Electrical double layer capacitors with sucrose derived carbon electrodes in ionic liquid electrolytes

Activated carbons were prepared via a pyrolysis of sucrose followed by activation in the stream of CO₂ gas for 2-6 h at 900 °C to tune the pore size distribution (PSD) and increase the specific surface area (SSA). The porosity of the activated sucrose derived carbons (ASCs) has been characterized using N₂ sorption measurements. Increasing activation time led to the significant increase in SSA and pore volume of ASCs, among which sucrose derived carbon with 6 h activation time (ASC-6 h) exhibited the highest SSA of 1941 m² g⁻¹ and the highest micropore volume of 0.87 cm³ g⁻¹. Cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and galvanostatic charge-discharge cycle tests have been applied to investigate the capacitive performance of the ASC electrodes in ionic liquids (ILs) at room and elevated temperatures. The ASC-6 h electrodes in ethyl-dimethyl-propyl-ammonium bis (trifluoromethylsulfonyl) imide (EdMPNTf₂N) showed specific capacitance in excess of 170 F g⁻¹ at 60 °C, whereas the same electrodes in 1-ethyl-3-methylimidazolium tetrafluoroborate (EMImBF₄) showed slightly lower capacitance but significantly better rate performance.     

Mesoporous activated carbon fiber as electrode material for high-performance electrochemical double layer capacitors with ionic liquid electrolyte

Activated carbon fibers (ACFs) with super high surface area and well-developed small mesopores have been prepared by pyrolyzing polyacrylonitrile fibers and NaOH activation. Their capacitive performances at room and elevated temperatures are evaluated in electrochemical double layer capacitors (EDLCs) using ionic liquid (IL) electrolyte composed of lithium bis(trifluoromethane sulfone)imide (LiN(SO₂CF₃)₂) and 2-oxazolidinone (C₃H₅NO₂). The surface area of the ACF is as high as 3291 m² g⁻¹. The pore volume of the carbon reaches 2.162 cm³ g⁻¹, of which 66.7% is the contribution of the small mesopores of 2-5 nm. The unique microstructures enable the ACFs to have good compatibility with the IL electrolyte. The specific capacitance reaches 187 F g⁻¹ at room temperature with good cycling and self-discharge performances. As the temperature increases to 60 °C, the capacitance increases to 196 F g⁻¹, and the rate capability is dramatically improved. Therefore, the ACF can be a promising electrode material for high-performance EDLCs.     

Room temperature molten salt as electrolyte for carbon nanotube-based electric double layer capacitors

Electric double layer capacitors (EDLCs) have been assembled with carbon nanotubes (CNTs) as the electrodes and a novel binary room temperature molten salt (RTMS) composed of lithium bis(trifluoromethane sulfone)imide (LiN(SO₂CF₃)₂, LiTFSI) and acetamide as the electrolyte. The electrochemical performances of the RTMS and the EDLC are evaluated with cyclic voltammetry (CV), ac impedance spectroscopy and galvanostatic charge/discharge, etc. The EDLC with these components show excellent electrochemical properties in specific capacitance, rate and cycling performances at ambient and elevated (60 °C) temperatures, indicating that RTMS is a promising electrolyte for advanced EDLCs.     

Application of activated carbon/DNA composite electrodes to aqueous electric double layer capacitors

A novel capacitor electrode auxiliary, deoxyribonucleic acid (DNA), is applied to an electric double layer capacitor (EDLC) containing an aqueous 3.5 M NaBr electrolyte. The present electrode is composed of activated carbon (95 wt.%) and DNA (2.5 wt.%) with polytetrafluoroethylene (PTFE) as a binder (2.5 wt.%). An EDLC cell with the DNA-loading electrodes exhibits improved rate capability and discharge capacitance. An EDLC cell with DNA-free electrodes cannot discharge above a current density of 3000 mA g⁻¹ (of the electrode), while a cell with the DNA-loading electrodes can work at least up to 6000 mA g⁻¹. Moreover, an open-circuit potential (OCP) of the DNA-loading electrode sifts negatively with ca. 0.2 V from an OCP of the corresponding electrode without DNA. It is noteworthy that a small amount of DNA loading (2.5 wt.%) to the activated carbon electrode not only improves the rate capability but also adjusts the working potential of the electrode to a more stable region.     

