The development of cobalt phosphate for bifunctional oxygen electrocatalysis in alkaline solution

Abstract

Most of the bifunctional, non-noble metal oxygen electrocatalysts for rechargeable metal-air batteries are based on transition metal oxides. Recent studies on metal phosphates have revealed their potential as another class of oxygen electrocatalysts. Among the metal phosphates Co3(PO4)2 has demonstrated fairly good activity for both the oxygen evolution reaction (OER) and the oxygen reduction reaction (ORR). However, the performance of native Co3(PO4)2 as a bifunctional oxygen catalyst is inferior to that of the noble metal catalysts. In this study we found out that the inadequacy of Co3(PO4)2 could be ameliorated by integrating it with reduced graphene oxide (RGO) nanosheets. The oxygen bifunctionality as measured by the potential difference (ΔE) between OER and ORR in alkaline solution can be lowered to 0.86V for an optimized catalyst, which even surpasses the performance of a 20wt% Ir/C benchmark catalyst (0.93V); making it one of the best non-noble metal oxygen electrocatalysts. The good bifunctional performance may be attributed to the strong coupling between the Co3(PO4)2 submicroparticles and the RGO nanosheets which synergizes the oxygen activity of Co3(PO4)2 with the (electron) transport properties of RGO. The experimental results also suggest that Co3(PO4)2 is more active than the popular CoOx catalysts since the performance degradation in OER and ORR was found to correlate positively with the in-situ transformation of surface Co3(PO4)2 to CoOx.