Oxygen Evolution Reaction Reaction Research Articles

Metal vacancies and self-reconstruction of high entropy metal borates to boost the oxygen evolution reaction

Exploring highly efficient oxygen evolution reaction (OER) electrocatalysts is important for industrial water electrolysis. Herein, a new high entropy material was reported, i.e., an amorphous high entropy metal borate (CrMnCoNiFe)0.2BOx was synthesized by a new simple solvothermal method. The key of this new synthesis method was to obtain (CrMnCoNiFe)0.2BOx with a high specific surface area (446 m2/g) and porous structure, being expected to promote OER reaction. (CrMnCoNiFe)0.2BOx provided a low overpotential of 236 mV at 10 mA cm−2, a reduced Tafel slope of 64 mV dec–1, and good stability. The outstanding OER performance of (CrMnCoNiFe)0.2BOx was attributed to the high entropy structure, generation of metal vacancies, and self-reconstruction during OER. The Cr vacancies generated during the OER process have been demonstrated by Cr 2P XPS and EPR experiments. Various in situ and Ex-situ analyses such as In situ Raman, XPS, and TEM were used to investigate the self-reconstruction of the (CrMnCoNiFe)0.2BOx during the OER. The resulting metal (oxy)hydroxide was believed to be the true active center for OER. The DFT calculation showed the high entropy structure can reduce the adsorption and conversion energy barriers of oxygen-containing intermediates, matching well with the catalytic performance results. This study provided a new simple strategy to construct high entropy metal borate (CrMnCoNiFe)0.2BOx and would inspire the design of high-performance OER catalysts.

Nanoengineered Cobalt Electrocatalyst for Alkaline Oxygen Evolution Reaction.

The alkaline oxygen evolution reaction (OER) remains a bottleneck in green hydrogen production owing to its slow reaction kinetics and low catalytic efficiencies of earth abundant electrocatalysts in the alkaline OER reaction. This study investigates the OER performance of hierarchically porous cobalt electrocatalysts synthesized using the dynamic hydrogen bubble templating (DHBT) method. Characterization studies revealed that electrocatalysts synthesized under optimized conditions using the DHBT method consisted of cobalt nanosheets, and hierarchical porosity with macropores distributed in a honeycomb network and mesopores distributed between cobalt nanosheets. Moreover, X-ray photoelectron spectroscopy studies revealed the presence of Co(OH)2 as the predominant surface cobalt species while Raman studies revealed the presence of the cubic Co3O4 phase in the synthesized electrocatalysts. The best performing electrocatalyst required only 360 mV of overpotential to initiate a current density of 10 mA cm-2, exhibited a Tafel slope of 37 mV dec-1, and stable OER activity over 24 h. The DHBT method offers a facile, low cost and rapid synthesis approach for preparation for highly efficient cobalt electrocatalysts.

Enhance the performance of OER electrocatalyst via synergistic oxygen vacancies and NiFe2O4-phosphorene heterostructure

The exfoliated black phosphorus nanosheets (EBP, phosphorene) is considered as a highly anticipated electrocatalyst for oxygen evolution reaction (OER) because of its unique electron structure. However, the poor stability and conductivity of EBP limit its application potential. In this paper, a heterostructure of EBP and nickel ferrite with oxygen vacancies (NFO-V-EBP) is designed by using a simple ultrasound and electronic regulation of EBP. NFO-V forms P-Ni bonds and P-O-M (M=Ni/Fe) bonds by interacting with the active lone pair electrons on the surface of EBP, which passivates EBP to improve its stability and provide more active sites. In situ Raman testing is used to assist in demonstrating the cooperative effect of EBP and oxygen vacancy, which reduced the potential barrier of OER reaction. NFO-V-EBP heterostructure exhibits excellent electrocatalytic performance for alkaline OER, showing overpotential of 243 mV at current density of 10 mA cm-2 and Tafel slope of 39 mV dec-1. It also has excellent long-term stability, at a density of 10 mA cm-2 with no significant increase in overpotential observed after nearly 100 hours under the chronoamperometric method.

