Nafion Adsorption in Proton Exchange Membrane Catalyst Inks and Its Impact on Fuel Cell Performance

Abstract

Electrochemical reactions in proton-exchange-membrane (PEM) fuel cells occur within catalyst layers (CL). The catalyst layer consists of a porous network of metal-activated catalyst particles coated with an ion-conducting polymer binder (ionomer) that enables proton transport. Nafion, a perfluorinated sulfonic-acid polymer, is the common binder used and consists of a hydrophobic backbone with negatively charged sulfonic-acid side chains. Prior studies have shown the Nafion distribution in catalyst layers is nonuniform on both the nano 1 and micrometer-scale. 2 This inhomogeneity is thought to impede catalyst-layer performance by limiting proton transport in Nafion-deficient regions and limiting oxygen transport in Nafion-enriched regions. The Nafion distribution in the catalyst layer is governed by the ink from which it is deposited. The catalyst ink is a colloidal slurry consisting of catalyst-activated particles and Nafion typically dispersed in a water-alcohol solvent mixture. Within this ink, some Nafion adsorbs to the surface of catalyst particles, while the remainder is dispersed in solution. Prior studies hypothesize that adsorption in the ink facilitates a more uniform Nafion distribution in the final catalyst layer, thereby improving transport in the catalyst layer. 3,4 Here, we characterize Nafion adsorption in catalyst inks by sedimenting catalyst particles with adsorbed Nafion from inks. The adsorbed Nafion content of the sediment is measured using thermogravimetric analysis. Nafion adsorption to catalyst particles is irreversible and is limited by electrostatic repulsion of the negatively charged Nafion sidechains. We demonstrate that one can control the extent of adsorption by adding mineral acid to the ink to increase the ionic strength and decrease electrostatic repulsion. Through adding acid, the extent of Nafion adsorption to catalyst particles increases by more than a factor of 2 as shown in Figure 1 below. Next, the effect of Nafion adsorption in the ink is correlated to final catalyst-layer performance and properties via cell diagnostics including sheet resistance, polarization performance, and limiting-current measurements. Overall, the talk will elucidate the governing interactions that control Nafion adsorption and how this adsorption affects performance. Acknowledgements This work was funded under the Million Mile Fuel Cell Truck Consortium. References [1] Girod, R., Lazaridis, T., Gasteiger, H. A., & Tileli, V. (2023). Three-dimensional nanoimaging of fuel cell catalyst layers. Nature Catalysis , 6 (5), 383–391. https://doi.org/10.1038/s41929-023-00947-y [2] Orfanidi, A., Rheinländer, P. J., Schulte, N., & Gasteiger, H. A. (2018). Ink Solvent Dependence of the Ionomer Distribution in the Catalyst Layer of a PEMFC. Journal of The Electrochemical Society , 165 (14), F1254–F1263. https://doi.org/10.1149/2.1251814jes [3] Ott, S., Orfanidi, A., Schmies, H., Anke, B., Nong, H. N., Hübner, J., Gernert, U., Gliech, M., Lerch, M., & Strasser, P. (2020). Ionomer distribution control in porous carbon-supported catalyst layers for high-power and low Pt-loaded proton exchange membrane fuel cells. Nature Materials , 19 (1), 77–85. https://doi.org/10.1038/s41563-019-0487-0 [4] Islam, M. N., Mansoor Basha, A. B., Kollath, V. O., Soleymani, A. P., Jankovic, J., & Karan, K. (2022). Designing fuel cell catalyst support for superior catalytic activity and low mass-transport resistance. Nature Communications , 13 (1), 1–11. https://doi.org/10.1038/s41467-022-33892-8 Figure 1

Key findings

• ▸Nafion distribution in catalyst layers is nonuniform on both nano and micrometer-scale.
• ▸Nafion adsorbs to the surface of catalyst particles and is dispersed in solution within the catalyst ink.
• ▸Inhomogeneity in Nafion distribution impedes catalyst-layer performance by limiting proton and oxygen transport.

Keywords

Identifiers

DOI

10.1149/ma2024-02443032mtgabs

Journal

ECS Meeting Abstracts

Year

2024