Recent progress in transition metal selenide electrocatalysts for water splitting

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

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

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

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

Partial Selenization Strategy for Fabrication of Ni0.85Se@NiCr-LDH Heterostructure as an Efficient Bifunctional Electrocatalyst for Overall Water Splitting

NiCr-LDH and its partial selenization as Ni0.85Se@NiCr-LDH heterostructure is established here as an alkaline water electrolyzer for achieving enhanced overall water splitting efficiency. The hydrothermally synthesized optimized batch of Ni0.85Se@NiCr-LDH is thoroughly characterized to elucidate its structure, morphology, and composition. Compared to pristine NiCr-LDH, the batch of Ni0.85Se@NiCr-LDH exhibits exceptional alkaline OER and HER activity with low overpotentials of 258 and 85 mV at 10 mA cm-2, respectively. Besides, Ni0.85Se@NiCr-LDH also exhibits excellent acidic HER with an overpotential of only 61 mV at 10 mA cm-2, indicating that Ni0.85Se@NiCr-LDH can operate effectively across a wide pH range. The excellent electrochemical stability of Ni0.85Se@NiCr-LDH for 24 h operation is attributed to the formation of a thin layer of SeOx during OER operation. The role of selenization and the effect of Cr in the LDH lattice toward enhanced electrocatalytic water splitting is discussed. The outstanding OER and HER performances of Ni0.85Se@NiCr-LDH are attributed to the higher electrochemical active surface area, favorable conditions for adsorption of HER/OER intermediates, low charge transfer resistance, and improved conductivity. The practical application of Ni0.85Se@NiCr-LDH as a bifunctional electrocatalyst for overall water splitting is reflected from the low cell voltage of 1.548 V at 10 mA cm-2.

Facile Synthesis of Selenium Nanoparticles for Enhanced Oxygen Evolution Reaction: Insights into Electrochemical and Photoelectrochemical Catalysis

Implementing a hydrogen economy on an industrial scale poses challenges, particularly in developing cost-effective and stable catalysts for water electrolysis. This study explores the catalytic potential of selenium nanoparticles (Se-NPs) synthesized via a simple chemical bath deposition method for electrochemical and photoelectrochemical (PEC) water splitting. The successful fabrication of Se-NPs on fluorine-doped tin oxide (FTO) electrodes has been confirmed using a wide range of analytical tools like X-ray diffraction, energy-dispersive X-ray spectroscopy, and scanning electron microscopy. Importantly, electrochemical measurements revealed superior electrocatalytic activity of the modified Se-NPs/FTO electrodes, with low overpotential (220 mV at 10 mA cm-2) and Tafel slope (90.13 mV dec-1), indicating faster reaction kinetics and reduced energy inputs for oxygen evolution reaction. Furthermore, the Se-NPs/FTO electrode was employed for PEC water splitting in Na2S electrolyte, showing a notable enhancement in photocurrent density with a difference of 700 μA cm-2 between light and dark conditions at 1.5 V vs RHE, demonstrating efficient light-driven hydrogen production. The overall findings of this work establish that the proposed Se-NPs/FTO electrodes are promising composites for both electrochemical and PEC performance, thereby providing insights into developing cost-effective catalysts for large-scale water splitting.

Ni2+ Ion-Induced Phase and Morphology Tailoring in Nix-CuMoSe Nanorod Catalysts for Overall Water Splitting

Copper molybdenum-based selenides (CuMoSe) as promising hydrogen evolution catalysts have been widely applied in the electrocatalytic splitting of water, while their limited active sites and relatively poor oxygen evolution activity restrict their further application as effective bifunctional electrocatalysts. Here, we report a simple hydrothermal method to fabricate rodlike NiCuMoSe arrays on nickel foam (Nix-CuMoSe/NF), the morphology of which is determined by Ni2+ ions because Ni2+ can promote the growth of the rodlike structure. Furthermore, the phase transformation from CuSe to CuSe2 is triggered by adjusting the Ni2+ ion content of the growth solution. Due to the morphology control and phase component modulation, the prepared Ni2.5-CuMoSe/NF possesses effective catalytic activities of the hydrogen/oxygen evolution reaction (HER/OER) with overpotentials of 89/183 mV at 10 mA cm-2 in 1 M KOH, and the cell (composed of Ni2.5-CuMoSe/NF as both the cathode and anode) can provide a current density of 10 mA cm-2 at a low cell voltage of 1.50 V with a stability of 110 h. This work will advance the development of transition metal electrocatalysts for effective overall water splitting.

Phase-Engineered Dichalcogenides/Fluorine-Free V4C3Tx (T = OH, O) Heterostructures for pH-Universal Hydrogen Evolution Reaction

This research addresses the pH-dependency limitation in electrocatalytic hydrogen evolution reactions (HER) by creating heterostructures through the chemical bonding between 2D-dichalcogenides and V4C3Tx (T = OH, O) planes. The one-step solvothermal synthesis employed in this study constructs a synergistically interacted 1T phase of, e.g., MoS2 and V4C3Tx MXene, demonstrating an omnidirectional improvement on catalytic stability, active site exposure, surface area enlargement, electrical conductivity, and hence enhancement of water dissociation activities. Despite the notable progress in creating hydrogen production catalysts with ground breaking performances, a significant gap remains in the availability of catalysts capable of functioning effectively under high current densities. The catalyst 1T MoS2@V4C3Tx shows remarkable activities under the current density of 1000 mA cm-2, which require overpotentials of 16, 24, and 37 mV in 0.5 m H2SO4, 1 m KOH, and 0.1 m PBS electrolytes, respectively at 10 mA cm-2, and exhibits excellent HER performance with small overpotentials of 103.16 and 138 mV to achieve current densities of 500 and 1000 mA cm-2, respectively, with outstanding stability for 1000 cylic voltammetric cycle HER test without degradation in acidic media. Enhanced HER performance has also been observed in other 2D-dichalcogenides/V4C3Tx heterostructures, providing prospects for phase-engineered dichalcogenides/fluorine-free V4C3Tx composites for pH-universal HER.

