Makoto Yamaguchi

Dispersion of Rod-like Particles of Nafion in Salt-Free Water/1-Propanol and Water/Ethanol Solutions.

The dispersion of perfluorinated sulfonic acid ionomers in catalyst inks is an important factor controlling the performance of catalyst layers in membrane electrode assemblies of proton exchange membrane fuel cells (PEMFCs). The effect of water/alcohol composition on the dispersion of H-Nafion in water/1-propanol and water/ethanol solutions was studied by dynamic light scattering (DLS), small-angle X-ray scattering (SAXS), and (19)F nuclear magnetic resonance ((19)F NMR) spectroscopy. Hydrodynamic radii calculated from DLS decay profiles and the radii and interparticle distance of rod-like particles derived from SAXS profiles showed almost the same dependence on alcohol concentration. 1-Propanol was more effective than ethanol to induce changes in the characteristic lengths of the rod-like particles. The motional narrowing in the (19)F NMR spectra by addition of 1-propanol indicates selective solvation of the rod-like particles. We suppose this might have decreased their radii and induced their elongation, which eventually led to extension of the ordered regions as observed in the hydrodynamic radii. Our study helps to clarify the dispersion of Nafion in aqueous alcohol solutions, which has implications for the performance of PEMFCs.

Ab Initio Molecular Dynamics Simulation of Infrared Absorption Spectra of H3O+ and H5O2+ in Nonaqueous Solutions of Trifluoromethanesulfonic Acid Hydrates

Infrared absorption spectra of hydrated protons are very sensitive to their different structures that change with the number of water molecules as well as their hydrogen bonds with anions and/or solvent molecules. Hydronium ion (H3O+) in nonaqueous solutions of trifluoromethanesulfonic (triflic) acid monohydrate shows largely red-shifted and very broad OH stretching bands in their infrared absorption spectra. We found that four triflate anions and four hydronium ions form a hydrogen-bonded cluster in which sulfur and oxygen atoms of anions and cations have a cubic structure with red-shifted OH stretching bands; from its ab initio molecular dynamics simulation large widths of the OH stretching absorption bands were successfully reproduced. Based on the very similar infrared absorption bands of perfluorinated sulfonic acid ionomer membranes with low water content their acid sites are supposed to have very similar cluster configurations.