Synthesis and characterization of para- and meta-polybenzimidazoles for high-temperature proton exchange membrane fuel cells

In this article, two polybenzimidazoles (PBIs) were synthesized by the polycondensation reaction of 3,3′-diaminobenzidine with terephthalic acid ( p-PBI) or isophthalic acid ( m-PBI) for application in high-temperature proton exchange membrane fuel cells. Two kinds of purification (P 1 , P 2 ) were applied to clean the polymers. The chemical structure of PBI polymers was confirmed using Fourier transform infrared (FTIR) spectroscopy. Polymers after P 1 can be soluble and cast into film only from hot sulfuric acid solution (H 2 SO 4 ), whereas both polymers after P 2 were soluble in dimethylacetamide solution. Polymer films cast from H 2 SO 4 were investigated using FTIR. The thermal stabilities of PBIs and PBI films were studied using the thermogravimetric analysis and the first derivative of the TG method. The temperature of 5% weight loss ( T 5% ) for both PBIs ranges from 136 to 312°C under nitrogen atmosphere, depending on the dicarboxylic acid used and the kind of purification. The PBI films exhibited T 5% at 55°C because of the protonation of nitrogen atom in PBI by hydrogen atom of hydroxyl group in H 2 SO 4 . Electrical behavior of the two polymers was tested using electrochemical impedance spectroscopy for gold (Au)/PBI/Au architecture. For both measured polymers, Nyquist plots are presented with fittings based on circuit models.