Clean and Affordable Hydrogen Fuel from Alkaline Water Splitting: Past, Recent Progress, and Future Prospects

A designed hierarchical porous Cu-Ni/Ni-Cu alloy converted from commercial nickel foam as versatile electrocatalysts for efficient and extremely stable water splitting

The lack of efficient and stable electrodes is a major issue hindering the wide application of alkaline water electrolysis (AWE). Here, through the designed gaseous oxidation-reduction (GOR) strategy, uniform nanopores are in-situ fabricated throughout each skeleton of commercial nickel foam (NF). Subsequently, an innovative combination of electrodeposition and a second GOR process is employed to simultaneously achieve nickel/copper alloying and the generation of more abundant nanopores. By integrating the hundred-micron pores within the skeleton of NF, a hierarchical porous Cu-Ni/Ni-Cu alloy (hp Cu-Ni/Ni-Cu) is synthesized. Three-dimensional hierarchical porous skeleton not only offers rapid electron transfer/mass transport but also provides abundant active sites with improved adsorption and desorption kinetics for reactive hydrogen intermediates. As a result, the hp Cu-Ni/Ni-Cu electrode exhibits superior alkaline HER electrocatalysis, achieving a current density of 10 mA cm-2 at a low overpotential of 31 mV. Moreover, the hp Cu-Ni/Ni-Cu-based alkaline electrolyzers also display a low voltage with 1.45 V at 20 mA cm-2 and retain excellent durability of more than 1700 h (∼2.5 months), outperforming alkaline electrolyzers composed of precious RuO2 and Pt, as well as most alkaline electrolyzers.

Synergizing ternary CoMoW alloy with CeO2 for enhancing electrocatalytic hydrogen evolution

Integrating metal materials with metal oxides to construct a multi-component electrocatalyst represents an effective strategy to develop high-performance electrocatalysts for hydrogen evolution reaction (HER). In this work, we design and synthesize a synergistic heterogeneous electrocatalyst comprising of CoMoW ternary metal alloys and ceria (CeO2) nanosheet supported on nickel foam (NF) via a facile and fast electrodeposition method. The strong electronic interaction between CoMoW and CeO2 not only improves charge transfer but also promotes water dissociation and optimizes hydrogen adsorption capability, thereby improving the kinetics of HER. Benefiting from this properly designed interface, the optimized CoMoW-CeO2/NF delivered a low overpotential of only 35.25 mV at 10 mA cm-2 for HER, which is superior to many reported similar catalysts. This work offers an effective approach for the design and construction of high-performance heterogeneous electrocatalyst comprising of ternary alloys and metal oxides.

Tailored electronic interaction between metal-support trigger reverse hydrogen spillover for efficient hydrogen evolution

The triggering of fast hydrogen spillover through regulating the charge rearrangement of the metal-support serves as a crucial mechanism for decoupling the activity of HER catalysts from the adsorption properties, which not only contributes to enhancing the performance of the catalysts but also facilitates the production of green hydrogen. Herein, we tailor the electronic interaction between two-dimensional (2D) nitrogen-doped MoC (N-MoC) nanosheets and anultra-low content of Pt nanoclusters (1 wt%) to trigger reverse hydrogen spillover and modulate the electronic structure of Pt, thus achieving efficient and stable HER. Compared to Pt/C (0.229 A mgPt-1), Pt/N-MoC demonstrates a mass activity of 12.945 A mgPt-1, representing an enhancement of nearly 57.5 times. Notably, the excellent electrocatalytic performance was verified in the proton exchange membrane water electrolyzer configuration. Combining experimental and theoretical analysis, anultra-low load of Pt nanocluster (1 wt%) integrated with N-MoC nanosheets can induce a charge transfer from N-MoC to Pt, thus modulating the d-band center of Pt to improve the hydrogen adsorption properties and achieving fast hydrogen desorption (ΔG = 0.019 eV); furthermore, a small difference in work function between Pt nanoclusters and the N-MoC were achieved to dilute charge accumulation between the metal-support interface, thus reducing the energy barrier of hydrogen spillover.

Single-source-precursor synthesis of porous Ni2Si/SiOC composites as oxygen evolution reaction electrocatalysts

In this study, a novel carbon-rich Ni2Si/SiOC ceramic composite was successfully synthesized via a polymer-derived ceramic approach, which was found to be catalytically active for the oxygen evolution reaction (OER). The synthesis involved the reaction of a commercially available carbon-rich polysiloxane precursor SPR-684 and nickel acetylacetonate, forming a Ni-containing single-source-precursor (SSP). Furthermore, to investigate the influence of porosity on OER performance, polystyrene (PS) was used as a sacrificial template for pore formation. Thermal treatment of the obtained mixture of SSP and PS at 1400 °C led to the encapsulation of Ni2Si particles by structurally ordered carbon, potentially enhancing the electrical conductivity of the composite. Additionally, the sample prepared at 1400 °C presented weight fractions of the crystalline phases of Ni2Si and SiC in the amounts of 39.6 wt% and 29.3 wt%, respectively. Owing to the support of increased conductive carbon formation, the sample obtained at 1400 °C demonstrated the best overpotential of 336 mV versus the reversible hydrogen electrode (RHE) at a current density of 10 mA cm-2 in 1 M KOH, indicating its enhanced performance for the OER.

Harnessing Work Function Modulation for Hydrogen Evolution Catalysis in Mesoporous Bimetallic Pt-M Alloys: The Role of Mesopores in Work Function Optimization

Work function (WF) influences electron transport and intermediates adsorption, enabling charge balance and catalytic optimization for the hydrogen evolution reaction (HER). However, the understanding of the role of mesopores and the relationship between composition and WF in pristine Pt-based alloys remains lacking. Herein, various mesoporous binary Pt-M alloy films (m-Pt-M, M = Pd, Rh, and Ru) with uniform pores and elemental distributions are synthesized, providing an experimental platform to investigate this relationship. It has been demonstrated that the WFs of m-Pt-M catalysts are strongly influenced by their compositions and mesoporous structures, thereby impacting HER activities. Among them, m-Pt-Ru with tailored WF lowers the thermodynamic energy barrier and accelerates the kinetic processes of HER. The mass activity of m-Pt-Ru in alkaline media is 17.8× and 5.1× higher, compared to Pt black and m-Pt, respectively. This work not only provides a simple method for the fabrication of well-defined binary metallic alloy films but also offers experimental insights into the rational design of highly efficient electrocatalysts with tunable WFs.

Delineating the multifunctional performance of Janus WSSe with nonmetals in water splitting and hydrogen fuel cell applications via first-principles calculations

The development of cost-effective and highly efficient multifunctional catalysts for water splitting and hydrogen fuel cells is crucial for advancing renewable energy technologies. This study employs density functional theory to investigate the electrocatalytic performance of Janus-type WSSe (JW) transition metal dichalcogenides in the hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and oxygen reduction reaction (ORR). Additionally, the impact of nonmetal doping (NM = C, O, N, P) at the S and Se sites in the JW structure is explored. A cohesive energy of -5.82 eV per atom and minimal fluctuations in AIMD simulations over 10 ps at 300 K and 500 K confirm its structural stability. Although the pristine structure exhibits a high overpotential, NM doping substantially improves catalytic performance, making it more suitable for efficient energy conversion applications. The N doped JW system demonstrates exceptional multifunctional performance, with NS@JW showing overpotentials of 0.34 V (HER), 0.18 V (OER), and 0.14 V (ORR), while NSe@JW exhibits overpotentials of 0.35 V (HER), 0.46 V (OER), and 0.24 V (ORR). This outstanding performance results from bonding-antibonding interactions in intermediate adsorption, as confirmed by crystal orbital Hamiltonian population analysis. This comprehensive study highlights the potential of Janus-type WSSe and emphasizes the crucial role of NM doping in boosting catalytic efficiency, offering key insights for designing cost-effective, high-performance multifunctional electrocatalysts for energy conversion.

