Oxygen-anion conductors

The working principles behind a solid oxide fuel cell (SOFC) are schematically illustrated in Figure 8.7, where, similar to the other fuel cell types, the three key parts of an SOFC, a cathode, an anode, and an electrolyte, are shown. The electrolyte is, in a majority of cases, an oxygen-anion ceramic conductor, which is, as well, an electronic insulator [5]. In the SOFC the fuel can be methane (CH4). Subsequently, in this case the oxidation reaction in the anode is given by... 

Proton-conducting materials [38-47], analogous to oxygen conductors but with stationary oxygen anions, can show mixed protonic-electronic conductivity, without considerable oxygen transport in hydrogen or water atmospheres [40,41], These materials have not been widely studied in comparison... 

Oxygen anion conductors solid electrolyte Oxygen, chemisorbed - chemisorption of oxygen Oxygen conducting solid electrolyte - solid electrolyte... 

As it is impossible to analyze all types of electrolyte tvithin the limitations of this chapter, the reader is directed towards many comprehensive reviews where the known solid electrolytes are classified according to their technological functions [38], the nature of their transition to a highly conducting state [13], the constituent chemical species [39], or their crystal structures [40]. Other recent surveys have been devoted to systems with 3-D ionic migration [41] and to the electrolytes with a certain type ofmobile species (e.g., oxygen anions [42]). Information on these groups of ionic conductors can be found in Chapters 7-9. Irrespective of classifications and microscopic mechanisms, the partial ionic conductivity (G,) of a solid can be expressed as... 

The pyrochlore-type compounds, where the crystal structure is usually considered as a cation-ordered fluorite derivative with % vacant oxygen site per fluorite formula unit, constitute another large family of oxygen anion conductors [9, 33, 41—43, 84—88]. The unoccupied sites provide pathways for oxygen migration furthermore, the pyrochlore structure may tolerate formation of cation and anion vacancies, doping in both cation sublattices, and antistructural cation disorder. Regardless of these factors. 

Monoclinic zirconia (baddeleyite structure) stable below 1197°C, tetragonal zirconia (rutile structure) stable between 1197 and 2300°C, cubic zirconia (fluorine structure) stable above 2300°C or at lower temperature if stabilized by addition of magnesia, calcia or yttria. Maximum service temperature 2400°C. Zirconia starts to act as an oxygen anion conductor at 1200°C. Highly... 

Oxygen Anion Transport in Solid Oxides Perovskite Proton Conductor Solid Electrol34 es Solid State Electrochemistry,... 

Some of these materials conduct both anions and cations, others are principally conductors of anions. It appears that solutions of small, polarizing ions, such as Mg(II) and Ca(II), are anion conductors. The cations are presumably trapped in strong associations with oxygen ions in the PEO chains, while the larger anions are less-strongly solvated by the... 

An entirely different selectivity principle known as phase equilibrium comes into play in high-temperature ionic conductors. Many important gases dissolve in ionic solids at elevated temperatures. However, the solubility is rather sharply defined for the gas and the solid by the lattice parameters and the size of the gas molecule. The best example is the solubility of oxygen in zirconium dioxide. When Z1O2 is doped with yttrium ions, it exhibits a high mobility for the O anion. The solubility and anion mobility then become the basis for several electrochemical gas sensors, using yttria-stabilized zirconia (YSZ). 

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