One-step electrodeposition synthesis of NiFePS on carbon cloth as self-supported electrodes for electrochemical overall water splitting

Electrocatalytic water splitting (EWS) for hydrogen production is considered an ideal strategy for utilizing renewable energy, reducing fossil fuel consumption, and addressing environmental pollution issues. Traditional noble metal electrocatalysts have excellent performance, but their cost is high. Developing efficient, stable, and relatively inexpensive dual functional electrocatalysts is crucial for promoting large-scale EWS hydrogen production processes. Herein, a simple one-step electrodeposition method was used to grow nickel-iron phosphorus-sulfides (NiFePS) on the surface of hydrophilic treated carbon cloth (CC). The resultant NiFePS/CC with a phosphorus to sulfur ratio of 1:4 exhibited the best electrocatalytic performance, requiring only -91 mV and 216 mV overpotentials to generate the current densities of 10 mA·cm-2 in hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), respectively. When it was used as a bifunctional electrocatalyst to overall water splitting (OWS), a voltage of 1.536 V can generate a current density of 10 mA·cm-2. The excellent electrocatalytic performance can be ascribed to two factors: 1) the CC with excellent conductivity serves as a growth substrate, reducing the impedance of charge transfer from the electrode to the electrolyte and accelerating the electron transfer rate; 2) The large number of ultra-thin nanosheets formed on the surface of the catalyst increase the electrochemical specific surface area, expose more reaction sites, and thus improve the electrocatalytic reaction performance. This work provides a new approach for designing efficient non-noble metal electrocatalysts for water splitting.

Bimetallic Sulfur-Doped Nickel-Cobalt Selenides as Efficient Bifunctional Electrocatalysts for the Complete Decomposition of Water

The creation and enhancement of non-precious metal bifunctional catalysts with superior stability and stabilizing activity is necessary to achieve water splitting in alkaline media. The paper presents a method for preparing nickel-cobalt bimetallic selenides (NiCo-Sex/CF) using a combination of hydrothermal and high-temperature selenization techniques. NiCo-Sex/CF shows great potential as a catalyst for water separation. The catalyst's electronic structure and active centre can be modified by double doping with sulfur and selenium, resulting in increased selectivity and activity under varying reaction conditions. This method also offers the benefits of a simple preparation process and applicability to a wide range of catalytic reactions. Experimental results demonstrate that an overpotential of 194 mV produces a current density of 10 mA cm-2 when using this electrocatalyst as an OER catalyst. When used as a HER catalyst, the electrocatalyst required an overpotential of only 76 mV to generate a current density of 10 mA cm-2.Furthermore, a voltage of 1.5 V can drive the overall decomposition of water to achieve a current density of 10 mA cm-2. This study highlights the potential of sulfur-selenide double-doped catalysts for both scientific research and practical applications.

NiMOF-Derived MoSe2/NiSe Hollow Nanoflower Structures as Electrocatalysts for Hydrogen Evolution Reaction in Alkaline Medium

Metal-organic frameworks (MOFs) are crystalline porous materials for storage and energy conversion applications with three-dimensional pore structure, high porosity, and specific surface area, which are widely utilized in electrocatalysis. Herein, MoSe2/NiSe composites were synthesized by selenization reaction using NiMOF as the precursor. The composites were hollow nanoflower structures with a synergistic effect between MoSe2 and NiSe to promote rapid electron transfer, which exhibited good hydrogen evolution reaction performance in an alkaline medium. At a current density of 10 mA/cm2, the HER overpotential reaches 80 mV, the Tafel slope is 33.86 mV/dec, and the material has good stability, with polarization curves remaining essentially unchanged after 3000 cycles. These results indicate that the method has a promising application in the preparation of efficient and sustainable catalysts for hydrogen production in alkaline medium.

Architecting Ni3Se4-NiSe2-Co3O4 Triple-Interface Heterostructure on MXene Nanosheets for Boosting Water Splitting by Electronic Modulation and Interface Effects

Strategically engineering electrocatalysts with optimized interfacial electronic architectures and accelerated reaction dynamics is pivotal for augmenting hydrogen generation via alkaline water electrolysis on an industrial scale. Herein, a novel triple-interface heterostructure Ni3Se4-NiSe2-Co3O4 nanoarrays are designed anchored on Ti3C2Tx MXene (Ni3Se4-NiSe2-Co3O4/MXene) with significant work function difference (ΔΦ) as bifunctional electrocatalysts for water electrolysis. Theoretical calculations combined with experiments uncover the pivotal role of the interface-induced electric field in steering charge redistribution, which in turn modulates the adsorption and desorption kinetics of reaction intermediates. Furthermore, the synergistic interaction between Ni3Se4-NiSe2-Co3O4 and Ti3C2Tx MXene nanosheets endows the hybrids with a large electrochemical surface area, abundantly active sites, and high conductivity. Thus, Ni3Se4-NiSe2-Co3O4/MXene manifests exceptional catalytic prowess for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). In addition, the Ni3Se4-NiSe2-Co3O4/MXene electrocatalyst in the water electrolyzer delivers excellent performance and maintains commendable stability beyond 100 h of electrocatalytic operation.

Phase-Induced Strain Effect to Synthesize an Iron-Doped Orthogonal Cobalt Selenide Electrocatalyst for the Oxygen Evolution Reaction

The etching effect has the capability to control atom doping and trigger phase transformation, thereby enhancing the electrocatalytic reaction. Herein, iron-doped cobalt selenide (Fe-CoSe2) nanoparticle-decorated carbon nanofibers (Fe-CoSe2/CNFs) are synthesized by assembling an FeCo-Prussian blue analogue (FeCo-PBA) cube precursor with polyacrylonitrile fibers and then treating with hydrochloric acid, followed by gas phase selenization. The Fe-CoSe2/CNFs catalyst exhibits a large surface area and a porous structure, facilitating the permeation of electrolytes. Moreover, orthorhombic CoSe2 is obtained, which is in favor of improving the oxygen evolution reaction (OER). By modulating the etching time, the ideal crystal phase and the optimal amount of the dopant (Fe) can be achieved, thus showing favorable OER activity. Specifically, the Fe-CoSe2/CNFs electrocatalyst enables high electrocatalytic activity for the OER with a low overpotential of 263 mV to drive a current density of 10 mA cm-2 in 1 M KOH. A small Tafel slope of 51 mV dec-1 shows fast charge transfer kinetics. Density functional theory (DFT) calculations reveal that Fe-doped orthorhombic CoSe2(111) can modulate the electron structure, contributing to OH- adsorption ability. Given this, a strategy for phase transformation induced by etching technology is proposed to improve the intrinsic activity of the catalyst.

