Ether-Bond-Containing Ionic Liquids as Supercapacitor Electrolytes

The Coiling Effect in Ether Ionic Liquids: Exploiting Acetate as a Probe for Transport Properties and Microenvironment Analysis

The inherently high viscosity of ionic liquids (ILs) can limit their potential applications. One approach to address this drawback is to modify the cation side chain with ether groups. Herein, we assessed the structure-property relationship by focusing on acetate (OAc), a strongly coordinating anion, with 1,3-dialkylimidazolium cations with different side chains, including alkyl, ether, and hydroxyl functionalized, as well as their combinations. We evaluated their viscosity, thermal stabilities, and microstructure using Raman and infrared (IR) spectroscopies, allied to density functional theory (DFT) and ab initio molecular dynamics (AIMD) simulations. The viscosity data showed that the ether insertion significantly enhances the fluidity of the ILs, consistent with the coiling effect of the cation chain. Through a combined experimental and theoretical approach, we analyzed how the OAc anion interacts with ether ILs, revealing a characteristic bidentate coordination, particularly in hydroxyl functionalized ILs due to specific hydrogen bonding with the OH group. IR spectroscopy showed subtle shifts in the acidic hydrogens of imidazolium ring C(2)-H and C(4,5)-H, suggesting weaker interactions between OAc and the imidazolium ring in ether-functionalized ILs. Additionally, spatial distribution functions (SDF) and dihedral angle distribution obtained via AIMD confirmed the intramolecular hydrogen bonding due to the coiling effect of the ether side chain.

Oxygen-Containing Quaternary Phosphonium Salts (oxy-QPSs): Synthesis, Properties, and Cellulose Dissolution

In the present study, the synthesis of oxygen-containing quaternary phosphonium salts (oxy-QPSs) was described. Within this work, structure-property relationships of oxy-QPSs were estimated by systematic analysis of physical-chemical properties. The influence of the oxygen-containing substituent was examined by comparing the properties of oxy-QPSs in homology series as well as with phosphonium analog-included alkyl side chains. The crystal structure analysis showed that the oxygen introduction influences the conformation of the side chain of the oxy-QPS. It was found that oxy-QPSs, using an aprotic co-solvent, dimethylsulfoxide (DMSO), can dissolve microcrystalline cellulose. The cellulose dissolution in oxy-QPSs appeared to be dependent on the functional group in the cation and anion nature. For the selected conditions, dissolution of up to 5 wt% of cellulose was observed. The antimicrobial activity of oxy-QPSs under study was expected to be low. The biocompatibility of oxy-QPSs with fermentative microbes was tested on non-pathogenic Saccharomyces cerevisiae, Lactobacillus plantarum, and Bacillus subtilis. This reliably allows one to safely address the combined biomass destruction and enzyme hydrolysis processes in one pot.

Structurally flexible pyrrolidinium- and morpholinium-based ionic liquid electrolytes

Ion transport measures and details as well as physico-chemical and electrochemical properties are presented for a small set of structurally flexible pyrrolidinium (Pyrr) and morpholinium (Morph) cation-based ionic liquids (ILs), all with oligoether phosphate-based anions. All have high thermal stabilities, low glass transition temperatures, and wide electrochemical stability windows, but rather moderate ionic conductivities, where both the anions and the cations of the Pyrr-based ILs diffuse faster than those of the Morph-based ILs. Overall, the Pyrr-based ILs show significantly more promise as high-temperature supercapacitor electrolytes, rendering a specific capacitance of 164 F g^-1 at 1 mV s^-1, a power density of 609 W kg^-1 and a specific energy density of 27 W h kg^-1 at 90 °C in a symmetric graphite supercapacitor.

Electrolyte selection for supercapacitive devices: a critical review

Electrolytes are one of the vital constituents of electrochemical energy storage devices and their physical and chemical properties play an important role in these devices' performance, including capacity, power density, rate performance, cyclability and safety. This article reviews the current state of understanding of the electrode-electrolyte interaction in supercapacitors and battery-supercapacitor hybrid devices. The article discusses factors that affect the overall performance of the devices such as the ionic conductivity, mobility, diffusion coefficient, radius of bare and hydrated spheres, ion solvation, viscosity, dielectric constant, electrochemical stability, thermal stability and dispersion interaction. The requirements needed to design better electrolytes and the challenges that still need to be addressed for building better supercapacitive devices for the competitive energy storage market have also been highlighted.

