An Artificial Interface for High Cell Voltage Aqueous-Based Electrochemical Capacitors (original) (raw)
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Electrode materials for ionic liquid-based supercapacitors
Journal of Power Sources, 2007
A Al lm ma a M Ma at te er r S St tu ud di io or ru um m --U Un ni iv ve er rs si it tà à d di i B Bo ol lo og gn na a DOTTORATO DI RICERCA IN Scienze Chimiche Ciclo XXII Settori scientifici-disciplinari di afferenza: CHIM/02, CHIM/03 Abstract The development of safe, high energy and power electrochemical energy-conversion systems can be a response to the worldwide demand for a clean and low-fuel-consuming transport. This thesis work, starting from a basic studies on the ionic liquid (IL) electrolytes and carbon electrodes and concluding with tests on large-size IL-based supercapacitor prototypes demonstrated that the IL-based asymmetric configuration (AEDLCs) is a powerful strategy to develop safe, high-energy supercapacitors that might compete with lithium-ion batteries in power assist-hybrid electric vehicles (HEVs). The increase of specific energy in EDLCs was achieved following three routes: i) the use of hydrophobic ionic liquids (ILs) as electrolytes; ii) the design and preparation of carbon electrode materials of tailored morphology and surface chemistry to feature high capacitance response in IL and iii) the asymmetric double-layer carbon supercapacitor configuration (AEDLC) which consists of assembling the supercapacitor with different carbon loadings at the two electrodes in order to exploit the wide electrochemical stability window (ESW) of IL and to reach high maximum cell voltage (V max ). Among the various ILs investigated the N-methoxyethyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR 1(2O1) TFSI) was selected because of its hydrophobicity and high thermal stability up to 350 °C together with good conductivity and wide ESW, exploitable in a wide temperature range, below 0°C. For such exceptional properties PYR 1(2O1) TFSI was used for the whole study to develop large size IL-based carbon supercapacitor prototype. This work also highlights that the use of ILs determines different chemical-physical properties at the interface electrode/electrolyte with respect to that formed by conventional electrolytes. Indeed, the absence of solvent in ILs makes the properties of the interface not mediated by the solvent and, thus, the dielectric constant and double-layer thickness strictly depend on the chemistry of the IL ions. The study of carbon electrode materials evidences several factors that have to be taken into account for designing performing carbon electrodes in IL. The heat-treatment in inert atmosphere of the activated carbon AC which gave ACT carbon featuring ca. 100 F g -1
Improved accessibility of porous carbon electrodes with surfactant ionic liquids for supercapacitors
Journal of Applied Electrochemistry
Ionic liquids (ILs) are promising electrolytes for supercapacitors due to their wide electrochemical window. However, most ILs are viscous in nature and require diffusional and rotational transformations to access the pore space of common supercapacitor electrodes. In this study, novel anionic surfactant ILs (ASILs) are synthesized to lubricate the electrode surface to improve pore accessibility by IL ions. ASIL composition (0-10 wt%) and temperature (22-150 °C)-dependent capacitances, as a measure of pore accessibility and wettability, are determined by cyclic voltammetry. 10 wt% 1-butyl-1-methylpyrrolidinium docusate, [PYR14][AOT], in the base IL 1-propyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl) imide, [PYR13][TFSI], exhibits the highest specific capacitance (202 F g −1 at 150 °C and 10 mV s −1), compared to [PYR13] [TFSI] (160 F g −1). Electrochemical impedance spectroscopy measurements indicate resistive charging for ASIL/IL electrolyte compared to the base IL at 22 °C due to reduced conductivity-a consequence of larger non-polar domains. However, at elevated temperatures (> 100 °C), electrolyte resistance is circumvented as the viscosity is reduced. The wide voltage window of ILs and improved wettability by ASILs can be coupled to maximize energy storage capability of supercapacitors for high-temperature power applications.
