Tunable phosphorization degree of CoxPy@N,P-doped carbon as a highly-active bifunctional electrocatalyst for rechargeable zinc–air batteries

Abstract

The commercial development of rechargeable metal–air batteries urgently requires us to investigate low-cost, durable and highly effective bifunctional electrocatalysts for the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). However, bifunctional electrocatalysts require different active sites, which inevitably increase the complexity of electrocatalyst construction. In this study, bifunctional ORR/OER electrocatalysts are prepared using Co_xP_y (x = 1 or 2, y = 0 or 1, i.e., Co, CoP and Co₂P) nanoparticles (NPs) anchored on a N,P-doped porous carbon framework (Co_xP_y@NPPC) through carbonizing the mixture of a melamine–phytic acid supermolecular aggregate (MPSA) and a CoCo-Prussian blue analogue (CoCo-PBA) after grinding in one step. By controlling the proportion of CoCo-PBA and MPSA, the stoichiometric proportion of Co_xP_y nanoparticles can be reasonably designed to achieve controllable preparation. The prepared cobalt-based nanocatalysts with different degrees of phosphorization (DoP) can be used for the OER and ORR. It is found that the as-synthesized Co₂P@NPPC exhibits the most excellent ORR/OER bifunctional activity among the Co_xP_y@NPPC analogues, with a half-wave potential of 0.85 V and an overpotential of 320 mV for the ORR and OER at 10 mA cm⁻². The reversible oxygen electrode index (ΔE = E_j10 − E_1/2) can reach ∼0.70 V. The assembled zinc–air battery based on Co₂P@NPPC delivers a wonderful charge–discharge stability with a recharge voltage of ∼2.05 V and a discharge voltage of ∼1.09 V for 160 h at 10 mA cm⁻² and a peak power density of 226 mW cm⁻², which is superior to Pt/C + RuO₂ and most of the Co_xP_y-based electrodes that have been reported. At the same time, this study provides an alternative and simple phosphating strategy for improving the catalytic activity of bifunctional electrocatalysts.