(585g) Interactions Governing Aggregation in PEM Fuel Cell Catalyst Inks

Electrochemical reactions in proton-exchange membrane fuel cells occur within catalyst layers. This layer consists of a porous network of metallic catalyst particles coated with an ion-conducting polymer binder (ionomer) that enables proton transport. The catalyst layer is formed by drying a colloidal slurry, called a catalyst ink, consisting of catalyst particles and ionomer typically dispersed in a water-alcohol solvent mixture. Interactions between components of this ink dictate the final catalyst layer structure.

Perfluorinated sulfonic acid polymer (PFSA) is the common ionomer and consists of a hydrophobic PTFE backbone with sulfonic-acid sidechains. In the ink, PFSA is dispersed in the solvent with a structure depending on the solvent composition: higher water contents cause PFSA to form a cylindrical structure with more charged sidechains extended into solution.¹ A portion of the PFSA adsorbs to the surface of catalyst particles driven by an increase in entropy possibly due to hydrophobic backbone rearrangement on the particle surface.² Adsorbed ionomer changes the surface properties of catalyst particles.

Figure 1a shows that as more ionomer is added to an ink, the zeta potential of catalyst particles becomes more negative due to the negatively charged sidechain groups of the ionomer. This is consistent with prior studies showing that adding even a small amount of ionomer to an ink significantly increases the ink stability.³ In Figure 1a, as the ionomer content increases past 0.1 g ionomer/g particle, roughly the maximum adsorption capacity of catalyst particles in the isotherm shown in Figure 1b, the zeta potentials remain constant. The changes in particle-particle interactions due to ionomer adsorption are expected to influence the aggregation behavior and consequently the final catalyst layer structure.

The connection between ionomer structure in solution, and ionomer adsorption behavior has not yet been thoroughly explored. Further, the role of adsorbed ionomer on particle aggregation has not been addressed. We elucidate these topics by investigating the role of particle functionalization, ionomer chemistry and solvent composition on adsorption and aggregation. We characterize ionomer adsorption to catalyst particles through isothermal titration calorimetry, quartz crystal microbalance, zeta potential and pH measurements and ink aggregation through particle size and rheology. Through our work we better quantify ionomer, particle, solvent interactions and develop physical insight into how these interactions govern adsorption and aggregation in catalyst inks. We thank the Million Mile Fuel Cell Truck Consortium for funding our work.

References:

1. 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 122, 7790–7796 (2018).
2. Berlinger, S. A., McCloskey, B. D. & Weber, A. Z. Probing Ionomer Interactions with Electrocatalyst Particles in Solution. ACS Energy Lett. 6, 2275–2282 (2021).
3. Shukla, S., Bhattacharjee, S., Weber, A. Z. & Secanell, M. Experimental and Theoretical Analysis of Ink Dispersion Stability for Polymer Electrolyte Fuel Cell Applications. J. Electrochem. Soc. 164, F600–F609 (2017).