Effects of structural disorder and nitrogen content on the oxygen reduction activity of polyvinylpyrrolidone-derived multi-doped carbon

Abstract

Multi-doped carbons, having high N-content (∼11 wt%, bulk) and the presence of C- and N-bound O/Co, were synthesized by pyrolysis of a polyvinylpyrrolidone (PVP) and cobalt nitrate (Co(NO₃)₂·6H₂O) mixture at various temperatures (T_p) ranging from 400 to 800 °C. To study the effects of surface modification on the oxygen reduction reaction (ORR) activity, doped carbon (CN_x) samples were further acid-treated with nitric acid (HNO₃). Structural characterization by Raman spectroscopy and X-ray diffraction reveals an amorphous phase of carbon for a T_p of 400 to 700 °C, and a partially graphitic structure for a T_p of 800 °C. The electrocatalytic performance of the as-synthesized and acid-treated CN_x determined by linear sweep as well as cyclic voltammetry in 0.1 mol L⁻¹ KOH electrolyte reveals significant catalytic activities with an ORR onset potential of −144 mV (vs. Ag/AgCl), as compared to −210 mV for a Pt electrode under similar conditions. Correlations between the electrochemical parameters and structural parameters such as the degree of disorder and N-content suggest high electrocatalytic activity for CN_x with low disorder and high N-content. In addition to their high ORR activity, the CN_x samples synthesized at lower T_p also exhibit catalytic activity for methanol oxidation, which makes them suitable catalyst supports for direct methanol fuel cells.