Novel proton exchange membrane for direct methanol fuel cell

Abstract

The proton exchange membrane for a direct methanol fuel cell was fabricated from sulfonated poly (aromatic imide-co-aliphatic imide) (SPI). 4,4'-diaminodiphenylmethane-2,2'-disulfonic acid disodium salt (SDDM) was synthesized from 4,4'-diaminodiphenylmethane (DDM), using concentrated sulfuric acid and fuming sulfuric acid. Then SPI was synthesized from SDDM, DDM, hexamethylenediamine (HDA), 3,3',4,4'-benzophenonetetracarboxylic dianhydride (BTDA), where dimethylsulfoxide (DMSO) was used as the solvent. SPI was synthesized at various degrees of sulfonation by varying the amount of SDDM in SPI. The membrane was obtained by solution casting. The sulfonated position of SDDM was characterized by 1H-NMR and FTIR. The chemical structure of SPI was characterized by FTIR. The characteristic peeks of SPI are at 1777, 1709, 1368, 1248 and 1157 cm-1 respectively. Thermogravimetric analysis was used to investigate the thermal stability of SPI. The sulfonated group of SPI decomposed around 200 °C and the degradation of polymer main chain occurs at around 480 °C. Sulfonated polyimide with the highest degree sulfonation possesses the proton conductivity of 0.003241 S/cm. The highest methanol permeability of sulfonated copolyimide is 2.7482 × 10-8 cm2/s which much lower than those of Nafion® (1.7400 × 10-6 cm2/s). The effects of degree of sulfonation on the properties of the membrane were also investigated and reported here: the water uptake (WU), the dynamic mechanical analysis (DMA) and the ion exchange capacity (IEC).