Preparation of main-chain-type and side-chain-type sulfonated poly(ether ether ketone) membranes for direct methanol fuel cell applications

Novel main-chain-type and side-chain-type sulphonated poly(ether ether ketone)s (MS-SPEEKs) are synthesised by reacting the sulphonic acid groups of pristine SPEEKs with 2-aminoethanesulphonic acid to improve the nano-phase separated morphology of the material. 1H NMR and FT-IR spectroscopy are employed to determine the structure and composition of main-chain-type and side-chain-type sulphonated polymers. Flexible and tough membranes with reasonable thermal properties are obtained. The MS-SPEEKs show good hydrolytic stability, and water uptake values ranging from 15% to 30% are observed. Compared to Nafion 117®, the methanol permeability of the MS-SPEEKs is dramatically reduced to 8.83 × 10−8 cm2 s−1 to 3.31 × 10−7 cm2 s−1. The proton conductivity increases with increasing temperature, reaching 0.013–0.182 S cm−1. A maximum power density and open circuit voltage of 115 mW cm−2 and 0.830 V are obtained at 80 °C, respectively, which is significantly greater than the values generated with Nafion 117®. The introduction of pendent side-chain-type sulphonic acid groups increases the single-cell performance by more than approximately 20%; thus, the lower water diffusivity, methanol permeability, electro-osmotic drag coefficient and high cell performance indicated that MS-SPEEK is a promising candidate for DMFC applications.

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► Novel MS-SPEEKs were synthesised to improve the nano-phase separated morphology of pristine SPEEKs.
► The introduction of side-chain-type sulphonic acid groups allowed for the formation of a well-defined nano-phase separated morphology.
► The introduction of pendent side-chain-type sulphonic acid groups increased the single-cell performance by approximately 20%.