Phase Change Materials for Renewable Energy Storage at Intermediate Temperatures.

Thermal energy storage technologies utilizing phase change materials (PCMs) that melt in the intermediate temperature range, between 100 and 220 °C, have the potential to mitigate the intermittency issues of wind and solar energy. This technology can take thermal or electrical energy from renewable sources and store it in the form of heat. This is of particular utility when the end use of the energy is also as heat. For this purpose, the material should have a phase change between 100 and 220 °C with a high latent heat of fusion. Although a range of PCMs are known for this temperature range, many of these materials are not practically viable for stability and safety reasons, a perspective not often clear in the primary literature. This review examines the recent development of thermal energy storage materials for application with renewables, the different material classes, their physicochemical properties, and the chemical structural origins of their advantageous thermal properties. Perspectives on further research directions needed to reach the goal of large scale, highly efficient, inexpensive, and reliable intermediate temperature thermal energy storage technologies are also presented.

A case study using spectroscopy and computational modelling for Co speciation in a deep eutectic solvent.

Cobalt has a vital role in the manufacturing of reliable and sustainable clean energy technologies. However, the forecasted supply deficit for cobalt is likely to reach values of 150 kT by 2030. Therefore, it is paramount to consider end-of-life devices as secondary resources for cobalt. Electrorecovery of cobalt from leached solutions has attracted attention due to the sustainability of the recovery process over solvent extraction followed by chemical precipitation. Recently, we reported Co electrorecovery from two different cobalt sources (CoCl2·6H2O and CoSO4·7H2O) using ethylene glycol : choline chloride (EG : ChCl) in a 4.5 : 1 molar ratio, leading to higher purity and easier electrodeposition when sulfate was present as an additive. Here, Co2+ speciation is reported for the two EG : ChCl systems depending on the cobalt source using several spectroscopic techniques (e.g. NMR, EPR, FTIR) in combination with molecular dynamics simulations. Monodentate coordination of SO42- to Co2+, forming the tetrahedral [CoCl3(SO4)]3- was observed as the dominant structure in the system containing CoSO4·7H2O, whereas the system comprising CoCl2·6H2O shows a homoleptic tetrahedral [CoCl4]2- as the dominant structure. This resulted in knowledge being gained regarding Co2+ speciation and the correlation with electrochemistry will contribute to the science required for designing safe electrolytes for efficient electrorecovery.

Zwitterionic materials with disorder and plasticity and their application as non-volatile solid or liquid electrolytes.

Zwitterionic materials can exhibit unique characteristics and are highly tunable by variation to the covalently bound cationic and anionic moieties. Despite the breadth of properties and potential uses reported to date, for electrolyte applications they have thus far primarily been used as additives or for making polymer gels. However, zwitterions offer intriguing promise as electrolyte matrix materials that are non-volatile and charged but non-migrating. Here we report a family of zwitterions that exhibit molecular disorder and plasticity, which allows their use as a solid-state conductive matrix. We have characterized the thermal, morphological and structural properties of these materials using techniques including differential scanning calorimetry, scanning electron microscopy, solid-state NMR and X-ray crystallography. We report the physical and transport properties of zwitterions combined with lithium salts and a lithium-functionalized polymer to form solid or high-salt-content liquid electrolytes. We demonstrate that the zwitterion-based electrolytes can allow high target ion transport and support stable lithium metal cell cycling. The ability to use disordered zwitterionic materials as electrolyte matrices for high target ion conduction, coupled with an extensive scope for varying the chemical and physical properties, has important implications for the future design of non-volatile materials that bridge the choice between traditional molecular and ionic solvent systems.

New organic ionic plastic crystals utilizing the morpholinium cation.

