Nuclear magnetic resonance studies of water in perfluorinated ion exchange membranes

Perfluorinated ion-exchange membranes such as NAFION (DuPont) serve as both electrolyte and separators employed in fuel cells. The presence of the water in these membranes is critical to fuel cell operation. Water molecular diffusion and charge transport across the membrane are correlated. Through the availability of water isotopically enriched in deuterium or {dollar}sp {lcub}17{rcub}{dollar}O, nuclear magnetic resonance (NMR) can be employed to study molecular dynamics by utilizing quadrupolar nuclei as probes. In this thesis, Deuteron and oxygen-17 NMR measurements in NAFION-117 membranes with variable water (D{dollar}sb{lcub}2{rcub}{dollar}O or H{dollar}sb{lcub}2{rcub}sp{lcub}17{rcub}{dollar}O) content (3{dollar}sim{dollar}18% by weight) have been carried out. Measurements were taken at variable temperature (room T down to 115 K), high pressure (up to 0.25 GPa), and on stretched samples. One of the main results concerns the observation of anisotropic molecular motion in the membrane plane, with dramatic enhancement of the anisotropy in modestly stretched membranes. Glassy behavior of the water domains at low temperature is evidenced by the specific nature of the {dollar}sp{lcub}2{rcub}{dollar}H NMR line shapes at 109 K. Activation energies extracted from both {dollar}sp{lcub}2{rcub}{dollar}H and {dollar}sp{lcub}17{rcub}{dollar}O spin-lattice relaxation data exhibit a steady increase with increasing water content. Activation volumes extracted from both {dollar}sp{lcub}1{rcub}{dollar}H and {dollar}sp{lcub}2{rcub}{dollar}H T{dollar}sb{lcub}1{rcub}{dollar} pressure dependence show a decrease with increasing water content, at room temperature. Analysis of these observations suggests a water cluster model for water organization in NAFION membranes. The relatively free water and motionally restricted (by interaction with the polymer host) water exchange each other rapidly yielding an averaged response at room T, while the hydrogen bonds become more rigid at low temperature. Deuteron and oxygen-17 NMR studies of NAFION-117 containing either deuterated methanol (CH{dollar}sb{lcub}3{rcub}{dollar}OD) or oxygen-17 enriched methanol (CH{dollar}sb{lcub}3{rcub} sp{lcub}17{rcub}{dollar}OH) demonstrate that the methanol molecular motion in NAFION-117 is considerably faster than for water in NAFION. This supports the conclusion that the methanol transport across the NAFION membrane is probably too high for fuel cell applications. Comparison of the water behavior in NAFION-127 with in NAFION-117 supports the conclusion that proton transport is more effective in NAFION-117.