Herein, we synthesized N- and P-doped carbons (PN-doped carbons) by controlled phosphoric acid treatment (CPAT) of folic acid (FA) and probed their ability to catalyze the oxygen reduction reaction at the cathode of a fuel cell. Precursors obtained by heating FA in the presence of phosphoric acid at temperatures of 400–1000 °C were further annealed at 1000 °C to afford PN-doped carbons. The extent of precursor P-doping was maximized at 700 °C, and the use of higher temperatures resulted in activation and increased porosity rather than in increased P content. The P/C atomic ratios of PN-doped carbons were well correlated with those of precursors, which indicated that CPAT was well suited for the preparation of PN-doped carbons. Carbon prepared using a CPAT temperature of 700 °C exhibited the highest oxygen reduction reaction (ORR) activity and was shown to contain –C–PO2 and –C–PO3 moieties as the major P species and pyridinic N as the major N species; moreover, no N–P bonds were detected. The presence of –C–PO2 and –C–PO3 units was concluded to decrease the work function and thus raise the Fermi level above the standard O2/H2O reduction potential, which resulted in enhanced ORR activity. Finally, CPAT was concluded to be applicable to the synthesis of PN-doped carbons from N-containing organic compounds other than FA.
Catalyst evaluation for oxygen reduction reaction in concentrated phosphoric acid at elevated temperatures
