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

Abstract

A novel polymer electrolyte membrane (PEM) based on sulfonated poly(aromatic imide-co-aliphatic imide) (S-coPI) is synthesized by the polycondensation of 3,3′,4,4′-benzophenonetetracarboxylic dianhydride (BPTDA) with 4,4′-diaminodiphenylmethane (DDM), 4,4′-diaminodiphenylmethane-3,3′-disulfonic acid disodium salt (S-DDM), and adipic acid dihydrazide (ADH). The ADH is utilized to increase the flexibility of the backbone structure due to its aliphatic structure. The sulfonation degree of S-coPI is controlled by varying the mole ratio of DDM, S-DDM, and ADH. The S-DDM containing two sulfonated groups is confirmed by ¹H-NMR. S-coPI membranes are characterized by FT-IR to identify the characteristic peaks of the sulfonated group at 1,298, 1,159, 1,098, and 1,065 cm⁻¹. Thermogravimetric analysis is used to investigate the thermal stability of S-coPI; it undergoes 2 steps in the degradation process, one at 320 °C which represents the decomposition of the sulfonated group and one at 560 °C which represents the polymer backbone degradation. The S-coPI-3, with the highest sulfonation degree, possesses proton conductivity value of about 1.16 × 10⁻² S cm⁻¹. The methanol permeability of the S-coPI films are in the range of 3.864 × 10⁻⁸–6.521 × 10⁻⁸ cm² s⁻¹, about 26–40 times lower than that of Nafion 117.