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High-entropy alloys (HEAs) comprising five or more elements in near-equiatomic proportions have attracted ever increasing attention for their distinctive properties, such as exceptional strength, corrosion resistance, high hardness, and excellent ductility. The presence of multiple adjacent elements in HEAs provides unique opportunities for novel and adaptable active sites. By carefully selecting the element configuration and composition, these active sites can be optimized for specific purposes. Recently, HEAs have been shown to exhibit remarkable performance in electrocatalytic reactions. Further activity improvement of HEAs is necessary to determine their active sites, investigate the interactions between constituent elements, and understand the reaction mechanisms. Accordingly, a comprehensive review is imperative to capture the advancements in this burgeoning field. In this review, we provide a detailed account of the recent advances in synthetic methods, design principles, and characterization technologies for HEA-based electrocatalysts. Moreover, we discuss the diverse applications of HEAs in electrocatalytic energy conversion reactions, including the hydrogen evolution reaction, hydrogen oxidation reaction, oxygen reduction reaction, oxygen evolution reaction, carbon dioxide reduction reaction, nitrogen reduction reaction, and alcohol oxidation reaction. By comprehensively covering these topics, we aim to elucidate the intricacies of active sites, constituent element interactions, and reaction mechanisms associated with HEAs. Finally, we underscore the imminent challenges and emphasize the significance of both experimental and theoretical perspectives, as well as the potential applications of HEAs in catalysis. We anticipate that this review will encourage further exploration and development of HEAs in electrochemistry-related applications. [ABSTRACT FROM AUTHOR]

Solar‐driven water electrolysis has been considered to be a promising route to produce green hydrogen, because the conventional water electrolysis system is not completely renewable as it requires power from nonrenewable fossil fuel sources. This review emphasizes the strategies for solar‐driven water electrolysis, including the construction of photovoltaic (PV)‐water electrolyzer systems, PV‐rechargeable energy storage device‐water electrolyzer systems with solar energy as the sole input energy, and photoelectrochemical water splitting systems. The basic discussions of the above strategies for solar‐driven water electrolysis are first presented. Meanwhile, replacing the oxygen evolution reaction with the electrooxidation of organic compounds can effectively improve the efficiency of water splitting. Also, solar‐driven seawater electrolysis greatly broadens the practical applications due to the abundant reserves of seawater. Recent years have witnessed great development in the field of solar‐driven water electrolysis. The recent research development in the area is subsequently reviewed. Finally, perspectives on the existing challenges along with some opportunities for the further development of solar‐driven water electrolysis are provided. [ABSTRACT FROM AUTHOR]

Water electrolysis, driven by renewable energy resources, is a promising energy conversion technology that has gained intensive interest in recent years. However, conventional water electrolysis faces a number of challenges, including large thermodynamic potential gaps, valueless anodic products, explosive hydrogen/oxygen mixtures, reactive oxygen species, and limited pure water. Hybrid water electrolysis, appending different electrolytes in the anode compartment to circumvent the above‐mentioned challenges in conventional water electrolysis, is a particularly attractive alternative. In this review, for the first time, a holistic and subtle description of hybrid water electrolysis is provided, focusing on the design of high‐activity/selectivity/stability anodic electrocatalysts for the electrochemical oxidation of various chemicals, such as alcohol, aldehyde, amine, urea and hydrazine, or the oxygen evolution reaction in seawater electrolytes. Comprehensive judging criteria for anodic oxidation reactions, electrocatalysts, and reaction parameters in hybrid water electrolysis are discussed. Some technoeconomic assessments, feasibility analyses, mechanism explorations, and correlation comparisons are involved. Finally, perspectives on and opportunities for future research directions in hybrid water electrolysis systems are outlined. [ABSTRACT FROM AUTHOR]

Over the past decade, transition metal phosphate materials have attracted enormous interest for various functional devices. In addition to being low‐cost, earth‐abundant and environmentally benign, transition metal phosphates have several unique advantages including high stability, unique chemical/physical characteristics and tunable multifunctionality, making them ideally suited for advanced highly‐efficiency energy conversion and storage applications. In this Review, the synthetic strategies for transition metal phosphates are summarized, and the most recent advances in the development of transition metal phosphates are described for efficient electrocatalysis such as oxygen evolution, hydrogen evolution and oxygen reduction reactions, highlighting the impact of their morphologies and structures on the electrochemical performance and practical applications in overall water splitting and rechargeable metal‐air batteries. Finally, the challenges facing the development of transition metal phosphates in the field of energy conversion and storage are outlined, together with directions of further research and perspectives. [ABSTRACT FROM AUTHOR]

