Electrocatalysts For Oxygen Evolution Reaction Research Articles

Oxygen-Deficient Thermal Treatment Boosts Performance of Co-Ni-Fe Oxide Electrocatalyst in Oxygen Evolution Reaction

The production of green hydrogen through water electrolysis using electricity generated from renewable sources is one of the most promising strategies toward achieving a carbon-neutral economy.[1-3] Water electrolysis is an effective and well-established water splitting process consisting of two half-cell reactions, the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) at cathode and anode, respectively. Although hydrogen as the value-added product is generated at the cathode, the anodic reaction plays an important role in determining the reaction kinetics of the entire water splitting process due to the multiple-electron transfer.[4] Therefore, developing low-cost and highly active OER electrocatalysts is one of the keys to further improve overall efficiency of water electrolysis. Among various water electrolysis technologies, alkaline water electrolysis (AWE) has been commercialized for many decades and standing for medium-scale hydrogen production and using non-noble metals as electrocatalysts.[5]Co-, Ni-, and Fe- oxides or (oxy)hydroxides are reported as auspicious OER electrocatalysts for AWE, as well as abundance on earth.[6] In addition, oxygen vacancies in metallic oxides are reported recently as an effective approach to enhance catalyst activity for OER.[7] Herein, we present trimetallic oxides (CoNiFe oxides) annealed in different atmospheres which are oxygen-rich (A; air), an oxygen-poor (M; 1 vol% air in argon), and oxygen-free (H; 5 vol% H2 in argon), respectively, The CoNiFe (2:2:1)/M with a molar ratio of Co : Ni : Fe = 2 : 2 : 1 treated under an oxygen-poor atmosphere (M) exhibits the best performance in 1 M KOH, with an overpotential of 291 mV at 10 mA cm-2 geo, a mass activity at 1.56 V versus the reversible hydrogen electrode (RHE) of ~345 A g-1 catalyst, and a small Tafel slope of 32.0 mV dec-1. Moreover, CoNiFe (2:2:1)/M shows excellent stability in the OER process for at least 50 hours at 100 mA cm-2 geo at lab scale and ~75 h at 400 mA cm-2 in an alkaline electrolyzer. This work presents a simple and effective method to design OER catalysts based on trimetallic oxides with oxygen deficiencies for alkaline water electrolysis.

Enhancing Stability and Activity of Transition Metal Chalcogenides: Development of Carbon-Based Hydrochar Supported Nickel-Cobalt Selenide Electrocatalyst for Oxygen Evolution Reaction

Transition metal chalcogenides (TMCs) have long been regarded as cost-substitutes for traditional noble metal electrocatalysts, exhibiting comparable activity in the oxygen evolution reaction (OER). However, stability and activity remain crucial factors influencing the broader applicability of the catalysts. Therefore, enhancing the stability of transition metal chalcogenides is a key area of interest. This study delves into the development of carbon-based hydrochar support, serving as an alternative scaffold for depositing TMCs. Specifically, it aims to explore the application of scaffold for nickel-cobalt selenide (NiCoSe), a TMC exhibiting significant potential in accelerating the OER rate by increasing extrinsic activity through higher catalyst loading, enhanced porosity, and ensuring stability of the supported catalyst. The activity and stability of the NiCoSe/hydrochar are determined from cyclic voltammogram and chronoamperometric curves. Characterization of the synthesized NiCoSe on hydrochar show porous microstructure and well-dispersed electrocatalyst on hydrochar scaffold which allowed enhanced transport of species and aided electron transport through the electrolyte-electrode interface. SEM-EDS spectra show uniform distribution of NiCoSe which is advantageous for enhanced performance of the catalyst. This study elucidated the effect of hydrochar to enhance the extrinsic activity of NiCoSe towards the oxygen evolution reaction.

MnCo2O4 Spinel & Carbon Black coated Nickel Foam Electrode. A Bifunctional Water-Splitting Catalyst?

Hydrogen has the potential to revolutionise how we produce and consume energy. We can meet energy demands without compromising the environment by using renewable sources such as wind, solar and wave power producing low-cost electricity to split the water molecule into its constituent parts. We could then convert Hydrogen back to electricity using a fuel cell, as illustrated in Figure 1. The by-product in this entire process is water, giving a clean technology with no toxic or greenhouses gases. Currently the best performing electrocatalytic materials for water splitting are noble metal based, namely platinum for the hydrogen evolution reaction (HER) and Ruthenium for the oxygen evolution reaction (OER). The use of these materials is unsustainable and makes fuel cells too expensive for commercial applications. We must engineer sustainable, cost-effective alternatives.Nickel foam electrodes display desirable electrocatalytic properties such as a large surface area compared to volume with highly conducting, porous networks, while transition metal dichalcogenides show very good electronic properties with high surface areas and large active sites. In this work we show that it is possible to electrochemically and solvothermally produce CoSex and CoSex composites on a porous nickel foam electrode (NFE). The best performing materials were selected and evaluated via electrochemical impedance spectroscopy. This analysis showed high electrochemical active surface area (ECSA) values and good stability across the selected materials. Scanning electron microscopy (SEM) images reveal that the surfaces of the synthesized materials exhibit distinct morphologies and have been significantly modified. Further electrochemical evaluation of the materials reveals low overpotentials for both hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) thus showing promise as a potential bi-functional electrocatalyst in overall water splitting.References(1) Chaudhari, N. K.; Jin, H.; Kim, B.; Lee, K. Nanostructured Materials on 3D Nickel Foam as Electrocatalysts for Water Splitting. Nanoscale. 2017. https://doi.org/10.1039/c7nr04187j.(2) Minadakis, M. P.; Tagmatarchis, N. Exfoliated Transition Metal Dichalcogenide-Based Electrocatalysts for Oxygen Evolution Reaction. Advanced Sustainable Systems. 2023. https://doi.org/10.1002/adsu.202300193.(3) Sukanya, R.; da Silva Alves, D. C.; Breslin, C. B. Review—Recent Developments in the Applications of 2D Transition Metal Dichalcogenides as Electrocatalysts in the Generation of Hydrogen for Renewable Energy Conversion. J Electrochem Soc 2022, 169 (6). https://doi.org/10.1149/1945-7111/ac7172. Figure 1

