RuO2 electronic structure and lattice strain dual engineering for enhanced acidic oxygen evolution reaction performance

Fabrication of cost-effective Ni-based metallic composite columnar catalyst on stainless steel electrode via stress release-assisted electrochemical micro-machining for water electrolyzer

This study investigates the preparation of NiCoCrMoW multicomponent alloy metallic composite on stainless steel mesh substrates via electrochemical micro-machining to fabricate columnar catalyst for enhanced hydrogen evolution reaction (HER) performance. Two simple steps of electrodeposition have successful prepared a Ni-NiCoCrMoW micro-metallic composite which are composed of the Ni anchor layer and the outer columnar grain structure layer with high internal stress. Designing the electrochemical micro-machining process that release the internal stresses by cathodic polarization in 0.5 mol/L H2SO4, triggers spontaneous deformation and structural changes in the crystalline and the formation of micro-cracks, thus the electrochemically active surface area and the high electrocatalytic sites are increased. Finally, the columnar catalyst exhibits excellent HER performance across acidic, neutral, and alkaline environments, achieving an overpotential of 57 mV at a current density of 10 mA cm-2 in 0.5 mol/L H2SO4. Additionally, electrochemical stability testing demonstrate that the electrodes maintain catalytic activity for over 200 h. This novel electrochemical fabrication process integrating stress engineering with 4D printing principles provides a scalable, cost-effective method for fabricating high-performance HER catalysts, with significant potential for industrial hydrogen production.

Breaking the Activity-Stability Trade-Off of RuO2 via Metallic Ru Bilateral Regulation for Acidic Oxygen Evolution Reaction

Developing highly efficient acidic oxygen evolution reaction (OER) electrocatalysts is crucial for proton exchange membrane water electrolyzer. RuO2 electrocatalysts, which followed a kinetically favorable lattice oxygen mechanism, perform a preferable intrinsic activity, but poor stability for acidic OER. Recent work often sacrifices the intrinsic activity of RuO2 to enhance stability. The balance between the activity and stability of RuO2-based catalysts is still overlooked in current research. Here, we report a controlled method to introduce metallic Ru onto RuO2 catalysts to form the Ru4+─O─Ru0 interfacial structure for decreasing the Ru4+ oxidation state and promoting deprotonation kinetics. Metallic Ru can serve as the electron donor to lower the oxidation state of *Vo-RuO4 2- in Ru/RuO2 for stabilizing the structure of *Vo-RuO4 2--Ru/RuO2 to favour the acidic OER stability. Moreover, the deprotonation kinetics via the interfacial oxygen site between Ru4+ and Ru0 is significantly enhanced on Ru/RuO2 catalysts to improve the acidic OER activity. This work offers a unique perspective to balance the acidic OER activity and stability of RuO2.

Regulating Ru-Ru Distance in RuO2 Catalyst by Lattice Hydroxyl for Efficient Water Oxidation

Highly active and durable electrocatalysts for the oxygen evolution reaction (OER) are crucial for proton exchange membrane water electrolysis (PEMWE). While doped RuO2 catalysts demonstrate good activity and stability, the presence of dopants limits the number of exposed active sites and complicates Ru recovery. Here, we present a monometallic RuO2 (d-RuO2) with lattice hydroxyl in the periodic structure as a high-performance OER electrocatalyst. The obtained d-RuO2 catalyst exhibits a low overpotential of 150 mV and long-term operational stability of 500 h at 10 mA cm-2, outperforming many Ru/Ir-based oxides ever reported. A PEMWE device using d-RuO2 sustains operation for 348 h at 200 mA cm-2. In-situ characterization reveals that the incorporation of lattice hydroxyl increases the Ru-Ru distance, which facilitates the turnover of the Ru oxidation state and promotes the formation of stable edge-sharing [RuO6] octahedra during the OER, thereby accelerating the formation of O-O bonds and suppressing the overoxidation of Ru sites. Additionally, the small particle size of the catalyst decreases the three-phase contact line and promotes bubble release. This study will provide insights into the design and optimization of catalysts for various electrochemical reactions.

Electron donation from carbon support enhances the activity and stability of ultrasmall ruthenium dioxide nanoparticles in acidic oxygen evolution reaction

Developing non-iridium (Ir)-based electrocatalysts with good stability and activity for acid oxygen evolution reaction (OER) is of great importance for electrocatalytic water splitting. Ruthenium dioxide (RuO2), which has lower price and higher OER activity, has been recognized as an attractive alternative to Ir-based electrocatalyst for acidic OER. However, the stability of most Ru-based electrocatalysts faces a great challenge in acidic condition. Here, a highly stable and active RuO2-based catalyst, tiny RuO2 nanoparticles inlaid onto carbon support (RuO2/C), is successfully prepared for acidic OER. Such a structure can efficiently inhibit the over-growth of RuO2 nanoparticles and prevent the agglomeration of RuO2 nanoparticles. Moreover, it is found that carbon support donate electron to RuO2 nanoparticles, which enhances the OER activity and stability of RuO2 during acidic OER. The RuO2/C exhibits an impressive OER performance with a low overpotential (197 mV at 10 mA cm-2) and low degradation rate (0.035 mV h-1) over a 450-h stability test in 0.5 M H2SO4, which are much better than the commercial Ir/C, RuO2 and the reported Ru-based electrocatalysts. This work provides an efficient strategy to simultaneously improve both stability and activity of Ru-based catalysts for acidic water oxidation.

Iridium-Free High-Entropy Alloy for Acidic Water Oxidation at High Current Densities

Designing active and cost-effective catalysts for acidic oxygen evolution reaction (OER) is critically important for improving proton exchange membrane water electrolyzers (PEMWEs) used in hydrogen production. In this study, we introduce a rapid and straightforward method to synthesize a quinary high-entropy ruthenium-based alloy (RuMnFeMoCo) for acidic OER. This iridium-free catalyst demonstrates a low overpotential of 170 mV and exceptional stability, enduring a 1000-hour durability test at 10 mA cm-2 in 0.5 M H2SO4. Microstructural analyses and density functional theory (DFT) calculations reveal that the incorporation of corrosion-resistant elements such as Ru, Mo, and Co enhances the overall stability of the catalyst under acidic conditions. Concurrently, the presence of Mn, Fe, and Co significantly reduces the energy barrier of the rate-determining step in the OER process, thus accelerating the OER kinetics and lowering the overpotential. The PEMWE employing the RuMnFeMoCo catalyst operates stably at high current densities of 500 and 1000 mA cm-2 for over 300 hours with negligible performance degradation. This work illustrates a strategy for designing high-performance OER electrocatalysts by synergistically integrating the benefits of multiple elements, potentially overcoming the activity-stability trade-off typically encountered in the catalyst development.

