Electrolytes For Electrochemical Double Layer Capacitors Research Articles

Water in Protic Ionic Liquids: Properties and Use of a New Class of Electrolytes for Energy-Storage Devices.

In this work, the properties of "water-in-PIL" (PIL=protic ionic liquid) electrolytes are reported based on 1-butylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PyrH4 TFSI). Taking advantage of experimental and theoretical investigations, it is shown that the amount of water inside the electrolyte has a dramatic effect on the viscosity, conductivity, density, cation-anion interplay, and electrochemical stability of PyrH4 TFSI. The impact of water on the properties of this ionic liquid also affects its use as an electrolyte for electrochemical double-layer capacitors (EDLCs). It is shown that the presence of water improves the transport properties of PyrH4 TFSI, with a beneficial effect on the capacitance retention of the devices in which these electrolytes are used. However, at the same time, water reduces the operative voltage of EDLCs containing this PIL as the electrolyte and, furthermore, it has a strong impact on the inactive components of these systems. To suppress this latter problem, and to realize EDLCs with high stability, the use of inactive components stable in aqueous environment appears necessary.

Influence of Water on the Properties of Protic Ionic Liquids

Protic ionic liquids (PILs) represent an interesting subclass of Ionic Liquids (ILs), with properties suitable for the realization of different kinds of electrochemical energy storage devices. [1-2] PIL have been proposed as electrolytes for electrochemical double layer capacitors (EDLCs) and lithium-ion batteries (LIBs) [ref]. In the case of EDLCs the use of these ILs appears not to be particularly advantageous due to the presence of a proton on their structure, limiting the operative voltage, and their relative high viscosity, limiting the power performance of these devices. In the case of LIBs, to the contrary, the presence of the proton has a positive influence on the lithium coordination and on the performance at high current densities of IL-based LIBs. [3]. The influence of the water content on the chemical-physical properties of PILs have been investigated in the past, and it has been found that even a few hundred ppm of residual water are strongly changing the properties of these ILs, e.g. they might significantly lower the melting point of PILs. Nevertheless, the impact of the water content on the electrochemical performance of electrochemical energy storage devices containing PILs as electrolytes has not been investigated in detail. These important aspects should be investigated because it could supply valuable information about the development of PIL-based devices. In this paper we report about the influence of water on the properties of the PIL butylpyrrolidinium bis(trifluoromethylsulfonyl)imide (PyrH4TFSI) containing different amounts of water (0, 1, 2, 3.8 wt%). The viscosity, ionic conductivity, density and electrochemical stability of these PIL have been investigated in detail. Furthermore, the mixtures were utilized as electrolytes for EDLCs containing activated carbon-based electrodes, as well as for LIBs containing graphite anodes and lithium iron phosphate (LFP) cathodes. The results of this study indicate that small amounts of water have a significant impact on the overall transport properties and stability of PyrH4TFSI. Moreover, they also show that the performance of EDLCs and LIBs containing PyrH4TFSI as electrolyte are considerably changing depending on the water content of the PIL. Literature: [1] Menne, Electrochem. Commun. 2013, 31, 39-41.[2] Vogl, Journal of Materials Chemistry A 2016, 4, 10472-10478.[3] Brandt, Electrochim. Acta 2013, 108, 226-231. Figure 1

A systematically comparative study on LiNO3 and Li2SO4 aqueous electrolytes for electrochemical double-layer capacitors

