Fast ion conductor
In materials science, fast ion conductors are solids in which ions are highly mobile. These materials are important in the area of solid-state ionics, and are also known as solid electrolytes and superionic conductors. These materials are useful in batteries and various sensors. Fast ion conductors are used primarily in solid oxide fuel cells. As solid electrolytes they allow the movement of ions without the need for a liquid or soft membrane separating the electrodes. The phenomenon relies on the hopping of ions through an otherwise rigid structure.
Fast ion conductors are intermediate in nature between crystalline solids which possess a regular structure with immobile ions, and liquid electrolytes which have no regular structure and fully mobile ions. Solid electrolytes find use in all solid state supercapacitors, batteries and fuel cells, and in various kinds of chemical sensors.
In solid electrolytes (glasses or crystals), the ionic conductivity Ωi is an arbitrary value but it should be greatly larger than electronic one. Usually, the solids where electronic conductivity Ωe is arbitrary value but Ωi is an order of 0.0001-0.1 Ohm−1 cm−1 (300 K) are called superionic conductors.
Unlike conventional solid electrolytes and superionic conductors.
Superionic conductors, where Ωi is more than 0.1 Ohm−1 cm−1 (300 K) and activation energy for ion transport Ei is small (about 0.1 eV), are called by advanced superionic conductors. The famous example of advanced superionic conductor-solid electrolyte is RbAg4I5 where Ωi > 0.25 Ohm−1 cm−1 and Ωe ~10−9 Ohm−1 cm−1 at 300 K. The Hall (drift) ionic mobility in RbAg4I5 is about 2x10−4 cm2/(V•s) at room temperatures. The Ωe – Ωi systematic diagram distinguishing the different types of solid state ionic conductors is given on the figure
Fig. Classification of solid state ionic conductors by the lg Ωe - lg Ωi diagram (Ohm−1 cm−1).
2, 4 and 6 – known solid electrolytes (SEs), materials with Ωi >> Ωe;
1, 3, and 5 – known mixed ion-electron conductors;
3 and 4 – superionic conductors (SICs), i.e. materials with Ωi > 0.001 Ohm−1cm−1, Ωe – arbitrary value;
4 – SIC and simultaneously SE, Ωi > 0.001 Ohm−1cm−1, Ωi >>Ωe;
5 and 6 – advanced superionic conductors (AdSICs), where Ωi > 10−1 Ohm−1cm−1 (300 K), energy activation Ei about 0.1 eV, Ωe – arbitrary value;
6 – AdSIC and simultaneously SE, Ωi > 10−1 Ohm−1cm−1, Ei about 0.1 eV, Ωi >>Ωe;
7 and 8 – hypothetical AdSIC with Ei ≈ kBT ≈0.03 eV (300 К);
8 – hypothetical AdSIC and simultaneously SE.
A common solid electrolyte is yttria-stabilized zirconia, YSZ. This material is prepared by doping Y2O3 into ZrO2. Oxide ions typically migrate only slowly in solid Y2O3 and in ZrO2, but in YSZ, the conductivity of oxide increases dramatically. These materials are used to allow oxide to move through the solid in certain kinds of fuel cells. Zirconium dioxide can also be doped with calcium oxide to give an oxide conductor that used in oxygen sensors in automobile controls. By doping only a few percent, the diffusion constant of oxide increases by nearly 1000x.
Other conductive ceramics function as ion conductors. One example is NASICON (Na3Zr2Si2PO12), a sodium super-ionic conductor
Another example of a popular fast ion conductor beta-alumina solid electrolyte. Unlike the usual forms of alumina, this modification has a layered structure with open galleries separated by pillars. Sodium ions (Na+) migrate through this material readily since the oxide framework provides an ionophilic, non-reducible medium. This material is considered as the sodium ion conductor for the sodium-sulfur battery.
Fluoride ion conductors
Lanthanum trifluoride (LaF3) is conductive for F- ions, used in some ion selective electrodes. Beta-lead fluoride exhibits a continuous growth of conductivity on heating. This property was first discovered by Michael Faraday.
A textbook case for a fast ion conductor is silver iodide (AgI). Upon heating the solid to 146 °C, this material adopts the alpha-polymorph. In this form, the iodide ions form a rigid cubic framework and the Ag+ centers are molten. The electrical conductivity of the solid increases by 4000x. Similar behavior is observed for copper iodide (CuI), rubidium silver iodide (RbAgI2), and Ag2HgI4.
Other Inorganic materials
- Silver sulfide, conductive for Ag+ ions, used in some ion selective electrodes
- Lead(II) chloride, conductive at higher temperatures
- Some perovskite ceramics - strontium titanate, strontium stannate - conductive for O2- ions
- Zr(HPO4)2.nH2O - conductive for H+ ions
- UO2HPO4.4H2O - conductive for H+ ions
Many gels, such polyacrylamides, agar, etc represent fast ion conductor A salt dissolved in a polymer - e.g. lithium perchlorate in polyethylene oxide. Polyelectrolytes / Ionomers - e.g. Nafion, a H+ conductor.
The important case of fast ionic conduction is one in a surface space-charge layer of ionic crystals. Such conduction was first predicted by Kurt Lehovec. As a space-charge layer has nanometer thickness, the effect is directly related to nanoionics (nanoionics-I). Lehovec's effect is used as a basis for developing nanomaterials for portable lithium batteries and fuel cells.
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