Ionomer Thin Films for PEM Fuel Cells

Abstract

Ion-conductive polymers (ionomers) such as perfluorosulfonic-acid (PFSA) have been widely adopted in polymer electrolyte fuel cells (PEFCs) as a polymer-electrolyte membrane (PEM) due to their ability to conduct ions in hydrated conditions without losing stability. The role of ionomers in PEFCs, however, is not limited to PEM as it also extends to the porous catalyst layers (CL) where ionomers exist as nanometer-thick electrolyte thin films binding the catalytic agglomerates and facilitating proton transport. Moreover, the ionomer’s properties differ from the bulk (PEM) when it is confined to these nanometer thicknesses where its behavior is influenced by its interfaces with the air and the catalytic sites, such as carbon/platinum. In particular, this phenomenon of confinement onto Pt sites could have important implications for the mass-transport limitations observed in fuel-cell catalyst layers, which have been related to the transport resistances at the ionomer thin-film. Thus, despite an ever-growing number of studies on ionomer membranes for PEM, for desired next-generation PEFCs, it is important to characterize catalyst ionomers and elucidate factors controlling their transport properties. In particular, if is of great interest to develop model systems for thin films that can mimic catalyst ionomer behavior and establish their structure/functionality to optimize ionomer interfaces and improve CL performance. This tutorial talk will present an overview of PFSA ionomer thin films and their current state of understanding with a focus on recent developments in advanced diagnostics and the information they can provide to establish their structure/function relationship. In addition, applicability of the various diagnostics techniques in the thin-film regime (5 to 500 nm) will be discussed, including the use of state-of-the-art techniques, such as Grazing-Incidence X-ray scattering (GIXS). First, thin films of dispersion-cast ionomers on model substrates will be explained in relation to catalyst ionomers. Swelling and water uptake behavior as well as various transport properties of these thin films will be examined. Then, it will be discussed how an ionomer’s phase-separated nanostructure and properties are correlated and impacted by film thickness (i.e., confinement effect) and substrate interactions, especially below 50 nm. Lastly, it will be explored how EW and side-chain chemistry influence the nature of ionomer-substrate interactions and thus the structure and properties of PFSA thin films. The collected data set will be analyzed to illustrate the bulk-to-film transition of ionomers of various equivalent weights used in fuel-cells. The results will be elucidated to highlight the key aspects of ionomer thin films, provide insight into optimization of their structure and functionalities for fuel-cell catalysts, and discuss implications for PEFC performance.