Structurally-Distorted RuIr-Based Nanoframes for Long-Duration Oxygen Evolution Catalysis

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.

Atomic Printing Strategy Achieves Precise Anchoring of Dual-Copper Atoms on C2N Structure for Efficient CO2 Reduction to Ethylene

Isolated metal sites catalysts (IMSCs) play crucial role in electrochemical CO2 reduction, with potential industrial applications. However, tunable synthesis strategies for IMSCs are limited. Herein, we present an atomic printing strategy that draws inspiration from the ancient Chinese "movable-type printing technology". Selecting customizable combinations of metal atoms as metal precursors from an extensive binuclear metal library. A series of dual-atom catalysts were prepared by utilizing the edge nitrogen atoms in the C2N cavity as anchoring "pincers" to capture metal atoms. To prove utility, the dual atom catalyst Cu2-C2N is investigated as electrocatalytic CO2RR catalyst. The synergistic interaction of dual Cu atoms promotes C-C coupling and guarantees FEC2+ (90.8 %) and FEC2H4. (71.7 %) at -1.10 V vs RHE. DFT calculations revealed the Cu2 site would be subtly flipped during CO2RR for enhancing *CO adsorption and dimerization. We validate that atomic printing strategies are applicable to wide range of metal combinations, representing a significant advancement in the development of IMSCs.

Ru Nanoparticles Encapsulated by Defective TiO2 Boost the Hydrogen Oxidation/ Evolution Reaction

The development of efficient and durable electrocatalysts for the alkaline hydrogen oxidation/evolution reaction is crucial for anion exchange membrane fuel cells/water electrolyzers. However, designing such electrocatalysts poses a challenge due to the need for optimizing various adsorbates. Herein, highly dispersed Ru nanoparticles catalysts is reported encapsulated and supported by defective anatase phase of titanium dioxide (named as Ru NPs/def-TiO2(A)) for boosting hydrogen-cycle electrocatalysis with robust anti-CO-poisoning in alkaline conditions. The Ru NPs/def-TiO2(A) achieves a high-quality activity of 7.65 A mgRu -1, which is 23.2 and 9.5-fold higher than commercial Ru/C and Pt/C in alkaline HOR. Moreover, this catalyst exhibits an outstanding overpotential of 21 mV at 10 mA cm-2 in alkaline HER. Hydrogen underpotential deposition (Hupd) and CO stripping experiments demonstrate that Ru NPs/def-TiO2(A) has the optimized H*, OH*, and CO* adsorption strength, enabling the Ru NPs/def-TiO2(A) catalyst to display excellent and robust HOR/HER performance under alkaline conditions. Using density functional theory calculations, the enhanced HOR performance mechanism for the Ru NPs/def-TiO2(A) catalyst originates from the TiO2 step face in contact with the Ru nanoparticles, indicating that the kinetics of water formation are considerably more favorable at the Ru NPs/def-TiO2(A) interface.

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.

Hollow Pt-Encrusted RuCu Nanocages Optimizing OH Adsorption for Efficient Hydrogen Oxidation Electrocatalysis

As one of the best candidates for hydrogen oxidation reaction (HOR), ruthenium (Ru) has attracted significant attention for anion exchange membrane fuel cells (AEMFCs), although it suffers from sluggish kinetics under alkaline conditions due to its strong hydroxide affinity. In this work, we develop ternary hollow nanocages with Pt epitaxy on RuCu (Pt-RuCu NCs) as efficient HOR catalysts for application in AEMFCs. Experimental characterizations and theoretical calculations confirm that the synergy in optimized Pt8.7-RuCu NCs significantly modifies the electronic structure and coordination environment of Ru, thereby balancing the binding strengths of H* and OH* species, which leads to a markedly enhanced HOR performance. Specifically, the optimized Pt8.7-RuCu NCs/C achieves a mass activity of 5.91 A mgPt+Ru -1, which is ~3.3, ~2.2, and ~15.0 times higher than that of RuCu NCs/C (1.38 A mgRu -1), PtRu/C (1.83 A mgPt+Ru -1) and Pt/C (0.37 A mgPt -1), respectively. Impressively, the specific peak power density of fuel cells reaches 15.9 W mgPt+Ru -1, significantly higher than those of most reported PtRu-based fuel cells.

