Carbon Thin-Layer-Protected Active Sites for ZIF-8-Derived Nitrogen-Enriched Carbon Frameworks/Expanded Graphite as Metal-Free Catalysts for Oxygen Reduction in Acidic Media

Generally, sluggish oxygen reduction reaction (ORR) kinetics on cathode limits the practical application of the direct methanol fuel cell (DMFC). In this study, zeolitic-imidazolate-framework-8 (ZIF-8)-derived carbon is used to decorate expanded graphite (EG) to prepare a nitrogen-rich porous metal-free carbon framework (Z8-NCF-P/EGs) as catalyst for ORR in DMFC. Through use of a simple one-pot method, ZIF-8 is firmly anchored on EG to obtain the ZIF-8/EG precursor via a polyvinypyrrolidone (PVP)-assisted route. After carbonization and acid etching, the formed micro-mesoporous structure contributes to the high specific surface area and well-distributed porous 3D bulk networks of Z8NCF-P/EGs, which provide short diffusion paths for rapid oxygen and mass transfer kinetics. Z8-NCF-P/EG-10 (ZIF-8/EG weight ratio of 10) exhibits activity comparable to that of commercial Pt/C (10 wt %) and methanol tolerance and stability much higher than those of Pt/C in acidic electrolyte. EG contributes to the uniform distribution and confined growth of Z8-NCF and the high electrical conductivity. PVP-derived carbon thin layer protects the deterioration of active sites. Pyridine N and graphitic N provide the highly active and fully accessible active sites to enhance the onset potential (0.663 V vs SCE) and charge transfer efficiency for ORR to promote the 4e(-) process for Z8-NCF-P/EG-10. Moreover, sufficient oxygen groups and N species maximize the exposure of catalytic centers for O-2 chemisorption and reduction to improve activity and methanol resistance. Therefore, metal-free Z8-NCF-P/EGs provide the reference and appreciable prospect for other ZIF-derived catalysts for enhancing ORR activity.