Silver nanoparticle-decorated NiFe2O4/CuWO4 heterostructure electrocatalyst for oxygen evolution reactions

In this work, Ag nanoparticles decorated with NiFe2O4/CuWO4 heterostructure were synthesized using the step-wise precipitation method. The influence of varying Ag loading on the NiFe2O4/CuWO4 heterostructure and its electrochemical OER performance was extensively studied in 1 M KOH electrolyte. The obtained LSV profile was analyzed to determine the overpotential, Tafel slope, and onset potential. The heterostructure with an optimal Ag loading of 5 wt% required the least overpotential (1.60 V vs. RHE) for generating a current density of 10 mA cm-2 with a lower Tafel slope of 44.5 mV dec-1, indicating its faster OER kinetics. Furthermore, the composite remained stable over a period of 24 hours with a minimum rise in the overpotential after the stability test. The enhanced OER performance of the as-prepared catalyst can be attributed to the presence of multiple metallic elements in the Ag-loaded NiFe2O4/CuWO4 composite, which created a diverse array of oxygen-vacant sites with varying reactivity, enhancing the charge-transfer kinetics; and thus contributing to the overall efficiency of OER. Therefore, optimizing the Ag concentration and engineering a microstructure represents an encouraging strategy for developing cost-effective catalysts for next-generation energy-conversion applications.

Three-dimensional bimodal pore-rich G/MXene sponge amalgamated with vanadium diselenide nanosheets as a high-performance electrode for electrochemical water-oxidation/reduction reactions

Exploring new strategies to design non-precious and efficient electrocatalysts can provide a solution for sluggish electrocatalytic kinetics and sustainable hydrogen energy. Transition metal selenides are potential contenders for bifunctional electrocatalysis owing to their unique layered structure, low band gap, and high intrinsic activities. However, insufficient access to active sites, lethargic water dissociation, and structural degradation of active materials during electrochemical reactions limit their activities, especially in alkaline media. In this article, we report a useful strategy to assemble vanadium diselenide (VSe2) into a 3D MXene/rGO-based sponge-like architecture (VSe2@G/MXe) using hydrothermal and freeze-drying approaches. The 3D hierarchical meso/macro-pore rich sponge-like morphology not only prevents aggregation of VSe2 nanosheets but also offers a kinetics-favorable framework and high robustness to the electrocatalyst. Synergistic coupling of VSe2 and a MXene/rGO matrix yields a heterostructure with a large specific surface area, high conductivity, and multi-dimensional anisotropic pore channels for uninterrupted mass transport and gas diffusion. Consequently, VSe2@G/MXe presented superior electrochemical activity for both the HER and OER compared to its counterparts (VSe2 and VSe2@G), in alkaline media. The overpotentials required to reach a cathodic and anodic current density of 10 mA cm-2 were 153 mV (Tafel slope = 84 mV dec-1) and 241 mV (Tafel slope = 87 mV dec-1), respectively. The Rct values at the open circuit voltage were as low as 9.1 Ω and 1.41 Ω for the HER and OER activity, respectively. Importantly, VSe2@G/MXe withstands a steady current output for a long 24 h operating time. Hence, this work presents a rational design for 3D microstructures with optimum characteristics for efficient bifunctional alkaline water-splitting.

Transition metal-based electrocatalysts for alkaline overall water splitting: advancements, challenges, and perspectives

Water electrolysis is a promising method for efficiently producing hydrogen and oxygen, crucial for renewable energy conversion and fuel cell technologies. The hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) are two key electrocatalytic reactions occurring during water splitting, necessitating the development of active, stable, and low-cost electrocatalysts. Transition metal (TM)-based electrocatalysts, spanning noble metals and TM oxides, phosphides, nitrides, carbides, borides, chalcogenides, and dichalcogenides, have garnered significant attention due to their outstanding characteristics, including high electronic conductivity, tunable valence electron configuration, high stability, and cost-effectiveness. This timely review discusses developments in TM-based electrocatalysts for the HER and OER in alkaline media in the last 10 years, revealing that the exposure of more accessible surface-active sites, specific electronic effects, and string effects are essential for the development of efficient electrocatalysts towards electrochemical water splitting application. This comprehensive review serves as a guide for designing and constructing state-of-the-art, high-performance bifunctional electrocatalysts based on TMs, particularly for applications in water splitting.

Spontaneous Built-In Electric Field in C3N4-CoSe2 Modified Multifunctional Separator with Accelerating Sulfur Evolution Kinetics and Li Deposition for Lithium-Sulfur Batteries

The discovery of the heterostructures that is combining two materials with different properties has brought new opportunities for the development of lithium sulfur batteries (LSBs). Here, C3N4-CoSe2 composite is elaborately designed and used as a functional coating on the LSBs separator. The abundant chemisorption sites of C3N4-CoSe2 form chemical bonding with polysulfides, provides suitable adsorption energy for lithium polysulfides (LiPSs). More importantly, the spontaneously formed internal electric field accelerates the charge flow in the C3N4-CoSe2 interface, thus facilitating the transport of LiPSs and electrons and promoting the bidirectional conversion of sulfur. Meanwhile, the lithiophilic C3N4-CoSe2 sample with catalytic activity can effectively regulate the uniform distribution of lithium when Li+ penetrates the separator, avoiding the formation of lithium dendrites in the lithium (Li) metal anode. Therefore, LSBs based on C3N4-CoSe2 functionalized membranes exhibit a stable long cycle life at 1C (with capacity decay of 0.0819% per cycle) and a large areal capacity of 10.30 mAh cm-2 at 0.1C (sulfur load: 8.26 mg cm-2, lean electrolyte 5.4 µL mgs -1). Even under high-temperature conditions of 60 °C, a capacity retention rate of 81.8% after 100 cycles at 1 C current density is maintained.

