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Polymer electrolyte membrane fuel cell (PEMFC) is an attractive alternative power source for next-generation vehicles due to its high energy density, high conversion efficiency and less environmental pollutants than other conventional energy sources. However one of the critical issues in commercializing fuel cell vehicles is the durability and cost of the PEMFC. The durability targets of United States Department of Energy (DOE) require operation time more than 5,000 hours. Although it is very difficult to clearly define the lifetime of PEMFC, it is generally accepted that current technologies cannot achieve the target lifespan. Particularly, repeated start up-shut down (SU/SD) conditions induce both carbon corrosion and platinum degradation seriously. The mechanism of carbon corrosion is shown in eq. (1) C + 2H2O → CO2 + 4H+ + 4e- Eo = 0.207 V (1) Although carbon is thermodynamically unstable to electrochemical carbon corrosion, the slow kinetics of this reaction makes it possible to use carbon in PEMFCs. In order to prevent this carbon corrosion phenomenon, a lot of researchers have suggested several solutions, i.e., to use of highly graphitized carbon and/or metal oxide as the supporting materials [1], to change the stack operation conditions for preventing air/fuel boundaries [2] and to use OER catalyst as the additional catalyst for protecting catalyst from the carbon corrosion.[3]. Here, we propose that the addition of an OER catalyst is one of the best choices for maintaining durable MEA because the addition doesn’t seem to severely alter overall components of electrode. This research is composed of three parts. Firstly, we establish an evaluation protocol mimicking the SU/SD phenomena in fuel cell vehicles. And then, the investigation of the OER catalyst with high activity and stability under PEMFC operating conditions is carried out. Lastly, the effect of an additional OER catalyst through the SU/SD test in single cell condition is evaluated. The 3M reports show that the SU conditions occur about 13,600 times and the abnormal increment of cathode potential occurs over 4,000 times during the expected life time, 6,000 hrs. In addition, as the cathode potential is increased over 1.4V, the oxidation current of 5mC/cm2 flows at the cathode electrode. This current causes water or carbon supports oxidized in cathode catalytic layer. From the analysis, 3M proposed the evaluation protocol of SU/SD (Fig 1(a)). However, it is difficult to apply the 3M protocol to our evaluation directly because the nano-structured thin film (NSTF), the distinctive catalytic layer employed in 3M does not have carbon material. Therefore, the modified protocol was proposed (Fig1 (b)). The voltage scan and oxidative current we used here are same as the values in 3M protocol, up to 1.4 V, 5 mC/cm2, respectively. With the protocol, catalytic activity and MEA durability were evaluated per every 1,000 cycle of ESCA. The purpose of second part is to investigate the high activity and stability OER catalyst. It is well-know that RuOx is the best catalyst for OER, but it dissolves out easily in acidic and high voltage conditions. Therefore, IrOx has been used as an OER catalyst in acidic conditions in spite of less activity compared to RuOx. Here we introduce the alloy compositions and forms between RuOx and IrOx in order to increase the OER activity and stability. Fig. 2 indicated that the mixing ratio of Ir and Ru, 1:1 is ideal to improve both activity and stability. Finally, the optimized OER catalyst was applied to the reference MEA of Samsung SDI. Although the reference MEA showed high performance, there was a large degradation in performance during the SU/SD cycling test. In comparison, the MEA applied OER catalyst of 2.5% maintained very high performance after 5,000 SU/SD cycles. References [1] Y. Shao, G. Yin, Y. Gao, P. Shi, J. Electrochem. Soc., 153, A1093 (2006). [2] K.S. Eom, Y. Y Jo, E. A. Cho, T-H Lim, J. H. Jang, H-J Kim, B. K. Hong, J. H. Lee, J. Power Sources, 198, 42 (2012). [3] J-G. Oh, W. H. Lee, H. Kim, Int. J. Hydrogen Energy, 37, 2455 (2012). |
