The stability number as a metric for electrocatalyst stability benchmarking

Complex Iridate Solid Solutions for Catalyzing Oxygen Evolution Reaction: Comparison of Elemental Leaching and Stability Numbers

As predicted by the Hume-Rothery rules, forming solid solutions of rutile IrO2 with other metal oxides that have different crystal structures is thermodynamically challenging. Consequently, achieving high solubility of foreign elements in Ir-based solid-solution oxides has been significantly limited. We demonstrate that hexagonal-perovskite BaIrO3 can serve as a flexible matrix oxide capable of incorporating a wide spectrum of (post)transition-metal cations with different electronic structures, ranging from d0 to d10 configurations. Among 12 cation solutes, Ta5+, Nb5+, and Zr4+ are found to be stable without substantial leaching during the oxygen evolution reaction (OER) under acidic condition. Acceptor-type trivalent cations, including Sc3+, In3+, and Fe3+, are identified to leach out gradually from the particle surface while enhancing the OER catalytic activity. Both X-ray absorption spectroscopy and ab initio molecular dynamics simulations consistently show that the robust face-sharing [IrO6] octahedral framework of the solid solutions remains unperturbed unless electrochemical leaching rapidly occurs. As a result, notably high S-numbers, on the order of 106, are achievable at pH = 1. Although our work focuses on single-element incorporation, it is suggested that the solid-solution methodology is an effective strategy for developing stable, long-lasting OER catalysts with further reduced Ir usage for acidic water oxidation.

Reactor operating parameters and their effects on the local reaction environment of CO(2) electroreduction

Low temperature aqueous electrochemical CO(2) reduction (ECR) emerged as a pathway to close the carbon cycle with the integration of renewable energy. However, activity, selectivity, and stability barriers prevent ECR from entering industrial scale operation. While catalyst design has made meaningful progress towards selective and active production of many products including CO, formate, and ethylene, operating conditions during catalyst testing have not been standardized. Operational parameters drastically impact the local reaction environment of the ECR and thus the performance of ECR. Herein, we summarize the prevailing operational variability of ECR and their interconnectedness. We first analyze reactant availability via tuning of cell geometry and CO(2) pressures. Then, optimization towards electrolyzer components including electrolyte, electrodes, and bipolar plates is discussed. We further assess the electrochemical protocols to enhance the performance or accelerate the degradation of ECR and the considerations required to scale up ECR to pilot scale. Finally, we provide perspectives on the current challenges of ECR and their promising solutions.

Oxidized Overlayers of Ruthenium and Iridium as Electrocatalysts for Anodic Reactions

Renewable energy technologies often rely on rutile tetravalent oxides, such as ruthenium(IV) oxide and iridium(IV) oxide, to catalyze anodic reactions that are paired with fuel formation. Herein, we report the synthesis of angstrom-scale and nanoscale oxidized overlayers of ruthenium (o-RuOx) and iridium (o-IrOx) from simple aqueous precursors grown on earth-abundant supports and state-of-the-art oxide electrocatalysts. The resulting overlayers exhibit distinct redox features and chemical states as indicated by cyclic voltammetry, X-ray photoelectron spectroscopy, and X-ray absorption spectroscopy. The electrocatalysts exhibit increased activity towards anodic reactions. In particular, annealed o-RuOx grown on TiO2 (a-TiO2/o-RuOx) results in an electrocatalyst with an overpotential of 213, 206, and 14 mV at 10 mA cm-2 for the oxygen evolution reaction (OER) in acid, the OER in base, and the chlorine evolution reaction, respectively. The activity of a-TiO2/o-RuOx corresponds to a 47.7×, 117.4×, and 1.3× increase in ruthenium mass activity compared to RuO2 towards the OER in acid, the OER in base, and the chlorine evolution reaction, respectively. These findings highlight the unique chemistry of oxidized overlayers and their potential to meet operational demands for renewable energy technologies.

Establishing the stability number descriptor for Fe-N-C fuel cell electrocatalysts

Fe-N-C electrocatalysts demonstrate high potential in catalyzing oxygen reduction reaction (ORR) in polymer electrolyte fuel cells, yet the bottleneck for their application is their moderate stabilities. In our previous work, we discovered a linear correlation between the rates of ORR and Fe dissolution in alkaline media at room temperature, and the stability (S-) number descriptor that reflects this correlation was introduced. On the way toward further generalization and establishment of this descriptor, we investigate the effect of pH, potential, current density, and temperature on the dissolution behavior of various representative Fe-N-C electrocatalysts. It is shown that the S-number concept is also applicable for ORR and Fe dissolution in alkaline electrolytes at 70 °C. It is more challenging to apply the S-number in acidic media, where the S-number is a function of ORR current density. A kinetic model is introduced, showing that the local pH inside the catalyst layer rises significantly with increasing current densities. The pH dependence of the S-number explains the results in acidic electrolytes. Accounting for such a dependence, the S-number descriptor can also benchmark Fe-N-C stability in acidic electrolytes. It is considered that this concept can also be extended to other reactions, allowing more rational activity and stability screening of electrocatalysts.

Deciphering Catalyst-Support Interaction via Doping for Highly Active and Durable Oxygen Evolution Catalysis

The design of oxygen evolution reaction (OER) electrocatalysts demands a delicate balance between activity and stability. In this study, we present a rational design approach that leverages catalyst-support interactions to enhance both the intrinsic activity and durability of Ir-based catalysts. Our study reveals that while Mo doping energetically promotes the formation of high-valent Ir species, enhancing intrinsic catalytic activity, it also leads to a reduction in electrical conductivity. These findings emphasize that supporting doping can introduce both beneficial and limiting effects, highlighting the need for a carefully balanced design strategy to optimize the overall OER performance. Simultaneously, in situ analytical techniques and comparative evaluation reveal the crucial role of oxide supports in stabilizing the catalyst. These findings highlight the pivotal role of interface engineering in maintaining catalyst integrity and the need for support materials that balance dopant-driven electronic promotion with structural and electrochemical robustness. These interconnected degradation pathways highlight the need to move beyond a catalyst-centric view and instead adopt a system-level understanding of the stability. Our approach offers a strong foundation for the rational design and evaluation of high-performance OER electrocatalysts for electrochemical energy applications.

Effect of ionic-bonding d0 cations on structural durability in barium iridates for oxygen evolution electrocatalysis

Iridium has the exclusive chemistry guaranteeing both high catalytic activity and sufficient corrosion resistance in a strong acidic environment under anodic potential. Complex iridates thus attract considerable attention as high-activity electrocatalysts with less iridium utilization for the oxygen evolution reaction (OER) in water electrolyzers using a proton-exchange membrane. Here we demonstrate the effect of chemical doping on the durability of hexagonal-perovskite Bax(M,Ir)yOz-type iridates in strong acid (pH ~ 0). Some aliovalent cations are directly visualized to periodically locate at the octahedral sites bridging the two face-sharing [Ir2O9] dimer or [Ir3O12] trimers in hexagonal-perovskite polytypes. In particular, highly ionic bonding of the d0 Nb5+ and Ta5+ cations with oxygen anions results in notable suppression of lattice oxygen participation during the OER and thus effectively preserves the connectivity between the [Ir3O12] trimers without lattice collapse. Providing an in-depth understanding of the correlation between the electronic structure and bonding nature in crystals, our work suggests that proper control of chemical doping in complex oxides promises a simple but efficient tool to realize OER electrocatalysts with markedly improved durability.

