Preparation and Properties of Branched Poly(aryl ether benzimidazole) High Temperature Proton Exchange Membranes

A series of novel branched poly(aryl ether benzimidazole) (OPBI) with 1,3,5-benzenetricarboxylic acid (BTA) as branching agent were synthesized and applied as proton exchange membranes for high temperature fuel cell. The introduction of branching points and aryl ether groups gave the branched OPBIs unique three-dimensional structures for acid absorption. Consequently, the proton conductivity was improved. The structure of branched OPBI was characterized by Fourier transform infrared spectroscopy (FTIR) and H-1-NMR spectra. The branched OPBI proton exchange membranes was directly produced by the solution casting process. Acid doping level (ADL), proton conductivity, solubility, thermal and oxidative stability, and mechanical properties of the proton exchange membranes were studied in detail. As the branching degree increased, the phosphoric acid (PA) doping level, proton conductivity and oxidative stability of the branched membranes were clearly improved. Compared with linear p-PBI and OPBI proton exchange membranes, the branched OPBI membranes had more free volume for acid absorption. The PA doping level of branched OPBI membranes with a 9% degree of branching exhibited the highest ADL of approximately 9.2 PA PRU-1, which was approximately 1.4 times greater than that of the linear OPBI membranes. The proton conductivity of the 9% branched OPBI membrane reached 0.0314 S/cm at 180 degrees C, which was nearly 1.8 times greater than that of the linear PBI membranes. Compared with linear PBIs, an excellent solubility of branched OPBI was observed. The improved solubility of branched OPBI in common solvents such as DMSO, DMAc at room temperature, may be derived from the introduction of aryl ether and the branching point. Moreover, branched OPBI membranes showed excellent thermal stability (decomposition temperature > 500 degrees C) and oxidative stability (weight loss < 12% in Fenton's reagent for 60 h). Even though the mechanical properties of branched OPBI membranes need further improvements, all of these properties indicate that branched OPBI membranes are promising candidates for high temperature proton exchange membranes fuel cells (HT-PEMFC).