materials doped with the elements, which have the larger (N) or smaller (P, B) electroneg. atoms than C such as N-doped C nanotubes (CNTs), P-doped graphite layers and B-doped CNTs, also showed pronounced catalytic activity. Herein, the graphenes doped with the elements, which have the similar electronegativity with C such as S and Se, can also exhibit better catalytic activity than the com. Pt/C in alk. media, indicating that these doped graphenes hold great potential for a substitute for Pt-based catalysts in FCs. The exptl. results are believed to be significant because they not only give further insight into the ORR mechanism of these metal-free doped C materials, but also open a way to fabricate other new low-cost NPMCs with high electrocatalytic activity by a simple, economical, and scalable approach for real FC applications.
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    Wong, W. Y.; Daud, W. R. W.; Mohamad, A. B.; Kadhum, A. A. H.; Loh, K. S.; Majlan, E. H. Recent progress in nitrogen-doped carbon and its composites as electrocatalysts for fuel cell applications. Int. J. Hydrogen Energy 2013, 38, 9370– 9386,  DOI: 10.1016/j.ijhydene.2012.12.095
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    Recent progress in nitrogen-doped carbon and its composites as electrocatalysts for fuel cell applications
    Wong, W. Y.; Daud, W. R. W.; Mohamad, A. B.; Kadhum, A. A. H.; Loh, K. S.; Majlan, E. H.
    International Journal of Hydrogen Energy (2013), 38 (22), 9370-9386CODEN: IJHEDX; ISSN:0360-3199. (Elsevier Ltd.)
    A review. The emergence of fuel cell technol. has created a new tool for the generation of clean, high efficiency alternative energy for humans. The research and development of new catalysts to replace the expensive and rare platinum (Pt) to reduce the overall cost of fuel cells is ongoing in this area. Nitrogen-doped carbon and its composites possess great potential for fuel cell catalyst applications esp. at the oxygen redn. cathode. It is proposed that the reaction mechanisms of nitrogen-doped carbon catalysts for oxygen redn. involve adsorption of oxygen at the partially polarized carbon atoms adjacent to the nitrogen dopants, different from the mechanism at platinum catalysts, which utilize d-bands filling at oxygen adsorption sites. Nitrogen doping in both carbon nanostructures and its composites with active metals or ceramics are reviewed. Nitrogen-doped carbon without composite metals, displays high catalytic activity in alk. fuel cells and exhibits significant activity in proton exchange membrane fuel cells and direct methanol fuel cells. Pt-based catalysts with nitrogen-doped carbon supports show enhanced catalytic activity towards oxygen redn., attributed to the enhanced anchoring of Pt to the support that results in better dispersion and stability of the electrodes. For nitrogen-doped carbon composites with non-noble metals (Fe, Co, etc), enhanced activity is seen in both proton exchange and alk. fuel cells. There are many ongoing debates about the nature of nitrogen-carbon bond in catalysis. Pyrrole- and pyridinic-type nitrogen generally considered to be responsible for the catalytic sites in acidic and alk. media, resp. In recent years, significant efforts have been made towards increasing the stability of nitrogen-doped carbon catalysts in acidic media through the formation of composites with ceramic or metal oxide materials. This article reviews the progress in the area of this new class of catalysts and their composites for greater enhancement of oxygen redn. activity and stability in various fuel cell applications.
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    Wang, T.; Chen, Z.-X.; Chen, Y.-G.; Yang, L.-J.; Yang, X.-D.; Ye, J.-Y.; Xia, H.-P.; Zhou, Z.-Y.; Sun, S.-G. Identifying the Active Site of N-Doped Graphene for Oxygen Reduction by Selective Chemical Modification. ACS Energy Lett. 2018, 3, 986– 991,  DOI: 10.1021/acsenergylett.8b00258
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    Identifying the Active Site of N-Doped Graphene for Oxygen Reduction by Selective Chemical Modification
    Wang, Tao; Chen, Zhi-Xin; Chen, Yu-Gang; Yang, Li-Jun; Yang, Xiao-Dong; Ye, Jin-Yu; Xia, Hai-Ping; Zhou, Zhi-You; Sun, Shi-Gang
    ACS Energy Letters (2018), 3 (4), 986-991CODEN: AELCCP; ISSN:2380-8195. (American Chemical Society)
    N-doped carbon materials are promising electrocatalysts for oxygen redn. reaction (ORR). However, the lack of knowledge in the nature of active sites limits the rational design of this type of catalysts. Although pyridinic N species are proposed to be active for ORR, little exptl. evidence is provided to reveal the reactive sites. Herein, a surface-modification method is developed to identify the ortho-carbon atom of the pyridinic ring as the reactive site for ORR on N-doped graphene. The pyridinic ring of N-doped graphene was selectively grafted by an acetyl group at pyridinic N and ortho-C atoms by electrophilic and radical substitution, resp. The former remained most of ORR catalytic activity, while the latter lost its activity completely. DFT calcns. confirm that O2 can get adsorbed and reduced favorably on the ortho-C atom of the pyridinic ring. This study provides new insight into the nature of active sites and the ORR mechanism for N-doped carbon materials.
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    Zhang, L.; Niu, J.; Dai, L.; Xia, Z. Effect of microstructure of nitrogen-doped graphene on oxygen reduction activity in fuel cells. Langmuir 2012, 28, 7542– 7550,  DOI: 10.1021/la2043262
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    Effect of Microstructure of Nitrogen-Doped Graphene on Oxygen Reduction Activity in Fuel Cells
    Zhang, Lipeng; Niu, Jianbing; Dai, Liming; Xia, Zhenhai
    Langmuir (2012), 28 (19), 7542-7550CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
    The development of fuel cells as clean-energy technol. is limited by the cost of the noble-metal catalysts needed for catalyzing the O redn. reaction (ORR) in fuel cells. A fundamental understanding of catalyst design principle that links material structure to the catalytic activity can accelerate the search for highly active and abundant non-metal catalysts to replace Pt. Here, the authors present a 1st-principles study of ORR on N-doped graphene in an acidic environment. The ORR activity primarily correlates to charge and spin densities of the graphene. The N doping and defects introduce high pos. spin and/or charge densities that facilitate the ORR on graphene surface. The identified active sites are closely related to doping cluster size and dopant-defect interactions. Generally speaking, a large doping cluster size (no. of N atoms >2) reduces the no. of catalytic active sites per N atom. In combination with N clustering, Stone-Wales defects can strongly promote ORR. For 4-electron transfer, the effective reversible potential ranges from 1.04 to 1.15 V/SHE, depending on the defects and cluster size. The catalytic properties of graphene could be optimized by introducing small N clusters in combination with material defects.
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    Zhang, L.; Xia, Z. Mechanisms of Oxygen Reduction Reaction on Nitrogen-Doped Graphene for Fuel Cells. J. Phys. Chem. C 2011, 115, 11170– 11176,  DOI: 10.1021/jp201991j
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    Mechanisms of Oxygen Reduction Reaction on Nitrogen-Doped Graphene for Fuel Cells
    Zhang, Lipeng; Xia, Zhenhai
    Journal of Physical Chemistry C (2011), 115 (22), 11170-11176CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    Graphene and its derivs. are attractive for electrocatalytic application in fuel cells because of their unique structures and electronic properties. The electrocatalytic mechanism of nitrogen-doped graphene in acidic environment was studied by using d. functional theory. The simulations demonstrate that the oxygen redn. reaction (ORR) on nitrogen-doped graphene is a direct four-electron pathway, which is consistent with the exptl. observations. The energy calcd. for each ORR step shows that the ORR can spontaneously occur on the nitrogen-doped graphene. The active catalytic sites on single nitrogen-doped graphene are identified, which have either high pos. spin d. or high pos. at. charge d. The nitrogen doping introduces asymmetry spin d. and at. charge d., making it possible for nitrogen-doped graphene to show high electrocatalytic activities for the ORR.
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    Ma, J.; Habrioux, A.; Luo, Y.; Ramos-Sanchez, G.; Calvillo, L.; Granozzi, G.; Balbuena, P. B.; Alonso-Vante, N. Electronic interaction between platinum nanoparticles and nitrogen-doped reduced graphene oxide: effect on the oxygen reduction reaction. J. Mater. Chem. A 2015, 3, 11891– 11904,  DOI: 10.1039/C5TA01285F
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    Electronic interaction between platinum nanoparticles and nitrogen-doped reduced graphene oxide: effect on the oxygen reduction reaction
    Ma, Jiwei; Habrioux, Aurelien; Luo, Yun; Ramos-Sanchez, Guadalupe; Calvillo, Laura; Granozzi, Gaetano; Balbuena, Perla B.; Alonso-Vante, Nicolas
    Journal of Materials Chemistry A: Materials for Energy and Sustainability (2015), 3 (22), 11891-11904CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
    Low-mass loadings (∼5%) Pt/C catalysts were synthesized using the carbonyl chem. route allowing for the heterogeneous deposition of Pt nanoparticles on different C-based substrates. N-doped reduced graphene oxide, reduced graphene oxide, graphene oxide, graphite and Vulcan XC-72 were used for the heterogeneous deposition of Pt nanoparticles. The effect of the chem. nature of the C-based substrate on the O Redn. Reaction (ORR) kinetics at Pt nanoparticles surfaces was studied. XPS results show that using N-doped reduced graphene oxide materials for the deposition of Pt nanoparticles gives Pt-N chem. bonds. This interaction between Pt and N allows for an electronic transfer from Pt to the C support. Ca. 25% of the total amt. of N atoms were bound to Pt ones. This chem. bond also revealed by the DFT anal., induces changes in the O adsorption energy at the Pt surface, engendering an enhancement of the catalyst activity towards ORR. In comparison with Vulcan XC-72, the mass activity at 0.9 V vs. RHE is 2.1 fold higher when N-doped reduced graphene oxide was used as substrate. In conjunction with the exptl. results, DFT calcns. describe the interaction between supported Pt clusters and O where the support was modeled accordingly with the C-based materials used as substrate. The presence of N-species in the support although leading to a weaker O2 adsorption, induces elongated O-O distances suggesting facilitated dissocn. Addnl., the strong interaction between Pt clusters and N-contg. substrates leads to very slight changes of the cluster-substrate distance even when O is adsorbed at the interfacial region, thus leading to a lower resistance for electron charge transfer and enabling electrochem. reactions.
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    Tao, L.; Dou, S.; Ma, Z.; Shen, A.; Wang, S. Simultaneous Pt deposition and nitrogen doping of graphene as efficient and durable electrocatalysts for methanol oxidation. Int. J. Hydrogen Energy 2015, 40, 14371– 14377,  DOI: 10.1016/j.ijhydene.2015.02.104
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    Simultaneous Pt deposition and nitrogen doping of graphene as efficient and durable electrocatalysts for methanol oxidation
    Tao, Li; Dou, Shuo; Ma, Zhaoling; Shen, Anli; Wang, Shuangyin
    International Journal of Hydrogen Energy (2015), 40 (41), 14371-14377CODEN: IJHEDX; ISSN:0360-3199. (Elsevier Ltd.)
    Nitrogen doping could effectively enhance the catalytical activity of graphene-supported Pt nano-electrocatalysts for methanol oxidn. reaction. Previously, the main strategy to the synthesis of Pt/N-graphene is the two-step reaction while it involves the complicated synthesis. In this work, we describe a facile and simple one-pot reaction including the redn. of graphene oxide, nitrogen doping of graphene, and uniform deposition of Pt nanoparticles on doped graphene. Compared with the Pt/graphene catalyst without nitrogen-doping, Pt/N-graphene exhibits excellent activity and durability towards methanol oxidn. reaction, which is mainly ascribed to the contribution of the improved dispersion of Pt nanoparticles and the enhanced interaction between it and nitrogen-doped graphene. It is expected that this facile, green and economic single-step synthesis approach for the Pt/N-graphene electrocatalyst could be widely utilized to the nitrogen-doped graphene catalysts.
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    Zhu, J.; Xiao, M.; Zhao, X.; Li, K.; Liu, C.; Xing, W. Nitrogen-doped carbon-graphene composites enhance the electrocatalytic performance of the supported Pt catalysts for methanol oxidation. Chem. Commun. 2014, 50, 12201– 12203,  DOI: 10.1039/C4CC04887C
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    Zheng, B.; Wang, J.; Wang, F.-B.; Xia, X.-H. Synthesis of nitrogen doped graphene with high electrocatalytic activity toward oxygen reduction reaction. Electrochem. Commun. 2013, 28, 24– 26,  DOI: 10.1016/j.elecom.2012.11.037
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    Synthesis of nitrogen doped graphene with high electrocatalytic activity toward oxygen reduction reaction
    Zheng, Bo; Wang, Jiong; Wang, Feng-Bin; Xia, Xing-Hua
    Electrochemistry Communications (2013), 28 (), 24-26CODEN: ECCMF9; ISSN:1388-2481. (Elsevier B.V.)
    A novel strategy for fabricating N doped graphene sheets was developed using graphite oxide as the C source and urea as the N source via hydrothermal approach. This method allows the authors to obtain high doping level of N in graphene. The doped N mainly exists as pyridinic and pyrrolic N bonding configurations. Subsequent thermal annealing will transfer the pyrrolic N to graphitic N significantly. Electrochem. results demonstrate that larger amt. of graphitic N configuration may play an active role in the excellent electrocatalytic activity toward O redn. reaction (ORR) in alk. electrolyte.
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    Li, X.; Wang, H.; Robinson, J. T.; Sanchez, H.; Diankov, G.; Dai, H. Simultaneous Nitrogen Doping and Reduction of Graphene Oxide. J. Am. Chem. Soc. 2009, 131, 15939– 15944,  DOI: 10.1021/ja907098f
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    Simultaneous Nitrogen Doping and Reduction of Graphene Oxide
    Li, Xiaolin; Wang, Hailiang; Robinson, Joshua T.; Sanchez, Hernan; Diankov, Georgi; Dai, Hongjie
    Journal of the American Chemical Society (2009), 131 (43), 15939-15944CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    We developed a simple chem. method to obtain bulk quantities of N-doped, reduced graphene oxide (GO) sheets through thermal annealing of GO in ammonia. XPS study of GO sheets annealed at various reaction temps. reveals that N-doping occurs at a temp. as low as 300°C, while the highest doping level of ∼5% N is achieved at 500°C. N-doping is accompanied by the redn. of GO with decreases in oxygen levels from ∼28% in as-made GO down to ∼2% in 1100 °C NH3-reacted GO. XPS anal. of the N binding configurations of doped GO finds pyridinic N in the doped samples, with increased quaternary N (N that replaced the carbon atoms in the graphene plane) in GO annealed at higher temps. (≥900°C). Oxygen groups in GO were found responsible for reactions with NH3 and C-N bond formation. Prereduced GO with fewer oxygen groups by thermal annealing in H2 exhibits greatly reduced reactivity with NH3 and a lower N-doping level. Elec. measurements of individual GO sheet devices demonstrate that GO annealed in NH3 exhibits higher cond. than those annealed in H2, suggesting more effective redn. of GO by annealing in NH3 than in H2, consistent with XPS data. The N-doped reduced GO shows clearly n-type electron doping behavior with the Dirac point (DP) at neg. gate voltages in three terminal devices. Our method could lead to the synthesis of bulk amts. of N-doped, reduced GO sheets useful for various practical applications.
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    Suzuki, K.; Morita, H.; Kaneko, T.; Yoshida, H.; Fujimori, H. Crystal structure and magnetic properties of the compound FeN. J. Alloys Compd. 1993, 201, 11– 16,  DOI: 10.1016/0925-8388(93)90854-G
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    Crystal structure and magnetic properties of the compound iron nitride (FeN)
    Suzuki, K.; Morita, H.; Kaneko, T.; Yoshida, H.; Fujimori, H.
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    FeN was prepd. as a single phase by d.c. reactive sputtering. Its crystal structure was detd. by x-ray diffraction measurements to be the Zn blende type fcc. structure. FeN is stable at ≤593 K and decomps. into FeN + Fe2N at >593 K. The present sample exhibits a mictomagnetic character at low temps., which is considered to be related to the antiferromagnetism of the FeN compd.
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    Zhang, H.; Gong, Q.; Ren, S.; Arshid, M. A.; Chu, W.; Chen, C. Implication of iron nitride species to enhance the catalytic activity and stability of carbon nanotubes supported Fe catalysts for carbon-free hydrogen production via low-temperature ammonia decomposition. Catal. Sci. Technol. 2018, 8, 907– 915,  DOI: 10.1039/C7CY02270K
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    Implication of iron nitride species to enhance the catalytic activity and stability of carbon nanotubes supported Fe catalysts for carbon-free hydrogen production via low-temperature ammonia decomposition
    Zhang, Hui; Gong, Qinmei; Ren, Shan; Arshid, Mahmood Ali; Chu, Wei; Chen, Chen
    Catalysis Science & Technology (2018), 8 (3), 907-915CODEN: CSTAGD; ISSN:2044-4753. (Royal Society of Chemistry)
    This study was aimed to boost the catalytic ability of C nanotubes (CNTs) supported Fe-based catalysts, prepd. using wet-impregnation and followed by nitrogenization, for C-free H prodn. from NH3 decompn. at a low temp. The nitrogenization temp. and Fe loading had significant effects on the size of the well-dispersed Fe2N crystallites. The Fe3O4/CNTs catalysts at a higher nitrogenation temp. under NH3 flow with a suitable Fe content led to the formation of the stable and uniformly distributed Fe2N species, which played an active role in the enhanced catalytic ability of the Fe3O4/CNTs catalysts. However, the nitrogenization of the Fe3O4/CNTs catalyst under either H or Ar led to the formation of the Fe4N and Fe2N species. In the presence of the Fe4N phases, the Fe3O4/CNTs catalyst exhibited an enhanced catalytic activity. The collaborative interaction of the active site Fe2N and carbon nanotubes in the Fe2N/CNTs catalysts resulted in a significant increase in the catalytic activity and durability ≤40 h. The effective control of the d. of the active sites Fe2N and the synergism between the carrier and the crystallite compn. of Fe nitrides are the key aspects for the efficient design of the transition nitride catalysts for carbon-free H prodn. via ammonia decompn.
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    Yin, H.; Zhang, C.; Liu, F.; Hou, Y. Hybrid of Iron Nitride and Nitrogen-Doped Graphene Aerogel as Synergistic Catalyst for Oxygen Reduction Reaction. Adv. Funct. Mater. 2014, 24, 2930– 2937,  DOI: 10.1002/adfm.201303902
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    Hybrid of Iron Nitride and Nitrogen-Doped Graphene Aerogel as Synergistic Catalyst for Oxygen Reduction Reaction
    Yin, Han; Zhang, Chenzhen; Liu, Fei; Hou, Yanglong
    Advanced Functional Materials (2014), 24 (20), 2930-2937CODEN: AFMDC6; ISSN:1616-301X. (Wiley-VCH Verlag GmbH & Co. KGaA)
    It is extremely desirable but challenging to create highly active, stable, and low-cost catalysts towards O redn. reaction to replace Pt-based catalysts to perform the commercialization of fuel cells. Here, a novel Fe nitride/N doped-graphene aerogel hybrid, synthesized by a facile 2-step hydrothermal process, in which Fe phthalocyanine is uniformly dispersed and anchored on graphene surface with the assist of π-π stacking and O-contg. functional groups, is reported. As a result, there exist strong interactions between FexN nanoparticles and graphene substrates, leading to a synergistic effect towards O redn. reaction. It is worth noting that the onset potential and c.d. of the hybrid are significantly better and the charge transfer resistance is much lower than that of pure N-doped graphene aerogel, free FexN and their phys. mixts. The hybrid also exhibits comparable catalytic activity as com. Pt/C at the same catalyst loading, while its stability and resistance to MeOH crossover are superior. Apart from the active nature of the hybrid, the large surface area and porosity are responsible for its excellent onset potential and the high d. of Fe-N-C sties and small size of FexN particles boost charge transfer rate.
  39. 39
    Hassan, F. M.; Chabot, V.; Li, J.; Kim, B. K.; Ricardez-Sandoval, L.; Yu, A. Pyrrolic-structure enriched nitrogen doped graphene for highly efficient next generation supercapacitors. J. Mater. Chem. A 2013, 1, 2904,  DOI: 10.1039/c2ta01064j
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    Pyrrolic-structure enriched nitrogen doped graphene for highly efficient next generation supercapacitors
    Hassan, Fathy M.; Chabot, Victor; Li, Jingde; Kim, Brian Kihun; Ricardez-Sandoval, Luis; Yu, Aiping
    Journal of Materials Chemistry A: Materials for Energy and Sustainability (2013), 1 (8), 2904-2912CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
    This study reports the prepn. of pyrrolic-structure enriched N doped graphene by hydrothermal synthesis at varied temp. The morphol., structure and compn. of the prepd. N doped graphene were confirmed with SEM, XRD, XPS and Raman spectroscopy. The material was tested for supercapacitive behavior. Doping graphene with N increased the elec. double layer supercapacitance to ≤194 F g-1. Also, d. functional theory (DFT) calcns. showed the proper level of binding energy found between the pyrrolic-N structure and the electrolyte ions, which may be used to explain the highest contribution of the pyrrolic-structure to the capacitance.
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    Ma, J.-h.; Wang, L.; Mu, X.; Li, L. Nitrogen-doped graphene supported Pt nanoparticles with enhanced performance for methanol oxidation. Int. J. Hydrogen Energy 2015, 40, 2641– 2647,  DOI: 10.1016/j.ijhydene.2014.12.080
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    Nitrogen-doped graphene supported Pt nanoparticles with enhanced performance for methanol oxidation
    Ma, Jun-hong; Wang, Li; Mu, Xue; Li, Li
    International Journal of Hydrogen Energy (2015), 40 (6), 2641-2647CODEN: IJHEDX; ISSN:0360-3199. (Elsevier Ltd.)
    A series of nitrogen-doped graphene nanocomposites (N-RGO) derived from pyrolysis of graphene oxide (GO)/polyaniline composites with varied mass ratio between GO and aniline were used as support for immobilization of Pt nanoparticles. Transmission electron microscopy, X-ray powder diffraction, Raman spectroscopy and XPS were used to characterize the morphol. and microstructure of the prepd. catalysts. The catalytic activity of the catalysts towards the oxidn. of methanol was evaluated by cyclic voltammetry. Compared to the undoped Pt/RGO catalyst, an enhanced electrocatalytic activity for methanol oxidn. was obtained in the case of the Pt/N-RGO with optimized compn. and nanostructure.
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    Choi, C. H.; Park, S. H.; Woo, S. I. Facile growth of N-doped CNTs on Vulcan carbon and the effects of iron content on electrochemical activity for oxygen reduction reaction. Int. J. Hydrogen Energy 2012, 37, 4563– 4570,  DOI: 10.1016/j.ijhydene.2011.08.086
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    Facile growth of N-doped CNTs on Vulcan carbon and the effects of iron content on electrochemical activity for oxygen reduction reaction
    Choi, Chang Hyuck; Park, Sung Hyeon; Woo, Seong Ihl
    International Journal of Hydrogen Energy (2012), 37 (5), 4563-4570CODEN: IJHEDX; ISSN:0360-3199. (Elsevier Ltd.)
    In order to develop cheap electrochem. oxygen redn. reaction (ORR) catalysts, N-doped CNTs grafted on Vulcan carbon were synthesized via pyrolysis of dicyandiamide on Fe2O3/C. Various contents of iron in Fe2O3/C (0, 20, 40 and 60 wt. %) were used as supports to investigate the effects and roles of iron content on ORR. It was shown that the iron acted as a promoter for doping nitrogen into carbon; as the iron content increased, the amt. of nitrogen doping also increased. TEM and element anal. results indicated that iron induced growth of CNTs and facilitated N-doping in carbon. However, further increase in iron content higher than 20 wt. % showed neg. effects on the ORR activity due to a decrease of the surface area of the prepd. catalysts. Hence, the catalyst with the highest performance was obsd. when dicyandiamide was pyrolyzed with Fe2O3/C 20 wt. % (Fe-N-C-20) and the order of activity towards ORR was Fe-N-C-20 > Fe-N-C-40 > Fe-N-C-60 > Fe-N-C-0 > Vulcan XC-72R.
  42. 42
    Zhang, Y.; Xie, Y.; Zhou, Y.; Wang, X.; Pan, K. Well dispersed Fe2N nanoparticles on surface of nitrogen-doped reduced graphite oxide for highly efficient electrochemical hydrogen evolution. J. Mater. Res. 2017, 32, 1770– 1776,  DOI: 10.1557/jmr.2017.138
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    Well dispersed Fe2N nanoparticles on surface of nitrogen-doped reduced graphite oxide for highly efficient electrochemical hydrogen evolution
    Zhang, Yi; Xie, Ying; Zhou, Yangtao; Wang, Xiuwen; Pan, Kai
    Journal of Materials Research (2017), 32 (9), 1770-1776CODEN: JMREEE; ISSN:2044-5326. (Cambridge University Press)
    It is important to fabricate iron-based nitride/conductive material composite to obtain good catalytic performance. In this work, Fe2N nanoparticles with diam. of approx. 30 nm have been successfully dispersed on the surface of nitrogen-doped graphite oxide (NrGO) by a facile sol-gel method and further ammonia atm. treatment. XPS, XRD, Raman, and TEM proved that Fe2N nanoparticles are well monodispersed, and nitrogen atoms are doped in NrGO. The composite possessed two merits, i.e., the more catalytic active site in Fe2N nanoparticles due to the well monodispersion, and fast electron transfer due to the nitrogen dope in rGO. With the proper ratio, the composite exhibited brilliant catalytic activity and durability in acidic media. It possesses overpotential of 94 mV to approach 10 mA/cm2, a small Tefel slope of 49 mV/dec, and maintains the good electrocatalytic activity for 10 h. Cyclic voltammetry and electrochem. impedance spectroscopy indicated that the electrocatalyst possessed high catalytic active area and fast electron transfer. Our work may provide a new avenue for the prepn. of low-cost iron-based nitride/NrGO composite for highly efficient electrochem. hydrogen evolution.
