Zirconia based electrolytes properties

Other oxygen ion conductors that have potential use as solid electrolytes in electrochemical devices are stabilized bismuth and cerium oxides and oxide compounds with the perovskite and pyrochlore crystal structures. The ionic conductivity and related properties of these compounds in comparison with those of the standard yttria-stabilized zirconia (YSZ) electrolyte are briefly described in this section. Many of the powder preparation and ceramic fabrication techniques described above for zirconia-based electrolytes can be adapted to these alternative conductors and are not discussed further. 

When zirconia-based electrolytes are exposed to the high temperatures (T > 1100°C) and low oxygen partial pressures P02 < 10 ° Pa), usually encountered in metal melts, they exhibit mixed ionic and n-type electronic conductivities. Under these conditions, the solid electrolyte sensor generates cm/that is influenced by the electrical properties of the solid electrolyte. Schmalzried [25] has analyzed the contribution of electronic conductivity in the zirconia electrolytes to the measured emf of an electrochemical cell in the P02 region less than 10 Pa and has shown that, in the presence of n-type electronic conductivity, the emf of the sensor can be expressed as... 

More types of oxides used for electrodes of the zirconia-based sensors both enable and encourage more possible combinations. Greater diversity in zirconia structures and types of oxide electrodes leads, in turn, to more incompatibilities in chemical, physical, electrochemical, and mechanical properties. The irony is that the more diversity achieved with advanced solid electrolyte and electrode materials, the bigger the challenges that arise for their joining. Beyond sheer diversity, modem... 

An excellent example of the advantages and limitations of IS is the use of this technique to examine the effect of various electrode materials on the properties of zirconia-based oxygen sensors at temperatures below 600°C (Matsui [1981], Badwal [1983], Mizusaki et al. [1983], Badwal et al. [1984]). The most common electrode material is platinum. However, the charge-transfer reaction (I) at the electrodeelectrolyte interface is restricted to regions at or near lines of three-phase (gas-electrode-electrolyte) contact ... 

As already discussed, the electrolyte is the most important component of a SOFC, the properties of which determine critical parameters such as cell performance and temperature of operation. Because of this, much time has been devoted to developing and understanding the materials and their properties and Raman spectroscopy has been a key tool. This section summarises the key results of studies that have used Raman spectroscopy to investigate electrolytes for SOFCs and is split into three parts. The first focuses on Zirconia based materials the conventional electrolyte of choice. The second will summarise the results of investigations on Ceria based electrolytes the frontrunner electrolyte for intermediate temperature SOFCs. Other novel electrolytes which have some potential for reduced temperature operation will be summarised in the final section. 

Properties and fabrication of two of the most common fluorite structured electrolyte materials, zirconia based and ceria based, are discussed below. 

During the past decades, many oxide formulations have been extensively examined in the search for candidate SOFC electrolyte materials. Zirconia-based compositions are still the best electrolytes at present owing to their good stability under reducing atmospheres, low electronic conductivity, and acceptable oxide ion conductivity above 800°C. The recent trend of SOFC development is to operate at lower temperatures. The lowest operation temperature limit of the cell, for thin YSZ electrolytes, is estimated to be about 700°C from YSZ conductivity and mechanical property data. Scandia-doped zirconia, which shows a higher conductivity than that of YSZ, could be preferred at temperatures below 700°C, if the cost of scandia was acceptable. 

When ceria oxides are used in anode materials in eombination with zireonia-based electrolytes, the calcination temperatures should be set as low as possible to prevent the formation of ceria-zirconia solid solutions which have a lower oxide ion conductivity. The detailed properties of those solid solutions are discussed in section 3.7 concerning the ehemieal stability of SOFC materials. 

On the other hand, the theoretical performance of concentration electrochemical cells, based on perovskite materials with protonic and oxygen ion conduction properties, has been described as well [191]. Besides, a sensor for the detection of oxidizable gases that employs the production of a non-Nemstian electrode potential, using zirconia as the solid electrolyte, has been developed [192],... 

The most commonly used electrolyte materials in SOFCs are based on zirconia and ceria doped with a suitable cation, normally a rare earth (see Chapter 9). The properties that make these two materials attractive for use in fuel cells are discussed in Section 4.4.4, and it is sufficient to note that the most important feature is that they are good oxygen ion conductors. We will focus here on some recent investigations of these materials, with emphasis placed on their methods of preparation. 

The oxygen-sensing properties of sensors based on the AljOj-ZrOj-YjOj eutectic composites and polycrystalline zirconia sensors with the same Y2O3 concentration in the electrolyte were also investigated [53]. All tests were carried out using nitrogen as a carrier gas. In all measurements, the airflow rate was -100 cmVmin. The oxygensensing properties of sensors based on the polycrystalline AljOj-ZrOj-YjOj solid electrolyte were shown for comparison. 

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