Dispersion-Solvent Control of Ionomer Aggregation in a Polymer Electrolyte Membrane Fuel Cell

Abstract

In this study, we examined the influence of the dispersion solvent in three dipropylene-glycol/ water (DPG/water) mixtures, with DPG contents of 0, 50, and 100 wt%, on ionomer morphology and distribution, using dynamic light scattering (DLS) and molecular-dynamics (MD) simulation techniques. The DLS results reveal that Nafion-ionomer aggregation increases with decreasing DPG content of the solvent. Increasing the proportion of water in the solvent also led to a gradual decrease in the radius of gyration (R-g) of the Nafion ionomer due to its strong backbone hydrophobicity. Correspondingly, MD simulations predict Nafion-ionomer solvation energies of - 147 +/- 9 kcal/mol in water, - 216 +/- 21 kcal/ mol in the DPG/water mixture, and - 444 +/- 9 kcal/mol in DPG. These results suggest that higher water contents in mixed DPG/water solvents result in increased Nafion-ionomer aggregation and the subsequent deterioration of its uniform dispersion in the solvent. Moreover, radial distribution functions (RDFs) reveal that the (-CF2CF2-) backbones of the Nafion ionomer are primarily enclosed by DPG molecules, whereas the sulfonate groups (SO3 (-)) of its side chains mostly interact with water molecules.