Structural Model and Effective Properties of Conventional CCL

The volumetric filling factor of the pore space by water is defined as the liquid water saturation, Sr. It depends on the PSD and wettability of pores. Moreover, it varies with environmental conditions and the current density of fuel cell operation. The amount of liquid water arriving in the CCL is roughly proportional to/o due to 

For conventional CCLs with thickness of Icl = 5-10 jam, adequate gas porosity is indispensable. They must be operated as gas diffusion electrodes (GDEs). In completely flooded CCLs with 1, reaction penetration depths would be in the range of 0.5 xm or smaller, which would render the major part ( 90%) inactive for the ORR. 

Prior to 2006, modeling of CCL neglected the impact of liquid water formation in pores. Results of these studies are valid in a regime of sufficiently low current density. In this regime, the amount of water accumulation in secondary pores is too small to exert a significant impact on performance, whereas water in hydrophilic primary pores is strongly bound by capillary forces. Therefore, the assumption of constant and uniform composition and effective properties is justified in this regime. 

More recent work explicitly incorporated the effect of the water accumulation in secondary pores, determined by porous structure, wettability, current density, and environmental conditions. In a regime of high current density, increasing jo leads to higher overall liquid water saturation and more nonuniform water distribution in porous electrode layers (Eikerling, 2006 Liu and Eikerling, 2008). 

A common characteristic of all performance models is that they treat the heterogeneous layer as a continuous effective medium. All processes in it are averaged 

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