Impact of Carbon Corrosion and Denitrogenation on the Deactivation of Fe–N–C Catalysts in Alkaline Media

Abstract

Fe−N−C catalysts are considered an earthabundant 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 remains 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 V RHE . It is shown that Fe dissolution accelerates with increased anodic potential and temperature, while it is independent of the presence/absence of O 2 . 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 CO x and NO x species, respectively. This correlation supports that the electrochemical oxidation of the N−C matrix triggers the observed catalyst demetalation 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 (O 2 -RT, O 2 -HT, and Ar-HT, where RT = 22 °C and HT = 62 °C) are rationalized. Combining the GDE-ICP-MS technique and post-mortem analyses, this work provides detailed 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.

Design and Impact: Navigating the Electrochemical Characterization Methods for Supported Catalysts

“…The GDE experiments can and should already be used to examine the impact of GDLs and membranes and to aid in optimizing variables before MEA testing. Additionally, accelerated stress tests (ASTs) in GDEs could be valuable alternatives to such tests in the MEAs, , but their limitations should be kept in mind . For example, drawing conclusions from degradation experiments might be difficult as the liquid electrolyte can flood the GDL of the GDE, which is not an issue in an MEA .…”

“…and Gebhard et al developed a setup for in-operando synchrotron X-ray and neutron imaging. Most GDE methods could also use in situ gas exhaust characterization methods such as differential electrochemical mass spectrometry (DEMS) , or conduct electrolyte characterization with ICP-MS. , A special GDE was built to perform membrane-less flow electrolyzer mass spectrometry (FEMS) . However, it must be kept in mind that the incorporated characterization methods are only reliable for the specific measurement conditions, and extrapolation to other conditions must be done carefully.…”