Customized reaction route for ruthenium oxide towards stabilized water oxidation in high-performance PEM electrolyzers

High-throughput screening of bifunctional catalysts for oxygen evolution/reduction reaction at the subnanometer regime

The development of low-cost, stable, and highly efficient electrocatalysts for the bifunctional oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) is crucial for advancing future renewable technologies. In this study, we systematically investigated the OER and ORR performance of subnano clusters across the 3d, 4d, and 5d transition metal (TM) series of varying sizes using first-principles calculations. The fluxional identity of these clusters in the subnanometer regime is reflected in their non-monotonic catalytic activity. We established a size-dependent scaling relationship between OER/ORR intermediates, leading to a reshaping of the activity volcano plot at the subnanometer scale. Our detailed mechanistic investigation revealed a shift in the apex of the activity volcano from the Pt(111) and IrO2 surfaces to the Au11 clusters for both OER and ORR. Late transition metal subnano clusters, specifically Au11, emerged as the best bifunctional electrocatalyst, demonstrating significantly lower overpotential values. Furthermore, we categorized our catalysts into three clusters and employed the Random Forest Regression method to evaluate the impact of non-ab initio electronic features on OER and ORR activities. Interestingly, d-band filling emerged as the primary contributor to the bifunctional activity of the subnano clusters. This work not only provides a comprehensive view of OER and ORR activities but also presents a new pathway for designing and discovering highly efficient bifunctional catalysts.

Regulating Ru-O Bond and Oxygen Vacancies of RuO2 by Ta Doping for Electrocatalytic Oxygen Evolution in Acid Media

Proton exchange membrane water electrolysis (PEMWE) is considered an ideal green hydrogen production technology with promising application prospects. However, the development of efficient and stable acid electroanalytic oxygen electrocatalysts is still a challenging bottleneck. This progress is achieved by adopting a strategic approach with the introduction of the high valence metal Ta to regulate the electronic configuration of RuO2 by manipulating its local microenvironment to optimize the stability and activity of the electrocatalysts. The Ta-RuO2 catalysts are notable for their excellent electrocatalytic activity, as evidenced by an overpotential of only 202 mV at 10 mA cm-2, which significantly exceeds that of homemade RuO2 and commercial RuO2. Furthermore, the Ta-RuO2 catalyst exhibits exceptional stability with negligible potential reduction observed after 50 h of electrolysis. Theoretical calculations show that the asymmetric configuration of Ru-O-Ta breaks the thermodynamic activity limitations usually associated with adsorption evolution, weakening the energy barrier for the formation of the OOH* formation. The strategic approach presented in this study provides an important reference for the development of a stable active center for acid water splitting.

Stability of electrocatalytic OER: from principle to application

Hydrogen energy, derived from the electrolysis of water using renewable energy sources such as solar, wind, and hydroelectric power, is considered a promising form of energy to address the energy crisis. However, the anodic oxygen evolution reaction (OER) poses limitations due to sluggish kinetics. Apart from high catalytic activity, the long-term stability of electrocatalytic OER has garnered significant attention. To date, several research studies have been conducted to explore stable electrocatalysts for the OER. A comprehensive review is urgently warranted to provide a concise overview of the recent advancements in the electrocatalytic OER stability, encompassing both electrocatalyst and device developments. This review aims to succinctly summarize the primary factors influencing OER stability, including morphological/phase change and electrocatalyst dissolution, as well as mechanical detachment, alongside chemical, mechanical, and operational degradation observed in devices. Furthermore, an overview of contemporary approaches to enhance stability is provided, encompassing electrocatalyst design (structural regulation, protective layer coating, and stable substrate anchoring) and device optimization (bipolar plates, gas diffusion layers, and membranes). Hopefully, more attention will be paid to ensuring the stable operation of electrocatalytic OER and the future large-scale water electrolysis applications. This review presents design principles aimed at addressing challenges related to the stability of electrocatalytic OER.

Atomic Manipulation to Create High-Valent Fe4+ for Efficient and Ultrastable Oxygen Evolution at Industrial-Level Current Density

Manipulating the electronic structure of a catalyst at the atomic level is an effective but challenging way to improve the catalytic performance. Here, by stretching the Fe-O bond in FeOOH with an inserted Mo atom, a Fe-O-Mo unit can be created, which will induce the formation of high-valent Fe4+ during the alkaline oxygen evolution reaction (OER). The highly active Fe4+ state has been clearly revealed by in situ X-ray absorption spectroscopy, which can both enhance the oxidation capability and lead to an efficient and stable adsorbate evolution mechanism (AEM) pathway for the OER. As a result, the obtained Fe-Mo-Ni3S2 catalyst exhibits both superior OER activity and outstanding stability, which can achieve an industrial-level current density of 1 A cm-2 at a low overpotential of 259 mV (at 60 °C) and can stably work at the large current for more than 2000 h. Moreover, by coupling with commercial Pt/C, the Fe-Mo-Ni3S2∥Pt/C system can be used in the anion exchange membrane cell to acquire 1 A cm-2 for overall water splitting at 1.68 V (2.03 V for 4 A cm-2), outperforming the benchmark RuO2∥Pt/C system. The efficient, low-cost, and ultrastable OER catalyst enabled by manipulating the atomic structure may provide potential opportunities for future practical water splitting.

Research Progress in Structure Evolution and Durability Modulation of Ir- and Ru-Based OER Catalysts under Acidic Conditions

Green hydrogen energy, as one of the most promising energy carriers, plays a crucial role in addressing energy and environmental issues. Oxygen evolution reaction catalysts, as the key to water electrolysis hydrogen production technology, have been subject to durability constraints, preventing large-scale commercial development. Under the high current density and harsh acid-base electrolyte conditions of the water electrolysis reaction, the active metals in the catalysts are easily converted into high-valent soluble species to dissolve, leading to poor structural durability of the catalysts. There is an urgent need to overcome the durability challenges under acidic conditions and develop electrocatalysts with both high catalytic activity and high durability. In this review, the latest research results are analyzed in depth from both thermodynamic and kinetic perspectives. First, a comprehensive summary of the structural deactivation state process of noble metal oxide catalysts is presented. Second, the evolution of the structure of catalysts possessing high durability is discussed. Finally, four new strategies for the preparation of stable catalysts, "electron buffer (ECB) strategy", combination strength control, strain control, and surface coating, are summarized. The challenges and prospects are also elaborated for the future synthesis of more effective Ru/Ir-based catalysts and boost their future application.

Ultrathin and Conformal Depletion Layer of Core/Shell Heterojunction Enables Efficient and Stable Acidic Water Oxidation

Ru-based electrocatalysts hold great promise for developing affordable proton exchange membrane (PEM) electrolyzers. However, the harsh acidic oxidative environment of the acidic oxygen evolution reaction (OER) often causes undesirable overoxidation of Ru active sites and subsequent serious activity loss. Here, we present an ultrathin and conformal depletion layer attached to the Schottky heterojunction of core/shell RuCo/RuCoOx that not only maximizes the availability of active sites but also improves its durability and intrinsic activity for acidic OER. Operando synchrotron characterizations combined with theoretical calculations elucidate that the lattice strain and charge transfer induced by Schottky heterojunction substantially regulate the electronic structures of active sites, which modulates the OER pathway and suppresses the overoxidation of Ru species. Significantly, the closed core/shell architecture of the RuCo/RuCoOx ensures the structure integrity of the Schottky heterojunction under acidic OER conditions. As a result, the core/shell RuCo/RuCoOx Schottky heterojunction exhibits an unprecedented durability up to 250 0 h at 10 mA cm-2 with an ultralow overpotential of ∼170 mV at 10 mA cm-2 in 0.5 M H2SO4. The RuCo/RuCoOx catalyst also demonstrates superior durability in a proton exchange membrane (PEM) electrolyzer, showcasing the potential for practical applications.