Bifunctional Electrocatalysts for Overall and Hybrid Water Splitting

Electrocatalytic water splitting driven by renewable electricity has been recognized as a promising approach for green hydrogen production. Different from conventional strategies in developing electrocatalysts for the two half-reactions of water splitting (e.g., the hydrogen and oxygen evolution reactions, HER and OER) separately, there has been a growing interest in designing and developing bifunctional electrocatalysts, which are able to catalyze both the HER and OER. In addition, considering the high overpotentials required for OER while limited value of the produced oxygen, there is another rapidly growing interest in exploring alternative oxidation reactions to replace OER for hybrid water splitting toward energy-efficient hydrogen generation. This Review begins with an introduction on the fundamental aspects of water splitting, followed by a thorough discussion on various physicochemical characterization techniques that are frequently employed in probing the active sites, with an emphasis on the reconstruction of bifunctional electrocatalysts during redox electrolysis. The design, synthesis, and performance of diverse bifunctional electrocatalysts based on noble metals, nonprecious metals, and metal-free nanocarbons, for overall water splitting in acidic and alkaline electrolytes, are thoroughly summarized and compared. Next, their application toward hybrid water splitting is also presented, wherein the alternative anodic reactions include sacrificing agents oxidation, pollutants oxidative degradation, and organics oxidative upgrading. Finally, a concise statement on the current challenges and future opportunities of bifunctional electrocatalysts for both overall and hybrid water splitting is presented in the hope of guiding future endeavors in the quest for energy-efficient and sustainable green hydrogen production.

Light-Assisted Lithium Metal Anode Enabled by In Situ Photoelectrochemical Engineering

Rechargeable battery devices with high energy density are highly demanded by the modern society. The use of lithium (Li) anodes is extremely attractive for future rechargeable battery devices. However, the notorious Li dendritic and instability of solid electrolyte interface (SEI) issues pose series of challenge for metal anodes. Here, based on the inspiration of in situ photoelectrochemical engineering, it is showed that a tailor-made composite photoanodes with good photoelectrochemical properties (Li affinity property and photocatalytic property) can significantly improve the electrochemical deposition behavior of Li anodes. The light-assisted Li anode is accommodated in the tailor-made current collector without uncontrollable Li dendrites. The as-prepared light-assisted Li metal anode can achieve the in situ stabilization of SEI layer under illumination. The corresponding in situ formation mechanism and photocatalytic mechanism of composite photoanodes are systematically investigated via DFT theoretical calculation, ex situ UV-vis and ex situ XPS characterization. It is worth mentioning that the as-prepared composite photoanodes can adapt to the ultra-high current density of 15 mA cm-2 and the cycle capacity of 15 mAh cm-2 under light, showing no dendritic morphology and low hysteresis voltage. This work is of great significance for the commercialization of new generation Li metal batteries.

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.

Exceptional green hydrogen production performance of a ruthenium-modulated nickel selenide

Developing low-cost, high-efficiency and stable electrocatalysts for the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) is crucial but highly challenging. Density functional theory (DFT) calculations reveal that doping ruthenium (Ru) into catalysts can effectively optimize their electronic structure, hence leading to an optimal Gibbs free energy on the catalyst surface. Herein, an ultra-low Ru (about 2.34 wt%)-doped Ni3Se2 nanowire catalyst (i.e., Ru/Ni3Se2) supported on nickel foam has been fabricated by a hydrothermal reaction followed by a chemical etching process. The unique three-dimensional (3D) interconnected nanowires not only endow Ru and Ni3Se2 with uniform distribution and coupling, but also provide higher electrical conductivity, more active sites, an optimized electronic structure and favorable reaction kinetics. Therefore, the as-obtained Ru/Ni3Se2 catalyst exhibits excellent electrocatalytic performance, with low overpotentials of 24 and 211 mV to supply a current density value of 10 mA cm-2 towards the HER and OER in an alkaline environment, respectively. Notably, the as-fabricated Ru/Ni3Se2 catalyst only requires a low voltage of 1.476 V to derive a current density of 10 mA cm-2 in the constructed two-electrode alkaline electrolyzer and exhibits exceptionally high stability. This work will provide a novel strategy for the design and fabrication of low-cost and high-performance bifunctional electrocatalysts for hydrogen production by water electrolysis.

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.

Probing Active Sites on Pristine and Defective MnPX3 (X: S and Se) Monolayers for Electrocatalytic Water Splitting

The state-of-the-art density functional theory approach was used to study the structural and electronic properties of pristine and defective MnPX3 monolayers as well as their activity toward water and hydrogen evolution reaction (HER) catalytic performance. The adsorption behavior of H2O on a pristine MnPX3 structure is of physisorption nature, whereas the adsorption energy is significantly increased for the defective structures. At the same time, the water dissociation process is more energetically favorable, and the reactivity of MnPX3 is determined by the vacancy configuration. Following Nørskov's approach, the HER catalytic performance is evaluated by calculating the hydrogen adsorption free energy on the respective MnPX3 surface. Our calculation results demonstrate that defective 2D MnPX3 with low coordinated P shows significantly higher HER performance compared to the pristine counterpart.

Dimetalloylene (M-E-M) Complexes of Heavier Main Group Elements Ge, Sn, Pb, Bi via Cleavage of E-X Bonds (X=N(SiMe3 )2 , OtBu) with an Iridium Hydride

Reactions of the IrV hydride [Me BDIDipp ]IrH4 {BDI=(Dipp)NC(Me)CH(Me)CN(Dipp); Dipp=2,6-iPr2 C6 H3 } with E[N(SiMe3 )2 ]2 (E=Sn, Pb) afforded the unusual dimeric dimetallotetrylenes ([Me BDIDipp ]IrH)2 (μ2 -E)2 in good yields. Moreover, ([Me BDIDipp ]IrH)2 (μ2 -Ge)2 was formed in situ from thermal decomposition of [Me BDIDipp ]Ir(H)2 Ge[N(SiMe3 )2 ]2 . These reactions are accompanied by liberation of HN(SiMe3 )2 and H2 through the apparent cleavage of an E-N(SiMe3 )2 bond by Ir-H. In a reversal of this process, ([Me BDIDipp ]IrH)2 (μ2 -E)2 reacted with excess H2 to regenerate [Me BDIDipp ]IrH4 . Varying the concentrations of reactants led to formation of the trimeric ([Me BDIDipp ]IrH2 )3 (μ2 -E)3 . The further scope of this synthetic route was investigated with group 15 amides, and ([Me BDIDipp ]IrH)2 (μ2 -Bi)2 was prepared by the reaction of [Me BDIDipp ]IrH4 with Bi(NMe2 )3 or Bi(OtBu)3 to afford the first example of a "naked" two-coordinate Bi atom bound exclusively to transition metals. A viable mechanism that accounts for the formation of these products is proposed. Computational investigations of the Ir2 E2 (E=Sn, Pb) compounds characterized them as open-shell singlets with confined nonbonding lone pairs at the E centers. In contrast, Ir2 Bi2 is characterized as having a closed-shell singlet ground state.

