Anode-supported cells

Figure C shows an electron photomicrograph of a broken planar SOFC. The thick portion on the left is the porous anode structure. This is an anode-supported cell, meaning that in addition to collecting current and supporting the anode reaction, the anode layer stiffens the whole cell. The layer on the right is the cathode, and the interface between the two is the thin electrolyte. One of the challenges of this design is to ensure that the rates of expansion of the cathode and the anode match. If the anode expands faster than the cathode, the planar cell tends to curl like a potato chip when the temperature changes.

The results suggest that the development of high performance catalyst and ASC (anode supported cell) is needed to improve the convasions of CO2 and CFLtand electrical performance. 

FIGURE 2.10 Impedance spectra measured at 750°C for anode-supported cells, for which a fuel mixture of 97% H2/3% H20 was introduced into the anode chamber (a) at 800°C after the cell was heated up to 800°C with the anode exposed to air, and (b) from room temperature during the heating process to 800°C. 

For anode-supported cells, the addition of pore former is a must, especially when fine starting materials are used. It was found that the anode porosity is very low when the starting materials are very fine. For example, Huebner et al. [31] found... 

Matsuzaki and Yasuda did not observe an obvious dependence of the time needed for the influence of the sulfur impurity to saturate with respect to sulfur concentration when H2S concentration was in the range of 2 to 15 ppm at 1000°C. Such dependence, however, was observed at lower temperatures (e.g., 750°C) in fuels with 0.1 to 10 ppm H2S by Waldbillig et al. [65] and Sprenkle et al. [73] on anode-supported cells, as shown in Figure 2.26. 

FIGURE 2.26 Change of cell performance versus time for anode-supported cells subject to different concentrations of H2S poison. (From Sprenkle, V. et al., Sulfur Poisoning Studies on the Delphi-Battelle SECA Program, 2007. Permission pending.)... 

The presence of a small amount of water vapor (up to pH20/pH2 = -0.03) in fuel reduces anode overpotential. For anode-supported cells, the use of pore formers is important to tailor the shrinkage during cofiring and to create adequate porosity for better performance. The difference in cell power output could differ by as much as 100% for cells as porosity changes from -30 to -50%. 

Fig. 14.19 Fracture surface of an anode-supported cell. From left to right, the porous Ni-YSZ anode, the dense 8YSZ electrolyte, and the porous LSM-YSZ cathode.

Any one of the three components in SOFC, the cathode, anode, or electrolyte, can provide the structural support for the cells. Traditionally, the electrolyte has been used as the support however, this approach requires the use of thick electrolytes, which in turn requires high operating temperatures. Electrode-supported cells allow the use of thin electrolytes. The Siemens—Westinghouse Corporation has developed a cathode-supported design,although this has required electrochemical vapor deposition of the YSZ electrolyte. Most other groups have focused on anode-supported cells. In all cases, it is important to maintain chemical compatibility of those parts that come in contact and to match the thermal expansion coefficients of the various components. A large amount of research has been devoted to these important issues, and we refer the interested reader to other reviews. 

However, the mechanical self support of cells is basically provided by the thickest PEN layer either one of the electrodes or the electrolyte (Sulzer Hexis, MHI, RR, CFCL). Thick porous ceramic (RR, MHI) and metallic substrates and interconnects (Ceres Power) onto which a thin PEN is applied have also been suggested to provide the mechanical support required. The electrolyte and anode supported cells are nowadays preferred in tubular and planar stacks. 

To solve the problem in the electrolyte-supporting cell, anode-supported cells have been developed. Since a thin electrolyte is fabricated on a thick anode, the ohmic resistance of the electrolyte can be reduced. The low resistance of the electrolyte implies that the anode-supported cell can be operated at a low temperature below 800°C. As a result, commercial alloys can be used as the interconnectors and auxiliaries. Using the alloys improves the mechanical reliability of the cell stack. For several years, Versa Power Systems have developed several kW systems with anode-supported cells and have demonstrated the performance of such systems [5],... 

Fig. 10.18 Photograph of the anode-supported cell (left figure) and its structure (right figure).

Fig. 10.19 Simulated residual stress at the electrolyte of the anode-supported cell at room temperature. Negative stress means that the stress is compressive.

Numerical calculations for the residual stresses in the anode-supported cells are carried out using ABAQUS. After modeling the geometry of the cell of the electro-lyte/anode bi-layer, the residual thermal stresses at room temperature are calculated. The cell model is divided into 10 by 10 meshes in the in-plane direction and 20 submeshes in the out-plane direction. In the calculation, it is assumed that both the electrolyte and anode are constrained each other below 1400°C and that the origin of the residual stresses in the cell is only due to the mismatch of TEC between the electrolyte and anode. The model geometry is 50 mm x 50 mm x 2 mm. The mechanical properties and cell size used for the stress calculation are listed in Table 10.5. 

