Conductors, ionic

It is now well established that solvent-free films can be cast from solutions of polyethers (such as poly(ethylene oxide)) and alkali metal salts, and that these films can display high ionic conductivity. Most of the effort devoted to this field has been based on the potential of such materials as solid-state electrolytes for battery applications. In this context, from viewpoints of both ionic mobility and weight, lithium salts in PEO have attracted the most intensive research and appear to offer the most promise such materials are discussed elsewhere. The preparation of materials displaying both electronic and ionic conductivity raises interesting possibilities both in the field of batteries and sensors and is beginning to attract attention (16). 

It is to be hoped that this brief summary of the active and constantly changing field of research generally labelled conducting polymers will be of use in introducing readers of this specialist text to a possibly relevant topic, and that it will allow access to the literature and an easier understanding for those who delve further. 

There are many good textbooks on polymer science a useful general introduction is provided in Polymers Chemistry and Physics of Modern Materials, J.M.G. Cowie, Intertext Books [Blackie Publishing Group] (1973). 

Gibson and J.M. Pochan, in Encyclopedia of Polymer Science and Engineering, vol. 1, 2nd edn., John Wiley, New York (1984) 87 H. W. Gibson, in Quasi-One-Dimensional Organics, E.M. Conwell (ed.), Academic Press, New York. 

A selection of articles on this and various other aspects of conducting polymers is to be found in Handbook of Conducting Polymers, T.A. Skotheim (ed.), Marel Dekker, New York (1986). An alternative view, probably more likely to appeal to chemists, has been presented by Wegner see, for example, G. Wegner, in Contemporary Topics in Polymer Science, E.J. Vandenberg (ed.), 5 (1984) 281 and Angew. Makromol. Chem. 145/146 (1986) 181, and references therein. 

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Electrochemistry is concerned with the study of the interface between an electronic and an ionic conductor and, traditionally, has concentrated on (i) the nature of the ionic conductor, which is usually an aqueous or (more rarely) a non-aqueous solution, polymer or superionic solid containing mobile ions (ii) the structure of the electrified interface that fonns on inunersion of an electronic conductor into an ionic conductor and (iii) the electron-transfer processes that can take place at this interface and the limitations on the rates of such processes. 

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. 

Phosphoric Acid Fuel Cell. Concentrated phosphoric acid is used for the electrolyte ia PAFC, which operates at 150 to 220°C. At lower temperatures, phosphoric acid is a poor ionic conductor (see Phosphoric acid and the phosphates), and CO poisoning of the Pt electrocatalyst ia the anode becomes more severe when steam-reformed hydrocarbons (qv) are used as the hydrogen-rich fuel. The relative stabiUty of concentrated phosphoric acid is high compared to other common inorganic acids consequentiy, the PAFC is capable of operating at elevated temperatures. In addition, the use of concentrated (- 100%) acid minimizes the water-vapor pressure so water management ia the cell is not difficult. The porous matrix used to retain the acid is usually sihcon carbide SiC, and the electrocatalyst ia both the anode and cathode is mainly Pt. 

A second type of soHd ionic conductors based around polyether compounds such as poly(ethylene oxide) [25322-68-3] (PEO) has been discovered (24) and characterized. These materials foUow equations 23—31 as opposed to the electronically conducting polyacetylene [26571-64-2] and polyaniline type materials. The polyethers can complex and stabilize lithium ions in organic media. They also dissolve salts such as LiClO to produce conducting soHd solutions. The use of these materials in rechargeable lithium batteries has been proposed (25). 

When two conducting phases come into contact with each other, a redistribution of charge occurs as a result of any electron energy level difference between the phases. If the two phases are metals, electrons flow from one metal to the other until the electron levels equiUbrate. When an electrode, ie, electronic conductor, is immersed in an electrolyte, ie, ionic conductor, an electrical double layer forms at the electrode—solution interface resulting from the unequal tendency for distribution of electrical charges in the two phases. Because overall electrical neutrality must be maintained, this separation of charge between the electrode and solution gives rise to a potential difference between the two phases, equal to that needed to ensure equiUbrium. 

The lead—acid battery is comprised of three primary components the element, the container, and the electrolyte. The element consists of positive and negative plates connected in parallel and electrically insulating separators between them. The container is the package which holds the electrochemically active ingredients and houses the external connections or terminals of the battery. The electrolyte, which is the Hquid active material and ionic conductor, is an aqueous solution of sulfuric acid. 

Fig. 6. Schematic of band gap energy. Eg, for the three types of electronic and ionic conductors. For electronic conductors the comparison is made of the relative occupancy of valence and conduction bands. For ionic conductors, the bands correspond to the relative occupancy of ionic sublattices. For (a),...

To put the above in perspective, it is necessary to point out that more humdrum ionic conductors (without the super cachet) have been known since the late 19th century, when Nernst developed a lamp based on the use of zirconia which is an ionic conductor (see Section 9.3.2). The use of zirconia for gas sensors is treated in Chapter 11. 

