Fused-salt solution

A nonaqueous solvent system particularly useful in coordination chemistry. Fused salts are a large class of liquids which are composed largely of ions. Many simple inorganic salts melt at rather high temperatures (greater than 600°C or 1000°F), forming liquids which have high specific conductivities, 1–6 ohm⁻¹ · cm⁻¹. There are, however, a number of exceptions to this generalization; for example, the electrical conductivity of AlCl₃ decreases sharply upon melting due to the formation of molecular liquid (Al₂Cl₆). See also High-temperature chemistry; Solvent.

The use of binary or ternary melt compositions results in liquids which typically have much lower melting temperatures and somewhat lower specific conductivities than do pure salts. The choice of melts for use as solvents is frequently based on such considerations as the availability and cost, the lowest melting temperature attainable, and the ease of purification of the solvent, as well as the width of the electrochemical span and the spectroscopic transparency of the melt. Several molten-salt solvents, such as the LiCl–KCl eutectic and the equimolar NaNO₃–KNO₃ melt, have been extensively studied, and many of their physical and chemical properties are well known. Many melt systems, particularly ternary and more complex compositions, have been only partially characterized; their physical properties are estimated by using the data available for less complex systems, such as the binary component melts.

One use of fused salts is as media for organic reactions. Not only does the fused-salt environment provide for a better thermal control of the reaction (heat dissipation is readily possible), but the fused salt may serve as a catalyst. For example, molten SbCl₃ and ZnCl₂ have been found to be effective hydrocracking catalysts for coal. It has been found that polycyclic hydrocarbons, such as anthracene, undergo several types of reactions in SbCl₃ melts, including formation of radical cations and protonated species which react further to form condensed systems, such as anthra[2.1-a]aceanthrylene. See also Catalysis.

Another technological area of great interest in which molten salts play a key role is that of advanced batteries and fuel cells. Thus, LiCl–KCl eutectic is the solvent in the rechargeable Li(Al)–FeS (or FeS₂) battery, and sodium polysulfide melts are employed in the sodium/sulfur battery which operates at about 350°C (660°F). The rechargeable cell Na/Na⁺ conductor/S(IV) in AlCl₃–NaCl has an open circuit voltage of 4.2 V at 200–250°C (390–480°F) as well as high energy densities.

Molten carbonates, such as the ternary Li₂CO₃–Na₂CO₃−K₂CO₃ melt, are used in fuel cells which employ H₂/CO mixtures and oxygen as electrochemical fuels. See also Fuel cell.