Cobalt Sulfide/Nickel Sulfide Heterostructure Directly Grown on Nickel Foam: An Efficient and Durable Electrocatalyst for Overall Water Splitting Application

Interface Charge Transfer of Heteroatom Boron Doping Cobalt and Cobalt Nitride for Boosting Water Oxidation

Designing high-performance transition-metal electrocatalysts with controlled active heterointerfacial sites for catalyzing the electrochemical oxygen evolution reaction (OER) is very desirable but remains a great challenge. Here, a facile strategy for the synthesis of transition-metal nitride-based interfacial electrocatalysts boron-doped cobalt/cobalt nitride (B-Co/Co2N) is demonstrated with optimal heterointerfaces between Co and Co2N electrocatalysts by introducing boron as a dopant to the former. Benefiting from the unique electronegativity of B, the obtained B-Co/Co2N electrocatalysts show excellent OER performance with overpotential inputs of as low as 262 and 310 mV for 10 and 100 mA cm-2, which are 1.4 and 6.6 times higher than those of Co/Co2N with the same potential input, respectively. The experimental and theoretical results demonstrate the role of the B dopant in inducing charge redistribution of Co active sites in the Co/Co2N interfacial region, which results in a downshift of the Co 3d band center, the optimal oxidation state of active sites for *OOH formation, and lower energy barriers. Furthermore, the assembled electrolyzer can steadily produce an industrial-grade current density of 1000 mA cm-2 at a cell voltage input of only 1.81 V for at least 100 h with a Faradaic efficiency near 100%. This study provides a promising strategy for heteroatom-doped interfacial electrocatalysts with high performance for energy and environmental applications.

Alloying Confined Regulation of Nanoparticles in a Hierarchically Directed Porous Carbon for Zinc-Air Batteries

The rational design of non-noble metal-based electrocatalysts with efficient bifunctional catalytic activity is critical for the widespread application of zinc-air batteries (ZABs). In this study, an FeNi alloy encapsulated three-dimensional honeycomb-like network structure of carbon aerogels (FeNi/CAs) electrocatalyst was constructed using directional freeze-drying technology. The innovative architecture, combined with the synergistic effect between Fe and Ni, endows the FeNi/CAs catalyst with outstanding bifunctional catalytic activity compared with the introduction of a single metal in carbon aerogels. Specifically, the catalyst achieves a high half-wave potential (E1/2) of 0.90 V for the oxygen reduction reaction (ORR) and excellent stability with a negligible shift of E1/2 (9 mV) after 2000 cycles. Moreover, the FeNi/CAs catalyst exhibits a smaller potential difference (ΔE = 0.68 V) between the ORR and oxygen evolution reaction (OER), highlighting its superior bifunctional activity. Furthermore, the rechargeable ZABs with FeNi/CAs catalysts show remarkable power density (226 mW cm-2) and energy density (985 mWh kg-1), as well as over 1200 h of cycling stability. Additionally, the discharge rate performance of the assembled flexible all-solid-state battery based on this catalyst remains stable under different bending angles, suggesting its robustness and potential for use in wearable electronic devices. This work provides a compelling strategy for the construction of advanced electrocatalysts by leveraging hierarchical structural features and metal synergy, paving the way for high-performance and durable ZABs in next-generation energy storage applications.

In-Situ Sulfuration of Ni(OH)2 to Heterostructured Ni3S2/Ni(OH)2@Ni Catalyst for Efficient Water Splitting

The exploring and developing non-precious transition metal-based catalysts for practical water electrolysis with the low cost, high efficiency and easy macroscopic preparation was still a challenge. Herein, Ni3S2/Ni(OH)2 heterojunction with different sulfuration time was proposed and hydrothermally synthesized using a simple two-step approach, which served as a bifunctional electrocatalyst for water splitting in alkaline solution at industrial temperature. Among these catalysts, Ni3S2/Ni(OH)2-5h displayed the smallest overpotentials (237 mV@100 mA cm-2 and 360 mV@100 mA cm-2) for OER and HER at room temperature, along with low Tafel slopes of 62.0 mV dec-1 and 80.8 mV dec-1 respectively. Furthermore, working at high temperature Ni3S2/Ni(OH)2-5h exhibited even lower overpotential of 82 mV@100 mA cm-2 at 70 °C for OER and 325 mV@100 mA cm-2 at 60 °C for HER. The excellent performance was ascribed to the heterojunction accelerating the charge transfer, hierarchical interconnected structure promoting the mass transfer and the synergistic effect between the components of Ni3S2 and Ni(OH)2. This work could provide a promising route for promoting the electrocatalytic performance of Ni-based catalyst with a simple sulfuration method for industrial water splitting, which could expand to other non-noble metal-based materials for enhancing the electrocatalytic activities.

Advancing Oxygen Evolution Catalysis with Dual-Phase Nickel Sulfide Nanostructures

The production, conversion and storage of energy based on electrocatalysis, mainly assisted by oxygen evolution reaction (OER), plays a crucial role in alkaline water electrolyzers (AWEs) and fuel cells. Nevertheless, the insufficient availability of highly efficient catalyst materials at a reasonable cost that overcome the sluggish electrochemical kinetics of the OER is one of the significant obstacles. Herein, we report a fast and facile synthesis of vapor phase deposition of dual-phase nickel sulfide (Ni-sulfide) using low-temperature annealing in the presence of H2S and demonstrated as an efficient catalyst for OER to address the issues with sluggish electrochemical kinetics. The dual-phase Ni-sulfide structures consist of densely packed 10-50 μm microcrystals with 40-50 individual dual-phase layers, such as NiS and Ni7S6. As an electrocatalyst, the dual-phase Ni-sulfide exhibits excellent OER activity by achieving a current density of 10 mA/cm2 at an overpotential (η10) of 0.29 V and excellent electrochemical stability over 50 h. Besides, the Ni-sulfide displays considerable electrochemical robustness in alkaline conditions and forms OER-active Ni-oxide/hydroxide species during the process. Using an energy-efficient synthesis method, the fabricated unique crystalline nanodesign of dual-phase Ni-sulfide could open new pathways for the controlled synthesis of a high-efficiency group of electrocatalysts for a long-time stable electrochemical catalytic activity.