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    Cui, Q.; Chao, S.; Wang, P.; Bai, Z.; Yan, H.; Wang, K.; Yang, L. Fe–N/C catalysts synthesized by heat-treatment of iron triazine carboxylic acid derivative complex for oxygen reduction reaction. RSC Adv. 2014, 4, 12168,  DOI: 10.1039/c3ra44958k
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    Fe-N/C catalysts synthesized by heat-treatment of iron triazine carboxylic acid derivative complex for oxygen reduction reaction
    Cui, Qian; Chao, Shujun; Wang, Panhao; Bai, Zhengyu; Yan, Huiying; Wang, Kui; Yang, Lin
    RSC Advances (2014), 4 (24), 12168-12174CODEN: RSCACL; ISSN:2046-2069. (Royal Society of Chemistry)
    4,4',4''-S-Triazine-1,3,5-triyltri-p-aminobenzoic acid (H3TATAB) was used as a ligand to prep. an iron-TATAB (Fe-TATAB) complex for the development of an effective oxygen redn. reaction (ORR) catalyst. The activity of the catalyst depended on wt. ratios between the Fe-TATAB complex and carbon black and heat-treated temps. The results showed that the Fe-N70%/C-800 catalyst (the wt. ratio of Fe complex to carbon black was 70 : 30 and the catalyst was pyrolyzed at 800 °C) had good catalytic activity toward ORR with the onset potential at 0.91 V vs. RHE and a kinetic c.d. of 4.3 mA cm-2 at 0.6 V vs. RHE in alk. medium. Moreover, the Fe-N70%/C-800 catalyst had better tolerance to methanol crossover effect in comparison with com. Pt/C (20%). The morphol. and compn. of the catalysts were characterized by high resoln. transmission electron microscopy (HRTEM), X-ray diffraction (XRD) as well as X-ray photoelectron spectroscopic (XPS). The electrocatalytic activities were demonstrated by cyclic voltammetry (CV), linear sweep voltammetry (LSV), chronoamperometric measurements and stability accelerated tests. According to rotating disk electrode (RDE) measurements and Koutecky-Levich anal., the overall electron transfer no. in the catalyzed ORR was found to be 3.7-3.9 and the ORR process was mainly a four-electron pathway. The results indicate that the Fe-N70%/C-800 catalyst may be a promising cathode catalyst for ORR.
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    Rohith Vinod, K.; Saravanan, P.; Sakar, M.; Balakumar, S. Insights into the nitridation of zero-valent iron nanoparticles for the facile synthesis of iron nitride nanoparticles. RSC Adv. 2016, 6, 45850– 45857,  DOI: 10.1039/C6RA04935D
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    Insights into the nitridation of zero-valent iron nanoparticles for the facile synthesis of iron nitride nanoparticles
    Rohith Vinod, K.; Saravanan, P.; Sakar, M.; Balakumar, S.
    RSC Advances (2016), 6 (51), 45850-45857CODEN: RSCACL; ISSN:2046-2069. (Royal Society of Chemistry)
    The process of nitridation of zero-valent iron nanoparticles (ZVINPs) is investigated by employing two different synthesis strategies such as solvothermal method and gas diffusion using N2 and NH3. It is obsd. that the phase formation of different iron nitrides mainly depends on the reaction chem. of the nitridation source and temp. Accordingly, N2 gas diffusion and solvothermal methods yield iron oxide phases, whereas NH3 gas diffusion yields pure iron nitride phase with particle sizes in the nanoscale. X-ray diffraction studies complemented by Rietveld refinement confirm the formation of ε-Fe3N and γ'-Fe4N nanoparticles. Field emission SEM images revealed spherical nanoparticles with av. particle sizes of 35 and 50 nm for ZVINPs and iron nitride NPs, resp. From the magnetization studies carried out using a superconducting quantum interference device magnetometer it is found that the field-dependent hysteresis curves indicated the ferromagnetic properties of ZVINPs, ε-Fe3N and γ'-Fe4N NPs with coercive fields of 160, 65 and 45 Oe, resp. Similarly, the temp.-dependent magnetization profiles revealed that the obsd. ferromagnetic properties of iron nitride phases can be attributed to the redistribution of electronic spin states due to both nitrogen populations and the confinement in the crystallites.
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    Oh, Y. J.; Yoo, J. J.; Kim, Y. I.; Yoon, J. K.; Yoon, H. N.; Kim, J.-H.; Park, S. B. Oxygen functional groups and electrochemical capacitive behavior of incompletely reduced graphene oxides as a thin-film electrode of supercapacitor. Electrochim. Acta 2014, 116, 118– 128,  DOI: 10.1016/j.electacta.2013.11.040
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    Oxygen functional groups and electrochemical capacitive behavior of incompletely reduced graphene oxides as a thin-film electrode of supercapacitor
    Oh, Young Joon; Yoo, Jung Joon; Kim, Yong Il; Yoon, Jae Kook; Yoon, Ha Na; Kim, Jong-Huy; Park, Seung Bin
    Electrochimica Acta (2014), 116 (), 118-128CODEN: ELCAAV; ISSN:0013-4686. (Elsevier Ltd.)
    For incompletely reduced graphene oxides (RGOs), an effect of O functional groups such as carboxyl, phenol, carbonyl, and quinone on electrochem. capacitive behavior was studied. To prep. RGO thin-film electrodes, a simple fabrication process by (i) dropping and evapg. the graphene oxide (GO) soln., (ii) irradiating pulsed light, and (iii) heat-treating at 200-360° was applied. It was notable that the pulsed light irradn. was effective to prevent the disfiguring of deposited GO thin-film during the thermal redn. From XRD analyses, interlayer distances of the RGOs were gradually decreased from 0.379 to 0.354 nm. As increasing the thermal redn. temp. from 200 to 360°, XPS O 1s spectra analyses showed that the at. percentages of carboxyl and phenol of the RGOs were sustained as 5.40 ± 0.36 and 4.77 ± 0.41 at.%, resp. Meanwhile, those of carbonyl and quinone of the RGOs were gradually declined from 3.10 to 1.81 and from 1.32 to 0.65 at.% with different thermal redn. temp., resp. For all RGO thin-film electrodes, the specific capacitance from the CV measurement in 6 M KOH was sustained as ∼220 F g-1 at the scan of 5 mV s-1. However, in 1 M H2SO4, the specific capacitance was gradually decreased from 171 to 136 F g-1. After 100,000 cycles in 6 M KOH, 1 M H2SO4, and 0.5 M Na2SO4, the RGO (200°) electrodes showed ∼92, 54, and 104% of the initial capacitances, resp. The at. percentages of the O functional groups involved in the pseudocapacitive faradaic reaction were decreased after the cycle test. Esp. in 1 M H2SO4, quinone group was decreased to ∼48% of initial at. percentage, which seems to be a main reason for the drastic redn. of capacitance. The specific pseudocapacitance per unit at. percentage for either carboxyl or phenol group in 6 M KOH was obtained as 12.59 F g-1 at%-1. For carbonyl group in 1 M H2SO4, it was a slightly deviated value as 13.55 F g-1 at%-1. For quinone group in 1 M H2SO4, it was 27.09 F g-1at%-1.
  46. 46
    Xing, Z.; Ju, Z.; Zhao, Y.; Wan, J.; Zhu, Y.; Qiang, Y.; Qian, Y. One-pot hydrothermal synthesis of Nitrogen-doped graphene as high-performance anode materials for lithium ion batteries. Sci. Rep. 2016, 6, 26146  DOI: 10.1038/srep26146
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    One-pot hydrothermal synthesis of Nitrogen-doped graphene as high-performance anode materials for lithium ion batteries
    Xing, Zheng; Ju, Zhicheng; Zhao, Yulong; Wan, Jialu; Zhu, Yabo; Qiang, Yinghuai; Qian, Yitai
    Scientific Reports (2016), 6 (), 26146CODEN: SRCEC3; ISSN:2045-2322. (Nature Publishing Group)
    Nitrogen-doped (N-doped) graphene has been prepd. by a simple one-step hydrothermal approach using hexamethylenetetramine (HMTA) as single carbon and nitrogen source. In this hydrothermal process, HMTA pyrolyzes at high temp. and the N-doped graphene subsequently self-assembles on the surface of MgO particles (formed by the Mg powder reacting with H2O) during which graphene synthesis and nitrogen doping are simultaneously achieved. The as-synthesized graphene with incorporation of nitrogen groups possesses unique structure including thin layer thickness, high surface area, mesopores and vacancies. These structural features and their synergistic effects could not only improve ions and electrons transportation with nanometer-scale diffusion distances but also promote the penetration of electrolyte. The N-doped graphene exhibits high reversible capacity, superior rate capability as well as long-term cycling stability, which demonstrate that the N-doped graphene with great potential to be an efficient electrode material. The exptl. results provide a new hydrothermal route to synthesize N-doped graphene with potential application for advanced energy storage, as well as useful information to design new graphene materials.
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    Azuma, M.; Kashihara, M.; Nakato, Y.; Tsubomura, H. Reduction of oxygen to water on cobalt-nitride thin film electrodes prepared by the reactive rf sputtering technique. J. Electroanal. Chem. Interfacial Electrochem. 1988, 250, 73– 82,  DOI: 10.1016/0022-0728(88)80193-1
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    Reduction of oxygen to water on cobalt-nitride thin film electrodes prepared by the reactive RF sputtering technique
    Azuma, Masashi; Kashihara, Minoru; Nakato, Yoshihiro; Tsubomura, Hiroshi
    Journal of Electroanalytical Chemistry and Interfacial Electrochemistry (1988), 250 (1), 73-82CODEN: JEIEBC; ISSN:0022-0728.
    Electrochem. redn. of O was carried out on amorphous Co-nitride (CoxN) thin film electrodes prepd. by the reactive RF sputtering technique. The onset potential of the O redn. current in the CoxN electrode in neutral electrolyte solns. was close to that on a Pt metal electrode, which is known to be the most efficient material for O redn. Measurements with rotating-disk electrodes (RDEs) and rotating ring-disk electrodes (RRDEs) showed that redn. of O to H2O (the 4-electron-transfer type) proceeded on the CoxN electrodes, unlike that on Co metal electrodes, where only the 2-electron-transfer type redn. to H2O2 took place. The reason for the improved electrocatalytic activity by nitridation is discussed briefly.
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    Gong, K.; Du, F.; Xia, Z.; Durstock, M.; Dai, L. Nitrogen-Doped Carbon Nanotube Arrays with High Electrocatalytic Activity for Oxygen Reduction. Science 2009, 323, 760– 764,  DOI: 10.1126/science.1168049
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    Nitrogen-Doped Carbon Nanotube Arrays with High Electrocatalytic Activity for Oxygen Reduction
    Gong, Kuanping; Du, Feng; Xia, Zhenhai; Durstock, Michael; Dai, Liming
    Science (Washington, DC, United States) (2009), 323 (5915), 760-764CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    The large-scale practical application of fuel cells will be difficult to realize if the expensive platinum-based electrocatalysts for oxygen redn. reactions (ORRs) cannot be replaced by other efficient, low-cost, and stable electrodes. Here, we report that vertically aligned nitrogen-contg. carbon nanotubes (VA-NCNTs) can act as a metal-free electrode with a much better electrocatalytic activity, long-term operation stability, and tolerance to crossover effect than platinum for oxygen redn. in alk. fuel cells. In air-satd. 0.1 M potassium hydroxide, we obsd. a steady-state output potential of -80 mV and a c.d. of 4.1 milliamps per square centimeter at -0.22 V, compared with -85 mV and 1.1 milliamps per square centimeter at -0.20 V for a platinum-carbon electrode. The incorporation of electron-accepting nitrogen atoms in the conjugated nanotube carbon plane appears to impart a relatively high pos. charge d. on adjacent carbon atoms. This effect, coupled with aligning the NCNTs, provides a four-electron pathway for the ORR on VA-NCNTs with a superb performance.
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    Nagaiah, T. C.; Kundu, S.; Bron, M.; Muhler, M.; Schuhmann, W. Nitrogen-doped carbon nanotubes as a cathode catalyst for the oxygen reduction reaction in alkaline medium. Electrochem. Commun. 2010, 12, 338– 341,  DOI: 10.1016/j.elecom.2009.12.021
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    Nitrogen-doped carbon nanotubes as a cathode catalyst for the oxygen reduction reaction in alkaline medium
    Nagaiah, Tharamani C.; Kundu, Shankhamala; Bron, Michael; Muhler, Martin; Schuhmann, Wolfgang
    Electrochemistry Communications (2010), 12 (3), 338-341CODEN: ECCMF9; ISSN:1388-2481. (Elsevier B.V.)
    A new approach to synthesize nitrogen-doped carbon nanotubes (NCNTs) as catalysts for oxygen redn. by treating oxidized CNTs with ammonia is presented. The surface properties and oxygen redn. activities were characterized by cyclic voltammetry, rotating disk electrode, and XPS. NCNTs treated at 800° show improved electrocatalytic activity for oxygen redn. as compared with com. available Pt/C catalysts.
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    Ren, G.; Gao, L.; Teng, C.; Li, Y.; Yang, H.; Shui, J.; Lu, X.; Zhu, Y.; Dai, L. Ancient Chemistry “Pharaoh’s Snakes” for Efficient Fe-/N-Doped Carbon Electrocatalysts. ACS Appl. Mater. Interfaces 2018, 10, 10778– 10785,  DOI: 10.1021/acsami.7b16936
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    Ancient Chemistry "Pharaoh's Snakes" for Efficient Fe-/N-Doped Carbon Electrocatalysts
    Ren, Guangyuan; Gao, Liangliang; Teng, Chao; Li, Yunan; Yang, Hequn; Shui, Jianglan; Lu, Xianyong; Zhu, Ying; Dai, Liming
    ACS Applied Materials & Interfaces (2018), 10 (13), 10778-10785CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
    The method of fabricating non-precious metal electrocatalysts with high activity and durability through a facile and eco-friendly procedure is of great significance to the development of low-cost fuel cells and metal-air batteries. Herein, the authors present that an ancient chem. reaction of Pharaoh's snakes can be a fast and convenient technique to prep. Fe-/N-doped C (Fe/N-C) nanosheet/nanotube electrocatalysts with sugar, soda, melamine, and Fe nitrate as precursors. The resultant Fe/N-C catalyst has a hierarchically porous structure, a large surface area, and uniformly distributed active sites. The catalyst shows high electrocatalytic activities toward both the O redn. reaction with a half-wave potential of 0.90 V (vs. reversible H electrode) better than that of Pt/C and the O evolution reaction with an overpotential of 0.46 V at the c.d. of 10 mA cm-2 comparable to that of RuO2. The activity and stability of the catalyst are also evaluated in primary and rechargeable Zn-air batteries. In both conditions, three-dimensional Fe/N-C exhibited performances superior to Pt/C. The authors' work demonstrates a success of using an ancient science to make a state-of-the-art electrocatalyst.
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    Susi, T.; Pichler, T.; Ayala, P. X-ray photoelectron spectroscopy of graphitic carbon nanomaterials doped with heteroatoms. Beilstein J. Nanotechnol. 2015, 6, 177– 192,  DOI: 10.3762/bjnano.6.17
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    X-ray photoelectron spectroscopy of graphitic carbon nanomaterials doped with heteroatoms
    Susi, Toma; Pichler, Thomas; Ayala, Paola
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    XPS is one of the best tools for studying the chem. modification of surfaces, and in particular the distribution and bonding of heteroatom dopants in carbon nanomaterials such as graphene and carbon nanotubes. Although these materials have superb intrinsic properties, these often need to be modified in a controlled way for specific applications. Towards this aim, the most studied dopants are neighbors to carbon in the periodic table, nitrogen and boron, with phosphorus starting to emerge as an interesting new alternative. Hundreds of studies have used XPS for analyzing the concn. and bonding of dopants in various materials. Although the majority of works has concd. on nitrogen, important work is still ongoing to identify its precise at. bonding configurations. In general, care should be taken in the prepn. of a suitable sample, consideration of the intrinsic photoemission response of the material in question, and the appropriate spectral anal. If this is not the case, incorrect conclusions can easily be drawn, esp. in the assignment of measured binding energies into specific at. configurations. Starting from the characteristics of pristine materials, this review provides a practical guide for interpreting X-ray photoelectron spectra of doped graphitic carbon nanomaterials, and a ref. for their binding energies that are vital for compositional anal. via XPS.
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    Wu, Z. S.; Yang, S.; Sun, Y.; Parvez, K.; Feng, X.; Mullen, K. 3D nitrogen-doped graphene aerogel-supported Fe3O4 nanoparticles as efficient electrocatalysts for the oxygen reduction reaction. J. Am. Chem. Soc. 2012, 134, 9082– 9085,  DOI: 10.1021/ja3030565
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    3D Nitrogen-Doped Graphene Aerogel-Supported Fe3O4 Nanoparticles as Efficient Electrocatalysts for the Oxygen Reduction Reaction
    Wu, Zhong-Shuai; Yang, Shubin; Sun, Yi; Parvez, Khaled; Feng, Xinliang; Muellen, Klaus
    Journal of the American Chemical Society (2012), 134 (22), 9082-9085CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Three-dimensional (3D) nitrogen-doped graphene aerogel (N-GA)-supported Fe3O4 nanoparticles (Fe3O4/N-GAs) as efficient cathode catalysts for the oxygen redn. reaction are reported. The graphene hybrids exhibit an interconnected macroporous framework of graphene sheets with uniform dispersion of Fe3O4 nanoparticles. In studying the effects of the carbon support on the Fe3O4 nanoparticles for the oxygen redn. reaction, we found that Fe3O4/N-GAs show a more pos. onset potential, higher cathodic d., lower H2O2 yield, and higher electron transfer no. for the oxygen redn. reaction in alk. media than Fe3O4 nanoparticles supported on N-doped carbon black or N-doped graphene sheets, highlighting the importance of the 3D macropores and high sp. surface area of the GA support for improving the oxygen redn. reaction performance. Furthermore, Fe3O4/N-GAs show better durability than the com. Pt/C catalyst.
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    Electrochimica Acta (1967), 12 (3), 287-97CODEN: ELCAAV; ISSN:0013-4686.
    The soly. of O in aq. KOH solns. was measured by a Van Slyke app. and by an adsorption technique developed by Hildebrand (Kobatake and H., CA 56: 988b). In the range of concn. of KOH between 0 and 12N, at 25°, the 2 methods yielded identical results; at 760 torr O partial pressure, log S = log 1.26 × 10-3 - 0.174 6C, where 0.1746 is the soly. coeff., S the concn. of O, g.-moles/l., and C the concn. of KOH, g./moles/l. Between 0° and 60° both the soly. and the soly. coeff. decrease with increasing temp. Diffusion coeffs. of O in aq. KOH were evaluated from the limiting current of O on a rotating disk electrode, and also by a stagnant tube technique similar to that used by von Stackelberg (S., et al., CA 47: 11908d). The diffusivity drops sharply with increasing KOH concn., and increases with temp. At 25° and for KOH concns. between 2 and 4N, the product of the diffusivity and the viscosity is const.: Dμ = 1.3 × 10-7 g.-cm./sec.2, where D is the diffusivity, cm.2/sec., and μ is the viscosity in poises. At 60° and for a normality of KOH between 1 and 8, the value of this product is: Dμ = 1.9 × 10-7 g.-cm./sec.2 28 references.
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    The CuII/CuI redox properties and electrochem. O2 redn. activity of CuII-complexes with pyridylalkylamine ligands were studied in a neutral buffer soln. The relation between CuII/CuI redox properties and O2 redn. activity was clearly demonstrated by voltammetric analyses.
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    Enhanced electrocatalytic activity of nitrogen-doped multi-walled carbon nanotubes towards the oxygen reduction reaction in alkaline media
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    Platinum Nanoparticles Supported on Nitrogen-Doped Graphene Nanosheets as Electrocatalysts for Oxygen Reduction Reaction
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    Molina-García, M. A.; Rees, N. V. Effect of catalyst carbon supports on the oxygen reduction reaction in alkaline media: a comparative study. RSC Adv. 2016, 6, 94669– 94681,  DOI: 10.1039/C6RA18894J
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    Effect of catalyst carbon supports on the oxygen reduction reaction in alkaline media: a comparative study
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    Some carbon materials commonly used as electronically-conductive supports for catalysts in fuel cell research are Carbon Black (CB), multi-walled carbon nanotubes (MWCNT), Graphene Oxide (GO) and reduced graphene oxide (rGO). Here we present a comparative study into the relative effects of each of these on the performance towards the oxygen redn. reaction (ORR) in alk. media. For the purposes of comparing the supports, a simple Pt catalyst is used and the performance is evaluated via Koutecky-Levich anal. and direct measurement of peroxide by rotating ring-disk electrode (RRDE) to det. the no. of electrons (n) transferred in the ORR. It is found that Pt/CB follows a quasi 4-electron mechanism due to that the ORR takes place mainly on the active Pt particles, whereas Pt/MWCNT, Pt/GO and Pt/rGO exhibit a mixed behavior between the two proposed mechanisms due to the higher activity of the graphene-derived supports towards the peroxide formation compared to CB. The effect of the oxide groups of GO and the metal impurities of MWCNT on the catalytic performance is also studied.
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    Wang, M.; Yang, Y.; Liu, X.; Pu, Z.; Kou, Z.; Zhu, P.; Mu, S. The role of iron nitrides in the Fe-N-C catalysis system towards the oxygen reduction reaction. Nanoscale 2017, 9, 7641– 7649,  DOI: 10.1039/C7NR01925D
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    The role of iron nitrides in the Fe-N-C catalysis system towards the oxygen reduction reaction
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    Fe-N-C series catalysts are always attractive for their high catalytic activity towards the oxygen redn. reaction (ORR). However, they usually consist of various components such as iron nitrides, metallic iron, iron carbides, N-doped carbon and Fe-N4 moieties, leading to controversial contributions of these components to the catalysis of the ORR, esp. iron nitrides. In this work, to investigate the function of iron nitrides, FexN nanoparticles (NPs) embedded in mesoporous N-doped carbon without Fe-N4 moieties are designed and constructed by a simple histidine-assisted method. Herein, the use of histidine can increase the N and Fe contents in the product. The obtained catalyst exhibits excellent ORR catalytic activity which is very close to that of the com. Pt/C catalyst in alk. electrolytes. Combining the catalytic activity, structural characterization (esp. from Mossbauer spectroscopy), and the results of DFT calcns. for adsorption energies of oxygen on the main surfaces of Fe2N including ε-Fe2N and ζ-Fe2N, it can be deduced that Fe2N NPs as active species make a contribution to the ORR catalysis, of which e-FexN (x = 2.1) is more active than ζ-Fe2N. In addn., we find that there exists an obvious synergistic effect between Fe2N NPs and N-doped carbon, leading to the greatly enhanced ORR catalytic activity.
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    Non-precious-metal catalysts have been investigated as alternatives to Pt-based oxygen redn. reaction catalysts for more than 50 years. While the incorporation of a metal is known to be necessary to generate a catalyst with high activity, the exact role of the metal is still not well-understood. In this work, we prep. an active oxygen redn. reaction catalyst contg. Fe and then selectively remove the Fe from the catalyst while preserving the carbon and nitrogen species. By comparing the oxygen redn. reaction activity of the catalyst before and after treatment, we show that in the absence of Fe the carbon and nitrogen sites in the catalyst exhibit a larger overpotential and lower selectivity for the 4e- redn. of oxygen in both acidic and alk. conditions. These findings reveal the direct involvement of the metal in the active site of non-precious-metal catalysts and provide important guidance for future catalyst improvements.
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    We present two different ways to fabricate nitrogen-doped graphene (N-graphene) and demonstrate its use as a metal-free catalyst to study the catalytic active center for the oxygen redn. reaction (ORR). N-graphene was produced by annealing of graphene oxide (G-O) under ammonia or by annealing of a N-contg. polymer/reduced graphene oxide (RG-O) composite (polyaniline/RG-O or polypyrrole/RG-O). The effects of the N precursors and annealing temp. on the performance of the catalyst were investigated. The bonding state of the N atom was found to have a significant effect on the selectivity and catalytic activity for ORR. Annealing of G-O with ammonia preferentially formed graphitic N and pyridinic N centers, while annealing of polyaniline/RG-O and polypyrrole/RG-O tended to generate pyridinic and pyrrolic N moieties, resp. Most importantly, the electrocatalytic activity of the catalyst was dependent on the graphitic N content which detd. the limiting c.d., while the pyridinic N content improved the onset potential for ORR. However, the total N content in the graphene-based non-precious metal catalyst does not play an important role in the ORR process.
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  36. Anquan Zhu, Lulu Qiao, Pengfei Tan, Yongjin Ma, Weixuan Zeng, Rui Dong, Cheng Ma, Jun Pan. Iron-nitrogen-carbon species for oxygen electro-reduction and Zn-air battery: Surface engineering and experimental probe into active sites. Applied Catalysis B: Environmental 2019, 254 , 601-611. https://doi.org/10.1016/j.apcatb.2019.05.037
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  • Abstract

