We investigated the durability of cathode catalysts during a start-up (SU) process similar to that used in actual polymer electrolyte fuel cell vehicle operation. The durability of Pt supported on graphitized carbon black (Pt/GCB) catalysts was evaluated in the practical SU process, i.e., the anode gas was successively cycled between air, hydrogen, and nitrogen. The effect of the SU process on the cell performance was evaluated using two types of catalysts (commercial Pt/GCB, and that prepared in house by the "nanocapsulemethod," n-Pt/GCB). The polarization curves and cyclic voltammetry were evaluated before and after the SU evaluation. The degradation of Pt nanoparticles and carbon supports was analyzed by transmission electron microscopy, scanning transmission electron microscopy, and micro-Raman spectroscopy. We also applied glancing incidence X-ray diffraction in order to observe the depth profiles of the Pt crystallite sizes in various interfacial regions. From these analyses, we found that the degradation of the Pt catalyst occurred not only in the gas outlet region but also the gas inlet region of the cathode. The degradation in the inlet region is ascribed to both the interim electrochemical evaluations and the potential fluctuations, which cause a dissolution of Pt nanoparticles used during the SU process. Polymer electrolyte fuel cells (PEFCs) are able to convert chemical energy directly to electrical energy with high efficiency and have shown promise to be an eco-friendly power source for residential cogeneration systems and fuel cell vehicles (FCV).1-3 Nevertheless, the widespread commercialization of PEFCs has been impeded because of the large amount of the platinum catalyst required, which must be minimized to reduce the system cost. 4 The minimization can be achieved, for example, by the improvement of utilization of Pt nanoparticles dispersed on carbon black supports (Pt/CB) with high surface area used as the cathode catalyst for PEFCs as well as the improvement of the durability. During the start-up and shutdown (SU/SD) cycles of the PEFCs, air and H 2 coexist transiently in the anode until the replacement of the former with the latter (or vice versa) is completed. Reiser et al. showed that this situation causes the cathode potential to climb to more than 1.5 V due to the so-called "reverse current mechanism", 5 which significantly accelerates the degradation of the catalyst due to the oxidation of the CB and the agglomeration or dissolution of the Pt nanoparticles. 6 In our previous research, we have developed a graphitized carbon black (GCB) supported Pt catalyst prepared by the "nanocapsule method" (n-Pt/GCB) to improve the performance of Pt catalysts. 7-9The high dispersion of the Pt nanoparticle catalyst particles on the GCB support prepared by this method provides enhanced oxygen reduction reaction (ORR) activity. It was also found that the durability of the n-Pt/GCB catalyst was improved during a potential cycling evaluation that simulated the variation of the cathode potential during the...
The use of a hydrogen purge for startup and shutdown (H 2 -SU/SD) process of polymer electrolyte fuel cells has been proposed, which suppresses the generation of internal current during the SU/SD, process so-called "reverse current", and the severe carbon oxidation reaction (COR) in the cathode. However it was found that the COR was still caused during this H2-SU/SD process, even though it was less severe than that during the usual SU/SD process, i.e., the anode gas was successively cycled between air and H 2 . In order to clarify the mechanisms of the COR, we investigated (1) the effect of the presence of Pt catalyst, (2) the timing, and (3) the effect of Pt oxidation state. These results indicated that the COR was accelerated by the Pt catalyst in the cathode and was decelerated with increasing cathode potential during the H 2 -SU/SD process. We propose that the COR is caused by a shortage of protons associated with both the reduction of the Pt oxide and the oxygen reduction reaction at the reduced Pt. Polymer electrolyte fuel cells (PEFCs) convert chemical energy directly to electrical energy with low emissions and high energy efficiency and have shown promise to be an eco-friendly power source for fuel cell vehicles (FCVs) and residential co-generation systems. [1][2][3] Nevertheless, PEFCs still have several problems to be solved, such as limited lifetime and reliability and high cost, before large-scale commercialization can be realized. [4][5][6] The minimization of the PEFC cost can be achieved by improving the specific mass activity (MA) of the catalyst for the oxygen reduction reaction (ORR) at the cathode. The conventional cathode catalysts have consisted of Pt nanoparticles dispersed on high surface area carbon black supports (Pt/CB) to maximize the electrochemically active surface area (ECSA) for the ORR. 4 However, as is widely known, Pt/CB cathode catalysts are degraded under PEFC operating conditions such as load change cycles and startup/shutdown (SU/SD) cycles due to a combination of processes, which include ECSA loss due to the agglomeration or dissolution of Pt nanoparticles, [6][7][8][9][10][11][12] and the corrosion of the CB support material. 7,[11][12][13][14][15] During the SU/SD cycles, air and H 2 coexist transiently in the anode until the replacement of the former with the latter (or vice versa) is completed. Reiser et al. showed that this situation causes the cathode potential to climb to more than 1.5 V due to the so-called "reverse current mechanism", which significantly accelerates the degradation of the catalyst due to the oxidation of the CB and the agglomeration or dissolution of the Pt nanoparticles. 12 The latest papers on this topic have been reviewed. 13 Several approaches have been taken to both understand and mitigate the decrease of PEFC performance during operation.13 One approach to mitigate the decrease of the cell performance is to use transition metal oxide support materials, for example, titanium-based oxides [16][17][18][19][20] and tin-based oxides. [21][22][23][...
