Preparation And Characterization Of Epoxy Cross-linked Fluorine Containing Polybenzimidazoles As High Temperature Proton Exchange Membrane Materials

High temperature proton exchange membrane fuel cells (HT-PEMFCs) working above 100 ℃ have superior properties including high tolerance to fuel impurities, e.g., CO, fast electrochemical reaction kinetics, simple water and thermal management, etc. The phosphoric acid (PA) doped polybenzimidazole (PBI) membranes play an important role in promoting the development of HT-PEMFCs, due to their high conductivity, excellent mechanical properties and low cost. The conductivity of acid doped proton exchange membranes depends on the phosphoric acid doping level (ADL). The higher ADL leads to the higher conductivity, but brings on the deteriorated mechanical strength of the membranes. Therefore, improvements in both conductivity and mechanical strength of electrolyte membrane materials has become the key issues to further development of the HT-PEMFCs.Crosslinking is an effective method to improve the mechanical properties of acid doped membranes. Fluorine containing polybenzimidazole (F6PBI) exhibits improved solubility in organic solvents, excellent chemical and thermal stability, as well as excellent mechanical strength at room temperature. However, its tensile stress decreases significantly at elevated temperatures, especially with high ADLs. The plastic distortion of the F6PBI membrane hinders its application in HT-PEMFC.In the present work, three different epoxy compounds (bisphenol A diglycidyl ether, BADE; bisphenol A propoxylate diglycidyl ether, BAPDE; and poly (ethylene glycol) diglycidyl ether, PEGDE) were used as crosslinkers to prepare cross-linked F6PBI (X-F6PBI, where X=BADE, BAPDE, PEGDE) membranes. The crosslinking reactions were performed with a facile heating process and the crosslinking degree of the X-F6PBI membranes were 5%, 10% and 20%, respectively. The results indicated that all the X-F6PBI membranes exhibited high thermal stability and improved mechanical strength, which satisfy the requirements of the HT-PEMFCs. For instance, the cross-linked membrane 10% PEGDE-F6PBI/14.0 PA showed a conductivity of 0.065 S cm-1 at 180 ℃; the tensile strength and elongation at break were 2.0 MPa and 43.4% at 130 ℃; the peak power density was found to be 409 mW cm-2 and the current density of 669 mA cm-2 at a cell voltage of 0.51 V was reached.