Gradient Co/Zn bimetallic coordinated polymer-derived hierarchically porous carbon for boosted oxygen electrocatalysts of rechargeable Zn–air batteries

The controllable design of low-cost oxygen electrocatalysts with outstanding activity and durability is a top priority for rechargeable metal–air batteries. Herein, a facile approach is proposed for obtaining hierarchically porous cobalt–nitrogen codoped carbon for reversible oxygen reduction and evolution reactions via the pyrolysis of Co/Zn bimetallic coordinated polymers with gradient Co:Zn ratio. The evaporation of Zn species during the pyrolysis process combined with the Kirkendall effect of Co species resulted in the formation of hierarchical porosity, which further creates highly accessible defective sites and improves multiscale mass transfer. The C-7Co93Zn catalyst with optimized defective Co–Nx and small-sized Co3O4 nanoparticles delivers an optimal bifunctional activity with a potential gap of 0.79 V for reversible oxygen reduction reaction (E1/2) and oxygen evolution reaction (Ej = 10). Moreover, it displays a superior power density of 122 mW cm−2 and long-term durability in a rechargeable Zn–air battery. Density functional theory calculations indicate that the defects adjacent to Co–Nx display improved electrocatalytic oxygen reduction with a smaller energy barrier. This work provides a facile method for optimizing the Co:Zn ratio towards the synthesis of bifunctional oxygen electrocatalyst derived from a class of bimetallic coordinated polymers resulting in hierarchically porous structure and defective cobalt-based active sites.