PFSA‐Ionomer Dispersions to Thin‐Films: Interplay Between Sidechain Chemistry and Dispersion Solvent

Tuning functionality of thin-film ionomers in fuel-cell catalyst layers requires an understanding how ionomer chemistry and dispersion solvent together impact ionomer interactions and properties. Varying the acid-group location on ionomer side-chain affects dispersion aggregation, with a secondary effect of solvent composition. The resulting dispersion-cast thin-film properties are affected primarily by chemistry and modulated by solvent. While side-chain chemistry can coarse-tune structure–property relationships, processing solvent allows for fine-tuning of dispersion-film interplay for controlling ionomer function.

Abstract

In fuel-cell catalyst layers (CLs), ionomers exists as nanoscale thin-films binding catalyst particles, wherein ionic and gaseous species transport to catalyst sites. To improve film function, ionomers have been designed based on the prototypical perfluorosulfonic acid (PFSA). Since CLs are fabricated through solution-based processing of inks, understanding how ink/dispersion solvent impacts ionomer interactions and function is important to guide future ionomer design. Herein, PFSA and its chemical derivatives are characterized from dispersion (in solvent) to thin film (cast on support) to investigate the impact of solvent water content on ionomer dispersion (structure, acidity) and resulting film properties. Dispersion characterization reveals that secondary aggregate structure depends on the sidechain chemistry, but all PFSA-based ionomers evolve similarly as dispersion becomes water-rich. The differences in aggregation alter ionomer adsorption and thin film morphology. Hydrophilic domain spacing and orientation are affected primarily by sidechain chemistry, and modulated by solvent composition. Thin-film conductivity correlates with hydration and nanodomain alignment, signifying the role of morphology in properties. Overall, this work provides insights into how chemistry can be used to coarse-tune structure–property relationships, while processing solvent is for fine-tuning of dispersion-film interplay for controlling CL-ionomer function, which can be translated to other chemistries and ink formulations.