Multistep Sulfur Leaching for the Development of a Highly Efficient and Stable NiS_x/Ni(OH)₂/NiOOH Electrocatalyst for Anion Exchange Membrane Water Electrolysis

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1. 1

   Buttler, A.; Spliethoff, H. Current Status of Water Electrolysis for Energy Storage, Grid Balancing and Sector Coupling via Power-to-Gas and Power-to-Liquids: a Review. Renewable Sustainable Energy Rev. 2018, 82, 2440– 2454,  DOI: 10.1016/j.rser.2017.09.003

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   Current status of water electrolysis for energy storage, grid balancing and sector coupling via power-to-gas and power-to-liquids: A review

   Buttler, Alexander; Spliethoff, Hartmut

   Renewable & Sustainable Energy Reviews (2018), 82 (Part_3), 2440-2454CODEN: RSERFH; ISSN:1364-0321. (Elsevier Ltd.)

   Water electrolysis has the potential to become a key element in coupling the electricity, mobility, heating and chem. sector via Power-to-Liqs. (PtL) or Power-to-Gas (PtG) in a future sustainable energy system. Based on an extensive market survey, discussions with manufacturers, project reports and literature, an overview of the current status of alk., PEM and solid oxide electrolysis on the way to large-scale flexible energy storage is presented. These main water electrolysis technologies were compared in terms of available capacity, nominal and part-load performance, flexibility (load range, load gradients, start-up time, stand-by losses) lifetime and investment costs. This review provides a basis of the parameters required and the necessary understanding of electrolysis fundamentals and technologies for a techno-economic anal. of water electrolysis-based concepts and an evaluation of PtG and PtL in energy system studies.
2. 2

   Carmo, M.; Fritz, D. L.; Mergel, J.; Stolten, D. A Comprehensive Review on PEM Water Electrolysis. Int. J. Hydrogen Energy 2013, 38, 4901– 4934,  DOI: 10.1016/j.ijhydene.2013.01.151

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   A comprehensive review on PEM water electrolysis

   Carmo, Marcelo; Fritz, David L.; Mergel, Juergen; Stolten, Detlef

   International Journal of Hydrogen Energy (2013), 38 (12), 4901-4934CODEN: IJHEDX; ISSN:0360-3199. (Elsevier Ltd.)

   A review. Hydrogen is often considered the best means by which to store energy coming from renewable and intermittent power sources. With the growing capacity of localized renewable energy sources surpassing the gigawatt range, a storage system of equal magnitude is required. PEM electrolysis provides a sustainable soln. for the prodn. of hydrogen, and is well suited to couple with energy sources such as wind and solar. However, due to low demand in electrolytic hydrogen in the last century, little research has been done on PEM electrolysis with many challenges still unexplored. The ever increasing desire for green energy has rekindled the interest on PEM electrolysis, thus the compilation and recovery of past research and developments is important and necessary. In this review, PEM water electrolysis is comprehensively highlighted and discussed. The challenges new and old related to electrocatalysts, solid electrolyte, current collectors, separator plates and modeling efforts will also be addressed. The main message is to clearly set the state-of-the-art for the PEM electrolysis technol., be insightful of the research that is already done and the challenges that still exist. This information will provide several future research directions and a road map in order to aid scientists in establishing PEM electrolysis as a com. viable hydrogen prodn. soln.
3. 3

   Ju, H.; Badwal, S.; Giddey, S. A Comprehensive Review of Carbon and Hydrocarbon Assisted Water Electrolysis for Hydrogen Production. Appl. Energy 2018, 231, 502– 533,  DOI: 10.1016/j.apenergy.2018.09.125

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   A comprehensive review of carbon and hydrocarbon assisted water electrolysis for hydrogen production

   Ju, HyungKuk; Badwal, Sukhvinder; Giddey, Sarbjit

   Applied Energy (2018), 231 (), 502-533CODEN: APENDX; ISSN:0306-2619. (Elsevier Ltd.)

   A review. Hydrogen is mainly produced by natural gas reforming, which is a highly efficient process with low feedstock costs. However, the rising interest in clean technologies will increase the demand for hydrogen, meaning that other sources will need to be explored. Although coal is currently the major source of power generation, its demand appears to be declining due to the rise in electricity generated from renewable energy sources and the worldwide quest for low-emission power generation. Coal reserves worldwide are abundant, but new technologies would be needed to produce hydrogen from this feedstock. Coal gasification is one well-established technol. for this purpose, but it is inefficient and produces high CO2 emissions. An alternative technol. that has been investigated over the past few decades is carbon assisted water electrolysis. The basic process is water/steam electrolysis, with part of the energy required for the electrolysis provided by the chem. energy of coal, which reduces the overall elec. energy input. In addn. to coal, the process can also use other carbon sources, such as biomass, alcs. or gaseous hydrocarbons. Several studies have investigated this electrochem. route of hydrogen prodn., employing different electrolytes in a wide temp. range (room temp. to 850 °C) under different process conditions. This paper presents a comprehensive review of carbon assisted water electrolysis, assocd. materials used and the challenges for the development of the technol. at the com. scale.
4. 4

   Lindquist, G. A.; Xu, Q.; Oener, S. Z.; Boettcher, S. W. Membrane Electrolyzers for Impure-Water Splitting. Joule 2020, 4, 2549– 2561,  DOI: 10.1016/j.joule.2020.09.020

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   Membrane Electrolyzers for Impure-Water Splitting

   Lindquist, Grace A.; Xu, Qiucheng; Oener, Sebastian Z.; Boettcher, Shannon W.

   Joule (2020), 4 (12), 2549-2561CODEN: JOULBR; ISSN:2542-4351. (Cell Press)

   Membrane electrolyzers enable the prodn. of high-purity H2 from pure water. There is growing interest in developing this technol. to operate in impure water. While proton-exchange-membrane (PEM) electrolyzers are the current industry leaders, anion-exchange-membrane (AEM) electrolyzers might be better suited for impure-water operation with further development. Here, the tech. barriers to developing AEM electrolyzers for impure-water splitting are discussed. We highlight progress in the field and present possible research approaches to advance the field.
5. 5

   Wang, M.; Wang, Z.; Gong, X.; Guo, Z. The Intensification Technologies to Water Electrolysis for Hydrogen Production -- a Review. Renewable Sustainable Energy Rev. 2014, 29, 573– 588,  DOI: 10.1016/j.rser.2013.08.090

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   The intensification technologies to water electrolysis for hydrogen production - A review

   Wang, Mingyong; Wang, Zhi; Gong, Xuzhong; Guo, Zhancheng

   Renewable & Sustainable Energy Reviews (2014), 29 (), 573-588CODEN: RSERFH; ISSN:1364-0321. (Elsevier Ltd.)

   A review. Water electrolysis derived by renewable energy such as solar energy and wind energy is a sustainable method for hydrogen prodn. due to high purity, simple and green process. One of the challenges is to reduce energy consumption of water electrolysis for large-scale application in future. Cell voltage, an important criterion of energy consumption, consists of theor. decompn. voltage (Uθ), ohmic voltage drop (i*.sum.R) and reaction overpotential (η). The kinetic and thermodn. roots of high cell voltage are analyzed systemically in this review. During water electrolysis, bubble coverage on electrode surface and bubble dispersion in electrolyte, namely bubble effect, result in high ohmic voltage drop and large reaction overpotential. Bubble effect is one of the most key factors for high energy consumption. Based on the theor. anal., we summarize and divide recent intensification technologies of water electrolysis into three categories: external field, new electrolyte compn. and new thermodn. reaction system. The fundamentals and development of these intensification technologies are discussed and reviewed. Reaction overpotential and ohmic voltage drop are improved kinetically by external field or new electrolyte compn. The thermodn. decompn. voltage of water is also reduced by new reaction systems such as solid oxide electrolysis cell (SOEC) and carbon assisted water electrolysis (CAWE).
6. 6

   Miller, H. A.; Bouzek, K.; Hnat, J.; Loos, S.; Bernäcker, C. I.; Weißgärber, T.; Röntzsch, L.; Meier-Haack, J. Green Hydrogen from Anion Exchange Membrane Water Electrolysis: a Review of Recent Developments in Critical Materials and Operating Conditions. Sustainable Energy Fuels 2020, 4, 2114– 2133,  DOI: 10.1039/C9SE01240K

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   Green hydrogen from anion exchange membrane water electrolysis: a review of recent developments in critical materials and operating conditions

   Miller, Hamish Andrew; Bouzek, Karel; Hnat, Jaromir; Loos, Stefan; Bernacker, Christian Immanuel; Weissgarber, Thomas; Rontzsch, Lars; Meier-Haack, Jochen

   Sustainable Energy & Fuels (2020), 4 (5), 2114-2133CODEN: SEFUA7; ISSN:2398-4902. (Royal Society of Chemistry)

   A review. Hydrogen prodn. using water electrolyzers equipped with an anion exchange membrane (AEM), a pure water feed and cheap components such as platinum group metal-free catalysts and stainless steel bipolar plates (BPP) can challenge proton exchange membrane (PEM) electrolysis systems as the state of the art. For this to happen the performance of the AEM electrolyzer must match the compact design, stability, H2 purity and high current densities of PEM systems. Current research aims at bringing AEM water electrolysis technol. to an advanced level in terms of electrolysis cell performance. Such technol. advances must be accompanied by demonstration of the cost advantages of AEM systems. The current state of the art in AEM water electrolysis is defined by sporadic reports in the academic literature mostly dealing with catalyst or membrane development. The development of this technol. requires a future roadmap for systematic development and commercialization of AEM systems and components. This will include basic and applied research, technol. development & integration, and testing at a lab. scale of small demonstration units (AEM electrolyzer shortstacks) that can be used to validate the technol. (from TRL 2-3 currently to TRL 4-5). This review paper gathers together recent important research in crit. materials development (catalysts, membranes and MEAs) and operating conditions (electrolyte compn., cell temp., performance achievements). The aim of this review is to identify the current level of materials development and where improvements are required in order to demonstrate the feasibility of the technol. Once the challenges of materials development are overcome, AEM water electrolysis can drive the future use of hydrogen as an energy storage vector on a large scale (GW) esp. in developing countries.
7. 7

   Gernaat, D. E. H. J.; de Boer, H. S.; Daioglou, V.; Yalew, S. G.; Müller, C.; an Vuuren, D. P. Climate Change Impacts on Renewable Energy Supply. Nat. Clim. Change 2021, 11, 119– 125,  DOI: 10.1038/s41558-020-00949-9

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   Liu, C.; Shviro, M.; Gago, A. S.; Zaccarine, S. F.; Bender, G.; Gazdzicki, P.; Morawietz, T.; Biswas, I.; Rasinski, M.; Everwand, A.; Schierholz, R.; Pfeilsticker, J.; Müller, M.; Lopes, P. P.; Eichel, R. A.; Pivovar, B.; Pylypenko, S.; Friedrich, K. A.; Lehnert, W.; Carmo, M. Exploring the Interface of Skin-Layered Titanium Fibers for Electrochemical Water Splitting. Adv. Energy Mater. 2021, 11, 2002926  DOI: 10.1002/aenm.202002926

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   Exploring the Interface of Skin-Layered Titanium Fibers for Electrochemical Water Splitting

   Liu, Chang; Shviro, Meital; Gago, Aldo S.; Zaccarine, Sarah F.; Bender, Guido; Gazdzicki, Pawel; Morawietz, Tobias; Biswas, Indro; Rasinski, Marcin; Everwand, Andreas; Schierholz, Roland; Pfeilsticker, Jason; Mueller, Martin; Lopes, Pietro P.; Eichel, Ruediger-A.; Pivovar, Bryan; Pylypenko, Svitlana; Friedrich, K. Andreas; Lehnert, Werner; Carmo, Marcelo

   Advanced Energy Materials (2021), 11 (8), 2002926CODEN: ADEMBC; ISSN:1614-6840. (Wiley-Blackwell)

   Water electrolysis is the key to a decarbonized energy system, as it enables the conversion and storage of renewably generated intermittent electricity in the form of hydrogen. However, reliability challenges arising from titanium-based porous transport layers (PTLs) have hitherto restricted the deployment of next-generation water-splitting devices. Here, it is shown for the first time how PTLs can be adapted so that their interface remains well protected and resistant to corrosion across ≈4000 h under real electrolysis conditions. It is also demonstrated that the malfunctioning of unprotected PTLs is a result triggered by addnl. fatal degrdn. mechanisms over the anodic catalyst layer beyond the impacts expected from iridium oxide stability. Now, superior durability and efficiency in water electrolyzers can be achieved over extended periods of operation with less-expensive PTLs with proper protection, which can be explained by the detailed reconstruction of the interface between the different elements, materials, layers, and components presented in this work.
9. 9

   Liu, C.; Wippermann, K.; Rasinski, M.; Suo, Y.; Shviro, M.; Carmo, M.; Lehnert, W. Constructing a Multifunctional Interface Between Membrane and Porous Transport Layer for Water Electrolyzers. ACS Appl. Mater. Interfaces 2021, 13, 16182– 16196,  DOI: 10.1021/acsami.0c20690

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   Constructing a Multifunctional Interface between Membrane and Porous Transport Layer for Water Electrolyzers

   Liu, Chang; Wippermann, Klaus; Rasinski, Marcin; Suo, Yanpeng; Shviro, Meital; Carmo, Marcelo; Lehnert, Werner

   ACS Applied Materials & Interfaces (2021), 13 (14), 16182-16196CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)

   The cell performance and durability of polymer electrolyte membrane (PEM) water electrolyzers are limited by the surface passivation of titanium-based porous transport layers (PTLs). In order to ensure stable performance profiles over time, large amts. (≥1 mg·cm-2) of noble metals (Au, Pt, Ir) are most widely used to coat titanium-based PTLs. However, their high cost is still a major obstacle toward commercialization and widespread application. In this paper, we assess different loadings of iridium, ranging from 0.005 to 0.05 mg·cm-2 in titanium PTLs, that consequently affect the investment costs of PEM water electrolyzers. Concerning a redn. in the precious metal costs, we found that Ir as a protective layer with a loading of 0.025 mg·cm-2 on the PTLs would be sufficient to achieve the same cell performance as PTLs with a higher Ir loading. This Ir loading is a 40-fold redn. over the Au or Pt loading typically used for protective layers in current com. PEM water electrolyzers. We show that the Ir protective layer here not only decreases the Ohmic resistance significantly, which is the largest part of the gain in performance, but moreover, the oxygen evolution reaction activity of the iridium layer makes it promising as a cost-effective catalyst layer. Our work also confirms that the proper construction of a multifunctional interface between a membrane and a PTL indeed plays a crucial role in guaranteeing the superior performance and efficiency of electrochem. devices.
10. 10

    Park, S.; Shviro, M.; Hartmann, H.; Besmehn, A.; Mayer, J.; Stolten, D.; Carmo, M. Nickel Structures as a Template Strategy to Create Shaped Iridium Electrocatalysts for Electrochemical Water Splitting. ACS Appl. Mater. Interfaces 2021, 13, 13576– 13585,  DOI: 10.1021/acsami.0c23026

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    10

    Nickel Structures as a Template Strategy to Create Shaped Iridium Electrocatalysts for Electrochemical Water Splitting

    Park, Seongeun; Shviro, Meital; Hartmann, Heinrich; Besmehn, Astrid; Mayer, Joachim; Stolten, Detlef; Carmo, Marcelo

    ACS Applied Materials & Interfaces (2021), 13 (11), 13576-13585CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)

    Low-cost, highly active, and highly stable catalysts are desired for the generation of hydrogen and oxygen using water electrolyzers. To enhance the kinetics of the oxygen evolution reaction in an acidic medium, it is of paramount importance to redesign iridium electrocatalysts into novel structures with organized morphol. and high surface area. Here, we report on the designing of a well-defined and highly active hollow nanoframe based on iridium. The synthesis strategy was to control the shape of nickel nanostructures on which iridium nanoparticles will grow. After the growth of iridium on the surface, the next step was to etch the nickel core to form the NiIr hollow nanoframe. The etching procedure was found to be significant in controlling the hydroxide species on the iridium surface and by that affecting the performance. The catalytic performance of the NiIr hollow nanoframe was studied for oxygen evolution reaction and shows 29 times increased iridium mass activity compared to com. available iridium-based catalysts. Our study provides novel insights to control the fabrication of iridium-shaped catalysts using 3d transition metal as a template and via a facile etching step to steer the formation of hydroxide species on the surface. These findings shall aid the community to finally create stable iridium alloys for polymer electrolyte membrane water electrolyzers, and the strategy is also useful for many other electrochem. devices such as batteries, fuel cells, sensors, and solar org. cells.
11. 11

    Liu, C.; Carmo, M.; Bender, G.; Everwand, A.; Lickert, T.; Young, J. L.; Smolinka, T.; Stolten, D.; Lehnert, W. Performance Enhancement of PEM Electrolyzers through Iridium-Coated Titanium Porous Transport Layers. Electrochem. Commun. 2018, 97, 96– 99,  DOI: 10.1016/j.elecom.2018.10.021

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    11

    Performance enhancement of PEM electrolyzers through iridium-coated titanium porous transport layers

    Liu, Chang; Carmo, Marcelo; Bender, Guido; Everwand, Andreas; Lickert, Thomas; Young, James L.; Smolinka, Tom; Stolten, Detlef; Lehnert, Werner

    Electrochemistry Communications (2018), 97 (), 96-99CODEN: ECCMF9; ISSN:1388-2481. (Elsevier B.V.)

