Optimizing Edge Active Sites via Intrinsic In-Plane Iridium Deficiency in Layered Iridium Oxides for Oxygen Evolution Electrocatalysis

Single-Atom Sb-Doped RuSbOx Bifunctional Catalysts for Ultra-Stable PEM Water Electrolyzers

Developing highly efficient and stable Pt/Ir-free bifunctional catalysts is very urgent for lowering the catalyst cost of proton exchange membrane water electrolyzer (PEMWE). Herein, a single-atom Sb-doped RuSbOx bifunctional catalyst is developed for ultra-stable PEMWE. RuSbOx exhibits excellent stability with a long-term operation of 150 h for oxygen evolution reaction (OER) and 300 h for hydrogen evolution reaction (HER) at 100 mA cm-2 in acidic media, respectively. Impressively, the PEMWE with RuSbOx as bifunctional catalysts only needs 1.72 to reach 1.0 A cm-2, and can maintain stable operation for 200 h at 200 mA cm-2. The in situ Raman and molecular probe methods reveal that the single-atom Sb doping can reconstruct the interfacial water structure on the surface of RuSbOx, resulting in an enriched supply of free water, accelerating the deprotonation process and reducing the local acidity of the catalyst surface, thereby improving the acidic OER activity and stability. Density functional theory calculations further confirm the above experimental results. In short, this work reveals that Sb is an outstanding structural stabilizer, and single-atom Sb-doping can maximize the OER stability of Ru-based catalysts in acid, which provides a useful strategy for designing ultra-stable electrocatalysts for PEMWE.

Modulating the covalency of Ru-O bonds by dynamic reconstruction for efficient acidic oxygen evolution

Developing ruthenium-based oxide catalysts capable of suppressing lattice oxygen participation in the catalytic reaction process is crucial for maintaining stable oxygen evolution reaction (OER) under acidic conditions. Herein, we delicately construct a RuO2 nanoparticle-anchored LiCoO2 nanosheet electrocatalyst (RuO2/LiCoO2), achieving dynamic optimization of RuO2 during the reaction process and improving catalytic stability. Benefiting from the unique electrochemical delithiation characteristics of the LiCoO2 support, the covalency of the Ru-O bond is effectively regulated during the OER process. The weakened Ru-O covalent bond inhibits the participation of lattice oxygen in the catalytic reaction and ensures the continuous operation of the Ru active sites. Moreover, the extended Ru-O bond in the optimized RuO2/LiCoO2 catalyst reduces the formation energy barrier of the *OOH intermediates, accelerating the progress of the OER. As a result, the RuO2/LiCoO2 catalyst requires only an overpotential of 150 ± 2 mV at 10 mA cm-2 in 0.5 M H2SO4 and operates stably for 2000 h at 1 A cm-2 in a proton exchange membrane water electrolysis. This work opens new avenues for designing efficient ruthenium-based catalysts.

Proton Exchange Membrane Water Splitting: Advances in Electrode Structure and Mass-Charge Transport Optimization

Proton exchange membrane water electrolysis (PEMWE) represents a promising technology for renewable hydrogen production. However, the large-scale commercialization of PEMWE faces challenges due to the need for acid oxygen evolution reaction (OER) catalysts with long-term stability and corrosion-resistant membrane electrode assemblies (MEA). This review thoroughly examines the deactivation mechanisms of acidic OER and crucial factors affecting assembly instability in complex reaction environments, including catalyst degradation, dynamic behavior at the MEA triple-phase boundary, and equipment failures. Targeted solutions are proposed, including catalyst improvements, optimized MEA designs, and operational strategies. Finally, the review highlights perspectives on strict activity/stability evaluation standards, in situ/operando characteristics, and practical electrolyzer optimization. These insights emphasize the interrelationship between catalysts, MEAs, activity, and stability, offering new guidance for accelerating the commercialization of PEMWE catalysts and systems.

