Proton exchange fuel cell membranes of poly(benzimidazole-amide)/sulfonated polystyrene/titania nanoparticles-grafted-multi-walled carbon nanotubes

Abstract

In this study, we have reported poly(benzimidazole-amide) containing flexible moieties such as ether, fluoro, and siloxane. The poly(benzimidazole-amide) synthesis was carried out by the condensation of 4‐(3,4-diaminophenoxy)benzene-1,2-diamine, bis(carboxypropyl)-tetramethyldisiloxane, and 2,2-bis(4-aminophenyl)hexafluoropropane in the presence of polyphosphoric acid at 160℃. Afterwards, poly(benzimidazole-amide) was blended with sulfonated polystyrene and 0.1–2 wt.% titania nanoparticles- grafted-multi-walled carbon nanotubes for the formation of hybrid proton exchange membranes for fuel cell (poly(benzimidazole-amide)/sulfonated polystyrene/titania nanoparticles- grafted-multi-walled carbon nanotubes) for fuel cell. Inclusion of titania modified multi-walled carbon nanotubes influenced the membrane performance by modifying the microstructure, mechanical properties, as well as water retention and proton conductivity properties of proton exchange membranes (PEM). The hybrid membranes were doped with phosphoric acid before subjecting to various characterizations. Field emission scanning electron microscopic study depicted typical blend morphology in which titania nanoparticles- grafted-multi-walled carbon nanotubes were partially embedded in the matrix and seemed to be pulled out of the matrix surface during fracturing. The phase separated structure was accountable for the water retention and higher proton conductivity. The tensile stress and modulus of acid doped poly(benzimidazole-amide)/sulfonated polystyrene/titania nanoparticles- grafted-multi-walled carbon nanotubes nanocomposites increased from 65.1 to 72.5 MPa and 9.5 to 15.2 GPa, respectively, with the increasing titania nanoparticles- grafted-multi-walled carbon nanotubes loading (0.1–2 wt.%). The glass transition temperature of phosphoric acid doped poly(benzimidazole-amide)/sulfonated polystyrene/titania nanoparticles-grafted-multi-walled carbon nanotubes 0.1–2 membranes increased from 227 to 236 C. They also had higher ion exchange capacity of 2.5–3.7 mmol/g and proton conductivity of 2.3–3.1 S/cm at 80℃ (higher than perfluorinated Nafion®117 membrane 1.1 × 10−1S/cm). A H2/O2 fuel cell using poly(benzimidazole-amide)/sulfonated polystyrene/titania nanoparticles- grafted-multi-walled carbon nanotubes 2 (ion exchange capacity 3.7 mmol/g) showed better performance than that of Nafion® 117 at 40℃ and 30% relative humidity.