Electrochimica Acta, 2019
This work shows the potential application of carbon materials prepared by three different ionic liquidbased methods, using 1-butyl-3-methylimidazolium methanesulfonate [bmim][MeSO 3 ], for electrochemical supercapacitors. The effects of [bmim][MeSO 3 ] on morphology, texture and surface chemistry of prepared materials has been explored by SEM/TEM, N 2 /CO 2 adsorption measurements and XPS. The results indicate the possibility of synthesis of carbon materials with tunable physicochemical properties using ionic liquid based methods. The charge storage behavior of all materials was studied in three different pH aqueous electrolytes. The pseudocapacitive and double layer contributions were estimated and discussed from the aspect of the textural changes and the changes of the chemical composition of surface functional groups containing heteroatoms. C]O type functional groups, with the contribution of COOH groups, were found to be responsible for a different amount of charge, which could be stored in alkaline and acidic electrolytic solution. The material prepared by direct carbonization of [bmim] [MeSO 3 ], showed the best electrochemical performance in alkaline electrolyte with a capacitance of 187 F g À1 at 5 mV s À1 (or 148 F g À1 at 1 A g À1), due to the contribution of both electric-double layer capacitance and pseudocapacitance which arises from oxygen, nitrogen and sulfur functional groups.
Faraday Discussions, 2012
Supercapacitors with two single-sheet graphene electrodes in the parallel plate geometry are studied via molecular dynamics (MD) computer simulations. Pure 1-ethyl-3-methylimidazolium tetrafluoroborate (EMI + BF À 4 ) and a 1.1 M solution of EMI + BF À 4 in acetonitrile are considered as prototypes of room-temperature ionic liquids (RTILs) and organic electrolytes. Electrolyte structure, charge density and associated electric potential are investigated by varying the charges and separation of the two electrodes. Multiple charge layers formed in the electrolytes in the vicinity of the electrodes are found to screen the electrode surface charge almost completely. As a result, the supercapacitors show nearly an ideal electric double layer behavior, i.e., the electric potential exhibits essentially a plateau behavior in the entire electrolyte region except for sharp changes in screening zones very close to the electrodes. Due to its small size and large charge separation, BF À 4 is considerably more efficient in shielding electrode charges than EMI + . In the case of the acetonitrile solution, acetonitrile also plays an important role by aligning its dipoles near the electrodes; however, the overall screening mainly arises from ions. Because of the disparity of shielding efficiency between cations and anions, the capacitance of the positively-charged anode is significantly larger than that of the negatively-charged cathode. Therefore, the total cell capacitance in the parallel plate configuration is primarily governed by the cathode. Ion conductivity obtained via the Green-Kubo (GK) method is found to be largely independent of the electrode surface charge. Interestingly, EMI + BF À 4 shows higher GK ion conductivity than the 1.1 M acetonitrile solution between two parallel plate electrodes.
Role of Carbon Porosity and Ion Size in the Development of Ionic Liquid Based Supercapacitors
Journal of The Electrochemical Society, 2011
The role played by carbon porosity and electrolyte chemistry in the development of double-layer supercapacitors based on solvent-free ionic liquids ͑ILs͒ of a wide electrochemical stability window is investigated. Voltammetric studies performed in N-methyl-N-butyl-pyrrolidinium bis͑trifluoromethanesulfonyl͒imide ͑PYR 14 TFSI͒, N-trimethyl-N-propylammonium bis͑trifluo-romethanesulfonyl͒imide, and N-methyl-N-butyl-pyrrolidinium tris͑pentafluoroethyl͒trifluorophosphate ionic liquids and PYR 14 TFSI-tetraethyl ammonium bis͑trifluoromethanesulfonyl͒imide solutions demonstrate that the pore-to-ion size ratio and the porous electrode/IL interface properties may have a higher impact on the electrode electrical response than do the inherent IL bulk properties. The effect of carbon porosity on the electrode capacitance and charge storage capability in both the positive and negative potential domains is discussed in relation to the IL properties, and an estimation of the upper limits of the performance of IL based supercapacitors with carbons of optimized porosity is reported.
