Water Dynamics in Nafion Fuel Cell Membranes: The Effects of Confinement and Structural Changes on the Hydrogen Bond Network

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Abstract
The complex environments experienced by water molecules in the hydrophilic channels of Nafion membranes are studied by ultrafast infrared pump−probe spectroscopy. A wavelength-dependent study of the vibrational lifetime of the O−D stretch of dilute HOD in H2O confined in Nafion membranes provides evidence of two distinct ensembles of water molecules. While only two ensembles are present at each level of membrane hydration studied, the characteristics of the two ensembles change as the water content of the membrane changes. Time-dependent anisotropy measurements show that the orientational motions of water molecules in Nafion membranes are significantly slower than in bulk water and that lower hydration levels result in slower orientational relaxation. Initial wavelength-dependent results for the anisotropy show no clear variation in the time scale for orientational motion across a broad range of frequencies. The anisotropy decay is analyzed using a model based on restricted orientational diffusion within a hydrogen bond configuration followed by total reorientation through jump diffusion.