Recent achievements in noble metal-based oxide electrocatalysts for water splitting

The search for sustainable energy sources has accelerated the exploration of water decomposition as a clean H2 production method. Among the methods proposed, H2 production via water electrolysis has garnered considerable attention. However, the process of H2 production from water electrolysis is severely limited by the slow kinetics of the anodic oxygen evolution reaction and large intrinsic overpotentials at the anode; therefore, suitable catalysts need to be found to accelerate the reaction rate. Noble metal-based oxide electrocatalysts retain the advantages of abundant active sites, high electrical conductivity of noble metals, and low cost, which make them promising electrocatalysts; however, they suffer from the challenge of an imbalance between catalytic activity and stability. This review presents recent research progress in noble metals and their oxides as electrocatalysts. In this review, two half-reactions (the hydrogen evolution reaction and the oxygen evolution reaction) of water electrolysis are described. Recently reported methods for the synthesis of noble metal-based oxide electrocatalysts, improvement strategies, and sources of enhanced activity and stability for these types of catalysts are presented. Finally, the challenges and future perspectives in the field are summarised. This review is expected to help improve the understanding of noble metal-based oxide electrocatalysts.

Bias-Induced Ga-O-Ir Interface Breaks the Limits of Adsorption-Energy Scaling Relationships for High-Performing Proton Exchange Membrane Electrolyzers

Rationally manipulating the in situ formed catalytically active surface of catalysts remains a significant challenge for achieving highly efficient water electrolysis. Herein, we present a bias-induced activation strategy to modulate in situ Ga leaching and trigger the dynamic surface restructuring of lamellar Ir@Ga2O3 for the electrochemical oxygen evolution reaction. The in situ reconstructed Ga-O-Ir interface sustains high water oxidation rates at oxygen evolution reaction (OER) overpotentials. We found that OER at the Ga-O-Ir interface follows a bi-nuclear adsorbate evolution mechanism with unsaturated IrOx as the active sites, while GaOx atoms play an indirect role in promoting water dissociation to form OH* and transferring OH* to Ir sites. This breaks the scaling relationship of the adsorption energies between OH* and OOH*, significantly lowering the energy barrier of the rate-limiting step and greatly increasing reactivity. The Ir@Ga2O3 catalyst achieves lower overpotentials, a current density of 2 A cm-2 at 1.76 V, and stable operation up to 1 A cm-2 in scalable proton exchange membrane water electrolyzer (PEMWE) at 1.63 V, maintaining stable operation at 1 A cm-2 over 1000 hours with a degradation rate of 11.5 μV h-1. This work prompted us to jointly address substrate-catalyst interactions and catalyst reconstruction, an underexplored path, to improve activity and stability in Ir PEMWE anodes.

Switchable Acidic Oxygen Evolution Mechanisms on Atomic Skin of Ruthenium Metallene Oxides

RuO2 has been considered as a promising, low-cost, and highly efficient catalyst in the acidic oxygen evolution reaction (OER). However, it suffers from poor stability due to the inevitable involvement of the lattice oxygen mechanism (LOM). Here, we construct a unique metallene-based core-skin structure and unveil that the OER pathway of atomic RuO2 skin can be regulated from the LOM to an adsorbate evolution mechanism by altering the core species from metallene oxides to metallenes. This switch is achieved without sacrificing the number of active sites, enabling Pd@RuO2 metallenes to exhibit outstanding acidic OER activity with a low overpotential of 189 mV at 10 mA cm-2, which is 54 mV lower than that of the counterpart PdO@RuO2 metallenes. Additionally, they also exhibit robust stability with negligible activity decay over 100 h at 50 mA cm-2, outperforming most reported RuO2-based catalysts. Multiple spectroscopic analyses and theoretical calculations demonstrate that the Pd-metallene core, acting as an electron donor, increases the migration energy of subsurface oxygen atoms and optimizes the adsorption energy of intermediates on the active Ru sites, enabling a switch in the reaction mechanism. Such a unique metallene-based core-skin structure offers a novel way for tuning the catalytic behaviors of electrocatalysts.