Electron deficient oxygen species in highly OER active iridium anodes characterized by X-ray absorption and emission spectroscopy

Water splitting is a promising technology for storing energy, yet it is challenged by the lack of stable anode materials that can overcome the sluggishness of the oxygen evolution reaction (OER). Iridium oxides are among the most active and stable OER catalysts, however how these materials achieve their performance remains under discussion. The activity of iridium based materials has been attributed to both high metal oxidation states and the appearance of O 2p holes. Herein we employ a combination of techniques-X-ray absorption at the Ir LII,III-edge, X-ray absorption and emission at the O K-edge, along with ab initio methods-to identify and characterize ligand holes present in highly OER-active X-ray amorphous oxides. We find, in agreement with the original proposition based on X-ray absorption measurement at the O K-edge, that O 2p holes are present in these materials and can be associated with the increased activity during OER.

Polyoxometalated metal-organic framework superstructure for stable water oxidation

Stable, nonprecious catalysts are vital for large-scale alkaline water electrolysis. Here, we report a grafted superstructure, MOF@POM, formed by self-assembling a metal-organic framework (MOF) with polyoxometalate (POM). In situ electrochemical transformation converts MOF into active metal (oxy)hydroxides to produce a catalyst with a low overpotential of 178 millivolts at 10 milliamperes per square centimeter in alkaline electrolyte. An anion exchange membrane water electrolyzer incorporating this catalyst achieves 3 amperes per square centimeter at 1.78 volts at 80°C and stable operation at 2 amperes per square centimeter for 5140 hours at room temperature. In situ electrochemical spectroscopy and theoretical studies reveal that the synergistic interactions between metal atoms create a fast electron-transfer channel from catalytic iron and cobalt sites, nickel, and tungsten in the polyoxometalate to the electrode, stabilizing the metal sites and preventing dissolution.

Sub-4 nm Ru-RuO2 Schottky Nanojunction as a Catalyst for Durable Acidic Water Oxidation

RuO2 with high intrinsic activity for water oxidation is a promising alternative to IrO2 in proton exchange membrane (PEM) electrolyzer, but it suffers from long-term stability issues due to overoxidation. Here, we report a sub-4 nm Ru-RuO2 Schottky nanojunction (Ru-RuO2-SN) prepared by a microwave reaction that exhibits high activity and long-term stability in both three-electrode systems and PEM devices. The lattice strain and charge transfer induced by the metal-oxide SN increase the work function of the Ru-RuO2-SN, optimize the local electronic structure, and reduce the desorption energy of the metal site to the oxygen-containing intermediates; as a result, it leads to the oxide path mechanism (OPM) and inhibits the excessive oxidation of surface ruthenium. The Ru-RuO2-SN requires only 165 mV overpotential to obtain 10 mA·cm-2 with 1400 h stability without obvious activity degradation, achieving a stability number (6.7 × 106) matching iridium-based catalysts. In a PEM electrolyzer with Ru-RuO2-SN as an anode catalyst, only 1.6 V is needed to reach 1.0 A·cm-2 and it shows long-term stability at 100 mA·cm-2 for 1100 h and at 500 mA·cm-2 for 100 h. The reaction mechanism for the high stability of Ru-RuO2-SN was analyzed by density functional theory calculations. This work reports a durable, pure Ru-based water-oxidation catalyst and provides a new perspective for the development of efficient Ru-based catalysts.

Leaching-Induced Ti Trapping Stabilizes Amorphous IrOx for Proton Exchange Membrane Water Electrolysis

Transition metal nitrides are promising conductive support materials in oxygen evolution reaction (OER), whereas it is unclear if their electrochemical oxidation during prolonged test would affect the OER catalysts. Herein, we demonstrate that the leaching-induced Ti trapping effect from TiN supports effectively stabilizes the electrochemically oxidized amorphous IrOx catalyst. Structural characterizations reveal that the TiN support experiences severe leaching during OER test, leaving minor amounts of Ti-O species trapping on IrOx clusters, which mitigate the overoxidation of Ir(III) species and elongate the Ir-O bonds. This leads to 70% reduction of lattice oxygen oxidation and the substantially reduced Ir leaching. Additionally, self-thickening of the catalyst layers is found during proton exchange membrane water electrolysis (PEMWE), enabling an ultralow catalyst loading of 87 µgIr cm-2 to obtain a stable operation at 1.0 A cm-2 for 500 h. This work deepens the understanding of TiN-supported catalysts for long-term stable OER electrocatalysis.

Proton-Conducting, Vacancy-Rich HxIrOy Nanosheets for the Fabrication of Low-Ionomer-Dependent Anode Catalyst Layer in PEM Water Electrolyzer

The anode catalyst layer is composed of catalytically functional IrOx and protonic conducting ionomer and largely dictates catalytic performance of proton exchange membrane water electrolyzer (PEMWE). Here, we report a new type of anode nanocatalyst that possesses both IrOx's catalytic function and high proton conductivity that traditional anode catalysts lack and demonstrate its ability to construct high-performance, low-ionomer-dependent anode catalyst layer, the interior of which-about 85% of total catalyst layer-is free of ionomers. The proton-conducting anode nanocatalyst is prepared via protonation of layered iridate K0.5(Na0.2Ir0.8)O2 and then exfoliation to produce cation vacancy-rich, 1 nm-thick iridium oxide nanosheets (labeled as □-HxIrOy). Besides being a proton conductor, the □-HxIrOy is found to have abundant catalytic active sites for the oxygen evolution reaction due to the optimization of both edge and in-plane iridium sites by multiple cation vacancies. The dual functionality of □-HxIrOy allows the fabrication of low-iridium-loading, low-ionomer-dependent anode catalyst layer with enhanced exposure of catalytic sites and reduced electronic contact resistance, in contrast to common fully mixed catalyst/ionomer layers in PEMWE. This work represents an example of realizing the structural innovation in anode catalyst layer through the bifunctionality of anode catalyst.

Recent Development of Ir- and Ru-Based Electrocatalysts for Acidic Oxygen Evolution Reaction

Proton exchange membrane (PEM) water electrolyzers are one type of the most promising technologies for efficient, nonpolluting and sustainable production of high-purity hydrogen. The anode catalysts account for a very large fraction of cost in PEM water electrolyzer and also determine the lifetime of the electrolyzer. To date, Ir- and Ru-based materials are types of promising catalysts for the acidic oxygen evolution reaction (OER), but they still face challenges of high cost or low stability. Hence, exploring low Ir and stable Ru-based electrocatalysts for acidic OER attracts extensive research interest in recent years. Owing to these great research efforts, significant developments have been achieved in this field. In this review, the developments in the field of Ir- and Ru-based electrocatalysts for acidic OER are comprehensively described. The possible OER mechanisms are first presented, followed by the introduction of the criteria for evaluation of the OER electrocatalysts. The development of Ir- and Ru-based OER electrocatalysts are then elucidated according to the strategies utilized to tune the catalytic performances. Lastly, possible future research in this burgeoning field is discussed.

A comprehensive activity-stability correlation study of tantalum-doped tin oxide as a support for iridium oxide in low loading water electrolysis cell anodes

A systematic study on the impact of the treatment temperature of IrO x supported onto doped-tin oxide (1 at% Ta-SnO2 and 10 at% Sb-SnO2) fibres led to electrocatalysts with high oxygen evolution reaction activity and resistance to degradation. The electrolytic performance was comparable to that of unsupported commercial IrO2 with seven times higher loading.

