Water in PEMs Classification Schemes

The existence of surface and bulk-like water can explain the effect of the water content on the microscopic mobility of protons, indicated by the dramatic increase 

Pore network models, based on Gierke s structural model and on cylindrical pore-type models, were developed to account for the transition from surface- to bulk-like conductivity (Eikerling et al., 2001). Relations of conductivity versus water content, calculated with pore network models, are in agreement with experimental data for PFSA-type membranes. 

A freezing point suppression has also been found by Cappadonia et al. in Arrhenius plots of conductivity data (Cappadonia et al., 1994,1995). Free water in PEMs possesses the same melting point as bulk water, and it sustains high bulk-like mobility of protons and water. In comparison of the different classification schemes, there seems to be a correlation between surface water and nonfreezable/freezable bound water, but the correspondence is not unique. The distinction of freezable-bound and free water is vague. 

The experimental basis of sorption studies includes isopiestic vapor sorption isotherms (Morris and Sun, 1993 Pushpaet al., 1988 Rivin et al., 2001 Zawodzinski et al., 1993c) and capillary isotherms, measured by standard porosimetry (Divisek et al., 1998 Vol fkovich and Bagotsky, 1994 Vol fkovich et al., 1980). A number of thermodynamic models of water uptake by vapor-equilibrated PEMs have been 

To build a model of water sorption and swelling in PEMs, three microscopic equilibrium conditions of water must be accounted for in the PEM and the adjacent medium. The global equilibrium state corresponds to the minimum of the appropriate thermodynamic free energy, in this case the Gibbs energy. 

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In general, pores swell nonuniformly, as seen in the section Water Sorption and Swelling of PEMs. As a simplification, the random network was assumed to consist of two types of pores. Nonswollen or dry pores (referred to as red pores) permit only a small residual conductance resulting from tightly bound surface water. Swollen or wet pores (referred to as blue pores) contain extra water with high bulklike conductance. Water uptake corresponds to the swelling of wet pores and to the increase of their relative fraction. In this model, proton transport in the PEM is mapped as a percolation problem, wherein randomly distributed sites represent pores of variable size and conductance. The distinction of red and blue pores accounts for variations of proton transport properties due to different water environments at the microscopic scale, as discussed in the section Water in PEMs Classification Schemes. ... 

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