Synergistic modulation of the d-band center in Ni3S2 by selenium and iron for enhanced oxygen evolution reaction (OER) and urea oxidation reaction (UOR)

Efficient production of green hydrogen energy is crucial in addressing the energy crisis and environmental concerns. The oxygen evolution reaction (OER) poses a challenge in conventional overall water electrolysis due to its slow thermodynamically process. Urea oxidation reaction (UOR) offers an alternative anodic oxidation method that is highly efficient and cost-effective, with favorable thermodynamics and sustainability. Recently, there has been limited research on bifunctional catalysts that exhibit excellent activity for both OER and UOR reactions. In this study, we developed a selenium and iron co-doped nickel sulfide (SeFe-Ni3S2) catalyst that demonstrated excellent Tafel slopes of 53.9 mV dec−1 and 16.4 mV dec−1 for OER and UOR, respectively. Density Functional Theory (DFT) calculations revealed that the introduction of metal (iron) and nonmetallic elements (selenium) was found to coordinate the d-band center, resulting in improved adsorption/desorption energies of the catalysts and reduced the overpotentials and limiting potentials for OER and UOR, respectively. This activity enhancement can be attributed to the altered electronic coordination structure after the introduction of selenium (Se) and iron (Fe), leading to an increase in the intrinsic activity of the catalyst. This work offers a new strategy for bifunctional catalysts for OER and UOR, presenting new possibilities for the future development of hydrogen production and novel energy conversion technologies. It contributes towards the urgent search for technologies that efficiently produce green hydrogen energy, providing potential solutions to mitigate the energy crisis and protect the environment.

Selectivity Study of Anodic Seawater Electrolysis Catalysts Using the Rotating Ring-Disc Electrode Method

Seawater Electrolysis is an exciting alternative to freshwater electrolysis; however, it suffers from a multitude of issues that must be resolved before the technology can be scaled. Corrosive hypochlorite forms at the anode of seawater electrolysis and can damage catalysts and electrolyzer components. Hypochlorite is unstable and can decay, particularly when exposed to heat and metal ions, which can lead to erroneously high oxygen evolution reaction (OER) selectivity conclusions, resulting in poor catalyst and electrolyzer component selection. In this study, we use the rotating ring-disc electrode technique to characterize the selectivity of IrO2, NiO, Co3O4, RuO2, Pt and PtRu electrocatalysts in situ at near-neutral pH (8.4) with linear scan voltammetry (LSV), potentiostatic and galvanostatic experiments. Our findings reveal that elevated temperatures are conducive to higher OER selectivity. Furthermore, when we increased the chloride concentration in the electrolyte with an IrO2 catalyst, we observed that although the overall selectivity towards the OER reaction decreased, the OER current independently increased, indicating a synergistic relationship.

Facet Dependence of the Oxygen Evolution Reaction on Co3O4, CoFe2O4, and Fe3O4 Epitaxial Film Electrocatalysts.

The main obstacle for the electrocatalytic production of "green hydrogen" is finding suitable electrocatalysts which operate highly efficiently over extended periods of time. The topic of this study is the oxygen evolution reaction (OER), one of the half-reactions of water splitting. It is complex and has intricate kinetics, which impairs the reaction efficiency. Transition metal oxides have shown potential as electrocatalysts for this reaction, but much remains unknown about the atomic scale processes. We have investigated structure-composition-reactivity correlations for Co3O4, CoFe2O4, and Fe3O4 epitaxial thin-film electrocatalysts exposing either the (001) or (111) surface facets. We found that for Co3O4, the (001) facet is more reactive, while for the other oxides, the (111) facet is more active. A Tafel-like evaluation reveals systematically smaller "Tafel" slopes for the (001) facets. Furthermore, the slopes are smaller for the iron-containing films. Additionally, we found that the oxyhydroxide skin layer which forms under OER reaction conditions is thicker on the cobalt oxides than on the other oxides, which we attribute to either a different density of surface defects or to iron hindering the growth of the skin layers. All studied skin layers were thinner than 1 nm.

