Dependence of Proton Conductivity on Cathode Degradation in PEFC

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I ntroduction Since the commercialization of FCVs, PEFCs are paid more attention to, and for further improvement of PEFC efficiency, raising the operating temperature is considered as one of the possible methods. Increase in the temperature above 100℃ leads to decrease in relative humidity, resulting in lower proton conductivity. Based on this background, we have been working on durability analyses of PEFC cathode under the low proton conductivity condition. Our study has suggested that the proton conductivity of ionomers in the cathode layer more dominantly influences on the degradation of PEFC cathode than that of electrolyte membranes [1, 2]. Therefore, in this study, dependence of the proton conductivity of ionomers on PEFC cathode degradation has been investigated by controlling the proton conductivity of ionomers. Experimental Three kinds of ionomers with different proton conductivity, AquivionD72>AquivionD83>Nafion, were used in this study. MEAs were made by spraying the slurry containing 46%Pt/KB (TEC10E50E) and each ionomer on the Nafion membrane (Nafion 212). Resulting MEAs were named as AqD72 MEA, AqD83 MEA, and Naf MEA, and evaluated under the high humidity condition (80℃-RH100%) and the low humidity condition (105℃-RH57%), where difference in proton conductivity influences less and more on the fuel cell reaction, respectively. Here, for durability analyses, degradation of carbon derived by the reaction with H2O in the cathode was focused on, and potential cycling between 1.0 and 1.5 V was performed as an accelerated degradation test. Then, changes in IV performance and each overvoltage were evaluated before and after the durability test. Changes in the cathode pore structure were also analyzed by FIB-SEM. Results and discussion Pore volume distribution before and after durability test was analyzed separately for the GDL side and the membrane side. Here, for each MEA, three different parts of the cathode were evaluated by FIB-SEM, and average pore volume with S.D. was analyzed based on the cathode image reconstructed from about 100 FIB-SEM pictures. Under low humidity condition, Naf MEA showed more localized increase in pore volume derived by carbon oxidation degradation at the membrane side than AqD72 MEA and AqD83 MEA, as shown in Figure 1. The reason is probably because only H2O generated by fuel cell reaction can contribute to carbon oxidation reaction in the low humidity condition, and such H2O is inhomogeneously produced only near the membrane side since Nafion has low proton conductivity (0.026 S/cm) comparing to AquivionD72 (0.090 S/cm) and AquivionD83 (0.080 S/cm). Unlike the low humidity condition, the pore volume in the cathode similarly increased on degradation among all MEAs under the high humidity condition. This is most likely because ionomers have enough high proton conductivity (over 0.060 S/cm) in all MEAs under high humidity. Consequently, dependence of proton conductivity on carbon oxidation degradation in the low humidity condition was observed, and controlling the proton conductivity of ionomers was found to be important especially under low humidity.