Self-Supported Transition-Metal-Based Electrocatalysts for Hydrogen and Oxygen Evolution

Advanced aqueous phenazine redox flow battery enhanced by selective interfacial water behavior on Co/NC modified electrode

Although aqueous organic redox flow battery (RFBs) is a highly promising energy storage device, the redox reaction kinetics of the anode organic electrolyte material, especially for phenazine derivatives, are limited by low electrochemical activity of traditional porous carbon electrodes. Herein, Co/NC composite electrocatalyst was elaborated to significantly enhance the redox reaction kinetics of phenazine derivatives, in which Co/NC electrocatalyst could improve energy efficiency of aqueous phenazine RFBs by 43.2 % compared to pure carbon felt electrodes at current density of 100 mA/cm2. Control experiments combined with density functional theory calculations identified that in addition to the more reactive sites provided by relatively large electrochemical active surface area accelerate the redox reaction of phenazine derivatives, the Co and CoNC reaction sites provided by Co modification not only reduce the energy barrier, but also provide new convenient reaction pathways based on selective interfacial water behavior, further promoting the reduction reaction of phenazine derivatives. This study gives an in-depth understanding of the synergistic effects between phases in composite electrocatalysts and proposes a new behavioral mechanism for the redox reactions of organic electroactive species on the surface of catalytic electrodes.

Unique hierarchical NiFe-LDH/Ni/NiCo2S4 heterostructure arrays on nickel foam for the improvement of overall water splitting activity

The development of environmentally friendly, high-efficiency, stable, earth-abundant and non-precious metal-based electrocatalysts with fast kinetics and low overpotential for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) is of exceeding significance but still challenging. Herein, a bifunctional electrode of unique hierarchical NiFe-LDH/Ni/NiCo2S4/NF (NiFe-LDH = nickel-iron layered double hydroxide and NF = nickel foam) electrocatalytic architecture, which is built up from NiFe-LDH nanosheets, Ni nanoparticles and NiCo2S4 nanoneedles sequentially arrayed on a porous NF substrate, has been prepared by a facile hydrothermal and electrodeposition method. This electrocatalytic architecture is binder-free and its outer NiFe-LDH nanosheets can effectively prevent the oxidation of inner Ni nanoparticles and corrosion of NiCo2S4 nanoneedles during water electrolysis. The integration of effective HER Ni nanoparticles into OER NiFe-LDH and NiCo2S4 electrocatalysts can not only overcome the disadvantage of their poor electrical conductivity, but also greatly improve their HER and OER activities, owing to the unique hierarchical heterostructure and multicomponent synergies among Ni, NiFe-LDH, and NiCo2S4. Consequently, this electrode shows better OER performance with ultra-low overpotential of 192 mV at 10 mA cm-2, a small Tafel slope of only 37 mV dec-1 and high stability in comparison with commercial RuO2, and also exhibits excellent HER activity with a low overpotential of 83 mV at 10 mA cm-2. More importantly, the NiFe-LDH/Ni/NiCo2S4/NF composite serves as anode and cathode for overall water splitting, and it requires a low cell potential of only 1.53 V to reach a current density of 10 mA cm-2, which evidently exceeds that of the standard Pt-C/NF//RuO2/NF cell of 1.61 V. This work provides a feasible strategy to prepare high-efficiency, low-cost and non-precious metal-based electrocatalysts for overall water splitting.

Carbonized Wood Decorated with Ternary Heterogeneous MoS2-MoP-Mo2C Nanoparticles for pH-Universal Hydrogen Evolution

The assembly of diverse active materials significantly enhances the efficiency of the electrocatalytic hydrogen evolution reaction (HER). Herein, we prepare a three-phase composite structure electrocatalyst on carbonized wood, in which molybdenum carbide and molybdenum phosphide are attached to molybdenum sulfide nanosheets (MoS2-MoP-Mo2C@CW) through the hydrothermal method in combination with high-temperature calcination for a pH-universal HER. MoS2 possesses abundant unsaturated coordination edge active sites, thus facilitating the adsorption and desorption of the hydrogen intermediate (H*). The synergistic effect between MoS2, MoP featuring favorable electronic conductivity, and Mo2C holding strong adsorption of H* enhances the catalytic activity for the pH-universal HER. Moreover, the carbonized wood with a hierarchical porous structure and aligned microchannels accelerates mass transport during the HER. As a result, the molybdenum-based self-supported composite electrode exhibits outstanding HER performance with the low overpotentials of 46, 84, and 65 mV to achieve the current density of 10 mA cm-2 and the corresponding Tafel slopes of 57, 111 mV, and 63 mV dec-1 in alkaline, neutral, and acidic environments, respectively. MoS2-MoP-Mo2C@CW also shows a long-term durability of 200 h over a broad pH range at 10 mA cm-2. This work provides an effective strategy for the development of multiphase carbonized wood-based electrocatalysts for the pH-universal HER.

Vanadium-Doped Ni3S2: Morphological Evolution for Enhanced Industrial-Scale Water and Urea Electrolysis

The development of an earth abundant, cost-effective, facile and multifunctional 3D-porous catalytic network for green hydrogen production is a tremendous challenge. Herein, we report the V-Ni3S2 self-supported catalytic network with optimized morphology grown directly on nickel foam (NF) by the one-step hydrothermal technique for water and urea electrolysis at industrial scale hydrogen generation. The morphology of Ni3S2 was modulated by doping of different concentrations of vanadium from granules to cross-linked wires to hierarchal nanosheets arrays, which is beneficial in electrochemical charge and mass transport, and generates more exposed active sites. The V-Ni3S2 catalyst requires the overpotential of 147 mV for hydrogen evolution reaction (HER). The OER and UOR half-cell reaction on V-Ni3S2 catalyst requires potential 1.57 V and 1.39 V (vs RHE), respectively to generate current 100 mA/cm2. The water electrolysis cell developed by V-Ni3S2 as both anode and cathode generates 100 mA/cm2 at cell voltage of 1.88 V in laboratory condition (1 M KOH, 25 °C) and 1.61 V at industrial condition (5 M KOH, 80 °C) and also shows considerable stability for 82 hr at current 300 mA/cm2. The urea electrolysis cell with 1 M KOH and 0.33 M urea generates 100 mA/cm2 at a cell voltage of 1.73 V, which is 150 mV less than that required for water electrolysis and demonstrate stability for 85 hr at a current of 100 mA/cm2. The results provide an innovative plan for the considerate synthesis and design of bifunctional catalysts for energy storage and water splitting.

Green H2 Generation from Seawater Deploying a Bifunctional Hetero-Interfaced CoS2-CoFe-Layered Double Hydroxide in an Electrolyzer

This work illustrates the practicality and economic benefits of employing a hetero-interfaced electrocatalyst (CoS2@CoFe-LDH), containing cobalt sulphide and iron-cobalt double-layer hydroxide for large-scale hydrogen generation. Here, the rational synthesis and detailed characterization of the CoS2@CoFe-LDH material to unravel its unique heterostructure are essayed. The CoS2@CoFe-LDH operates as a bifunctional electrocatalyst to trigger both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) in alkaline seawater (pH 14.0) while showcasing low overpotential requirement for HER (311 mV) and OER (450 mV) at 100 mA cm- 2 current density. The identical CoS2@CoFe-LDH on either electrode in an H-cell setup results in simultaneous H2 and O2 production from seawater with a ≈98% Faradaic efficiency with an applied potential of 1.96V@100 mA cm- 2. Next, this CoS2@CoFe-LDH catalyst is deployed on both sides of a membrane electrode assembly in a one-stack electrolyzer, which retains the intrinsic bifunctional reactivity of the catalyst to generate H2 and O2 in tandem from alkaline seawater with an impeccable energy efficiency (50 kWh kg-1-of-H2). This electrolyzer assembly can be directly linked with a Si-solar cell to produce truly green hydrogen with a solar-to-hydrogen generation efficiency of 15.88%, highlighting the potential of this converting seawater to hydrogen under solar irradiation.

