Cobalt sulfide nanoparticles grown on nitrogen and sulfur codoped graphene oxide: An efficient electrocatalyst for oxygen reduction and evolution reactions

Electrochemical oxygen evolution and reduction reactions have received great attention due to their importance in several key technologies such as fuel cells, electrolyzers, and metal-air batteries. Here, we present a simple approach to the preparation of cobalt sulfide nanoparticles in situ grown on a nitrogen and sulfur codoped graphene oxide surface. The particle size and phase were controlled by changing the treatment temperature. Cobalt sulfide nanoparticles dispersed on graphene oxide hybrids were successfully prepared by a solid-state thermolysis approach at different temperatures (400, 500, and 600 °C) using cobalt thiourea and graphene oxide. X-ray diffraction studies revealed that hybrids prepared at 400 and 500 °C result in pure CoS₂ phase, whereas the hybrid prepared at 600 °C exhibits Co₉S₈ phase. X-ray photoelectron spectroscopy studies revealed that nitrogen and sulfur simultaneously codoped on the graphene oxide surface, and these sites act to anchor the CoS₂ nanoparticles strongly on the GO surface. The strong coupling between CoS₂ and N,S-GO was reflected in the improvement of the oxygen electrode potential. CoS₂(400)/N,S-GO showed an outstanding oxygen electrode activity with a potential of about 0.82 V against a reversible hydrogen electrode in alkaline medium, which is far better than the performance of precious catalysts such as Pt/C (1.16 V), Ru/C (1.01 V), and Ir/C (0.92 V).