Regulating Co-O covalency to manipulate mechanistic transformation for enhancing activity/durability in acidic water oxidation

Developing earth-abundant electrocatalysts with high activity and durability for acidic oxygen evolution reaction is essential for H2 production, yet it remains greatly challenging. Here, guided by theoretical calculations, the challenge of overcoming the balance between catalytic activity and dynamic durability for acidic OER in Co3O4 was effectively addressed via the preferential substitution of Ru for the Co2+ (Td) site of Co3O4. In situ characterization and DFT calculations show that the enhanced Co-O covalency after the introduction of Ru SAs facilitates the generation of OH* species and mitigates the unstable structure transformation via direct O-O coupling. The designed Ru SAs-CoO x catalyst (5.16 wt% Ru) exhibits enhanced OER activity (188 mV overpotential at 10 mA cm-2) and durability, outperforming most reported Co3O4-based and Ru-based electrocatalysts in acidic media.

Optimal Electrocatalyst Design Strategies for Acidic Oxygen Evolution

Hydrogen, a clean resource with high energy density, is one of the most promising alternatives to fossil. Proton exchange membrane water electrolyzers are beneficial for hydrogen production because of their high current density, facile operation, and high gas purity. However, the large-scale application of electrochemical water splitting to acidic electrolytes is severely limited by the sluggish kinetics of the anodic reaction and the inadequate development of corrosion- and highly oxidation-resistant anode catalysts. Therefore, anode catalysts with excellent performance and long-term durability must be developed for anodic oxygen evolution reactions (OER) in acidic media. This review comprehensively outlines three commonly employed strategies, namely, defect, phase, and structure engineering, to address the challenges within the acidic OER, while also identifying their existing limitations. Accordingly, the correlation between material design strategies and catalytic performance is discussed in terms of their contribution to high activity and long-term stability. In addition, various nanostructures that can effectively enhance the catalyst performance at the mesoscale are summarized from the perspective of engineering technology, thus providing suitable strategies for catalyst design that satisfy industrial requirements. Finally, the challenges and future outlook in the area of acidic OER are presented.

Ruthenium Single-Atom Modulated Protonated Iridium Oxide for Acidic Water Oxidation in Proton Exchange Membrane Electrolysers

Proton exchange membrane water electrolysers promise to usher in a new era of clean energy, but they remain a formidable obstacle in designing active and durable electrocatalysts for the acidic oxygen evolution reaction (OER). In this study, a protonated iridium oxide embedded with single-atom dispersed ruthenium atoms (H3.8Ir1- xRuxO4) that demonstrates exceptional activity and stability in acidic water oxidation is introduced. The single Ru dopants favorably induce localized oxygen vacancies in the Ir─O lattice, synergistically strengthening the adsorption of OOH* intermediates and enhancing the intrinsic OER activity. In addition, the preferential oxidation of Ru and the electronegativity of the oxygen vacancies significantly stabilize the Ir─O active sites, improving the OER stability. Consequently, the H3.8Ir1─ xRuxO4 catalyst shows an overpotential of 255 mV at 10 mA cm-2 and displays exceptional catalytic endurance in acidic electrolytes, surpassing 1100 h, representing a remarkable one-order-of-magnitude increase in stability compared to that of pristine H3.8IrO4. A proton exchange membrane electrolyser utilizing the H3.8Ir1- xRuxO4 catalyst as an anode exhibits stable performance for more than 1280 h under a high current density of 2 A cm-2.

Breaking the Bottleneck of Activity and Stability of RuO2-Based Electrocatalysts for Acidic Oxygen Evolution

Electrochemical acidic oxygen evolution reaction (OER) is an important part for water electrolysis utilizing a proton exchange membrane (PEM) apparatus for industrial H2 production. RuO2 has garnered considerable attention as a potential acidic OER electrocatalyst. However, the overoxidation of Ru active sites under high potential conditions is usually harmful for activity and stability, thereby posing a challenge for large-scale commercialization, which needs effective strategies to circumvent the leaching of Ru and further activate Ru sites. Herein, a Mini-Review is presented to summarize the recent developments regarding the activation and stabilization of the Ru active sites and lattice oxygen through the modulation of the d-band center, coordination environment, bridged heteroatoms, and vacancy engineering, as well as structural protection strategies and reaction pathway optimization to promote the acidic OER activity and stability of RuO2-based electrocatalysts. This Mini-Review offers a profound understanding of the design of RuO2-based electrocatalysts with greatly enhanced acidic OER performances.

Stepwise-Induced Synthesis and Excellent Corrosion Protection of Ce/Eu Codoped ZnO Solid Solution Materials

Under purely inorganic conditions, a synthesis route was devised wherein elements were introduced stepwise via coprecipitation based on differences in compound solubility. This synthesis method can change the microscopic morphology of the material without relying on a templating agent, resulting in the formation of the multilayered lamellar Ce/Eu codoped zinc oxide solid solution (ZCEOSS) with a self-assembled nested imbrication structure. The study improves the critical matter of corrosion by focusing on the electron and energy transfer mechanisms. By introduction of the bandgap modulator cerium element and fluorescence enhancer europium element into the ZnO material, the anticorrosion material has been successfully endowed with both photocathodic protection and luminescent initiative/stress dual corrosion defense functions. Due to the energy level staircase protection mechanism synergistically generated by the 4f electron shell of rare-earth elements in concert with semiconductor zinc oxide, the energy band positions were modulated to progressively guide the direction of electron flow, thereby suppressing corrosion reactions. In particular, the ZCEOSS material synthesized by doping 1% cerium and 7% europium and adding rare-earth elements at pH 7 exhibited the best corrosion inhibition performance. After immersion in simulated seawater for 96 h, Tafel polarization test results showed that compared to epoxy resin and ZnO anticorrosion systems, the ZCEOSS anticorrosion system exhibited significantly improved corrosion inhibition efficiency with enhancements of 1028.3 and 402.9%, respectively. This study provides new insights into the development of highly efficient inorganic anticorrosion materials.

Operando identification of the oxide path mechanism with different dual-active sites for acidic water oxidation

The microscopic reaction pathway plays a crucial role in determining the electrochemical performance. However, artificially manipulating the reaction pathway still faces considerable challenges. In this study, we focus on the classical acidic water oxidation based on RuO2 catalysts, which currently face the issues of low activity and poor stability. As a proof-of-concept, we propose a strategy to create local structural symmetry but oxidation-state asymmetric Mn4-δ-O-Ru4+δ active sites by introducing Mn atoms into RuO2 host, thereby switching the reaction pathway from traditional adsorbate evolution mechanism to oxide path mechanism. Through advanced operando synchrotron spectroscopies and density functional theory calculations, we demonstrate the synergistic effect of dual-active metal sites in asymmetric Mn4-δ-O-Ru4+δ microstructure in optimizing the adsorption energy and rate-determining step barrier via an oxide path mechanism. This study highlights the importance of engineering reaction pathways and provides an alternative strategy for promoting acidic water oxidation.