Spin Polarization and Phase Transformation-Aided Efficient Overall Water Splitting Using Ni50Mn18Ga25Cu7 Ferromagnetic Shape Memory Heusler Alloy

We demonstrate experimentally that the combination of half-metallic property and shape memory features of the Ni50Mn18Ga25Cu7 (NMGC) alloy can synergistically catalyze both the oxygen and hydrogen evolution reactions, leading to excellent water splitting. NMGC, a copper-doped nickel-based ferromagnetic shape memory alloy, undergoes first-order martensite to austenite phase transition with temperature variations. The martensite phase of NMGC demonstrates remarkable efficiency for both the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). With a large current density of 414 ± 3.8 mA/cm2 at 2.9 V, an OER overpotential of only 220 ± 1.7 mV at 20 mA/cm2, and a HER overpotential of 282 ± 2.2 mV at -10 mA/cm2, NMGC (martensite) exhibits superior electrocatalytic performance compared to the austenite phase. Additionally, under a 5000 Oe external magnetic field, NMGC (martensite) shows a significant reduction of 52 mV in OER overpotential and 6 mV for HER, highlighting the promising role of spin and phase in enhancing the water-splitting kinetics.

Electrodeposition and Optimisation of Amorphous NixSy Catalyst for Hydrogen Evolution Reaction in Alkaline Environment

Anion exchange membrane (AEM) water electrolysers have shown their potential in green hydrogen production. One of the crucial tasks is to discover novel cost-effective and sustainable electrocatalyst materials. In this study, a low-cost Ni-S-based catalyst for hydrogen evolution reaction was prepared via a simple electrodeposition process from a modified Watts bath recipe. Physical characterisation methods suggest this deposit film to be amorphous. Optimisation of the electrodeposition parameters of the NixSy catalyst was carried out using a rotating disk electrode setup. The optimised catalyst exhibited excellent catalytical performance in 1 M KOH on a microelectrode, with overpotentials of 41 mV, 111 mV and 202 mV at 10, 100 and 1000 mA cm-2 with Tafel slope of 67.9 mV dec-1 recorded at 333 K. Long-term testing of the catalyst demonstrated steady performance over a 24 h period on microelectrode at 100 mA cm-2 with only 71 mV and 37 mV overpotential increase at 293 K and 333 K respectively. Full cell testing with the optimised NixSy as cathode and NiFe(OH)2 as anode showed 1.88 V after 1 h electrolysis at 500 mA cm-2 in 1 M KOH under 333 K with FAA-3-30 membrane.

Superhydrophilic and Underwater Superaerophobic Dual-Function Peony-Shaped Selenide Micro-nano Array Self-Supported Electrodes for High-Efficiency Overall Water Splitting Driven by Renewable Energy

With the intensification of global environmental pollution and resource scarcity, hydrogen has garnered significant attention as an ideal alternative to fossil fuels due to its high energy density and nonpolluting nature. Consequently, the urgent development of electrocatalytic water-splitting electrodes for hydrogen production is imperative. In this study, a superwetting selenide catalytic electrode with a peony-flower-shaped micronano array (MoS2/Co0.8Fe0.2Se2/NixSey/nickel foam (NF)) was synthesized on NF via a two-step hydrothermal method. The optimal catalytic activity of cobalt-iron selenide was achieved by adjusting the Co/Fe ratio. The intrinsic catalytic activity of the electrodes was enhanced by incorporating transition metal selenides, which then served as a precursor for the subsequent loading of MoS2 nanoflowers on the surface to fully expose the active sites. Furthermore, the superwetting properties of the electrode accelerated electrolyte penetration and electron/mass transfer, while also facilitating bubble detachment from the electrode surface, thereby preventing "bubble shielding effect". This resulted in superior oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) performance, as well as overall water splitting capabilities. In a 1.0 M KOH solution, the electrode required only 166 and 195 mV overpotential to achieve a current density of 10 mA cm-2 for OER and HER, respectively. When functioning as a bifunctional catalytic electrode, only 1.60 V of voltage was necessary to drive the electrolyzer to reach a current density of 10 mA cm-2. Moreover, laboratory simulations of wind and solar energy-driven water splitting validated the feasibility of establishing a sustainable energy-to-hydrogen production chain. This work provides new insights into the preparation of low-overpotential, high-catalytic-activity superhydrophilic and underwater superaerophobic catalytic electrodes by rationally adjusting elemental ratios and exploring changes in electrode surface wettability.

Incorporation of Ag in Co9S8-Ni3S2 for Predominantly Enhanced Electrocatalytic Activities for Oxygen Evolution Reaction: A Combined Experimental and DFT Study

Electrodeposition of abundant metals to fabricate efficient and durable electrodes indicate a viable role in advancing renewable electrochemical energy tools. Herein, we deposit Co9S8-Ag-Ni3S2@NF on nickel foam (NF) to produce Co9S8-Ag-Ni3S2@NF as a exceedingly proficient electrode for oxygen evolution reaction (OER). The electrochemical investigation verifies that the Co9S8-Ag-Ni3S2@NF electrode reveals better electrocatalytic activity to OER because of its nanoflowers' open-pore morphology, reduced overpotential (η10=125 mV), smaller charge transfer resistance, long-term stability, and a synergistic effect between various components, which allows the reactants to be more easily absorbed and subsequently converted into gaseous products during the water electrolysis route. Density functional theory (DFT) calculation as well reveals the introduction of Ag (222) surface into the Co9S8 (440)-Ni3S2 (120) structure increases the electronic density of states (DOS) per unit cell of a system and increases the electrocatalytic activity of OER by considerably lowering the energy barriers of its intermediates. This study provides the innovation of employing trimetallic nanomaterials immobilized on a conductive, continuous porous three-dimensional network formed on a nickel foam (NF) substrate as a highly proficient catalyst for OER.