    Titanium-based porous transport layers (PTL) used in polymer electrolyte membrane (PEM) water electrolyzers suffer from surface passivation (titanium oxidn.), which increases the interface resistance between the PTL and electrode. For long-term operation, PTLs are typically coated with considerable amts. of platinum or gold to ensure reasonable performance profiles over time. Moreover, it is well known that the oxide forms of platinum and gold are not stable under electrolysis conditions. In this study, an easy and scalable method is introduced to protect the titanium PTL from passivation by sputtering very thin layers of iridium onto com.-available titanium PTLs. The iridium layer reduces the overall ohmic resistance of the PTL/catalyst layer interface and improves the cell's performance to that achieved with carbon-based PTLs. The coating process homogeneously deposited iridium throughout the inner structure of the PTL. The findings of this study may lead to the use of iridium as a protective layer for titanium PTLs, potentially enable operation at increased cell voltages and lead to increased electrolyzer durability.
12. 12

    Arges, C. G.; Zhang, L. Anion Exchange Membranes’ Evolution toward High Hydroxide Ion Conductivity and Alkaline Resiliency. ACS Appl. Energy Mater. 2018, 1, 2991– 3012,  DOI: 10.1021/acsaem.8b00387

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    12

    Anion Exchange Membranes' Evolution toward High Hydroxide Ion Conductivity and Alkaline Resiliency

    Arges, Christopher G.; Zhang, Le

    ACS Applied Energy Materials (2018), 1 (7), 2991-3012CODEN: AAEMCQ; ISSN:2574-0962. (American Chemical Society)

    A review is given. The development of alk. fuel cells over the past decade has led to exciting developments in low resistant and alk. stable anion exchange membranes (AEMs). This paper highlights new material chemistries and macromol. designs that have fueled AEMs with ionic conductivities >100 mS/cm, while demonstrating stability for extended periods in base bath solns. of 1M KOH or NaOH solns. at ≥80°. The new AEMs have led to AEM fuel cells (AEMFCs) with power d. values that exceed 1 W/cm2 with H and O. AEM research activities are motivated in large part by their prospect to realize fuel cells free of Pt group metals, which is paramount for cost redn. of fuel cell technol. In addn. to highlighting the remarkable achievements of AEMs in the past 4 yr, this paper discusses future priorities for the scientific community to address in AEM development. These priorities include stability and cond. under low humidity or dry conditions, resisting carbonation and oxidn., and AEMFC device stability studies.
13. 13

    Cao, Y.-C.; Wu, X.; Scott, K. A Quaternary Ammonium Grafted Poly Vinyl Benzyl Chloride Membrane for Alkaline Anion Exchange Membrane Water Electrolysers with No-Noble-Metal Catalysts. Int. J. Hydrogen Energy 2012, 37, 9524– 9528,  DOI: 10.1016/j.ijhydene.2012.03.116

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    13

    A quaternary ammonium grafted poly vinyl benzyl chloride membrane for alkaline anion exchange membrane water electrolysers with no-noble-metal catalysts

    Cao, Yuan-Cheng; Wu, Xu; Scott, Keith

    International Journal of Hydrogen Energy (2012), 37 (12), 9524-9528CODEN: IJHEDX; ISSN:0360-3199. (Elsevier Ltd.)

    This work reported an alk. anion exchange membrane water electrolyzer (AAEMWE) without noble metal as the catalyst. Methylated melamine grafted poly vinyl benzyl chloride (mm-qPVBz/Cl-) was synthesized and cast as the membrane. The cond. of this hydroxide ion exchange membrane increased from 1.6 × 10-2 S cm-1 to 2.7 × 10-2 S cm-1 when the temp. was increased from 25 °C to 60 °C. Membranes were examd. using TEM. The oxygen evolution catalyst used was based on Cu0.7Co2.3O4 particle 20-30 nm in size, synthesized through a thermal decompn. method. A membrane electrode assembly was prepd. with the resultant membrane as electrolyte, the Cu0.7Co2.3O4 nano-particles as the anode catalyst and Ni nano-powders as the hydrogen evolution catalyst. SEM observations showed that the catalysts were well dispersed on the electrodes. The polarization curves exhibited onset voltages for water electrolysis of around 1.5 V. The MEA polarization in deionized water exhibited voltages of 2.19 V, 2.05 V, 1.99 V at a c.d. of 100 mA cm-2 at temps. of 25 °C, 40 °C and 55 °C resp.
14. 14

    Chu, X.; Shi, Y.; Liu, L.; Huang, Y.; Li, N. Piperidinium-Functionalized Anion Exchange Membranes and Their Application in Alkaline Fuel Cells and Water Electrolysis. J. Mater. Chem. A 2019, 7, 7717– 7727,  DOI: 10.1039/C9TA01167F

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    14

    Piperidinium-functionalized anion exchange membranes and their application in alkaline fuel cells and water electrolysis

    Chu, Xiaomeng; Shi, Yan; Liu, Lei; Huang, Yingda; Li, Nanwen

    Journal of Materials Chemistry A: Materials for Energy and Sustainability (2019), 7 (13), 7717-7727CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)

    To produce a stable anion exchange membrane (AEM) for deployable electrochem. devices with a long lifespan, we here present the synthesis and properties of a series of piperidinium-functionalized poly(2,6-di-Me phenylene oxide)s with different locations of piperidinium groups along the polymer backbones. A distinct phase sepd. morphol. was obsd. for long side-chain-type AEMs (LSCPi) as confirmed by AFM anal., which in turn enabled its higher hydroxide cond. over side-chain-type (SCPi) and std. benzylmethyl piperidinium AEMs (BPi). A hydroxide cond. of 29.0 mS cm-1 at 20°C was achieved for the LSCPi membrane with an IEC value of 1.57 meq. g-1. This level of cond. was lower than that of the corresponding QA-based AEMs (LSCQA) (38.7 mS cm-1 at 20°C), probably as a result of its low IEC accompanied by low water uptake. The LSCPi membrane displayed excellent alk. stability with 98% retention in cond. after 560 h of testing in 1 M NaOH at 80°C, and no obvious degrdn. was detected by NMR anal. of the aged sample. To demonstrate the feasibility of piperidinium-functionalized AEMs, both SCPi and LSCPi membranes were fabricated into a membrane electrode assembly for the H2/O2 alk. fuel cell and AEM water electrolyzer applications. The highly conductive LSCPi membrane showed good cell performance with a peak power d. of 116 mW cm-2 at 60°C in alk. fuel cells and 300 mA cm-2 at 1.80 V at 50°C in AEM water electrolysis working with pure water. Although a gradual drop in performance was obsd. for both the alk. fuel cell and water electrolyzer durability testing at a const. current during the test of 8.7 h and 35 h resp., the high durability of AEMs having piperidinium cations was verified by post-mortem anal. of aged AEMs by NMR spectroscopy. The current findings provided fundamental insights into the durability of AEMs under ex situ and in situ operating conditions and demonstrated that the piperidinium-functionalized AEM appears to be a promising material for durable AEM-based devices.
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    Fortin, P.; Khoza, T.; Cao, X.; Martinsen, S. Y.; Oyarce Barnett, A.; Holdcroft, S. High-performance Alkaline Water Electrolysis Using Aemion Anion Exchange Membranes. J. Power Sources 2020, 451, 227814  DOI: 10.1016/j.jpowsour.2020.227814

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    15

    High-performance alkaline water electrolysis using Aemion anion exchange membranes

    Fortin, Patrick; Khoza, Thulile; Cao, Xinzhi; Martinsen, Stig Yngve; Oyarce Barnett, Alejandro; Holdcroft, Steven

    Journal of Power Sources (2020), 451 (), 227814CODEN: JPSODZ; ISSN:0378-7753. (Elsevier B.V.)

    We report the use of all-hydrocarbon anion exchange membranes capable of achieving high current densities through the optimization of MEA fabrication and operating conditions. The electrochem. behavior of com.-available Aemion anion exchange membranes with various ion-exchange capacities and thicknesses is investigated, and the impact of these properties on electrolyzer performance and short-term stability are discussed. The hydrocarbon anion exchange membrane AF1-HNN8-50, having an ion exchange capacity of 2.1-2.5 meq OH- g-1 and a thickness of 50 μm, was able to achieve current densities of 2 A cm-2 at a potential of 1.82 V using 1 M KOH at 60 °C.
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    Hagesteijn, K. F. L.; Jiang, S.; Ladewig, B. P. A Review of the Synthesis and Characterization of Anion Exchange Membranes. J. Mater. Sci. 2018, 53, 11131– 11150,  DOI: 10.1007/s10853-018-2409-y

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    16

    A review of the synthesis and characterization of anion exchange membranes

    Hagesteijn, Kimberly F. L.; Jiang, Shanxue; Ladewig, Bradley P.

    Journal of Materials Science (2018), 53 (16), 11131-11150CODEN: JMTSAS; ISSN:0022-2461. (Springer)

    This review highlights advancements made in anion exchange membrane (AEM) head groups, polymer structures and membrane synthesis methods. Limitations of current anal. techniques for characterizing AEMs are also discussed. AEM research is primarily driven by the need to develop suitable AEMs for the high-pH and high-temp. environments in anion exchange membrane fuel cells and anion exchange membrane water electrolysis applications. AEM head groups can be broadly classified as nitrogen based (e.g. quaternary ammonium), nitrogen free (e.g. phosphonium) and metal cations (e.g. ruthenium). Metal cation head groups show great promise for AEM due to their high stability and high valency. Through "rational polymer architecture", it is possible to synthesize AEMs with ion channels and improved chem. stability. Heterogeneous membranes using porous supports or inorg. nanoparticles show great promise due to the ability to tune membrane characteristics based on the ratio of polymer to porous support or nanoparticles. Future research should investigate consolidating advancements in AEM head groups with an optimized polymer structure in heterogeneous membranes to bring together the valuable characteristics gained from using head groups with improved chem. stability, with the benefits of a polymer structure with ion channels and improved membrane properties from using a porous support or nanoparticles.
17. 17

    Chen, P.; Hu, X. High-Efficiency Anion Exchange Membrane Water Electrolysis Employing Non-Noble Metal Catalysts. Adv. Energy Mater. 2020, 10, 2002285  DOI: 10.1002/aenm.202002285

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    17

    High-Efficiency Anion Exchange Membrane Water Electrolysis Employing Non-Noble Metal Catalysts

    Chen, Pengzuo; Hu, Xile

    Advanced Energy Materials (2020), 10 (39), 2002285CODEN: ADEMBC; ISSN:1614-6840. (Wiley-Blackwell)

    Alk. anion exchange membrane (AEM) water electrolysis is a promising technol. for producing hydrogen using renewable energies. However, current AEM electrolyzers still employ noble-metal-contg. electrocatalysts, or have significant overpotential loss, or both. Here non-noble-metal electrocatalysts for both the hydrogen and oxygen evolution reactions (HER and OER) are developed. Both catalysts are made of a same NiMo oxide. Judicious processing of these materials in a mixed NH3/H2 atmosphere results in a NiMo-NH3/H2 catalyst, which has superior activity in HER, delivering 500 mA cm-2 at an overpotential of 107 mV. Doping Fe ions into the NiMo-NH3/H2 catalyst yields an Fe-NiMo-NH3/H2 catalyst, which is highly active for the OER, delivering 500 mA cm-2 at an overpotential of 244 mV. These catalysts are integrated into an AEM electrolyzer, which delivers 1.0 A cm-2 at 1.57 V at 80 &°C in 1 M KOH. The energy conversion efficiency at this c.d. is as high as 75%. This work demonstrates high-efficiency AEM electrolysis using earth-abundant catalytic materials.
18. 18

    Cossar, E.; Oyarce Barnett, A.; Seland, F.; Baranova, E. A. The Performance of Nickel and Nickel-Iron Catalysts Evaluated as Anodes in Anion Exchange Membrane Water Electrolysis. Catalysts 2019, 9, 814  DOI: 10.3390/catal9100814

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    18

    The performance of nickel and nickel-iron catalysts evaluated as anodes in anion exchange membrane water electrolysis

    Cossar, Emily; Barnett, Alejandro Oyarce; Seland, Frode; Baranova, Elena A.

    Catalysts (2019), 9 (10), 814CODEN: CATACJ; ISSN:2073-4344. (MDPI AG)

    Anion exchange membrane water electrolysis (AEMWE) is an efficient, cost-effective soln. to renewable energy storage. The process includes oxygen and hydrogen evolution reactions (OER and HER); the OER is kinetically unfavorable. Studies have shown that nickel (Ni)- iron (Fe) catalysts enhance activity towards OER, and cerium oxide (CeO2) supports have shown pos. effects on catalytic performance. This study covers the preliminary evaluation of Ni, Ni90Fe10 (at%) and Ni90Fe10/CeO2 (50 wt%) nanoparticles (NPs), synthesized by chem. redn., as OER catalysts in AEMWE using com. membranes. Transmission electron microscopy (TEM) images of the Ni-based NPs indicate NPs roughly 4-6 nm in size. Three-electrode cell measurements indicate that Ni90Fe10 is the most active non-noble metal catalyst in 1 and 0.1 M KOH. AEMWE measurements of the anodes show cells achieving overall cell voltages between 1.85 and 1.90 V at 2 A cm-2 in 1 M KOH at 50°C, which is comparable to the selected iridium-black ref. catalyst. In 0.1 M KOH, the AEMWE cell contg. Ni90Fe10 attained the lowest voltage of 1.99 V at 2 A cm-2. Electrochem. impedance spectroscopy (EIS) of the AEMWE cells using Ni90Fe10/CeO2 showed a higher ohmic resistance than all catalysts, indicating the need for support optimization.
19. 19

    Henkensmeier, D.; Najibah, M.; Harms, C.; Žitka, J.; Hnát, J.; Bouzek, K. Overview: State-of-the-Art Commercial Membranes for Anion Exchange Membrane Water Electrolysis. J. Electrochem. Energy Convers. Storage 2021, 18, 024001  DOI: 10.1115/1.4047963

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    Overview: state-of-the art commercial membranes for anion exchange membrane water electrolysis

    Henkensmeier, Dirk; Najibah, Malikah; Harms, Corinna; Zitka, Jan; Hnat, Jaromir; Bouzek, Karel

    Journal of Electrochemical Energy Conversion and Storage (2021), 18 (2), 024001CODEN: JEECAJ; ISSN:2381-6910. (American Society of Mechanical Engineers)

    A review. The intermittent availability of renewal energy makes it difficult to integrate it with established alk. water electrolysis technol. Proton exchange membrane (PEM) water electrolysis (PEMEC) is promising, but limited by the necessity to use expensive platinum and iridium catalysts. The expected soln. is anion exchange membrane (AEM) water electrolysis, which combines the use of cheap and abundant catalyst materials with the advantages of PEM water electrolysis, namely, a low foot print, large operational capacity, and fast response to changing operating conditions. The key component for AEM water electrolysis is a cheap, stable, gas tight and highly hydroxide conductive polymeric AEM. Here, we present target values and tech. requirements for AEMs, discuss the chem. structures involved and the related degrdn. pathways, give an overview over the most prominent and promising com. AEMs (Fumatech Fumasep FAA3, Tokuyama A201, Ionomr Aemion, Dioxide materials Sustainion, and membranes commercialized by Orion Polymer), and review their properties and performances of water electrolyzers using these membranes. One promising way to store and distribute large amts. of renewable energy is water electrolysis, coupled with transport of hydrogen in the gas grid and storage in tanks and caverns.
20. 20

