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.