Single-atom catalysts with full utilization of metal centers can bridge the gap between molecular and solid-state catalysis. Metal-nitrogen-carbon materials prepared via pyrolysis are promising single-atom catalysts but often also comprise metallic particles. Here, we pyrolytically synthesize a Co–N–C material only comprising atomically dispersed cobalt ions and identify with X-ray absorption spectroscopy, magnetic susceptibility measurements and density functional theory the structure and electronic state of three porphyrinic moieties, CoN4C12, CoN3C10,porp and CoN2C5. The O2 electro-reduction and operando X-ray absorption response are measured in acidic medium on Co–N–C and compared to those of a Fe–N–C catalyst prepared similarly. We show that cobalt moieties are unmodified from 0.0 to 1.0 V versus a reversible hydrogen electrode, while Fe-based moieties experience structural and electronic-state changes. On the basis of density functional theory analysis and established relationships between redox potential and O2-adsorption strength, we conclude that cobalt-based moieties bind O2 too weakly for efficient O2 reduction.

Nitrogen-doped carbon-based catalysts are increasingly being studied as Pt-free electrodes for oxygen reduction in polymer electrolyte membrane fuel cells. Here, we study the oxygen reduction activity of stoichiometric carbon nitride, which has much higher nitrogen content and is synthesized at lower temperatures, without using ionic or metallic iron. Carbon nitride was studied and characterized via X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, BET specific surface area analysis, and thermogravimetric analysis. Rotating electrode voltammetry in oxygen-saturated sulfuric acid was used to determine the catalytic activity. The onset potential for oxygen reduction by carbon nitride electrodes was 0.69 V (vs NHE) compared to 0.45 V for a carbon black reference electrode. However, the current density was low, possibly due to the low surface area of the material. Blending the carbon nitride with a high surface area carbon black support resulted in a significant improvement in current density and in an increase in onset potential to 0.76 V. The role of surface area was elucidated via cyclic voltammetry. This work confirms that stoichiometric carbon nitride has improved inherent oxygen reduction activity compared to pure carbon, and suggests that Fe coordination sites are not essential for electrochemical oxygen reduction in nitrogen-containing carbon materials.