    Jiao, S.; Fu, X.; Wang, S.; Zhao, Y. Perfecting the Electrocatalysts via Imperfections: towards Large-Scale Deployment of Water Electrolysis Technology. Energy Environ. Sci. 2021, 14, 1722– 1770,  DOI: 10.1039/D0EE03635H

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    20

    Perfecting electrocatalysts via imperfections: towards the large-scale deployment of water electrolysis technology

    Jiao, Shilong; Fu, Xianwei; Wang, Shuangyin; Zhao, Yong

    Energy & Environmental Science (2021), 14 (4), 1722-1770CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)

    A review. As a potential energy carrier, hydrogen has surged up the priority list as part of broader decarbonization efforts and strategies to build or acquire clean energy economies. Driven by renewable electricity, electrochem. water splitting (WS) promises an ideal long-term, low-carbon way to produce hydrogen, with the ability to tackle various crit. energy challenges. To improve the efficiency of electrocatalytic water splitting, electrocatalysts with enhanced cond., more exposed active sites, and high intrinsic activity are crucial for decreasing the energy gap for the rate-detg. step (RDS) and subsequently improving the conversion efficiency. The incorporation of multidimensional imperfections has been demonstrated to be efficient for modulating the electron distribution and speeding up the electrocatalysis kinetics during electrocatalytic processes and this is now attracting ever-increasing attention. Herein, in this review, we summarize recent progress relating to the regulation of elec. behavior and electron distributions for the optimization of electrocatalytic water-splitting performance via defect engineering. With an emphasis on the beneficial aspects of the hydrogen economy and an in-depth understanding of electron redistribution caused by defect effects, we offer a comprehensive summary of the progress made in the last three to five years. Finally, we also offer future perspectives on the challenges and opportunities relating to water-splitting electrocatalysts in this attractive field.
21. 21

    Kwon, C. Y.; Jeong, J. Y.; Yang, J.; Park, Y. S.; Jeong, J.; Park, H.; Kim, Y.; Choi, S. M. Effect of Copper Cobalt Oxide Composition on Oxygen Evolution Electrocatalysts for Anion Exchange Membrane Water Electrolysis. Front. Chem. 2020, 8, 600908  DOI: 10.3389/fchem.2020.600908

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    Effect of copper cobalt oxide composition on oxygen evolution electrocatalysts for anion exchange membrane water electrolysis

    Kwon, Chae-Yeon; Jeong, Jae-Yeop; Yang, Juchan; Park, Yoo Sei; Jeong, Jaehoon; Park, Honghyun; Kim, Yangdo; Choi, Sung Mook

    Frontiers in Chemistry (Lausanne, Switzerland) (2020), 8 (), 600908CODEN: FCLSAA; ISSN:2296-2646. (Frontiers Media S.A.)

    Copper cobalt oxide nanoparticles (CCO NPs) were synthesized as an oxygen evolution electrocatalyst via a simple co-pptn. method, with the compn. being controlled by altering the precursor ratio to 1:1, 1:2, and 1:3 (Cu:Co) to investigate the effects of compn. changes. The effect of the ratio of Cu2+/Co3+ and the degree of oxidn. during the co-pptn. and annealing steps on the crystal structure, morphol., and electrocatalytic properties of the produced CCO NPs were studied. The CCO1:2 electrode exhibited an outstanding performance and high stability owing to the suitable electrochem. kinetics, which was provided by the presence of sufficient Co3+ as active sites for oxygen evolution and the uniform sizes of the NPs in the half cell. Furthermore, single cell tests were performed to confirm the possibility of using the synthesized electrocatalyst in a practical water splitting system. The CCO1:2 electrocatalyst was used as an anode to develop an anion exchange membrane water electrolyzer (AEMWE) cell. The full cell showed stable hydrogen prodn. for 100 h with an energetic efficiency of >71%. In addn., it was possible to mass produce the uniform, highly active electrocatalyst for such applications through the co-pptn. method.
22. 22

    Liao, H.; Guo, X.; Hou, Y.; Liang, H.; Zhou, Z.; Yang, H. Construction of Defect-Rich Ni-Fe-Doped K_0.23MnO₂ Cubic Nanoflowers via Etching Prussian Blue Analogue for Efficient Overall Water Splitting. Small 2020, 16, 1905223  DOI: 10.1002/smll.201905223

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    22

    Construction of Defect-Rich Ni-Fe-Doped K0.23MnO2 Cubic Nanoflowers via Etching Prussian Blue Analogue for Efficient Overall Water Splitting

    Liao, Huanyun; Guo, Xingzhong; Hou, Yang; Liang, Hao; Zhou, Zheng; Yang, Hui

    Small (2020), 16 (10), 1905223CODEN: SMALBC; ISSN:1613-6810. (Wiley-VCH Verlag GmbH & Co. KGaA)

    Designing elaborate nanostructures and engineering defects have been promising approaches to fabricate cost-efficient electrocatalysts toward overall water splitting. In this work, a controllable Prussian-blue-analog-sacrificed strategy followed by an annealing process to harvest defect-rich Ni-Fe-doped K0.23MnO2 cubic nanoflowers (Ni-Fe-K0.23MnO2 CNFs-300) as highly active bifunctional catalysts for oxygen and hydrogen evolution reactions (OER and HER) is reported. Benefiting from many merits, including unique morphol., abundant defects, and doping effect, Ni-Fe-K0.23MnO2 CNFs-300 shows the best electrocatalytic performances among currently reported Mn oxide-based electrocatalysts. This catalyst affords low overpotentials of 270 (320) mV at 10 (100) mA cm-2 for OER with a small Tafel slope of 42.3 mV dec-1, while requiring overpotentials of 116 and 243 mV to attain 10 and 100 mA cm-2 for HER resp. Moreover, Ni-Fe-K0.23MnO2 CNFs-300 applied to overall water splitting exhibits a low cell voltage of 1.62 V at 10 mA cm-2 and excellent durability, even superior to the Pt/C||IrO2 cell at large c.d. D. functional theory calcns. further confirm that doping Ni and Fe into the crystal lattice of δ-MnO2 can not only reinforce the cond. but also reduces the adsorption free-energy barriers on the active sites during OER and HER.
23. 23

    Xi, W.; Yan, G.; Lang, Z.; Ma, Y.; Tan, H.; Zhu, H.; Wang, Y.; Li, Y. Oxygen-Doped Nickel Iron Phosphide Nanocube Arrays Grown on Ni Foam for Oxygen Evolution Electrocatalysis. Small 2018, 14, 1802204  DOI: 10.1002/smll.201802204

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    There is no corresponding record for this reference.
24. 24

    Xuan, C.; Zhang, J.; Wang, J.; Wang, D. Rational Design and Engineering of Nanomaterials Derived from Prussian Blue and Its Analogs for Electrochemical Water Splitting. Chem. – Asian J. 2020, 15, 958– 972,  DOI: 10.1002/asia.201901721

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    24

    Rational Design and Engineering of Nanomaterials Derived from Prussian Blue and Its Analogs for Electrochemical Water Splitting

    Xuan, Cuijuan; Zhang, Jian; Wang, Jie; Wang, Deli

    Chemistry - An Asian Journal (2020), 15 (7), 958-972CODEN: CAAJBI; ISSN:1861-4728. (Wiley-VCH Verlag GmbH & Co. KGaA)

    A review. Electrochem. water splitting (EWS) is a sustainable and promising technol. for producing hydrogen as an ideal energy carrier to address environmental and energy issues. Developing highly-efficient electrocatalysts for hydrogen and oxygen evolution reactions (HER and OER) is crit. for increasing the efficiency of water electrolysis. Recently, nanomaterials derived from Prussian blue (PB) and its analogs (PBA) have received increasing attention in EWS applications owing to their unique compn. and structure properties. In this Minireview, the latest progress of PB/PBA-derived materials for EWS is presented. Firstly, the catalyst design principles and the advantages of prepg. electrocatalysts with PB/PBA as precursors are briefly introduced. Then, strategies for enhancing the electrocatalytic performance (HER, OER or overall water splitting) were discussed in detail, and the recent development and applications of PB/PBA-derived catalysts for EWS were summarized. Finally, major challenges and possible future trends related to PB/PBA-derived functional materials are proposed.
25. 25

    Zhao, D.; Lu, Y.; Ma, D. Effects of Structure and Constituent of Prussian Blue Analogs on Their Application in Oxygen Evolution Reaction. Molecules 2020, 25, 2304  DOI: 10.3390/molecules25102304

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    25

    Effects of structure and constituent of prussian blue analogs on their application in oxygen evolution reaction

    Zhao, Dongni; Lu, Yuezhen; Ma, Dongge

    Molecules (2020), 25 (10), 2304CODEN: MOLEFW; ISSN:1420-3049. (MDPI AG)

    The importance of advanced energy-conversion devices such as water electrolysis has manifested dramatically over the past few decades because it is the current mainstay for the generation of green energy. Anodic oxygen evolution reaction (OER) in water splitting is one of the biggest obstacles because of its extremely high kinetic barrier. Conventional OER catalysts are mainly noble-metal oxides represented by IrO2 and RuO2, but these compds. tend to have poor sustainability. The attention on Prussian blue (PB) and its analogs (PBA) in the field of energy conversion systems was concd. on their open-framework structure, as well as its varied compn. comprised of Earth-abundant elements. The unique electronic structure of PBA enables its promising catalytic potential, and it can also be converted into many other talented compds. or structures as a precursor. This undoubtedly provides a new approach for the design of green OER catalysts. This article reviews the recent progress of the application of PBA and its derivs. in OER based on in-depth studies of characterization techniques. The structural design, synthetic strategy, and enhanced electrochem. properties are summarized to provide an outlook for its application in the field of OER. Moreover, due to the similarity of the reaction process of photo-driven electrolysis of water and the former one, the application of PBA in photoelectrolysis is also discussed.
26. 26

    Cao, L. M.; Hu, Y. W.; Tang, S. F.; Iljin, A.; Wang, J. W.; Zhang, Z. M.; Lu, T. B. Fe-CoP Electrocatalyst Derived from a Bimetallic Prussian Blue Analogue for Large-Current-Density Oxygen Evolution and Overall Water Splitting. Adv. Sci. 2018, 5, 1800949  DOI: 10.1002/advs.201800949

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    26

    Fe-CoP Electrocatalyst Derived from a Bimetallic Prussian Blue Analogue for Large-Current-Density Oxygen Evolution and Overall Water Splitting

    Cao Li-Ming; Hu Yu-Wen; Wang Jia-Wei; Lu Tong-Bu; Tang Shang-Feng; Zhang Zhi-Ming; Lu Tong-Bu; Iljin Andrey

    Advanced science (Weinheim, Baden-Wurttemberg, Germany) (2018), 5 (10), 1800949 ISSN:2198-3844.

    Industrial application of overall water splitting requires developing readily available, highly efficient, and stable oxygen evolution electrocatalysts that can efficiently drive large current density. This study reports a facile and practical method to fabricate a non-noble metal catalyst by directly growing a Co-Fe Prussian blue analogue on a 3D porous conductive substrate, which is further phosphorized into a bifunctional Fe-doped CoP (Fe-CoP) electrocatalyst. The Fe-CoP/NF (nickel foam) catalyst shows efficient electrocatalytic activity for oxygen evolution reaction, requiring low overpotentials of 190, 295, and 428 mV to achieve 10, 500, and 1000 mA cm(-2) current densities in 1.0 m KOH solution. In addition, the Fe-CoP/NF can also function as a highly active electrocatalyst for hydrogen evolution reaction with a low overpotential of 78 mV at 10 mA cm(-2) current density in alkaline solution. Thus, the Fe-CoP/NF electrode with meso/macropores can act as both an anode and a cathode to fabricate an electrolyzer for overall water splitting, only requiring a cell voltage of 1.49 V to afford a 10 mA cm(-2) current density with remarkable stability. This performance appears to be among the best reported values and is much better than that of the IrO2-Pt/C-based electrolyzer.
27. 27

    Cui, Y.; Xue, Y.; Zhang, R.; Zhang, J.; Li, X.; Zhu, X. Vanadium-Cobalt Oxyhydroxide Shows Ultralow Overpotential for the Oxygen Evolution Reaction. J. Mater. Chem. A 2019, 7, 21911– 21917,  DOI: 10.1039/C9TA07918A

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    27

    Vanadium-cobalt oxyhydroxide shows ultralow overpotential for the oxygen evolution reaction

    Cui, Yan; Xue, Yuan; Zhang, Rui; Zhang, Jian; Li, Xing'ao; Zhu, Xinbao

    Journal of Materials Chemistry A: Materials for Energy and Sustainability (2019), 7 (38), 21911-21917CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)

    Transition metal oxyhydroxides are one of the most effective electrocatalysts for the oxygen evolution reaction (OER), which is often considered as a bottleneck in the water splitting process. Herein, a cation-modulated V-Co-based oxyhydroxide one-dimensional scaffold configuration (Co1-xVxOOH) assembled using uniform ∼4 nm thick nanosheets is reported, which demonstrates superb OER activity and durability. Theor., the Co sites adjacent to V in the bimetal samples have the max. required energy of 0.37 V (*O → *OOH) when combined with 20% V (Co0.8V0.2OOH), which is favorable for enhancing the OER activity with the ultralow overpotential of 190 mV vs. the c.d. of 10 mA cm-2, Tafel slope of 39.6 mV dec-1 and remarkable durability over 100 h. To the best of our knowledge, the overpotential of 190 mV at 10 mA cm-2 is the best value reported to date for Co or V (oxy)hydroxide-based OER catalysts. The outstanding activity is ascribed to the hierarchically stable scaffold configuration, high electrochem. active surface area, enhanced oxygen vacancies on the surface and the synergistic effect of the active metal atoms. This study affords a strategy for the rational design of earth-abundant electrocatalysts for energy conversion applications.
28. 28

    Jiang, M.; Li, Y.; Lu, Z.; Sun, X.; Duan, X. Binary Nickel-Iron Nitride Nanoarrays as Bifunctional Electrocatalysts for Overall Water Splitting. Inorg. Chem. Front. 2016, 3, 630– 634,  DOI: 10.1039/C5QI00232J

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    Binary nickel-iron nitride nanoarrays as bifunctional electrocatalysts for overall water splitting

    Jiang, Ming; Li, Yingjie; Lu, Zhiyi; Sun, Xiaoming; Duan, Xue

    Inorganic Chemistry Frontiers (2016), 3 (5), 630-634CODEN: ICFNAW; ISSN:2052-1553. (Royal Society of Chemistry)

    Electrochem. water splitting provides a facile method for high-purity hydrogen prodn., but electro-catalysts with a stable bifunctional activity towards both oxygen and hydrogen evolution have been rarely developed. Herein we report a Fe2Ni2N material with a vertically aligned nanoplate array architecture as a bifunctional catalyst for overall water splitting in an alk. environment. This advanced catalyst affords small onset overpotentials and fast c.d. increase, resulting in an excellent water splitting performance (requiring 1.65 V for achieving 10 mA cm-2), superior to the combination of benchmark noble metal catalysts.
29. 29

    Li, P.; Duan, X.; Kuang, Y.; Li, Y.; Zhang, G.; Liu, W.; Sun, X. Tuning Electronic Structure of NiFe Layered Double Hydroxides with Vanadium Doping toward High Efficient Electrocatalytic Water Oxidation. Adv. Energy Mater. 2018, 8, 1703341  DOI: 10.1002/aenm.201703341

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    There is no corresponding record for this reference.
30. 30

    Wang, D.; Li, Q.; Han, C.; Lu, Q.; Xing, Z.; Yang, X. Atomic and Electronic Modulation of Self-Supported Nickel-Vanadium Layered Double Hydroxide to Accelerate Water Splitting Kinetics. Nat. Commun. 2019, 10, 3899  DOI: 10.1038/s41467-019-11765-x

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    Atomic and electronic modulation of self-supported nickel-vanadium layered double hydroxide to accelerate water splitting kinetics

    Wang Dewen; Li Qun; Han Ce; Lu Qingqing; Xing Zhicai; Yang Xiurong; Wang Dewen; Li Qun; Yang Xiurong; Lu Qingqing

    Nature communications (2019), 10 (1), 3899 ISSN:.

