Durable Sulfonated Poly(benzothiazole-co-benzimidazole) Proton Exchange Membranes

Abstract

Two series of random sulfonated poly-(benzothiazole-co-benzimidazole) polymers (sPBT-BI) with 70% and 60% degree of sulfonation were evaluated as proton exchange membranes. sPBT was also prepared for a comparative study. The mechanical properties of sPBT-BI were greatly enhanced by incorporation of benzimidazole (BI); sPBT-BI70-10 showed a tensile strength of 125 MPa and elongation at break of 38.9%, an increase of 56.5% and 145%, respectively, compared with sPBT. The solubility, dimensional stability, thermal properties, and oxidative stability of sPBT-BI were also improved. The ionic clusters of sPBT-BI membranes in both AFM phase images and TEM images became narrower with increasing amounts of BI while containing the same molar amount of sulfonic acid groups. This resulted in lower dimensional swelling and higher mechanical strength, but the proton conductivity decreased. However, high proton conductivity was achieved by incorporating an appropriate content of BI. PEMFC H 2 /air single cell performances and durabilities were improved by incorporation of 5% of BI units in sPBT.

Soluble sulfonated polybenzothiazoles containing naphthalene for use as proton exchange membranes

“…The sPBT-6FA membranes were immersed in Fenton's reagent (3% H 2 O 2 containing 2 ppm FeSO 4 ) at 80 1C to compare the oxidative stability [7][8][9][10][11]24]. Before testing, the membranes were dried under vacuum at 120 1C for 24 h and cut to the same amount of weight ( $ 0.02 g).…”

“…The crosshead speed was set at 1 mm min À 1 [28][29][30]. Before testing the mechanical properties of the sPBT-6FA membranes, they were dried at 120 1C for 24 h under vacuum [11,20,32].…”

“…PBT has usually been used only as a structural material, because of its poor solubility and typically low molecular weight [7][8][9][10][11]. sPBT exhibits no softening or glass transition temperature (T g ) before thermal degradation occurs.…”

“…sPBTs have rigid aromatic and heterocyclic benzothiazole backbone structures that exhibit strong intermolecular interactions. It is difficult to prepare sPBT polymers with high molecular weights that are soluble in common polar aprotic solvents [7][8][9][10][11]. To the best of our knowledge, only a few sPBTs have been utilized as PEMs.…”

“…Yan et al prepared soluble sPBT for PEMs for the first time, derived from bis(3-sulfonate-4-carboxyphenyl) sulfone (BSCPS), by incorporating bulky pendent groups or flexible linkages into the backbone [10]. Flexible hexafluoroisopropylidene units were incorporated into the polymer backbone to prepare more flexible sPBT polymers that were soluble in polar aprotic solvents [7][8][9][10][11]. In addition, the incorporation of naphthalene units into the backbone of aromatic polymers reduce water uptake and dimensional swelling of PEMs because the rigid and planar nature of the naphthalene units enhance the aggregation of the polymer chain, inducing the formation of larger and more developed hydrophilic ionic clusters [14,18].…”