Cobalt-doped electrolytic manganese dioxide as an efficient bifunctional catalyst for oxygen evolution/reduction reactions

Abstract

In this study, commercially available γ-type manganese dioxide, namely electrolytic manganese dioxide (EMD), was transformed to a bifunctional catalyst that exhibits activity not only for the oxygen reduction reaction (ORR) but also for the oxygen evolution reaction (OER), by the doping of cobalt into EMD without changing its framework structure. The use of EMD, which has long been proven in the primary battery industry, would be advantageous in terms of manufacturing, supply, and quality control. The process involves a simple wet treatment of EMD in the presence of N2H4 and Co2+. EMD is intrinsically active for the ORR, more than chemically-synthesized MnO2, but inactive for the inverse reaction, the OER. Here, the extremely high mass activity of a small amount of Co that was introduced into the framework of γ-MnO2 boosted its OER activity, while did not offset the ORR activity. As a result, the overpotential at 10 mA cm−2 in the OER was measured to be 440 mV, while that at −1 mA cm−2 in the ORR was 447 mV. Their sum (ηOER+ηORR) of 887 mV was much smaller than that (1040 mV) of pristine EMD and differed from that of Pt/C by only 26 mV.