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

Abstract

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.