Crystalline ionic conductors. Superionic conductors have already been briefly introduced in Section 1.2.2.2. They have been known for quite a long time, and a major NATO Advanced Study Institute on such conductors was held as early as 1972 (van Gool 1973). Of course, all ionic crystals are to a greater or lesser extent ionically conducting - usually they are cationic conductors, because cations are smaller than anions. Superionic conductors typically have ionic conductivities lO" times higher than do ordinary ionic crystals such as KCl or AgCl. 

Polymeric ionic conductors. One of the most unexpected developments in recent decades in the whole domain of electrochemistry has been the invention of and gradual improvements in ionically conducting polymeric membranes, to the... 

Sudworth, J.L. et al. (2000) Toward commercialization of the beta-alumina family of ionic conductors, MRS Bull. 25(3), 22. 

M. Armand, M. Gauthier in High Conductivity Solid Ionic Conductors (Ed. T. Takahashi), World Scientific, Singapore, 1989, p. 114. 

Figure 8. Arrhenius diagram for various fast ion conductors. For each indicated monovalent mobile ion, the given ionic conductors are the fastest ones known (Na Na 1 - / "-Al203 Cu+, CulflRb4I7Cll3 K+, K+-/T-A120, H H3Moi2P04(, -30H2O Ag, Ag Rbls F, La0 95Sr005F295 Li, ...

Figure 9. Compilation of the solid ionic conductors known at present. For references see Table 1.

Dispersing a dielectric substance such as A1203 in Lil [34] enhances the ionic conductivity of Lil about two orders of magnitude. The smaller the particle size of the dielectrics, the larger is the effect. This phenomenon is explained on the basis that the space-charge layer consists of or Li, generated at the interface between the ionic conductor (Lil) and the dielectric material (A1203) [35],... 

In most cases of practically useful ionic conductors one may assume a very large concentration of mobile ionic defects. As a result, the chemical potential of the mobile ions may be regarded as being essentially constant within the material. Thus, any ionic transport in such a material must be predominantly due to the influence of an internal electrostatic potential gradient,... 

Measurements of photoconductivity and of the Hall potential [367] are accurate and unambiguous methods of detecting electronic conduction in ionic solids. Kabanov [351] emphasizes, however, that the absence of such effects is not conclusive proof to the contrary. From measurements of thermal potential [368], it is possible to detect solid-solution formation, to distinguish between electronic and positive hole conductivity in semi-conductors and between interstitial and vacancy conductivity in ionic conductors. 

In principle, reaction schemes similar to that given in the preceding paragraph may be developed for other comparable rate processes, for example spinel formation. However, Stone [27] has pointed out that, where the barrier phase is not an efficient ionic conductor, the overall reaction may be controlled by the movement of a single cation and anion. In addition, there is the probability that lattice imperfections (internal surfaces, cracks, leakage paths [1172], etc.) may provide the most efficient route to product formation.]... 

Other useful solid-state electrodes are based on silver compounds (particularly silver sulfide). Silver sulfide is an ionic conductor, in which silver ions are the mobile ions. Mixed pellets containing Ag2S-AgX (where X = Cl, Br, I, SCN) have been successfiilly used for the determination of one of these particular anions. The behavior of these electrodes is determined primarily by the solubility products involved. The relative solubility products of various ions with Ag+ thus dictate the selectivity (i.e., kt] = KSp(Agf)/KSP(Aw)). Consequently, the iodide electrode (membrane of Ag2S/AgI) displays high selectivity over Br- and Cl-. In contrast, die chloride electrode suffers from severe interference from Br- and I-. Similarly, mixtures of silver sulfide with CdS, CuS, or PbS provide membranes that are responsive to Cd2+, Cu2+, or Pb2+, respectively. A limitation of these mixed-salt electrodes is tiiat the solubility of die second salt must be much larger than that of silver sulfide. A silver sulfide membrane by itself responds to either S2- or Ag+ ions, down to die 10-8M level. 

Most electrochemical reactions occur at an interface between an electronic conductor system and an ionic conductor system. An interface has three components the two systems and the surface of separation. The electronic conductor stores one of the required chemicals electrons or wide electronic levels. The ionic conductor stores the other chemical needed for an electrochemical reaction the electroactive substance. A reaction occurs only if both components meet physically at the interface separating the two systems. 

The substitution of the two-sided tape with a film of an ionic conductor gives (Fig. 24) a triple-layered muscle working in air.114 The tape now acts as a solid electrolyte. Nevertheless, the system only works if the relative humidity in air surpasses 60%. Under these conditions, movements and rates similar to those shown by a triple layer working in aqueous solution were obtained. This device was developed in cooperation with Dr. M. A. De Paoli from the Campinnas University (Campinnas, Brazil). At the moment several groups are developing actuators, muscles, and electrochemomechanical devices based on bilayer or multilayer structures.115-125... 

Figure 24. An all solid muscle working in air. An ionic conductor elastomer was substituted for the nonconducting tape from Fig. 23. (From Ref. 105).

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