Organic ionic plastic crystals (OIPCs) are emerging candidates as safer, quasi solid-state ion conductors for various applications, especially for next-generation batteries. However, a fundamental understanding of these OIPC materials is required, particularly concerning how the choice of cation and anion can affect the electrolyte properties. Here, we report the synthesis and characterisation of a range of new morpholinium-based OIPCs and demonstrate the benefit of the ether functional group in the cation ring. Specifically, we investigate the 4-ethyl-4-methylmorpholinium [C2mmor]+ and 4-isopropyl-4-methylmorpholinium [C(i3)mmor]+ cations paired with bis(fluorosulfonyl)imide [FSI]- and bis(trifluoromethanesulfonyl)imide [TFSI]- anions. A fundamental study of the thermal behaviour and transport properties was performed using differential scanning calorimetry (DSC), thermal gravimetric analysis (TGA) and electrochemical impedance spectroscopy (EIS). The free volume within the salts has been investigated by positron annihilation lifetime spectroscopy (PALS) and the ion dynamics using solid-state nuclear magnetic resonance (NMR) analysis. Finally, the electrochemical stability window was studied using cyclic voltammetry (CV). Out of the four morpholinium salts, [C2mmor][FSI] exhibits the widest phase I range from 11 to 129 °C, which is advantageous for their application. [C(i3)mmor][FSI] displayed the highest conductivity of 1 × 10-6 S cm-1 at 30 °C, whereas the largest vacancy volume of 132 Å3 was found for [C2mmor][TFSI]. These insights into the properties of new morpholinium-based OIPCs will be important for developing new electrolytes with optimised thermal and transport properties for a range of clean energy applications.

Halide-Free Synthesis of New Difluoro(oxalato)borate [DFOB]- -Based Ionic Liquids and Organic Ionic Plastic Crystals.

The implementation of next-generation batteries requires the development of safe, compatible electrolytes that are stable and do not cause safety problems. The difluoro(oxalato)borate ([DFOB]- ) anion has been used as an electrolyte additive to aid with stability, but such an approach has most commonly been carried out using flammable solvent electrolytes. As an alternative approach, utilisation of the [DFOB]- anion to make ionic liquids (ILs) or Organic Ionic Plastic Crystals (OIPCs) allows the advantageous properties of ILs or OIPCs, such as higher thermal stability and non-volatility, combined with the benefits of the [DFOB]- anion. Here, we report the synthesis of new [DFOB]- -based ILs paired with triethylmethylphosphonium [P1222 ]+ , and diethylisobutylmethylphosphonium [P122i4 ]+ . We also report the first OIPCs containing the [DFOB]- anion, formed by combination with the 1-ethyl-1-methylpyrrolidinium [C2 mpyr]+ cation, and the triethylmethylammonium [N1222 ]+ cation. The traditional synthetic route using halide starting materials has been successfully replaced by a halide-free tosylate-based synthetic route that is advantageous for a purer, halide free product. The synthesised [DFOB]- -based salts exhibit good thermal stability, while the ILs display relatively high ionic conductivity. Thus, the new [DFOB]- -based electrolytes show promise for further investigation as battery electrolytes both in liquid and solid-state form.

Plastic crystal-based electrolytes using novel dicationic salts.

The unique structures of dications increase the number of possible combinations of cations and anions that can be used to obtain new materials with a wide range of physicochemical properties. However, structure-property relationships related to dicationic organic salts are seldom explored. Here, we report the synthesis and characterization of two new dicationic salts, 1,2-bis(N-ethylpyrrolidinium)ethane bis(trifluoromethanesulfonyl)imide ([C2-Pyrr2][TFSI]2) and 1,2-bis(N-n-propylpyrrolidinium)ethane bis(trifluoromethanesulfonyl)imide ([C2-Pyrr3][TFSI]2). To investigate the physicochemical properties of the organic salts, local structure and dynamics were investigated by variable temperature solid-state NMR and correlated with the thermal analysis and ionic conductivity. These studies revealed that [C2-Pyrr3][TFSI]2, with the longer alkyl-side chain on the dication, showed improved transport properties compared to [C2-Pyrr2][TFSI]2. Further exploration of the organic salts as potential electrolyte materials was conducted by mixing with 10 mol% lithium bis(trifluoromethanesulfonyl)imide (LiTFSI). This study demonstrates the effect that lithium salt addition has on thermal and ionic conductivity properties, where the largest increase in conductivity was found for [C2-Pyrr3][TFSI]2/LiTFSI (10 mol% LiTFSI). Solid-state NMR analysis revealed that Li+ and [TFSI]- ions acted as the major contributors to ionic conductivity while the dications in the bulk structure showed lower mobility.

Probing the molecular interactions and physicochemical properties of a cobalt-based redox electrolyte system for thermo-electrochemical cells.

