Vol.5 No.1 2012
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Research paper : Durable polymer electrolyte fuel cells (PEFC) for residential co-generation application (K. Tanimoto et al.)−59−Synthesiology - English edition Vol.5 No.1 (2012) factors would progress. In the actual project, the confirmation of 40,000-hour lifetime by the accelerated test was not the only objective, but the clarification of the degradation factors was an important issue of PEFC development. In this project, to establish the technology for practical accelerated tests for the fuel cell cogeneration system expected to be commercialized in 2008, the accelerated test to estimate the use of 40,000 hours in one year was developed. Led by the energy supply companies that were close to the end users, roundtable discussions were held among the energy supply companies and the fuel cell system maker on the issues pertaining to the PEFC degradation in the fuel cell cogeneration system. The priority issues for the PEFC degradation were extracted, and the objectives were set to clarify the degradation mechanism and to develop the accelerated test method.[1]4 Investigation of the degradation mechanism4.1 Degradation factors of the actual cell/stack and the accelerated aging methodFigure 3 shows the data for the time profile of the cell performance that the participating members possessed at the start of the project. The degradation due to the decreased cell performance were categorized into four patterns and investigated.In the accelerated degradation pattern of (c) the constant output is continued in normal operation, but the folding point cannot be estimated. It was judged to be most fatal for the cogeneration system because the standard output would no longer be obtained immediately after the folding point. In the linear performance decline of (a), the end life can be estimated from the rate of voltage decrease. If this period is faster than 40,000 hours, it must be controlled. For pattern (d), it was determined that this is caused by the problems in the management of cell manufacture or the failure in system configuration. Based on the above patterns, and considering the time profile data of the cell performance of the system makers and the information pertaining to the cell material degradation, it was determined that the most important point was to cover the following three requirements, and priority should be given on solving them as well as clarifying the mechanism. The three requirements were: 1) to obtain the accelerated degradation pattern of (c); 2) considering the case of 40,000 hour durability with no saturation in the decrease of cell voltage, the effect is fatal; and 3) at this point, the evaluation method of degradation and countermeasures are not clear. The “decline of CO poisoning resistance” and the “decrease of gas diffusion due to the flooding at the electrode” shown in Fig. 4 were selected as main topics for the practical degradation factors, and were shared commonly as main topics among the members.As the test protocol to accelerate the degradation factor, we proposed the introduced gas switching method. This test involved the two techniques: 1) the method of introducing air and nitrogen (inactive atmosphere) alternatively to the cathode (air electrode) of PEFC (method 1); and 2) the method of repeating the cycle of hydrogen nitrogen air nitrogen (hydrogen) to the anode (fuel electrode) of PEFC (method 2). In both methods, it was predicted that the surface of the supported carbon of the cathode catalyst layer would become oxidized by the retention of high potential at the cathode, flooding by water accumulated on the catalyst surface would progress, and gas diffusion at the electrode would decrease. For the establishment of the accelerated test protocol, the clarification of the corrosion reaction mechanism and the carbon corrosion behavior at the material surface was important.For the “decrease of CO poisoning resistance of the catalyst at the fuel electrode” that was the other degradation factor, we considered including this as the accelerating condition. However, in the actual system, it was confirmed that the effect was seen when the CO concentration in the fuel gas was at several 10 ppm level. It was determined that a practical accelerated test would not be possible unless the factors that caused the CO poisoning could be controlled with high accuracy, and it was also determined that it was not appropriate to conduct the accelerated test on the base of this factor. Since this phenomenon occurred as a result, it could be used as the index for determining the degree of degradation. In the actual PEFC anode catalyst, normally, to add resistance to CO poisoning, the alloy of platinum (Pt) and ruthenium (Ru) is used instead of the Pt catalyst that is readily poisoned. However, the decrease of CO poisoning resistance occurs with this alloy catalyst, and it is thought to occur as the Ru dissolute from the anode catalyst in long-Fig. 3 Types of degradation behavior in the time change pattern of cell propertyTimeTimeTimeTimeCell voltageCell voltageCell voltageCell voltage(d) Sudden degradation type(c) Accelerated degradation type(b) Saturated degradation type(a) Linear degradation typePatterns of decrease of cell performance profileCan be estimated since decrease is proportional to time. If the degree of decrease is great, the reduction of degradation rate is necessary.Fatal degradation

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