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Structure-Property Relationships at Nafion Thin-Film Interfaces: Thickness Effects on Hydration and Anisotropic Ion Transport

Published

Author(s)

Steven C DeCaluwe, Andrew M. Baker, Pavan Bhargava, John E. Fischer, Joseph Dura

Abstract

The effect of film thickness on water uptake and structure in ultra-thin Nafion films is probed via in situ neutron reflectometry for a series of 10 samples with thicknesses ranging from 5–153 nm. Emphasis is given to the lamellae that form at hydrophilic substrate interfaces and their anisotropic effect on ionic conductivity. 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 free surface; (iii) in the thick-film regime (greater than or equal to 60 nm), the bulklike layer thickness exceeds the radius of gyration for thin-film Nafion. The water uptake in the lamellae and the bulk-like layer varies non-monotonically, and can be ordered as: thin-film < truncated < thick-film. In the thin-film regime, the water uptake in the bulklike layer increases with thickness, while in the lamellar region it equals that in the bulklike layer, except for the more hydrated layer adjacent to the SiO2 substrate. In the thickfilm regime, the bulk-like layer water uptake equals that in macroscopic Nafion membranes, and is invariant with film thickness, while the lamella water uptake greatly exceeds this. Composition depth profiles are used to predict the anisotropic ionic conductivities, which are compared to published results to estimate the ion mobility in the sample layers. Results demonstrate that the lamellar structure is required for accurate conductivity predictions, and that ion mobility decreases as a function of proximity to the substrate. Conductivities at elevated temperatures (relevant to operating polymer electrolyte membrane fuel cells) remain accurate, and provide insights for minimizing transport losses in fuel cell catalyst layers.
Citation
Nano Energy
Volume
46

Keywords

Thin film ionomer, Nafion, Lamellae, confinement, PEM fuel cell, neutron reflectometry, ionic conductivity

Citation

DeCaluwe, S. , Baker, A. , Bhargava, P. , Fischer, J. and Dura, J. (2018), Structure-Property Relationships at Nafion Thin-Film Interfaces: Thickness Effects on Hydration and Anisotropic Ion Transport, Nano Energy, [online], https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=923903 (Accessed April 14, 2024)
Created April 2, 2018, Updated October 12, 2021