In-Depth Analysis of the Impact of Ionomer Content on PEMFC Degradation By Combining Electrochemical Characterization Techniques and Transmission Line Modeling

The future success of the polymer electrolyte membrane fuel cell (PEMFC) technology depends on a further increase in performance and durability as well as a decrease in costs. The catalyst layer plays an important role in these aims, as the employed catalyst, carbon and ionomer materials and their composition strongly influence these three success factors. One key parameter, which has a strong effect on the cathode catalyst layer (CCL), is the ionomer to carbon (I/C) ratio. The I/C ratio has shown a direct impact on mass transport processes, performance [1] and cold start behavior [2] of PEMFCs. It also has an influence on the degradation during cycling of relative humidity [3], temperature [4] and high potential cycling [5]. One of the most crucial operating states of PEMFCs is the start-up/shut-down condition, during which high potentials arise due to an oxygen-hydrogen front, which lead to a corrosion of the electrode carbon structure. Accelerated stress tests (ASTs) commonly emulated this condition by cycling at high potentials (1.0-1.5 V) [6,7]. As the I/C ratio strongly affects the microstructure and mass transport processes in PEMFCs, catalyst layer reconstruction due to carbon corrosion and its effect on mass transport losses may have an interaction with the I/C ratio. This investigation aims to analyze the influence of the I/C ratio on the carbon corrosion behavior by applying high potential cycles using an incremental 1 cm 2 single-cell setup. The incremental cell setup enables a gradient-free operation of the cell concerning humidification, gas composition, temperature, pressure and degradation. For the in-depth analysis of the carbon corrosion behavior, several electrochemical in- and ex-operando techniques are combined. The electrochemical impedance spectroscopy (EIS) under H 2 /air conditions combined with the distribution of relaxation times (DRT) method (see figure 1) and the subsequent fit of a physicochemically motivated transmission line model (TLM) enable a deconvolution and quantification of the ohmic, charge transfer, mass transport and ionic resistances in the CCL [8]. To better understand the degradation mechanisms within the CCL, these results are complemented by EIS under H 2 /N 2 conditions, cyclic voltammetry (CV) and limiting current measurements separating the molecular diffusion (GDL/gas channel) from the Knudsen/film diffusion (MPL/CCL/ionomer and water film) resistances [9]. The combined results from different in- and ex-op