Surface Engineered Ni-Fe Foam As a Bi-Functional Electrode for Water Electrolysis in Alkaline Media

Using fossil fuels as a main source of energy has resulted in various environmentally hazardous effects, including global warming, climate change, and environmental pollution. Consequently, the development of a sustainable and clean source of energy became of urgent need. Water electrolysis has captured considerable attention as the main technology of hydrogen fuel production. It is particularly enticing due to its eco-friendliness and sustainability. Self-supported electrocatalysts for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) have prominent roles in water electrolysis. Growing self-supported metal oxides, hydroxides1, phosphates2, sulfides3, etc. that possess high electrical conductivity, electrocatalytic activity, and structural stability have attracted attention lately. FeNi-based catalysts are considered one of the most promising electrocatalysts for water electrolysis in alkaline solutions. However, improving their catalytic activity and stability is one of the research targets before further practical applications. Nickel foam is widely used as a self-supported electrocatalyst due to its high surface area, low cost, and exceptional catalytic activity, with several studies focused on the deposition of nickel-iron materials on nickel foam for the development of a bifunctional catalyst for water electrolysis4. However, very few studies used NiFe mesh or foam to grow a self-supported catalyst material5.In this study, surface-engineered NiFe foam electrodes were developed by facile control of the Ni/Fe surface ratio via chemical leaching and oxidation with NaOH and NaClO4 followed by electrochemical activation, aiming to design a bifunctional electrocatalyst for both OER and HER. Results revealed that varying the ratios of NaOH and NaClO4 induced significant alterations to the surface composition (different Ni/Fe), and hence the active sites for OER and HER. The physicochemical and structural properties of the NiFe foam were characterized using glancing angle X-ray diffraction (GAXRD), X-ray photoelectron spectroscopy (XPS), and scanning electron microscope (SEM). Further, the catalytic activity of the modified electrode was assessed utilizing cyclic voltammetry, linear sweep voltammetry, chronopotentiometry, and electrochemical impedance spectroscopy. References F. Dionigi and P. Strasser, Advanced Energy Materials, 6, 1600621 (2016).Y. Yu et al., Journal of Colloid and Interface Science, 607, 1091–1102 (2022).A. Shankar, R. Elakkiya, and G. Maduraiveeran, New J. Chem., 44, 5071–5078 (2020).H.-S. Hu, S. Si, R.-J. Liu, C.-B. Wang, and Y.-Y. Feng, International Journal of Energy Research, 44, 9222–9232 (2020).C. Sun et al., ACS Appl. Energy Mater., 4, 8791–8800 (2021).

Electrooxidation of Poly(methyl-methacrylate) and Poly(ethylene glycol): Hydrogen Production from Plastic Wastes Electrolysis

Introduction. Green H2 production by water electrolysis at low temperature has some bottlenecks mostly dealing with the anodic reaction, the oxygen evolution reaction (OER), given its thermodynamic and kinetic limitations [1]. One strategy to reduce the energy requirements for this H2 production route is the replacement of the conventional OER (with a thermodynamic potential of 1.23 V vs. Reversible Hydrogen Electrode (RHE)) by a less-energy intensive reaction like the electrooxidation of organic molecules (in some cases presenting a thermodynamic potential as low as c.a. 0 V vs RHE). This approach also addresses the possibility of converting organic wastes into value-added chemicals avoiding the CO2 emissions derived from conventional treatments (e.g., incineration) [2, 3]. Our research group is focused on the electrochemical valorization of several organic wastes like polyethylene glycol (PEG) or poly(methyl-methacrylate) (PMMA). Both are commonly found in urban and industrial water effluents and concern has been raised about the true fate of these polymers in the environment. Herein, the possibility of generating H2 and value-added organic molecules from these plastic wastes has been explored. Experimental. A binary solvent strategy was followed to dissolve the PMMA polymer, by using 1 M H2SO4 in 80% v/v 2-propanol/water electrolyte. Then the electrooxidation of PMMA was studied in a 2-electrode batch cell at 70ºC with different PMMA concentrations (0.1-2 wt.%). On the other hand, the electrooxidation of ethylene glycol (EG) and PEG with various molecular weights (from 200 to 4000 g mol-1), which are soluble in water, was carried out in aqueous solutions (PEG concentrations from 1 to 20 g/L) between 30 and 80 °C by using a Polymer Electrolyte Membrane (PEM) reactor. Pt/C electrodes were used in all cases. A commercial Pt cathode was employed and the anode material was prepared by the spray-deposition of a Pt(20 wt.%)/C catalyst powder onto a carbon paper gas-diffusion layer. The liquid products at the anodic compartment were analysed by size-exclusion chromatography (SEC) and nuclear magnetic resonance (NMR) spectroscopy while the production of hydrogen in the cathodic compartment was online monitored with a mass spectroscopy. Results and discussion. In the case of PMMA electrooxidation, despite the dissolution challenge, polymer macromolecules showed to partially block the accessibility of the Pt active sites on the electrode and strongly degraded its electrochemical performance [4]. In the case of PEG, it was much more easily electrooxidised, at low cell voltages, from around 0.4 V at 80°C (Figure 1a), near to values recorded with EG, suggesting a first step linked with the electrooxidation of the terminal groups of the macromolecules chain. Current densities larger than 100 mA.cm-2 at 0.8 V and 80°C were achieved for low molecular mass molecules (200 and 400 g/mol). A decrease of the PEG molecular weight in the anodic solution was demonstrated by SEC, confirming that the electro-oxidation process likely follows two main mechanisms, involving the consumption of terminal –OH or intermediate -C-O-C- bonds cleavage (Figure 1b), depending on the polymer molecular weight. This study demonstrates the proof-of-concept of the low-temperature electro-oxidation of polymers containing C-O bonds in their main chain. Conclusions. With a further macroporous electrode enegineering and catalyst development, the electrolysis of plastic wastes stands as a promising technology not only for the production of hydrogen from macromolecules by electrochemical means, but also as a potential pathway to depolymarize plastic wastes, thus increasing their degradability. References. [1]. Y. Cheng, S.P. Jiang, Advances in electrocatalysts for oxygen evolution reaction of water electrolysis-from metal oxides to carbon nanotubes, Prog. Nat. Sci.: Mater. 25 (2015) 545-553.[2]. M. Simões, S. Baranton, C. Coutanceau, Electrochemical valorisation of glycerol, ChemSusChem. 5 (2012) 2106–2124[3]. Y.X. Chen, A. Lavacchi, H.A. Miller, M. Bevilacqua, J. Filippi, M. Innocenti, et al. Nanotechnology makes biomass electrolysis more energy efficient than water electrolysis. Nat. Commun. 5 (2014) 1-6.[4]. N. Grimaldos-Osorio, F. Sordello, M. Passananti, J. González-Cobos, A. Bonhommé, P. Vernoux, A. Caravaca. From plastic-waste to H2: A first approach to the electrochemical reforming of dissolved Poly(methyl methacrylate) particles. Int. J. Hydrogen Energy. 48 (2023) 11899-11913. Acknowledgments The authors gratefully acknowledge the French institution “Ecole Urbaine de Lyon” (EUL - Institut de Convergences) for funding a PhD grant of Dr. N. Grimaldos-Osorio. Figure 1