Significantly enhanced charge conduction in electric double layer capacitors using carbon nanotube-grafted activated carbon electrodes

Carbon nanotube (CNT)-grafting by chemical vapor deposition was conducted to reduce the resistance of activated carbon fiber serving as an electrode for electric double layer capacitors. Sputtering deposition of Ni catalyst particles led to a uniform growth of CNTs on the carbon fiber surface through the tip-growth mechanism. Because sputtering deposition ensures little pore blockage (in comparison with wet-impregnation), the surface area decrease of the carbon fiber due to Ni loading was minimized. By using H₂SO₄ aqueous solution as the electrolyte, a capacitor cell assembled with the CNT-grafted fiber showed higher electron and electrolyte-ion conductivities relative to a cell assembled with the bare fiber. By increasing the discharging current density from 1 to 150 mA cm⁻², the bare fiber exhibited a capacitance loss of 17% while the CNT-grafted fiber showed a mitigated capacitance loss of only 7%. This developed CNT-grafting technique renders activated carbon fiber a promising electrode material for a variety of electrochemical applications.     

3D-honeycomb carbons for high performance electrical double layer capacitors electrodes

 Sodium fulvic acid based hierarchical porous carbons (SFA-HPCs) with a specific surface area of 1919 m2/g and total volume of 1.7 cm3·g^–1 has been synthesized by a simple self-template method. The carbon skeleton can be formatted by the decomposition process of sodium fulvic acid (SFA) in a N₂ atmosphere. The sodium compund in SFA is used as a self-template to create the hierarchical porous structure. The unique hierarchical structure of SFA-HPCs provides an efficient pathway for electrolyte ions to be diffused into the internal surfaces of bulk electrode particles. It results in a high charge storage capacitance of 186 F·g^–1 at current load of 40 A·g^–1. The capacitance 230 F·g^–1 at 0.05 A·g^–1 and 186 F·g^–1 at 40 A·g^–1 show its high rate capability. Besides, it also achieves desirable cycling stability, 99.4% capacitance remained after 10000 cycles at 40 A·g^–1.     

High performance electrochemical capacitors from aligned carbon nanotube electrodes and ionic liquid electrolytes

We report a new class of electrochemical capacitors by utilizing vertically aligned carbon nanotubes as the electrodes and environmentally friendly ionic liquids (ILs) as the electrolytes. With their vertically aligned structures and well spacing, aligned carbon nanotubes showed a strong capacitive behavior in the ionic liquid electrolyte. Plasma etching played an important role in opening the end tips of nanotubes and in introducing defects and oxygenated functionalization to the nanotubes, further enhancing the capacitive behavior of carbon nanotubes. With the combined contribution from double-layer capacitance and redox pseudocapacitance, carbon nanotubes showed a remarkable capacitance in ionic liquid electrolyte. Combining the highly capacitive behavior of carbon nanotube electrodes with the large electrochemical window of ionic liquid electrolytes, the resultant capacitors showed a high cell voltage, high energy density, and high power density, potentially outperforming the current electrochemical capacitor technology. The device configuration incorporating vertically aligned nanostructured electrodes and inherently safe electrolytes would be useful for improving performances for new energy storage technologies.     

New design of electric double layer capacitors with aqueous LiOH electrolyte as alternative to capacitor with KOH solution

Activated carbon (AC) fiber cloths and a hydrophobic microporous polypropylene (PP) membrane, both modified with lithiated acetone oligomers, were used as electrodes and a separator in electric double layer capacitors (EDLCs) with aqueous lithium hydroxide (LiOH) as the electrolyte. Electrochemical characteristics of EDLCs were investigated by cyclic voltammetry (CV), galvanostatic charge-discharge cycle tests and impedance spectroscopy (EIS), compared with a case of the capacitor with aqueous potassium hydroxide (KOH) as an electrolyte. As a result, the capacitor with LiOH aqueous solution and a modified separator and electrodes was found to exhibit higher specific capacitance, maximum energy stored and maximum power than that with KOH aqueous solution.     

