(439e) Exploring Solvent/PFSA Interactions in Solution

Solution-processing approaches to manufacture electrodes are utilized in a variety of electrochemical device technologies, from batteries to fuel cells. The electrode inks are colloidal dispersions that are typically composed of catalyst particles, conductive support, (functional) polymer binder, and solvent, and which form agglomerates in solution of adsorbed polymer/particle complexes. The structure of these agglomerates dictates final electrode morphology and performance. Therefore, in order to better control and engineer these inks, solution-phase behavior of all components must be understood, including synergistic multi-component interactions.

In hydrogen fuel cells and electrolyzers, these inks typically contain perfluorosulfonic acid (PFSA) as the polymer. PFSA is a charged polymer consisting of a hydrophobic polytetrafluoroethylene backbone and pendant side-chains that terminate in sulfonic-acid groups. The solvent/polymer interaction in these dispersions is complex: different solvents cause PFSA to adopt dramatically different conformations in solution.¹ Properties of the dispersion (e.g., pH) also change with solvent composition, altering the electrostatic interaction between PFSA and the conductive support/catalyst particles.²

In this presentation, we focus on experimentally exploring the intricacies of the solvent/PFSA interaction in order to lend insight into the aggregation behavior of the composite ink. Specifically, we investigate the influence of temperature and age of the dispersion on properties such as pH, conductivity, viscosity, and adsorption behavior to particles. Using a combination of electrochemical impedance spectroscopy, rheology, and quartz crystal microbalance techniques, we find these properties are strongly affected by age and temperature. Furthermore, the solvent type as well as the PFSA charge density are critical parameters which modulate the solution interaction.

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.

References

1. Welch, C.; Labouriau, A.; Hjelm, R.; Orler, B.; Johnston, C.; Kim, Y. S., Nafion in Dilute Solvent Systems: Dispersion or Solution? ACS Macro. Lett. 2012, 1 (12), 1403-1407.
2. Berlinger, S. A.; McCloskey, B. D.; Weber, A. Z., Inherent Acidity of Perfluorosulfonic Acid Ionomer Dispersions and Implications for Ink Aggregation. J. Phys. Chem. B. 2018, 122 (31), 7790-7796.