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Chapter 4 Nafion®/H-ZSM-5 composite membranes with superior performance for DMFCs 3.6. Single cell performance In literature [8, 15-17] studies, which are dealing with polymer-zeolite composite membranes for DMFCs, use the characteristic number, β (proton conductivity/methanol permeability), for prediction of DMFC performance instead of the real fuel cell. In this study, we combine the regular characterization methods with DMFC measurements. The DEZ25-5 and DEZ50-1 composite membranes were selected for further tests in DMFC due to their low methanol cross-over and high proton conductivity, respectively. Their performance is compared to that of pure DE2020. MEAs were prepared by hot pressing two E-TEK commercial electrodes on both sides of the membranes at 10bar. At least two MEAs were prepared and measured for each type of membrane. Every day minimum 20 polarization curves made for each MEA and then the system switched off and no methanol and oxygen fed to the system. The next day the system was switched on again and another 20 polarization curves were obtained. This procedure was followed for 5 days. Figure 9 presents typical results of maximum power density of each polarization curve as a function of time for DE2020, DEZ50-1, DEZ25-5 and N117 membranes. The concentration of methanol at the anode side was 1M. Each symbol in the graphs corresponds to a different MEA. All three composite membranes show an increase in their maximum power density (MPD) up to 400th minute. It seems that in the beginning of the experiment, maximum power densities increase due to the activation of all catalyst particles and also due to wetting of the membrane. From that point on DE2020 shows an increase in its MPD in the beginning of the measurement and then decreases continuously till the end of the measurement for that day. Next day, it again shows high MPD value and then decreases again and if one waits long enough, MPD reaches around 275mW/cm2 (Figure 9a), which is in agreement with our previous study for DE2020 membrane [18]. DEZ50-1 shows similar phenomena, perhaps, due to its low zeolite loading. Its MPD reaches 250-275mW/cm2 (Figure 9b). The DEZ25-5 shows same trends (high MPD in the beginning and drop later) like the other two mem- branes. However, the drop is less sharp than the other two and most impor- tantly reaches high MPD in the end of the 5th day at around 300mW/cm2. One can see that the performance of N117 MEA (taken from our earlier work [18], prepared under the same conditions as DEZ MEAs) is very stable without drop throughout the day (Figure 9d). It seems that those fluctua- 66PDF Image | Development of membrane materials for direct methanol fuel cells
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