Conventional Electric Double-layer Capacitors Research Articles

Direct Ink Writing (DIW) printed high-performance asymmetric supercapacitor based on 0D@2D silver-nanoparticles@MXene as anode and 0D@2D MnO2-nanoparticles@MXene as cathode materials

Direct Ink Writing (DIW) printed high-performance asymmetric supercapacitor based on 0D@2D silver-nanoparticles@MXene as anode and 0D@2D MnO2-nanoparticles@MXene as cathode materials

Thermally stable bulk-type all-solid-state capacitor with a highly deformable oxide solid electrolyte

The development of all-solid-state capacitors (ASSCs) based on inorganic solid electrolytes (SEs) with high thermal stability is desired. However, because of their high cell resistance, such capacitors have lower capacitances and rate capabilities than conventional electric double-layer capacitors (EDLCs) that use aqueous solutions and organic liquid electrolytes. The high resistance is caused by the mechanical and electrochemical properties of the SE. In this study, a highly deformable Li-ion conducting oxide SE was investigated to improve the electrochemical performance of ASSCs. Bulk-type symmetric ASSCs, with a 33Li 3 BO 3 ·33Li 2 SO 4 ·33Li 2 CO 3 (LBSC) SE layer between two electrode layers of an LBSC-CNT composite, showed low resistance and were operable at 100–300 °C. Additionally, the highest capacitance at the highest current reported for ASSCs till date was achieved. The highly deformable SE will facilitate the design of ASSCs and expand the potential range of applications of EDLCs. • Thermally stable all-solid-state capacitor with oxide electrolyte is developed. • Highly-deformable electrolyte and carbon nanotube are mixed to obtain electrode. • The fabricated capacitor is operable at 100–300 °C with high capacity.

Production of a hybrid capacitive storage device via hydrogen gas and carbon electrodes coupling

Conventional electric double-layer capacitors are energy storage devices with a high specific power and extended cycle life. However, the low energy content of this class of devices acts as a stumbling block to widespread adoption in the energy storage field. To circumvent the low-energy drawback of electric double-layer capacitors, here we report the assembly and testing of a hybrid device called electrocatalytic hydrogen gas capacitor containing a hydrogen gas negative electrode and a carbon-based positive electrode. This device operates using pH-universal aqueous electrolyte solutions (i.e., from 0 to 14) in a wide temperature range (i.e., from − 70 °C to 60 °C). In particular, we report specific energy and power of 45 Wh kg−1 and 458 W kg−1 (both values based on the electrodes’ active materials mass), respectively, at 1 A g−1 and 25 °C with 9 M H3PO4 electrolyte solution. The device also enables capacitance retention of 85% (final capacitance of about 114 F g−1) after 100,000 cycles at 10 A g−1 and 25 °C with 1 M phosphate buffer electrolyte solution.

Surface capacitive charge storage in carbon nanodots-anchored hybrid halide perovskites

Abstract Surface faradaic process can bring about storage (pseudocapacitive) mechanism that store much more energy than conventional electric double-layer capacitors (ELDCs) quite rapidly. Many transition metal oxides possess pseudocapacitor properties, but only few are capable of providing a high power capability because of their low intrinsic conductivities. Here we demonstrate that lead- and bismuth-based hybrid halide perovskites, anchored with carbon nanodots, can operate at rates exceeding those of ELDCs and provide higher area normalized and volumetric capacitances than standalone halide perovskites and carbon dots. We report carbon nanodots in methyl ammonium bismuth iodide (CNDs/MBI) and methyl ammonium lead iodide (CNDs/MPI) hybrid electrodes, designed to facilitate electron transfer and cation accessibility to faradaic active sites. Devices based on CNDs/MBI and CNDs/MPI electrodes are able delivered up to 550 Fg−1 and 402 Fg-1, with corresponding energy densities of 76.5 Wh/kg and 55.8 Wh/kg at scan rates of 0.01 V−1. Deconvolution of the cyclic voltanogramm also provide insight to fraction of charge storage due to surface- and diffusion-controlled process.

Reduced graphene oxide decorated with SnO2 nanoparticles as negative electrode for lithium ion capacitors

Abstract The effort to increase the energy density of conventional electric double-layer capacitors (EDLCs) goes through the development of lithium-ion capacitors (LICs). Herein, we report a self-standing, binder-free composite as the battery-type negative electrode obtained by a low-cost and easily scalable method. Tin(IV) oxide nanoparticles (

Electrochemical Performance of Activated Carbon Based Electric Double Layer Capacitors Using 1-Ethyl-3-Methylimidazolium Dicyanamide Ionic Liquid and Its Gel Polymer Electrolyte

