Surface Defects, Ni3+ Species, Charge Transfer Resistance, and Surface Area dictate the Oxygen Evolution Reaction Activity of Mesoporous NiCo2O4 Thin Films

Abstract

NiCo2O4 has been proven to show high electrocatalytic activity, however facile and inexpensive techniques for preparation of this compound as mesostructured thin film is lacking. In this study, the sol‐gel synthesis of nanocrystalline mesoporous spinel NiCo2O4 thin films by dip‐coating and soft‐templating using the evaporation‐induced self‐assembly approach is reported. The morphology and crystallographic structure were thoroughly probed by various physicochemical characterization techniques collectively validating the development of uniform mesoporous NiCo2O4 architectures crystallizing exclusively in the cubic spinel phase after calcination in air. The surface area of the thin films depended on the calcination temperature. XPS investigations showed that the amount of Ni3+ and defective oxygen species increased for decreasing calcination temperatures. The overall electrochemical surface area, Ni3+ content, charge transfer resistance, and amount of defective oxygen sites were found to collectively control the oxygen evolution reaction performance. After an optimized annealing procedure at 300°C and chronopotentiometric analysis at 10 mA/cm2 for 1.5 h, a low overpotential of 330 mV vs. RHE at 10 mA/cm2 in alkaline solution was achieved. The results highlight the necessity of precise selection of the appropriate calcination temperature and electrochemical pre‐treatment conditions for adjusting the concentration of electrocatalytically active reaction sites of sol‐gel‐derived NiCo2O4 thin films.