Influence of Ionomer Chemistry on Oxygen Permeability in Hydrocarbon-Based Proton-Exchange Membrane Fuel Cells

Abstract

The pursuit of alternatives to perfluorinated sulfonic acid (PFSA) materials is of particular interest in light of the global and European mid-term goals to avoid fluorinated materials, and the technical target to operate proton-exchange membrane (PEM) fuel cells at temperatures of 120°C. Non-fluorinated hydrocarbon ionomers offer improved chemical stability at temperatures above 100°C, are less environmentally hazardous, exhibit lower gas crossover, and have the potential to reduce production costs.1 Recent years have seen significant progress in the development of hydrocarbon-based membranes and electrodes, with performance on par with that of state-of-the-art PFSA-based fuel cells under optimized conditions.2 However, a performance gap between hydrocarbon- and PFSA-based PEM fuel cells persists under application-relevant conditions, necessitating an understanding of the limitations that arise. Mass transport-related phenomena, particularly at the ionomer/catalyst interfaces, play a crucial role in controlling PEM fuel cell performance.3 While extensive studies have been conducted for PFSA-based fuel cells to deconvolute the local from the total transport resistance of the cathode catalyst layer, no such studies have been conducted for hydrocarbon ionomers. In this study, we investigated gas transport resistance and its components in hydrocarbon-based PEM fuel cells using H2- and O2 limiting current methods and ex-situ investigations in ionomer thin film behavior, while varying the ionomer's ion-exchange capacity. Our results show that local transport resistances are similar to those measured with PFSA-based cells with respect to different ionomer to carbon mass ratios (I/C). However, higher transport-induced resistances are observed under the potential formation of liquid water in the range of high current densities. We attribute these effects to the more pronounced water uptake of the ionomer thin film compared to the PFSA based reference. Thus, reducing ionomer swelling while maintaining the necessary proton conductivity of the ionomer thin film is crucial for next-generation hydrocarbon PEM fuel cells.References(1) H. Nguyen, C. Klose, L. Metzler, S. Vierrath and M. Breitwieser, Adv. Energy Mater., 2022, 12, 2103559.(2) H. Nguyen, F. Lombeck, C. Schwarz, P. Heizmann, M. Adamski, H. F. Lee, B. Britton, S. Holdcroft, S. Vierrath and M. Breitwieser, Sustainable Energy Fuels, 2021, 5, 3687–3699.(3) T. Schuler, A. Chowdhury, A. T. Freiberg, B. Sneed, F. B. Spingler, M. C. Tucker, K. L. More, C. J. Radke and A. Z. Weber, J. Electrochem. Soc ., 2019, 166(9), F3020.