Electrocatalytic enhancement mechanism of cobalt single atoms anchored on different MXene substrates in oxygen and hydrogen evolution reactions

Decorating single atoms of transition metals on MXenes to enhance the electrocatalytic properties of the resulting composites is a useful strategy for developing efficient electrocatalysts, and the mechanisms behind this enhancement are under intense scrutiny. Herein, we anchored Co single atoms onto several commonly used MXene substrates (V₂CT_x, Nb₂CT_x and Ti₃C₂T_x) and systematically studied the electrocatalytic behavior and the mechanisms of oxygen and hydrogen evolution reactions (OER and HER, respectively) of the resulting composites. Co@V₂CT_x composite displays an OER overpotential of 242 mV and an HER overpotential of 35 mV at 10 mA cm⁻² in 1.0 M KOH electrolyte, which is much lower than for Co@Nb₂CT_x and Co@Ti₃C₂T_x, making it comparable to the commercial noble metal Pt/C and RuO₂/C electrocatalysts. The experimental and theoretical results point out that the enhanced bifunctional catalytic performance of Co@V₂CT_x benefits from the stronger hybridization between Co 3d and surface terminated O 2p orbitals which optimized the electronic structure of Co single atoms in the composite. This, in turn, results in lowering the OER and HER energy barriers and acceleration of the catalytic kinetics in case of the Co@V₂CT_x composite. The advantage of Co@V₂CT_x was further validated by its high overall water splitting performance (1.60 V to deliver 10 mA cm⁻²). Our study sheds light on the origins of the catalytic activity of single transition metals atoms on MXene substrates, and provides guidelines for designing efficient bifunctional MXene-based electrocatalysts. © 2022 The Authors. EcoMat published by The Hong Kong Polytechnic University and John Wiley & Sons Australia, Ltd.