Promoting Oxygen Reduction Reaction Activity of Fe-N/C Electrocatalysts by Silica-Coating-Mediated Synthesis for Anion-Exchange Membrane Fuel Cells

Abstract

Iron- and nitrogen-codoped carbon (FeN/C) catalysts have emerged as promising alternatives to Pt-based catalysts for the oxygen reduction reaction (ORR) owing to their prominent ORR activity among nonprecious metal catalysts (NPMCs). This high ORR activity originates from atomically dispersed Fe coordinated with nitrogen atoms (FeNx site). However, the rational design of FeN/C catalysts with abundant FeNx active sites remains a challenge. In this work, we demonstrate that a silica-coating-mediated synthetic strategy enables the preparation of FeN/C catalysts enriched with active FeNx 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 FeN/C catalysts with various carbon supports and a wide range of Fe contents and pyrolysis temperatures. The strategy could be further extended to S- or P-doped FeN/C catalysts, in which the formation of inactive FeS and Fe2P species was suppressed. As a result, FeN/C catalysts prepared with the silica coating exhibited improved ORR activity up to a factor of 11 compared to silica-uncoated counterparts. Significantly, the S-doped FeN/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 (AEMFC) tests, the S-doped FeN/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 FeN/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 monatomic catalysts exhibit high catalytic activities.