Dispersions of carbon nanotubes in sulfonated poly[bis(benzimidazobenzisoquinolinones)] and their proton-conducting composite membranes

A sulfonated poly[bis(benzimidazobenzisoquinolinones)] (SPBIBI) possessing a conjugated pyridinone ring was shown to be effective for dispersing multiwalled carbon nanotubes (MWCNTs) in DMSO. The dispersions in which the SPBIBI to MWCNTs mass ratio was 4:1 demonstrated the highest MWCNTs concentrations, i.e., 1.5-2.0 mg mL⁻¹, and were found to be stable for more than six months at room temperature. Through casting of these dispersions, MWCNTs/SPBIBI composite membranes were successfully fabricated on substrates as proton exchange membranes for fuel cell applications and showed no signs of macroscopic aggregation. The properties of composite membranes were investigated, and it was found that the homogeneous dispersion of the MWCNTs in the SPBIBI matrix altered the morphology structures of the composite membranes, which lead to the formation of more regular and smaller cluster-like ion domains. As a result, and in comparison to a pristine SPBIBI membrane, the composite membranes displayed more significant proton conductivities, especially at low relative humidity, without sacrificing other excellent properties, such as thermal, dimensional and oxidative stabilities. For instance, the composite membranes with an MWCNTs content only of 0.5 wt% exhibited proton conductivities of 0.021 S cm⁻¹ at 50 RH% and 70 °C, a value almost fourfold as high as that of the pristine SPBIBI membranes under identical conditions (0.005 S cm⁻¹). The result was comparable to Nafion 117 (0.021 S cm⁻¹). The homogenous dispersion of the MWCNTs and the efficient enhancement the SPBIBI performance were attributed to the π-π interaction between the pyridinone ring and the sidewalls of the MWCNTs which changed the morphological structure of composite membranes as revealed by TEM. A combination of a low methanol crossover with excellent thermo-oxidative and water stabilities indicated that the SPBIBI composite membranes were good candidate materials for proton exchange membranes in fuel cell applications.