Unraveling the efficacy of defect engineered mesoporous Ni–Co spinel oxide nanowires as an energy efficient electrocatalyst for the oxygen reduction reaction and fuel cell applications

Abstract

Defect engineering and morphology control are crucial in designing advanced transition metal oxide (TMO)-based electrocatalysts with superior activity towards the oxygen reduction reaction (ORR). Following this blueprint, one-dimensional mesoporous Ni–Co oxide spinel (NCO) nanowires were synthesized using a simple solvothermal method, where defects were introduced by modulating the intrinsic metal ion ratio. Extensive structural, morphological, and compositional characterization of the nanowires was performed using various experimental techniques. The nanowires exhibited appreciably large surface area and porous architecture. X-ray absorption and positron annihilation spectroscopic studies revealed an exclusive Co-ion vacancy in Co-deficient nanowires and oxygen vacancies in Co-rich and -deficient nanowires. X-ray photoelectron spectroscopic studies unraveled preferential surface enrichment of the Co-rich NCO catalyst with Co³⁺ and Ni²⁺ ions. Co-excess NCO catalysts showed outstanding electrocatalytic activity towards the ORR, with a Tafel slope as low as ∼50 mV per decade with an overpotential of ∼0.35 V, comparable to benchmark noble metal-based catalysts, and periodic DFT studies delineated the crucial role of excess Co on the catalyst surface for demonstrating such superior activity. The NCO catalyst with a Co/Ni content of 2.6 : 1 exhibited the highest selectivity towards O₂/H₂O conversion, and it was further deployed in a real-life operational alkaline anion exchange membrane fuel cell (AEMFC) at room temperature. This study successfully demonstrates practical utilization of judiciously designed NCO catalysts as suitable alternatives to noble metal-based catalysts in sustainable energy applications.