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

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 H(2)O 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.

Figure 1

Repeat structure of Nafion. n can vary between 5 and 14 but for 1100 EW Nafion, n = 7. m is typically on the order of 1000.

Figure 2

a. Pump-probe spectrum of λ = 3 Nafion at 0.2 ps. b. Population dynamics (vibrational lifetimes) at four different frequencies in the 0-1 corresponding to the frequencies labeled in a. The solid lines are biexponential fits to the data at each frequency with the time constants fixed at τ₁ = 3.2 ps and τ₂ = 8.6 ps. Only the relative amplitude of the two components is varied.

Figure 3

The fraction of the total amplitude in each component of the vibrational population decay at frequencies in the 0-1 region of λ = 3 Nafion.

Figure 4

Anisotropy decays at the peak of the absorption spectrum for λ = 1 (circles), 3 (squares), 5 (triangles), and 7.5 (diamonds). The anisotropy decay of bulk water (hexagons) is shown for comparison. Solid lines are fits to the data. All Nafion samples were fit using biexponential decays. Water was fit using a single exponential decay.

Figure 5

Anisotropy decays for λ = 3 at eight frequencies ranging from 2568 cm^-1 to 2630 cm^-1. Within experimental error there is no frequency dependence.