Developing highly efficient and cost‐effective electrocatalysts to boost the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is vital for the sustainable energy systems. The nitrogen and phosphorus codoped vesica‐like carbons with hierarchical nanoporous structure are synthesized through an efficient self‐polymerization strategy with the template of g‐C3N4 monolith. Featuring sufficient heteroatom‐doping, well‐developed porous hierarchy, and high electrochemical surface area, the fabricated carbon materials exhibit considerable activity and robust stability in both electrocatalyzing ORR and OER, in competition with the precious metal electrocatalysts, which are thus capable of using as air cathode catalysts for rechargeable Zn‐air batteries, affording large discharge current density and impressive operation stability. Get on top of your electrocat: Hierarchically porous heteroatoms‐doped vesica‐like carbons were synthesized through a polymerization‐carbonization approach using g‐C3N4 as template, exhibiting considerable performance towards ORR and OER. [ABSTRACT FROM AUTHOR]

Abstract: The growing energy and environmental crisis calls for renewable systems for energy production and conversion. Herein, polyaniline‐derived porous carbon nanorods tri‐doped with N, P, and S (NPSC) are directly fabricated by the one‐pot polymerization of aniline and diphenylamine‐4‐sulfonic acid induced by organophosphonic acid and subsequent pyrolysis at 900 °C. The nanorods exhibit high‐efficiency electrochemical oxygen reduction and oxygen evolution activity in alkaline electrolyte, along with strong durability and methanol tolerance. The synergistic effect among the tri‐doped elements and the related porosity can account for the superior electrocatalytic capacity of the fabricated catalysts. Particularly, a Zn–air battery assembled with the fabricated NPSC as the cathode shows a peak power density of 122 mW cm−2 and superior stability, presenting a promising potential for sustainable energy devices, particularly regenerative metal–air batteries and fuel cells. [ABSTRACT FROM AUTHOR]

Development of catalysts with high activity and durability is essential for the cathodic hydrogen evolution reaction (HER) and anodic oxygen evolution reaction (OER) of water splitting. Three-dimensional catalysts have large surface areas and good mechanical and antipoisoning properties and can directly act as working electrodes; these are especially important factors from the point of view of practical applications. Here we review recent significant progress in the field of three-dimensional catalysts for water splitting. Various three-dimensional catalysts that can be applied in electrocatalytic water splitting are presented. The fabricating methods, electrocatalytic performances, and the catalytic mechanisms of these catalysts are introduced. Lastly, the major challenges in this particular field and their prospective solutions are also discussed. [ABSTRACT FROM AUTHOR]

A facile one-step route has been developed to electrodeposite P Co nanoparticles on a nickel foam in deep eutectic solvents. The as-prepared catalyst exhibits excellent performance towards both hydrogen evolution reaction and oxygen evolution reaction. Only 62 mV and 320 mV overpotentials were required to reach a current density of 10 mA cm −2 for hydrogen evolution reaction and oxygen evolution reaction, respectively. That current density is measured at the voltage of 1.59 V for an overall water splitting when used as both anode and cathode. The scanning electron microscopy images indicate a high dispersion of the P Co sample on the Ni foam. The prepared material possesses a relative high ECSA and a low charge transfer resistance, indicating a large number of active sites for water splitting. [ABSTRACT FROM AUTHOR]

• Al-doped Ni 2 P nanoflowers exhibit excellent HER, HzOR and hydrazine-assisted water splitting activity. • Al induced electron redistribution and intermediate energy optimization contribute to the enhanced activity. • An integrated system consisting of Zn-hydrazine battery and hydrazine-assisted water splitting driven by sustainable solar energy is proposed. Hydrazine-assisted water electrolysis emerges as a promising strategy to replace conventional device for effective hydrogen generation. Limited to the great difference of intermediate adsorption energy for hydrogen evolution reaction (HER) and hydrazine oxidation reaction (HzOR), the design and fabrication of bifunctional catalysts is still a knotty challenge in industrial conditions. Herein, Al-doped Ni 2 P nanoflower architecture grown in-situ on nickel foam (Al-Ni 2 P/NF) is fabricated through the combination of hydrothermal process and subsequent phosphorization treatment. Ascribed to the redistribution of electron state and optimization of reaction intermediate energy induced by Al, Al-Ni 2 P/NF expresses distinguished bifunctional catalytic performance with ultralow potentials of −205 and 300 mV at 500 mA cm−2 for HER and HzOR, respectively. When integrated into hydrazine-assisted water splitting as both the anode and cathode, only 0.717 V is required to reach a large current density of 500 mA cm−2. Specially, a novel integrated system driven entirely by sustainable solar energy is proposed, generating energy-saving hydrogen and purify hydrazine-containing sewage simultaneously. This work provides an attracting and practical path for accelerating the development of hydrogen economy. [ABSTRACT FROM AUTHOR]