Amorphous conversion in pyrolytic symmetric trinuclear nickel clusters trigger trifunctional electrocatalysts

The pursuit of multifunctional electrocatalysts holds significant importance due to their comprehension of material chemistry. Amorphous materials are particularly appealing, yet they pose challenges in terms of rational design due to their structural disorder and thermal instability. Herein, we propose a strategy that entails the tandem (low-temperature/250-350 °C) pyrolysis of molecular clusters, enabling preservation of the local short-range structures of the precursor Schiff base nickel (Ni3[2(C21H24N3Ni1.5O6)]). The temperature-dependent residuals demonstrate exceptional activity and stability for at least three distinct electrocatalytic processes, including the oxygen evolution reaction (η10 = 197 mV), urea oxidation reaction (η10 = 1.339 V), and methanol oxidation reaction (1358 mA cm-2 at 0.56 V). Three distinct nickel atom motifs are discovered for three efficient electrocatalytic reactions (Ni1 and Ni1' are preferred for UOR/MOR, while Ni2 is preferred for OER). Our discoveries pave the way for the potential development of multifunctional electrocatalysts through disordered engineering in molecular clusters under tandem pyrolysis.

Low-temperature synthesis of high-entropy amorphous metal oxides (HEOs) for enhanced oxygen evolution performance

The rational design of multiple metal ions into high-entropy oxide electrode material via a single-step hydrothermal process is applicable to the evolution of oxygen molecules (O2) through simple water electrolysis. Their cost-effectiveness, high performance, and durable nature are the key factors of non-precious high-entropy multiple metal-based electrocatalysts, which can be used as replaceable catalysts instead of precious ones. This article reports a low-temperature synthesis of the cauliflower-type morphology of high-entropy amorphous metal oxides, and their electrochemical performances towards the oxygen evolution reaction (OER) are investigated. The multiple metal ion (Mn2+, Fe3+, Co2+, Ni2+, Cu2+) oxide electrode material shows an acceptable oxygen evolution reaction (OER) with an overpotential of 290 mV at a current density of 10 mA cm-2 and a lower Tafel slope value of 85 mV dec-1, respectively. Moreover, the 20 h durability test with negligible change in overpotential shows the efficacy of the modified electrode material in harsh alkaline media. The observed electrochemical results towards the OER correspond to the amorphous nature of the active material that displayed a cauliflower-type morphology, having a large specific surface area (240 m2 g-1) and providing higher electrochemical active sites as well. Consequently, post-stability characterization studies (such as PXRD, FESEM, TEM, and XPS) provide more information for understanding the post-structural and morphological results of the high-entropy amorphous metal oxide.

Integrated Catalyst-Substrate Electrodes for Electrochemical Water Splitting: A Review on Dimensional Engineering Strategy

Water splitting (or, water electrolysis) is considered as a promising approach to produce green hydrogen and relieve the ever-increasing energy consumption as well as the accompanied environmental impact. Development of high-efficiency, low-cost practical water-splitting systems demands elegant design and fabrication of catalyst-loaded electrodes with both high activity and long-life time. To this end, dimensional engineering strategies, which effectively tune the microstructure and activity of electrodes as well as the electrochemical kinetics, play an important role and have been extensively reported over the past years. Here, a type of most investigated electrode configurations is reviewed, combining particulate catalysts with 3D porous substrates (aerogels, metal foams, hydrogels, etc.), which offer special advantages in the field of water splitting. It is analyzed the design principles, structural and interfacial characteristics, and performance of particle-3D substrate electrode systems including overpotential, cycle life, and the underlying mechanism toward improved catalytic properties. In particular, it is also categorized the catalysts as different dimensional particles, and show the importance of building hybrid composite electrodes by dimensional control and engineering. Finally, present challenges and possible research directions toward low-cost high-efficiency water splitting and hydrogen production is discussed.

Selective Reduction of Multivariate Metal-Organic Frameworks for Advanced Electrocatalytic Cathodes in High Areal Capacity and Long-Life Lithium-Sulfur Batteries

Lithium-sulfur batteries hold great promise as next-generation high-energy-density batteries. However, their performance has been limited by the low cycling stability and sulfur utilization. Herein, we demonstrate that a selective reduction of the multivariate metal-organic framework, MTV-MOF-74 (Co, Ni, Fe), transforms the framework into a porous carbon decorated with bimetallic CoNi alloy and Fe3O4 nanoparticles capable of entrapping soluble lithium polysulfides while synergistically facilitating their rapid conversion into Li2S. Electrochemical studies on coin cells containing 89 wt % sulfur loading revealed a reversible capacity of 1439.8 mA h g-1 at 0.05 C and prolonged cycling stability for 1000 cycles at 1 C/1060.2 mA h g-1 with a decay rate of 0.018% per cycle. At a high areal sulfur loading of 6.9 mg cm-2 and lean electrolyte/sulfur ratio (4.5 μL:1.0 mg), the battery based on the 89S@CoNiFe3O4/PC cathode provides a high areal capacity of 6.7 mA h cm-2. The battery exhibits an outstanding power density of 849 W kg-1 at 5 C and delivers a specific energy of 216 W h kg-1 at 2 C, corresponding to a specific power of 433 W kg-1. Density functional theory shows that the observed results are due to the strong interaction between the CoNi alloy and Fe3O4, facilitated by charge transfer between the polysulfides and the substrate.

