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

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.