Generalized Silica-Coating-Mediated Synthesis Toward Enhancing the Catalytic Activity of Fe−N/C Oxygen Reduction Electrocatalysts

Abstract

Iron and nitrogen codoped carbon (Fe−N/C) catalysts have emerged as promising substitutes for the Pt-based catalysts for the oxygen reduction reaction (ORR) in polymer electrolyte fuel cells, owing to their high ORR activity among nonprecious metal catalysts. This high ORR activity originates from the atomically dispersed Fe coordinated with nitrogen atoms (Fe−Nx site). However, the rational design that can create abundant active Fe−Nx site remains an important challenge. In this work, we demonstrate that a silica-coating-mediated synthetic strategy enables the preparation of Fe−N/C catalysts enriched with active Fe−Nx sites while mitigating the formation of less active Fe and Fe3C species. The silica coating-mediated strategy was generally applicable to various types of Fe and N precursors, including iron porphyrin, iron acetate/1,10-phenanthroline, and iron chloride/polyaniline. This strategy was also effective in the preparation of Fe−N/C catalysts with various carbon supports and with a wide range of Fe contents and pyrolysis temperatures. The strategy could be further extended to S- or P-doped Fe−N/C catalysts, in which the formation of inactive FeS and Fe2P species was suppressed. Significantly, the S-doped Fe−N/C catalyst exhibited very high ORR activity with half-wave potential at 0.91 V (vs. RHE) in alkaline media. In anion-exchange membrane fuel cell tests, the S-doped Fe−N/C based cathode showed a current density of 977 mA cm−2 at 0.6 V, which is the highest performance among reported AMEFCs with NPMC-based cathodes. The S-doped Fe−N/C-based cathode also demonstrated promising volumetric current density in an acidic proton exchange membrane fuel cell. Thus, the silica-coating-mediated strategy is generally effective in preparing atomically dispersed catalytic entities and may be applicable to other catalytic reactions whereby atomically dispersed catalysts exhibit high catalytic activities.