In situ fabrication of Ni3S2/Cu2S heterojunction on nickel foam as a highly efficient and durable electrocatalyst for overall water splitting

The development of cost-efficient bifunctional electrocatalysts is significant for overall water splitting. Herein, we report the in situ fabrication of heterogeneous NF/Ni3S2/Cu2S-X (where X refers to Cu2+ concentrations of 50, 75, and 100 mM) on nickel foam (NF) using an electrodeposition-hydrothermal method. The in situ electrodeposited metallic Cu0 layers on the NF conferred higher stability to the resulting bimetallic sulfide of Ni3S2/Cu2S. In alkaline media (1 M KOH), the optimized NF/Ni3S2/Cu2S-75 exhibited ultra-low overpotentials of 108 and 166 mV during the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) at 10 mA·cm-2. For overall water splitting, the catalyst showed a significantly low cell voltage of 1.50 V and long stabilization time (≥150h)at15mA·cm-2. Density functional theory calculations revealed that the formation of Ni3S2/Cu2S heterojunction reduced the Gibbs free energy of hydrogen adsorption (ΔGH*) on the S site, thus facilitating H2 generation. This study serves as a guide for tailoring transition metal-based catalysts with enhanced activity and long-term durability, thereby contributing to highly efficient water electrolysis for large-scale hydrogen production.

Unlocking the Electrocatalytic Behavior of Cu2MnS2 Nanoflake-Anchored rGO for the Oxygen Evolution Reaction in an Alkaline Medium

A catalyst of the oxygen evolution reaction (OER) that is viable, affordable, and active for effective water-splitting applications is critical. A variety of electrocatalysts have been discovered to replace noble metal-based catalysts. Of these, transition metal-based sulfides are essential for incorporating carbonaceous materials to improve electrical conductivity, resulting in better electrocatalytic performance. Our study illustrates the synthesis of Cu2MnS2 (CMS) nanoflakes and their different rGO composites (10 to 40 wt %) via a hydrothermal technique for an effective water oxidation reaction. The X-ray diffraction pattern reveals that the prepared Cu2MnS2 nanoflakes exhibit a cubic crystal structure. The high-resolution scanning electron microscopy and the high resolution transmission electron microscopy images corroborate the formation of the nanoflake-like morphology of Cu2MnS2 with the strong interaction of rGO. The selected area electron diffraction analysis pattern reveals a polycrystalline nature. The Fourier transform infrared spectrum shows the existence of a metal sulfur vibrational band at 590 cm-1, and Raman analysis infers the presence of rGO. The X-ray photoelectron spectroscopy spectra reveal the oxidation states of the elements present in the samples. Using Brunauer-Emmett-Teller analysis, the surface area of CMS-20 is found to be 117.04 m2/g. The measured OER overpotentials using linear sweep volammetry and the values are 380, 370, 340, 376, and 400 mV at 10 mA/cm2 for CMS, CMS-10, CMS-20, CMS-30, and CMS-40, respectively, and the corresponding Tafel slope values are 179, 158, 149, 206, and 240 mV/decade, respectively. The electrochemical active surface area is estimated using cyclic voltammetry for all of the catalysts, where CMS-20 showed a larger surface area. Also, the same catalyst exhibits good stability for ∼24 h at a constant potential, which is confirmed via chronoamperometry. Thus, combining transition metal-based sulfides with carbonaceous materials indicates improved catalytic behavior for the preparation of high-performance OER electrocatalysts. Overall, the prepared CMS-20 performed as an efficient OER electrocatalyst and can be utilized for practical applications in energy conversion.

Chemical Epitaxy of Iridium Oxide on Tin Oxide Enhances Stability of Supported OER Catalyst

Significantly reducing the iridium content in oxygen evolution reaction (OER) catalysts while maintaining high electrocatalytic activity and stability is a key priority in the development of large-scale proton exchange membrane (PEM) electrolyzers. In practical catalysts, this is usually achieved by depositing thin layers of iridium oxide on a dimensionally stable metal oxide support material that reduces the volumetric packing density of iridium in the electrode assembly. By comparing two support materials with different structure types, it is shown that the chemical nature of the metal oxide support can have a strong influence on the crystallization of the iridium oxide phase and the direction of crystal growth. Epitaxial growth of crystalline IrO2 is achieved on the isostructural support material SnO2, both of which have a rutile structure with very similar lattice constants. Crystallization of amorphous IrOx on an SnO2 substrate results in interconnected, ultrasmall IrO2 crystallites that grow along the surface and are firmly anchored to the substrate. Thereby, the IrO2 phase enables excellent conductivity and remarkable stability of the catalyst at higher overpotentials and current densities at a very low Ir content of only 14 at%. The chemical epitaxy described here opens new horizons for the optimization of conductivity, activity and stability of electrocatalysts and the development of other epitaxial materials systems.