A Literature Survey of Transition Metal Boride, Carbide, Pnictide, and Chalcogenide Water Oxidation Electrocatalysts

Transition metal borides, carbides, pnictides, and chalcogenides (X-ides) are a new class of oxygen evolution reaction (OER) electrocatalysts, which have become a relevant topic for electrocatalytic energy conversion and storage research. This is because of their earth abundance, high electrical conductivity, and resultant high OER performance. Furthermore, some X-ide electrocatalysts demonstrate various degrees of oxidation resistance under OER potentials due to their differences in chemical composition, crystal structure, and morphology. Interestingly, there are three possible post-OER electrocatalyst models: a fully oxidized oxide/(oxy)hydroxide material, a partially oxidized core@shell structure, and an unoxidized material.1 In the past ten years (from 2013 to 2022), over 890 peer-reviewed research papers have focused on X-ide OER electrocatalysts. Recently, several topical reviews have been published. However, typically these literature reviews have provided limited conclusions targeted at only certain X-ides. Additionally, certain reviews omit key points, especially regarding the “catalytically active species” in X-ide OER electrocatalysts.In this work, we provide a comprehensive summary of (i) the experimental parameters (e.g., substrates, electrocatalyst loading amounts, electrocatalyst shapes, geometric overpotentials, Tafel slopes, etc.) and (ii) the details of the electrochemical stability tests and following post analyses used in all the X-ide OER electrocatalyst publications from 2013 (the birth of this research area) to 2022.2 In particular, based on the reported post-OER analytical results, we attempt to exhaustively classify all the X-ide electrocatalysts into four groups: no oxidation, partial oxidation, full oxidation, and unknown, which is relevant to the nature of the “catalytically active species” in X-ide OER electrocatalysts. After investigating the simpler X-ide systems, transition metal-based polyanion compounds/composites are also surveyed. Finally, we introduce the reported novel and special analytical techniques employed for observing X-ide reconstruction (i.e., oxidation and other phase transformations) during OER testing, which can be useful in selecting the analytical technique of maximum utility for the researchers’ purpose. Additionally, for each material sub-group (e.g., borides, carbides, nitrides, phosphides, sulfides, selenides, and tellurides), we also point out further challenges to be addressed and propose further questions yet to be answered, which may provide clues pointing to future research tasks. References B. R. Wygant, K. Kawashima, and C. B. Mullins, ACS Energy Lett., 3, 2956–2966 (2018).K. Kawashima et al., Chem. Rev., 123, 12795–13208 (2023).

Role of Electrochemical Activation in Evaluating Oxygen Evolution Activity of Ni-Incorporated Bimetallic Tungstate in Alkaline Media

Hydrogen is considered the future fuel and promising alternative towards carbon neutrality. In this context, water electrolysis using renewable electricity is an environmentally benign approach for clean hydrogen production. In water electrolysis, among the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), OER is the rate-limiting due to slow kinetics, high overpotential, and complex pathways. To catalyze the OER, choice of non-precious electrocatalysts is preferred over the precious platinum group metal (PGM)-based catalysts owing to their relatively higher abundance and cost-effectiveness. The feasibility of using non-precious electrocatalysts in alkaline water electrolysis is also favorable for the low-capital cost compared to the acidic one. In lab scale research, the preliminary evaluation of OER electrocatalysts is usually performed using thin-film rotating disk electrodes (TF-RDEs) in a three-electrode system. In this process, the very first step involves multiple cyclic voltammetry (CV) cycles for better wettability of the catalyst layer to the electrolyte, which is considered as electrochemical activation (EA). Incomplete activation of the catalyst layer would lead to inconsistent OER performance of the electrocatalysts. This is much more pronounced in non-precious electrocatalysts compared to the precious ones in alkaline media. Despite that the importance of EA is overlooked most of the time and rarely studied. In this work, the critical role of EA is systematically investigated for Ni-containing bimetallic tungstate in alkaline OER and also compared with that of monometallic counterparts. This study demonstrates more than 1.2-fold performance improvement in OER for bimetallic tungstate. The corresponding insights on the interfacial structure and oxidative surface reconstruction are also noted by using in-situ electrochemical analysis and ex-situ characterizations. The present study is believed to be beneficial for the precise analysis and development of non-precious alkaline OER electrocatalysts.

