## 3 Materials
### Electrolyte  
An electrolyte is a component in a solid oxide fuel cell used for the ion transportation between electrodes. High oxygen conductivity (_i.e._, >0.1 S- cm\({}^{-1}\)) over a wide range of oxygen partial pressure is needed by electrolyte. A material can be a good electrolyte if it meets multifaceted property criteria. The electrolyte must be fully dense with no open porosity in order to avoid any cross diffusion of fuel and oxidant constituents. The electrolyte must be chemically inert with respect to electrodes and sealing materials as it is in contact with these materials. It must also have comparable thermal expansion coefficient (TEC) and fine mechanical properties. The electrolyte must be stable both chemically and structurally in order to make ionic transport number close to unity since it is exposed to extremely oxidizing and reducing environments at electrodes. To withstand high mechanical and thermal stress during operation and fabrication it must contain good tensile stress and toughness [10; 12; 60; 61; 62; 63; 64; 65; 66; 67; 68; 69]. A great deal of consideration has been given to four types of electrolyte, based on fully or partially stained zirconia, doped ceria, doped LaGaO\({}_{3}\) and doped Bi\({}_{2}\)O\({}_{3}\)[10; 12; 35; 59; 61; 62]. Each system includes different sub-categories of materials with one or more dopant contents which improves their ionic conduction or phase stability. The type and concentration of dopant defines effect on phase assemblage, chemical stability, ionic conductivity, thermal and mechanical properties. A careful attention must be given to the fact that many materials are unstable under the SOFC operating conditions, exhibit low ionic conductivity and develop conductivity in reducing environment [12; 35; 50; 59; 61]. The electrolyte that fulfills most of the characteristics required for the operation of SOFC is based on Y\({}_{2}\)O\({}_{3}\)-ZrO\({}_{2}\) with Y\({}_{2}\)O\({}_{3}\) (3-10 mol % range). The 8 mol % Y\({}_{2}\)O\({}_{3}\) doped ZrO\({}_{2}\) (YSZ) composition with cubic structure has been known to have the highest ionic conductivity during SOFC operation at about 1000 \({}^{\circ}\)C, with about 225-300 MPa mechanical strength at room temperature and 150 MPa above 500 \({}^{\circ}\)C [61]. On the other hand, 3 YSZ has a very high mechanical strength of about 1000 MPa at room temperature and 400 MPa above 500 \({}^{\circ}\)C but at the expense of ionic conductivity, and it also contains unstable and complex phase assemblage [61]. A lot of emphasis is being given to reducing SOFCs operating temperature, their cost and the severity of the materials degradation [62; 63]. However, with a decrease in operating temperature from 1000 \({}^{\circ}\)C to 800 \({}^{\circ}\)C the ionic conductivity of 8 YSZ reduces from 0.178 S- cm\({}^{-1}\) to 0.052 S- cm\({}^{-1}\)[61]. Therefore, alternative materials have been explored that can have higher ionic conductivity at lower temperatures [10; 12; 58; 59; 60; 61; 62; 63]. Other electrolyte materials that are considered due to their higher ionic conductivity include Sc\({}_{2}\)O\({}_{3}\)-ZrO\({}_{2}\), doped ceria, doped Bi\({}_{2}\)O\({}_{3}\), lanthanum strontium magnesium gallate perovskite (LSGM) [10; 12; 35; 61; 62; 63; 64; 65; 66; 67]. The thermal and mechanical properties of Scandia doped Zirconia are fairly similar to Yttria-Zirconia and the ionic conductivity of 9 mol % Sc\({}_{2}\)O\({}_{3}\)-ZrO\({}_{3}\) is almost twice that of 8 YSZ at 1000 \({}^{\circ}\)C. However, Sc\({}_{2}\)O\({}_{3}\) is limited and expensive for practical use as an electrolyte in a large scale SOFC [59; 61]. Beside Sc\({}_{2}\)O\({}_{3}\)-ZrO\({}_{2}\), the other electrolyte families, doped ceria and doped Bi\({}_{2}\)O\({}_{3}\), develop electronic conductivity above 600 \({}^{\circ}\)C and therefore often need a protection of YSZ thin layer [10; 35; 61; 63].