PHENOMENOLOGICAL THEORY OF ELECTROOSMOTIC EFFECT AND WATER MANAGEMENTIN POLYMER ELECTROLYTE PROTON-CONDUCTING MEMBRANES

Partial dehydration of the proton-conducting membrane under working co nditions is one of the major problems in low-temperature fuel cell tec hnology. In this paper a model, which accounts for the electro-osmotic ally induced drag of water from anode to cathode and the counterflow i n a hydraulic pressure gradient is proposed. A balance of these flows determines a gradient: of water content across the membrane, which cau ses a decline of the current-voltage performance. Phenomenological tra nsport equations coupled with the capillary pressure isotherm are used , involving the conductivity, permeability. and electro-osmotic drag c oefficients dependent on the local water content. The effects of membr ane parameters on current-voltage performance are investigated. A.univ ersal feature of the obtained current-voltage plots is the existence o f a critical current at which the potential drop across the membrane i ncreases dramatically due to the dehydration of membrane layers close to the anode. For a membrane with zero residual conductivity in its dr y parts, the critical current is a limiting current. Well below the cr itical current the effect of dehydration is negligible and the current -voltage plot obeys Ohm's law. The shape of the capillary pressure iso therm determines the nonohmic corrections. A comparison of the results of this study to those of the pertinent diffusion-type models reveals qualitatively different features, the convection model is found to be closer to experimental observations.