CLs are made from precursor inks that are colloidal dispersions of a catalyst (typically platinum on carbon), an ion-conducting polymer (traditionally perfluorinated sulfonic acid or PFSA, the most widely used of which is Nafion), and a solvent traditionally composed of different low-weight alcohols and water. Ink properties directly impact the resultant morphology, transport, and electrochemical properties of the CL. While there has been much work on ink composition, the research to date has been almost entirely empirical, resulting in limited understanding of fundamental species interactions in the ink and their effect on the resultant CL structure. Furthermore, though the structure of nafion in solution has been extensively studied,[2-4] there has been little work on applying this information to decribe interactions with other ink components.
In this study, we create a series of inks composed of commerical PFSA dispersions of different equivalent weights and side chains, a variety of solvents, and model compound particles. Effects of solvent composition, ionomer loading, and processing conditions are investigated on both the solution phase and the resulting cast solid layers including ionomer thin films and micrometer layers with and without solid particles. Using microscopy, rheology, and dynamic light scattering, we explore the aggregation behavior and fundamental forces governing the behavior of these ionomer/particle, particle/solvent, and solvent/ionomer interactions. Using these insights, design metrics and governing design rules are elucidated, which can help in fabrication of next-generation CLs.
This work was mainly funded under the Fuel Cell Performance and Durability Consortium (FC PAD) funded by the Energy Efficiency and Renewable Energy, Fuel Cell Technologies Office, of the U. S. Department of Energy under contract number DE-AC02-05CH11231. S.B also acknowledges support from National Science Foundation Graduate Research Fellowship under Grant No. DGE 1106400.
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2. Lee, S.-J., et al., Solution properties of nafion in methanol/water mixture solvent. Polymer, 2004. 45(8): p. 2853-2862.
3. Welch, C., et al., Nafion in Dilute Solvent Systems: Dispersion or Solution? ACS Macro Letters, 2012. 1(12): p. 1403-1407.
4. Kim, Y.S., et al., Origin of Toughness in Dispersion-Cast Nafion Membranes. Macromolecules, 2015. 48(7): p. 2161-2172.