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

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 FeN_xSe_y 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 FeN_xSe_y 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 FeN_xSe_y catalyst are only one-quarter and one-third of that on the Fe-N-C counterpart, respectively. Therefore, the FeN_xSe_y catalyst exhibits outstanding cyclic stability, losing only 10 mV in half-wave potential E_1/2 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 FeN_xSe_y catalyst in the presence of H₂O₂. As a result, the FeN_xSe_y 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.