Water in Catalyst Layers Preliminary Considerations

Accounting for the role of water in physical models of CCLs requires knowledge of the following two aspects in theory and experiment. 

Local equilibrium of water By which mechanism does water attain local equilibrium The approach in Eikerling (2006) and Liu and Eikerling (2008) presumes that capillary forces at the liquid-gas interfaces in pores equilibrate the local water content. This approach neglects surface film formation or droplet formation in pores of CLs. Ex situ diagnostics, probing porous structures, and water sorption characteristics under equilibrium condition, are needed to establish relations between porous structure, operating conditions, and local water distribution. 

Water transport and transformation What mechanisms of water transport are involved and what are the values of the transport parameters The relevant 

On the cathode side of a PEFC, electro-osmotic influx of water from the PEM and water production in the ORR create an excess of liquid water under normal conditions, even if the reactant at the cathode inlet is dry. Under normal conditions, it is reasonable to assume that primary hydrophilic pores and ionomer in the CCL are well hydrated. The proton conductivity can be assumed to be relatively constant. At high rate of water formation and insufficient water removal, excessive accumulation of water occurs in diffusion media and flow fields, which blocks critical pathways for gas diffusion of reactants. 

The main mechanism of equilibration of water with the porous medium is by capillary condensation. Pore filling is therefore determined by the Young-Laplace equation that relates the capillary pressure, jf, or the capillary radius, r, to the local 

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