Promoting Oxygen Reduction Reaction Activity of Active-Site-Rich Fe−N/C Electrocatalysts by Silica Coating Strategy

Abstract

Oxygen reduction reaction (ORR) is an important half-reaction in clean energy conversion devices such as polymer electrolyte fuel cells (PEFCs). Sluggish reaction kinetics of the ORR renders the use of Pt-based electrocatalysts that are efficient; however, they are scarce and expensive, thus precluding the wide employment of PEFCs. Iron and nitrogen codoped carbon (Fe−N/C) catalysts have demonstrated promise as substitutes for the Pt-based catalysts due to their high ORR activity, which has been found to stem from the atomically dispersed Fe coordinated with nitrogen atoms (Fe−Nx site). Rational design that can create abundant active Fe−Nx site remains an important challenge in Fe−N/C catalysts. Recently, we have developed the silica-coating-assisted synthetic strategy for high-performance Fe−N/C catalysts using metalloporphyrin as Fe and N precursors. In this work, we generalize this synthetic concept with more affordable Fe and N precursors, such as iron acetate/1,10-phenanthroline and iron acetate/polyaniline. In addition, we extensively explored the impacts of pyrolysis temperature and Fe contents, and the type of carbon support on the ORR activity of the resulting Fe−N/C catalysts. The silica coating step during the preparation of Fe−N/C catalysts was generally effective to make the Fe−N/C catalysts predominantly composed of active Fe−Nx site while suppressing the generation of inactive Fe-based particles, thus significantly improving the ORR activity of the Fe−N/C catalysts. Significantly, the addition of S-containing precursor further boosted the ORR activity of Fe−N/C catalysts.