Impact of Titanium Dioxide on Water Uptake and Diffusion in Nafion

Inorganic fillers are commonly integrated into ion-exchange membranes to alter water hydration, improve ion conductivity, and tune transport properties. Here, we measure water uptake and diffusion in Nafion, which contains various concentrations of TiO₂ (0-5 wt %). With adsorption-desorption isotherms measured by dynamic vapor sorption, we probe the moisture kinetics and find a maximum change in hydrated Nafion mass by water vapor adsorption at 2 wt % TiO₂. Pulsed field gradient nuclear magnetic resonance (NMR) reveals that with increasing TiO₂ concentration, the two measured components of water diffusion are also maximized at 2 wt % TiO₂. The first component had a diffusion coefficient, on the order of 1 × 10^-9 m²/s, which approaches the diffusion of free water. Observing root-mean-square displacement as a function of increasing observation time indicates that this water population is affected by the ionomer-TiO₂ morphology. The second component had a diffusion coefficient on the order of 1× 10^-11 m²/s, which is attributed to water molecules that interact more strongly with sulfonic acid groups. With solid-state proton NMR relaxometry under low (near 0%) and high (near 100%) relative humidity, we measure T₁ and T₂ relaxation of water molecules on different time scales. Hydrated Nafion+2 wt % titania offers the highest water relaxation times with T₁ = 413 ms and T₂ = 4.77 ms. This high T₁/T₂ ratio further indicates a two-component water environment and semirestricted water diffusion.