Construction of Ultrafine PtIr Clusters Supported on Co3O4 Nanoflowers for Enhanced Overall Water Splitting.

Green hydrogen production through electrochemical overall water splitting has suffered from sluggish oxygen evolution reaction (OER) kinetics, inferior conversion efficiency, and high cost. Herein, ultrafine PtIr clusters are synthesized via an electrodeposition method and decorated on the Co3O4 nanoflowers assembled by nanowires (PtIr-Co3O4). The encouraging performances in electrochemical OER and hydrogen evolution reaction (HER) are achieved over the PtIr-Co3O4 catalyst, with the overpotentials as low as 410 and 237 mV at 100 mA cm-2, respectively, outperforming the commercial IrO2 and Pt/C catalysts. Due to the ultralow loading of PtIr clusters, the PtIr-Co3O4 catalyst exhibits 1270 A gIr -1 for OER at the overpotential of 400 mV. Our detailed analyses also show that the strong interactions between the ultrafine PtIr clusters and the Co3O4 nanoflowers enable the PtIr-Co3O4 catalyst to afford 10 mA cm-2 for the overall water splitting at the potential of 1.57 V, accompanied by high durability for 100 h.

Fluorine-decoration induced deep reconstruction of iron cobalt oxide for high-efficiency oxygen evolution reaction

The oxygen evolution reaction (OER) is a pivotal process in water electrolysis systems, yet developing cost-effective and highly active OER electrocatalysts from earth-abundant materials through straightforward methods remains a formidable challenge. In this study, FeCo oxides and their fluorides are synthesized using a one-step room-temperature process followed by fluorination with fluorine gas. We compared the OER performance of Fe, Co, and FeCo oxides and investigated the impact of fluorine doping on their physical properties and OER efficiency. Results revealed that the FeCo–O bimetallic oxide exhibited an overpotential of 370 mV in an alkaline electrolyte at a current density of 10 mA/cm2, significantly surpassing the performance of monometallic Fe–O (1 V) and Co–O (467 mV). Fluorine modification markedly enhanced the OER activity of these materials, with Fe-OF at 570 mV, Co-OF at 330 mV, and FeCo-OF at 285 mV. Notably, FeCo-OF demonstrated a stable overpotential lower than commercial RuO2 (289 mV) during a continuous 12 h OER test. Electrochemical analyses and spectral characterizations indicated that fluorine ion electronic regulation facilitated efficient electrocatalytic active sites in the spent FeCo-OF catalyst through in situ self-reconstruction, leading to its stable and superior OER performance. Density functional theory (DFT) calculations confirmed that fluorine modification of the bimetallic oxides decreased the O* Gibbs free energy from 4.86 to 2.44 eV, thereby enhancing the kinetics of the OER reaction.

Co-Ni co-sputter deposited bimetallic thin film catalysts for alkaline hydrogen and oxygen evolution reactions

Co-Ni bi-metallic thin films with different stoichiometries have been deposited by magnetron co-sputtering technique on Ni foam (NF), Si and carbon paper substrates. The structural and morphological characterisations of these films have been done by X-ray diffraction, X-ray absorption spectroscopy and Field Emission Scanning Electron Microscope measurements. The performances of these films for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) have been evaluated through electrochemical measurements like Linear Sweep Voltammetry, Cyclic Voltammetry and Electrochemical Impedance Spectroscopy measurements. Operando X ray absorption near edge spectroscopy measurements have also been carried out during HER and OER to decipher the redox reaction mechanisms. It has been observed that the HER and OER performance of all the Co Ni bi-metallic thin films on NF are found to be better than that of bare NF. The CoNi sample with composition of 58.7 % Co and 41.3 % Ni shows the best catalytic activity and stability towards HER and OER reactions. Though the HER activity is not the best but is comparable to many results reported in the literature, however the catalyst shows excellent OER activity with overpotential of 214 mV for 10 mA/cm2 current density. This is much better than that of the OER benchmark catalyst RuO2 and most of those reported in the literature. The OER is kinetically slower than HER and requires more overpotential, hence a significant improvement in the catalytic OER activity will definitely enhance the overall water splitting activity and hydrogen production efficiency. Thus the Co-Ni bi-metallic catalyst prepared by an easily scalable and highly reproducible magnetron co-sputtering technique has significant potential to emerge out as a technologically viable water splitting catalyst.

