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...
Sulfur species are major impurities in practical hydrocarbon-basedfuels, chemisorption of which on Ni anode catalysts results in possible performance degradation of SOFC systems. In the present study, degradation of SOFC performance caused by sulfur in prereformed CH 4 has been evaluated under various operational conditions such as current density, fuel utilization, pre-reforming ratio, steam-to-carbon ratio, and operational temperature.
Long-term durability is one of the most important technological issues for SOFC system development. Life time of SOFCs can be critically affected by foreign species, including impurities and minor constituents in fuels and oxidants, from the raw materials of cell components, and from system components. This paper summarizes an effort to understand the impurity poisoning analyzed by adding specific impurity species into fuels under various operational conditions to clarify poisoning mechanisms and their concentration threshold. Possible acceleration procedures for poisoning phenomena have also been discussed.
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.
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