General synthesis of nitrogen-doped metal (M = Co2+, Mn2+, Ni2+, or Cu2+) phosphates

Metal (M = Co2+, Mn2+, Ni2+, or Cu2+) ammonium phosphate was synthesized by a simple process of chemical precipitation. By controllably calcining the metal (M = Co2+, Mn2+, Ni2+, or Cu2+) ammonium phosphates (NH4MPO4·H2O), the corresponding nitrogen-doped metal (M = Co2+, Mn2+, Ni2+, or Cu2+) phosphate (N-MPO) were obtained. The nitrogen content of these materials can be controlled by varying the calcination temperature. Interestingly, the obtained amorphous nitrogen-doped cobalt phosphate (A-NCoPO) with a porous structure showed extraordinary and lasting properties for the oxygen evolution reaction (OER). At 10 mA cm−2, the overpotential was 181 mV, and the Tafel slope was 94 mV dec−1. The effect of calcination temperature on the various materials was compared and analyzed. The experimental results demonstrated that the nitrogen-doped cobalt phosphate calcined at 400 °C can promote catalytic activity by increasing the electronic conductivity and OH− adsorption strength owing to the disordered crystal structure and nitrogen doping. Using a simple synthesis strategy, A-NCoPO retains the structural advantages of transition metal phosphate and shows a fascinating potential as an OER catalyst with favorable stability.