Study of active sites on Se-MnS/NiS heterojunctions as highly efficient bifunctional electrocatalysts for overall water splitting

Mn-based sulfides have been used in oxygen reduction reactions, lithium-ion batteries and supercapacitors. However, only a few Mn-based catalysts have been reported for OER or HER due to their poor catalytic activity. Mn is commonly used as a dopant to tune the electronic structure of electrocatalysts. It is thus a challenge to achieve highly efficient OER and HER activities for overall water splitting using Mn-based catalysts. In this work, for the first time, we synthesized a Se-MnS/NiS electrocatalyst through facile hydrothermal and chemical deposition treatments. The introduction of the Se dopant could tailor the structure and increase the electrochemically active surface area, favoring the electrocatalysis of HER and OER. Besides, the synergistic effect of the Se-MnS/NiS heterojunctions promoted the adsorption of hydrogen atoms on the surface of the catalyst compared to the NiSe, NiS, and Se-NiS catalysts. The resulting Se-NiS/MnS catalyst required overpotentials as low as 56 mV and 211 mV to deliver a 10 mA cm(-2) current density for HER and OER in alkaline media, respectively. Furthermore, Se-MnS/NiS, when directly used as bifunctional electrodes for overall water splitting, exhibited a relatively low voltage of 1.47 V at 10 mA cm(-2) and remarkable durability for 48 h. Therefore, it can be considered as one of the promising bifunctional Mn-based electrocatalyst candidates that can replace precious-metal-based electrocatalysts for efficient water splitting.