Investigation of Plasmon-Mediated Oxygen Evolution Reaction

To achieve a carbon-neutral future, sustainable energy is needed through storing energy through its chemical bond. Water electrolysis, an exemplary form of electrocatalysis, presents an example of storing energy within chemical bonds of the high-energy hydrogen gas. Nevertheless, the anodic reaction involved in the oxygen evolution reaction (OER) poses limitations on the overall rate of the process. Plasmonic gold nanoparticles (Au NPs) have been added to enhance the charge transfer at the interface of the OER electrocatalysts and electrolyte under light illumination.1-3 However, the mechanistic understanding of how the Au NPs on the photo-assisted electrochemical process is still lacking. We applied a model system of plasmonic Au electrode with nickel (Ni) and cobalt (Co)-based OER electrocatalyst to investigate the plasmon-mediated OER process. The composite system has been characterized with XPS, AFM, (photo)electrochemical and spectroscopic characterizations. Our preliminary data shows that the electrodeposited plasmonic Au electrode is free of surfactant and the resonant light illumination could modulate the electrochemical properties of the Au electrode and the Ni- and Co-based electrocatalysts in the alkaline electrolytes. Liu, G.; Li, P.; Zhao, G.; Wang, X.; Kong, J.; Liu, H.; Zhang, H.; Chang, K.; Meng, X.; Kako, T.; Ye, J. Promoting Active Species Generation by Plasmon-Induced Hot-Electron Excitation for Efficient Electrocatalytic Oxygen Evolution. Am. Chem. Soc. 2016, 138(29), 9128–9136.Wang, M.; Wang, P.; Li, C.; Li, H.; Jin, Y. Boosting Electrocatalytic Oxygen Evolution Performance of Ultrathin Co/Ni-MOF Nanosheets via Plasmon-Induced Hot Carriers. ACS Appl. Mater. Interfaces 2018, 10(43), 37095–37102.Zeng, X.; Choi, S. M.; Bai, Y.; Jang, M. J.; Yu, R.; Cho, H.-S.; Kim, C.-H.; Myung, N. V.; Yin, Y. Plasmon-Enhanced Oxygen Evolution Catalyzed by Fe2N-Embedded TiOxNy ACS Appl. Energy Mater. 2020, 3(1), 146–151.

Development of Ba3TiO7-Supported IrOx Electrocatalysts for Enhanced Mass Activity in the Acidic Oxygen Evolution Reaction

Overcoming the challenge of developing highly durable and conductive support materials for Ir-based electrocatalysts in the acidic oxygen evolution reaction (OER) is difficult due to the highly corrosive conductions experienced during anodic process. In this research, we develop IrOx/Ba3TiO7 electrocatalysts, which employ Ba3TiO7 as a new support material, to minimize loading amount of iridium. We also try to fabricated Ba3TiO(7-x) support material with abundant oxygen vacancies for enhanced conductivity of the support material. As a results, the IrOx/Ba3TiO7 electrocatalysts demonstrated remarkable OER performances and enhanced mass activity compared to unsupported IrOx electrocatalysts. Through a combination of physical and chemical analyses, It was elucidated that the excellent thermodynamic stability and electrical conductivity of Ba3TiO7 effectively enhanced the mass activity of IrOx/Ba3TiO7 electrocatalysts. These finding provide further insight into new category of anode support materials for proton exchange membrane water electrolyzer.

A Spray-Drying Approach for Highly Active Amorphous NiFe2O4 for AEM Water Electrolysis