Unraveling Compressive Strain and Oxygen Vacancy Effect of Iridium Oxide for Proton-Exchange Membrane Water Electrolyzers

Iridium-based electrocatalysts are commonly regarded as the sole stable operating acidic oxygen evolution reaction (OER) catalysts in proton-exchange membrane water electrolysis (PEMWE), but the linear scaling relationship (LSR) of multiple reaction intermediates binding inhibits the enhancement of its activity. Herein, the compressive strain and oxygen vacancy effect exists in iridium dioxide (IrO2)-based catalyst by a doping engineering strategy for efficient acidic OER activity. In situ synchrotron characterizations elucidate that compressive strain can enhance Ir─O covalency and reduce the Ir─Ir bond distance, and oxygen vacancy (Ov) as an electronic regulator causes rapid adsorption of water molecules on the Ir and adjacent Ov (Ir─Ov) pair site to be coupled directly into *O─O* intermediates. Importantly, hence, volcano-shape curves are established between the compressive strain/oxygen vacancy and OER current using OER as the probe reaction. Theoretical calculation reveals Ni dopant can modulate Ir 5d- and O 2p-band centers for increasing overlap of Ir 5d and O 2p orbits to trigger a continuous metal site-oxygen vacancy synergistic mechanism (MS-OVSM) pathway, successfully breaking the LSR of intermediates binding during OER. Therefore, the resultant proton-exchange membrane water electrolysis (PEMWE) device fabricated using T-0.24Ni/IrO2 delivers a current density of 500 mA cm-2 and operates stably for 500 h.

Microkinetic Modelling of Electrochemical Oxygen Evolution Reaction on Ir(111)@N-Graphene Surface

We have explored the thermodynamics and microkinetic aspects of oxygen evolution catalysis on low loading of Ir(111) on nitrogen-doped graphene at constant potential. The electronic modification induced by N-doping is the reason for the reduced overpotential of OER. The N-induced defect in the charge density is observed with increasing charge-depleted region around the Ir atoms. The lattice contraction shifts the d-band center away from the Fermi level, which increases the barrier for OH* and O* formation on Ir(111) supported on NGr (Ir(111)@NGr). Thus, highly endothermic O* formation reduces the OOH* formation, which is the potential determining step. For comparison, all electronic and binding energy calculations were also performed against Ir NP supported on Gr (Ir(111)@Gr). The stepwise potential-dependent activation barrier (G a ${{G}_{a}}$ ) was obtained using the charge extrapolation method. The third step remains the RDS in all ranges of water oxidation potentials. The potential dependentG a ${{G}_{a}}$ is further applied to the Eyring rate equation to obtain the current density (j O E R ${{j}_{OER}}$ ) and correlation betweenj O E R ${{j}_{OER}}$ and pH dependence, i. e., OH- concentration. The microkineticj O E R ${{j}_{OER}}$ progression leads to a Tafel slope value of 30 mV dec-1 at pH=14.0, requiringη k i n e t i c = 0 . 33 V ${{\eta }_{kinetic}=0.33\ V}$ .

Oxidation of interfacial cobalt controls the pH dependence of the oxygen evolution reaction

Transition metal oxides often undergo dynamic surface reconstruction under oxygen evolution reaction conditions to form the active state, which differs in response to the electrolyte pH. The resulting pH dependency of catalytic activity is commonly observed but poorly understood. Herein we track Co oxidation state changes at different pH-directed (hydr)oxide/electrolyte interfaces using operando X-ray absorption spectroscopy characterizations. Combined with in situ electrochemical analyses, we establish correlations between Co redox dynamics, the flat band potential and Co oxidation state changes to explain the pH dependency of the oxygen evolution activity. Alkaline environments provide a low flat band potential that yields a low-potential Co redox transformation, which favours surface reconstruction. Neutral and acidic environments afford an anodic shift of the Co redox transformation that increases the catalytic overpotential. The larger overpotential in neutral environments is attributable to poor Co atom polarizability and slow Co oxidation state changes. These findings reveal that interfacial Co oxidation state changes directly determine the pH dependency of the oxygen evolution reaction activity.

Local Oxygen Vacancy-Mediated Oxygen Exchange for Active and Durable Acidic Water Oxidation

Developing an active and durable acidic oxygen evolution reaction (OER) catalyst is vital for implementing a proton exchange membrane water electrolyzer (PEMWE) in sustainable hydrogen production. However, it remains dauntingly challenging to balance high activity and long-term stability under harsh acidic and oxidizing conditions. Herein, through developing the universal rare-earth participated pyrolysis-leaching approach, we customized the active and long lifespan pseudo-amorphous IrOx with locally ordered rutile IrO2 and unique defect sites (IrOx-3Nd). IrOx-3Nd achieved a low overpotential of 206 mV and long-term durability of 2200 h with a slow degradation rate of 0.009 mV  h-1 at 10 mA cm-2, and, more importantly, high efficiency in PEMWE (1.68 V at 1 A cm-2 for 1000 h) for practical hydrogen production. Utilizing in situ characterizations and theoretical calculations, we found that lattice oxygen vacancies (Ov) and contracted Ir-O in locally ordered rutile IrO2 induced the Ov-modulated lattice oxygen exchange process, wherein thermodynamically spontaneous occupation of surface hydroxyl groups on Ov and effective promotion of O─O coupling and lattice oxygen recovery accounted for enhanced activity and durability. This work underscores the importance of tailor-made local configuration in boosting activity and durability of OER catalyst and different insights into the promotion mechanism.

Establishing the Link Between Oxygen Vacancy and Activity Enhancement in Acidic Water Oxidation of Trigonal Iridium Oxide

Developing durable IrO2-based electrocatalysts with high oxygen evolution reaction (OER) activity under acidic condition is crucial for proton exchange membrane electrolyzers. While oxygen defects are considered potentially important in OER, their direct relationship with catalytic activity has yet to be established. In this study, we introduced abundant oxygen vacancies through Re doping in 2D IrO2 (Re0.03Ir0.97O2), demonstrating their decisive role in enhancing OER performance. The Re0.03Ir0.97O2 catalyst exhibited excellent OER performance with an overpotential of 193 mV at 10 mA cm-2 and sustained activity for over 650 hours, significantly surpassing the undoped catalyst. Moreover, it maintained operation at a cell voltage of 1.70 V (~1200 mA cm-2) for over 140 hours without significant performance degradation. Theoretical calculations coupled with cyclic voltammetry, transient potential scanning and in situ characterizations confirmed the adsorbate evolving mechanism on Re0.03Ir0.97O2, as well as the critical role of Re-induced oxygen vacancies in enhancing OER performance. These findings highlight that oxygen defects directly influence OER activity, providing guidance for the application of oxygen vacancy engineering in electrocatalyst design.

Optimizing Acidic Oxygen Evolution Reaction via Modulation Doping in Van der Waals Layered Iridium Oxide

Anodic oxygen evolution reaction (OER) exhibits a sluggish four-electron transfer process, necessitating catalysts with exceptional catalytic activity to enhance its kinetic rate. Van der Waals layered oxides are ideal materials for catalyst design, yet its stability for acidic OER remains large obstacle. Doping provides a crucial way to improve the activity and stability simultaneously. However, doping in Van der Waals layered oxides remains a great challenge since it easily leads to lattice distortion or even the crystal structure damage. In this work, we successfully doping acid-resistant niobium (Nb) into Van der Waals layered edge-shared 1T phase iridium oxide (1 T-IrO2) via alkali-assisted thermal method. 1 T-IrO2 with a 5 % Nb doping (Nb0.05Ir0.95O2) only required an overpotential of 191 mV to achieve a current density of 10 mA cm-2 in 0.5 M H2SO4, 56 mV lower than that of 1T-IrO2. When applied in proton exchange membrane water electrolyzer, Nb0.05Ir0.95O2 show stable operation at a high current density of 1.2 A cm-2 for over 50 days. Density functional theory calculation reveals that doping Nb changes the potential-determining step from the *OOH deprotonation process in 1 T-IrO2 to the *O-OH coupling process in Nb0.05Ir0.95O2.