Enhancing the electrochemical activity of zinc cobalt sulfide via heterojunction with MoS2 metal phase for overall water splitting

The integration of diverse components into a single heterostructure represents an innovative approach that boosts the quantity and variety of active centers, thereby enhancing the catalytic activity for both hydrogen evolution reactions (HER) and oxygen evolution reactions (OER) in the water splitting process. In this study, a novel, hierarchically porous one-dimensional nanowire array comprising zinc cobalt sulfide and molybdenum disulfide (MoS2@Zn0.76Co0.24S) was successfully synthesized on a Ni foam substrate using an efficient and straightforward hydrothermal synthesis strategy. The incorporation of the metallic phase of molybdenum disulfide elevates the electronic conductivity of MoS2@Zn0.76Co0.24S, resulting in impressively low overpotentials. At 20, 50, and 100 mA cm-2, the overpotentials for oxygen evolution reaction (OER) are merely 90 mV, 170 mV, and 240 mV, respectively. Similarly, for hydrogen evolution reaction (HER), the overpotentials are 169 mV, 237 mV, and 301 mV at the same current densities in 1.0 M potassium hydroxide solution. The utilization of the MoS2@Zn0.76Co0.24S /NF electrolyzer demonstrates its exceptional performance as a catalyst in alkaline electrolyzers. Operating at a mere 1.45 V and 10 mA cm-2, it showcases outstanding efficiency. Achieving a current density of 405 mA cm-2, the system generates hydrogen at a rate of 3.1 mL/min with a purity of 99.997%, achieving an impressive cell efficiency of 68.28% and a voltage of 1.85 V. Furthermore, the MoS2@Zn0.76Co0.24S /NF hybrid exhibits seamless integration with solar cells, establishing a photovoltaic electrochemical system for comprehensive water splitting. This wireless assembly harnesses the excellent performance of the hybrid nanowire, offering a promising solution for efficient, durable, and cost-effective bifunctional electrocatalysts in the realm of renewable energy.

Activating Commercial Nickel Foam to a Highly Efficient Electrocatalyst for Oxygen Evolution Reaction through a Three-Step Surface Reconstruction

It is highly desired to directly use commercial nickel foam (CNF) as an electrocatalyst for the oxygen evolution reaction (OER) via simple surface reconstruction. In our research, a simple three-step preactivation process was proposed to reconstruct CNF as an efficient OER catalyst, including calcination, high-voltage treatment, and immersing in electrolyte. The optimal CNF after three-step activation reaches an excellent OER performance of 228 and 267 mV at η10 and η100 in alkaline media and can tolerate long-term tests under a large current density of 500 mA·cm-2. The promotion of each step was explored. The calcination step leads to a reconstructive surficial morphology with an enlarged active surface, providing a prerequisite for the following construction steps. The high-voltage treatment changes the valence of surface Ni species, generating phases with higher catalytic activity, and the immersing process introduces Fe heteroatoms into the surface of CNF, boosting the catalytic performance of CNF through Ni-Fe interactions. This research provides a simple method of making high-performance catalysts with accessible nickel foam, a potential for large-scale application in practical industry, and new thinking for the manipulation of Ni-based catalysts.

Low-cost flexible textile electrocatalyst for overall water splitting

Through the braidability of cotton fiber and the richness of surface functional groups, cotton fiber can be woven into any shape, and catalytically active centers can be stably anchored on the fibers. During the electrocatalytic overall water splitting (OWS) process, catalyst shedding and activity reduction can be effectively avoided.

A Review of Transition Metal Boride, Carbide, Pnictide, and Chalcogenide Water Oxidation Electrocatalysts

Transition metal borides, carbides, pnictides, and chalcogenides (X-ides) have emerged as a class of materials for the oxygen evolution reaction (OER). Because of their high earth abundance, electrical conductivity, and OER performance, these electrocatalysts have the potential to enable the practical application of green energy conversion and storage. Under OER potentials, X-ide electrocatalysts demonstrate various degrees of oxidation resistance due to their differences in chemical composition, crystal structure, and morphology. Depending on their resistance to oxidation, these catalysts will fall into one of three post-OER electrocatalyst categories: fully oxidized oxide/(oxy)hydroxide material, partially oxidized core@shell structure, and unoxidized material. In the past ten years (from 2013 to 2022), over 890 peer-reviewed research papers have focused on X-ide OER electrocatalysts. Previous review papers have provided limited conclusions and have omitted the significance of "catalytically active sites/species/phases" in X-ide OER electrocatalysts. In this review, a comprehensive summary of (i) experimental parameters (e.g., substrates, electrocatalyst loading amounts, geometric overpotentials, Tafel slopes, etc.) and (ii) electrochemical stability tests and post-analyses in X-ide OER electrocatalyst publications from 2013 to 2022 is provided. Both mono and polyanion X-ides are discussed and classified with respect to their material transformation during the OER. Special analytical techniques employed to study X-ide reconstruction are also evaluated. Additionally, future challenges and questions yet to be answered are provided in each section. This review aims to provide researchers with a toolkit to approach X-ide OER electrocatalyst research and to showcase necessary avenues for future investigation.