    Herein, ruthenium (Ru) and iridium (Ir) are introduced to tailor the atomic and electronic structure of self-supported nickel-vanadium (NiV) layered double hydroxide to accelerate water splitting kinetics, and the origin of high hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) activities are analyzed at atomic level. X-ray photoelectron spectroscopy and X-ray absorption near-edge structure spectroscopy studies reveal synergistic electronic interactions among Ni, V, and Ru (Ir) cations. Raman spectra and Fourier and wavelet transform analyses of the extended X-ray absorption fine structure indicate modulated local coordination environments around the Ni and V cations, and the existence of V vacancies. The Debye-Waller factor suggests a severely distorted octahedral V environment caused by the incorporation of Ru and Ir. Theoretical calculations further confirm that Ru or Ir doping could optimize the adsorption energy of intermediates in the Volmer and Heyrovsky steps for HER and accelerate the whole kinetic process for OER.
31. 31

    Xuan, C.; Peng, Z.; Xia, K.; Wang, J.; Xiao, W.; Lei, W.; Gong, M.; Huang, T.; Wang, D. Self-Supported Ternary Ni-Fe-P Nanosheets Derived from Metal-Organic Frameworks as Efficient Overall Water Splitting Electrocatalysts. Electrochim. Acta 2017, 258, 423– 432,  DOI: 10.1016/j.electacta.2017.11.078

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    31

    Self-supported ternary Ni-Fe-P nanosheets derived from metal-organic frameworks as efficient overall water splitting electrocatalysts

    Xuan, Cuijuan; Peng, Zongkai; Xia, Kedong; Wang, Jie; Xiao, Weiping; Lei, Wen; Gong, Mingxing; Huang, Ting; Wang, Deli

    Electrochimica Acta (2017), 258 (), 423-432CODEN: ELCAAV; ISSN:0013-4686. (Elsevier Ltd.)

    Developing cost-effective, highly-efficient and stable electrocatalysts is of significance to replace noble metal based materials for overall H2O electrolysis. Ni-Fe phosphide nanosheets on Ni foam (Ni-Fe-P/NF) were synthesized through in-situ chem. etching and subsequently phosphating treatment. The Ni foam used in this work not only serves as conductive substrate and metal current collector, but also as Ni source for the growth of metal org. frameworks (i.e. Prussian blue analog pyramids), which were then converted to Ni-Fe-P nanosheets via phosphating. Benefiting from the unique nanostructure, Fe incorporation, and the high cond. of Ni foam, the resulting Ni-Fe-P/NF could be used as self-supported and binder-free working electrode with superior overall electrochem. H2O splitting performance. Electrochem. measurement demonstrates that the Ni-Fe-P/NF exhibits excellent electrocatalytic activities with overpotentials of 98 mV for HER and 168 mV for OER to deliver current densities of 10 mA cm-2 in 1 M KOH soln. Also, the Ni-Fe-P/NF catalyst was also employed as both as anode and cathode for overall H2O electrolysis, and shows extraordinary activities with low voltage of only 1.486 V to yield 10 mA cm-2 and outstanding cycling stability with negligible voltage elevation after chronopotentiometry detn. for 200 h. This work highlights that direct growth of metal org. frameworks on conductive substrates is an effective method to explore electrocatalysts for multifunctional electrochem. applications.
32. 32

    Zhang, B.; Xiao, C.; Xie, S.; Liang, J.; Chen, X.; Tang, Y. Iron-Nickel Nitride Nanostructures in Situ Grown on Surface-Redox-Etching Nickel Foam: Efficient and Ultrasustainable Electrocatalysts for Overall Water Splitting. Chem. Mater. 2016, 28, 6934– 6941,  DOI: 10.1021/acs.chemmater.6b02610

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    32

    Iron-Nickel Nitride Nanostructures in Situ Grown on Surface-Redox-Etching Nickel Foam: Efficient and Ultrasustainable Electrocatalysts for Overall Water Splitting

    Zhang, Bo; Xiao, Chunhui; Xie, Sanmu; Liang, Jin; Chen, Xu; Tang, Yuhai

    Chemistry of Materials (2016), 28 (19), 6934-6941CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)

    Water splitting is widely considered to be a promising strategy for clean and efficient energy prodn. For the 1st time the authors report an in situ growth of iron-nickel nitride nanostructures on surface-redox-etching Ni foam (FeNi3N/NF) as a bifunctional electrocatalyst for overall water splitting. This method does not require a specially added nickel precursor nor an oxidizing agent, but achieves well-dispersed iron-nickel nitride nanostructures that are grown directly on the nickel foam surface. The com. Ni foam in this work not only acts as a substrate but also serves as a slow-releasing nickel precursor that is induced by redox-etching of Fe3+. FeCl2 is a more preferable iron precursor than FeCl3 for no matter quality of FeNi3N growth or its electrocatalytic behaviors. The obtained FeNi3N/NF exhibits extraordinarily high activities for both oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) with low overpotentials of 202 and 75 mV at 10 mA/cm2, Tafel slopes of 40 and 98 mV/dec, resp. The presented FeNi3N/NF catalyst has an extremely good durability, reflecting in >400 h of consistent galvanostatic electrolysis without any visible voltage elevation.
33. 33

    Yu, F.; Zhou, H.; Huang, Y.; Sun, J.; Qin, F.; Bao, J.; Goddard, W. A., 3rd; Chen, S.; Ren, Z. High-Performance Bifunctional Porous Non-Noble Metal Phosphide Catalyst for Overall Water Splitting. Nat. Commun. 2018, 9, 2551  DOI: 10.1038/s41467-018-04746-z

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    High-performance bifunctional porous non-noble metal phosphide catalyst for overall water splitting

    Yu Fang; Zhou Haiqing; Sun Jingying; Chen Shuo; Ren Zhifeng; Yu Fang; Zhou Haiqing; Huang Yufeng; Goddard William A 3rd; Qin Fan; Bao Jiming

    Nature communications (2018), 9 (1), 2551 ISSN:.

    Water electrolysis is an advanced energy conversion technology to produce hydrogen as a clean and sustainable chemical fuel, which potentially stores the abundant but intermittent renewable energy sources scalably. Since the overall water splitting is an uphill reaction in low efficiency, innovative breakthroughs are desirable to greatly improve the efficiency by rationally designing non-precious metal-based robust bifunctional catalysts for promoting both the cathodic hydrogen evolution and anodic oxygen evolution reactions. We report a hybrid catalyst constructed by iron and dinickel phosphides on nickel foams that drives both the hydrogen and oxygen evolution reactions well in base, and thus substantially expedites overall water splitting at 10 mA cm(-2) with 1.42 V, which outperforms the integrated iridium (IV) oxide and platinum couple (1.57 V), and are among the best activities currently. Especially, it delivers 500 mA cm(-2) at 1.72 V without decay even after the durability test for 40 h, providing great potential for large-scale applications.
34. 34

    Thangavel, P.; Ha, M.; Kumaraguru, S.; Meena, A.; Singh, A. N.; Harzandi, A. M.; Kim, K. S. Graphene-Nanoplatelets-Supported NiFe-MOF: High-Efficiency and Ultra-Stable Oxygen Electrodes for Sustained Alkaline Anion Exchange Membrane Water Electrolysis. Energy Environ. Sci. 2020, 13, 3447– 3458,  DOI: 10.1039/D0EE00877J

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    34

    Graphene-nanoplatelets-supported NiFe-MOF: high-efficiency and ultra-stable oxygen electrodes for sustained alkaline anion exchange membrane water electrolysis

    Thangavel, Pandiarajan; Ha, Miran; Kumaraguru, Shanmugasundaram; Meena, Abhishek; Singh, Aditya Narayan; Harzandi, Ahmad M.; Kim, Kwang S.

    Energy & Environmental Science (2020), 13 (10), 3447-3458CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)

    Practical hydrogen prodn. using high-efficiency, low-cost, and stable oxygen electrodes is crucial for a sustainable clean energy future. Herein we report a graphene-nanoplatelets-supported (Ni,Fe) metal-org. framework (MOF) as a superior and ultra-durable (>1000 h) anode for alk. water electrolysis. The MOF on carbon-fiber paper electrodes requires an overpotential η = 220 mV to achieve a c.d. j = 10 mA cm-2 (η = 180 mV on nickel foam for j = 20 mA cm-2) with a Tafel slope of 51 mV per decade, high turnover frequency (1.22 s-1), high faradaic efficiency (99.1%), and long-term durability of >1000 h in continuous electrolysis. In an alk. anion exchange membrane water electrolyzer (AAEMWE), it exhibits a record c.d. of 540 mA cm-2 at 1.85 V at 70°C, outperforming the state-of-the-art Pt/C//IrO2. A breakthrough strategy introduced in membrane electrode assembly fabrication by extending the elec. contact with an aq. electrolyte offers an addnl. OH- transport pathway to regenerate the original cond. of the AAEMWE in continuous electrolysis, without any significant change in the pH of the electrolyte. These findings open up durable, high-performance AAEMWE and direct solar-to-fuel conversion, esp. to replace high-cost proton exchange membrane water electrolysis that already works with ultra-pure water.
35. 35

    Harzandi, A. M.; Shadman, S.; Nissimagoudar, A. S.; Kim, D. Y.; Lim, H. D.; Lee, J. H.; Kim, M. G.; Jeong, H. Y.; Kim, Y.; Kim, K. S. Ruthenium Core-Shell Engineering with Nickel Single Atoms for Selective Oxygen Evolution via Nondestructive Mechanism. Adv. Energy Mater. 2021, 11, 2003448  DOI: 10.1002/aenm.202003448

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    35

    Ruthenium Core-Shell Engineering with Nickel Single Atoms for Selective Oxygen Evolution via Nondestructive Mechanism

    Harzandi, Ahmad M.; Shadman, Sahar; Nissimagoudar, Arun S.; Kim, Dong Yeon; Lim, Hee-Dae; Lee, Jong Hoon; Kim, Min Gyu; Jeong, Hu Young; Kim, Youngsik; Kim, Kwang S.

    Advanced Energy Materials (2021), 11 (10), 2003448CODEN: ADEMBC; ISSN:1614-6840. (Wiley-Blackwell)

    To develop effective electrocatalytic splitting of acidic water, which is a key reaction for renewable energy conversion, the fundamental understanding of sluggish/destructive mechanism of the oxygen evolution reaction (OER) is essential. Through investigating atom/proton/electron transfers in the OER, the distinctive acid-base (AB) and direct-coupling (DC) lattice oxygen mechanisms (LOMs) and adsorbates evolution mechanism (AEM) are elucidated, depending on the surface-defect engineering condition. The designed catalysts are composed of a compressed metallic Ru-core and oxidized Ru-shell with Ni single atoms (SAs). The catalyst synthesized with hot acid treatment selectively follows AB-LOM, exhibiting simultaneously enhanced activity and stability. It produces a c.d. of 10/100 mA cm-2 at a low overpotential of 184/229 mV and sustains water oxidn. at a high c.d. of up to 20 mA cm-2 over ≈200 h in strongly acidic media.
36. 36

    Meena, A.; Thangavel, P.; Jeong, D. S.; Singh, A. N.; Jana, A.; Im, H.; Nguyen, D. A.; Kim, K. S. Crystalline-Amorphous Interface of Mesoporous Ni₂P@FePO_xH_y for Oxygen Evolution at High Current Density in Alkaline-Anion-Exchange-Membrane Water-Electrolyzer. Appl. Catal., B 2022, 306, 121127  DOI: 10.1016/j.apcatb.2022.121127

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    36

    Crystalline-amorphous interface of mesoporous Ni2P @ FePOxHy for oxygen evolution at high current density in alkaline-anion-exchange-membrane water-electrolyzer

    Meena, Abhishek; Thangavel, Pandiarajan; Jeong, Da Sol; Singh, Aditya Narayan; Jana, Atanu; Im, Hyunsik; Nguyen, Duc Anh; Kim, Kwang S.

    Applied Catalysis, B: Environmental (2022), 306 (), 121127CODEN: ACBEE3; ISSN:0926-3373. (Elsevier B.V.)

    For industrial high-purity hydrogen prodn., it is essential to develop low-cost, earth-abundant, highly-efficient, and stable electrocatalysts which deliver high c.d. (j) at low overpotential (η) for oxygen evolution reaction (OER). Herein, we report an active mesoporous Ni2P @ FePOxHy pre-electrocatalyst, which delivers high j = 1 A cm-2 at η = 360 mV in 1 M KOH with long-term durability (12 days), fulfilling all the desirable com. criteria for OER. The electrocatalyst shows abundant interfaces between cryst. metal phosphide and amorphous phosphorus-doped metal-oxide, improving charge transfer capability and providing access to rich electroactive sites. Combined with an excellent non-noble metal-based HER catalyst, we achieve com. required j = 500/1000 mA cm-2 at 1.65/1.715 V for full water-splitting with excellent stability in highly corrosive alk. environment (30% KOH). The alk.-anion-exchange-membrane water-electrolyzer (AAEMWE) fabricated for com. viability exhibits high j of 1 A cm-2 at 1.84 V with long-term durability as an economical hydrogen prodn. method, outperforming the state-of-the-art Pt/C-IrO2 catalyst.
37. 37

    Thangavel, P.; Kim, G.; Kim, K. S. Electrochemical Integration of Amorphous NiFe (Oxy)Hydroxides on Surface-Activated Carbon Fibers for High-Efficiency Oxygen Evolution in Alkaline Anion Exchange Membrane Water Electrolysis. J. Mater. Chem. A 2021, 9, 14043– 14051,  DOI: 10.1039/D1TA02883A

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    Electrochemical integration of amorphous NiFe (oxy)hydroxides on surface-activated carbon fibers for high-efficiency oxygen evolution in alkaline anion exchange membrane water electrolysis

    Thangavel, Pandiarajan; Kim, Guntae; Kim, Kwang S.

    Journal of Materials Chemistry A: Materials for Energy and Sustainability (2021), 9 (24), 14043-14051CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)

    Developing practical water-splitting devices that convert earth-abundant solar energy and water into renewable fuel holds promise for a sustainable energy future; however, its successful commercialization for practical applications is limited by the sluggish kinetics of the oxygen evolution reaction (OER). Herein, we developed a high-efficiency and low-cost three-dimensional (3D) OER electrode via electrochem. integration of amorphous NiFeOOH on surface activated carbon fiber paper (CFP). The as-synthesized 3D-a-NiFeOOH/N-CFP electrode exhibits an ultra-low overpotential η(O2) of 170 mV to afford 10 mA cm-2 c.d., together with a Tafel slope of 39 mV per decade, and excellent stability under OER conditions. Apart from the synergistic effect, the excellent OER activity of a-NiFeOOH/N-CFP is attributed to the unique 3D structure with enriched active sites and the improved elec. cond. that facilitates the fast OER kinetics and mass transport properties. As a result, the catalyst achieves a high turnover frequency (TOF) of 0.99/s and mass activity (jm) of 2527 A g-1 at η(O2) 270 mV, which outperforms so far reported state-of-the-art OER catalysts and com. IrO2. Besides, an alk. anion exchange membrane water electrolyzer fabricated with the a-NiFeOOH/N-CFP anode delivers 1 A current at 1.88 V with a long-term durability of 240 h. These findings highlight the design of high-efficiency OER catalysts and significant advancements towards the utilization of NiFeOOH catalysts for com. applications.
38. 38

    Zhou, W.; Wu, X.-J.; Cao, X.; Huang, X.; Tan, C.; Tian, J.; Liu, H.; Wang, J.; Zhang, H. Ni₃S₂ Nanorods/Ni Foam Composite Electrode with Low Overpotential for Electrocatalytic Oxygen Evolution. Energy Environ. Sci. 2013, 6, 2921– 2924,  DOI: 10.1039/c3ee41572d

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    38

    Ni3S2 nanorods/Ni foam composite electrode with low overpotential for electrocatalytic oxygen evolution

    Zhou, Weijia; Wu, Xue-Jun; Cao, Xiehong; Huang, Xiao; Tan, Chaoliang; Tian, Jian; Liu, Hong; Wang, Jiyang; Zhang, Hua

    Energy & Environmental Science (2013), 6 (10), 2921-2924CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)

    A Ni3S2 nanorods/Ni foam composite electrode was prepd. as a high-performance catalyst for the oxygen evolution reaction (OER), which exhibits excellent OER activity with a small overpotential of ∼157 mV based on the onset of catalytic current.
39. 39

    Shang, X.; Li, X.; Hu, W.-H.; Dong, B.; Liu, Y.-R.; Han, G.-Q.; Chai, Y.-M.; Liu, Y.-Q.; Liu, C.-G. In Situ Growth of Ni_xS_y Controlled by Surface Treatment of Nickel Foam as Efficient Electrocatalyst for Oxygen Evolution Reaction. Appl. Surf. Sci. 2016, 378, 15– 21,  DOI: 10.1016/j.apsusc.2016.03.197

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    39

    In situ growth of NixSy controlled by surface treatment of nickel foam as efficient electrocatalyst for oxygen evolution reaction

    Shang, Xiao; Li, Xiao; Hu, Wen-Hui; Dong, Bin; Liu, Yan-Ru; Han, Guan-Qun; Chai, Yong-Ming; Liu, Yun-Qi; Liu, Chen-Guang

    Applied Surface Science (2016), 378 (), 15-21CODEN: ASUSEE; ISSN:0169-4332. (Elsevier B.V.)