Layered-Hierarchical Dual-Lattice Strain Suppresses NixSe Surface Reconstruction for Stable OER in Alkaline Fresh/Seawater Splitting

Transition metal selenides (TMSe) are promising oxygen evolution reaction (OER) electrocatalysts but act as precursors rather than the actual active phase, transforming into amorphous oxyhydroxides during OER. This transformation, along with the formation of selenium oxyanions and unstable heterointerfaces, complicates the structure-activity relationship and reduces stability. This work introduces novel "layered-hierarchical dual lattice strain engineering" to inhibit the surface reconstruction of NixSe by modulating both the nickel foam (NF) substrate with Mo2N nanosheets (NM) and the NixSe nanorods-nanosheets catalytic layer (NiSe-Ni0.85Se-NiO, NSN) with ultrafast interfacial bimetallic amorphous NiFeOOH coating, achieving the optimized NM/NSN/NiFeOOH configuration. The NM substrate induces lattice strain, enhancing OER activity by improving electron transport and adhesion, while the NiFeOOH coating induces additional lattice strain, mitigating the surface reconstruction and oxidative degradation, reinforcing structural integrity. The NM/NSN/NiFeOOH catalyst demonstrates exceptional OER performance with low overpotentials of 208 mV@10 mA cm-2 and outstanding stability over 100 h at 100 mA cm-2 in alkaline freshwater and seawater. Theoretical analysis shows that NiFeOOH effectively prevents surface reconstruction and oxidative degradation by preserving Ni sites for optimal OER intermediate interactions while stabilizing the electronic environment. This work provides a novel strategy for enhancing the OER stability of TMSe and beyond.

Highly Coupled Dynamically Modulated Electrocatalysts on Wafer-Scale InGaN/GaN Nanowires on Silicon for Successive Acidic Photoelectrochemical Water Oxidation

Photoelectrochemical water splitting is considered one of the most promising paths for sustainable hydrogen production. However, the sluggish kinetics of the water oxidation reaction and poor stability of the photoanode significantly limit the overall performance of the photoelectrochemical device, particularly under acidic conditions, which poses great challenges for practical applications. Herein, the coupling of unique CoRuOx nanoclusters with dynamic electronic modulation effects to wafer-scale InGaN nanowires is proposed, demonstrating superior photoelectrochemical activity and stability for acidic water oxidation. Compared with InGaN nanowires loaded with typical RuO₂ cocatalysts, CoRuOx/InGaN photoanodes achieve a remarkable improvement in applied bias photon-to-current efficiency from 0.77% to 2.25%, with stable operation for over 500 min under strongly acidic conditions. Such boosted performance is attributed mainly to Co induced dynamic electronic modulation, which enhances oxygen evolution while maintaining the stable operation of CoRuOx/InGaN photoanodes. Initially, the Co sites increased the oxidation state of Ru, enhancing the activity of oxygen evolution. Moreover, during PEC operation, the Co sites stabilized the Ru sites, preventing dissolution of cocatalyst. This unique self-adaptive process significantly enhances the stability and activity of the photoanode, opening an effective avenue to achieve efficient and durable photoanodes for PEC applications.

Iodine-Induced Redirection of Active Sources in Cu-Based Catalysts during Efficient and Stable Water Oxidation

Enhancing the mechanistic regulation of the oxygen evolution reaction (OER) is crucial for developing efficient and stable electrocatalysts. However, the dynamic variation of surface structure during the electrocatalytic process limits the accurate identification of the active source and underlying reaction mechanism. Herein, we report an iodine-doping strategy to direct the reconstruction of active species in CuS catalysts toward an unconventional oxygen vacancy oxidation mechanism, thereby overcoming the activity and stability limitations. Mechanistic analysis indicates that the electronic manipulation, weak coordination of Cu-S bonds, and lattice distortion induced by iodine-doping facilitate the thermodynamically favorable Cu2+ to Cu3+ oxidation during OER. The decisively formed oxygen vacancies are emphasized as a genuine active source to promote hydroxyl adsorption, with hypervalent Cu species acting as auxiliary sites to accelerate deprotonation by strengthening Cu-O covalent. Consequently, the optimal iodine-doped CuS exhibits a reduced overpotential of 189 mV at 10 mA cm-2 and superb stability prolonging to 1250 h. When used as a bifunctional electrode in a membrane electrode assembly electrolyzer, it also exhibits a low voltage of 1.65 V at 1 A cm-2, with electrolysis durability of 480 h and a low hydrogen cost of US$1.70/kg H2, outperforming the 2026 targets set by the U.S. Department of Energy.

Spin-controlled electrocatalysis: an out-of-the-box strategy for the advancement of electrochemical water splitting

Spin-polarized catalysts have garnered significant interest in electrocatalysis, namely in the electrocatalytic oxidation of water, which has very sluggish kinetics due to its high overpotential. After the groundbreaking discovery that the electron's spin employing the chiral-induced spin selectivity (CISS) effect can control the kinetics of the oxygen evolution reaction (OER), numerous studies have been carried out to demonstrate the impact of electron's spin on reducing the overpotential of the OER. Apart from CISS, various magnetic materials have been explored as OER catalysts, and the outcomes are found to be very promising for the development of spin-based OER catalyst materials. This review highlights the remarkable journey of the evolution of the spin-polarized catalyst, starting from chiral materials to magnetic materials, which has happened in the last decade and its contribution toward the enhancement of OER kinetics, which is a very essential process for the advancement of renewable energy technologies.

Interface engineering of hollow Janus-structured NiCoP/P-MoS2 heterojunction as self-supported electrode enables boosted alkaline hydrogen evolution reaction

Transition metal phosphorus (TMPs) and sulfides have attracted extensive attention as important candidates to replace noble metal-based hydrogen evolution (HER) catalysts. However, the insufficient specific surface area, low conductivity and easy detachments from electrode seriously affect the HER catalytic activity and stability. Herein, a novel self-supported hollow Janus-structured NiCoP/P-MoS2 heterojunction is designed on carbon cloth (CC) as high-performance electrocatalyst for alkaline HER. The binder-free NiCoP/P-MoS2/CC electrode with well-dispersed hollow structure exhibits acceptable durability and low overpotential, which requires overpotential of 52.6 mV to reach 10 mA cm-2, far superior to that of NiCoP/CC (111.2 mV), P-MoS2/CC (213.3 mV) electrode and also the corresponding NiCoP/P-MoS2 powder catalyst (113.1 mV). Experimental and theoretical results confirm that heterointerface interaction can improve the electronic state, accelerate charge transfer and optimize hydrogen adsorption energy, resulting in boosted HER kinetic process. Additionally, self-supported strategy is conducive to tightly anchoring high-quality active substances with well-organized hollow array structure, which significantly prevents the catalyst agglomeration and shedding, leading to the improved HER stability. This work offers valuable insights into the catalytic mechanisms and provides an avenue for designing hierarchical architecture for highly efficient and stable HER electrocatalysts.