In this work, highly soluble LiNO3 was adopted as the neutrally aqueous electrolyte for active carbon (AC)-based electrochemical double-layer capacitors (EDLCs), of which the electrochemical performances were evaluated. Simultaneously, the physicochemical properties such as the ionic conductivity and viscosity of the LiNO3 aqueous solution were investigated. As compared with the most studied Li2SO4 aqueous solution, the LiNO3 aqueous solution displays more favorable physicochemical properties and electrochemical performances as the neutral electrolytes for EDLCs. To be specific, the conductivity of the 5.0 M LiNO3 aqueous solution can reach up to 154.8 mS cm−1 at 25 °C, which is nearly two times of the maximum conductivity of 77.6 mS cm−1 for the 2.0 M Li2SO4 aqueous solution under the identical testing conditions. Even at a concentration as high as 9.0 M, the absolute viscosity of the LiNO3 aqueous solution is only 2.4, while that of the Li2SO4 aqueous solution achieves 3.0 at the maximum concentration of 2.5 M. Additionally, 5.0 M LiNO3 aqueous solution exhibits a wide electrochemical potential stability window from −0.9 to 0.9 V (vs. SCE) at the AC electrode, giving rise to an operating cell voltage of 1.8 V, which is comparable to that of 2.0 M Li2SO4 aqueous solution. Further, with the 5.0 M LiNO3 aqueous solution as the electrolyte, the as-fabricated EDLC delivers an energy density up to 21.16 Wh Kg−1 at a power density of 100.09 W kg−1, which is higher than 18.43 Wh Kg−1 for the EDLC with the 2.0 M Li2SO4 aqueous electrolyte at the identical power density. Even though the power density reaches as high as 5970 W kg−1, the energy density of the EDLC with the 5.0 M LiNO3 aqueous electrolyte can still remain at 13.1 Wh Kg−1, substantially higher than 8.71 Wh Kg−1 of the EDLC with the 2.0 M Li2SO4 aqueous electrolyte at the same power density. Moreover, the EDLC with the 5.0 M LiNO3 aqueous electrolyte also holds good cyclic stability. After 10000 charge–discharge cycles at a current density of 1 A g−1 and a cut-off voltage of 1.8 V, the capacity retention of this EDLC retains more than 90%. These results can render an insight to explore safe, eco-friendly, inexpensive and neutrally aqueous electrolytes for supercapacitors.

The Influence of Ion-Solvent Interactions on the Performance of Electrochemical Double Layer Capacitors Containing Non-Conventional Solvents

In the state of the art electrochemical double layer capacitors (EDLCs), solutions of tetraethylammonium tetrafluroroborate (Et4NBF4) in propylene carbonate (PC) or acetonitrile (ACN) are used as electrolytes. Using these electrolytes in combination with activated carbons (AC) based electrodes is possible to realize EDLCs with operative voltages of 2.7-2.8 V and energy in the order of 5Whkg-1. The power of these devices is up 10 kW kg-1 and their cycle life higher than 500.000 cycles [1]. Presently the realization of high energy EDLCs is seen as one of the most important goal for an increase of the market of these devices [1]. The most straightforward strategy to increase the EDLC´s energy is to increase their operative voltage. Since such desirable increase is not possible when the state-of-the-art electrolytes are used, the introduction of new electrolytic solution is presently considered of great importance [1] In order to realize advanced electrolytes for EDLC, the introduction of new conducting salts and/or new solvents appears a viable strategy [1,2]. In the past years we showed that using non-conventional salts is possible to realize PC-based EDLCs with operative voltages higher than 3 V [1,2]. More recently, taking advantaged of computational screening [3], we identified cyano esters as new solvent for the realization of high voltage and high energy EDLCs [4,5]. We showed that cyano ester-based EDLCs display operative voltage as high as 3.5 V and good performance in term of specific capacitance and capacitance retention. After 500 h of float tests carried out at 3.2 V EDLCs containing this type of solvent are able to retain almost 80% of their initial capacitance [4]. It is well known that the ion-solvent interaction might have a dramatic effect on the behaviour of EDLCs [1,2]. While these interactions have been considered for conventional salt-solvent combinations, only little information is available on the case of non-conventional electrolyte components [2]. Herein, we report a systematic investigation about the chemical-physical properties of electrolytes containing the salts tetraethylammonium tetrafluroroborate (Et4NBF4), tetraethylammonium bis(trifluoromethanesulfonyl) imide (Et4NTFSI) and N-butyl-1-methylpyrrolidinium tetrafluoroborate (Pyr14BF4), in the state of the art solvent ACN, and in the alternative solvents Adiponitrile (ADN), 2-methylglutaronitril (MGN) and 3-cyanopropionic acid methyl ester (CPAME). The ionic conductivity, viscosity, density and electrochemical stability windows (ESW) of these electrolytic solutions are considered. Furthermore, the electrochemical performance of EDLCs containing these electrolytes are investigate in details. Finally, also the anodic dissolution of the aluminium current collector in these electrolytes is analysed and discussed [6].

