A discussion of two-phase flow in the gas channel and porous transport layer regions of polymer electrolyte cells

This talk is about an ongoing programme of research on detailed performance calculations for two-phase liquid-gas flow in the gas channel and porous transport layer, as found, for example, on the cathode side of a polymer electrolyte fuel cell. The porous geometry is typically obtained by digital reconstruction from nano-computer tomography images. The domain may then tessellated with a computational mesh, whereupon the equations of mass and momentum are solved ,e.g., by means of a volume-of-fluid method. Liquid water is produced at the same time as gaseous oxygen is consumed by electrochemical reduction at the electrode surface, which is to be considered a boundary condition in the present problem. The problem was originally formulated in ref. [1]. Liquid-gas counter flow in the porous transport layer results in liquid drops being entrained in co-flow in the gas channels and convected downstream by the gas. The flow in the channels and adjacent parts of the porous transport layer is transient-periodic, but with some significant randomness, associated with the interactions between the different fluid streams percolating into the channel from the pores.

In this presentation, the complex micro-scale flow field is described in some detail. Consideration is also given as to the mechanisms for construction of macro-homogeneous properties such as absolute and relative permeability, capillary pressure vs. saturation for porous media, as well as macroscopic drag laws for two-phase channel flows based on calculations on a micro-scale. These are required for macro-scale homogeneous models, as typically employed at a cell-level. A discussion is also given of the impact of salient physical properties such as surface tension, and how these may be manipulated to improve future performance for electrochemical conversion devices such as fuel cells and electrolysers.