Recent Progress in Balancing the Activity, Durability, and Low Ir Content for Ir-Based Oxygen Evolution Reaction Electrocatalysts in Acidic Media

Proton exchange membrane (PEM) electrolysis faces challenges associated with high overpotential and acidic environments, which pose significant hurdles in developing highly active and durable electrocatalysts for the oxygen evolution reaction (OER). Ir-based nanomaterials are considered promising OER catalysts for PEM due to their favorable intrinsic activity and stability under acidic conditions. However, their high cost and limited availability pose significant limitations. Consequently, numerous studies have emerged aimed at reducing iridium content while maintaining high activity and durability. Furthermore, the research on the OER mechanism of Ir-based catalysts has garnered widespread attention due to differing views among researchers. The recent progress in balancing activity, durability, and low iridium content in Ir-based catalysts is summarized in this review, with a particular focus on the effects of catalyst morphology, heteroatom doping, substrate introduction, and novel structure development on catalyst performance from four perspectives. Additionally, the recent mechanistic studies on Ir-based OER catalysts is discussed, and both theoretical and experimental approaches is summarized to elucidate the Ir-based OER mechanism. Finally, the perspectives on the challenges and future developments of Ir-based OER catalysts is presented.

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.

Designing Ru-B-Cr Moieties to Activate the Ru Site for Acidic Water Electrolysis under Industrial-Level Current Density

Developing highly efficient non-iridium-based active sites for acidic water splitting is still a huge challenge. Herein, unique Ru-B-Cr moieties have been constructed in RuO2 nanofibers (NFs) to activate Ru sites for water electrolysis, which overcomes the bottleneck of RuO2-based catalysts usually possessing low activity for the hydrogen evolution reaction (HER) and poor stability for the oxygen evolution reaction (OER). The fabricated Cr, B-doped RuO2 NFs exhibit low overpotentials of 205 and 379 mV for acidic HER and OER at 1 A cm-2 with outstanding stability lasting 1000 and 188 h, respectively. The assembled acidic electrolyzer also possesses great hydrogen production efficiency and durability at a high current density. Experimental and theoretical explorations reveal that the formation of Ru-B-Cr moieties effectively optimizes the atomic configurations and modulates the adsorption/desorption free energy of reaction intermediates to achieve exceptional HER and OER performance.

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.

Rational design of water splitting electrocatalysts through computational insights

Electrocatalytic water splitting is vital for the sustainable production of green hydrogen. Electrocatalysts, including those for the hydrogen evolution reaction at the cathode and the oxygen evolution reaction at the anode, are crucial in determining the overall performance of water splitting. Traditional methods for electrocatalyst development often rely on trial-and-error, which can be time-consuming and inefficient. Recent advancements in computational techniques provide more systematic and predictive strategies for catalyst design. This review article explores the role of computational insights in the development of water-splitting electrocatalysts. We start by giving an introduction of electrocatalytic water splitting mechanisms. Then, fundamental theories such as the Sabatier principle and scaling relationships are reviewed, which provide a theoretical basis for catalytic activity. We also discuss thermodynamic, electronic, and geometric descriptors used to guide catalyst design and provide an in-depth discussion of their applications and limitations. Advanced computational approaches, including high-throughput screening, machine learning, solvation models and Ab initio molecular dynamics, are also highlighted for their ability to accelerate catalyst discovery and simulate realistic reaction conditions. Finally, we propose future research directions aimed at searching universal descriptors, expanding data sets, and integrating developing interpretable models with catalyst design.

Copper Sulfide based Photocatalysts, Electrocatalysts and Photoelectrocatalysts: Innovations in Structural Modulation and Application

Copper sulfides (CuxS, 1 ≤ x ≤ 2) are notable for their unique photoelectric properties and potential applications, particularly in photo/electrocatalysis. These materials are valued for their tunable band gap, near-infrared optical characteristics, and plasmonic resonance effects. However, challenges such as low catalytic activity and limited stability impede their practical applications. This review addresses these issues by exploring advanced strategies for electronic structure modulation, including atomic doping, shape alteration, heterojunction construction, and defect introduction to enhance catalytic efficiency. A detailed analysis of the optical and electrical properties of CuxS across various stoichiometric ratios and crystal structures is provided, offering a comprehensive overview of their applications in photocatalysis, electrocatalysis, and photo/electrocatalysis. Additionally, the review synthesizes current knowledge and highlights the potential of these strategies to optimize CuxS-based photo/electrocatalysts, proposing future research directions to bridge the gap between theoretical studies and practical applications. This work underscores the importance of CuxS in photo/electrocatalysis and aims to inspire further innovation and exploration in this field, emphasizing its significance in material science and engineering.