In polymer electrolyte fuel cells (PEFC), Pt nanoparticles on high surface area carbon black (CB) supports are used as the cathode catalyst. However, it has been found that Pt/CB catalysts are degraded under PEFC operating conditions due to a combination of processes, which include loss of electrochemically active surface area (ECA) due to agglomeration or dissolution of Pt nanoparticles, and corrosion of the CB supports1). During the start-up of fuel cell vehicles (FCVs), the cathode potential can momentarily climb to more than 1.5 V due to the “reverse current mechanism”, which significantly accelerates the carbon corrosion2). In order to reduce the cost of PEFCs, it is necessary to reduce the amount of Pt by improving the durability, especially for the cathode catalysts. To mitigate the corrosion of the carbon support, the use of graphitized carbon blacks (GCB), for which there is a high degree of graphitization, was found to be effective3). In this study, we investigated the cathode catalyst durability during a start-up process like that in actual FCV operation. Pt/graphitized carbon black catalysts were tested in this practical start-up process, i.e., the anode gas was successively cycled between air, hydrogen, and nitrogen. First, air was flowed to the anode and cathode for 90 s. Second, H2 gas was flowed to the anode to start up the fuel cell for 90 s. Finally, N2 gas was flowed to the anode and cathode for 60 s. These three steps are defined as one cycle. The effect of the start-up process on the cell performance was evaluated using three types of catalysts (commercial 50 wt% Pt/GCB and commercial 30 wt% Pt/GCB supplied by Tanaka Kikinzoku Kogyo K.K., and 30 wt% n-Pt/GCB prepared in house by the nanocapsule-method). The average Pt particle sizes of 50 wt% Pt/GCB, 30 wt% Pt/GCB and n-Pt/GCB were 3.4 ± 0.7 nm, 3.3 ± 0.5 nm and 2.6 ± 0.4 nm, respectively. All of the catalysts were tested for 1000 cycles. The I-V performances were evaluated under O2 and air at 65 °C with 100% RH before and after the durability test, which consisted of given numbers of start-up cycles (N). Cyclic voltammetry (CV) was also examined to estimate the ECA under N2 at the cathode and H2at the anode at 65 °C and 100% RH every 200 cycles. Figure 1 shows the changes of ECA values for each catalyst. Before the durability test, n-Pt/GCB showed the highest ECA. This is due to the highly uniform dispersion of small Pt particles on the surface of the GCB support by the nanocapsule method. After 1000 cycles of durability testing, the ECA advantage of n-Pt/GCB was maintained. Figure 2 shows the changes in mass activity at 0.85 V before and after the durability test for each catalyst. The 30 wt% Pt/GCB and the n-Pt/GCB showed nearly the same initial mass activity. However, n-Pt/GCB exhibited a higher mass activity than that of Pt/GCB after 1000 cycles. From these electrochemical measurements, it was found that n-Pt/GCB had higher performance and durability compared with those for both commercial Pt/GCB catalysts. It is considered that the both Pt/GCB easily underwent agglomeration and/or increase of particle size by electrochemical Ostwald ripening, because the Pt particles were situated quite closely together. However, the particle agglomeration of n-Pt/GCB was relatively suppressed due to the uniformity both of the Pt dispersion and of the Pt particle size, as described in our previous report4). We also investigated the degradation of the cathode catalysts in detail by using Raman spectroscopy, transmission electron microscopy (TEM), scanning electron microscopy (SEM), and the glancing incidence X-ray diffraction (GIXD) method. This work was supported by funds for the “Research on Nanotechnology for High Performance Fuel Cells (HiPer-FC)” project from the New Energy and Industrial Technology Development Organization (NEDO) of Japan. References: 1) A. Iiyama, K. Shinohara, S. Iguchi, and A. Daimaru, Handbook of Fuel Cells: Fundamentals, Technology and Applications, Vol. 6, John Wiley & Sons Ltd., Hoboken, NJ (2009). 2) C. A. Reiser, L. Bregoli, T. W. Patterson, J. S. Yi, J. Deliang, M. L. Perry, and T. D Jarvi, Electrochem. Solid-State Lett., 9 (2006) A183. 3) M. Hara, M. Lee, C. Liu, B. Chen, Y. Yamashita, M. Uchida, H. Uchida, M. Watanabe, Electrochim. Acta 70 (2012) 171. 4) M. Uchida, Y.C. Park, K. Kakinuma, H. Yano, D.A. Tryk, T. Kamino, H. Uchida, M. Watanabe, Phys. Chem. Chem. Phys. 15 (2013) 11236.
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