Efficient Hydrogen Oxidation Catalyzed by Strain‐Engineered Nickel Nanoparticles

Sub-2 nm Microstrained High-Entropy-Alloy Nanoparticles Boost Hydrogen Electrocatalysis

High-entropy alloys (HEAs) confine multifarious elements into the same lattice, leading to intense lattice distortion effect. The lattice distortion tends to induce local microstrain at atomic level and thus affect surface adsorptions toward different adsorbates in various electrocatalytic reactions, yet remains unexplored. Herein, this work reports a class of sub-2 nm IrRuRhMoW HEA nanoparticles (NPs) with distinct local microstrain induced by lattice distortion for boosting alkaline hydrogen oxidation (HOR) and evolution reactions (HER). This work demonstrates that the distortion-rich HEA catalysts with optimized electronic structure can downshift the d-band center and generate uncoordinated oxygen sites to enhance the surface oxophilicity. As a result, the IrRuRhMoW HEA NPs show a remarkable HOR kinetic current density of 8.09 mA µg-1 PGM at 50 mV versus RHE, 8.89, 22.47 times higher than those of IrRuRh NPs without internal strain and commercial Pt/C, respectively, which is the best value among all the reported non-Pt based catalysts. IrRuRhMoW HEA NPs also display great HER performances with a turnover frequency (TOF) value of 5.93 H2 s-1 at 70 mV versus RHE, 4.6-fold higher than that of Pt/C catalyst, exceeding most noble metal-based catalysts. Experimental characterizations and theoretical studies collectively confirm that weakened hydrogen (Had) and enhanced hydroxyl (OHad) adsorption are achieved by simultaneously modulating the hydrogen adsorption binding energy and surface oxophilicity originated from intensified ligand effect and microstrain effect over IrRuRhMoW HEA NPs, which guarantees the remarkable performances toward HOR/HER.

Enabling Internal Electric Field in Heterogeneous Nanosheets to Significantly Accelerate Alkaline Hydrogen Electrocatalysis

Efficient bifunctional hydrogen electrocatalysis, encompassing both hydrogen evolution reaction (HER) and hydrogen oxidation reaction (HOR), is of paramount significance in advancing hydrogen-based societies. While non-precious-metal-based catalysts, particularly those based on nickel (Ni), are essential for alkaline HER/HOR, their intrinsic catalytic activity often falls short of expectations. Herein, an internal electric field (IEF) strategy is introduced for the engineering of heterogeneous nickel-vanadium oxide nanosheet arrays grown on porous nickel foam (Ni-V2O3/PNF) as bifunctional electrocatalysts for hydrogen electrocatalysis. Strikingly, the Ni-V2O3/PNF delivers 10 mA cm-2 at an overpotential of 54 mV for HER and a mass-specific kinetic current of 19.3 A g-1 at an overpotential of 50 mV for HOR, placing it on par with the benchmark 20% Pt/C, while exhibiting enhanced stability in alkaline electrolytes. Density functional theory calculations, in conjunction with experimental characterizations, unveil that the interface IEF effect fosters asymmetrical charge distributions, which results in more thermoneutral hydrogen adsorption Gibbs free energy on the electron-deficient Ni side, thus elevating the overall efficiency of both HER and HOR. The discoveries reported herein guidance are provided for further understanding and designing efficient non-precious-metal-based electrocatalysts through the IEF strategy.

Compressive Strain in Platinum-Iridium-Nickel Zigzag-Like Nanowire Boosts Hydrogen Catalysis

Strain effect in the structurally defective materials can contribute to the catalysis optimization. However, it is challenging to achieve the performance improvement by strain modulation with the help of geometrical structure because strain is spatially dependent. Here, a new class of compressively strained platinum-iridium-metal zigzag-like nanowires (PtIrM ZNWs, M = nickel (Ni), cobalt (Co), iron (Fe), zinc (Zn) and gallium (Ga)) is reported as the efficient alkaline hydrogen evolution reaction (HER) and hydrogen oxidation reaction (HOR) catalysts. Particularly, the optimized PtIrNi ZNWs with 3% compressive strain (cs-PtIrNi ZNWs) can achieve the highest HER/HOR performances among all the catalysts investigate. Their HOR mass and specific activities are 3.2/14.4 and 2.6/32.7 times larger than those of PtIrNi NWs and commercial Pt/C, respectively. Simultaneously, they can exhibit the superior stability and high CO resistance for HOR. Further, experimental and theoretical studies collectively reveal that the compressive strain in cs-PtIrNi ZNWs effectively weakens the adsorption of hydroxyl intermediate and modulates the electronic structure, resulting in the weakened hydrogen binding energy (HBE) and moderate hydroxide binding energy (OHBE), beneficial for the improvement of HOR performance. This work highlights the importance of strain tuning in enhancing Pt-based nanomaterials for hydrogen catalysis and beyond.

