Preparation of PBI/PTFE composite membranes from PBI in N,N′-dimethyl acetamide solutions with various concentrations of LiCl

In this report, properties of 2 mg ml−1 PBI in N,N′-dimethyl acetamide (DMAc) solutions containing LiCl with molar ratios of [LiCl]/[BI] = 3.62–14.51 (where [BI] is the concentration of benzimidazole repeat unit in the solutions) were investigated. We show the solutions properties of PBI in DMAc mixed with LiCl (PBI/DMAc/LiCl) are strongly influenced by the molar ratio of [LiCl]/[BI] in the solutions. Thus, the properties of membranes prepared by solutions castings also depend on the LiCl concentration in the solutions. Both viscosity of PBI/DMAc/LiCl solutions and hydrodynamic radius of PBI in PBI/DMAc/LiCl solutions decrease when the molar ratio of [LiCl]/[BI] is increased from 0.0 to ~8.0 and then increase when the molar ratio of [LiCl]/[BI] is increased from ~8.0 to 14.5. These results suggest a lowest polymer aggregation of PBI in DMAc/LiCl solutions when the [LiCl]/[BI] molar is~ 8.0. Using a dialysis method with conductivity measurements, we found around 2.5 LiCl molecules were bonded on each BI repeat unit when the [LiCl]/[BI] fed molar ratio was 8.0 in PBI/DMAc/LiCl solutions. The value “2.5” of “2.5 LiCl molecules” bonded on each BI was close to the value “2” of “2 –NH groups” and “2 –N=C– groups” consisted in the chemical structure of a BI repeat unit. The IR spectra also show the hydrogen bonds between –NH and –N=C– of BI structures are dissociated by the presence of LiCl in PBI/DMAC solutions. These results suggest that all the –NH and –N=C groups of PBI are bonded by LiCl when the [LiCl]/[BI] fed molar ratio is at ~8.0. The porous poly(tetrafluoro ethylene) (PTFE) reinforced PBI (PBI/PTFE) composite membranes prepared from PBI/DMAc/LiCl solutions with [LiCl]/[BI] molar ratios of 3.6, 8.0, and 9.0 were used to prepare membrane electrode assemblies (MEA). The fuel cells performances of these MEAs were investigated at 150 °C and revealed a highest fuel cell performance when the composite membrane was prepared from a solution with a [LiCl]/[BI] molar ratio of 8.0.