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.

Cr dopant mediates hydroxyl spillover on RuO2 for high-efficiency proton exchange membrane electrolysis

Simultaneously improving the activity and stability of catalysts for anodic oxygen evolution reaction (OER) in proton exchange membrane water electrolysis (PEMWE) remains a notable challenge. Here, we report a chromium-doped ruthenium dioxide with oxygen vacancies, termed Cr0.2Ru0.8O2-x, that drives OER with an overpotential of 170 mV at 10 mA cm-2 and operates stably over 2000 h in acidic media. Experimental and theoretical studies show that the synergy of Cr dopant and oxygen vacancy induces an unconventional dopant-mediated hydroxyl spillover mechanism. Such dynamic hydroxyl spillover from Cr dopant to Ru active site changes the rate-determining step from OOH* formation to O2 formation and thus greatly improves the OER performance. Moreover, the Cr dopant and oxygen vacancy also play a crucial role in stabilizing surface Ru and lattice oxygen in the Ru-O-Cr structural motif. When assembled into the anode of a practical PEMWE device, Cr0.2Ru0.8O2-x enables long-term durability of over 200 h at an ampere-level current density and 60 degrees centigrade.

Interpretable Data-Driven Descriptors for Establishing the Structure-Activity Relationship of Metal-Organic Frameworks Toward Oxygen Evolution Reaction

The development of readily accessible and interpretable descriptors is pivotal yet challenging in the rational design of metal-organic framework (MOF) catalysts. This study presents a straightforward and physically interpretable activity descriptor for the oxygen evolution reaction (OER), derived from a dataset of bimetallic Ni-based MOFs. Through an artificial-intelligence (AI) data-mining subgroup discovery (SGD) approach, a combination of the d-band center and number of missing electrons in eg states of Ni, as well as the first ionization energy and number of electrons in eg states of the substituents, is revealed as a gene of a superior OER catalyst. The found descriptor, obtained from the AI analysis of a dataset of MOFs containing 3-5d transition metals and 13 organic linkers, has been demonstrated to facilitate in-depth understanding of structure-activity relationship at the molecular orbital level. The descriptor is validated experimentally for 11 Ni-based MOFs. Combining SGD with physical insights and experimental verification, our work offers a highly efficient approach for screening MOF-based OER catalysts, simultaneously providing comprehensive understanding of the catalytic mechanism.

Accelerated Proton Transfer in Asymmetric Active Units for Sustainable Acidic Oxygen Evolution Reaction

The poor durability of Ru-based catalysts limits the practical application in proton exchange membrane water electrolysis (PEMWE). Here, we report that the asymmetric active units in Ru1-xMxO2 (M = Sb, In, and Sn) binary solid solution oxides are constructed by introducing acid-resistant p-block metal sites, breaking the activity and stability limitations of RuO2 in acidic oxygen evolution reaction (OER). Constructing highly asymmetric Ru-O-Sb units with a strong electron delocalization effect significantly shortens the spatial distance between Ru and Sb sites, improving the bonding strength of the overall structure. The unique two-electron redox couples at Sb sites in asymmetric active units trigger additional chemical steps at different OER stages, facilitating continuous proton transfer. The optimized Ru0.8Sb0.2O2 solid solution requires a superlow overpotential of 160 mV at 10 mA cm-2 and a record-breaking stability of 1100 h in an acidic electrolyte. Notably, the scale-prepared Ru0.8Sb0.2O2 achieves efficient PEMWE performance under industrial conditions. General mechanism analysis shows that the enhanced proton transport in the asymmetric Ru-O-M unit provides a new working pathway for acidic OER, breaking the scaling relationship without sacrificing stability.

Oxyanion Engineering on RuO2 for Efficient Proton Exchange Membrane Water Electrolysis

In acidic proton exchange membrane water electrolysis (PEMWE), the anode oxygen evolution reaction (OER) catalysts rely heavily on the expensive and scarce iridium-based materials. Ruthenium dioxide (RuO2) with lower price and higher OER activity, has been explored for the similar task, but has been restricted by the poor stability. Herein, we developed an anion modification strategy to improve the OER performance of RuO2 in acidic media. The designed multicomponent catalyst based on sulfate anchored on RuO2/MoO3 displays a low overpotential of 190 mV at 10 mA cm-2 and stably operates for 500 hours with a very low degradation rate of 20 μV h-1 in acidic electrolyte. When assembled in a PEMWE cell, this catalyst as an anode shows an excellent stability at 500 mA cm-2 for 150 h. Experimental and theoretical results revealed that MoO3 could stabilize sulfate anion on RuO2 surface to suppress its leaching during OER. Such MoO3-anchored sulfate not only reduces the formation energy of *OOH intermediate on RuO2, but also impedes both the surface Ru and lattice oxygen loss, thereby achieving the high OER activity and exceptional durability.

Amorphous MnRuOx Containing Microcrystalline for Enhanced Acidic Oxygen-Evolution Activity and Stability

Compared to Ir, Ru-based catalysts often exhibited higher activity but suffered significant and rapid activity loss during the challenging oxygen evolution reaction (OER) in a corrosive acidic environment. Herein, we developed a hybrid MnRuOx catalyst in which the RuO2 microcrystalline regions serve as a supporting framework, and the amorphous MnRuOx phase fills the microcrystalline interstices. In particular, the MnRuOx-300 catalyst from an annealing temperature of 300 °C contains an optimal amorphous/crystalline heterostructure, providing substantial defects and active sites, facilitating efficient adsorption and conversion of OH-. In addition, the heterostructure leads to a relative increase of the d-band center close to the Fermin level, thus accelerating electron transfer with reduced charge transfer resistance at the active interface between crystalline and amorphous phases during the OER. The catalyst was further thoroughly evaluated under various operating conditions and demonstrated exceptional activity and stability for the OER, representing a promising solution to replace Ir in water electrolyzers.

Understanding the catalytic performances of metal-doped Ta2O5 catalysts for acidic oxygen evolution reaction with computations

The design of stable and active alternative catalysts to iridium oxide for the anodic oxygen evolution reaction (OER) has been a long pursuit in acidic water splitting. Tantalum pentoxide (Ta2O5) has the merit of great acidic stability but poor OER performance, yet strategies to improve its intrinsic OER activity are highly desirable. Herein, by using density functional theory (DFT) calculations combined with aqueous stability assessment from surface Pourbaix diagrams, we systematically evaluated the OER activity and acidic stability of 14 different metal-doped Ta2O5 catalysts. Apart from the experimentally reported Ir-doped Ta2O5, we computationally identified Ru- and Nb-doped Ta2O5 catalysts as another two candidates with reasonably high stability and activity in acidic OER. Our study also underscores the essence of considering stable surface states of catalysts under working conditions before a reasonable activity trend can be computationally achieved.