Unveiling the Promotion of Fe in Ni3S2 Catalyst on Charge Transfer for the Oxygen Evolution Reaction

In recent years, catalysts based on transition metal sulfides have garnered extensive attention due to their low cost and excellent electrocatalytic activity in the alkaline oxygen evolution reaction. Here, the preparation of Fe-doped Ni3S2 via a one-step hydrothermal approach is reported by utilizing inexpensive transition metals Ni and Fe. In an alkaline medium, Fe-Ni3S2 exhibits outstanding electrocatalytic activity and stability for the OER, and the current density can reach 10 mA cm-2 with an overpotential of 163 mV. In addition, Pt/C||Fe-Ni3S2 is used as the membrane electrode of the anion exchange membrane water electrolyzer, which is capable of providing a current density of 650 mA cm-2 at a cell voltage of 2.0 V, outperforming the benchmark Ir/C. The principle is revealed that the doping of Fe enhances the electrocatalytic water decomposition ability of Ni3S2 by in situ Raman and in situ X-ray absorption fine structure. The results indicate that the doping of Fe decreases the charge density near Ni atoms, which renders Fe-Ni3S2 more favorable for the adsorption of OH- and the formation of *OO- intermediates. This work puts forward an effective strategy to significantly improve both the alkaline OER activity and stability of low-cost electrocatalysts.

Porphyrin-Confined Supported Ultrasmall Ir Clusters as Oxygen Evolution Catalysts for Water Electrolysis

Metalloporphyrin ligands themselves can participate in the redox process, making them beneficial in promoting the multielectron catalytic process of the oxygen evolution reaction (OER). However, OER catalysts synthesized by traditional chemical strategies face challenges in water electrolysis. We synthesized high-performance and stable alkaline and acidic OER electrocatalysts loaded with ultrasmall iridium clusters by taking advantage of the attraction and confinement of Ir atoms by the Ir-N bonds formed by the porphyrin cavity. The N in the porphyrin cavity forms an Ir-N bond with Ir so that Ir carries a negative charge and attracts Ir atoms to form ultrasmall Ir clusters above the cavity to adjust the electronic structure of the Ir clusters. The resulting catalyst Tpyp-Ir(IrOX) exhibits a small overpotential (242 and 259 mV) at a current density of 10 mA cm-2 in alkaline and acidic conditions and demonstrates good long-term operational stability. In addition, Tpyp-Ir(IrOX) exhibits a higher transition frequency (TOF) (1.69 O2 s-1 at 300 mV) in 1 M KOH, which is 7 times that of Ir/C.

Solvation Effect-Determined Mechanisms of Cation Exchange Reactions for Efficient Multicomponent Nanocatalysts

Cation exchange (CE) reaction is a classical synthesis method for creating complex structures. A lock of study on intrinsic mechanism limits its understanding and practical application. Using X-ray absorption spectroscopy, we observed that the evolution from Ru-Cl to Ru-O/OH occurs during the CE between K2RuCl6 and CoSn(OH)6 in aqueous solution, while CE between K2PtCl6 and CoSn(OH)6 is inhibited due to the failure of structural evolution from Pt-Cl to Pt-O/OH. Theoretical simulations imply that the interaction between Ru-O and CoSn(OH)6 with Co vacancy (CoVCoSn(OH)6) endows the electron transfer, as a result of strengthened adsorption on CoVCoSn(OH)6. Moreover, this mechanism is validated for CE between K2RuCl6 and ASn(OH)6 (A=Mg, Ca, Mn, Co, Cu, Zn), and CE between K2PdCl6/Na3RhCl6/K2IrCl6 and CoSn(OH)6. Impressively, the Pt-free CoRuSn(OH)x produced via CE displays a mass activity and a power density of 15.0 A mgRu -1 and 11.6 W mgRu -1, respectively, for anion exchange membrane fuel cell (AEMFC) exceeding the values of commercial PtRu/C (11.8 A mgRu+Pt -1 and 9.0 W mgRu+Pt -1). This work, for the first time, reveals the intrinsic mechanism of CE as structural evolution of target ion breaking through the traditional classic etch-adsorption mechanism and will promote fundamental research and practical application in various fields.

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.

