Biomass-derived nitrogen self-doped porous carbon as effective metal-free catalysts for oxygen reduction reaction

Abstract

Biomass-derived nitrogen self-doped porous carbon was synthesized by a facile procedure based on simple pyrolysis of water hyacinth (eichhornia crassipes) at controlled temperatures (600–800 °C) with ZnCl₂ as an activation reagent. The obtained porous carbon exhibited a BET surface area up to 950.6 m² g⁻¹, and various forms of nitrogen (pyridinic, pyrrolic and graphitic) were found to be incorporated into the carbon molecular skeleton. Electrochemical measurements showed that the nitrogen self-doped carbons possessed a high electrocatalytic activity for ORR in alkaline media that was highly comparable to that of commercial 20% Pt/C catalysts. Experimentally, the best performance was identified with the sample prepared at 700 °C, with the onset potential at ca. +0.98 V vs. RHE, that possessed the highest concentrations of pyridinic and graphitic nitrogens among the series. Moreover, the porous carbon catalysts showed excellent long-term stability and much enhanced methanol tolerance, as compared to commercial Pt/C. The performance was also markedly better than or at least comparable to the leading results in the literature based on biomass-derived carbon catalysts for ORR. The results suggested a promising route based on economical and sustainable biomass towards the development and engineering of value-added carbon materials as effective metal-free cathode catalysts for alkaline fuel cells.