Polybenzimidazole films doped with phosphoric acid are being investigated as potential polymer electrolytes for use in hydrogen/air and direct methanol fuel cells. In this paper, we present experimental findings on the proton conductivity, water content, and methanol vapor permeability of this material, as well as preliminary fuel cell results. The low methanol vapor permeability of these electrolytes significantly reduces the adverse effects of methanol crossover typically observed in direct methanol polymer electrolyte membrane fuel cells.

In developing advanced fuel cells and other electrochemical reactors, it is desirable to combine the advantages of solid polymer electrolytes with the enhanced catalytic activity associated with temperatures above 100~ This will require polymer electrolytes which retain high ionic conductivity at temperatures above the boiling point of water. One possibility is to equilibrate standard perfluorosulfonic acid polymer electrolytes such as Nafion TM, with a high boiling point Bronsted base such as phosphoric acid. The Nafion/H3P04 electrolyte has been evaluated with respect to water content, ionic conductivity and transport of oxygen, and methanol vapor. The results show that at elevated temperatures reasonably high conductivity (>0.05 ~Q-1 cm-1) can be obtained. Methanol permeability is shown to be proportional to the methanol vapor activity and thus decreases with increasing temperature for a given methanol partial pressure. Comparisons and distinctions between this electrolyte and pure phosphoric acid are also considered.

The electro-osmotic drag coefficient of water in two polymer electrolytes was experimentally determined as a function of water activity and current density for temperatures up to 200°C. The results show that the electro-osmotic drag coefficient varies from 0.2 to 0.6 in Nafion®/H3P04 membrane electrolyte, but is essentially zero in phosphoric acid-doped FBI (polybenzimidazole) membrane electrolyte over the range of water activity considered. The near-zero electro-osmotic drag coefficient found in FBI indicates that this electrolyte should lessen the problems associated with water redistribution in proton exchange membrane fuel cells.

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Acid-Doped Polybenzimidazoles: A New Polymer Electrolyte.-The proton conductivity and MeOH vapor permeability are measured for polybenzimidazole (PBI) films doped with H3PO4 and the applicability of the films as potential polymer electrolytes for hydrogen/air and direct MeOH fuel cells is studied. Long-term tests show that the PBI electrolyte is stable at elevated temp. even in the presence of H2, O2, and Pt. A H2/O2 cell with the PBI-based electrolyte and Pt/C electrodes was successfully operated at 200 mA/cm2 for over 200 h without decay of the performance. The max. power observed for an unoptimized cell was 0.25 W/cm2 at 700 mA/cm2. The MeOH/O2 fuel cell with PBI-H3PO4 membrane, Pt-Ru anode, and Pt black cathode produces over 0.1 W/cm2 at 250-500 mA/cm2. The low MeOH permeability of the electrolyte reduces the adverse effects of MeOH crossover. -(WAINRIGHT, J. S.; WANG, J.-T.; WENG, D.; SAVINELL, R. F.; LITT, M.; J. Electrochem. Soc. 142 (1995) 7, L121-L123; Dep. Chem. Eng., Case West. Reserve Univ., Cleveland, OH 44106-7217, USA; EN)