Water Transport in the Ionomer-Phase and Across its Interfaces in Catalyst Coated Membranes for Proton Exchange Membrane Fuel Cells

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 the structural components of the catalyst layer control the water management. 

This study requires the consideration of a hierarchy of length scales over which significant changes in the parameters related to structure, processes and operating conditions occur. The fuel cell operation, control and optimization is modeled at the macroscopic-scale, where the catalyst layer thickness (~10µm) represents the characteristic length and where the parameters associated with this level, such as current density, water concentration or water flux represent macroscopic, averaged quantities. The macroscopic equation for water transport in CCMs has been frequently derived for use in macroscopic models intuitively, or by performing a water flux balance over a control volume representing 100% ionomer-phase. The resulting equation does not capture properly the interfacial water fluxes across the distributed interfaces representing various mechanisms of water transfer between ionomer and catalyst layer pores at microscopic scale, which are specific for catalyst layers and which control the water management.

Based on a multi-scale analysis of the catalyst layer morphology and starting from the microscopic, point equations for the transport of protons and water in the ionomer-phase (microscopic scale), we present a continuum model for water transport in the catalyst layers for PEMFCs  using the method of volume averaging1,2. This equation allows the consideration of the catalyst layer as a macro-homogeneous domain but in the same time retains the information regarding interfacial transport across ionomer-phase boundaries. The scope of this analysis is to emphasize and quantify various mechanisms of water transfer between the ionomer-phase and its surroundings. We show that during fuel cell operation, water is exchanged between the ionomer and the catalyst layer pores by sorption and desorption at non-equilibrium and between ionomer-phase and the three-phase boundaries (TPB) by electro-osmotic discharge (EOD), which is the electro-osmotic drag of water by protons when they engage in electrochemical reactions. The second mechanism does not affect water accumulation in the ionomer-phase, but affects water accumulation in the catalyst layer pores.  The model indicates that water also accumulates in the ionomer-phase of the CCMs by “stripping-off” water molecules in the peripheral solvation shells of the protonated complexes when their hydrodynamic motion occurs along a gradient of water content (along a gradient of hydrophilic domain sizes).