UC Berkeley

Reducing the loading of expensive Pt-based catalysts in polymer-electrolyte fuel-cell (PEFC) catalyst layers (CLs) is imperative for wide-scale commercialization. However, a local resistance close to the Pt reaction sites becomes significant at low Pt loadings and hinders adequate performance.1,2 In our previous study, we demonstrated that the local resistance is primarily related to the proton-conducting ionomer thin-film within the CL and consists of: (i) reactant transport resistance due to low permeability of ionomer thin-film (compared to bulk ionomer) and (ii) interfacial resistance due to Pt/sulfonic-moiety interactions.3 Thus, there is need to optimize ionomer design to minimize CL resistance. In this talk, we investigate the impact of ionomer chemistry, content, and equivalent weight (EW) on CL resistance. Using H2 limiting-current measurements, the total CL resistance is segregated into transport and interfacial components. CL resistance increases monotonically with ionomer content, primarily due to thicker ionomer films. Low EW ionomers demonstrate higher interfacial resistance due to high density of sulfonic groups, whereas higher EW ionomers exhibit higher transport resistance due to low-reactant permeability associated with low-water uptake. These findings are rationalized through comparison of properties and structure/function relationships elucidated by ionomer thin-film studies on planar substrates, thus bringing together model and in-situ studies. Further, time-permitting, the impact of carbon support (low versus high surface area carbon) on local CL resistance will also be discussed, mainly via modeling water uptake and proton pathways. Overall, the knowledge gained demonstrate pathways towards ameliorating the local resistance and enabling high performing low Pt-loaded PEFCs. Acknowledgements We would like to thank Sarah A. Berlinger and Robert Darling for helpful discussions. This work was funded under the Fuel Cell Performance and Durability Consortium (FC-PAD) funded by the Energy Efficiency and Renewable Energy, Fuel Cell Technologies Office, of the U. S. Department of Energy under contract number DE-AC02-05CH11231. References: 1. T. A. Greszler, D. Caulk and P. Sinha, J Electrochem Soc, 159, F831 (2012). 2. A. Z. Weber and A. Kusoglu, J Mater Chem A, 2, 17207 (2014). 3. T. Schuler, A. Chowdhury, A. T. S. Freiberg, B. T. Sneed, F. B. Spingler, M. C. Tucker, K. More, C. J. Radke and A. Z. Weber, J Electrochem Soc, 166, F3020 (2019).