To simulate realistic operating conditions in SOFC systems, we investigate the influence of thermal cycling on the performance of electrolyte-supported planar SOFCs. Thermal cycling is often associated with interruption of fuel supply, with three main modes; hot standby, cold standby, and shutdown. Cell performance degradation is most significant during shutdown cycles. Nickel oxidation and agglomeration are more pronounced when SOFCs are subjected to lower temperatures for longer periods of time, leading to significant performance degradation. Ostwald ripening at the anode leads to degradation as Ni grains increase in size with cycling. Ni particle precipitation on the anode zirconia grains and along electrolyte grain boundaries is found for the first time in shutdown cycling tests. When H 2 S is mixed with the fuel, the internal reforming reactions and electrode reactions are inhibited by sulfur poisoning of the Ni anodes, accelerating degradation. The SOFC cycling degradation mechanisms are discussed in detail. Solid oxide fuel cells (SOFCs) have several advantages including high efficiency, fuel flexibility, and utilization of non-noble metal, Pt-free catalysts, due to their relatively high operation temperature. Commercialization of SOFC systems for residential electric power applications began in Japan in 2011. Such systems are frequently stopped and restarted in normal operation, e.g. when power is not required, or in an emergency. Such start-stop operation results in thermal cycling, and is often associated with an interruption in fuel supply. Although it is well known that thermal and redox cycling under startstop operation deteriorates SOFC electrochemical performance, [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15] there are only a limited number of studies that systematically focus on this technologically relevant issue.Continuous SOFC stack operation with constant power output generally results in a gradual degradation in performance during long-term operation (SOFCs should run for up to a decade). However, SOFCs can suffer to a greater degree from changes in operation conditions. Due to thermal expansion mismatch between the different components, the cells can suffer from mechanical degradation mechanisms, such as delamination and crack formation with simple thermal cycling. Changes in atmosphere can result in more serious degradation. The influences of thermal cycling and current density cycling on cell degradation have been previously investigated.1-4 The change in volume causes Ni agglomeration. [5][6][7] Redox cycles are typically associated with oxidation of Ni particles at the anode. [8][9][10][11][12] Furthermore, redox cycling results in the formation of Ni hydroxides at a certain vapor pressure in oxidizing atmosphere, with a high water vapor concentration. 13,14 In real residential SOFC power units, cells and stacks are not subjected to these different conditions independently; the changes occur much more dynamically. Therefore, cycle durability studies should be performed using reali...
Durability of cells and stacks against thermal cycling and redox cycling is essential for practical SOFCs, in which the system experiences various kinds of cycling conditions, including the shutoff of fuel supply. In this study, we have investigated the influence of thermal cycling conditions, such as hot-standby, cold-standby, and shut-down, on the cell performance degradation.
Introduction Solid oxide fuel cell (SOFC) is the promising type of fuel cells with e.g. high electric efficiency and fuel flexibility. Durability and reliability are the most important technological issues on such an early commercialization stage. In our research group, we are focusing on the chemical degradation of SOFCs, determining the lifetime of SOFCs operating at high temperatures using various kinds of practical fuels. In this paper, various extrinsic and intrinsic degradation phenomena are systematically classified and their degradation mechanisms are discussed [1-3]. Extrinsic chemical degradation phenomena While the advantage of SOFCs is their fuel flexibility, impurity species can often flow into the SOFC system, causing degradation phenomena. Due to their high operational temperatures, species with a high vapor pressure can also evaporate and flow into the SOFC stacks from system components. In addition, the use of low-purity raw materials could cause impurity poisoning by the contaminants in such raw materials. Possible extrinsic degradation phenomena are summarized in Fig. 1, including mechanisms associated with surface adsorption/desorption in the case of sulfur at a relatively low concentration, but associated with accumulation and reaction product formation for other impurities. Intrinsic chemical degradation phenomena External impurities can be, in principle, removed by using a getter as a desulfurization reactor or by using fuels with better purity. However, chemical degradation associated with diffusion from neighboring components may become important in long-term operation. As compiled in Fig. 2, various intrinsic degradation phenomena have been revealed by evaluating SOFC samples after long-term tests beyond one thousand or several thousand hours. Future perspectives are also presented including the importance of long-term / cycle testing followed by detailed microstructural analysis. Acknowledgements: We thank for the financial support by METI to establish Next-Generation Fuel Cell Research Center. Financial supported by the NEDO SOFC project is gratefully acknowledged. References [1] K. Sasaki et al., J. Power Sources, 196[22], 9130 (2011). [2] K. Sasaki et al., J. Electrochem. Soc., 153 [11], A2023 (2006). [3] K. Sasaki et al., ECS Trans., 57 [1], 315 (2013).
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