THERMAL-STABILITY OF PROTON CONDUCTING ACID DOPED POLYBENZIMIDAZOLE IN SIMULATED FUEL-CELL ENVIRONMENTS

Recently, polybenzimidazole membrane doped with phosphoric acid (PBI) was found to have promising properties for use as a polymer electrolyt e in a high temperature (ca. 150 to 200 degrees C) proton exchange mem brane direct methanol fuel cell. However, operation at 200 degrees C i n strongly reducing and oxidizing environments introduces concerns of the thermal stability of the polymer electrolyte. To simulate the cond itions in a high temperature fuel cell, PBI samples were loaded with f uel cell grade platinum black, doped with ca. 480 mole percent phospho ric acid (i.e., 4.8 H3PO4 molecules per PBI repeat unit) and heated un der atmospheres of either nitrogen, 5% hydrogen, or air in a thermal g ravimetric analyzer. The products of decomposition were taken directly into a mass spectrometer for identification. In all cases weight loss below 400 degrees C was found to be due to loss of water. Judging fro m the results of these tests, the thermal stability of PBI is more tha n adequate for use as a polymer electrolyte in a high temperature fuel cell.