Oxygen stabilized zirconia electrolytes

A conductivity cell is set up using an yttria-stabilized zirconia electrolyte. At 900°C the equilibrium pressure in the cell was 1.02 x 10-10 atm, and the reference pressure outside the cell was 7.94 x 10 18 atm. (a) What is the cell voltage The temperature was dropped to 800°C and the reference pressure changed to 1.61 x 10-19 atm. The measured equilibrium voltage was 946 mV. (b) What is the equilibrium oxygen pressure in the cell [Data adapted from D-K. Lee et al., J. Solid State Chem., 178, 185-193 (2005).]... 

Oxygen sensors, in low volume use as part of a closed loop emission control system for automotive applications since 1977, have seen wide-spread use starting with the 1981 model year. At the present time, a partially stabilized zirconia electrolyte using yttrium oxide as the stabilizer appears to be the most common choice for this application. 

Solid-state devices based on stabilized zirconia electrolytes are of considerable significance in the energy industry. In this sense, the elevated ionic conductivity, and mechanical and chemical stabilities of zirconia have been applied in oxygen sensing, for prolonged periods, to monitor the operation of internal combustion engines in automobiles, to raise fuel efficiency and minimize emissions [186],... 

High-temperature stabilized NO-, zirconia potentiometric sensors are also being utilized [187], The electrochemical reactions on zirconia devices take place at the triple-phase boundary, that is, the junction between the electrode, electrolyte, and gas [186], It has been reported that sensors composed of a W03 electrode, yttria-stabilized zirconia electrolyte, and Pt-loaded zeolite filters demonstrate high sensitivity toward NO,, and are free from interferences from CO, propane, and ammonia, and are subject to minimal interferences from humidity and oxygen, at levels typically present in combustion environments [188], In this sensor, a steady-state potential arises when the oxidation-reduction reaction [186,188]... 

C.B. Alcock and J.C. Chan, The oxygen permeability of stabilized zirconia electrolytes at high temperatures. Can. Metall. Quart., 11(4) (1972) 559-567. 

NEI/POW] O Neill, H. S. C., Pownceby, M. I., Thermodynamic data from redox reactions at high temperatures. I. An experimental and theoretical assessment of the electrochemical method using stabilized zirconia electrolytes, with revised values for the Fe-FeO, Co-CoO, Ni-NiO and CU-CU2O oxygen buffers, and new data for the W-WO2 buffer, Contrib. Mineral. Petrol., 114, (1993), 296-314. Cited on pages 105,425. 

Laguna-Bercero MA, Kilner JA, Skinner SJ (2011) Development of oxygen electrodes for reversible solid oxide fuel cells with scandia stabilized zirconia electrolytes. Solid State Ionics 192 501-504. doi 10.1016/j.ssi.2010.01.003... 

Mainly thanks to Neuymin s activity, the first in the world 50 W solid oxide fuel cell based on yttrium-stabilized zirconia electrolyte was built in 1965 and tested for 1000 h in the Laboratory of the Electrolytes. All kinds of oxygen sensors were developed by him or under his supervision. On the base of these scientific achievements, the Soviet industry began the production of the first sensors and laboratory oxygen analyzers, e.g., Agate, ANG, and SIVE. Sensors for detecting oxygen in the copper and iron melts were also developed by Neuymin and his co-workers. 

FIGURE 2.15 Oxygen detector using a stabilized zirconia electrolyte. " To detect oxygen, gas to be measured is passed through heated fuel cell. Voltage generated by cell is displayed on the voltmeter. 

Oxygen sensors with stabilized zirconia electrolytes can reliably measure oxygen pressure. However, at very low oxygen pressures, in the zirconia electrolyte is reduced, which results in an increased electronic conduction. An n-type electronic conduction appears in the electrolyte body and the sensor s output becomes unreliable. In the oxide ionic conductors, thorium oxide holds pure ionic conducting characteristics at lower oxygen partial pressures, compared to stabilized zirconia. However, as mentioned above, thorium oxide is radioactive and its commercial application is quite limited. For the purpose of measuring low oxygen partial pressures, a more suitable solid electrolyte is required. Perovskite oxides have been examined for this use, since they are based on oxides and are also very stable. 

W. Araki, Y. Imai, T. Adachi, Mechanical stress effect on oxygen ion mobility in 8 mol % yttria-stabilized zirconia electrolyte. J. Eur. Ceram. Soc. 29, 2275-2279 (2009)... 

Ionic conductivity is used in oxygen sensors and in batteries (qv). Stabilized zirconia, Zr Ca 02 has a very large number of oxygen vacancies and very high conductivity. P-Alurnina/72(9(9j5 -4< -(y, NaAl O y, is an excellent cation conductor because of the high mobiUty of Na" ions. Ceramics of P-alurnina are used as the electrolyte in sodium-sulfur batteries. 

Another application is in tire oxidation of vapour mixtures in a chemical vapour transport reaction, the attempt being to coat materials with a tlrin layer of solid electrolyte. For example, a gas phase mixture consisting of the iodides of zirconium and yttrium is oxidized to form a thin layer of ytnia-stabilized zirconia on the surface of an electrode such as one of the lanthanum-snontium doped transition metal perovskites Lai j.Srj.M03 7, which can transmit oxygen as ions and electrons from an isolated volume of oxygen gas. 

In general Zr02 oxygen sensors consist of a tube-like solid-state Zr02 electrolyte where the electronic conductivity is based on oxygen ion charge carrier transport. The inner and outer surface of the yttrium-doped and stabilized zirconia tube is covered by porous platinum electrodes. 

Four solid oxide electrolyte systems have been studied in detail and used as oxygen sensors. These are based on the oxides zirconia, thoria, ceria and bismuth oxide. In all of these oxides a high oxide ion conductivity could be obtained by the dissolution of aliovalent cations, accompanied by the introduction of oxide ion vacancies. The addition of CaO or Y2O3 to zirconia not only increases the electrical conductivity, but also stabilizes the fluorite structure, which is unstable with respect to the tetragonal structure at temperatures below 1660 K. The tetragonal structure transforms to the low temperature monoclinic structure below about 1400 K and it is because of this transformation that the pure oxide is mechanically unstable, and usually shatters on cooling. The addition of CaO stabilizes the fluorite structure at all temperatures, and because this removes the mechanical instability the material is described as stabilized zirconia (Figure 7.2). 

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