Non-Noble Metal High-Entropy Alloy-Based Catalytic Electrode for Long-Life Hydrogen Gas Batteries

The development of efficient, stable, and low-cost bifunctional catalysts for the hydrogen evolution/oxidation reaction (HER/HOR) is critical to promote the application of hydrogen gas batteries in large scale energy storage systems. Here we demonstrate a non-noble metal high-entropy alloy grown on Cu foam (NNM-HEA@CF) as a self-supported catalytic electrode for nickel-hydrogen gas (Ni-H2) batteries. Experimental and theoretical calculation results reveal that the NNM-HEA catalyst greatly facilitates the HER/HOR catalytic process through the optimized electronic structures of the active sites. The assembled Ni-H2 battery with NNM-HEA@CF as the anode shows excellent rate capability and exceptional cycling performance of over 1800 h without capacity decay at an areal capacity of 15 mAh cm-2. Furthermore, a scaled-up Ni-H2 battery fabricated with an extended capacity of 0.45 Ah exhibits a high cell-level energy density of ∼109.3 Wh kg-1. Moreover, its estimated cost reaches as low as ∼107.8 $ kWh-1 based on all key components of electrodes, separator and electrolyte, which is reduced by more than 6 times compared to that of the commercial Pt/C-based Ni-H2 battery. This work provides an approach to develop high-efficiency non-noble metal-based bifunctional catalysts for hydrogen batteries in large-scale energy storage applications.

Construction of Heterostructure between Ni17W3 and WO2 to Boost the Hydrogen Oxidation Reaction in Alkaline Medium

The inferior intrinsic performance of Ni-based catalysts for the hydrogen oxidation reaction (HOR) in an alkaline medium seriously restricts the utilization of emerging anion-exchange membrane fuel cells (AEMFCs). This is because the hydrogen and hydroxyl binding energies on Ni need to be optimized. Although electrocatalysts obtained by alloying Ni with Mo or W reportedly exhibit enhanced activity, they are still far from industrial requirements based on unbalanced HBE and OHBE. Herein, we report to further enhance alkaline HOR activity by constructing a heterostructure between NiW alloy and metal oxide (Ni17W3/WO2), which is synthesized through solvothermal treatment combined with annealing. The as-fabricated reduced graphene oxide (rGO)-supported Ni17W3/WO2 (Ni17W3/WO2/rGO) exhibits state-of-the-art catalytic activity (current density of 2.9 mA cm-2 at 0.1 V vs RHE), faster kinetics (geometric kinetics current density of 4.0 mA cm-2 that can be comparable to Pt/C), and high stability (maintaining the current density for more than 80 h) toward HOR in alkaline media. The detailed characterizations reveal that the charge transfer across the boundary arising from constructing the as-prepared heterostructure tunes the electronic structures, ultimately facilitating the HOR process.

Optimizing Hydrogen and Hydroxyl Adsorption over Ru/WO2.9 Metal/Metalloid Heterostructure Electrocatalysts for Highly Efficient and Stable Hydrogen Oxidation Reactions in Alkaline Media

The development of high-performance, stable and platinum-free electrocatalysts for the hydrogen oxidation reaction (HOR) in alkaline media is crucial for the commercial application of anion exchange membrane fuel cells (AEMFCs). Ruthenium, as an emerging HOR electrocatalyst with a price advantage over platinum, still needs to solve the problems of low intrinsic activity and easy oxidation. Herein, Ru nanoparticles are anchored on the oxygen-vacancy-rich metalloid WO2.9 by interfacial engineering to create abundant and efficient Ru and WO2.9 interfacial active sites for accelerated HOR in alkaline media. Ru/WO2.9 /C displays excellent catalytic activity with mass activity (8.29 A mgNM -1 ) and specific activity (1.32 mA cmNM -2 ), which are 2.5/3.3 and 21.8/8.3 times that of PtRu/C and Pt/C, respectively. Moreover, Ru/WO2.9 /C exhibits excellent CO tolerance and operational stability. Experimental and theoretical studies reveal that the improved charge transfer from Ru to WO2.9 in the metal/metalloid heterostructure significantly tune the electronic structure of Ru sites and optimize the hydrogen binding energy (HBE) of Ru. While, WO2.9 provides abundant hydroxyl adsorption sites. Therefore, the equilibrium adsorption of hydrogen and hydroxyl at the interface of Ru/WO2.9 will be realized, and the oxidation of metal Ru would be avoided, thereby achieving excellent HOR activity and durability.

High CO and sulfur tolerant proton exchange membrane fuel cell anodes enabled by "work along both lines" mechanism of 2,6-dihydroxymethyl pyridine molecule blocking layer

Proton exchange membrane fuel cells (PEMFCs) are hindered by their poor tolerance to CO and H2S poisoning. Herein, we report an effective method, via engineering 2,6-dihydroxymethyl pyridine (DhmPy) molecule blocking layers on Pt surface, aiming to save the poisoning issue for PEMFC anode reaction. The PEMFCs assembled by this catalyst produce a power density of 1.18 W cm-2 @ 2.0 A cm-2 and 1.32 W cm-2 @ 2.0 A cm-2, far exceeding commercial Pt/C after H2/10 ppm CO poisoning and H2/5 ppm H2S poisoning tests, respectively. Density functional theory (DFT) indicates that a coronal molecule layer with a steric confinement height (1.82 Å), constructed by DhmPy, emerges more intensive adsorption energy compared to 2,6-pyridinedicarboxamide (DcaPy) and 2,6-diacetylpyridine (DAcPy), thereby more effectively inhibits the adsorption of large-sized CO and H2S on Pt surface without affecting H2 traverse. This "work along both lines" mechanism with the resistance of both CO and H2S provides a new and promising design thought for high CO and sulfur tolerant PEMFC anodes.

