Structure-property relationships at Nafion thin-film interfaces: Thickness effects on hydration and anisotropic ion transport

The effect of film thickness on water uptake and structure in ultra-thin Nafion is probed via in situ neutron reflectometry for a series of 10 films with thicknesses ranging from 5 to 153 nm. Observed interfacial lamellae are used to understand anomalous transport limitations in such films. Results show three distinct thickness regimes: (i) in the truncated regime (< 12 nm), the entire film consists of lamellae; ( ii) in the thin-film regime (12-42 nm), a non-lamellar bulk-like layer forms between the lamellae and vapor; (iii) in the thick-film regime (>= 60 nm), the bulk-like layer thickness exceeds the radius of gyration for thin-film Nafion. The water uptake in the sample varies non-monotonically with thickness, and can be ordered as: thin-film < truncated < thick-film. In the thin-film regime, the water uptake in the bulk-like layer and lamellae are equal, and both increase with thickness, except for a well-hydrated layer adjacent to the SiO2 substrate. In the thick-film regime, the bulk-like layer water uptake equals that in macroscopic Nafion membranes, and is invariant with film thickness, while the lamellar water uptake greatly exceeds this. Composition depth profiles are used to predict the anisotropic ionic conductivities. These are fitted to previously published experimental results to demonstrate that the lamellar structure is required for accurate conductivity predictions. These results provide key insights for minimizing transport losses in fuel cell catalyst layers.