Fabrication of high-power electric double-layer capacitors

The electrochemical behavior of activated carbon/carbon (AC/C) composite electrodes was investigated for high-power electric doublelayer capacitors (EDLCs). It was found that high-rate charge/discharge characteristics are affected by the resistance of the electrolyte phase in the pores of the electrode. The charge/discharge characteristics were improved by optimizing the pore-size distribution of the electrodes. The size and total volume of the macro-pores in the electrodes were controlled by mixing and burning out polymer spheres. A high-power EDLC (15V, 470 F), which can discharge as much as 500 A, was fabricated by using improved AC/C composite electrodes.     

Structures and electrochemical performances of pyrolized carbons from graphite oxides for electric double-layer capacitor

The structural features and the electrochemical performances of pyrolized needle cokes from oxidized cokes are examined and compared with those of KOH-activated needle coke. The structure of needle coke is changed to a single phase of graphite oxide after oxidation treatment with an acidic solution having an NaClO₃/needle coke composition ratio of above 7.5, and the inter-layer distance of the oxidized needle coke is expanded to 6.9 Å with increasing oxygen content. After heating at 200 °C, the oxidized needle coke is reduced to a graphite structure with an inter-layer distance of 3.6 Å. By contrast, a change in the inter-layer distance in KOH-activated needle coke is not observed.     

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.     

Effect of treatment of activated carbon fiber cloth electrodes with cold plasma upon performance of electric double-layer capacitors

Charge/discharge behavior of electric double-layer capacitors composed of activated carbon fiber cloth (ACFC) electrodes and an organic electrolyte was investigated. The modification of the ACFC electrodes was performed using cold plasma generated in argon-oxygen atmosphere. The effect of the cold plasma treatment of the ACPC electrodes on the capacitor performance was discussed on the basis of the physical and chemical properties of the ACFC surface such as pore radius distribution and surface atom concentration.     

Cooperation of micro- and meso-porous carbon electrode materials in electric double-layer capacitors

The capacitive characteristics of micro- and meso-porous carbon materials have been compared in cyclic voltammetric studies and galvanostatic charge-discharge tests. Meso-porous carbon can keep certain high capacitance values at high scan rates, whereas micro-porous carbon possesses very high capacitance values at low scan rates but fades quickly as the scan rate rises up. For better performance of electric double-layer capacitors (EDLCs), the cooperative application of both kinds of carbon materials has been proposed in the following two ways: mixing both kinds of carbons in the same electrode or using the asymmetric configuration of carbon electrodes in the same EDLC. The cooperative effect on the electrochemical performance has also been addressed.     

Activated carbon prepared from PVDC by NaOH activation as electrode materials for high performance EDLCs with non-aqueous electrolyte

Mesoporous activated carbons with high surface area have been prepared from PVDC by NaOH activation for non-aqueous electric double layer capacitors (EDLCs). The BET surface area and pore volume of the carbon reach as high as 2675 m² g⁻¹ and 1.683 cm³ g⁻¹, respectively. The pore size of the carbon distributes mainly in small mesopore of 2∼4 nm, which is ideal for non-aqueous electrolyte EDLCs. The unique microstructure features, i.e. very high surface area and optimized pore size make the carbon present both a high capacitance of 155 F/g and outstanding rate capability in non-aqueous electrolytes. As the current density increases to 18?000 mA/g, it remains 109 F/g, an attractive value for EDLCs.     

Recycled waste paper—A new source of raw material for electric double-layer capacitors

For the first time, a new carbon–carbon composite electrode material for supercapacitors is prepared by simple KOH activation of waste newspaper. The amorphous nature and surface morphology of the carbon composite are investigated by X-ray diffraction (XRD), N₂ adsorption/desorption and scanning electron microscopy. The surface area and pore diameter are 416 m² g⁻¹ and 5.9 nm, respectively. Electrochemical characteristics are evaluated by cyclic voltammetry (CV) and charge–discharge tests in 6.0 M KOH at a 1 mA cm⁻² current density. The CV results reveal a maximum specific capacitance of 180 F g⁻¹ at a 2 mV s⁻¹ scan rate and the data explore a development of new use for waste paper into a valuable energy storage material.     

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.     

Electric double-layer capacitor using organic electrolyte

Electric double-layer capacitors (EDLCs) based on charge storage at the interface between a high surface area carbon electrode and a propylene carbonate solution are widely used as maintenance-free power sources for 1C memories and microcomputers. New applications for electric double-layer capacitors have been recently proposed. The popularity of these devices is derived from their high energy density relative to conventional capacitors and their long cycle life and high power density relative to batteries. The performance of the capacitor depends not only on the material used but also on the construction of the cells. The material, construction and performance of coin-type capacitors for memory protection and power capacitors for large power sources are described.     

