(Energy Technology Division Supramaniam Srinivasan Young Investigator Award Address) New Insights into PFSA Ionomers - from Membranes to Thin Films

Abstract

Ionomers are ion-conducting polymers commonly used as the solid-electrolyte/separator in electrochemical energy-conversion and storage devices, where they provide multiple functionalities such as ion conductivity, gas separation, and solvent transport. In polymer-electrolyte fuel-cell (PEFCs), ionomers play a key role not only as a proton-exchange membrane (PEM), but also as an nanometer-thick electrolyte “thin” films within porous catalyst structures, where they provide transport pathways to the catalytic particles. Throughout this thickness range, intrinsic morphology of the ionomer and its transport properties are controlled by the interplay between the chemical and mechanical properties, controlling its transport and stability. It is this interplay that ultimately affects the performance and durability of these ionomers in electrochemical devices. Ionomers such as perfluorosulfonic-acid (PFSA) are widely-used in many electrochemical-energy devices, including PEFCS, due its high conductivity and chemical-mechanical stability. Yet, for the desired new clean-energy paradigm, PEFCs require further improvements in PFSA’s stability and transport functionalities. In particular, the latter phenomenon could have implications on the mass-transport limitations in catalyst layers (CLs), where the transport resistances at the ionomer thin-film becomes critical. Hence, it is of great interest to elucidate how changes in chemistry and environmental stressors control PFSA behavior across lengthscales, not only as a bulk membrane (i.e., PEM), but also as a catalyst-ionomer film (i.e., thin-film). This talks presents an overview of the structure/function relationship of PFSA ionomers of various chemistries with a focus on correlations between their phase-separated nanomorphology and properties – from micrometer to nanometer lengthscales. First, it will be examined how the transport and stability of these membranes are correlated through the interactions between chemical structure, solvent uptake, and morphology, studied by small- and wide-angle X-ray scattering (SAXS/WAXS) techniques. The role of equivalent weight and side-chain chemistry of PFSAs will also be explored to delineate the key factors controlling a PFSA membrane’s behavior. Our investigations demonstrate the existence of universal correlations between the water-uptake and conductivity (chemical energies), and backbone crystallinity (mechanical energies), with a key role of the side-chain chemistry altering this balance between the chemical-mechanical energies. We will then present results showing how an ionomer's structural features and transport properties deviate from the bulk when it is confined to nanometer-thick films and how these deviations are controlled by the substrate/film interactions, chemistry and environment. The collected data set are analyzed to illustrate the bulk-to-film transition of PFSA ionomers of various equivalent weights used in fuel-cell membranes and CLs. The results will be discussed to develop a holistic view of the ionomers as membranes and thin-films and to provide insight into optimization of their functionalities within the context of PEFC performance.