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 –NC– groups” consisted in the chemical structure of a BI repeat unit. The IR spectra also show the hydrogen bonds between –NH and –NC– of BI structures are dissociated by the presence of LiCl in PBI/DMAC solutions. These results suggest that all the –NH and –NC 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.
Available from: Shanfu Lu
- "Considering the performance and cost of various strategies, a promising approach to overcome this problem is preparing PTFEbased composite membranes. Previously, PTFE/PBI composite membranes    have been prepared with minimal thickness and good mechanical properties after PA doping, and have proved to be promising materials for HT-PEMFCs. "
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ABSTRACT: A high temperature proton-exchange membrane (PEM), polytetrafluoroethylene-poly(ethersulphone)-poly(vinyl pyrrolidone) (PES-PVP/PTFE) composite membranes, was prepared by impregnating PES-PVP solutions into the porous PTFE films to reinforce the mechanical properties thereof. After doping with phosphoric acid (PA), the tensile strength and Young's modulus of PES-PVP/PTFE composite membranes were much higher than those of the PA doped PES-PVP membrane. PA doping level of the composite membrane was positively related to the impregnating degree of PES-PVP into PTFE. In this study, the PES-PVP/PTFE composite membranes with the optimized impregnating degree exhibited a proton conductivity of 0.26 S cm(-1) at 180 degrees C with 560 wt% H3PO4 doped. The high-temperature PEMFCs with PES-PVP/PTFE-5 composite membrane demonstrated a maximum power density of 607 mW cm(-2) at 180 degrees C with H-2/O-2 system. Furthermore, the PES-PVP/PTFE composite membranes show a low H-2 crossover current density in high-temperature PEMFCs.
Available from: Dah-Shyang Tsai
- "A proven technique is to crosslink the linear heterocyclic PBI chains, using either small molecules, such as epoxy agents, pyridine, and divinylsulfone [13e16], or much longer PBI segments . Other structure-tailoring strategies also generate positive results, including grafting benzimidazole moieties , thermal curing , and PBI/PTFE composite  . "
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ABSTRACT: The proton conductivity of phosphoric acid (PA) doped polybenzimidazole (PBI) membrane is improved through imbibing antimony pentachloride before doping PA. The antimony involvement diminishes the temperature dependence of proton conductivity in the targeted temperature range 160–260 °C. Among the compositions being prepared, the most conductive one exhibits 8.08 × 10−2 S cm−1 at 180 °C, with a small temperature sensitivity 6.3 × 10−5 S cm−1 °C−1. Several membrane-electrode assemblies (MEAs) have been prepared with the Pt/C catalyst layers to evaluate the practicality of co-doped PBI membrane. After proper activation, the hydrogen/air cell generates substantial electric power, denoted by its peak value over 500 mW cm−2 at 180 °C. Impedance analysis indicates carbon monoxide poisoning affects overall MEA kinetics, as evidenced in the rising resistances of electrolyte, cathode, and anode. But the poisoned performance due to 3% CO/H2 fuel can be rejuvenated after the fuel is switched back to pure hydrogen in one or two hours.
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ABSTRACT: It has been reported that a thin polybenzimidazole (PBI)/poly(tetrafluoro ethylene) (PTFE) composite membrane (thickness 25–30 μm) can be prepared by impregnating a thin porous PTFE (thickness 15–20 μm) in a PBI/N,N′-dimethyl actamide (DMAc) solution. In this paper, a 400 h life test of a fuel cell prepared from PBI/PTFE composite membrane was carried out at 160 °C with a current density i = 200 mA cm−2. During long time test, the i–V curve and AC-impedance measurements were conducted every 12 h. The experiment data showed a 240 h period of activation. After 240 h the cell voltage started to decay. AC-impedance measurements showed internal resistance (Rs) and charge transfer resistance (Rc) decreased in the initial 240 h life test and then Rs remained almost constant and Rc increased after 240 h. The decay of fuel cell performance can be attributed to the migration of phosphoric acid out from membrane leading to the delamination between membrane and electrodes.
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