Methanol flux

The methanol permeation measurement illustrates the relationship between the methanol permeation rate and the temperature versus the thickness of the membrane. For thinner samples, the permeation rate changes by a factor of 4 or 5 with a temperarnre change from 25°C to 65°C, while in the thicker membranes this factor is about 3. At lower temperatures, the variation of the methanol flux through the membranes versus thickness is small compared to that at 65°C, where this dependence is pronounced. As anticipated, a strong decrease in permeation with thickness is observed in all cases. For the reciprocal quantity, a linear relationship between permeation rate and thickness can be derived approximately as shown in the inset of Figure 27.64. The comparison between the composite membranes and the commercial Nafion (Nafion 112, 115, and 117) reveals a similar methanol permeation rate at elevated temperature with slightly higher values of the composites. 

Under most common DMFC operation conditions, the oxygen reduction process is under interfacial kinetics control and the methanol oxidation process is determined by the methanol flux across the membrane,. /crossover, (Eqs 59-61). The increase in cathode overpotential is therefore be given by... 

Figure 9. Fluxes of methanol measured with a fast response PTR-MS instrument during hay harvesting at a field site in western Austria. The data shown are for methanol fluxes on the second day after hay cutting, and for periods when the prevailing wind was suitable. Methanol fluxes largely correlated with air temperature, declined sharply in the afternoon, and approached zero as the hay harvest began after 15 45. Also plotted are sensible heat flux (i.e. transfer of heat due to conduction and convection) and latent heat flux (i.e. heat loss due to evaporation of liquid water). Data redrawn from Ref. [45].

A packed-bed distillation column is used to adiabatically separate a mixture of methanol and water at a total pressure of 1 atm. Methanol—the more volatile of the two components—diffuses from the liquid phase toward the vapor phase, while water diffuses in the opposite direction. Assuming that the molar latent heat of vaporization is similar for the two components, this process is usually modeled as one of equimolar counterdiffusion. At a point in the column, the mass-transfer coefficient is estimated as 1.62 x 10-5 kmol/m2-s-kPa. The gas-phase methanol mole fraction at the interface is 0.707, while at the bulk of the gas it is 0.656. Estimate the methanol flux at that point. 

Therefore, the points N (0.172, 0.600) and P (0.200, 0.360) completely define the straight line that contains the segment PM. The intersection of line PN with the equilibrium curve yields the interfacial concentrations at point M. They are very similar to those obtained by the rigorous method using the E-type coefficients, namely, xA i = 0.178, yAi = 0.550. The local methanol flux is... 

At a different point in the packed distillation column of Example 3.6, the methanol content of the bulk of the gas phase is 76.2 mol% that of the bulk of the liquid phase is 60 mol%. The temperature at that point in the tower is around 343 K. The packing characteristics and flow rates at that point are such that Fg = 1.542 x 10-3 kmol/m2-s and Fr = 8.650 x 10-3 kmol/m2-s. Calculate the interfacial compositions and the local methanol flux. To calculate the latent heat of vaporization at the new temperature, modify the values given in Example 3.6 using Watson s method (Smith et al., 1996) ... 

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... 

In liquid-fed DMFCs, methanol typically also constitutes a small fraction of water-methanol solution and Eq. (1.52) can be written for methanol flux... 

In DMFCs, the methanol flux through the membrane is large and it cannot be ignored. The flux of methanol in the anode backing layer of DMFC obeys an equation... 

However, oxygen and methanol fluxes in the DMFC are affected by crossover. Thus, to construct a full ID model of this cell we should write the balance of methanol and oxygen fluxes taking crossover into account. From this balance we will deduce the methanol (cf) and oxygen (cj) concentrations in the respective catalyst layer. Making a substitution similar to (3.4), we will obtain the half-cell polarization voltages and then construct the polarization curve of the whole cell. 

To avoid confusion it should be noted that jcross is simply the methanol flux expressed in units of electric current physically this is not a current since no charge is transported with this flirx. 

Condition 7 = 1 is equivalent to = j, or 6FD c °// = AFDld fP/ll- In other words, at the inlet the methanol flux in the anode backing layer equals the oxygen flux in the cathode backing layer. Both fluxes are maximal since they provide limiting current density. The concentrations of oxygen and methanol in the respective catalyst layer thus tend to zero. 

Setting in Equation 4.211 x = = 0, and assuming that the methanol flux... 

Ideally, the potential drop between the auxiliary electrode and the cathode should provide complete oxidation of the methanol flux in the membrane. This potential drop is easy to control by varying the load resistor R. Thus, for any given current in the main load, the value of R should be selected to provide the highest cell potential. Note that the resistor R could also be a useful load, so that the current in R is not wasted. 

The methanol concentration Cmt in the anode depends on the cell current density. A balance of methanol flux through the DMEC leads to the following relation between Cmt and the methanol concentration in the feed channel Ch (Kulikovsky, 2002b) ... 

New organic-inorganic composite membranes based on sulfonated polyether-ketone (SPEK) and SPEEK were synthesized with SiO, TiO, and ZrO. The modification of SPEK and SPEEK with ZrO reduced the methanol flux by 60-fold. On the other hand, there was a big compromise on conductivity, which was reduced by 13-fold, while modification of PEK and SPEEK with silane (SiO ) led to a 40-fold decrease of water permeability without a large decrease of protonic conductivity [35]. With some encouraging results of the modification of PEK with SiO, TiOj, and ZrO, modification of PEK with heteropolyacid further yielded some notable results. Actually the composite membranes were prepared using an organic matrix... 

Nafion/PTFE composite membranes were fabricated [17] by impregnation of a porons PI LE film with a solution of Nafion in a 2-propanol/water mixture. After solvent evaporation, the impregnated film was annealed at 120°C for 1 h. The resultant membrane was 20 pm in thickness, with a proton conductivity of 0.033 S cm at 25°C (30% that of Nafion 117), while the methanol flux was 4.43x10 mol cm s (as compared with 1.62x10 mol cm s for Nafion 117 and 6.20x10 mol cm s for Nafion 112). DMEC performance of the composite membrane, measured at 70 C with 2.0 M methanol feed and pure oxygen, was superior to that of both Nafion 112 and 117. 

The simplest and most straightforward way to decrease the methanol flux across a Nafion membrane is to increase its thickness. The effect of Nafion thickness on DMFC performance was studied by fabricating membrane electrode assemblies (MEAs) with multiple (stacked) Nafion 112 films (where each Nafion 112 layer had a wet thickness of 60 pm). Using this approach, one can effectively create Nafion fuel cell membranes that are thicker than that which is available commercially. Figure 14.4 shows the effect of Nafion membrane thickness (stacking... 

Fig. 14.6 The effect of methanol concentration and membrane thickness on DMFC performance curves. Methanol crossover fluxes are given relative to Nafion 117 (215 um) at 1.0 M and 60°C. Crossover is expressed as a relative fraction of the methanol flux observed for a Nafion 117 membrane at the fuel cell operation conditions with Nafion 117 and 1.0 M methanol...

Methanol flux from the reservoir compartment (A) to receiving compartment (B) Vg= Volume of compartment B A = Membrane area... 

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