Lattice strain controlled Ni@NiCu efficient anode catalysts for direct borohydride fuel cells

We successfully fabricated a novel tensile lattice strained Ni@NiCu catalyst with a popcorn-like morphology, which is composed of a crystalline Ni core and a NiCu alloy shell. It exhibits outstanding catalytic activity, selectivity, and stability towards borohydride electrooxidation. Moreover, a direct borohydride fuel cell (DBFC) with a Ni@NiCu anode can deliver a power density of 433 mW cm-2 and an open circuit voltage of 1.94 V, much better than the performances of DBFCs employing other anode catalysts reported in the literature. This could be attributed to the fact that the tensile lattice strain generated by the introduction of Cu leads to a rise in the d-band center of the Ni metal and promotes the final B-H decoupling, which is the rate-determining step in the borohydride oxidation reaction, thus improving remarkably the catalytic performances of Ni@NiCu.

Solid-Liquid-Gas Management for Low-Cost Hydrogen Gas Batteries

Aqueous nickel-hydrogen gas (Ni-H₂) batteries with excellent durability (>10,000 cycles) are important candidates for grid-scale energy storage but are hampered by the high-cost Pt electrode with limited performance. Herein, we report a low-cost nickel-molybdenum (NiMo) alloy as an efficient bifunctional hydrogen evolution and oxidation reaction (HER/HOR) catalyst for Ni-H₂ batteries in alkaline electrolytes. The NiMo alloy demonstrates a high HOR mass-specific kinetic current of 28.8 mA mg^-1 at 50 mV as well as a low HER overpotential of 45 mV at a current density of 10 mA cm^-2, which is better than most nonprecious metal catalysts. Furthermore, we apply a solid-liquid-gas management strategy to constitute a conductive, hydrophobic network of NiMo using multiwalled carbon nanotubes (NiMo-hydrophobic MWCNT) in the electrode to accelerate HER/HOR activities for much improved Ni-H₂ battery performance. As a result, Ni-H₂ cells based on the NiMo-hydrophobic MWCNT electrode show a high energy density of 118 Wh kg^-1 and a low cost of only 67.5 $ kWh^-1. With the low cost, high energy density, excellent durability, and improved energy efficiency, the Ni-H₂ cells show great potential for practical grid-scale energy storage.

Heterostructural Ni-Ni0.2 Mo0.8 N Interface Engineering Boosts Alkaline Hydrogen Electrocatalysis

Exploring efficient and low-cost bifunctional catalysts for hydrogen evolution reaction (HER) and hydrogen oxidation reaction (HOR) is highly desirable for the achievement of unitized regenerative fuel cells. Herein, a facile method to prepare hetero-interfacial Ni-Ni_0.2 Mo_0.8 N nanosheets with tailored d-band for efficient alkaline hydrogen electrocatalysis is presented. Mechanism studies indicate that interface engineering can downshift the d-band center of Ni-Ni_0.2 Mo_0.8 N nanosheets due to the electron transfer from Ni to Ni_0.2 Mo_0.8 N, which weakens the binding strength of reaction intermediates, thereby boosting the catalytic performance. Relative to pure Ni, Ni-Ni_0.2 Mo_0.8 N nanosheets show a lower overpotential of 83 mV at -10 mA cm^-2 and good stability during 2,000 cycles for HER. Meanwhile, Ni-Ni_0.2 Mo_0.8 N nanosheets exhibit an improved exchange current density for HOR with a 10.2-fold enhancement in comparison with that of pure Ni. This work provides valuable insight into the reasonable design of efficient energy-related electrocatalysts based on the tailoring of d-band center by interface engineering.

Pseudo-Pt Monolayer for Robust Hydrogen Oxidation

Heteroepitaxial core-shell structure is conducive to combining the advantages of the epilayer and the substrate, creating a novel multifunctionality for catalysis application. Herein, we report a pseudomorphic-Pt atomic layer (PmPt) epitaxially growing on an IrPd-core matrix (PmPt@IrPd/C) as an efficient and stable catalyst for alkaline hydrogen oxidation reaction that exhibits ∼29.2 times more mass activity enhancement than that of benchmark Pt/C. The PmPt@IrPd/C catalyst also gives rise to ∼25.0 times more enhancement than Pt/C during a 50,000-cycle accelerated stability test. This robust stability originates from the resistance to carbon corrosion owing to the stronger H2O interaction instead of carbon oxide (COx) poison species, and the modulated hydroxyl (OH*) adsorption could inhibit the OH* species from shuffling the surface Pt atoms away from the substrate. Moreover, the anion-exchange membrane fuel cells assembled by PmPt@IrPd/C with an ultralow Pt loading of 0.009 mgPt cm-2 in the anode can deliver a power density of 1.27 W cm-2.

