**1.4.3 Electrolyte**

The design of fuel cells with solid oxide electrolyte must be based on the concept of oxygen ion conduction through the electrolyte, with ions O2- migrating from the cathode to the anode, where they react with the fuel (H2, CO, etc..) generating an electrical current.

The materials that have been studied the most are: yttria stabilized zirconia, doped ceria with gadolinium and lanthanum gallate doped with strontium and magnesium. The solid oxide fuel cells can, in principle, operate in a wide temperature range between 500 °C and 1000 °C. Thus, they can be divided into two types: operating at high temperatures (> 750 ºC) and intermediate temperatures (500 ºC to 750 ºC). One of the determinant factors for the operating temperature is the characteristic of the solid electrolyte. The ohmic losses associated with the electrolyte are important for the cell performance. In order to reduce the operating temperature of SOFC, aiming the use of more conventional steel alloys as interconnects at temperatures around 700 °C (Horita et al., 2008; Perednis & Gauckler, 2004), it is necessary to employ electrolytes with high oxygen ionic conductivity at low temperatures.

YSZ is so far the most widely used solid electrolyte for application in high temperature SOFC. For many years, the zirconium oxide is already known as a conductor of oxygen ions.

The yttria addition to the zirconia-yttria solid solution has two functions: to stabilize the cubic structure type fluorite and to form oxygen vacancies in concentrations proportional to the yttria content. These vacancies are responsible for high ionic conductivity. Yttria stabilized zirconia is a suitable ionic conductor at temperatures above 800 °C, since thin dense membranes (less than 20 µm) can be manufactured. These membranes should be free of impurities. The stabilized zirconia is chemically inert to most reactive gases and electrode materials.

In view of the limitations encountered in using other types of ceramic conductors than yttria stabilized zirconia, its efficiency at low temperatures had to be improved. To reduce the operating temperature of the cell without affecting the efficiency of oxygen ion conduction, the electrolyte has to be as thin as possible in order to compensate the increase in ohmic losses (Huijsmans, 2001). Other advantages of fuel cells with thin electrolytes are reduction in material costs and improvement in the characteristics of the cells (Perednis & Gauckler, 2004). Therefore, the yttria stabilized zirconia is still a material with great prospects in the application as electrolyte in solid oxide fuel cells. Research about this type of material is aimed to improve its characteristics in order to adapt the needs of current applications.
