Fluorine-ion conductors

Some fluorine-ion conductors exhibit high ionic conductivities, even at room temperature [4], which are not equaled by other halide-ion conductors. However, there is a lack of known electrode materials. Further research on this topic is very worthwhile. 

Reau, J.-M. and Portier. J. (1978) Fluorine ion conductors, in Solid Electrolytes General Principles, Characterization, Materials, Applications (eds P. Hagenmuller and W. Van Gool), Academic Press, New York, pp. 313-33. 

Rare-earth fluorides are good fluorine-ion conductors (see sect 5.2). Addition of aliovalent cations significantly increases the fluorine-ion conductivity even fiulher. For example, single crystals of Lap3 doped with Eup2 are widely used in commercial apphcations of ion-selective electrodes (ISE) as specific electrodes (Frant and Ross 1966). In the field of ISE, only the pH-sensing glass electrode is more widely used. Fluoride-ion detection is important in sea water, water minerals, rocks, fossils and minerals, biomedical applications, potable water and plant and animal metabolism. 

Apart from crystalline phases, various glassy solid F conductors have also been developed. A glass in the ZrF4-BaF2 system, the first example of such conductors, exhibits a conductivity of 10" S cm" at 200°C.2 Extensive studies have been performed with this system, including short-range stractural analyses, T NMR, simulation based on molecular dynamics, etc. According to the results, there are basically two kinds of fluorine ions, i.e., a... 

Table 7.1 reproduces a selection of equilibrium cells based on fluorine, silver and oxygen ion conductors, which have been used to determine relevant thermodsmamic data (such as were treated in Chapter 4) [6,550]. A cell analogous [365] to the cell in Eq. (7.21)... 

Polymer Electrolyte Fuel Cell. The electrolyte in a PEFC is an ion-exchange (qv) membrane, a fluorinated sulfonic acid polymer, which is a proton conductor (see Membrane technology). The only Hquid present in this fuel cell is the product water thus corrosion problems are minimal. Water management in the membrane is critical for efficient performance. The fuel cell must operate under conditions where the by-product water does not evaporate faster than it is produced because the membrane must be hydrated to maintain acceptable proton conductivity. Because of the limitation on the operating temperature, usually less than 120°C, H2-rich gas having Htde or no (

The PEFC was first developed for the Gemini space vehicle by General Electric, USA. In this fuel cell type, the electrolyte is an ion-exchange membrane, specifically, a fluorinated sulfonic acid polymer or other similar solid polymer. In general, the polymer consists of a polytetrafluoroethylene (Teflon) backbone with a perfluorinated side chain that is terminated with a sulfonic acid group, which is an outstanding proton conductor. Hydration of the membrane yields dissociation and solvation of the proton of the acid group, since the solvated protons are mobile within the polymer. Subsequently, the only liquid necessary for the operation of this fuel cell type is water [7,8],... 

Summary. We have shown that ion transport in "Nafion" per-fluorinated membrane is controlled by percolation, which means that the connectivity of ion clusters is critical. This basically reflects the heterogeneous nature of a wet membrane. Although transport across a membrane is usually perceived as a one-dimensional process, our analysis suggests that it is distinctly three-dimensional in "Nafion". (Compare the experimental values of c and n with those listed in Table 7.) This is not totally unexpected since ion clusters are typically 5.0 nm, whereas a membrane is normally several mils thick. We have also uncovered an ionic insulator-to-conductor transition at 10 volume % of electrolyte uptake. Similar transitions are expected in other ion-containing polymers, and the Cluster-Network model may find useful application to ion transport in other ion containing polymers. Finally, our transport and current efficiency data are consistent with the Cluster-Network model, but not the conventional Donnan equilibrium. 

Many other systems have subsequently been found to display anion conductivity. The electrical conductivity of lead fluoroborate and fluorogermanate glasses is similar to that of the lead fluorosilicate glasses. Calcium fluoroaluminate glasses also conduct by fluorine migration. Fluorophosphate glasses have also proven to be good anionic conductors. All of these systems are free of alkali ions. 

The electrolyte in this fuel cell is an ion exchange membrane (fluorinated sulfonic acid polymer or other similar polymer) that is an excellent proton conductor. The only liquid in this fuel cell is water thus, corrosion problems are minimal. Typically, carbon electrodes with platinum electrocatalyst are used for both anode and cathode, and with either carbon or metal interconnects. 

Another common fluoride ionic conductor is calcium fluoride. La order to increase the fluoride ionic vacancies, an aliovalent cation has to be mixed with CaF2. In this case, trivalent rare-earth cations such as La +, ate used as the mixing ion. La addition to these fluoride electrolytes, P-PbF2 also shows a good F" ionic conduction. The details of these two fluorine conducting electrolytes are described below. 

The HF vapour was condensed and electrolysed in a platinum U-tube with platinum-iridium alloy electrodes at -23 °C. Convinced that fluorine would react violently with silicon, Moissan had long tested likely samples with a crystal of silicon. His crystal had survived many tests unscathed, but on this occasion, when it was exposed to the gas evolved at the positive electrode, it exploded and burst into flames fluorine had at last been isolated. The French Academy of Science sent a deputation to verify this important discovery. Moissan took no chances determined that his HF should be as pure as possible, he performed Reaction 6.5 with especial care. But the crucial demonstration was a disaster no current flowed, and no electrolysis took place. When the embarrassment subsided, Moissan realized that he had been too careful. Pure liquid HF is a very poor conductor, but his first and productive sample had been contaminated by KF from Reaction 6.5. Like K2O in water, KF in HF creates ions ... 

---