Pyrrolic N wrapping strategy to maximize the number of single-atomic Fe-N-x sites for oxygen reduction reaction

Abstract

Iron-nitrogen-carbon (Fe-N-C) catalysts with a representative single-atomic structure are promising platinum group metal-free catalysts for the oxygen reduction reaction (ORR) as they exhibit comparable activity to commercial catalysts. To enhance the ORR activity of Fe-N-C catalysts, the number of single Fe atoms coordinated N (Fe-N-x) should be maximized. In this study, a strategy is devised to increase the number of Fe-N-x sites using electrostatic interactions between electronegative pyrrolic-N and electropositive Fe ions. Pyrrolic N-rich carbon (pNC) is dispersed on the surface of the metal-organic framework (MOF) to form composite supports (pNC@MOF). Owing to the well-dispersed pNC and electrostatic interactions, the number of Fe-N-x sites on the pNC@MOF-derived hollow carbon framework (Fe/pNC@HCF) increases dramatically compared to that on the pristine MOF (Fe/HCF). The original shape of the Fe-absorbed MOF is maintained by the conversion of pNC into carbon layer within the framework by pyrolysis at 1000 degrees C even though pure Fe-absorbed MOF collapses. An anion exchange membrane fuel cell (AEMFC) with Fe/pNC@HCF is fabricated, and it shows a high current density of 437 mA cm(-2) at 0.6 V and a power density of 343 mW cm(-2). This performance suggests that the synthesized catalysts are excellent potential cathodic catalysts for AEMFCs.