Vol.5 No.1 2012

Research paper : Durable polymer electrolyte fuel cells (PEFC) for residential co-generation application (K. Tanimoto et al.)−61−Synthesiology - English edition Vol.5 No.1 (2012) is started when the air is remaining in the anode, a transient state appears where the region with fuel and the region with oxygen exist simultaneously within a cell, the reverse current is produced in some areas with remaining oxygen, and the electrode material carbon becomes corroded as the potential of the cathode becomes high in some areas. Since such phenomenon could not be measured by observing the cell from the outside, to verify the “reverse current decay mechanism,” we conducted measurements by fabricating the “100 segment cell” (Fig. 6). The electrode was divided into minute segments, settings were done to measure the time change of the generated current distribution and the local potential distribution within a cell, and transient states were created by switching several types of gases. As a result, we succeeded in measuring the abnormal high potential of about 1.6 V locally (in normal operation, the material will not be subject to potential higher than 1 V) in the transient state in which the “reverse current decay mechanism” was expected to occur.[6] From the measurements of other types of transient states, we found two phenomena that were closely related to the conditions of the proposed accelerated aging test. One was the phenomenon in which the high potential that may cause cathode degradation like the “reverse current decay mechanism” could occur when the anode gas was switched from nitrogen to fuel. Another phenomenon was when the cathode was switched from nitrogen to air, certain anode had high local potential (about 0.7 V) in the potential range of 1 V or less. These results offered logical explanation for the phenomena that occurred in the “accelerated aging method 2” and “accelerated aging method 1” respectively. The phenomenon in which the anode local potential increased, for example, to 0.7 V in the cathode gas switching test was certainly a degradation factor as the ruthenium in the anode catalyst eluted. However, it was considered harmless for the carbon material since it was 1 V or less. However, when a basic test using beaker cells was conducted for the purpose of studying the factors that affected the rate of carbon corrosion, it was found that the change in potential was the factor that promoted carbon corrosion even in the range of 1 V or less that was thought to not cause much corrosion.[7] Combining these findings, it was found that the degradation in the “accelerated aging method 1” accelerated not only the ruthenium elution but also the carbon corrosion.In the PEFC cathode, the surface area of the effective platinum catalysis decreased with degradation, and this was because of the increased particle size and loss of platinum due to dissolving, as well as the shedding and aggregation due to the corrosion of the supported carbon. To investigate these phenomena, we devised the “identical position observation” that allowed almost “in-site observation.” Microscopic observation (AFM and SEM) of the shedding and aggregation of the platinum particles on the model electrode was conducted, and the phenomenon where the presence of platinum promoted the carbon corrosion was captured.[8][9] These results made basic contributions in accurately understanding the degradation phenomena that occurred under the accelerated aging condition. 5 From the development of accelerated aging method to commercializationAs the test method for accelerating the degradation process by enhancing the “decrease of gas diffusion by the flooding at the electrode,” we proposed two gas switching methods. The phenomenon of flooding due to increasing the wettability of the cathode was hypothesized as being caused by the production of hydrophilic functional group at the surface of the catalyst support carbon. For the production of this hydrophilic group, the carbon oxidation would not be enhanced greatly even if open voltage 1 V was retained, but such functional group might be yielded by shifting the potential by switching Fig. 6 External appearance of the segmented electrode of the “100 segment cell” fabricated to measure partial current and local potentialFig. 5 TEM images of the Pt/C electrode catalyst layer and the electrolyte membrane interface after conducting the aging test by applying constant voltage(a) Using the 50 m electrolyte membrane, nitrogen was supplied to the cathode and 1.0 V was applied for 87 hours. (b) Using the 175 m electrolyte membrane, nitrogen was supplied to the cathode and 1.0 V was applied for 87 hours. (c) Using the 175 m electrolyte membrane, air was supplied to the cathode and 1.0 V was applied for 30 hours. (d) Using the 175 m electrolyte membrane, air was supplied to the cathode and 1.0 V was applied for 87 hours.500 nmPt/C50 µm N2 87h175 µm N2 87h175 µm Air 30h175 µm Air 87h


page 64