Optimizing Interfacial Charge Dynamics and Quantum Effects in Heterodimensional Superlattices for Efficient Hydrogen Production

Superlattice materials have emerged as promising candidates for water electrocatalysis due to their tunable crystal structures, electronic properties, and potential for interface engineering. However, the catalytic activity of transition metal-based superlattice materials for the hydrogen evolution reaction (HER) is often constrained by their intrinsic electronic band structures, which can limit charge carrier mobility and active site availability. Herein, a highly efficient electrocatalyst based on a VS2-VS heterodimensional (2D-1D) superlattice with sulfur vacancies is designed addressing the limitations posed by the intrinsic electronic structure. The enhanced catalytic performance of the VS2-VS superlattice is primarily attributed to the engineered heterojunction, where the work function difference between the VS2 layer and VS chain induces a charge separation field that promotes efficient electron-hole separation. Introducing sulfur vacancies further amplifies this effect by inducing quantum localization of the separated electrons, thereby significantly boosting HER activity. Both theoretical and experimental results demonstrate that the superlattice achieves a ΔGH* of -0.06 eV and an impressively low overpotential of 46 mV at 10 mA·cm-2 in acidic media, surpassing the performance of commercial Pt/C while maintaining exceptional stability over 15 000 cycles. This work underscores the pivotal role of advanced material engineering in designing catalysts for sustainable energy applications.

Dynamic evolution of metal-nitrogen-codoped carbon catalysts in electrocatalytic reactions

Atomic metal-nitrogen-codoped carbon (M-N-C) catalysts are highly efficient for various electrocatalytic reactions because of their high atomic utilization efficiency. However, the high surface energy of M-N-C catalysts often results in stability issues in electrochemical reactions. Therefore, understanding the stability and dynamic evolution of M-N-C catalysts is crucial for elucidating the active centers and the composition/structure-activity relationship. This review summarizes the factors affecting the durability of atomic catalysts in electrochemical reactions and discusses possible changes in catalysts during these electrochemical processes. Finally, advanced characterization techniques are described, with a focus on tracking the dynamic evolution of M-N-C catalysts during electrocatalysis. This review offers insights into the rational optimization of M-N-C electrocatalysts and provides a framework for linking their composition and structure with their catalytic activity in future research.

Arrayed metal phosphide heterostructure by Fe doping for robust overall water splitting

Transition metal phosphides (TMPs) show promise in water electrolysis due to their electronic structures, which activate hydrogen/oxygen reaction intermediates. However, TMPs face limitations in catalytic efficiency due to insufficient active sites, poor conductivity, and multiple intermediate steps in water electrolysis. Here, we synthesize a highly efficient bifunctional self-supported electrocatalyst, which consists of an N-doped carbon shell anchored on Fe-doped CoP/Co2P arrays on nickel foam (NC@Fe-CoxP/NF) using hydrothermal and phosphorization techniques. Experimental and theoretical results indicate that the modified morphology, with increased active site density and a tunable electronic structure induced by Fe doping in the CoP/Co2P heterostructure, leads to superior water electrolysis performance. The resulting NC@Fe0.1-CoP/Co2P/NF catalyst exhibits overpotentials of 122 mV for the hydrogen evolution reaction (HER) and 270 mV for the oxygen evolution reaction (OER) at 100 mA cm-2. Furthermore, using NC@Fe0.1-CoP/Co2P/NF as both the cathode and anode in an alkaline electrolyzer enables the cell system to achieve 100 mA cm-2 at a voltage of 1.70 V, while maintaining long-term catalytic durability. This work may pave the way for designing self-supported, highly efficient electrocatalysts for practical water electrolysis applications.

Cation-Modulated Ni/Ni3N Compound Heterojunctions as Highly Efficient Bifunctional Electrocatalysts for Water Splitting

The exploration and rational design of high-performance, durable, and non-precious-metal bifunctional oxygen electrocatalysts are highly desired for the large-scale application of overall water splitting. Herein, an effective and straightforward coupling approach was developed to fabricate high-performance bifunctional OER/HER electrocatalysts based on core-shell nanostructure comprising a Ni/Ni3N core and a NiFe(OH)x shell. The as-prepared Ni/Ni3N@NiFe(OH)x-4 catalyst exhibited low overpotentials of 57 and 243 mV at 10 mA cm-2 for the HER and OER in 1.0 m KOH, respectively, superior to most bifunctional oxygen electrocatalysts reported so far. Compared to the unmodified Ni/Ni3N, the Ni/Ni3N@NiFe(OH)x-4 catalyst exhibited a 43.3-fold increase in mass activity for the OER and an 8.7-fold increase for the HER, as well as a 29.5-fold increase in intrinsic activity for the OER and a 2.6-fold increase for the HER. When employed as both the cathode and the anode of the electrolyzer for the overall water splitting reaction, its voltage was reduced to 1.58 V at 10 mA cm-2. This surface reconstruction method increased the electrochemically active surface area and enhanced the catalytic activity. Furthermore, in situ Raman spectroscopy revealed that the Fe etching reduced the onset potential for the active phase NiOOH, promoted its formation, and accelerated the reaction kinetics, thereby enhancing the overall electrocatalytic performance of the catalyst.

Dual chalcogenide coordination engineering on a self-supported alloy electrode for enhanced hydrogen evolution reaction

The electrochemical hydrogen evolution reaction (HER) holds substantial promise for large-scale green hydrogen production due to its cost-effectiveness, high performance, and scalability for repeated implementation. A promising strategy that involves a novel transition-metal chalcogenide with a self-supported framework can improve the reaction kinetics in the HER. We synthesize a nano three-dimensional self-supported HER catalyst via a straightforward one-step chemical vapor deposition process. This method incorporates a dual co-reaction with sulfur (S) and selenium (Se) onto a commercially available Monel alloy (CNSSe). The dynamics ion exchange redox reactions promote the formation of heterogeneous catalyst structures. Density functional theory (DFT) calculations indicate that the CNSSe catalyst exhibits low hydrogen coverage, as evidenced by a thermoneutral free energy of adsorbed hydrogen (ΔGH*) of 0.105 eV, which can be attributed to the dual introduction of S and Se. Consequently, the optimized CNSSe electrocatalyst achieves an enhancement in HER performance exceeding 100% compared to catalysts introduced exclusively with either S or Se. These results underscore the substantial potential of the optimized CNSSe electrocatalyst to improve both the performance and economic feasibility of HER technologies in alkaline water electrolysis.

Engineering Lattice Oxygen Regeneration of NiFe Layered Double Hydroxide Enhances Oxygen Evolution Catalysis Durability

The lattice oxygen mechanism (LOM) endows NiFe layered double hydroxide (NiFe-LDH) with superior oxygen evolution reaction (OER) activity, yet the frequent evolution and sluggish regeneration of lattice oxygen intensify the dissolution of active species. Herein, we overcome this challenge by constructing the NiFe hydroxide/Ni4Mo alloy (NiFe-LDH/Ni4Mo) heterojunction electrocatalyst, featuring the Ni4Mo alloy as the oxygen pump to provide oxygenous intermediates and electrons for NiFe-LDH. The released lattice oxygen can be timely offset by the oxygenous species during the LOM process, balancing the regeneration of lattice oxygen and assuring the enhancement of the durability. In consequence, the durability of NiFe-LDH is significantly enhanced after the modification of Ni4Mo with an impressive durability for over 60 h, much longer than that of NiFe-LDH counterpart with only 10 h. In situ spectra and first-principle simulations reveal that the adsorption of OH- is significantly strengthened owing to the introduction of Ni4Mo, ensuring the rapid regeneration of lattice oxygen. Moreover, NiFe-LDH/Ni4Mo-based anion exchange membrane water electrolyzer (AEMWE) presents an impressive durability for over 150 h at 100 mA cm-2. The oxygen pump strategy opens opportunities to balance the evolution and regeneration of lattice oxygen, enhancing the durability of efficient OER catalysts.

Achieving Excess Hydrogen Output via Concurrent Electrochemical and Chemical Redox Reactions on P-Doped Co-Based Catalysts with Electron Manipulation and Kinetic Regulation

Electrolytic hydrogen production is of great significance in energy conversion and sustainable development. Traditional electrolytic water splitting confronts high anode voltage with oxygen generation and the amount of hydrogen produced at cathode depends entirely on the quantity of electric charge input. Herein, excess hydrogen output can be achieved by constructing a spontaneous hydrazine oxidation reaction (HzOR) coupled hydrogen evolution reaction (HER) system. For the hydrazine oxidation-assisted electrolyzer in this work, both the external input electrons and the electrons produced by spontaneous chemical redox reaction can reduce water, producing more hydrogen than traditional electrolytic water splitting system. The ultrafast kinetics of bifunctional P-doped Co-based catalysts plays a key role in the spontaneous feature of HzOR/HER redox reaction and low working voltage of hydrazine oxidation-assisted electrolyzer (12 mV@100 mA cm-2). Theoretical calculation results and ex situ/in situ spectra demonstrate that doped P could optimize electronic structure, regulate adsorption energy of intermediates, and thus endows catalysts with ultrafast kinetics. This work provides a new pathway for the development of spontaneous oxidation-assisted hydrogen production, to achieve excess hydrogen output via concurrent electrochemical and chemical redox reactions.

