Synthesis and characterization of sulfonated poly (aromatic imide-co-aliphatic imide) (S-coPl) membrane for direct methanol fuel cell

Abstract

A novel polymer electrolyte membrane (PEM) was based on the sulfonated poly(aromatic imide-co-aliphatic imide) (S-coPI) was synthesized by the poly condensation of 3,3ʹ,4,4ʹ-benzophenonetetracarboxylic dianhydride(BPTDA) with 4,4ʹ-diaminodiphenylmethane (DDM), 4,4ʹ-diaminodiphenylmethane-2,2ʹ-disulfonic acid disodium salt (S-DDM), and adipic acid dihydrazide (ADH). ADH helped to increase the flexibility of the backbone structure due to its aliphatic structure. The sulfonation degree of S-coPI was controlled by varying the mole ratio of DDM, S-DDM and ADH. The S-DOM containing two sulfonated groups was confirmed by H-NMR. S-coPI membranes were characterized by FTIR to confirm the characteristic peaks of the sulfonated group at 1298, 1159, 1098, and 1065 cm⁻¹. TGA was used to investigate the thermal stability of S-coPI, it underwent a 2 steps degradation process, one at 320°C which represented the decomposition of the sulfonated group and one at 560°C which represented the polymer backbone degradation. The S-coPI-3, with the highest sulfonation degree, had the proton conductivity about 1.16x10⁻² S cm⁻¹. The methanol permeability of the S-coPI films is in the range of 3.864x10⁻⁸ cm²/s which is lower than that of Nafion 117 about 26-40 times.