(Invited) Ionomer Membrane and Thin-Film Diagnostics

Abstract

Ionomers are used as the solid-electrolyte in polymer-electrolyte fuel-cells (PEFCs) where they provide multiple functionalities such as ion conductivity, gas separation, and water transport in a mechanically robust polymer matrix. In addition, these ionomers are found as nanometer-thick electrolyte thin films within fuel-cell catalyst layers (CLs) wherein electrochemical reactions occur.1 The performance and durability of the ionomers in PEFCs are related to their properties governed by the physiochemical interactions between chemical structure, water uptake, and morphology, all of which change with the environment. Moreover, the ionomer behaves differently from the bulk (polymer-electrolyte) when it is confined to nanometer thicknesses (as in CL),2,3 where its behavior seems to be strongly influenced by its interfaces with the air and the catalytic sites, such as carbon and platinum.1,2 Thus, it is of great interest to measure in order to understand and optimize transport functionalities and mechanical stability across the lengthscales, i.e. from bulk to thin film. In this talk, the structure/function relationship of perfluorosulfonic acid (PFSA) ionomers will be examined from micrometer to nanometer lengthscales, with a focus on the related advanced diagnostics and the information they can provide to enable correlations between an ionomer’s phase-separated nanostructure and its transport and mechanical properties. The morphology of PFSA ionomers of different equivalent weights and side-chain chemistries will be discussed based on small- and wide-angle X-ray scattering (SAXS/WAXS) experiments. We will then present an overview of the membrane's hydration, mechanical and transport properties in response to humidity, time, temperature, and thermal history. Such measurements utilize various traditional diagnostics like impedance spectroscopy and diffusion cells, as well as advanced diagnostic methods, including time-resolved SAXS, computed x-ray tomography, and conductive AFM. In addition, applicability of the diagnostics to the thin-film regime (4 to 400 nm) will be discussed, including the use of state-of-the-art in-situ measurement techniques, such as Grazing-Incidence SAXS (GISAXS). We will report results showing how an ionomer's structure and swelling properties deviate from the bulk when it is thinner than 50 nm due to confinement2,3 and how these deviations are controlled by the substrate/film interactions.2 The collected data set will be analyzed to illustrate the bulk-to-film transition of ionomers of various equivalent weights used in fuel-cell membranes and CLs.