Activity of Transition-Metal (Manganese, Iron, Cobalt, and Nickel) Phosphates for Oxygen Electrocatalysis in Alkaline Solution

Although transition-metal oxides are common non-platinum group metal catalysts for the industrially important oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), the performance gap between transition-metal oxides and platinum group metal catalysts is still substantial and there is a continuing need to search for alternatives. In this study, transition-metal (Mn, Fe, Co, and Ni) phosphates prepared by a solution chemistry method under ambient conditions are found to display interesting electrocatalytic properties for the ORR and OER in alkaline solution. Among them, manganese phosphate is more active than most state-of-the-art manganese oxides for the ORR, and nickel phosphate is as active as the best Ni-based catalysts for the OER. Hence these phosphates can be used as tandem catalysts for rechargeable metal-air batteries in which both the ORR and OER take place. The good performance may be attributed to the stabilization of the catalytic centers by the phosphate framework. This study establishes phosphates as yet another class of highly active low-cost non-platinum group metal alternatives for oxygen electrocatalysis in alkaline solution.