Ionically Conducting Solid Electrolytes

93 lONICALLY CONDUCTING SOLID ELECTROLYTES 93.1 Ionic Semiconductors 

The conductivity of solid salts and oxides was first investigated by M. Faraday in 1833. It was not yet known at that time that the nature of conduction in solid salts is different from that in metals. A number of fundamental studies were performed between 1914 and 1927 by Carl Tubandt in Germany and from 1923 onward by Abram F. Ioffe and coworkers in Russia. These studies demonstrated that a mechanism of ionic migration in the lattice over macroscopic distances is involved. It was shown that during current flow in such a solid electrolyte, electtochemical changes obeying Faraday s laws occur at the metal/electrolyte interface. 

In some cases (particularly at elevated temperatures) mixed electtonic and ionic conduction is observed in solid salts. Typical materials with purely ionic conduction are the halides and sulfides of a number of metals, viz., AgBr, Ag2S, PbCl2, CuCU, and many others. 

The conductivity a of such materials is usually low at room temperature. The values of T strongly increase with temperature (Fig. 9.1). The conductivities of ionic crystals strongly depend on their purity. Impurities in the crystals markedly raise the values of r, particularly at lower temperatures when the intrinsic conductivity of the pure material is still low. 

All these features low values of u, a strong temperature dependence, and the effect of impurities, are reminiscent of the behavior of p-type and n-type semiconductors. By analogy, we can consider these compounds as ionic semiconductors with intrinsic or impurity-type conduction. 

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Increasing numbers of advanced batteries for all purposes depend on ionically conducting solid electrolytes, so it will be helpful to discuss these before continuing. It should be remembered that any battery can be described as an electron pump, and the role of the electrolyte is to block the passage of electrons, letting ions through instead. 

Solid ionic conductors can also be used in the fabrication of solid state double-layer supercapacitors. An example is the device developed in the late 1960s by Gould Ionics which adopted a cell system using a silver-carbon electrode couple separated by the highly ionically conducting solid electrolyte RbAg4I5 (see Section 9.1) ... 

For an ionic conducting solid electrolyte to be seriously considered for use in a practical electrochemical device, which operates at a given temperature T, the maximum value for the area-specific resistance f as should be about 0.5 Q-cm. J as is the product of the electrolyte resistivity p at T in ohm-centimeters and the membrane thickness t (cm) in the direction of current flow. Table 2 lists maximum limits on electrolyte resistivity for various electrolyte membrane thicknesses. 

Metal Oxides with Ionic Conductivity Solid Electrolytes. 53... 

Metal Oxides with ionic Conductivity Solid Electrolytes... 

As an example, the working principle of a DEFC is illustrated in Fig. 9.1. The electrochemical cell consists of two electronic conducive electrodes, an anode and a cathode separated by an ionic conductive solid electrolyte (a proton exchange membrane generally of Nation type). At the anode the electro-oxidation of alcohol takes place as follows ... 

Another way of SO2 sensing, without the sulfate electrolyte, is to use an excellent alkali metal ionic conducting solid electrolyte such as (3-alumina (Na ) (Itoh et al. 

Today, the term solid electrolyte or fast ionic conductor or, sometimes, superionic conductor is used to describe solid materials whose conductivity is wholly due to ionic displacement. Mixed conductors exhibit both ionic and electronic conductivity. Solid electrolytes range from hard, refractory materials, such as 8 mol% Y2C>3-stabilized Zr02(YSZ) or sodium fT-AbCb (NaAluOn), to soft proton-exchange polymeric membranes such as Du Pont s Nafion and include compounds that are stoichiometric (Agl), non-stoichiometric (sodium J3"-A12C>3) or doped (YSZ). The preparation, properties, and some applications of solid electrolytes have been discussed in a number of books2 5 and reviews.6,7 The main commercial application of solid electrolytes is in gas sensors.8,9 Another emerging application is in solid oxide fuel cells.4,5,1, n... 

Intermediate Temperature Solid Oxide Fuel Cell (ITSOFC) The electrolyte and electrode materials in this fuel cell are basically the same as used in the TSOFC. The ITSOFC operates at a lower temperature, however, typically between 600 to 800°C. For this reason, thin film technology is being developed to promote ionic conduction alternative electrolyte materials are also being developed. 