Measurements of Local pH Gradients for Electrocatalysts in the Oxygen Evolution Reaction by Electrochemiluminescence

An accurate understanding of the mechanism of the oxygen evolution reaction (OER) is crucial for catalyst design in the hydrogen energy industry. Despite significant advancements in microscopic pH detection, selective, sensitive, speedy, and reliable detection of local pH gradients near the catalysts during the OER remains elusive. Here, we pioneer an electrochemiluminescence (ECL) method for local pH detection during the OER. For this purpose, a new class of ECL emitters based on ECL resonance energy transfer was theoretically predicted and facilely synthesized by grafting functional fluorescent dyes onto noble 2D carbon nitride. By positioning one of the as-prepared ECL emitters with pH-responsibility neighboring the OER catalysts, local pH gradient generation near the catalysts could be qualitatively measured in real-time with a subsecond resolution. It provided details of the reaction mechanism of the OER and unveiled the catalyst degrading pathway caused by proton accumulation. Besides, the average proton generation rate on the catalyst was also extractable from the local pH measurement as a quantitative descriptor of the OER reaction rate. Owing to the high designability of the grafting method, this study opens up new strategies for studying reaction mechanisms and detecting intermediates.

Enhancing hydrogen evolution by heterointerface engineering of Ni/MoN catalysts

Molybdenum nitrides have garnered significant attention for their potential in the hydrogen evolution reaction (HER) due to their metallic behavior, abundant reserves, and pH-universal stability. However, their unsatisfactory hydrogen adsorption limits industrial applications. Heterostructures can be designed to introduce defects and modulate the electronic structure of catalysts to optimize hydrogen adsorption to enhance HER. Nevertheless, the exact active sites at the heterointerface and the fundamental mechanisms underlying the HER process remain inadequately understood. Herein, a composite electrocatalyst in which metallic Ni and MoN phases (Ni/MoN) form the heterointerface between them is fabricated. The heterointerface produces strong electronic interactions between Ni and MoN to facilitate electron transfer from MoN to Ni, and the built-in electric field facilitates charge transfer during electrocatalysis. This optimized electronic configuration with abundant active sites delivers excellent performance in alkaline HER. Density-functional theory calculations demonstrate that H2O dissociates at the Ni site, whereas H2 desorption occurs at the Mo site. As a result, Ni/MoN/CC requires an overpotential of only 95 mV to achieve a current density of 10 mA cm-2 and a Tafel slope of 104 mV dec-1. Moreover, it maintains a high current density of 100 mA cm-2 for 100 h with negligible morphological or compositional changes. The strategy of modulating the electronic structure of low-cost, transition metal-based heterostructured electrocatalysts is an effective and commercially viable means to design and develop high-performance electrocatalysts for water splitting.

Synergistic Effect of NiMoO4 Nanorods with Polyaniline for Efficient Electrochemical Water Splitting

Electrochemical water splitting has emerged as a promising solution for sustainable hydrogen production, but the development of efficient, durable, and cost-effective bifunctional electrocatalysts remains a critical challenge. In this work, we report the novel fabrication of composite materials consisting of nickel molybdate (NiMoO4) coated with polyaniline (PANI). NiMoO4 nanorods were initially synthesized on nickel foam (NF) using a hydrothermal technique and subsequently coated with PANI via UV-assisted polymerization. The resulting NiMoO4@PANI nanostructures demonstrate increased active sites for improved efficiency in electron transfer and catalytic activity. This combination enhanced hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) performance, achieving reduced overpotential values of 88 and 167 mV for HER and OER at 10 mA cm-2, respectively. Comprehensive electrochemical evaluations, including Tafel slope, electrochemical impedance spectroscopy (EIS), and electrochemical double-layer capacitance (Cdl) measurements, validate the enhancements in kinetics and charge transfer facilitated by the PANI coating. Density functional theory (DFT) calculations offer further insights into the improved catalytic efficiency, showing reduced barrier for water splitting (ΔGb = 0.45 eV), nearly negligible hydrogen adsorption energy (ΔG*H = 0.08 eV), appropriate adsorption energy of oxygen evolution (ΔG*OOH - ΔG*OH = 2.58 eV), and high density of states close to the Fermi level. The NiMoO4@PANI nanostructures exhibit excellent stability for 310 h without interruption, suggesting the potential for sustainable hydrogen production.

Confined RuP2 Nanoparticles in N,P,S-Tridoped Carbon as Superior Electrocatalyst for pH-Wide Hydrogen Evolution

Hydrogen has been deemed as the ideal energy source and carrier due to its unmatched energy efficiency and sustainability. Nevertheless, there is a pressing need to develop cost-effective materials to replace costly Pt in the hydrogen evolution reaction (HER), and the electrocatalysts with low overpotential and robust stability under various conditions is a particularly significant concern. In this study, a straightforward and effective approach was proposed for the precise synthesis of RuP2 nanoparticles encapsulated in N, P, S-tridoped carbon, which involves utilizing zinc pyrithione, phytic acid and Ru salt as starting materials. The effect of different Ru loadings on the morphology and structures of the composite catalysts was examined carefully. The obtained composites exhibit superior alkaline activity surpassing commercial Pt/C and comparable acidic and neutral activity as well as excellent pH-wide stability. DFT computations reveal the integration of RuP2 with tridoped carbon can tailor the electronic structure of Ru active sites by interfacial electron transfer, thus optimizing the adsorption energy and promoting the HER activity. The benign graphitization of doped carbon and porous structures ensure the smooth charge and mass transfer during HER process. The cost-effective and straightforward synthesis methods presented in this work offer a promising alternative to commercial Pt/C for practical hydrogen-related applications.

Heterogeneous Interfaces of Ni3Se4 Nanoclusters Decorated on a Ni3N Surface Enhance Efficient and Durable Hydrogen Evolution Reactions in Alkaline Electrolyte

Transition metal selenides (TMSes) have been identified as cost-efficient alternatives to platinum (Pt) for the alkaline hydrogen evolution reaction (HER) owing to their distinct electronic properties and excellent conductivity. However, they encounter challenges such as sluggish water dissociation and severe oxidative degradation, requiring further optimizations. In this study, we developed a dual-site heterogeneous catalyst, Ni3Se4-Ni3N, by decorating Ni3Se4 nanoclusters on a Ni3N substrate. This catalyst design promoted significant interfacial electronic interactions, modulated electronic structures, and enhanced the adsorption of the intermediates. Various spectroscopic analyses and theoretical calculations revealed that the nitride surfaces improved water adsorption and dissociation, enriching the surface with adsorbed hydrogen (H*) atoms, while the Se sites facilitated hydrogen coupling and subsequent release of H2. Following a hydrogen spillover mechanism, the surface-adsorbed hydrogen atoms were transferred to nearby electron-dense selenide sites for H2 formation and release. Consequently, the optimized catalyst demonstrated improved HER activity, requiring only an ∼60 mV overpotential at 10 mA cm-2 current density and maintained stability under higher potential conditions.

Unprecedented Hydrogen Evolution Reactions Based on the Accelerating Effect of [Co-Tb]-Supramolecular Complex-Anchored CdS Heterojunctions

This study presents a series of 1-Tb/CdS binary heterojunctions, synthesized by combining 1-Tb (a Co3+-Tb3+-based supramolecular complex) and CdS in various weight ratios via a solvothermal process. These heterojunctions have been thoroughly characterized to elucidate their chemical, structural, and optoelectronic properties. The catalytic effectiveness of these heterojunctions was evaluated with respect to the hydrogen evolution reaction (HER), spanning photocatalytic, electrocatalytic, and photoelectrocatalytic processes. Significantly outperforming individual photocatalysts (CdS and 1-Tb) and other binary heterostructures, the 7.5-1-Tb/CdS heterojunction exhibited the highest catalytic HER efficiency. The exceptional catalytic performance of 7.5-1-Tb/CdS is attributed to a synergistic integration of 1-Tb and CdS, enabling a highly efficient Z-scheme heterojunction. This unique architecture enhances charge separation and transfer by leveraging the complementary electronic properties of the individual CdS and 1-Tb semiconductors while minimizing recombination losses. X-ray photoelectron spectroscopy, band structure analysis, photoluminescence, and electrochemical impedance spectroscopy further validated the Z-scheme mechanism, highlighting the optimal alignment of energy levels. Overall, this study highlights the potential of 1-Tb/CdS binary heterojunctions as robust and efficient catalysts for the HER. The simplicity of the synthesis process, coupled with the exceptional catalytic activity, offers a significant advancement in clean energy technologies, paving the way for sustainable hydrogen production.

