The controllable loading of Fe/Co alloy on heteroatom doped hollow graphene spheres realized by small molecule regulation for rechargeable zinc-air batteries

Abstract

Although transition metals as the bifunctional catalysts of zinc-air batteries (ZABs) have obvious price advantages, their performance in ZABs still fails to meet expectations due to the uncontrollable loading caused by the rapid nucleation rate of transition metals. In this study, the controllable loading of Fe/Co alloy on heteroatom doped hollow graphene spheres (FeCo@NGHS) realized by the regulation of small molecule.Sodium citrate, which acted as metal complexing agent and reaction buffer, can effectively suppress the excessive loading of Fe/Co alloy particles and facilitate the formation of Fe(Co)Nx active sites. Melamine, which acted as precursor of N atoms doping, can provide anchor points for the loading of Fe/Co alloy particles and participate in the generation of Fe(Co)Nx. The fabricated catalyst has active sites with different chemical structures, such as pyridine-N, graphite-N, Fe(Co)Nx and Fe/Co alloy particle, all of which will benefit the improvement of oxygen reduction reaction/oxygen evolution reaction (ORR /OER) performance. The result shows that the fabricated FeCo@NGHS, which possesses the appropriate amount of Fe/Co alloy particles combined the highest amount of formed Fe(Co)Nx active sites, exhibits the best ORR/OER bifunctional catalytic performance in alkaline electrolyte and excellent electrocatalytic stability. The ORR onset potential and half-wave potential are 0.961 V and 0.846 V (vs. RHE), respectively. And the overpotential of OER can achieve a low level of 391 mV at current density of 10 mA cm-2. Furthermore, the rechargeable liquid ZAB and flexible all-solid-state (ASS) ZAB assembled by FeCo@NGHS exhibits higher discharge power density and longer charge-discharge cycle performance. FeCo@NGHS based air cathodes exhibit outstanding performance in flexible ASS-ZABs, showing high open circuit voltage (1.45 V) and peak power density (74.06 mW cm-2). In clean energy storage and conversion technologies, a new synthetic strategy for constructing excellent bifunctional oxygen electrocatalysts is proposed in this work.