Synthesis of a Cross-Linked Polymer Electrolyte Membrane with an Ultra-High Density of Sulfonic Acid Groups

Abstract

Sulfonated cross-linked polystyrene membranes, one of the hydrocarbon-based polymer electrolytes membranes (PEMs), are typically synthesized via sulfonation of cross-linked polystyrene. However, it is difficult to prepare a sulfonated cross-linked polystyrene with a high sulfonation level, i.e., a high mole fraction of styrenesulfonic acid unit (fsSA), by using conventional synthetic methods. In this study, to synthesize a relatively homogeneous cross-linked poly(styrenesulfonic acid) (CL-sSA) membrane with an fsSA of 90 mol % or higher, sulfonic acid groups in a styrenesulfonate monomer were protected by esterification, and then a cross-linked polymer was synthesized using the monomer and divinylbenzene. Finally, the cross-linked polymer was converted to CL-sSA by deprotecting the alkyl groups with a base. The conductivity of the CL-sSA membrane at 80 °C and 90% RH was 0.93 S cm-1, which is extremely high and much higher than those of commercially available Nafion 212 and Selemion HSFN membranes with a low fsSA. The synthetic scheme of the CL-sSA membrane used in this study is a very effective route for preparing highly conductive cross-linked PEMs with a high fsSA