Excellent Bifunctional Oxygen Evolution and Reduction Electrocatalysts (5A_1/5)Co₂O₄ and Their Tunability

Hastening the progress of rechargeable metal-air batteries and hydrogen fuel cells necessitates the advancement of economically feasible, earth-abundant, inexpensive, and efficient electrocatalysts facilitating both the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). Herein, a recently reported family of nano (5A_1/5)Co₂O₄ (A = combinations of transition metals, Mg, Mn, Fe, Ni, Cu, and Zn) compositionally complex oxides (CCOs) [Wang et al., Chemistry of Materials, 2023,35 (17), 7283-7291.] are studied as bifunctional OER and ORR electrocatalysts. Among the different low-temperature soft-templating samples, those subjected to 600 °C postannealing heat treatment exhibit superior performance in alkaline media. One specific composition (Mn_0.2Fe_0.2Ni_0.2Cu_0.2Zn_0.2)Co₂O₄ exhibited an exceptional overpotential (260 mV at 10 mA cm^-2) for the OER, a favorable Tafel slope of 68 mV dec^-1, excellent onset potential (0.9 V) for the ORR, and lower than 6% H₂O₂ yields over a potential range of 0.2 to 0.8 V vs the reversible hydrogen electrode. Furthermore, this catalyst displayed stability over a 22 h chronoamperometry measurement, as confirmed by X-ray photoelectron spectroscopy analysis. Considering the outstanding performance, the low cost and scalability of the synthesis method, and the demonstrated tunability through chemical substitutions and processing variables, CCO ACo₂O₄ spinel oxides are highly promising candidates for future sustainable electrocatalytic applications.