Water electrolysis, a key process for hydrogen production, holds immense potential in mitigating carbon dioxide emissions when powered by renewable energy sources, thereby contributing to a sustainable and clean energy future. Commercially available and widely recognized technologies for water electrolysis include the proton exchange membrane water electrolysis (PEMWE) and the traditional alkaline water electrolysis (AWE). In order to use the benefits of both systems, the anion exchange membrane water electrolysis (AEMWE) method is proposed, enabling the utilization of non-noble catalyst materials and achieving high current densities at low cell voltages [1].In the present work, the influence of a spray-drying approach compared to a co-precipitation method on the bimetallic non-noble NiFe2O4 catalysts on the oxygen evolution reaction (OER) activity is systematically investigated. State of the art catalyst for OER is Iridium oxide and so far, the best non-noble catalyst has not been found yet. A widely used non-noble catalyst is NiFe- layered double hydroxide but it suffers from low electric conductivity [2]. Another promising catalyst is the NiFe2O4, which also provides low OER overvoltage. However, amorphous oxides are said to have a higher activity than crystalline [3]. Furthermore, the use of a spray drying approach is highly reproducible, with which the crystallinity can be set very easily [4, 5].The experimental study of the catalysts was carried out in a commercial Baltic-Fuel Cell with a geometrical area of 25 cm² at 60 °C in 1 M KOH electrolyte and a contact pressure of 1.25 MPa. The electrodes were prepared by directly spray coating the desired amount of catalyst onto the anode and cathode substrates. A Bekipor® 2NI 18-0.25 nickel felt was used as the substrate for the anode, while AvCarb MGL370 carbon paper was employed for the cathode. As cathode catalyst Pt/C from Tanaka (TEC10E40E, 37.2 % Pt) was used, while a loading of 0.5 mg cm-2 Pt was targeted during the spraying procedure. On the anode, the NiFe2O4 loading was set to 2 mg cm-2. Both electrodes were separated by a Sustainion® X37-50 Grade T membrane. For determination of relevant catalyst activity data chronopotentiometry (CP) and electrochemical impedance spectroscopy (EIS) were carried out.Figure 1 reveals that the spray-dried NiFe2O4 (amorphous, red curve) has a superior performance compared to the precipitated NiFe2O4 (green curve), which is partly crystalline. With the spray-dried catalyst, a reduction in overvoltage of approx. 60 mV at 1 A cm-2 can be achieved. With a cell voltage of less than 1.7 V at a current density of 1 A cm-2, our recently developed catalyst ranks among the most outstandingly described in the literature to date [6]. These findings can contribute to a further development of amorphous catalysts with even higher activity.[1] Cossar, E., Murphy, F. and Baranova, E.A. (2022), Nickel-based anodes in anion exchange membrane water electrolysis: a review. J Chem Technol Biotechnol, 97: 1611-1624.[2] Jeon, S.S., Lim, J. Kang, P.W., Lee, J.W., Kang, G., Lee, H. (2021), Design Principles of NiFe-Layered Double Hydroxide Anode Catalysts for Anion Exchange Membrane Water Electrolyzers. ACS Appl Mater Interfaces,13 (31): 37179-37186.[3] Shi, G., Arata, C., Tryk, D., Tano, T., Yamaguchi, M., Iiyama, A., Uchia, M., Iida, K., Watanabe, S., Kakinuma, K. (2023), NiFe Alloy Integrated with Amorphous/Crystalline NiFe Oxide as an Electrocatalyst for Alkaline Hydrogen and Oxygen Evolution Reactions. ACS Omega, 8: 13068−13077.[4] Kreitz, B., Arias, A. M., Martin, J., Weber, A. P., Turek, T. (2020), Spray-Dried Ni Catalysts with Tailored Properties for CO2 Methanation, Catalysts, 10 (12): 1410.[5] Arias, A. M., Weber, A. P. (2019), Aerosol synthesis of porous SiO2-cobalt-catalyst with tailored pores and tunable metal particle size for Fischer-Tropsch synthesis (FTS), J Aerosol Sci, 131: 1-12.[6] Du, N., Roy, C. Peach, R., Turnbull, M., Thiele, S. and Bock, C. (2022), Anion-Exchange Membrane Water Electrolyzers. Chem Rev, 122: 11830-11895. Figure 1

Correlating atomic-scale structural and compositional details of Ca-doped LaCoO3 perovskite nanoparticles with activity and stability towards the oxygen evolution reaction

Developing efficient oxygen evolution reaction (OER) electrocatalysts requires a thorough understanding of structure–activity-stability relationships, ideally at the atomic scale. Herein, we employed atom probe tomography and transmission electron microscopy to reveal compositional and structural changes on LaCoO3 and Ca-doped LaCoO3 surfaces during OER. We reveal that the topmost surfaces of pristine perovskite are terminated by the A-site element (La). After OER, amorphous La(OH)3 is formed on the surfaces of LaCoO3, which leads to significant activity deterioration. For Ca-doped LaCoO3, enhanced intercalation and penetration of hydroxide ions, along with the appearance of Co3+/4+ redox couple, are observed, contributing to its enhanced OER activity and stability. Our study demonstrates how atomic-scale compositional and structural details of electrocatalyst surfaces deepen our understanding of their activity and stability.

Dual-Site Bridging Mechanism for Bimetallic Electrochemical Oxygen Evolution.

Heterogeneous dual-site electrocatalysts are emerging cutting-edge materials for efficient electrochemical water splitting. However, the corresponding oxygen evolution reaction (OER) mechanism on these materials is still unclear. Herein, based on a series of in situ spectroscopy experiments and density function theory (DFT) calculations, a new heterogeneous dual-site O-O bridging mechanism (DSBM) is proposed. This mechanism is to elucidate the sequential appearance of dual active sites through in situ construction (hybrid ions undergo reconstruction initially), determine the crucial role of hybrid dual sites in this mechanism (with Ni sites preferentially adsorbing hydroxyls for catalysis followed by proton removal at Fe sites), assess the impact of O-O bond formation on the activation state of water (inducing orderliness of activated water), and investigate the universality (with Co doping in Ni(P4O11)). Under the guidance of this mechanism, with Fe-Ni(P4O11) as pre-catalyst, the in situ formed Fe-Ni(OH)2 electrocatalyst has reached a record-low overpotential of 156.4 mV at current density of 18.0 mA cm-2. Successfully constructed Fe-Ni(P4O11)/Ti uplifting the overall efficacy of the phosphate from moderate to superior, positioning it as an innovative and highly proficient electrocatalyst for OER.

(Invited) Boosting Oxygen Evolution Reaction by Stabilization of Ultrasmall Ruthenium Nanoclusters on Thiol-Functionalized Metal-Organic Framework