Interfacial "Single-Atom-in-Defects" Catalysts Accelerating Li+ Desolvation Kinetics for Long-Lifespan Lithium-Metal Batteries

The lithium-metal anode is a promising candidate for realizing high-energy-density batteries owing to its high capacity and low potential. However, several rate-limiting kinetic obstacles, such as the desolvation of Li+ solvation structure to liberate Li+ , Li0 nucleation, and atom diffusion, cause heterogeneous spatial Li-ion distribution and fractal plating morphology with dendrite formation, leading to low Coulombic efficiency and depressive electrochemical stability. Herein, differing from pore sieving effect or electrolyte engineering, atomic iron anchors to cation vacancy-rich Co1- x S embedded in 3D porous carbon (SAFe/CVRCS@3DPC) is proposed and demonstrated as catalytic kinetic promoters. Numerous free Li ions are electrocatalytically dissociated from the Li+ solvation complex structure for uniform lateral diffusion by reducing desolvation and diffusion barriers via SAFe/CVRCS@3DPC, realizing smooth dendrite-free Li morphologies, as comprehensively understood by combined in situ/ex situ characterizations. Encouraged by SAFe/CVRCS@3DPC catalytic promotor, the modified Li-metal anodes achieve smooth plating with a long lifespan (1600 h) and high Coulombic efficiency without any dendrite formation. Paired with the LiFePO4 cathode, the full cell (10.7 mg cm-2 ) stabilizes a capacity retention of 90.3% after 300 cycles at 0.5 C, signifying the feasibility of using interfacial catalysts for modulating Li behaviors toward practical applications.

Nickel Sulfide/Hierarchical Porous Carbon from Spent Residue Hydrocracking Catalyst as Electrocatalyst for the Oxygen Evolution Reaction

Spent residue slurry-phase hydrocracking catalyst coated with coke have been classified as hazardous solid waste, presenting serious economic and environmental issues to refiners. Herein, the spent catalysts with a nickel sulfide nanoparticle/coke hierarchical structure (NiSX /C) from our previous work were used to prepare nickel sulfide/hierarchical porous carbon (NiSX /HPC) for the oxygen evolution reaction (OER) through the method of carbonization, activation, and sulfurization. The results indicate that the NiSX /C converts into Ni/HPC after carbonization and activation, and then transform into NiSX /HPC by sulfurization. The optimized NiSX /HPC-8 possesses the crystal phase of NiS2 , and the high specific surface area of 1134.9 m2  g-1 with the hierarchical micro-mesoporous structure. Besides, NiSX /HPC-8 achieves a low overpotential of 236 mV at 10 mA cm-2 , a low Tafel slope of 64.1 mV dec-1 , and excellent stability. This work provides a viable method for upcycling spent catalysts to re-constructed OER catalysts with high catalytic performance and durability.

Rational Design of a Copper Cobalt Sulfide/Tungsten Disulfide Heterostructure for Excellent Overall Water Splitting

Integrating different components into a heterostructure is a novel approach that increases the number of active centers to enhance the catalytic activities of a catalyst. This study uses an efficient, facile hydrothermal strategy to synthesize a unique heterostructure of copper cobalt sulfide and tungsten disulfide (CuCo2S4-WS2) nanowires on a Ni foam (NF) substrate. The nanowire arrays (CuCo2S4-WS2/NF) with multiple integrated active sites exhibit small overpotentials of 202 (299) and 240 (320) mV for HER and OER at 20 (50) mA cm-2 and 1.54 V (10 mA cm-2) for an electrolyzer in 1.0 M KOH, surpassing commercial and previously reported catalysts. A solar electrolyzer composed of CuCo2S4-WS2 bifunctional electrodes also produced significant amounts of hydrogen through a water splitting process. The remarkable performance is accredited to the extended electroactive surface area, reasonable density of states near the Fermi level, optimal adsorption free energies, and good charge transfer ability, further validating the excellent dual function of CuCo2S4-WS2/NF in electrochemical water splitting.

Manganese cobalt sulfide/molybdenum disulfide nanowire heterojunction as an excellent bifunctional catalyst for electrochemical water splitting

Heterointerface engineering enhances catalytic active centers and charge transfer capabilities to increase oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) kinetics. In this study, a novel heterostructure of manganese cobalt sulfide-molybdenum disulfide on nickel foam (MnCo₂S₄-MoS₂/NF) was synthesized via a two-step hydrothermal process. The nanowire-shaped MnCo₂S₄-MoS₂ on NF displayed accelerated charge transfer ability and multiple integrated active sites. When tested in one molar (1 M) potassium hydroxide (KOH) electrolyte, it furnished low overpotentials of 105 and 171 mV for the HER and 220 and 300 mV for the OER at the current densities of 20 and 50 mA cm^-2, respectively. An electrolyzer based on MnCo₂S₄-MoS₂/NF required low operating potentials of 1.41 and 1.49 V to yield the current densities of 10 and 20 mA cm^-2, respectively, surpassing commercial and previously reported catalysts. Density functional theory (DFT) analysis revealed that the MnCo₂S₄-MoS₂ heterostructure possesses the optimal adsorption free energies for the reactants, an extended electroactive surface area, good charge transfer ability, and reasonable density of electronic states close to the Fermi level, all of which contribute to the high activity of catalyst. Thus, heterointerface engineering is a promising strategy for creating efficient catalysts for overall water splitting.

Recent Advances and Future Perspectives of Metal-Based Electrocatalysts for Overall Electrochemical Water Splitting

Recently, the growing demand for a renewable and sustainable fuel alternative is contingent on fuel cell technologies. Even though it is regarded as an environmentally sustainable method of generating fuel for immediate concerns, it must be enhanced to make it extraordinarily affordable, and environmentally sustainable. Hydrogen (H₂ ) synthesis by electrochemical water splitting (ECWS) is considered one of the foremost potential prospective methods for renewable energy output and H₂ society implementation. Existing massive H₂ output is mostly reliant on the steaming reformation of carbon fuels that yield CO₂ together with H₂ and is a finite resource. ECWS is a viable, efficient, and contamination-free method for H₂ evolution. Consequently, developing reliable and cost-effective technology for ECWS was a top priority for scientists around the globe. Utilizing renewable technologies to decrease total fuel utilization is crucial for H₂ evolution. Capturing and transforming the fuel from the ambient through various renewable solutions for water splitting (WS) could effectively reduce the need for additional electricity. ECWS is among the foremost potential prospective methods for renewable energy output and the achievement of a H₂ -based economy. For the overall water splitting (OWS), several transition-metal-based polyfunctional metal catalysts for both cathode and anode have been synthesized. Furthermore, the essential to the widespread adoption of such technology is the development of reduced-price, super functional electrocatalysts to substitute those, depending on metals. Many metal-premised electrocatalysts for both the anode and cathode have been designed for the WS process. The attributes of H₂ and oxygen (O₂ ) dynamics interactions on the electrodes of water electrolysis cells and the fundamental techniques for evaluating the achievement of electrocatalysts are outlined in this paper. Special emphasis is paid to their fabrication, electrocatalytic performance, durability, and measures for enhancing their efficiency. In addition, prospective ideas on metal-based WS electrocatalysts based on existing problems are presented. It is anticipated that this review will offer a straight direction toward the engineering and construction of novel polyfunctional electrocatalysts encompassing superior efficiency in a suitable WS technique.