DFT-metadynamics insights on the origin of the oxygen evolution kinetics at the (100)-WSe2 surface

Water oxidation or oxygen evolution reaction (OER) in electrochemical cells is considered to be a major bottleneck in the way of hydrogen production by electro-synthesis, mainly due to a sluggish kinetics that characterizes the OER steps. Layered transition metal dichalcogenides, such as WSe2, are emerging as promising non-precious electrocatalysts for water splitting due to their excellent activity and stability. This paper aims to shed light on the (100) WSe2-aqueous interface in catalyzing the slow kinetics of the OER in the context of water splitting electro-catalysis. We employ state-of-the-art DFT-metadynamics to explore reaction mechanisms, activation free energies, and catalytic sites. This study reveals an energetically preferred water-assisted OER mechanism, where proton transfer is facilitated by the surrounding aqueous environment. Our findings not only provide insights into the OER process but also offer a design strategy for optimizing WSe2-based catalysts and a modeling protocol for future DFT-based OER investigations.

Support-free iridium hydroxide for high-efficiency proton-exchange membrane water electrolysis

The large-scale implementation of proton-exchange membrane water electrolyzers relies on high-performance membrane-electrode assemblies that use minimal iridium (Ir). In this study, we present a support-free Ir catalyst developed through a metal-oxide-based molecular self-assembly strategy. The unique self-assembly of densely isolated single IrO6H8 octahedra leads to the formation of μm-sized hierarchically porous Ir hydroxide particles. The support-free Ir catalyst exhibits a high turnover frequency of 5.31 s⁻¹ at 1.52 V in the membrane-electrode assembly. In the corresponding proton-exchange membrane water electrolyzer, notable performance with a cell voltage of less than 1.75 V at 4.0 A cm⁻² (Ir loading of 0.375 mg cm⁻²) is achieved. This metal-oxide-based molecular self-assembly strategy may provide a general approach for the development of advanced support-free catalysts for high-performance membrane-electrode assemblies.

Oxygen Atom Transfer Reactions of Colloidal Metal Oxide Nanoparticles

Redox transformations at metal oxide (MOx)/solution interfaces are broadly important, and oxygen atom transfer (OAT) is one of the simplest and most fundamental examples of such reactivity. OAT is a two-electron transfer process, well-known in gas/solid reactions and catalysis. However, OAT is rarely directly observed at oxide/water interfaces, whose redox reactions are typically proposed to occur in one-electron steps. Reported here are stoichiometric OAT reactions of organic molecules with aqueous colloidal titanium dioxide and iridium oxide nanoparticles (TiO2 and IrOx NPs). Me2SO (DMSO) oxidizes reduced TiO2 NPs with the formation of Me2S, and IrOx NPs transfer O atoms to a water-soluble phosphine and a thioether. The reaction stoichiometries were established and the chemical mechanisms were probed using typical solution spectroscopic techniques, exploiting the high surface areas and transparency of the colloids. These OAT reactions, including a catalytic example, utilize the ability of the individual NPs to accumulate many electrons and/or holes. Observing OAT reactions of two different materials, in opposite directions, is a step toward harnessing oxide nanoparticles for valuable multi-electron and multi-hole transformations.

Insight into the structural reconstruction of alkaline water oxidation electrocatalysts

The oxygen-evolution reaction (OER) is an indispensable component of various energy storage and conversion electrocatalytic systems. However, the slow reaction kinetics have forced the development of advanced, efficient, and inexpensive OER electrocatalysts to break through the bottleneck of its application. Recently, the structural reconstruction of precatalysts has provided a promising avenue to boost the catalytic activity of electrocatalysts. Structural reconstruction implies atomic rearrangement and composition change of the pristine catalytic materials, which is a very complex process. Therefore, it is very crucial to have a deep understanding of the reconstruction chemical process and then modulate the reconstruction by deliberate design of electrochemical conditions and precatalysts. However, a systematic review of the structural reconstruction process, research methods, influencing factors and structure-performance relationship remains elusive, significantly impeding the further developments of efficient electrocatalysts based on structural reconstruction chemistry. This critical review is dedicated to providing a deep insight into the structural reconstruction during alkaline water oxidation, comprehensively summarizing the basic research methods to understand the structural evolution process and various factors affecting the structural reconstruction process, and providing a reference and basis for regulating the dynamic reconstruction. Moreover, the impact of reconstruction on the structure and performance is also covered. Finally, challenges and perspectives for the future study on structural reconstruction are discussed. This review will offer future guidelines for the rational development of state-of-the-art OER electrocatalysts.

An Up-Scalable Solid-State Approach to Synthesize Iridium Nanoparticles on ATO for Water Splitting

Proton exchange membrane water electrolysis (PEMWE) technology commercialization strongly relies on developing efficient and cost-effective anode catalysts. One key challenge is the high cost associated with IrO2, which can be mitigated by reducing iridium (Ir) loading. A promising approach to achieving this is using a conductive support material to anchor Ir/IrO2. In this study, we explored depositing metallic Ir on antimony-doped tin oxide (ATO) using a solid-state method. This approach is straightforward and time-efficient. Among four samples with 50 wt % Ir loading, one prepared with NaOH in 100% ethanol (Ir/ATO-NE) exhibited the highest specific oxygen evolution reaction (OER) performance. The Ir/ATO-NE catalyst achieved 340 A gIr-1 at 1.6 V (versus RHE), surpassing a commercial IrO2 catalyst, which showed 282 A gIr-1. Additionally, Ir/ATO-NE demonstrated the lowest Tafel slope, indicating enhanced oxygen evolution kinetics and long-term durability comparable to commercial catalysts. Electron microscopy revealed uniform Ir nanoparticle (NP) sizes and a complete layer of Ir NPs on the support, in contrast to other samples. This study introduces a synthesis protocol for Ir catalysts that is efficient, simple, and effective for oxygen evolution in acidic media.

Mn0.75Ru0.25O2 with Low Ru Concentration for Active and Durable Acidic Oxygen Evolution

Ruthenium has emerged as a promising alternative to iridium in water-splitting anodes. However, it becomes overoxidized and dissolves at industry-relevant working conditions. To enhance the activity and stability of electrocatalysts for oxygen evolution reaction, an isostructural rutile MnRu oxide with low Ru concentration (Mn0.75Ru0.25O2) is synthesized and an asymmetric Mn-O-Ru dual-site active center is developed. It exhibits 154 mV overpotential at 10 mA cm-2 and can operate stably at 200 mA cm-2 for 670 h with a degradation rate of 29 uV/h-1. A proton exchange membrane water electrolyzer achieves stable operation at 1 A cm-2 for 700 h with a degradation rate of 53 uV h-1. Structural analysis and isotopic labeling correlate the asymmetric nature of the Mn-O-Ru dual-site active center, which facilitates the oxygen evolution reaction along the radical coupling pathway, with the stabilization of the cations and the lattice oxygen in isostructural rutile Mn0.75Ru0.25O2.