Advances in Noble Metal Electrocatalysts for Acidic Oxygen Evolution Reaction: Construction of Under-Coordinated Active Sites

Renewable energy-driven proton exchange membrane water electrolyzer (PEMWE) attracts widespread attention as a zero-emission and sustainable technology. Oxygen evolution reaction (OER) catalysts with sluggish OER kinetics and rapid deactivation are major obstacles to the widespread commercialization of PEMWE. To date, although various advanced electrocatalysts have been reported to enhance acidic OER performance, Ru/Ir-based nanomaterials remain the most promising catalysts for PEMWE applications. Therefore, there is an urgent need to develop efficient, stable, and cost-effective Ru/Ir catalysts. Since the structure-performance relationship is one of the most important tools for studying the reaction mechanism and constructing the optimal catalytic system. In this review, the recent research progress from the construction of unsaturated sites to gain a deeper understanding of the reaction and deactivation mechanism of catalysts is summarized. First, a general understanding of OER reaction mechanism, catalyst dissolution mechanism, and active site structure is provided. Then, advances in the design and synthesis of advanced acidic OER catalysts are reviewed in terms of the classification of unsaturated active site design, i.e., alloy, core-shell, single-atom, and framework structures. Finally, challenges and perspectives are presented for the future development of OER catalysts and renewable energy technologies for hydrogen production.

Developing Catalysts for Membrane Electrode Assemblies in High Performance Polymer Electrolyte Membrane Water Electrolyzers

Extensive research is underway to achieve carbon neutrality through the production of green hydrogen via water electrolysis, powered by renewable energy. Polymer membrane water electrolyzers, such as proton exchange membrane water electrolyzer (PEMWE) and anion exchange membrane water electrolyzer (AEMWE), are at the forefront of this research. Developing highly active and durable electrode catalysts is crucial for commercializing these electrolyzers. However, most research is conducted in half-cell setups, which may not fully represent the catalysts' effectiveness in membrane-electrode-assembly (MEA) devices. This review explores the catalysts developed for high-performance PEMWE and AEMWE MEA systems. Only the catalysts reporting on the MEA performance were discussed in this review. In PEMWE, strategies aim to minimize Ir use for the oxygen evolution reaction (OER) by maximizing activity, employing metal oxide-based supports, integrating secondary elements into IrOx lattices, or exploring non-Ir materials. For AEMWE, the emphasis is on enhancing the performance of NiFe-based and Co-based catalysts by improving electrical conductivity and mass transport. Pt-based and Ni-based catalysts for the hydrogen evolution reaction (HER) in AEMWE are also examined. Additionally, this review discusses the unique considerations for catalysts operating in pure water within AEMWE systems.

Grain-Boundary-Rich RuO2 Porous Nanosheet for Efficient and Stable Acidic Water Oxidation

RuO2 has been considered as the most likely acidic oxygen evolution reaction (OER) catalyst to replace IrO2, but its performance, especially long-term stability under harsh acidic conditions, is still unacceptable. Here, we propose a grain boundary (GB) engineering strategy by fabricating the ultrathin porous RuO2 nanosheet with abundant of grain boundaries (GB-RuO2) as an efficient acid OER catalyst. The involvement of GB induces significant tensile stress and creates an unsaturated coordination environment, effectively optimizing the adsorption of intermediates and stabilizing active site structure during OER process. Notably, the GB-RuO2 not only exhibits a low overpotential (η10=187 mV) with an ultra-low Tafel slope (34.5 mV dec-1), but also steadily operates for over 550 h in 0.1 M HClO4. Quasi in situ/operando methods confirm that the improved stability is attributed to GB preventing Ru dissolution and greatly inhibiting the lattice oxygen oxidation mechanism (LOM). A proton exchange membrane water electrolysis (PEMWE) using the GB-RuO2 catalyst operates a low voltage of 1.669 V at 2 A cm-2 and operates stably for 100 h at 100 mA cm-2.

Lanthanide-regulating Ru-O covalency optimizes acidic oxygen evolution electrocatalysis

Precisely modulating the Ru-O covalency in RuOx for enhanced stability in proton exchange membrane water electrolysis is highly desired. However, transition metals with d-valence electrons, which were doped into or alloyed with RuOx, are inherently susceptible to the influence of coordination environment, making it challenging to modulate the Ru-O covalency in a precise and continuous manner. Here, we first deduce that the introduction of lanthanide with gradually changing electronic configurations can continuously modulate the Ru-O covalency owing to the shielding effect of 5s/5p orbitals. Theoretical calculations confirm that the durability of Ln-RuOx following a volcanic trend as a function of Ru-O covalency. Among various Ln-RuOx, Er-RuOx is identified as the optimal catalyst and possesses a stability 35.5 times higher than that of RuO2. Particularly, the Er-RuOx-based device requires only 1.837 V to reach 3 A cm-2 and shows a long-term stability at 500 mA cm-2 for 100 h with a degradation rate of mere 37 μV h-1.

Designing 3d Transition Metal Cation-Doped MRuOx As Durable Acidic Oxygen Evolution Electrocatalysts for PEM Water Electrolyzers

The continuous dissolution and oxidation of active sites in Ru-based electrocatalysts have greatly hindered their practical application in proton exchange membrane water electrolyzers (PEMWE). In this work, we first used density functional theory (DFT) to calculate the dissolution energy of Ru in the 3d transition metal-doped MRuOx (M = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn) to evaluate their stability for acidic oxygen evolution reaction (OER) and screen out ZnRuOx as the best candidate. To confirm the theoretical predictions, we experimentally synthesized these MRuOx materials and found that ZnRuOx indeed displays robust acidic OER stability with a negligible decay of η10 after 15 000 CV cycles. Of importance, using ZnRuOx as the anode, the PEMWE can run stably for 120 h at 200 mA cm-2. We also further uncover the stability mechanism of ZnRuOx, i.e., Zn atoms doped in the outside of ZnRuOx nanocrystal would form a "Zn-rich" shell, which effectively shortened average Ru-O bond lengths in ZnRuOx to strengthen the Ru-O interaction and therefore boosted intrinsic stability of ZnRuOx in acidic OER. In short, this work not only provides a new study paradigm of using DFT calculations to guide the experimental synthesis but also offers a proof-of-concept with 3d metal dopants as RuO2 stabilizer as a universal principle to develop high-durability Ru-based catalysts for PEMWE.

Ru/Ir-Based Electrocatalysts for Oxygen Evolution Reaction in Acidic Conditions: From Mechanisms, Optimizations to Challenges

The generation of green hydrogen by water splitting is identified as a key strategic energy technology, and proton exchange membrane water electrolysis (PEMWE) is one of the desirable technologies for converting renewable energy sources into hydrogen. However, the harsh anode environment of PEMWE and the oxygen evolution reaction (OER) involving four-electron transfer result in a large overpotential, which limits the overall efficiency of hydrogen production, and thus efficient electrocatalysts are needed to overcome the high overpotential and slow kinetic process. In recent years, noble metal-based electrocatalysts (e.g., Ru/Ir-based metal/oxide electrocatalysts) have received much attention due to their unique catalytic properties, and have already become the dominant electrocatalysts for the acidic OER process and are applied in commercial PEMWE devices. However, these noble metal-based electrocatalysts still face the thorny problem of conflicting performance and cost. In this review, first, noble metal Ru/Ir-based OER electrocatalysts are briefly classified according to their forms of existence, and the OER catalytic mechanisms are outlined. Then, the focus is on summarizing the improvement strategies of Ru/Ir-based OER electrocatalysts with respect to their activity and stability over recent years. Finally, the challenges and development prospects of noble metal-based OER electrocatalysts are discussed.

