Electrolytes For Double Layer Capacitors Research Articles

Electrochemical Performance and Molecular Structure of Diluted 1-Alkyl-3-methylimidazolium Tetrafluoroborate Ionic Liquids and Their Mixture as Electrolytes for Double-Layer Capacitors: An Integrated Approach by Electrochemical Characterization and Molecular Dynamics Simulation

This paper reports the electrochemical studies of two room-temperature ionic liquids (RTILs), namely, 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM][BF4]) and 1-butyl-3-methylimidazolium tet...

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

We investigated the use of fluorine free ionic liquids (ILs) containing the tricyanomethanide anion ([C(CN)3]) as an electrolyte in electrochemical double-layer capacitors (EDLCs). Three cations were used; 1-butyl-3-methylimidazolium ([Im1,4]), N-butyl-N-methylpyrrolidinium ([Pyr1,4]) and N-butyl-N-methylpiperidinium ([Pip1,4]). Their physicochemical properties are discussed alongside with their performance as electrolytes. We found that the cyano-based ILs present higher ionic conductivity (9.4, 8.7 and 4.2 mS cm-1 at 25 °C for [Im1,4], [Pyr1,4] and [Pip1,4], respectively) than the widely studied IL containing the bis(trifluoromethylsulfonyl)imide anion, namely [Pyr1,4][Tf2N] (2.7 mS cm-1 at 25 °C). Of the three ILs investigated, [Pip1,4][C(CN)3] presents the widest electrochemical stability window, 3.0 V, while [Pyr1,4][C(CN)3] is stable up to 2.9 V and its [Tf2N] analogue can operate at 3.5 V. Despite operating at a lower voltage, [Pyr1,4][C(CN)3] EDLC is capable of delivering up to 4.5 W h kg-1 when operating at high specific power of 7.2 kW kg-1, while its [Pyr1,4][Tf2N] counterpart only delivered 3.0 W h kg-1 when operated at similar power.

Novel ionic liquid based electrolyte for double layer capacitors with enhanced high potential stability

Developing electrolyte with high electrochemical stability is the most effective way to improve the energy density of double layer capacitors (DLCs), and ionic liquid is a promising choice. Herein, a novel ionic liquid based high potential electrolyte with a stabilizer, succinonitrile, was proposed to improve the high potential stability of the DLC. The electrolyte with 7.5 wt% succinonitrile added has a high ionic conductivity of 41.1 mS cm-1 under ambient temperature, and the DLC adopting this electrolyte could be charged to 3.0 V with stable cycle ability even under a discharge current density of 6 A g-1. Moreover, the energy density could be increased by 23.4% when the DLC was charged to 3.0 V compared to that charged to 2.7 V.

Thin-film electrochemical sensor electrode for rapid evaluation of electrolytic conductivity, cyclic voltammetry, and temperature measurements

A thin-film electrochemical sensor electrode capable of electrolytic conductivity, cyclic voltammetry, and temperature measurements of electrolyte solutions has been fabricated and characterized. The electrode fabrication and calibration is detailed, showing accuracies of $$\pm$$ 6 % for electrolytic conductivity measurements over the range of 0.1–100 mS·cm $$^{-1}$$ and temperature measurement within $$\pm$$ 2 $$^{\circ }$$ C from $$-$$ 60 to 95 $$^{\circ }$$ C. The electrode’s capabilities are verified with standard Li-ion battery and electrochemical double-layer capacitor electrolytes, to which the measured data match well with literature values. Lastly, demonstration of the electrode’s capabilities is shown with determination of cyclic voltammetry curves and electrolytic conductivity over a range of temperatures for other electrolyte solutions. Integration of these three measurement tools into a single, low-cost, high accuracy, rapid measurement electrode can greatly simplify quantitative characterization of novel electrolytes developed in the electrochemical energy storage field.

Anti-freezing aqueous electrolytes for electric double-layer capacitors

Different types of alcohol have been proposed as the anti-freezing additives for aqueous electrolytes in double-layer capacitors (EDLCs). The influence of ion solvation on the capacitive behavior of porous carbon electrode in the anti-freezing aqueous electrolytes has been studied. The low-temperature performance of EDLCs composed of either micro- or meso-porous carbon in these electrolytes has been fully investigated. Deduced from cyclic voltammetry, electrochemical impedance spectroscopy and galvanostatic charge–discharge measurements, it has been discovered that the anti-freezing electrolyte of methanol/water-concentrated NaClO4 possesses low equivalent series resistance at −40°C. EDLCs using the electrolytes of methanol/water-concentrated NaClO4 solutions deliver improved energy density and power density at the temperature as low as −40°C.

Substituted phosphonium cation based electrolytes for nonaqueous electrical double-layer capacitors

Tetrakis(diethylamino)phosphonium tetrafluoroborate (TDENPBF4), tetrakis(diethylamino)phosphonium hexafluorophosphate (TDENPPF6), and tetrakis(dimethylamino)phosphonium tetrafluoroborate (TDMNPBF4) in acetonitrile (AN) have been studied as electrical double-layer capacitor electrolytes in a two-electrode test cell using titanium carbide derived carbon, C(TiC), as an electrode material. Electrochemical characteristics for C(TiC)|1 M TDENPBF4 + AN, C(TiC)|1 M TDENPPF6 + AN, and C(TiC)|1 M TDMNPBF4 + AN interfaces have been obtained by cyclic voltammetry, constant current charging/discharging, and electrochemical impedance spectroscopy. High-capacitance (85 °F/g) and gravimetric power (269 kW/kg) have been achieved at cell voltage 3.2 V. Data obtained have been compared with results published previously.

