Metal–Organic Framework-Derived Cu-Doped Co₉S₈ Nanorod Array with Less Low-Valence Co Sites as Highly Efficient Bifunctional Electrodes for Overall Water Splitting

Abstract

One of the main goals of current renewable energy research is continuously developing low-cost, robust, and environmentally friendly electrocatalysts to replace scarce and unstable precious metal catalysts. Hence, a novel high-performance electrocatalytic water-splitting material based on a metal–organic framework (MOF)-derived Cu-doped Co9S8 nanorod array was successfully prepared. Using 1,3,5-benzenetricarboxylic acid (H3BTC) as a ligand, a CuCo MOF (CuCo-MOF) nanobelt array precursor was prepared and then transformed into a Cu-Co9S8 nanorod by a simple sulfurization process using thioacetamide as a sulfur source. It is worth noting that the intermediate CuCo-MOF nanobelts play a key role in the generation of nanostructures of the Cu-doped Co9S8 nanorod array. As one of the most promising bifunctional electrocatalysts reported, the Cu-doped Co9S8-6h only needs 260 mV to drive 50 mA cm–2 for oxygen evolution reaction and 62 mV to drive 10 mA cm–2 for hydrogen evolution reaction. When the Cu-doped Co9S8-6h was used as an electrode in a self-made two-electrode system, it requires only 1.49 V of cell voltage to drive 10 mA cm–2, which is one of the smallest open-circuit voltages reported. Density functional theory results show that the superior performance of the Cu-doped Co9S8-6h is attributed to increased conductivity and increased water adsorption energy because of Cu doping. By comparing the water adsorption energy of Co2+, Cu2+, and Co3+, it is proved that the active Cu2+ replaces the inert Co2+, and less low-valence Co2+ sites make the Cu-doped Co9S8-6h show superior catalytic activity. These results demonstrate that the Cu-doped Co9S8-6h nanorod array can be used as an excellent bifunctional electrocatalyst for overall water splitting, and this work will provide an excellent synergistic strategy to energy storage and conversion.