Nafion water electroosmotic coefficient

The electroosmotic transport coefficient for water through Nafion 295 and 1150 membranes is typical and is shown to be highly dependent on the anolyte concentration to the exclusion of all the other variables studied. The water transport coefficient varies almost linearly with anolyte concentration from 6 to 17 molar caustic, giving 2.9 to 0.8 moles/F, as shown in Figure 3. The sodium ion transport number goes through a maximum of 0.82 eq/F in the 7 to 13 molar caustic range (27). 

Springer and others were the first to use detailed, experimentally derived diffusion and electroosmotic drag coefficients of water in Nafion in a model for steady-state water profile and the resulting protonic conductivity in the membrane of an operating PEFC [87]. The distribution of water in a PEFC at steady, state (at constant current and reactant/water fluxes) was calculated in this model by considering water flow through five regions of unit cross-sectional area within the fuel cell two inlet... 

Hydrated Nafion 117 (175 pm thick) has conductivities in the range of 10" S cm" at 80°C. Nafion has a high electroosmotic drag coefficient that can lead to problems with water management in a PEMFC. Nafion has been... 

The last assumption means that local electroosmotic flux of water in membrane is exactly counterbalanced by back diffusion. Recent studies [14,27] have shown that in a wide range of operating conditions total transfer coefficient of water from the anode to the cathode does not exceed 0.2. Since electroosmotic drag coefficient in Nafion is 1.5 [28], we conclude that the average over the cell surface electroosmotic flux in the membrane is almost fully compensated for by back diffusion. Note that the local value of total water flux in the membrane may significantly deviate from the surface-averaged value, e.g. close to the outlet of the oxygen channel [27]. Nevertheless, assumption 5 seems to be a reasonable approximation. 

Nafion 1135 membrane. Nafion 1135 membranes possesses a much higher fraction of freezable water than BPSH membranes with similar total X. This property affects not only the proton conductivity of membranes but also transport properties, such as the electroosmotic drag coefficient, and gas and methanol permeabiUty. 

Fig. 21 Water permeation from anode to cathode in the direct methanol fuel cell for radiation grafted membranes based on FEP with different initial film thickness (25 and 75 ixm) and Nafion 117. The electroosmotic drag coefficient H20/H is calculated from the slope of the regression line. Conditions cell temperature 90 °C, pressime 2 bar, 20mLmin 0.5 M methanol, air stoichiometry is 2.0 for FEP and 3.0 for Nafion 117...

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