Excellent Bifunctional Oxygen Evolution and Reduction Electrocatalysts (5A)CoO 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)CoO (A = combinations of transition metals, Mg, Mn, Fe, Ni, Cu, and Zn) compositionally complex oxides (CCOs) [Wang et al., , , 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 (MnFeNiCuZn)CoO exhibited an exceptional overpotential (260 mV at 10 mA cm) for the OER, a favorable Tafel slope of 68 mV dec, excellent onset potential (0.9 V) for the ORR, and lower than 6% HO 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 ACoO spinel oxides are highly promising candidates for future sustainable electrocatalytic applications.