Efficient Alkaline Water Oxidation Electrocatalysis Enabled via the Modulation of Sn‐containing Anions towards NiFe Oxyhydroxide

Despite being considered as efficient non‐noble metal‐based electrocatalysts for alkaline oxygen evolution reaction (OER), NiFe (oxy)hydroxides (NiFeOxHy) typically fall short of meeting practical requirements. To address this challenge, herein, we introduce a facile in situ electrochemical incorporation technique to form Sn‐doped NiFe‐layered double hydroxide (NiFeSn‐LDH) precatalysts. Subsequently, under the alkaline OER process, the precatalyst evolves into stannate ion‐adsorbed NiFe oxyhydroxides (NiFeOOH). The presence of these stable stannate oxyanions plays a key role in mitigating Fe leaching, optimizing the electronic structure of NiFeOOH, and improving its reaction kinetics, thereby significantly enhancing alkaline OER performance. Notably, the nickel foam‐supported NiFeSn‐LDH demonstrated impressive results delivering a large current density of 100 mA cm−2 at an overpotential as low as ~260 mV and maintaining the industrial‐relevant ~500 mA cm−2 current density over 5 days with negligible activity decay, surpassing the performance of most of the transition metal‐based electrocatalysts. Comprehensive advanced characterization of the precatalysts, before and after the OER reactions, has been performed to uncover stable residual stannate ions at the surface of NiFeOxHy and to determine their pivotal role in promoting alkaline OER.

Four Generations of Volcano Plots for the Oxygen Evolution Reaction: Beyond Proton-Coupled Electron Transfer Steps?

ConspectusDue to its importance for electrolyzers or metal-air batteries for energy conversion or storage, there is huge interest in the development of high-performance materials for the oxygen evolution reaction (OER). Theoretical investigations have aided the search for active material motifs through the construction of volcano plots for the kinetically sluggish OER, which involves the transfer of four proton-electron pairs to form a single oxygen molecule. The theory-driven volcano approach has gained unprecedented popularity in the catalysis and energy communities, largely due to its simplicity, as adsorption free energies can be used to approximate the electrocatalytic activity by heuristic descriptors.In the last two decades, the binding-energy-based volcano method has witnessed a renaissance with special concepts being developed to incorporate missing factors into the analysis. To this end, this Account summarizes and discusses the different generations of volcano plots for the example of the OER. While first-generation methods relied on the assessment of the thermodynamic information for the OER reaction intermediates by means of scaling relations, the second and third generations developed strategies to include overpotential and kinetic effects into the analysis of activity trends. Finally, the fourth generation of volcano approaches allowed the incorporation of various mechanistic pathways into the volcano methodology, thus paving the path toward data- and mechanistic-driven volcano plots in electrocatalysis.Although the concept of volcano plots has been significantly expanded in recent years, further research activities are discussed by challenging one of the main paradigms of the volcano concept. To date, the evaluation of activity trends relies on the assumption of proton-coupled electron transfer steps (CPET), even though there is experimental evidence of sequential proton-electron transfer (SPET) steps. While the computational assessment of SPET for solid-state electrodes is ambitious, it is strongly suggested to comprehend their importance in energy conversion and storage processes, including the OER. This can be achieved by knowledge transfer from homogeneous to heterogeneous electrocatalysis and by focusing on the material class of single-atom catalysts in which the active center is well defined. The derived concept of how to analyze the importance of SPET for mechanistic pathways in the OER over solid-state electrodes could further shape our understanding of the proton-electron transfer steps at electrified solid/liquid interfaces, which is crucial for further progress toward sustainable energy and climate neutrality.

