Cobalt-Doped Ba₂In₂O₅ Brownmillerites: An Efficient Electrocatalyst for Oxygen Reduction in Alkaline Medium

A series of compounds with cobalt doping in the indium site of Ba₂In₂O₅ brownmillerites exhibited excellent oxygen reduction activity under alkaline conditions. Doping (25%) retains the brownmillerite structure with disorder in the O3 site in the two-dimensional alternate layer along the ab plane. Further substitution of cobalt in the indium site leads to the loss of a brownmillerite structure, and the compound attains a perovskite structure. Cobalt-doped samples exhibited far better oxygen reduction reaction (ORR) activity when compared to the parent Ba₂In₂O₅ brownmillerite. Among the series of compounds, BaIn_0.25Co_0.75O_3-δ with the highest Co doping and oxygen vacancies randomly distributed in the lattice exhibited the best ORR activity. BaIn_0.25Co_0.75O_3-δ showed a 40 mV positive shift in the onset potential with better limiting current density and a nearly four-electron-transfer reduction pathway when compared to the parent Ba₂In₂O₅ brownmillerite.