## 4 Fuel Cell Designs
### Cell Design  
The fuel cell performance can be limited by three key factors, namely the ohmic losses caused by transportation of charges, catalytic activity and losses due to mass transport due to either by flow of gas or transportation of ions through components of the cell. The limitations of mass and ohmic transportations are directly related to the thickness of the different cell components, which means they are directly connected to the fuel cell design, whereas materials used in the components of cell affect the catalytic activity of electrodes. Beside performance, other factors that are taken into account for the fuel cell design are: cost of materials and mechanical strength [8; 102; 103]. Generally, four different types of cell designs are used, namely: cathode supported, electrolyte supported, anode supported and externally supported designs shown in Figure 4. In each of the above mentioned designs, one cell component is usually thicker by about 1-5 mm than other components in order to provide mechanical support to the cell. In order to increase the cell performance, its mechanical strength and to reduce the cell cost, the thickness of different components of the cell other than support materials are small, _i.e._, in the range of 5-50 \(\upmu\)m. The mechanical strength of the cell can be increased by reducing the cracks or porosity, thermo-mechanical stress that develops due to mismatched thermal expansion coefficient of the different components of the cell [8; 12; 102; 103; 104]. The electrolyte supported fuel cell is the most commonly used design. Electrolyte supported design has advantages over the other designs in terms of fabrication simplicity and mechanical strength. In this design the thickness of non porous electrolyte is of the range of 50-500 \(\upmu\)m. The electrodes are deposited on either side by wet methods like dip coating or screen printing. Normally, electrolyte material is fired at a higher temperature on which electrodes are than deposited and fired at lower temperatures. This method helps in avoiding the reaction of materials combination with dense electrolytes at higher temperature like in the case of LSM and LSCF materials (cathode) that reacts at higher temperature with most commonly used materials. The disadvantage of this type of design is the reduced overall performance due to thicker electrolyte required for mechanical strength of the cell [102; 105]. The fuel cell designs based on cathode support are considered to be an excellent design on the basis of system size being developed and operating hours with nominal degradations. This design was abandoned in early 2000 due to high cost of support materials and manufacturing cost [106]. The rare earth oxide cathode materials, _i.e._, lanthanum oxide, are the main cause of its high cost. The high cost along with cost of support structures developed LSM due to its low mechanical strength and high density resulted into extremely high manufacturing cost as compared to structure developed from NiO/YSZ or pristine zirconia. Also, at the high temperature that is a requirement for the conventional ceramic processing, lanthanum oxide reacts with zirconia [103; 105]. In order to avoid high temperature processing routes so that reaction of lanthanum oxide with zirconia can be avoided, simple temperature processing techniques like vapor deposition is used but it increases the manufacturing cost [107]. At present, no large scale work on the basis of this design is going on but the recent advancements in the cathode materials like lesser rare earth content and development of low cost cathode manufacturing techniques like cathode infiltration may result in the revival of cathode supported design in the near future [106; 108]. Fuel cell designs based on anode support in which especially NiO-YSZ cermets are extremely popular because of their high strength, low solubility of NiO in Zirconia, ease in fabrication and its high electrical conductivity with small ohmic losses.