Ex Situ Water Fluxes in PEM

The resistance to water flux in the membrane interior and vaporization-condensation kinetics at its interfaces are important factors for the operation of PEFCs (Eikerling et al 1998,2007a Weber and Newman, 2004b). Specifically, developed models can be applied to water flux measurements (Adachi et al., 2009 Majsztrik et al., 2007 Monroe et al., 2008 Romero and Merida, 2009) to distinguish interfacial sorption-desorption kinetics from the mechanisms of water transport inside of the PEM. 

When modeling dynamic water sorption phenomena, information about evaporation and condensation is contained in the boundary conditions that account for water exchange across membrane-gas interfaces. The rate of interfacial water exchange is determined by values of the instantaneous water content on the PEM side of the interface, Xm, and by the vapor pressure, of the adjacent gas. The deviation of these local variables from their chemical equilibrium establishes the driving force of interfacial vaporization exchange. 

In the past, it has been a common albeit dubious practice to adopt an equihb-rium sorption isotherm for the relation between Xm and This approach demands an infinite rate constant of vaporization exchange. It is problematic for two reasons. First, the relative importance of interfacial water exchange grows with decreasing membrane thickness. Below a critical thickness, interfacial kinetics, rather than bulk transport, will limit the net water flux, implying an out-of-equilibrium condition. Second, if gases adjacent to the membrane are moving, water may be convected away from its surfaces. It is inherently contradictory to assume equilibrium in the presence of any kinetic or convective process. 

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