Impact of Equivalent Weight and Side-Chain on Structure/Functionality of PFSA Ionomers and Thin Films

Abstract

Perfluorosulfonic-acid (PFSA) ionomers play key roles in polymer-electrolyte fuel-cell (PEFCs), primarily as a proton-exchange membrane (PEM), and also as an electrolyte film in porous catalyst structures binding the catalytic agglomerates.1 Despite their wide adoption in PEFCs, most studies on PFSAs have thus far focused on Nafion, and studies on other PFSAs, in particular with varying side-chain chemistries and equivalent weights (EWs), have been relatively scarce. Therefore, it is of interest to elucidate how such changes in chemistry and EW influence the PFSA behavior, not only as a bulk membrane (i.e., PEM), but also as a catalyst-ionomer film (i.e., thin-film). In particular, the latter phenomenon could have important implications on explaining the mass-transport limitations in catalyst layers, which have been recently shown to be strongly related to the transport resistances at the ionomer thin-film.1,2 In this talk, the impact of EW and side-chain chemistry on structure/property relationship of various PFSAs will be explored as bulk membranes (> 20 μm) and dispersion-cast thin films (< 100 nm). PFSAs owe their combination of good transport properties and mechanical stability to their phase-separated morphology of conductive hydrophilic ionic domains and the hydrophobic backbone, which are connected via side-chains. It will be shown how this PFSA morphology is controlled by the fraction of backbone (via EW) and the side-chain length (via chemistry), which together govern the interrelation between transport and stability. Moreover, 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 length in altering this balance between the chemical-mechanical energies. We will report the results of a systematic investigation on hydration, conductivity, mechanical properties and crystallinity of various types and EWs of PFSA ionomers to develop universal structure/function relationships. Lastly, it will be explored how these EW and side-chain effects influence the structure and properties of PFSA thin films when confined to nanometer thicknesses, and control their deviation from bulk membrane behavior. Our results provide new insights into the understanding and optimizing the PFSA ionomers' functionalities in fuel-cells, both as bulk membranes and as thin-films.