Impact of Exposed Crystal Facets on Oxygen Reduction Reaction Activity in Zeolitic Imidazole Frameworks

Oxygen reduction reactions (ORR) are critical reactions for efficient sustainable and renewable energy storage. However, the limitation of ORR is a slow kinetic reaction due to the multi-step transfer mechanism of electrons. Therefore, electrocatalysts with high ORR activity and stability are needed to overcome the reaction limitations. Zeolitic imidazole framework-67 (ZIF-67) has attracted attention in electrochemical applications due to its multiple redox-active sites and high stability in alkali conditions. The Co-doped porous carbon catalysts derived from ZIF-67 have the potential to be used as ORR catalysts with high performance, while pure phase ZIF-67 electrocatalyst for ORR is rarely studied. The work studies the importance of crystal facets on ORR reactivity. The results show that ZIF-67 nanocube exhibits excellent ORR activity with lower half wave-potential and faster kinetics process compared to its bulk structure. It is proposed that ZIF-67 nanocube provides improvement of selectivity for the four-electron pathway, which is a favorable process with higher reaction efficiency. The density functional theory (DFT) calculations suggest unsaturated Co2+ active sites with enriched (100) facets in the cubic crystal. Therefore, the fast diffusion of oxygen molecules to active sites and strong interaction with intermediate are observed in a cubic structure. The correspondence between experiment and calculation offers enhancement of ORR activity and selectivity from control-specific growth of (100) facet with cubic shape. The work introduces a new strategy for designing highly efficient ORR electrocatalysts with facet-controlled, allowing further study in other electrochemical reactions.