Crossover problems

Moreover, well away from the critical point, the range of correlations is much smaller, and when this range is of the order of the range of the intenuolecular forces, analytic treatments should be appropriate, and the exponents should be classical . The need to reconcile the nonanalytic region with tlie classical region has led to attempts to solve the crossover problem, to be discussed in section A2.5.7.2. 

K. Bode, Dr. J. Ensling, Dr. J. Fleisch, Dr. P. Ganguli, Dr. K. M. Hasselbach, H. Koppen, Dr. R. Link, E.W. Muller, I. Sanner, Dr. M. Sorai, Dr. H. Spiering, H.G. Steinhauser, and G. Sudheimer, who have collaborated with me on spin crossover problems with great enthousiasm. I owe particular thanks to my colleague Dr. H. Spiering, who prepared the chapter on spin crossover models. 

Abnormal hemoglobulins can be detected by electrophoresis, as shown in Figure 7.4, which includes a pattern observed in /3+-thalassemia and one in a newborn with a-thalassemia (possibly HbH disease). It should also be mentioned that, unless there is a coexisting hemoglobin abnormality resulting from a point mutation or crossover problem, the globin chains of classic a- and /3-thalassemia are perfectly normal. It is usually the quantities of either the a or the /3 chains that are decreased. Some frameshifts have been found near the terminus of the /3 chain that lead to frameshift mutations in certain areas. 

The principle is shown in Figure 5.5. The electrolyte is KOH solution, with a fuel, such as hydrazine, or ammonia, mixed with it. The fuel anode is along the lines discussed in Section 5.4.4, with a platinum catalyst. The fuel is also fully in contact with the cathode. This makes the fuel crossover problem discussed in Section 3.5 very severe. However, in this case, it does not matter greatly, as the cathode catalyst is not platinum, and so the rate of reaction of the fuel on the cathode is very low. There is thus only one seal that could leak, a very low-pressure joint around the cathode. The cell is refuelled simply by adding more fuel to the electrolyte. A possible variation is to put a membrane across the electrolyte region and only provide fuel below the membrane - a suitably chosen membrane will stop the fuel from reaching the air cathode. 

The second major problem is that of fuel crossover. This was discussed briefly in Section 3.5. It is particularly acute in the DMFC because the electrolyte used is usually a proton exchange membrane (PEM), as described in Chapter 4. These readily absorb methanol, which mixes well with water, and so quickly reaches the cathode. This shows itself as a reduced open circuit voltage but affects the performance of the fuel cell at all currents. DMFC electrolytes and this fuel crossover problem are further discussed in Section 6.3. 

However, there are problems with this approach. The first is that all catalysts that do not promote the fuel oxidation tend only to very slowly promote the reaction of oxygen with the H+ ions. Thus, the activation losses on the cathode are made even worse than normal, and there is no increase in performance. Another problem is that although the mixed-potential problem may be solved, the fuel is still crossing over, and while it may not be reacting on the cathode, it will probably just evaporate instead. Thus, it will still be wasted. So, although it may be possible in the future to find selective cathode catalysts that amehorate the fuel crossover problem, this approach does not offer a complete solution. 

Scaling is not able to predict exactly how the concentration profile merges respectively into surface and bulk concentrations. This can be considered as a simple crossover problem, but it may turn out to be a major issue. 

Methanol crossover depends on a number of factors, the most important ones being the membrane permeability/thickness, the concentration of methanol in the fuel feed, the operating temperature, and the performance of the anode itself The membrane is a very important factor regarding the methanol crossover problem. Thinner membranes give lower resistances in the ceU, but tend to have a higher permeability for liquid methanol. For methanol fuel cells, a thicker membrane such as Nafion 120 is advantageous [63]. 

For DMFC, two alternatives can be used as approaches for the fuel feed the methanol-water mixture can be fed into the cell/stack as a liquid or as a vapor. Gas feeding of the fuel minimizes the crossover problem, but it can give more problems with humidification of the ceU. Both systems have been investigated and no conclusive arguments have been found as to which of the two systems is better. Methanol crossover is still the main problem for both configurations, so the development of methanol-tolerant cathodes and better membranes remains the biggest issue for these systems [61]. 

For DAFCs using a liquid feed, like the DMFC, the water balance and fuel crossover problem are more acute than the hydrogen fuel cell. Dilute Uquid solutions, thicker membranes, and capillary pressure management are used to control these two issues. As a result of the high methanol and water crossover in the DMFC, the open-circuit potential is very low, and performance is also low compared to the H2 PEFC. However, the use of diffusion barriers in the anode and capillary pressure management eliminates the need for highly dilute methanol solutions, and these systems may ultimately be more appropriate than their H2 PEFC counterparts for portable applications. 

Interaction mechanism illustrated in the model of Figure 19.S could also further reduce the methanol crossover problem related to DMFC. Figure 19.6 illustrates the water, methanol, and proton transport model in SPEEK/CloisitelSA nanocomposite membrane that is attached by TAP. From the transport model, illustrated in the... 

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