Two-phase transport in porous gas diffusion electrodes

Abstract

The accumulation of liquid water in electrodes can severely hinder the performance of PEMFCs. The accumulated water reduces the ability of reactant gas to reach the reaction zone. Current understanding of the phenomena involved is limited by the inaccessibility of PEMFC electrodes to in situ experimental measurements, and numerical models continue to gain acceptance as an essential tool to overcome this limitation. This Chapter provides a review of the transport phenomena in the electrodes of PEM fuel cells and of the physical characteristics of such electrodes. The review draws from the polymer electrolyte membrane fuel cell literature as well as relevant literature in a variety of fields. The focus is placed on two-phase flow regimes in porous media, with a discussion of the driving forces and the various flow regimes. Mathematical models ranging in complexity from multi-fluid, to mixture formulation, to porosity correction are summarized. The key parameters of each model are identified and, where possible, quantified, and an assessment of the capabilities, applicability to fuel cell simulations and limitations is provided for each approach. The needs for experimental characterization of porous electrode materials employed in PEMFCs are also highlighted.

On Modeling of Heat and Mass Transfer and Other Transport Phenomena in Fuel Cells

“…Instead, much attention has been paid on numerical investigations to reveal the relationships between the water saturation, proton conductivity (ohmic loss), the level of catalyst flooding (activation loss), and the effective diffusivity of the porous layer (concentration loss), see Lister and Djilali (2005), You and Liu (2001), He et al (2000), Nguyen (2000), Wang et al (2001), and Djilali and Lu (2002).…”

“…The maximum rate of evaporation for liquid water can then be determined based on the approximation of the evaporation/condensation coefficient. While for the second method, the liquid water is assumed to exist in the form of a spherical droplet, and the mass transfer is determined from the diffusion rate between the bulk water vapour and the surface of the droplet by employing a mass transfer Nusselt number as a measure of the droplet's ability to exchange mass (Lister and Djilali (2005)). …”

“…However, this model does not account for liquid water motion at all. The second approach, namely the moisture diffusion model, treats the liquid water motion by the surface tension, and the liquid water phase is considered as a scalar species with no convection terms when it is incorporated into a CFD code (Lister and Djilali (2005)). This approach is rather simple since only one additional equation is required compared to the one-phase model.…”

Fluid Flow in Polymer Electrolyte Membrane Fuel Cells

“…The need for an effective diffusivity stems from the fact that porous media presents important geometric restrictions (pores, etc.). The Bruggemann correction is the most used correction factor for the diffusion coefficient (Sundén & Faghri, 2005), where:…”

“…The diffusion coefficient is then determined from the kinetic theory of gases (Sundén & Faghri, 2005).…”