Synthesis and properties of water stable poly[bis(benzimidazobenzisoquinolinone)] ionomers for proton exchange membranes fuel cells

A novel sulfonated tetraamine, di(triethylammonium)-4,4′-bis(3,4-diaminophenoxy)biphenyl-3,3′-disulfonate (BAPBDS), was successfully synthesized by nucleophilic aromatic substitution of 4,4′-dihydroxybiphenyl with 5-chloro-2-nitroaniline, followed by sulfonation and reduction. A high-temperature polycondensation of sulfonated tetraamine, non-sulfonated tetraamine (4,4′-bis(3,4-aminophenoxy)biphenyl) and 1,4,5,8-naphthalenetetracarboxylic dianhydride (a) or 4,4′-binaphthyl-1,1′,8,8′-tetracarboxylic dianydride (b) gave the poly[bis(benzimidazobenzisoquinolinones)] ionomers SPBIBI-a(x) or SPBIBI-b(x), where x refers to the molar percentage of the sulfonated tetraamine monomer. Flexible and tough membranes of high mechanical strength were obtained by solution casting and the electrolyte properties of the polymers were intensively investigated. The ionomer membranes displayed excellent dimensional and hydrolytic stabilities. Moreover, these novel membranes showed proton conductivities comparable to that of Nafion 117, especially at high temperature. In addition, the proton conductivities of the SPBIBI-a ionomer membranes were found to be higher than those of the SPBIBI-b ones due to the weakened acid–base interactions between the pyridinone ring and the sulfonic acid groups. The highest proton conductivity (0.174 S/cm) was obtained for the SPBIBI-a(100) membrane at 100 °C, with an IEC of 2.65 mequiv./g. A combination of excellent dimensional and hydrolytic stabilities indicated that the SPBIBI ionomers were good candidate materials for proton exchange membrane in fuel cell applications.