    In situ growth of NixSy with different crystal phases supported on different surface-treated (acidification or oxidn.) Ni foam (NF) was successfully achieved by a facile solvothermal process. XRD and SEM results show that crystal phase and morphol. of NixSy were greatly affected by the surface treatment of NF. XRD results show that the mixt. crystal phases of NixSy were obtained on both acid-treated NF (NF(a)) and oxidant treated NF (NF(o)). NixSy/NF(a) contains Ni3S2 and NiS, whereas NixSy/NF(o) has Ni3S2 and NiS2, implying different crystal phases derived from different surface treatment of NF. SEM images also reveal the different morphol. of two samples based on pre-treatment support. NixSy/NF(a) displays unique conical agglomeration surrounded by porous structure. NixSy/NF(o) has the disorder stacking structure of nanosheets. Electrochem. measurements for O evolution reaction (OER) show the enhanced performances of NixSy/NF(a) than NixSy/NF(o) and pure Ni3S2/NF as contrast samples, implying that NiS outperforms other types of NixSy. The mechanisms of sulfurization path of different surface-treated NF are discussed. The facile surface treatment of NF may provide a new strategy to prep. excellent electrocatalysts for OER.
40. 40

    Zhang, W.; Song, H.; Cheng, Y.; Liu, C.; Wang, C.; Khan, M. A. N.; Zhang, H.; Liu, J.; Yu, C.; Wang, L.; Li, J. Core-Shell Prussian Blue Analogs with Compositional Heterogeneity and Open Cages for Oxygen Evolution Reaction. Adv. Sci. 2019, 6, 1801901  DOI: 10.1002/advs.201801901

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    Core-Shell Prussian Blue Analogs with Compositional Heterogeneity and Open Cages for Oxygen Evolution Reaction

    Zhang Wuxiang; Liu Chao; Wang Chaohai; Khan Muhammad Abdul Nasir; Zhang Hao; Wang Lianjun; Li Jiansheng; Song Hao; Yu Chengzhong; Cheng Yan; Liu Chao; Yu Chengzhong; Liu Jizi

    Advanced science (Weinheim, Baden-Wurttemberg, Germany) (2019), 6 (7), 1801901 ISSN:2198-3844.

    Here, a reduction-cation exchange (RCE) strategy is proposed for synthesizing Fe-Co based bimetallic Prussian blue analogs (PBAs) with heterogeneous composition distribution and open cage nanocage architecture. Specially, bivalent cobalt is introduced into a potassium ferricyanide solution containing hydrochloric acid and polyvinyl pyrrolidone. The uniform PBAs with opened cages are formed tardily after hydrothermal reaction. Time-dependent evolution characterization on composition elucidating the RCE mechanism is based on the sequential reduction of ferric iron and cation exchange reaction between divalent iron and cobalt. The PBA structures are confirmed by electron tomography technology, and the heterogeneous element distribution is verified by energy-dispersive X-ray spectroscopy elemental analysis, leading to the formation of core-shell PBAs with compositional heterogeneity (Fe rich shell and Co rich core) and open cage architecture. When the PBA catalysts are used to boost the oxygen evolution reaction (OER), superior OER activity and long-term stability (low overpotential of 271 mV at 10 mA cm(-2) and ≈5.3% potential increase for 24 h) are achieved, which is attributed to the unique compositional and structural properties as well as high special surface areas (576.2 m(2) g(-1)). The strategies offer insights for developing PBAs with compositional and structural multiplicity, which encourages more practical catalytic applications.
41. 41

    Pavel, C. C.; Cecconi, F.; Emiliani, C.; Santiccioli, S.; Scaffidi, A.; Catanorchi, S.; Comotti, M. Highly Efficient Platinum Group Metal Free Based Membrane-Electrode Assembly for Anion Exchange Membrane Water Electrolysis. Angew. Chem., Int. Ed. 2014, 53, 1378– 1381,  DOI: 10.1002/anie.201308099

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    41

    Highly Efficient Platinum Group Metal Free Based Membrane-Electrode Assembly for Anion Exchange Membrane Water Electrolysis

    Pavel, Claudiu C.; Cecconi, Franco; Emiliani, Chiara; Santiccioli, Serena; Scaffidi, Adriana; Catanorchi, Stefano; Comotti, Massimiliano

    Angewandte Chemie, International Edition (2014), 53 (5), 1378-1381CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)

    Low-temp. electricity-driven H2O splitting is an established technol. for H prodn. However, the two main types, proton exchange membrane (PEM) and liq. alk. electrolysis, have limitations. For instance, PEM electrolysis requires a high amt. of costly Pt-group-metal (PGM) catalysts, and liq. alk. electrolysis is not well suited for intermittent operation. Herein the authors report a highly efficient alk. polymer electrolysis design, which uses a membrane-electrode assembly (MEA) based on low-cost transition-metal catalysts and an anion exchange membrane (AEM). This system exhibited similar performance to the one achievable with PGM catalysts. Also, it is very suitable for intermittent power operation, durable, and able to efficiently operate at differential pressure up to 3 MPa. This system combines the benefits of PEM and liq. alk. technologies allowing the scalable prodn. of low-cost H from renewable sources.
42. 42

    Wang, L.; Weissbach, T.; Reissner, R.; Ansar, A.; Gago, A. S.; Holdcroft, S.; Friedrich, K. A. High Performance Anion Exchange Membrane Electrolysis Using Plasma-Sprayed, Non-Precious-Metal Electrodes. ACS Appl. Energy Mater. 2019, 2, 7903– 7912,  DOI: 10.1021/acsaem.9b01392

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    42

    High Performance Anion Exchange Membrane Electrolysis Using Plasma-Sprayed, Non-Precious-Metal Electrodes

    Wang, Li; Weissbach, Thomas; Reissner, Regine; Ansar, Asif; Gago, Aldo S.; Holdcroft, Steven; Friedrich, K. Andreas

    ACS Applied Energy Materials (2019), 2 (11), 7903-7912CODEN: AAEMCQ; ISSN:2574-0962. (American Chemical Society)

    The prodn. of green hydrogen by a cost-effective electrolysis technol. is of paramount importance for future energy supply systems. In this regard, proton exchange membrane (PEM) electrolysis is the technol. of choice due to its compactness and high efficiency, however its dependence on the scarce iridium catalyst jeopardizes the deployment at large scale. Here, a low cost electrolyzer is presented consisting of an assembly of an anion exchange membrane (AEM) and plasma-sprayed electrodes without any precious metals. Several electrode materials are developed and tested in this configuration at 60° and feeding 1M KOH electrolyte. The AEM electrolyzer with NiAlMo electrodes is able to achieve a potential of 2.086 V at a c.d. of 2 A cm-2, which is comparable to the performances of industrial MW-size PEM electrolyzers. The cell potential with NiAl anode and NiAlMo cathode is 0.4 V higher at the same c.d., but it keeps a stable operation for more than 150 h. Through different post-mortem analyses on the aged electrodes, the degrdn. mechanism of NiAlMo anode is elucidated. The efficiencies of the developed AEM electrolyzer concept reported herein are close to those of the com. PEM systems, and thus a cost-effective alternative to this technol. is provided based on the results.
43. 43

    Xuan, C.; Lei, W.; Wang, J.; Zhao, T.; Lai, C.; Zhu, Y.; Sun, Y.; Wang, D. Sea Urchin-Like Ni-Fe Sulfide Architectures as Efficient Electrocatalysts for the Oxygen Evolution Reaction. J. Mater. Chem. A 2019, 7, 12350– 12357,  DOI: 10.1039/C9TA02761K

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    43

    Sea urchin-like Ni-Fe sulfide architectures as efficient electrocatalysts for the oxygen evolution reaction

    Xuan, Cuijuan; Lei, Wen; Wang, Jie; Zhao, Tonghui; Lai, Chenglong; Zhu, Ye; Sun, Yubao; Wang, Deli

    Journal of Materials Chemistry A: Materials for Energy and Sustainability (2019), 7 (19), 12350-12357CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)

    Exploring highly efficient and cost-effective electrocatalysts for the oxygen evolution reaction (OER) is becoming increasingly important in the field of sustainable energy systems. In this work, a three dimensional (3D) hierarchical porous nickel and iron based sulfide (Ni-Fe-S) with a sea urchin-like morphol. is synthesized by a facile sulfurization of Prussian blue analog (PBA) precursors with a hydrothermal reaction and post-calcination treatment. The mass ratio of PBA and sulfur sources, the hydrothermal temp. and time, and the presence of hydrazine hydrate are found to be important factors for the formation of the unique sea urchin-like materials with the porous carbon layer and mixed phases of Fe5Ni4S8 and NiS, which are conducive to fast mass and charge transfer along various directions, endow the materials with mixed valences, improve the electronic cond. and prevent the agglomeration of nanostructured sulfides. Benefiting from these fascinating advantages, the optimal Ni-Fe-S catalyst exhibits excellent catalytic activities with an overpotential as low as 200 mV to attain a c.d. of 10 mA cm-2 and good stability toward the OER. This work not only offers a facile strategy to prep. efficient transition metal based sulfides with excellent electrocatalytic activity for the OER but also extends the synthesis and application of PBA-derived nanostructured materials.
44. 44

    Wu, Q.; Xiao, M.; Wang, W.; Cui, C. In Situ Coordination Environment Tuning of Cobalt Sites for Efficient Water Oxidation. ACS Catal. 2019, 9, 11734– 11742,  DOI: 10.1021/acscatal.9b03762

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    44

    In Situ Coordination Environment Tuning of Cobalt Sites for Efficient Water Oxidation

    Wu, Qianbao; Xiao, Mengjun; Wang, Wei; Cui, Chunhua

    ACS Catalysis (2019), 9 (12), 11734-11742CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)

    Most transition metal-based oxygen-evolving catalysts surface typically experiences irreversible compositional and structural variations during oxygen evolution reaction (OER) in hydrolytic and corrosive alk. media, degrading the coordination environment of active metal sites into unified (oxy)hydroxides. Here an in situ electrochem. coordination tuning is presented of cobalt sites for OER in strong base, where electrolyzing sol. cobalt-2,2'-bipyridine (Co-bpy) complex partially splits bpy ligand, leading to the deposition of active Co sites with fine coordination at room temp. The Co sites are deposited while catalyzing water oxidn. at the same condition so that this catalyst can adapt the hostile alk. condition. This robust coordination environment involving remaining bpy and generated (oxy)hydroxide ligands (Co-BH catalyst) sustains the highly improved OER activity over 500 h at 200 mA cm-2 outperforming other fragile Co sites with only (oxy)hydroxides. In addn., this work presents an efficient tuning of metal coordination environments to in situ generate highly active and stable metal sites in alk. electrolytes for water splitting.
45. 45

    Mabayoje, O.; Shoola, A.; Wygant, B. R.; Mullins, C. B. The Role of Anions in Metal Chalcogenide Oxygen Evolution Catalysis: Electrodeposited Thin Films of Nickel Sulfide as “Pre-Catalysts. ACS Energy Lett. 2016, 1, 195– 201,  DOI: 10.1021/acsenergylett.6b00084

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    The Role of Anions in Metal Chalcogenide Oxygen Evolution Catalysis: Electrodeposited Thin Films of Nickel Sulfide as "Pre-catalysts"

    Mabayoje, Oluwaniyi; Shoola, Ahmed; Wygant, Bryan R.; Mullins, C. Buddie

    ACS Energy Letters (2016), 1 (1), 195-201CODEN: AELCCP; ISSN:2380-8195. (American Chemical Society)

    O evolution catalysts composed of a metal (Ni, Co, or Fe) and a pnictide or chalcogenide (P, S, or Se) counterion are a promising class of electrocatalysts for the O evolution reaction (OER), an important reaction for the photoelectrochem. splitting of H2O. The authors synthesized a Ni-based O evolution catalyst derived from pulse-electrodeposited Ni sulfide. This catalyst was found to produce current densities of 10 mA/cm2 at the relatively low overpotential of 320 mV in alk. electrolyte (1 M KOH). Importantly, the S anion in the Ni sulfide is depleted in the active form of the electrocatalyst and the NiS is converted into an amorphous Ni oxide in the potential range where H2O is oxidized to O. The superior catalytic activity of this Ni sulfide is thus unrelated to the S anions in the active catalyst but is instead related to the metal sulfide's ability to act as a precursor to a highly active Ni oxide OER electrocatalyst. The Ni oxide derived from Ni sulfide is amorphous with a relatively high surface area, 2 factors that have been previously shown to be important in O evolution electrocatalysis.
46. 46

    Jin, S. Are Metal Chalcogenides, Nitrides, and Phosphides Oxygen Evolution Catalysts or Bifunctional Catalysts?. ACS Energy Lett. 2017, 2, 1937– 1938,  DOI: 10.1021/acsenergylett.7b00679

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    46

    Are Metal Chalcogenides, Nitrides, and Phosphides Oxygen Evolution Catalysts or Bifunctional Catalysts?

    Jin, Song

    ACS Energy Letters (2017), 2 (8), 1937-1938CODEN: AELCCP; ISSN:2380-8195. (American Chemical Society)

    There is no expanded citation for this reference.
47. 47

    Lee, M.; Oh, H.-S.; Cho, M. K.; Ahn, J.-P.; Hwang, Y. J.; Min, B. K. Activation of a Ni Electrocatalyst through Spontaneous Transformation of Nickel Sulfide to Nickel Hydroxide in an Oxygen Evolution Reaction. Appl. Catal., B 2018, 233, 130– 135,  DOI: 10.1016/j.apcatb.2018.03.083

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    47

    Activation of a Ni electrocatalyst through spontaneous transformation of nickel sulfide to nickel hydroxide in an oxygen evolution reaction

    Lee, Minoh; Oh, Hyung-Suk; Cho, Min Kyung; Ahn, Jae-Pyoung; Hwang, Yun Jeong; Min, Byoung Koun

    Applied Catalysis, B: Environmental (2018), 233 (), 130-135CODEN: ACBEE3; ISSN:0926-3373. (Elsevier B.V.)

