Temperature effects cell performance, humidification

FIGURE 4.9 Effect of cell temperature on cell performance at a humidification temperature of 50 °C [71],... 

Recently, the temperature effect on the performance of an HT-PEM fuel cell with a PBI-membrane-based MEA (purchased from PEMEAS Fuel Cell Technologies) in the temperamre range of 120-200 °C was also reported [2]. As the proton conductivity of a PBI membrane does not rely on its water content, the fuel cell was operated without extra humidification (0% RH) at ambient backpressure. 

By using the impedance method, Freire et al. also demonstrated that thinner membranes not only show better performance but also are much less sensitive to humidification conditions, cell temperature, and current density. The dependence of the real resistance at high-frequency intercepts on membrane thickness at different current densities is illustrated in Figure 6.14. Linear dependence of the high-frequency resistance, RhJ. on the membrane thickness was observed at 80°C, whereas non-linear dependence of RhJ on membrane thickness was shown at 40°C and 60°C. They explained that this was due to better hydration of the membrane at higher temperatures. It is also observable that Rhf almost does not depend on current density at 80°C and at low current densities rather, Rhf dependence on membrane thickness is almost linear, which indicates that for thicker membranes at high current densities it no longer behaves as a pure resistor due to a capacitive effect caused by less effective back transport of water [9],... 

On the other hand, the use of external humidification is essential at high temperatures, because the concentration gradient of H2O in the membrane of individual cells would be more uniform if both air and hydrogen streams were humidified externally. The external water supply helps to balance the combined effects of electro-osmotic drag and back diffusion permitting to maintain the performance of the membrane. External humidification is practically useful also below 60°C, at least for medium large-size FCS. 

Based on the Model 3, a 10-cell stack of microfuel cells was assembled. It was reported that an output of near 10 W was achieved, as shown in Fig. 8.23 [47]. Recently, through membrane electrode assembly (MEA) improvement, a power density 290 mW cm of the cell with an air cathode has been achieved. A 5-cell stack with effective area of 67 cm demonstrated that the power reached IlOW when the operating temperature reached 60 °C, though the stack started at room temperature without humidification. The performances of single ceU and 5-ceU stacks are shown in Fig. 8.24. 

The temperature of operation has one of the strongest influences on durability and contamination effects. It can also have a strong influence on performance when coupled with humidification effects. Typical fuel cell operation is in the range of 60 to 80°C, but the overall drive in the fuel cell industry is to run at higher temperatures in order to reduce system cooling requirements and catalyst contamination issues. 

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