Polarization curve DMFC current

Nevertheless, Eqs (3.32) with (3.27) and (3.28) can be used to analyse the DMFC polarization curves at low current densities. Moreover, these equations provide the basis for the construction of a more detailed quasi-2D model of DMFC (Section 4.7). 

In the general case of arbitrary A and A, DMFC performance is governed by the system (4.197), (4.198) and (4.233). Parameters J and E determine the point on the polarization curve. The mean current density in a cell is J the solution to (4.233) must, therefore, obey the relation... 

Figure 4.32 (a) Model polarization curves of DMFC for the indicated values of oxygen stoichiometry A° and A = 100. Shown is the dimensionless total voltage loss E as a function of the mean current density in a cell J. Crossover parameter / = 1, and the other parameters are the same as in Figure 4.29. (b) Experimental polarization curves of DMFC for the indicated values of oxygen stoichiometry A° and A A°. Diamonds—the points where the jumper forms. 

To verify these predictions the measurements of DMFC polarization curves in conditions -C A , J 1 were performed (Kulikovsky et al., 2005b). The results are shown in Figure 4.32(b). Comparing Figures 4.32(a) and 4.32(b), we see that the experimental polarization curves reproduce, remarkably well, the two main features of the model curves the drastic change in the slope at a current density J, where the jumper forms (large diamonds in Figure 4.32(b)) and the constancy of the product A°J J s for A° = 8,4 and 2 are related nearly as 1 2 4. 

Graphitic nanofibers with herringbone structure (for description and preparation method see [279]) were also employed as supports for PtRu (1 1) [280]. The total metal eontent was 42 wt% and the catalyst particle size was 6 nm. DMFC polarization experiments showed an approximately 50% improvement in superficial current density at constant cell voltage for the supported vs. unsupported catalyst over the entire polarization curve [280]. 

Yang et al. (2010) developed an EIS technique to characterize a DMFC under various operating conditions. A silver/silver chloride electrode was used as an external reference electrode to probe the anode and cathode during fuel cell operation. The external reference was sensitive to the anode and cathode as current was passed in the working DMFC. The impedance spectra and DMFC polarization curves were investigated systematically as a function of air and methanol flow rates, methanol concentration, temperature, and current density. Water flooding in the cathode was also examined. 

The model below shows that this approach is correct unless the cell current is not large (Kulikovsky, 2012c). At small currents, the MOR runs close to the membrane, while the ORR is shifted toward the GDL. In this regime, the DMFC cathode represents a complete short-circuited fuel cell. However, at large currents, the MOR and ORR share the same domain of the CCL, leading to a rapidly decreasing, resistive-like polarization curve. 

Figure 4.24 shows the CCL polarization curves for the range of parameter between zero (no crossover) and 0.5 (large crossover). Note that Figure 4.24 shows the electrode potential calculated according to Ecath = E oi — t]ox,o. The crossover dramatically (by 300-600 mV) lowers the electrode potential at the OCP (Figure 4.24). Note that for jS between 0 and 0.2, there is a range of currents (50 to 150 mA cm ), where the polarization curve is nearly flat (Figure 4.24). This is a feature of DMFC an increase in the useful current jo lowers the crossover current jcross,o so that the sum jo + jcross.o and hence the cell potential do not change significantly.

FIGURE 4.27 (a) The points the DMFC polarization curves for the indicated working temperatures (data from Argyropoulos et al. (2(X)2)). The dotted lines the linear fit according to Equation 4.215. The slopes of the straight line (the CCL resistivities) are collected in Table 4.3. (b) The calculated electrode polarization curve versus the sum of the useful and crossover current densities jo + jcrossfl for tbe indicated parameter jS. ... 

Fig. 4.23 DMFC performance of using various membranes with 3 M methanol in feed (a) polarization curves and (b) the relationship of power density and current density, cell 70°C MeOH(aq) feeding rate, 2 mL min rate of humidified O, 150 mL min" [73]. Reprinted from Y. L. Liu et al, Using silica nanopaiticles for modifying sulfonated poly(phthalazinone ether ketone) membrane for direct methanol fuel cell A significant improvement on cell performance. J. Power Sources (2005), with permission from Elsevier...

V for H2/O2 PEMFC and down to around 0.6 V for DMFC. The PEMFC (Figure 8.3) and SOFC (Figure 8.4) polarization curves at low current densities are quite different due to significant temperature dependence of the charge transfer reaction. At elevated temperatures, the electron transfer reaction is much faster, and therefore, the charge transfer resistance is much smaller. As a result, the experimental and theoretical (0.977 V) OCPs at 800°C are very close. 

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