The Transition from Electronic to Ionic Conduction

If one were to describe the essence of electrode kinetics in one short phrase, it woxild be the transition from electronic to ionic conduction, and the phenomena associated with, and controlling it. Conduction in the solution is ionic, whereas in the electrodes and the connecting wires it is electronic. The transition from one mode of conduction to the other requires charge transfer across the interfaces. This is a kinetic process. 

Its rate is controlled by the catalytic properties of the surface and adsorption on it, the concentration and the nature of the reacting species and all other parameters that control the rate of heterogeneous chemical reactions. In addition, the potential plays an important role. This is not surprising, since charge transfer is involved, which may be accelerated by applying a potential difference of the right polarity across the interface. 

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As described in the previous section, both electronic and ionic conductions take place in mesophases when the material contains trace amounts of chemical impurities. These types of conduction exhibit different charge carrier transport properties, as demonstrated by the transient photocurrents the different mesophases measured under the same conditions as shown in Fig. 2.10. In these transient photocurrents, the fast transit times are shifted to shorter times as the molecular order in the mesophases is increased from SmA to SmE and from Coin to the plastic phase, while the slow transits stay in the same time range of 1,000 (xs irrespective of the mesophase in both smectics and discotics. Figiue 2.11 shows Arrhenius plots of the mobilities for fast and slow transits of the 2-phenylnaphthalene derivative 8PNP-012. The mobility for the fast transit hardly depends on the temperature, while the mobility for the slow transit does depend on temperature, with an activation energy... 

Ionic fluorides with large optical gaps exhibit high transparency to electromagnetic radiation. MgF2, for instance, is transparent from 10 cm (corresponding to the energy threshold for the electronic transition from the valence band to the conduction band) to 10 cm (maximum frequency of lattice vibrations). The transparency of metal fluorides has led to their use as windows and prisms in optical instruments (see... 

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). 

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