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.

Heteroatom-Induced Accelerated Kinetics on Nickel Selenide for Highly Efficient Hydrazine-Assisted Water Splitting and Zn-Hydrazine Battery

Hydrazine-assisted water electrolysis is a promising energy conversion technology for highly efficient hydrogen production. Rational design of bifunctional electrocatalysts, which can simultaneously accelerate hydrogen evolution reaction (HER)/hydrazine oxidation reaction (HzOR) kinetics, is the key step. Herein, we demonstrate the development of ultrathin P/Fe co-doped NiSe2 nanosheets supported on modified Ni foam (P/Fe-NiSe2) synthesized through a facile electrodeposition process and subsequent heat treatment. Based on electrochemical measurements, characterizations, and density functional theory calculations, a favorable "2 + 2" reaction mechanism with a two-step HER process and a two-step HzOR step was fully proved and the specific effect of P doping on HzOR kinetics was investigated. P/Fe-NiSe2 thus yields an impressive electrocatalytic performance, delivering a high current density of 100 mA cm-2 with potentials of - 168 and 200 mV for HER and HzOR, respectively. Additionally, P/Fe-NiSe2 can work efficiently for hydrazine-assisted water electrolysis and Zn-Hydrazine (Zn-Hz) battery, making it promising for practical application.

Unveiling the Effect of Organic Sulfur Sources on Synthesized MoS2 Phases and Electrocatalytic Hydrogen Evolution Performances

Metallic-phase MoS₂ exhibits Pt-comparable electrocatalytic hydrogen evolution reaction (HER) performance in acidic conditions. However, the controllable synthesis of metallic-phase MoS₂ is quite challenging because the key factor determining the phase types of MoS₂ during synthesis is still unclear. Herein, the effect of organic sulfur sources on the formed MoS₂ phase is studied by use of thioacetamide (TAA), l-cysteine, and thiourea as sulfur sources. The TAA and l-cysteine produce metallic MoS₂, while thiourea gives rise to semiconducting MoS₂. Owing to the metallic phase and smaller size, the MoS₂ prepared with TAA and l-cysteine has a higher electrocatalytic HER activity than the MoS₂ obtained from thiourea. The HER overpotential of MoS₂ synthesized with TAA is only 210 mV for reaching the current density of 10 mA/cm², and the corresponding Tafel slope is 44 mV/decade. Further studies find that the decomposition temperature of sulfur precursors is the key factor for the formation of metallic MoS₂. Sulfur precursors with a lower decomposition temperature release sulfur ions quickly, which in turn stabilize the metallic phase and inhibit the growth of MoS₂ into large sizes. Our findings unveil the key factor for controlling the phase type of MoS₂ synthesized from organic sulfur precursors and will be very helpful for the synthesis of MoS₂ with high electrocatalytic activity.

Recent advances in interface engineering of Fe/Co/Ni-based heterostructure electrocatalysts for water splitting

Among various methods of developing hydrogen energy, electrocatalytic water splitting for hydrogen production is one of the approaches to achieve the goal of zero carbon emissions. It is of great significance to develop highly active and stable catalysts to improve the efficiency of hydrogen production. In recent years, the construction of nanoscale heterostructure electrocatalysts through interface engineering can not only overcome the shortcomings of single-component materials to effectively improve their electrocatalytic efficiency and stability but also adjust the intrinsic activity or design synergistic interfaces to improve catalytic performance. Among them, some researchers proposed to replace the slow oxygen evolution reaction at the anode with the oxidation reaction of renewable resources such as biomass to improve the catalytic efficiency of the overall water splitting. The existing reviews in the field of electrocatalysis mainly focus on the relationship between the interface structure, principle, and principle of catalytic reaction, and some articles summarize the performance and improvement schemes of transition metal electrocatalysts. Among them, few studies are focusing on Fe/Co/Ni-based heterogeneous compounds, and there are fewer summaries on the oxidation reactions of organic compounds at the anode. To this end, this paper comprehensively describes the interface design and synthesis, interface classification, and application in the field of electrocatalysis of Fe/Co/Ni-based electrocatalysts. Based on the development and application of current interface engineering strategies, the experimental results of biomass electrooxidation reaction (BEOR) replacing anode oxygen evolution reaction (OER) are discussed, and it is feasible to improve the overall electrocatalytic reaction efficiency by coupling with hydrogen evolution reaction (HER). In the end, the challenges and prospects for the application of Fe/Co/Ni-based heterogeneous compounds in water splitting are briefly discussed.

In Situ Surface Restructuring of Amorphous Ni-Doped CoMo Phosphate-Based Three-Dimensional Networked Nanosheets: Highly Efficient and Durable Electrocatalyst for Overall Alkaline Water Splitting

Developing cost-efficient bifunctional electrocatalysts with high efficiency and durability for the production of green hydrogen and oxygen is a demanding and challenging research area. Due to their high earth abundance, transition metal-based electrocatalysts are alternatives to noble metal-based water splitting electrocatalysts. Herein, binder-free three-dimensional (3D) networked nanosheets of Ni-doped CoMo ternary phosphate (Pi) were prepared using a facile electrochemical synthetic strategy on flexible carbon cloth without any high-temperature heat treatment or complicated electrode fabrication. The optimized CoMoNiPi electrocatalyst delivers admirable hydrogen (η₁₀ = 96 mV) and oxygen (η₁₀ = 272 mV) evolution performances in 1.0 M KOH electrolyte. For overall water splitting in a two-electrode system, the present catalyst demands only 1.59 and 1.90 V to reach current densities of 10 and 100 mA/cm², respectively, which is lower than that of the Pt/C||RuO₂ couple (1.61 V @ 10 mA/cm², 2 V > @ 100 mA/cm²) and many other catalysts reported previously. Furthermore, the present catalyst delivers excellent long-term stability in a two-electrode system continuously over 100 h at a high current density of 100 mA/cm², exhibiting nearly 100% faradic efficiency. The unique 3D amorphous structure with high porosity, a high active surface area, and lower charge transfer resistance provides excellent overall water splitting. Notably, the amorphous structure of the present catalyst favors the in situ surface reconstruction during electrolysis and generates very stable surface-active sites capable of long-term performance. The present work provides a route for the preparation of multimetallic-Pi nanostructures for various electrode applications that are easy to prepare and have superior activity, high stability, and low cost.