A Review of Surface Reconstruction and Transformation of 3d Transition-Metal (oxy)Hydroxides and Spinel-Type Oxides during the Oxygen Evolution Reaction

Developing efficient and sustainable electrocatalysts for the oxygen evolution reaction (OER) is crucial for advancing energy conversion and storage technologies. 3d transition-metal (oxy)hydroxides and spinel-type oxides have emerged as promising candidates due to their structural flexibility, oxygen redox activity, and abundance in earth's crust. However, their OER performance can be changed dynamically during the reaction due to surface reconstruction and transformation. Essentially, multiple elementary processes occur simultaneously, whereby the electrocatalyst surfaces undergo substantial changes during OER. A better understanding of these elementary processes and how they affect the electrocatalytic performance is essential for the OER electrocatalyst design. This review aims to critically assess these processes, including oxidation, surface amorphization, transformation, cation dissolution, redeposition, and facet and electrolyte effects on the OER performance. The review begins with an overview of the electrocatalysts' structure, redox couples, and common issues associated with electrochemical measurements of 3d transition-metal (oxy)hydroxides and spinels, followed by recent advancements in understanding the elementary processes involved in OER. The challenges and new perspectives are presented at last, potentially shedding light on advancing the rational design of next-generation OER electrocatalysts for sustainable energy conversion and storage applications.

Durable Acidic Oxygen Evolution Via Self-Construction of Iridium Oxide/Iridium-Tantalum Oxide Bi-Layer Nanostructure with Dynamic Replenishment of Active Sites

Proton exchange membrane (PEM) water electrolysis presents considerable advantages in green hydrogen production. Nevertheless, oxygen evolution reaction (OER) catalysts in PEM water electrolysis currently encounter several pressing challenges, including high noble metal loading, low mass activity, and inadequate durability, which impede their practical application and commercialization. Here we report a self-constructed layered catalyst for acidic OER by directly using an Ir-Ta-based metallic glass as the matrix, featuring a nanoporous IrO2 surface formed in situ on the amorphous IrTaOx nanostructure during OER. This distinctive architecture significantly enhances the accessibility and utilization of Ir, achieving a high mass activity of 1.06 A mgIr-1 at a 300 mV overpotential, 13.6 and 31.2 times greater than commercial Ir/C and IrO2, respectively. The catalyst also exhibits superb stability under industrial-relevant current densities in acid, indicating its potential for practical uses. Our analyses reveal that the coordinated nature of the surface-active Ir species is effectively modulated through electronic interaction between Ir and Ta, preventing them from rapidly evolving into high valence states and suppressing the lattice oxygen participation. Furthermore, the underlying IrTaOx dynamically replenishes the depletion of surface-active sites through inward crystallization and selective dissolution, thereby ensuring the catalyst's long-term durability.

Highly conductive composite anode catalysts featuring a fused Ir nano-network towards proton exchange membrane electrocatalysis

A boride-assisted method has been presented to synthesize a fused Ir nano-network supported on TiO2 as highly conductive composite catalysts (Ir NN@TiO2) for the reaction of oxygen evolution in acid. The Ir NN@TiO2 can be utilized to construct an anode catalyst layer of a proton exchange membrane water electrolyzer (PEMWE) with a low iridium content of 0.3 mgIr cm-2. The low-iridium-loading PEMWE exhibits excellent performance, i.e., 2.9 A cm-2@1.9 V with Nafion 115 membrane, and operates stably at a current density of 1.0 A cm-2 for over 1000 h.

Spillover of active oxygen intermediates of binary RuO2/Nb2O5 nanowires for highly active and robust acidic oxygen evolution

Over-oxidation of surface ruthenium active sites of RuOx-based electrocatalysts leads to the formation of soluble high-valent Ru species and subsequent structural collapse of electrocatalysts, which results in their low stability for the acidic oxygen evolution reaction (OER). Herein, a binary RuO2/Nb2O5 electrocatalyst with abundant and intimate interfaces has been rationally designed and synthesized to enhance its OER activity in acidic electrolyte, delivering a low overpotential of 179 mV at 10 mA cm-2, a small Tafel slope of 73 mV dec-1, and a stabilized catalytic durability over a period of 750 h. Extensive experiments have demonstrated that the spillover of active oxygen intermediates from RuO2 to Nb2O5 and the subsequent participation of lattice oxygen of Nb2O5 instead of RuO2 for the acidic OER suppressed the over-oxidation of surface ruthenium species and thereby improved the catalytic stability of the binary electrocatalysts.

Impacts of Surface Reconstruction and Metal Dissolution on Ru1-x Ti x O2 Acidic Oxygen Evolution Electrocatalysts

Improved oxygen evolution reaction (OER) electrocatalysts based on an additional understanding of surface changes that occur upon metal dissolution are needed to enable the efficient use of electrochemical water splitting. This work integrates theoretical and experimental studies of the effects of metal dissolution from the RuO2 and Ru1-x Ti x O2 surfaces on the OER activity and electrochemical stability. Our computational analysis shows that the energetic barriers for metal dissolution depend highly on the surface site and Ti-substituent location. Metal dissolution induces the formation of new active surface sites with different electronic density distributions. In addition to dissolution-induced changes to the surface composition, electron density changes occur in the interfacial electrolyte components. Surface reconstruction changes the activation barriers for the OER steps. Our experimental analysis of RuO2 and Ru0.8Ti0.2O2 using a two-step durability test in acidic electrolytes shows that the OER activity, surface, and metal dissolution change over the durability tests. Ti-substitution exhibits improved electrochemical stability with cycling. For RuO2, changes in the mass activity of RuO2 with cycling are directly correlated with Ru dissolution and lowering of the electrochemical surface area (ECSA). In contrast, Ru0.8Ti0.2O2 showed a 19 times lower Ru dissolution rate, and metal dissolution results in increasing the ECSA and new active sites. Our STEM and EELS analysis supports that repeated cycling under OER conditions results in surface reconstruction for both RuO2 and Ru0.8Ti0.2O2, with the formation of a disordered RuO2 surface and changes to the distribution of Ru and Ti at the Ru0.8Ti0.2O2 surface. The experimentally observed changes in activity and surface structure after cycling are consistent with computational analysis, which shows how metal dissolution may alter the OER activation barriers. Combining experimental and computational insights, this work reveals the effects of metal dissolution on the surface atomic and electronic structure and OER activity and advances our comprehension of metal dissolution dynamics and surface reconstruction, which may have implications for other catalytic processes.

Effectiveness of strain and dopants on breaking the activity-stability trade-off of RuO2 acidic oxygen evolution electrocatalysts

Ruthenium dioxide electrocatalysts for acidic oxygen evolution reaction suffer from mediocre activity and rather instability induced by high ruthenium-oxygen covalency. Here, the tensile strained strontium and tantalum codoped ruthenium dioxide nanocatalysts are synthesized via a molten salt-assisted quenching strategy. The tensile strained spacially elongates the ruthenium-oxygen bond and reduces covalency, thereby inhibiting the lattice oxygen participation and structural decomposition. The synergistic electronic modulations among strontium-tantalum-ruthenium groups both optimize deprotonation on oxygen sites and intermediates absorption on ruthenium sites, lowering the reaction energy barrier. Those result in a well-balanced activity-stability profile, confirmed by comprehensive experimental and theoretical analyses. Our strained electrode demonstrates an overpotential of 166 mV at 10 mA cm-2 in 0.5 M H2SO4 and an order of magnitude higher S-number, indicating comparable stability compared to bare catalyst. It exhibits negligible degradation rates within the long-term operation of single cell and PEM electrolyzer. This study elucidates the effectiveness of tensile strain and strategic doping in enhancing the activity and stability of ruthenium-based catalysts for acidic oxygen evolution reactions.

