Fuel cells fluorite

Yttria-stabilized zirconia f[Zrlj YJ02, /2) is known in the literature as YSZ and has a fluorite-type structure [67] (see Figure 2.16). This material has a high oxygen ion conductivity and is, therefore, applied as a high-temperature electrolyte material, for example, in high-temperature fuel cells [68,73],... 

Pale yellow cerium dioxide (ceria, ceric oxide) has the fluorite structure and is used in catalysis" ", solid oxide fuel cells (SOFC)", thin film optical waveguides" , reversible oxygen storage materials for automobile catalysts" and for doping copper oxide superconductors". The diverse cerium enolate precursors and deposition methods used in the formation of cerium oxide thin films are summarized in Table 6, whereby the most common precursor for ceria is Ce(thd)4. 

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, - ... 

The principles behind this membrane technology originate from solid-state electrochemistry. Conventional electrochemical halfceU reactions can be written for chemical processes occurring on each respective membrane surface. Since the general chemistry under discussion here is thermodynamically downhill, one might view these devices as short-circuited solid oxide fuel cells (SOFCs), although the ceramics used for oxygen transport are often quite different. SOFCs most frequently use fluorite-based solid electrolytes - often yttria stabUized zirco-nia (YSZ) and sometimes ceria. In comparison, dense ceramics for membrane applications most often possess a perovskite-related lattice. The key fundamental... 

The highly conducting Zr02 crystalhzes in the fluorite structure, which stabilizes by doping with Y2O3 or CaO. Zr02 is used in several electrochemical developments, e.g., as electrolyte in solid oxide fuel cells (SOFCs) or for an electrochemical oxygen sensor in the car industry (A,-probe). 

Rare-earth elements are vital constituents of several prominent high-temperature solid electrolytes ranging from oxygen- or fluoride-ion conductors in the fluorite structures to protonic conductors in the doped perovskite phases and trivalent-ion conduction in Sc2(W04)3 and 3-alumina-type compounds. Solid electrolytes are considered as important for scientific studies and technological applications in vital areas such as fuel cells, batteries, sensors, process control and environmental protection. 

The high temperature electrolytes are mostly oxides of composition MO based upon the fluorite, structure. The best investigated is "calcia stabilized zirconia (CSZ) which consists of a solid solution of 12-15% CaO in ZrO. The addition of calcia transforms ZrO from the monoclinic to tne cubic (fluorite) structure and also introduces anion vacancies for charge compensation. Conduction is by 0 ion diffusion through anion vacancies and ZrO -CaO has a resistivity of 30 ohm-cm at 950 C. Trivalent cations may also be used to stabilise ZrO with resistivities at 950 C of 12 ohm-cm for ZrO -Y 0 and - 6 ohm-cm for ZrO -Yb O or ZrO -Sc O (Figure l). Staoilized zirconia is of interest as an electrolyte for fuel cells, but no battery applications have been proposed and the temperature of conduction is too high to be of real interest. 

Yttria-stabilized Z1O2, discovered by Nemst [39], is still one of the state-of-the-art SOFC electrolyte materials which was used to demonstrate the first SOFC (and the first solid electrolyte fuel cell) in 1937 at ETH-Ziirich [40]. Electronic defect concentrations are negligibly low [41]. As can be observed in Fig. 6.1, the ionic conductivity of fluorite-type oxides stabilized with hypovalent elements exhibits a maximum at a certain dopant concentration above which defect interactions occur [42-45]. As shown in Fig. 6.2, it is known that the peak value of the ionic... 

A SOFC was proposed by Baur and Preis as far back as 1937 based upon an electrolyte of stabilised zirconia with metallic electrodes. Since then stabilised zirconia has been the electrolyte that has received most attention by fuel cell developers. Most zirconia electrolytes are based upon either yttria or scandia stabilisation of the tetragonal poly-morph, commonly referred to as YSZ and ScSZ, respectively, although a number of alternative dopants have been investigated (Tables 2.1 and 2.2). Conventionally the substitution level is between 3mol% and 8 mol% for the yttria-based materials and at 10-12 mol% for the scan-dia-based materials. The choice of the dopant level is dictated by a compromise between mechanical robustness and overall conductivity, as summarised in Table 2.1. Substitution of zirconia results in the stabilisation of either the tetragonal or cubic polymorphs adopting the fluorite type structure as shown in Figure 2.2. This substitution... 

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