Activation of raw pitch coke with alkali hydroxide to prepare high performance carbon for electric double layer capacitor

Powder of raw pitch coke was activated with alkali hydroxides at 500–900 °C to prepare carbon electrode of high capacitance for electric double layer capacitor (EDLC). KOH provided very high surface area of 2320 m²/g at 800 °C, while NaOH did moderate surface area of 1000 m²/g at 650–750 °C. High surface area provided by KOH led to a high capacitance per weight of 39 F/g. However, its capacitance per volume was as low as 16 F/ml. Although the coke of moderate surface area activated with NaOH showed a similar capacitance per weight, its capacity per volume was as high as 28 F/ml because of its high density. Adequate porosity must be selectively introduced by NaOH activation to the coke to obtain moderate surface area. Much smaller expansion of layers in the present needle type coke activated by NaOH than that by KOH is indicative for the higher density of the former activated coke.     

Polymer/graphite oxide composites as high-performance materials for electric double layer capacitors

A single graphene sheet represents a carbon material with the highest surface area available to accommodating molecules or ions for physical and chemical interactions. Here we demonstrate in an electric double layer capacitor the outstanding performance of graphite oxide for providing a platform for double layer formation. Graphite oxide is generally the intermediate compound for obtaining separated graphene sheets. Instead of reduction with hydrazine, we incorporate graphite oxide with a poly(ethylene oxide)-based polymer and anchor the graphene oxide sheets with poly(propylene oxide) diamines. This polymer/graphite oxide composite shows in a “dry” gel-electrolyte system a double layer capacitance as high as 130 F g⁻¹. The polymer incorporation developed here can significantly diversify the application of graphene-based materials in energy storage devices.     

Effects of the cationic structures of fluorohydrogenate ionic liquid electrolytes on the electric double layer capacitance

Electric double layer capacitance of an activated carbon electrode has been measured for fluorohydrogenate ionic liquids (FHILs) based on five different cations (1,3-dimethylimidazolium (DMIm⁺), 1-ethyl-3-methylimidazolium (EMIm⁺), 1-butyl-3-methylimidazolium (BMIm⁺), 1-ethyl-1-methylpyrrolidinium (EMPyr⁺), and 1-methoxymethyl-1-methylpyrrolidinium (MOMMPyr⁺)) at 25 °C. For all the FHILs, the capacitance increases with increase in charging voltage, and exhibits the maximum value around 2.7 V. The capacitances for FHILs are higher than those for EMImBF₄ or 1 M tetraethylammonium tetrafluoroborate in propylene carbonate (TEABF₄/PC) in the measured range (1.0 < V < 3.2). For the three imidazolium-based FHILs, the maximum capacitance decreases with increase in the size of the cation in the order, DMIm(FH)_2.3F (178 F g⁻¹) > EMIm(FH)_2.3F (162 F g⁻¹) > BMIm(FH)_2.3F (135 F g⁻¹). On the other hand, the maximum capacitance observed for MOMMPyr(FH)_2.3F (152 F g⁻¹) is larger than that for EMPyr(FH)_2.3F (134 F g⁻¹) in spite of the larger size of MOMMPyr⁺ than EMPyr⁺, which is derived from introduction of the methoxy group. Some FHILs with low melting points exhibit a sufficient capacitance even at −40 °C (64 F g⁻¹ for EMIm(FH)_2.3F).     

Matrix circuit model for an electric double layer capacitor

A mathematical model of an electric double layer capacitor is developed treating the capacitance as a matrix instead of a scalar. Model explicitly demonstrates that when two electrodes are immersed an electrolyte and a potential difference applied, a stable double layer that store energy is created. It is suggested that supercapacitors could be modeled on the basis of capacitance matrices whose elements parameterize the geometry of the porous electrode.     

Electrochemical cell studies based on non-aqueous magnesium electrolyte for electric double layer capacitor applications

Performances of electric double layer capacitors (EDLCs) based on an activated carbon electrode with acetonitrile (ACN), propylene carbonate (PC), or a ternary electrolyte, i.e., PC:ethylene carbonate (EC):diethyl carbonate (DEC), at 1 mol dm⁻³ of magnesium perchlorate [Mg(ClO₄)₂] salt have been investigated. The electrochemical responses were studied by impedance spectroscopy, cyclic voltammetry, and galvanostatic charge-discharge experiments at 25 °C in a three-electrode configuration. For a comparative evaluation, lithium perchlorate (LiClO₄) salt-based systems were also evaluated. All the observed results showed typical EDLC characteristics within the potential range between 0 and 1 V vs. an Ag/Ag⁺ reference electrode. The Mg-based systems exhibited similar or rather better performances than the corresponding Li-based electrolytes; in particular, the rate capability of Mg-based ACN and PC electrolytes was much better than the corresponding Li-based electrolytes, indicating the high accessibility and utility of activated carbon pores by solvated Mg ions.     