Leveraging Iron in the Electrolyte to Improve Oxygen Evolution Reaction Performance: Fundamentals, Strategies, and Perspectives

Electrochemical water splitting is a pivotal technology for storing intermittent electricity from renewable sources into hydrogen fuel. However, its overall energy efficiency is impeded by the sluggish oxygen evolution reaction (OER) at the anode. In the quest to design high-performance anode catalysts for driving the OER under non-acidic conditions, iron (Fe) has emerged as a crucial element. Although the profound impact of adventitious electrolyte Fen+ species on OER catalysis had been reported forty years ago, recent interest in tailoring the electrode-electrolyte interface has spurred studies on the controlled introduction of Fe ions into the electrolyte to improve OER performance. During the catalytic process, scenarios where the rate of Fen+ deposition on a specific host material outruns that of dissolution pave the way for establishing highly efficient and dynamically stable electrochemical interfaces for long-term steady operation. This review systematically summarizes recent endeavors devoted to elucidating the behaviors of in situ Fe(aq.) incorporation, the role of incorporated Fe sites in the OER, and critical factors influencing the interplay between the electrode surface and Fe ions in the electrolyte environment. Finally, unexplored issues related to comprehensively understanding and leveraging the dynamic exchange of Fen+ at the interface for improved OER catalysis are summarized.

Strong Heteroatomic Bond-Induced Confined Restructuring on Ir-Mn Intermetallics Enable Robust PEM Water Electrolyzers

Low-iridium acid-stabilized electrocatalysts for efficient oxygen evolution reaction (OER) are crucial for the market deployment of proton exchange membrane (PEM) water electrolysis. Manipulating the in situ reconstruction of Ir-based catalysts with favorable kinetics is highly desirable but remains elusive. Herein, we propose an atomic ordering strategy to modulate the dynamic surface restructuring of catalysts to break the activity/stability trade-off. Under working conditions, the strong heteroatom-bonded structure triggers rational surface-confined reconstruction to form self-stabilizing amorphous (oxy)hydroxides on the model Ir-Mn intermetallic (IMC). Combined in situ/ex situ characterizations and theoretical analysis demonstrate that the induced strong covalent Ir-O-Mn units in the catalytic layer weaken the formation barrier of OOH* and promote the preferential dynamic replenishment/conversion pathway of H2O molecules to suppress the uncontrollable participation of lattice oxygen (about 2.6 times lower than that of pure Ir). Thus, a PEM cell with Ir-Mn IMC as anode "pre-electrocatalyst" (0.24 mgIr cm-2) delivers an impressive performance (3.0 A cm-2@1.851 V@80 °C) and runs stably at 2.0 A cm-2 for more than 2,000 h with the cost of USD 0.98 per kg H2, further validating its promising application. This work highlights surface-confined evolution triggered by strong heteroatom bonds, providing insights into the design of catalysts involving surface reconstruction.

Electrocatalysis: From Planar Surfaces to Nanostructured Interfaces

The reactions critical for the energy transition center on the chemistry of hydrogen, oxygen, carbon, and the heterogeneous catalyst surfaces that make up electrochemical energy conversion systems. Together, the surface-adsorbate interactions constitute the electrochemical interphase and define reaction kinetics of many clean energy technologies. Practical devices introduce high levels of complexity where surface roughness, structure, composition, and morphology combine with electrolyte, pH, diffusion, and system level limitations to challenge our ability to deconvolute underlying phenomena. To make significant strides in materials design, a structured approach based on well-defined surfaces is necessary to selectively control distinct parameters, while complexity is added sequentially through careful application of nanostructured surfaces. In this review, we cover advances made through this approach for key elements in the field, beginning with the simplest hydrogen oxidation and evolution reactions and concluding with more complex organic molecules. In each case, we offer a unique perspective on the contribution of well-defined systems to our understanding of electrochemical energy conversion technologies and how wider deployment can aid intelligent materials design.

Synthesis, Characterization, and Electrocatalytic Properties of PrMn0.5M0.5O3 (M = Cr, Fe, Co, Ni) Perovskites

In this paper, the synthesis, characterization, and investigation of electrocatalytic properties of perovskites of general formula PrMn0.5M0.5O3 (M = Cr, Fe, Co, Ni) are presented. The synthesis was conducted by the solution combustion method using glycine as a fuel. The perovskite with the formula PrMn0.5Fe0.5O3 was also synthesized by the sol-gel combustion method with citric acid as fuel. The obtained perovskites were investigated by X-ray powder diffraction (XRPD), scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDX), infrared spectroscopy, and cyclic voltammetry. The XRPD patterns showed that the compounds are pure and isostructural within the series. The unit cell parameters of the compounds were determined within the Pnma space group, and several crystallochemical parameters were calculated and discussed. The recorded SEM images of the perovskites revealed a porous morphology, while the EDX analysis confirmed the 2:1:1 atomic percentage ratio of Pr:Mn:M. Within this investigation, the electrocatalytic properties of the obtained perovskites towards oxidation of OH- ions and H2O2 oxidation in phosphate buffer were studied by cyclic voltammetry, using a paraffin-impregnated graphite electrode (PIGE) modified with microcrystals of the investigated perovskites. PrMn0.5Fe0.5O3 showed high electrocatalytic activity for OH- oxidation, while both PrMn0.5Fe0.5O3 and PrMn0.5Co0.5O3 exhibited significant efficiency for H2O2 oxidation, with a distinct oxidation peak with a peak potential of 0.6 V.

Strontium Doped IrOx Triggers Direct O-O Coupling to Boost Acid Water Oxidation Electrocatalysis

The discovery of efficient and stable electrocatalysts for the oxygen evolution reaction (OER) in acidic conditions is crucial for the commercialization of proton-exchange membrane water electrolyzers. In this work, we propose a Sr(OH)2-assisted method to fabricate a (200) facet highly exposed strontium-doped IrOx catalyst to provide available adjacent iridium sites with lower Ir-O covalency. This design facilitates direct O-O coupling during the acidic water oxidation process, thereby circumventing the high energy barrier associated with the generation of *OOH intermediates. Benefiting from this advantage, the resulting Sr-IrOx catalyst exhibits an impressive overpotential of 207 mV at a current density of 10 mA cm-2 in 0.5 M H2SO4. Furthermore, a PEMWE device utilizing Sr-IrOx as the anodic catalyst demonstrates a cell voltage of 1.72 V at 1 A cm-2 and maintains excellent stability for over 500 hours. Our work not only provides guidance for the design of improved acidic OER catalysts but also encourages the development of iridium-based electrocatalysts with novel mechanisms for other electrocatalytic reactions.

Highly dispersed Ir nanoparticles on Ti3C2Tx MXene nanosheets for efficient oxygen evolution in acidic media

The industrialization of hydrogen production technology through polymer electrolyte membrane water splitting faces challenges due to high iridium (Ir) loading on the anode catalyst layer. While rational design of oxygen evolution reaction (OER) electrocatalysts aimed at effective iridium utilization is promising, it remains a challenging task. Herein, we present exfoliated Ti3C2Tx MXene as a highly conductive and corrosion-resistant support for acidic OER. We develop an alcohol reduction method to achieve uniform and dense loading of ultrafine Ir nanoparticles on the MXene surface. The IrO2/TiOx heterointerface is formed in situ on the Ir@Ti3C2Tx MXene surface, acting as a catalytically active phase for OER during electrocatalysis. The electron interactions at the IrO2/TiOx heterointerface create electron-rich Ir sites, which reduce the adsorption properties of oxygen intermediates and enhance intrinsic OER activity. Consequently, the prepared Ir@Ti3C2Tx exhibits a mass activity that is 7 times greater than that of the benchmark IrO2 catalyst for OER in acidic media. In addition, the /Ti3C2Tx MXene support can stabilize the Ir nanoparticles, so that the stability number of Ir@Ti3C2Tx MXene is about 2.4 times higher than that of the IrO2 catalyst.