Redox-active materials play a primary role in the high-performance electrochemical device research field. Their bulk ion dynamics and performances can be studied using different electrochemical analysis methods, but their molecular level interactions and dynamics on which these depend are often not well understood. Here, nuclear magnetic resonance (NMR) relaxation and double-stimulated echo pulsed field gradient (PFG) techniques have been used to gain insights into the molecular level interactions, exchange dynamics and self-diffusivity of the various species present in a cobalt-based redox active electrolyte system used for thermo-electrochemical applications, including how these factors depend on the oxidation state and concentration of the redox species. A series of liquid electrolyte samples consisting of a Co2+/3+(bpy)3(NTf2)2/3 redox couple (where bpy = bipyridyl and NTf2 = bis(trifluoromethanesulfonyl)imide) in 3-methoxypropionitrile (MPN) have been investigated using NMR as well as viscosity and conductivity measurements carried out over a temperature range 293 to 353 K. The results provide insights into the mobilities and interactions between the various species present, including the exchange of the NTf2- anions between the solvation shells of the Co(bpy)3 species. Such information will be useful in understanding the behaviour of these electrolytes in devices such as thermo-electrochemical cells.

Exploration of phase diagram, structural and dynamic behavior of [HMG][FSI] mixtures with NaFSI across an extended composition range.

Hexamethylguanidinium bis(fluorosulfonyl)imide ([HMG][FSI]) has recently been shown to be a promising solid state organic ionic plastic crystal with potential application in advanced alkali metal batteries. This study provides a detailed exploration of the structural and dynamic behavior of [HMG][FSI] mixtures with the sodium salt NaFSI across the whole composition range from 0 to 100 mol%. All mixtures are solids at room temperature. A combination of differential scanning calorimetry (DSC), synchrotron X-ray diffraction (SXRD) and multinuclear solid state NMR spectroscopy is employed to identify a partial phase diagram. The 25 mol% NaFSI/75 mol% [HMG][FSI] composition presents as the eutectic composition with the eutectic transition temperature at 44 °C. Both DSC and SXRD strongly support the formation of a new compound near 50 mol% NaFSI. Interestingly, the 53 mol% NaFSI [HMG][FSI] composition was consistently found to display features of a pure compound whereas the 50 mol% materials always showed a second phase. Many of the compositions examined showed unusual metastable behaviour. Moreover, the ion dynamics as determined by NMR, indicate that the Na+ and FSI- anions are signifcantly more mobile than the HMG cation in the liquid state (including the metastable state) for these materials.

The influence of alkyl chain branching on the properties of pyrrolidinium-based ionic electrolytes.

Ionic liquids and plastic crystals based on pyrrolidinium cations are recognised for their advantageous properties such as high conductivity, low viscosity, and good electrochemical and thermal stability. The pyrrolidinium ring can be substituted with symmetric or asymmetric alkyl chain substituents to form a range of ionic liquids or plastic crystals depending on the anion. However, reports into the use of branched alkyl chains and how this influences the material properties are limited. Here, we report the synthesis of six salts - ionic liquids and organic ionic plastic crystals - where the typically used linear propyl chain substituent is replaced by the branched alternative, isopropyl, to form the cation [C(i3)mpyr]+, in combination with six different anions: dicyanamide, (fluorosulfonyl)(trifluoromethanesulfonyl)imide, bis(trifluoromethanesulfonyl)imide, bis(fluorosulfonyl)imide, tetrafluoroborate and hexafluorophosphate. The thermal and transport properties of these salts are compared to those of the analogous N-propyl-N-methylpyrrolidinium and N,N-diethylpyrrolidinium-based salts. Finally, a high lithium salt content ionic liquid electrolyte based on the bis(fluorosulfonyl)imide salt was developed. This electrolyte showed high coulombic efficiencies of lithium plating/stripping and high lithium ion transference number, making it a strong candidate for use in lithium metal batteries.

Organic salts utilising the hexamethylguanidinium cation: the influence of the anion on the structural, physical and thermal properties.

