Abstract
We study the problem of water transport in the ionomer-phase of catalyst coated membranes (CCMs) for proton exchange membrane fuel cells (PEMFCs), where microscopic-scale phenomena at the distributed interfaces between structural components control the water management. Existing models for water transport in CCMs describe the transport in systems which consist exclusively of an ionomer-phase. Interfacial water fluxes across distributed interfaces representing various mechanisms of water transfer between ionomer and catalyst layer pores are not captured properly in these models. Here we develop a continuum model for water transport in CCMs using the method of volume averaging. Water is exchanged between ionomer and the catalyst layer pores by electro-osmotic discharge (EOD) through the three-phase boundary (TPB) regions and by sorption and desorption across the ionomer-pore interfaces. While the former mechanism does not affect directly the water content in the ionomer-phase, it represents an effective mechanism for water transfer during fuel cell operation and controls directly the water saturation in the catalyst pores.
Highlights
Catalyst layers for proton exchange membrane fuel cells (PEMFCs) are heterogeneous porous structures consisting of interpenetrating phases for the transport of electrical charges, electroactive gaseous species, and water
The misinterpretation of the effect that various terms have on the prediction of water accumulation in ionomer may be attributed in part to the lack in the literature of a formal derivation of a continuum model for water transport in catalyst coated membranes (CCMs)
An objective of this paper is to present a continuum model for water transport in the ionomer-phase of catalyst layers for PEMFCs, starting from the point equations for conservation of charge and for water concentration in ionomer-phase
Summary
Catalyst layers for proton exchange membrane fuel cells (PEMFCs) are heterogeneous porous structures consisting of interpenetrating phases for the transport of electrical charges, electroactive gaseous species, and water. We will present the local, microscopic equations governing proton and water transport in ionomer-phase (Level II) along with their boundary conditions and we will derive the volume-averaged equations for conservation of charge and water transport in catalyst layers (Level I). The scope of this analysis is to emphasize various mechanisms of water transfer between the ionomerphase and its surroundings, some of which have not been conceptualized and quantified until very recently. Application of the averaging theorems (5b) and (5c) to the microscopic equation for water
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