Dynamic Redeposition Over Bidirectional Amorphous NiFe-Oxides toward Surface Self-Healing for the Alkaline Oxygen Evolution Reaction

The NiFe-oxy/hydroxides (NiFeOxHy) have emerged as promising candidates for alkaline oxygen evolution reaction (OER) but suffer from irreversible metal dissolution to pose a great challenge to long-term stability. Here, a self-supported electrode of NiFeOxHy/FeNiOx/SS-A (substrate-etched stainless steel, SS-A; interlayer-amorphous Fe3Ni1Ox oxide; catalytic layer-amorphous Ni3Fe1OxHy) is fabricated and presents rare self-healing property of surface cracks, as well excellent activity and stability. It is found that crack repair is driven by the redeposition of dissolved Fe and Ni ions from the amorphous interlayer involved in the OER process because no similar behavior is observed in Fe-free and crystalline interlayer-supported NiFeOxHy. Moreover, the repair performance is dependent on current density and electrolysis time, with 71% of surface cracks being repaired after 72 h operated on an industrial level of 500 mA cm-2. It needs to be emphasized that the irreversible dissolution of Fe and Ni from the catalytic layer of NiFeOxHy still occurs but is effectively suppressed. It is demonstrated that the construction of an amorphous FeNiOx oxide interlayer in self-supported electrodes plays an important role in improving the stability and is expected to open up an opportunity for the design and develop highly efficient and durable alkaline OER catalytic electrodes.

Alkaline Hydrogen Evolution Reaction Electrocatalysts for Anion Exchange Membrane Water Electrolyzers: Progress and Perspective

For the aim of achieving the carbon-free energy scenario, green hydrogen (H2) with non-CO2 emission and high energy density is regarded as a potential alternative to traditional fossil fuels. Over the last decades, significant breakthroughs have been realized on the alkaline hydrogen evolution reaction (HER), which is a fundamental advancement and efficient process to generate high-purity H2 in the laboratory. Based on this, the development of the practical industry-oriented anion exchange membrane water electrolyzer (AEMWE) is on the rise, showing competitiveness with the incumbent megawatt-scale H2 production technologies. Still, great challenges lie in exploring the electrocatalysts with remarkable activity and stability for alkaline HER, as well as bridging the gap of performance difference between the three-electrode cell and AEMWE devices. In this perspective, we systematically discuss the in-depth mechanisms for activating alkaline HER electrocatalysts, including electronic modification, defect construction, morphology control, synergistic function, field effect, etc. In addition, the current status of AEMWE is reviewed, and the underlying bottlenecks that impede the application of HER electrocatalysts in AEMWE are summarized. Finally, we share our thoughts regarding the future development directions of electrocatalysts toward both alkaline HER and AEMWE, in the hope of advancing the commercialization of water electrolysis technology for green H2 production.

Amorphous High-entropy Phosphide Nanosheets With Multi-atom Catalytic Sites for Efficient Oxygen Evolution

The alkaline oxygen evolution reaction (OER) mainly encompasses four elementary reactions, involving intermediates such as HO*, O*, and HOO*. Balancing the Gibbs free energies of these intermediates at a single active site is a challenging task. In this work, a high-entropy metal-organic framework incorporating Fe, Ni, Co, Cu, and Y metal elements is synthesized using an electrodeposition method, which then serves as a template for preparing a high-entropy phosphide/carbon (FeCoNiCuYP/C) composite. Notably, the obtained composite exhibits an amorphous structure with multiple catalytically active sites. Combined theoretical calculations and experimental measurements reveal the critical roles of Co/Ni and Fe atoms in tuning the electronic structure of FeCoNiCuYP and optimizing the binding strength of intermediates. Furthermore, Fe and Ni/Co sites prefer to stabilize the HO* and HOO* intermediates respectively, conducive to breaking their scaling relation of Gibbs free energy during OER. Owing to its fine-tuned composition and the synergistic effect of multiple active sites, the FeCoNiCuYP/C electrocatalyst demonstrates superior OER performance in alkaline solutions, requiring a mere 316 mV overpotential to yield 100 mA cm-2 current density with excellent stability. This work provides an innovative route to design efficient high-entropy electrocatalysts, holding significant promise for cutting-edge electrocatalytic applications.

Development and application of ordered membrane electrode assemblies for water electrolysis

With the development of hydrogen energy, there has been increasing attention toward fuel cells and water electrolysis. Among them, the zero-gap membrane electrode assembly (MEA) serves as an important triple-phase reaction site that determines the performance and efficiency of the reaction system. The development of efficient and durable MEAs plays a crucial role in the development of hydrogen energy. Consequently, a great deal of effort has been devoted to developing ordered MEAs that can effectively increase catalyst utilization, maximize triple-phase boundaries, enhance mass transfer and improve stability. The research progress of ordered MEAs in recent advances is highlighted, involving hydrogen fuel cells and low temperature water electrolysis technology. Firstly, the fundamental scientific understanding and structural characteristics of MEAs based on one-dimensional nanostructures such as nanowires, nanotubes and nanofibers are summarized. Then, the classification, preparation and development of ordered MEAs based on three-dimensional structures are summarized. Finally, this review presents current challenges and proposes future research on ordered MEAs and offers potential solutions to overcome these obstacles.

Self-Supported Porous Carbon Monoliths for Electrocatalytic Hydrogen Evolution in Alkaline Freshwater and Seawater

Developing efficient catalysts for seawater electrolysis hydrogen evolution reaction (HER) is crucial for producing green hydrogen. Carbonized wood (CW), a porous carbon monolith, is a promising self-supporting electrocatalytic electrode owing to its environmentally friendly, sustainable, and hierarchically porous properties. However, the impact of different tree species on the hydrogen evolution performance remains unclear. In this study, various types of CWs, including carbonized poplar (PoCW), carbonized balsa (BaCW), carbonized fir (FiCW), and carbonized pine (PiCW), have been selected to investigate their electrocatalytic performance in hydrogen evolution. Among these, the PoCW exhibits superior electrocatalytic HER performance compared to the other CWs, attributed to its electrochemically active surface area, resistance, and the content of oxygen-containing functional groups. PoCW demonstrates a low overpotential of 284 mV and 356 mV at 10 mA cm-2 in alkaline freshwater and seawater, respectively. Moreover, PoCW shows long-term durability for 100 h in both alkaline freshwater and seawater. This work guides the selection of wood-based carbon monoliths and demonstrates that metal-free, CW-based self-supporting electrodes hold great potential for electrocatalytic hydrogen evolution in both freshwater and seawater.

Silk Fibroin-Derived N-Doped Carbon Cloth Coupling with Molybdenum Carbide Nanoparticles as a Self-Supported Electrode for Hydrogen Evolution Reaction

Handling tons of waste textiles is a big challenge in the textile and apparel industry. The recycling of waste textiles, especially expensive silk fabrics, is of great significance for resource conservation and green, low-carbon cycle development. In addition, the development of precious metal-free electrocatalysts for the hydrogen evolution reaction (HER) remains a major challenge. In this study, molybdenum carbide nanoparticle-decorated carbonized silk fabrics (Mo2C@CSF) were fabricated using a simple process, which can be directly employed as an HER electrode. The obtained materials exhibit remarkable electrocatalytic HER performance in acidic media with low overpotential (60 mV at 10 mA/cm2) and long-term stability (more than 24 h). This work will provide a fresh perspective on the treatment of waste silk fabrics and the fabrication of high-performance electrocatalysts.