Plasma-assisted synthesis of hierarchical defect N-doped iron–cobalt sulfide@Co foam as an efficient bifunctional electrocatalyst for overall water splitting

N-doped CoFeS was synthesized via an ion exchange method to prepare a precursor, followed by sulphidation and plasma-assisted engraving in nitrogen gas.

Tuning Electronic Structure and Composition of FeNi Nanoalloys for Enhanced Oxygen Evolution Electrocatalysis via a General Synthesis Strategy

Developing low-cost and efficient oxygen evolution electrocatalysts is key to decarbonization. A facile, surfactant-free, and gram-level biomass-assisted fast heating and cooling synthesis method is reported for synthesizing a series of carbon-encapsulated dense and uniform FeNi nanoalloys with a single-phase face-centered-cubic solid-solution crystalline structure and an average particle size of sub-5 nm. This method also enables precise control of both size and composition. Electrochemical measurements show that among Fe_x Ni_{(1- x )} nanoalloys, Fe_0.5 Ni_0.5 has the best performance. Density functional theory calculations support the experimental findings and reveal that the optimally positioned d-band center of O-covered Fe_0.5 Ni_0.5 renders a half-filled antibonding state, resulting in moderate binding energies of key reaction intermediates. By increasing the total metal content from 25 to 60 wt%, the 60% Fe_0.5 Ni_0.5 /40% C shows an extraordinarily low overpotential of 219 mV at 10 mA cm^-2 with a small Tafel slope of 23.2 mV dec^-1 for the oxygen evolution reaction, which are much lower than most other FeNi-based electrocatalysts and even the state-of-the-art RuO₂ . It also shows robust durability in an alkaline environment for at least 50 h. The gram-level fast heating and cooling synthesis method is extendable to a wide range of binary, ternary, quaternary nanoalloys, as well as quinary and denary high-entropy-alloy nanoparticles.

Nanoengineering of Catalysts for Enhanced Hydrogen Production

Hydrogen (H2) has emerged as a sustainable energy carrier capable of replacing/complementing the global carbon-based energy matrix. Although studies in this area have often focused on the fundamental understanding of catalytic processes and the demonstration of their activities towards different strategies, much effort is still needed to develop high-performance technologies and advanced materials to accomplish widespread utilization. The main goal of this review is to discuss the recent contributions in the H2 production field by employing nanomaterials with well-defined and controllable physicochemical features. Nanoengineering approaches at the sub-nano or atomic scale are especially interesting, as they allow us to unravel how activity varies as a function of these parameters (shape, size, composition, structure, electronic, and support interaction) and obtain insights into structure–performance relationships in the field of H2 production, allowing not only the optimization of performances but also enabling the rational design of nanocatalysts with desired activities and selectivity for H2 production. Herein, we start with a brief description of preparing such materials, emphasizing the importance of accomplishing the physicochemical control of nanostructures. The review finally culminates in the leading technologies for H2 production, identifying the promising applications of controlled nanomaterials.

Highly Effective OER Electrocatalysts Generated from a Two-Dimensional Metal-Organic Framework Including a Sulfur-Containing Linker without Doping

Metal-organic frameworks (MOFs) with different topologies formed by the self-assembly of sulfur-containing inorganic ligands, cobalt ions, and large ligands can be used to prepare electrocatalysts for water splitting in order to fully explore the advantages of MOFs in terms of structural tailoring and quantitative assembly. It is possible to avoid using an extradoped sulfur source to reduce waste as well as to disperse Co and sulfur elements evenly and controllably throughout the final material to maximize the overall synergistic effect. In this work, different kinds of bimetallic MOF materials containing sulfur can be synthesized very conveniently by using an economical and practical diffusion method. These materials are directly used as OER electrocatalysts, and the bimetallic MOFs have the best electrocatalytic performance when the ratio of Co to Fe is 6:4. The overpotential at a current density of 10 mA cm^-2 was 260 mV, with a Tafel slope of 56 mV dec^-1 and good stability. It was assembled with 20% commercial Pt/C material into a two-electrode system for all-water decomposition, and the decomposition voltage at 10 mA cm^-2 was 1.81 V. From the electronic configuration microscopic point of view, the introduction of iron ions changed the original synergistic effect for Co-S-Co, which more easily led to the formation of high-valence Co³⁺ and finally produced highly active electrocatalytic sites. From a macroscopic point of view, the material produced in situ during the electrochemical reaction process not only retains the original 2D layered structure but also utilizes bubbles to produce a loose structure with defective sites. These structural features are advantageous because they provide not only an abundance of active sites and permeable channels but also the necessary interfaces and electron-transport channels for the formation of electrostatic charge-separation layers, making it easier to intercalate and delaminate the hydroxide ions. Furthermore, the changed hydroxyl ions and nitrogen and sulfur atoms on the channel surface may operate as interaction sites, increasing the surface characteristics, facilitating electron transfer, and reducing electron-transfer resistance. To summarize, the rational design of sulfur-containing layered MOF materials directly as water-splitting catalysts is a crucial next step in developing cost-effective, environmentally friendly, and low-energy-consumption electrocatalysts based on the findings of this study.