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    Figure 1

    Figure 1. TEM images of the FeNx/NG samples with various weight percents of iron (a–d), and the pristine iron nitride particles synthesized without the presence of graphene oxide (e).

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    Figure 2

    Figure 2. (a) X-ray diffractograms and (b) Raman spectra of the FeNx/NG samples with varying iron contents.

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    Figure 3

    Figure 3. Fe 2p (a) spectrum of the 20 wt % iron-containing FeNx/NG sample. N 1s (b) spectrum of the pure iron nitride particles synthesized without the presence of graphene oxide. O 1s (c) and (d) N 1s spectra of the FeNx/NG composite with 20 wt % iron content.

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    Figure 4

    Figure 4. (a) LSV curves of the 20% iron-containing FeNx/NG using rotating disk electrode at various rotation rates. (b) Voltammograms recorded at fixed 1500 rpm for the FeNx/NG and Pt/CB composites on glassy carbon electrode (GCE). (c) Reduction current densities (at 0 V (vs RHE) and 1500 rpm) and (d) electron-transfer numbers were determined from LSV measurements. Voltammograms were recorded in alkaline conditions (0.1 M KOH) applying a scan rate of 10 mV s–1.

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    Figure 5

    Figure 5. Koutecký–Levich plots determined from linear sweep voltammetry. Composites with (a) 5%, (b) 10%, (c) 20%, and (d) 50% iron contents were deposited onto a glassy carbon electrode.

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    Figure 6

    Figure 6. Schematic of the possible ORR mechanism in FeNx/NG composite via the four-electron pathway.

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    Figure 7

    Figure 7. Alkaline (0.1 M KOH) chronoamperometric methanol tolerance ORR response at 0.51 V (vs RHE) of commercial Pt/CB catalyst and the 20 wt % Fe-containing composite deposited onto a glassy carbon electrode.

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  • References

    This article references 67 other publications.

