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.

When disregarding the current COVID19 crisis the climate change is one of the most pressing global issues with its well-known challenges concerning the energy economy and climate policy. In this context due to their many benefits, fuel cells and batteries are being discussed as potential applications. These technologies are not without some disadvantages themselves, though. Most polymer electrolyte membrane (PEM) fuel cells work with Nafion®, a sulfonated fluorocopolymer, as their membrane, which loses conductivity at higher temperatures due to dehydration, which limits the operation temperature to around 80 °C at ambient pressure. [1] As a result alternative membrane materials are needed which can preferably be designed with defined geometries and sizes and show high ion mobilities and corresponding ionic conductivities at elevated temperatures.As promising components in energy devices such as fuel or solar cells and batteries ionic liquids (ILs) are discussed due to their promising properties. These salts with melting points below 100 °C show high thermal and electrochemical stability, non-flammability and high ionic conductivities. For the applications mentioned above the immobilization of the ILs is necessary to realize proper function and prevent leaking. [2] The immobilization of the ILs can be realized in three different ways: 1. swelling of polymers in the IL, 2. polymerization of polymerizable ILs and 3. polymerization of vinyl-monomers in an IL. [3] The resulting ionogels (IGs) can then combine for example the characteristics of the respective IL with the useful properties of the polymer, i.e. its mechanical stability. To realize precise shapes and geometries for the membrane materials stereolithography and 3D-printing of the respective IGs provide a suitable method. [4]The aim of this study is the synthesis and characterization of ILs for proton-conduction. The ionic conductivities of these compounds range between 10-2 - 10-4 S/cm at elevated temperatures. Moreover, these ILs exhibit wide electrochemical (e. g. ΔE up to 3 V) and thermal stability windows (Td ≥ 230 °C) and are therefore promising for ion transport at elevated temperatures. NMR-studies also provide information about their ion mobility. Furthermore the ILs are immobilized via two of the aforementioned methods to provide flexible and transparent IGs that contain up to 80 wt% of IL. These IGs display promising thermal and mechanical stability and reach ionic conductivities of up to 10-3 S/cm at elevated temperatures. Successful 3D-printing and therefore structuring of IGs can also be demonstrated.[1] Martinez, et al., Journal of Power Sources, 2010, 195, 5829–5839.[2] Le Bideau, L. Viau, A. Vioux, Chem. Soc. Rev., 2011, 40, 907–925.[3] Lu, F. Yan, J. Texter, Progress in Polymer Science, 2009, 34, 431-448.[4] Zehbe, A. Lange, A. Taubert, Energy Fuels, 2019, 33, 12885-12893.

Hybrid materials based on ionic or polymer components are very useful in electrochemical devices like battery systems or fuel cells due to their unique properties like thermal and electrochemical stability and polarity. For all advances over the past, lithium ion based materials are still costly, limited and not very stable. Currently, solid-state polymer electrolytes (SPEs) and gel polymer electrolytes (GPEs) [1] have ion conductivities below 10-4 S/cm at room temperature. Polymer electrolytes can be established by ionic liquids (ILs) which combine beneficial properties like high ionic conductivity, wide electrochemical potential window and high viscosity. These kinds of electrolytes can be very beneficial in solid-state batteries for example. This technology has garnered immense excitement from investors and large corporations. The main advantage of all-solid-state to liquid electrolyte-based batteries is that the size can be easily made smaller e.g. solid‐state electrolytes have a typical thickness of approximately 1.0 μm, whereas the separator in liquid electrolyte‐based cells typically has a thickness of 20 μm. Several investigators are exploring 3D architectures for thin-film rechargeable battery electrodes. Nevertheless additive manufacturing, also known as 3D printing, could revolutionize the structural properties of batteries. In addition, this technology could improve the adaptability of the electrolyte to the electrodes. This scientific approach introduces the possibility to combine ionic liquids with a commercial photoreactive resin to realise a well-defined structure by using stereolithography (SLA). This provides, for the first time, access to ionogels (IGs) with complex 3D shapes (figure 1a) with potential application in battery or fuel cell technology. For first experiments new IGs were prepared by adding 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 clear photoreactive resin which were used in a Form 2 3D printer [2]. Moreover, the IGs are thermally stable up to about 200 °C and mechanically robust. In addition to this synthesized protic conductive IGs, sulfobataine, metal and lanthanides containing IGs were prepared and analysed. These IGs exhibit higher conductivities compared to the sulfonated based IGs in the range from 10-4 to 10-2 S/cm at room temperature. 3D Printable metal-lanthanide IGs could possibly be an adequate replacement for lithium-ion electrolytes. Figure 1: a) Well-defined structures of IGs by SLA, b) Rhd battery cell for electrochemical measurements. Literature: [1] Stephan, A.M., Review on gel polymer electrolytes for lithium batteries. European polymer journal, 2006. 42(1): p. 21-42. [2] Zehbe, K.; Lange,A.; Taubert, A., Stereolithography provides access to 3D printed ionogels with high ionic conductivity, Energy and Fuels, accepted. Figure 1

As promising components in energy devices such as fuel or solar cells and batteries ionic liquids (ILs) are discussed due to their promising properties. These salts with melting points below 100 °C show e.g., high thermal and electrochemical stability, high ionic conductivities and non-flammability, making them interesting candidates for use as electrolytes.[1] One of the most pressing global issues of our time, when disregarding the current COVID19 crisis, is the climate change with its well-known challenges concerning the climate policy and the energy economy. In this context due to their many benefits, the aforementioned fuel cells and batteries are being discussed as potential appliances, although there are some disadvantages attached to these technologies. Most polymer-electrolyte-membrane (PEM) fuel cells work with Nafion®, a sulfonated fluorocopolymer, as their membrane. Nafion® however loses conductivity at higher temperatures due to dehydration, limiting its operation temperature to around 80 °C at ambient pressure.[2] ILs are considered compounds that can overcome these limitations as they offer high conductivities even at elevated temperatures. For the applications as electrolytes in the aforementioned systems the immobilization of the ILs is necessary, however, to prevent leaking and realize proper function.[1] The resulting ionogels (IGs) then combine the properties of the polymer matrix, i.e. its mechanical stability, with the characteristics of the respective IL. A suitable method to realize precise geometries and shapes for these membranes is given by 3D printing, which offers adaptable electrolyte design.[3] The synthesis and characterization of ILs for ion (specifically proton) conduction is the aim of this work. These ILs exhibit wide electrochemical and thermal stability windows. The ionic conductivities of the investigated compounds range between 10-2 - 10-4 S/cm at elevated temperatures. The ILs are furthermore investigated under aspects of ion and proton transport via different spectroscopy methods.[3,4] Moreover, the ILs are immobilized in different matrix materials to provide flexible and transparent IGs that contain up to 80 wt% of IL. Additionally, successful 3D-printing and structuring of the IGs is also demonstrated.[3] [1] M. Martinez, et al, Journal of Power Sources, 2010, 195, 5829-5839. [2] J. Le Bideau, et al., Chem. Soc. Rev., 2011, 40, 907-925. [3] K. Zehbe, et al., Energy Fuels, 2019, 33, 12885-12893. [4] Z. Wojnarowska, et al., ACS Appl. Mater. Interfaces, 2021, 13, 30614–30624.