Controllable loading of an Fe/Co alloy on heteroatom-doped hollow graphene spheres realized via regulation of small molecules for rechargeable zinc–air batteries

Abstract

Although the use of transition metals as bifunctional catalysts for zinc–air batteries (ZABs) has obvious economic 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, controllable loading of an Fe/Co alloy on heteroatom-doped hollow graphene spheres (FeCo@NGHS) was realized via the regulation of small molecules. Sodium citrate, which served as a metal complexing agent and reaction buffer, effectively suppressed the excessive loading of Fe/Co alloy particles and facilitated the formation of Fe(Co)N_x active sites. Melamine, which served as a precursor for doping N atoms, provided anchor points for the loading of Fe/Co alloy particles and participated in the generation of Fe(Co)N_x. The fabricated catalyst had active sites with different chemical structures, such as pyridine-N, graphite-N, Fe(Co)N_x and Fe/Co alloy particles, all of which benefit the improvement of the oxygen reduction reaction/oxygen evolution reaction (ORR/OER) performance. Results showed that the fabricated FeCo@NGHS, which possesses the appropriate amount of Fe/Co alloy particles combined with the highest amount of formed Fe(Co)N_x active sites, exhibited the best ORR/OER bifunctional catalytic performance in alkaline electrolytes and excellent electrocatalytic stability. The ORR onset potential and half-wave potential were 0.961 V and 0.846 V (vs. RHE), respectively. The OER could achieve a low overpotential level of 391 mV at a current density of 10 mA cm⁻². Furthermore, the rechargeable liquid ZAB and flexible all-solid-state (ASS) ZAB assembled by FeCo@NGHS exhibited higher discharge power density and longer charge–discharge cycle performance. FeCo@NGHS-based air cathodes exhibited outstanding performance in flexible ASS–ZABs, showing high open circuit voltage (1.45 V) and peak power density (74.06 mW cm⁻²). Thus, in clean energy storage and conversion technologies, a new synthetic strategy for constructing excellent bifunctional oxygen electrocatalysts is proposed in this work.