Highly Selective Pd Nanosheet Aerogel Catalyst with Hybrid Strain Induced by Laser Irradiation and P Doping Postprocess

Strain engineering of electrocatalysts provides an effective strategy to improve the intrinsic catalytic activity. Here, the defect-rich crystalline/amorphous Pd nanosheet aerogel with hybrid microstrain and lattice strain is synthesized by combining laser irradiation and phosphorus doping methods. The surface strain exhibited by the microstrain and lattice strain shifts the d-band center of the electrocatalyst, enhancing the adsorption of intermediates in the ethanol oxidation reaction and thus improving the catalytic performances. The measured mass activity, specific activity and C1-path selectivity of the Pd nanosheet aerogel are 4.48, 3.06, and 5.06 times higher than those of commercial Pd/C, respectively. These findings afford a new strategy for the preparation of highl activity and C1 pathway selective catalysts and provide insight into the catalytic mechanism of strain-rich heterojunction materials based on tunable surface strain values.

Competitive Adsorption: Reducing the Poisoning Effect of Adsorbed Hydroxyl on Ru Single-Atom Site with SnO2 for Efficient Hydrogen Evolution

Ruthenium (Ru) has been theoretically considered a viable alkaline hydrogen evolution reaction electrocatalyst due to its fast water dissociation kinetics. However, its strong affinity to the adsorbed hydroxyl (OH_ad ) blocks the active sites, resulting in unsatisfactory performance during the practical HER process. Here, we first reported a competitive adsorption strategy for the construction of SnO₂ nanoparticles doped with Ru single-atoms supported on carbon (Ru SAs-SnO₂ /C) via atomic galvanic replacement. SnO₂ was introduced to regulate the strong interaction between Ru and OH_ad by the competitive adsorption of OH_ad between Ru and SnO₂ , which alleviated the poisoning of Ru sites. As a consequence, the Ru SAs-SnO₂ /C exhibited a low overpotential at 10 mA cm^-2 (10 mV) and a low Tafel slope of 25 mV dec^-1 . This approach provides a new avenue to modulate the adsorption strength of active sites and intermediates, which paves the way for the development of highly active electrocatalysts.

Unlocking Interfacial Electron Transfer of Ruthenium Phosphides by Homologous Core-Shell Design toward Efficient Hydrogen Evolution and Oxidation

Developing high-efficiency electrocatalysts for the hydrogen evolution and oxidation reactions (HER/HOR) in alkaline electrolytes is of critical importance for realizing renewable hydrogen technologies. Ruthenium phosphides (RuP_x ) are promising candidates to substitute Pt-based electrodes; however, great challenges still remain in their electronic structure regulation for optimizing intermediate adsorption. Herein, it is reported that a homologous RuP@RuP₂ core-shell architecture constructed by a phosphatization-controlled phase-transformation strategy enables strong electron coupling for optimal intermediate adsorption by virtue of the emergent interfacial functionality. Density functional theory calculations show that the RuP core and RuP₂ shell present efficient electron transfer, leading to a close to thermoneutral hydrogen adsorption Gibbs free energy of 0.04 eV. Impressively, the resulting material exhibits superior HER/HOR activities in alkaline media, which outperform the benchmark Pt/C and are among the best reported bifunctional hydrogen electrocatalysts. The present work not only provides an efficient and cost-effective bifunctional hydrogen electrocatalyst, but also offers a new synthetic protocol to rationally synthesize homologous core-shell nanostructures for widespread applications.

Strain Engineering: A Boosting Strategy for Photocatalysis

Whilst the photocatalytic technique is considered to be one of the most significant routes to address the energy crisis and global environmental challenges, the solar-to-chemical conversion efficiency is still far from satisfying practical industrial requirements, which can be traced to the suboptimal bandgap and electronic structure of photocatalysts. Strain engineering is a universal scheme that can finely tailor the bandgap and electronic structure of materials, hence supplying a novel avenue to boost their photocatalytic performance. Accordingly, to explore promising directions for certain breakthroughs in strained photocatalysts, an overview on the recent advances of strain engineering from the basics of strain effect, creations of strained materials, as well as characterizations and simulations of strain level is provided. Besides, the potential applications of strain engineering in photocatalysis are summarized, and a vision for the future controllable-electronic-structure photocatalysts by strain engineering is also given. Finally, perspectives on the challenges for future strain-promoted photocatalysis are discussed, placing emphasis on the creation and decoupling of strain effect, and the modification of theoretical frameworks.