    Ni-sulfide compds. synthesized on Ni foam by simple thermal sulfurization are employed as electrocatalysts for water oxidn., resulting in superior activity in alk. electrolyte media. The role of sulfur in Ni-sulfide was found to be an activator that transformed sulfide into hydroxide, which was eventually transformed into (oxy)hydroxide. The Ni-(oxy)hydroxide phase was also found to be layered and/or amorphous. This activated catalyst showed significant enhancement in water oxidn. performance with a low overpotential value of 256 mV at c.d. of 10 mA cm-2. Our observation could offer important insight into metal-chalcogenide electrocatalyst for water oxidn.
48. 48

    Li, W.; Xiong, D.; Gao, X.; Liu, L. The Oxygen Evolution Reaction Enabled by Transition Metal Phosphide and Chalcogenide Pre-Catalysts with Dynamic Changes. Chem. Commun. 2019, 55, 8744– 8763,  DOI: 10.1039/C9CC02845E

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    The oxygen evolution reaction enabled by transition metal phosphide and chalcogenide pre-catalysts with dynamic changes

    Li, Wei; Xiong, Dehua; Gao, Xuefei; Liu, Lifeng

    Chemical Communications (Cambridge, United Kingdom) (2019), 55 (60), 8744-8763CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)

    The oxygen evolution reaction represents an important electrochem. reaction in several energy storage and conversion devices such as water electrolyzers and metal-air batteries. Developing efficient, inexpensive and durable electrocatalysts for the oxygen evolution reaction (OER) has been one of the major focuses of applied electrochem. and has attracted considerable research attention in the past decades. Non-oxide based transition metal compds., typically transition metal phosphides (TMPs) and chalcogenides (TMCs), have recently emerged as new categories of OER pre-catalysts, demonstrated outstanding electrocatalytic performance as compared to the conventional oxide- or hydroxide-based OER catalysts for alk. water electrolysis, and even shown promise to replace noble metals for proton-exchange membrane (PEM) water electrolysis. In this feature article, the latest advances are summarized in the development of TMP- and TMC-based OER electrocatalysts. In particular, the electrochem. stability is discussed of TMPs and TMCs predicted using Pourbaix diagrams and their morphol., structural and compositional evolution under OER conditions. Some challenges are also pointed out to be addressed in this specific area of research and propose further investigations yet to be done.
49. 49

    Fan, K.; Zou, H.; Lu, Y.; Chen, H.; Li, F.; Liu, J.; Sun, L.; Tong, L.; Toney, M. F.; Sui, M.; Yu, J. Direct Observation of Structural Evolution of Metal Chalcogenide in Electrocatalytic Water Oxidation. ACS Nano 2018, 12, 12369– 12379,  DOI: 10.1021/acsnano.8b06312

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    49

    Direct Observation of Structural Evolution of Metal Chalcogenide in Electrocatalytic Water Oxidation

    Fan, Ke; Zou, Haiyuan; Lu, Yue; Chen, Hong; Li, Fusheng; Liu, Jinxuan; Sun, Licheng; Tong, Lianpeng; Toney, Michael F.; Sui, Manling; Yu, Jiaguo

    ACS Nano (2018), 12 (12), 12369-12379CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)

    As one of the most remarkable O evolution reaction (OER) electrocatalysts, metal chalcogenides were intensively reported during the past few decades because of their high OER activities. It is reported that electron-chem. conversion of metal chalcogenides into oxides/hydroxides would take place after the OER. However, the transition mechanism of such unstable structures, as well as the real active sites and catalytic activity during the OER for these electrocatalysts, was not understood yet; therefore a direct observation for the electrocatalytic H2O oxidn. process, esp. at nano or even angstrom scale, is urgently needed. In this research, by employing advanced Cs-cor. TEM, a step by step oxidational evolution of amorphous electrocatalyst CoSx into crystd. CoOOH in the OER was in situ captured: irreversible conversion of CoSx to crystd. CoOOH is initiated on the surface of the electrocatalysts with a morphol. change via Co(OH)2 intermediate during the OER measurement, where CoOOH is confirmed as the real active species. Besides, this transition process also was confirmed by multiple applications of XPS, in situ FTIR spectroscopy, and other ex situ technologies. Also, from this discovery, a high-efficiency electrocatalyst of a N-doped graphene foam (NGF) coated by CoSx was explored through a thorough structure transformation of CoOOH. The authors believe this in situ and in-depth observation of structural evolution in the OER measurement can provide insights into the fundamental understanding of the mechanism for the OER catalysts, thus enabling the more rational design of low-cost and high-efficient electrocatalysts for H2O splitting.
50. 50

    Zhao, G.; Zhang, Y.; Yang, L.; Jiang, Y.; Zhang, Y.; Hong, W.; Tian, Y.; Zhao, H.; Hu, J.; Zhou, L.; Hou, H.; Ji, X.; Mai, L. Nickel Chelate Derived NiS₂ Decorated with Bifunctional Carbon: An Efficient Strategy to Promote Sodium Storage Performance. Adv. Funct. Mater. 2018, 28, 1803690  DOI: 10.1002/adfm.201803690

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51. 51

    Zeng, L.; Liu, Z.; Sun, K.; Chen, Y.; Zhao, J.; Chen, Y.; Pan, Y.; Lu, Y.; Liu, Y.; Liu, C. Multiple Modulations of Pyrite Nickel Sulfides via Metal Heteroatom Doping Engineering for Boosting Alkaline and Neutral Hydrogen Evolution. J. Mater. Chem. A 2019, 7, 25628– 25640,  DOI: 10.1039/C9TA08030A

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    51

    Multiple modulations of pyrite nickel sulfides via metal heteroatom doping engineering for boosting alkaline and neutral hydrogen evolution

    Zeng, Lingyou; Liu, Zhi; Sun, Kaian; Chen, Yanju; Zhao, Jinchong; Chen, Yinjuan; Pan, Yuan; Lu, Yukun; Liu, Yunqi; Liu, Chenguang

    Journal of Materials Chemistry A: Materials for Energy and Sustainability (2019), 7 (44), 25628-25640CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)

    Rational design of alternative, cost-effective and highly active electrocatalysts for the hydrogen evolution reaction (HER) in alk. and neutral media is greatly desirable and challenging. The authors developed a simple but effective manganese-metal-heteroatom doping strategy to realize the simultaneous modulations of the active site no., water dissocn., and hydrogen adsorption free energy in pyrite NiS2 hierarchical nanosheets to significantly boost alk. and neutral HER catalysis. Specifically, the incorporation of Mn heteroatoms into the NiS2 system, as revealed by HRTEM, XPS, XANES spectra and theor. studies, not only induce lattice distortions and defects for increasing the exposure of active sites, but also effectively optimize the electronic structure configuration of Ni sites, leading to optimal hydrogen adsorption free energy. The doped Mn heteroatom itself can act as a water-activated site to lower the energy barrier of water dissocn. As a result, the synergistic regulation of active sites and HER kinetics brings nearly 9-fold enhancement of alk. HER activity for Mn-doped NiS2/Ni foam (NF) with a quite low overpotential of 71 mV to reach 10 mA/cm2 in 1 M KOH, which is among the most active HER electrocatalysts reported to date. Despite few reports about the effective neutral HER on transition-metal sulfides so far, a small overpotential of 84 mV at 10 mA/cm2 can be achieved in 1 M phosphate-buffered saline (PBS, pH 7). Also, the Mn-doped NiS2/NF electrode also exhibits efficient and stable HER performances in near-neutral real seawater and has no obvious catalytic degrdn. after various extreme bending tests, verifying its high flexibility and robustness under severe conditions, which vastly broadens its application prospects.
52. 52

    Nesbitt, H.; Legrand, D.; Bancroft, G. Interpretation of Ni2p XPS Spectra of Ni Conductors and Ni Insulators. Phys. Chem. Miner. 2000, 27, 357– 366,  DOI: 10.1007/s002690050265

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    Interpretation of Ni2p XPS spectra of Ni conductors and Ni insulators

    Nesbitt, H. W.; Legrand, D.; Bancroft, G. M.

    Physics and Chemistry of Minerals (2000), 27 (5), 357-366CODEN: PCMIDU; ISSN:0342-1791. (Springer-Verlag)

    Ni2p3/2 X-ray photoelectron spectral peak binding energies of Ni metal, NiS, and NiAs (all conductors) span a range of about 0.5 eV and are, consequently, insensitive to formal Ni oxidn. state and to the nature of the ligand to which Ni is bonded, relative to other metals (e.g., Fe). Ni2p3/2 peak structures and binding energies reflect two energetic contributions. The major contribution is that assocd. with the electrostatic field produced by ejection of the Ni(2p) photoelectron, the minor contribution is the relaxation energy assocd. with filling unoccupied, conduction band 3d9 and 4s Ni metal orbitals. These conduction band orbitals become localized on the Ni photoion (and sometimes filled) in response to the field created by the photoemission event. Because only the core Ni2p electron and nonbonding orbitals of predominantly metallic character are affected, the main peak of all three conductors are affected similarly, leading to similar Ni2p3/2 main peak binding energies.NiO, Ni(OH)2, and NiSO4 are insulators in which Ni is divalent and is bonded to oxygen. Although Ni is bonded to oxide in these phases, Ni2p binding energies differ substantially, and reflect primarily the nature of the ligand (O2-, OH-, SO42-) to which Ni is bonded. The influence of the ligand is the result of charge (electron) transfer from valence band bonding orbitals of dominantly ligand character, to unoccupied conduction band orbitals localized on Ni photoions. Relaxation energy resulting from charge transfer is acquired by the emitted photoelectron, thus Ni2p3/2 photopeak binding energies of these insulators reflect the nature of the ligand to which Ni is bonded.The Ni2p main peak binding energy of these conductors and insulators is a poor guide to Ni oxidn. states. The Ni2p3/2 binding energies of insulators reflect, however, the nature of the ligand in the first coordination sphere of Ni.The intensity of the Doniach-Sunjic contribution to Ni2p XPS spectra of NiS and NiAs is dependent on the nature of the ligand. The Doniach-Sunjic contribution to ligand XPS core-level photopeaks (e.g., S2p of NiS and As3d of NiAs) has not been explained and is poorly understood.
53. 53

    Huang, S.; Li, Y.; Chen, S.; Wang, Y.; Wang, Z.; Fan, S.; Zhang, D.; Yang, H. Y. Regulating the Breathing of Mesoporous Fe_0.95S_1.05 Nanorods for Fast and Durable Sodium Storage. Energy Storage Mater. 2020, 32, 151– 158,  DOI: 10.1016/j.ensm.2020.06.039

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    Wang, L.; Zhu, Y.; Li, H.; Li, Q.; Qian, Y. Hydrothermal Synthesis of NiS Nanobelts and NiS₂ Microspheres Constructed of Cuboids Architectures. J. Solid State Chem. 2010, 183, 223– 227,  DOI: 10.1016/j.jssc.2009.10.021

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    54

    Hydrothermal synthesis of NiS nanobelts and NiS2 microspheres constructed of cuboids architectures

    Wang, Lili; Zhu, Yongchun; Li, Haibo; Li, Qianwen; Qian, Yitai

    Journal of Solid State Chemistry (2010), 183 (1), 223-227CODEN: JSSCBI; ISSN:0022-4596. (Elsevier B.V.)

    NiS nanobelts of hexagonal phase were hydrothermally synthesized starting from Ni(CH3COO)2·4H2O and Na2S2O3·5H2O at 200° for 12 h. The as-prepd. nanobelts were 50 nm thick, 70-200 nm wide and >10 μm long. As EDTA was added, in similar condition, 2 μm NiS2 microspheres of cubic phase were prepd. However, as Ni2+/ S2O32- ratio was 1:1 and the temp. was decreased to 160°, 5 μm NiS2 microspheres constructed of cuboids were formed.
55. 55

    Wan, K.; Luo, J.; Zhou, C.; Zhang, T.; Arbiol, J.; Lu, X.; Mao, B. W.; Zhang, X.; Fransaer, J. Hierarchical Porous Ni₃S₄ with Enriched High-Valence Ni Sites as a Robust Electrocatalyst for Efficient Oxygen Evolution Reaction. Adv. Funct. Mater. 2019, 29, 1900315  DOI: 10.1002/adfm.201900315

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56. 56

    He, Y.; Zhang, X.; Wang, S.; Meng, J.; Sui, Y.; Wei, F.; Qi, J.; Meng, Q.; Ren, Y.; Zhuang, D. Rubik’s Cube-Like Ni₃S₄/CuS₂ Nanocomposite for High-Performance Supercapacitors. J. Alloys Compd. 2020, 847, 156312  DOI: 10.1016/j.jallcom.2020.156312

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    Rubik's cube-like Ni3S4/CuS2 nanocomposite for high-performance supercapacitors

    He, Yezeng; Zhang, Xiaolong; Wang, Shitong; Meng, Jiaxi; Sui, Yanwei; Wei, Fuxiang; Qi, Jiqiu; Meng, Qingkun; Ren, Yaojian; Zhuang, Dongdong

    Journal of Alloys and Compounds (2020), 847 (), 156312CODEN: JALCEU; ISSN:0925-8388. (Elsevier B.V.)

    Transition metal sulfide, a promising electrode material using for supercapacitors, has attracted broad attention. In this work, a Rubik's cube-like Ni3S4/CuS2 nanocomposite has been successfully fabricated by vulcanizing the Ni(OH)2/CuS2 precursor prepd. via sacrificial template method. The obtained Ni3S4/CuS2 have a high specific capacitance of 888 F g-1 at 1 A g-1 and superior cycling stability of 83.33% retention after 2000 circles. Furthermore, the asym. supercapacitor, based on the Ni3S4/CuS2 nanocomposite and the reduced graphene oxide, shows an ultra-high energy d. of 49.68 Wh kg-1 at the power d. of 400 W kg-1.
57. 57

    Nandhini, S. Facile Microwave-Hydrothermal Synthesis of NiS Nanostructures for Supercapacitor Applications. Appl. Surf. Sci. 2018, 449, 485– 491,  DOI: 10.1016/j.apsusc.2018.01.024

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    Facile microwave-hydrothermal synthesis of NiS nanostructures for supercapacitor applications

    Nandhini, S.; A., Juliet Christina Mary; Muralidharan, G.

    Applied Surface Science (2018), 449 (), 485-491CODEN: ASUSEE; ISSN:0169-4332. (Elsevier B.V.)

    Here, NiS nanostructures (M, H and MH) were synthesized through 3 different methods: microwave, hydrothermal and a combination of microwave and hydrothermal. The effect of prepn. method on NiS nanostructures was studied through structural, morphol. and electrochem. studies. XRD patterns reveal orthorhombic phase of Ni9S8 in the case of M while H and MH exhibit hexagonal NiS structure. SEM micrographs of M, H and MH indicate the nanoflake, spherical and layered structure, resp. The electrochem. studies were carried out via cyclic voltammetry, charge-discharge studies and electrochem. impedance anal. MH provides the largest specific capacitance of 964 F/g (from galvanostatic charge-discharge studies at a specific current of 1 A/g in 2M KOH electrolyte) combined with a charge transfer resistance of 0.6Ω. The MH electrode could provide undiminished capacity retention after 2000 cycles. A sym. supercapacitor device provides max. specific capacitance of 119 F/g at 1 A/g with energy and power densities of 16.5 Wh/Kg and 250 W/Kg. These results imply that MH nanostructure is well suited as a supercapacitor electrode material.
58. 58

    Wang, H.-Y.; Hsu, Y.-Y.; Chen, R.; Chan, T.-S.; Chen, H. M.; Liu, B. Ni³⁺-Induced Formation of Active NiOOH on the Spinel Ni-Co Oxide Surface for Efficient Oxygen Evolution Reaction. Adv. Energy Mater. 2015, 5, 1500091  DOI: 10.1002/aenm.201500091

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    58

    Ni3+-Induced Formation of Active NiOOH on the Spinel Ni-Co Oxide Surface for Efficient Oxygen Evolution Reaction

    Wang, Hsin-Yi; Hsu, Ying-Ya; Chen, Rong; Chan, Ting-Shan; Chen, Hao Ming; Liu, Bin

    Advanced Energy Materials (2015), 5 (10), 1500091/1-1500091/8CODEN: ADEMBC; ISSN:1614-6840. (Wiley-Blackwell)

    Efficient and earth abundant electrocatalysts for high-performance oxygen evolution reaction (OER) are essential for the development of sustainable energy conversion technologies. Here, a new hierarchical Ni-Co oxide nanostructure, composed of small secondary nanosheets grown on primary nanosheet arrays, is synthesized via a topotactic transformation of Ni-Co layered double hydroxide. The Ni3+-rich surface benefits the formation of NiOOH, which is the main redox site as revealed via in situ X-ray absorption near edge structure and extended X-ray absorption fine structure spectroscopy. The Ni-Co oxide hierarchical nanosheets (NCO-HNSs) deliver a stable c.d. of 10 mA cm-2 at an overpotential of ≈0.34 V for OER with a Tafel slope of as low as 51 mV dec-1 in alk. media. The improvement in the OER activity can be ascribed to the synergy of large surface area offered by the 3D hierarchical nanostructure and the facile formation of NiOOH as the main active sites on the surface of NCO-HNSs to decrease the overpotential and facilitate the catalytic reaction.
59. 59

    Tahir, M.; Pan, L.; Zhang, R.; Wang, Y.-C.; Shen, G.; Aslam, I.; Qadeer, M. A.; Mahmood, N.; Xu, W.; Wang, L.; Zhang, X.; Zou, J.-J. High-Valence-State NiO/Co₃O₄ Nanoparticles on Nitrogen-Doped Carbon for Oxygen Evolution at Low Overpotential. ACS Energy Lett. 2017, 2, 2177– 2182,  DOI: 10.1021/acsenergylett.7b00691

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    59

    High-Valence-State NiO/Co3O4 Nanoparticles on Nitrogen-Doped Carbon for Oxygen Evolution at Low Overpotential

    Tahir, Muhammad; Pan, Lun; Zhang, Rongrong; Wang, Yi-Cheng; Shen, Guoqiang; Aslam, Imran; Qadeer, M. A.; Mahmood, Nasir; Xu, Wei; Wang, Li; Zhang, Xiangwen; Zou, Ji-Jun

    ACS Energy Letters (2017), 2 (9), 2177-2182CODEN: AELCCP; ISSN:2380-8195. (American Chemical Society)

    The electrocatalytic oxygen evolution reaction (OER) plays a crit. role in sustainable energy conversion and storage, but OER is severely hampered owing to the lack of highly efficient catalysts. Here, an efficient electrocatalyst, with NiO/Co3O4 nanoparticles decorated on nitrogen-doped carbon (NiO/Co3O4@NC) is reported. Abundant high-valence Ni3+ and Co3+ species were obsd. on the surface of the hybrid due to the strong NC-metal oxide and NiO-Co3O4 interactions. This unique structure leads to excellent OER performance, delivering a very low overpotential of 240 mV@10 mA·cm-2 on glassy carbon and 200 mV@10 mA·cm-2 on Ni foam in KOH and having a turnover frequency (@350 mV overpotential) 6 and 16 times higher than that of IrO2 and RuO2, resp.
60. 60

    Ma, Q.; Hu, C.; Liu, K.; Hung, S.-F.; Ou, D.; Chen, H. M.; Fu, G.; Zheng, N. Identifying the Electrocatalytic Sites of Nickel Disulfide in Alkaline Hydrogen Evolution Reaction. Nano Energy 2017, 41, 148– 153,  DOI: 10.1016/j.nanoen.2017.09.036

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    Identifying the electrocatalytic sites of nickel disulfide in alkaline hydrogen evolution reaction

    Ma, Qiuyu; Hu, Chengyi; Liu, Kunlong; Hung, Sung-Fu; Ou, Daohui; Chen, Hao Ming; Fu, Gang; Zheng, Nanfeng

    Nano Energy (2017), 41 (), 148-153CODEN: NEANCA; ISSN:2211-2855. (Elsevier Ltd.)

