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

For catalyzing the oxygen evolution reaction, earth‐abundant materials with high activity and stability need to be developed. NiCo 2 O 4 has been proven to show high OER activity, however facile and inexpensive techniques for preparation of this compound as mesostructured thin film, possessing a high surface area, is lacking. In this study, the sol‐gel synthesis of nanocrystalline, mesoporous spinel NiCo 2 O 4 thin films by dip‐coating and soft‐templating using the evaporation‐induced self‐assembly approach and utilizing the tri‐block‐copolymer Pluronic® F‐127 as structure‐directing agent is reported. The morphology and crystallographic structure were thoroughly probed by various physicochemical characterization techniques collectively validating the development of uniform mesoporous NiCo 2 O 4 architectures crystallizing exclusively in the cubic spinel phase after calcination in air at ether 300 °C, 400 °C, or 500 °C. The surface area of thin films increased from 300 °C to 400 °C owing to degradation of the organic template, while the growth of the mesopores from 400 °C to 500 °C resulted in significant decline of the overall (electrochemical) surface area. XPS investigations showed that the amount of octahedrally coordinated Ni 3+ and defective (low‐coordinated) oxygen species increased for decreasing calcination temperatures. The nanomorphology and presence of catalytically active surface sites of the mesoporous NiCo 2 O 4 electrodes were correlated with the electrochemical properties, presenting that the overall surface area, Ni 3+ content, charge transfer resistance, and amount of defective oxygen sites collectively control the OER performance. After an optimized annealing procedure at 300 °C and chronopotentiometric analysis at 10 mA/cm 2 for 1.5 h, a low overpotential of 330 mV vs. RHE at 10 mA/cm 2 in alkaline solution was achieved. The results highlight the necessity of precise selection of the appropriate calcination temperature and tailoring of the nanostructure and electrochemical pre‐treatment conditions of NiCo 2 O 4 sol‐gel thin films for adjusting the concentration of electrocatalytically active reaction sites.