Morphological Effects on Ionic Conductivity in Solid Polymer Nanocomposite Electrolytes

Perfluorosulfonic acid (PFSA) polymers, such as the benchmark Nafion®, are consistently used as proton exchange membranes (PEMs) in fuel cells and batteries. However, PFSA polymers’ high price, environmental safety issues and reduced lifetime have motivated the search for adequate alternatives. This work investigates two systems: a neat sulfonated polystyrene random copolymer (PS-S_x) and a blend of PS-Sx mixed with sulfonated PS grafted iron oxide nanoparticles (PS-S_x NP). Random copolymer films were characterized with electrochemical impedance spectroscopy (EIS). Conductivity values were determined for PS-S_x with sulfonation levels ranging from 0.7mol% to 47.9mol% as a function of relative humidity (RH, 30%, 60%, 90%) at 40°C. The highest sulfonation level of PS showed a conductivity value of 0.04 S/cm at 90% RH, half that of Nafion®. The conductivity of the binary composite is investigated as a function of film morphology (i.e. discrete or percolated NP domains) and characterized through small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). These morphologies aim to provide increased mechanical strength and ionic conductivity compared to pure PS-S_x systems, and offer insights into ion mobility through percolated domains.