There is an urgent need for new energy storage and conversion systems in order to tackle the environmental problems we face today and to make the transition to a fossil fuel-free society. New batteries, supercapacitors, and fuel cells have the potential to be key devices for large-scale energy storage systems for load leveling and electric vehicles. In many cases, the concepts are known, but the right materials solutions are lacking. Ionic liquids (ILs) have been highlighted as suitable materials to be included in new devices, most commonly as electrolytes. Attractive features of ILs such as high ionic conductivity, low vapor pressure, high thermal and electrochemical stability, large temperature range for the liquid phase, and flexibility in molecular design have drawn the attention of researchers from many different fields. In addition, there is the possibility of designing new materials and morphologies using electrochemical synthesis with ILs. In this article, we provide an introduction to ILs and their properties, serving as a base for the topical articles in this issue.

Ionic liquids (ILs) are a very interesting new class of fluid materials because of their unique characteristics, such as wide chemical, thermal, and electrochemical stability, high ion conduction, non-detectable vapor pressure, nonflammability, and good-to-excellent capability to dissolve inorganic, organic, and polymer compounds. ILs are proposed for a very wide variety of applications, including electrochemical devices. However, high purity ILs, particularly for high-energy electrochemical applications, are not widely available commercially. In addition, solvent restriction and environmental impact, as well as the possibility to fully recycle chemicals and reagents, represent the most stringent requirements for the future synthesis processes of ILs. This article reviews synthesis route improvements in terms of environment impact solvents, chemical recycling and cost, and process yield for obtaining high purity (below 50 ppm) ILs.

This article reports the search for nonflammable, stable electrolytes based on ionic liquid (IL) compounds, able to effectively improve the needed safety and reliability of lithium batteries. The most significant results are reviewed with the aim of elucidating critical aspects governing the properties of IL electrolytes, including (1) transport properties affecting ionic conductivity and the cycling rate of battery systems, (2) electrochemical/chemical stability toward most conventional electrode materials, and (3) thermal properties determining the range of applicability. Both liquid and polymer electrolytes, adopting ILs as the main component or as an additive to standard electrolyte solutions, are considered. Very promising results, in terms of battery prototype performances in scaled-up configurations, demonstrate the validity of the use of ILs for practical applications. Even though further improvements are necessary, particularly at high current density operations in both lithium-metal and lithium-ion systems, the realization of safer, high-performance batteries based on IL electrolytes is certainly possible. It can be concluded that ILs represent a viable solution to disappointing compromises between energy density and acceptable safety features in lithium batteries.

Supercapacitors are nowadays considered to be one of the most important electrochemical storage devices. These devices display high power and extraordinary cycle life, and they are currently used in an increasing number of applications. However, in order to further increase the applications of supercapacitors, an increase in their energy capacity appears to be necessary. Moreover, the development of safe and environmentally friendly supercapacitors is also required. In this article, we illustrate the contributions ionic liquids (ILs) might play in the development of high energy and safe supercapacitors. First, the use of ILs as electrolytes in supercapacitors is considered, and the advantages as well as challenges related to the use of this kind of electrolyte are analyzed. Next, the interaction between ILs and electrode materials is taken into account, with particular attention paid to inactive components of supercapacitor electrodes. The introduction of natural cellulose as a binder is used as an example of the contribution ILs might provide to the development of environmentally friendly supercapacitors.

We have investigated protic ionic liquids (PILs) as proton conductors for non-humidified intermediate-temperature fuel cells. PILs exhibit proton conductivity and activity in fuel cell electrode reactions, as seen in acidic aqueous solutions and acidic polymer membranes. The wide molecular designability of PILs enabled the finding of a promising candidate, diethylmethylammonium trifluoromethanesulfonate ([dema][ TfO]), which exhibits favorable bulk properties and electrochemical activity. Solid thin films containing [dema][ TfO] were fabricated using sulfonated polyimide as a matrix polymer. By using the composite membrane, non-humidifying fuel cell operation at 120°C succeeded. The fuel cell performance can be further improved by the optimization of the catalyst layer and with further research on PILs.

Ionic liquids are well suited to the electrochemical synthesis of freestanding metallic nanowires as well as macroporous metals and semiconductors. Such materials are potentially interesting for future generation Li-ion batteries. As the energy density of current Li-ion batteries barely exceeds 0.15 kWh/kg (in contrast to the 12 kWh/kg of hydrocarbons), there is a need for new anode and cathode materials if electrically driven cars are to have more than a 150 km cruising range at an affordable price. Freestanding aluminum nanowires and macroporous aluminum are easily feasible from AlCl3-based ionic liquids and show promising charge/discharge behavior even with ionic liquids as electrolytes. The challenges and the potential to make nanowires or macroporous structures of semiconductors (Si, Ge) are also briefly discussed.