Figure 10.19 shows the simulated stress distribution in the electrolyte for the anode-supported cell at room temperature. Except for the edge, the stresses are al-... 

Here, the evaluation of the residual stresses in the electrolyte of the anode-supported cell at room temperature is reported. The X-ray diffraction method is used to measure the residual stresses in the electrolyte of the anode-supported cell, and a synchrotron radiation is used as an excellent X-ray source this enables the estimation of the residual stresses in the electrolyte with a high accuracy. It is clarified that the measured stresses are close to the calculated stresses. 

The X-ray diffraction pattern for the electrolyte of the anode-supported cell is measured and the residual stresses were estimated by the sin2 sustaining stresses, the inter-planar spacing d between the specified diffracting planes of a crystallite in a microscopic grain can be expressed as... 

Fig. 10.46 Side view of the warped anode-supported cell. The electrolyte is located on the upper side.

Fig. 10.50 X-ray diffraction pattern for the electrolyte of the anode-supported cell. The diffraction peaks by arrows are used for the stress measurements.

Fig. 10.53 Measured surface shape of the anode-supported cell (a) on the anode side and (b) on the cathode side.

Fig. 10.54 Calculated change in the magnitude of the warp for the anode-supported cell at room temperature. The warp is plotted with a relative thickness between the electrolyte and anode.

The impedance is dependent on temperature, as can be seen in Figure 4, which shows the area specific resistance (ASR) of a cell as a function of cell temperature for different gas flow rates. For the same cell temperatures, lower ASR was observed for increasing gas flow rates due to the increased gas diffusion near the electrodes that effectively reduced the overpotential resistances [4], Because the anode and cathode are often conductive, the impedance of the cell is dependent largely on the thickness of the electrolyte. Using an anode supported cell structure, a YSZ electrolyte can be used as thin as 10-20 pm or even 1-2 pm [32, 33] as compared to 0.5 mm for a typical electrolyte supported cell [26],... 

Single-cell tests - anode-supported cells... 

Planar SOFCs are composed of flat, ultra-thin ceramic plates, which allow them to operate at 800°C or even less, and enable less exotic construction materials. P-SOFCs can be either electrode- or electrolyte- supported. Electrolyte-supported cells use YSZ membranes of about 100 pm thickness, the ohmic contribution of which is still high for operation below 900°C. In electrode-supported cells, the supporting component can either be the anode or the cathode. In these designs, the electrolyte is typically between 5-30 pm, while the electrode thickness can be between 250 pm - 2 mm. In the cathode-supported design, the YSZ electrolyte and the LSM coefficients of thermal expansion are well matched, placing no restrictions on electrolyte thickness. In anode-supported cells, the thermal expansion coefficient of Ni-YSZ cermets is greater than that of the YSZ... 

Self-supported SOFC can be classified into anode-supported and cathode-supported fuel cells. The SOFC assembly for laboratory testing has a shape of button with 1 - 2 cm in diameter and less than 500 im in thickness. The majority of these button cells are anode-supported cells due to the easy of their fabrication as compared with that of the cathode-supported cell. These self-supported fuel cell usually possess thin (5-20 p,m) electrolyte and can operate at reduced temperatures (< 800 °C). The low temperature operation is the key to decrease... 

Similar concepts are pursued e.g. by Global Thermoelectric, PNNL/Delphi and Ris0 National Laboratory. ECN and its spin-off company InDEC (Innovative Dutch Electrochemical Cells) manufacture electrolyte supported cells as well as anode supported cells. 

In the field of cell development many activities are ongoing, especially at various universities. Therefore it is quite difficult to compile comparison data, especially if they are supposed to be based on comparable operating conditions. In Fig. 9 this has been attempted for anode supported cells at 750°C operating temperature, comparing the most common cathode materials. 

Figure 9. Anode supported cells- power density at 0.7 V and 750°C, using + 3%H20...

The major contribution to ohmic overpotential is the ionic resistance of the electrolyte material. In the state of the art anode supported cell the ohmic losses are minimized by the use of thin film electrolytes which are usually 5-10 pm thick and high temperature operation. However, the high temperature operation is not preferred because of the detrimental effects on the cell life time and the cost of ceramic materials required for high temperature operation [80]. 

Though SOFC can be either of anode, electrolyte or cathode supported, in the case of cells running on hydrocarbon fuels, anode supported cells may be preferable to the others for the reasons of internal reforming. However, the optimal anode thickness required to support the cell mechanically and to achieve the desired level of internal reforming and optimal cell performance is rather a difficult task. 

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