Ruthenium-Based Binary Alloy with Oxide Nanosheath for Highly Efficient and Stable Oxygen Evolution Reaction in Acidic Media

Traditional noble metal oxide, such as RuO2, is considered a benchmark catalyst for acidic oxygen evolution reaction (OER). However, its practical application is limited due to sluggish activity and severe electrochemical corrosion. In this study, Ru-Fe nanoparticles loading on carbon felt (RuFe@CF) is synthesized via an ultrafast Joule heating method as an active and durable OER catalyst in acidic conditions. Remarkably low overpotentials of 188 and 269 mV are achieved at 10 and 100 mA cm-2, respectively, with a robust stability up to 620 h at 10 mA cm-2. When used as an anode in a proton exchange membrane water electrolyzer, the catalyst shows more than 250 h of stability at a water-splitting current of 200 mA cm-2. Experimental characterizations reveal the presence of a Ru-based oxide nanosheath on the surface of the catalyst during OER tests, suggesting a surface reconstruction process that enhances the intrinsic activity and inhibits continuous metal dissolution. Moreover, density functional theory calculations demonstrate that the introduction of Fe into the RuFe@CF catalyst reduces the energy barrier and boosts its activities. This work offers an effective and universal strategy for the development of highly efficient and stable catalysts for acidic water splitting.

Dopamine-modified cobalt spinel nanoparticles as an active catalyst for the acidic oxygen evolution reaction

The development of efficient non-noble metal electrocatalysts for the oxygen evolution reaction (OER) under acidic conditions remains a critical challenge. Herein, we report a N-doped carbonaceous component-engineered Co3O4 (NCEC) catalyst synthesized via the sol-gel method. Dopamine hydrochloride (DA)-derived nitrogen-doped carbonaceous components were found to boost the OER performance of Co3O4. The optimized catalyst can reach an overpotential as low as 330 mV in 1 M H2SO4 at a current density of 10 mA cm-2 and maintains a good long-term stability of 60 hours. In particular, we found that the thermodynamic overpotential was inversely proportional to the content of oxidized N and pyridinic N, whereas it was directly proportional to the pyrrolic-N content. Our experiments and density functional theory (DFT) calculations confirm that the optimized catalyst exhibits enhanced charge transfer and the oxidized N species on Co3O4 is responsible for the high catalytic activity. Our study suggests that the performance of NCEC in acidic media can be further optimized by enhancing the content of oxidized N species.

Enhancing Ni/Co Activity by Neighboring Pt Atoms in NiCoP/MXene Electrocatalyst for Alkaline Hydrogen Evolution

Density functional theory (DFT) calculations demonstrate neighboring Pt atoms can enhance the metal activity of NiCoP for hydrogen evolution reaction (HER). However, it remains a great challenge to link Pt and NiCoP. Herein, we introduced curvature of bowl-like structure to construct Pt/NiCoP interface by adding a minimal 1 ‰-molar-ratio Pt. The as-prepared sample only requires an overpotential of 26.5 and 181.6 mV to accordingly achieve the current density of 10 and 500 mA cm-2 in 1 M KOH. The water dissociation energy barrier (Ea) has a ~43 % decrease compared with NiCoP counterpart. It also shows an ultrahigh stability with a small degradation rate of 10.6 μV h-1 at harsh conditions (500 mA cm-2 and 50 °C) after 3000 hrs. X-ray photoelectron spectroscopy (XPS), soft X-ray absorption spectroscopy (sXAS), and X-ray absorption fine structure (XAFS) verify the interface electron transfer lowers the valence state of Co/Ni and activates them. DFT calculations also confirm the catalytic transition step of NiCoP can change from Heyrovsky (2.71 eV) to Tafel step (0.51 eV) in the neighborhood of Pt, in accord with the result of the improved Hads at the interface disclosed by in situ electrochemical impedance spectroscopy (EIS) and scanning electrochemical microscopy (SECM) tests.

Bicontinuous RuO2 nanoreactors for acidic water oxidation

Improving activity and stability of Ruthenium (Ru)-based catalysts in acidic environments is eager to replace more expensive Iridium (Ir)-based materials as practical anode catalyst for proton-exchange membrane water electrolyzers (PEMWEs). Here, a bicontinuous nanoreactor composed of multiscale defective RuO2 nanomonomers (MD-RuO2-BN) is conceived and confirmed by three-dimensional tomograph reconstruction technology. The unique bicontinuous nanoreactor structure provides abundant active sites and rapid mass transfer capability through a cavity confinement effect. Besides, existing vacancies and grain boundaries endow MD-RuO2-BN with generous low-coordination Ru atoms and weakened Ru-O interaction, inhibiting the oxidation of lattice oxygen and dissolution of high-valence Ru. Consequently, in acidic media, the electron- and micro-structure synchronously optimized MD-RuO2-BN achieves hyper water oxidation activity (196 mV @ 10 mA cm-2) and an ultralow degradation rate of 1.2 mV h-1. A homemade PEMWE using MD-RuO2-BN as anode also conveys high water splitting performance (1.64 V @ 1 A cm-2). Theoretical calculations and in-situ Raman spectra further unveil the electronic structure of MD-RuO2-BN and the mechanism of water oxidation processes, rationalizing the enhanced performance by the synergistic effect of multiscale defects and protected active Ru sites.

Design of Superior Electrocatalysts for Proton-Exchange Membrane-Water Electrolyzers: Importance of Catalyst Stability and Evolution

By virtue of the high energy conversion efficiency and compact facility, proton exchange membrane water electrolysis (PEMWE) is a promising green hydrogen production technology ready for commercial applications. However, catalyst stability is a challenging but often-ignored topic for the electrocatalyst design, which retards the device applications of many newly-developed electrocatalysts. By defining catalyst stability as the function of activity versus time, we ascribe the stability issue to the evolution of catalysts or catalyst layers during the water electrolysis. We trace the instability sources of electrocatalysts as the function versus time for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in acid and classify them into internal and external sources. Accordingly, we summarize the latest studies for stability improvements into five strategies, i. e., thermodynamic stable active site construction, precatalyst design, support regulation, superwetting electrode fabrication, and catalyst-ionomer interface engineering. With the help of ex-situ/ in-situ characterizations and theoretical calculations, an in-depth understanding of the instability sources benefits the rational development of highly active and stable HER/OER electrocatalysts for PEMWE applications.

Quantitative measurement and comparison of breakthroughs inside the gas diffusion layer using lattice Boltzmann method and computed tomography scan

In Proton Exchange Membrane Fuel Cells (PEMFCs), the presence of residual water within the Gas Diffusion Layer (GDL) poses challenges during cold starts and accelerates degradation. A computational model based on the Lattice Boltzmann Method (LBM) was developed to consider the capillary pressure inside the PEMFC and to analyze the exact geometries of the GDLs, which were obtained using the Computed Tomography scan. The novelty of this study is to suggest a methodology to compare the quantitative water removal performance of the GDLs without long-term experimental testing. Two different samples of GDLs were considered, pristine and aged. The results of quantitative measurements revealed the amount of water columns (breakthroughs) inside each sample. Considering the volume of 12,250,000 µm3 for each sample, the pristine and the aged samples are prone to have 774,200 µm3 (6.32%) and 1,239,700 µm3 (10.12%) as water columns in their porous domain. Micro-structural properties such as connectivity, mean diameter, effective diffusivity, etc. were also compared to observe the impacts of aging on the properties of the GDL.

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.