Design of NiCoP nanorod loaded on cocoon carbon substrate and its non-metal doping for efficient hydrogen evolution

The quest for effective and sustainable electrocatalysts for hydrogen evolution is crucial in advancing the widespread use of H2. In this study, we utilized silkworm cocoons as the source material to produce porous N-doped carbon (PNCC) substrates through a process involving degumming and annealing. Subsequently, NiCoP nanorod (NiCoP@PNCC) is deposited onto the substrates via a simple impregnation and calcination method to enhance the catalytic performance for the hydrogen evolution reaction (HER). The optimal spacing between the silk fibers of PNCC facilitates longitudinal growth, increases the active surface area, and balances the adsorption and desorption of reaction intermediates, thereby accelerating HER kinetics. Consequently, NiCoP@PNCC demonstrates impressive performance, with 44 mV overpotential to achieve a current density of 10 mA cm-2. Additionally, density functional theory (DFT) calculations reveal that the electronic structure and energy band of NiCoP@PNCC can be modified through the doping of elements such as B, C, N, O, F, and S. In addition, with the electronegativity enhancement of the doping elements, the interaction between Co atoms in NiCoP@PNCC and O atoms in adsorbed H2O molecules gradually enhanced, which is conducive to the dissociation of water in alkaline solution. This research introduces a novel approach for fine-tuning the catalytic activity of transition metal phosphides.

Waxberry-like hydrophilic Co-doped ZnFe2O4 as bifunctional electrocatalysts for water splitting

Water splitting is a promising technique for clean hydrogen production. To improve the sluggish hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), the development of efficient bifunctional electrocatalysts for both HER and OER is urgent to approach the scale-up applications of water splitting. Nowadays transition metal oxides (TMOs) are considered as the promising electrocatalysts due to their low cost, structural flexibility and stability, however, their electrocatalytic activities are eager to be improved. Here, we synthesized waxberry-like hydrophilic Co-doped ZnFe2O4 electrocatalysts as bifunctional electrocatalysts for water splitting. Due to the enhanced active sites by electronic structure tuning and modified super-hydrophilic characteristics, the spinel ZFO-Co0.5 electrocatalyst exhibits excellent catalytic activities for both OER and HER. It exhibits a remarkable low OER overpotential of 220 mV at a current density of 10 mA cm-2 and a Tafel slope of 28.2 mV dec-1. Meanwhile, it achieves a low overpotential of 73 mV at a current density of 10 mA cm-2 with the Tafel slope of 87 mV dec-1 for HER. In addition, for water electrolysis device, the electrocatalytic performance of ZFO-Co0.5||ZFO-Co0.5 surpasses that of commercial IrO2||Pt/C. Our work reveals that the hydrophilic morphology regulation combined with metallic doping strategy is a facile and effective approach to synthesize spinel TMOs as excellent bifunctional electrocatalyst for water splitting.

Recent Progress in Non-Noble Metal Catalysts for Oxygen Evolution Reaction: A Focus on Transition and Rare-Earth Elements

The demand for renewable energy sources has become more urgent due to climate change and environmental pollution. The oxygen evolution reaction (OER) plays a crucial role in green energy sources. This article primarily explores the potential of using non-noble metals, such as transition and rare earth metals, to enhance the efficiency of the OER process. Due to their cost-effectiveness and unique electronic structure, these non-noble metals could be a game-changer in the field. 'Doping,' which is the process of adding a small amount of impurity to a material to alter its properties, and 'synergistic effects,' which refer to the combined effect of two or more elements that is greater than the sum of their individual effects, are two key concepts in this field. Transition and rare earth metals can reduce the overpotential, a measure of the excess potential required to drive a reaction, thus enhancing the OER process by engineering the electronic and surface molecular structure. This article summarizes the roles of various non-noble metals in the OER process and highlights opportunities for researchers to propose innovative ways to optimize the OER process.

Sputtering induced the architecture of "needle mushroom" shaped Cu2O-NiCo2O4 heterostructure with novel morphology and abundant interface for high-efficiency electrochemical water oxidation

Oxygen evolution reaction (OER) is widely recognized as a bottleneck in the kinetics and activity of decomposition water. Unique geometric design and compositional regulation are important technologies for achieving significant activity and excellent kinetics, but they continue to face obstacles in reaction thermodynamics and kinetic response. Here, a "needle mushroom" shaped Cu2O-NiCo2O4 heterostructure with abundant active sites and optimized conductivity that is grown on the Nickel-foam (NF) (labeled as Cu2O-NiCo2O4/NF-2) is prepared using advanced magnetron sputtering strategies for electrochemical water oxidation. Based on the excellent geometric advantages and efficient charge transfer capabilities, the catalyst of Cu2O-NiCo2O4/NF-2 shows superior electrocatalytic activity (low overpotential) and kinetics (low electrochemical impedance) compared with nanoneedle shaped Cu2O-NiCo2O4/NF-1 and NiCo2O4/NF for OER in alkaline medium. This work demonstrates a practical and economical strategy toward the fabrication of ternary transition metal oxides for water oxidation.

Self-Supporting Hierarchical Carbon Network Loaded with NiW Nanoparticles for Efficient Hydrogen Evolution

Water splitting technology can convert renewable energies such as solar and wind into hydrogen energy, which is key to achieving a low-carbon hydrogen economy cycle. However, Pt-based catalysts for hydrogen evolution reaction (HER) are too expensive, thus it needs to develop efficient non-noble metal catalysts as alternatives. Herein, Ni-BDC-loaded carbon cloth (CC) is co-pyrolyzed with urea to obtain a composite structure of carbon nanotubes (CNT) and porous carbon (PC) embedded with W-doped Ni nanoparticles on CC, resulting in NiW-CNT/PC/CC. Benefiting from the synergistic effect between Ni and W, the high conductivity of CNT, and the high mass transfer rate of PC, NiW-CNT/PC/CC exhibits excellent HER activity in KOH, which only requires a low overpotential of 45 mV to drive a current density of 10 mA cm-2 with stability exceeding 40 h. Simulation calculations confirm that the W doping in metal Ni can optimize its electronic structure by lowering the d-band center and weakening hydrogen adsorption, thus reducing its HER barrier.

Routes to Bidirectional Cathodes for Reversible Aprotic Alkali Metal-CO2 Batteries

Aprotic alkali metal-CO2 batteries (AAMCBs) have garnered significant interest owing to fixing CO2 and providing large energy storage capacity. The practical implementation of AAMCBs is constrained by the sluggish kinetics of the CO2 reduction reaction (CO2RR) and the CO2 evolution reaction (CO2ER). Because the CO2ER and CO2RR take place on the cathode, which connects the internal catalyst with the external environment. Building a bidirectional cathode with excellent CO2ER and CO2RR kinetics by optimizing the cathode's internal catalyst and environment has attracted most of the attention to improving the electrochemical performance of AAMCBs. However, there remains a lack of comprehensive understanding. This review aims to give a route to bidirectional cathodes for reversible AAMCBs, by systematically discussing engineering strategies of both the internal catalyst (atomic, nanoscopic, and macroscopic levels) and the external environment (photo, photo-thermal, and force field). The CO2ER and CO2RR mechanisms and the "engineering strategies from internal catalyst to the external environment-cathode properties-CO2RR and CO2ER kinetics and mechanisms-batteries performance" relationship are elucidated by combining computational and experimental approaches. This review establishes a fundamental understanding for designing bidirectional cathodes and gives a route for developing reversible AAMCBs and similar metal-gas battery systems.

Pristine Wood-Supported Electrodes With Intrinsic Superhydrophilic/Superaerophobic Surface Intensify Hydrogen Evolution Reaction

Wood, as a renewable material, has been regarded as an emerging substrate for self-supporting electrodes in large-scale water electrolysis due to numerous merits such as rich pore structure, abundant hydroxyl groups, etc. However, poor conductivity of wood can greatly suppress the performance of wood-based electrodes. Carbonization process can improve wood's conductivity, but the loss of hydroxyl groups and the required high energy consumption are the drawbacks of such a process. Here, a facile strategy is developed to prepare pristine wood-supported electrode (Ni-NiP/W) for enhanced hydrogen evolution reaction (HER); this improves electrical conductivity of wood while retaining its excellent intrinsic properties. The preparation process involves the deposition of copper on the untreated wood followed with the loading of Ni-NiP catalyst at room temperature. Encouragingly, the Ni-NiP/W exhibits conductive and inherited pristine wood's superhydrophilic and superaerophobic properties, that effectively boost mass and charge transfer. It demonstrates high activity and excellent stability in acidic, alkali, and seawater conditions as well as high current densities of up to 2000 mA cm-2; particularly a record-low HER overpotential of 206 mV in acidic conditions at 1000 mA cm-2. This work fully unlocks the admiring potential of pristine wood as superior substrate for high-performance electrochemical electrodes.