An efficient nickel hydrogen oxidation catalyst for hydroxide exchange membrane fuel cells

The hydroxide exchange membrane fuel cell (HEMFC) is a promising energy conversion technology but is limited by the need for platinum group metal (PGM) electrocatalysts, especially for the hydrogen oxidation reaction (HOR). Here we report a Ni-based HOR catalyst that exhibits an electrochemical surface area-normalized exchange current density of 70 μA cm^-2, the highest among PGM-free catalysts. The catalyst comprises Ni nanoparticles embedded in a nitrogen-doped carbon support. According to X-ray and ultraviolet photoelectron spectroscopy as well as H₂ chemisorption data, the electronic interaction between the Ni nanoparticles and the support leads to balanced hydrogen and hydroxide binding energies, which are the likely origin of the catalyst's high activity. PGM-free HEMFCs employing this Ni-based HOR catalyst give a peak power density of 488 mW cm^-2, up to 6.4 times higher than previous best-performing analogous HEMFCs. This work demonstrates the feasibility of efficient PGM-free HEMFCs.

Janus bimetallic materials as efficient electrocatalysts for hydrogen oxidation and evolution reactions

The development of hydrogen energy is limited by the high cost of platinum group metals (PGM). There is an urgent need to design efficient PGM-free electrocatalysts in the hydrogen electrode. Herein, Janus Ni/W bimetallic materials are proposed as an effective PGM-free bifunctional hydrogen electrocatalyst. By constructing the bimetallic materials, a synergistic effect is realized to enhance the reaction kinetics and improve the catalytic performance. In general, Ni can provide excellent H_ad sites, and W serves as OH_ad sites. Therefore, the synergistic effect of Ni and W can improve the kinetics of hydrogen evolution reaction and the hydroxide oxidation reaction. Ni/W@NF can obtain the hydrogen evolution reaction current density of 10 mA cm^-2 with an overpotential of only 62.6 mV, and the exchange current density of hydroxide oxidation reaction can reach 1.83 mA cm^-2. This work provides a new idea for the design of high-efficiency and low-cost PGM-free bifunctional hydrogen electrocatalysts.

Design of Ru-Ni diatomic sites for efficient alkaline hydrogen oxidation

Anion exchange membrane fuel cells are limited by the slow kinetics of alkaline hydrogen oxidation reaction (HOR). Here, we establish HOR catalytic activities of single-atom and diatomic sites as a function of *H and *OH binding energies to screen the optimal active sites for the HOR. As a result, the Ru-Ni diatomic one is identified as the best active center. Guided by the theoretical finding, we subsequently synthesize a catalyst with Ru-Ni diatomic sites supported on N-doped porous carbon, which exhibits excellent catalytic activity, CO tolerance, and stability for alkaline HOR and is also superior to single-site counterparts. In situ scanning electrochemical microscopy study validates the HOR activity resulting from the Ru-Ni diatomic sites. Furthermore, in situ x-ray absorption spectroscopy and computational studies unveil a synergistic interaction between Ru and Ni to promote the molecular H₂ dissociation and strengthen OH adsorption at the diatomic sites, and thus enhance the kinetics of HOR.

Electrocatalytic Hydrogen Oxidation in Alkaline Media: From Mechanistic Insights to Catalyst Design

With the potential to circumvent the need for scarce and cost-prohibitive platinum-based catalysts in proton-exchange membrane fuel cells, anion-exchange membrane fuel cells (AEMFCs) are emerging as alternative technologies with zero carbon emission. Numerous noble metal-free catalysts have been developed with excellent catalytic performance for cathodic oxygen reduction reaction in AEMFCs. However, the anodic catalysts for hydrogen oxidation reaction (HOR) still rely on noble metal materials. Since the kinetics of HOR in alkaline media is 2-3 orders of magnitude lower than that in acidic media, it is a major challenge to either improve the performance of noble metal catalysts or to develop high-performance noble metal-free catalysts. Additionally, the mechanisms of alkaline HOR are not yet clear and still under debate, further hampering the design of electrocatalysts. Against this backdrop, this review starts with the prevailing theories for alkaline HOR on the basis of diverse activity descriptors, i.e., hydrogen binding energy theory and bifunctional theory. The design principles and recent advances of HOR catalysts employing the aforementioned theories are then summarized. Next, the strategies and recent progress in improving the antioxidation capability of HOR catalysts, a thorny issue which has not received sufficient attention, are discussed. Moreover, the significance of correlating computational models with real catalyst structure and the electrode/electrolyte interface is further emphasized. Lastly, the remaining controversies about the alkaline HOR mechanisms as well as the challenges and possible research directions in this field are presented.

Atomic-precision Pt6 nanoclusters for enhanced hydrogen electro-oxidation

The discord between the insufficient abundance and the excellent electrocatalytic activity of Pt urgently requires its atomic-level engineering for minimal Pt dosage yet maximized electrocatalytic performance. Here we report the design of ultrasmall triphenylphosphine-stabilized Pt₆ nanoclusters for electrocatalytic hydrogen oxidation reaction in alkaline solution. Benefiting from the self-optimized ligand effect and atomic-precision structure, the nanocluster electrocatalyst demonstrates a high mass activity, a high stability, and outperforms both Pt single atoms and Pt nanoparticle analogues, uncovering an unexpected size optimization principle for designing Pt electrocatalysts. Moreover, the nanocluster electrocatalyst delivers a high CO-tolerant ability that conventional Pt/C catalyst lacks. Theoretical calculations confirm that the enhanced electrocatalytic performance is attributable to the bifold effects of the triphenylphosphine ligand, which can not only tune the formation of atomically precise platinum nanoclusters, but also shift the d-band center of Pt atoms for favorable adsorption kinetics of *H, *OH, and CO.