Interstitial-Substitutional-Mixed Solid Solution of RuO2 Nurturing a New Pathway Beyond the Activity-Stability Linear Constraint in Acidic Water Oxidation

The acidic oxygen evolution reaction (OER) electrocatalysts for proton exchange membrane electrolyzer (PEMWE) often face a trade-off between activity and stability due to inherent linear relationship and overoxidation of metal atoms in highly oxidative environments, while following the conventional adsorbate evolution mechanism (AEM). Herein, a favorable AEM-derived proton acceptor-electron donor mechanism (PAEDM) is proposed in RuO2 by constructing interstitial-substitutional mixed solid solution structure (denoted as C,Ta-RuO2), which can effectively break the activity-stability trade-off of RuO2 in acidic OER. In situ spectroscopy experiments and theoretical calculations reveal that the interstitial C as the proton acceptor reduces the deprotonation energy barrier, enhancing catalytic activity, while the substitutional Ta as the electron donor donates electrons to the Ru sites via bridging oxygen, weakening the Ru─O bond covalency and preventing over-oxidation of surface Ru, thereby ensuring long-term stability. Under the guidance of this mechanism, the optimized C,Ta-RuO2 simultaneously achieves far low overpotential (η10 = 171 mV) and ultra-long stability (over 1300 h) for the acidic OER. More remarkably, a homemade PEMWE using C,Ta-RuO2 as the anode also shows high water splitting performance (1.63 V@1 A cm-2). This work supplies a novel strategy to guide future developments on efficient OER electrocatalysts toward water oxidation.

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.

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.

Cerium regulating Ru-O covalency enhances the RuO2 acidic oxygen evolution reaction

Ruthenium-based catalysts suffer from poor stability owing to the soluble high-valent Ru species. This study proposes a Ce doping strategy to modulate the Ru-O covalency, enhancing its activity and stability. This discovery presents a new perspective for enhancing the catalytic performance of RuO2-based catalysts in acidic electrolytes.

W-Mediated Electron Accumulation in Ru-O-W Motifs Enables Ultra-Stable Oxygen Evolution Reaction in Acid

The development of efficient and durable oxygen evolution reaction (OER) catalysts is crucial for advancing proton exchange membrane water electrolysis (PEMWE) technology, especially in the pursuit of non-iridium alternatives. Herein, we report a Zn, W co-doped Ru3Zn0.85W0.15Ox (RZW) ternary oxide catalyst that exhibits a low overpotential of 200 mV and remarkable stability for over 4000 hours at 10 mA cm-2 in 0.1 M HClO4. The incorporation of highly electronegative W facilitates the efficient capture of electrons released from the sacrificial Zn species during OER, and subsequently mediated to Ru sites. The observed enhancement in electron density within the stable Ru-O-W motifs substantially improves the anti-overoxidation properties of the Ru active sites. Our findings highlight the importance of strategic metal doping in modulating the electronic structure of OER catalysts during operation, thereby facilitating the development of practical and long-lasting water electrolysis technologies.

Anode Alchemy on Multiscale: Engineering from Intrinsic Activity to Impedance Optimization for Efficient Water Electrolysis

The past decade has seen significant progress in proton exchange membrane water electrolyzers (PEMWE), but the growing demand for cost-effective electrolytic hydrogen pushes for higher efficiency at lower costs. As a complex system, the performance of PEMWE is governed by a combination of multiscale factors. This review summarizes the latest progress from quantum to macroscopic scales. At the quantum level, electron spin configurations can be optimized to enhance catalytic activity. At the nano and meso scales, advancements in atomic structure optimization, crystal phase engineering, and heterostructure design improve catalytic performance and mass transport. At the macro scale, innovative techniques in gas bubble management and internal resistance reduction drive further efficiency gains under ampere-level operating conditions. These modifications at the quantum level cascade through meso- and macro-scales, affecting charge transfer, reaction kinetics, and gas evolution management. Unlike conventional approaches that focus solely on one scale-either at the catalyst level (e.g., atomic, or crystal modifications) or at the device level (e.g., porous transport layers design)-combining multiscale optimizations unlocks greater performance improvements. Finally, a perspective on future opportunities for multiscale engineering in PEMWE anode design toward commercial viability is offered.

Feasibility of Active and Durable Lattice Oxygen-Mediated Oxygen Evolution Electrocatalysts in Proton Exchange Membrane Water Electrolyzers Through d0 Metal Ion Incorporation

The primary hurdle faced in the practical application of proton exchange membrane water electrolyzer (PEMWE) involves improving the intrinsic kinetic activity of oxygen evolution reaction (OER) electrocatalysts while concurrently enhancing their durability. Although electrocatalysts based on lattice oxygen-mediated mechanism (LOM) have the potential to significantly enhance the activity in OER without being restricted by scaling relationships, they are neglected in acidic electrolytes due to limited durability. In this study, an innovative approach is presented to simultaneously promote the activation of lattice oxygen and improve the durability of LOM-based OER electrocatalysts by incorporating d0 metal ions into the RuO2 electrocatalyst. Leveraging the unique electronic properties of the d0 metal ion, the O 2p band center and Ru-O covalency of the electrocatalyst are successfully engineered, resulting in the change in OER mechanism. Furthermore, in a single cell of PEMWE, the LOM-based electrocatalyst demonstrates outstanding performance, achieving 3.0 A cm-2 at 1.81 V and maintaining durability for 100 h at 200 mA cm-2, surpassing commercial RuO2. This innovative strategy challenges the traditional viewpoint that suppressing lattice oxygen activation in OER is essential for enhancing PEMWE durability, offering new perspectives for the development of OER electrocatalysts in acidic electrolytes.

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.