The synthesis and characterisation of new solid-state electrolytes is a key step in advancing the development of safer and more reliable electrochemical energy storage technologies. Organic ionic plastic crystals (OIPCs) are an increasingly promising class of material for application in devices such as lithium or sodium metal batteries as they can support high ionic conductivity, with good electrochemical and thermal stability. However, the choice of OIPC-forming ions is still relatively limited. Furthermore, understanding of the influence of different cations and anions on the thermal, structural and transport properties of these materials is still in its infancy. Here we report the synthesis and in-depth characterisation of a range of new OIPCs utilising the hexamethylguanidinium cation ([HMG]) with five different anions. The thermal, structural, transport properties and free volume in the different salts have been investigated. The free volume within the salts has been investigated by positron annihilation lifetime spectroscopy, and the single crystal and powder X-ray diffraction analysis of [HMG] bis(trifluoromethanesulfonyl)imide ([TFSI]) in phase I and II, [HMG] hexafluorophosphate ([PF6]) and [HMG] tetrafluoroborate ([HMG][BF4]) are reported. The HMG cation can exhibit significant disorder, which is advantageous for plasticity and future use of these materials as high ionic conductivity matrices. The bis(fluorosulfonyl)imide salt, [HMG][FSI], is identified as particularly promising for use as an electrolyte, with good electrochemical stability and soft mechanical properties. The findings introduce a range of new materials to the solid-state electrolyte arena, while the insights into the physico-chemical relationships in these materials will be of importance for the future development and understanding of other ionic electrolytes.

New dimensions in salt-solvent mixtures: a 4th evolution of ionic liquids.

In the field of ionic liquids (ILs) it has long been of fundamental interest to examine the transition from salt-in-solvent behaviour to pure liquid-salt behaviour, in terms of structures and properties. At the same time, a variety of applications have beneficially employed IL-solvent mixtures as media that offer an optimal set of properties. Their properties in many cases can be other than as expected on the basis of simple mixing concepts. Instead, they can reflect the distinct structural and interaction changes that occur as the mixture passes through the various stages from pure coulombic medium, to "plasticised" coulombic medium, into a meso-region where distinct molecular and ionic domains can co-exist. Such domains can persist to quite a high dilution into the salt-in-solvent regime and their presence manifests itself in a number of important synergistic interaction effects in diverse areas such as membrane transport and corrosion protection. Similarly, the use of ionic liquids in synthetic processes where there is a significant volume fraction of molecular species present can produce a variety of distinct and unexpected effects. The range of these salt-solvent mixtures is considerably broader than just those based on ionic liquids, since there is only minor value in the pure salt being a liquid at the outset. In other words, the extensive families of organic and metal salts become candidates for study and use. Our perspective then is of an evolution of ionic liquids into a broader field of fundamental phenomena and applications. This can draw on an even larger family of tuneable salts that exhibit an exciting combination of properties when mixed with molecular liquids.

N-ethyl-N-methylpyrrolidinium bis(fluorosulfonyl)imide-electrospun polyvinylidene fluoride composite electrolytes: characterization and lithium cell studies.

Using the organic ionic plastic crystal N-ethyl-N-methylpyrrolidinium bis(fluorosulfonyl)imide ([C2mpyr][FSI]) with electrospun nanofibers, LiFSI doped [C2mpyr][FSI]-PVdF composites were developed as solid state, self-standing electrolyte membranes. Different lithium salt concentration were investigated, with 10 mol% LiFSI found to be optimal amongst those assessed. Composites with different weight ratios of plastic crystal and polymer were prepared and 10 wt% polymer gave the highest conductivity. In addition, the effects of PVdF incorporation on the morphological, thermal, and structural properties of the organic ionic plastic crystal were investigated. Ion mobilities were also studied using solid-state nuclear magnetic resonance techniques. The electrolytes were then assembled into lithium symmetric cells and cycled galvanostatically at 0.13 mA cm-2 at both ambient temperature and at 50 °C, for more than 500 cycles.

Molecular simulation study of CO2 and N2 absorption in a phosphonium based organic ionic plastic crystal.

An organic ionic plastic crystal (OIPC), methyl(diethyl)isobutylphosphonium hexafluorophosphate [P122i4][PF6], was investigated for CO2 and N2 absorption using molecular simulations. Ab initio calculations showed that both the cation and anion exhibit larger binding energy for CO2 compared with N2. The CO2 absorption, as calculated from classical molecular dynamics simulations, increased by a factor of 7.5 from 275 K to 325 K, while that of N2 showed low absorption at both temperatures. The simulations suggest that the significant increase in CO2 absorption at 325 K is attributed to a higher degree of disorder and increase in the free volume due to the gas/solid interfaces. While the ab initio calculations were helpful in identifying specific interaction sites on the constituent ions, the classical MD simulations elucidated the importance of interfaces in gas absorption studies in this material. The results show that the OIPC can be a promising material for CO2 separations from CO2/N2 mixture.

Temperature dependence of the electrode potential of a cobalt-based redox couple in ionic liquid electrolytes for thermal energy harvesting.

