Perovskite structured electrolytes

The second category of solid-state ion conductor is perovskites with general formula ABO3. AB can possess different combinations to yield total charge of +6, i.e., 1+5, 3 + 3, 2 + 4 and in complex ways as Ti03 (A = Li,Na A = Pr, La) and 

Initial work was based on investigating the oxide ion conductivity of LaAlOs materials. Mizusaki and co-workers reported the oxide ion conductivity for Laj cCa cA103 single crystal sample with (x = 0.0027-0.008). For this material, oxygen vacancies are major defects whereas electron holes are the minor defects. The 

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. 

SOFCs [52], Only few perovskites are purely Ionic in their conduction behaviour. Properties of selected ionic-conducting perovskites are discussed below. 

9Bflo 1/I/O3, (6) SrTlo.fAlo.iO],(/) CaTio.iifMgo.osO. (8)CaTig yAlo.yO, (9) CaTig gAlg jOs). 

The oxide ion conductivity strongly depends on the particular cation on the A site, and increases in the following order, Pr La Nd Sm. Electrical conductivity of all Ga-based perovskites is almost independent of the oxygen partial pressure, indicating that oxide ion conduction is dominant in these materials. 

The oxide ion conductivity of Sr and Mg doped LaGa03 is higher than that of typical YSZ or ceria based materials and somewhat lower than Bi203-based oxides. However, electronic conduction and thermal instability are problems for Bi based electrolytes. Doubly doped LaGaOs formulations are very promising electrolytes for SOFCs in terms of ionic conductivity. 

---

The perovskite structure, ABO3 (where A represents a large cation and B a medium-size cation) is adopted by many solids and solid solutions between them can readily be prepared. Vacancy-containing systems with the perovskite structure are of interest as electrolytes in solid-state batteries and fuel cells. Typical representatives of this type of material can be made by introducing a higher valence cation into the A sites or a lower valance cation into the B sites. 

Certain oxides with a perovskite structure are generally applied to the cathode. For high temperature type SOFCs, doped-LaMn03 is used as the typical cathode. For low temperature SOFCs, LaSr(CoFe)03 or La(NiFe)03 are used as the cathode. Doped LaCo03 has a high electric conductivity and shows an excellent catalytic performance. However, the TEC of LaSrCo03 is larger than that of the electrolyte, and so Fe is substituted to reduce the TEC of the cathode. 

Bonanos, N., Knight, K.S., and Ellis, B., Perovskite solid electrolytes Structure, transport properties and fuel cell applications. Solid State Ionics, 79, 61,1995. 

Perovskite-structured oxides with high electronic and oxygen ion conductivities could be used as a membrane alternative to solid electrolytes for oxygen separation. In such materials, both oxygen ions and electronic defects are transported in an internal circuit in the membrane material. 

In the search for new oxygen ion conductors to be used as electrolytes in SOFCs, oxides have been extensively studied in recent years, which crystallize in the perovskite structure and in structures derived therefrom e.g. the brownmillerite... 

Perovskite (ABO3 in which A is divalent and B is tetravalent) and pyrochlore (AaBaOi in which A is trivalent and B is tetravalent) oxide compounds have been proposed as oxygen ion conducting electrolytes for electrochemical devices. Some of the perovskite structures (e.g., BaCeO and SrCeOs) are generating interest because of... 

Many perovskite-structured oxides exhibit high oxide-ion conductivities at elevated temperatures, and have attracted significant interest for use as sohd electrolytes in, for example, SOFCs (see Chapters 9, 12 and 13). The compounds can be divided into camps with compositions or A + B + O3, of which LaGaO3 and... 

In 2000, the details of a new trivalent Y ion-conducting solid electrolyte with an A-site-defident perovskite structure, Y (Ta3 Wi 3jO3 (0<%<0.33) [115], were reported. By substituting the pentavalent Ta site for hexavalent W in Yi/rTaOj, A-site cations (such as Y +) could be completely moved into alternate layers, and Y +vacancies introduced. In the Y (Ta3xWi 3x)O3 series, Yo.o6(Tao.i8Wo.82)03 (x —0.06) exhibited the highest conductivity (ca. 2.6 x 10 Scm at 362 °C), this... 

Solid oxide electrolyzer cells (SOEC) have a solid oxide ion conductor as electrolyte, often yttria-stabilized zirconia (YSZ). The cathode (CO evolution, negative) is often a Ni-YSZ composite called a cermet. The anode (O2 evolution, positive) most often consists of a composite of YSZ electrolyte and an electron-conducting perovskite-structured oxide, e.g., (Lao.75Sro.25)o.95Mn03 [1]. 

---