Multiple Ether-Functionalized Phosphonium Ionic Liquids as Highly Fluid Electrolytes

Ionic liquids (ILs) are promising electrolytes, although their often high viscosity remains a serious drawback. The latter can be addressed by the introduction of multiple ether functionalization. Based on the highly atom efficient synthesis of tris(2-ethoxyethyl) phosphine, several new phosphonium ionic liquids were prepared, which allows studying the influence of the ether side chains. Their most important physicochemical properties have been determined and will be interpreted using established approaches like ionicity, hole theory, and the Walden plot. There is striking evidence that the properties of phosphonium ionic liquids with the methanesulfonate anion are dominated by aggregation, whereas the two triple ether functionalized ILs with the highest fluidity show almost ideal behavior with other factors being dominant. It is furthermore found that the deviation from ideality is not significantly changed upon introduction of the ether side chains, although a very beneficial impact on the fluidity of ILs is observed. Multiple ether functionalization therefore proves as a powerful tool to overcome the disadvantages of phosphonium ionic liquids with large cations.

Ionic Liquids as Environmentally Benign Electrolytes for High-Performance Supercapacitors

Electrochemical capacitors (ECs) are a vital class of electrical energy storage (EES) devices that display the capacity of rapid charging and provide high power density. In the current era, interest in using ionic liquids (ILs) in high-performance EES devices has grown exponentially, as this novel versatile electrolyte media is associated with high thermal stability, excellent ionic conductivity, and the capability to withstand high voltages without undergoing decomposition. ILs are therefore potentially useful materials for improving the energy/power performances of ECs without compromising on safety, cyclic stability, and power density. The current review article underscores the importance of ILs as sustainable and high-performance electrolytes for electrochemical capacitors.

High-Voltage and Low-Temperature Aqueous Supercapacitor Enabled by "Water-in-Imidazolium Chloride" Electrolytes

Symmetric aqueous high voltage supercapacitors up to 3 V have been demonstrated using concentrated aqueous 1-butyl-3-methylimidazolium chloride ([BMIm]Cl), namely, "water-in-imidazolium chloride", as working electrolytes, and graphene nanoplatelets-coated carbon paper as electrodes. Performance enhancement was further achieved either through adding redox species such as 4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl (4hT) into the electrolytes (110 Wh kg^-1 for a 20 m [BMIm]Cl/H₂ O with 0.1 m 4hT) or by pre-inserting ClO₄ ^- anions into the graphene platelets. Moreover, the newly studied aqueous electrolytes allow low-temperature operation at -20 °C and even at -32 °C, retaining competitive energy storage capability (maximum energy densities of 36 and 21 Wh kg^-1 , respectively).

Electrocatalytic studies on imidazolium based ionic liquids: defining experimental conditions

The number of publications devoted to studying electrochemical reactions in room temperature ionic liquids (RTILs) is constantly growing, but very few of them have been devoted to defining proper experimental conditions to obtain reproducible electrochemical results. In this work, we demonstrate that the combination of a proper RTIL purification treatment and a filtered Ar gas stream allow us to obtain featureless voltammograms in [C4mim][BF4], [C4mim][NTf2], and [C4m2im][NTf2], which otherwise present signals associated with different types of impurities such as water and some minor electroactive impurities acquired during the RTIL synthesis process. Moreover, we demonstrate that bubbling Ar, or another inert gas, through the electrolyte in order to purge O2 dissolved in RTILs is one of the major sources of water and O2 impurities incorporated in RTILs within the electrochemical cell. To overcome this source of water uptake, we have incorporated a gas stream purification filter before the gas reaches the RTIL in the electrochemical cell. To illustrate the effect of these impurities in relevant electrocatalytic studies, we study the electrocatalytic reduction of CO2 on Pt nanoparticles and the key role of an appropiate filter when the CO2 gas stream is bubbled within imidazolium based RTILs. Our cyclic voltammetric studies point out that CO2 electroreduction on Pt nanoparticles only presents activity in [C4mim][NTf2] and [C4m2im][NTf2], thus suggesting that the C-2 position on the imidazolium ring is not the key position in CO2 electrochemical reduction. In contrast, the same Pt nanoparticles are inactive towards CO2 electroreduction in [C4mim][BF4], which is a more hydrophilic RTIL.