While Pt is an active fuel cell catalyst, it’s low abundance and high cost spurs research into boosting performances from lesser Pt amounts. Here, authors design atomically precise triphenylphosphine-stabilized Pt nanoclusters with high activities and durabilities for electrocatalytic H₂ oxidation.

Strain Relaxation in Metal Alloy Catalysts Steers the Product Selectivity of Electrocatalytic CO2 Reduction

Strain engineering in bimetallic alloy structures is of great interest in electrochemical CO₂ reduction reactions (CO₂RR), in which it simultaneously improves electrocatalytic activity and product selectivity by optimizing the binding properties of intermediates. However, a reliable synthetic strategy and systematic understanding of the strain effects in the CO₂RR are still lacking. Herein, we report a strain relaxation strategy used to determine lattice strains in bimetal MNi alloys (M = Pd, Ag, and Au) and realize an outstanding CO₂-to-CO Faradaic efficiency of 96.6% and show the outstanding activity and durability toward a Zn-CO₂ battery. Molecular dynamics (MD) simulations predict that the relaxation of strained PdNi alloys (s-PdNi) is correlated with increases in synthesis temperature, and the high temperature activation energy drives complete atomic mixing of multiple metal atoms to allow for regulation of lattice strains. Density functional theory (DFT) calculations reveal that strain relaxation effectively improves CO₂RR activity and selectivity by optimizing the formation energies of *COOH and *CO intermediates on s-PdNi alloy surfaces, as also verified by in situ spectroscopic investigations. This approach provides a promising approach for catalyst design, enabling independent optimization of formation energies of reaction intermediates to improve catalytic activity and selectivity simultaneously.

Electrocatalysis in Alkaline Media and Alkaline Membrane-Based Energy Technologies

Hydrogen energy-based electrochemical energy conversion technologies offer the promise of enabling a transition of the global energy landscape from fossil fuels to renewable energy. Here, we present a comprehensive review of the fundamentals of electrocatalysis in alkaline media and applications in alkaline-based energy technologies, particularly alkaline fuel cells and water electrolyzers. Anion exchange (alkaline) membrane fuel cells (AEMFCs) enable the use of nonprecious electrocatalysts for the sluggish oxygen reduction reaction (ORR), relative to proton exchange membrane fuel cells (PEMFCs), which require Pt-based electrocatalysts. However, the hydrogen oxidation reaction (HOR) kinetics is significantly slower in alkaline media than in acidic media. Understanding these phenomena requires applying theoretical and experimental methods to unravel molecular-level thermodynamics and kinetics of hydrogen and oxygen electrocatalysis and, particularly, the proton-coupled electron transfer (PCET) process that takes place in a proton-deficient alkaline media. Extensive electrochemical and spectroscopic studies, on single-crystal Pt and metal oxides, have contributed to the development of activity descriptors, as well as the identification of the nature of active sites, and the rate-determining steps of the HOR and ORR. Among these, the structure and reactivity of interfacial water serve as key potential and pH-dependent kinetic factors that are helping elucidate the origins of the HOR and ORR activity differences in acids and bases. Additionally, deliberately modulating and controlling catalyst-support interactions have provided valuable insights for enhancing catalyst accessibility and durability during operation. The design and synthesis of highly conductive and durable alkaline membranes/ionomers have enabled AEMFCs to reach initial performance metrics equal to or higher than those of PEMFCs. We emphasize the importance of using membrane electrode assemblies (MEAs) to integrate the often separately pursued/optimized electrocatalyst/support and membranes/ionomer components. Operando/in situ methods, at multiscales, and ab initio simulations provide a mechanistic understanding of electron, ion, and mass transport at catalyst/ionomer/membrane interfaces and the necessary guidance to achieve fuel cell operation in air over thousands of hours. We hope that this Review will serve as a roadmap for advancing the scientific understanding of the fundamental factors governing electrochemical energy conversion in alkaline media with the ultimate goal of achieving ultralow Pt or precious-metal-free high-performance and durable alkaline fuel cells and related technologies.