The slow reaction kinetics of oxygen electrode is the bottleneck restricting the development of rechargeable Zn-air batteries (ZABs). In order to further improve the energy conversion efficiency of ZABs, a novel urea-assisted ZAB system was proposed by replacing oxygen evolution reaction (OER) with urea oxidation reaction (UOR) with lower theoretical thermodynamic potential. Inspired by nature's ingenious structure-function, a "leaf-branch" CoNi@NCNTs-LDH/CC binary-site electrocatalyst was designed for the above system, where spatially separated branches and leaves serve as the main active sites for ORR and UOR, respectively. A build-in electric field from CoNi-LDH to CoNi@NCNTs is formed at the interface of the composite, which plays a command effect, effectively triggering the accumulation of O 2 and urea reactants around the CoNi and LDH sites, respectively, and thus improving the efficiency of both ORR and UOR reactions. Based on that, a conceptual urea-assisted rechargeable ZAB is demonstrated to have significantly decreased charging voltage, higher energy conversion efficiency (74.6 %) compared with the conventional ZAB, as well as high urea elimination rates. Considering the urea-assisted ZAB with high energy conversion efficiency and wastewater treatment versatility as an energy transfer station applied in a high value-added agricultural system, uninterrupted hydrogen production and wastewater treatment of agricultural wastewater can be simultaneously achieved using sustainable energy sources. [Display omitted] • A novel urea-assisted rechargeable ZAB system is innovatively proposed. • Through biomimetic design, a "leaf-branch" binary-site electrocatalyst is prepared. • CoNi@NCNTs-LDH/CC exhibits excellent ORR and UOR bifunctional activity. • The built-in electric field regulates the adsorption of reactants at different sites. • Hydrogen production and wastewater treatment of agricultural wastewater can be realized simultaneously. [ABSTRACT FROM AUTHOR]

[Display omitted] • Interfacial heterogeneous Ni(OH) 2 /Ni 2 P microstructures are constructed on nickel foam. • Ni(OH) 2 /Ni 2 P/NF exhibits excellent HER, HzOR and hydrazine-assisted water splitting activity. • The heterogeneous interfaces contribute to the enhanced activity. • An innovative intermittent hydrazine-assisted water splitting electrolysis system is proposed. Hydrazine oxidation reaction (HzOR) has been proposed to replace the sluggish oxygen evolution reaction (OER) for energy-saving hydrogen generation. However, the rational design of bifunctional electrocatalysts that can simultaneously accelerate HER/HzOR kinetics and the source of the hydrazine chemical substrate are still challenging. Herein, interfacial heterogeneous nickel hydroxide/nickel phosphide microstructures are in-situ grown on nickel foam (Ni(OH) 2 /Ni 2 P/NF) via a combined electrodeposition-phosphorization-electrodeposition strategy. After Ni(OH) 2 modification, a charge redistribution is triggered between Ni(OH) 2 and Ni 2 P, reducing the charge-transfer resistance and optimizing the adsorption energy of reaction intermediates. Ni(OH) 2 /Ni 2 P/NF thus yields an impressive bifunctional electrocatalytic activity for HER and HzOR with potentials of −72 and −14 mV at 10 mA cm−2, respectively. When using Ni(OH) 2 /Ni 2 P/NF as both electrodes, decreased cell voltage of 0.357 V is required to drive a current density of 100 mA cm−2 in 0.5 M hydrazine-containing alkaline electrolyte. Furthermore, an intermittent hydrazine-assisted water electrolysis system is proposed to make the combination of hydrazine sewage purification and energy-saving hydrogen production realistic. [ABSTRACT FROM AUTHOR]