Progress in Experimental Methods Using Model Electrodes for the Development of Noble-Metal-Based Oxygen Electrocatalysts in Fuel Cells and Water Electrolyzers

Hydrogen plays a key role in maximizing the benefits of renewable energy, and the widespread adoption of water electrolyzers and fuel cells, which convert the chemical energy of hydrogen and electrical energy into each other, is strongly desired. Electrocatalysts used in these devices, typically in the form of nanoparticles, are crucial components because they significantly affect cell performance, but their raw materials rely on limited resources. In catalyst research, electrochemical experimental studies using model catalysts, such as single-crystal electrodes, have provided valuable information on reaction and degradation mechanisms, as well as catalyst development strategies aimed at overcoming the trade-off between activity and durability, across spatial scales ranging from the atomic to the nanoscale. Traditionally, these experiments are conducted using well-defined, simple model surfaces like bare single-crystal electrodes in pure systems. However, in recent years, experimental methods using more complex interfaces-while still precisely controlling elemental distribution, microstructure, and modification patterns-have been established. This paper reviews the history of those studies focusing on noble-metal-based electrocatalysts for oxygen reduction reactions and oxygen evolution reactions, which account for the majority of efficiency losses in fuel cells and water electrolyzers, respectively. Furthermore, potential future research themes in experimental studies using model electrodes are identified.

Be Aware of Transient Dissolution Processes in Co3O4 Acidic Oxygen Evolution Reaction Electrocatalysts

Recently, cobalt-based oxides have received considerable attention as an alternative to expensive and scarce iridium for catalyzing the oxygen evolution reaction (OER) under acidic conditions. Although the reported materials demonstrate promising durability, they are not entirely intact, calling for fundamental research efforts to understand the processes governing the degradation of such catalysts. To this end, this work studies the dissolution mechanism of a model Co3O4 porous catalyst under different electrochemical conditions using online inductively coupled plasma mass spectrometry (online ICP-MS), identical location scanning transmission electron microscopy (IL-STEM), and differential electrochemical mass spectrometry (DEMS). Despite the high thermodynamics tendency reflected in the Pourbaix diagram, it is shown that the cobalt dissolution kinetics is sluggish and can be lowered further by modifying the electrochemical protocol. For the latter, identified in this study, several (electro)chemical reaction pathways that lead to the dissolution of Co3O4 must be considered. Hence, this work uncovers the transient character of cobalt dissolution and provides valuable insights that can help to understand the promising stability of cobalt-based materials in already published works and facilitate the knowledge-driven design of novel, stable, abundant catalysts toward the OER in an acidic environment.

Controlled Structural Activation of Iridium Single Atom Catalyst for High-Performance Proton Exchange Membrane Water Electrolysis

Iridium single atom catalysts are promising oxygen evolution reaction (OER) electrocatalysts for proton exchange membrane water electrolysis (PEMWE), as they can reduce the reliance on costly Ir in the OER catalysts. However, their practical application is hindered by their limited stability during PEMWE operation. Herein, we report on the activation of Ir-doped CoMn2O4 in acidic electrolyte that leads to enhanced activity and stability in acidic OER for long-term PEMWE operation. In-depth material characterization combined with electrochemical analysis and theoretical calculations reveal that activating Ir-doped CoMn2O4 induces controlled restructuring of Ir single atoms to IrOx nanoclusters, resulting in an optimized Ir configuration with outstanding mass activity of 3562 A gIr-1 at 1.53 V (vs RHE) and enhanced OER stability. The PEMWE using activated Ir-doped CoMn2O4 exhibited a stable operation for >1000 h at 250 mA cm-2 with a low degradation rate of 0.013 mV h-1, demonstrating its practical applicability. Furthermore, it remained stable for more than 400 h at a high current density of 1000 mA cm-2, demonstrating long-term durability under practical operation conditions.

Beyond conventional structures: emerging complex metal oxides for efficient oxygen and hydrogen electrocatalysis

The core of clean energy technologies such as fuel cells, water electrolyzers, and metal-air batteries depends on a series of oxygen and hydrogen-based electrocatalysis reactions, including the oxygen reduction reaction (ORR), oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), which necessitate cost-effective electrocatalysts to improve their energy efficiency. In the recent decade, complex metal oxides (beyond simple transition metal oxides, spinel oxides and ABO3 perovskite oxides) have emerged as promising candidate materials with unexpected electrocatalytic activities for oxygen and hydrogen electrocatalysis owing to their special crystal structures and unique physicochemical properties. In this review, the current progress in complex metal oxides for ORR, OER, and HER electrocatalysis is comprehensively presented. Initially, we present a brief description of some fundamental concepts of the ORR, OER, and HER and a detailed description of complex metal oxides, including their physicochemical characteristics, synthesis methods, and structural characterization. Subsequently, we present a thorough overview of various complex metal oxides reported for ORR, OER, and HER electrocatalysis thus far, such as double/triple/quadruple perovskites, perovskite hydroxides, brownmillerites, Ruddlesden-Popper oxides, Aurivillius oxides, lithium/sodium transition metal oxides, pyrochlores, metal phosphates, polyoxometalates and other specially structured oxides, with emphasis on the designed strategies for promoting their performance and structure-property-performance relationships. Moreover, the practical device applications of complex metal oxides in fuel cells, water electrolyzers, and metal-air batteries are discussed. Finally, some concluding remarks summarizing the challenges, perspectives, and research trends of this topic are presented. We hope that this review provides a clear overview of the current status of this emerging field and stimulate future efforts to design more advanced electrocatalysts.

Honeycomb-Structured IrOx Foam Platelets as the Building Block of Anode Catalyst Layer in PEM Water Electrolyzer

Achieving robust long-term durability with high catalytic activity at low iridium loading remains one of great challenges for proton exchange membrane water electrolyzer (PEMWE). Herein, we report the low-temperature synthesis of iridium oxide foam platelets comprising edge-sharing IrO6 octahedral honeycomb framework, and demonstrate the structural advantages of this material for multilevel tuning of anodic catalyst layer across atomic-to-microscopic scales for PEMWE. The integration of IrO6 octahedral honeycomb framework, foam-like texture and platelet morphology into a single material system assures the generation and exposure of highly active and stable iridium catalytic sites for the oxygen evolution reaction (OER), while facilitating the reduction of both mass transport loss and electronic resistance of catalyst layer. As a proof of concept, the membrane electrode assembly in single-cell PEMWE based on honeycomb-structured IrOx foam platelets, with a low iridium loading (~0.3 mgIr/cm2), is demonstrated to exhibit high catalytic activity at ampere-level current densities and to remain stable for more than 2000 hours.

Research Progress of High-entropy Oxides for Electrocatalytic Oxygen Evolution Reaction

High-entropy oxides (HEOs), similar to high-entropy materials (HEMs), have "four-core effects", i. e., high-entropy effect, delayed diffusion effect, lattice distortion effect and cocktail effect, which have attracted more and more attention in the scientific field of renewable energy technology due to their unique structural characteristics, variable chemical composition and corresponding functional properties. HEOs have become potential candidates for electrocatalytic oxygen evolution reaction (OER), which is a key half reaction for electrolytic CO2, nitrogen reduction, and water electrolysis. However, the precise synthesis of HEOs with a wide range of components and structures is challenging, not to mention their active and stable operation for OER. In this paper, we review the recent advancements in the electrocatalytic oxygen evolution facilitated by HEOs in water electrolysis. We analyze these developments from the perspectives of activity and stability in acid and alkaline conditions, respectively. Furthermore, we summarize the design from the aspect of element composition, structure, morphology, and catalyst-support interactions, along with related reaction mechanism of HEOs. Additionally, we discuss the current challenges faced by HEOs in the field of OER and suggest potential directions for the future development of HEOs beyond water electrolysis application.