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      Catalysts for oxygen redn. reaction (ORR) are crucial for the commercialization of proton exchange membrane fuel cells. In this work, nanocrystal tungsten nitride decorated carbon nanotubes hybrid (WN/CNTs-M) introduced as Pt catalyst supports (Pt-WN/CNTs-M) is reported for the first time. X-ray diffraction (XRD), transmission electron microscopy (TEM) and XPS measurements are adopted to investigate the physicochem. properties of the prepd. catalysts. Cyclic voltammetry and rotating disk electrode techniques are employed to study their corresponding electrocatalytic properties. The results demonstrate that the prepd. Pt-WN/CNTs-M catalysts exhibit significantly improved activity toward ORR in acid aq. solns., which also display better stability in comparison with the other catalysts in the present study. The reason could be predominantly ascribed to the synergistic effect between tungsten nitride and Pt in the Pt-WN/CNTs-M catalysts and the nitrogen-doping effect of carbon nanotubes with ammonia. The tungsten nitride decorated carbon nanotubes hybrid might be a promising alternative for low-Pt or non-Pt catalysts for oxygen redn.
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      Raj, C. R.; Samanta, A.; Noh, S. H.; Mondal, S.; Okajima, T.; Ohsaka, T. Emerging new generation electrocatalysts for the oxygen reduction reaction. J. Mater. Chem. A 2016, 4, 11156– 11178,  DOI: 10.1039/C6TA03300H
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      Raj, C. Retna; Samanta, Arpan; Noh, Seung Hyo; Mondal, Siniya; Okajima, Takeyoshi; Ohsaka, Takeo
      Journal of Materials Chemistry A: Materials for Energy and Sustainability (2016), 4 (29), 11156-11178CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
      The design and development of a new economically viable electrocatalyst for the cathodic redn. of oxygen in fuel cells and metal-air batteries is of significant interest. The high cost, scarcity and lack of durability of traditional Pt-based electrocatalysts limit the widespread implementation of fuel cells for practical applications. The emergence of non-Pt and metal-free electrocatalysts for the oxygen redn. reaction (ORR) is promising in the development of energy conversion devices. In this review, we discuss the emerging new electrocatalysts, non-precious transition metals, metal nitrides and carbides and the nanoscale carbon-based metal-free electrocatalysts, for the ORR. Although the actual ORR mechanism and the active site of these catalysts are not well understood, their catalytic activity is undoubtful. The porosity and chem. and electronic environments of the catalysts control their activity. The activity of these catalysts is discussed in terms of onset potential, durability and their tolerance towards anode fuels. The metal-free heteroatom-doped carbon-based electrocatalysts are highly active in alk. medium, paving the way for the development of alk. fuel cells, though their long time durability in an actual fuel cell stack is not well explored. The challenges in the use of these catalysts and the lack of fundamental understanding of the catalytic activity are addressed.
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      Nanostructured nonprecious metal catalysts for oxygen reduction reaction
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      Review. Platinum-based catalysts represent a state of the art in the electrocatalysis of oxygen redn. reaction (ORR) from the point of view of their activity and durability in harnessing the chem. energy via direct electrochem. conversion. However, because platinum is both expensive and scarce, its widespread implementation in such clean energy applications is limited. Recent breakthroughs in the synthesis of high-performance nonprecious metal catalysts (NPMCs) make replacement of Pt in ORR electrocatalysts with earth-abundant elements, such as Fe, Co, N, and C, a realistic possibility. In this Account, we discuss how we can obtain highly promising M-N-C (M: Fe and/or Co) catalysts by simultaneously heat-treating precursors of nitrogen, carbon, and transition metals at 800-1000 °C. The activity and durability of resulting catalysts depend greatly on the selection of precursors and synthesis chem. In addn., they correlate quite well with the catalyst nanostructure. While chemists have presented no conclusive description of the active catalytic site for this class of NPMCs, they have developed a designed approach to making active and durable materials, focusing on the catalyst nanostructure. The approach consists of nitrogen doping, in situ carbon graphitization, and the usage of graphitic structures (possibly graphene and graphene oxides) as carbon precursors. Various forms of nitrogen, particularly pyridinic and quaternary, can act as n-type carbon dopants in the M-N-C catalysts, assisting in the formation of disordered carbon nanostructures and donating electrons to the carbon. The CNx structures are likely a crucial part of the ORR active site(s). Noteworthy, the ORR activity is not necessarily governed by the amt. of nitrogen, but by how the nitrogen is incorporated into the nanostructures. Apart from the possibility of a direct participation in the active site, the transition metal often plays an important role in the in situ formation of various carbon nanostructures by catalyzing the decompn. of the nitrogen/carbon precursor. We can control the formation of different nanostructures during the synthesis of M-N-C catalysts. For example, in situ formed nitrogen-doped graphene-sheets can only be derived from polyaniline (PANI), probably due to structural similarities between the arom. structures of PANI and graphene. Highly-graphitized carbon nanostructures may serve as a matrix for the formation of ORR-active groups with improved catalytic activity and durability, contg. nitrogen and most probably also metal atoms. In the future, we will likely focus NPMC synthesis approaches on precise control of interactions between precursors of the metal and carbon/nitrogen during the heat treatment. The main purposes will be to maximize the no. of active sites, optimize nitrogen doping levels, and generate morphologies capable of hosting active and stable ORR sites.
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      Review of performance of anion exchange membrane fuel cells
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      A review of the performance and performance stability of AEMFCs through the years since the 1st reports in the early 2000s. Anion exchange membrane fuel cells (AEMFCs) have recently received increasing attention since in principle they allow for the use of non-precious metal catalysts, which dramatically reduces the cost per kW of power in fuel cell devices. Until not long ago, the main barrier in the development of AEMFCs was the availability of highly conductive anion exchange membranes (AEMs); however, improvements on this front in the past decade show that newly developed AEMs have already reached high levels of cond., leading to satisfactory cell performance. In recent years, a growing no. of research studies have reported AEMFC performance results. In the last 3 years, new records in performance were achieved. Most of the literature reporting cell performance is based on H-AEMFCs, although an increasing no. of studies have also reported the use of fuels others than H - such as alcs., nonalc. C-based fuels, as well as N-based fuels.
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      Zhang, J.; He, D.; Su, H.; Chen, X.; Pan, M.; Mu, S. Porous polyaniline-derived FeNxC/C catalysts with high activity and stability towards oxygen reduction reaction using ferric chloride both as an oxidant and iron source. J. Mater. Chem. A 2014, 2, 1242– 1246,  DOI: 10.1039/C3TA14065B
      13
      Porous polyaniline-derived FeNxC/C catalysts with high activity and stability towards oxygen reduction reaction using ferric chloride both as an oxidant and iron source
      Zhang, Jian; He, Daping; Su, Hao; Chen, Xu; Pan, Mu; Mu, Shichun
      Journal of Materials Chemistry A: Materials for Energy and Sustainability (2014), 2 (5), 1242-1246CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
      A nonprecious metal catalyst (NPMC), with nano-porous structure and high BET surface area, was prepd. by pyrolyzing the polyaniline on C nanospheres using ferric chloride both as an oxidant and Fe source. Electrochem. test results show that the catalyst has a high activity and much better stability than that of com. Pt/C in acid medium.
    14. 14
      Wang, S.; Yu, D.; He, X. A facile strategy to fabricate nitrogen-doped graphene aerogel-supported Fe3N nanoparticles as efficient electrocatalysts for the oxygen reduction reaction. New J. Chem. 2017, 41, 1755– 1764,  DOI: 10.1039/C6NJ02679F
      14
      A facile strategy to fabricate nitrogen-doped graphene aerogel-supported Fe3N nanoparticles as efficient electrocatalysts for the oxygen reduction reaction
      Wang, Shufeng; Yu, Dingling; He, Xingquan
      New Journal of Chemistry (2017), 41 (4), 1755-1764CODEN: NJCHE5; ISSN:1144-0546. (Royal Society of Chemistry)
      In this study, a novel hybrid composed of iron nitride and nitrogen-functionalized graphene aerogel (Fe3N/N-GA) was fabricated and used as an electrode material for the oxygen redn. reaction (ORR). The Fe3N/N-GA obtained at the pyrolysis temp. of 900 °C, denoted as Fe3N/N-GA-900, exhibits outstanding catalytic activity towards the ORR compared to other Fe3N/N-GA hybrids, which may be due to the highest abs. content of Fe-N and graphitic-N species in the Fe3N/N-GA-900. Remarkably, the fabricated Fe3N/N-GA-900 hybrid is comparable to the benchmark Pt/C catalyst in terms of the onset potential and half-wave potential, but it possesses a larger kinetic energy, which limits the c.d., and better methanol tolerance and operational stability than those of the com. Pt/C catalyst for the ORR in an alk. medium. Therefore, it holds great promise as a replacement for the Pt/C catalyst in alk. direct methanol fuel cells (DMFCs).
    15. 15
      Chen, Z.-Y.; Li, Y.-N.; Lei, L.-L.; Bao, S.-J.; Wang, M.-Q.; Heng-Liu, H.-L.; Zhao, Z.-L.; Xu, M.-w. Investigation of Fe2N@carbon encapsulated in N-doped graphene-like carbon as a catalyst in sustainable zinc–air batteries. Catal. Sci. Technol. 2017, 7, 5670– 5676,  DOI: 10.1039/C7CY01721A
      15
      Investigation of Fe2N@carbon encapsulated in N-doped graphene-like carbon as a catalyst in sustainable zinc-air batteries
      Chen, Zhao-Yang; Li, Ya-Nan; Lei, Ling-Li; Bao, Shu-Juan; Wang, Min-Qiang; Heng-Liu; Zhao, Zhi-Liang; Xu, Mao-wen
      Catalysis Science & Technology (2017), 7 (23), 5670-5676CODEN: CSTAGD; ISSN:2044-4753. (Royal Society of Chemistry)
      The rational construction of low cost, efficient, and stable oxygen redn. reaction (ORR) electrocatalysts is important for the commercialization of fuel cells and metal-air batteries. In this article, an easy and effective soft-template method is reported to in situ assemble Fe2N nanoparticles on the surface of N-doped graphene-like carbon (NC). The prepd. Fe2N nanoparticles were covered by a few carbon layers, which promoted the connection of Fe-NX clusters with graphene to facilitate the formation of Fe-N-C active sites. Fe-NX and NC units were found to resp. fulfill different functionalities and commonly afford the sample with excellent performance. The electrochem. data show that the Fe2N@NC composite with high-purity and good cryst. displays a synergistic enhanced catalytic activity for ORR, including a pos. onset potential (0.084 V), a half-wave potential (-0.036 V) and a high electron transfer no. (∼4e-), as compared to 20% Pt/C. Addnl., the existence of carbon shells wrapped around Fe2N nanoparticles can restrain their expansion and dissoln. In addn., the as-prepd. catalyst was implemented as an air catalyst for zinc-air batteries and was found to display a comparable open circuit voltage of ca. 1.48 V and a max. power d. of 82.3 mW cm-2. These results demonstrate that the Fe2N@NC catalyst may serve as a good alternative to precious Pt for ORR in practical applications.
    16. 16
      Park, M.; Lee, J.; Hembram, K.; Lee, K.-R.; Han, S.; Yoon, C.; Nam, S.-W.; Kim, J. Oxygen Reduction Electrocatalysts Based on Coupled Iron Nitride Nanoparticles with Nitrogen-Doped Carbon. Catalysts 2016, 6, 86,  DOI: 10.3390/catal6060086
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      Qin, Y.; Li, J.; Yuan, J.; Kong, Y.; Tao, Y.; Lin, F.; Li, S. Hollow mesoporous carbon nitride nanosphere/three-dimensional graphene composite as high efficient electrocatalyst for oxygen reduction reaction. J. Power Sources 2014, 272, 696– 702,  DOI: 10.1016/j.jpowsour.2014.09.017
      17
      Hollow mesoporous carbon nitride nanosphere/three-dimensional graphene composite as high efficient electrocatalyst for oxygen reduction reaction
      Qin, Yong; Li, Juan; Yuan, Jie; Kong, Yong; Tao, Yongxin; Lin, Furong; Li, Shan
      Journal of Power Sources (2014), 272 (), 696-702CODEN: JPSODZ; ISSN:0378-7753. (Elsevier B.V.)
      Hollow mesoporous carbon nitride nanosphere (HMCN) is prepd. via an etching route using hollow mesoporous silica as a sacrificial template. The as-obtained HMCN is a uniform spherical particle with a diam. of ∼300 nm, and possesses a high sp. surface area up to 439 m2/g. Hollow mesoporous carbon nitride nanosphere/three-dimensional graphene composite (HMCN-G) is subsequently fabricated via a hydrothermal treatment of HMCN with graphene oxide. As an electrocatalyst for oxygen redn. reaction, the HMCN-G shows significantly enhanced electrocatalytic activity compared to bulk graphitic carbon nitride (g-C3N4) and HMCN in terms of the electron-transfer no., c.d. and onset potential. Increased d. of catalytically active sites and improved accessibility to electrolyte enabled by the hollow and mesoporous architecture of HMCN, and high cond. induced from graphene are considered to contribute to the remarkable electrocatalytic performance of the HMCN-G. Furthermore, HMCN-G exhibits superior methanol tolerance to Pt/C catalyst, suggesting that it is a promising metal-free electrocatalyst for polymer electrolyte membrane fuel cell.
    18. 18
      Zheng, Y.; Jiao, Y.; Jaroniec, M.; Jin, Y.; Qiao, S. Z. Nanostructured Metal-Free Electrochemical Catalysts for Highly Efficient Oxygen Reduction. Small 2012, 8, 3550– 3566,  DOI: 10.1002/smll.201200861
      18
      Nanostructured Metal-Free Electrochemical Catalysts for Highly Efficient Oxygen Reduction
      Zheng, Yao; Jiao, Yan; Jaroniec, Mietek; Jin, Yonggang; Qiao, Shi Zhang
      Small (2012), 8 (23), 3550-3566CODEN: SMALBC; ISSN:1613-6810. (Wiley-VCH Verlag GmbH & Co. KGaA)
      A review. Replacing precious and nondurable Pt catalysts with cheap and com. available materials to facilitate sluggish cathodic oxygen redn. reaction (ORR) is a key issue in the development of fuel cell technol. The recently developed cost effective and highly stable metal-free catalysts reveal comparable catalytic activity and significantly better fuel tolerance than that of current Pt-based catalysts; therefore, they can serve as feasible Pt alternatives for the next generation of ORR electrocatalysts. Their promising electrocatalytic properties and acceptable costs greatly promote the R and D of fuel cell technol. This review provides an overview of recent advances in state-of-the-art nanostructured metal-free electrocatalysts including nitrogen-doped carbons, graphitic-carbon nitride (g-C3N4)-based hybrids, and 2-dimensional graphene-based materials. A special emphasis is placed on the mol. design of these electrocatalysts, origin of their electrochem. reactivity, and ORR pathways. Finally, some perspectives are highlighted on the development of more efficient ORR electrocatalysts featuring high stability, low cost, and enhanced performance, which are the key factors to accelerate the commercialization of fuel cell technol.
    19. 19
      Chen, X.; Chang, J.; Yan, H.; Xia, D. Boron Nitride Nanocages as High Activity Electrocatalysts for Oxygen Reduction Reaction: Synergistic Catalysis by Dual Active Sites. J. Phys. Chem. C 2016, 120, 28912– 28916,  DOI: 10.1021/acs.jpcc.6b08560
      19
      Boron Nitride Nanocages as High Activity Electrocatalysts for Oxygen Reduction Reaction: Synergistic Catalysis by Dual Active Sites
      Chen, Xin; Chang, Junbo; Yan, Huijun; Xia, Dingguo
      Journal of Physical Chemistry C (2016), 120 (51), 28912-28916CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      The O redn. reaction (ORR) catalytic mechanism and activity on B12N12 and B60N60 nanocages were studied in detail by d. functional theory methods. The calcd. results indicate that all the adsorption energies of ORR intermediates on B12N12 are close to those known for the Pt(111) catalyst, implying that it can be an effective catalyst for the ORR, with catalytic properties similar to Pt. A relative energy profile suggests that the ORR process could spontaneously take place on the studied two BN nanocages, with a four-electron redn. mechanism. More importantly, during the entire redn. process, the BN nanocages can provide dual-catalytic sites, esp. in the 2nd and 3rd H transfer step, further accelerating the ORR pathways. Thus, the synergistic catalytic effect between B and N atoms is considerable in BN nanocages.
    20. 20
      Uosaki, K.; Elumalai, G.; Noguchi, H.; Masuda, T.; Lyalin, A.; Nakayama, A.; Taketsugu, T. Boron nitride nanosheet on gold as an electrocatalyst for oxygen reduction reaction: theoretical suggestion and experimental proof. J. Am. Chem. Soc. 2014, 136, 6542– 6525,  DOI: 10.1021/ja500393g
      20
