(208a) Transport in Constrained Polymer-Electrolyte Membranes

Water transport in polymer-electrolyte membrane fuel cells is known to play a vital role in the efficient operation of these devices. The polymer-electrolyte membrane must be hydrated, to facilitate conduction of hydrogen ions; water formed as a by-product of the cell reaction at the cathode needs to be removed to avoid flooding. Moreover, PEMFCs operate in conditions where the membrane is not allowed to swell freely upon water uptake. This physical constraint can generate a stress differential in the membrane associated with water flux.

Water transport in the membrane is commonly acknowledged to occur by molecular diffusion, pressure diffusion, or electro-osmotic drag [1]; at higher water contents, external stress or pressure gradients across the membrane can also cause fluid flow directly by hydraulic permeation [2]. Most mathematical models, as well as experiments, treat the diffusive and hydraulic-permeation effects in isolation. There is no inherent contradiction in establishing a system where both these forces act in parallel, however. In addition to charge and material balances, we have added a local momentum balance that couples stress-induced compression and flow (permeation) into the transport system, as well as a thermodynamic equation of state that accounts for the local distribution of swelling incurred by variations in water content. Also, we argue that vapor-equilibrated and liquid-equilibrated cases can be handled by a single set of transport laws.

This talk will focus on reconciling all modes of transport in a single, thermodynamically rigorous macroscopic transport model accounting for simultaneous charge, water, and momentum transport in ionomer membranes. Composition and pressure dependences of properties such as activity coefficients, membrane compressibility and partial molar volumes will be explored from the perspective of thermodynamic constraints, and will be elucidated using experimental data from the literature.

The results help to rationalize phenomena associated with membrane swelling upon water sorption, as well as the consequences of constraining a membrane physically as water flux occurs. A comprehensive model will not only depict more accurate water profiles in the membrane, which can aid the design of effective water management schemes for fuel cells, but can also inform future modeling of swelling-induced membrane fatigue.

1. Weber A.Z., & Newman J. (2004). Transport in Polymer-Electrolyte Membranes. II. Mathematical Model. Journal of The Electrochemical Society. 151(2), A311-A325.

2. Duan Q., Wang H., & Benziger J. (2012). Transport of Liquid Water through Nafion Membranes. Journal of Membrane Science. 392-393, 88-94.