Phenomenological Theory of Electro‐osmotic Effect and Water Management in Polymer Electrolyte Proton‐Conducting Membranes

Abstract

Partial dehydration of the proton-conducting membrane under working conditions is one of the major problems in low-temperature fuel cell technology In this paper a model, which accounts for the electro-osmotically induced drag of water from anode to cathode and the counterfiow in a hydraulic pressure gradient is proposed. A balance of these flows determines a gradient of water content across the membrane, which causes a decline of the current-voltage performance. Phenomenological transport equations coupled with the capillary pressure isotherm are used, involving the conductivity permeability and electro-osmotic drag coefficients dependent on the local water content. The effects of membrane param-eters on current-voltage performance are investigated. A universal feature of the obtained current-voltage plots is the existence of a critical current at which the potential drop across the membrane increases dramatically due to the dehy-dration of membrane layers close to the anode. For a membrane with zero residual conductivity in its dry parts, the crit-ical current is a limiting current. Well below the critical current the effect of dehydration is negligible and the current-voltage plot obeys Ohm's law. The shape of the capillary pressure isotherm determines the nonohmic corrections. A comparison of the results of this study to those of the pertinent diffusion-type models reveals qualitatively different fea-tures, the convection model is found to be closer to experimental observations.