A thin layer including a carbon material improves the rate capability of an electric double layer capacitor

We present a new method to improve the rate capability of an electric double layer capacitor (EDLC) using a thin polymer layer having a high concentration of carbon material on a current collector (CLC). A novel thermocuring coating composed of a glycol-chitosan, a pyromellitic acid and a conductive carbon powder can form stable CLC on a metal foil current collector simply by spreading and curing at 160 °C for a couple of minutes. We compared the performance of some demonstration EDLC cells using three kinds of current collector: a conventional aluminum oxide foil for EDLC, an aluminum foil and an aluminum foil with CLC. The cell with the CLC had a much higher rate capability than the cell without CLC. Only the CLC cell was able to discharge at a current density of 500C. This cell shows a slight deterioration in capacity in a high temperature, continuous charging, life test, and the CLC has a suppressing effect on the internal resistance increase of EDLCs. The use of a CLC film current collector is one of the most effective and simple methods for the improvement of EDLC rate performance. In particular, a current collector consisting of aluminum foil coupled with a CLC promises to be a low cost alternative to the aluminum oxide foil commonly used in EDLCs.     

LiTFSI-BEPyTFSI as an improved ionic liquid electrolyte for rechargeable lithium batteries

We report an electrochemical study of solutions of lithium bis(trifluoromethanesulfonyl)imide, LiTFSI, in a N-n-butyl-N-ethylpyrrolidinium bis(trifluoromethanesulfonyl)imide, BEPyTFSI. We show that these ionic liquid solutions have stability towards lithium metal electrode which allows various electrochemical tests, including impedance spectroscopy and voltammetry. The ionic conductivity and lithium transference number, of the order of 10⁻³ S cm⁻¹ and 0.4, respectively, make these solutions suitable for application as electrolytes in advanced lithium batteries. A prototype of these batteries, having lithium iron phosphate as the cathode, showed good performance in terms of charge–discharge efficiency and rate capability. The results reported in this work, although preliminary, are encouraging in supporting the practical interest of this LiTFSI-BEPyTFSI class of lithium conducting ionic liquids.     

POSS based ionic liquid as an electrolyte for hybrid electrochromic devices

The main objective of this study was to broaden the assortment of I⁻/I₃⁻ redox ionic liquids using polyhedral oligomeric silsesquioxanes (POSS) acting as nanobuilding blocks for the construction of functionalized 1,3-alkylimidazolium iodide solid (melting temperature 150-200 °C) and room temperature (RT) ionic liquids.The structural characteristics of the synthesised final ionic liquids and the corresponding intermediates were determined using ¹H, ²⁹Si NMR and infrared spectroscopic measurements. Raman spectra were next reported, in order to demonstrate the presence of polyiodides formed after the addition of iodine and the formation of redox electrolytes. Ionic conductivity values obtained from the impedance (EIS) spectra were determined in the temperature interval from room temperature up to 100 °C. Finally, a hybrid electrochromic cell was constructed from room temperature MePrIm⁺I_x⁻ IO₇ T₈ POSS (x=1, 1.2, 3 and 5) ionic liquids encapsulated between a lithiated WO₃ working and Pt counter-electrode, and colouring-bleaching changes assessed for cells cycled up to 1000 repetitive cycles.     

Electrochemical characteristics of new electric double layer capacitor with acidic polymer hydrogel electrolyte

The acidic polymer hydrogel electrolyte was prepared from 1 M H₂SO₄ aqueous solution, poly(vinyl alcohol) (PVA) and glutaraldehyde (GA). A new electric double layer capacitor (EDLC) with the polymer hydrogel electrolyte was assembled, and its electrochemical characteristics were investigated. As a result, the EDLC cell with the polymer hydrogel electrolyte exhibited almost the same discharge capacitance and high-rate dischargeability as that with a 1 M H₂SO₄ aqueous solution as an electrolyte. It was also found that the self-discharge was remarkably suppressed by using the polymer hydrogel electrolyte.     