Preparation and Electrochemical Applications of Magnéli Phase Titanium Suboxides: A Review

Magnéli phase titanium suboxides (M-TSOs) belong to a type of sub-stoichiometric titanium oxides based on the crystal structure of rutile TiO2. They possess a unique shear structure, granting them exceptional electrical conductivity and corrosion resistance. These two advantages are crucial for electrode materials in electrochemistry, hence the significant interest from numerous researchers. However, the preparation of M-TSOs is uneconomic due to high temperature reduction and other complex synthesis process, thus limiting their practical application in electrochemical fields. This review delves into the crystal structure, properties, and synthesis methods of M-TSOs, and touches on their applications as electrocatalysts in wastewater treatment and electrochemical water splitting. Furthermore, it highlights the research challenges and potential future research directions in M-TSOs.

Boron/nitrogen-trapping and regulative electronic states around Ru nanoparticles towards bifunctional hydrogen production

Developing a straightforward and general strategy to regulate the surface microenvironment of a carbon matrix enriched with N/B motifs for efficient atomic utilization and electronic state of metal sites in bifunctional hydrogen production via ammonia-borane hydrolysis (ABH) and water electrolysis is a persistent challenge. Herein, we present a simple, green, and universal approach to fabricate B/N co-doped porous carbons using ammonia-borane (AB) as a triple functional agent, eliminating the need for hazardous and explosive functional agents and complicated procedures. The pyrolysis of AB induces the regulation of the surface microenvironment of the carbon matrix, leading to the formation of abundant surface functional groups, defects, and pore structures. This regulation enhances the efficiency of atom utilization and the electronic state of the active component, resulting in improved bifunctional hydrogen evolution. Among the catalysts, B/N co-doped vulcan carbon (Ru/BNC) with 2.1 wt% Ru loading demonstrates the highest performance in catalytic hydrogen production from ABH, achieving an ultrahigh turnover frequency of 1854 min-1 (depending on the dispersion of Ru). Furthermore, this catalyst shows remarkable electrochemical activity for hydrogen evolution in alkaline water electrolysis with a low overpotential of 31 mV at 10 mA cm-2. The present study provides a simple, green, and universal method to regulate the surface microenvironment of various carbons with B/N modulators, thereby adjusting the atomic utilization and electronic state of active metals for enhanced bifunctional hydrogen evolution.

Synthesis of P-(NiCo)CO3/TiO2/Ti Self-Supported Electrode with High Catalytic Activity and Stability for Hydrogen Evolution

Hydrogen is considered an ideal clean energy due to its high mass-energy density, and only water is generated after combustion. Water electrolysis is a sustainable method of obtaining a usable amount of pure hydrogen among the various hydrogen production methods. However, its development is still limited by applying expensive noble metal catalysts. Here, the dissolution-recrystallization process of TiO2 nanotube arrays in water with the hydrothermal reaction of a typical nickel-cobalt hydroxide synthesis process followed by phosphating to prepare a self-supported electrode with (NiCo)CO3/TiO2 heterostructure named P-(NiCo)CO3/TiO2/Ti electrode is combined. The electrode exhibits an ultra-low overpotential of 31 mV at 10 mA  cm-2 with a Tafel slope of 46.2 mV dec-1 in 1 m KOH and maintained its stability after running for 500 h in 1 m KOH. The excellent catalytic activity can be attributed to the structure of nanotube arrays with high specific surface area, superhydrophilicity, and super aerophobicity on the electrode surface. In addition, the uniform (NiCo)CO3/TiO2 heterostructure also accelerates the electron transfer on the electrode surface. Finally, DFT calculations demonstrate that phosphating also improves the ΔGH* and ΔGH2O of the electrode. The synthesis strategy also promotes the exploration of catalysts for other necessary electrocatalytic fields.

Unlocking Efficiency: Minimizing Energy Loss in Electrocatalysts for Water Splitting

Catalysts play a crucial role in water electrolysis by reducing the energy barriers for hydrogen and oxygen evolution reactions (HER and OER). Research aims to enhance the intrinsic activities of potential catalysts through material selection, microstructure design, and various engineering techniques. However, the energy consumption of catalysts has often been overlooked due to the intricate interplay among catalyst microstructure, dimensionality, catalyst-electrolyte-gas dynamics, surface chemistry, electron transport within electrodes, and electron transfer among electrode components. Efficient catalyst development for high-current-density applications is essential to meet the increasing demand for green hydrogen. This involves transforming catalysts with high intrinsic activities into electrodes capable of sustaining high current densities. This review focuses on current improvement strategies of mass exchange, charge transfer, and reducing electrode resistance to decrease energy consumption. It aims to bridge the gap between laboratory-developed, highly efficient catalysts and industrial applications regarding catalyst structural design, surface chemistry, and catalyst-electrode interplay, outlining the development roadmap of hierarchically structured electrode-based water electrolysis for minimizing energy loss in electrocatalysts for water splitting.

Coordination engineering of single-atom ruthenium in 2D MoS2 for enhanced hydrogen evolution

This study investigates the enhancement of catalytic activity in single-atom catalysts (SACs) through coordination engineering. By introducing non-metallic atoms (X = N, O, or F) into the basal plane of MoS2 via defect engineering and subsequently anchoring hetero-metallic Ru atoms, we created 10 types of non-metal-coordinated Ru SACs (Ru-X-MoS2). Computations indicate that non-metal atom X significantly modifies the electronic structure of Ru, optimizing the hydrogen evolution reaction (HER). Across acidic, neutral, and alkaline electrolytes, Ru-X-MoS2 catalysts exhibit significantly improved HER performance compared with Ru-MoS2, even surpassing commercial Pt/C catalysts. Among these, the Ru-O-MoS2 catalyst, characterized by its asymmetrically coordinated O2-Ru-S1 active sites, demonstrates the most favorable electrocatalytic behavior and exceptional stability across all pH ranges. Consequently, single-atom coordination engineering presents a powerful strategy for enhancing SAC catalytic performance, with promising applications in various fields.

Electronic Engineering of Crystalline/Amorphous CoP/FeCoPx Nanoarrays for Efficient Water Electrolysis

The development of bifunctional, non-noble metal-based electrocatalysts for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) through morphology and electronic engineering is highly attractive for efficient water splitting. Herein, hierarchical nanoarrays consisting of crystalline cobalt phosphide nanorods covered by amorphous Fe-doped cobalt phosphide nanocuboids (CoP/FeCoPx) are constructed as bifunctional catalysts for both HER and OER. Experimental results and theoretical calculations reveal that the catalysts exhibit balanced dual-catalytic properties due to simultaneous introduction of Fe doping and phosphorus vacancies, leading to an optimized electronic structure of the CoP/FeCoPx. Furthermore, the hierarchical nanoarrays made of crystalline/amorphous heterostructures significantly enhance the performance of the electrocatalysts. As a result, the CoP/FeCoPx catalyst demonstrates remarkable performance in both HER and OER, with overpotentials of 74 and 237 mV at 10 mA cm-2 in 1 m KOH, respectively, as well as a low cell voltage of 1.53 V at 10 mA cm-2 for alkaline overall water splitting. This work integrates the morphology engineering involving design of hierarchical crystalline/amorphous nanoarrays and the electronic engineering through Fe doping and phosphorus vacancies for efficient water electrolysis. It may open a new route toward rational design and feasible fabrication of high-performance, multifunctional, non-noble metal-based electrocatalysts for energy conversion.

One-step Sintering Synthesis of Ni3Se2-Ni Electrode with Robust Interfacial Bonding for Ultra-stable Hydrogen Evolution Reaction

Exploring efficient and robust self-supporting hydrogen evolution reaction (HER) electrodes using simple, accessible, and low-cost synthetic processes is crucial for the commercial application of water electrolysis at high current densities. Ni-based self-supporting electrodes are widely studied owing to their low cost and good catalytic performance. However, to date, the preparation of Ni-based electrodes requires multistep and complex preparation processes. In this study, a novel one-step in situ sintering method to synthesize mechanically stable and highly active Ni3Se2-Ni electrodes with well-controlled morphologies and structures is developed. Their excellent performance and durability can be attributed to the numerous highly active nano-Ni3Se2 catalysts embedded on the surface of the Ni skeleton, the excellent conductivity of the interconnected conductive network, and the strong interfacial bonding between Ni3Se2 and Ni. As a result, the Ni3Se2-Ni600 electrode can operate stably at 85 and 400 mA cm-2 for more than 800 and 300 h, respectively. Moreover, the Ni3Se2-Ni600 electrode displays outstanding stability for over 500 h in a commercial two-electrode system. This study provides a feasible one-step synthesis method for low-cost, high-efficiency metal selenide-metal self-supporting electrodes for water electrolysis.