Solar-to-Hydrogen Conversion Efficiency for Photovoltaic Water Electrolysis to Produce Green Hydrogen

Hydrogen is widely regarded as a key energy source for the 21st century, offering sustainability, environmental benefits, and energy storage capacity. Solar-powered water splitting is a frontier technology for green hydrogen production, circumventing reliance on fossil fuels. Advances in solar cells and electrocatalysis have significantly improved hydrogen production via photovoltaic-electrolysis (PV-EC). However, solar-to-hydrogen (STH) conversion efficiency is still limited by factors such as solar cell performance, electrolysis efficiency, and system integration. Optimizing these elements is essential for enhancing overall efficiency. This review focuses on the critical technologies influencing STH efficiency in PV-EC systems. Specifically, the efficiency of photovoltaic devices in harnessing solar energy, the catalytic performance of electrocatalytic materials for efficient water splitting, and the integration of solar cells with electrolyzer systems to optimize overall energy conversion. Furthermore, the latest developments and ongoing challenges in PV-EC water splitting research are explored, evaluating their economic feasibility and offering a perspective on future advancements in photovoltaic water electrolysis.

Revisiting Water Oxidation Reaction with Micro Bubble Lithography (MBL) Printed ZIF-67 MOF Electrocatalysts

Microbubble-based micro-lithographic techniques have developed rapidly over the last ten years and are capable of reproducibly patterning a wide variety of soft materials and colloids, including polymers, metals, and proteins. Zeolitic imidazolate framework (ZIF) materials have attracted a great deal of research and application interest in the field of materials science because of their chemical and thermal stabilities. Furthermore, ZIF-67 has demonstrated significant potential for applications in gas adsorption, molecule separation, electrochemistry, and catalysis, which when converted into "lab-on-a-chip" platforms might produce remarkable and diverse application-oriented outcomes. This is due to their highly adjustable nanostructures. Using Co(OAc)2.4H2O and Co(NO3)2.6H2O as the metal ion sources and 2-methylimidazole as the ligand, To design ZIF-67 (composed of Co2+ ions and imidazolate ligands) is attempted. Inspired by previous results, the Micro-Bubble Lithography (MBL) approach is used to successfully demonstrate an instantaneous in situ green synthesis and micro-patterning of ZIF-67 MOFs in this work. With reasonable stability and an over-potential of 440 mV, these micro-patterns are used as microelectrodes for the electrocatalytic oxygen evolution reaction (OER) in media having different pH.

Co3O4-RuO2/Ti3C2Tx MXene Electrocatalysts for Oxygen Evolution Reaction in Acidic and Alkaline Media

MXene 2D materials and non-noble transition metal oxide nanoparticles have been proposed as novel pH-universal platforms for oxygen evolution reaction (OER), owing to the enhancement of active site exposures and conductivity. Herein, Co3O4-RuO2 /Ti3C2Tx/carbon cloths (CRMC) were assembled in a facile way as an efficient OER platform through a hydrothermal process. The Co3O4-RuO2/Ti3C2Tx demonstrated prominent OER catalytic performance under acidic and alkaline conditions, which showed overpotential values of 195 and 247 mV at 10 mA cm-2 with Tafel slopes of 93 and 97 mVdec-1, respectively. The experimental results demonstrated that the electron transfer from Co3O4-RuO2 to Ti3C2Tx/carbon cloth played a remarkable role in promoting OER catalytic activity. Further OER characterization indicated that the enhanced multi-electron reaction kinetics are attributed to Co and Ru acting as the primary active places for O2 adsorption and activation, which facilitated the generation of *OOH intermediate.

Construction of Ni2P/WS2/CoWO4@C multi-heterojunction electrocatalysis derived from heterometallic clusters for superior overall water splitting

The reasonable design of an economical and robust bifunctional electrocatalyst for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is both essential but challenging. Herein, we synthesized a multi-interfacial Ni2P/WS2/CoWO4@C hybrid electrocatalyst devived from the heterometallic clusters [Co24(TC4A)6(WO4)8Cl6][HPW12O40], in which Ni2P was incorporated into WS2/CoWO4@C nanosheets via interfacial interactions by in situ phosphorization processes. Theoretical calculations revealed that moderate electron transfer from CoWO4 and Ni2P to WS2 induced by the multi-heterojunction significantly regulate the binding energies of the reactive intermediates, thus enhacing its intrinsic activity. Under alkaline medium, the overpotential of the optimized Ni2P/WS2/CoWO4@C electrocatalyst for OER and HER is only 206 mV and 90 mV at 10 mA cm-2, respectively, with extraordinary stability more than 100 h, and the potential of overall water splitting is only 1.46 V. This work motivates further research and presents a reliable design route for other heterojunction engineered cost-effective bifuctional electrocatalysts.

Doping Mo Triggers Charge Distribution Optimization and P Vacancy of Ni2P@Ni12P5 Heterojunction for Industrial Electrocatalytic Production of Adipic Acid and H2

Synchronous electrosynthesis of value-added adipic acid (AA) and H2 is extremely crucial for carbon neutrality. However, accomplishing the preparation of AA and H2 at large current density with high selectivity is still challenging. Herein, a robust Mo-doped Ni2P@Ni12P5 heterojunction with more P vacancies on Ni foam is proposed for accomplishing simultaneous electrooxidation of cyclohexanol (CHAOR) to AA and hydrogen evolution reaction (HER) at large current density. Combined X-ray photoelectron spectroscopy, X-ray absorption fine structure, and electron spin resonance confirm that Mo incorporation induces the charge redistribution of Ni2P@Ni12P5, where Mo adjusts electrons from Ni to P, and triggers more P vacancies. Further experimental and theoretical investigations reveal that the d-band center is upshifted, optimizing adsorption energies of water and hydrogen on electron-rich P site for boosting HER activity. Besides, more Ni3+ generated from electron-deficient Ni induced by Mo, alongside more OH* triggered from more P vacancies concurrently promote CHA dehydrogenation and C─C bond cleavage, decreasing energy barrier of CHAOR. Consequently, a two-electrode flow electrolyzer achieves industrial current density (>230 mA cm-2) with 85.7% AA yield, 100% Faradaic efficiency of H2 production. This study showcases an industrial bifunctional electrocatalyst for AA and H2 production with high productivity.

Highly Stable Bifunctional Heterostructured Electrocatalyst Integrated with LDPE-Derived Spherical Carbon for Longevous Alkaline Seawater Splitting

The development of innovative electrocatalysts for seawater splitting shows great potential for large-scale green energy. Specifically, interface engineering plays a vital role in improving surface properties and charge transfer. However, seawater electrolysis encounters considerable challenges like chloride-induced corrosion, impurities, and microorganisms that hinder efficiency. Herein, we design a highly durable electrocatalyst based on selenium-enriched NiMn-Sx  supported on low-density polyethylene-derived spherical carbon-Ni foam (Se-NiMnSx@SC/NF) using combination of pyrolysis and hydrothermal processes. The resulting Se-NiMnSx@SC/NF bifunctional catalyst with hollow cycas cone structure exhibited exceptional electrochemical performance and corrosion resistance in alkaline seawater with an ultralow overpotential of 146 and 262 mV for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) to achieve a large current density of 500 mA cm⁻2. In a simulated alkaline seawater splitting setup, the Se-NiMnSx@SC/NF catalyst maintained a cell voltage of 2.07 V at 500 mA cm⁻2, demonstrating outstanding durability for over 100 h with ≈100% Faradaic efficiency. Se and S doping in the heterostructured electrocatalyst refines the electronic structure and boosts reaction kinetics, while the hollow cycas cone design increases the exposure of active sites. Additionally, the carbon layer provided strong resistance to seawater corrosion, making Se-NiMnSx@SC/NF an excellent bifunctional catalyst for alkaline seawater electrolysis.