Enhanced electrochemical double-layer capacitive performance with CO2 plasma treatment on activated carbon prepared from pyrolysis of pistachio shells

This study reports an original approach based on the CO₂ plasma treatment on modification of the chemical or physical properties of activated carbon(AC) from the pistachio shells as a waste for application as electrochemical double-layer capacitors(EDLC). In the AC production experiments, impregnation ratio, impregnation pre-treatment temperature, activation temperature and activation time are investigated. In the AC modification experiments with plasma treatment, the effects of plasma gases, plasma power and plasma time are performed. The results of the different conditions indicated that the structural properties of the obtained AC were significantly dependent on the plasma and pyrolysis parameters. The surface properties of the raw AC and plasma-treated AC (PTAC) with X-ray photoelectron spectroscopy (XPS), nitrogen adsorption technique, and scanning electron microscope (SEM) are characterized. Surface area values for the raw AC and PTAC are 768 and 1250 m² g⁻¹, respectively. A change in the peak positions and an increase in the percentage of oxygen of the AC treated with CO₂ plasma were obtained from XPS results. After 15 min of CO₂ plasma activation, a significant increase in the capacitance of up to about 141% was obtained as a 118.4 F g ⁻¹ compared to 49.98 F g ⁻¹ for untreated AC. The results show that the plasma treatment on the specific surface area and surface functional groups of AC has a significant impact.     

Carbon materials for electrochemical capacitors

The carbon materials used for electrochemical capacitors were reviewed and discussed the contribution of the surfaces owing to micropores and other larger pores to the capacitance and rate performance of the electric double-layer capacitors. The necessity to have an internationally accepted specification for the measurement of capacitor performance was emphasized.     

Concentrated NaClO4 aqueous solutions as promising electrolytes for electric double-layer capacitors

Concentrated NaClO₄ aqueous solutions have been proposed as the electrolytes in electric double-layer capacitors. The advantages of this kind of electrolytes have been addressed in the terms of enlarged specific capacitance, enhanced rate capability and elevated low-temperature performance of porous carbon electrodes. Thermal analysis, ionic conductivity measurement and Raman spectroscopic investigations have been performed on the NaClO₄ aqueous solutions in conjunction with the electrochemical study of porous carbon electrodes in this kind of electrolytes. The correlation between the hydration number of ions in the solutions and capacitive behavior of porous carbon has been clarified. The rate performance improvement in porous carbon electrode has also been connected to the increase in ionic conductivity of the electrolytes. The enhanced capacitance retention of porous carbon electrode at low temperatures in concentrated solutions has been ascribed to a fall of freezing point.     

Electrochemical performance of activated carbons prepared from rice husk in different types of non-aqueous electrolytes

Activated carbons (ACs) prepared from rice husk (RH), an agricultural byproduct, have mesoporosity that is obtainable from leaching of the mineral component of silica. To verify the suitability of RH-derived ACs for the use of electrode materials of electrical double-layer capacitors, we evaluated the electrochemical performance of three RH-derived ACs (two micro- and mesoporous ACs and one mesoporous AC). Evaluation was done by using the non-aqueous ionic electrolyte solutions 1 mol dm⁻³ triethylmethyl ammonium tetrafluoroborate/propylene carbonate (PC) solution, 1.5 mol dm⁻³ spiro-(1,1′)-bipyrrolidinium tetrafluoroborate/PC (SBP·BF₄/PC) solution, and the ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIm·BF₄). Under low voltage scan rate (1 mV s⁻¹) and low current density (<1 mA cm⁻²), mesoporous AC, which had the highest specific surface area, showed the highest specific capacitance (120 F g⁻¹) in EMIm·BF₄. However, its specific capacitance considerably decreased because of the increase in scan rate and current density. Under high scan rate (10 and 100 mV s⁻¹) and high current density (>10 mA cm⁻²), micro- and mesoporous AC in 1.5 mol dm⁻³ SBP·BF₄/PC showed the highest specific capacitance and highest retention of specific capacitance, even though its specific surface area was not the highest. Mesoporous AC showed voltage-dependent specific capacitance, indicating that ionic transport in the mesoporous structure was sensitive to electric field. It was finally shown that micro- and mesoporosity developed by utilizing natural structure and composition of RH was useful for the electrode materials of advanced electrical double-layer capacitors requiring more viscous non-aqueous electrolytes.     

Electric double-layer capacitors with sheet-type polarizable electrodes and application of the capacitors

Sheet-type polarizable electrodes with low sheet resistances for electric double-layer capacitors were outlined. The sheet-type electrodes consisted of activated carbon layers on aluminium foils. The sheet resistance of the sheet-type electrode mainly correlated with a filling density of activated carbons in the carbon layer. The species of activated carbons and the particle size of activated carbons affected the filling density of activated carbons in the layer. High filling ratio of activated carbons with small particle size resulted in the sheet-type electrodes with very low sheet resistance and high capacitance. Capacitors employing sheet-type electrodes showed very low internal resistance. Some application examples of the capacitor are described.     

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.     

Performance of mesoporous carbons derived from poly(vinyl alcohol) in electrochemical capacitors

The present work shows that mesoporous materials obtained by the carbonization of mixtures of poly(vinyl alcohol) with magnesium citrate are very promising candidates for electrodes in supercapacitors. Their high performance arises essentially from a double-layer mechanism through the extent of the total surface area and one obtains at low current density (1 mA cm⁻²) values as high as 180 F g⁻¹ in aqueous 2 M H₂SO₄ electrolyte and around 100 F g⁻¹ in 1 M (C₂H₅)₄NBF₄ in acetonitrile. Moreover, in most cases the specific capacitance is reduced only by 15% at 100 mA cm⁻², as opposed to many other types of carbons which display much higher reductions.     

