Solid ionic conductors, high temperature cells

Solid ionic conductors that can be used in electrochemical cells as an electrolyte are called solid electrolytes. In such compotmds only one ion is mobile (see entry. Solid State Electrochemistry, Electrochemistry Using Solid Electrolytes). Generally, any conductor with a high ionic transference number can serve as an electrolyte. Often, the definition after Patterson is used who described solids with a transference number > 0.99 as solid electrolytes [1]. The transference number is not a fixed value. It depends on the temperature and the partial pressure of the gas involved in the chemical reaction with the mobile ion. Therefore, all solids are more or less conductors with a mixed ionic and electronic conductivity, so-called mixed conductors. For the application in sensors and fuel cells, only a window concerning temperature and partial pressure is suitable. This is also called as electrolytic domain. The phenomenon that solids exhibit a high ionic conductivity is also designed as fast ion transport. 

Another very similar method can also be used - particularly to measure the activity of an element in a metal alloy at high temperature. We create a cell with two electrodes, one of which is the pure metal and the other is the solid solution, using a solid ionic conductor as an electrolyte. By measuring the electromotive force of the cell, at the chosen temperature, we will be able to calculate the activity of an element of the alloy at the desired temperature. 

SOE cells utilize solid ceramic electrolytes (e.g. yttria stabilized zirconia) that are good oxygen ion (0 ) conductors at very high temperatures in the range of 1000°C [8]. The operating temperature is decided by the ionic conductivity of the electrolyte. The feed gas, steam mixed with hydrogen, is passed through the cathode compartment. At the cathode side, the reaction is... 

Solid Polymer Electrolyte Fuel Cell Here, there is no apparent liquid solution, or high-temperature ionic conductor. The usual ionic solution between the electrodes is replaced by a well-humidified membrane made of a perfluorosulfonic acid polymer that conducts protons. 

Iwahara, H. et al.. High temperature type proton conductor based on SrCeOj and its application to solid electrolyte fuel cells. Solid State Ionics, 9/10, 1021-1026 (1983). 

The difficulties in the development of HTSO fuel cells are in the area of stability of materials rather than in catalysis. Different materials, some of them ionic conductors with no electronic conductivity and others electronic conductors with no ionic conductivity, must be compatible with each other chemically at a high temperature and mechanically during temperature cycling. Improvements in materials are steadily made, but the more sophisticated materials developed for this purpose tend to increase the cost. Once the materials problems have been overcome, the inherent simplicity of the design and operation of high temperature solid oxide fuel cells may make them the most useful... 

High-energy batteries with a lithium anode are classified < with regard to the type of their ionic conductor. This can be a fast solid Li -ion conductor, a fused lithium salt, a lithium-potassium-salt eutectic mixture, or a non-aqueous lithium salt solution. If inorganic solvents are used, e.g. SOj, SOClj, SOjClj, the solvent itself is the depolarizer and then a solid catalytic electrode is needed, e.g. carbon. The type of ionic conductor determines the internal resistance of the cell and the working temperature range and hence the possible technical applications. 

Every galvanic cell consists of an electrolyte which must be predominantly an ionic conductor and two electrodes which must also conduct electronically. Although the e.m.f. is determined when no current flows, in the actual measurement the transportation of some ions to the electrodes cannot be avoided. Unless diffusion rates are high, transport of materials may lead to changes in concentration in the electrodes and the electrolyte and thus falsify the measurements. This applies particularly when any part of the cell is a solid, whereas diffusion rates in liquids often suffice to secure equilibration. Hence amalgam electrodes have found a wide application at room temperature and have also been used to measure activities of metals dissolved in mercury. 

---