Inhibiting Overoxidation of Dynamically Evolved RuO2 to Achieve a Win-Win in Activity-Stability for Acidic Water Electrolysis

Proton exchange membrane (PEM) water electrolysis offers an efficient route to large-scale green hydrogen production, in which the RuO2 catalyst exhibits superior activity but limited stability. Unveiling the atomic-scale structural evolution during operando reaction conditions is critical but remains a grand challenge for enhancing the durability of the RuO2 catalyst in the acidic oxygen evolution reaction (a-OER). This study proposes an adaptive machine learning workflow to elucidate the potential-dependent state-to-state global evolution of the RuO2(110) surface within a complex composition and configuration space, revealing the correlation between structural patterns and stability. We identify the active state with distorted RuO5 units that self-evolve at low potential, which exhibits minor Ru dissolution and an activity self-promotion phenomenon. However, this state exhibits a low potential resistance capacity (PRC) and evolves into inert RuO4 units at elevated potential. To enhance PRC and mitigate the overevolution of the active state, we explore the metal doping engineering and uncover an inverse volcano-type doping rule: the doped metal-oxygen bond strength should significantly differ from the Ru-O bond. This rule provides a theoretical framework for designing stable RuO2-based catalysts and clarifies current discrepancies regarding the roles of different metals in stabilizing RuO2. Applying this rule, we predict and confirm experimentally that Na can effectively stabilize RuO2 in its active state. The synthesized Na-RuO2 operates in a-OER for over 1800 h without any degradation and enables long-term durability in PEM electrolysis. This work enhances our understanding of the operando structural evolution of RuO2 and aids in designing durable catalysts for a-OER.

Stabilizing Lattice Oxygen of Bi2O3 by Interstitial Insertion of Indium for Efficient Formic Acid Electrosynthesis

Bismuth oxide (Bi2O3) emerges as a potent catalyst for converting CO2 to formic acid (HCOOH), leveraging its abundant lattice oxygen and the high activity of its Bi-O bonds. Yet, its durability is usually impeded by the loss of lattice oxygen causing structure alteration and destabilized active bonds. Herein, we report an innovative approach via the interstitial incorporation of indium (In) into the Bi2O3, significantly enhancing bond stability and preserving lattice oxygen. The optimized In-Bi2O3-100 catalyst achieves over 90 % Faradaic efficiency for HCOOH production across a wide potential range, in both H-cells and flow cells, maintaining robust stability after 100 hours of continuous operation. In situ surface-enhanced infrared absorption spectroscopy and theoretical calculations reveal that the interstitial In doping precisely tunes the adsorption of CO2* and OCHO* intermediate, facilitating rapid conversion. Further in situ Raman spectroscopy confirms the role of In bolstering the oxidized structure's stability within Bi2O3, critical for sustaining lattice oxygen during electrochemical CO2 reduction.

CeO2 Modification Promotes the Oxidation Kinetics for Adipic Acid Electrosynthesis from KA Oil Oxidation at 200 mA cm-2

Electrocatalytic oxidation of cyclohexanol/cyclohexanone in water provides a promising strategy for obtaining adipic acid (AA), which is an essential feedstock in the polymer industry. However, this process is impeded by slow kinetics and limited Faradaic efficiency (FE) due to a poor understanding of the reaction mechanism. Herein, NiCo2O4/CeO2 is developed to enable the electrooxidation of cyclohexanol to AA with a 0.0992 mmol  h-1 cm-2 yield rate and 87 % Faradaic efficiency at a lower potential. Mechanistic investigations demonstrate that cyclohexanol electrooxidation to AA is a gradual oxidation process involving the dehydrogenation of cyclohexanol to cyclohexanone, the generation of 2-hydroxy cyclohexanone, and subsequent C-C cleavage. Theoretical calculations reveal that electronic interactions between CeO2 and NiCo2O4 decrease the energy barrier of cyclohexanone oxidation to 2-hydroxy cyclohexanone and inhibit the *OH to *O step, leading to AA electrosynthesis with a high yield rate and FE. Kinetic analysis further elucidates the effect of CeO2 on promoting cyclohexanone adsorption and activation on the electrode surface, thus facilitating the reaction kinetics. Moreover, a two-electrode flow reactor is constructed to produce 72.1 mmol AA and 10.4 L H2 by using KA oil as the anode feedstock at 2.5 A (200 mA cm-2), demonstrating promising potential.

Constructing weak Ru-Mo metallic bonds to suppress Ru overoxidation for durable acidic water oxidation

Although reducing the Ru-O covalency suppresses the loss of lattice oxygen, it also weakens the electron transfer of the Ru-Obri-Mo configuration, leading to Ru overoxidation. Herein, doping Mo into RuO2 weakens the Ru-O covalency and forms weak Ru-Mo metallic bonds to compensate for the electron density of Ru, where the Mo0.125Ru0.875O2 catalyst exhibits stable PEM performance at 300 mA cm-2 for 500 h.

La-Doping-Induced Lattice Strain and Electronic State Modulation in RuO2 for Electrocatalytic Oxygen Evolution in Acidic Solutions

Pursuing highly active and stable Ru-based catalysts for the oxygen evolution reaction (OER) under acidic conditions is important in advancing proton exchange membrane (PEM) water electrolyzers. Unfortunately, the inadequate stability, especially under a large current density of Ru-based catalysts, still hinders its practical application. Herein, we report a La doping strategy that simultaneously enhances both OER activity and stability of RuO2 in acidic media. The introduction of La into RuO2 induces tensile strain, which effectively weakens the covalency of Ru-O bonds. This structural modification significantly inhibits Ru dissolution, thereby substantially enhancing the stability of RuO2. Meanwhile, La doping modulates the electronic structure of RuO2 and optimizes the adsorption energy of the reaction intermediates, thereby enhancing the electrocatalytic OER activity. Notably, the optimized La0.05-RuO2 electrocatalyst presents an excellent OER performance in 0.5 M H2SO4 electrolyte, which delivers a low overpotential of 190 mV at 10 mA cm-2 and sustains 150 h without obvious decay at 50 mA cm-2. More importantly, a PEM electrolyzer is constructed by using our La0.05-RuO2 as the anode catalyst, which acquires 200 h stability at 1 A cm-2, highlighting its strong potential for practical industrial applications. This work sheds new light on designing high-performance OER catalysts toward PEM electrolyzer applications.

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.

