Enhanced Oxygen Evolution Reaction Activity of Co Ions Isolated in the Interlayer Space of Buserite MnO₂

We have fabricated a thin film of layered manganese dioxide (MnO₂) that accommodates cobalt ions in its interlayer space, constructing the so-called buserite structure, via electrodeposition and the subsequent ion-exchange. The MnO₂ layers could isolate Co²⁺ ions to provide an environment beneficial for oxygen evolution reactions (OERs) in alkaline electrolyte, where fast electron transfer and high utilization efficiency were achieved. The catalyst with isolated Co²⁺ ions exhibited a mass activity as high as 63.5 A/g_Co at an overpotential (η) of 0.4 V, which was much larger than those of Co ions bound in the oxide network. Moreover, it also exhibited excellent stability for long-term OER operation. Namely, the potential needed to generate a current density of 10 mA/cm² increased only 0.073 V during 100 h operation, and no significant change was seen after 100 consecutive potential cycles between +1.0 and +2.0 V versus the reversible hydrogen electrode.