Nanocoral CuCoNiS4: Enhancement in the electrochemical capacitance by Ni incorporation in CuCo2S4

Nanocoral structures of CuCo<inf>2</inf>S<inf>4</inf> and CuCoNiS<inf>4</inf> were synthesized using hydrothermal method. Excellent electrocatalytic response with oxygen evolution reaction (OER) at an onset potential of 1.27 V (vs. RHE) is observed in CuCo<inf>2</inf>S<inf>4</inf>. On replacement of half of Co ions by Ni, the electrocatalytic activity reduced with the disappearance of OER. Electrochemical impedance studies indicated that on Ni substitution, the impedance behaviour changes from resistive to a nearly capacitive or pseudocapacitive behaviour. EIS equivalent circuit fitting model showed bulk resistance (Ω·cm²) values of 2.16 and 1.65 for CuCo<inf>2</inf>S<inf>4</inf> and CuCoNiS<inf>4</inf> respectively. However, specific capacitance increased to 308 F/g in Ni substituted thiospinel, CuCoNiS<inf>4</inf> compared to 83 F/g in CuCo<inf>2</inf>S<inf>4</inf> at a current density of 3 A/g and at a potential of 0.85 V (vs. Ag/AgCl). For a higher current loading of 10 A/g, CuCoNiS<inf>4</inf> showed a specific capacitance of 177 F/g. In this work, we report the key role played by octahedral Ni ions in Cu – Co based thiospinels.