Decoupling Electronic Activation and Pore Connectivity in Acidic Oxygen Reduction on Metal-Free Biomass-Derived Carbon Catalysts

Abstract

Acidic oxygen reduction is often discussed as if catalytic activation and mass transport were inseparable. Here, biomass-derived carbon was used to examine these contributions through oxidation, ammonia annealing, and KOH activation. The onset potential is associated primarily with defect-derived p_z/π electronic states, whereas the maximum slope in the transport-influenced region is associated with hierarchical pore accessibility and connectivity. Apparent electrochemically accessible surface area does not correlate significantly with onset potential in the six-sample series, although it covaries with maximum slope; this covariance is attributed to pore accessibility rather than surface area itself. Multiscale X-ray tomography of the optimized catalyst visualizes connected pore pathways from macro- to submicrometer scales, while the connected framework may be partly inherited from the rice-husk precursor. The combined results support preferentially separate consideration of electronic activation and pore-network accessibility for the acidic oxygen reduction reaction.