CNT-Supported RuNi Composites Enable High Round-Trip Efficiency in Regenerative Fuel Cells

Regenerative fuel cells hold significant potential for efficient, large-scale energy storage by reversibly converting electrical energy into hydrogen and vice versa, making them essential for leveraging intermittent renewable energy sources. However, their practical implementation is hindered by the unsatisfactory efficiency. Addressing this challenge requires the development of cost-effective electrocatalysts. In this study, a carbon nanotube (CNT)-supported RuNi composite with low Ru loading is developed as an efficient and stable catalyst for alkaline hydrogen and oxygen electrocatalysis, including hydrogen evolution, oxygen evolution, hydrogen oxidation, and oxygen reduction reaction. Furthermore, a regenerative fuel cell using this catalyst composite is assembled and evaluated under practical relevant conditions. As anticipated, the system exhibits outstanding performance in both the electrolyzer and fuel cell modes. Specifically, it achieves a low cell voltage of 1.64 V to achieve a current density of 1 A cm- 2 for the electrolyzer mode and delivers a high output voltage of 0.52 V at the same current density in fuel cell mode, resulting in a round-trip efficiency (RTE) of 31.6% without further optimization. The multifunctionality, high activity, and impressive RTE resulted by using the RuNi catalyst composites underscore its potential as a single catalyst for regenerative fuel cells.

Suppressing Mo-Species Leaching in MoOx/A-Ni3S2 Cathode for Stable Anion Exchange Membrane Water Electrolysis at Industrial-Scale Current Density

The development of non-noble metal-based hydrogen evolving reaction (HER) electrocatalysts operating under high current density plays a critical role in the large-scale application of anion exchange membrane water electrolysis (AEM-WE). Herein, a porous and hybrid MoS2/Ni3S2 is synthesized on nickel foam (NF) via a one-step hydrothermal method and studied its reconstruction process during alkaline HER conditions. Experimental results indicated that the MoS2 underwent an oxidative dissolution followed by a dynamic equilibrium between dissolution and redeposition of the amorphous MoOx during HER. Meanwhile, S-vacancy-rich Ni3S2 (A-Ni3S2) is exposed and acts as the real active site for HER. The obtained MoOx/A-Ni3S2 catalyst exhibited high catalytic performance in three-electrode systems and single-cell AEM-WE. Finally, for a long-term durability test in the AEM electrolyzer, a dry cathode method is applied to suppress the Mo species leaching from the MoOx/A-Ni3S2 electrode. Remarkably, the device assembled by MoOx/A-Ni3S2 as the cathode catalyst and NiFe as the anode catalyst demonstrated a high stability of 2500 h at 2 A cm-2 and 40 °C with a small aging rate of 30 µV h-1.

Hollow CoNiFe ternary metal selenide electrocatalysts derived from Prussian blue analogues for boosting the oxygen evolution reaction

The development of effective electrocatalysts is a top priority for the oxygen evolution reaction (OER), which is the crucial half-reaction of water electrolysis, since electrocatalytic water splitting for hydrogen production offers a practical solution to the upcoming energy crisis. Herein, we report a strategy to fabricate hollow ternary metal selenide (CoNiFe-Se) nanocubes derived from Prussian blue analogues (PBAs) by phytic acid etching and low-temperature gas-phase selenization. Due to the advantages of its multi-component composition and hollow structure, CoNiFe-Se exhibited a low overpotential of 275 mV@10 mA cm-2, a small Tafel slope of 62.3 mV dec-1 and a long-term stability of more than 80 h in 1.0 M KOH. This research offers an innovative idea and a straightforward technique for preparing hollow multimetallic selenide electrocatalysts derived from PBAs.

Interfacial-Free-Water-Enhanced Mass Transfer to Boost Current Density of Hydrogen Evolution

The advancement of water electrolysis highlights the growing importance of electrolyzers capable of operating at high current densities, where mass transfer dynamics plays a crucial role. In the electrode reactions, the interfacial water is a key factor in regulating these dynamics. However, the potential of utilizing interfacial-free water (IFW) to modulate electrode behavior remains underexplored. Herein, we investigate the effect of interfacial water structure on hydrogen evolution reaction (HER) performance across different current density ranges, using designed platinum-coated nickel hydroxide on nickel foam (Pt@Ni(OH)2-NF) electrodes. We reveal that with increasing current density, changes in interfacial water structure alter the rate-determining step of the HER. Pt@Ni(OH)2-NF exhibited excellent performance in alkaline electrolytes, achieving 1000 mA cm-2 at 114 mV overpotential. This study provides a novel approach to optimizing alkaline water electrolysis dynamics by enhancing mass transfer, further paving the way for more efficient and energy-saving hydrogen production.

Zr-doped porous Ni2P nanoarray as a highly efficient electrocatalyst for hydrogen evolution reaction in alkaline seawater

The utilization of seawater electrolysis is recognized as a promising method for generating hydrogen as a substitute for conventional technology. Herein, the electrodes were fabricated byin-situgrowth of Zr-Ni2P arrays on a nickel foam substrate (Zr-Ni2P/NF) through a low-temperature hydrothermal and phosphating process. The Zr-Ni2P/NF can achieve efficient hydrogen evolution reaction (HER) performance in alkaline seawater electrolyte. This Zr-Ni2P/NF shows overpotentials as low as 197 and 274 mV at 100 and 500 mA cm-2for HER in alkaline seawater, respectively. Moreover, it exhibits strong stability for at least 50 h of electrolysis at a current density of 500 mA cm-2in alkaline seawater with the overpotential attenuation of 38 mV. The excellent performance and hierarchical structure advantages of Zr-Ni2P/NF provide new ideas for designing efficient seawater splitting electrocatalysts.

Visualization of the Key Proton Activities in Hydrogen Evolution Reaction by Electrochromic Catalyst

The electrocatalytic hydrogen evolution reaction (HER) is a promising route to produce sustainable hydrogen energy carrier for global carbon neutrality. The HER performance is largely determined by the overall proton activities, but the identification of such key proton activities in microscopic HER process is rather difficult. Herein, the study demonstrates a visualized HER concept by integrating the fundamental HER process with electrochromic technology on a well-designed Pt@WO3 platform in acidic electrolyte, where the overall proton activities in HER process can be rapidly discriminated by the color changes of Pt@WO3 electrochromic electrode. In contrast to bare WO3 counterpart, the Pt@WO3 electrochromic electrode displays a rather more positive potential of initial-coloration state and faster decoloration rate associated with significantly improved reaction kinetics of hydrogen intercalation and deintercalation within WO3 component. Correspondingly, the as-prepared Pt@WO3 catalyst electrode exhibits a remarkable HER activity with a lower onset-potential (45 mV, proton adsorption and accumulation) and smaller Tafel slope (50 mV dec-1, proton desorption), nearly 11.1- and 3.5-fold enhancement than those of bare WO3 counterpart. It is believed that the work in integrating the interesting visualization functionality into fundamental HER process may improve the readability of such microscopic electrocatalytic reaction and advance the exploration of more intelligent electrocatalysts.

Modulating Support Effect in High-Entropy Sulfide via La Single-Atom for Boosted Oxygen Evolution

Reduced energy barrier induced enhanced oxygen evolution reaction (OER) kinetics can be achieved by implementing an efficient electrocatalyst. Herein, positive effect of lanthanum (La) single-atom modified hollow carbon sphere (HCS) support on OER activity of high-entropy sulfide (HES) material (FeCoNiCrCuAl)S has been reported. Briefly, La single-atom boosts the aggregation of electrons at adjacent Fe, Co, Ni, Cr, and Cu sites and dissipation of electrons at Al site in HES material, facilitating reconstruction of electronic structure and down-shifting their d-band center away from Fermi level, resulting in reduced adsorption energy of OER intermediates. As developed (FeCoNiCrCuAl)S@La-HCS depicts high OER performance with an overpotential of only 297 mV at 100 mA cm-2, surpassing (FeCoNiCrCuAl)S@HCS (324 mV) and commercial RuO2 catalyst (419 mV). This work provides an insight into the integration of single atom with high-entropy sulfide toward efficient oxygen evolution.