Acidic Oxygen Evolution Reaction: Fundamental Understanding and Electrocatalysts Design.

Water electrolysis driven by "green electricity" is an ideal technology to realize energy conversion and store renewable energy into hydrogen. With the development of proton exchange membrane (PEM), water electrolysis in acidic media suitable for many situations with an outstanding advantage of high gas purity has attracted significant attention. Compared with hydrogen evolution reaction (HER) in water electrolysis, oxygen evolution reaction (OER) is a kinetic sluggish process that needs a higher overpotential. Especially in acidic media, OER process poses higher requirements for the electrocatalysts, such as high efficiency, high stability and low costs. This review focuses on the acidic OER electrocatalysis, reaction mechanisms, and critical parameters used to evaluate performance. Especially the modification strategies applied in the design and construction of new-type electrocatalysts are also summarized. The characteristics of traditional noble metal-based electrocatalysts and the noble metal-free electrocatalysts developed in recent decades are compared and discussed. Finally, the current challenges for the most promising acidic OER electrocatalysts are presented, together with a perspective for future water electrolysis.

Theoretical study on oxygen evolution reaction mechanism of double rare earth europium-doped graphene under hydroxyl modification in alkaline environment

The development and design of high performance OER catalyst is the key to electrocatalysis technology. Herein, based on density functional theory (DFT), the oxygen evolution reaction mechanism of double rare earth europium-doped graphene under hydroxyl modification in alkaline environment has been systematically studied. Through thermodynamic and kinetic stability analysis, and the optimal reaction path and different adsorption sites of intermediates were compared. Four catalysts with good catalytic activity for OER reaction were selected. The results show that when the two hydroxyl groups are modified, the overpotentials on the optimal pathway for the four catalyst configurations are 0.54 V, 0.50 V, 0.54 V, and 0.61 V, respectively. These catalysts demonstrate excellent catalytic activity for the oxygen evolution reaction (OER). Moreover, these catalysts have good electrical conductivity, and the active site and adsorption intermediates can be stably bonded together. In addition, the scaling relationship between oxygen adsorption free energy and overpotential is described. This work may provide new insights and guidance for future research on rare-earth atom-based OER catalysts.

2D Metal Porphyrin-Based MOFs and ZIF-8 Composite-Derived Carbon Materials Containing M-Nx Active Sites as Bifunctional Electrocatalysts for Zinc-Air Batteries.

The main impediment to the development of zinc-air batteries is the sluggish kinetics of the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER). Transition metal N-doped carbon catalysts offer a promising alternative to noble metal catalysts, with metal-organic framework (MOF)-derived carbon material catalysts being particularly noteworthy. Here, we synthesized MxP-Z-C carbon catalysts by combining two-dimensional (2D) metal porphyrin-based MOFs (MxPMFs, x = Fe, Co, Ni, Mn) and three-dimensional zeolitic imidazole framework-8 (ZIF-8) through electrostatic interaction, followed by carbonization. ZIF-8 was inserted between the layers of MxPMFs to prevent its Π-Π stacking, allowing the active sites to become fully exposed. MxP-Z-C demonstrated an impressive catalytic activity for both the ORR and the OER reactions. Among them, FeP-Z-C showed the best catalytic activity. The half-wave potential for ORR was 0.92 V (vs the reversible hydrogen electrode (RHE)), while the overpotential for the OER was 290 mV. In addition, the zinc-air battery assembled by FeP-Z-C exhibited high power density (133.14 mW cm-2) and significant specific capacity (816 mAh gZn-1), indicating considerable potential as a bifunctional catalyst for electronic devices.

