Anion dependency of spinel type cobalt catalysts for efficient overall water splitting in an acid medium

Developing effective electrocatalysts for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is an important task in sustainable water splitting for manufacturing green hydrogen. In the present study, we report a bifunctional catalyst prepared with an easy and cost-effective synthesis method for designing a spinel-structured, cobalt-based, and carbon-supported electrocatalyst (Co3O4@rGO and Co3S4@rGO). X-ray absorption spectroscopy reveals that Co-O and Co-S bonds are formed at the edge between cations and anions in the Co3O4, Co3S4 on the surface of reduced graphene oxide, which can alter the electronic structure of electrocatalysts. Fabricated Co3S4@rGO nanostructure catalysts showed best catalytic activity toward HER (ƞ10: 151 mV) and OER (ƞ10: 350 mV) comparable to those of commercial Pt-C and IrO2, respectively. This showed a significant improvement of water splitting performance in 0.5 M H2SO4 electrolyte as compared to Co3O4@rGO electrocatalyst, with an operating voltage of 1.82 V at a current density 10 mA cm−2. These spinel cobalt electrocatalysts greatly enhanced the water splitting performance depending on the choice of anion.