Binary Co9S8–NiCo2S4 anchored on N-doped rGO backbone as an efficient bifunctional and durable hetero-catalyst for overall water-splitting in alkaline medium

Abstract

Developing cost-efficient, stable, and environmentally benign electrocatalysts is a strong enticement towards the realization of commercial clean energy devices. Ni–Co-based systems have been particularly interesting owing to the well-recognized catalytically active redox pairs of Co³⁺/Co²⁺ and Ni³⁺/Ni²⁺. The sulphides of the latter have been frequently investigated catalyst systems with a particular focus on the sulphide-based heterostructured unions that offer better interfacial contacts. In this study, we report an electrochemically efficient hetero-catalyst (Co₉S₈–NiCo₂S₄/N-rGO) consisting of a binary sulphide–sulphide system, Co₉S₈/NiCo₂S₄, anchored on N-doped rGO) through a simple sulphurization strategy. The hetero-catalyst with ideally integrated interfaces ensures facile interfacial electron movement within the sulphide components and the rGO backbone, rendering excellent electrocatalytic properties. The highly exposed surface and N-doping in rGO further offer to expedite the mass transfer rate, and the suitable integration with the latter reduces the electrolyte corrosion to improve the activity and stability. Co₉S₈–NiCo₂S₄/N-rGO exhibits impressive bifunctional activity with an overpotential requirement of 149 mV for a benchmark hydrogen evolution reaction (HER) current of 10 mA cm⁻² and an overpotential requirement of 230 mV for a benchmark oxygen evolution reaction (OER) current of 20 mA cm⁻². The enhanced HER and OER kinetics are further supported by low Tafel slopes of 89 mV dec⁻¹ and 93 mV dec⁻¹, respectively. The full cell constructed from Co₉S₈–NiCo₂S₄/N-rGO exhibits water-splitting to the tune of 10 mA cm⁻² and 100 mA cm⁻² at cell voltages of 1.59 V and 2.1 V, respectively. Moreover, excellent catalytic stability was demonstrated with negligible loss in the catalytic performance for 15 continuous hours of electrocatalysis.