Lamellar Nanoporous Intermetallic Cobalt-Titanium Multisite Electrocatalyst with Extraordinary Activity and Durability for the Hydrogen Evolution Reaction

Constructing well-defined multisites with high activity and durability is crucial for the development of highly efficient electrocatalysts toward multiple-intermediate reactions. Here we report negative mixing enthalpy caused intermetallic cobalt-titanium (Co3Ti) nanoprecipitates on a lamellar hierarchical nanoporous cobalt skeleton as a high-performance nonprecious multisite electrocatalyst for an alkaline hydrogen evolution reaction. The intermetallic Co3Ti as a robust multisite substantially boosts the reaction kinetics of water dissociation and hydrogen adsorption/combination by unisonous adsorptions of hydrogen and hydroxyl intermediates with proper binding energies. By virtue of a bicontinuous and hierarchical nanoporous cobalt skeleton that enables sufficiently accessible Co3Ti multisites and facilitates electron transfer and ion/molecule transportation, a self-supported nanoporous cobalt-titanium heterogeneous electrode exhibits extraordinary electrocatalytic activity and durability toward the hydrogen evolution reaction in 1 M KOH. It reaches a current density of as high as ∼3.31 A cm-2 at a low overpotential of 200 mV and maintains exceptional stability at ∼1.33 A cm-2 for >1000 h.

Recent Strategies for Ni3S2-Based Electrocatalysts with Enhanced Hydrogen Evolution Performance: A Tutorial Review

Water electrolysis represents one of the most environmentally friendly methods for hydrogen production, while its overall efficiency is primarily governed by the electrocatalyst. Nickel sulfides, e.g., Ni3S2, are considered to be highly promising catalysts for the hydrogen evolution reaction (HER) due to their distinctive chemical structure. However, the practical application of Ni3S2-based electrocatalysts is hindered by unsatisfactory high overpotential in the HER and weakened catalytic performance under alkaline conditions. Therefore, in this regard, further research on Ni3S2-based catalysts is being carried out to tackle these challenges. This review provides a comprehensive survey of the latest advancements in Ni3S2-based in improving the HER performance of Ni3S2-based electrocatalysts. The review may offer some inspiration for the rational design and synthesis of novel transition metal-based catalysts with enhanced water electrolysis performance.

Effects of Hydration Level and Hydrogen Bonds on Hydroxide Transport Mechanisms in Anion Exchange Membranes

The transport of hydroxide in anion exchange membranes (AEMs) is generally determined by multiple factors, including hydration levels, pore morphologies, and the hydration shells of cationic groups and hydroxides. Thus, clarifying the working mechanisms benefits the proposal of strategies for enhancing the hydroxide transport, thereby enabling a rational design of high-performance AEMs. Herein, by using ReaxFF molecular dynamics (MD) simulations and RDAnalyzer, this study explores the straightforward but effective correlations for steric hindrance versus hydration shell, hydration level versus free/associated diffusion, and strong (short) hydrogen bond (SHB) versus vehicular/Grotthuss diffusion. The theoretical investigations indicate that higher steric hindrance of cationic groups results in less water in the first hydration shell of cationic groups in AEMs. Meanwhile, a higher hydration level facilitates wider hydrophilic pores of AEMs and increases the ratio of the free diffusion mechanism of hydroxides. Interestingly, this study finds a strong correlation between the number of SHBs and the Grotthuss diffusion, thereby enhancing the understanding of the high conductivity of covalent organic framework (COF)-based AEMs that contain obvious SHBs. This work provides a theoretical view for fine-tuning the free/associated and vehicular/Grotthuss transport of hydroxide in AEMs.

Synergistic Reconstruction of Defect-Enriched NiFe-LDH Hierarchical Structures toward Large-Current and Stable Oxygen Evolution Reaction

NiFe layered double hydroxide (LDH) is the benchmark electrocatalyst toward alkaline oxygen evolution reaction (OER), however, it remains a grand challenge to develop NiFe LDH catalysts with higher intrinsic catalytic activity and abundant active sites by a simple and facile method. In this study, a synergistic reconstruction approach is introduced to fabricate defect-enriched NiFe layered double hydroxide (d-NiFe LDH) with three-dimensional (3D) hierarchical structures. Through in situ synergistic reconstruction of molybdates and phytic acid ligands, rapid generation of d-NiFe LDH two-dimensional nanosheets on one-dimensional nanorods is achieved. The d-NiFe LDH displays elevated intrinsic catalytic activity, with the 3D hierarchical structures exposing a greater number of active sites. Leveraging these characteristics, the electrode demonstrates outstanding OER catalytic performance with minimal overpotentials of 204 and 282 mV to reach current densities of 10 and 500 mA cm-2. Notably, this electrode maintains excellent stability for over 350 h at 500 mA cm-2. When coupled with a NiMoN electrode in a two-electrode system, low voltages of 1.47 and 1.73 V are needed to achieve 10 and 500 mA cm-2, respectively. The work paves a fresh doorway for developing defects and 3D structures to construct advanced electrocatalysts toward various catalytic communities beyond OER.

Isolated p-Block Antimony Atoms Activated CuO@Co-CN Enable High Performances for Water Splitting and Zn-Air Batteries

This study reports an effective strategy for designing 3D electrocatalyst via the deposition of ZIF67-derived Co-CN shell layer over CuO nanoarrays to form a CuO@Co-CN hybrid, followed by incorporation with p-block Sb single atoms (CuO@Co-CN/Sb) to obtain highly activated catalytic behaviors. Inheriting both the excellent intrinsic catalytic activity of the components and their synergy, the CuO@Co-CN/Sb material serves as a high-efficiency multifunctional catalyst for overall water splitting and zinc (Zn)-air batteries. The material yields a current density of 10 mA cm-2 at a low overpotential of 72 and 250 mV for the hydrogen evolution reaction and oxygen evolution reaction, respectively. Furthermore, an electrolyzer based on CuO@Co-CN/Sb shows remarkable performance with a derived current density of 0.5 A cm-2 at low cell voltage of 2.67 V and good stability for 50 h continuous operation at a high current density of 0.5 A cm-2. Simultaneously, Zn-air battery using the CuO@Co-CN/Sb material as air cathode yields a high open circuit voltage of 1.455 V and a discharge power density of 131.07 mW cm-2.

Photonic Sintering as an Electrode Structuring Process to Improve Electrocatalytic Activity and Durability in Anion Exchange Membrane Water Electrolysis

Hydrogen production via water electrolysis is essential for achieving carbon-free energy. However, enhancing the performance of these systems, particularly at the electrode level, remains challenging. Photonic sintering (PS) is proposed as a highly effective post-treatment method for electrodes, highlighting the importance of electrode design and optimization. PS significantly enhances the catalytic activity and durability of spinel-type copper-cobalt oxide-based anodes for the oxygen evolution reaction and Pt@C-based cathodes for the hydrogen evolution reaction, which are attributed to structural and chemical modifications, including active site control, optimized surface chemical bonding, improved catalyst-substrate adhesion, and generation of a reduced surface. PS-treated electrodes maintain well-preserved electrochemical active sites and pore structures, which are crucial for activation polarization and mass transport kinetics. Consequently, an anion exchange membrane water electrolysis cell with PS-treated electrodes achieved 89.57% cell efficiency, 3.91 W cm-2 area-specific power at 1.8 V, and a low degradation rate of 0.049 mV h-1 (at 0.5 A cm-2) and 0.136 mV h-1 (at 1.0 A cm-2) over 500 h. This research overcomes the traditional trade-off between activity and durability, indicating that PS can be widely applied across various energy fields, including electrochemical storage and conversion.