Rational designing of efficient nanocatalysts plays a crucial role in catalyzing kinetically sluggish reactions like Oxygen Evolution Reaction (OER). However, the uncontrolled nucleation and growth of nanostructures poses major effectiveness and economic challenges in the implementation and commercialization of noble metal-based electrocatalysts. Functionalized Metal-Organic Frameworks (MOFs) have the properties to stabilize such unstable nanoclusters in extremely small sizes by compensating for their high surface energy and Ostwald’s ripening effect. Here, we have reported the synthesis of ultrasmall Ruthenium nanoclusters stabilized through thiol-functionalized Ni-MOF (Ru@Ni-M-SH). The stabilization of ruthenium under reduced conditions on the MOF surface was facilitated by the lower electronegativity and increased orbital overlapping effect of sulfur, resulting in an average size of 1.5 nm. A perturbed electronic structure confirmed from XPS as well as XAS studies revealed the fundamental understanding behind electronic redistribution. Backed with such a favorable electronic structure, the catalytic OER activity for Ru@Ni-M-SH outperformed the state-of-the-art RuO2 with three times higher current density (242 mA/cm2) and 143 mV reduced overpotential. With 95% Faradaic efficiency, the Turnover Frequency (TOF) and Mass activity of Ru@Ni-M-SH were found to be several orders higher as compared to RuO2. Unlike other Ru-based catalysts, Ru@Ni-M-SH showed extremely high stability with over 24 h of chronoamperometry study. With thorough in-situ analysis providing information regarding catalytically active sites and intermediates, Ru@Ni-M-SH established itself as an economically sustainable OER electrocatalyst.

Electrolyte-Induced Fe-Doping of LaNiO3 Model Surfaces for Water Electrolysis

Renewable energy needs to be store in different energy forms for the energy transition away from fossil fuels. One of most promising forms is chemicals such as H2 and C-based fuels obtained via electrochemical reactions. The common hinderance in such reactions is the sluggish oxygen evolution reaction (OER), which causes low energy efficiency in conversion devices. Developing highly active, earth-abundant OER electrocatalysts in alkaline media is necessary to boost the performance of such devices. Fe ion content in electrolyte is known to influence the OER-activity of metal oxide electrocatalysts. For example, the Fe incorporation in NiOOH is a well-established and important effect for OER.[1] Here, we present the effect of absorbed Fe on the OER activity of the epitaxial Ni-surface terminated LaNiO3 thin films. The correlation between surface species and OER activity of electrocatalysts was investigated using surface-sensitive low-energy ion scattering and X-ray photoemission spectroscopies. We observe more than an order-of-magnitude boost in OER activity during initial cycling, from 0.15 mAcm-2 to 3.40 mAcm-2 at 1.60 V vs. reversible hydrogen electrode. The activity increase was mainly caused by the incorporation of up to 5 % Fe at the top surface of LaNiO3 film (less than ~1 nm) even though only a very trace amount of Fe ion, 30 ppb, is present in the 0.1 M electrolyte of high purity KOH 99.99 %. Our results demonstrate that the disordered active surface phase, which develops even on single crystalline surfaces, is a complex interplay between the as-prepared (surface) crystalline structure, composition of the solid electrocatalyst and the liquid electrolyte, and has strong impact on the final OER activity, implying that we need to think differently about the crystalline electrocatalysts: crystalline bulk and dynamically changing surface layer.[1] L. Trotochaud, S.L. Young, J.K. Ranney, S.W. Boettcher, Nickel–Iron Oxyhydroxide Oxygen-Evolution Electrocatalysts: The Role of Intentional and Incidental Iron Incorporation, J. Am. Chem. Soc. 136 (2014) 6744–6753.

(Invited) Graphene-Supported Fe/Ni Single Atoms and Feni Alloy Nanoparticles as Bifunctional Oxygen Electrocatalysts for Rechargeable Zinc-Air Batteries

To alleviate the severe energy crisis and environmental problems caused by the massive and unsustainable consumption of fossil fuels, it is urgent to develop renewable energy conversion and energy storage technologies to reform the traditional energy structure. Batteries are considered as one of the most promising energy storage technologies due to their sustainability, high efficiency and less pollution. Among various battery storage devices, rechargeable zinc-air batteries (ZABs) have attracted widespread attention on account of their high theoretical energy density (1086 Wh kg– 1), low cost, abundant zinc resources, low operating temperature, and good stability and safety in aqueous environments. However, the kinetics of oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) occurred on the air electrode during the discharging and charging processes are extremely sluggish, leading to low energy efficiency, which significantly limits the large-scale application of rechargeable ZABs. Pt-based electrocatalysts exhibit high ORR activities, while Ru- and Ir-based oxides are excellent OER electrocatalysts, but natural scarcity, high cost and poor durability seriously reduce the marketability and commercial competitiveness of rechargeable ZABs. Therefore, the development of robust, cost-effective and high-efficiency non-noble metal bifunctional ORR/OER electrocatalysts is of crucial significance for the practical application of rechargeable ZABs.As a promising alternative, earth-abundant transition metal (Fe, Co, Ni, etc.)-based materials have been extensively studied due to their relatively high electrocatalytic activity and durability. Thereinto, transition metal and nitrogen co-doped carbon (M-NC), especially Fe-NC, has been demonstrated to be the most effective ORR catalyst, and the active sites are generally considered as atomically dispersed metal coordinated to N atoms (M-Nx sites, x indicates the number of N atoms coordinating the metal). Nevertheless, M-NC catalysts only possess good ORR activity with poor OER performance. Bimetallic FeNi nanocomposites (metals, oxides, carbides, sulfides) are promising for OER. At high potentials, highly active FeNi oxyhydroxide species are generated on the surface of the FeNi-based electrocatalysts. Since ORR and OER depend on different catalytic active sites, an ideal bifunctional oxygen electrocatalysts should integrate M-NC complex with high ORR activity and bimetallic FeNi nanocomposite with a high OER activity to achieve bifunctionality. However, due to the intense competition between transition-metal single atoms and nanocomposites in the preparation process, it is challenging to combine them properly for superior bifunctional ORR/OER catalytic activity.Herein, we report a bifunctional oxygen electrocatalyst consisting of ZIF-derived carbon-anchored Fe/Ni single atoms and FeNi alloy nanoparticles; meanwhile, graphene is also introduced as carbon support to improve the uniformity of active sites (denoted as Fe/Ni-NC||FeNi@G). Benefiting from this unique composition and structure, Fe/Ni-NC||FeNi@G electrocatalyst displays excellent bifunctional ORR/OER activity and durability in rechargeable ZABs. This work provides an effective approach to designing highly efficient non-noble metal multifunctional catalysts for practical applications in rechargeable metal-air batteries. Acknowledgements Thanks for the financial support from the Shenzhen Science and Technology Innovation Committee (JCYJ20180507183818040).