    Transition-metal chalcogenides have attracted great attention for their superior catalytic activity towards hydrogen evolution reaction (HER) as an alternative to platinum. Here we report a facile method for synthesizing two-dimensional nickel disulfide (NiS2) by using Ni(OH)2 on nickel foam as substrate. The as-synthesized NiS2 displayed an activation period during HER with a remarkable structural and compositional change under alk. conditions. Electrochem. in situ X-ray absorption spectroscopy revealed that metallic Ni acted as catalytic active species with superior activity of 67 mV to reach 10 mA cm-2. The in situ generated metallic Ni were easily oxidized to large-area ultrathin Ni(OH)2 when exposed to air. The overall water splitting device was fabricated by using NiS2-derived metallic Ni and Fe doped NiS2-derived hydroxide as HER and OER electrode with a potential of 1.52 V to reach 10 mA cm-2.
61. 61

    Kamyshny, A.; Jenny, G.; Dan, R.; Ovadia, L. Equilibrium Distribution of Polysulfide Ions in Aqueous Solutions at 25°C: a New Approach for the Study of Polysulfides’ Equilibria. Environ. Sci. Technol. 2004, 38, 6633– 6644,  DOI: 10.1021/es049514e

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    61

    Equilibrium distribution of polysulfide ions in aqueous solutions at 25°C: A new approach for the study of polysulfides' equilibria

    Kamyshny, Alexey, Jr.; Goifman, Anatoly; Gun, Jenny; Rizkov, Dan; Lev, Ovadia

    Environmental Science and Technology (2004), 38 (24), 6633-6644CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)

    A new approach based on rapid, chem. derivatization in a single phase was used to det. the disproportionation consts. and the underlying thermodn. of inorg. polysulfides in aq. solns. This method resolves the dispute over the existence of hexasulfide in aq. solns. and establishes the presence of even higher polysulfide chains in water. The Gibbs free energies of formation (G°Sn2-) for the polysulfide species are 77.4, 71.6, 67.4, 66.1, 67.2, 70.5, and 73.6 kJ/mol for n = 2-8, resp. Our approach is based on single phase, fast methylation of polysulfides with Me trifluoromethanesulfonate (Me triflate) and subsequent detn. of the dimethylpolysulfides by HPLC. Two independent methods were used in order to confirm quant. equivalence between the obsd. distribution of dimethylpolysulfides and the polysulfide distribution in the water: (1) Kinetic studies of each competing reaction step showed that the kinetics of the derivatization are faster than each of the competing reactions that may lead to disproportionation and deviation of the obsd. distribution of dimethylpolysulfides from that of the aq. polysulfides. (2) Detn. of isotope mixing during the derivatization of a mixt. of two solns., one contg. polysulfide of natural isotopic distribution and the second contg. 34S-rich polysulfide revealed that polysulfide mixing during derivatization is rather low. The systematic error due to redistribution of pentasulfide during derivatization is 3% based on isotope diln. tests and less than 5% of total zero-valent sulfur based on kinetic considerations.
62. 62

    Wang, R.; Xu, C.; Lee, J.-M. High Performance Asymmetric Supercapacitors: New NiOOH Nanosheet/Graphene Hydrogels and Pure Graphene Hydrogels. Nano Energy 2016, 19, 210– 221,  DOI: 10.1016/j.nanoen.2015.10.030

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    62

    High performance asymmetric supercapacitors: New NiOOH nanosheet/graphene hydrogels and pure graphene hydrogels

    Wang, Ronghua; Xu, Chaohe; Lee, Jong-Min

    Nano Energy (2016), 19 (), 210-221CODEN: NEANCA; ISSN:2211-2855. (Elsevier Ltd.)

    NiOOH nanosheet/graphene hydrogels (H-NiOOH/GS), with mesoporous NiOOH nanosheets uniformly dispersed within the highly interconnected 3D graphene network, are constructed and studied for the first time by a mixed solvothermal and hydrothermal reaction. The effect of solvent compn. on the morphol., phase, dispersibility of nanocrystal and hydrogel strength is systematically studied. As binder-free electrodes of supercapacitors, H-NiOOH/GS delivers high capacitance of 1162 F g-1 at 1 A g-1 with excellent rate capability (981 F g-1 at 20 A g-1). The charge-storage mechanisms of H-NiOOH/GS are in-depth investigated by quantifying the kinetics of charge storage, which reveals that NiOOH exhibits both capacitive effects and diffusion-controlled battery-type behavior during charge storage. Addnl., solvothermal-induced pure graphene hydrogels (H-GS) are also prepd. and used as the neg. electrode for the first time, which show an impressive specific capacitance of 425 and 368 F g-1 at 5 and 40 mV s-1, resp. Benefitting from the synergistic contribution of both pos. and neg. electrodes, the assembled H-NiOOH/GS//H-GS asym. supercapacitors achieve a remarkable energy d. of 66.8 W h kg-1 at a power d. of 800 W kg-1, and excellent cycling stability with 85.3% capacitance retention after 8000 cycles, holding great promise for energy storage applications.
63. 63

    Xiao, M.; Tian, Y.; Yan, Y.; Feng, K.; Miao, Y. Electrodeposition of Ni(OH)₂/NiOOH in the Presence of Urea for the Improved Oxygen Evolution. Electrochim. Acta 2015, 164, 196– 202,  DOI: 10.1016/j.electacta.2015.02.205

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    63

    Electrodeposition of Ni(OH)2/NiOOH in the Presence of Urea for the Improved Oxygen Evolution

    Xiao, Mingshu; Tian, Yanping; Yan, Yuhua; Feng, Kai; Miao, Yuqing

    Electrochimica Acta (2015), 164 (), 196-202CODEN: ELCAAV; ISSN:0013-4686. (Elsevier Ltd.)

    To lower the energy barrier and improve the energy efficiency for H prodn. by splitting H2O, highly efficient OER catalysts are required. Here, the authors report the electrodeposition of Ni(OH)2/NiOOH in the presence of urea showing the improved electrocatalysis for the H prodn. in KOH electrolyte. By potential scanning, the Ni(OH)2 colloid in alkali soln. was transformed into the deposited Ni(OH)2/NiOOH. The presence of urea facilitates to deposit Ni(OH)2/NiOOH with higher electron transfer than the absence of urea, leading to a higher electrocatalytic OER. As supporting electrolyte, KOH exhibits better performance on the electrodeposition of Ni(OH)2/NiOOH and their OER electrocatalysis than NaOH. The work shows the potential application using cheap, nonprecious metal-based electrocatalysts to overcome the known bottlenecks limiting power d. of H fuel cell and energy efficiency of splitting H2O.
64. 64

    Zhang, Y.; Sun, W.; Rui, X.; Li, B.; Tan, H. T.; Guo, G.; Madhavi, S.; Zong, Y.; Yan, Q. One-Pot Synthesis of Tunable Crystalline Ni₃S₄@Amorphous MoS₂ Core/Shell Nanospheres for High-Performance Supercapacitors. Small 2015, 11, 3694– 3702,  DOI: 10.1002/smll.201403772

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    64

    One-Pot Synthesis of Tunable Crystalline Ni3S4@Amorphous MoS2 Core/Shell Nanospheres for High-Performance Supercapacitors

    Zhang, Yu; Sun, Wenping; Rui, Xianhong; Li, Bing; Tan, Hui Teng; Guo, Guilue; Madhavi, Srinivasan; Zong, Yun; Yan, Qingyu

    Small (2015), 11 (30), 3694-3702CODEN: SMALBC; ISSN:1613-6810. (Wiley-VCH Verlag GmbH & Co. KGaA)

    Transition metal sulfides gain much attention as electrode materials for supercapacitors due to their rich redox chem. and high elec. cond. Designing hierarchical nanostructures is an efficient approach to fully use merits of each component. Amorphous MoS2 is firstly demonstrated to show specific capacitance 1.6 times as that of the cryst. counterpart. Then, cryst. core@amorphous shell (Ni3S4@MoS2) is prepd. by a facile 1-pot process. The diam. of the core and the thickness of the shell can be independently tuned. Taking advantages of flexible protection of amorphous shell and high capacitance of the conductive core, Ni3S4@amorphous MoS2 nanospheres are tested as supercapacitor electrodes, which exhibit high specific capacitance of 1440.9 F g-1 at 2 A g-1 and a good capacitance retention of 90.7% after 3000 cycles at 10 A g-1. This design of cryst. core@amorphous shell architecture may open up new strategies for synthesizing promising electrode materials for supercapacitors.
65. 65

    Kaspar, J.; Bazarjani, M. S.; Schitco, C.; Gurlo, A.; Graczyk-Zajac, M.; Riedel, R. Electrochemical Study of NiO Nanosheets: toward the Understanding of Capacity Fading. J. Mater. Sci. 2017, 52, 6498– 6505,  DOI: 10.1007/s10853-017-0885-0

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    Electrochemical study of NiO nanosheets: toward the understanding of capacity fading

    Kaspar, Jan; Bazarjani, Mahdi Seifollahi; Schitco, Cristina; Gurlo, Aleksander; Graczyk-Zajac, Magdalena; Riedel, Ralf

    Journal of Materials Science (2017), 52 (11), 6498-6505CODEN: JMTSAS; ISSN:0022-2461. (Springer)

    NiO nanosheets are prepd. by calcination of nickel hydroxide nanosheets, obtained by the hydrolysis of trans-bis(acetato-κO)bis(2-aminoethanol-κ2N,O)nickel(II) complex. BET anal. reveals the presence of a high sp. surface area of 48 m2g-1 and a pore vol. of 0.26 cm3g-1 after calcination at 400 °C. The two-dimensional NiO nanostructure undergoes a reversible lithium ion uptake and release revealing an initial unexpectedly high capacity of ∼1100 mAhg-1 at a cycling current of 400 mAg-1, exceeding the theor. capacity of NiO (718 mAhg-1). We attribute this high storage capacity to the advantageous two-dimensional morphol. of the sample, namely to the presence of agglomerates composed of NiO nanosheets, allowing a pronounced Li-ion storage through the insertion mechanism and by the formation of a polymer-like layer at the samples internal surfaces. However, after 20 cycles the recovered capacity diminishes rapidly due to the onset of Li-ion intercalation into NiO, which is found less reversible. In addn., an increase in the charge transfer resistance and increase in the electrode polarization, measured by differential capacity, contribute to the analyzed capacity decay upon continuous cycling.
66. 66

    Yavuz, A.; Ozdemir, N.; Erdogan, P. Y.; Zengin, H.; Zengin, G.; Bedir, M. Nickel-Based Materials Electrodeposited from a Deep Eutectic Solvent on Steel for Energy Storage Devices. Appl. Phys. A 2019, 125, 494  DOI: 10.1007/s00339-019-2787-2

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    Nickel-based materials electrodeposited from a deep eutectic solvent on steel for energy storage devices

    Yavuz, Abdulcabbar; Ozdemir, Naime; Erdogan, Perihan Yilmaz; Zengin, Huseyin; Zengin, Gulay; Bedir, Metin

    Applied Physics A: Materials Science & Processing (2019), 125 (8), 494CODEN: APAMFC; ISSN:0947-8396. (Springer)

    Nickel film composed of agglomerated nanoparticles was electrodeposited cathodically on stainless steel current collectors from choline chloride and urea-based deep eutectic solvent for charge storage electrodes. The electrochem. modified electrodes were investigated at pos. potential regions in alk. soln. Nickel-based electrode cycled in KOH was NiOOH in the oxidized form and Ni(OH)2 in the reduced form. Compositional, structural and morphol. studies of the electrodes were characterized by means of FTIR, XRD and SEM, resp. The porous NiOOH/Ni(OH)2 electrode with KOH electrolyte can provide a high electrode/electrolyte interface for fast charge transfer reactions. The charge storage mechanism was the mixed surface-controlled and diffusional-controlled processes. The as-prepd. binder-free nickel-based electrode illustrates a high specific capacity of 986 F g-1 at 5 mV s-1. The cycling stability test gave 86% of initial capacity retained after 550 cycles. The use of deep eutectic solvent for the growth of nickel-based nanoparticles presented herein may offer promising potential in electrodeposition for the prepn. of high-performance supercapacitors.
67. 67

    Sivakumar, S.; Soundhirarajan, P.; Venkatesan, A.; Khatiwada, C. P. Spectroscopic Studies and Antibacterial Activities of Pure and Various Levels of Cu-Doped BaSO₄ Nanoparticles. Spectrochim. Acta, Part A 2015, 151, 895– 907,  DOI: 10.1016/j.saa.2015.07.048

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    Spectroscopic studies and antibacterial activities of pure and various levels of Cu-doped BaSO4 nanoparticles

    Sivakumar, S.; Soundhirarajan, P.; Venkatesan, A.; Khatiwada, Chandra Prasad

    Spectrochimica Acta, Part A: Molecular and Biomolecular Spectroscopy (2015), 151 (), 895-907CODEN: SAMCAS; ISSN:1386-1425. (Elsevier B.V.)

