Fast ionic transport

Shown in Figure 1.1 is the oxygen ion conductivity of selected oxides. Among these oxides, only a few materials have been developed as SOFC electrolytes due to numerous requirements of the electrolyte components. These requirements include fast ionic transport, negligible electronic conduction, and thermodynamic stability over a wide range of temperature and oxygen partial pressure. In addition, they must... 

In particular, the small ionic radius of Ag(r g + = 1.00 A) obviously favors the cation diffusion in a-Agl via migration in the sublattice formed by large 1 anions. However, Na has a comparable size (rNa+ =0.99 A) while Nal is not a solid electrolyte, even at elevated temperature. On the other hand, if the aforementioned requirement of a small ionic radius were crucial, then solid electrolytes with mobile anions would be rather uncommon, as anions are usually larger than cations. As a matter of fact, F and conductors may outnumber cationic conductors thus, the requirements mentioned above are somewhat interdependent, and there is no separate factor which can be used solely to explain fast ionic transport. 

Some vinyl fluoride-based polymers with side chains of perfluorosulfonic acid (the Nation family) are important ion-exchange membrane materials used in practice for electrolysis of NaCl and in certain fuel cells. They show a proton conductivity of 0.01 S cm- at room temperature. However, such fast ionic transport occurs only when they are swollen with water. It is therefore not appropriate to call them solid electrolytes in the tme sense of the word. It was in 1970 that anionic conductivity, though not high, was reported for crown ether complexes such as dibenzo-18-crown-6 KSCN, in which cations are trapped by the ligand. " A few years later much higher cationic (instead of anionic) conduction was found in complexes of a chain-like polyether such as PEO or PPO with alkaline salts here, PEO stands for poly(ethyleneoxide), (CHjCHj-O), and PPO for poly(propyleneoxide)."2>"3 These were the flrst examples of tme polymer solid electrolytes and were followed by a great number of studies. Polymeric electrolytes are advantageous in practice because they are easily processed and formed into flexible Aims. 

We summarize what is special with these prototype fast ion conductors with respect to transport and application. With their quasi-molten, partially filled cation sublattice, they can function similar to ion membranes in that they filter the mobile component ions in an applied electric field. In combination with an electron source (electrode), they can serve as component reservoirs. Considering the accuracy with which one can determine the electrical charge (10 s-10 6 A = 10 7 C 10-12mol (Zj = 1)), fast ionic conductors (solid electrolytes) can serve as very precise analytical tools. Solid state electrochemistry can be performed near room temperature, which is a great experimental advantage (e.g., for the study of the Hall-effect [J. Sohege, K. Funke (1984)] or the electrochemical Knudsen cell [N. Birks, H. Rickert (1963)]). The early volumes of the journal Solid State Ionics offer many pertinent applications. 

Proton conducting copolymers of vinylphosphonic acid and 4-vinylimidazole have been reported recently by Bozkurt et al. [7]. Since the imidazole ring can act as a proton hopping site in the polymer matrix [8-10], these copolymers had ionic conductivity of about 10 S cm at 60°C without solvent or salt. To realize fast proton transport in copolymer systems, it is essential to design an ion conductive paths that uses the IL domain. 

Kilner, J.A., Fast oxygen transport in acceptor doped oxides. Solid State Ionics, 2000, 129, 13-23. 

Perovskite membranes are interesting systems not only for their possible applications (e.g., fuel cells, oxygen generators, oxidation catalysts) but also for the fundamental fascination of fast oxygen transport in solid-state ionic. 

Pietraszko, A., Hilczer, B. and Pawlowski, A. (1999). Structural aspects of fast proton transport in (NH4)3H(Se04)2 single crystals. Solid State Ionics 119, 281-288. 

Transport through surface films and coatings, including membranes, ionic and electronic conducting polymers, fast ionic conducting solids, and passive corrosion films... 

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