Nanocoral CuCo2S4 thiospinels: Oxygen evolution reaction via redox interaction of metal ions

Understanding the electrocatalytic activity and redox mechanism in mixed metal sulfides has steered the present work. Mixed metal sulfide, CuCo<inf>2</inf>S<inf>4</inf>, synthesised by hydrothermal method, reveals the presence of highly crystalline nanocoral sheets of CuCo<inf>2</inf>S<inf>4</inf> with mixed oxidation states of Co and Cu ions. Electrochemical measurements show superior oxygen evolution reaction (OER) activity and high exchange current density over CuCo<inf>2</inf>S<inf>4</inf> in comparison with pristine (Co<inf>3</inf>S<inf>4</inf> and Cu<inf>2</inf>S) and sintered CuCo<inf>2</inf>S<inf>4</inf>. In 0.5 M KOH, the nanocoral CuCo<inf>2</inf>S<inf>4</inf> catalyst and its sintered form show a current density of 10 mA cm⁻² at overpotential of 368 mV and 511 mV, respectively. A close inspection of the electrocatalytic performance in correlation with XPS spectra reveals the synergistic effect of Co and Cu ions through a redox interaction. Density Functional Theory calculations also indicate strong dependence of redox activity on redox couples as accompanied with changes in the partial charges of different constituent ions in the catalyst upon adsorption of hydroxyl ions which is a key step in OER. The study not only details sulfide-based catalytic compound for OER but also provides conclusive insights into the surface redox processes thereby providing an alternative direction to the search on novel OER catalysts.