Increasing the application of technologies for harvesting waste heat could make a significant contribution to sustainable energy production. Thermoelectrochemical cells are one such emerging technology, where the thermal response of a redox couple in an electrolyte is used to generate a potential difference across a cell when a temperature gradient exists. The unique physical properties of ionic liquids make them ideal for application as electrolytes in these devices. One of the keys to utilizing these media in efficient thermoelectrochemical cells is achieving high Seebeck coefficients, Se: the thermodynamic quantity that determines the magnitude of the voltage achieved per unit temperature difference. Here, we report the Se and cell performance of a cobalt-based redox couple in a range of different ionic liquids, to investigate the influence of the nature of the IL on the thermodynamics and cell performance of the redox system. The results reported include the highest Se to-date for an IL-based electrolyte. The effect of diluting the different ILs with propylene carbonate is also reported, which results in a significant increase in the output powers and current densities of the device.

Enhanced thermal energy harvesting performance of a cobalt redox couple in ionic liquid-solvent mixtures.

Thermoelectrochemical cells are increasingly promising devices for harvesting waste heat, offering an alternative to the traditional semiconductor-based design. Advancement of these devices relies on new redox couple/electrolyte systems and an understanding of the interplay between the different factors that dictate device performance. The Seebeck coefficient (Se) of the redox couple in the electrolyte gives the potential difference achievable for a given temperature gradient across the device. Prior work has shown that a cobalt bipyridyl redox couple in ionic liquids (ILs) displays high Seebeck coefficients, but the thermoelectrochemical cell performance was limited by mass transport. Here we present the Se and thermoelectrochemical power generation performance of the cobalt couple in novel mixed IL/molecular solvent electrolyte systems. The highest power density of 880 mW m(-2), at a ΔT of 70 °C, was achieved with a 3 : 1 (v/v) MPN-[C2mim][B(CN)4] electrolyte combination. The significant power enhancement compared to the single solvent or IL systems results from a combination of superior ionic conductivity and higher diffusion coefficients, shown by electrochemical analysis of the different electrolytes. This is the highest power output achieved to-date for a thermoelectrochemical cell utilising a high boiling point redox electrolyte.

New insights into the thermal behaviour of organic ionic plastic crystals: magnetic resonance imaging of polycrystalline morphology alterations induced by solid-solid phase transitions.

Organic ionic plastic crystals (OIPCs) show strong potential as solid-state electrolytes for lithium battery applications, demonstrating promising electrochemical performance and eliminating the need for a volatile and flammable liquid electrolyte. The ionic conductivity (σ) in these systems has recently been shown to depend strongly on polycrystalline morphology, which is largely determined by the sample's thermal history. [K. Romanenko et al., J. Am. Chem. Soc., 2014, 136, 15638]. Tailoring this morphology could lead to conductivities sufficiently high for battery applications, so a more complete understanding of how phenomena such as solid-solid phase transitions can affect the sample morphology is of significant interest. Anisotropic relaxation of nuclear spin magnetisation provides a new MRI based approach for studies of polycrystalline materials at both a macroscopic and molecular level. In this contribution, morphology alterations induced by solid-solid phase transitions in triisobutyl(methyl)phosphonium bis(fluorosulfonyl)imide (P1444FSI) and diethyl(methyl)(isobutyl)phosphonium hexafluorophosphate (P1224PF6) are examined using magnetic resonance imaging (MRI), alongside nuclear magnetic resonance (NMR) spectroscopy, diffusion measurements and conductivity data. These observations are linked to molecular dynamics and structural behaviour crucial for the conductive properties of OIPCs. A distinct correlation is established between the conductivity at a given temperature, σ(T), and the intensity of the narrow NMR signal that is attributed to a mobile fraction, fm(T), of ions in the OIPC. To explain these findings we propose an analogy with the well-studied relationship between permeability (k) and void fraction (θ) in porous media, with k(θ) commonly quantified by a power-law dependence that can also be employed to describe σ(fm).

Anisotropic MRI contrast reveals enhanced ionic transport in plastic crystals.

