Zirconia based electrolytes

J. Xue, and R. Dieckmann. Oxygen partial pressure dependence of the oxygen content of zirconia-based electrolytes in Ionic and Mixed Conducting Ceramics Second International Symposium 94-12, 191-208 (1994) ES Meeting San Francisco, California. 

Activation Energy (Ea) and Conductivity (o) Values at High- and Low-Temperature Range for Some Zirconia-Based Electrolytes [10]... 

The solid oxide electrolyte must be free of porosity that permits gas to permeate from one side of the electrolyte layer to the other, and it should be thin to minimize ohmic loss. In addition, the electrolyte must have a transport number for O as close to unity as possible, and a transport and a transport number for electronic conduction as close to zero as possible. Zirconia-based electrolytes are suitable for SOFCs because they exhibit pure anionic conductivity over a wide... 

Oxides exhibiting only high ion conductivity are mainly fluorite-related structures based on zirconia or ceria. Zirconia-based electrolytes are currently used in solid oxide fuel cells (SOFCs). The MIEC oxides are more attractive for separative membrane applications, and these oxides mainly belong to the following types fluorite-related oxides doped to improve their electron conduction, - ... 

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. 

Figure 13.20 Outputs of mixed potential C3H5 sensors with gold electrodes and various ceria- and zirconia-based electrolytes [228, 229, 231, 233, 236-238],...

For the zirconia-based electrolytes in the region where Pq2 < 10 Pa. Equation... 

After substitution of (1.24) into (1.26) and considering (1.22) for the zirconia-based electrolytes, the following Wagner appears ... 

If X < 0.16, the solid solution dissociates on two phases with a following sharp decrease of its electroconductivity. It was shown [33] that the correlation between the change in the phase composition and the decrease of electroconductivity in time does exist for the zirconia-based electrolytes. 

In contrast from the single-phase electrolytes, the two-phase electrolytes are characterized by the deeper aging, and the duration of aging is usually much longer. The absence of equilibrium was reported for the zirconia-based electrolytes held at the temperature of 1100°C for 2000-3000 hours [13], for the Z1O2-SC2O3 electrolyte part of the aging curve has the 5-shape form [34],... 

Kuzin, B.L. and Bronin, D.I., Electrical double-layer capacitance of M, 2/0 interfaces (M=Pt, Au, Pd, hijOj 0 -zirconia-based electrolyte). Solid State Ionics 136-137 (2000) 45-50. 

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... 

The most advanced SOFC s employ oxide ion conducting zirconia-based electrolytes. The conductivity of the electrolyte determines their operation temperature. The temperature dependence of the electrical conductivity for zirconia-hased oxides [12] is shown in Fig. 2. 

Room temperature monoclinic zirconia has little use as a SOFC electrolyte because it is predominantly an electronic conductor with low oxygen ion conductivity [15]. Cubic zirconia has high ionic conductivity but needs to be stabilized so that it retains its cubic structure at room temperature. Nemst discovered and reported in 1899 that mixtures of zirconia with other oxides such as magnesia showed high ionic conduction at high temperatures [16]. Two years later, he patented his further observation that the material composition (15% yttria and 85% zirconia) was suitable for electric-lamp glowers [17]. Westinghouse Electric Corporation has used a similar zirconia-based electrolyte in their SOFC development since 1962 [18]. 

The electrolytic domain for a ceria-based electrolyte, at any given temperature, is narrower than that of a zirconia-based electrolyte, such that electronic conductivity is acquired in reducing atmospheres. As shown in fig. 8 (Steele et al. 1994), the electrolytic domain of Ce02-0d0i.5 (10 mol%) at 1000 K is such that the electrolyte is not suitable for use at oxygen pressures below 10 atm. The high ionic conductivity, however, makes the electrolyte of considerable interest at low temperatures and/or in environments of moderate oxygen potential. 

The most successful SECSs are those which use zirconia-based electrolytes to measure oxygen concentrations. The three most common applications of these electrolytes are to measure oxygen concentrations of steel melts and in combustion gas environments and to control the air-fuel ratio in automobile engines. In the latter two applications, there is increasing interest in lowering the sensor temperature below 600°C, the current minimum temperature of operation because of low ionic conductivity and slow charge-transfer reactions at electrode-electrolyte interfaces. 

It can be concluded therefore that at 1273°K, a cathodic overpotential of 1.228V (with reference to an electrode maintained at an oxygen partial pressure of 1 atm.) should be sufficient to initiate a reaction between the platinum electrode and the zirconia based electrolyte... 

The reaction between platinum and zirconia based electrolyte obviously merits further investigation and is a further example of the propensity of most materials to react when brought into contact at elevated temperature. It was this type of behaviour that produced many of the technological problems associated with the development of commercial high temperature fuel cells and electrolytes and still provides the incentive for developing systems that can operate at ambient or moderate temperatures. 

Much has been published on ceria-based fuel cells however, to date, only Ceres Power Ltd., Crawly, UK, is developing SOFCs with ceria-based electrolytes, whereas numerous companies are developing SOFCs with zirconia-based electrolytes. Ceres Power Ltd. is developing a ceria-based electrolyte on a metal support, where the target operating temperature is 500-600 Ceres Power Ltd. is focus-... 

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