Nanosheet-Assembled Hierarchical Carbon Nanoframeworks Bearing a Multiactive Center for Oxygen Reduction Reaction

Designing noble metal-free electrocatalysts with novel structure and high performance for the oxygen reduction reaction (ORR) has been long pursued in many electrochemical energy storage and conversion technologies but remains a great challenge. Here, a novel zirconium-porphyrin metal–organic framework (MOF) hierarchical nanoframework structure assembled by nanosheets is synthesized. Subsequent pyrolysis and acid etching of such material generates N-doped hierarchical graphitic carbon nanoframework (N-GCF) equipped with M-N₄ units. The N-GCF catalyst demonstrates high ORR activity and stability with large diffusion-limited current density (6.19 mA cm⁻²) and half-wave potential (0.83 V vs reversible hydrogen electrode), which is one of the best carbon-based ORR catalysts in basic electrolyte. Systematic experiments and characterizations reveal that favorable internal spaces and pores, unique hierarchical structure, and numerous nitrogen-doped and metal-N₄ species synergistically contribute to the remarkably enhanced catalytic activity for ORR. This work is expected to expand the toolbox for the design and synthesis of MOF-derived functional materials for electrochemical applications.