Locking the lattice oxygen in RuO2 to stabilize highly active Ru sites in acidic water oxidation

Ruthenium dioxide is presently the most active catalyst for the oxygen evolution reaction (OER) in acidic media but suffers from severe Ru dissolution resulting from the high covalency of Ru-O bonds triggering lattice oxygen oxidation. Here, we report an interstitial silicon-doping strategy to stabilize the highly active Ru sites of RuO2 while suppressing lattice oxygen oxidation. The representative Si-RuO2-0.1 catalyst exhibits high activity and stability in acid with a negligible degradation rate of ~52 μV h-1 in an 800 h test and an overpotential of 226 mV at 10 mA cm-2. Differential electrochemical mass spectrometry (DEMS) results demonstrate that the lattice oxygen oxidation pathway of the Si-RuO2-0.1 was suppressed by ∼95% compared to that of commercial RuO2, which is highly responsible for the extraordinary stability. This work supplied a unique mentality to guide future developments on Ru-based oxide catalysts' stability in an acidic environment.

Surface engineering for stable electrocatalysis

In recent decades, significant progress has been achieved in rational developments of electrocatalysts through constructing novel atomistic structures and modulating catalytic surface topography, realizing substantial enhancement in electrocatalytic activities. Numerous advanced catalysts were developed for electrochemical energy conversion, exhibiting low overpotential, high intrinsic activity, and selectivity. Yet, maintaining the high catalytic performance under working conditions with high polarization and vigorous microkinetics that induce intensive degradation of surface nanostructures presents a significant challenge for commercial applications. Recently, advanced operando and computational techniques have provided comprehensive mechanistic insights into the degradation of surficial functional structures. Additionally, various innovative strategies have been devised and proven effective in sustaining electrocatalytic activity under harsh operating conditions. This review aims to discuss the most recent understanding of the degradation microkinetics of catalysts across an entire range of anodic to cathodic polarizations, encompassing processes such as oxygen evolution and reduction, hydrogen reduction, and carbon dioxide reduction. Subsequently, innovative strategies adopted to stabilize the materials' structure and activity are highlighted with an in-depth discussion of the underlying rationale. Finally, we present conclusions and perspectives regarding future research and development. By identifying the research gaps, this review aims to inspire further exploration of surface degradation mechanisms and rational design of durable electrocatalysts, ultimately contributing to the large-scale utilization of electroconversion technologies.

Strain-modulated Ru-O Covalency in Ru-Sn Oxide Enabling Efficient and Stable Water Oxidation in Acidic Solution

RuO2 is one of the benchmark electrocatalysts used as the anode material in proton exchange membrane water electrolyser. However, its long-term stability is compromised due to the participation of lattice oxygen and metal dissolution during oxygen evolution reaction (OER). In this work, weakened covalency of Ru-O bond was tailored by introducing tensile strain to RuO6 octahedrons in a binary Ru-Sn oxide matrix, prohibiting the participation of lattice oxygen and the dissolution of Ru, thereby significantly improving the long-term stability. Moreover, the tensile strain also optimized the adsorption energy of intermediates and boosted the OER activity. Remarkably, the RuSnOx electrocatalyst exhibited excellent OER activity in 0.1 M HClO4 and required merely 184 mV overpotential at a current density of 10 mA cm-2 . Moreover, it delivered a current density of 10 mA cm-2 for at least 150 h with negligible potential increase. This work exemplifies an effective strategy for engineering Ru-based catalysts with extraordinary performance toward water splitting.

Rare Earth Evoked Subsurface Oxygen Species in Platinum Alloy Catalysts Enable Durable Fuel Cells

Alleviating the degradation issue of Pt based alloy catalysts, thereby simultaneously achieving high mass activity and high durability in proton exchange membrane fuel cells (PEMFCs), is highly challenging. Herein, we provide a new paradigm to address this issue via delaying the place exchange between adsorbed oxygen species and surface Pt atoms, thereby inhibiting Pt dissolution, through introducing rare earth bonded subsurface oxygen atoms. We have succeeded in introducing Gd-O dipoles into Pt3 Ni via a high temperature entropy-driven process, with direct spectral evidence attained from both soft and hard X-ray absorption spectroscopies. The higher rated power of 0.93 W cm-2 and superior current density of 562.2 mA cm-2 at 0.8 V than DOE target for heavy-duty vehicles in H2 -air mode suggest the great potential of Gd-O-Pt3 Ni towards practical application in heavy-duty transportation. Moreover, the mass activity retention (1.04 A mgPt -1 ) after 40 k cycles accelerated durability tests is even 2.4 times of the initial mass activity goal for DOE 2025 (0.44 A mgPt -1 ), due to the weakened Pt-Oads bond interaction and the delayed place exchange process, via repulsive forces between surface O atoms and those in the sublayer. This work addresses the critical roadblocks to the widespread adoption of PEMFCs.

Stabilizing Highly Active Ru Sites by Electron Reservoir in Acidic Oxygen Evolution

Proton exchange membrane water electrolysis is hindered by the sluggish kinetics of the anodic oxygen evolution reaction. RuO2 is regarded as a promising alternative to IrO2 for the anode catalyst of proton exchange membrane water electrolyzers due to its superior activity and relatively lower cost compared to IrO2. However, the dissolution of Ru induced by its overoxidation under acidic oxygen evolution reaction (OER) conditions greatly hinders its durability. Herein, we developed a strategy for stabilizing RuO2 in acidic OER by the incorporation of high-valence metals with suitable ionic electronegativity. A molten salt method was employed to synthesize a series of high-valence metal-substituted RuO2 with large specific surface areas. The experimental results revealed that a high content of surface Ru4+ species promoted the OER intrinsic activity of high-valence doped RuO2. It was found that there was a linear relationship between the ratio of surface Ru4+/Ru3+ species and the ionic electronegativity of the dopant metals. By regulating the ratio of surface Ru4+/Ru3+ species, incorporating Re, with the highest ionic electronegativity, endowed Re0.1Ru0.9O2 with exceptional OER activity, exhibiting a low overpotential of 199 mV to reach 10 mA cm-2. More importantly, Re0.1Ru0.9O2 demonstrated outstanding stability at both 10 mA cm-2 (over 300 h) and 100 mA cm-2 (over 25 h). The characterization of post-stability Re0.1Ru0.9O2 revealed that Re promoted electron transfer to Ru, serving as an electron reservoir to mitigate excessive oxidation of Ru sites during the OER process and thus enhancing OER stability. We conclude that Re, with the highest ionic electronegativity, attracted a mass of electrons from Ru in the pre-catalyst and replenished electrons to Ru under the operating potential. This work spotlights an effective strategy for stabilizing cost-effective Ru-based catalysts for acidic OER.

The Promising Seesaw Relationship Between Activity and Stability of Ru-Based Electrocatalysts for Acid Oxygen Evolution and Proton Exchange Membrane Water Electrolysis

The development of electrocatalysts that are not reliant on iridium for efficient acid-oxygen evolution is a critical step towards the proton exchange membrane water electrolysis (PEMWE) and green hydrogen industry. Ruthenium-based electrocatalysts have garnered widespread attention due to their remarkable catalytic activity and lower commercial price. However, the challenge lies in balancing the seesaw relationship between activity and stability of these electrocatalysts during the acid-oxygen evolution reaction (OER). This review delves into the progress made in Ru-based electrocatalysts with regards to acid OER and PEMWE applications. It highlights the significance of customizing the acidic OER mechanism of Ru-based electrocatalysts through the coordination of adsorption evolution mechanism (AEM) and lattice oxygen oxidation mechanism (LOM) to attain the ideal activity and stability relationship. The promising tradeoffs between the activity and stability of different Ru-based electrocatalysts, including Ru metals and alloys, Ru single-atomic materials, Ru oxides, and derived complexes, and Ru-based heterojunctions, as well as their applicability to PEMWE systems, are discussed in detail. Furthermore, this paper offers insights on in situ control of Ru active sites, dynamic catalytic mechanism, and commercial application of PEMWE. Based on three-way relationship between cost, activity, and stability, the perspectives and development are provided.