Physical–Chemical Characterization of Binary Mixtures of 1-Butyl-1-methylpyrrolidinium Bis{(trifluoromethyl)sulfonyl}imide and Aliphatic Nitrile Solvents as Potential Electrolytes for Electrochemical Energy Storage Applications

In the scope of improving the energy and power densities of electrochemical double layer capacitors (EDLCs), the development of high performance electrolytes with enhanced operative voltages is imperative. The formulation of mixtures containing ionic liquids with organic molecular solvents is an important strategy in the pursuit of developing highly electrochemically stable and safe materials while retaining fast transport properties for high power applications. In this work, we report on the physical–chemical investigations into binary mixtures containing the ionic liquid 1-butyl-1-methylpyrrolidinium bis{(trifluoromethyl)sulfonyl}imide with one mononitrile solvent, butyronitrile, and two dinitrile solvents, glutaronitrile and adiponitrile, as potential electrolytes for EDLCs. The thermal, volumetric, and transport properties of the binary mixtures are investigated as functions of the electrolyte composition and temperature. Furthermore, the electrolyte composition which exhibits the highest conductivity...

(Invited) Binary Mixtures of 1-Butyl-1-Methylpyrrolidinium Bis{(trifluoromethyl)Sulfonyl}Imide and Aliphatic Nitrile Solvents As Electrolyte for Electrochemical Double Layer Capacitors

Electrochemical double layer capacitors (EDLCs), also known as supercapacitors, are promising energy storage devices, especially when considering high power applications [1]. EDLCs can be charged and discharged within seconds [1], feature high power (10 kW·kg-1) and an excellent cycle life (>500,000 cycles). All these properties are a result of the energy storage process of EDLCs, which relies on storing energy by charge separation instead of chemical redox reactions, as utilized in battery systems. Upon charging, double layers are forming at the electrode/electrolyte interface consisting of the electrolyte’s ions and electric charges at the electrode surface. In state-of-the-art EDLC systems activated carbons (AC) are used as active materials and tetraethylammonium tetrafluoroborate ([Et4N][BF4]) dissolved in organic solvents like propylene carbonate (PC) or acetonitrile (ACN) are commonly used as the electrolyte [2]. These combinations of materials allow operative voltages up to 2.7 V - 2.8 V and an energy in the order of 5 Wh·kg-1[3]. The energy of EDLCs is dependent on the square of the operative voltage, thus increasing the usable operative voltage has a strong effect on the delivered energy of the device [1]. Due to their high electrochemical stability, ionic liquids (ILs) were thoroughly investigated as electrolytes for EDLCs, as well as, batteries, enabling high operating voltages as high as 3.2 V - 3.5 V for the former [2]. While their unique ionic structure allows the usage of neat ILs as electrolyte in EDLCs, ILs suffer from low conductivity and high viscosity increasing the intrinsic resistance and, as a result, a lower power output of the device. In order to overcome this issue, the usage of blends of ionic liquids and organic solvents has been considered a feasible strategy as they combine high usable voltages, while still retaining good transport properties at the same time. In our recent work the ionic liquid 1-butyl-1-methylpyrrolidinium bis{(trifluoromethyl)sulfonyl}imide ([Pyrr14][TFSI]) was combined with two nitrile-based organic solvents, namely butyronitrile (BTN) and adiponitrile (ADN), and the resulting blends were investing regarding their usage in electrochemical double layer capacitors [4,5]. Firstly, the physicochemical properties were investigated, showing good transport properties for both blends, which are similar to the state-of-the-art combination of [Et4N][BF4] in PC. Secondly, the electrochemical properties for EDLC application were studied in depth revealing a high electrochemical stability with a maximum operative voltage as high as 3.7 V. In full cells these high voltage organic solvent based electrolytes have a good performance in terms of capacitance and an acceptable equivalent series resistance at cut-off voltages of 3.2 and 3.5 V. However, long term stability tests by float testing revealed stability issues when using a maximum voltage of 3.5 V for prolonged time, whereas at 3.2 V no such issues are observed (Fig. 1). Considering the obtained results, the usage of ADN and BTN blends with [Pyrr14][TFSI] in EDLCs appears to be an interesting alternative to state-of-the-art organic solvent based electrolytes, allowing the usage of higher maximum operative voltages while having similar transport properties to 1 mol∙dm-3 [Et4N][BF4] in PC at the same time.