Preparation of ordered mesoporous carbon nanopipes with controlled nitrogen species for application in electrical double-layer capacitors

Nitrogen-doped mesoporous carbon nanopipes with various nitrogen states are prepared by controlling the carbonization temperature. Nitrogen adsorption-desorption and transmission electron microscopy (TEM) analyses reveal that the optimum carbonization temperature is 1123 K. A carbonization temperature below 1073 K is insufficient to form a mesoporous carbon framework, while collapse of the carbon structure is observed above 1173 K. X-ray photoelectron spectroscopy measurements clearly show that nitrogen species are chemically transformed into pyrrolic and quaternary state species with higher binding energies. In cyclic voltammetry measurements, polar species of quaternary nitrogen on a carbon surface show a positive effect to enhance the capacitance via an increase in hydrophilicity and wettability of carbon by the electrolyte.     

Characterization of equivalent series resistance of electric double-layer capacitor electrodes using transient analysis

In this study, transient equations based on chronoamperometry (CA), chronopotentiometry (CP), electrochemical impedance spectroscopy (EIS) and imaginary capacitance analysis (ICA) are proposed using two equivalent circuit models for the purpose of accurate estimation of the equivalent series resistance (ESR) in electric double-layer capacitor (EDLC) electrodes. After examining transient equations based on a simple resistance-capacitance series connection, alternative equations with a more complicated form are proposed using the transmission line model. From these equations, it is theoretically predicted that one-third of the electrolyte resistance within the pores contributes to the total ESR, irrespective of the electrochemical analysis method employed. As EDLC electrode materials, mesoporous carbons with different pore structure (size, surface area) are prepared by the direct template method. After fabrication of EDLC electrodes using these materials, transient experiments using CA, CP, EIS, and ICA are conducted, and a consistent ESR is obtained. From ESR comparison, it is observed that the increase in ESR is mostly attributable to the electrolytic resistance in the pores and is highly correlated with the pore structure of the carbon electrodes. Additionally, it is found that a mesoporous carbon electrode with a 2-h reaction time exhibits an improved rate performance comparable with that of ordered mesoporous carbon electrodes prepared by the templating of ordered mesoporous silica.     

Influence of physical properties of activated carbons on characteristics of electric double-layer capacitors

Electrochemical characterization has been carried out for several activated carbons used as polarizable electrodes of electric double-layer capacitors in an aqueous electrolytic solution. The rest potential of the activated carbon was proportional to the logarithm of the oxygen content or to the concentration of the acidic surface functional groups of the activated carbon. The result of triangular voltage-sweep cyclic voltammetry was the same as that of the residual current measurement. The oxygen content and concentration of the acidic surface groups of activated carbon influenced the electrochemical characteristics of the activated carbon. Under anodic polarization, gas evolution was observed at the electrode surface of activated carbon with high oxygen content at 0.8 V versus saturated calomel electrode ( SCE). Gas evolution was not observed at the electrode surface of activated carbon with low oxygen content even to 1.0 V versus SCE. Under cathodic polarization of activated carbon with high oxygen content, the peak was observed at approximately −0.2 V versus SCE, but there was no gas evolution at the electrode surface of the activated carbon. Bubbles were not observed at the electrode surface of activated carbon with low oxygen content at −0.5 V versus SCE. Electric double-layer capacitors were made from activated carbons used for electrochemical measurements; load-life tests have been carried out. Thickness and internal resistance of the capacitor composed of activated carbon with high oxygen content increased. The changes in thickness and internal resistance of the capacitor composed of activated carbon with low oxygen content were small.     

Application of Si-C-O glass-like compounds as negative electrode materials for lithium hybrid capacitors

The Si-C-O glass-like compound (a-SiCO) was applied to a negative electrode of a lithium hybrid capacitor (LHC) with activated carbon positive electrodes. The performance as a negative electrode (by a three-electrode system) and LHC (by a two-electrode system) was evaluated in LiClO₄ (EC-DEC) and LiBF₄ (PC) electrolytes. With a-SiCO reversible insertion/extraction of lithium ions at high current densities (0.5-2.0 A g⁻¹) was possible. By prior short-circuiting of the negative electrode with lithium metal in the electrolytes for appropriate periods, the charge/discharge performance of the assembled LHC compared favorably with an electric double layer capacitor (EDLC) made of the activated carbon used for LHC. The cycle performance of the LHC was better but the capacitance was smaller in the LiBF₄ (PC) electrolyte than in LiClO₄ (EC-DEC) electrolyte. Smaller capacitance is mainly due to lower electric conductivity and higher viscosity of LiBF₄ (PC) electrolyte than LiClO₄ (EC-DEC) electrolyte. The energy density of the assembled LHC reached a maximum of about three times that of EDLC, with the power density comparable to that of the EDLC.