The Effect of pH on Fuel-Cell Catalyst-Layer Inks

Abstract

Fuel cell catalyst layers are cast from inks containing a solvent, an ionomer, and catalyst particles. Though there are generally accepted material classes (water/alcohol mixtures for the solvent, perfluorosulfonic acid (PFSA) polymers, and Pt-alloy based catalyst particles supported on carbon), there are no standard ratios or metrics for ink formulation. As such, ink recipes have been highly empirical to date, with little understanding of the governing interactions controlling resultant properties and behavior. This study therefore aims to systematically investigate the effect of pH on ink aggregation and rheology. Solvent choice greatly influences the morphology of PFSAs in solution1-4, which in turn alters pH. Starting with a solvent-ionomer compositional matrix, solvent versus pH effects are decoupled. Using dynamic and electrophoretic light scattering, rheology, and direct visualization with cryo electron microscopy, particle-solvent-ionomer systems are then studied to understand aggregation, stability, and rheology under various pH conditions. AcknowledgementsThis 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.A.B also acknowledges support from National Science Foundation Graduate Research Fellowship under Grant No. DGE 1106400. References Welch, C.; Labouriau, A.; Hjelm, R.; Orler, B.; Johnston, C.; Kim, Y. S., Nafion in Dilute Solvent Systems: Dispersion or Solution? ACS Macro. Lett. 2012, 1 (12), 1403-1407.Ma, C.-H.; Yu, T. L.; Lin, H.-L.; Huang, Y.-T.; Chen, Y.-L.; Jeng, U. S.; Lai, Y.-H.; Sun, Y.-S., Morphology and properties of Nafion membranes prepared by solution casting. Polymer 2009, 50 (7), 1764-1777.Lee, S.-J.; Yu, T. L.; Lin, H.-L.; Liu, W.-H.; Lai, C.-L., Solution properties of nafion in methanol/water mixture solvent. Polymer 2004, 45 (8), 2853-2862.Gebel, G.; Loppinet, B., Colloidal structure of ionomer solutions in polar solvents. J. Mol. Struct. 1996, 383 (1), 43-49.