The effect of the side-chain ratio on main-chain-type and side-chain-type sulfonated poly(ether ether ketone) for direct methanol fuel cell applications

A series of side-chain ratios of novel main-chain-type and side-chain-type sulfonated poly(ether ether ketone)s (MS-SPEEKs) were synthesised by reaction of the sulfonic acid groups of pristine SPEEKs with 2-aminoethanesulfonic acid to improve the nano-phase separated morphology of the material. 1H NMR and FT-IR spectroscopy was employed to determine the structure and composition of main-chain-type and side-chain-type sulfonated polymers. Flexible and tough membranes with reasonable thermal properties were obtained. The MS-SPEEKs showed good hydrolytic stability, and water uptake values that range from 17% to 25% were observed. Compared to Nafion® 117, the methanol permeability of the MS-SPEEKs was dramatically reduced to between 9.97 × 10−8 cm2 s−1 and 1.51 × 10−7 cm2 s−1. The proton conductivity increased with increasing temperature and reached 0.031–0.099 S cm−1. A maximum power density and open-circuit voltage of 142 mW cm−2 and 0.79 V, respectively, were obtained at 70 °C, which were significantly greater than the values generated with Nafion® 117. The introduction of pendent side-chain-type sulfonic acid groups with a side-chain ratio of 8% showed the best cell performance; thus, the lower water diffusivity, lower methanol permeability and higher cell performance indicated that MS-SPEEK was a promising candidate for DMFC applications.