(Invited) Cost Reduction Strategies in Low Temperature Water Electrolysis with Improved Catalytic Activity

Proton exchange membrane water electrolysis (PEMWE) technology is especially advantageous for green H2 production as a clean energy vector. During the water electrolysis process, the oxygen evolution reaction (OER) requires a large amount of iridium (2-3 mgIr cm−2) as catalyst. This material is scarce and expensive, representing a major bottleneck for large-scale deployment of electrolyzers. Recently, Retuerto et al. have synthesized different Ir double perovskites (Sr2CaIrO6) with the Ir atoms in a high oxidation state (Ir6+/5+) 1 and their catalytic activity was extensively investigated. Specifically, a Ca-based double perovskite (Sr2CaIrO6) has showed the highest OER activity similar to those of Ir or Ir oxides-based catalysts while being stable for the OER in half cell measurements. We report the development of a membrane electrode assembly taking advantage of the high activity of the Sr2CaIrO6 catalyst that has only 0.2 mgIr cm−2, around 10 times lower compared to what it is used in the anodes of commercial PEMWE, showing superior performance and stability. The morphology/structure of the developed anode and Sr2CaIrO6 catalyst, before and after operation in PEMWE, were extensively characterized by X-ray diffraction (XRD, high-resolution transmission electron microscopy (HRTEM), scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and X-ray absorption near edge structure (XANES). We discuss in this contribution how the finding of this work can provide a strategy to lower the Ir content in commercial PEMWE.Anion exchange membrane water electrolysis (AEMWE) combines the advantages of proton exchange membrane water electrolysis (PEMWE), i.e., high current density while maintaining high efficiency and fast dynamic response with the advantages of alkaline water electrolysis (AWE), i.e., low-cost materials. However, AEMWE technology is still depends on a supporting electrolyte and its durability is low. In this regard, the German Aerospace Center (DLR) is developing novel approaches and cell components. Raney-nickel electrodes were developed via vacuum plasma spraying, reaching performance and stability comparable to SoA MW size PEMWE. The electrodes were further refined, switching to lower cost atmospheric plasma spraying while maintaining high performance. Studying the effect of supporting electrolyte concentration it was found, that lowering the ohmic resistance is by far the largest challenge for high performance AEMWE, especially at low KOH concentrations and electrode kinetics are less impactful. Moreover, a 17% efficiency uplift was achieved by applying a carefully tuned, thermally sprayed, nickel-based macro porous layer (MPL) onto a traditional mesh porous transport layer. Both ohmic and mass transport resistances were found to be lowered by the MPL. FIB-SEM and µCT analysis revealed an ideal structure, i.e., 30-40% porosity, almost exclusively open pores, and a broad pore size distribution2. In a fundamental study3, the mechanism of the OER activity was investigated as well as the effect of trace amounts of iron in Ni(OH)2. X-ray diffraction (XRD) and Thermogravimetric analysis (TGA) TGA analysis lead to the conclusion, that iron stabilized the α-Ni(OH)2which is the most active. CV analysis showed that an important redox switching step is also accelerated by Fe-doping. M. Retuerto, L. Pascual, J. Torrero, M. A. Salam, Á. Tolosana-Moranchel, D. Gianolio, P. Ferrer, P. Kayser, V. Wilke, S. Stiber, V. Celorrio, M. Mokthar, A. S. Gago, K. A. Friedrich, M. A. Peña, J. A. Alonso, D. G. Sanchez, S. Rojas, Highly Active and Stable OER Electrocatalysts Derived from Sr2MIrO6 for Proton Exchange Membrane Water Electrolyzers. Nat. Commun. 2022, 13, 7935. Razmjooei, F.; Morawietz, T.; Taghizadeh, E.; Hadjixenophontos, E.; Mues, L.; Gerle, M.; Wood, B. D.; Harms, C.; Gago, A. S.; Ansar, S. A.; Friedrich, K. A., Increasing the performance of an anion-exchange membrane electrolyzer operating in pure water with a nickel-based microporous layer. Joule 2021, 5 (7), 1776-1799. Wang, L.; Saveleva, V. A.; Eslamibidgoli, M. J.; Antipin, D.; Bouillet, C.; Biswas, I.; Gago, A. S.; Hosseiny, S. S.; Gazdzicki, P.; Eikerling, M. H.; Savinova, E. R.; Friedrich, K. A., Deciphering the Exceptional Performance of NiFe Hydroxide for the Oxygen Evolution Reaction in an Anion Exchange Membrane Electrolyzer. ACS Applied Energy Materials 2022, 5 (2), 2221-2230.

