In-Depth Analysis of the Impact of Ionomer Content on PEMFC Degradation By Combining Electrochemical Characterization Techniques and Transmission Line Modeling
Authors: Raab Sebastian , Chuang Po-Ya Abel , Weber Andre 
Published: December 2024

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 cm2 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 H2/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 H2/N2 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-operando characterization techniques enable us to obtain a more comprehensive understanding of the dominating degradation mechanisms and the interaction with varying I/C ratios. The insights gained from this study can guide future material design for enabling fuel cell commercialization.

References:

[1] Lee et al., Effects of Ionomer Carbon Ratio and Ionomer Dispersity on the Performance and Durability of MEAs, Fuel Cells, 18: 129-36 (2018).

[2] Xie et al., Experimental and theoretical analysis of ionomer/carbon ratio effect on PEM fuel cell cold start operation, International Journal of Hydrogen Energy, 42: 12521-30 (2017).

[3] Du et al., Effects of PEMFC Operational History under Dry/Wet Conditions on Additional Voltage Losses due to Ionomer Migration, Journal of The Electrochemical Society, 167 (2020).

[4] Sabawa et al., Investigation of degradation mechanisms in PEM fuel cells caused by low-temperature cycles, International Journal of Hydrogen Energy, 46: 15951-64 (2021).

[5] Babu et al., Effect of Catalyst and Catalyst Layer Composition on Catalyst Support Durability, Journal of The Electrochemical Society, 168 (2021).

[6] U.S. DRIVE, Fuel Cell Technical Team Roadmap Hydrogen Storage Technologies Roadmap (2017).

[7] Labata et al., Insights on platinum-carbon catalyst degradation mechanism for oxygen reduction reaction in acidic and alkaline media, Journal of Power Sources, 487 (2021)

[8] Heinzmann et al., Impedance modelling of porous electrode structures in polymer electrolyte membrane fuel cells, Journal of Power Sources, 444 (2019).

[9] Reshetenko et al., Separation Method for Oxygen Mass Transport Coefficient in Gas and Ionomer Phases in PEMFC GDE, Journal of The Electrochemical Society, 161: F1089-F100 (2014).

Figure 1