RuO2 Catalysts for Electrocatalytic Oxygen Evolution in Acidic Media: Mechanism, Activity Promotion Strategy and Research Progress

Proton Exchange Membrane Water Electrolysis (PEMWE) under acidic conditions outperforms alkaline water electrolysis in terms of less resistance loss, higher current density, and higher produced hydrogen purity, which make it more economical in long-term applications. However, the efficiency of PEMWE is severely limited by the slow kinetics of anodic oxygen evolution reaction (OER), poor catalyst stability, and high cost. Therefore, researchers in the past decade have made great efforts to explore cheap, efficient, and stable electrode materials. Among them, the RuO2 electrocatalyst has been proved to be a major promising alternative to Ir-based catalysts and the most promising OER catalyst owing to its excellent electrocatalytic activity and high pH adaptability. In this review, we elaborate two reaction mechanisms of OER (lattice oxygen mechanism and adsorbate evolution mechanism), comprehensively summarize and discuss the recently reported RuO2-based OER electrocatalysts under acidic conditions, and propose many advanced modification strategies to further improve the activity and stability of RuO2-based electrocatalytic OER. Finally, we provide suggestions for overcoming the challenges faced by RuO2 electrocatalysts in practical applications and make prospects for future research. This review provides perspectives and guidance for the rational design of highly active and stable acidic OER electrocatalysts based on PEMWE.

Assessing Challenges of 2D-Molybdenum Ditelluride for Efficient Hydrogen Generation in a Full-Scale Proton Exchange Membrane (PEM) Water Electrolyzer

Proton exchange membrane (PEM) water electrolyzers are critical enablers for sustainable green hydrogen production due to their high efficiency. However, nonplatinum catalysts are rarely evaluated under actual electrolyzer operating conditions, limiting knowledge of their feasibility for H2 production at scale. In this work, metallic 1T'-MoTe2 films were synthesized on carbon cloth supports via chemical vapor deposition and tested as cathodes in PEM electrolysis. Initial three-electrode tests revealed that at 100 mA cm-2, the overpotential of 1T'-MoTe2 approached that of leading 1T'-MoS2 systems, confirming its promise as a hydrogen evolution catalyst. However, when tested in a full-scale PEM electrolyzer, 1T'-MoTe2 delivered only 150 mA cm-2 at 2 V, far below expectations. Postelectrolysis analysis revealed an unexpected passivating tellurium layer, likely inhibiting catalytic sites. While initially promising, the unanticipated passivation caused 1T'-MoTe2 to underperform in practice. This highlights the critical need to evaluate emerging electrolyzer catalysts in PEM electrolyzers, revealing limitations of the idealized three-electrode configuration. Moving forward, validation of model systems in actual electrolyzers will be key to identifying robust nonplatinum catalysts for sustainable green hydrogen production.

Breaking the Ru-O-Ru Symmetry of a RuO2 Catalyst for Sustainable Acidic Water Oxidation

Proton exchange membrane water electrolysis is a highly promising hydrogen production technique for sustainable energy supply, however, achieving a highly active and durable catalyst for acidic water oxidation still remains a formidable challenge. Herein, we propose a local microenvironment regulation strategy for precisely tuning In-RuO2 /graphene (In-RuO2 /G) catalyst with intrinsic electrochemical activity and stability to boost acidic water oxidation. The In-RuO2 /G displays robust acid oxygen evolution reaction performance with a mass activity of 671 A gcat -1 at 1.5 V, an overpotential of 187 mV at 10 mA cm-2 , and long-lasting stability of 350 h at 100 mA cm-2 , which arises from the asymmetric Ru-O-In local structure interactions. Further, it is unraveled theoretically that the asymmetric Ru-O-In structure breaks the thermodynamic activity limit of the traditional adsorption evolution mechanism which significantly weakens the formation energy barrier of OOH*, thus inducing a new rate-determining step of OH* absorption. Therefore, this strategy showcases the immense potential for constructing high-performance acidic catalysts for water electrolyzers.

Tuning Coordination Structures of Zn Sites Through Symmetry-Breaking Accelerates Electrocatalysis

Manipulating the coordination environment of individual active sites in a precise manner remains an important challenge in electrocatalytic reactions. Herein, inspired by theoretical predictions, a facile procedure to synthesize a series of symmetry-breaking zinc metal-organic framework (Zn-MOF) catalysts with well-defined structures is presented. Benefiting from the optimized coordination microenvironment regulated by symmetry-breaking, Zn-N2 S2 -MOF exhibits the best performance of nitrogen (N2 ) reduction reaction (NRR) with NH3 yield rate of 25.07 ± 1.57 µg h-1  cm-2 and Faradaic efficiency of 44.57 ± 2.79% compared with reported Zn-based NRR catalysts. X-ray absorption near-edge structure shows that the symmetry-breaking distorts the coordination environment and modulates the delocalized electrons around the Zn sites, which favors the formation of unpaired low-valence Znδ+ , thereby facilitating the adsorption/activation of N2 . Theoretical calculations elucidate that low-valence Znδ+ in Zn-N2 S2 -MOF can effectively lower the energy barrier of potential determining step, promoting the kinetics and boosting the NRR activity. This work highlights the relationship between the precise coordination environment of metal sites and the catalytic activity, which offers insightful guidance for rationally designing high-efficiency electrocatalysts.

Stabilizing ruthenium dioxide with cation-anchored sulfate for durable oxygen evolution in proton-exchange membrane water electrolyzers

Ruthenium dioxide is the most promising alternative to the prevailing but expensive iridium-based catalysts for the oxygen evolution reaction in proton-exchange membrane water electrolyzers. However, the under-coordinated lattice oxygen of ruthenium dioxide is prone to over-oxidation, and oxygen vacancies are formed at high oxidation potentials under acidic corrosive conditions. Consequently, ruthenium atoms adjacent to oxygen vacancies are oxidized into soluble high-valence derivatives, causing the collapse of the ruthenium dioxide crystal structure and leading to its poor stability. Here, we report an oxyanion protection strategy to prevent the formation of oxygen vacancies on the ruthenium dioxide surface by forming coordination-saturated lattice oxygen. Combining density functional theory calculations, electrochemical measurements, and a suite of operando spectroscopies, we showcase that barium-anchored sulfate can greatly impede ruthenium loss and extend the lifetime of ruthenium-based catalysts during acidic oxygen evolution, while maintaining the activity. This work paves a new way for designing stable and active anode catalysts toward acidic water splitting.

