Methanol electro-osmotic flux

According to Eq. 6.11, the DMFC determination of the methanol permeability requires the knowledge of the methanol drag factor, because the electro-osmotic flux could afford for a considerable fraction of the methanol flow, particularly at high methanol concentrations. An important drawback of this method is that the methanol drag coefficient is not well known, so Ren et al. [299] assumed that it was similar to the water drag coefficient ( =2.5). However, some recent NMR [300] and electro-osmosis [301] studies would indicate that this assumption is not valid, leading to considerable uncertainties in the methanol permeability coefficients determined by this method. 

Figure 7.2. Non-Linear dependence of electro-osmotic flux on potential difference for IRC-50 (H+ form)/methanol-water system.

For a typical one-molar methanol concentration, the electro-osmotic flux of methanol in the membrane is small as compared to the methanol diffusion. The expression for the crossover current follows from the methanol mass balance in the cell (Kulikovsky, 2002b)... 

Table 2.1 lists the measured value of the DMFC current density, the equivalent current density of methanol crossover, the total water flux at a DMFC cathode and the calculated water electro-osmotic drag coefficient from Equation 2.2 at various DMFC operating temperatures. 

It should be noted that in this method the electro-osmotic drag of methanol by the protons is opposite to the drag effects taking place in a DMFC under normal operation, where protons travel in the same direction than the methanol drived by diffusion from the anode to the cathode. The net methanol flux in this DMFC... 

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