Recent advances in the pre-oxidation process in electrocatalytic urea oxidation reactions

The electrocatalytic urea oxidation reaction (UOR) has attracted substantial research interests over the past few years owing to its critical role in coupled electrochemical systems for energy conversion, for example, coupling with the hydrogen evolution reaction (HER) to realize urea-assisted hydrogen production and assembling direct urea fuel cells (DUFC) by coupling with the oxygen reduction reaction (ORR). The UOR process has been proved to be a two-step process which involves an electrochemical pre-oxidation reaction of the metal sites and a subsequent chemical oxidation of the urea molecules on the as-formed high-valence metal sites. Hence, designing advanced (pre-)catalysts with a boosted pre-oxidation reaction is of great importance in improving the UOR performance and thus accelerating the coupled reactions. In this feature article, we discuss the significant role of the pre-oxidation process during the urea electro-oxidation reaction, and summarize detailed strategies and recent advances in promoting the pre-oxidation reaction, including the modulation of the crystallinity, active phase engineering, defect engineering, elemental incorporation and constructing hierarchical nanostructures. We anticipate that this feature article will offer helpful guidance for the design and optimization of advanced (pre-)catalysts for UOR and related energy conversion applications.

Anion-exchange membrane water electrolyzers and fuel cells

The key components, working management, and operating techniques of anion-exchange membrane water electrolyzers and fuel cells are reviewed for the first time.

Catalyst overcoating engineering towards high-performance electrocatalysis

Clean and sustainable energy needs the development of advanced heterogeneous catalysts as they are of vital importance for electrochemical transformation reactions in renewable energy conversion and storage devices. Advances in nanoscience and material chemistry have afforded great opportunities for the design and optimization of nanostructured electrocatalysts with high efficiency and practical durability. In this review article, we specifically emphasize the synthetic methodologies for the versatile surface overcoating engineering reported to date for optimal electrocatalysts. We discuss the recent progress in the development of surface overcoating-derived electrocatalysts potentially applied in polymer electrolyte fuel cells and water electrolyzers by correlating catalyst intrinsic structures with electrocatalytic properties. Finally, we present the opportunities and perspectives of surface overcoating engineering for the design of advanced (electro)catalysts and their deep exploitation in a broad scope of applications.

Electron Density Modulation of MoO2/Ni to Produce Superior Hydrogen Evolution and Oxidation Activities

Hydrogen evolution reaction (HER) and hydrogen oxidation reaction (HOR) have aroused great interest, but the high price of platinum group metals (PGMs) limits their development. The electronic reconstruction at the interface of a heterostructure is a promising strategy to enhance their catalytic performance. Here, MoO₂/Ni heterostructure was synthesized to provide effective HER in an alkaline electrolyte and exhibit excellent HOR performance. Theoretical and experimental analyses prove that the electron density around the Ni atom is reduced. The electron density modulation optimizes the hydrogen adsorption and hydroxide adsorption free energy, which can effectively improve the activity of both HER and HOR. Accordingly, the prepared MoO₂/Ni@NF catalyst reveals robust HER activity (η₁₀ = 50.48 mV) and HOR activity (j₀ = ∼1.21 mA cm^-2). This work demonstrates an effective method to design heterostructure interfaces and tailor the surface electronic structure to improve HER/HOR performance.

MOFs-Derived Carbon-Based Metal Catalysts for Energy-Related Electrocatalysis

Electrochemical devices, as renewable and clean energy systems, display a great potential to meet the sustainable development in the future. However, well-designed and highly efficient electrocatalysts are the technological dilemmas that retard their practical applications. Metal-organic frameworks (MOFs) derived electrocatalysts exhibit tunable structure and intriguing activity and have received intensive investigation in recent years. In this review, the recent progress of MOFs-derived carbon-based single atoms (SAs) and metal nanoparticles (NPs) catalysts for energy-related electrocatalysis is summarized. The effects of synthesis strategy, coordination environment, morphology, and composition on the catalytic activity are highlighted. Furthermore, these SAs and metal NPs catalysts for the applications of electrocatalysis (hydrogen evolution reaction, oxygen evolution reaction, oxygen reduction reaction, carbon dioxide reduction reaction, and nitrogen reduction reaction) are overviewed. Finally, some current challenges and foresighted ideas for MOFs-derived carbon-based metal electrocatalysts are presented.

Alloying Nickel with Molybdenum Significantly Accelerates Alkaline Hydrogen Electrocatalysis

Bifunctional hydrogen electrocatalysis (hydrogen-oxidation and hydrogen-evolution reactions) in alkaline solution is desirable but challenging. Among all available electrocatalysts, Ni-based materials are the only non-precious-metal-based candidates for alkaline hydrogen oxidation, but they generally suffer from low activity. Here, we demonstrate that properly alloying Ni with Mo could significantly promote its electrocatalytic performance. Ni₄ Mo alloy nanoparticles are prepared from the reduction of molybdate-intercalated Ni(OH)₂ nanosheets. The final product exhibits an apparent hydrogen-oxidation activity exceeding that of the Pt benchmark and a record-high mass-specific kinetic current of 79 A g^-1 at an overpotential of 50 mV. A superior hydrogen-evolution performance is also measured in alkaline solution. These experimental data are rationalized by our theoretical simulations, which show that alloying Ni with Mo significantly weakens its hydrogen adsorption, improves the hydroxyl adsorption and decreases the reaction barrier for water formation.

