The Molten Salts at Room Temperature: A Narrative Review of Ionic Liquids from Walden's Ethylammonium Nitrate to Energy Applications

Abstract

Ionic liquids---the salts whose melting points fell below the room's temperature---moved from Walden's 1914 ethylammonium nitrate through the chloroaluminate melts and the imidazolium family to the air-stable hydrophobic solvents, Welton's synthesis charter, Rogers and Seddon's vision, the electrochemical challenges, the synthesis's second survey, and the energy applications' review. This article presents a narrative review of that arc's canonical line: Walden's 1914 molten salts, Hurley and Wier's 1951 electrodeposition, Robinson and Osteryoung's 1979 chloroaluminate study, Wilkes and colleagues's 1982 dialkylimidazolium melts, Bonhote and colleagues's 1996 hydrophobic salts, Welton's 1999 Chemical Reviews charter, Holbrey and Seddon's 1999 phase behaviour, Earle and Seddon's 2000 green solvents, Rogers and Seddon's 2003 Science vision, Armand and colleagues's 2009 electrochemical challenges, Hallett and Welton's 2011 synthesis update, and MacFarlane and colleagues's 2014 energy applications. The review is organized around three themes: the founding salts, in which the first melt, the electroplating's quaternary salts, and the imidazolium's chloroaluminates built the field's first materials; the solvent's charter, in which the air-stable turn, the synthesis's and the catalysis's definition, the phase behaviour's crystals, and the green solvents' program made the liquids the chemistry's solvents; and the applications' era, in which the solvents-of-the-future's vision, the electrochemical devices' materials, the synthesis's second survey, and the energy's batteries and cells carried the liquids into the technology. It is concluded that ionic liquids are the solvent's third phase---the water's, the organic's, and now the molten's---and that their arc is the volatility's abolition whose the design's the applications' the discipline's.