Ionic liquid crystals (ILCs), that is, ionic liquids exhibiting mesomorphism, liquid crystalline phases, and anisotropic properties, have received intense attention in the past years. Among others, this is due to their special properties arising from the combination of properties stemming from ionic liquids and from liquid crystalline arrangements. Besides interesting fundamental aspects, ILCs have been claimed to have tremendous application potential that again arises from the combination of properties and architectures that are not accessible otherwise, or at least not accessible easily by other strategies. The current review highlights recent developments in ILC research, starting with some key fundamental aspects. Further subjects covered include the synthesis and variations of modern ILCs, including the specific tuning of their mesomorphic behavior. The review concludes with reflections on some applications that may be within reach for ILCs and finally highlights a few key challenges that must be overcome prior and during true commercialization of ILCs.

New ionogels (IGs) were prepared by combination of a series of sulfonate-based ionic liquids (ILs), 1-methyl-3-(4-sulfobutyl)­imidazolium para-toluenesulfonate [BmimSO3H]­[pTS], 1-methyl-1-butylpiperidiniumsulfonate para-toluenesulfonate [BmpipSO3]­[pTS], and 1-methyl-3-(4-sulfobutyl) imidazolium methylsulfonate [BmimSO3H]­[MeSO3] with a commercial stereolithography photoreactive resin. The article describes both the fundamental properties of the ILs and the resulting IGs. The IGs obtained from the ILs and the resin show high ionic conductivity of up to ca. 0.7·10–4 S/cm at room temperature and 3.4·10–3 S/cm at 90 °C. Moreover, the IGs are thermally stable to about 200 °C and mechanically robust. Finally, and most importantly, the article demonstrates that the IGs can be molded three-dimensionally using stereolithography. This provides, for the first time, access to IGs with complex 3D shapes with potential application in battery or fuel cell technology.

The use of acidic ionic liquids and solids as electrolytes in fuel cells is an emerging field due to their efficient proton conductivity and good thermal stability. Despite multiple reports describing conducting properties of acidic ILs, little is known on the charge-transport mechanism in the vicinity of liquid–glass transition and the structural factors governing the proton hopping. To address these issues, we studied two acidic imidazolium-based ILs with the same cation, however, different anions—bulk tosylate vs small methanesulfonate. High-pressure dielectric studies of anhydrous and water-saturated materials performed in the close vicinity of T g have revealed significant differences in the charge-transport mechanism in these two systems being undetectable at ambient conditions. Thereby, we demonstrated the effect of molecular architecture on proton hopping, being crucial in the potential electrochemical applications of acidic ILs.

In light of the well-known challenges regarding the global energy economy and climate issues fuel cells are widely discussed for their benefits and potential applications in the future. One of the main problems of PEM-fuel cells is that Nafion®-membranes have a limited application temperature window of ≤ 80 °C at ambient pressure due to dehydration and corresponding loss of conductivity at higher temperatures. [1] As a result there is a significant need for new membranes of a precise and defined size and geometry exhibiting significant ion mobilities far above 100 °C. Based on their highly useful properties like thermal stability, non-flammability, and high ionic conductivities ionic liquids (ILs) are promising components in energy devices such as batteries, solar cells or fuel cells. For proper function of, for example, fuel cell membranes, it is, however, necessary to immobilize the ILs in a polymer matrix, resulting in ionogels (IGs) combining the characteristics of the respective IL with e. g. the mechanical stability of the polymer. [2] The combination of 3D-printing with suitable polymer scaffolds and suitable ILs enables the design of membrane materials with precisely controlled sizes, shapes and geometries along with the necessary electrochemical performance. [3] The aim of this work therefore is the synthesis and characterization of protic amine-based ILs for proton conduction. All compounds are ILs at room temperature. Their ionic conductivities range between 10-2 – 10-4 S/cm. Moreover, with wide electrochemical and thermal stability windows (ΔE up to 3 V, Tg around -90 °C and Td over 200 °C) these ILs are promising for ion transport in fuel cell membranes above 100 °C. The corresponding transparent and flexible IGs display promising thermal and mechanical stability and reach ionic conductivities of up to 10-3 S/cm at elevated temperatures. Moreover, this study also demonstrates that the IGs can be obtained and structured using 3D-printing; this clearly enables the design of many materials with different requirements by simply adapting the size and shape. [1] A. Martinelli, A. Matic, P. Jacobsson, L. Börjesson, A. Fernicola, S. Panero, B. Scrosati, H. Ohno, J. Phys. Chem. B, 2007, 111, 12462. [2] Y.-S. Ye, J. Rick, B.-J. Hwang, J. Mater. Chem. A, 2013, 1, 2719. [3] K. Zehbe, A. Lange, A. Taubert, Sustainable Energy Fuels, submitted.

Considering the well-known, global challenges the energy economy and climate policy face, batteries and fuel cells are widely discussed for potential applications in the future due to their many benefits. There are some difficulties connected with these technologies though. For PEM-fuel cells one of the main problems is linked to their membranes. These are based on Nafion®, a sulfonated fluorocopolymer and therefore have a limited application temperature of 80 °C at ambient pressure due to dehydration and corresponding loss of conductivity at higher temperatures. [1] Due to this limit in operating temperature there is a significant need for alternative membrane materials, preferably with precise and defined size and geometry that can exhibit high ion mobilities and ionic conductivities above 100 °C. In recent years research on ionic liquids (ILs) has experienced a revival. These salts with melting points below 100 °C are promising components in energy devices such as batteries, solar or fuel cells, owing to their high thermal and electrochemical stability, non-flammability and high ionic conductivities. However, to prevent leaking and realise proper function of these devices immobilizing the IL in a matrix is necessary. [2] The resulting ionogels (IGs) then combine the characteristics of the respective IL with the useful properties of the polymer, i.e. its mechanical stability. This immobilization can be realized in three different ways: doping of polymers with the IL, polymerization of vinyl monomers in the IL, and polymerization of polymerizable ILs. [3] To obtain membrane materials with precisely controlled sizes, shapes and geometries along with the necessary performance stereolithography and 3D-printing of suitable materials are a promising method. [4] The aim of this work therefore is the synthesis and characterization of ILs for ion- and especially proton-conduction. The ionic conductivities of these compounds range between 10-2 – 10-4 S/cm at elevated temperatures. Moreover, with wide electrochemical and thermal stability windows (e. g. ΔE up to 3 V, Tg around -90 °C and Td over 200 °C (for some of them)) these ILs are promising for ion transport in fuel cell membranes above 100 °C and their properties are in addition studied under aspects of ion mobility. The immobilization of these ILs is furthermore realized via different methods, as mentioned above. The corresponding transparent and flexible IGs, in part containing high wt% of IL, display promising thermal and mechanical stability and reach ionic conductivities of up to 10-3 S/cm at elevated temperatures. This study also demonstrates successful 3D-printing and structuring of IGs, which clearly enables the design of materials with different requirements by simply adapting the size and shape. [1] A. Martinelli, A. Matic, P. Jacobsson, L. Börjesson, A. Fernicola, S. Panero, B. Scrosati, H. Ohno, J. Phys. Chem. B, 2007, 111, 12462. [2] Y.-S. Ye, J. Rick, B.-J. Hwang, J. Mater. Chem. A, 2013, 1, 2719. [3] J. Lu, F. Yan, J. Texter, Progress in Polymer Science, 2009, 34, 431. [4] K. Zehbe, A. Lange, A. Taubert, Energy Fuels, 2019, 33, 12885.