Bimetallic Mg/Zn-based zeolitic imidazolate frameworks for zinc–air batteries: disclosing the role of defective imidazole-Mg sites in the electrocatalytic performance†

Abstract

The development of new noble metal-free, non-toxic and low-cost materials with efficient catalytic properties for the oxygen reduction reaction (ORR) is a key issue for improving energy storage devices, such as fuel cells or metal–air batteries. Herein, taking inspiration from the function of Mg in nature as a cofactor in many catalytic reactions, we have synthesized bimetallic Mg/Zn-based zeolitic imidazolate frameworks (Mg-doped ZIF-8), which resulted in a significant improvement in the electrocatalytic activity for ORR compared to pristine ZIF-8, especially when prepared as nanosized rather than microsized particles. Under optimized synthetic conditions, we succeeded in incorporating a large amount of Mg within the ZIF-structure (17.5% mol Mg doping), which was critical for improving the ORR response. Importantly, this work demonstrates for the first time the role of Mg as a dopant in ZIFs to boost the ORR performance, revealing that di-coordinated imidazole-Mg species (Im₂Mg) are the key active sites that enhance the adsorption of O₂ and water in the ORR process, as evidenced by our computational studies. Exploiting the exceptional electrocatalytic performance of the as-prepared Mg-doped ZIF-8 nanoparticles, we built zinc–air batteries that exhibited a specific capacity of 4.95 A h g⁻¹, significantly surpassing the values reported previously for other catalysts containing single-atom M–N–C sites.