Investigation of Degradation at the PEFC Cathode Layer under Higher Temperature Operation

Abstract

Introduction Elevation of the PEFC operation temperature over 100 oC is expected for the next generation FCVs in order to further increase the efficiency. Even so, only a few reports studying MEA performance and durability over 100 oC are available. In our laboratory, we have been examining MEAs at the high temperature and low humidity condition, and particularly paying attention to the durability of electrocatalysts. During our study, carbon corrosion was found to be promoted at the high temperature and low humidity condition when the accelerating degradation test simulating the start/stop cycles of FCVs was performed. This carbon deterioration was observed at the interface between the cathode layer and the Nafion electrolyte membrane in particular. We believe that this phenomenon is derived from the decreased proton conductivity of Nafion under the low humidity and carbon corrosion homogeneously occurs over the cathode layer if the proton conductivity is high enough through the cathode layer. Therefore, the aim of this study is clarifying the phenomenon of cathode layer deterioration using the electrolyte with higher proton conductivity Experimental Aquivion membrane was used in this study. Here, proton conductivity of Aquivion is higher than that of Nafion, and is assumed to be high enough even over 100 oC. Then, MEAs were made through spray-printing catalyst layers containing 46.2%Pt/KB electrocatalyst (TEC10E50E) and Aquivion ionomer. IV performance was evaluated at 105 oC-RH57 % by supplying 100 cc/min of hydrogen and 100 cc/min of air to the anode and the cathode, respectively. Then, the durability test was carried out by fluctuating the potential between 1.0 and 1.5 V based on the protocol1simulating the start/stop cycles of FCVs. Besides IV characteristics, ECSA and impedance measurements were also performed. Finally, the change in the cathode layer structure was evaluated three dimensionally using the FIB-SEM technique. Results and discussion Durability tests were performed up to 10,000 cycles at 105 oC.-RH57 %. Then, the change in IV characteristics, ECSA, and cathode structures was compared between Aquivion MEA and Nafion MEA. Even after 2000 cycles, with Aquivion MEA, large increase in activation overvoltage and also in ECSA was observed. Based on this observation, the degradation phenomenon is assumed to be different between the two MEAs as shown in Fig. 1. For Nafion MEA, carbon oxidation reactions only occur at the interface between Nafion membrane and the cathode layer because protons cannot easily transfer into the cathode layer owing to decreased proton conductivity of Nafion (Fig. 1a). On the other hand, in Aquivion MEA, protons can more easily move into the cathode layer, and then deterioration of carbon is promoted (Fig. 1b). In order to confirm our assumption, the structure change in the cathode layer was analyzed by the FIB-SEM techique. After the three dimensional reconstruction based on 100 SEM images, relatively large pores produced by carbon corrosion were more often observed near Nafion membrane in Nafion MEA. On the other hand, in Aquivion MEA, the pores were homogeneously distributed. In other words, deflection of cathode deterioration observed in Nafion MEA does not take place even at the high temperature if the proton conductivity is high enough. (1)A. Ohma et al., ECS Trans, 41 (2011) 755. Figure 1