Analysis of Transient Water Flux Data

The vaporization exchange model for water sorption and flux in Nafion-type PEMs has been modified and applied to treat transient water flux data (Rinaldo et al., 2011). A decisive modification was the inclusion of transport coefficients that depend on the water concentration or water content in the PEM. A simple form of this dependence is a step function change from slow to fast transport at a given transition concentration c. For this purpose, a hyperbolic tangent function was introduced  

The flux boundary condition at the interface to the flow chamber is 

For a PEM in contact with liquid water as the stagnant fluid (LE), the boundary condition is 0m (r, 0) = 1, and for a PEM in contact with water vapor (VE), the boundary condition is 

The molar concentration of sulfonic acid groups in Nafion 117 is approximately Cf = 1.2 kmol m . Maximal water content, defined as the number of moles of water molecules per moles of ion exchange sites, can be taken to be A.max = 14. This corresponds to a maximal water concentration of = kiass.Cf = 16.8 kmol m .  

The extracted parameters are vital for rationalizing mechanisms and amounts of water fluxes in PEFCs. The model could be applied for the analysis of sorption data at varying PEM thickness and equilibrium water content. Experiments running at varying T would provide activation energies of the vaporization-exchange rate constant and bulk transport coefficients. Similar modeling tools can be developed for the study of water sorption and fluxes in catalyst layers. They can be extended, furthermore, 

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