Advances in Oxygen Evolution Reaction Electrocatalysts via Direct Oxygen-Oxygen Radical Coupling Pathway

Oxygen evolution reaction (OER) is a cornerstone of various electrochemical energy conversion and storage systems, including water splitting, CO2/N2 reduction, reversible fuel cells, and rechargeable metal-air batteries. OER typically proceeds through three primary mechanisms: adsorbate evolution mechanism (AEM), lattice oxygen oxidation mechanism (LOM), and oxide path mechanism (OPM). Unlike AEM and LOM, the OPM proceeds via direct oxygen-oxygen radical coupling that can bypass linear scaling relationships of reaction intermediates in AEM and avoid catalyst structural collapse in LOM, thereby enabling enhanced catalytic activity and stability. Despite its unique advantage, electrocatalysts that can drive OER via OPM remain nascent and are increasingly recognized as critical. This review discusses recent advances in OPM-based OER electrocatalysts. It starts by analyzing three reaction mechanisms that guide the design of electrocatalysts. Then, several types of novel materials, including atomic ensembles, metal oxides, perovskite oxides, and molecular complexes, are highlighted. Afterward, operando characterization techniques used to monitor the dynamic evolution of active sites and reaction intermediates are examined. The review concludes by discussing several research directions to advance OPM-based OER electrocatalysts toward practical applications.

Modulation of RuO2 Nanocrystals with Facile Annealing Method for Enhancing the Electrocatalytic Activity on Overall Water Splitting in Acid Solution

RuO2-based materials are considered an important kind of electrocatalysts on oxygen evolution reaction and water electrolysis, but the reported discrepancies of activities exist among RuO2 electrocatalysts prepared via different processes. Herein, a highly efficient RuO2 catalysts via a facile hydrolysis-annealing approach is reported for water electrolysis. The RuO2 catalyst dealt with at 200 °C (RuO2-200) performs the highest activities on both oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) in acid with overpotentials of 200 mV for OER and 66 mV for HER to reach a current density of 100 mA cm-2 as well as stable operation for100 h. The high-resolution transmission electron microscopy (HRTEM) and X-ray photoelectron spectroscopy (XPS) characterizations show that the activities of as-prepared RuO2 rely on the hydroxide group/lattice oxygen (OH-/O2-) ratio, size, and crystalline of RuO2. The density functional theory (DFT) calculation also reveal that the OH- would enhance the activities of RuO2 for HER and OER via modifying the electronic structure to facilitate intermediate adsorption, thereby reducing the energy barrier of the rate-determining step. The water electrolysis by using RuO2-200 as the catalyst on both anode and cathode demonstrates a stable generation of hydrogen and oxygen with high Faradic efficiency at a current density of ≈30 mA cm-2 and a potential of below 1.47 V.

Upcycling of Multi-Metal Contaminated Wastewater into High-Entropy Layered Double Hydroxide for Oxygen Evolution Reaction

The rapid growth of the electric vehicle industry has driven up nickel demand for batteries. However, the release of various metals during the smelting of nickel-containing ore leads to complex multi-metal contaminated smelting wastewater. Herein, CaFe layered double hydroxide (denoted as CaFe) is synthesized for the treatment of multi-metal contaminated wastewater, achieving removal efficiencies of 98.0%, 98.6%, 100%, and 100% for Co2+, Ni2+, Cu2+, and Zn2+, respectively. The quasi-situ X-ray diffraction (XRD) and X-ray absorption fine structure (XAFS) results indicate the formation of high-entropy LDH of CaCoNiCuZnFe by the isomorphic substitution of Ca2+ in CaFe. Meanwhile, lattice distortion and the formation of metal vacancies can be observed due to the introduction of metals with different ionic radii and the dissolution of Ca2+. Given the stability and abundant active sites of high-entropy material, the CaCoNiCuZnFe shows good OER performance with an overpotential of 310.7 mV at 10 mA cm-2 and long-term stability of 250 h. Density functional theory (DFT) calculations reveal that lattice distortion optimizes intermediate adsorption energy by enhancing M─O covalency and metal vacancy activates lattice oxygen by generating non-bonding oxygen, which synergistically triggers the lattice oxygen mechanism (LOM). This strategy converts multi-metal contaminated wastewater resources into valuable products and achieves dual goals of environmental remediation and resource utilization.

Cost-Effective RuNi Solid Solutions Prepared by Electrodeposition for Efficient Alkaline Hydrogen Evolution

The development of efficient hydrogen evolution reaction (HER) catalysts is crucial for water electrolysis. Currently, Ru-based catalysts are considered top contenders, but issues with stability, activity, and cost remain. In this work, RuNi alloys possessing a solid solution structure within the Ru lattice are prepared via straightforward electrodeposition on various substrates and assessed as HER catalysts in alkaline media. A RuNi solid solution containing 9.8 at. % Ni deposited on Ti substrate, wherein the Ni content greatly surpasses the solubility limit of Ni in Ru at room temperature, exhibits a considerably low overpotential of 28 mV at a current density of 10 mA cm- 2, along with good long-term stability (less than 100 mV increase in overpotential after 600 h). The enhancement in HER performance results from the increased electron density around Ru atoms due to Ni coordination, which facilitates the desorption of H* from the catalyst surface to produce H2. Concurrently, incorporating Ni reduces the Ru usage, rendering the RuNi alloy a viable cost-effective HER catalyst for practical applications.

Phase-Engineered Bi-RuO2 Single-Atom Alloy Oxide Boosting Oxygen Evolution Electrocatalysis in Proton Exchange Membrane Water Electrolyzer

Engineering nanomaterials at single-atomic sites can enable unprecedented catalytic properties for broad applications, yet it remains challenging to do so on RuO2-based electrocatalysts for proton exchange membrane water electrolyzer (PEMWE). Herein, the rational design and construction of Bi-RuO2 single-atom alloy oxide (SAAO) are presented to boost acidic oxygen evolution reaction (OER), via phase engineering a novel hexagonal close packed (hcp) RuBi single-atom alloy. This Bi-RuO2 SAAO electrocatalyst exhibits a low overpotential of 192 mV and superb stability over 650 h at 10 mA cm-2, enabling a practical PEMWE that needs only 1.59 V to reach 1.0 A cm-2 under industrial conditions. Operando differential electrochemical mass spectroscopy analysis, coupled with density functional theory studies, confirmed the adsorbate-evolving mechanism on Bi-RuO2 SAAO and that the incorporation of Bi1 improves the activity by electronic density optimization and the stability by hindering surface Ru demetallation. This work not only introduces a new strategy to fabricate high-performance electrocatalysts at atomic-level, but also demonstrates their potential use in industrial electrolyzers.

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.

