Nitrogen-Doped Annealed Nanodiamonds with Varied sp2/sp3 Ratio as Metal-Free Electrocatalyst for the Oxygen Reduction Reaction

Nitrogen-doped nanocarbon materials as metal-free electrocatalysts for the oxygen reduction reaction (ORR) have attracted great attention, owing to their catalytic activity close to Pt/C in alkaline media. Annealed nanodiamonds (AND) possess a special structure, because we can finely control the carbon sp(2)/sp(3) ratio. This provides a good opportunity to study the relationship between ORR performance (activity and stability) and the special structure of AND. We have used nitrogen-doped annealed nanodiamonds (NAND) with varied carbon sp(2)/sp(3) ratio as metal-free electrocatalysts for ORR in alkaline media. NAND showed higher catalytic activity for ORR with higher content of sp(2) carbon and larger amount of nitrogen species present as pyridinic groups. More importantly, NAND displayed stability superior to other nitrogen-doped nanocarbon materials as a result of transformation of carbon from sp(3) to sp(2) during electrochemical cycling.