Yttrium-stabilised zirconia

For the HTE process, the electrochemical cell consists of a tri-layer ceramic, well known for its brittleness, which limits applied loads. In addition, the relatively low ionic conduction properties of the electrolyte materials (3% yttrium-stabilised zirconia) requires an operating temperature above 700°C to reduce ohmic losses. This creates difficulties for the involved metallic materials, including bipolar plates and seals. 

The sensor (a sensor) is designed to deliver measurements of the [O j in the exhaust gas. P(02) in the exhaust can be taken as a direct measure of the A/F ratio in the combustion chamber s inlet. The sensor is essentially a yttrium-stabilised zirconia electrode whose potential depends on P(02). This electrode (which is composed of the same material as the electrol)i e in a solid oxide fuel cell) transports as O " ions generating an electrical signal whose strength is proportional to P(02). The signal is sent to the fuel injection system which increases or decreases the A/F ratio as desired in order to keep the mixture stoichiometric [12]. 

Selection of an appropriate solute is important for the formulation of an effective electrolyte. Maximum conductivity, for example, seems to be associated with a size homogeneity between the substituting species and the majority cation in the cubic structure, as well as its concentration in solid solution. Figure 3 presents the effects on the ionic conductivity of stabilised zirconia at a fixed temperature, on variation of the cationic substituting species. It is evident that the optimised yttrium solid solution has a conductivity of about 0.015 S cm at 800°C, so that only a very thin electrolyte membrane can provide a technically acceptable current density at that temperature. The well-established Westinghouse SOFC system therefore operates closer to 1000°C to take advantage of the rapid increase of electrolyte conductivity with temperature (7) (see also Fig. 7). This dependance, particularly steep for YSZ, is presented for several solid ionic conducting materials in Fig. 4. 

Carter, G.A., Hart, R.D., Rowles, M., Ogden, M.I., and Buckley, C.E. (2009) Industrial precipitation of yttrium chloride and zirconyl chloride effect of pH on ceramic properties for yttria partially stabilised zirconia. J. Alloys Compd., 480 (2),... 

In the late 1970s, electrochemical vapour deposition began to be used to make tubular cells at Westinghouse [9.10]. A porous tubular substrate, around 15-20 mm in diameter, made originally from calcia-stabilised zirconia but later from the cathode material, doped lanthanum manganite, was placed in a low-pressure furnace chamber, and zirconium chloride plus yttrium chloride... 

In addition to fluorite structure electrolytes such as stabilised zirconia and ceria, there are many non-fluorite structure oxides which are potentially attractive for SOFC electrolyte application. These include perovskites like lanthanum gallate and to a lesser degree calcium titanate. Alternative oxides are the pyrochlores such as yttrium zirconate (YZr207)and gadolinium titanate (Gd2Ti207) [49,50], but these are only suitable in very limited oxygen pressure ranges. Therefore, the main discussion here focuses on the perovskites. 

---