The p/n-junction in heteroatoms-doped carbon might be engineered to adjust the population of the charge carriers to perform the oxygen evolution reaction (OER) with holes in a p-type region and hydrogen evolution reaction (HER) with electrons in a n-type region. The carbons (AC) with introduced Fe, Ni and Co species of hierarchical porous structures were derived by pyrolysis of melted poly (styrenesulfonic acid-co-maleic acid) salts. The obtained materials were used as HER, OER and water splitting catalysts. Although all samples were catalytically active, their activity differed depending on the kind of metal. The most promising results were obtained on Ni-AC. An overpotential of Ni-AC to reach 10 mA cm−2 was 259, 382 and 640 mV for HER, OER and water splitting, respectively. Mott-Schottky tests indicated that Ni-AC has p / n -type semiconductor features. While its n -type region was responsible for HER, the p -type region governed OER. A high dispersion of the nickel-based catalytic centers in the hierarchical pore system resulted in an efficient utilization of the active sites for the electron transfer processes. Besides the satisfactory performance, the advantage of the derived catalyst is in its simple synthesis from a commodity polymer. The Ni doped Hierarchical porous carbon was obtained through the melting assisted method. The Ni-AC electrode represented a high stability in HER, OER, and water splitting with a p/n-junction Image 1 • Hierarchical porous carbon was derived from melted polymer salts by pyrolysis. • Highly dispersed transition metals (Ni, Fe and Co) were dispersed within the carbon matrix. • Addition of Ni promoted a high surface area of 832 m2 g-1 with preserved a hierarchical porous structure. • Ni-AC exhibited an overpotential of 640 mV for water splitting due to its p/n -junction. [ABSTRACT FROM AUTHOR]

• Sisal-like porous MoO 2 nanosheet arrays grown on Ni foam are synthesized by a facile process. • The MoO 2 /NF exhibits exceptional activity and durability for both NRR and OER. • A remarkable ammonia production performance is achieved in two-electrode full N 2 electrolysis. • The valence state of Mo species plays a key role in promoting NRR activity. Developing efficient electrocatalysts for direct N 2 electrolysis that includes two key reactions of oxygen evolution reaction (OER) at the anode and nitrogen reduction reaction (NRR) at the cathode is an appealing yet challenging task. Herein, Ni foam (NF) supported sisal-like porous MoO 2 nanosheet array (MoO 2 /NF) was synthesized through a facile hydrothermal and subsequent annealing process. It is shown that in the NRR process the Mo4+ in MoO 2 /NF is more favorable in an associative mechanism of NRR than Mo6+ in MoO 3 /NF, thus yielding an impressive ammonia electrosynthesis activity (NH 3 yield rate: 9.56 μg h−1 cm−2; Faradaic efficiency: 5.14%) in 0.1 M KOH. Moreover, the sisal-like porous nanosheet array structure not only provides a high surface area for abundant accessible active sites, but also limits the coalescence of gas bubbles, enabling active sites better exposed to nitrogen by removing the small gas bubbles rapidly. The MoO 2 /NF also shows excellent performance for the OER in 0.1 M KOH, exhibiting a small overpotential of 340 mV to realize a current density of 10 mA cm−2. In two-electrode full N 2 electrolysis, a remarkable ammonia production performance with an ammonia yield of 5.73 μg h−1 cm−2, an FE of 15.8%, and good long-term stability for at least 24 h is obtained. [ABSTRACT FROM AUTHOR]

FeNiP nanoparticles dispersed on carbon nanosheets were fabricated by direct pyrolysis, which achieved the improved electrochemical activities through optimizing Fe/Ni ratio. • FeNiP nanoparticles dispersed on N,P-doped carbon nanosheets fabricated by a facile method. • The electrochemical activity of the FeNiP/NPCS improved through optimizing Fe/Ni ratio. • High electrocatalytic activity and stability for HER, OER, and ORR. • High performance for overall water splitting and rechargeable Zn-air batteries. Hybridization of non-precious metals with advanced carbon monolith provides a promising approach to substitute noble metal catalysts for various electrochemical reactions. The structural and electronic properties of non-precious metals significantly affect electrocatalytic performance, which depends on the physicochemical structure of carbon substrates and the types of metal components. Herein, binary FeNi phosphide nanoparticles coupled with N,P-modified carbon nanosheets were fabricated through an universal carbonization-phosphorization approach. Besides the structural advantage of 2D nanosheets, the electronic structure change of bimetallic FeNi phosphides via modulating Fe/Ni ratio is another reason responded for the exceptional activities with reduced overpotentials of this hybridized catalyst for both hydrogen evolution, oxygen evolution and oxygen reduction reactions. Furthermore, by employing the fabricated materials as electrode catalysts for overall water splitting and rechargeable Zn-air batteries with liquid and solid-state electrolytes, excellent performances are achieved in these energy conversion and storage devices, revealing the significant feasibility for practical applications. [ABSTRACT FROM AUTHOR]