(534e) QCM-D Study of the Solvent-Polymer-Catalyst Particle Interface in Fuel-Cell Inks

Quartz-crystal microbalance with dissipation (QCM-D) is a widely-applicable tool capable of measuring mass uptake in liquid media. The Sauerbrey equation is routinely used to analyze QCM-D data. In many instances, however, the assumptions of the Sauerbrey equation (namely that there exists a thin, rigid, homogeneous adsorbed layer) are not met. This is particularly the case when studying polymer adsorption to a surface, primarily because mass change is due to both adsorbed polymer and trapped water. Furthermore, the dissipation signal is often affected by hydrodynamic complications as a result inhomogeneous film formation.¹ Therefore, deconvolution of the data to isolate the contribution of adsorbed polymer is nontrivial. Herein, we explain the complexities of analyzing polymer adsorption data obtained with QCM-D by studying polymer adsorption to model surfaces, with application to fuel-cell inks.

Fuel-cell electrodes are made from inks that are mixtures of polymer and catalyst nanoparticles dispersed in a mixed solvent (typically water and alcohols). The polymer is a perfluorinated sulfonic acid polymer (PFSA), which has a Teflon backbone with sidechains that terminate in sulfonic-acid moieties. The duality between the strongly acidic sidechains and hydrophobic backbone of the polymer cause it to adopt unique conformations in solution, which are a strong function of the amount of water present in the ink.^2-3 Understanding PFSA adsorption to the catalyst particle surface is critical for understanding ink (and electrode) structure and performance. In this study, we explore the polymer/particle interaction using QCM-D with model functionalized surfaces that can be related to the more complex catalyst-particle system. PFSAs of different ion-exchange capacities in a variety of solvents are investigated to understand how polymer adsorption and desorption behavior is affected by solvent quality. Results reveal that hydrophobic interactions are a major driving force for PFSA adsorption.

Acknowledgements

This study was mainly 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. S.B. also acknowledges support from the National Science Foundation Graduate Research Fellowship under grant number DGE 1752814.

References

1. Reviakine, I.; Johannsmann, D.; Richter, R. P., Analytical Chemistry 2011, 83 (23), 8838-8848.
2. Welch, C.; Labouriau, A.; Hjelm, R.; Orler, B.; Johnston, C.; Kim, Y. S., ACS Macro. Lett. 2012, 1 (12), 1403-1407.
3. Berlinger, S. A.; McCloskey, B. D.; Weber, A. Z., J. Phys. Chem. B. 2018.