Atomic-level Ru-Ir mixing in rutile-type (RuIr)O2 for efficient and durable oxygen evolution catalysis

The success of proton exchange membrane water electrolysis (PEMWE) depends on active and robust electrocatalysts to facilitate oxygen evolution reaction (OER). Heteroatom-doped-RuOx has emerged as a promising electrocatalysts because heteroatoms suppress lattice oxygen participation in the OER, thereby preventing the destabilization of surface Ru and catalyst degradation. However, identifying suitable heteroatoms and achieving their atomic-scale coupling with Ru atoms are nontrivial tasks. Herein, to steer the reaction pathway away from the involvement of lattice oxygen, we integrate OER-active Ir atoms into the RuO2 matrix, which maximizes the synergy between stable Ru and active Ir centers, by leveraging the changeable growth behavior of Ru/Ir atoms on lattice parameter-modulated templates. In PEMWE, the resulting (RuIr)O2/C electrocatalysts demonstrate notable current density of 4.96 A cm-2 and mass activity of 19.84 A mgRu+Ir-1 at 2.0 V. In situ spectroscopic analysis and computational calculations highlight the importance of the synergistic coexistence of Ru/Ir-dual-OER-active sites for mitigating Ru dissolution via the optimization of the binding energy with oxygen intermediates and stabilization of Ru sites.

Activity-Stability Relationships in Oxygen Evolution Reaction

The oxygen evolution reaction (OER) is a critical process in various sustainable energy technologies. Despite substantial progress in catalyst development, the practical application of OER catalysts remains hindered by the ongoing challenge of balancing high catalytic activity with long-term stability. We explore the inverse trends often observed between activity and stability, drawing on key insights from both experimental and theoretical studies. Special focus is placed on the performance of different electrodes and their interaction with acidic and alkaline media across a range of electrochemical conditions. This Perspective integrates recent advancements to present a thorough framework for understanding the mechanisms underlying the activity-stability relationship, offering strategies for the rational design of next-generation OER catalysts that successfully meet the dual demands of activity and durability.

Tantalum-stabilized ruthenium oxide electrocatalysts for industrial water electrolysis

The iridium oxide (IrO2) catalyst for the oxygen evolution reaction used industrially (in proton exchange membrane water electrolyzers) is scarce and costly. Although ruthenium oxide (RuO2) is a promising alternative, its poor stability has hindered practical application. We used well-defined extended surface models to identify that RuO2 undergoes structure-dependent corrosion that causes Ru dissolution. Tantalum (Ta) doping effectively stabilized RuO2 against such corrosion and enhanced the intrinsic activity of RuO2. In an industrial demonstration, Ta-RuO2 electrocatalyst exhibited stability near that of IrO2 and had a performance decay rate of ~14 microvolts per hour in a 2800-hour test. At current densities of 1 ampere per square centimeter, it had an overpotential 330 millivolts less than that of IrO2.

Ensuring Stability of Anode Catalysts in PEMWE: From Material Design to Practical Application

Proton Exchange Membrane Water Electrolysis (PEMWE) has emerged as a clean and effective approach for the conversion and storage of renewable electricity, particularly due to its compatibility with fluctuating photovoltaic and wind power. However, the high cost and limited performance of iridium oxide catalysts (i. e. IrO2) used as anode catalyst in industrial PEM electrolyzers remain significant obstacles to widespread application. Although numerous low-cost and efficient alternative catalysts have been developed in laboratory research, comprehensive stability studies critical for industrial use are often overlooked. This leads to the failure of performance transfer from catalysts tested in liquid half-cell systems to those employed in PEM electrolyzers. This concept presents a thorough overview for the stability issues of anode catalysts in PEMWE, and discuss their degradation mechanisms in both liquid half-cell systems and PEM electrolyzers. We summarize the comprehensive protocols for assessment and characterization, analyze the effective strategies for stability optimization, and explore the opportunities for designing viable anode catalysts for PEM electrolyzers.

Boosting the durability of RuO2 via confinement effect for proton exchange membrane water electrolyzer

Ruthenium dioxide has attracted extensive attention as a promising catalyst for oxygen evolution reaction in acid. However, the over-oxidation of RuO2 into soluble H2RuO5 species results in a poor durability, which hinders the practical application of RuO2 in proton exchange membrane water electrolysis. Here, we report a confinement strategy by enriching a high local concentration of in-situ formed H2RuO5 species, which can effectively suppress the RuO2 degradation by shifting the redox equilibrium away from the RuO2 over-oxidation, greatly boosting its durability during acidic oxygen evolution. Therefore, the confined RuO2 catalyst can continuously operate at 10 mA cm-2 for over 400 h with negligible attenuation, and has a 14.8 times higher stability number than the unconfined RuO2 catalyst. An electrolyzer cell using the confined RuO2 catalyst as anode displays a notable durability of 300 h at 500 mA cm-2 and at 60 °C. This work demonstrates a promising design strategy for durable oxygen evolution reaction catalysts in acid via confinement engineering.

Ir-O-Mn embedded in porous nanosheets enhances charge transfer in low-iridium PEM electrolyzers

Using metal oxides to disperse iridium (Ir) in the anode layer proves effective for lowering Ir loading in proton exchange membrane water electrolyzers (PEMWE). However, the reported low-Ir-based catalysts still suffer from unsatisfying electrolytic efficiency and durability under practical industrial working conditions, mainly due to insufficient catalytic activity and mass transport in the catalyst layer. Herein we report a class of porous heterogeneous nanosheet catalyst with abundant Ir-O-Mn bonds, achieving a notable mass activity of 4 A mgIr-1 for oxygen evolution reaction at an overpotential of 300 mV, which is 150.6 times higher than that of commercial IrO2. Ir-O-Mn bonds are unraveled to serve as efficient charge-transfer channels between in-situ electrochemically-formed IrOx clusters and MnOx matrix, fostering the generation and stabilization of highly active Ir3+ species. Notably, Ir/MnOx-based PEMWE demonstrates comparable performance under 10-fold lower Ir loading (0.2 mgIr cm-2), taking a low cell voltage of 1.63 V to deliver 1 A cm-2 for over 300 h, which positions it among the elite of low Ir-based PEMWEs.

Constructing Highly Porous Low Iridium Anode Catalysts Via Dealloying for Proton Exchange Membrane Water Electrolyzers

Iridium (Ir) is the most active and durable anode catalyst for the oxygen evolution reaction (OER) for proton exchange membrane water electrolyzers (PEMWEs). However, their large-scale applications are hindered by high costs and scarcity of Ir. Lowering Ir loadings below 1.0 mgcm-2 causes significantly reduced PEMWE performance and durability. Therefore, developing efficient low Ir-based catalysts is critical to widely commercializing PEMWEs. Herein, an approach is presented for designing porous Ir metal aerogel (MA) catalysts via chemically dealloying IrCu alloys. The unique hierarchical pore structures and multiple channels of the Ir MA catalyst significantly increase electrochemical surface area (ECSA) and enhance OER activity compared to conventional Ir black catalysts, providing an effective solution to design low-Ir catalysts with improved Ir utilization and enhanced stability. An optimized membrane electrode assembly (MEA) with an Ir loading of 0.5 mgIr cm-2 generated 2.0 A cm-2 at 1.79 V, higher than the Ir black at a loading of 2.0 mgIr cm-2 (1.63 A cm-2). The low-Ir MEA demonstrated an acceptable decay rate of ≈40 µV h-1 during durability tests at 0.5 (>1200 h) and 2.0 A cm-2 (400 h), outperforming the commercial Ir-based MEA (175 µV h-1 at 2.0 mgIr cm-2).