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 (H2SO4), whereas both polymers after P-2 were soluble in dimethylacetamide solution. Polymer films cast from H2SO4 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 degrees C under nitrogen atmosphere, depending on the dicarboxylic acid used and the kind of purification. The PBI films exhibited T-5% at 55 degrees C because of the protonation of nitrogen atom in PBI by hydrogen atom of hydroxyl group in H2SO4. 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.