High-performance oxygen evolution reaction via self-optimizing interface engineering with simultaneous activation of dual-sites of surface oxyhydroxides

Metal oxyhydroxides produced by the surface reconstruction were widely considered as active catalytic species in the oxygen evolution reaction (OER). However, simultaneous activation of metal sites in surface oxyhydroxides remains a great challenge. In this study, the interface self-activation strategy was utilized to simultaneously activate both the iron and nickel sites at the surface oxyhydroxides of (Fe,Ni)OOH-NiSe2 nano heterostructure. The OER activity was greatly boosted by the dual activation of active sites, resulting in an overpotential of 245 mV@100 mA cm−2 with a small Tafel slope of 44 mV dec−1. The finely constructed FeOOH-NiSe2 heterostructure was transformed into (Fe,Ni)OOH-NiSe2 through the formation of distinct bonds of M(Fe,Ni)-O-Se during the OER process, which was discovered through a combination of experimental studies with DFT calculations. A fast OER reaction dynamic was achieved due to the unique self-optimized interface structure which produced a dual synergistic effect between the interface structure and the active sites of Fe and Ni of oxyhydroxides, modulated the electronic structure and d band center of active sites, and increased the number of optimum active sites. This work paves a way to design high-performance electrocatalysts with multiple active sites for other electrochemical reactions.

Modified NiFe2O4-Supported Graphene Oxide for Effective Urea Electrochemical Oxidation and Water Splitting Applications.

The production of green hydrogen using water electrolysis is widely regarded as one of the most promising technologies. On the other hand, the oxygen evolution reaction (OER) is thermodynamically unfavorable and needs significant overpotential to proceed at a sufficient rate. Here, we outline important structural and chemical factors that affect how well a representative nickel ferrite-modified graphene oxide electrocatalyst performs in efficient water splitting applications. The activities of the modified pristine and graphene oxide-supported nickel ferrite were thoroughly characterized in terms of their structural, morphological, and electrochemical properties. This research shows that the NiFe2O4@GO electrode has an impact on both the urea oxidation reaction (UOR) and water splitting applications. NiFe2O4@GO was observed to have a current density of 26.6 mA cm-2 in 1.0 M urea and 1.0 M KOH at a scan rate of 20 mV s-1. The Tafel slope provided for UOR was 39 mV dec-1, whereas the GC/NiFe2O4@GO electrode reached a current of 10 mA cm-2 at potentials of +1.5 and -0.21 V (vs. RHE) for the OER and hydrogen evolution reaction (HER), respectively. Furthermore, charge transfer resistances were estimated for OER and HER as 133 and 347 Ω cm2, respectively.

Revolutionizing energy evolution: SnS-Sn2S3 layered structures as exceptional electrocatalytic materials for H2 and O2 generation

In this paper, we prepared the SnS-Sn2S3 layered structure using the in-situ solvothermal method for hydrogen evolution (HER) and oxygen evolution reaction (OER). XRD and HRTEM confirm the successful synthesis of the orthorhombic crystal structure of the SnS-Sn2S3. Further, SEM images demonstrate the layered shape of the SnS-Sn2S3 nanostructure, promoting the formation of more active sites on the surface for better electrocatalytic activities. Interestingly, during OER investigation, the SnS-Sn2S3 catalyst depicts an overpotential of 359 mV (10 mA cm−2) and a Tafel slope of 90.1 mV dec-1. However, during HER investigation, the SnS-Sn2S3 catalyst describes an overpotential of 162 mV (-10 mA cm−2) and a Tafel slope of 107.6 mV dec-1. Moreover, the SnS-Sn2S3 catalyst unveils superb stability for 2.3 h at 10 mA cm−2 for OER and 4.0 h at −10 mA cm−2 for HER. The OER and HER reaction mechanisms were also discussed to explore the kinetics rate of SnS-Sn2S3 catalyst.

Increased Readiness for Water Splitting: NiO-Induced Weakening of Bonds in Water Molecules as Possible Cause of Ultra-Low Oxygen Evolution Potential.

