Metal Oxides with Ionic Conductivity Solid Electrolytes

3 Metal Oxides with Ionic Conductivity Solid Electrolytes 

It is also necessary to take into acconnt that the process of solid electrolyte doping differs from the doping process peculiar to semiconductors. Studies of doping solid electrolytes resulted in some general rules with regard to the effect on the transport and phase stability properties (Weppner 2000). 

p- and n-type condncting dopants increase the partial hole and electron conductivity, respectively, in the case where there is abont the same activity of the aifionic component, e.g., oxygen. This rule is quite independent of the crystal structure and chemical environment. 

The ionic condnctivity commonly decreases by the addition of dopants because of entropy effects and local lattice distortions. The magnitude of solid solubility of the dopant seems not to be related directly to the lattice distortion, however. 

The diffnsion coefficients of the electronic charge carriers and mobile ions are related to each other. Both decrease with increasing amounts of dopants. 

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Metal Oxides with ionic Conductivity Solid Electrolytes... 

A controlled modification of the rate and selectivity of surface reactions on heterogeneous metal or metal oxide catalysts is a well-studied topic. Dopants and metal-support interactions have frequently been applied to improve catalytic performance. Studies on the electric control of catalytic activity, in which reactants were fed over a catalyst interfaced with O2--, Na+-, or H+-conducting solid electrolytes like yttrium-stabilized zirconia (or electronic-ionic conducting supports like Ti02 and Ce02), have led to the discovery of non-Faradaic electrochemical modification of catalytic activity (NEMCA, Stoukides and Vayenas, 1981), in which catalytic activity and selectivity were both found to depend strongly on the electric potential of the catalyst potential, with an increase in catalytic rate exceeding the rate expected on the basis of Faradaic ion flux by up to five orders of... 

As mentioned, the solid electrolytes are sintered metal oxides with mobility of ions where the ionic conductivity is influenced by both the microstructure and geometry. The effects of composition, structure, microstructure, and strain on ionic transport at grain boundary provided complementary tools for futiu-e developments in solid electrolyte materials. Among these, a particular attention was given to the impact on ionic transport of defects in various types of structures, dislocations, grain boundaries, and heterostructure interfaces. The design of such structural properties also considered the achievements of the development in nanotechnologies. 

Polymer electrolytes (e.g., poly (ethylene oxide), poly(propylene oxide)) have attracted considerable attention for batteries in recent years. These polymers form complexes with a variety of alkali metal salts to produce ionic conductors that serve as solid electrolytes. Their use in batteries is still limited due to poor electrode/electrolyte interface and poor room temperature ionic conductivity. Because of the rigid structure, they can also serve as the separator. Polymer electrolytes are discussed briefly in section 6.2. 

A solid oxide fuel cell (SOFC) consists of two electrodes anode and cathode, with a ceramic electrolyte between that transfers oxygen ions. A SOFC typically operates at a temperature between 700 and 1000 °C. at which temperature the ceramic electrolyte begins to exhibit sufficient ionic conductivity. This high operating temperature also accelerates electrochemical reactions therefore, a SOFC does not require precious metal catalysts to promote the reactions. More abundant materials such as nickel have sufficient catalytic activity to be used as SOFC electrodes. In addition, the SOFC is more fuel-flexible than other types of fuel cells, and reforming of hydrocarbon fuels can be performed inside the cell. This allows use of conventional hydrocarbon fuels in a SOFC without an external reformer. 

Hitherto we have dealt with model FICs that are mostly useful as solid electrolytes. The other class of compounds of importance as electrode materials in solid state batteries is mixed electronic-ionic conductors (with high ionic conductivity). The conduction arises from reversible electrochemical insertion of the conducting species. In order for such a material to be useful in high-energy batteries, the extent of insertion must be large and the material must sustain repeated insertion-extraction cycles. A number of transition-metal oxide and sulphide systems have been investigated as solid electrodes (Murphy Christian, 1979). 

In 1973, Peter Wright and coworkers first reported [39-41] the ionic conductivity of poly(ethylene oxide), [CH2CH20]n, (PEO), with alkali metal salts. This was followed by the visionary suggestion of M. Armand for the use of PEO as a solid electrolyte system for the transport of ions [42-43]. Since then, the area of polymer electrolytes has attracted considerable interest. In the following account, first a discussion is presented on the general features applicable to polymer electrolytes. This is followed by an account on individual polymer electrolytes, par-... 

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