(Poster Award - Honorable Mention) Probing the Catalyst-Ionomer Interface in Catalyst Layers with Sulfo-Phenylated Polyphenylene Ionomers Using CO-Displacement and Stripping Techniques

Abstract

Sulfo-phenylated polyphenylene ionomers (sPPP) are a class of hydrocarbon Ionomer being explored as a replacement for the perfluorinated sulfonic acid (PFSA) ionomer in fuel cell membranes and catalyst layers because they do not contain fluorinated compounds. Integration of sPPP ionomers into catalyst layers is challenging, as they have significantly different morphology compared to PFSA-based ionomers. Furthermore, little is known about the resulting ionomer-catalyst interface and whether characterization techniques developed for catalysts layers with PFSA-based ionomers will perform well for catalyst layers containing an sPPP ionomer. In this work, we examine the usefulness of CO displacement and stripping techniques to probe both the electrochemically active surface area (ECSA) of Pt/C in the catalyst layer and adsorbed species on the Pt catalyst sites. For catalyst layers containing PFSA, we measure a strong CO displacement reduction peak at potentials above potential of zero charge, when CO is introduced to the system, corresponding to sulfonic acid groups being displaced from active sites. The range of sulfonic acid groups adsorbed on the surface is 15- 25 % depending on the carbon support used. When the catalyst layers contain an sPPP ionomer instead of PFSA, the CO displacement peak is almost entirely suppressed, resulting in less than 10 % of sulfonic acid group adsorption. This is indicative that sPPP ionomer orientation near the Pt surface is very different compared to PFSA-based ionomer and that sulfonic acid groups do not “poison” Pt when in contact with the sPPP ionomer. However, mass activity increase is not observed for the sPPP ionomer-based catalyst layers, indicating there might be different “poisoning” mechanisms present, perhaps associated with the phenyl rings adsorption [1]. ECSA measurements for MEAs containing the non-fluorinated ionomer catalyst layers align with those measured for PFSA MEAs with the same catalysts type, loading, and comparable ionomer content, indicating that CO stripping is still a useful tool for assessing the ECSA for MEAs.[1] Zhai, Y., Baturina, O., Ramaker, D., Farquhar, E., St-Pierre, J., & Swider-Lyons, K. (2015). Chlorobenzene Poisoning and Recovery of Platinum-Based Cathodes in Proton Exchange Membrane Fuel Cells. Journal of Physical Chemistry C, 119(35), 20328–20338. https://doi.org/10.1021/acs.jpcc.5b06362