      Boron Nitride Nanosheet on Gold as an Electrocatalyst for Oxygen Reduction Reaction: Theoretical Suggestion and Experimental Proof
      Uosaki, Kohei; Elumalai, Ganesan; Noguchi, Hidenori; Masuda, Takuya; Lyalin, Andrey; Nakayama, Akira; Taketsugu, Tetsuya
      Journal of the American Chemical Society (2014), 136 (18), 6542-6545CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Boron nitride (BN), which is an insulator with a wide band gap, supported on Au is theor. suggested and exptl. proved to act as an electrocatalyst for oxygen redn. reaction (ORR). D.-functional theory calcns. show that the band gap of a free h-BN monolayer is 4.6 eV but a slight protrusion of the unoccupied BN states toward the Fermi level is obsd. if BN is supported on Au(111) due to the BN-Au interaction. A theor. predicted metastable configuration of O2 on h-BN/Au(111), which can serve as precursors for ORR, and free energy diagrams for ORR on h-BN/Au(111) via two- and four-electron pathways show that ORR to H2O2 is possible at this electrode. It is exptl. proved that overpotential for ORR at the gold electrode is significantly reduced by depositing BN nanosheets. No such effect is obsd. at the glassy carbon electrode, demonstrating the importance of BN-substrate interaction for h-BN to act as the ORR electrocatalyst. A possible role of the edge of the BN islands for ORR is also discussed.
    21. 21
      Alexeyeva, N.; Shulga, E.; Kisand, V.; Kink, I.; Tammeveski, K. Electroreduction of oxygen on nitrogen-doped carbon nanotube modified glassy carbon electrodes in acid and alkaline solutions. J. Electroanal. Chem. 2010, 648, 169– 175,  DOI: 10.1016/j.jelechem.2010.07.014
      21
      Electroreduction of oxygen on nitrogen-doped carbon nanotube modified glassy carbon electrodes in acid and alkaline solutions
      Alexeyeva, N.; Shulga, E.; Kisand, V.; Kink, I.; Tammeveski, K.
      Journal of Electroanalytical Chemistry (2010), 648 (2), 169-175CODEN: JECHES; ISSN:1572-6657. (Elsevier B.V.)
      The electrochem. redn. of oxygen was studied on vertically aligned N-doped carbon nanotube (NCNT) modified glassy carbon (GC) electrodes in 0.5 M H2SO4 and in 0.1 M KOH solns. using the rotating disk electrode (RDE) method. For comparison purposes, the oxygen redn. behavior of undoped carbon nanotube material has been also investigated. Both catalysts were prepd. by chem. vapor deposition (CVD). Acetonitrile was used as the precursor in the synthesis of the NCNT material. The surface morphol. and chem. compn. of the NCNT catalysts were studied by SEM and XPS. The NCNT/GC electrodes showed a significant enhancement of the kinetics of oxygen redn. in both solns. In acid media the half-wave potential of O2 redn. on NCNT-modified electrodes shifted by 250 mV to more pos. potentials as compared to that of vertically aligned undoped CNT materials. The factors that det. the high electrocatalytic activity of nitrogen-doped carbon nanotubes towards oxygen redn. are discussed.
    22. 22
      Chen, L.; Cui, X.; Wang, Y.; Wang, M.; Qiu, R.; Shu, Z.; Zhang, L.; Hua, Z.; Cui, F.; Wei, C.; Shi, J. One-step synthesis of sulfur doped graphene foam for oxygen reduction reactions. Dalton Trans. 2014, 43, 3420– 3423,  DOI: 10.1039/c3dt52253a
      There is no corresponding record for this reference.
    23. 23
      Jiang, S.; Ma, Y.; Jian, G.; Tao, H.; Wang, X.; Fan, Y.; Lu, Y.; Hu, Z.; Chen, Y. Facile Construction of Pt-Co/CNx Nanotube Electrocatalysts and Their Application to the Oxygen Reduction Reaction. Adv. Mater. 2009, 21, 4953– 4956,  DOI: 10.1002/adma.200900677
      23
      Facile Construction of Pt-Co/CNx Nanotube Electrocatalysts and Their Application to the Oxygen Reduction Reaction
      Jiang, Shujuan; Ma, Yanwen; Jian, Guoqiang; Tao, Haisheng; Wang, Xizhang; Fan, Yining; Lu, Yinong; Hu, Zheng; Chen, Yi
      Advanced Materials (Weinheim, Germany) (2009), 21 (48), 4953-4956CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Taking advantage of the inherent chem. activity of CNx nanotubes that arise from nitrogen incorporation, a facile strategy was developed for the construction of Pt-Co/CNx electrocatalysts. Pt-based alloyed nanoparticles were highly and homogeneously dispersed on CNx nanotubes with a size about 3 nm. Compared with the com. Pt/C and the monometallic Pt/CNx catalysts, the binary Pt-Co/CNx catalysts show much higher electrocatalytic activities and similar stabilities for oxygen redn. in acidic electrolyte but with much less consumption of precious Pt. The good performance of the catalysts mainly arise from the high dispersion of Pt-based species, the alloying effect of Pt-Co and the intrinsic catalytic capacity of CNx nanotubes with high cond.
    24. 24
      Li, M.; Zhang, L.; Xu, Q.; Niu, J.; Xia, Z. N-doped graphene as catalysts for oxygen reduction and oxygen evolution reactions: Theoretical considerations. J. Catal. 2014, 314, 66– 72,  DOI: 10.1016/j.jcat.2014.03.011
      24
      N-doped graphene as catalysts for oxygen reduction and oxygen evolution reactions: Theoretical considerations
      Li, Mingtao; Zhang, Lipeng; Xu, Quan; Niu, Jianbing; Xia, Zhenhai
      Journal of Catalysis (2014), 314 (), 66-72CODEN: JCTLA5; ISSN:0021-9517. (Elsevier Inc.)
      Electrocatalysts are essential to two key electrochem. reactions, oxygen evolution reaction (OER) and oxygen redn. reaction (ORR) in renewable energy conversion and storage technologies such as regenerative fuel cells and rechargeable metal-air batteries. Here, we explored N-doped graphene as cost-effective electrocatalysts for these key reactions by employing d. functional theory (DFT). The results show that the substitution of carbon at graphene edge by nitrogen results in the best performance in terms of overpotentials. For armchair nanoribbons, the lowest OER and ORR overpotentials were estd. to be 0.405 V and 0.445 V, resp., which are comparable to those for Pt-contg. catalysts. OER and ORR with the min. overpotentials can occur near the edge on the same structure but different sites. These calcns. suggest that engineering the edge structures of the graphene can increase the efficiency of the N-doped graphene as efficient OER/ORR electrocatalysts for metal-air batteries, water splitting, and regenerative fuel cells.
    25. 25
      Qu, L.; Liu, Y.; Baek, J.-B.; Dai, L. Nitrogen-Doped Graphene as Efficient Metal-Free Electrocatalyst for Oxygen Reduction in Fuel Cells. ACS Nano 2010, 4, 1321– 1326,  DOI: 10.1021/nn901850u
      25
      Nitrogen-Doped Graphene as Efficient Metal-Free Electrocatalyst for Oxygen Reduction in Fuel Cells
      Qu, Liangti; Liu, Yong; Baek, Jong-Beom; Dai, Liming
      ACS Nano (2010), 4 (3), 1321-1326CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      Nitrogen-doped graphene (N-graphene) was synthesized by CVD of methane in the presence of ammonia. The resultant N-graphene was demonstrated to act as a metal-free electrode with a much better electrocatalytic activity, long-term operation stability, and tolerance to crossover effect than platinum for oxygen redn. via a four-electron pathway in alk. fuel cells. This is the 1st report on the use of graphene and its derivs. as metal-free catalysts for oxygen redn. The important role of N-doping to oxygen redn. reaction (ORR) can be applied to various carbon materials for the development of other metal-free efficient ORR catalysts for fuel cell applications, even new catalytic materials for applications beyond fuel cells.
    26. 26
      Yang, Z.; Yao, Z.; Li, G.; Fang, G.; Nie, H.; Liu, Z.; Zhou, X.; Chen, X.; Huang, S. Sulfur-Doped Graphene as an Efficient Metal-free Cathode Catalyst for Oxygen Reduction. ACS Nano 2012, 6, 205– 211,  DOI: 10.1021/nn203393d
      26
      Sulfur-Doped Graphene as an Efficient Metal-free Cathode Catalyst for Oxygen Reduction
      Yang, Zhi; Yao, Zhen; Li, Guifa; Fang, Guoyong; Nie, Huagui; Liu, Zheng; Zhou, Xuemei; Chen, Xi'an; Huang, Shaoming
      ACS Nano (2012), 6 (1), 205-211CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      Tailoring the electronic arrangement of graphene by doping is a practical strategy for producing significantly improved materials for the O-redn. reaction (ORR) in fuel cells (FCs). Recent studies proved that the C materials doped with the elements, which have the larger (N) or smaller (P, B) electroneg. atoms than C such as N-doped C nanotubes (CNTs), P-doped graphite layers and B-doped CNTs, also showed pronounced catalytic activity. Herein, the graphenes doped with the elements, which have the similar electronegativity with C such as S and Se, can also exhibit better catalytic activity than the com. Pt/C in alk. media, indicating that these doped graphenes hold great potential for a substitute for Pt-based catalysts in FCs. The exptl. results are believed to be significant because they not only give further insight into the ORR mechanism of these metal-free doped C materials, but also open a way to fabricate other new low-cost NPMCs with high electrocatalytic activity by a simple, economical, and scalable approach for real FC applications.
    27. 27
      Wong, W. Y.; Daud, W. R. W.; Mohamad, A. B.; Kadhum, A. A. H.; Loh, K. S.; Majlan, E. H. Recent progress in nitrogen-doped carbon and its composites as electrocatalysts for fuel cell applications. Int. J. Hydrogen Energy 2013, 38, 9370– 9386,  DOI: 10.1016/j.ijhydene.2012.12.095
      27
      Recent progress in nitrogen-doped carbon and its composites as electrocatalysts for fuel cell applications
      Wong, W. Y.; Daud, W. R. W.; Mohamad, A. B.; Kadhum, A. A. H.; Loh, K. S.; Majlan, E. H.
      International Journal of Hydrogen Energy (2013), 38 (22), 9370-9386CODEN: IJHEDX; ISSN:0360-3199. (Elsevier Ltd.)
      A review. The emergence of fuel cell technol. has created a new tool for the generation of clean, high efficiency alternative energy for humans. The research and development of new catalysts to replace the expensive and rare platinum (Pt) to reduce the overall cost of fuel cells is ongoing in this area. Nitrogen-doped carbon and its composites possess great potential for fuel cell catalyst applications esp. at the oxygen redn. cathode. It is proposed that the reaction mechanisms of nitrogen-doped carbon catalysts for oxygen redn. involve adsorption of oxygen at the partially polarized carbon atoms adjacent to the nitrogen dopants, different from the mechanism at platinum catalysts, which utilize d-bands filling at oxygen adsorption sites. Nitrogen doping in both carbon nanostructures and its composites with active metals or ceramics are reviewed. Nitrogen-doped carbon without composite metals, displays high catalytic activity in alk. fuel cells and exhibits significant activity in proton exchange membrane fuel cells and direct methanol fuel cells. Pt-based catalysts with nitrogen-doped carbon supports show enhanced catalytic activity towards oxygen redn., attributed to the enhanced anchoring of Pt to the support that results in better dispersion and stability of the electrodes. For nitrogen-doped carbon composites with non-noble metals (Fe, Co, etc), enhanced activity is seen in both proton exchange and alk. fuel cells. There are many ongoing debates about the nature of nitrogen-carbon bond in catalysis. Pyrrole- and pyridinic-type nitrogen generally considered to be responsible for the catalytic sites in acidic and alk. media, resp. In recent years, significant efforts have been made towards increasing the stability of nitrogen-doped carbon catalysts in acidic media through the formation of composites with ceramic or metal oxide materials. This article reviews the progress in the area of this new class of catalysts and their composites for greater enhancement of oxygen redn. activity and stability in various fuel cell applications.
    28. 28
      Wang, T.; Chen, Z.-X.; Chen, Y.-G.; Yang, L.-J.; Yang, X.-D.; Ye, J.-Y.; Xia, H.-P.; Zhou, Z.-Y.; Sun, S.-G. Identifying the Active Site of N-Doped Graphene for Oxygen Reduction by Selective Chemical Modification. ACS Energy Lett. 2018, 3, 986– 991,  DOI: 10.1021/acsenergylett.8b00258
      28
      Identifying the Active Site of N-Doped Graphene for Oxygen Reduction by Selective Chemical Modification
      Wang, Tao; Chen, Zhi-Xin; Chen, Yu-Gang; Yang, Li-Jun; Yang, Xiao-Dong; Ye, Jin-Yu; Xia, Hai-Ping; Zhou, Zhi-You; Sun, Shi-Gang
      ACS Energy Letters (2018), 3 (4), 986-991CODEN: AELCCP; ISSN:2380-8195. (American Chemical Society)
      N-doped carbon materials are promising electrocatalysts for oxygen redn. reaction (ORR). However, the lack of knowledge in the nature of active sites limits the rational design of this type of catalysts. Although pyridinic N species are proposed to be active for ORR, little exptl. evidence is provided to reveal the reactive sites. Herein, a surface-modification method is developed to identify the ortho-carbon atom of the pyridinic ring as the reactive site for ORR on N-doped graphene. The pyridinic ring of N-doped graphene was selectively grafted by an acetyl group at pyridinic N and ortho-C atoms by electrophilic and radical substitution, resp. The former remained most of ORR catalytic activity, while the latter lost its activity completely. DFT calcns. confirm that O2 can get adsorbed and reduced favorably on the ortho-C atom of the pyridinic ring. This study provides new insight into the nature of active sites and the ORR mechanism for N-doped carbon materials.
    29. 29
      Zhang, L.; Niu, J.; Dai, L.; Xia, Z. Effect of microstructure of nitrogen-doped graphene on oxygen reduction activity in fuel cells. Langmuir 2012, 28, 7542– 7550,  DOI: 10.1021/la2043262
      29
      Effect of Microstructure of Nitrogen-Doped Graphene on Oxygen Reduction Activity in Fuel Cells
      Zhang, Lipeng; Niu, Jianbing; Dai, Liming; Xia, Zhenhai
      Langmuir (2012), 28 (19), 7542-7550CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
      The development of fuel cells as clean-energy technol. is limited by the cost of the noble-metal catalysts needed for catalyzing the O redn. reaction (ORR) in fuel cells. A fundamental understanding of catalyst design principle that links material structure to the catalytic activity can accelerate the search for highly active and abundant non-metal catalysts to replace Pt. Here, the authors present a 1st-principles study of ORR on N-doped graphene in an acidic environment. The ORR activity primarily correlates to charge and spin densities of the graphene. The N doping and defects introduce high pos. spin and/or charge densities that facilitate the ORR on graphene surface. The identified active sites are closely related to doping cluster size and dopant-defect interactions. Generally speaking, a large doping cluster size (no. of N atoms >2) reduces the no. of catalytic active sites per N atom. In combination with N clustering, Stone-Wales defects can strongly promote ORR. For 4-electron transfer, the effective reversible potential ranges from 1.04 to 1.15 V/SHE, depending on the defects and cluster size. The catalytic properties of graphene could be optimized by introducing small N clusters in combination with material defects.
    30. 30
      Zhang, L.; Xia, Z. Mechanisms of Oxygen Reduction Reaction on Nitrogen-Doped Graphene for Fuel Cells. J. Phys. Chem. C 2011, 115, 11170– 11176,  DOI: 10.1021/jp201991j
      30
      Mechanisms of Oxygen Reduction Reaction on Nitrogen-Doped Graphene for Fuel Cells
      Zhang, Lipeng; Xia, Zhenhai
      Journal of Physical Chemistry C (2011), 115 (22), 11170-11176CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      Graphene and its derivs. are attractive for electrocatalytic application in fuel cells because of their unique structures and electronic properties. The electrocatalytic mechanism of nitrogen-doped graphene in acidic environment was studied by using d. functional theory. The simulations demonstrate that the oxygen redn. reaction (ORR) on nitrogen-doped graphene is a direct four-electron pathway, which is consistent with the exptl. observations. The energy calcd. for each ORR step shows that the ORR can spontaneously occur on the nitrogen-doped graphene. The active catalytic sites on single nitrogen-doped graphene are identified, which have either high pos. spin d. or high pos. at. charge d. The nitrogen doping introduces asymmetry spin d. and at. charge d., making it possible for nitrogen-doped graphene to show high electrocatalytic activities for the ORR.
    31. 31
      Ma, J.; Habrioux, A.; Luo, Y.; Ramos-Sanchez, G.; Calvillo, L.; Granozzi, G.; Balbuena, P. B.; Alonso-Vante, N. Electronic interaction between platinum nanoparticles and nitrogen-doped reduced graphene oxide: effect on the oxygen reduction reaction. J. Mater. Chem. A 2015, 3, 11891– 11904,  DOI: 10.1039/C5TA01285F
      31
      Electronic interaction between platinum nanoparticles and nitrogen-doped reduced graphene oxide: effect on the oxygen reduction reaction
      Ma, Jiwei; Habrioux, Aurelien; Luo, Yun; Ramos-Sanchez, Guadalupe; Calvillo, Laura; Granozzi, Gaetano; Balbuena, Perla B.; Alonso-Vante, Nicolas
      Journal of Materials Chemistry A: Materials for Energy and Sustainability (2015), 3 (22), 11891-11904CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
