ELECTRO-OSMOTIC DRAG COEFFICIENT OF WATER IN POLYMER ELECTROLYTES AT ELEVATED TEMPERATURES D. Weng, J. S. Wainright, U. Landau* and R. F. Savinell Ernest B. Yeager Center for Electrochemical Sciences Department of Chemical Engineering Case Western Reserve University Cleveland, OH 44106 ABSTRACT The electro-osmotic drag coefficient of water in two polymer electrolytes was experimentally determined as a function of water vapor 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®/H3PO4 membrane electrolyte, but is essentially zero in a phosphoric acid doped PBI (polybenzimidazole) membrane electrolyte over the range of water vapor activity considered. The near zero electro-osmotic drag coefficient found in PBI indicates that this electrolyte should lessen the problems associated with water redistribution in PEM fuel cells. INSTRUCTION In order to assure long term stable cell performance, it is important to maintain an optimal chemical species distribution in solid electrolyte fuel cells. We are currently investigating the use of polymer electrolytes for direct methanol fuel cells operating at temperature exceeding 100 °C1,2. It is believed that operation at these temperature can minimize the effects of catalyst poisoning associated with strongly adsorbed intermediates e.g., CO and/or impurities, and significantly enhance electrochemical performance by increasing the catalyst activity. In PEM (proton exchange membrane) fuel cells, it is particularly critical to manage the water content in the polymer electrolytes to maintain sufficient proton conductivity and avoid cathode flooding and anode dehydration. The motion of water through a polymer electrolyte in a methanol/air fuel cell can be ascribed according to concentrated solution theory3,4 to the gradient in the chemical potentials of water and methanol, and to water drag due to the movement of protons (proportional to the DC current). This latter water drag is referred to as the “electro- osmotic drag” and is quantified in term of the electro-osmotic drag coefficient, • Author to whom correspondence should be addressed. Proceedings of The Electrochemical Society, PV 1995-23, 214-225 (1995) DOI: 10.1149/199523.0214PV © The Electrochemical Society ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 207.241.231.83 Downloaded on 2019-04-29 to IP