A Self-Templating Redox-Mediated Synthesis of Hollow Phosphated Manganese Oxide Nanospheres as Noble-Metal-like Oxygen Electrocatalysts

The development of low-cost, high-performance electrocatalysts for the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) to replace the use of noble metals is a challenge for the rechargeable metal–air batteries. Although manganese compounds, oxides in particular, have drawn the most interest, they rarely deliver the same performance as the noble metals in oxygen electrocatalysis, notwithstanding the enhancements introduced by nanosizing and adjuvant metal doping. Herein, we report a noble-metal-like performance for manganese oxide catalysts by combining phosphate modification with a hollow nanostructure. A simple and scalable self-templating method based on phosphate-mediated Mn redox reactions was developed for the preparation of hollow phosphated manganese oxide nanospheres at room temperature. A series of simple (h-MnO_xP_y) and complex phosphated manganese oxide (h-Me-MnO_xP_y, Me = Co, Ni, Cu) hollow nanospheres can be produced more efficiently than normal hollow nanostructure construction techniques based on hard and soft templates and hydrothermal Ostwald ripening. Among the hollow phosphated manganese oxides h-MnO_xP_0.21 delivered the best ORR performance (half-wave potential of 0.85 V vs RHE, similar to 20 wt % Pt/C) and h-Co-MnO_xP_0.21 the best OER performance (1.60 V vs RHE for 10 mA cm^–2, marginally higher than 20 wt % Ir/C). Small charge–discharge voltage gaps (ΔV) were shown in both alkaline (ΔV = 0.72 V at 5 mA cm^–2) and neutral (ΔV = 1.28 V at 1 mA cm^–2) rechargeable Zn–air batteries with the combined use of these catalysts, similar to the 20 wt % Pt/C + 20 wt % Ir/C combined catalytic systems.