Ionic Liquids as Electrolytes for Electrochemical Double-Layer Capacitors: Structures that Optimize Specific Energy.

Key parameters that influence the specific energy of electrochemical double-layer capacitors (EDLCs) are the double-layer capacitance and the operating potential of the cell. The operating potential of the cell is generally limited by the electrochemical window of the electrolyte solution, that is, the range of applied voltages within which the electrolyte or solvent is not reduced or oxidized. Ionic liquids are of interest as electrolytes for EDLCs because they offer relatively wide potential windows. Here, we provide a systematic study of the influence of the physical properties of ionic liquid electrolytes on the electrochemical stability and electrochemical performance (double-layer capacitance, specific energy) of EDLCs that employ a mesoporous carbon model electrode with uniform, highly interconnected mesopores (3DOm carbon). Several ionic liquids with structurally diverse anions (tetrafluoroborate, trifluoromethanesulfonate, trifluoromethanesulfonimide) and cations (imidazolium, ammonium, pyridinium, piperidinium, and pyrrolidinium) were investigated. We show that the cation size has a significant effect on the electrolyte viscosity and conductivity, as well as the capacitance of EDLCs. Imidazolium- and pyridinium-based ionic liquids provide the highest cell capacitance, and ammonium-based ionic liquids offer potential windows much larger than imidazolium and pyridinium ionic liquids. Increasing the chain length of the alkyl substituents in 1-alkyl-3-methylimidazolium trifluoromethanesulfonimide does not widen the potential window of the ionic liquid. We identified the ionic liquids that maximize the specific energies of EDLCs through the combined effects of their potential windows and the double-layer capacitance. The highest specific energies are obtained with ionic liquid electrolytes that possess moderate electrochemical stability, small ionic volumes, low viscosity, and hence high conductivity, the best performing ionic liquid tested being 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide.

Electrochemical Characterization of the Interface Between Pyrolytic Graphite Electrode and 1-Butyl-3-Methylimidazolium Iodide

