Exploring the Dichotomy of Ionomer and Membrane Properties

Abstract

Ion-conductive polymers are critical components of polymer-electrolyte fuel cells, where they their primary role is to facilitate selective proton transport as the polymer electrolyte membrane (PEM). Another important role they play is in the catalyst-layer of the PEFC, where they extend their proton-transport functionality to the catalytic sites, where they are found as nanometer-thick electrolyte “thin film” covering carbon and platinum nanoparticles. Although ionomers are responsible for efficient proton conductivity as both PEM and CL electrolyte, their functionality differs. While a PEM ionomer is expected to inhibit gas crossover, a CL ionomer should be able to transport these gases, in particular oxygen, to the catalytic sites as well. Any limitation to the oxygen transport through the ionomer film to the Pt sites results in mass-transport losses with dramatic decrease in cell performance. Thus, improving fuel-cell performance requires ionomers tuned specifically for each component, i.e. PEM and CL, with distinctly different gas transport functionalities. Such a modification presents a challenge for the same ionomer, whose transport properties are governed by its physicochemical properties and morphology. Thus, for the desired next-generation fuel cells, a new paradigm is needed for the design and optimization of ionomers, with alternative chemistries beyond Nafion, the prototypical fuel-cell ionomer. In PEFCs, Nafion, a perfluorosulfonic acid (PFSA) ionomer, is the most widely used material and research on other PFSAs, in particular with varying side-chain chemistries and equivalent weights (EWs), have been relatively scarce. Thus, by exploring the effect of chemistry and EW on ionomer’s structure/functionality, one could work toward identifying key factors controlling transport properties, and how they can be tuned for a bulk membrane (i.e., PEM) and a catalyst thin film. This talk presents a comparison of various side-chain chemistry effects on structure/function relationships of PFSA ionomers from bulk membranes to thin films confined to nanometer thickness. The interplay between the side-chain chemistry and film thickness will be examined for several PFSAs, including multi-acid chain (MASC) ionomers to elucidate the factors controlling their swelling behavior and various transport properties. In addition, these properties will be correlated to the morphological features studied by small- and wide-angle X-ray scattering (SAXS/WAXS) for membranes and grazing-incidence SAXS (GISAXS) for thin films. It will be shown how an ionomer's nanostructure and transport properties deviate from the bulk when it is confined to nm-thick films and how these deviations are impacted by the side-chain. The results will be used to highlight design guidelines for ionomers as fuel-cell membranes and catalyst films, and then correlated with the fuel-cell performance to provide insight into optimization of their functionalities for PEFCs and other energy storage and conversion technologies. Acknowledgement We thank Mike Yandrasits and Andrew Haug of 3M for providing the ionomers and helpful discussions. This work was funded under the Fuel Cell Performance and Durability Consortium (FC-PAD), by the Fuel Cell Technologies Office (FCTO), Office of Energy Efficiency and Renewable Energy (EERE), of the U.S. Department of Energy under contract number DE-AC02-05CH11231. X-ray experiments were performed in the beamline 7.3.3 at Advanced Light Source (ALS), Lawrence Berkeley National Laboratory, which is a national user facility funded by the Department of Energy, Office of Basic Energy Sciences, under contract number DE-AC02-05CH11231.