Different nanostructured CoP microcubes derived from metal formate frameworks with enhanced oxygen evolution reaction performance

Abstract

Transition metal phosphides have been widely used as electrocatalysts for water splitting. In particular, cobalt phosphide electrocatalysts play an important part in the oxygen evolution reaction. Here, two kinds of CoP with different nanostructures, CoP-MA and CoP-GC (MA = methylamine and GC = guanidine carbonate), were successfully prepared by a two-step method, including calcination of the corresponding Co-based metal formate frameworks, [Amine][Co(HCOO)₃] (Amine = CH₃NH₃ and C(NH₂)₃), in air and the subsequent low temperature phosphating process. As OER catalysts, the CoP nanostructures show strong electrochemical structure-dependent properties. Electrochemical tests show that nanoparticle based CoP-MA micro-cubes required an overpotential of 340 mV at a current density of 10 mA cm⁻², while nanosheet based CoP-GC micro-cubes only required an overpotential of 310 mV at the same current density. Importantly, the two CoP micro-cubes have a similar morphology but different structures, providing a direct side-by-side evaluation of their OER activities. The comparisons of the same metal phosphide OER catalysts with the same morphology but different structures are of great significance in identifying the key material characteristics that lead to high activity.