A solid electrolyte is a material in which the electrolytic, or ionic, conductivity is much greater than the electronic conductivity (for solid electrolytes to be practically useful the ratio of electrolytic to electronic conductivities should be of the order of 100 or greater1,2). Solid electrolytes with conduction ions of 02 , H+, Li+, Na+, Ag+, F, Cl- have all been reported. Much attention has been devoted to oxygen-ion conducting solid electrolytes, many of which show appreciable oxygen-ion conductivities in the range of 200-1200°C. 

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... 

In the technology of ceramics, electronic conductors (semiconductors), ionic conductors (solid electrolytes) and mixed electronic-ionic conductors are encountered. In all cases the conductivity is likely to vary with temperature according to... 

UV-Vis spectroscopy — Electronic absorption in the UV-Vis range by species generated during electrochemical reactions or being present at the electrochemical interface between the electronically conducting electrode and an ionically conducting phase (electrolyte solution, molten electrolyte, ionic liquid, solid electrolyte) can be studied with in situ UV-Vis spectroscopy in various modes [i-iii] ... 

Hammond and DeLongchamp [255] reported the development of a highly ionic, conductive, solid polymer electrolyte film from hydrogen bonding LBL... 

The idea that ions can diffuse as rapidly in a solid as in an aqueous solution or in a molten salt may seem astonishing. However, since the 1960s, a variety of solids that include crystalline compounds, glasses, polymers, and composite materials with exceptionally high ionic conductivities have been discovered. Materials that conduct anions (e.g. and 0 ) and cations including monovalent (e.g. H+, Fi+, Na+, Cu+, Ag+), divalent, and even trivalent and tetravalent ions have been synthesized. A variety of names that have been used for these materials include solid electrolytes, superionic conductors, and fast-ionic conductors. Solid electrolytes arguably provides the least misleading and broadest description for this class of materials. 

Shuk, P., Wiemhofer, H.D., Guth, U., Gopel, W., and Greenblatt, M., Oxide ion conducting solid electrolytes based on BijOj, Solid State Ionics, 1996, 89, 179-196. 

Forsyth, M., Wong, S., Nairn, K., Best, A., Newman, P., and MacFarlane, D., NMR studies of modified NASICON-like, lithium conducting solid electrolytes. Solid State Ionics, 124, 213, 1999. 

What makes the sodium-sulfur cell possible is a remarkable property of a compound called beta-alumina, which has the composition NaAlnOiy. Beta-alumina allows sodium ions to migrate through its structure very easily, but it blocks the passage of polysulfide ions. Therefore, it can function as a semipermeable medium like the membranes used in osmosis (see Section 11.5). Such an ion-conducting solid electrolyte is essential to prevent direct chemical reaction between sulfur and sodium. The lithium-sulfur battery operates on similar principles, and other solid electrolytes such as calcium fluoride, which permits ionic transport of fluoride ion, may find use in cells based on those elements. 

The system (Figure 12) is based on sulfuric acid (H2SO4) synthesis and decomposition process (the Westinghouse process) developed earlier. The sulfur trioxide (SO3) decomposition process is facilitated by electrolysis with ionic oxygen conductive solid electrolyte to reduce the operation temperature 200°C-300°C lower than the Westinghouse process. 

Only 7.8% SO3 thermally decomposes in equilibrium condition at 500°C as shown in Figure 1, and also only about 20% in the case membrane reactor technique is used to increase decomposition fraction of SO3 at 500°C. Therefore, some other energy is required to obtain higher decomposition fraction of SO3. Electrolysis by ionic oxygen conductive solid electrolyte is applied to increase decomposition fraction of SO3 in HHLT. HHLT is composed of the reactions shown below. 

Kavan [28] and Kijima et al. [29] have used the electrochemical method to synthesize carbyne. This technique may be realized by classical electrochemistry whereby the charge transfer reaction occurs at interface of a metal electrode and liquid electrolyte solution. Electrons in reaction were supplied either through redox active molecules or through an electrode, which contacts an ionically conducting solid or liquid phase and the precursor. In general, the structure and properties of electrochemical carbon may differ considerably from those of usual pyrolytic carbons. The advantage of this technique is the synthesis of carbyne at low (room) temperature. It was shown that the best product was prepared by cathodic defluorination of poly(tetrafluoroethylene) and some other perhalo-//-alkanes. The carbyne... 

Inaba. Y, Tamura, S. and Imanaka, N. (2007) New type of sulfur dioxide gas sensor based on trivalent Al ion-conducting solid electrolyte. Solid State Ionics, 179, 1625-7. 

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