Promotion of the bifunctional electrocatalytic oxygen activity of manganese oxides with dual-affinity phosphate

The search for low cost and high activity bifunctional electrocatalysts for the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) is a research priority in the development of rechargeable metal-air batteries. Manganese oxides are the current favorite, but their native activity needs to be substantially improved before they can be considered as the substitute of noble metals. We discovered phosphate modification as an effective strategy to increase the bifunctional oxygen activity of MnO2 in ORR and OER. Specifically, the half-wave potential of phosphate-promoted α-MnO2 (PMO) for ORR is 0.85 V vs. RHE, ∼70 mV more positive than α-MnO2 and the same as the benchmark 20 wt% Pt/C catalyst. The OER potential to maintain 10 mA cm−2 of current density is 1.63 V, ∼40 mV more negative than α-MnO2 and about the same as the benchmark 20 wt% Ir/C catalyst (1.60 V). The potential difference between ORR (at −3 mA cm−2) and OER (at 10 mA cm−2) is 0.79 V, a standard measure of bifunctional oxygen activity, notably surpasses the performance of α-MnO2 (0.91 V) and the noble metals (0.95 V for Pt/C and 0.93 V for Ir/C). Additional electrochemical measurements and density functional theory calculations suggest that the promoted bifunctional activity of PMO is due to the dual-affinity of phosphate for O and OH groups, which assists O2/OH adsorption in ORR/OER.