Improved Alkaline Hydrogen Evolution Performance of Dealloying Fe75-xCoxSi12.5B12.5 Electrocatalyst

The electrocatalytic performance of a Fe65Co10Si12.5B12.5 Fe-based compounds toward alkaline hydrogen evolution reaction (HER) is enhanced by dealloying. The dealloying process produced a large number of nanosheets on the surface of NS-Fe65Co10Si12.5B12.5, which greatly increased the specific surface area of the electrode. When the dealloying time is 3 h, the overpotential of NS-Fe65Co10Si12.5B12.5 is only 175.1 mV at 1.0 M KOH and 10 mA cm-2, while under the same conditions, the overpotential of Fe65Co10Si12.5B12.5 is 215 mV, which is reduced. In addition, dealloying treated electrodes also show better HER performance than un-dealloying treated electrodes. With the increase in Co doping amount, the overpotential of the hydrogen evolution reaction decreases, and the hydrogen evolution activity is the best when the addition amount of Co is 10%. This work not only provides a basic understanding of the relationship between surface activity and the dealloying of HER catalysts, but also paves a new way for doping transition metal elements in Fe-based electrocatalysts working in alkaline media.

Growing highly ordered Pt and Mn bimetallic single atomic layers over graphdiyne

Controlling the precise growth of atoms is necessary to achieve manipulation of atomic composition and atomic position, regulation of electronic structure, and an understanding of reactions at the atomic level. Herein, we report a facile method for ordered anchoring of zero-valent platinum and manganese atoms with single-atom thickness on graphdiyne under mild conditions. Due to strong and incomplete charge transfer between graphdiyne and metal atoms, the formation of metal clusters and nanoparticles can be inhibited. The size, composition and structure of the bimetallic nanoplates are precisely controlled by the natural structure-limiting effect of graphdiyne. Experimental characterization clearly demonstrates such a fine control process. Electrochemical measurements show that the active site of platinum-manganese interface on graphdiyne guarantees the high catalytic activity and selectivity (~100%) for alkene-to-diol conversion. This work lays a solid foundation for obtaining high-performance nanomaterials by the atomic engineering of active site.

Microwave-Assisted Ultrafast Synthesis of Bimetallic Nickel-Cobalt Metal-Organic Frameworks for Application in the Oxygen Evolution Reaction

Herein, a series of monometallic Ni-, Co- and Zn-MOFs and bimetallic NiCo-, NiZn- and CoZn-MOFs of formula M2(BDC)2DABCO and (M,M')2(BDC)2DABCO, respectively, (M, M'=metal) with the same pillar and layer linkers 1,4-diazabicyclo[2.2.2]octane (DABCO) and benzene-1,4-dicarboxylate (BDC) were prepared through a fast microwave-assisted thermal conversion synthesis method (MW) within only 12 min. In the bimetallic MOFs the ratio M:M' was 4 : 1. The mono- and bimetallic MOFs were selected to systematically explore the catalytic-activity of their derived metal oxide/hydroxides for the oxygen evolution reaction (OER). Among all tested bimetallic MOF-derived catalysts, the NiCoMOF exhibits superior catalytic activity for the OER with the lowest overpotentials of 301 mV and Tafel slopes of 42 mV dec-1 on a rotating disk glassy carbon electrode (RD-GCE) in 1 mol L-1 KOH electrolyte at a current density of 10 mA cm-2. In addition, NiCoMOF was insitu grown in just 25 min by the MW synthesis on the surface of nickel foam (NF) with, for example, a mass loading of 16.6 mgMOF/gNF, where overpotentials of 313 and 328 mV at current densities of 50 and 300 mA cm-2, respectively, were delivered and superior long-term stability for practical OER application. The low Tafel slope of 27 mV dec-1, as well as a low reaction resistance from electrochemical impedance spectroscopy (EIS) measurement (Rfar=2 Ω), confirm the excellent OER performance of this NiCoMOF/NF composite. During the electrocatalytic processes or even before upon KOH pre-treatment, the MOFs are transformed to the mixed-metal hydroxide phase α-/β-M(OH)2 which presents the active species in the reactions (turnover frequency TOF=0.252 s-1 at an overpotential of 320 mV). Compared to the TOF from β-M(OH)2 (0.002 s-1), our study demonstrates that a bimetallic MOF improves the electrocatalytic performance of the derived catalyst by giving an intimate and uniform mixture of the involved metals at the nanoscale.

Advances in Inorganic Foam Materials Fabricated Via Blowing Strategy: A Comprehensive Review

Two-dimensional (2D) materials with excellent properties and widespread applications have been explosively investigated. However, their conventional synthetic methods exhibit concerns of limited scalability, complex purification process, and incompetence of prohibiting their restacking. The blowing strategy, characterized by gas-template, low-cost, and high-efficiency, presents a valuable avenue for the synthesis of 2D-based foam materials and thereby addresses these constraints. Whereas, its comprehensive introduction has been rarely outlined so far. This review commences with a synopsis of the blowing strategy, elucidating its development history, the statics and kinetics of the blowing process, and the choice of precursor and foaming agents. Thereafter, we dwell at length on across-the-board foams enabled by the blowing route, like BxCyNz foams, carbon foams, and diverse composite foams consisting of carbon and metal compounds. Following that, a wide-ranging evaluation of the functionality of the foam products in fields such as energy storage, electrocatalysis, adsorption, etc. is discussed, revealing their distinctive strength originated from the foam structure. Finally, after concluding the current progress, we provide some personal discussions on the existing challenges and future research priorities in this rapidly developing method.

Enhancement of acidic hydrogen evolution reaction efficiency through Cu/Ni-doped MFI-type protozeolite layered nanoclusters

We have prepared a highly active and stable copper-doped nickel electrocatalyst. Cu/Ni-doped MFI-type protozeolite layered nanoclusters electrodes have a large electrochemically active surface area (ECSA) and good HER activity, as well as excellent durability. The addition of Cu greatly increases hydrogen evolution reaction (HER) activity under acidic conditions. At the same time, the in situ grown Cu2+1O provides some activity, and in addition, the interface constructed between Cu and Ni further generates sufficient electrochemically active surface area. The activated Cu/Ni-doped MFI-type layered nanoclusters required only a 385 mV overpotential to generate 10 mA cm-2, demonstrating efficient and stable activity with potential practical applications.

Structural Reconstruction of a Cobalt- and Ferrocene-Based Metal-Organic Framework during the Electrochemical Oxygen Evolution Reaction

Metal-organic frameworks (MOFs) are increasingly being investigated as electrocatalysts for the oxygen evolution reaction (OER) due to their unique modular structures that present a hybrid between molecular and heterogeneous catalysts, featuring well-defined active sites. However, many fundamental questions remain open regarding the electrochemical stability of MOFs, structural reconstruction of coordination sites, and the role of in situ-formed species. Here, we report the structural transformation of a surface-grown MOF containing cobalt nodes and 1,1'-ferrocenedicarboxylic acid linkers (denoted as CoFc-MOF) during the OER in alkaline electrolyte. Ex situ and in situ investigations of CoFc-MOF film suggest that the MOF acts as a precatalyst and undergoes a two-step restructuring process under operating conditions to generate a metal oxyhydroxide phase. The MOF-derived metal oxyhydroxide catalyst, supported on nickel foam electrodes, displays high activity toward the OER with an overpotential of 190 mV at a current density of 10 mA cm-2. While this study demonstrates the necessity of investigating structural evolution of MOFs during electrocatalysis, it also shows the potential of using MOFs as precursors in catalyst design.

