Defects, Doping, and Curvature: Structure–Property–Function Design of Carbon Nano-Onions for Metal-Free ORR Electrocatalysis

The interplay between defects, heteroatom doping, and surface curvature in carbon nanostructures governs their electronic transport and catalytic properties, yet it remains poorly understood. Here, we elucidate how structural defects, N/O functionalities, and high curvature collectively modulate morphology, conductivity, and oxygen reduction reaction (ORR) activity in carbon nano-onions (CNOs) derived from nanodiamonds (NDs). Ultradispersed NDs with tailored surface terminations are thermally transformed at 1,150 and 1,650 °C to generate partially graphitized core–shell nanostructures and fully converted, highly graphitized CNOs, respectively. X-ray photoelectron spectroscopy and electrochemical analysis reveal that pyridinic- and graphitic-N and carbonyl/phenolic O at curvature-induced defect sites define the defect chemistry, enabling fine control over charge transport and interfacial reactivity. Rotating ring-disk electrode studies show that optimally graphitized, defect-accessible CNOs (1,650 °C) deliver high specific capacitances, near-diffusion-limited ORR currents, with the number of electrons transferred per O2 molecule during ORR approximately 4.0, and a ≤10% H2O2 selectivity in alkaline media. Within a curvature-engineering framework, concentric graphenic layers introduce a Gaussian curvature that concentrates local fields and tunes *OOH binding, while continuous sp2 networks minimize resistive losses. This work establishes a structure–property–function concept for “curved” nanocarbons and defines design rules for next-generation, metal-free CNO electrocatalysts.