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.

Nanostructured hexagonal boron nitride (h-BN)/reduced graphene oxide (RGO) composite is prepared by insertion of h-BN into the graphene oxide through hydrothermal reaction. Formation of the super lattice is confirmed by the existence of two separate UV-visible absorption edges corresponding to two different band gaps. The composite materials show enhanced electrical conductivity as compared to the bulk h-BN. A high specific capacitance of ∼824 F g(-1) is achieved at a current density of 4 A g(-1) for the composite in three-electrode electrochemical measurement. The potential window of the composite electrode lies in the range from -0.1 to 0.5 V in 6 M aqueous KOH electrolyte. The operating voltage is increased to 1.4 V in asymmetric supercapacitor (ASC) device where the thermally reduced graphene oxide is used as the negative electrode and the h-BN/RGO composite as the positive electrode. The ASC exhibits a specific capacitance of 145.7 F g(-1) at a current density of 6 A g(-1) and high energy density of 39.6 W h kg(-1) corresponding to a large power density of ∼4200 W kg(-1). Therefore, a facile hydrothermal route is demonstrated for the first time to utilize h-BN-based composite materials as energy storage electrode materials for supercapacitor applications.

Sulfanilic acid azocromotrop (SAC) modified reduced graphene oxide (SAC-RGO) was prepared by simple non-covalent functionalization of graphene oxide (GO) followed by post reduction using hydrazine monohydrate. Spectral analysis (Fourier transform infrared, Raman and X-ray photoelectron spectroscopy) revealed that successful modification had occurred of GO with SAC through p-p interaction. The electrical conductivity of SAC-RGO was found to be $551 S m À1 . The capacitive performance of SAC-RGO was recorded using a three electrode set up with 1 (M) aqueous H 2 SO 4 as the electrolyte. The -SO 3 H functionalities of SAC contributed pseudocapacitance as evidenced from the redox peaks (at $0.43 and 0.27 V) present in the cyclic voltammetric (CV) curves measured for SAC-RGO. The contribution of electrical double layer capacitance was evidenced from the near rectangular shaped CV curves and resulted in a high specific capacitance of 366 F g À1 at a current density of 1.2 A g À1 for SAC-RGO electrode. An asymmetric device (SAC-RGO//RGO) was designed with SAC-RGO as the positive electrode and RGO as the negative electrode. The device showed an energy density of $25.8 W h kg À1 at a power density of $980 W kg À1 . The asymmetric device showed retention in specific capacitance of $72% after 5000 charge-discharge cycles. The Nyquist data of the device was fitted with Z-view and different components (solution resistance, charge-transfer resistance and Warburg elements) were calculated from the fitted curves.

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.