Preparation And Characterization Of Proton Exchange Membrane Of Phosphate Acid Doped Polybenzimidazole By Sol-Gel Method

Polybenzimidazole (PBI) membranes are used as membrane electrolyte materials for high temperature pronton exchange membrane fuel cells (HT-PEMFCs) working at 120-200℃due to its excellent thermal and chemical stabilities. The proton conductivity of the pure PBI membrane is as low as about 10-7 S/cm at room temperature, therefore PBI membranes need to be modified by doping with phosphoric acid for using as the proton exchange membrane electrolyte. PBI membranes are usually prepared by means of solution casting method. However, the PBI solubility in organic solvents, such as DMAc, decreases obviously with the increase of PBI molecular weight owing to its very rigid molecular structure. The restricted solubility of PBI in organic solvents results in a difficulty of preparation of PBI membranes especially when polymer has a high molecular weight. Therfore, direct casting acid doped PBI membrane was proposed by preparation of PBI solutions in mixed acids of triluoroacetic acid and phosphoric acid. N-substituted polybenzimidazoles by introducing ether linkage, side-chain alkyl and hexafluoropropyl in the polymer backbones have also been synthesized in order to improve the solubility. But these methods may increase the cost of preparing membranes or sacrifice the brand PBI properties to improve the processibility.Jana et al. studied the thermoreversible gelation of PBI in phosphoric acid. This provides us a route to prepare the acid doped PBI membranes by conveniently dissolving PBI powders in phosphoric acid without organic solvents. Based on this sdudy, we prepared PBI membranes with various acid doping levels controlled by means of acid de-doping. The PBI-H3PO4 Sol-gel method shortens experiment period and doesn't use organic solwents at the same time. For a PBI membrane with an acid doping level of 5.2, the stress at break and proton conductivity at 120℃without humidification were 5.1 MPa and 2.4×10-2 S/cm, respectively.The process for acid de-doping of the membranes was investigated. The process of the acid de-doping from the membrane was faster than of acid doping in the membrane. The de-doped acid molecules caused the shrinkage of the PBI membrane and thus the increase of the membrane strength due to the closeness of PBI chains according to the X-ray diffraction analysis. The acid doping level of PBI membranes only depends on the concentration of phosphoric acid used for acid de-doping. When acid de-doping process occurred in H3PO4 solvent at boiling point, it made membranes denser and cost less time than at room temperature. The increased separation of PBI backbones for PBI membranes by doped with phosphoric acids is confirmed by the analysis of XRD patterns.