Fe-N-C in Proton Exchange Membrane Fuel Cells: Impact of Ionomer Loading on Degradation and Stability

Fe single atoms in N-doped C (Fe-N-C) present the most promising replacement for carbon-supported Pt-based catalysts for the O₂ reduction reaction at the cathode of proton exchange membrane fuel cells (PEMFCs). However, it remains unclear how the I/C ratio affects Fe-N-C degradation and the stability of single Fe atom active sites (FeN_x). Here, an accelerated stress test (AST) protocol is combined with emerging electrochemical techniques for a porous Fe-N-C in PEMFC with a range of I/C ratios. The PEMFC current density degradation rates are found to be comparable; however, with increased I/C ratio the additional FeN_x sites accessed are more stable, as shown by their higher active site stability number (electrons passed per FeN_x lost) at the end of the AST protocol. Meanwhile, the initial rate of TOF decay is suppressed with increasing I/C. Electrochemical process changes are studied via distribution of relaxation times analysis. Minor changes in H⁺ and O₂ transport resistance at low current density prove kinetic degradation dominants at high potentials. These findings demonstrate how electrochemical techniques can be combined with stability metrics to determine and deconvolute changes from the active site to device level electrochemical processes in PEMFCs.