Developments in Nanostructured MoS2-Decorated Reduced Graphene Oxide Composite Aerogel as an Electrocatalyst for the Hydrogen Evolution Reaction

Developing lightweight, highly active surfaces with a high level of performance and great stability is crucial for ensuring the dependability of energy harvesting and conversion devices. Aerogel-based electrocatalysts are an efficient option for electrocatalytic hydrogen production because of their numerous benefits, such as their compatibility with interface engineering and their porous architecture. Herein, we report on the facile synthesis of a nanorod-like molybdenum sulfide-reduced graphene oxide (M-rG) aerogel as an electrocatalyst for the hydrogen evolution reaction (HER). The 3D architecture of the network-like structure of the M-rG hybrid aerogel was created via the hydrothermal technique, using a saturated NaCl solution-assisted process, where the MoS2 was homogeneously incorporated within the interconnected rGO aerogel. The optimized M-rG-300 aerogel electrocatalyst had a significantly decreased overpotential of 112 mV at 10 mA/cm2 for the HER in alkaline conditions. The M-rG-300 also showed a higher level of reliability. The remarkable efficiency of the HER involving the M-rG-300 is principally attributed to the excellent connectivity between the rGO and MoS2 in the aerogel structure. The efficient interconnection influenced the achievement of a larger electrochemically active surface area, increased electrical conductivity, and the exposure of more active sites for the HER. Furthermore, the creation of a synergistic effect in the M-rG-300 aerogel is the most probable mechanism to boost the electrocatalytic activity.

Oxalic Acid-Assisted Vacancy Engineering Promotes Iron-Copper Sulfide Nanosheets for High-Current Density Water Oxidation

The effective defect and interface coupling are pivotal for the promotion of the catalytic activity for the oxygen evolution reaction. Herein, we report novel hybrid nanosheets with sulfur vacancies composed of FeS2 and Cu39S28 grown on Cu foam (Vs-FeS2/Cu39S28). The optimal Vs-FeS2/Cu39S28 exhibits a high current output of 500 mA cm-2 at a low overpotential of 370 mV and robust stability for 60 h at 100 mA cm-2, surpassing the values of most previously reported Cu-based catalysts. Furthermore, a two-electrode electrolyzer made by pairing the prepared catalyst with commercial Pt/C requires a low cell voltage of 1.75 V at 100 mA cm-2 and is retained over 80 h. Key to its excellent performance is the synergism between intertwined FeS2 and Cu39S28 domains, enriched by the deliberate introduction of sulfur vacancies, thus optimizing the electronic structure and causing the proliferation of catalytic active sites. This work presents a potent Cu-based electrocatalyst and emphasizes the leveraging of non-precious metals for efficient water oxidation.

Morphological and Coordination Modulations in Iridium Electrocatalyst for Robust and Stable Acidic OER Catalysis

Proton exchange membrane water splitting (PEMWS) technology has high-level current density, high operating pressure, small electrolyzer-size, integrity, flexibility, and has good adaptability to the volatility of wind power and photovoltaics, but the development of both active and high stability of the anode electrocatalyst in acidic environment is still a huge challenge, which seriously hinders the promotion and application of PEMWS. In recent years, researchers have made tremendous attempts in the development of high-quality active anode electrocatalyst, and we summarize some of the research progress made by our group in the design and synthesis of PEMWS anode electrocatalysts with different nanostructures, and makes full use of electrocatalytic activity points to increase the inherent activity of Iridium (Ir) sites, and provides optimization strategies for the long-term non-decay of catalysts under high anode potential in acidic environments. At this stage, these research advances are expected to facilitate the research and technological progress of PEMWS, and providing some research ideas and references for future research on efficient and inexpensive PEMWS anode electrocatalysts.