      Low-mass loadings (∼5%) Pt/C catalysts were synthesized using the carbonyl chem. route allowing for the heterogeneous deposition of Pt nanoparticles on different C-based substrates. N-doped reduced graphene oxide, reduced graphene oxide, graphene oxide, graphite and Vulcan XC-72 were used for the heterogeneous deposition of Pt nanoparticles. The effect of the chem. nature of the C-based substrate on the O Redn. Reaction (ORR) kinetics at Pt nanoparticles surfaces was studied. XPS results show that using N-doped reduced graphene oxide materials for the deposition of Pt nanoparticles gives Pt-N chem. bonds. This interaction between Pt and N allows for an electronic transfer from Pt to the C support. Ca. 25% of the total amt. of N atoms were bound to Pt ones. This chem. bond also revealed by the DFT anal., induces changes in the O adsorption energy at the Pt surface, engendering an enhancement of the catalyst activity towards ORR. In comparison with Vulcan XC-72, the mass activity at 0.9 V vs. RHE is 2.1 fold higher when N-doped reduced graphene oxide was used as substrate. In conjunction with the exptl. results, DFT calcns. describe the interaction between supported Pt clusters and O where the support was modeled accordingly with the C-based materials used as substrate. The presence of N-species in the support although leading to a weaker O2 adsorption, induces elongated O-O distances suggesting facilitated dissocn. Addnl., the strong interaction between Pt clusters and N-contg. substrates leads to very slight changes of the cluster-substrate distance even when O is adsorbed at the interfacial region, thus leading to a lower resistance for electron charge transfer and enabling electrochem. reactions.
    32. 32
      Tao, L.; Dou, S.; Ma, Z.; Shen, A.; Wang, S. Simultaneous Pt deposition and nitrogen doping of graphene as efficient and durable electrocatalysts for methanol oxidation. Int. J. Hydrogen Energy 2015, 40, 14371– 14377,  DOI: 10.1016/j.ijhydene.2015.02.104
      32
      Simultaneous Pt deposition and nitrogen doping of graphene as efficient and durable electrocatalysts for methanol oxidation
      Tao, Li; Dou, Shuo; Ma, Zhaoling; Shen, Anli; Wang, Shuangyin
      International Journal of Hydrogen Energy (2015), 40 (41), 14371-14377CODEN: IJHEDX; ISSN:0360-3199. (Elsevier Ltd.)
      Nitrogen doping could effectively enhance the catalytical activity of graphene-supported Pt nano-electrocatalysts for methanol oxidn. reaction. Previously, the main strategy to the synthesis of Pt/N-graphene is the two-step reaction while it involves the complicated synthesis. In this work, we describe a facile and simple one-pot reaction including the redn. of graphene oxide, nitrogen doping of graphene, and uniform deposition of Pt nanoparticles on doped graphene. Compared with the Pt/graphene catalyst without nitrogen-doping, Pt/N-graphene exhibits excellent activity and durability towards methanol oxidn. reaction, which is mainly ascribed to the contribution of the improved dispersion of Pt nanoparticles and the enhanced interaction between it and nitrogen-doped graphene. It is expected that this facile, green and economic single-step synthesis approach for the Pt/N-graphene electrocatalyst could be widely utilized to the nitrogen-doped graphene catalysts.
    33. 33
      Zhu, J.; Xiao, M.; Zhao, X.; Li, K.; Liu, C.; Xing, W. Nitrogen-doped carbon-graphene composites enhance the electrocatalytic performance of the supported Pt catalysts for methanol oxidation. Chem. Commun. 2014, 50, 12201– 12203,  DOI: 10.1039/C4CC04887C
      There is no corresponding record for this reference.
    34. 34
      Zheng, B.; Wang, J.; Wang, F.-B.; Xia, X.-H. Synthesis of nitrogen doped graphene with high electrocatalytic activity toward oxygen reduction reaction. Electrochem. Commun. 2013, 28, 24– 26,  DOI: 10.1016/j.elecom.2012.11.037
      34
      Synthesis of nitrogen doped graphene with high electrocatalytic activity toward oxygen reduction reaction
      Zheng, Bo; Wang, Jiong; Wang, Feng-Bin; Xia, Xing-Hua
      Electrochemistry Communications (2013), 28 (), 24-26CODEN: ECCMF9; ISSN:1388-2481. (Elsevier B.V.)
      A novel strategy for fabricating N doped graphene sheets was developed using graphite oxide as the C source and urea as the N source via hydrothermal approach. This method allows the authors to obtain high doping level of N in graphene. The doped N mainly exists as pyridinic and pyrrolic N bonding configurations. Subsequent thermal annealing will transfer the pyrrolic N to graphitic N significantly. Electrochem. results demonstrate that larger amt. of graphitic N configuration may play an active role in the excellent electrocatalytic activity toward O redn. reaction (ORR) in alk. electrolyte.
    35. 35
      Li, X.; Wang, H.; Robinson, J. T.; Sanchez, H.; Diankov, G.; Dai, H. Simultaneous Nitrogen Doping and Reduction of Graphene Oxide. J. Am. Chem. Soc. 2009, 131, 15939– 15944,  DOI: 10.1021/ja907098f
      35
      Simultaneous Nitrogen Doping and Reduction of Graphene Oxide
      Li, Xiaolin; Wang, Hailiang; Robinson, Joshua T.; Sanchez, Hernan; Diankov, Georgi; Dai, Hongjie
      Journal of the American Chemical Society (2009), 131 (43), 15939-15944CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      We developed a simple chem. method to obtain bulk quantities of N-doped, reduced graphene oxide (GO) sheets through thermal annealing of GO in ammonia. XPS study of GO sheets annealed at various reaction temps. reveals that N-doping occurs at a temp. as low as 300°C, while the highest doping level of ∼5% N is achieved at 500°C. N-doping is accompanied by the redn. of GO with decreases in oxygen levels from ∼28% in as-made GO down to ∼2% in 1100 °C NH3-reacted GO. XPS anal. of the N binding configurations of doped GO finds pyridinic N in the doped samples, with increased quaternary N (N that replaced the carbon atoms in the graphene plane) in GO annealed at higher temps. (≥900°C). Oxygen groups in GO were found responsible for reactions with NH3 and C-N bond formation. Prereduced GO with fewer oxygen groups by thermal annealing in H2 exhibits greatly reduced reactivity with NH3 and a lower N-doping level. Elec. measurements of individual GO sheet devices demonstrate that GO annealed in NH3 exhibits higher cond. than those annealed in H2, suggesting more effective redn. of GO by annealing in NH3 than in H2, consistent with XPS data. The N-doped reduced GO shows clearly n-type electron doping behavior with the Dirac point (DP) at neg. gate voltages in three terminal devices. Our method could lead to the synthesis of bulk amts. of N-doped, reduced GO sheets useful for various practical applications.
    36. 36
      Suzuki, K.; Morita, H.; Kaneko, T.; Yoshida, H.; Fujimori, H. Crystal structure and magnetic properties of the compound FeN. J. Alloys Compd. 1993, 201, 11– 16,  DOI: 10.1016/0925-8388(93)90854-G
      36
      Crystal structure and magnetic properties of the compound iron nitride (FeN)
      Suzuki, K.; Morita, H.; Kaneko, T.; Yoshida, H.; Fujimori, H.
      Journal of Alloys and Compounds (1993), 201 (1-2), 11-16CODEN: JALCEU; ISSN:0925-8388.
      FeN was prepd. as a single phase by d.c. reactive sputtering. Its crystal structure was detd. by x-ray diffraction measurements to be the Zn blende type fcc. structure. FeN is stable at ≤593 K and decomps. into FeN + Fe2N at >593 K. The present sample exhibits a mictomagnetic character at low temps., which is considered to be related to the antiferromagnetism of the FeN compd.
    37. 37
      Zhang, H.; Gong, Q.; Ren, S.; Arshid, M. A.; Chu, W.; Chen, C. Implication of iron nitride species to enhance the catalytic activity and stability of carbon nanotubes supported Fe catalysts for carbon-free hydrogen production via low-temperature ammonia decomposition. Catal. Sci. Technol. 2018, 8, 907– 915,  DOI: 10.1039/C7CY02270K
      37
      Implication of iron nitride species to enhance the catalytic activity and stability of carbon nanotubes supported Fe catalysts for carbon-free hydrogen production via low-temperature ammonia decomposition
      Zhang, Hui; Gong, Qinmei; Ren, Shan; Arshid, Mahmood Ali; Chu, Wei; Chen, Chen
      Catalysis Science & Technology (2018), 8 (3), 907-915CODEN: CSTAGD; ISSN:2044-4753. (Royal Society of Chemistry)
      This study was aimed to boost the catalytic ability of C nanotubes (CNTs) supported Fe-based catalysts, prepd. using wet-impregnation and followed by nitrogenization, for C-free H prodn. from NH3 decompn. at a low temp. The nitrogenization temp. and Fe loading had significant effects on the size of the well-dispersed Fe2N crystallites. The Fe3O4/CNTs catalysts at a higher nitrogenation temp. under NH3 flow with a suitable Fe content led to the formation of the stable and uniformly distributed Fe2N species, which played an active role in the enhanced catalytic ability of the Fe3O4/CNTs catalysts. However, the nitrogenization of the Fe3O4/CNTs catalyst under either H or Ar led to the formation of the Fe4N and Fe2N species. In the presence of the Fe4N phases, the Fe3O4/CNTs catalyst exhibited an enhanced catalytic activity. The collaborative interaction of the active site Fe2N and carbon nanotubes in the Fe2N/CNTs catalysts resulted in a significant increase in the catalytic activity and durability ≤40 h. The effective control of the d. of the active sites Fe2N and the synergism between the carrier and the crystallite compn. of Fe nitrides are the key aspects for the efficient design of the transition nitride catalysts for carbon-free H prodn. via ammonia decompn.
    38. 38
      Yin, H.; Zhang, C.; Liu, F.; Hou, Y. Hybrid of Iron Nitride and Nitrogen-Doped Graphene Aerogel as Synergistic Catalyst for Oxygen Reduction Reaction. Adv. Funct. Mater. 2014, 24, 2930– 2937,  DOI: 10.1002/adfm.201303902
      38
      Hybrid of Iron Nitride and Nitrogen-Doped Graphene Aerogel as Synergistic Catalyst for Oxygen Reduction Reaction
      Yin, Han; Zhang, Chenzhen; Liu, Fei; Hou, Yanglong
      Advanced Functional Materials (2014), 24 (20), 2930-2937CODEN: AFMDC6; ISSN:1616-301X. (Wiley-VCH Verlag GmbH & Co. KGaA)
      It is extremely desirable but challenging to create highly active, stable, and low-cost catalysts towards O redn. reaction to replace Pt-based catalysts to perform the commercialization of fuel cells. Here, a novel Fe nitride/N doped-graphene aerogel hybrid, synthesized by a facile 2-step hydrothermal process, in which Fe phthalocyanine is uniformly dispersed and anchored on graphene surface with the assist of π-π stacking and O-contg. functional groups, is reported. As a result, there exist strong interactions between FexN nanoparticles and graphene substrates, leading to a synergistic effect towards O redn. reaction. It is worth noting that the onset potential and c.d. of the hybrid are significantly better and the charge transfer resistance is much lower than that of pure N-doped graphene aerogel, free FexN and their phys. mixts. The hybrid also exhibits comparable catalytic activity as com. Pt/C at the same catalyst loading, while its stability and resistance to MeOH crossover are superior. Apart from the active nature of the hybrid, the large surface area and porosity are responsible for its excellent onset potential and the high d. of Fe-N-C sties and small size of FexN particles boost charge transfer rate.
    39. 39
      Hassan, F. M.; Chabot, V.; Li, J.; Kim, B. K.; Ricardez-Sandoval, L.; Yu, A. Pyrrolic-structure enriched nitrogen doped graphene for highly efficient next generation supercapacitors. J. Mater. Chem. A 2013, 1, 2904,  DOI: 10.1039/c2ta01064j
      39
      Pyrrolic-structure enriched nitrogen doped graphene for highly efficient next generation supercapacitors
      Hassan, Fathy M.; Chabot, Victor; Li, Jingde; Kim, Brian Kihun; Ricardez-Sandoval, Luis; Yu, Aiping
      Journal of Materials Chemistry A: Materials for Energy and Sustainability (2013), 1 (8), 2904-2912CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
      This study reports the prepn. of pyrrolic-structure enriched N doped graphene by hydrothermal synthesis at varied temp. The morphol., structure and compn. of the prepd. N doped graphene were confirmed with SEM, XRD, XPS and Raman spectroscopy. The material was tested for supercapacitive behavior. Doping graphene with N increased the elec. double layer supercapacitance to ≤194 F g-1. Also, d. functional theory (DFT) calcns. showed the proper level of binding energy found between the pyrrolic-N structure and the electrolyte ions, which may be used to explain the highest contribution of the pyrrolic-structure to the capacitance.
    40. 40
      Ma, J.-h.; Wang, L.; Mu, X.; Li, L. Nitrogen-doped graphene supported Pt nanoparticles with enhanced performance for methanol oxidation. Int. J. Hydrogen Energy 2015, 40, 2641– 2647,  DOI: 10.1016/j.ijhydene.2014.12.080
      40
      Nitrogen-doped graphene supported Pt nanoparticles with enhanced performance for methanol oxidation
      Ma, Jun-hong; Wang, Li; Mu, Xue; Li, Li
      International Journal of Hydrogen Energy (2015), 40 (6), 2641-2647CODEN: IJHEDX; ISSN:0360-3199. (Elsevier Ltd.)
      A series of nitrogen-doped graphene nanocomposites (N-RGO) derived from pyrolysis of graphene oxide (GO)/polyaniline composites with varied mass ratio between GO and aniline were used as support for immobilization of Pt nanoparticles. Transmission electron microscopy, X-ray powder diffraction, Raman spectroscopy and XPS were used to characterize the morphol. and microstructure of the prepd. catalysts. The catalytic activity of the catalysts towards the oxidn. of methanol was evaluated by cyclic voltammetry. Compared to the undoped Pt/RGO catalyst, an enhanced electrocatalytic activity for methanol oxidn. was obtained in the case of the Pt/N-RGO with optimized compn. and nanostructure.
    41. 41
      Choi, C. H.; Park, S. H.; Woo, S. I. Facile growth of N-doped CNTs on Vulcan carbon and the effects of iron content on electrochemical activity for oxygen reduction reaction. Int. J. Hydrogen Energy 2012, 37, 4563– 4570,  DOI: 10.1016/j.ijhydene.2011.08.086
      41
      Facile growth of N-doped CNTs on Vulcan carbon and the effects of iron content on electrochemical activity for oxygen reduction reaction
      Choi, Chang Hyuck; Park, Sung Hyeon; Woo, Seong Ihl
      International Journal of Hydrogen Energy (2012), 37 (5), 4563-4570CODEN: IJHEDX; ISSN:0360-3199. (Elsevier Ltd.)
      In order to develop cheap electrochem. oxygen redn. reaction (ORR) catalysts, N-doped CNTs grafted on Vulcan carbon were synthesized via pyrolysis of dicyandiamide on Fe2O3/C. Various contents of iron in Fe2O3/C (0, 20, 40 and 60 wt. %) were used as supports to investigate the effects and roles of iron content on ORR. It was shown that the iron acted as a promoter for doping nitrogen into carbon; as the iron content increased, the amt. of nitrogen doping also increased. TEM and element anal. results indicated that iron induced growth of CNTs and facilitated N-doping in carbon. However, further increase in iron content higher than 20 wt. % showed neg. effects on the ORR activity due to a decrease of the surface area of the prepd. catalysts. Hence, the catalyst with the highest performance was obsd. when dicyandiamide was pyrolyzed with Fe2O3/C 20 wt. % (Fe-N-C-20) and the order of activity towards ORR was Fe-N-C-20 > Fe-N-C-40 > Fe-N-C-60 > Fe-N-C-0 > Vulcan XC-72R.
    42. 42
      Zhang, Y.; Xie, Y.; Zhou, Y.; Wang, X.; Pan, K. Well dispersed Fe2N nanoparticles on surface of nitrogen-doped reduced graphite oxide for highly efficient electrochemical hydrogen evolution. J. Mater. Res. 2017, 32, 1770– 1776,  DOI: 10.1557/jmr.2017.138
      42
      Well dispersed Fe2N nanoparticles on surface of nitrogen-doped reduced graphite oxide for highly efficient electrochemical hydrogen evolution
      Zhang, Yi; Xie, Ying; Zhou, Yangtao; Wang, Xiuwen; Pan, Kai
      Journal of Materials Research (2017), 32 (9), 1770-1776CODEN: JMREEE; ISSN:2044-5326. (Cambridge University Press)
      It is important to fabricate iron-based nitride/conductive material composite to obtain good catalytic performance. In this work, Fe2N nanoparticles with diam. of approx. 30 nm have been successfully dispersed on the surface of nitrogen-doped graphite oxide (NrGO) by a facile sol-gel method and further ammonia atm. treatment. XPS, XRD, Raman, and TEM proved that Fe2N nanoparticles are well monodispersed, and nitrogen atoms are doped in NrGO. The composite possessed two merits, i.e., the more catalytic active site in Fe2N nanoparticles due to the well monodispersion, and fast electron transfer due to the nitrogen dope in rGO. With the proper ratio, the composite exhibited brilliant catalytic activity and durability in acidic media. It possesses overpotential of 94 mV to approach 10 mA/cm2, a small Tefel slope of 49 mV/dec, and maintains the good electrocatalytic activity for 10 h. Cyclic voltammetry and electrochem. impedance spectroscopy indicated that the electrocatalyst possessed high catalytic active area and fast electron transfer. Our work may provide a new avenue for the prepn. of low-cost iron-based nitride/NrGO composite for highly efficient electrochem. hydrogen evolution.
    43. 43
      Cui, Q.; Chao, S.; Wang, P.; Bai, Z.; Yan, H.; Wang, K.; Yang, L. Fe–N/C catalysts synthesized by heat-treatment of iron triazine carboxylic acid derivative complex for oxygen reduction reaction. RSC Adv. 2014, 4, 12168,  DOI: 10.1039/c3ra44958k
      43
      Fe-N/C catalysts synthesized by heat-treatment of iron triazine carboxylic acid derivative complex for oxygen reduction reaction
      Cui, Qian; Chao, Shujun; Wang, Panhao; Bai, Zhengyu; Yan, Huiying; Wang, Kui; Yang, Lin
      RSC Advances (2014), 4 (24), 12168-12174CODEN: RSCACL; ISSN:2046-2069. (Royal Society of Chemistry)