Identification of Interface Structure for a Topological CoS2 Single Crystal in Oxygen Evolution Reaction with High Intrinsic Reactivity

Transition metal chalcogenides such as CoS₂ have been reported as competitive catalysts for oxygen evolution reaction. It has been well confirmed that surface modification is inevitable in such a process, with the formation of different re-constructed oxide layers. However, which oxide species should be responsible for the optimized catalytic efficiencies and the detailed interface structure between the modified layer and precatalyst remain controversial. Here, a topological CoS₂ single crystal with a well-defined exposed surface is used as a model catalyst, which makes the direct investigation of the interface structure possible. Cross-sectional transmission electron microscopy of the sample reveals the formation of a 2 nm thickness Co₃O₄ layer that grows epitaxially on the CoS₂ surface. Thick CoO pieces are also observed and are loosely attached to the bulk crystal. The compact Co₃O₄ interface structure can result in the fast electron transfer from adsorbed O species to the bulk crystal compared with CoO pieces as evidenced by the electrochemical impedance measurements. This leads to the competitive apparent and intrinsic reactivity of the crystal despite the low surface geometric area. These findings are helpful for the understanding of catalytic origins of transition metal chalcogenides and the designing of high-performance catalysts with interface-phase engineering.

Promising Electrocatalytic Water and Methanol Oxidation Reaction Activity by Nickel Doped Hematite/Surface Oxidized Carbon Nanotubes Composite Structures

Tailoring the precise construction of non-precious metals and carbon-based heterogeneous catalysts for electrochemical oxygen evolution reaction (OER) and methanol oxidation reaction (MOR) is crucial for energy conversion applications. Herein, this work reports the composite of Ni doped Fe₂ O₃ (Ni-Fe₂ O₃ ) with mildly oxidized multi-walled CNT (O-CNT) as an outstanding Mott-Schottky catalyst for OER and MOR. O-CNT acts as a co-catalyst which effectively regulates the charge transfer in Ni-Fe₂ O₃ and thus enhances the electrocatalytic performance. Ni-Fe₂ O₃ /O-CNT exhibits a low onset potential of 260 mV and overpotential 310 mV @ 10 mA cm^-2 for oxygen evolution. Being a Mott-Schottky catalyst, it achieves the higher flat band potential of -1.15 V with the carrier density of 0.173×10²⁴  cm^-3 . Further, in presence of 1 M CH₃ OH, it delivers the MOR current density of 10 mA cm^-2 at 1.46 V vs. RHE. The excellent electrocatalytic OER and MOR activity of Ni-Fe₂ O₃ /O-CNT could be attributed to the synergistic interaction between Ni-doped Fe₂ O₃ and O-CNT.

Modified, Solvothermally Derived Cr-doped SnO2 Nanostructures for Enhanced Photocatalytic and Electrochemical Water-Splitting Applications

Cr-doped SnO₂ nanostructures with a dopant concentration ranging from 1 to 5% have been successfully prepared using low-temperature modified solvothermal synthesis. The as-prepared nanoparticles showed a rutile tetragonal structure with a rough undefined morphology having no other elemental impurities. The particle shape and size, band gap, and specific surface area of the samples were investigated by scanning electron microscopy, transmission electron microscopy (TEM), high-resolution TEM, UV-visible diffused reflectance spectroscopy, and Brunauer-Emmett-Teller surface area studies. The optical band gap was found in the range of 3.23-3.67 eV and the specific surface area was in the range of 108-225 m²/g, which contributes to the significantly enhanced photocatalytic and electrochemical performance. Photocatalytic H₂ generation of as-prepared Cr-doped SnO₂ nanostructures showed improved effect of the increasing dopant concentration with narrowing of the band gap. Electrochemical water-splitting studies also stressed upon the superiority of Cr-doped SnO₂ nanostructures over pristine SnO₂ toward hydrogen evolution reaction and oxygen evolution reaction responses.

Metallic Co and crystalline Co-Mo oxides supported on graphite felt for bifunctional electrocatalytic hydrogen evolution and urea oxidation

Oxygen evolution reaction (OER) and urea oxidation reaction (UOR) play important roles in the field of hydrogen energy preparation and pollution treatment. In this work, by merging bimetallic Co-Mo oxides with metallic Co on the graphite felt (GF), we effectively manufacture a 3D bifunctional and highly efficient electrocatalyst (CoMoO@Co/GF) with multi-site functionality for the simultaneous reduction of water and the oxidation of urea in an alkaline medium. The presence of metallic Co causes Co-Mo oxides to evolve from amorphous to crystalline structures. The coupling interface produced between metallic Co and Co-Mo oxides is proven to facilitate electron transport in addition to extensively accessible and highly electroactive Co-Mo oxide nanoflower architecture. The experimental results reveal that the overpotentials for OER and UOR in the CoMoO@Co/GF electrode require only 269 and 115 mV to obtain a current density of 10 mA cm^-2, respectively. Furthermore, with the aid of urea, the overpotential for HER at the current density of 10 mA cm^-2 is lowered to 155 mV. Most notably, the constructed CoMoO@Co/GF-based electrolytic cell only requires a 1.5 V dry battery to achieve effective H₂ evolution and noteworthy stability, outperforming the commercial catalyst-based device and many previous results. The combination of experiments and theoretical calculations further clarifies the active sites in the catalyst. What's more, the pathway of electron transfer in the catalytic process is defined.

