Solution-blended sulfonated polyphenylene and branched poly(arylene ether sulfone): Synthesis, state of water, surface energy, proton transport, and fuel cell performance

Abstract

A poly(arylene ether sulfone) (PAES) was synthesized with a sulfonated poly(phenyl sulfone) side-chain (sB) to create a branched sBPAES ionomer. This sBPAES ionomer was solution blended with sulfonated polyphenylene (sPP). The sulfonated side-chain dramatically improved miscibility between these distinctly different ionomers. This functional group facilitated their solution-casting into robust films that were flexible and tough. Ionomer film proton conductivity, water uptake, density, state-of-water, contact angle, surface energy, wide angle x-ray scattering, and hydrogen fuel-cell function were studied based upon blend concentration. Results showed that sPP blended with 5 wt% sBPAES had a relative free water content that increased by 97% as compared to unblended sPP. Ionomer blends containing 10 wt% sBPAES reduced the film's water uptake by 37.5% and only caused a 3.3% decrease in proton conductivity from 97.5 to 94.3 mS/cm. The relative free water concentration improved within the blend up to 10 wt% sBPAES. These improvements were attributed to changes in ion clustering noted using wide angle x-ray scattering. The film's apparent water contact angle revealed that introducing 10 wt % sBPAES into sPP lowered its surface energy from 27.2 mJ/m2 to 24.3 mJ/m2. This led to a higher membrane-electrode interfacial resistance for these ionomer-blended films. An optimal ionomer-blend containing 5 wt% sBPAES had a H2/O2 peak-power output of 630 mW/cm2 at 1490 mA/cm2, which was greater than its unmodified sPP version of 570 mW/cm2 and 1220 mA/cm2.