Surface Reconstruction of Covellite CuS Nanocrystals for Enhanced OER Catalytic Performance in Alkaline Solution

Oxygen evolution reaction (OER) is one of the important half-reactions in energy conversion equipment such as water-spitting devices, rechargeable metal-air batteries, and so on. It is beneficial to develop efficient and low-cost catalysts that understand the reaction mechanism of OER and analyze the reconstruction phenomenon of transition metal sulfide. Interestingly, copper sulfide and cuprous sulfide with the same components possess different reconstruction behaviors due to their different metal ion valence states and different atomic arrangement modes. Because of a unique atomic arrangement sequence and certain cationic defects, the reconstruction phenomenon of CuS nanomaterials are that S2- is firstly oxidized to SO4 2- and then Cux + is converted into CuO via Cu(OH)2 . In addition, the specific "modified hourglass structure" of CuS with excellent conductivity is easier to produce intermediates. Compared with Cu2 S, CuS exhibits excellent OER activity with a lower overpotential of 192 mV at 10 mA cm-2 and remarkable electrochemical stability in 1.0 m KOH for 120 h. Herein, this study elucidates the reconstruction modes of CuS and Cu2 S in the OER process and reveals that CuS has a stronger CuS bond and a faster electronic transmission efficiency due to "modified hourglass structure," resulting in faster reconstruction of CuS than Cu2 S.

An inclusive review and perspective on Cu-based materials for electrochemical water splitting

In recent years, there has been a resurgence of interest in developing green and renewable alternate energy sources as a solution to the energy and environmental problems produced by conventional fossil fuel use. As a very effective energy transporter, hydrogen (H2) is a possible candidate for the future energy supply. Hydrogen production by water splitting is a promising new energy option. Strong, efficient, and abundant catalysts are required for increasing the efficiency of the water splitting process. Cu-based materials as an electrocatalyst have shown promising results for application in the Hydrogen Evolution Reaction (HER) and Oxygen Evolution Reaction (OER) in water splitting. In this review, our aim is to cover the latest developments in the synthesis, characterisation, and electrochemical behaviour of Cu-based materials as a HER, and OER electrocatalyst, highlighting the impact that these advances have had on the field. It is intended that this review article will serve as a roadmap for developing novel, cost-effective electrocatalysts for electrochemical water splitting based on nanostructured materials with particular emphasis on Cu-based materials for electrocatalytic water splitting.

Vacancy-mediated transition metals as efficient electrocatalysts for water splitting

Water splitting using renewable electricity provides a promising way for large-scale hydrogen production due to its zero-carbon emission properties. However, the development of highly efficient, low-cost and durable electrocatalysts remains an ongoing challenge in industrial applications. Herein, a strategy integrating vacancy engineering and metal doping was proposed to design and screen M@CuS catalysts with excellent catalytic activity via density functional theory (DFT) calculations. TM single atoms anchored by the vacancy of the CuS surface show high stability, and serve as the active centers for water splitting. Ti@CuS and Co@CuS exhibit exceptional performance towards the hydrogen evolution reaction (HER). Ti@CuS and Co@CuS can achieve hydrogen adsorption free energies (ΔG_H*) of 0.01 eV and -0.03 eV, respectively. The HER process of Ti@CuS is controlled by the Heyrovsky mechanism. Co@CuS also shows superior catalytic activity towards the oxygen evolution reaction (OER), and presents a relatively lower OER overpotential of 0.41 V. Co@CuS serves as a promising candidate of bifunctional HER/OER electrocatalysts. This work not only provides highly efficient electrocatalysts for water splitting, but also inspires a novel concept to guide the extending design of catalysts in other catalysis fields.

Electrochemically Derived Crystalline CuO from Covellite CuS Nanoplates: A Multifunctional Anode Material

In the present era, electrochemical water splitting has been showcased as a reliable solution for alternative and sustainable energy development. The development of a cheap, albeit active, catalyst to split water at a substantial overpotential with long durability is a perdurable challenge. Moreover, understanding the nature of surface-active species under electrochemical conditions remains fundamentally important. A facile hydrothermal approach is herein adapted to prepare covellite (hexagonal) phase CuS nanoplates. In the covellite CuS lattice, copper is present in a mixed-valent state, supported by two different binding energy values (932.10 eV for Cu^I and 933.65 eV for Cu^II) found in X-ray photoelectron spectroscopy analysis, and adopted two different geometries, that is, trigonal planar preferably for Cu^I and tetrahedral preferably for Cu^II. The as-synthesized covellite CuS behaves as an efficient electro(pre)catalyst for alkaline water oxidation while deposited on a glassy carbon and nickel foam (NF) electrodes. Under cyclic voltammetry cycles, covellite CuS electrochemically and irreversibly oxidized to CuO, indicated by a redox feature at 1.2 V (vs the reversible hydrogen electrode) and an ex situ Raman study. Electrochemically activated covellite CuS to the CuO phase (termed as CuS_EA) behaves as a pure copper-based catalyst showing an overpotential (η) of only 349 (±5) mV at a current density of 20 mA cm^-2, and the TOF value obtained at η₃₄₉ (at 349 mV) is 1.1 × 10^-3 s^-1. A low R_ct of 5.90 Ω and a moderate Tafel slope of 82 mV dec^-1 confirm the fair activity of the CuS_EA catalyst compared to the CuS precatalyst, reference CuO, and other reported copper catalysts. Notably, the CuS_EA/NF anode can deliver a constant current of ca. 15 mA cm^-2 over a period of 10 h and even a high current density of 100 mA cm^-2 for 1 h. Post-oxygen evolution reaction (OER)-chronoamperometric characterization of the anode via several spectroscopic and microscopic tools firmly establishes the formation of crystalline CuO as the active material along with some amorphous Cu(OH)₂ via bulk reconstruction of the covellite CuS under electrochemical conditions. Given the promising OER activity, the CuS_EA/NF anode can be fabricated as a water electrolyzer, Pt(-)//(+)CuS_EA/NF, that delivers a j of 10 mA cm^-2 at a cell potential of 1.58 V. The same electrolyzer can further be used for electrochemical transformation of organic feedstocks like ethanol, furfural, and 5-hydroxymethylfurfural to their respective acids. The present study showcases that a highly active CuO/Cu(OH)₂ heterostructure can be constructed in situ on NF from the covellite CuS nanoplate, which is not only a superior pure copper-based electrocatalyst active for OER and overall water splitting but also for the electro-oxidation of industrial feedstocks.