On the addition of conducting ceramic nanoparticles in solvent-free ionic liquid electrolyte for dye-sensitized solar cells

Titanium carbide (TiC) is an extremely hard conducting ceramic material often used as a coating for titanium alloys as well as steel and aluminum components to improve their surface properties. In this study, conducting ceramic nanoparticles (CCNPs) have been used, for the first time, in dye-sensitized solar cells (DSSCs), and the incorporation of TiC nanoparticles in a binary ionic liquid electrolyte on the cell performance has been investigated.Cell conversion efficiency with 0.6 wt% TiC reached 1.68%, which was higher than that without adding TiC (1.18%); however, cell efficiency decreased when the TiC content reached 1.0 wt%. The electrochemical impedance spectroscopy (EIS) technique was employed to analyze the interfacial resistance in DSSCs, and it was found that the resistance of the charge-transfer process at the Pt counter electrode (R_ct1) decreased when up to 1.0 wt% TiC was added. Presumably, this was due to the formation of the extended electron transfer surface (EETS) which facilitates electron transfer to the bulk electrolyte, resulting in a decrease of the dark current, whereby the open-circuit potential (V_OC) could be improved. Furthermore, a significant increase in the fill factor (FF) for all TiC additions was related to the decrease in the series resistance (R_S) of the DSSCs. However, at 1.0 wt% TiC, the largest charge-transfer resistance at the TiO₂/dye/electrolyte interface was observed and resulted from the poor penetration of the electrolyte into the porous TiO₂. The long-term stability of DSSCs with a binary ionic liquid electrolyte, which is superior to that of an organic solvent-based electrolyte, was also studied.     

Evaluation of nafion based double layer capacitors by electrochemical impedance spectroscopy

In this work some electrochemical characteristics of all solid double layer capacitors prepared by high surface carbon and Nafion polymer electrolyte are reported. Carbon composite electrodes with a Nafion loading of 30 wt.% were prepared and evaluated. Nafion 115 membrane, recast Nafion membrane and 1 M H₂SO₄ solution in a matrix of glass fiber have been used as electrolyte, in the double layer capacitors. The different double layer capacitors (DLCs) have been evaluated by electrochemical impedance spectroscopy. The capacitor with a recast Nafion electrolyte exhibits a proton conductivity of about 3×10⁻² S cm⁻¹ at ambient temperature, that is higher of that reported for solid electrolytes (10⁻³ to 10⁻⁴ S cm⁻¹) in the current literature on capacitors. A maximum of specific capacitance of 13 F/g of active materials (carbon+Nafion) corresponding to 52 F/g for a single electrode measured in a three-electrode arrangement has been achieved with the capacitor with recast Nafion. The capacitance of the capacitor with recast Nafion electrolyte, evaluated in low-frequency region below 10 mHz, was practically equivalent at that with sulphuric acid electrolyte. The interpretation of the characteristics of the microporous structure of carbon material of the electrodes by impedance analysis is also discussed.     

Chitosan/polyaniline/MWCNT nanocomposite fibers as an electrode material for electrical double layer capacitors

Polyaniline–MWCNT nanocomposite has successfully been synthesized on the surface of chitosan wet-spun fibers by chemical oxidative polymerization. Morphological characterization of the nanocomposite fibers was performed by scanning electron microscopy (SEM). Electrochemical properties of the nanocomposite fibers as electrode material for electrical double layer capacitors (supercapacitors) in 0.5 M H₂SO₄ were studied by cyclic voltammetry (CV), galvanostatic charge/discharge, and electrochemical impedance spectroscopy (EIS) methods. The results showed that the nanocomposite fibers possess a specific capacitance of 14.48 F cm⁻² and a specific energy of 0.0013 Wh cm⁻² corresponding to a specific power of 0.011 W cm⁻². Total capacitance of the nanocomposite fiber consists of pseudocapacitance produced by the polyaniline and electrical double-layer capacitance produced by fiber|electrolyte interface and chitosan.