In Situ Grown RuNi Alloy on ZrNiNx as a Bifunctional Electrocatalyst Boosts Industrial Water Splitting

Alkaline water electrolysis represents a pivotal technology for green hydrogen production yet faces critical challenges including limited current density and high energy input. Herein, a heterostructured bimetallic nitrides supported RuNi alloy (RuNi/ZrNiNx) is developed through in situ epitaxial growth under ammonolysis, achieving exceptional bifunctional activity and durability for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in 1 m KOH electrolyte. The RuNi/ZrNiNx exhibits a HER current density of -2 A cm-2 at an overpotential of 392.8 mV, maintaining initial overpotential after 1000 h continuous electrolysis at -500 mA cm-2. For OER, it delivers a current density of 2 A cm-2 at 1.822 V versus RHE, and sustains stable operation for 705 h at 500 mA cm-2. Experimental and theoretical studies unveil that the charge redistribution-induced high-valence Zr centers effectively polarize H─O bonds and promote water dissociation, and the electron-deficient interface Ru sites optimize hydrogen desorption kinetics. Dynamic OH spillovers from Zr sites to the adjacent tri-coordinated Ni hollow sites in NiNx promote rapid *OH intermediate desorption and active site regeneration. Notably, the tri-coordinated Ni hollow sites in NiNx proximal to Zr atoms exhibit tailored adsorption strength for oxo-intermediates, enabling a more energetically favorable pathway for O2 production.

Progress in Cu-Based Catalyst Design for Sustained Electrocatalytic CO2 to C2+ Conversion

The electrocatalytic conversion of CO2 into valuable multi-carbon (C2+) products using Cu-based catalysts has attracted significant attention. This review provides a comprehensive overview of recent advances in Cu-based catalyst design to improve C2+ selectivity and operational stability. It begins with an analysis of the fundamental reaction pathways for C2+ formation, encompassing both established and emerging mechanisms, which offer critical insights for catalyst design. In situ techniques, essential for validating these pathways by real-time observation of intermediates and material evolution, are also introduced. A key focus of this review is placed on how to enhance C2+ selectivity through intermediates manipulation, particularly emphasizing catalytic site construction to promote C─C coupling via increasing *CO coverage and optimizing protonation. Additionally, the challenge of maintaining catalytic activity under reaction conditions is discussed, highlighting the reduction of active charged Cu species and materials reconstruction as major obstacles. To address these, the review describes recent strategies to preserve active sites and control materials evolution, including novel catalyst design and the utilization and mitigation of reconstruction. By presenting these developments and the challenges ahead, this review aims to guide future materials design for CO2 conversion.

Formation of electron-deficient Ni in a Nb/NiFe-layered double hydroxide nanoarray via electrochemical activation for efficient water oxidation

The intrinsically sluggish kinetics of the oxygen evolution reaction (OER) at the anode poses a formidable challenge to the industrial application of water electrolysis, although NiFe-based oxides and hydroxides have emerged as promising anodic candidates. Within this framework, we report the synthesis of Nb-doped NiFe-layered double hydroxides (Nb/NiFe-LDH) via a straightforward one-step hydrothermal approach. Notably, Nb doping maintained the structural integrity of the NiFe-LDH framework and it enhanced the valence state of the active Ni species during the electrochemical activation process, which accelerated the concomitant reconstruction kinetics of the LDH phase. As a result, Nb/NiFe-LDH demonstrated a remarkable overpotential of 198 mV to attain a current density of 10 mA cm-2 in an alkaline electrolyte. This work proposes a novel doping strategy for enhancing the performance of OER electrocatalysts.

Enhanced Hydrogen Evolution Reaction in Alkaline Media via Ruthenium-Chromium Atomic Pairs Modified Ruthenium Nanoparticles

Precisely optimizing the electronic metal support interaction (EMSI) of the electrocatalysts and tuning the electronic structures of active sites are crucial for accelerating water adsorption and dissociation kinetics in alkaline hydrogen evolution reaction (HER). Herein, an effective strategy is applied to modify the electronic structure of Ru nanoparticles (RuNPs) by incorporating Ru single atoms (RuSAs) and Ru and Cr atomic pairs (RuCrAPs) onto a nitrogen-doped carbon (N-C) support through optimized EMSI. The resulting catalyst, RuNPs-RuCrAPs-N-C, shows exceptional performance for alkaline HER, achieving a six times higher turnover frequency (TOF) of 13.15 s⁻¹ at an overpotential of 100 mV, compared to that of commercial Pt/C (2.07 s⁻¹). Additionally, the catalyst operates at a lower overpotential at a current density of 10 mA·cm⁻2 (η10 = 31 mV), outperforming commercial Pt/C (η10 = 34 mV). Experimental results confirm that the RuCrAPs modified RuNPs are the main active sites for the alkaline HER, facilitating the rate-determining steps of water adsorption and dissociation. Moreover, the Ru-Cr interaction also plays a vital role in modulating hydrogen desorption. This study presents a synergistic approach by rationally combining single atoms, atomic pairs, and nanoparticles with optimized EMSI effects to advance the development of efficient electrocatalysts for alkaline HER.

A Library of Polymetallic Alloy Nanotubes: From Binary to Septenary

The polymetallic alloy nanostructure has received widespread attention in electrocatalytic reactions due to the variability in composition and excellent performance. However, the controllable synthesis of one-dimensional (1D) polymetallic alloy nanomaterials remains a significant challenge. Herein, we propose a new and general low-temperature method to prepare a library of binary to septenary polymetallic alloy nanotubes, and this method can be used to synthesize 18 kinds of polymetallic alloy nanotube (NT), including eight kinds of high-entropy alloy (HEA) NT. A representative Cu30Ni26Co19Ru14Ir11 HEA NT demonstrates efficient and stable catalytic performance for both oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). The Cu30Ni26Co19Ru14Ir11 HEA NT exhibits an overpotential of only 121 mV for the HER and 272 mV for the OER, respectively, when measured at a current density of 100 mA cm-2. In addition, the two-electrode system comprising the Cu30Ni26Co19Ru14Ir11 HEA NT exhibits an impressive efficiency of 100 mA cm-2 during overall water splitting, requiring only a potential of 1.67 V. The high catalytic activity of the Cu30Ni26Co19Ru14Ir11 HEA NT is attributed to the downward shift of the d-band center. In the HER, the downward shift of the d-band center can reduce the binding energy with *H, which is beneficial for the desorption process of hydrogen. In the OER, the downward shift can also reduce the reaction energy barrier associated with the rate-determining step from *O to *OOH. This work seeks to offer a new and general method for synthesizing polymetallic alloy nanotubes with controllable structures and compositions under mild conditions.

Partially Interstitial Silicon-Implanted Ruthenium as an Efficient Electrocatalyst for Alkaline Hydrogen Evolution

To enhance the alkaline hydrogen evolution reaction (HER), it is crucial, yet challenging, to fundamentally understand and rationally modulate potential catalytic sites. In this study, we confirm that despite calculating a low water dissociation energy barrier and an appropriate H adsorption free energy (ΔG*H) at Ru-top sites, metallic Ru exhibits a relatively inferior activity for the alkaline HER. This is primarily because the Ru-top sites, which are potential H adsorption sites, are recessive catalytic sites, compared with the adjacent Ru-hollow sites that have a strong ΔG*H. To promote the transformation of Ru-top sites from recessive to dominant catalytic sites, interstitial Si atoms are implanted into the hollow sites. However, complete interstitial implantation leads to a high water dissociation energy barrier at the RuSi intermetallic surface. Thus, we present a partial interstitial incorporation strategy to form a Ru-RuSi heterostructure that not only converts the Ru-top sites from recessive to dominant catalytic sites but also preserves the low water dissociation energy barrier at the Ru surface. Moreover, the spontaneously formed built-in electric fields bidirectionally optimize the adsorption ability of the Ru sites, thereby greatly reducing the thermodynamic energy barrier and enhancing the alkaline HER.