    The present study was made to design the pure and various levels of Cu-doped (0.025 M, 0.05 M and 0.075 M) BaSO4 NPs synthesized by chem. pptn. method. The synthesized products have been characterized by X-ray Diffractometry (XRD), FTIR spectrometry, TG-DTA, UV-Vis-diffused reflectance spectroscopy (UV-Vis-DRS), field emission-SEM with energy dispersive spectroscopy (FE-SEM with EDS), transmission electron microscopy (TEM) and antibacterial activity. The result detd. from XRD was affirmed by the results obtained from electron microscopy measurements. XRD study revealed that the synthesized products were composed of orthorhombic structure and highly cryst. in nature. Furthermore, flake-like morphol. of pure and Cu-BaSO4 nanoparticles have been obsd. The existence of Cu2+ was confirmed by EDS anal. The functional groups of the synthesized samples were analyzed by FT-IR study. The band gap energies of pure and doped samples were accomplished using UV-Vis-DRS anal. Also, the kinetic parameters were evaluated and reported from the thermal stability of nanoparticles. Gram-neg. bacteria is less affected compared to gram-pos. bacteria due to adsorption of BaSO4 nanoparticles on the surface of the used bacteria.
68. 68

    Chen, W.; Wang, H.; Li, Y.; Liu, Y.; Sun, J.; Lee, S.; Lee, J. S.; Cui, Y. In Situ Electrochemical Oxidation Tuning of Transition Metal Disulfides to Oxides for Enhanced Water Oxidation. ACS Cent. Sci. 2015, 1, 244– 251,  DOI: 10.1021/acscentsci.5b00227

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    In Situ Electrochemical Oxidation Tuning of Transition Metal Disulfides to Oxides for Enhanced Water Oxidation

    Chen, Wei; Wang, Haotian; Li, Yuzhang; Liu, Yayuan; Sun, Jie; Lee, Sanghan; Lee, Jang-Soo; Cui, Yi

    ACS Central Science (2015), 1 (5), 244-251CODEN: ACSCII; ISSN:2374-7951. (American Chemical Society)

    The development of catalysts with earth-abundant elements for efficient oxygen evolution reactions is of paramount significance for clean and sustainable energy storage and conversion devices. Our group demonstrated recently that the electrochem. tuning of catalysts via lithium insertion and extn. has emerged as a powerful approach to improve catalytic activity. Here we report a novel in situ electrochem. oxidn. tuning approach to develop a series of binary, ternary, and quaternary transition metal (e.g., Co, Ni, Fe) oxides from their corresponding sulfides as highly active catalysts for much enhanced water oxidn. The electrochem. tuned cobalt-nickel-iron oxides grown directly on the three-dimensional carbon fiber electrodes exhibit a low overpotential of 232 mV at c.d. of 10 mA cm-2, small Tafel slope of 37.6 mV dec-1, and exceptional long-term stability of electrolysis for over 100 h in 1 M KOH alk. medium, superior to most non-noble oxygen evolution catalysts reported so far. The materials evolution assocd. with the electrochem. oxidn. tuning is systematically investigated by various characterizations, manifesting that the improved activities are attributed to the significant grain size redn. and increase of surface area and electroactive sites. This work provides a promising strategy to develop electrocatalysts for large-scale water-splitting systems and many other applications.
69. 69

    Xu, X.; Song, F.; Hu, X. A Nickel Iron Diselenide-Derived Efficient Oxygen-Evolution Catalyst. Nat. Commun. 2016, 7, 12324  DOI: 10.1038/ncomms12324

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    A nickel iron diselenide-derived efficient oxygen-evolution catalyst

    Xu, Xiang; Song, Fang; Hu, Xile

    Nature Communications (2016), 7 (), 12324CODEN: NCAOBW; ISSN:2041-1723. (Nature Publishing Group)

    Efficient oxygen-evolution reaction catalysts are required for the cost-effective generation of solar fuels. Metal selenides have been reported as promising oxygen-evolution catalysts; however, their active forms are yet to be elucidated. Here we show that a representative selenide catalyst, nickel selenide, is entirely converted into nickel hydroxide under oxygen-evolution conditions. This result indicates that metal selenides are unstable during oxygen evolution, and the in situ generated metal oxides are responsible for their activity. This knowledge inspired us to synthesize nanostructured nickel iron diselenide, a hitherto unknown metal selenide, and to use it as a templating precursor to a highly active nickel iron oxide catalyst. This selenide-derived oxide catalyzes oxygen evolution with an overpotential of only 195 mV for 10 mA cm-2. Our work underscores the importance of identifying the active species of oxygen-evolution catalysts, and demonstrates how such knowledge can be applied to develop better catalysts.
70. 70

    Ni, B.; He, T.; Wang, J. O.; Zhang, S.; Ouyang, C.; Long, Y.; Zhuang, J.; Wang, X. The Formation of (NiFe)S₂ Pyrite Mesocrystals as Efficient Pre-catalysts for Water Oxidation. Chem. Sci. 2018, 9, 2762– 2767,  DOI: 10.1039/C7SC05452A

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    The formation of (NiFe)S2 pyrite mesocrystals as efficient pre-catalysts for water oxidation

    Ni, Bing; He, Ting; Wang, Jia-ou; Zhang, Simin; Ouyang, Chen; Long, Yong; Zhuang, Jing; Wang, Xun

    Chemical Science (2018), 9 (10), 2762-2767CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)

    Designing intricate structures and searching for functional materials has attracted wide interest in nanoscience. Herein we have fabricated (NiFe)S2 pyrite mesocrystals in the form of nearly-single cryst. porous cubes, and studied their self-optimization to realize efficient activity toward water oxidn. under electrochem. conditions. The growth mechanism of the mesocrystals was a non-classical mechanism, which was initiated by the formation of a large quantity of small nickel sulfide clusters, followed by the aggregation and transformation of these small clusters in an oriented manner. When these mesocrystals were tested for water oxidn. under electrocatalytic conditions, the materials served as pre-catalysts and immediately self-optimized to form amorphous S-doped metal (oxy)hydroxides, which are the real catalytically active materials. As a result, the obsd. overpotential to reach a c.d. of 10 mA cm-2 on glassy carbon electrodes was less than 260 mV. The growth mechanism studied here may provide opportunities for constructing intricate sulfide structures, and the self-optimization process during water oxidn. can inspire new thoughts on electrocatalysis.
71. 71

    Michael, J. D.; Demeter, E. L.; Illes, S. M.; Fan, Q.; Boes, J. R.; Kitchin, J. R. Alkaline Electrolyte and Fe Impurity Effects on the Performance and Active-Phase Structure of NiOOH Thin Films for OER Catalysis Applications. J. Phys. Chem. C 2015, 119, 11475– 11481,  DOI: 10.1021/acs.jpcc.5b02458

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    Alkaline Electrolyte and Fe Impurity Effects on the Performance and Active-Phase Structure of NiOOH Thin Films for OER Catalysis Applications

    Michael, John D.; Demeter, Ethan L.; Illes, Steven M.; Fan, Qingqi; Boes, Jacob R.; Kitchin, John R.

    Journal of Physical Chemistry C (2015), 119 (21), 11475-11481CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)

    The effects of varying alk. electrolyte and electrolyte Fe levels on the performance and active-phase structure of NiOOH thin films for catalysis of the oxygen evolution reaction were studied. An electrolyte effect on catalytic performance was obsd. Under purified conditions, current densities followed the trend Cs+ > K+ ≈ Na+ ≈ Li+ at current densities > 1 mA/cm2. Under Fe-satd. conditions, current densities followed the trend K+ ≈ Na+ > Cs+ > Li+ at all current densities. Voltammetry was coupled with Raman spectroscopy for studies in LiOH and CsOH. Raman spectra were fit to Gaussian functions and analyzed quant. based on mean peak positions. Both purified and Fe-satd. CsOH promoted slightly lower peak positions than purified and Fe-satd. LiOH, indicating that CsOH promoted a NiOOH active-phase structure with longer Ni-O bonds. Both Fe-satd. CsOH and LiOH promoted slightly lower Raman peak positions than purified CsOH and LiOH, but only for one of the two Raman peaks. These results indicate that Fe promoted an active-phase structure with slightly longer Ni-O bonds. This study shows that the catalytic performance and active-phase structure of NiOOH can be tuned by simply varying the alk. electrolyte and electrolyte Fe levels.
72. 72

    Klaus, S.; Cai, Y.; Louie, M. W.; Trotochaud, L.; Bell, A. T. Effects of Fe Electrolyte Impurities on Ni(OH)₂/NiOOH Structure and Oxygen Evolution Activity. J. Phys. Chem. C 2015, 119, 7243– 7254,  DOI: 10.1021/acs.jpcc.5b00105

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    Effects of Fe Electrolyte Impurities on Ni(OH)2/NiOOH Structure and Oxygen Evolution Activity

    Klaus, Shannon; Cai, Yun; Louie, Mary W.; Trotochaud, Lena; Bell, Alexis T.

    Journal of Physical Chemistry C (2015), 119 (13), 7243-7254CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)

    Ni-(oxy)hydroxide-based materials are promising earth-abundant catalysts for electrochem. H2O oxidn. in basic media. Recent findings demonstrate that incorporation of trace Fe impurities from commonly used KOH electrolytes significantly improves O evolution reaction (OER) activity over NiOOH electrocatalysts. Because nearly all previous studies detailing structural differences between α-Ni(OH)2/γ-NiOOH and β-Ni(OH)2/β-NiOOH were completed in unpurified electrolytes, it is unclear whether these structural changes are unique to the aging phase transition in the Ni-(oxy)hydroxide matrix or if they arise fully or in part from inadvertent Fe incorporation. Here, the authors report a study of the effects of Fe incorporation on structure-activity relations in Ni-(oxy)hydroxide. Electrochem., in situ Raman, XPS, and electrochem. quartz crystal microbalance measurements were employed to study Ni(OH)2 thin films aged in Fe-free and unpurified (reagent-grade) 1 M KOH (<1 ppm Fe). Ni films aged in unpurified electrolyte can incorporate ≥20% Fe after 5 wk of aging, and the max. catalyst activity is comparable to that reported for optimized Ni1-xFexOOH catalysts. Conversely, Fe-free Ni(OH)2 films exhibit a substantially lower activity and higher Tafel slope for the OER. Films aged in Fe-free electrolyte are predominantly disordered β-Ni(OH)2/β-NiOOH if maintained <0.7 V vs. Hg/HgO in 1 M KOH and will overcharge to form a mixt. of γ- and β-NiOOH above this potential. Fe-contg. Ni(OH)2 films evidence a lesser extent of β-Ni(OH)2 formation and instead exhibit NiOOH structural changes in accordance with the formation of a NiFe-layered double hydroxide phase. Also, turnover frequency calcns. indicate that Fe is the active site within this phase, and ⪆11% Fe content, a sep., Fe-rich phase forms. These findings are the 1st to demonstrate the in situ changes in the catalyst structure resulting from the incorporation of Fe electrolyte impurities within Ni-(oxy)hydroxide, providing direct evidence that a Ni-Fe layered double (oxy)hydroxide (LDH) phase is crit. for high OER activity.
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    Gocyla, M.; Kuehl, S.; Shviro, M.; Heyen, H.; Selve, S.; Dunin-Borkowski, R. E.; Heggen, M.; Strasser, P. Shape Stability of Octahedral PtNi Nanocatalysts for Electrochemical Oxygen Reduction Reaction Studied by In Situ Transmission Electron Microscopy. ACS Nano 2018, 12, 5306– 5311,  DOI: 10.1021/acsnano.7b09202

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    Shape Stability of Octahedral PtNi Nanocatalysts for Electrochemical Oxygen Reduction Reaction Studied by in situ Transmission Electron Microscopy

    Gocyla, Martin; Kuehl, Stefanie; Shviro, Meital; Heyen, Henner; Selve, Soeren; Dunin-Borkowski, Rafal E.; Heggen, Marc; Strasser, Peter

    ACS Nano (2018), 12 (6), 5306-5311CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)

    Octahedral faceted nanoparticles are highly attractive fuel cell catalysts as a result of their activity for the oxygen redn. reaction (ORR). However, their surface compositional and morphol. stability currently limits their long-term performance in real membrane electrode assemblies. Here, we perform in situ heating of compositionally segregated PtNi1.5 octahedral nanoparticles inside a transmission electron microscope, in order to study their compositional and morphol. changes. The starting PtNi1.5 octahedra have Pt-rich edges and concave Ni-rich {111} facets. We reveal a morphol. evolution sequence, which involves transformation from concave octahedra to particles with atomically flat {100} and {111} facets, ideally representing truncated octahedra or cuboctahedra. The flat {100} and {111} facets are thought to comprise a thin Pt layer with a Ni-rich subsurface, which may boost catalytic activity. However, the transformation to truncated octahedra/cuboctahedra also decreases the area of the highly active {111} facets. The morphol. and surface compositional evolution, therefore, results in a compromise between catalytic activity and morphol. stability. Our findings are important for the design of more stable faceted PtNi nanoparticles with high activities for the ORR.
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    Shviro, M.; Gocyla, M.; Schierholz, R.; Tempel, H.; Kungl, H.; Eichel, R. A.; Dunin-Borkowski, R. E. Transformation of Carbon-Supported Pt-Ni Octahedral Electrocatalysts into Cubes: toward Stable Electrocatalysis. Nanoscale 2018, 10, 21353– 21362,  DOI: 10.1039/C8NR06008H

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    Transformation of carbon-supported Pt-Ni octahedral electrocatalysts into cubes: toward stable electrocatalysis

    Shviro, Meital; Gocyla, Martin; Schierholz, Roland; Tempel, Hermann; Kungl, Hans; Eichel, Ruediger-A.; Dunin-Borkowski, Rafal E.

    Nanoscale (2018), 10 (45), 21353-21362CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)

    Octahedral Pt-Ni catalyst nanoparticles (NPs) are predicted to exhibit high activity for the oxygen redn. reaction. However, until now this class of catalysts has been limited by its long-term performance, as a result of compositional and morphol. instabilities of the NPs. In situ transmission electron microscopy (TEM) is a powerful technique for understanding morphol. and compositional evolution under controlled conditions. It is of great importance to study the evolution of the morphol. and elemental distribution in bimetallic NPs and their interaction with the support in reducing and oxidizing treatments at the at. scale for the rational design of catalysts. Here, we use in situ TEM to follow dynamic changes in the NP morphol., faceting and elemental segregation under working conditions in previously unreported Pt-Ni core-shell octahedral structures. We follow changes in the Pt-Ni catalyst from a segregated structure to an alloyed shell configuration and then a more spherical structure as a function of temp. under reducing conditions. Exposure to an oxidizing environment then leads to oxidn. of the C support, while the spherical NPs undergo a cycle of transformations into cubic NPs followed by the reaction to spherical NPs. The formation of the cubic NPs results from CO formation during C oxidn., before it is finally oxidized to CO2. Our observations may pave the way towards the design of optimized structure-stability electrocatalysts and highlight the importance of TEM visualization of degrdn. and transformation pathways in bimetallic Pt-Ni NPs under reducing and oxidizing conditions.
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    Meena, A.; Thangavel, P.; Nissimagoudar, A. S.; Narayan Singh, A.; Jana, A.; Sol Jeong, D.; Im, H.; Kim, K. S. Bifunctional Oxovanadate Doped Cobalt Carbonate for High-Efficient Overall Water Splitting in Alkaline-Anion-Exchange-Membrane Water-Electrolyzer. Chem. Eng. J. 2022, 430, 132623  DOI: 10.1016/j.cej.2021.132623

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    Bifunctional oxovanadate doped cobalt carbonate for high-efficient overall water splitting in alkaline-anion-exchange-membrane water-electrolyzer

    Meena, Abhishek; Thangavel, Pandiarajan; Nissimagoudar, Arun S.; Narayan Singh, Aditya; Jana, Atanu; Sol Jeong, Da; Im, Hyunsik; Kim, Kwang S.

    Chemical Engineering Journal (Amsterdam, Netherlands) (2022), 430 (Part_1), 132623CODEN: CMEJAJ; ISSN:1385-8947. (Elsevier B.V.)

    Large scale, cost-efficient, durable, and non-noble metal catalysts for overall water splitting in alk.-anion-exchange-membrane-water-electrolyzer (AAEMWE) are highly demanded for the clean hydrogen economy. Meanwhile, V- and Co-based bimetallic oxide materials were rarely reported for overall water splitting in AAEMWE. Herein, we demonstrate that the self-supported oxovanadate-doped cobalt carbonate (VCoCOx@NF) on nickel foam (NF) is a high-performance overall water-splitting catalyst in AAEMWE. The as-prepd. VCoCOx@NF catalyst demonstrates high activity for both hydrogen and oxygen evolution reactions (HER and OER) in alk. media, with a c.d. (j) of 10 mA cm-2 at overpotentials of 63 mV and 240 mV, resp. Assembled as a conventional electrolyzer for overall water splitting, VCoCOx@NF as both anode and cathode in 1 M KOH operates at low cell voltages of 1.54 and 1.74 V at 10 and 100 mA cm-2, resp., superior to the Ir/C-Pt/C@NF electrolyzer (1.59 and 1.86 V, resp.). First principle calcns. show that the remarkable HER and OER at the Co site are due to the doping of V species, which reduces the overpotential by shifting the d-electron states of Co towards the Fermi-level. Besides, an AAEMWE cell fabricated with the VCoCOx@NF catalyst delivers j = 200 mA cm-2 at 2.01 V in deionized water, lower than the expensive com. IrOx-Pt/C@Au/Ti electrolyzer (2.06 V). This finding provides the stage for large-scale hydrogen prodn. by utilizing the V- and Co-based bimetallic oxide materials.