In situ phosphoselenization induced heterointerface engineering endow NiSe2/Ni2P/FeSe2 hollow nanocages with efficient water oxidation electrocatalysis performance

Exploiting Earth-abundant and highly effective electrocatalysts toward the oxygen evolution reaction (OER) is critical for boosting water splitting efficiency. Herein, we proposed a novel in situ phosphoselenization strategy to fabricate heterostructured NiSe₂/Ni₂P/FeSe₂ (NiFePSe) nanocages with a modified electronic structure and well-defined nanointerfaces. Owing to the strong interfacial coupling and synergistic effect among the three components, the prepared NiFePSe nanocages exhibit superior OER performance with an ultralow overpotential of 242 mV at 10 mA cm^-2 and a small Tafel slope of 55.8 mV dec^-1 along with robust stability in 1 M KOH. Remarkably, the highly open 3D porous architecture, delicate internal voids, and numerous surface defects endow the NiFePSe nanocages with abundant active sites and enhanced electron mobility. In addition, the super-hydrophilic surface is conducive to facilitating mass transfer between the electrolyte and electrode and rapidly releasing the bubbles. This work may lead to new breakthroughs in the tuning of multi-component transition metal catalysts and the designing of highly active and durable materials for water splitting.

A comprehensive review on the electrochemical parameters and recent material development of electrochemical water splitting electrocatalysts

Electrochemical splitting of water is an appealing solution for energy storage and conversion to overcome the reliance on depleting fossil fuel reserves and prevent severe deterioration of the global climate. Though there are several fuel cells, hydrogen (H2) and oxygen (O2) fuel cells have zero carbon emissions, and water is the only by-product. Countless researchers worldwide are working on the fundamentals, i.e. the parameters affecting the electrocatalysis of water splitting and electrocatalysts that could improve the performance of the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) and overall simplify the water electrolysis process. Noble metals like platinum for HER and ruthenium and iridium for OER were used earlier; however, being expensive, there are more feasible options than employing these metals for all commercialization. The review discusses the recent developments in metal and metalloid HER and OER electrocatalysts from the s, p and d block elements. The evaluation perspectives for electrocatalysts of electrochemical water splitting are also highlighted.

Recent advances in epitaxial heterostructures for electrochemical applications

Construction of epitaxial heterostructures is crucial for boosting the electrochemical properties of various materials, however a review dedicated to this attractive topic is still lacking. In this Minireview, a timely summary on the achievements of epitaxial heterostructure design for electrochemical applications is provided. We first introduce the synthesis strategies to provide fundamental understanding on how to create epitaxial interfaces between different components. Secondly, the superiorities of epitaxial heterostructures in electrocatalysis, supercapacitors and batteries are highlighted with the underlying structure-property relationship elucidated. Finally, a discussion on the challenges and future prospects of this field is presented.

PdO@CoSe2 composites: efficient electrocatalysts for water oxidation in alkaline media

In this study, we have prepared cobalt selenide (CoSe2) due to its useful aspects from a catalysis point of view such as abundant active sites from Se edges, and significant stability in alkaline conditions. CoSe2, however, has yet to prove its functionality, so we doped palladium oxide (PdO) onto CoSe2 nanostructures using ultraviolet (UV) light, resulting in an efficient and stable water oxidation composite. The crystal arrays, morphology, and chemical composition of the surface were studied using a variety of characterization techniques, including X-ray diffraction (XRD), scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared (FTIR) spectroscopy. It was also demonstrated that the composite systems were heterogeneous in their morphology, undergoing a shift in their diffraction patterns, suffering from a variety of metal oxidation states and surface defects. The water oxidation was verified by a low overpotential of 260 mV at a current density of 20 mA cm-2 with a Tafel Slope value of 57 mV dec-1. The presence of multi metal oxidation states, rich surface edges of Se and favorable charge transport played a leading role towards water oxidation with a low energy demand. Furthermore, 48 h of durability is associated with the composite system. With the use of PdO and CoSe2, new, low efficiency, simple electrocatalysts for water catalysis have been developed, enabling the development of practical energy conversion and storage systems. This is an excellent alternative approach for fostering growth in the field.

Interfacial Electronic Engineering of NiSe–Anchored Ni–N–C Composite Electrocatalyst for Efficient Hydrogen Evolution

Rational design and construction of cost–effective electrocatalysts for efficient hydrogen production has attracted extensive research attention worldwide. Herein, we report the construction of a transition metal selenide/carbon composite catalyst featuring uniform NiSe nanoparticles anchored to single Ni atom doped porous carbon structure (NiSe/Ni–N–C) via a facile one–pot pyrolysis of low–cost solid mixtures. NiSe/Ni–N–C exhibits remarkable catalytic performance towards hydrogen evolution reaction (HER) in 1.0 M KOH, requiring a low overpotential of 146 mV to reach a current density of 10 mA cm−2. The unique carbon layer encapsulation derived from the enwrapping of fluid catalytic cracking slurry further renders NiSe/Ni–N–C excellent for long–term durability in electrolyte corrosion and nanostructure aggregation. This work paves the way for the design and synthesis of highly efficient composite HER electrocatalysts.