Critical Review, Recent Updates on Zeolitic Imidazolate Framework-67 (ZIF-67) and Its Derivatives for Electrochemical Water Splitting

Design and construction of low-cost electrocatalysts with high catalytic activity and long-term stability is a challenging task in the field of catalysis. Metal-organic frameworks (MOF) are promising candidates as precursor materials in the development of highly efficient electrocatalysts for energy conversion and storage applications. This review starts with a summary of basic concepts and key evaluation parameters involved in the electrochemical water-splitting reaction. Then, different synthesis approaches reported for the cobalt-based Zeolitic imidazolate framework (ZIF-67) and its derivatives are critically reviewed. Additionally, several strategies employed to enhance the electrocatalytic activity and stability of ZIF-67-based electrocatalysts are discussed in detail. The present review provides a succinct insight into the ZIF-67 and its derivatives (oxides, hydroxides, sulfides, selenides, phosphide, nitrides, telluride, heteroatom/metal-doped carbon, noble metal-supported ZIF-67 derivatives) reported for oxygen evolution reaction (OER), hydrogen evolution reaction (HER), and overall water splitting applications. Finally, this review concludes with the associated challenges and the perspectives on developing the best economic, durable electrocatalytic materials.

Self-supported Co9S8-Ni3S2-CNTs/NF electrode with superwetting multistage micro-nano structure for efficient bifunctional overall water splitting

Electrochemical water splitting for hydrogen production using cost-effective and high-efficiency electrocatalysts in alkaline electrolytes is of great significance for solving energy crisis and environmental pollution. Herein, we reported a superhydrophilic and underwater superaerophobic multistage layered micro-nano structure ofCo₉S₈-Ni₃S₂-CNTs/NF on nickel foam (NF) prepared by a simple one-step hydrothermal procedure. Particularly, the multistage layered micro-nano structure makes the electrode superhydrophilic and superaerophobic, which can facilitate the exposure of active sites, accelerate the tansfer of electrolyte and the release of gas bubbles. Consequently, the rough electrode demonstrated excellent catalytic performance in alkaline condition, which only need a low overpotential 127 mV for oxygen evolution reaction (OER) and 243 mV for hydrogen evolution reaction (HER) at 10 mA cm^-2 and can keep a long durability for 10 h at 10 mA cm^-2. In addition, the production of hydrogen in an electrolytic water device with Co₉S₈-Ni₃S₂-CNTs/NF as bifunctional electrode prowered by the electricity derived from solar and wind energy in laboratory condition was artificially simulated. This work represents a perspective in improving the electrocatalytic performance of water splitting by structure and wettability regulation and opens a new avenue for clean energy generation.

Fe doped NiS nanosheet arrays grown on carbon fiber paper for a highly efficient electrocatalytic oxygen evolution reaction

Developing efficient and low-cost non-noble metal catalysts for the oxygen evolution reaction (OER) is important for hydrogen production through water electrolysis. Herein, Fe doped NiS nanosheets directly grown on conductive carbon fiber paper (Fe-NiS@CFP) were fabricated through a two-step hydrothermal process. The microstructure, interface and electronic states of the prepared sample were modulated by Fe doping, exhibiting small internal and interface charge-transfer resistance. Benefiting from these factors, Fe-NiS@CFP shows superior electrocatalytic performance with an overpotential of 275 mV at 100 mA cm^-2 and maintains the activity for at least 50 h as a working electrode for the OER. This work may provide insights into the design and fabrication of non-noble metal sulfide electrocatalysts.

Dumbbell-Shaped Ternary Transition-Metal (Cu, Ni, Co) Phosphate Bundles: A Promising Catalyst for the Oxygen Evolution Reaction

Development of economical and high-performance electrocatalysts for the oxygen evolution reaction (OER) is of tremendous interest for future applications as sustainable energy materials. Here, a unique member of efficient OER electrocatalysts has been developed based upon structurally versatile dumbbell-shaped ternary transition-metal (Cu, Ni, Co) phosphates with a three-dimensional (3D) (Cu₂(OH)(PO₄)/Ni₃(PO₄)₂·8H₂O/Co₃(PO₄)₂·8H₂O) (CNCP) structure. This structure is prepared using a simple aqueous stepwise addition of metal ion source approach. Various structural investigations demonstrate highly crystalline nature of the composite structure. Apart from the unique structural aspect, it is important that the CNCP composite structure has proved to be an excellent electrocatalyst for OER performance in comparison with its binary or constituent phosphate under alkaline and neutral conditions. Notably, the CNCP electrocatalyst displays a much lower overpotential of 224 mV at a current density of 10 mA cm^-2 and a lower Tafel slope of 53 mV dec^-1 with high stability in alkaline medium. In addition, X-ray photoelectron spectroscopy analysis suggested that the activity and long-term durability for the OER of the ternary 3D metal phosphate are due to the presence of electrochemically dynamic constituents such as Ni and Co and their resulting synergistic effects, which was further supported by theoretical studies. Theoretical calculations also reveal that the incredible OER execution was ascribed to the electron redistribution set off in the presence of Ni and Cu and the most favorable interaction between the *OOH intermediate and the active sites of CNCP. This work may attract the attention of researchers to construct efficient 3D ternary metal phosphate catalysts for various applications in the field of electrochemistry.

Ru@Ni3S2 nanorod arrays as highly efficient electrocatalysts for the alkaline hydrogen evolution reaction

Ru-decorated Ni3S2 nanorod arrays demonstrate an superior alkaline hydrogen evolution performance. Further modification with polyaniline could significantly enhance the long-term stability for continuous hydrogen generation.

Hexagonal NiMoO4-MoS2 nanosheet heterostructure as a bifunctional electrocatalyst for urea oxidation assisted overall water electrolysis

A hexagonal NiMoO4-MoS2 nanosheet heterostructure on nickel foam (NiMoO4-MoS2/NF) was synthesized by simple hydrothermal and annealing treatment.