      4,4',4''-S-Triazine-1,3,5-triyltri-p-aminobenzoic acid (H3TATAB) was used as a ligand to prep. an iron-TATAB (Fe-TATAB) complex for the development of an effective oxygen redn. reaction (ORR) catalyst. The activity of the catalyst depended on wt. ratios between the Fe-TATAB complex and carbon black and heat-treated temps. The results showed that the Fe-N70%/C-800 catalyst (the wt. ratio of Fe complex to carbon black was 70 : 30 and the catalyst was pyrolyzed at 800 °C) had good catalytic activity toward ORR with the onset potential at 0.91 V vs. RHE and a kinetic c.d. of 4.3 mA cm-2 at 0.6 V vs. RHE in alk. medium. Moreover, the Fe-N70%/C-800 catalyst had better tolerance to methanol crossover effect in comparison with com. Pt/C (20%). The morphol. and compn. of the catalysts were characterized by high resoln. transmission electron microscopy (HRTEM), X-ray diffraction (XRD) as well as X-ray photoelectron spectroscopic (XPS). The electrocatalytic activities were demonstrated by cyclic voltammetry (CV), linear sweep voltammetry (LSV), chronoamperometric measurements and stability accelerated tests. According to rotating disk electrode (RDE) measurements and Koutecky-Levich anal., the overall electron transfer no. in the catalyzed ORR was found to be 3.7-3.9 and the ORR process was mainly a four-electron pathway. The results indicate that the Fe-N70%/C-800 catalyst may be a promising cathode catalyst for ORR.
    44. 44
      Rohith Vinod, K.; Saravanan, P.; Sakar, M.; Balakumar, S. Insights into the nitridation of zero-valent iron nanoparticles for the facile synthesis of iron nitride nanoparticles. RSC Adv. 2016, 6, 45850– 45857,  DOI: 10.1039/C6RA04935D
      44
      Insights into the nitridation of zero-valent iron nanoparticles for the facile synthesis of iron nitride nanoparticles
      Rohith Vinod, K.; Saravanan, P.; Sakar, M.; Balakumar, S.
      RSC Advances (2016), 6 (51), 45850-45857CODEN: RSCACL; ISSN:2046-2069. (Royal Society of Chemistry)
      The process of nitridation of zero-valent iron nanoparticles (ZVINPs) is investigated by employing two different synthesis strategies such as solvothermal method and gas diffusion using N2 and NH3. It is obsd. that the phase formation of different iron nitrides mainly depends on the reaction chem. of the nitridation source and temp. Accordingly, N2 gas diffusion and solvothermal methods yield iron oxide phases, whereas NH3 gas diffusion yields pure iron nitride phase with particle sizes in the nanoscale. X-ray diffraction studies complemented by Rietveld refinement confirm the formation of ε-Fe3N and γ'-Fe4N nanoparticles. Field emission SEM images revealed spherical nanoparticles with av. particle sizes of 35 and 50 nm for ZVINPs and iron nitride NPs, resp. From the magnetization studies carried out using a superconducting quantum interference device magnetometer it is found that the field-dependent hysteresis curves indicated the ferromagnetic properties of ZVINPs, ε-Fe3N and γ'-Fe4N NPs with coercive fields of 160, 65 and 45 Oe, resp. Similarly, the temp.-dependent magnetization profiles revealed that the obsd. ferromagnetic properties of iron nitride phases can be attributed to the redistribution of electronic spin states due to both nitrogen populations and the confinement in the crystallites.
    45. 45
      Oh, Y. J.; Yoo, J. J.; Kim, Y. I.; Yoon, J. K.; Yoon, H. N.; Kim, J.-H.; Park, S. B. Oxygen functional groups and electrochemical capacitive behavior of incompletely reduced graphene oxides as a thin-film electrode of supercapacitor. Electrochim. Acta 2014, 116, 118– 128,  DOI: 10.1016/j.electacta.2013.11.040
      45
      Oxygen functional groups and electrochemical capacitive behavior of incompletely reduced graphene oxides as a thin-film electrode of supercapacitor
      Oh, Young Joon; Yoo, Jung Joon; Kim, Yong Il; Yoon, Jae Kook; Yoon, Ha Na; Kim, Jong-Huy; Park, Seung Bin
      Electrochimica Acta (2014), 116 (), 118-128CODEN: ELCAAV; ISSN:0013-4686. (Elsevier Ltd.)
      For incompletely reduced graphene oxides (RGOs), an effect of O functional groups such as carboxyl, phenol, carbonyl, and quinone on electrochem. capacitive behavior was studied. To prep. RGO thin-film electrodes, a simple fabrication process by (i) dropping and evapg. the graphene oxide (GO) soln., (ii) irradiating pulsed light, and (iii) heat-treating at 200-360° was applied. It was notable that the pulsed light irradn. was effective to prevent the disfiguring of deposited GO thin-film during the thermal redn. From XRD analyses, interlayer distances of the RGOs were gradually decreased from 0.379 to 0.354 nm. As increasing the thermal redn. temp. from 200 to 360°, XPS O 1s spectra analyses showed that the at. percentages of carboxyl and phenol of the RGOs were sustained as 5.40 ± 0.36 and 4.77 ± 0.41 at.%, resp. Meanwhile, those of carbonyl and quinone of the RGOs were gradually declined from 3.10 to 1.81 and from 1.32 to 0.65 at.% with different thermal redn. temp., resp. For all RGO thin-film electrodes, the specific capacitance from the CV measurement in 6 M KOH was sustained as ∼220 F g-1 at the scan of 5 mV s-1. However, in 1 M H2SO4, the specific capacitance was gradually decreased from 171 to 136 F g-1. After 100,000 cycles in 6 M KOH, 1 M H2SO4, and 0.5 M Na2SO4, the RGO (200°) electrodes showed ∼92, 54, and 104% of the initial capacitances, resp. The at. percentages of the O functional groups involved in the pseudocapacitive faradaic reaction were decreased after the cycle test. Esp. in 1 M H2SO4, quinone group was decreased to ∼48% of initial at. percentage, which seems to be a main reason for the drastic redn. of capacitance. The specific pseudocapacitance per unit at. percentage for either carboxyl or phenol group in 6 M KOH was obtained as 12.59 F g-1 at%-1. For carbonyl group in 1 M H2SO4, it was a slightly deviated value as 13.55 F g-1 at%-1. For quinone group in 1 M H2SO4, it was 27.09 F g-1at%-1.
    46. 46
      Xing, Z.; Ju, Z.; Zhao, Y.; Wan, J.; Zhu, Y.; Qiang, Y.; Qian, Y. One-pot hydrothermal synthesis of Nitrogen-doped graphene as high-performance anode materials for lithium ion batteries. Sci. Rep. 2016, 6, 26146  DOI: 10.1038/srep26146
      46
      One-pot hydrothermal synthesis of Nitrogen-doped graphene as high-performance anode materials for lithium ion batteries
      Xing, Zheng; Ju, Zhicheng; Zhao, Yulong; Wan, Jialu; Zhu, Yabo; Qiang, Yinghuai; Qian, Yitai
      Scientific Reports (2016), 6 (), 26146CODEN: SRCEC3; ISSN:2045-2322. (Nature Publishing Group)
      Nitrogen-doped (N-doped) graphene has been prepd. by a simple one-step hydrothermal approach using hexamethylenetetramine (HMTA) as single carbon and nitrogen source. In this hydrothermal process, HMTA pyrolyzes at high temp. and the N-doped graphene subsequently self-assembles on the surface of MgO particles (formed by the Mg powder reacting with H2O) during which graphene synthesis and nitrogen doping are simultaneously achieved. The as-synthesized graphene with incorporation of nitrogen groups possesses unique structure including thin layer thickness, high surface area, mesopores and vacancies. These structural features and their synergistic effects could not only improve ions and electrons transportation with nanometer-scale diffusion distances but also promote the penetration of electrolyte. The N-doped graphene exhibits high reversible capacity, superior rate capability as well as long-term cycling stability, which demonstrate that the N-doped graphene with great potential to be an efficient electrode material. The exptl. results provide a new hydrothermal route to synthesize N-doped graphene with potential application for advanced energy storage, as well as useful information to design new graphene materials.
    47. 47
      Azuma, M.; Kashihara, M.; Nakato, Y.; Tsubomura, H. Reduction of oxygen to water on cobalt-nitride thin film electrodes prepared by the reactive rf sputtering technique. J. Electroanal. Chem. Interfacial Electrochem. 1988, 250, 73– 82,  DOI: 10.1016/0022-0728(88)80193-1
      47
      Reduction of oxygen to water on cobalt-nitride thin film electrodes prepared by the reactive RF sputtering technique
      Azuma, Masashi; Kashihara, Minoru; Nakato, Yoshihiro; Tsubomura, Hiroshi
      Journal of Electroanalytical Chemistry and Interfacial Electrochemistry (1988), 250 (1), 73-82CODEN: JEIEBC; ISSN:0022-0728.
      Electrochem. redn. of O was carried out on amorphous Co-nitride (CoxN) thin film electrodes prepd. by the reactive RF sputtering technique. The onset potential of the O redn. current in the CoxN electrode in neutral electrolyte solns. was close to that on a Pt metal electrode, which is known to be the most efficient material for O redn. Measurements with rotating-disk electrodes (RDEs) and rotating ring-disk electrodes (RRDEs) showed that redn. of O to H2O (the 4-electron-transfer type) proceeded on the CoxN electrodes, unlike that on Co metal electrodes, where only the 2-electron-transfer type redn. to H2O2 took place. The reason for the improved electrocatalytic activity by nitridation is discussed briefly.
    48. 48
      Gong, K.; Du, F.; Xia, Z.; Durstock, M.; Dai, L. Nitrogen-Doped Carbon Nanotube Arrays with High Electrocatalytic Activity for Oxygen Reduction. Science 2009, 323, 760– 764,  DOI: 10.1126/science.1168049
      48
      Nitrogen-Doped Carbon Nanotube Arrays with High Electrocatalytic Activity for Oxygen Reduction
      Gong, Kuanping; Du, Feng; Xia, Zhenhai; Durstock, Michael; Dai, Liming
      Science (Washington, DC, United States) (2009), 323 (5915), 760-764CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
      The large-scale practical application of fuel cells will be difficult to realize if the expensive platinum-based electrocatalysts for oxygen redn. reactions (ORRs) cannot be replaced by other efficient, low-cost, and stable electrodes. Here, we report that vertically aligned nitrogen-contg. carbon nanotubes (VA-NCNTs) can act as a metal-free electrode with a much better electrocatalytic activity, long-term operation stability, and tolerance to crossover effect than platinum for oxygen redn. in alk. fuel cells. In air-satd. 0.1 M potassium hydroxide, we obsd. a steady-state output potential of -80 mV and a c.d. of 4.1 milliamps per square centimeter at -0.22 V, compared with -85 mV and 1.1 milliamps per square centimeter at -0.20 V for a platinum-carbon electrode. The incorporation of electron-accepting nitrogen atoms in the conjugated nanotube carbon plane appears to impart a relatively high pos. charge d. on adjacent carbon atoms. This effect, coupled with aligning the NCNTs, provides a four-electron pathway for the ORR on VA-NCNTs with a superb performance.
    49. 49
      Nagaiah, T. C.; Kundu, S.; Bron, M.; Muhler, M.; Schuhmann, W. Nitrogen-doped carbon nanotubes as a cathode catalyst for the oxygen reduction reaction in alkaline medium. Electrochem. Commun. 2010, 12, 338– 341,  DOI: 10.1016/j.elecom.2009.12.021
      49
      Nitrogen-doped carbon nanotubes as a cathode catalyst for the oxygen reduction reaction in alkaline medium
      Nagaiah, Tharamani C.; Kundu, Shankhamala; Bron, Michael; Muhler, Martin; Schuhmann, Wolfgang
      Electrochemistry Communications (2010), 12 (3), 338-341CODEN: ECCMF9; ISSN:1388-2481. (Elsevier B.V.)
      A new approach to synthesize nitrogen-doped carbon nanotubes (NCNTs) as catalysts for oxygen redn. by treating oxidized CNTs with ammonia is presented. The surface properties and oxygen redn. activities were characterized by cyclic voltammetry, rotating disk electrode, and XPS. NCNTs treated at 800° show improved electrocatalytic activity for oxygen redn. as compared with com. available Pt/C catalysts.
    50. 50
      Ren, G.; Gao, L.; Teng, C.; Li, Y.; Yang, H.; Shui, J.; Lu, X.; Zhu, Y.; Dai, L. Ancient Chemistry “Pharaoh’s Snakes” for Efficient Fe-/N-Doped Carbon Electrocatalysts. ACS Appl. Mater. Interfaces 2018, 10, 10778– 10785,  DOI: 10.1021/acsami.7b16936
      50
      Ancient Chemistry "Pharaoh's Snakes" for Efficient Fe-/N-Doped Carbon Electrocatalysts
      Ren, Guangyuan; Gao, Liangliang; Teng, Chao; Li, Yunan; Yang, Hequn; Shui, Jianglan; Lu, Xianyong; Zhu, Ying; Dai, Liming
      ACS Applied Materials & Interfaces (2018), 10 (13), 10778-10785CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
      The method of fabricating non-precious metal electrocatalysts with high activity and durability through a facile and eco-friendly procedure is of great significance to the development of low-cost fuel cells and metal-air batteries. Herein, the authors present that an ancient chem. reaction of Pharaoh's snakes can be a fast and convenient technique to prep. Fe-/N-doped C (Fe/N-C) nanosheet/nanotube electrocatalysts with sugar, soda, melamine, and Fe nitrate as precursors. The resultant Fe/N-C catalyst has a hierarchically porous structure, a large surface area, and uniformly distributed active sites. The catalyst shows high electrocatalytic activities toward both the O redn. reaction with a half-wave potential of 0.90 V (vs. reversible H electrode) better than that of Pt/C and the O evolution reaction with an overpotential of 0.46 V at the c.d. of 10 mA cm-2 comparable to that of RuO2. The activity and stability of the catalyst are also evaluated in primary and rechargeable Zn-air batteries. In both conditions, three-dimensional Fe/N-C exhibited performances superior to Pt/C. The authors' work demonstrates a success of using an ancient science to make a state-of-the-art electrocatalyst.
    51. 51
      Susi, T.; Pichler, T.; Ayala, P. X-ray photoelectron spectroscopy of graphitic carbon nanomaterials doped with heteroatoms. Beilstein J. Nanotechnol. 2015, 6, 177– 192,  DOI: 10.3762/bjnano.6.17
      51
      X-ray photoelectron spectroscopy of graphitic carbon nanomaterials doped with heteroatoms
      Susi, Toma; Pichler, Thomas; Ayala, Paola
      Beilstein Journal of Nanotechnology (2015), 6 (), 177-192, 16 pp.CODEN: BJNEAH; ISSN:2190-4286. (Beilstein-Institut zur Foerderung der Chemischen Wissenschaften)
      XPS is one of the best tools for studying the chem. modification of surfaces, and in particular the distribution and bonding of heteroatom dopants in carbon nanomaterials such as graphene and carbon nanotubes. Although these materials have superb intrinsic properties, these often need to be modified in a controlled way for specific applications. Towards this aim, the most studied dopants are neighbors to carbon in the periodic table, nitrogen and boron, with phosphorus starting to emerge as an interesting new alternative. Hundreds of studies have used XPS for analyzing the concn. and bonding of dopants in various materials. Although the majority of works has concd. on nitrogen, important work is still ongoing to identify its precise at. bonding configurations. In general, care should be taken in the prepn. of a suitable sample, consideration of the intrinsic photoemission response of the material in question, and the appropriate spectral anal. If this is not the case, incorrect conclusions can easily be drawn, esp. in the assignment of measured binding energies into specific at. configurations. Starting from the characteristics of pristine materials, this review provides a practical guide for interpreting X-ray photoelectron spectra of doped graphitic carbon nanomaterials, and a ref. for their binding energies that are vital for compositional anal. via XPS.
    52. 52
      Wu, Z. S.; Yang, S.; Sun, Y.; Parvez, K.; Feng, X.; Mullen, K. 3D nitrogen-doped graphene aerogel-supported Fe3O4 nanoparticles as efficient electrocatalysts for the oxygen reduction reaction. J. Am. Chem. Soc. 2012, 134, 9082– 9085,  DOI: 10.1021/ja3030565
      52
      3D Nitrogen-Doped Graphene Aerogel-Supported Fe3O4 Nanoparticles as Efficient Electrocatalysts for the Oxygen Reduction Reaction
      Wu, Zhong-Shuai; Yang, Shubin; Sun, Yi; Parvez, Khaled; Feng, Xinliang; Muellen, Klaus
      Journal of the American Chemical Society (2012), 134 (22), 9082-9085CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Three-dimensional (3D) nitrogen-doped graphene aerogel (N-GA)-supported Fe3O4 nanoparticles (Fe3O4/N-GAs) as efficient cathode catalysts for the oxygen redn. reaction are reported. The graphene hybrids exhibit an interconnected macroporous framework of graphene sheets with uniform dispersion of Fe3O4 nanoparticles. In studying the effects of the carbon support on the Fe3O4 nanoparticles for the oxygen redn. reaction, we found that Fe3O4/N-GAs show a more pos. onset potential, higher cathodic d., lower H2O2 yield, and higher electron transfer no. for the oxygen redn. reaction in alk. media than Fe3O4 nanoparticles supported on N-doped carbon black or N-doped graphene sheets, highlighting the importance of the 3D macropores and high sp. surface area of the GA support for improving the oxygen redn. reaction performance. Furthermore, Fe3O4/N-GAs show better durability than the com. Pt/C catalyst.
    53. 53
      Davis, R. E.; Horvath, G. L.; Tobias, C. W. The solubility and diffusion coefficient of oxygen in potassium hydroxide solutions. Electrochim. Acta 1967, 12, 287– 297,  DOI: 10.1016/0013-4686(67)80007-0
      53
      Solubility and diffusion coefficient of oxygen in potassium hydroxide solutions
      Davis, Russ Erik; Horvath, George L.; Tobias, Charles W.
      Electrochimica Acta (1967), 12 (3), 287-97CODEN: ELCAAV; ISSN:0013-4686.
      The soly. of O in aq. KOH solns. was measured by a Van Slyke app. and by an adsorption technique developed by Hildebrand (Kobatake and H., CA 56: 988b). In the range of concn. of KOH between 0 and 12N, at 25°, the 2 methods yielded identical results; at 760 torr O partial pressure, log S = log 1.26 × 10-3 - 0.174 6C, where 0.1746 is the soly. coeff., S the concn. of O, g.-moles/l., and C the concn. of KOH, g./moles/l. Between 0° and 60° both the soly. and the soly. coeff. decrease with increasing temp. Diffusion coeffs. of O in aq. KOH were evaluated from the limiting current of O on a rotating disk electrode, and also by a stagnant tube technique similar to that used by von Stackelberg (S., et al., CA 47: 11908d). The diffusivity drops sharply with increasing KOH concn., and increases with temp. At 25° and for KOH concns. between 2 and 4N, the product of the diffusivity and the viscosity is const.: Dμ = 1.3 × 10-7 g.-cm./sec.2, where D is the diffusivity, cm.2/sec., and μ is the viscosity in poises. At 60° and for a normality of KOH between 1 and 8, the value of this product is: Dμ = 1.9 × 10-7 g.-cm./sec.2 28 references.