Two-Phase Synthesis of Fe Doped Cerium Phosphate Ultra-fine Nanocrystals for Efficient Oxygen Evolution

It is very important and challenging to develop electrocatalysts with high performance and economic benefits. In this work, ultra-fine Fe doped CePO4 nanocrystals (Fe-CePO4) were prepared by a simple two-step...

Amorphous nickel tungstate films prepared by SILAR method for electrocatalytic oxygen evolution reaction

Development of electrocatalyst using facile way from non-noble metal compounds with high efficiency for effective water electrolysis is highly demanding for production of hydrogen energy. Nickel based electrocatalysts were currently developed for electrochemical water oxidation in alkaline pH. Herein, amorphous nickel tungstate (NiWO₄) was synthesized using the facile successive ionic layer adsorption and reaction method. The films were characterized by X-ray diffraction, Raman spectroscopy, Fourier transfer infrared spectroscopy, scanning electron microscopy, X-ray photoelectron spectroscopy, and transmission electron microscopy techniques. The electrochemical analysis showed 315 mV of overpotential at 100 mA cm^-2 with lowest Tafel slope of 32 mV dec^-1 for oxygen evolution reaction (OER) making films of NiWO₄ compatible towards electrocatalysis of water in alkaline media. The chronopotentiometry measurements at 100 mA cm^-2 over 24 h showed 97% retention of OER activity. The electrochemical active surface area (ECSA) of NW120 film was 25.5 cm^-2.

Coupling SnS2 and rGO aerogel to CuS for enhanced light-assisted OER electrocatalysis

In order to harvest more light wavelengths to improve the light-assisted electrochemical water splitting capacity, we developed a novel heterostructure of three-dimensional (3D) flower-like CuS architecture with accompanying SnS2 nanoparticles and reduced graphene oxide (rGO) aerogel for outstanding light-assisted electrocatalytic OER performance and good stability. The excellent catalytic kinetics, effective capturing of visible light, and rapid charge transfer of the CuS/SnS2/rGO (CSr) heterostructure were demonstrated. The overpotential (264 mV@10 mA cm-2) under light-assisted conditions is 20% lower than that under light-chopped conditions. SnS2 can harvest more light wavelengths and this boosts its intrinsic activity. However, with the increase of the SnS2 content, the OER activity decreases. The combination of the CS heterostructure and the rGO conductive aerogel achieves rapid charge transfer. Furthermore, the possible mechanism of the light-assisted electrocatalytic OER was also proposed. Overall, this work provides new insights into the simple and scalable fabrication of a highly efficient, low-cost, and stable non-noble-metal-based electrocatalyst.

CuS concave polyhedral superstructures enabled efficient N2 electroreduction to NH3 at ambient conditions

CuS concave polyhedral superstructures with high symmetry offer an appealing V_NH3 of 18.18 μg h⁻¹ mg⁻¹_cat. and a faradaic efficiency of 5.63% at −0.15 V vs. RHE in 0.1 M HCl.

Investigation on nanostructured Cu-based electrocatalysts for improvising water splitting: a review

In this review, various forms of Cu based nanostructures have been explored in terms of improvise and enhancing their activity and durability with vast investigation for OER, HER and TWS applications.