Converting Fe−N−C Single-atom Catalyst to a New FeNxSey Cluster Catalyst for Proton-exchange Membrane Fuel Cells

Abstract

Iron-nitrogen-carbon (Fe−N−C) single-atom catalyst is the most promising alternative to platinum catalyst for proton-exchange membrane fuel cells (PEMFCs), however its high performance cannot be maintained for a long enough time in device operation. The construction of a new Fe coordination environment that is completely different from the square-planar Fe−N4 configuration in classic Fe−N−C catalyst is expected to break the current stability limits of Pt-free catalysts, which however remains unexplored. Here, we report, for the first time, the conversion of Fe−N−C catalyst to a new FeNxSey cluster catalyst, where the active Fe sites are three-dimensionally (3D) co-coordinated by N and Se atoms. Due to this unique Fe coordination configuration, the FeNxSey catalyst exhibits much better 4e− ORR activity and selectivity than the state-of-the-art Fe−N−C catalyst. Specifically, the yields of hydrogen peroxide (H2O2) and ⋅OH radicals on the FeNxSey catalyst are only one-quarter and one-third of that on the Fe−N−C counterpart, respectively. Therefore, the FeNxSey catalyst exhibits outstanding cyclic stability, losing only 10 mV in half-wave potential E1/2 after 10,000 potential cycles, much smaller than that of the Fe−N−C catalyst (56 mV), representing the most stable Pt-free catalysts ever reported for PEMFCs. More significantly, the 3D co-coordination structure effectively inhibits the Fe demetallization of the FeNxSey catalyst in the presence of H2O2. As a result, the FeNxSey based PEMFC shows excellent durability, with the current density attenuation significantly lower than that of the Fe−N−C based device after accelerated durability testing. Our work provides guidance for the development of next-generation Pt-free catalysts for PEMFCs.