Comparative Study of Proton Conducting Ionic Liquid Doped Nafion Membranes Elaborated by Swelling and Casting Methods: Processing Conditions, Morphology, and Functional Properties

Abstract

Trifluoromethanesulfonate of triethylammmonium (TFTEA) doped Nafion-TEA membranes have been evaluated to understand the impact of different elaboration methods, i.e., swelling and casting, on their nanoscale morphology using SANS and resulting functional properties such as electrochemical, thermomechanical, and transport properties. The effect of transformation of acidic side chains of Nafion into ammonium side chains (Nafion-TEA) was first compared for the Nafion-TEA membrane elaborated by casting with the one elaborated by modification of extruded commercial Nafion117 membrane. Concerning TFTEA doped membranes, their nanostructuration is very similar whatever the processing mode despite some key differences. The casting based membranes exhibit higher and/or more heterogeneous swelling of ionic domains of Nafion-TEA by TFTEA and lower impact of TFTEA on the long-range crystalline order in Nafion-TEA. Casting based membranes present better thermomechanical properties compared to swelling based membranes above 150 °C and sustain a storage modulus of 1 MPa in the temperature range of 150–180 °C. With increasing TFTEA content as well as temperature, both types of membranes show an increase in the ionic conductivity of a similar order but no significant changes in hydrogen and oxygen-permeability coefficients. Both kinds of doped membranes exhibit similar sorption behavior (e.g., an increase in water uptake with increasing TFTEA content), though the results from GAB modeling suggest that morphology developed by casting is more favorable for the sorption of water molecules in middle range activity. Current density of 0.85 A/cm2 at 0.6 V was obtained with a membrane containing 20% TFTEA.