Valence Oscillation of Ru Active Sites for Efficient and Robust Acidic Water Oxidation

The continuous oxidation and leachability of active sites in Ru-based catalysts hinder practical application in proton-exchange membrane water electrolyzers (PEMWE). Herein, robust inter-doped tungsten-ruthenium oxide heterostructures [(Ru-W)Ox ] fabricated by sequential rapid oxidation and metal thermomigration processes are proposed to enhance the activity and stability of acidic oxygen evolution reaction (OER). The introduction of high-valent W species induces the valence oscillation of the Ru sites during OER, facilitating the cyclic transition of the active metal oxidation states and maintaining the continuous operation of the active sites. The preferential oxidation of W species and electronic gain of Ru sites in the inter-doped heterostructure significantly stabilize RuOx on WOx substrates beyond the Pourbaix stability limit of bare RuO2 . Furthermore, the asymmetric Ru-O-W active units are generated around the heterostructure interface to adsorb the oxygen intermediates synergistically, enhancing the intrinsic OER activity. Consequently, the inter-doped (Ru-W)Ox heterostructures not only demonstrate an overpotential of 170 mV at 10 mA cm-2 and excellent stability of 300 h in acidic electrolytes but also exhibit the potential for practical applications, as evidenced by the stable operation at 0.5 A cm-2 for 300 h in PEMWE.

Unveiling the Oxygen Evolution Mechanism with FeNi (Hydr)oxide under Neutral Conditions

Large-scale solar-driven water splitting is a way to store energy, but it requires the development of practical and durable oxygen evolution reaction (OER) catalysts. The present paper aims to investigate the mechanism of the OER, local pH, high-valent metal ions, limitations, conversions, and details during the OER in the presence of FeNi foam using in situ surface-enhanced Raman spectroscopy. This research also explores the use of in situ surface-enhanced Raman spectroscopy for detecting species on foam surfaces during the OER. The acidic media around the electrode not only limit the process but also affect the phosphate ion protonation and overall catalysis effectiveness. The study proposes that FeNi hydroxides serve as true catalysts for OER under neutral conditions, rather than FeNi phosphates. However, phosphate species remain crucial for proton transfer and water molecule adsorption. Changes observed in pH at the open-circuit potential suggest new insights concerning the coordination of Ni(II) to phosphate ions under certain conditions. By extrapolating the Tafel plot, the overpotential for the onset of OER was determined to be 470 mV. Furthermore, the overpotentials for current densities of 1 and 5 mA/cm2 were 590 and 790 mV, respectively. These findings offer valuable insights into the advancement of the OER catalysts and our understanding of the underlying mechanism for efficient water splitting; both are crucial elements for the purpose of energy storage.

Stable and oxidative charged Ru enhance the acidic oxygen evolution reaction activity in two-dimensional ruthenium-iridium oxide

The oxygen evolution reactions in acid play an important role in multiple energy storage devices. The practical promising Ru-Ir based catalysts need both the stable high oxidation state of the Ru centers and the high stability of these Ru species. Here, we report stable and oxidative charged Ru in two-dimensional ruthenium-iridium oxide enhances the activity. The Ru0.5Ir0.5O2 catalyst shows high activity in acid with a low overpotential of 151 mV at 10 mA cm-2, a high turnover frequency of 6.84 s-1 at 1.44 V versus reversible hydrogen electrode and good stability (618.3 h operation). Ru0.5Ir0.5O2 catalysts can form more Ru active sites with high oxidation states at lower applied voltages after Ir incorporation, which is confirmed by the pulse voltage induced current method. Also, The X-ray absorption spectroscopy data shows that the Ru-O-Ir local structure in two-dimensional Ru0.5Ir0.5O2 solid solution improved the stability of these Ru centers.

Direct Dioxygen Radical Coupling Driven by Octahedral Ruthenium-Oxygen-Cobalt Collaborative Coordination for Acidic Oxygen Evolution Reaction

The acidic oxygen evolution reaction (OER) has long been the bottleneck of proton exchange membrane water electrolyzers given its harsh oxidative and corrosive environments. Herein, we suggest an effective strategy to greatly enhance both the acidic OER activity and stability of Co3O4 spinel by atomic Ru selective substitution on the octahedral Co sites. The resulting highly symmetrical octahedral Ru-O-Co collaborative coordination with strong electron coupling effect enables the direct dioxygen radical coupling OER pathway. Indeed, both experiments and theoretical calculations reveal a thermodynamically breakthrough heterogeneous diatomic oxygen mechanism. Additionally, the active Ru-O-Co units are well-maintained upon the acidic OER thanks to the electron transfer from surrounding electron-enriched tetrahedral Co atoms via bridging oxygen bonds that suppresses the overoxidation and thus dissolution of active Ru and Co species. Consequently, the prepared catalyst, even with a low Ru mass loading of ca. 42.8 μg cm-2, exhibits an attractive acidic OER performance with a low overpotential of 200 mV and a low potential decay rate of 0.45 mV h-1 at 10 mA cm-2. Our work suggests an effective strategy to significantly enhance both the acidic OER activity and stability of low-cost electrocatalysts.

Manipulating electron redistribution induced by asymmetric coordination for electrocatalytic water oxidation at a high current density

Electronic structure manipulation with regard to active site coordination is an effective strategy to improve the electrocatalytic oxygen evolution reaction (OER) activity. Herein, we present the structure-activity relationship between oxygen-atom-mediated electron rearrangement and active site coordination asymmetry. Ni²⁺ ions are introduced to FeWO₄ on Ni foam (NF) via self-substitution to break the symmetry of the FeO₆ octahedron and regulate d-electron structure of Fe sites. Structural regulation optimizes the adsorption energy of hydroxyl on the Fe sites and promotes the partial formation of hydroxyl oxide with high OER activity on the tungstate surface. Fe_0.53Ni_0.47WO₄/NF with the asymmetric FeO₆ octahedron of Fe sites can achieve an ultralow overpotential of 170 mV at 10 mA cm^-2 and 240 mV at 1000 mA cm^-2 with robust stability for 500 h at high current density under alkaline conditions. This research develops novel electrocatalysts with impressive OER performance and provides new insights into the design of highly active catalytic systems.

Activity-Stability Balance: The Role of Electron Supply Effect of Support in Acidic Oxygen Evolution

Developing efficient and durable electrocatalysts for the oxygen evolution reaction (OER) in proton exchange membrane (PEM) electrolyzers represents a significant challenge. Herein, the cobalt-ruthenium oxide nano-heterostructures are successfully synthesized on carbon cloth (CoO_x /RuO_x -CC) for acidic OER through a simple and fast solution combustion strategy. The rapid oxidation process endows CoO_x /RuO_x -CC with abundant interfacial sites and defect structures, which enhances the number of active sites and the charge transfer at the electrolyte-catalyst interface, promoting the OER kinetics. Moreover, the electron supply effect of the CoO_x support allows electrons to transfer from Co to Ru sites during the OER process, which is beneficial to alleviate the ion leaching and over-oxidation of Ru sites, improving the catalyst activity and stability. As a self-supported electrocatalyst, CoO_x /RuO_x -CC displays an ultralow overpotential of 180 mV at 10 mA cm^-2 for OER. Notably, the PEM electrolyzer using CoO_x /RuO_x -CC as the anode can be operated at 100 mA cm^-2 stably for 100 h. Mechanistic analysis shows that the strong catalyst-support interaction is beneficial to redistribute the electronic structure of RuO bond to weaken its covalency, thereby optimizing the binding energy of OER intermediates and lowering the reaction energy barrier.