Spontaneous ORR Activity Drop of Fe-N-C Catalysts upon Atmospheric Storage

Abstract

Iron-Nitrogen-Carbon (Fe-N-C) catalysts are promising materials to replace the scarce and expensive platinum-group materials in proton exchange membrane fuel cells (PEMFC) cathodes. Fe-N-C catalysts can be prepared by multiple ways [1-5], and each synthesis method can be fine-tuned to produce single Fe atoms coordinated to nitrogen atoms (FeNx), identified as the most active sites toward the oxygen reduction reaction (ORR). However, despite these notable efforts, a rapid drop in ORR activity is noticed within the first hours of operation in thin-film rotating disk electrode (RDE) (liquid electrolyte) or in PEMFC systems (solid electrolyte) [6]. Unravelling the degradation mechanisms at stake and developing strategies to improve the durability of Fe-N-C catalysts has thus become an important objective for researchers worldwide [7].In this work, we provide evidences for spontaneous ageing of Fe-N-C catalysts synthesized using different synthetic approaches, and thus featuring distinct chemical, textural and structural properties. For all materials, a pronounced drop of ORR activity is measured, indicating that the “spontaneous ageing” phenomenon is universal. The drop of ORR mass activity over time may reach as high as 90 % of the initial value, and relates to a drop of the active site density and/or the turnover frequency. These adverse effects can be mitigated using storage under dry and oxygen-free atmosphere.