Tuning and understanding the electronic effect of Co–Mo–O sites in bifunctional electrocatalysts for ultralong-lasting rechargeable zinc–air batteries

Abstract

Herein, we report a post-assembly strategy by growing bimetallic Co/Zn zeolitic imidazolate frameworks (BIMZIF) on the surface of customized Mo metal–organic frameworks (MOFs) (Mo-MOFs) to prepare core–shell structured Mo-MOF@BIMZIF, which results in a porous Co–MoO₂ polyhedral nanocage (Co–MoO₂-NC) structure and abundant Co–Mo–O active sites after high temperature pyrolysis. Co–MoO₂-NC-900 which was obtained by pyrolysis at 900 °C displays a low overpotential (370 mV) for the oxygen evolution reaction (OER) and a half-wave potential (E_1/2 ≈ 0.89 V) for the oxygen reduction reaction (ORR), exhibiting bifunctional features. A Zn–air battery with Co–MoO₂-NC-900 shows a high power density of 176.5 mW cm⁻² at 217.1 mA cm⁻², which is better than that of reference Pt/C + Ir/C electrocatalysts, and is capable of exhibiting a smaller discharge–charge overpotential (0.85 V) and excellent stability (1145 h). Density functional theory calculations and experiments reveal that the structure and electron transfer between the Co and MoO₂ components reduce the free energy of the intermediates, resulting in bifunctional electrocatalytic activity.