Ionic liquids as electrolytes for modern electrochemical energy conversion/storage devices (electrochemical double layer capacitors (EDLC), hybrid capacitors, batteries, etc.) have been intensively investigated over the past few years. However, still the kinetics of formation/rearrangement of the electrical double layer (edl) between electrode and ionic liquid (IL) needs an extended examination. Our previous works were focused on developing of better electrolyte for EDLCs by mixing chemically different ionic liquids EMImBF4 and EMImI1. It has already been successfully tested in a system using microporous-mesoporous carbon electrodes2. The main aim of this work was to establish the electrochemical characteristics of adsorption of iodide ions from ionic liquid media without added electrolytes. In this work electrochemical impedance spectroscopy and cyclic voltammetry (CV) methods were applied to study the electrochemical characteristics of the interface between pyrolytic graphite (PG) and 1-butyl-3-methylimidazolium iodide (BMImI) at 22±1 oC. All measurements were carried out inside a glove box in a 3-electrode electrochemical cell using a PG as a working, Pt net as a counter, and Ag wire coated with AgCl as a reference electrodes. Current density (i) vs. electrode potential (E) dependencies show that there is clear dependence of capacitance on the composition of IL if we compare 1-ethyl-3-methylimidazolium tetrafluoroborate (EMImBF4) and PG electrode interface to the BMImI containing system. There is an influence of the alkyl chain length and in the case of BMImI, the I- anion demonstrates specific adsorption behaviour. The electrochemical characteristics of BMImI|Bi(111) interface show similarities to EMImBF4 + EMImI interface. The PG|BMImI interface characteristics were measured within the potential region from ─1.0 V to +1.5 V (vs. Ag | AgCl). The shape of the complex impedance plane, i.e. Nyquist plots, total impedance and phase angle vs. frequency plots shape depend strongly on the electrode potential and on the ac frequency (f) applied. The shape of the phase angle vs. ac f dependencies show that at low frequency range (f < 1 Hz) and potential region from ─0.8 V to +0.4 V the system has nearly ideally capacitive behaviour. At more positive potentials the mixed kinetics behaviour can be seen. The values of series differential capacitance (C s) were calculated from Nyquist dependencies at different fixed ac frequencies (C s = (Z''i2πf) −1; where i = √−1 and Z’’ is the imaginary part of impedance). C s depends strongly on ac frequency and on electrode potential applied, increasing at lower ac frequencies and at anodic range of potentials.

Almond shell-based activated nanoporous carbon electrode for EDLCs

Almond shell-based chemically treated and activated nanoporous charcoal powder (AS(T)) has been successfully prepared by chemical activation method using potassium hydroxide (KOH) as an activating agent. The as-synthesized AS(T) was systematically characterized by various techniques like N2 adsorption, scanning electron microscopy (SEM), X-ray diffraction, and thermogravimetric analysis. The AS(T)-based nanoporous activated charcoal is tested as an electrode material with ionic liquid-based polymer gel electrolyte for electrochemical double-layer capacitors (EDLCs). EDLCs prepared from AS(T) exhibit specific capacitance of 986.3 mF cm−2 (equivalent to single-electrode specific capacitance of 563.6 F g−1). The energy density of 62.8 Wh kg−1 and power density of 2.1 kW kg−1 have been observed for nanoporous AS(T)-based EDLCs.

An investigation about the use of mixtures of sulfonium-based ionic liquids and propylene carbonate as electrolytes for supercapacitors

This study describes the use of mixtures of the ionic liquid (IL) trimethyl-sulfonium bis[(trifluoromethyl)sulfonyl]imide (Me3STFSI) and propylene carbonate (PC) as electrolytes for carbon-based electrochemical double layer capacitors (EDLCs). Two different mixtures have been investigated. The first one contained the maximum amount of Me3STFSI (3.8 mol L−1) soluble in PC at room temperature (RT). The second one contained 1.9 mol L−1 of Me3STFSI, which is the concentration featuring the highest conductivity among all possible mixtures between PC and Me3STFSI at RT. The physicochemical properties, including conductivity, viscosity, and electrochemical and thermal stability of both mixtures, have been initially investigated. Afterward, the use of these mixtures as electrolytes for EDLCs has been considered. This study showed that by using these innovative electrolytes, it is possible to develop EDLCs with an operative voltage as high as 3.0 V. It was clearly demonstrated that the EDLC cycle life strongly depends on the concentration of Me3STFSI present in the mixture. Moreover, it was also proved that mixtures containing high concentrations of IL are able to suppress anodic oxidation of the Al current collector at high potential. When a solution with a high salt concentration is used, an EDLC with high energy, high cycle life and a broad temperature range of operation can be realized.