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

Abstract

For catalyzing the oxygen evolution reaction, earth-abundant materials with high activity and stability need to be developed. NiCo₂O₄ 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 NiCo₂O₄ 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 NiCo₂O₄ 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 Ni³⁺ and defective (low-coordinated) oxygen species increased for decreasing calcination temperatures. The nanomorphology and presence of catalytically active surface sites of the mesoporous NiCo₂O₄ electrodes were correlated with the electrochemical properties, presenting that the overall surface area, Ni³⁺ 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/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 tailoring of the nanostructure and electrochemical pre-treatment conditions of NiCo₂O₄ sol-gel thin films for adjusting the concentration of electrocatalytically active reaction sites.