Cobalt-Doped Ru@RuO2 Core-Shell Heterostructure for Efficient Acidic Water Oxidation in Low-Ru-Loading Proton Exchange Membrane Water Electrolyzers

Proton exchange membrane water electrolysis (PEMWE) is a highly promising hydrogen production technology for enabling a sustainable energy supply. Herein, we synthesize a single-atom Co-doped core-shell heterostructured Ru@RuO2 (Co-Ru@RuO2) catalyst via a combination of ultrafast pulse-heating and calcination methods as an iridium (Ir)-free and durable oxygen evolution reaction (OER) catalyst in acidic conditions. Co-Ru@RuO2 exhibits a low overpotential of 203 mV and excellent stability over a 400 h durability test at 10 mA cm-2. When implemented in industrial PEMWE devices, a current density of 1 A cm-2 is achieved with only 1.58 V under an extremely low catalyst loading of 0.34 mgRu cm-2, which is decreased by 4 to 6 times as compared to other reported Ru-based catalysts. Even at 500 mA cm-2, the PEMWE device could work stably for more than 200 h. Structural characterizations and density functional theory (DFT) calculations reveal that the single-atom Co doping and the core-shell heterostructure of Ru@RuO2 modulate the electronic structure of pristine RuO2, which reduce the energy barriers of OER and improve the stability of surface Ru. This work provides a unique avenue to guide future developments on low-cost PEMWE devices for hydrogen production.

Chloride Residues in RuO2 Catalysts Enhance Its Stability and Efficiency for Acidic Oxygen Evolution Reaction

. Ruthenium dioxide (RuO2) is a benchmark electrocatalyst for proton exchange membrane water electrolyzers (PEMWE), but its stability during the oxygen evolution reaction (OER) is often compromised by lattice oxygen involvement and metal dissolution. Despite that the typical synthesis of RuO2 produces chloride residues, the underlying function of chloride have not well investigated. In this study, we synthesized chlorine-containing RuO2 (RuO2-Cl) and pure RuO2 catalysts with similar morphology and crystallinity. RuO2-Cl demonstrated superior stability, three times greater than that of pure RuO2, and a lower overpotential of 176 mV at 10 mA cm-2. Furthermore, the RuO2-Cl catalysts that were in situ synthesized on a platinum-coated titanium felt could maintain high performance for up to 1200 hours at 100 mA cm-2. Computational and experimental analyses show that chloride stabilizes RuO2 by substituting the bridging oxygen atoms, which subsequently inhibits lattice oxygen evolution and Ru demetallation. Notably, this substitution also lowers the energy barrier of the rate-determining step by strengthening the binding of *OOH intermediates. These findings offer new insights into the previously unknown role of chloride residues and how to improve RuO2 stability.

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.

Atomic-Level Tin Regulation for High-Performance Zinc-Air Batteries

The trade-off between the performances of the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) presents a challenge in designing high-performance aqueous rechargeable zinc-air batteries (a-r-ZABs) due to sluggish kinetics and differing reaction requirements. Accurate control of the atomic and electronic structures is crucial for the rational design of efficient bifunctional oxygen electrocatalysts. Herein, we designed a Sn-Co/RuO2 trimetallic oxide utilizing dual-active sites and tin (Sn) regulation strategy by dispersing Co (for ORR) and auxiliary Sn into the near-surface and surface of RuO2 (for OER) to enhance both ORR and OER performances. Both theoretical calculations and advanced dynamic monitoring experiments revealed that the auxiliary Sn effectively regulated the atomic/electronic environment of Ru and Co dual-active sites, which optimized the *OOH/*OH adsorption behavior and promoted the release of the final products, thus breaking the reaction limits. Therefore, the as-designed Sn-Co/RuO2 catalysts exhibited superb bifunctional performance with an oxygen potential difference (ΔE) of 0.628 V and negligible activity degradation after 200,000 (ORR) or 20,000 (OER) CV cycles. The a-r-ZABs based on the Sn-Co/RuO2 catalyst exhibited a higher performance at a wide temperature range of -30 to 65 °C. They demonstrated an ultralong lifespan of 138 days (20,000 cycles) at 5 mA cm-2, 39.7 times higher than that of Pt/C + IrO2 coupled catalysts at a low temperature of -20 °C. Additionally, they maintained an initial power density of 85.8% after long-term tests, significantly outperforming previously reported catalysts. More importantly, the a-r-ZABs also showed excellent stability of 766.45 h (about 4598 cycles) at a high current density of 10 mA cm-2.

Enhanced Cooperative Generalized Compressive Strain and Electronic Structure Engineering in W-Ni3N for Efficient Hydrazine Oxidation Facilitating H2 Production

As promising bifunctional electrocatalysts, transition metal nitrides are expected to achieve an efficient hydrazine oxidation reaction (HzOR) by fine-tuning electronic structure via strain engineering, thereby facilitating hydrogen production. However, understanding the correlation between strain-induced atomic microenvironments and reactivity remains challenging. Herein, a generalized compressive strained W-Ni3N catalyst is developed to create a surface with enriched electronic states that optimize intermediate binding and activate both water and N2H4. Multi-dimensional characterizations reveal a nearly linear correlation between the hydrogen evolution reaction (HER) activity and the d-band center of W-Ni3N under strain state. Theoretically, compressive strain enhances the electron transfer capability at the surface, increasing donation into antibonding orbitals of adsorbed species, which accelerates the HER and HzOR. Leveraging both compressive strain and the modified electronic structure from W incorporation, the W-Ni3N catalysts demonstrate outstanding bifunctional performance, achieving overpotentials of 46 mV for HER at 10 mA cm-2 and 81 mV for HzOR at 100 mA cm-2. Furthermore, W-Ni3N catalyst achieves efficient overall hydrazine splitting at a low cell voltage of 0.185 V for 50 mA cm-2, maintaining stability for ≈450 h. This work provides new insights into the dual engineering of strain and electronic structure in the design of advanced catalysts.

Oxidative reconstructed Ru-based nanoclusters forming heterostructures with lanthanide oxides for acidic water oxidation

Achieving rapid anodic oxygen evolution reaction (OER) kinetics and improving the stability of the corresponding ruthenium (Ru)-based catalysts is a current priority for the realisation of industrial water splitting. However, the activity and stability of O2 evolution in electrocatalysis are largely inhibited by the insufficient adsorption of the reactant H2O and too strong adsorption of the intermediate OOH*, as well as by the dissolution of the active site due to excessive oxidation. To solve this challenge, herein, we developed a regulatory strategy combining lanthanide oxides and metal oxidative reconfiguration. The introduction of Eu2O3 effectively promotes the adsorption of H2O, optimizes the adsorption energy of OOH*, and reduces the reaction energy barrier of acidic OER process. And the metal oxidation remodeling process exposed more active sites and prevented the peroxidation process. The optimized Ru/Eu2O3@CNT catalyst showed the highest catalytic activity and stability in acidic OER. Its mass activity was 1219.1 A gRu-1 and the TOF value reached 4.4 s-1 at 1.48 V. Additionally, Ru/Eu2O3@CNT after oxidative reconstruction demonstrates the industrially needed current density of 1.0 A cm-2 at 1.71 V in PEM electrolyser, achieving stability in excess of 200 h.

