Ionic liquids are room-temperature molten salts, composed mostly of organic ions that may undergo almost unlimited structural variations. This review covers the newest aspects of ionic liquids in applications where their ion conductivity is exploited; as electrochemical solvents for metal/semiconductor electrodeposition, and as batteries and fuel cells where conventional media, organic solvents (in batteries) or water (in polymer-electrolyte-membrane fuel cells), fail. Biology and biomimetic processes in ionic liquids are also discussed. In these decidedly different materials, some enzymes show activity that is not exhibited in more traditional systems, creating huge potential for bioinspired catalysis and biofuel cells. Our goal in this review is to survey the recent key developments and issues within ionic-liquid research in these areas. As well as informing materials scientists, we hope to generate interest in the wider community and encourage others to make use of ionic liquids in tackling scientific challenges.

Ionic liquids are defined today as liquids which solely consist of cations and anions and which by definition must have a melting point of 100 degrees C or below. Originating from electrochemistry in AlCl(3) based liquids an enormous progress was made during the recent 10 years to synthesize ionic liquids that can be handled under ambient conditions, and today about 300 ionic liquids are already commercially available. Whereas the main interest is still focussed on organic and technical chemistry, various aspects of physical chemistry in ionic liquids are discussed now in literature. In this review article we give a short overview on physicochemical aspects of ionic liquids, such as physical properties of ionic liquids, nanoparticles, nanotubes, batteries, spectroscopy, thermodynamics and catalysis of/in ionic liquids. The focus is set on air and water stable ionic liquids as they will presumably dominate various fields of chemistry in future.

The double layer structure of two ionic liquids (ILs), 1-butyl-1-methylpyrrolidinium tris(pentafluoroethyl)trifluorophosphate ([Py1,4]FAP) and 1-ethyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate ([EMIm]FAP) at the polarized Au(111) electrode interface is probed using Atomic Force Microscopy force measurements. The force-separation profiles suggest a multilayered morphology is present at the electrified Au(111)−IL interface, with more near surface layers detected at higher potentials. At the (slightly negative) open circuit potential, multiple ion layers are present, and the innermost layer, in contact with the Au(111) surface, is enriched in the cation due to electrostatic adsorption. Upon applying negative electrode potentials (−1.0 V, −2.0 V), stronger IL near surface structure is detected: both the number of ion layers and the force required to rupture these layers increases. Positive electrode potentials (+1.0 V, +2.0 V) also enhance IL near surface structure, but not as much as negative potentials, because surface-adsorbed anions are less effective at templating structure in subsequent layers than cations. This interfacial structure is not consistent with a double layer in the Stern−Gouy−Chapman sense, as there is no diffuse layer. The structure is consistent with a capicitative double-layer model, with a very small separation distance between the planes of charge.

The surface interaction of two air- and water-stable ionic liquids (ILs), 1-ethyl-3-methylimidazolium bis[(trifluoromethylsulfonyl]amide ([EMIm]TFSA) and 1-butyl-1-methylpyrrolidinium bis[(trifluoromethyl)sulfonyl]amide ([BMP]TFSA), with Au(111) has been investigated using atomic force microscopy (AFM), cyclic voltammetry, and scanning tunneling microscopy (STM) measurements. AFM experiments reveal that multiple solvation layers are present at the gold interface for both ILs and that the strength of the interaction between the innermost layer and the substrate is dependent on the cation type; the force required to rupture the innermost solvation layer is greater for [BMP]TFSA than for [EMIm]TFSA, attributed to stronger cation surface interactions. In situ STM elucidates the influence of IL species on restructuring of the Au(111) interface. In the presence of [BMP]TFSA, the Au(111) surface restructures to produce a wormlike pattern, but this unusual morphology is not observed for the [EMIm]TFSA−Au(111) system. This remarkable difference in electrochemical behavior is ascribed to the greater strength of the interaction of [BMP]+ compared to [EMIm]+ with the Au(111) surface. These results demonstrate that such interfacial effects have to be considered for all electrochemical reactions and provide insight into the electrical double-layer structure in IL systems.

In a wider sense, ionic liquids are molten salts that melt below 100 °C. As their name suggests, they are solely composed of ions and many combinations of organic and/or inorganic cations and anions exist. Depending on the systems they can reach electrochemical windows of more than 4 V and thus they give access to a number of elements that cannot be electrodeposited from aqueous solutions, such as the light and refractory metals, as well as elemental and compound semiconductors. Presumably, ionic liquids will become important for electrochemical nanotechnology.