Computational and Experimental Investigations of the Au and NiOOH Interface in the Oxygen Evolution Reaction

The transition to renewable energy sources is crucial for combating climate change, and hydrogen (H2) is a promising clean fuel. Electrolysis of water, specifically the oxygen evolution reaction (OER), is a key process for producing H2. Prior experimental results have shown that gold (Au) could improve the apparent activities of the OER electrocatalysts such as nickel hydroxide (Ni(OH)2) in alkaline electrolytes.1-3 However, the detailed interfacial atomic structure and mechanisms of the increased catalytic activity remain poorly understood.To understand the interfacial effect between Au and Ni(OH)2 in the OER process, our research uses the model system of Au nanoparticles (NPs) and nickel oxyhydroxide (NiOOH), which is the active phase of Ni(OH)2 layers in the OER, to investigate their interfacial interactions with both experimental and computational approaches. We hypothesize that the Au NPs on NiOOH enhances the OER activity by promoting electron transfer at the interface. Our studies involve computational analysis using Density Functional Theory (DFT) calculations to obtain the local density of states (LDOS), which can help us better visualize and compare the local electronic structure of NiOOH in the presence and in the absence of AuNPs. Two architectures of the Au and NiOOH interface will be investigated: Au NPs on a NiOOH film and NiOOH NPs on a Au film.Additionally, Bader analysis is employed to determine the Bader charge, offering a more detailed understanding of how electrons are distributed across the AuNPs/NiOOH interface. Furthermore, the catalytic efficacy of AuNPs on NiOOH is assessed through electrodepositions followed by cyclic voltammetry(CV) measurements. The experimental and computational results will be directly compared, and this research is expected to contribute to the insights of substrate and catalyst interactions for design of more efficient electrocatalysts.

Unveiling the Synergistic Effect of Two-Dimensional Heterostructure NiFeP@FeOOH as Stable Electrocatalyst for Oxygen Evolution Reaction

Introducing multiple active sites and constructing a heterostructure are efficient strategies to develop high-performance electrocatalysts. Herein, two-dimensional heterostructure NiFeP@FeOOH nanosheets supported by nickel foam (NF) are prepared by a hydrothermal–phosphorization–electrodeposition process. The synthesis of self-supporting heterostructure NiFeP@FeOOH nanosheets on NF increases the specific surface region, while bimetallic phosphide realizes rapid charge transfer, improving the electron transfer rate. The introduction of FeOOH and the construction of a heterostructure result in a synergistic effect among the components, and the surface-active sites are abundant. In situ Raman spectroscopy showed that the excellent oxygen evolution reaction (OER) performance was due to reconstruction-induced hydroxyl oxide, which achieved a multi-active site reaction. The NiFeP@FeOOH/NF electrocatalytic activity was then significantly improved. The findings indicate that in a 1.0 M KOH alkaline solution, NiFeP@FeOOH/NF showed an OER overpotential of 235 mV at 100 mA cm−2, a Tafel slope of 46.46 mV dec−1, and it worked stably at 50 mA cm−2 for 80 h. This research proves that constructing heterostructure and introducing FeOOH are of great significance to the study of the properties of OER electrocatalysts.

Noble Metal‐Free High Entropy Perovskite Oxide with High Cationic Dispersion Enhanced Oxygen Redox Reactivity

AbstractHigh entropy perovskite oxide (HEPO) materials have received significant interest as efficient electrocatalysts owing to their chemical and structural stability, effective interaction of multiple metal active sites with the reaction intermediate species resulting in enhanced catalytic activity. The effect of high‐entropy strategy could achieve the regulation of the structural durability by tailoring the composition. Herein, we design a novel transition metal and rare‐earth metal based high entropy perovskite oxide (Co0.11Mn0.11Ni0.11Fe0.14La0.13Nd0.13Gd0.13Pr0.13)O3, showing superior catalytic activity in the oxygen reduction reaction (ORR) and also promising electrocatalyst in oxygen evolution reaction (OER). High degree of cationic dispersion in the lattice and multiple electrocatalytic active sites enhance the oxygen ion migration at the surface and facilitate electron transfer, manifesting good ORR performance. Notably, the as‐prepared high entropy material exhibits onset and half‐wave potential of 1.02 V and 0.89 V versus RHE, respectively, exceeding the performance of the benchmarked Pt/C catalyst in the ORR process with reasonable redox kinetics. Moreover, the material also presents faster kinetics with reasonably low overpotential and excellent electrochemical stability in the OER process as well. This study shows the viability of designing non‐precious high entropy materials as highly efficient oxide electrocatalyst by utilizing its broad compositional space.

Designing Highly Efficient Electrocatalyst for ORR and OER Based on Nb2CO2 MXene: The Role of Transition Metals and N-Doping Content.

Finding efficient and stable electrocatalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is imperative for advancing zinc-air batteries. Herein, the effect of transition metal anchored on Nb2CO2 with different N content to form TM-Nx-Nb2CO2 on the catalytic activity of ORR and OER is investigated by density functional theory. Among all the designed TM-Nx-Nb2CO2, Pt-N12.50%-Nb2CO2, Pt-N37.50%-Nb2CO2, Pt-N50.00%-Nb2CO2, Pd-N68.75%-Nb2CO2, and Pd-N100%-Nb2CO2 are excellent ORR electrocatalysts with ηORR values of 0.38, 0.36, 0.38, 0.38, and 0.34 V, respectively. Rh-Nb2CO2, Rh-N12.50%-Nb2CO2, Rh-N31.25%-Nb2CO2, Rh-N37.50%-Nb2CO2, Rh-N50.00%-Nb2CO2, Pt-N50.00%-Nb2CO2, Rh-N68.75%-Nb2CO2, and Rh-N81.25%-Nb2CO2 are excellent OER electrocatalysts with ηOER values of 0.33, 0.37, 0.34, 0.36, 0.37, 0.34, 0.38, and 0.33 V, respectively. Notably, Rh-Nb2CO2 and Pt-N50.00%-Nb2CO2 exhibit outstanding ORR and OER bifunctional catalytic activity with potential gap values of 0.80 and 0.72 V, respectively, which are higher than the activities of most reported bifunctional catalysts. Furthermore, electronic structure analysis indicates that the moderate adsorption strength of oxygen-containing intermediates on active centers is crucial for achieving highly active bifunctional catalysts for ORR and OER. This study provides a strategy for the design of novel ORR and OER catalysts using 2D MXene materials.