Allyl-functionalized ionic liquids as electrolytes for electric double-layer capacitors

Double-layer capacitor electrolytes employing allyl-functionalized ionic liquids as electrolytes with solvents have been evaluated. Imidazolium cations with allyl groups enabled the high capacitances and low resistances of electric double-layer capacitor (EDLC) cells at a wide range of temperature in spite of the large cation sizes and low ionic conductivities of the electrolytes compared to imidazolium with saturated alkyl groups, 1-ethyl-3-methylimidazolium (EMIm). The improvement of EDLC performance was noted particularly in the case of diallylimidazolium (DAIm) cation and TFSA anion. The substitution of the vinyl group increased the high capacitance only at 298 K and decreased the capacitance at low temperature and direct current resistance (DC-IR) at 243 and 298 K. The butenyl group deteriorated the capacitance and DC-IR at 243 and 298 K. The stability of EDLC cell of DAIm–BF 4/PC was inferior to that of EMIm–BF 4/PC. The addition of DMC to PC improved the stability.

Application of sulfonium-, thiophenium-, and thioxonium-based salts as electric double-layer capacitor electrolytes

The purpose of this paper is to develop novel molten salts based on sulfonium, thiophenium, and thioxonium cations as electrolytes for EDLCs. We evaluated various kinds of the salts, with tetrafluoroborate (BF 4) and bis(trifluorometanesulfonyl)amide (TFSA) anions, including several kinds of ionic liquids. The cell using the electrolyte containing diethylmethylsulfonium (DEMS)–BF 4 salt had the higher capacitance and the lower DC-IR value than those containing conventional salts such as 1-ethyl-3-methylimidazolium (EMI)–BF 4 and N, N, N-triethyl- N-methylammonium (TEMA)–BF 4 at 243 K and 2 V. The capacitance and DC-IR at low temperature depended strongly on the structure, particularly on the size of each ion. We also examined the durability of cells by continuous charging at 333 K and 2.5 V. The stabilities of sulfonium-, thiophenium-, and thioxonium-based electrolytes were much inferior to that of EMI–BF 4.

Double-Layer Capacitor Electrolytes Using 1,3-D ioxolane for Low Temperature Operation

Double-layer capacitor electrolytes employing 1,3-dioxolane as a cosolvent with acetonitrile have been evaluated in coin cells using electrochemical impedance spectroscopy and dc charging and discharging tests. Addition of the lower-melting-point 1,3-dioxolane to the standard acetonitrile solvent was found to extend the low-temperature operational range of test cells beyond that of commercially available cells. By adjusting the concentration of the tetraethylammonium tetrafluoroborate salt used, the equivalent series resistance can be minimized to enable optimal power delivery at a given temperature.

γ-Butyrolactone-acetonitrile solution of triethylmethylammonium tetrafluoroborate as an electrolyte for double-layer capacitors

Properties of a γ-butyrolactone-acetonitrile solution of triethylmethylammonium tetrafluoroborate were systematically measured for its potential to be used as an electrolyte in double-layer capacitors and for demonstrating the ways in which these properties can be optimized through choosing an appropriate solvent mixture and an appropriate salt and adjusting the electrolyte composition. These properties included the liquid range of the solvent and the solution at different compositions, the dielectric constant of the binary solvent as a function of solvent composition and temperature, the conductivity of the solution as a function of salt content, solvent composition, and temperature and the oxidative stability of the electrolytes at different solvent compositions. To correlate the properties of the electrolytes with the properties and performance of the double-layer capacitors utilizing the electrolytes, such capacitors were built and their equivalent series resistance, cyclic voltammogram, and operating voltage were measured for different solvent compositions of the capacitor electrolytes. The results revealed a clear correlation between the conductivity and the oxidative stability of the electrolytes and the equivalent series resistance and the operating voltage of the capacitors, respectively. Furthermore, it is clearly demonstrated that by using a binary solvent and adjusting its composition properly, the electrolyte it makes with an appropriate salt can be effectively optimized with regard to its liquid range, conductivity, and electrochemical stability window, which in turn imparts the double-layer capacitor utilizing such an electrolyte with a set of optimal properties such as a wide range of operating temperature, low equivalent series resistance, and high operating voltage.

Ionic Liquids with Low Melting Points and Their Application to Double-Layer Capacitor Electrolytes

Ionic liquids, 1-ethyl-3-methylimidazolium hexafluorotantalate (EMITaF 6 ) and hexafluoroniobate (EMINbF 6 ) were synthesized by a new method. They showed lower melting points and higher decomposition temperatures than popular EMIBF 4 due to larger anion size. Electrolytic conductivities of these ionic liquids were about 6 mS cm - 1 at 25°C. Lowering the melting point of ionic liquids did not result in the improvement of low-temperature characteristics of carbon double-layer capacitors, however, the new ionic liquids showed long life compared with EMICF 3 SO 3 , EMI(CF 3 SO 2 ) 2 N, and EMI(C 2 F 5 SO 2 ) 2 N at 3.0 V, 70°C. The derivatives of the new ionic liquids having a methyl group in 2-position (l-ethyl-2,3-dimethylimidazolium salts) were not ionic liquids at room temperature.

A better quantification of electrochemical stability limits for electrolytes in double layer capacitors

An improved quantification of electrochemical stability limits is proposed for electrolytes working in capacitors, where the enormous surface area of the electrode material renders the conventional determination of electrolyte decomposition inappropriate. Using the proposed new approach, reliable stability data are obtained for the description of non-aqueous electrolytes operating in double layer capacitors. With these new stability data it is also found that the electrolyte decomposition correlates with the surface concentration of the functional group rather than the total surface area of the electrode. This new method could be used to evaluate electrolyte/electrode interaction in more general systems in which the electrode has a high surface area without any irreversibility being desired.