Abstract
Herein, we synthesized P- and N-doped carbon materials (PN-doped carbon materials) through controlled phosphoric acid treatment (CPAT) of folic acid (FA) and probed their ability to catalyze the oxygen reduction reaction (ORR) 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 carbon materials. 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 carbon materials correlated well with those of the precursors, which indicated that CPAT is well suited for the preparation of PN-doped carbon materials. The carbon material prepared using a CPAT temperature of 700 °C exhibited the highest 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. It was concluded that the presence of –C–PO2 and –C–PO3 units decreases the work function and thus raises 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 carbon materials from N-containing organic compounds other than FA.
Highlights
The widespread application of fuel cells as clean energy sources is the most desirable way of realizing a low-CO2-emission society
To establish a more generalized PN-doping method allowing for the use of more common compounds, we developed the technique of controlled phosphoric acid treatment (CPAT) that is potentially applicable to non-special N-containing organic compounds and applied it to folic acid (FA) as a commonly occurring N-containing organic compound
The CPAT temperature affected both the BET specific surface area (BET-SSA) and surface elemental composition of the precursors, as exemplified by values derived from X-ray photoelectron spectra of P-series precursors (Table 1, for the naming scheme of the samples see section “Experimental”)
Summary
The widespread application of fuel cells as clean energy sources is the most desirable way of realizing a low-CO2-emission society. In conventional polymer electrolyte fuel cells (PEFCs), both anode and cathode reactions are catalyzed by Pt. Compared to the anode reaction, the cathode reaction, namely the oxygen reduction reaction (ORR), is rather slow and requires the use of larger amounts of Pt [1], which increases the cost of PEFCs and prevents their wide application as domestic, Beilstein J. The cost of cathode catalysts can be reduced in a number of ways, e.g., by alloying Pt with base metals [2], forming core–shell particles with basemetal cores covered by thin Pt layers [3], and developing non-Pt catalysts. The implementation of non-Pt or nonprecious-metal cathode catalysts is the ultimate goal of PEFC development
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