Molecular weight effect of partially sulfonated PS-b-PDMS diblock copolymers as proton exchange membrane for direct methanol fuel cell

Abstract

Partially sulfonated polystyrene-b-poly(dimethylsiloxane) (sPS-b-PDMS) block copolymer membranes were prepared using three different polymers with various molecular weights and block ratios. Eleven different products were obtained with controlled degree of sulfonation (DS) of PS block ranging from 22% to 48%, and they were cast into proton exchange membranes (PEMs). Each PEM was rigorously characterized to see the effect of molecular weight and PS content to PEM for direct methanol fuel cells (DMFC). The first set of sPS-b-PDMS with the highest molecular weight and 50% PS block ratio generally showed high mechanical strength and selectivity (proton conductivity/ methanol permeability), and the second set with medium molecular weight and 60% PS ratio showed high proton conductivity. The last PEM with the lowest molecular weight and 80% PS ratio exhibited a poor tensile strength which is not suitable for PEM application. When the membrane/electrode assemblies (MEAs) were fabricated using the sPS-b-PDMS PEMs with moderately high DS (~40%), one from the second set (moderate molecular weight, 60% PS content) showed the best power performance (90 mW/cm2 at 70 °C) in active mode DMFC operation, which was found to be 20% higher than that of Nafion 115 MEA.