Improved Performance of Ionic Liquid Supercapacitors by using Tetracyanoborate Anions

Supercapacitors are energy storage devices designed to operate at higher power densities than conventional batteries, but their energy density is still too low for many applications. Efforts are made to design new electrolytes with wider electrochemical windows than aqueous or conventional organic electrolytes in order to increase energy density. Ionic liquids (ILs) with wide electrochemical stability windows are excellent candidates to be employed as supercapacitor electrolytes. ILs containing tetracyanoborate anions [B(CN)₄] offer wider electrochemical stability than conventional electrolytes and maintain a high ionic conductivity (6.9 mS cm⁻¹). Herein, we report the use of ILs containing the [B(CN)₄] anion for such an application. They presented a high maximum operating voltage of 3.7 V, and two‐electrode devices demonstrate high specific capacitances even when operating at relatively high rates (ca. 20 F g⁻¹ @ 15 A g⁻¹). This supercapacitor stored more energy and operated at a higher power at all rates studied when compared with cells using a commonly studied ILs.

Reviving lithium cobalt oxide-based lithium secondary batteries-toward a higher energy density

This review summarizes the key challenges, effective modification strategies and perspectives regarding reviving lithium cobalt oxide-based lithium secondary batteries-toward a higher energy density.

Thermophysical and Electrochemical Properties of Ethereal Functionalised Cyclic Alkylammonium-based Ionic Liquids as Potential Electrolytes for Electrochemical Applications

A series of hydrophobic room temperature ionic liquids (ILs) based on ethereal functionalised pyrrolidinium, piperidinium and azepanium cations bearing the bis[(trifluoromethyl)sulfonyl]imide, [TFSI]- , anion were synthesized and characterized. Their physicochemical properties such as density, viscosity and electrolytic conductivity, and thermal properties including phase transition behaviour and decomposition temperature have been measured. All of the ILs showed low melting point, low viscosity and good conductivity and the latter properties have been discussed in terms of the IL fragility, an important electrolyte feature of the transport properties of glass-forming ILs. Furthermore, the studied [TFSI]- -based ILs generally exhibit good electrochemical stabilities and, by coupling electrochemical experiments and DFT calculations, the effect of ether functionalisation at the IL cation on the electrochemical stability of the IL is discussed. Preliminary investigations into the Li-redox chemistry at a Cu working electrode are also reported as a function of ether-functionality within the pyrrolidinium-based IL family. Overall, the results show that these ionic liquids are suitable for electrochemical devices such as battery systems, fuel cells or supercapacitors.

Experimental and theoretical studies on solvation in aqueous solutions of ionic liquids carrying different side chains: the n-butyl-group versus the methoxyethyl group

We used solvatochromic compounds to probe solvation in mixtures of water, W, and four ionic liquids (ILs), 1-R-3-methylimidazoliumX, where R =n-butyl or methoxyethyl and X = acetate and chloride.

Ionic liquids containing tricyanomethanide anions: physicochemical characterisation and performance as electrochemical double-layer capacitor electrolytes

Fluorine free ionic liquids with low density and high ionic conductivity for high energy electrochemical double-layer capacitors.

Influence of Particle Size Distribution on the Performance of Ionic Liquid-based Electrochemical Double Layer Capacitors

Electrochemical double layer capacitors (EDLCs) employing ionic liquid electrolytes are the subject of much research as they promise increased operating potentials, and hence energy densities, when compared with currently available devices. Herein we report on the influence of the particle size distribution of activated carbon material on the performance of ionic liquid based EDLCs. Mesoporous activated carbon was ball-milled for increasing durations and the resultant powders characterized physically (using laser diffraction, nitrogen sorption and SEM) and investigated electrochemically in the form of composite EDLC electrodes. A bi-modal particle size distribution was found for all materials demonstrating an increasing fraction of smaller particles with increased milling duration. In general, cell capacitance decreased with increased milling duration over a wide range of rates using CV and galvanostatic cycling. Reduced coulombic efficiency is observed at low rates (<25 mVs⁻¹) and the efficiency decreases as the volume fraction of the smaller particles increases. Efficiency loss was attributed to side reactions, particularly electrolyte decomposition, arising from interactions with the smaller particles. The effect of reduced efficiency is confirmed by cycling for over 15,000 cycles, which has the important implication that diminished performance and reduced cycle life is caused by the presence of submicron-sized particles.

