Ideal Solid Electrolytes

A large number of the ions of one species should be mobile. This requires a large number of empty sites, either vacancies or accessible interstitial sites. As is well known, empty sites are needed for ions to move through the lattice. 

The empty and occupied sites should have similar potential energies with a low activation energy barrier for jumping between neighboring sites. High activation energy decreases carrier mobility and can lead to carrier localization. 

The structure should have a solid framework, preferably 3D, permeated by open channels. The migrating ion lattice should be molten, so that a sohd framework of other ions is needed in order to prevent the entire material from melting. 

The framework ions, usually anions, should be highly polarizable. Such ions can deform to stabilize transition-state geometries of the migration ion through covalent interactions. 

However, as was shown by Weppner (2000), any material used for gas sensor design taken by itself has no meaning. As in semiconductor electronics, only combinations of materials play a practical role in electrochemical gas sensors, which as a rule include both ion and electron conductors. At that, material combinations have to be considered individually to evaluate stability and performance of the galvanic cell. For example, according to Dell (1975), four distinct cases may be identified depending on the nature of electrode material used  

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In order for an emf to be completely defined, the phase rule must be obeyed for each electrode side. Cells involving ideal solid electrolytes (ti0B = 1) can be usually measured over a wider temperature range and are hence convenient means to deduce thermodynamic data such as formation enthalpies and formation entropies. The cell given in Eq. (82)... 

This method is very popular for measuring transference numbers of lithium electrolytes (see references in Table 17.18) because of the very easy procedure and low time costs. But it must be taken into account that the measurement was developed for binary and ideal solid electrolytes, which is often not the case, especially for solid polymer electrolytes, where a large amount of ion-pairs is probable. 

II. Ease of electrical connection Here the main problem is that of efficient electrical current collection, ideally with only two electrical leads entering the reactor and without an excessive number of interconnects, as in fuel cells. This is because the competitor of an electrochemically promoted chemical reactor is not a fuel cell but a classical chemical reactor. The main breakthrough here is the recent discovery of bipolar or wireless NEMCA,8 11 i.e. electrochemical promotion induced on catalyst films deposited on a solid electrolyte but not directly connected to an electronic conductor (wire). 

The electrical double layer has also been investigated at the interface between two immiscible electrolyte solutions and at the solid electrolyte-electrolyte solution interface. Under certain conditions, the interface between two immiscible electrolyte solutions (ITIES) has the properties of an ideally polarized interphase. The dissolved electrolyte must have the following properties ... 

For a battery to give a reasonable power output, the ionic conductivity of the electrolyte must be substantial. Historically, this was achieved by the use of liquid electrolytes. However, over the last quarter of a century there has been increasing emphasis on the production of batteries and related devices employing solid electrolytes. These are sturdy and ideal for applications where liquid electrolytes pose problems. The primary technical problem to overcome is that of achieving high ionic conductivity across the solid. 

Solid electrolytes are frequently used in studies of solid compounds and solid solutions. The establishment of cell equilibrium ideally requires that the electrolyte is a pure ionic conductor of only one particular type of cation or anion. If such an ideal electrolyte is available, the activity of that species can be determined and the Gibbs energy of formation of a compound may, if an appropriate cell is constructed, be derived. A simple example is a cell for the determination of the Gibbs energy of formation of NiO ... 

A solid electrolyte is an ionic conductor and an electronic insulator. Ideally, it conducts only one ionic species. Aside from a few specialty applications in the electronics industry, solid electrolytes are used almost exclusively in electrochemical cells. They are particularly useful where the reactants of the electrochemical cell are either gaseous or liquid however, they may be used as separators where one or both of the reactants are solids. Used as a separator, a solid electrolyte permits selection of two liquid or elastomer electrolytes each of which is matched to only the solid reactant with which it makes contact. 

Electrochemical cells are of two types power cells and sensors. In an ideal power cell, the ionic current through the electrolyte inside the cell matches an electronic current through an external load. The solid electrolyte is in the form of a membrane of thickness L and area A that separates electronically the two electrodes of the cell. Any internal electronic current across the electrolyte reduces the power output. The internal resistance to the ionic current is... 