Advances in Stability of NiFe-Based Anodes toward Oxygen Evolution Reaction for Alkaline Water Electrolysis

Alkaline electrolysis plays a crucial role in sustainable energy solutions by utilizing electrolytic cells to produce hydrogen gas, providing a clean and efficient method for energy storage and conversion. Efficient, stable, and low-cost electrocatalysts for the oxygen evolution reaction (OER) are essential to facilitate alkaline water electrolysis on a commercial scale. Nickel-iron-based (NiFe-based) transition metal electrocatalysts are considered the most promising non-precious metal catalysts for alkaline OER due to their low cost, abundance, and tunable catalytic properties. Nevertheless, the majority of existing NiFe-based catalysts suffer from limited activity and poor stability, posing a significant challenge in meeting industrial applications. This also highlights a common situation where the emphasis on material activity receives significant attention, while the equally critical stability aspect is often underemphasized. Initiating with a comprehensive exploration of the stability of NiFe-based OER materials, this article first summarizes the debate surrounding the determination of active sites in NiFe-based OER electrocatalysts. Subsequently, the degradation mechanisms of recently reported NiFe-based electrocatalysts are outlined, encompassing assessments of both chemical and mechanical endurance, along with essential approaches for enhancing their stability. Finally, suggestions are put forth regarding the essential considerations for the design of NiFe-based OER electrocatalysts, with a focus on heightened stability.

Regulating Strain and Electronic Structure of Indium Tin Oxide Supported IrOx Electrocatalysts for Highly Efficient Oxygen Evolution Reaction in Acid

The development of proton exchange membrane water electrolysis is a promising technology for hydrogen production, which has always been restricted by the slow kinetics of the oxygen evolution reaction (OER). Although IrOx is one of the benchmark acidic OER electrocatalysts, there are still challenges in designing highly active and stable Ir-based electrocatalysts for commercial application. Herein, a Ru-doped IrOx electrocatalyst with abundant twin boundaries (TB-Ru0.3Ir0.7Ox@ITO) is reported, employing indium tin oxide with high conductivity as the support material. Combing the TB-Ru0.3Ir0.7Ox nanoparticles with ITO support could expose more active sites and accelerate the electron transfer. The TB-Ru0.3Ir0.7Ox@ITO exhibits a low overpotential of 203 mV to achieve 10 mA cm-2 and a high mass activity of 854.45 A g-1noble metal at 1.53 V vs RHE toward acidic OER, which exceeds most reported Ir-based OER catalysts. Moreover, improved long-term stability could be obtained, maintaining the reaction for over 110 h at 10 mA cm-2 with negligible deactivation. DFT calculations further reveal the activity enhancement mechanism, demonstrating the synergistic effects of Ru doping and strains on the optimization of the d-band center (εd) position and the adsorption free energy of oxygen intermediates. This work provides ideas to realize the trade-off between high catalytic activity and good stability for acidic OER electrocatalysts.

Boron-Induced Interstitial Effects Drive Water Oxidation on Ordered Ir-B Compounds

Interstitial filling of light atoms strongly affects the electronic structure and adsorption properties of the parent catalyst due to ligand and ensemble effects. Different from the conventional doping and surface modification, constructing ordered intermetallic structures is more promising to overcome the dissolution and reconstruction of active sites through strong interactions generated by atomic periodic arrangement, achieving joint improvement in catalytic activity and stability. However, for tightly arranged metal lattices, such as iridium (Ir), obtaining ordered filling atoms and further unveiling their interstitial effects are still limited by highly activated processes. Herein, we report a high-temperature molten salt assisted strategy to form the intermetallic Ir-B compounds (IrB1.1) with ordered filling by light boron (B) atoms. The B residing in the interstitial lattice of Ir constitutes favorable adsorption surfaces through a donor-acceptor architecture, which has an optimal free energy uphill in rate-determining step (RDS) of oxygen evolution reaction (OER), resulting in enhanced activity. Meanwhile, the strong coupling of Ir-B structural units suppresses the demetallation and reconstruction behavior of Ir, ensuring catalytic stability. Such B-induced interstitial effects endow IrB1.1 with higher OER performance than commercial IrO2, which is further validated in proton exchange membrane water electrolyzers (PEMWEs).