    54. 54
      Marković, N. M.; Gasteiger, H. A.; Ross, P. N. Oxygen Reduction on Platinum Low-Index Single-Crystal Surfaces in Alkaline Solution: Rotating Ring DiskPt(hkl) Studies. J. Phys. Chem. 1996, 100, 6715– 6721,  DOI: 10.1021/jp9533382
      There is no corresponding record for this reference.
    55. 55
      Asahi, M.; Yamazaki, S.; Itoh, S.; Ioroi, T. Electrochemical reduction of dioxygen by copper complexes with pyridylalkylamine ligands dissolved in aqueous buffer solution: the relationship between activity and redox potential. Dalton Trans. 2014, 43, 10705– 10709,  DOI: 10.1039/c4dt00606b
      55
      Electrochemical reduction of dioxygen by copper complexes with pyridylalkylamine ligands dissolved in aqueous buffer solution: the relationship between activity and redox potential
      Asahi, Masafumi; Yamazaki, Shin-ichi; Itoh, Shinobu; Ioroi, Tsutomu
      Dalton Transactions (2014), 43 (28), 10705-10709CODEN: DTARAF; ISSN:1477-9226. (Royal Society of Chemistry)
      The CuII/CuI redox properties and electrochem. O2 redn. activity of CuII-complexes with pyridylalkylamine ligands were studied in a neutral buffer soln. The relation between CuII/CuI redox properties and O2 redn. activity was clearly demonstrated by voltammetric analyses.
    56. 56
      Li, Y.; Kuttiyiel, K. A.; Wu, L.; Zhu, Y.; Fujita, E.; Adzic, R. R.; Sasaki, K. Enhancing Electrocatalytic Performance of Bifunctional Cobalt–Manganese-Oxynitride Nanocatalysts on Graphene. ChemSusChem 2017, 10, 68– 73,  DOI: 10.1002/cssc.201601188
      There is no corresponding record for this reference.
    57. 57
      Vikkisk, M.; Kruusenberg, I.; Ratso, S.; Joost, U.; Shulga, E.; Kink, I.; Rauwel, P.; Tammeveski, K. Enhanced electrocatalytic activity of nitrogen-doped multi-walled carbon nanotubes towards the oxygen reduction reaction in alkaline media. RSC Adv. 2015, 5, 59495– 59505,  DOI: 10.1039/C5RA08818F
      57
      Enhanced electrocatalytic activity of nitrogen-doped multi-walled carbon nanotubes towards the oxygen reduction reaction in alkaline media
      Vikkisk, Merilin; Kruusenberg, Ivar; Ratso, Sander; Joost, Urmas; Shulga, Eugene; Kink, Ilmar; Rauwel, Protima; Tammeveski, Kaido
      RSC Advances (2015), 5 (73), 59495-59505CODEN: RSCACL; ISSN:2046-2069. (Royal Society of Chemistry)
      In this work multi-walled carbon-nanotubes (MWCNTs) were doped with nitrogen using cyanamide (CM) or dicyandiamide (DCDA). To incorporate nitrogen into the CNT structure, high-temp. pyrolysis in an inert atm. was performed. For surface characterization of nitrogen-doped CNTs (NCNTs) XPS, Raman spectroscopy, SEM (SEM) and transmission electron microscopy (TEM) were used. According to the results of XPS anal., nitrogen was successfully incorporated into the carbon nanotube network. The electrocatalytic activity of NCNT catalysts for oxygen redn. reaction (ORR) in alk. media was examd. using the rotating disk electrode (RDE) and linear sweep voltammetry (LSV) measurements. The NCNT-DCDA material showed a better ORR performance than the NCNT-CM catalyst. The RDE results reveal that the NCNT materials studied could be considered as interesting alternatives to Pt-based catalysts in alk. membrane fuel cells.
    58. 58
      Jukk, K.; Kongi, N.; Rauwel, P.; Matisen, L.; Tammeveski, K. Platinum Nanoparticles Supported on Nitrogen-Doped Graphene Nanosheets as Electrocatalysts for Oxygen Reduction Reaction. Electrocatalysis 2016, 7, 428– 440,  DOI: 10.1007/s12678-016-0322-1
      58
      Platinum Nanoparticles Supported on Nitrogen-Doped Graphene Nanosheets as Electrocatalysts for Oxygen Reduction Reaction
      Jukk, Kristel; Kongi, Nadezda; Rauwel, Protima; Matisen, Leonard; Tammeveski, Kaido
      Electrocatalysis (2016), 7 (5), 428-440CODEN: ELECCF; ISSN:1868-2529. (Springer)
      This paper deals with nitrogen-doped graphene nanosheets prepd. using dicyandiamide precursor as a catalyst support for oxygen redn. reaction (ORR). Platinum nanoparticles supported on N-doped graphene nanosheets (Pt/NG) were studied as electrocatalysts for ORR in acid and alk. solns. employing the rotating disk electrode (RDE) technique. Pt/NG nanomaterials were synthesized by chem. redn. of hexachloroplatinic acid using sodium borohydride or ethylene glycol as reducing agents. Surface morphol. and compn. of the prepd. catalysts were examd. by transmission electron microscopy (TEM) and XPS. The TEM images showed a high dispersion of Pt nanoparticles on N-doped graphene nanosheets due to strong interaction of Pt with nitrogen functionalities. The av. nitrogen content was between 6 and 7 at.% according to the XPS anal. In acidic soln., 20 wt% Pt/NG catalyst prepd. by borohydride redn. showed the highest specific activity for O2 redn. from all the Pt/NG materials studied. Pt/NG nanomaterials exhibited excellent electrocatalytic activity in alk. media, and their half-wave potentials were similar to that of com. Pt/C catalyst. The RDE data anal. showed that the ORR on the Pt/NG catalysts proceeded via four-electron pathway.
    59. 59
      Molina-García, M. A.; Rees, N. V. Effect of catalyst carbon supports on the oxygen reduction reaction in alkaline media: a comparative study. RSC Adv. 2016, 6, 94669– 94681,  DOI: 10.1039/C6RA18894J
      59
      Effect of catalyst carbon supports on the oxygen reduction reaction in alkaline media: a comparative study
      Molina-Garcia, Miguel A.; Rees, Neil V.
      RSC Advances (2016), 6 (97), 94669-94681CODEN: RSCACL; ISSN:2046-2069. (Royal Society of Chemistry)
      Some carbon materials commonly used as electronically-conductive supports for catalysts in fuel cell research are Carbon Black (CB), multi-walled carbon nanotubes (MWCNT), Graphene Oxide (GO) and reduced graphene oxide (rGO). Here we present a comparative study into the relative effects of each of these on the performance towards the oxygen redn. reaction (ORR) in alk. media. For the purposes of comparing the supports, a simple Pt catalyst is used and the performance is evaluated via Koutecky-Levich anal. and direct measurement of peroxide by rotating ring-disk electrode (RRDE) to det. the no. of electrons (n) transferred in the ORR. It is found that Pt/CB follows a quasi 4-electron mechanism due to that the ORR takes place mainly on the active Pt particles, whereas Pt/MWCNT, Pt/GO and Pt/rGO exhibit a mixed behavior between the two proposed mechanisms due to the higher activity of the graphene-derived supports towards the peroxide formation compared to CB. The effect of the oxide groups of GO and the metal impurities of MWCNT on the catalytic performance is also studied.
    60. 60
      Wang, M.; Yang, Y.; Liu, X.; Pu, Z.; Kou, Z.; Zhu, P.; Mu, S. The role of iron nitrides in the Fe-N-C catalysis system towards the oxygen reduction reaction. Nanoscale 2017, 9, 7641– 7649,  DOI: 10.1039/C7NR01925D
      60
      The role of iron nitrides in the Fe-N-C catalysis system towards the oxygen reduction reaction
      Wang, Min; Yang, Yushi; Liu, Xiaobo; Pu, Zonghua; Kou, Zongkui; Zhu, Peipei; Mu, Shichun
      Nanoscale (2017), 9 (22), 7641-7649CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
      Fe-N-C series catalysts are always attractive for their high catalytic activity towards the oxygen redn. reaction (ORR). However, they usually consist of various components such as iron nitrides, metallic iron, iron carbides, N-doped carbon and Fe-N4 moieties, leading to controversial contributions of these components to the catalysis of the ORR, esp. iron nitrides. In this work, to investigate the function of iron nitrides, FexN nanoparticles (NPs) embedded in mesoporous N-doped carbon without Fe-N4 moieties are designed and constructed by a simple histidine-assisted method. Herein, the use of histidine can increase the N and Fe contents in the product. The obtained catalyst exhibits excellent ORR catalytic activity which is very close to that of the com. Pt/C catalyst in alk. electrolytes. Combining the catalytic activity, structural characterization (esp. from Mossbauer spectroscopy), and the results of DFT calcns. for adsorption energies of oxygen on the main surfaces of Fe2N including ε-Fe2N and ζ-Fe2N, it can be deduced that Fe2N NPs as active species make a contribution to the ORR catalysis, of which e-FexN (x = 2.1) is more active than ζ-Fe2N. In addn., we find that there exists an obvious synergistic effect between Fe2N NPs and N-doped carbon, leading to the greatly enhanced ORR catalytic activity.
    61. 61
      Varnell, J. A.; Sotiropoulos, J. S.; Brown, T. M.; Subedi, K.; Haasch, R. T.; Schulz, C. E.; Gewirth, A. A. Revealing the Role of the Metal in Non-Precious-Metal Catalysts for Oxygen Reduction via Selective Removal of Fe. ACS Energy Lett. 2018, 3, 823– 828,  DOI: 10.1021/acsenergylett.8b00144
      61
      Revealing the Role of the Metal in Non-Precious-Metal Catalysts for Oxygen Reduction via Selective Removal of Fe
      Varnell, Jason A.; Sotiropoulos, James S.; Brown, Therese M.; Subedi, Kiran; Haasch, Richard T.; Schulz, Charles E.; Gewirth, Andrew A.
      ACS Energy Letters (2018), 3 (4), 823-828CODEN: AELCCP; ISSN:2380-8195. (American Chemical Society)
      Non-precious-metal catalysts have been investigated as alternatives to Pt-based oxygen redn. reaction catalysts for more than 50 years. While the incorporation of a metal is known to be necessary to generate a catalyst with high activity, the exact role of the metal is still not well-understood. In this work, we prep. an active oxygen redn. reaction catalyst contg. Fe and then selectively remove the Fe from the catalyst while preserving the carbon and nitrogen species. By comparing the oxygen redn. reaction activity of the catalyst before and after treatment, we show that in the absence of Fe the carbon and nitrogen sites in the catalyst exhibit a larger overpotential and lower selectivity for the 4e- redn. of oxygen in both acidic and alk. conditions. These findings reveal the direct involvement of the metal in the active site of non-precious-metal catalysts and provide important guidance for future catalyst improvements.
    62. 62
      Lai, L.; Potts, J. R.; Zhan, D.; Wang, L.; Poh, C. K.; Tang, C.; Gong, H.; Shen, Z.; Lin, J.; Ruoff, R. S. Exploration of the active center structure of nitrogen-doped graphene-based catalysts for oxygen reduction reaction. Energy Environ. Sci. 2012, 5, 7936,  DOI: 10.1039/c2ee21802j
      62
      Exploration of the active center structure of nitrogen-doped graphene-based catalysts for oxygen reduction reaction
      Lai, Linfei; Potts, Jeffrey R.; Zhan, Da; Wang, Liang; Poh, Chee Kok; Tang, Chunhua; Gong, Hao; Shen, Zexiang; Lin, Jianyi; Ruoff, Rodney S.
      Energy & Environmental Science (2012), 5 (7), 7936-7942CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)
      We present two different ways to fabricate nitrogen-doped graphene (N-graphene) and demonstrate its use as a metal-free catalyst to study the catalytic active center for the oxygen redn. reaction (ORR). N-graphene was produced by annealing of graphene oxide (G-O) under ammonia or by annealing of a N-contg. polymer/reduced graphene oxide (RG-O) composite (polyaniline/RG-O or polypyrrole/RG-O). The effects of the N precursors and annealing temp. on the performance of the catalyst were investigated. The bonding state of the N atom was found to have a significant effect on the selectivity and catalytic activity for ORR. Annealing of G-O with ammonia preferentially formed graphitic N and pyridinic N centers, while annealing of polyaniline/RG-O and polypyrrole/RG-O tended to generate pyridinic and pyrrolic N moieties, resp. Most importantly, the electrocatalytic activity of the catalyst was dependent on the graphitic N content which detd. the limiting c.d., while the pyridinic N content improved the onset potential for ORR. However, the total N content in the graphene-based non-precious metal catalyst does not play an important role in the ORR process.
    63. 63
      Ge, X.; Sumboja, A.; Wuu, D.; An, T.; Li, B.; Goh, F. W. T.; Hor, T. S. A.; Zong, Y.; Liu, Z. Oxygen Reduction in Alkaline Media: From Mechanisms to Recent Advances of Catalysts. ACS Catal. 2015, 5, 4643– 4667,  DOI: 10.1021/acscatal.5b00524
      63
      Oxygen Reduction in Alkaline Media: From Mechanisms to Recent Advances of Catalysts
      Ge, Xiaoming; Sumboja, Afriyanti; Wuu, Delvin; An, Tao; Li, Bing; Goh, F. W. Thomas; Hor, T. S. Andy; Zong, Yun; Liu, Zhaolin
      ACS Catalysis (2015), 5 (8), 4643-4667CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
      A review. The oxygen redn. reaction (ORR) is an important electrode reaction for energy storage and conversion devices based on oxygen electrocatalysis. This paper introduces the thermodn., reaction kinetics, reaction mechanisms, and reaction pathways of ORR in aq. alk. media. Recent advances of the catalysts for ORR were extensively reviewed, including precious metals, nonmetal-doped carbon, carbon-transition metal hybrids, transition metal oxides with spinel and perovskite structures, and so forth. The applications of those ORR catalysts to zinc-air batteries and alk. fuel cells were briefly introduced. A concluding remark summarizes the current status of the reaction pathways, advanced catalysts, and the future challenges of the research and development of ORR.
    64. 64
      Ramaswamy, N.; Mukerjee, S. Fundamental Mechanistic Understanding of Electrocatalysis of Oxygen Reduction on Pt and Non-Pt Surfaces: Acid versus Alkaline Media. Adv. Phys. Chem. 2012, 2012, 1– 17,  DOI: 10.1155/2012/491604
      There is no corresponding record for this reference.
    65. 65
      Liu, J.; Li, E.; Ruan, M.; Song, P.; Xu, W. Recent Progress on Fe/N/C Electrocatalysts for the Oxygen Reduction Reaction in Fuel Cells. Catalysts 2015, 5, 1167– 1192,  DOI: 10.3390/catal5031167
      65
      Recent progress on Fe/N/C electrocatalysts for the oxygen reduction reaction in fuel cells
      Liu, Jing; Li, Erling; Ruan, Mingbo; Song, Ping; Xu, Weilin
      Catalysts (2015), 5 (3), 1167-1192CODEN: CATACJ; ISSN:2073-4344. (MDPI AG)
      In order to reduce the overall system cost, the development of inexpensive, high-performance and durable oxygen redn. reaction (ORR)N, Fe-codoped carbon-based (Fe/N/C) electrocatalysts to replace currently used Pt-based catalysts has become one of the major topics in research on fuel cells. This review paper lays the emphasis on introducing the progress made over the recent five years with a detailed discussion of recent work in the area of Fe/N/C electrocatalysts for ORR and the possible Fe-based active sites. Fe-based materials prepd. by simple pyrolysis of transition metal salt, carbon support, and nitrogen-rich small mol. or polymeric compd. are mainly reviewed due to their low cost, high performance, long stability and because they are the most promising for replacing currently used Pt-based catalysts in the progress of fuel cell commercialization. Addnl., Fe-base catalysts with small amt. of Fe or new structure of Fe/Fe3C encased in carbon layers are presented to analyze the effect of loading and existence form of Fe on the ORR catalytic activity in Fe-base catalyst. The proposed catalytically Fe-centered active sites and reaction mechanisms from various authors are also discussed in detail, which may be useful for the rational design of high-performance, inexpensive, and practical Fe-base ORR catalysts in future development of fuel cells.
    66. 66
      Liu, Y.-L.; Xu, X.-Y.; Shi, C.-X.; Ye, X.-W.; Sun, P.-C.; Chen, T.-H. Iron–nitrogen co-doped hierarchically mesoporous carbon spheres as highly efficient electrocatalysts for the oxygen reduction reaction. RSC Adv. 2017, 7, 8879– 8885,  DOI: 10.1039/C6RA26917F
      66
      Iron-nitrogen co-doped hierarchically mesoporous carbon spheres as highly efficient electrocatalysts for the oxygen reduction reaction
      Liu, You-Lin; Xu, Xue-Yan; Shi, Cheng-Xiang; Ye, Xin-Wei; Sun, Ping-Chuan; Chen, Tie-Hong
      RSC Advances (2017), 7 (15), 8879-8885CODEN: RSCACL; ISSN:2046-2069. (Royal Society of Chemistry)
      By using hierarchically mesoporous silica spheres as hard template, 2-aminopyridine and FeCl3 as carbon, nitrogen and iron sources, resp., iron and nitrogen co-doped hierarchically mesoporous carbon spheres (Fe-N-CS) were successfully prepd. The sample Fe-N-CS-900 obtained at a carbonization temp. of 900 °C exhibited a highly efficient electrocatalytic activity with pos. half-wave potential (-0.11 V), high limiting c.d. (-4.79 mA cm-2) and high selectivity (electron transfer no. around 4) for the oxygen redn. reaction (ORR) in alk. media. Moreover, Fe-N-CS-900 shows higher stability and better methanol tolerance in comparison to com. Pt/C catalyst in both alk. and acidic media. Its highly efficient ORR activity could be ascribed to its high sp. surface area, unique porous structure and homogeneous distribution of Fe-Nx active sites formed during pyrolysis.
    67. 67
      Hummers, W. S.; Offeman, R. E. Preparation of Graphitic Oxide. J. Am. Chem. Soc. 1958, 80, 1339,  DOI: 10.1021/ja01539a017
      67
      Preparation of graphitic oxide
      Hummers, Wm. S., Jr.; Offeman, Richard E.
      Journal of the American Chemical Society (1958), 80 (), 1339CODEN: JACSAT; ISSN:0002-7863.
      See U.S. 2,798,878 (C.A. 51, 15080a).

  • Supporting Information

    Supporting Information

    The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.8b02646.

    • XPS, CV, and LSV results of the composites, and TEM, XRD, and LSV data for the etched FeNx/NG composites (PDF)


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