Electrostatically Limited PFSA Ionomer Adsorption to Catalyst Particles in Catalyst Inks

Abstract

Understanding and controlling ionomer adsorption in catalyst inks is critical to optimizing catalyst-layer structure and performance in membrane-electrode assemblies. Here, we systematically quantify perfluorosulfonic-acid (PFSA) ionomer adsorption across a diverse set of catalyst particles, including carbon and Pt/C, functionalized carbons, and iridium oxide (IrOx). We find that the extent of adsorption varies by nearly an order of magnitude among these diverse particles. Using potentiometric titration, the measured particle surface-charge densities are related to adsorption behavior using a kinetic adsorption model. A free-energy barrier arises in the kinetic theory because of repulsive electric double-layer interactions between the anionic ionomer in solution and the solid substrate. We calculate these interactions from DLVO theory adapted to PFSA systems. The energy barrier grows as the ionomer adsorbs to support-particle surfaces and imparts a negative surface charge through the adsorbed sulfonic-acid moieties. Ionomer adsorption self-quenches when the energy barrier is larger than about 25 kBT. We find excellent agreement between theory and experiment, and show that, for a given ionomer concentration, particles adsorb ionomer until reaching a common limiting surface-charge density, independent of particle type. By leveraging this insight, we demonstrate that adsorption can be systematically tuned via surface functionalization that modulates support-particle charge. The findings establish electrostatic interactions as the dominant mechanism limiting the extent of PFSA ionomer adsorption, providing a general framework for the rational design of catalyst inks and improved ionomer distributions in catalyst layers.