Tuning the electronic-state of metal cobalt/cobalt iron alloy hetero-interface embedded in nitrogen-doped carbon nanotube arrays for boosting electrocatalytic overall water splitting

Maximizing the utilization of active sites and tuning the electronic-state are crucial yet extremely challenging in enhancing the ability of alloy-based catalysts to catalyze hydrogen and oxygen evolution reactions (HER and OER). Here, the 3D self-supported N-doped carbon nanotube arrays (NCNTAs) was synthesized on Ni foam by the drop-casting and calcination method, where the metal Co and Co7Fe3 alloy were enclosed at the NCNT tip (denoted as Co/Co7Fe3@NCNT/NF). The Co/Co7Fe3 hetero-interface formation led to changes in the electronic state, which can optimize the adsorption free energy of reaction intermediates and thereby boost the intrinsic catalytic performance. The well-dispersed carbon nanotube arrays with superhydrophilic and superaerophobic characteristic promotes electrolyte permeation and bubbles escape. Therefore, the optimized Co/Co7Fe3-10@NCNT/NF exhibits superior bifunctional activities with overpotential of 93 and 174 mV at 10 mA cm-2 for HER and OER, respectively. For overall water splitting (OWS), the assembled dual electrode device with Co/Co7Fe3-10@NCNT/NF only requires a low voltage of 1.56 V to achieve 10 mA cm-2 and stabilizes for 24 h at 100 mA cm-2. The result underscores the importance of hetero-interface electronic effect and carbon nanotube arrays in catalytic water splitting, providing valuable insights for the design of more advanced bifunctional electrocatalysts for OWS.

Synergistic core-shell boosts P-CoNiMoO@Co2P-Ni2P bifunctional catalyst for efficient and robust overall water splitting

Optimizing hydrogen adsorption and enhancing water absorption are essential for the design of effective hydrogen evolution reaction (HER) electrocatalysts. Herein, a well-defined core-shell-structured P-CoNiMoO@Co2P-Ni2P catalyst was synthesized on nickel foam via high-temperature phosphidation of heterostructured precursor CoMoO4·xH2O/NiMoO4·xH2O with hydrogen (H2) assistance. This catalyst exhibits good HER performance, requiring only 24 mV of overpotential to achieve a current density of 10 mA cm-2, and long-term stability, maintaining a current density of 100 mA cm-2 for over 100 h. Density functional theory calculations indicate that the molybdenum site is highly favorable for water adsorption in phosphorus-doped cobalt nickel molybdate (P-CoNiMoO), while the trigonal Ni3 site is optimal for hydrogen adsorption. These findings indicate that the cooperative interactions and functional division between the core and shell substantially enhance HER performance. In addition, P-CoNiMoO@Co2P-Ni2P demonstrates high oxygen evolution reaction performance, achieving a current density of 10 mA cm-2 at an overpotential of 243 mV. When functioning as a bifunctional electrocatalyst, it requires only 1.49 V to drive overall water splitting at a current density of 10 mA cm-2, with a durability of over 200 h at current densities of 100 and 300 mA cm-2. This study provides significant insights into the development of HER catalysts with potential applications in other fields.

Coordination Tuning of FeNi-HMT Frameworks Derived Effective Hybrid Catalysts for Water Oxidation

FeNi-based hybrid materials are promising oxygen evolution reaction (OER) catalysts for water electrolysis in hydrogen generation. In this work, the coordination tuning of FeNi-HMT frameworks was achieved by simply changing the Fe/Ni ratios using hexamethylenetetramine (HMT) as an organic ligand, and the derived hybrid FeNi catalysts with varied compositions were probed for OER. Incorporating varying amounts of Fe3+ by adjusting the Ni/Fe ratio results in different metal-organic framework (MOF) structures, and higher Fe feed leads to the formation of amorphous structures due to the coordination structure destruction from the weaker coordination capacity of Fe3+ compared to Ni2+ combining with the tertiary amine ligand. Among them, the FeNi-HMT (with the Fe/Ni molar ratio of 1/1) derived catalyst, consisting of Fe0.36Ni0.64 alloy/Ni0.4Fe2.6O4 spinel oxide heterostructures supported by graphitized carbon matrix, exhibits the highest OER performance. The unique structure facilitates significant electron transfer at the alloy/spinel interface due to the large work function difference between each phase. This strong electronic effect downshifts the d-band center of the catalyst and optimizes the binding energies to the crucial oxygenated intermediates, thereby promoting the OER kinetics. This work highlights the importance of the coordination tuning of FeNi-HMT frameworks for highly efficient catalyst development.

Revealing the promoting effect of heterojunction on NiSx/MoO2 in urea oxidation assisted water electrolysis

Investigating efficient non-precious metal-based catalysts for water electrolysis to produce hydrogen is a significant and urgent need in the field of clean energy technologies. Moreover, utilizing transition metal dichalcogenides (TMDs) to replace the oxygen evolution reaction (OER) with the urea oxidation reaction (UOR), coupled with the hydrogen evolution reaction (HER), is an effective energy-saving hydrogen production method. A heterostructure NiSx/MoO2 catalyst was prepared by a simple method, which exhibits excellent activity for UOR, requiring only 1.4 V to reach 100 mA cm-2. The high performance is attributed to the presence of the heterostructure, which effectively promotes charge redistribution and optimizes the electronic structure of the catalyst, thereby enhancing its adsorption capacity for intermediates. As a result, an electrolyzer assembled with NiSx/MoO2 as a bifunctional catalyst demonstrates excellent catalytic activity, ensures stability for over 200 h at a current density of 10 mA cm-2, and achieves a hydrogen production rate of 0.402 mmol h-1 at a potential of 1.8 V.

Core-shell cobalt-iron silicide electrocatalysts with enhanced bifunctional performance in hydrogen and oxygen evolution reactions

To satisfy the growing demand for green energy, hydrogen production through water electrolysis has emerged as a promising approach, making the design and synthesis of efficient and durable bifunctional electrocatalysts both critical and challenging for the advancement of hydrogen energy. In this study, we synthesized core-shell structured bifunctional transition metal silicide electrocatalysts using a magnesium thermal reduction method. During the exothermic reduction, a silicon oxide (SiOx) shell was formed, coating the active centers of the silicide and providing a protective core-shell structure. The overpotentials of oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) of bimetallic cobalt iron silicide (CFS-2, Co: Fe = 1:1) catalyst in 1 M KOH at 10 mA cm-2 were 291 mV and 242 mV, respectively, with Tafel slopes of 65 and 164 mV dec-1, which were superior to single metal electrocatalysts of cobalt silicide (CS) and iron silicide (FS). The core-shell structure, with a metal silicide core and a passivating silica shell, enhances electron transfer while preventing silicon leaching and improving catalyst stability. Remarkably, after continuous operation for 24 h at a fixed current density of 10 mA cm-2, it remained stable at 1.66 V. This work represents the first successful synthesis of cobalt-iron bimetallic silicide catalysts for overall water splitting, demonstrating their significant potential for electrocatalytic applications and promoting the broader use of silicides in hydrogen production.

Nanoporous high-entropy alloys and metallic glasses: advanced electrocatalytic materials for electrochemical water splitting

Electrochemical water splitting is a promising approach to convert renewable energy into hydrogen energy and is beneficial for alleviating environmental pollution and energy crises, and is considered a clean method to achieve dual-carbon goals. Electrocatalysts can effectively reduce the reaction energy barrier and improve reaction efficiency. However, designing electrocatalysts with high activity and stability still faces significant challenges, which are closely related to the structure and electronic configuration of catalysts. Nanoporous high-entropy alloys (np-HEAs) and metallic glasses (np-MGs), characterized by long-range chemical disorder intertwined with local chemical order combined with three-dimensional, interconnected nanoporous structure, exhibit distinctive electrocatalytic properties and application potential for electrochemical water splitting. To promote the widespread application of np-HEAs and np-MGs, it is of great significance to rationally design and apply them in the field of electrolytic water splitting. In this review, the basic principles of hydrogen evolution reaction and oxygen evolution reaction as well as the fabrication techniques of np-HEAs and np-MGs are introduced. The recent progress in the efficient application of np-HEAs and np-MGs in electrochemical water splitting, and the current challenges and prospects are summarized. This review will provide theoretical guidance for the development of np-HEAs and np-MGs in electrochemical water splitting applications.

