Mechanisms of performance degradation induced by thermal cycling in solid oxide fuel cell stacks with flat-tube anode-supported cells based on double-sided cathodes
Introduction
Solid oxide fuel cell (SOFC) is an all solid-state energy conversion device that directly converts the chemical energy in fuels into electrical energy through electrochemical reactions. An SOFC is usually operated under a temperature range from 650 to 1000 °C, and it does not necessitate any precious metal catalyst. As a result, SOFC has many advantages, such as high energy efficiency, fuel flexibility, and low pollution emission [1,2]. In addition, when an SOFC runs in a reverse direction, it can convert the electric energy into chemical energy by directly electrolyzing water or carbon dioxide to fuels, such as hydrogen and carbon monoxide [3,4]. Therefore, the SOFC technology has become a promising energy solution. With the continuous maturity of SOFC technology, SOFC power systems have gradually entered the stage of commercial application, and have a wide range of application prospects in various fields, such as small combined-heat and power (CHP), auxiliary power unit (APU), individual power supply, ship power supply, and other mobile applications [5,6]. A common trait of these applications is the necessity for a start-up, which coincides with thermal cycles. Therefore, research is needed to address the effect of thermal cycling on the performance durability of SOFC systems.
The components of an SOFC single cell or a stack possess different thermal expansion coefficients, so the interface between two components is subjected to mechanical and thermal stresses during thermal cycling [7,8]. Frequent thermal cycling may cause damage to the structure of the cell or damage the interface between the interconnect and the electrode, and eventually lead to stack degradation. Many researchers have carried out multiple thermal cycles experiments on a short stack. During thermal cycling, the open circuit voltage (OCV) can remain stable, while the discharge voltage of the cell or the short stack under a constant current density gradually decreases with thermal cycles, with a large decay rate of 1%–1.5%/cycle [9,10]; however, these reports did not offer any specific explanation for the performance degradation of the stack during thermal cycles.
In recent years, some researchers have found that the damage to the cell structure is one of the main reasons for the performance degradation by thermal cycling. For example, with increasing thermal cycles, the average size of the Ni particles in the Ni-YSZ anode increases, and the conductivity of the Ni-YSZ anode decreases [11,12]. In addition, the uneven temperature distribution in SOFC during the thermal cycle may also induce delamination between the electrode and the electrolyte or microcracks in the electrolyte layer, resulting in a rapid decrease in the cell performance [13,14]. Through literature review, it is found that the degradation caused by the growth and coarsening of Ni particles often takes a long time, and the rate is relatively small, often less than 0.5%/1000h [15], while the degradation induced by the damage to the cell structure often shows a greater rate, which is mainly caused by the stress generated by the asymmetric structure of the cell [16].
On the other hand, the degradation rate of tubular structure is often very small under thermal cycles. For example, Bujalski et al. [15] reported the cycling performance of SOFCs with three different structures: flat, segmented-series, and tubular cells. Their results showed that the cell performance of flat and segmented cells exhibited performance degradation only after a few thermal cycles with a heating and cooling rate of 2 °Cmin−1, while the performance of the tubular cell was only slightly reduced even after 50 cycles. Furthermore, an SOFC stack includes interconnect, sealant, and interface composed of cell and interconnect. Our previous research results showed that the interface was an important factor affecting the output performance of SOFC stack [[17], [18], [19]]. Obviously, for an SOFC stack, the performance degradation and its degradation mechanism caused by thermal cycling are related to factors besides the cell structure. Thus, a more comprehensive evaluation and in-depth investigation to improve the cycling performance are needed.
In recent years, Guan et al. [20,21] proposed a flat-tube anode-supported cell based on double-sided cathodes, which effectively reduced the high-temperature thermal stresses during operation. This type of structure also shows strong redox tolerance and higher mechanical strength, which ensures the cell maintaining its structural integrity under thermal shock, and thus significantly improves the thermal cycling performance. In this work, the cycling-induced degradation and its mechanism are studied.
Section snippets
Experimental procedure
The structure of the anode-supported flat-tube cell is LSCF-GDC/GDC/8YSZ/NiO-8YSZ/NiO-3YSZ/NiO-8YSZ/8YSZ/GDC/LSCF-GDC, as shown in Fig. 1(a) and (b). The cell size is 102 mm × 47 mm × 5 mm with a total effective area on both sides of 56 cm2. The detailed description of the cell preparation process has been reported in our previous publications [22,23]. The cell is assembled into one-cell stack, as shown in Fig. 1(c). To enhance electric collection on the cathode side, 10 μm thick silver paste
Results and discussion
After reduction at 750 °C for about 4 h, NiO in the anode is completely reduced to Ni, and the cell OCV is stable at about 1.102 V, indicating that the stack is well sealed. The initial discharge performance before thermal cycling is shown in Fig. 2. It can be seen that the maximum output power density reaches 0.188 Wcm−2 when the current density is 0.339 Acm−2. The output power of the stack 1 is slightly low, but it does not affect the ability to carry out research on the effect of thermal
Conclusions
In this work, the changes of electrochemical performance of the anode-supported flat-tube SOFCs based on double-sided cathodes under thermal cycling were studied. In the process of 100 thermal cycles between 750–200 °C with a heating and cooling rate of 3 °Cmin−1, the stack performance shows a declining trend, and the power degradation mainly occurs before the first 34 cycles with a degradation rate of 0.89% per cycle. During the following 66 cycles, the degradation rate is smaller, ~0.38% per
Acknowledgments
This work was supported by National Key R&D Program, China (No. 2018YFB1502600), National Natural Science Foundation of China (No. 11932005) and Ningbo Major Special Projects of the Plan “Science and Technology Innovation 2025 ”(2019B10043). The authors thank Dr. Subhash C. Singhal for reviewing and editing the manuscript.
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2021, International Journal of Hydrogen EnergyCitation Excerpt :The interfacial contact resistance is smaller with a higher interfacial adhesion strength or a better interface bonding, and thus the cell performance is improved. Guan [17] et al. studied the mechanisms of performance degradation induced by thermal cycling in SOFCs with flat-tube anode-supported cells based on double-sided cathodes. The main reason for the cycling performance degradation was the increase of ohmic impedance caused by the weakening interface between the cathode and the interconnect, which could be mitigated by a moderate secondary loading on the cathode side of the cell stack.