Morphology effect of sulfonated triblock copolymers for water retention and proton conductivity at high temperature

Abstract

Control of ion transfer paths in ion conducting polymers is an important factor in increasing efficiency in a variety of applications such as the use of ion exchange resins, sensors, secondary batteries, and fuel cells. 1 In particular, the fuel cell has recently gained attention as a promising candidate in generating environmental-friendly energy because of its direct energy conversion by electrochemical reactions. 2 For this application, many researchers have tried to increase efficiency by designing catalysts, electrodes, proton exchange membranes (PEMs), etc. Among these components in the fuel cell, the PEM has a direct relationship with the increase of proton conductivity because the ionic channel formed in the PEM matrix creates a proton migration path from the anode to the cathode. 3 The most famous PEM, Nafion ® , a perfluorosulfonic acid polymer, has been known to give sufficient proton conductivity, as well as good chemical and mechanical stability under 80 o C. However, it suffers from a decrease of proton conductivity and mechanical stability at high temperature, probably due to dehydration in the ionic channel. 4 Among various alternative proton conducting polymers such as polyarylene, polyimide, polyphosphazene, polystyrene-based copolymer, etc., a block copolymer is an interesting candidate for use in overcoming such problems at high temperatures, 5 because the hydrophilic domain (ionic channel) and hydrophobic domain (non ionic matrix) can be controlled by the self assembly of the block copolymer. This is achieved through the control of the molecular weights of the hydrophilic and hydrophobic block segments. 6