Boosting the overall electrochemical water splitting performance of pentlandites through non-metallic heteroatom incorporation

We report on synthesis of the heterotrimetallic pentlandite-type material Fe3Co3Ni3S8 (FCNS) in presence of suitable phosphorus-(FCNSP) and nitrogen-(FCNSN) donors for the overall electrochemical water splitting. Throughout the experiments, a preferential incorporation of N into the FCNS-lattice is observed whereas the addition of phosphorus generally leads to metal-phosphate-FCNS composites. The obtained FCNSP, FCNSN, and FCNSNP facilitate the oxygen evolution reaction (OER) at 100 mAcm-2 in 1.0M KOH with overpotentials of 479, 440, and 427 mV, respectively, outperforming the benchmark IrO2 (564 mV) and commercial Ni metal powder (>600 mV). Likewise, FCNSN and FCNSNP reveal an improved performance toward the hydrogen evolution reaction (HER) in 0.5M H2SO4, outperforming the pristine FCNS. All materials revealed high stability and morphological robustness during OER and HER. Notably, DFT calculation suggests that N and P doping boost the OER activity of the pristine FCNS, whereas N doping enhances the HER activity.

Heterostructures of tin and tungsten selenides for robust overall water splitting

Layered transition metal selenides have garnered increased attention in recent times as non-noble metal bifunctional electrocatalysts for electrochemical water splitting. Tungsten diselenide @ tin diselenide heterostructures in the present study significantly increase the electrochemical performance of oxygen evolution reaction with a low overpotential of 250 mV at 10 mA cm^-2 and high stability for 16 h (8.9 % loss), hydrogen evolution reaction with a low overpotential of 180 mV at 10 mA cm^-2 with a 21.9% loss in 16 h. The overall water splitting using a lab-size electrolyzer shows a low cell voltage (1.52 V @ 10 mA cm^-2) and high durability for 50 h (15.2% loss @ 10 mA cm^-2 and 4.4% loss @ 50 mA cm^-2). As a result, the heterostructures have demonstrated their ability to handle multiple challenges in energy conversion systems due to their unique properties.

Sol-Gel Synthesized High Entropy Metal Oxides as High-Performance Catalysts for Electrochemical Water Oxidation

Hexanary high-entropy oxides (HEOs) were synthesized through the mechanochemical sol-gel method for electrocatalytic water oxidation reaction (WOR). As-synthesized catalysts were subjected to characterization, including X-ray diffraction (XRD), Fourier transforms infrared (FTIR) analysis, and scanning electron microscopy (SEM). All the oxide systems exhibited sharp diffraction peaks in XRD patterns indicating the defined crystal structure. Strong absorption between 400–700 cm−1 in FTIR indicated the formation of metal-oxide bonds in all HEO systems. WOR was investigated via cyclic voltammetry using HEOs as electrode platforms, 1M KOH as the basic medium, and 1M methanol (CH3OH) as the facilitator. Voltammetric profiles for both equiatomic (EHEOs) and non-equiatomic (NEHEOs) were investigated, and NEHEOs exhibited the maximum current output for WOR. Moreover, methanol addition improved the current profiles, thus leading to the electrode utility in direct methanol fuel cells as a sequential increase in methanol concentration from 1M to 2M enhanced the OER current density from 61.4 to 94.3 mA cm−2 using NEHEO. The NEHEOs comprising a greater percentage of Al, ([Al0.35(Mg, Fe, Cu, Ni, Co)0.65]3O4) displayed high WOR catalytic performance with the maximum diffusion coefficient, D° (10.90 cm2 s−1) and heterogeneous rate constant, k° (7.98 cm s−1) values. These primary findings from the EC processes for WOR provide the foundation for their applications in high-energy devices. Conclusively, HEOs are proven as novel and efficient catalytic platforms for electrochemical water oxidation.

MOF-derived nanoarrays as advanced electrocatalysts for water splitting

Developing efficient, nanostructured electrocatalysts with the desired compositions and structures is of great significance for improving the efficiency of water splitting toward hydrogen production. In this regard, metal-organic framework (MOF) derived nanoarrays have attracted great attention as promising electrocatalysts because of their diverse compositions and adjustable structures. In this review, the recent progress in MOF-derived nanoarrays for electrochemical water splitting is summarized, highlighting the structural design of the MOF-derived nanoarrays and the electrocatalytic performance of the derived composite carbon materials, oxides, hydroxides, sulfides, and phosphides. In particular, the structure-performance relationships of the MOF-derived nanoarrays and the modulation strategies toward enhanced catalytic activity for both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) are discussed, providing insights into the development of advanced catalysts for the HER and OER. The challenges and prospects in this promising field for future industrial applications are also addressed.

Recent advances in solution assisted synthesis of transition metal chalcogenides for photo-electrocatalytic hydrogen evolution

Hydrogen evolution from water splitting is considered to be an important renewable clean energy source and alternative to fossil fuels for future energy sustainability. Photocatalytic and electrocatalytic water splitting is considered to be an effective method for the sustainable production of clean energy, H₂. This perspective especially emphasizes research advances in the solution-assisted synthesis of transition metal chalcogenides for both photo and electrocatalytic hydrogen evolution applications. Transition metal chalcogenides (CdS, MoS₂, WS₂, TiS₂, TaS₂, ReS₂, MoSe₂, and WSe₂) have received intensified research interest over the past two decades on account of their unique properties and great potential across a wide range of applications. The photocatalytic activity of transition metal chalcogenides can further be improved by elemental doping, heterojunction formation with noble metals (Au, Pt, etc.), non-chalcogenides (MoS₂, In₂S₃, NiS_1-X), morphological tuning, through various solution-assisted synthesis processes, including liquid-phase exfoliation, heat-up, hot-injection methods, hydrothermal/solvothermal routes and template-mediated synthesis processes. In this review we will discuss recent developments in transition metal chalcogenides (TMCs), the role of TMCs for hydrogen production and various strategies for surface functionalization to increase their activity, different synthesis methods, and prospects of TMCs for hydrogen evolution. We have included a brief discussion on the effect of surface hydrogen binding energy and Gibbs free energy change for HER in electrocatalytic hydrogen evolution.

Covalent and Non-covalent Functionalized Nanomaterials for Environmental Restoration

Nanotechnology has emerged as an extraordinary and rapidly developing discipline of science. It has remolded the fate of the whole world by providing diverse horizons in different fields. Nanomaterials are appealing because of their incredibly small size and large surface area. Apart from the naturally occurring nanomaterials, synthetic nanomaterials are being prepared on large scales with different sizes and properties. Such nanomaterials are being utilized as an innovative and green approach in multiple fields. To expand the applications and enhance the properties of the nanomaterials, their functionalization and engineering are being performed on a massive scale. The functionalization helps to add to the existing useful properties of the nanomaterials, hence broadening the scope of their utilization. A large class of covalent and non-covalent functionalized nanomaterials (FNMs) including carbons, metal oxides, quantum dots, and composites of these materials with other organic or inorganic materials are being synthesized and used for environmental remediation applications including wastewater treatment. This review summarizes recent advances in the synthesis, reporting techniques, and applications of FNMs in adsorptive and photocatalytic removal of pollutants from wastewater. Future prospects are also examined, along with suggestions for attaining massive benefits in the areas of FNMs.

