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

In this report, properties of 2mgml(-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 similar 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/LjCl solutions when the [LiCl]/[BI] molar is similar to 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 "25" 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 similar to 8.0. The porous poly(tetrafluorc, 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 degrees C and revealed a highest fuel cell performance when the composite membrane was prepared from a solution with a [LiCl]/[BI] molar ratio of similar to 8.0. (c) 2008 Elsevier B.V. All rights reserved.