Synergistic Sr Activation and Cr Buffering Effect on RuO2 Electronic Structures for Enhancing the Acidic Oxygen Evolution Reaction

The oxygen evolution reaction (OER) performance of ruthenium-based oxides strongly correlates with the electronic structures of Ru. However, the widely adopted monometal doping method unidirectionally regulates only the electronic structures, often failing to balance the activity and stability. Here, we propose an "elastic electron transfer" strategy to achieve bidirectional optimization of the electronic structures of Sr, Cr codoped RuO2 catalysts for acidic OER. The introduction of electron-withdrawing Sr intrinsically activates the Ru sites by increasing the oxidation state of Ru. Simultaneously, Cr acts as an electron buffer, donating electrons to Ru in the presence of Sr in the as-prepared catalysts and absorbing excess electrons from Sr leaching during the OER. Such a bidirectional regulation feature of Cr prevents overoxidation of Ru and maintains its high oxidation state during the OER. The optimal Ru3Cr1Sr0.175 catalyst exhibits a low overpotential (214 mV @ 10 mA cm-2) and excellent stability (over 300 h).

A Long-Range Disordering RuO2 Catalyst for Highly Efficient Acidic Oxygen Evolution Electrocatalysis

Non-iridium acid-stabilized electrocatalysts for oxygen evolution reaction (OER) are crucial to reducing the cost of proton exchange membrane water electrolyzers (PEMWEs). Here, we report a strategy to modulate the stability of RuO2 by doping boron (B) atoms, leading to the preparation of a RuO2 catalyst with long-range disorder (LD-B/RuO2). The structure of long-range disorder endowed LD-B/RuO2 with a low overpotential of 175 mV and an ultra-long stability, which can maintain OER for about 1.6 months at 10 mA cm-2 current density in 0.5 M H2SO4 with almost invariable performance. More importantly, a PEM electrolyzer using LD-B/RuO2 as the anode demonstrated excellent performance, reaching 1000 mA cm-2 at 1.63 V with durability exceeding 300 h at 250 mA cm-2 current density. The introduction of B atoms induced the formation of a long-range disordered structure and symmetry-breaking B-Ru-O motifs, which enabled the catalyst structure to a certain toughness while simultaneously inducing the redistribution of electrons on the active center Ru, which jointly promoted and guaranteed the activity and long-term stability of LD-B/RuO2. This study provides a strategy to prepare long-range disordered RuO2 acidic OER catalysts with high stability using B-doping to perturb crystallinity, which opens potential possibilities for non-iridium-based PEMWE applications.

Acidic Oxygen Evolution Reaction: Fundamental Understanding and Electrocatalysts Design

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

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.

Recent Advances in Iridium-based Electrocatalysts for Acidic Electrolyte Oxidation

Ongoing research to develop advanced electrocatalysts for the oxygen evolution reaction (OER) is needed to address demand for efficient energy conversion and carbon-free energy sources. In the OER process, acidic electrolytes have higher proton concentration and faster response than alkaline ones, but their harsh strongly acidic environment requires catalysts with greater corrosion and oxidation resistance. At present, iridium oxide (IrO2) with its strong stability and excellent catalytic performance is the catalyst of choice for the anode side of commercial PEM electrolysis cells. However, the scarcity and high cost of iridium (Ir) and the unsatisfactory activity of IrO2 hinder industrial scale application and the sustainable development of acidic OER catalytic technology. This highlights the importance of further research on acidic Ir-based OER catalysts. In this review, recent advances in Ir-based acidic OER electrocatalysts are summarized, including fundamental understanding of the acidic OER mechanism, recent insights into the stability of acidic OER catalysts, highly efficient Ir-based electrocatalysts, and common strategies for optimizing Ir-based catalysts. The future challenges and prospects of developing highly effective Ir-based catalysts are also discussed.

Rapid Conversion from Alloy Nanoparticles to Oxide Nanowires: Strain Wave-Driven Ru-O-Mn Collaborative Catalysis for Durable Oxygen Evolution Reaction

Metal-doped ruthenium oxides with low prices have gained widespread attention due to their editable compositions, distorted structures, and diverse morphologies for electrocatalysis. However, the mainstream challenge lies in breaking the so-called seesaw relationship between activity and stability during acidic oxygen evolution reaction (OER). Herein, strain wave-featured Mn-RuO2 nanowires (NWs) with asymmetric Ru-O-Mn bonds are first fabricated by thermally driven rapid solid phase conversion from RuMn alloy nanoparticles (NPs) at moderate temperature (450 °C). In 0.5 M H2SO4, the resultant NWs display a surprisingly ultralow overpotential of 168 mV at 10 mA cm-2 and run at a stable cell voltage (1.67 V) for 150 h at 50 mA cm-2 in PEMWE, far exceeding IrO2||Pt/C assemble. The simultaneous enhancement of both activity and stability stems from the presence of dense strain waves composed of alternating compressive and tensile ones in the distorted NWs, which collaboratively activate the Ru-O-Mn sites for faster OER. More importantly, the atomic strain waves trigger dynamic Ru-O-Mn regeneration via the refilling of oxygen vacancies by oxyanions adsorbed on adjacent Mn and Ru sites, achieving long-term stability. This work opens a door to designing non-precious metal-assisted ruthenium oxides with unique strains for practical application in commercial PEMWE.