Ether-Bond-Containing Ionic Liquids as Supercapacitor Electrolytes
The Journal of Physical Chemistry Letters, 2013
Electrochemical capacitors (ECs) are electrical energy storage devices that have the potential to be very useful in a wide range of applications, especially where there is a large disparity between peak and average power demands. The use of ionic liquids (ILs) as electrolytes in ECs can increase the energy density of devices; however, the viscosity and conductivity of ILs adversely influence the power density of the device. We present experimental results where several ILs containing different cations have been employed as the electrolyte in cells containing mesoporous carbon electrodes. Specifically, the behavior of ILs containing an ether bond in an alkyl side chain are compared with those of a similar structure and size but containing purely alkyl side chains. Using electrochemical impedance spectroscopy and constant current cycling, we show that the presence of the ether bond can dramatically increase the specific capacitance and reduce device resistance. These results have the important implication that such ILs can be used to tailor the physical properties and electrochemical performance of IL-based electrolytes. SECTION: Energy Conversion and Storage; Energy and Charge Transport E lectrochemical capacitors (ECs), often referred to as super-
Capacitance response of carbons in solvent-free ionic liquid electrolytes
Electrochemistry Communications, 2007
Ionic liquids (IL) are very promising ''solvent-free'' electrolytes for high-voltage double-layer supercapacitors (EDLCs) and to this purpose they are generally selected on the basis of their bulk properties, such as electrochemical stability and ion conductivity, without taking into account those of the electrified electrode-IL interface. This interface, which has yet to be well characterized, has features that notably affect electrode capacitance, and our paper for the first time highlights the importance of the molecular chemistry and structure of the ions for the double-layer capacitive response of carbonaceous electrodes in IL. The double-layer capacitive responses of negatively charged electrodes based on activated carbons and aero/cryo/xerogel carbons in two ILs featuring the same anion and different cations of almost the same size, i.e. the N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR 14 TFSI) and 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (EMITFSI) are reported. The porosity, structure and surface chemistry of the carbons are compared to their capacitive response to evince the role played by these carbon properties and by the chemistry and structure of the IL ions in the electric double-layer.
Protic ionic liquids (PILs) were implemented as electrolytes for supercapacitors using activated carbons with various porous textures as electrode material. The carbon with the largest specific surface area and highest amount of narrow mesopores (pore diameter: 2-7 nm) was found to give the highest specific capacitance in pyrrolidinium nitrate (PyNO 3 ) ionic liquid. However, it should be noted that when the pH value of this ionic liquid was adjusted around 11, higher specific capacitance was achieved, revealing a better electrochemical performance of carbon electrodes in basic media (i.e., capacitance values of 121 and 208 F g −1 for an electrolyte based on PyNO 3 with a pH value of 7 and 11, respectively). This ionic liquid contained a small amount of water, which restricted the maximum voltage of symmetric capacitors to a value of 1.2 V, even after PyNO 3 had been partially dried (H 2 O content around 1110 ppm). Therefore, the triethylammonium bis(trifluoromethylsufonyl)imide -NEt 3 H TFSI -PIL was prepared in order to expand the potential window; after drying this PIL contained 200 ppm water. The results obtained with NEt 3 H TFSI suggest that maximum voltages as high as 2.5 V can be achieved. This clearly shows that the presence of water in PILs has a negative effect on the performance of supercapacitors.
ACS applied materials & interfaces, 2016
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 (imidazoli...
The Journal of Physical Chemistry Letters, 2013
The potential pathways to increase the energy storage in electric double-layer (EDL) supercapacitors using room-temperature ionic liquid electrolytes and carbon-based nanostructured electrodes are explored by molecular dynamics simulations. A systematic comparison of capacitances obtained on nanoparticles of various shape and dimensions showed that when the electrode curvature and the length scale of the surface roughness are comparable to ion dimensions, a noticeable improvement in the capacitive storage is observed. The nanoconfinement of the electrolyte in conductive electrode pores further enhances the capacitance due to mismatch in ion−electrode surface interactions and strong electrostatic screening. We show that nanoporous structures made of arrays of conductive carbon chains represent a synergy of all three favorable factors (that is, high curvature, atomic scale roughness, and nanoconfinement) and can generate non-Faradic capacitance ranging from 260 to 350 F/g, which significantly exceeds the performance of the current generation of nanostructured electrodes. SECTION: Energy Conversion and Storage; Energy and Charge Transport E lectric double-layer capacitors (EDLCs or supercapacitors) have been extensively explored as promising energy storage devices in which the charge/energy is stored through ion rearrangement in the interfacial layer between the electrode and electrolyte. Substantial efforts have been dedicated to improve the energy density of EDLCs through design of nanostructured electrodes with a high specific surface area and understanding the interplay between various phenomena defining the performance of these devices. Several experimental, theoretical, and simulation studies conducted on this topic over the past few years clearly demonstrated the complexity of physical phenomena and correlations governing the EDL capacitance. For example, it was shown that the capacitance could be strongly influenced by thermodynamic conditions, 1 electrolyte chemical structure, 2−4 and nanoconfinement, 5,6 as well as by the electrode surface roughness/nanostructure 7,8 and conductivity. While our detailed understanding of these complex systems is still in its infancy, there is sufficient evidence that envisioned breakthroughs for supercapacitors must come from the optimal design of nanostructured electrode/electrolyte combinations.