Influence of the preparation conditions on the properties of polymeric and hybrid cation exchange membranes

The present work deals with the influence of the membrane preparation conditions on the properties of polymeric and hybrid cation exchange membranes prepared by casting and solvent evaporation method. The polymer material used was the sulphonated derivative of an amorphous polyetheretherketone, known as PEEK-WC (sulphonated polymer is indicated as SPEEK-WC). Hybrid SPEEK-WC membrane containing inorganic proton conductors were also prepared at various loading (acid silicotungstate, ammonium and cesium silicotungstate). Different properties of the polymer electrolyte membranes (PEMs), like proton conductivity, ion exchange capacity, methanol permeability, water content and thermal stability, were examined as a function of solvent type, casting temperature and membrane post-treatment. The membrane cast using dimethyl sulphoxide (DMSO) as solvent, at higher temperature (45 °C vs 30 °C) and with acid post-treatments, resulted to be more conductive. It was found that polymer-solvent interactions play a determining role on the PEM microstructural organization and, as a consequence, on the proton transport pathway. The electrochemical properties of the PEMs are indeed closely related to their microstructure, especially the spatial distribution and nature of their ionic sites. The DMSO use as solvent increased also the thermal stability of the membranes (higher decomposition temperature of the sulphonic groups).