Molecular Strain Accelerates Electron Transfer for Enhanced Oxygen Reduction

Fe-N-C materials are emerging catalysts for replacing precious platinum in the oxygen reduction reaction (ORR) for renewable energy conversion. However, their potential is hindered by sluggish ORR kinetics, leading to a high overpotential and impeding efficient energy conversion. Using iron phthalocyanine (FePc) as a model catalyst, we elucidate how the local strain can enhance the ORR performance of Fe-N-Cs. We use density functional theory to predict the reaction mechanism for the four-electron reduction of oxygen to water. Several key differences between the reaction mechanisms for curved and flat FePc suggest that molecular strain accelerates the reductive desorption of *OH by decreasing the energy barrier by ∼60 meV. Our theoretical predictions are substantiated by experimental validation; we find that strained FePc on single-walled carbon nanotubes attains a half-wave potential (E1/2) of 0.952 V versus the reversible hydrogen electrode and a Tafel slope of 35.7 mV dec-1, which is competitive with the best-reported Fe-N-C values. We also observe a 70 mV change in E1/2 and dramatically different Tafel slopes for the flat and curved configurations, which agree well with the calculated energies. When integrated into a zinc-air battery, our device affords a maximum power density of 350.6 mW cm-2 and a mass activity of 810 mAh gZn-1 at 10 mA cm-2. Our results indicate that molecular strain provides a compelling tool for modulating the ORR activities of Fe-N-C materials.

Surface S-Doped Nanostructured RuO2 and Its Anion Passivating Effect for Efficient Overall Seawater Splitting

Electrolysis of seawater for hydrogen (H2) production to harvest clean energy is an appealing approach. In this context, there is an urgent need for catalysts with high activity and durability. RuO2 electrocatalysts have shown efficient activity in the hydrogen and oxygen evolution reactions (HER and OER), but they still suffer from poor stability. Herein, surface S-doped nanostructured RuO2 (S-RuO2) is rationally fabricated for efficient overall seawater splitting. Doping with S enhances the activity (overpotentials of 25 mV for the HER and 243 mV for the OER), long-term durability (1000 h at 100 mA cm-2), and achieves nearly 100% Faraday efficiency (FE). Moreover, the S-RuO2-based anion exchange membrane seawater electrolyzer requires 2.01 V to reach 1.0 A cm-2 under demanding industrial conditions. Experimental analysis and theoretical calculations indicate that surface S introduction could lower the valence state of Ru, thereby conferring enhanced activity and stability. Furthermore, the nanostructured S-RuO2 electrocatalyst is highly protected by the S-doped surface, which repels Cl- in alkaline seawater. This investigation presents a feasible strategy for designing RuO2-based seawater splitting catalysts with both high performance and good resistance to anodic corrosion.

Electron-phonon coupling and coherent energy superposition induce spin-sensitive orbital degeneracy for enhanced acidic water oxidation

The development of acid-stable water oxidation electrocatalysts is crucial for high-performance energy conversion devices. Different from traditional nanostructuring, here we employ an innovative microwave-mediated electron-phonon coupling technique to assemble specific Ru atomic patterns (instead of random Ru-particle depositions) on Mn0.99Cr0.01O2 surfaces (RuMW-Mn1-xCrxO2) in RuCl3 solution because hydrated Ru-ion complexes can be uniformly activated to replace some Mn sites at nearby Cr-dopants through microwave-triggered energy coherent superposition with molecular rotations and collisions. This selective rearrangement in RuMW-Mn1-xCrxO2 with particular spin-differentiated polarizations can induce localized spin domain inversion from reversed to parallel direction, which makes RuMW-Mn1-xCrxO2 demonstrate a high current density of 1.0 A cm-2 at 1.88 V and over 300 h of stability in a proton exchange membrane water electrolyzer. The cost per gallon of gasoline equivalent of the hydrogen produced is only 43% of the 2026 target set by the U.S. Department of Energy, underscoring the economic significance of this nanotechnology.

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.

Electron-Donating Zr Induces Suppressed Ru Over-Oxidation and Accelerated Deprotonation Process Toward Efficient and Durable Water Electrolysis

The scarcity of cost-effective and durable iridium-free anode electrocatalysts for the oxygen evolution reaction (OER) poses a significant challenge to the widespread application of the proton exchange membrane water electrolyzer (PEMWE). To address the electrochemical oxidation and dissolution issues of Ru-based electrocatalysts, an electron-donating modification strategy is developed to stabilize WRuOx under harsh oxidative conditions. The optimized catalyst with a low Zirconium doping (Zr, 1 wt.%) enhances durability noticeably, with a 77% reduction in degradation rate in the durability test of 10 mA cm-2 in 0.5 m H2SO4. When integrated into a homemade PEMWE device, the Zr-doped catalyst achieves excellent long-term stability, lasting up to 650 h at 100 mA cm⁻2. Additionally, the electronic modulation from the Zr modification leads to superior activity with a low overpotential of 208 mV at 10 mA cm-2. Theoretical calculation results further reveal that electron-donating Zr modification effectively suppresses Ru overoxidation and lattice oxygen participation, maintaining a robust structure during acidic OER. This modification also promotes deprotonation through stronger Brønsted acid sites, significantly improving both long-term stability and activity.

Hydrogel Electrolyte-Mediated In Situ Zn-Anode Modification and the Ru-RuO2/NGr-Coated Cathode for High-Performance Solid-State Rechargeable Zn-Air Batteries

This work aims to deal with the challenges associated with designing complementary bifunctional electrocatalysts and a separator/membrane that enables rechargeable zinc-air batteries (RZABs) with nearly solid-state operability. This solid-state RZAB was accomplished by integrating a bifunctional electrocatalyst based on Ru-RuO2 interface nanoparticles supported on nitrogen-doped (N-doped) graphene (Ru-RuO2/NGr) and a dual-doped poly(acrylic acid) hydrogel (d-PAA) electrolyte soaked in KOH with sodium stannate additive. The catalyst shows enhanced activity and stability toward the two oxygen reactions, i.e., oxygen reduction and evolution reactions (ORR and OER), with a very low potential difference (ΔE) of 0.64 V. The computational insights bring out the electronic factors contributing to the enhanced catalytic activity of Ru-RuO2/NGr based on the charge density difference (CDD) between the interfaces. The disadvantages of the existing solid-state RZABs, such as their limited lifespan brought on by passivation, dendritic growth, corrosion, and shape change, have also been taken into account. The introduction of the stannate additive to the electrolyte induced an in situ Zn-anode modification, which subsequently improved the interfacial stability of the ZABs and, hence, the battery life cycles. The experimental observations reveal that, during the charging process, the Sn nanoparticles enable the homogeneous Zn deposition on the surface of the anode. Thus, the in situ Zn-anode surface modification assisted in achieving a high-rate cycle capability, viz., the homemade catalyst-based system exhibited continuous charge-discharge cycles for 20 h at a current density of 2.0 mA cm-2, with each cycle lasting for 5 min.

