Résumé
Fe-N-C catalysts are considered an earth-abundant alternative to Pt in cathodes of anion exchange membrane fuel cells, although their stability still requires improvement for further commercialization. The degradation of Fe-N-C during both load cycles and start-stop events must be understood and mitigated to minimize system costs. Several approaches have recently been proposed to improve the durability of Fe active species during the oxygen reduction reaction in acidic media. On the other hand, knowledge of the degradation of Fe-N-C catalysts during start-stop events of anion exchange membrane fuel cells is scarce. In this work, we use a gas diffusion electrode half-cell coupled with inductively coupled plasma mass spectrometry (GDE-ICP-MS) to quantify the Fe dissolution rates in the potential range between 0.93 and 1.5 VRHE. It is shown that Fe dissolution accelerates with increased anodic potential and temperature while it is independent on the presence/absence of O2. The onset potential of Fe dissolution at room temperature agrees with the reported onset potentials of carbon corrosion and denitrogenation, C and N being oxidized to gaseous COx and NOx species, respectively. This correlation supports that the electrochemical oxidation of the N-C matrix triggers the observed catalyst demetallation in these conditions. Using a set of ex situ physicochemical characterization techniques, including spectroscopy and microscopy, the various degrees of degradation under three sets of experimental conditions of interest (O2-RT, O2-HT, and Ar-HT, where RT = 22℃ and HT = 62℃) are rationalized. Combining the GDE-ICP-MS technique and post mortem analyses, this work provides novel insights into the degradation pathways of various Fe, N, and C species during start-stop events, which may inspire the next generation of durable Fe-N-C catalysts for anion exchange membrane fuel cells.