A review of electrolyte materials and compositions for electrochemical supercapacitors

Electrolytes have been identified as some of the most influential components in the performance of electrochemical supercapacitors (ESs), which include: electrical double-layer capacitors, pseudocapacitors and hybrid supercapacitors. This paper reviews recent progress in the research and development of ES electrolytes. The electrolytes are classified into several categories, including: aqueous, organic, ionic liquids, solid-state or quasi-solid-state, as well as redox-active electrolytes. Effects of electrolyte properties on ES performance are discussed in detail. The principles and methods of designing and optimizing electrolytes for ES performance and application are highlighted through a comprehensive analysis of the literature. Interaction among the electrolytes, electro-active materials and inactive components (current collectors, binders, and separators) is discussed. The challenges in producing high-performing electrolytes are analyzed. Several possible research directions to overcome these challenges are proposed for future efforts, with the main aim of improving ESs' energy density without sacrificing existing advantages (e.g., a high power density and a long cycle-life) (507 references).

The smallest quaternary ammonium salts with ether groups for high-performance electrochemical double layer capacitors

The smallest quaternary ammonium salts (QASs) with ether groups on tails and tetrafluoroborate (BF₄) as an anion exhibit high performance in electrochemical double layer capacitors (EDLCs).

Electrochemical double layer capacitors (EDLCs) are energy storage devices that have been used for a wide range of electronic applications. In particular, the electrolyte is one of the important components, directly related to the capacitance and stability. Herein, we first report a series of the smallest quaternary ammonium salts (QASs), with ether groups on tails and tetrafluoroborate (BF₄) as an anion, for use in EDLCs. To find the optimal structure, various QASs with different sized head groups and ether-containing tail groups were systematically compared. Comparing two nearly identical structures with and without ether groups, QASs with oxygen atoms showed improved capacitance, proving that ions with oxygen atoms move more easily than their counterparts at lower electric fields. Moreover, the ether containing QASs showed low activation energy values of conductivities, leading to smaller IR drops during the charge and discharge processes, resulting in an overall higher capacitance.

Influence of imidazolium-based ionic liquid electrolytes on the performance of nano-structured MnO2 hollow spheres as electrochemical supercapacitor

Activated carbon//MnO₂ hollow sphere asymmetric supercapacitor shows an energy density of 163 W h kg⁻¹ in EMIMBF₄ ionic liquid as electrolyte.

Ionic liquid based EDLCs: influence of carbon porosity on electrochemical performance

Electrochemical double layer capacitors (EDLCs) are a category of supercapacitors; devices that store charge at the interface between electrodes and an electrolyte. Currently available commercial devices have a limited operating potential that restricts their energy and power densities. Ionic liquids (ILs) are a promising alternative electrolyte as they generally exhibit greater electrochemical stabilities and lower volatility. This work investigates the electrochemical performance of EDLCs using ILs that combine the bis(trifluoromethanesulfonyl)imide anion with sulfonium and ammonium based cations. Different activated carbon materials were employed to also investigate the influence of varying pore size on electrochemical performance. Electrochemical impedance spectroscopy (EIS) and constant current cycling at different rates were used to assess resistance and specific capacitance. In general, greater specific capacitances and lower resistances were found with the sulfonium based ILs studied, and this was attributed to their smaller cation volume. Comparing electrochemical stabilities indicated that significantly higher operating potentials are possible with the ammonium based ILs. The marginally smaller sulfonium cation performed better with the carbon exhibiting the largest pore width, whereas peak performance of the larger sulfonium cation was associated with a narrower pore size. Considerable differences between the performance of the ammonium based ILs were observed and attributed to differences not only in cation size but also due to the inclusion of a methoxyethyl group. The improved performance of the ether bond containing IL was ascribed to electron donation from the oxygen atom influencing the charge density of the cation and facilitating cation–cation interactions.