Fabrication of high-performance noble-metal-free bifunctional electrocatalysts for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in water is a promising strategy toward future carbon-neutral economy. Herein, a one-pot hydrothermal synthesis of cobalt sulfide/nickel sulfide heterostructure supported by nickel foam (CoS /NiS@NF) was performed. The Ni foam acted as the three-dimensional conducting substrate as well as the source of nickel for NiS. The formation of CoS /NiS@NF was confirmed by X-ray diffraction and X-ray photoelectron spectroscopy. The formation of CoS /NiS@NF facilitated easy charge transport and showed synergistic electrocatalytic effect toward HER, OER, and overall water splitting in alkaline medium. Remarkably, CoS /NiS@NF showed catalytic activity comparable with that of benchmarking electrocatalysts Pt/C and RuO. For CoS /NiS@NF, overpotentials of 204 and 280 mV were required to achieve current densities of 10 and 20 mA cm for HER and OER, respectively, in 1.0 M KOH solution. A two-electrode system was formulated for overall water splitting reaction, which showed current densities of 10 and 50 mA cm at 1.572 and 1.684 V, respectively. The prepared catalyst exhibited excellent durability in HER and OER catalyzing conditions and also in overall water splitting operation. Therefore, CoS /NiS@NF could be a promising noble-metal-free electrocatalyst for overall water splitting application.

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Rechargeable aqueous Zn-ion batteries (ZiBs) have received significant attention owing to their low cost and environmental friendliness. The charge storage mechanism of ZiBs generally depends on the cationic redox conversion. The anodic redox conversion is still rare due to the lack of suitable cathode materials and electrolytes, which may impede the ZiB performance. The present investigation focuses on enhancing the ZiB performance through in situ involvement of disulfide redox chemistry from a patronite Mn-doped VS4 crystal in a 1 M Zn­(OTf)2 water/acetonitrile (ACN) hybrid electrolyte. Simultaneous involvement of cationic and anionic redox conversion during the charging/discharging process enhances the specific capacity as high as ∼547 mA h g–1 at 0.2 A g–1 current density. The water/ACN hybrid solvent effectively improves the working voltage and suppresses Zn dendrite formation, resulting in superior cycling performance. The conversion of the VS4 crystal phase and intercalation of H+/Zn2+ in the cathode have been investigated through ex situ X-ray diffraction, field emission scanning electron microscopy, X-ray photoelectron spectroscopy, and Raman studies.

The electrocatalyst comprising two different metal atoms is found suitable for overall water splitting in alkaline medium. Hydrothermal synthesis is an extensively used technique for the synthesis of various metal sulfides. Time-dependent diffusion of the constituting ions during hydrothermal synthesis can affect the crystal and electronic structure of the product, which in turn would modulate its electrocatalytic activity. Herein, cobalt molybdenum bimetallic sulfide was prepared via hydrothermal method after varying the duration of reaction. The change in crystal structure, amount of Co−S−Mo moiety, and electronic structure of the synthesized materials were thoroughly investigated using different analytical techniques. These changes modulated the charge transfer at the electrode−electrolyte interface, as evidenced by electrochemical impedance spectroscopy. The Tafel plots for the prepared materials were investigated considering a less explored approach and it was found that different materials facilitated different electrocatalytic pathways. The product obtained after 12 h reaction showed superior catalytic activity in comparison to the products obtained from 4, 8, and 16 h reaction, and it surpassed the overall water splitting activity of the RuO 2 −Pt/C couple. This study demonstrated the ion diffusion within the bimetallic sulfide during hydrothermal synthesis and change in its electrocatalytic activity due to ion diffusion.

The development of non‐noble metal based electrocatalysts for overall water splitting is a potent strategy towards a carbon‐neutral and clean energy economy. Herein, hierarchical CoSx@MoS2 was synthesized via a one‐pot solvothermal process. Formation of the heterostructure was confirmed by electron microscopy and spectroscopic techniques. CoSx@MoS2 showed competent electrocatalytic activity towards both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in alkaline medium. Superior electrocatalytic activity was attributed to the increase in number of active sites, betterment in charge transfer and facilitation of H‐ and O‐ containing active species adsorption‐desorption at the active sites. Overall water splitting efficiency of CoSx@MoS2 was found to be superior in comparison to the state‐of‐the‐art RuO2‐Pt/C couple. Along with efficiency the heterostructure also exhibited long‐term operational durability. Thus, hierarchical CoSx@MoS2 is a potential non‐noble metal based bifunctional electrocatalyst towards overall water splitting.

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The polyol method is an efficient procedure for metal sulfide preparation where polyol not only acts as a solvent but also as reducing and morphology‐modulating agent. Herein, iron sulfide particles were prepared via a modified polyol method by changing the ethylene glycol (EG) : water (H2O) ratio in the mixed solvent. Analytical techniques and electronic microscopy studies confirmed that the change in EG : H2O ratio modulated the crystal structure, morphology, and electronic structure of the prepared iron sulfide particles. The electrocatalytic activity of iron sulfide changed owing to these modulations. EG helped in the formation of a sheet‐like structure – a morphology that favours a higher accessibility to the catalytically active sites. As evidenced form electrochemical impedance studies, an increased electron density near the Fermi level, a faster substrate adsorption‐desorption rate at the active sites, and a faster charge transfer at the electrode‐electrolyte interface were the key factors for the amplification in catalytic activity. The prepared iron sulfide particles showed an overall water splitting efficiency that is comparable to that of the state‐of‐the‐art RuO2‐Pt/C couple in alkaline medium. This study shows the potential of the polyol method in the preparation and catalytic‐activity modulation of Fe−S‐based electrocatalysts.

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