Polyfunctional conductor

These ideas, which come from electrochemistry of aqueous solutions, have been further developed and applied to the processes in molten salts media [24,25]. It was found that the properties of the intermediate film could be represented in terms of the concept of a polyfunctional conductor (PFC), which was invented and described by Velikanov [26]. This phenomenological concept is based on the theory of amorphous [27] and liquid [28-30] semiconductors. Since it is important for our purposes, it will be discussed in more detail below in Chap. 4. Here we should only emphasise a few main features of the object... 

Hence, our model of EFS is represented by consecutively connecting a conductor of 1st kind (electronic)—the PFC—and a conductor of second kind (ionic). The description of the EFS is thus focused on the concept of polyfunctional conductors. For this reason, we use the earlier introduced term film system as more appropriate than the bifunctional one. 

We have seen that the idea of an electrode film system is useful for electrochemistry of molten salts including low-temperature ionic liquids. It is not restricted, however, to this field only. As an example, the protective layer on lithium metal in aprotic organic electrolytes could be mentioned. This layer, so-called solid electrolyte interphase (SEl), exhibits properties of a polyfunctional conductor with high ionic conductivity (Li ions are the carriers) and low electronic conductivity of semiconductive nature. Some peculiarities of film systems with semiconductive character of electronic conductivity are considered below. 

As follows from the previous chapters, a complex interface Metal/MIEC/Electrolyte (MIEC = mixed ion-electron conductor) appears in many processes related to the electrochemistry of polyvalent metals. The model of MIEC in terms of the concept of polyfunctional conductor (PFC) can be a useful approach to deal with the mechanisms of the processes in such systems. The qualitative classification of EPS has been given based on this approach. Further on, we are going to demonstrate that this concept is useful for quantitative (or at least, semi-quantitative) modelling of macrokinetics (dynamics) of the processes in highly non-equilibrium systems. Before doing this, it is worthwhile to outline some basic ideas related to the MIEC. These considerations will also show some restrictions and approximations that are commonly applied in electrochemical practice and which are no longer valid in such kind of systems. 

Electrolytical production of metals from chalcogenide (in particular, sulphide) compounds was, in fact, the first problem where the researchers faced the essential effect of mixed conductivity in electrochemical practice. Owing to the studies of Velikanov and his team [1-7], we had got the term polyfunctional conductor (PFC) and the main ideas about physico-chemical properties of this object. According to his theory, the electronic conductance of PFC can undergo the semiconductor to metal transformation (Mott transition), which can be detected from the ccaiductivity-temperature dependency. The possibihty had been found for the enhancement of ionic conductivity and, thus, for the improvement of electrochemical behaviour of the melt. It was achieved by means of so-called heteropolar additives— compounds with ionic chemical bond. 

It is shown that the Velikanov model [6] of polyfunctional conductor (PEC) is a good approximation for the properties of the LVl film. The electrochemistry of PEC had attracted considerable attention in the 1970s with regard to the practical problem of electrochemical processing of chalcogenide (sulphide) compounds, the components of natural polymetallic ores. 

To improve proton transport of PFSA membranes at high temperatures and in order to operate PFSA membranes at temperatures above 100°C, inorganic compounds such as Si02 and Ti02 were employed as additives to retain water in the Naflon membranes for an acceptable proton conductivity. Proton conductors such as zirconium phosphate, heteropolyacid/ and heterocycle compounds including imidazole,benzimidazole, triazole, and polyfunctional phosphonic acid were also added in the Naflon membrane. In addition, ionic liquids were applied to fabricate composite Naflon membranes due to their anhydrous high conductivity and good thermal stability. Besides, Naflon/Naflon-functionalized multiwalled carbon nanotube composite membrane exhibits a remarkable improvement in proton conductivity compared to the pristine Naflon membrane. ... 

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