Superionic conductor Ag2Se modulated CoSe2 nanosheets prepared via monometallic cation release for efficient pH-universal water electrolysis into hydrogen

Superionic conductors regulated transition metal chalcogenides are the newly emerged electrocatalyst in water electrolysis into clean hydrogen and oxygen. However, there is still much room for the development of structural design, electronic modulation and heterogeneous interface construction to improve the overall water splitting performance in pH-universal solutions, especially in alkaline and neutral mediums. Herein, using β-cyclodextrin (β-CD) and citric acid (CA) organics with abundant hydroxyl (-OH) and carboxyl (-COOH), a special Ag₂Se nanoparticles-decorated CoSe₂ flower-like nanosheets loaded on porous and conductive nickel foam substrate (Ag₂Se-CoSe₂/NF) was successfully constructed by a new method of monometallic cation release of coordinated cobalt. The Ag₂Se phase exerts the nature characteristics of superionic conductors to modulate the morphological and electronic structures of CoSe₂ as well as improve its conductivity. The generated rich active interfaces and abundant Se vacancy defects facilitate numerous active sites exposure to accelerate the hydrogen ion transport and charge transfer. Compared to the single-phase Ag₂Se/NF-8 and CoSe₂/NF, the prepared Ag₂Se-CoSe₂/NF-8 with a two-phase synergistic effect achieves an outstanding pH-universal electrocatalytic hydrogen production performance by water electrolysis, as evidenced by a lower overpotential (60 mV, 212 mV and 85 mV vs RHE at 10 mA cm^-2 for pH = 0.36, 7.00 and 13.70, respectively). Only a voltage of 1.55 V at 10 mA cm^-2 is required to implement the overall water splitting in an alkaline electrolyzer. This work provides significant guidance for the future designation and practical development of transition metal chalcogenides with superionic conductors applied in the electrocatalytic field.

Single-Step Solid-State Scalable Transformation of Ni-Based Substrates to High-Oxidation State Nickel Sulfide Nanoplate Arrays as Exceptional Bifunctional Electrocatalyst for Overall Water Splitting

Hydrogen, undoubtedly the next-generation fuel for supplying the world's energy demands, needs economically scalable bifunctional electrocatalysts for its sustainable production. Non-noble transition metal-based electrocatalysts are considered an economic solution for water splitting applications. A single-step solid-state approach for the economically scalable transformation of Ni-based substrates into single-crystalline nickel sulfide nanoplate arrays is developed. X-ray diffraction and transmission electron microscopy measurements reveal the influence of the transformation temperature on the crystal growth direction, which in turn can manipulate the chemical state at the catalyst surface. Ni-based sulfide formed at 450 °C exhibits an enhanced concentration of electrocatalytically-active Ni³⁺ at their surface and a reduced electron density around sulfur atoms, optimal for efficient H₂ production. The Ni-based sulfide electrocatalysts display exceptional electrocatalytic performance for both oxygen and hydrogen evolution, with overpotentials of 170 and 90 mV respectively. Remarkably, the two-electrode cell for overall electrolysis of alkaline water demonstrates an ultra-low cell potential of 1.46 V at 10 mA cm^-2 and 1.69 V at 100 mA cm^-2 . In addition to the exceptionally low water-splitting cell voltage, this self-standing electrocatalyst is of binderfree nature, with the electrode preparation being a low-cost and single-step process, easily scalable to industrial scales.

Review on 2D Molybdenum Diselenide (MoSe2) and Its Hybrids for Green Hydrogen (H2) Generation Applications

Hydrogen (H2) is a green and economical substitute to traditional fossil fuels due to zero carbon emissions. Water splitting technology is developing at a rapid speed to sustainably generate H2 through electro- and photolysis of water without the harmful emissions associated with steam methane reforming. Development of efficient catalysts for the hydrogen evolution reaction (HER) is pertinent for economical green H2 generation. In this regard, 2D transition metal dichalcogenides (TMDCs) are considered to be excellent alternatives to noble metal catalysts. Among other TMDCs, 2D MoSe2 is preferred due to the low Gibbs free energy for hydrogen adsorption, good electrical conductivity, and more metallic nature. Moreover, the physicochemical and electronic properties of MoSe2 can be easily tailored to suit HER application by simple synthetic strategies. Herein, we comprehensively review the application of 2D MoSe2 in the electrocatalytic HER, focusing on recent advancements in the modulation of the MoSe2 properties through nanostructure design, phase transformation, defect engineering, doping, and formation of heterostructures. We also discuss the role of 2D MoSe2 as a cocatalyst in the photocatalytic HER. The article concludes with a synopsis of current progress and prospective future trends.

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.

Engineering transition metal catalysts for large-current-density water splitting

Electrochemical water splitting plays a crucial role in transferring electricity to hydrogen fuel and appropriate electrocatalysts are crucial to satisfy the strict industrial demand. However, the successfully developed non-noble metal catalysts have a small tested range and the current density is usually less than 100 mA cm^-2, which is still far away from the practical application standards. Aiming to provide guidance for the fabrication of more advanced electrocatalysts with a large current density, we herein systematically summarize the recent progress achieved in the field of cost-efficient and large-current-density electrocatalyst design. Beginning by illustrating the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) mechanisms, we elaborate on the concurrent issues of non-noble metal catalysts that are required to be addressed. In view of large-current-density operating conditions, some distinctive features with regard to good electrical conductivity, high intrinsic activity, rich active sites, and porous architecture are also summarized. Next, some representative large-current-density electrocatalysts are classified. Finally, we discuss the challenges associated with large-current-density water electrolysis and future pathways in the hope of guiding the future development of more efficient non-noble-metal catalysts to boost large-scale hydrogen production with less electricity consumption.