Description
Proton exchange membrane fuel cells (PEMFCs) are subjected, during their lifetime, to freeze/thaw cycles of water produced in the catalytic layers and remaining after cell shutdown. However, in situ freeze/thaw experiments show that, for the specific types of PEMFCs investigated here, these cycles have no measurable impact on electrochemical performance and suggest that there is no significant degradation of the internal structure. This observation raises a fundamental question: where is the water located when the cell is shut down, and how is it redistributed within the porous medium constituting the catalytic layer at the nanoscopic scale?
To answer this question, we use a two-phase computational fluid dynamics approach based on a Cahn–Hilliard phase-field model implemented in Comsol Multiphysics, while neglecting disjoining pressure and evaporation in a first step. This approach allows us to explicitly describe the dynamics of liquid–gas interfaces. Particular attention is paid to modelling the dynamic contact angle through the introduction of a molecular self-layering contact angle model that depends on both the interface front velocity and the equilibrium contact angle. This approach makes it possible to go beyond static descriptions of wetting and simplified pore network model formulations that rely on a single equivalent contact angle for all surfaces, which are often insufficient to describe complex geometries involving multiple solid surfaces with distinct properties.
Two-dimensional simulations are performed on idealized geometries representing simple patterns, at scales characteristic of the porous media of catalytic layers. These configurations allow us to systematically study the influence of the coexistence of two distinct equilibrium contact angles, associated with different solid surfaces, on interface displacement and water transport.
We also identify an ideal geometric case of a cylindrical pore in which the contact angle varies locally and for which an analytical solution is available. This case highlights the limitations of Cassie's law, which is commonly used to predict an equivalent contact angle. Three-dimensional numerical simulations are then carried out to explore and propose an alternative law for defining a relevant equivalent contact angle in complex geometries.
These results shed new light on the mechanisms of water redistribution in porous catalyst media in PEMFCs and highlight the importance of a dynamic and geometrically consistent description of wetting to understand two-phase transport phenomena.