Modulating Built-In Electric Field Strength in Ru/RuO2 Interfaces through Ni Doping to Enhance Hydrogen Conversion at Ampere-level Current

Improving the alkaline hydrogen evolution reaction (HER) efficiency is essential for developing advanced anion exchange membrane water electrolyzers (AEMWEs) that operate at industrial ampere-level currents. Herein, we employ density functional theory (DFT) calculations to identify Ni-RuO2 as the leading candidate among various 3d transition metal-doped M-RuO2 (where metal M includes Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn). The incorporation of Ni atoms facilitates the partial reduction of RuO2, resulting in the formation of a Ni-Ru/RuO2 interface having a significant built-in electric field (BIEF) during electrochemical reactions. The resulted BIEF enhances electron transfer across the interface, which is critical in lowering energy barriers and accelerating the hydrogen evolution reaction (HER) kinetics. As a result, the Ni-RuO2 catalyst exhibits an overpotential of 134 mV at 1 A cm-2 and a low Tafel slope of 20.85 mV dec-1, with just 0.03 mg cm-2 of Ru loading. The highly effective BIEF, therefore, plays a pivotal role in the catalyst's remarkable performance, allowing the Ni-RuO2-based AEMWE to require only 1.71 V to maintain stable operation at 1 A cm-2 over a 1000-hour period.

Tailoring Lewis Acidity of Metal Oxides on Nickel to Boost Electrocatalytic Hydrogen Evolution in Neutral Electrolyte

Neutral-pH water splitting for hydrogen production features a benign environment that could alleviate catalyst and electrolyzer corrosion but calls for the corresponding high-efficiency and earth-abundant hydrogen evolution reaction (HER) catalysts. Herein, we first designed a series of metal oxides decorated on Ni as the model catalysts and found a volcano-shaped relationship between the Lewis acidity of Ni/metal oxides and HER activity in neutral media. The Ni/ZnO with the optimum Lewis acidity could balance water dissociation and hydroxyl desorption, thereby greatly boosting the HER. On the basis of this finding, we further in situ grew the Ni/ZnO heterostructure on a three-dimensional conductive support. The resulting catalyst requires overpotentials of merely 34 and 194 mV to deliver the current densities of 10 and 200 mA cm-2, respectively, and can stably operate at these current densities for 2000 h in 1 M phosphate buffer solution (pH 7), representing the most active and durable HER catalyst in neutral electrolyte reported thus far. Our work provides an effective design scheme for low-cost and high-performance neutral HER catalysts.

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

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

Interstitial Manganese-Tuned Nickel-Iron Diselenide Anode for Efficient and Durable Anion Exchange Membrane Water Electrolysis

Anion exchange membrane water electrolysis (AEMWE) employing Ir/Ru-free anodes emerges as a bright prospect for green hydrogen society. Here, a Ni0.8Fe0.2Mn0.1Se2 nanosheet electrocatalyst is reported, in situ grown on stainless-steel paper, as an efficient and durable self-supporting AEMWE anode for oxygen evolution reaction (OER). The interstitial [MnSe4] tetrahedra elevate the Fermi level and narrows the band gap of the electrocatalyst, thereby expediting electrode reaction kinetics and increasing the electrical conductivity. In addition, the interstitial Mn atoms attenuate the electron density of Ni and Fe and motivate phase transition to actual active (Mn, Fe)-doped γ-NiOOH species. The downward d-band center of Ni active center facilitates the rate-limiting *OOH desorption step, refreshing the active center, and reducing the free energy barriers for OER. Accordingly, the Ni0.8Fe0.2Mn0.1Se2 electrode achieves OER overpotentials of 149 and 232 mV at 10 and 100 mA cm-2 in 1 m KOH. The AEMWE cell incorporating Ni0.8Fe0.2Mn0.1Se2 anode demonstrates high performance (1.0 A cm-2 at 1.68 Vcell) and durability (at 1 A cm-2 for 300 h), surpassing most AEMWE cells that use NiFe-based anodes. This work highlights the potential of noble-metal-free anodes for efficient and durable AEMWE.

Dominant Role of Coexisting Ruthenium Nanoclusters Over Single Atoms to Enhance Alkaline Hydrogen Evolution Reaction

Developing efficient and cost-effective electrocatalysts to replace expensive carbon-supported platinum nanoparticles for the alkaline hydrogen evolution reaction remains an important challenge. Recently, an innovative catalyst, composed of ruthenium single atoms (Ru1) integrated with small Ru nanoclusters (RuNC), has attracted considerable attention from the scientific community. However, because of its complexity, this catalyst remains a topic of some debate. Here, a method is reported of precisely controlling the ratios of Ru1 to RuNC on a nitrogenated carbon (NC)-based porous organic framework to produce Ru/NC catalysts, by using different amounts (0, 5, 10 wt.%) of reducing agent. The Ru/NC-10 catalyst, formed with 10 wt.% reducing agent, delivered the best performance under alkaline conditions, indicating that RuNC played a significant role in actual alkaline hydrogen evolution reaction (HER). An anion exchange membrane water electrolyzer (AEMWE) system using the Ru/NC-10 catalyst required a significantly lower operating voltage (1.72 V) than the commercial Pt/C catalyst (1.95 V) to achieve 500 mA cm-2. Moreover, the system can be operated at 100 mA cm-2 without notable performance decay for over 180 h. Theoretical calculations supported these experimental findings that Ru1 contributed to the water dissociation process, while RuNC is more actively associated with the hydrogen recombination process.

A photovoltaic-electrolysis system with high solar-to-hydrogen efficiency under practical current densities

The photovoltaic-alkaline water (PV-AW) electrolysis system offers an appealing approach for large-scale green hydrogen generation. However, current PV-AW systems suffer from low solar-to-hydrogen (STH) conversion efficiencies (e.g., <20%) at practical current densities (e.g., >100 mA cm-2), rendering the produced H2 not economical. Here, we designed and developed a highly efficient PV-AW system that mainly consists of a customized, state-of-the-art AW electrolyzer and concentrator photovoltaic (CPV) receiver. The highly efficient anodic oxygen evolving catalyst, consisting of an iron oxide/nickel (oxy)hydroxide (Fe2O3-NiOxHy) composite, enables the customized AW electrolyzer with unprecedented catalytic performance (e.g., 1 A cm-2 at 1.8 V and 0.37 kgH2/m-2 hour-1 at 48 kWh/kgH2). Benefiting from the superior water electrolysis performance, the integrated CPV-AW electrolyzer system reaches a very high STH efficiency of up to 29.1% (refer to 30.3% if the lead resistance losses are excluded) at large current densities, surpassing all previously reported PV-electrolysis systems.

Undercoordinated Two-Dimensional Pt Nanoring Stabilized by a Ring-on-Sheet Nanoheterostructure for Highly Efficient Alkaline Hydrogen Evolution Reaction

Platinum (Pt) is a state-of-the-art electrocatalyst for green hydrogen production in alkaline electrolytes. The delicate design and fabrication of two-dimensional (2D) Pt nanocatalysts can significantly enhance atomic utilization efficiency, while further improving intrinsic catalytic performance by modulating the density of surface active sites. However, the high surface energy and morphology complexity of 2D nanostructures often result in poor structural stability under the working conditions. Here, we report the synthesis of a 2D ring-on-sheet nanoheterostructure featuring abundant low-coordination Pt sites in which a defect-rich Pt nanoring is stabilized by an ultrathin 2D rhodium (Rh) support. The Rh@Pt nanoring exhibits remarkably enhanced activity and stability in an electrocatalytic hydrogen evolution reaction in alkaline media compared to defect-free Rh@Pt core-shell nanoplates and commercial Pt/C. This work provides new insights for the design and synthesis of 2D nanoheterostructures with abundant surface active sites for efficient and durable electrocatalysis.