Anode-supported cells cathodes

Electrolyte-Supported Cell Anode-Supported Cell Cathode-Supported Cell... 

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.

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. 

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

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],... 

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... 

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. 

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. 

This equation is an approximation for anode supported cells with very small cathode thickness). In general, the limiting ciurent density at the cathode is given by imposing to zero... 

Figure 9.11 Current density at 700 mV of anode-supported cells with an LSC(F) cathode as a function of the sintering temperature of the screen-printed CGO diffusion barrier...

Mechanical strength If anode-supported cell configuration If cathode-supported cell configuration... 

The ohmic polarization in Eq. (26.12) represents the total area specific ohmic resistance of the cell. Ri is the sum of the anode, cathode, electrolyte, interconnect, and contact ohmic resistances. Typically, the ohmic resistance is dominated by the electrolyte resistance and decreases with increasing operating temperature. The reduction in ohmic polarization is part of the reason why anode-supported cells have become the standard design in current high-performance SOFCs. 

Metal Supported-Solid Oxide Fuel Cells (MS-SOFC) represent a promising new design for fuel cells which may overcome the limitations of anode-supported cells (such as poor thermal cycling resistance and brittleness. Nickel phase re-oxidation upon exposure to transient uncontrolled conditions) due to the much better mechanical properties of the support that is represented by a porous thick metal substrate, the thickness of the ceramic layers (anode/electrolyte/cathode) being in the order of 10-50 pm, only. In addition, in this design (Fig. 1), the replacement of the thick Ni/YSZ cermet with ferritic stainless steel leads to several benefits in term of fabrication cost and safety. 

In many cases, achievement of reliability is in trade-off relation to cost reduction. To overcome this, it is required to make breakthroughs in the fabrication technologies. Cells to be operated in the temperature region of around 800°C. Metal intercoruiects are used together with anode-support cells and sealing materials. The active cathode such as LSCF (lanthanum strontium cobaltite ferrites) is used so that the durability is one of the main issues. 

These features have been found to be highly correlated with the fabrication method/sequence as well as materials selected. For example, anode support cells have stable anodes but there remain several points to be optimized for a cathode-complex-layer structure. On contrary, cathode-support cells have the stable performance for cathodes, but anodes may have some changed in microstructure because of nickel sintering [63]. 

The current state-of-the-art SOFC anode-supported cells based on doped zircona ceramic electrolytes, ceramic LSM cathodes, and Ni/YSZ cermet anodes are operated in the temperature range 700-800°C with a cell area specific resistance (ASR) of about 0.5 O/cm at 750°C. Using the more active ceramic lanthanum strontium cobalt ferrite (LSFC)-based cathodes, the ASR is decreased to about 0.25 Q/cm at this temperature, which is a more favorable value regarding overall stack power density and cost-effectiveness. 

Anode-Supported. Advances in manufacturing techniques have allowed the production of anode-supported cells (supporting anode of 0.5 to 1 mm thick) with thin electrolytes. Electrolyte thicknesses for such cells typically range from around 3 to 15 im (thermomechanically, the limit in thickness is about 20 to 30 im (the cathode remains around 50 am thick), given the difference in thermal expansion between the anode and the electrolyte). Such cells provide potential for very high power densities (up to 1.8 W/cm under laboratory conditions, and about 600 to 800 mW/cm under commercially-relevant conditions). 

Barnett, Perry, and Kaufmann (75) found that fuel cells using 8 im thick yttria-stabiUzed zirconia (YSZ) electrolytes provide low ohmic loss. Furthermore, adding thin porous yttria-doped ceria (YDC) layers on either side of the YSZ yielded much-reduced interfacial resistance at both LSM cathodes and Ni-YSZ anodes. The cells provided higher power densities than previously reported below 700 °C, e g., 300 and 480 mW/cm at 600 and 650 °C, respectively (measured in 97 percent H2 and 3 percent H2O and air), and also provided high power densities at higher temperatures, e g., 760 mW/cm at 750 °C. Other data (Figure 7-25) from the University of Utah (73) show power densities of 1.75 W/cm with H2/air and 2.9 W/cm with H2/O2 at 800 °C for an anode-supported cell. However, no data is presented with regard to electrodes or electrolyte thickness or composition. 

One of the problems with anode-supported cells is that any difference in thermal expansion between anode and electrolyte becomes more significant than in conventional high-temperature SOFCs. For this reason many developers use porous nickel cermet anodes with interfacial regions made of NiA SZ doped with ceria. Operating at temperatures below about 700°C means that metallic bipolar plates can be used, and the lower the temperature, the less exotic the steel needs to be. Ferritic stainless steels can be used below about 600°C, and these have the advantage that they have a low thermal expansion coefficient. Conventional doped LSM-YSZ cathodes can be used but there is much development in progress to improve cathode materials as the cathode overpotentials become more significant as the cell temperatures are lowered. A recent review of cathode materials has been published by Ralph (2001). 

Planar SOFCs are generally manufactured in three different configurations depending on the structure-supported cell element and operating temperature range as shown in Figure 9.19. These configurations are referred to as (i) electrolyte-supported cell with thick electrolyte layer, (ii) anode-supported cell with thick anode layer, and (iii) cathode-supported cell with thick cathode layer. 

Different configurations of planar SOFC designs, (a) Electrolyte-supported cell, (b) Anode-supported cell, (c) Cathode-supported cell. 

Currently, electrolyte-supported, cathode-supported, anode-supported, and metallic substrate-supported planar SOFCs are tmder development. In electrolyte-supported cells, the thickness of the electrolyte, typically YSZ, is 50-150 pm, making then-ohmic resistance high, and such cells are suitable only for operation at 1,000°C. In electrode-supported designs, the electrolyte thickness can be much lower, typically 5-20 pm, which decreases their ohmic resistance and makes them better suited for operation at lower temperatures. The anode (Ni/YSZ cermet) is selected as the supporting electrode, because it provides superior thermal and electrical conductivity, superior mechanical strength, and minimal chemical interaction with the electrolyte. Kim et al. [83] have reported power densities as high as 1.8 W/cm at 800°C for such anode-supported SOFCs. At Pacific Northwest National Laboratory [84, 85], similar anode-supported cells have been developed using 10 pm... 

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