Solid electrolytes ionic conduction

Generally, in solid electrolytes, ionic conductivity is predominant (( = 1) only over a limited chemical potential. The electrolytic conductivity domain is an important factor limiting the application of solid electrolytes in electrochemical sensors. 

Like the cathode, the anode must combine catalytic activity for fuel oxidation with electrical conductivity. Catalytic properties of the anode are necessary for the kinetics of the fuel oxidation with the oxide ions coming through the solid electrolyte. Ionic conductivity allows the anode to spread the oxide ions across a broader region of anode/electrolyte interface, and electronic conductivity is necessary to convey the electrons resulting from the electrode reaction out into the external circuit. 

Figure 3.1. Temperature dependence of the ionic conductivity of some solid electrolytes. The conductivity of concentrated H2S04 (37 wt%) is included for comparison.16 Reprinted with permission from WILEY-VCH.

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. 

In a broad sense, electrochemical phenomena involve electron transfer processes through a two-dimensional boundary (interface) separating the electrode (metal-type conductor) and the electrolyte (ionically conducting). In the study of such phenomena, one can distinguish between electrodics, focused on the heterogeneous elec-trode/electrolyte charge transfer process, and ionics, devoted to the study of ionically conducting liquid or solid phases (Bockris and Reddy, 1977). 

Rhodes, C.R, J.W Long, M.S. Doescher, J.J. FontaneUa, and D.R. Rolison, Nanoscale polymer electrolytes Ultrathin electrodepositedpoly(phenylene oxide) with solid-state ionic conductivity. Journal of Physical Chemistry B, 2004. 108(35) pp. 13079-13087 Petiii, O.A. and G.A. Tsirlina, Size effects in electrochemistry. Uspekhi Khimii, 2001. 70(4) pp. 330-344... 

In spite of these analogies the electrochemistry of solids is more complex than the electrochemistry in aqueous solutions. So it must be noted that apart from ionic conduction, solids often show an electronic conductivity, caused by electrons or electron defects, which may be predominant in many cases over the ionic conduction. In good solid electrolytes the conduction of the electrical current is caused exclusively by the ions—in most cases practically by only one kind of ion present in a crystal. 

Solid-state electrolyte Ionic conductivity Zirconium oxide... 

A possible classification of solids where ionic conductivity plays an important role is given in Table 5.1. In contrast to the situation in solutions, ionic transport in solids is accompanied by an electronic counterpart. For solid electrolytes the ionic contribution to the total electrical conductivity is predominant. Other cases represent MIECs. 

Ionic conductivity is electrical conductivity due to the motion of ionic charge. Elementary science introduces this phenomenon as a property of liquid electrolyte solutions. In the solid state, ionic conductivity has recently been somewhat overshadowed by electronic, but nevertheless was recognized by Faraday, who observed electrical conductivity in solid lead fluoride at high temperature. The conductivity in this case was due to the motion of fluoride anions within the structure. This type of conductivity in solids has long been of fundamental interest as well as being applied in the interpretation of corrosion. More recently, applications have been found in energy conversion devices and chemical sensors. ... 

Ionic conductors arise whenever there are mobile ions present. In electrolyte solutions, such ions are nonually fonued by the dissolution of an ionic solid. Provided the dissolution leads to the complete separation of the ionic components to fonu essentially independent anions and cations, the electrolyte is tenued strong. By contrast, weak electrolytes, such as organic carboxylic acids, are present mainly in the undissociated fonu in solution, with the total ionic concentration orders of magnitude lower than the fonual concentration of the solute. Ionic conductivity will be treated in some detail below, but we initially concentrate on the equilibrium stmcture of liquids and ionic solutions. 

An example of an ionically conductive polymer is polyethylene oxide containing LiC104, which is used as a solid phase electrolyte in batteries. 

A completely separate family of conducting polymers is based on ionic conduction polymers of this kind (Section 11.3.1.2) are used to make solid electrolyte membranes for advanced batteries and some kinds of fuel cell. 

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. 

Electrolyte a substance, liquid or solid, which conducts electrical current by movement of ions (not of electrons). In corrosion science, an electrolyte is usually a liquid solution of salts dissolved in a solvent, or a molten salt. The term also applies to polymers and ceramics which are ionically conductive. 

When an ionic solid such as NaCl dissolves in water the solution formed contains Na+ and Cl- ions. Since ions are charged particles, the solution conducts an electric current (Figure 2.12) and we say that NaCl is a strong electrolyte. In contrast, a water solution of sugar, which is a molecular solid, does not conduct electricity. Sugar and other molecular solutes are nonelectrolytes. 

One leading prototype of a high-temperature fuel cell is the solid oxide fuel cell, or SOFC. The basic principle of the SOFC, like the PEM, is to use an electrolyte layer with high ionic conductivity but very small electronic conductivity. Figure B shows a schematic illustration of a SOFC fuel cell using carbon monoxide as fuel. 

This relationship makes it possible to calculate the maximum ionic conductivity of solid electrolytes. Assuming that the mobile ions are moving with thermal velocity v without resting and oscillating at any lattice site, this results in a jump frequency... 

Generally, solid electrolytes for battery applications require high ionic conductivities and wide ranges of appropriate thermodynamic stability. 

Though solid electrolytes for multivalent ions offer the advantage of a larger charge transfer, their conductivities are much lower than those of monovalent ions at ambient temperature because of a higher activation enthalpy for the ionic motion... 

Another way of looking at high ionic conductivities of solid electrolytes is to consider the activation enthalpy as illustrated in Fig. 8. Generally, the activation enthalpy is strongly correlated with the room-temperature ionic conductivity the higher the room-temperature ionic conductivity, the lower the activation enthalpy. The straight lines in the Arrhenius... 

Traditionally, the chemical stability of the electrode/electrolyte interface and its electronic properties have not been given as much consideration as structural aspects of solid electrolytes, in spite of the fact that the proper operation of a battery often depends more on the interface than on the solid electrolyte. Because of the high ionic conductivity in the electrolyte and the high electronic conductivity in the electrode, the voltage falls completely within a very narrow region at the electrolyte/electrode interface. 

Thin-film solid electrolytes in the range of lpm have the advantage that the material which is inactive for energy storage is minimized and the resistance of the solid electrolyte film is drastically decreased for geometrical reasons. This allows the application of a large variety of solid electrolytes which exhibit quite poor ionic conductivity but high thermodynamic stability. The most important thin-film preparation methods for solid electrolytes are briefly summarized below. 

There is a wide variety of solid electrolytes and, depending on their composition, these anionic, cationic or mixed conducting materials exhibit substantial ionic conductivity at temperatures between 25 and 1000°C. Within this very broad temperature range, which covers practically all heterogeneous catalytic reactions, solid electrolytes can be used to induce the NEMCA effect and thus activate heterogeneous catalytic reactions. As will become apparent throughout this book they behave, under the influence of the applied potential, as active catalyst supports by becoming reversible in situ promoter donors or poison acceptors for the catalytically active metal surface. 

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