Electronically modulated cobalt–nitrogen/carbon catalyst via ligand displacement of metal–organic frameworks toward efficient oxygen reduction†

Abstract

The electronic environment of metal–nitrogen/carbon catalysts derived from metal–organic frameworks (MOFs) strongly correlates with their catalytic performance in the oxygen reduction reaction (ORR). Here, we report a novel ligand displacement approach for exchanging the 2-methylimidazole (2-MI) ligand in zeolitic imidazolate framework-67 (ZIF-67) with mercapto-5-nitrobenzimidazole (MNBI), followed by pyrolysis at 800 °C to generate ZIF-67/MNBI-800 as a Co–N/C catalyst. The ligand displacement efficiently tuned the electronic environment of the Co center through different configurations of nitrogen and sulfur elements, thereby affecting the oxygen-binding force of the Co–N/C catalysts. Further acid etching eliminates the impact of symbiotic Co nanocrystal in raw ZIF-67/MNBI-800 catalysts, allowing the Co–N/C catalysts to achieve an exceptional performance. Acid etched ZIF-67/MNBI-800 (A-ZIF-67/MNBI-800) shows an optimal oxygen adsorption strength, of which the half-wave potential is 39 mV more positive, and mass activity increases by a factor of 5.9, as compared with the benchmark platinum catalyst. This impressive performance makes A-ZIF-67/MNBI-800 a robust air-breathing electrode for a rechargeable zinc–air battery that exhibits high peak power density (302.0 mW cm⁻²) and amazing cycle life (2220 cycles for 750 h of operation). This work provides a simple but effective approach for tuning the electronic density of metal centers and an in-depth insight into its correlation with catalytic properties, thereby paving the way for rational ORR catalyst design.