(Invited) Ultra-Low EW Ionomers and Membranes for Fuel Cells

Abstract

Perfluorosulfonic acid (PFSA) ionomers have long been used to fabricate membranes for proton exchange membrane fuel cells (PEMFC). When used in combination with mechanical reinforcements and peroxide scavenging additives, this class of ionomers has demonstrated excellent durability and performance in both accelerated lab testing and in end-use applications.Automotive traction applications, among others, have very demanding performance targets at relatively high temperature and low humidity, driven by the need to reduce overall system cost. In order to meet these targets, ionomers with increased proton conductivity are required. Under dry conditions, conductivities of commercial ionomers are insufficient to achieve several of the targets, as they are limited to equivalent weights (EWs) of about 700 g/mol. Not only are low EW PFSA ionomers difficult to polymerize, they also tend to become water soluble as the amount of the tetrafluoroethylene (TFE) co-monomer is reduced.One strategy for achieving lower equivalent weight, while maintaining suitable TFE content, is a multi-acid side chain (MASC) approach. We have developed a synthetic process to use the 3M precursor polymer to prepare a new ionomer containing both bis(sulfonyl)imide and sulfonic acid groups in each sidechain, to form the perfluoro-imide acid (PFIA) ionomer. Both functional groups are strong acids, and their combination results in EWs in the range of 620-650 g/mol, with increased conductivity under all conditions, including low humidity. This approach has been extended to include multiple bis(sulfonyl)imide groups per side chain to form materials classified as perfluoro-ionene chain extended (PFICE) ionomers, with EWs as low as 440 g/mol.Characterization of these new ionomers for conductivity, solubility, and performance will be presented, along with a discussion of the practical limits of the MASC approach. Like PFSA ionomers, the PFIA and PFICE ionomers require both mechanical reinforcement and stabilizing additives in order to fabricate a viable PEMFC membrane. Development of membrane design considerations will be presented.