Characterization of Ionomer Dispersions Towards Improved Catalyst Ink Design for Proton Exchange Membrane Fuel Cells

A key indicator of coated catalyst membrane (CCM) and membrane electrode assembly (MEA) performance in proton exchange membrane fuel cell systems is the formation of the catalyst-ionomer interface. 1,2 Ionomer coverage and thickness dictate the ion transport to the catalyst material (i.e., platinum, Pt). A thick layer of ionomer can block pores and slow oxygen permeability, resulting in lower electrochemically active surface area (ECSA) and activity whereas a thin layer of ionomer can result in a poor ionic network and slow transport of protons. 3 Further, inconsistencies in the layer can result in inconsistent ion transport throughout the catalyst layer. The conformation of ionomer can influence the formation of the catalyst-ionomer interface and thus the distribution of ionomer within the catalyst layer. 4 Due to both hydrophilic and hydrophobic regions present in ionomer, it is known to change conformation depending on the solvent system used; this can result in rod-like or spherical aggregates of ionomer particles. 5 Water content of the solvent system and pH have been found to correlate to increasing ionomer size and exposure of accessible protons respectively. 6,7 Ionomer conformation has previously been determined through both small-angle X-Ray scattering (SAXS) and small-angle neutron scattering (SANS) which has been used to study the rod-like and spherical shapes of the particles depending on the solvent system used. 8,9 While SAXS and SANS are useful techniques to assess ionomer conformation, further understanding of the size and accessibility of protons is important to elucidate further details on the ionomer-catalyst interface, and thus the effect of ionomer conformation on ionic conductivity and functionality (e.g., coatability, performance) of the catalyst layer. In this work, ionomer dispersions are investigated in both aqueous and mixed alcoholic solvent systems to gain an understanding of the size, conformation and viscosity through dynamic light scattering (DLS), SAXS and rheology. The use of DLS has previously been demonstrated for use of ionomer particles. 10 However, DLS size distribution models are built for spherical, monodispersed nanoparticles and are non-ideal for polymeric materials. 11 This work investigates the challenges, limitations and methods for consistent, reproducible measurements to evaluate size and shape of ionomer dispersions, with supporting techniques of SAXS and rheology. Continued work will investigate the relationship between the morphology of ionomer dispersions and the formation of the ionomer-catalyst interface and improved electrochemical performance. Acknowledgements This work was supported by Mitacs through the Mitacs Elevate program and performed in collaboration with Unilia (Canada) Fuel Cells. References 1) Li, C. et al. Energy Environ. Sci. , 2023 , 16 , 2977 2) Choi, WJ. et al. Korean J. Chem. Eng. 2024 . 3) Woo, S. et al . Curr. Opin. Electrochem. 2020 , 21 , 289-296 4) Yang, D. et al. Int. J. Hydron. Ener. 2021 , 46 , 66, 33300-33313 5) Tarokh, A. et al. Macromolecules , 2020 , 53 , 1 6) Berlinger, S. A. et al. J. Phys. Chem. B , 2018 , 122 , 31 7) Srivastav, H. et al. Langmuir , 2024 , 40 , 13, 8) Bird, A. et al. Adv. Energy Mater. 2025 , 2404242 9) Song, J. et al. Giant , 2024 , 20 , 100332 10) Orfanidi, A. et al. J. Electrochem. Soc. 2018 , 165 , F1254 11) Fischer, K.; Schmidt, M. Biomaterials , 2016 , 98 , 79-91

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