ESR and ENDOR of Cu 2+ in Nafion solutions and in reconstituted membranes

Abstract

Electron spin resonance (ESR) and electron nuclear double resonance (ENDOR) spectroscopies have been applied to the study of the local environment of paramagnetic counterions in solutions of Nafion perfluorinated membranes in various solvents, and in membranes reconstituted from solutions by evaporation of the solvents at different temperatures. Improved resolution in the ESR and ENDOR spectra was possible by using selectively deuterated solvents and Cu 2+ ions enriched with 63Cu. In contrast to the preferential salvation of the counterions by water in membranes swollen by a water—alcohol mixed solvent, the counterions in Nafion dissolved in the mixed solvent are preferentially solvated by the alcohol. This conclusion is based on the analysis of the solvent-specific ESR parameters and is strengthened by the detection of splittings due to the methanol protons in the ENDOR experiments. The ESR parameters measured in Naflon solutions containing N-methylformamide (NMF) can be explained by assuming 14N ligands around the counterion. These results also suggest that NMF is retained in the reconstituted films obtained by drying ionomer solutions at or below 373 K; drying at 493 K, however, restores the original membrane. The effectiveness of the mixed solvent (water—ethanol) for solubilization of the Naflon membranes appears to be due to preferential salvation of the sulfonic groups by water, and of the perfluorinated chains by methanol or ethanol.