(217fr) Effects of Polymer Solid Content and Chemical Structure On High Temperature Creep Compliance of Phosphoric Acid-Doped Polybenzimidazole Gel Membranes

Phosphoric acid (PA) doped polybenzimidazole (PBI) gel membranes are promising candidate materials for high temperature PEM fuel cells.  PA-doped PBI gel membranes are Flory type III physical gels made by a polyphosphoric acid (PPA) mediated sol-gel process.  They have demonstrated outstanding fuel cell performance under anhydrous, ambient pressure operation conditions and at temperatures up to 200°C.  Long-term durability is a major concern for these gel membranes.  In addition to chemical stability, mechanical durability is equally important. The current low-solids PA-doped PBI gel membranes are susceptible to creep at high temperatures.  The creep resistance of the gel membranes may be improved by increasing the polymer solid content or by altering the chemical structure of the PBI backbone.  In this work, we investigated the high temperature creep behavior of para-PBI and meta-PBI homopolymers as well as copolymers based on dihydroxy-PBI, para-PBI, meta-PBI, 2,5-pyridine-PBI and 3,5-pyridine-PBI moieties.  Varying the type and amount of each component enables us to balance PBI solubility and gel thermal stability.  We investigated the effects of varying polymer solid content and PBI chemical structure on the compression creep properties of these new PBI gel membranes, evaluated at a standard operating temperature of 180°C.  The results of this work demonstrate the feasibility of improving the mechanical properties of PA-doped PBI gel membranes by rational design of membrane composition and structure.