Ceria-Optimized Oxygen-Species Exchange in Hierarchical Bimetallic Hydroxide for Electrocatalytic Water Oxidation

The utilization of rare earth elements to regulate the interaction between catalysts and oxygen-containing species holds promising prospects in the field of oxygen electrocatalysis. Through structural engineering and adsorption regulation, it is possible to achieve high-performance catalytic sites with a broken activity-stability tradeoff. Herein, this work fabricates a hierarchical CeO2/NiCo hydroxide for electrocatalytic oxygen evolution reaction (OER). This material exhibits superior overpotentials and enhanced stability. Multiple potential-dependent experiments reveal that CeO2 promotes oxygen-species exchange, especially OH- ions, between catalyst and environment, thereby optimizing the redox transformation of hydroxide and the adsorption of oxygen-containing intermediates during OER. This is attributed to the reduction in the adsorption energy barrier of Ni to *OH facilitated by CeO2, particularly the near-interfacial Ni sites. The less-damaging adsorbate evolution mechanism and the CeO2 hierarchical shell significantly enhance the structural robustness, leading to exceptional stability. Additionally, the observed "self-healing" phenomenon provides further substantiation for the accelerated oxygen exchange. This work provides a neat strategy for the synthesis of ceria-based complex hollow electrocatalysts, as well as an in-depth insight into the co-catalytic role of CeO2 in terms of oxygen transfer.

Confined growth of Ultrathin, nanometer-sized FeOOH/CoP heterojunction nanosheet arrays as efficient self-supported electrode for oxygen evolution reaction

Self-supported electrodes, featuring abundant active species and rapid mass transfer, are promising for practical applications in water electrolysis. However, constructing efficient self-supported electrodes with a strong affinity between the catalytic components and the substrate is of great challenge. In this study, by combining the ideas of in-situ construction and space-confined growth, we designed a novel self-supported FeOOH/cobalt phosphide (CoP) heterojunctions grown on a carefully modified commercial Ni foam (NF) with three-dimensional (3D) hierarchically porous Ni skeleton (FeOOH/CoP/3D NF). The specific porous structure of 3D NF directs the confined growth of FeOOH/CoP catalyst into ultra-thin and small-sized nanosheet arrays with abundant edge active sites. The active FeOOH/CoP component is stably anchored on the rough pore wall of 3D NF support, leading to superior stability and improved conductivity. These structural advantages contributed to a highly facilitated oxygen evolution reaction (OER) activity and enhanced durability of the FeOOH/CoP/3D NF electrode. Herein, the FeOOH/CoP/3D NF electrode afforded a low overpotential of 234 mV at 10 mA cm-2 (41 mV smaller than FeOOH/CoP grown on unmodified Ni foam) and high stability for over 90 h, which is among the top reported OER catalysts. Our study provides an effective idea and technique for the construction of active and robust self-supported electrodes for water electrolysis.

Unraveling the Role of the Stoichiometry of Atomic Layer Deposited Nickel Cobalt Oxides on the Oxygen Evolution Reaction

Nickel cobalt oxides (NCOs) are promising, non-precious oxygen evolution reaction (OER) electrocatalysts. However, the stoichiometry-dependent electrochemical behavior makes it crucial to understand the structure-OER relationship. In this work, NCO thin film model systems are prepared using atomic layer deposition. In-depth film characterization shows the phase transition from Ni-rich rock-salt films to Co-rich spinel films. Electrochemical analysis in 1 m KOH reveals a synergistic effect between Co and Ni with optimal performance for the 30 at.% Co film after 500 CV cycles. Electrochemical activation correlates with film composition, specifically increasing activation is observed for more Ni-rich films as its bulk transitions to the active (oxy)hydroxide phase. In parallel to this transition, the electrochemical surface area (ECSA) increases up to a factor 8. Using an original approach, the changes in ECSA are decoupled from intrinsic OER activity, leading to the conclusion that 70 at.% Co spinel phase NCO films are intrinsically the most active. The studies point to a chemical composition dependent OER mechanism: Co-rich spinel films show instantly high activities, while the more sustainable Ni-rich rock-salt films require extended activation to increase the ECSA and OER performance. The results highlight the added value of working with model systems to disclose structure-performance mechanisms.

Co─Mn Bimetallic Nanowires by Interfacial Modulation with/without Vacancy Filling as Active and Durable Electrocatalysts for Water Splitting

Active and stable nonnoble electrocatalysts for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) are required for water splitting by sustainable electricity. Here, Mn bonded with O and P is incorporated to modulate Co3S4 and Co2P respectively to enhance the catalytic activity and extend the catalyst lifetime. Mn3O4 adjusts the electronic structure of Co3S4 and Co atom fills the oxygen vacancy in Mn3O4. The interfacial interaction endows Co3S4/Mn3O4 to a lower reaction barrier due to ideal binding energies for OER intermediates. Structure stability of active sites and enhanced Co─S bonds by Operando Raman spectroscopy and theoretical calculations reduce the dissolution of Co3S4/Mn3O4, resulting in a lifetime of 500 h at 50 mA cm-2 for OER. The modulation of Co2P by MnP weakens the interaction between Co sites and adsorbed H*, achieving a high activity under a large current for HER. The assembled electrolyzer affords 50 mA cm-2 at 1.58 V and exhibits a lifetime of 350 h at 50 mA cm-2. The calculations disclose the electron interaction for the activity and stability, as well as the enhanced conductivity. The findings develop new avenues toward promoting catalytic activity and stability, making Co─Mn bimetallic nanowires efficient electrocatalysts for nonnoble water electrolyzers.

Rationally designing of Co-WS2 catalysts with optimized electronic structure to enhance hydrogen evolution reaction

Designing and developing cost-effective, high-performance catalysts for hydrogen evolution reaction (HER) is crucial for advancing hydrogen production technology. Tungsten-based sulfides (WSx) exhibit great potential as efficient HER catalysts, however, the activity is limited by the larger energy required for water dissociation under alkaline conditions. Herein, we adopt a top-down strategy to construct heterostructure Co-WS2 nanofiber catalysts. The experimental results and theoretical simulations unveil that the work functions-induced built-in electric field at the interface of Co-WS2 catalysts facilitates the electron transfer from Co to WS2, significantly reducing water dissociation energy and optimizing the Gibbs free energy of the entire reaction step for HER. Besides, the self-supported catalysts of Co-WS2 nanoparticles confining 1D nanofibers exhibit an increased number of active sites. As expected, the heterostructure Co-WS2 catalysts exhibit remarkable HER activity with an overpotential of 113 mV to reach 10 mA cm-2 and stability with 30 h catalyzing at 23 mA cm-2. This work can provide an avenue for designing highly efficient catalysts applicable to the field of energy storage and conversion.

Molybdenum and Vanadium-Codoped Cobalt Carbonate Nanosheets Deposited on Nickel Foam as a High-Efficient Bifunctional Catalyst for Overall Alkaline Water Splitting

To address issues of global energy sustainability, it is essential to develop highly efficient bifunctional transition metal-based electrocatalysts to accelerate the kinetics of both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). Herein, the heterogeneous molybdenum and vanadium codoped cobalt carbonate nanosheets loaded on nickel foam (VMoCoCOx@NF) are fabricated by facile hydrothermal deposition. Firstly, the mole ratio of V/Mo/Co in the composite is optimized by response surface methodology (RSM). When the optimized composite serves as a bifunctional catalyst, the water-splitting current density achieves 10 mA cm-2 and 100 mA cm-2 at cell voltages of 1.54 V and 1.61 V in a 1.0 M KOH electrolyte with robust stability. Furthermore, characterization is carried out using field emission scanning electron microscopy-energy dispersive spectroscopy (FESEM-EDS), high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). Density functional theory (DFT) calculations reveal that the fabricated VMoCoCOx@NF catalyst synergistically decreases the Gibbs free energy of hydrogen and oxygen-containing intermediates, thus accelerating OER/HER catalytic kinetics. Benefiting from the concerted advantages of porous NF substrates and clustered VMoCoCOx nanosheets, the fabricated catalyst exhibits superior electrocatalytic performance. This work presents a novel approach to developing transition metal catalysts for overall water splitting.

