MOF-Derived Carbon Spheres with Moderate N, O Co-Doping and High-Temperature Carbon Defect Repair for Boosted ORR and Zinc-Air Batteries

Abstract

Nitrogen-doped carbon materials derived from metal–organic framework (MOF) have been commonly regarded as promising metal-free electrocatalysts for the oxygen reduction reaction (ORR). However, their performance is severely limited by nanoparticle agglomeration-induced sluggish mass transfer, insufficient ORR intermediate adsorption regulation by sole N doping, and structural damage from excessive heteroatom doping. Herein, we report a mesoscopic superstructure engineering strategy to fabricate moderately nitrogen–oxygen codoping MOF-derived carbon sphere superstructures (N/O-CNS) for high-performance ORR. A three-dimensional (3D) spherical MOF superstructure precursor was first constructed via urea-induced self-assembly of one-dimensional (1D) rod-like Zn-MOF-74, followed by one-step controlled pyrolysis to preserve the 3D hierarchical architecture and achieve uniform nitrogen–oxygen doping. The 3D superstructure provides abundant accessible catalytic centers and accelerated mass/electron transport rates. Density functional theory (DFT) calculations verify that, compared with sole N doping, nitrogen–oxygen codoping further optimizes the adsorption energy of the rate-determining *OOH intermediate and lowers the ORR reaction energy barrier. Regulated pyrolysis enables continuous tuning of nitrogen–oxygen codoping content and the graphitization degree of the carbon matrix, achieving a favorable trade-off among catalytic site population, charge transport efficiency, and structural integrity. Notably, the optimized N/O-CNS-1000 with the lowest doping content among all samples exhibits exceptional ORR activity in alkaline electrolyte, with an onset potential of 0.98 V and a half-wave potential (E1/2) of 0.83 V, outperforming commercial Pt/C. When assembled into rechargeable Zinc-air batteries (ZABs), the N/O-CNS cathode achieves a maximum power density of 140.55 mW cm–2 and good cycling lifespan of over 750 h.