Engineering the Near-Surface of PtRu3 Nanoparticles to Improve Hydrogen Oxidation Activity in Alkaline Electrolyte

Tailoring the near-surface composition of Pt-based alloy can optimize the surface chemical properties of a nanocatalyst and further improve the sluggish H₂ electrooxidation performance in an alkaline electrolyte. However, the construction of alloy nanomaterials with a precise near-surface composition and smaller particle size still needs to overcome huge obstacles. Herein, ultra-small PtRu₃ binary nanoparticles (<2 nm) evenly distributed on porous carbon (PtRu₃ /PC), with different near-surface atomic compositions (Pt-increased and Ru-increased), are successfully synthesized. XPS characterizations and electrochemical test confirm the transformation of a near-surface atomic composition after annealing PtRu₃ /PC-300 alloy; when annealing in CO atmosphere, forming the Pt-increased near-surface structure (500 °C), while the Ru-increased near-surface structure appears in an Ar heat treatment process (700 °C). Furthermore, three PtRu₃ /PC nanocatalysts all weaken the hydrogen binding strength relative to the Pt/PC. Remarkably, the Ru-increased nanocatalyst exhibits up to 38.8-fold and 9.2-fold HOR improvement in mass activity and exchange current density, compared with the Pt/PC counterpart, respectively. CO-stripping voltammetry tests demonstrate the anti-CO poisoning ability of nanocatalysts, in the sequence of Ru-increased ≥ PtRu₃ /PC-300 > Pt-increased > Pt/PC. From the perspective of engineering a near-surface structure, this study may open up a new route for the development of high-efficiency electrocatalysts with a strong electronic effect and oxophilic effect.

Interface engineering of heterostructured electrocatalysts towards efficient alkaline hydrogen electrocatalysis

Boosting the alkaline hydrogen evolution and oxidation reaction (HER/HOR) kinetics is vital to practicing the renewable hydrogen cycle in alkaline media. Recently, intensive research has demonstrated that interface engineering is of critical significance for improving the performance of heterostructured electrocatalysts particularly toward the electrochemical reactions involving multiple reaction intermediates like alkaline hydrogen electrocatalysis, and the research advances also bring substantial non-trivial fundamental insights accordingly. Herein, we review the current status of interface engineering with respect to developing efficient heterostructured electrocatalysts for alkaline HER and HOR. Two major subjects-how interface engineering promotes the reaction kinetics and what fundamental insights interface engineering has brought into alkaline HER and HOR-are discussed. Specifically, heterostructured electrocatalysts with abundant interfaces have shown substantially accelerated alkaline hydrogen electrocatalysis kinetics owing to the synergistic effect from different components, which could balance the adsorption/desorption behaviors of the intermediates at the interfaces. Meanwhile, interface engineering can effectively tune the electronic structures of the active sites via electronic interaction, interfacial bonding, and lattice strain, which would appropriately optimize the binding energy of targeted intermediates like hydrogen. Furthermore, the confinement effect is critical for delivering high durability by sustaining high density of active sites. At last, our own perspectives on the challenges and opportunities toward developing efficient heterostructured electrocatalysts for alkaline hydrogen electrocatalysis are provided.

Design Strategies of Transition-Metal Phosphate and Phosphonate Electrocatalysts for Energy-Related Reactions

The key challenge to developing renewable energy conversion and storage devices lies in the exploration and rational engineering of cost-effective and highly efficient electrocatalysts for various energy-related electrochemical reactions. Transition-metal phosphates and phosphonates have shown remarkable performances for these reactions based on their unique physicochemical properties. Compared with transition-metal oxides, phosphate groups in transition-metal phosphates and phosphonates show flexible coordination with diverse orientations, making them an ideal platform for designing active electrocatalysts. Although numerous efforts have been spent on the development of transition-metal phosphate and phosphonate electrocatalysts, some urgent issues, such as low intrinsic catalytic efficiency and low electronic conductivity, have to be resolved in accordance with their applications. In this Review, we focus on the design strategies of highly efficient transition-metal phosphate and phosphonate electrocatalysts, with special emphasis on the tuning of transition-metal-center coordination environment, optimization of electronic structures, increase of catalytically active site densities, and construction of heterostructures. Guided by these strategies, recently developed transition-metal phosphate and phosphonate materials have exhibited excellent activity, selectivity, and stability for various energy-related electrocatalytic reactions, showing great potential for replacing noble-metal-based catalysts in next-generation advanced energy techniques. The existing challenges and prospects regarding these materials are also presented.

Inter-regulated d-band centers of the Ni3B/Ni heterostructure for boosting hydrogen electrooxidation in alkaline media

Ni₃B/Ni heterostructures have been constructed, which exhibit exceptional catalytic performance toward the hydrogen oxidation reaction (HOR) under alkaline media, with the mass activity being about 10 times greater than that of Ni₃B and Ni, respectively, ranking among the most active platinum-group-metal-free electrocatalysts. Experimental results and theoretical calculations confirm electron transfer from Ni₃B to Ni at the Ni₃B/Ni interface, resulting in inter-regulated d-band centers of these two components. This inter-regulation gives rise to optimized binding energies of intermediates, which together contribute to enhanced alkaline HOR activity.