Enhancement of Ionic Conductivity

The features described above that lead to high ionic conductivity are uncommon and most solids have low ionic conductivity. However, it is possible to enhance ionic conductivity if a substantial population of defects can be introduced into the crystal. There are a number of strategies that can be employed to achieve this. 

A straightforward method is to incorporate ahovalent impurity ions into the crystal. These impurities can, in principle, be compensated structurally, by the incorporation of interstitials or vacancies, or by electronic defects, holes, or electrons. The possibility of electronic compensation can be excluded by working with insulating solids that contain ions with a fixed valence. 

The fluoride ion interstitials again lead to an increase in ionic conductivity. At lower temperatures this increase is modest because the interstitials aggregate into clusters, thus impeding ionic diffusion. At higher temperatures the clusters tend to dissociate, resulting in a substantial increase in conductivity. 

The stabilized zirconia family of oxides, especially calcia-stabilized zirconia, are solids in which oxide ion conductivity has been increased to the extent that they are widely used solid electrolytes (Section 1.11.6, Section 4.4.5, and Section 6.8). 

In many nonstoichiometric solids high-temperature structures contain disordered defects, but, as in the previous example, these order or form aggregates at lower temperatures. If this aggregation could be suppressed, low-temperature ionic conductivity would be enhanced. An example of this strategy is provided by compounds with the 

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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. 

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Sanna, S., Esposito, V., Tebano, A., Licoccia, S., Traversa, E., and Balestrino, G. (2010) Enhancement of ionic conductivity in Sm-doped ceria/yttria-stabilized zirconia heteroepitaxial structures. Small, 6, 1863-1867. 

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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],... 

There are three broad categories of materials that have been utilized in this endeavor. In the first, even in fully stoichiometric compounds, the ionic conductivity is high enough to be useful in devices because the cation or anion substructure is mobile and behaves rather like a liquid phase trapped in the solid matrix. A second group have structural features such as open channels that allow easy ion transport. In the third group the ionic conductivity is low and must be increased by the addition of defects, typically impurities. These defects are responsible for the enhancement of ionic transport. 

In some experiments, we need to enhance the ionic conductivity of a solution, so we add an additional ionic compound to it. Rather confusingly, we call both the compound and the resultant solution an electrolyte . 

Ohmic Polarization Ohmic losses occur because of resistance to the flow of ions in the electrolyte and resistance to flow of electrons through the electrode materials. The dominant ohmic losses, through the electrolyte, are reduced by decreasing the electrode separation and enhancing the ionic conductivity of the electrolyte. Because both the electrolyte and fuel cell electrodes obey Ohm s law, the ohmic losses can be expressed by the equation... 

The doping of a solid is similar to the enhancement of the H30+ or OH- concentration in water by adding a strong acid or base. However, while in water mobilities of dopant ions are frequently similar to those of the native defects H30+ and OH-[69, 70], dopant ions in solids (e.g. CdXg in AgCl) are almost immobile. This is also why supporting electrolytes (i.e. electrolytes with dissolved dopants that enhance the ionic conductivity, but do not influence electrochemical electrode reactions [71, 72], are unknown in solid state electrochemistry. 

The self-assembly of LC block molecules composed of two or more covalently bonded immiscible molecular parts including an ion-conductive moiety leads to the formation of well-defined nano-segregated structures such as layer and columnar structures [4,5,26,27,30]. These anisotropic structures formed for LC block molecules having PEO and ionic liquid moieties would be useful for the anisotropic transportation of ions. Moreover, we considered that the macroscopic orientation of the nano-segregated LC structures plays a key role in the enhancement of the conductivities because the boundary in randomly oriented polydomains disturbs the anisotropic transportation of ions. 

There have been several attempts to reduce the crystallinity of PEO and enhance the ionic conductivity at ambient temperatures. These include (a) addition of plasticizers and other related additives to PEO, (b) use of plasticized salts, (c) cross linking of PEO by grafting and other methods. Some of these modifications are discussed below. 

The most important aspects of the study of oxygen conductors are the abilities to enhance their ionic conductivity and reaction kinetics. Both features are essential for the development of electrochemical devices including fuel cells, gas sensors and ionic membranes. These devices have the potential to deliver high economic and ecological benefits however to achieve satisfactory performance, it is necessary to optimize the ionic conductivity of the solid electrolytes. 

The ionic conductivity of YSZ can be also enhanced by the introduction of high density of dislocations [17] or interfaces that act as rapid diffusion paths for oxygen vacancies. Such an idea has been discussed for BaF2/CaF2/Bap2 superlattices where a substantial increase of ionic conductivity was observed [18]. In this system a progressive increase in the conductivity was correlated with the increase of interfacial density. 

The ionic transport in solids is attributed to the hopping of ionic carriers between the equivalent positions in the crystal lattice. This mechanism is known as lattice diffusion and depends on the jumping distance and frequency of moved ions. The understanding of the influence of these factors on the ionic conductivity is very important for the development of material with enhanced ionic transport. The question of what is the limit of ionic conductivity in solids will be addressed by analyzing the ionic transport in cubic stabilized zirconia systems with different acceptor dopants. 

In the present paper the question of how the microstructure can influence the electrical transport of nanocrystalline stabilized zirconia was discussed. The reviewed results obtained from the study of nanocrystalline YSZ and ScSZ thin films have shown the possibility of the enhancement of ionic or electronic conductivity. 

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