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.

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.

Metal oxide plating for maximizing the performance of ruthenium(IV) oxide-catalyzed electrochemical oxygen evolution reaction

Hydrogen production by proton exchange membrane water electrolysis requires an anode with low overpotential for oxygen evolution reaction (OER) and robustness in acidic solution. While exploring new electrode materials to improve the performance and durability, optimizing the morphology of typical materials using new methods is a big challenge in materials science. RuO2 is one of the most active and stable electrocatalysts, but further improvement in its performance and cost reduction must be achieved for practical use. Herein, we present a novel technology, named "metal oxide plating", which can provide maximum performances with minimum amount. A uniform single-crystal RuO2 film with thickness of ∼2.5 nm was synthesized by a solvothermal-post heating method at an amount (x) of only 18 μg cm-2 (ST-RuO2(18)//TiO2 NWA). OER stably proceeds on ST-RuO2(18)//TiO2 NWA with ∼100% efficiency to provide a mass-specific activity (MSA) of 341 A gcat-1 at 1.50 V (vs. RHE), exceeding the values for most of the state-of-the-art RuO2 electrodes.

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.

A one-pot hydrothermal synthesis of morphologically controllable yolk-shell structured CoFe glycerate spheres for oxygen evolution reaction

CoFe-based catalysts are efficient electrocatalysts for the oxygen evolution reaction (OER) in alkaline media. Here, we present a simple one-pot hydrothermal method for synthesizing a series of CoFe glycerates with controllable surface morphologies and investigate their potential as highly efficient catalysts for the OER in alkaline media. These CoFe glycerates exhibit a unique yolk-shell microsphere structure assembled from ultrathin nanosheets. The adjustment of the surface nanosheet size is achieved by varying the CoFe ratio, ensuring a more efficient electrocatalytic system for the OER process. Due to the abundant active sites provided by the yolk-shell structure and interleaved ultrathin nanosheets, Co3Fe1 glycerate (Co3Fe1 gly) demonstrates a low overpotential (283 mV) and a small Tafel slope (44.61 mV dec-1) at 10 mA cm-2. Additionally, Co3Fe1 gly exhibits excellent durability in alkaline electrolytes. Moreover, a series of characterizations demonstrate that the active sites of Co3Fe1 gly are the high-valence Co species generated during the OER process. This study opens a promising avenue for utilizing efficient and low-cost electrocatalysts to enhance OER performance.

Enriched Electrophilic Oxygen Species on Ru Optimize Acidic Water Oxidation

Ruthenium oxide (RuO2) is considered one of the most promising catalysts for replacing iridium oxide (IrO2) in the acidic oxygen evolution reaction (OER). Nevertheless, the performance of RuO2 remains unacceptable due to the dissolution of Ru and the lack of *OH in acidic environments. This paper reports a grain boundary (GB)-rich porous RuO2 electrocatalyst for the efficient and stable acidic OER. The involvement of GB regulates the valence state of Ru and weakens the interaction between Ru and O, effectively facilitating *OH adsorption and *OOH formation. Notably, achieved a record-high catalytic activity (145 mV at 10 mA cm-2) with a low Tafel slope (40.9 mV dec-1) and a remarkable mass activity of 332 mA mg-1 Ru at 1.5 V versus reversible hydrogen electrode is achieved. Additionally, the porous RuO2 exhibits superb stability with an ultra-low degradation rate of 26 µV h-1 over a 50-day durability test. This study opens a viable pathway for the development of efficient and robust Ru-based acidic OER electrocatalysts.

Dynamic-Cycling Zinc Sites Promote Ruthenium Oxide for Sub-Ampere Electrochemical Water Oxidation

Although iridium-based electrocatalysts are commonly regarded as the sole stable operating acidic oxygen evolution reaction (OER) catalysts in proton-exchange membrane water electrolysis (PEMWE) devices, their exorbitant cost and scarcity severely restrict their widespread application. Herein, we introduce a promising alternative to iridium: zinc-doped ruthenium dioxide (TE-Zn/RuO2), which exhibits remarkable and enduring activity for acidic OER. In situ characterizations elucidate that the dynamic cycling of zinc dopants serves as both electron acceptors and donors, facilitating the activation of Ru sites at low overpotentials while thwarting peroxidation at high overpotentials, thus concurrently achieving heightened activity and robust stability. Additionally, the incorporation of zinc induces weakened Ru-O covalency, thereby stabling *OOH intermediates and instigating a sustained adsorbate evolution mechanism, dramatically stabilizing the RuO2 lattice. Importantly, the TE-Zn/RuO2 catalyst as an anode exhibits good stability over 300 h at a water-splitting current of 500 mA cm-2 in the PEMWE device, underscoring its considerable promise for practical applications.

Ruthenium clusters decorated on lattice expanded hematite Fe2O3 for efficient electrocatalytic alkaline water splitting

Electrocatalytic water splitting in alkaline media plays an important role in hydrogen production technology. Normally, the catalytic activity of commonly used transition metal oxides usually suffers from unsatisfactory electron conductivity and unfavorable binding strength for transition intermediates. To boost the intrinsic catalytic activity, we propose a rational strategy to construct lattice distorted transition metal oxides decorated with noble-metal nanoclusters. This strategy is verified by loading ruthenium clusters onto lithium ion intercalated hematite Fe2O3, which leads to significant distortion of the FeO6 unit cells. A remarkable overpotential of 21 mV with a Tafel slope of 39.8 mV dec-1 is achieved at 10 mA cm-2 for the hydrogen evolution reaction in 1.0 M KOH aqueous electrolyte. The assembled alkaline electrolyzer can catalyse overall water splitting for as long as 165 h at a current density of 250 mA cm-2 with negligible performance degradation, indicating great potential in the field of sustainable hydrogen production.