Research progress on layered metal oxide electrocatalysts for an efficient oxygen evolution reaction

Hydrogen, highly valued for its pristine cleanliness and remarkable efficiency as an emerging energy source, is anticipated to ascend to a preeminent status within the forthcoming energy landscape. Electrocatalytic water splitting is considered a pivotal, eco-friendly, and sustainable strategy for hydrogen production. The substantial energy consumption stemming from oxygen evolution side reactions significantly impedes the commercial viability of water electrolysis. Consequently, the pursuit of a cost-effective and efficacious oxygen evolution reaction (OER) catalyst stands as an imperative strategy for realizing hydrogen production via water electrolysis. Layered metal oxides, owing to their robust anisotropic properties, versatile adjustability, and extensive surface area, have emerged as suitable candidates for OER catalysts. However, owing to the distinctive attributes of layered metal oxides, ongoing investigations into these materials are slightly fragmented, lacking universal consensus. This article comprehensively surveys the recent advancements in layered metal oxide-based OER catalysts, categorized into single metal oxides, alkali cobalt oxides, perovskites, and miscellaneous metal oxides. Initially, the main OER intermediate reaction steps of layered metal oxides are scrutinized. Subsequently, the design, mechanism, and application of several pivotal layered metal oxides in the OER are systematically delineated. Finally, a summary is provided, alongside the proposal of future research trajectories and challenges encountered by layered metal oxides, with the aspiration that this paper may serve as a valuable reference for scholars in the field.

Janus electronic state of supported iridium nanoclusters for sustainable alkaline water electrolysis

Metal-support electronic interactions play crucial roles in triggering the hydrogen spillover (HSo) to boost hydrogen evolution reaction (HER). It requires the supported metal of electron-rich state to facilitate the proton adsorption/spillover. However, this electron-rich metal state contradicts the traditional metal→support electron transfer protocol and is not compatible with the electron-donating oxygen evolution reaction (OER), especially in proton-poor alkaline conditions. Here we profile an Ir/NiPS3 support structure to study the Ir electronic states and performances in HSo/OER-integrated alkaline water electrolysis. The supported Ir is evidenced with Janus electron-rich and electron-poor states at the tip and interface regions to respectively facilitate the HSo and OER processes. Resultantly, the water electrolysis (WE) is efficiently implemented with 1.51 V at 10 mA cm-2 for 1000 h in 1 M KOH and 1.44 V in urea-KOH electrolyte. This research clarifies the Janus electronic state as fundamental in rationalizing efficient metal-support WE catalysts.

Vertically Stacked Amorphous Ir/Ru/Ir Oxide Nanosheets for Boosted Acidic Water Splitting

Integrating multiple functional components into vertically stacked heterostructures offers a prospective approach to manipulating the physicochemical properties of materials. The synthesis of vertically stacked heterogeneous noble metal oxides remains a challenge. Herein, we report a surface segregation approach to create vertically stacked amorphous Ir/Ru/Ir oxide nanosheets (NSs). Cross-sectional high-angle annular darkfield scanning transmission electron microscopy images demonstrate a three-layer heterostructure in the amorphous Ir/Ru/Ir oxide NSs, with IrOx layers located on the upper and lower surfaces, and a layer of RuOx sandwiched between the two IrOx layers. The vertically stacked heterostructure is a result of the diffusion of Ir atoms from the amorphous IrRuOx solid solution to the surface. The obtained A-Ir/Ru/Ir oxide NSs display an ultralow overpotential of 191 mV at 10 mA cm-2 toward acid oxygen evolution reaction and demonstrate excellent performance in a proton exchange membrane water electrolyzer, which requires only 1.63 V to achieve 1 A cm-2 at 60 °C, with virtually no activity decay observed after a 1300 h test.