The )5-aluminas are described in some detail in Chapter 2, only a few specific features are noted here. In the "-aluminas, the spinel blocks are stacked in such a way that the energetically equivalent sites occupied by Na" ions are ideally just half-filled in the -aluminas the spinel blocks are stacked so as to distinguish two types of Na" -ion sites of different potential energy, the Beevers-Ross (BR) and anti-Beevers-Ross (aBR) sites. In the Na-O planes, the shortest bottleneck distance 2.7 A is just a little greater than the sum of the ionic radii, 2.4 A, at room temperature, so a small value of AH can be anticipated. The discovery of fast Na -ion conductivity in the Na j5-aluminas (Yao and Kummer, 1967 Kummer and Weber, 1967) led to the invention of the Na/S battery that triggered extensive interest in the solid-electrolyte problem. 

Figure 53. Idealized half-cell response of a thin solid electrolyte cell, (a) Cell geometry including working electrodes A and B and reference electrode (s). (b) Equivalent circuit model for the cell in a, where the electrolyte and two electrodes have area-specific resistances and capacitances as indicated, (c) Total cell and half-cell impedance responses, calculated assuming the reference electrode remains equipotential with a planar surface located somewhere in the middle of the active region, halfway between the two working electrodes, as shown in a.

Gas sensors — (b) Gas sensors with solid electrolytes — Figure 4. Voltage characteristics of an idealized hydrocarbon electrode vs. Pt-air reference electrode in propylene-containing mixtures... 

Schenck et al. (1929) had earlier used the same technique but used CO/CO2 gas mixture for equilibration. They had reported moderate positive deviation from ideality, which was also confirmed by Swerdtfeger andMuan (1967). However, Engell (1962) reported a much higher positive deviation using the solid electrolyte galvanic cell technique, about which we would discuss in the next section. 

By measuring the equitibrium EMF of this cell, Engell (1962) reported high positive deviation from ideality in this system. Seetharaman Abraham (1968) used a similar technique but used Thoria doped with Yttria as the solid electrolyte. They used the following cell... 

Hydrocarbons Oxidation by heterogeneous catalysis C02 (ideally) Used in solid-electrolyte cells... 

Although the basic principles of type III potentiometric sensors are apphcable for gaseous oxide detection, this should not obscure the fact that these sensors still require further development. This is especially true in view of the kinetics of equilibria and charged species transport across the solid electrolyte/electrode interfaces where auxiliary phases exist. Real life situations have shown that, in practice, gas sensors rarely work under ideal equilibrium conditions. The transient response of a sensor, after a change in the measured gas partial pressure, is in essence a non-equilibrium process at the working electrode. Consequently, although this kind of sensor has been studied for almost 20 years, practical problems still exist and prevent its commercialization. These problems include slow response, lack of sensitivity at low concentrations, and lack of long-term stability. " It has been reported " that the auxiliary phases were the main cause for sensor drift, and that preparation techniques for electrodes with auxiliary phases were very important to sensor performance. ... 

The theoretical treatment presented (Eqs 4.1-4.5) is applicable also for direct wet electrochemistry on Pt cathode in aprotic electrolyte solution [12,13] (Table 4.1) and for some other chemical reductants, Rj, viz. benzoin dianion [14] and sodium dihydronaphthylide [15] (Table 4.1). Apparently, the decision between chemical and electrochemical carbonization may not be straightforward. The latter scenario requires a compact solid electrolyte with mixed electron/ion conductivity to be present at the interface. This occurs almost ideally in the reactions of solid fluoropolymers with diluted alkali metal amalgams [3]. If the interfacial layer is mechanically cracked, both electrochemical and chemical carbonization may take place, and the actual kinetics deviates from that predicted by Eq. 4.4 [10]. There is, however, another mechanism, leading to the perturbations of the Jansta and Dousek s electrochemical model (Eq. 4.4). This situation typically occurs if gaseous perfluorinated precursors react with Li-amalgam [4,5], and it will be theoretically treated in the next section. 

The halogen hydracids are all weaker than perchloric acid. Actually they are not ideal strong electrolytes, although they a pproach this behavior when water is used as the solvent. Certainly, these compounds differ distinctly from typical strong electrolytes such as potassium chloride and other neutral salts. The difference probably originates in the structure of the solid form. Neutral salts in the solid crystalline state possess a coordination lattice. Simple molecules do not exist in this type of lattice since the constituents of the salt are present. solely in the ionic form. Each ion is surrounded in a uniform manner by a definite number of other ions of opposite charge. Indeed it is no longer correct to speak of undissociated molecules in the solid state. 

In ideal semiconductor/electrolyte junctions, the presence of an energetic barrier in the semiconductor phase, originated from the equilibrium of the Fermi level in the solid and liquid phases, can be approached by the semiconductor depletion layer capacitance or space charge capacitance, Cgc- Measurement of capacitance versus... 

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