Metal-free porous nitrogen-doped carbon nanotubes for enhanced oxygen reduction and evolution reactions

Abstract: Developing efficient metal-free bi-functional electrocatalysts is required to reduce costs and improve the slow oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) kinetics in electrochemical systems. Porous N-doped carbon nanotubes (NCNTs) were fabricated by KOH activation and pyrolysis of polypyrrole nanotubes. The NCNTs possessed a large surface area of more than 1,000 m² g⁻¹. NCNT electrocatalysts, particularly those annealed at 900 °C, exhibited excellent ORR electrocatalytic performance. Specifically, they yielded a more positive onset potential, higher current density, and long-term operation stability in alkaline media, when compared with a commercially available 20 wt% Pt/C catalyst. This resulted from the synergetic effect between the dominant pyridinic/graphitic-N species and the porous tube structures. The NCNT electrocatalyst also exhibited good performance for the OER. The metal-free porous nitrogen-doped carbon nanomaterials were prepared from low cost and environmentally friendly precursors. They are potential alternatives to Pt/C catalysts, for electrochemical energy conversion and storage.