(Invited) Structure-Property Relathionships of Proton Conducting Membranes Studied By Synchrotron Radiation

Abstract

The properties of ion conducting polymers (ionomers) used as proton conducting membranes are strongly dictated by their nanostructure. Due to the strong difference in polarity between the acidic sulfonic groups and the apolar main polymer chain, these materials exhibit strong phase separation. X-ray and neutron scattering are among the best experimental techniques to investigate the ionomer nanostructure. In particular, synchrotron based small and wide angle X-ray scattering (SAXS and WAXS) are particularly suited to perform in-situ investigation on ionomers. In this contribution, I will show our most recent results obtained by using synchrotron SAXS and WAXS on the investigation of the structure-property relationships in different ionomers. The structural knowledge acquired on the benchmark Nafion membranes is compared to that of its short-side chain counterpart (Aquivion) and to other non-perfluorosulfonated ionomers like sulfonated poly(ether ether ketone)s and sulfonated polysulfones. The nano morphology of Nafion and Aquivion can be qualitatively and (almost) quantitatively explained by a locally flat morphology, describing the morphology as an alternated assembly of water/ions and polymeric 2D domains. The same morphological model can be applied successfully to other fluorine-free ionomers, suggesting that a local flat morphology could be common to many proton conducting ionomers. In the second part of my talk, I will show some recent in-situ SAXS experiments on microstructural changes occurring in different ionomer membranes under controlled temperature and humidity conditions. The data suggest local re-distribution of water inside the membranes that can lead either to membrane swelling (low temperature and high humidity) or to local ionic structure collapse (high temperature and low humidity). These experiments mimic very closely what may happen in a real fuel cell and the understanding of the results is crucial to improve the membrane performances.