The development of non-precious metal-based electrodes that actively and stably support the oxygen evolution reaction (OER) in water electrolysis systems remains a challenge, especially at low pH levels. The recently published study has conclusively shown that the addition of haematite to H2 SO4 is a highly effective method of significantly reducing oxygen evolution overpotential and extending anode life. The far superior result is achieved by concentrating oxygen evolution centres on the oxide particles rather than on the electrode. However, unsatisfactory Faradaic efficiencies of the OER and hydrogen evolution reaction (HER) parts as well as the required high haematite load impede applicability and upscaling of this process. Here it is shown that the same performance is achieved with three times less metal oxide powder if NiO/H2 SO4 suspensions are used along with stainless steel anodes. The reason for the enormous improvement in OER performance by adding NiO to the electrolyte is the weakening of the intramolecular O─H bond in the water molecules, which is under the direct influence of the nickel oxide suspended in the electrolyte. The manipulation of bonds in water molecules to increase the tendency of the water to split is a ground-breaking development, as shown in this first example.

Operando Tracking the Interactions between CoOx and CeO2 during Oxygen Evolution Reaction

AbstractCeO2 greatly enhances the electrocatalytic oxygen evolution reaction (OER) activity of CoOx, though the enhancement mechanism beyond this synergy is yet to be understood. Here, operando hard X‐ray absorption spectroscopy (hXAS) is applied to monitor the Co K edge and Ce L3 edge in CoOx/CeO2 to shed light on the evolution of the Co and Ce oxidation states during OER. In addition, ex situ soft XAS (sXAS) characterizations provide information on the irreversible surface‐specific transformations of the Co L3 edge as well as of the O K edge. Combining the operando and ex situ spectroscopic characterizations with comprehensive electrochemical analyses, it is confirmed that CeO2 is not the active center for the OER. However, coupling CeO2 with CoOx introduces significant modifications in the Co and O species at the CoOx surface and alters the flat band potential (Efb), leading to more favorable Co oxidation state transformations during OER and possibly modifying the preferential reaction pathway. This work establishes the connections between electronic structures, Co oxidation state and the OER reaction mechanism for CoOx/CeO2 composites electrodes.

Alkaline Water Electrolysis for Green Hydrogen Production.

ConspectusThe global energy landscape is undergoing significant change. Hydrogen is seen as the energy carrier of the future and will be a key element in the development of more sustainable industry and society. However, hydrogen is currently produced mainly from fossil fuels, and this needs to change. Alkaline water electrolysis with advanced technology has the most significant potential for this transition to produce large-scale green hydrogen by utilizing renewable energy. The assembly of industrial electrolyzer plants is more complex on a larger scale, but it follows a basic working principle, which involves two half-cells of anode and cathode sites where the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) occur. Out of the two reactions, the OER is more challenging both thermodynamically and kinetically. Besides having access to renewable electricity, developing durable and abundant electrocatalysts for the OER remains a challenge in large-scale alkaline water electrolysis. Among different physicochemical properties, the electrocatalyst surface and its interaction with water and reaction intermediates, as well as formed molecular hydrogen and oxygen, play an essential role in the catalytic performance and the reaction mechanism. In particular, the binding strengths between the catalyst surface and intermediates determine the rate-limiting step and electrocatalytic performance.This Account gives some insights into the status of the hydrogen economy and basic principles of alkaline water electrolysis by covering its fundamentals as well as industrial developments. Further, the HER and OER reaction mechanisms of alkaline water electrolysis and selected electrocatalyst progress for both half-reactions are briefly discussed. The Adsorbate Evolution Mechanism and the Lattice Oxygen Mechanism for the OER are explained with specific references. This Account also deliberates on the author's selected contributions to the development of transition metal-based electrocatalysts for alkaline water electrolysis with an emphasis on OER. The focus is particularly given to the enhancement of intrinsic activity, the role of eg-filling, phase segregation, and defect structure of cobalt-based electrocatalysts for OER. Structural modification and phase transformation of the cobalt oxide electrocatalyst under working conditions are further deliberated. In addition, the creation of new active surface species and the activation of cobalt- and nickel-based electrocatalysts through iron uptake from the alkaline electrolyte are discussed. In the end, this Account provides a brief overview of challenges related to large-scale production and utilization of green hydrogen.