Direct Current Pulse Atmospheric Pressure Plasma Jet Treatment on Electrochemically Deposited NiFe/Carbon Paper and Its Potential Application in an Anion-Exchange Membrane Water Electrolyzer

An atmospheric pressure plasma jet (APPJ) is used to process electrochemically deposited NiFe on carbon paper (NiFe/CP). The reactive oxygen and nitrogen species (RONs) of the APPJ modify the surface properties, chemical bonding types, and oxidation states of the material at the self-sustained temperature of the APPJ. The APPJ treatment further enhances the hydrophilicity and creates a higher disorder level in the carbon material. Moreover, the metal carbide bonds of NiFe/CP formed in the electrochemical deposition (ED) process are converted to metal oxide bonds after APPJ processing. The potential application of APPJ treatment on NiFe/CP in alkaline water electrolysis is demonstrated. With more oxygen-containing species and better hydrophilicity after APPJ treatment, APPJ-treated NiFe/CP is applied as the electrocatalyst for the oxygen evolution reaction (OER) in alkaline water electrolysis. APPJ-treated NiFe/CP is also used in a custom-made anion-exchange membrane water electrolyzer (AEMWE); this should contribute toward realizing the practical large-scale application of AEM for hydrogen production.

Synergistic Effect Enables the Dual-Metal Doped Cobalt Telluride Particles as Potential Electrocatalysts for Oxygen Evolution in Alkaline Electrolyte

Cobalt (Co)-based materials have been widely investigated as hopeful noble-metal-free alternatives for the oxygen evolution reaction (OER) in alkaline electrolytes, which is crucial for generating hydrogen by water electrolysis. Herein, cobalt-based telluride particles with good electronic conductivity as anodic electrocatalysts were prepared under vacuum by the solid-state strategy, which display remarkable activities toward the OER. Nickel (Ni) and iron (Fe) codoped cobalt telluride (NiFe-CoTe) exhibits an overpotential of 321 mV to achieve a current density of 10 mA cm-2 and a Tafel slope of 51.8 mV dec-1, outperforming the performances of CoTe, CoTe2, and IrO2. According to the DFT calculation, the adsorbed hydroxyl-assisted adsorbate evolution mechanism was proposed for the OER process of NiFe-CoTe, which reveals the synergetic effect toward OER induced by codoping of the Ni and Fe atoms. This work proposes a rational strategy to prepare cobalt-based tellurides as efficient OER catalysts in alkaline electrolytes, providing a new strategy to prepare and regulate metal-based tellurides for catalysis and beyond.

Laser Additive Manufacturing of Three-Dimensional Ti/TiN Nanotube Arrays with Hierarchical Pore Structures and Promoted Supercapacitor Performances

Titanium-based composites hold great promise in versatile functional application fields, including supercapacitors. However, conventional subtractive methods for preparing complex-shaped titanium-based composites generally suffer from several significant shortcomings, including low efficiency, strictly simple geometry, low specific surface area, and poor electrochemical performance of the products. Herein, three-dimensional composites of Ti/TiN nanotube arrays with hierarchically porous structures were prepared using the additive manufacturing method of selective laser melting combined with anodic oxidation and nitridation. The resultant Ti/TiN nanotube array composites exhibit good electrical conductivity, ultrahigh specific surface areas, and outstanding supercapacitor performances featuring the unique combination of a large specific capacitance of 134.4 mF/cm2 and a high power density of 4.1 mW/cm2, which was remarkably superior to that of their counterparts. This work is anticipated to provide new insights into the facile and efficient preparation of high-performance structural and functional devices with arbitrarily complex geometries and good overall performances.

Cr-doping promoted surface reconstruction of Ni3N electrocatalysts toward efficient overall water splitting

Understanding and utilizing the dynamic changes of electrocatalysts under working conditions are important for advancing the sustainable hydrogen production. Here, we for the first time report that Cr-doping can promote the in situ reconstruction of a self-supported Ni3N electrocatalyst (Cr-Ni3N/NF) during oxygen and hydrogen evolution reactions (OER and HER), and therefore improve the electrocatalytic water splitting performance. As identified by in situ measurements and theoretical calculations, Cr-doping enhances OH- adsorption during OER at anode and thereby boosts the transformation of Ni3N pre-catalysts to defect-rich nickel oxyhydroxide (NiOOH) active species. Meanwhile, it facilitates the generation of Ni3N/Ni(OH)2 at cathodes due to effective H2O activation, leading to the fast HER kinetics on the Ni3N/Ni(OH)2 interfaces. Notably, the optimal Cr-Ni3N/NF displays good OER and HER performance in 1.0 M KOH electrolytes, with low overpotentials of 316 and 188 mV to achieve the current density of ± 100 mA cm-2, respectively. Benefiting from its bi-functionality and self-supporting property, an alkaline electrolyzer equipped with Cr-Ni3N/NF as both anode and cathode affords a small voltage of 1.72 V at 100 mA cm-2, along with 100 h operation stability. Elucidating that Cr-doping can boost in situ reconfiguration and consequently the electrocatalytic activity, this work would shed new light on the rational design and synthesis of electrocatalysts via directional reconstructions.

Metal-insulator-semiconductor photoelectrodes for enhanced photoelectrochemical water splitting

Photoelectrochemical (PEC) water splitting provides a scalable and integrated platform to harness renewable solar energy for green hydrogen production. The practical implementation of PEC systems hinges on addressing three critical challenges: enhancing energy conversion efficiency, ensuring long-term stability, and achieving economic viability. Metal-insulator-semiconductor (MIS) heterojunction photoelectrodes have gained significant attention over the last decade for their ability to efficiently segregate photogenerated carriers and mitigate corrosion-induced semiconductor degradation. This review discusses the structural composition and interfacial intricacies of MIS photoelectrodes tailored for PEC water splitting. The application of MIS heterostructures across various semiconductor light-absorbing layers, including traditional photovoltaic-grade semiconductors, metal oxides, and emerging materials, is presented first. Subsequently, this review elucidates the reaction mechanisms and respective merits of vacuum and non-vacuum deposition techniques in the fabrication of the insulator layers. In the context of the metal layers, this review extends beyond the conventional scope, not only by introducing metal-based cocatalysts, but also by exploring the latest advancements in molecular and single-atom catalysts integrated within MIS photoelectrodes. Furthermore, a systematic summary of carrier transfer mechanisms and interface design principles of MIS photoelectrodes is presented, which are pivotal for optimizing energy band alignment and enhancing solar-to-chemical conversion efficiency within the PEC system. Finally, this review explores innovative derivative configurations of MIS photoelectrodes, including back-illuminated MIS photoelectrodes, inverted MIS photoelectrodes, tandem MIS photoelectrodes, and monolithically integrated wireless MIS photoelectrodes. These novel architectures address the limitations of traditional MIS structures by effectively coupling different functional modules, minimizing optical and ohmic losses, and mitigating recombination losses.

Porous Flower-Like Nanoarchitectures Derived from Nickel Phosphide Nanocrystals Anchored on Amorphous Vanadium Phosphate Nanosheet Nanohybrids for Superior Overall Water Splitting

Transition metal phosphides (TMPs) and phosphates (TM-Pis) nanostructures are promising functional materials for energy storage and conversion. Nonetheless, controllable synthesis of crystalline/amorphous heterogeneous TMPs/TM-Pis nanohybrids or related nanoarchitectures remains challenging, and their electrocatalytic applications toward overall water splitting (OWS) are not fully explored. Herein, the Ni2P nanocrystals anchored on amorphous V-Pi nanosheet based porous flower-like nanohybrid architectures that are self-supported on carbon cloth (CC) substrate (Ni2P/V-Pi/CC) are fabricated by conformal oxidation and phosphorization of pre-synthesized NiV-LDH/CC. Due to the unique microstructures and strong synergistic effects of crystalline Ni2P and amorphous V-Pi components, the obtained Ni2P/V-Pi/CC owns abundant active sites, suitable surface/interface electronic structure and optimized adsorption-desorption of reaction intermediates, resulting in outstanding electrocatalytic performances toward hydrogen and oxygen evolution reactions in alkaline media. Correspondingly, the assembled Ni2P/V-Pi/CC||Ni2P/V-Pi/CC electrolyzer only needs an ultralow cell voltage (1.44 V) to deliver 10 mA cm-2 water-splitting currents, exceeding its counterparts, recently reported bifunctional catalysts-based devices, and Pt/C/CC||IrO2/CC pairs. Moreover, the Ni2P/V-Pi/CC||Ni2P/V-Pi/CC manifests remarkable stability. Also, such device shows a certain prospect for OWS in acidic media. This work may spur the development of TMPs/TMPis-based nanohybrid architectures by combining structure and phase engineering, and push their applications in OWS or other clean energy options.