Organic ionic plastic crystals (OIPCs) are attractive as solid-state electrolytes for electrochemical devices such as lithium-ion batteries and solar and fuel cells. OIPCs offer high ionic conductivity, nonflammability, and versatility of molecular design. Nevertheless, intrinsic ion transport behavior of OIPCs is not fully understood, and their measured properties depend heavily on thermal history. Solid-state magnetic resonance imaging experiments reveal a striking image contrast anisotropy sensitive to the orientation of grain boundaries in polycrystalline OIPCs. Probing triethyl(methyl)phosphonium bis(fluorosulfonyl)imide (P1222FSI) samples with different thermal history demonstrates vast variations in microcrystallite alignment. Upon slow cooling from the melt, microcrystallites exhibit a preferred orientation throughout the entire sample, leading to an order of magnitude increase in conductivity as probed using impedance spectroscopy. This investigation describes both a new conceptual window and a new characterization method for understanding polycrystalline domain structure and transport in plastic crystals and other solid-state conductors.

Investigation of the kinetic and mass transport limitations in thermoelectrochemical cells with different electrode materials.

Thermoelectrochemical cells (TECs) have the potential to offer a continuous renewable electricity supply from a variety of thermal energy sources. Because of the thermal gradient, the device characteristics are a complex function of temperature dependent electrolyte transport properties, electrode electro-catalytic properties and the Seebeck coefficient of the redox couple. Understanding the interplay between these functions is critical to identifying the limiting factors that need to be overcome to produce more advanced devices. Thus, in this work we have developed a theoretical model for TECs and have measured a range of properties required by the model. We focused attention on the Co(n)(bpy)3(NTf2)n in a [C2mim][B(CN)4] ionic liquid electrolyte as one of the optimal systems for >100 °C operation. The exchange current densities on a range of electrode materials were measured in order to explore the role of electrode function in the simulation. Alternatives to platinum electrodes (maximum output power, Pmax = 183 mW m(-2)), including platinized stainless steel, Pt-SS (Pmax = 188 mW m(-2)) and poly(3,4-ethylenedioxythiophene) deposited on stainless steel, PEDOT-SS (Pmax = 179 mW m(-2)), were shown to be viable options. From the simulations we conclude that for further development of ionic liquid TECs, modifications to the redox couple to increase the Seebeck coefficient, and increasing the rate of diffusion of the redox couple to minimize mass transport resistance, will yield the greatest improvements in device performance.

Insights into the Carbon Dioxide Separation Performance of Bis(trifluoromethylsulfonyl)imide-based Plastic Crystal Composite Membranes with Fluorinated Polar Polymers.

Membrane-based gas separation technologies are one solution towards mitigating global emissions of CO2. New membrane materials with improved separation performance are still highly sought after. Composite membranes based on organic ionic plastic crystals (OIPCs) have shown preferential interaction for CO2 over N2, leading in some cases to competitive CO2/N2 selectivities. However, these ionic materials have been scarcely studied in the field of gas separation. Here, OIPCs based on the bis(trifluoromethylsulfonyl)imide ([TFSI]-) anion were investigated for use as gas separation membranes for the first time. The effect of the polymer type was also investigated, through the comparison of poly(vinylidene fluoride) (PVDF) and poly(vinylidene fluoride)-hexafluoropropylene (PVDF-HFP) OIPC membranes. A strong temperature dependence of the gas separation performance was found, particularly in the N-methyl-N-ethylpyrrolidinium-based composites where the material undergoes a solid-solid phase transition within the testing temperature range. The polymer type was noted to induce a strong effect on the structure of the composites, as well as affecting the gas and ionic transport. Thus, this research provides insights on the influence of the [TFSI]- anion on the structure and separation properties of OIPC-based composites, and new information towards the development of novel OIPC-based membranes with enhanced gas separation performance.

Unexpected Energy Applications of Ionic Liquids.

Ionic liquids and their various analogues are without doubt the scientific sensation of the last few decades, paving the way to a more sustainable society. Their versatile suite of properties, originating from an almost inconceivably large number of possible cation and anion combinations, allows tuning of the structure to serve a desired purpose. Ionic liquids hence offer a myriad of useful applications from solvents to catalysts, through to lubricants, gas absorbers, and azeotrope breakers. The purpose of this review is to explore the more unexpected of these applications, particularly in the energy space. It guides the reader through the application of ionic liquids and their analogues as i) phase change materials for thermal energy storage, ii) organic ionic plastic crystals, which have been studied as battery electrolytes and in gas separation, iii) key components in the nitrogen reduction reaction for sustainable ammonia generation, iv) as electrolytes in aluminum-ion batteries, and v) in other emerging technologies. It is concluded that there is tremendous scope for further optimizing and tuning of the ionic liquid in its task, subject to sustainability imperatives in line with current global priorities, assisted by artificial intelligence.