Electrocatalytic properties of metal hydroxy-phosphonoacetate derivatives for OER, ORR and HER

Abstract

Although Pt and Ru/Ir-based noble metal materials are the state-of-the-art electrocatalysts for energy storage and conversion, their applications are still limited by their scarcity and high price. Therefore, the design of low-cost and efficient electrocatalysts is highly desired and full of challenge. Herein we report the electrochemical properties of several metal phosphonates derived from the reaction of (R,S)-2-hydroxyphosphonoacetic acid (HPAA) with transition metals (M = Fe2+, Co2+, Mn2+, Ni2+), including their corresponding solid solutions. By pyrolytic treatment under N2 or 5% H2/Ar, materials having a wide range of compositions (phosphate, pyrophosphate, phosphides…) have been prepared and their electrochemical properties tested for OER and ORR and HER. Pyrolyzing in N2 at 500 °C, led to amorphous pyrophosphates, with Fe2+/Co2+ solid solutions displaying the lowest overpotential for the OER, whereas the Mn2+ derivative showed the best performance for the ORR. At higher temperature, pyrolyzed materials were crystalline pyrophosphates (N2) or metal phosphides (H2-Ar). In general, the latter compounds displayed better performances for the HER, Ni2P being the most active electrocatalyst. Addition of reduced graphene oxide (rGO) significantly improved the behaviour of the electrocatalysts for OER and ORR.