KIr4O8 Nanowires with Rich Hydroxyl Promote Oxygen Evolution Reaction in Proton Exchange Membrane Water Electrolyzer

The sluggish kinetics for anodic oxygen evolution reaction (OER) and insufficient catalytic performance over the corresponding Ir-based catalysts are still enormous challenges in proton exchange membrane water electrolyzer (PEMWE). Herein, it is reported that KIr4O8 nanowires anode catalyst with more exposed active sites and rich hydroxyl achieves a current density of 1.0 A cm-2 at 1.68 V and possesses excellent catalytic stability with 1230 h in PEMWE. Combining in situ Raman spectroscopy and differential electrochemical mass spectroscopy results, the modified adsorbate evolution mechanism is proposed, wherein the rich hydroxyl in the inherent structure of KIr4O8 nanowires directly participates in the catalytic process for favoring the OER. Density functional theory calculation results further suggest that the enhanced proximity between Ir (d) and O (p) band center in KIr4O8 can strengthen the covalence of Ir-O, facilitate the electron transfer between adsorbents and active sites, and decrease the energy barrier of rate-determining step from OH* to O* during the OER.

Sub-2 nm IrRuNiMoCo High-Entropy Alloy with Iridium-Rich Medium-Entropy Oxide Shell to Boost Acidic Oxygen Evolution

Ensuring high catalytic activity and durability at low iridium (Ir)usage is still a big challenge for the development of electrocatalysts toward oxygen evolution reaction (OER) in proton exchange membrane water electrolysis (PEMWE). Here, a rapid liquid-reduction combined with surface galvanic replacement strategy is reported to synthesize the sub 2 nm high-entropy alloy (HEA) nanoparticles featured with Ir-rich IrRuNiMo medium-entropy oxide shell (Ir-MEO) and a IrRuCoNiMo HEA core (HEA@Ir-MEO). Advanced spectroscopies reveal that the Ir-rich MEO shell inhibits the severe structural evolution of transition metals upon the OER, thus guaranteeing the structural stability. In situ differential electrochemical mass spectrometry, activation energy analysis and theoretical calculations unveil that the OER on HEA@Ir-MEO follows an adsorbate evolution mechanism pathway, where the energy barrier of rate-determining step is substantially lowered. The optimized catalyst delivers the excellent performance (1.85 V/3.0 A cm-2@80 °C), long-term stability (>500 h@1.0 Acm-2), and low energy consumption (3.98 kWh Nm-3 H2 @1.0 A cm-2) in PEMWE with low Ir usage of ≈0.4 mg cm-2, realizing the dramatical reduction of hydrogen (H2) production cost to 0.88 dollar per kg (H2).

Polarized Ultrathin BN Induced Dynamic Electron Interactions for Enhancing Acidic Oxygen Evolution

Developing ruthenium-based heterogeneous catalysts with an efficient and stable interface is essential for enhanced acidic oxygen evolution reaction (OER). Herein, we report a defect-rich ultrathin boron nitride nanosheet support with relatively independent electron donor and acceptor sites, which serves as an electron reservoir and receiving station for RuO2, realizing the rapid supply and reception of electrons. Through precisely controlling the reaction interface, a low OER overpotential of only 180 mV (at 10 mA cm-2) and long-term operational stability (350 h) are achieved, suggesting potential practical applications. In situ characterization and theoretical calculations have validated the existence of a localized electronic recycling between RuO2 and ultrathin BN nanosheets (BNNS). The electron-rich Ru sites accelerate the adsorption of water molecules and the dissociation of intermediates, while the interconnection between the O-terminal and B-terminal edge establishes electronic back-donation, effectively suppressing the over-oxidation of lattice oxygen. This study provides a new perspective for constructing a stable and highly active catalytic interface.