Coupling nano-Fe3O4 with oxygen vacancies on a hypercrosslinked iron porphyrin-coated ZIF-8 as a high-efficiency oxygen reduction reaction electrocatalyst

In this study, a novel and highly efficient ORR catalyst is designed via the pyrolysis of zeolitic imidazole framework-8 (ZIF-8) under the protection of hypercrosslinked iron porphyrins. Fe3O4 nanoparticles with oxygen vacancies are formed in situ and promoting catalytic efficiency. Benefiting from the synergistic effect among the Fe3O4 and ZIF-8 skeleton, the hybrid catalyst exhibits excellent ORR activity even surpassing commercial Pt/C. The pyrolyzation temperature is studied in detail and determined to be a crucial parameter for ORR performance. Nano-Fe3O4, which has high catalytic efficiency, is formed in situ at 800 ℃. Amorphous iron is formed at 600 ℃ and 700 ℃, and zero-valent iron is formed at 900 ℃. The content of oxygen vacancies, Fe–N, and graphite–N, which are beneficial to the ORR reaction, are all reached to a maximum at 800 ℃. This study provides a new approach to design highly active Fe3O4-based electrocatalytic materials. [Display omitted] •A catalyst was synthesized by pyrolysis of hypercrosslinked FeTPP-coated ZIF-8.•The catalyst showed excellent ORR activity even surpassing commercial Pt/C.•Nano-Fe3O4 with oxygen vacancies was suggested to be the critical active species.•Mechanism how pyrolyzation temperature regulate ORR activity was clarified.