The supercapacitors have emerged as an important energy storage device as they can deliver a high specific power, long cycle life, and much higher charge-discharge rates than a battery. In particular, solid-state supercapacitors have gained worldwide attention as an emerging candidate for smart and efficient energy storage devices due to the increasing demand for miniaturized electronics. Electric double layer capacitors (EDLCs) are the most widely studied class of supercapacitors which physically stores charge by forming an electric double layer at the interfaces between the high surface area carbon electrode and the electrolyte. The conventional EDLCs using high surface area activated carbon electrodes and non-aqueous liquid electrolytes represent the current state of the art. The most common cause of failure of such devices is related to the electrolyte leakage. Most of the past work on EDLCs have been carried out using the liquid electrolytes (aqueous, organic and pure ionic liquid). The gel polymer electrolytes with high ionic mobility and good dimensional/mechanical stability, offer a viable substitute for the liquid electrolytes and they are being used for flexible and solid-state electrochemical energy storage devices. Here, we report the electrochemical performance of high surface area activated carbon based EDLCs using 1-ethyl-3-methylimidazolium dicyanamide (EMImDCA) ionic liquid and its poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF-HFP) gel counterpart as electrolytes. The gel electrolyte films were prepared by immobilizing the EMImDCA ionic liquid in PVdF-HFP copolymer matrix by solution casting method. The activated carbon electrodes were prepared by mixing the activated carbon powder with conductive additive (Super C65) and PVdF-HFP binder in a ratio of 8:1:1 (w/w/w), and then the mixture was drop-casted onto a flexible thin graphite sheet which serves as current collector. The solid-state EDLC cells were fabricated by sandwiching the gel electrolyte film between two identical activated carbon electrodes whereas EDLC cells with pure IL were fabricated by using fiberglass separator soaked with IL. The EDLC cells show highly capacitive behavior as evident from the cyclic voltammogram of the cells measured at a scan rate of 20 mV s-1 which is rectangular in shape (Fig 1a). However, gel electrolyte based solid-state cell shows slightly higher resistance, obvious from slight deviation from rectangular shape at both turning potential. Typical galvanostatic charge-discharge characteristic of the EDLC cells at current density of 5 mA cm-2is shown in Fig. 1b. The linear discharge characteristics of the cell further confirms the capacitive behavior of the cells due to formation of an electric double layer at the interfaces. The EDLC cell with pure IL shows specific capacitance of ~98 F g-1 whereas IL-gel based EDLC cell shows specific capacitance of ~86 F g-1, derived from charge-discharge curves at 5 mA cm-2. A comparative performance of the EDLC cells was carried our using the impedance spectroscopy, cyclic voltammetry, and galvanostatic charge-discharge techniques. This paper discusses the results of these comparative studies with pure IL and its PVdF-HFP gel electrolytes. Figure 1

Electrochemical Performance of Lithium-Ion Capacitors Using Pre-Lithiated Multiwalled Carbon Nanotubes as Anode

Pre-lithiated multiwalled carbon nanotube anode was prepared by internal short circuit approach(ISC) for 5[Formula: see text]min, 30[Formula: see text]min, 60[Formula: see text]min and 120[Formula: see text]min respectively. Lithium ion capacitors (LICs) were assembled by using pre-lithiated multiwalled carbon nanotubes as anodes and activated carbon (AC) as cathodes. The structure of multiwalled carbon nanotubes and electrodes were investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The electrochemical performance of pre-lithiated multiwalled carbon nanotube electrodes and pristine carbon nanotube electrodes were tested by galvanostatic charge/discharge and electrochemical impedance. The results indicated that pre-lithiation carbon nanotubes greatly improved the charge/discharge performance of LICs. The energy density was four times than conventional electric double-layer capacitors (EDLCs) at the current density of 100[Formula: see text]mA/g. The LICs achieved a specific capacitance of 59.3[Formula: see text]F/g at the current density of 100[Formula: see text]mA/g with 60[Formula: see text]min pre-lithiatiation process. The maximum energy density and power density was 96[Formula: see text]Wh/kg and 4035[Formula: see text]W/kg, respectively. The energy density still remained about 89.0% after 1000 cycles. The LIC showed excellent supercapacitor performance.

The High-Performance Asymmetric Capacitors with Lithium-Intercalated Metal-Organic Framework Electrodes

Text: High-power energy storage devices can provide an improvement of drivability and a load-levelling system in an electric vehicle, a hybrid electric vehicle, and a fuel-cell vehicle.1 Asymmetric capacitors known as lithium-ion capacitors (LIC), in which a faradaic, battery-type electrode is combined with a non-faradaic, capacitor-type electrode, are up-and-coming devices because they show faster charge and discharge responses than lithium ion batteries and larger energy densities than electric double-layer capacitors (EDLC).1 Safety is also an important factor for the onboard devices. However, LICs using conventional graphite carbon negative electrodes operating at 0.1 V have the risk of a metallic Li deposition causing internal short circuits. Thus, we have studied lithium-intercalated metal-organic frameworks (iMOFs)2 and reported the novel negative electrode of 2,6-naphthalene dicarboxylate dilithium (2,6-Naph(COOLi)2), whose operating potential (0.8 V vs. Li/Li+) is expected to suppress a metallic Li deposition. In this report, we will introduce a novel iMOF negative electrode for asymmetric capacitors, a 4,4’-biphenyl dicarboxylate dilithium (4,4’-Bph(COOLi)2), which is fast responsible because of the remarkably low internal resistances. 4,4’-Bph(COOLi)2 can be prepared from biphenyl-4,4’-dicarboxilic acid by the deprotonation in LiOH under methanol solution. The powder X-ray diffraction of 4,4’-Bph(COOLi)2 and the corresponding Rietvelt refinement revealed the repeating structure of organic p-stacked biphenyl moieties and inorganic tetrahedral LiO4 units (Figure 1a). 4,4’-Bph(COOLi)2 has a larger lattice parameter of the a-axis direction than 2,6-Naph(COOLi)2because of the longer aromatic unit. The charge/discharge curve of a Li/4,4’-Bph(COOLi)2 cell indicates that the 4,4’-Bph(COOLi)2 electrode shows reversible two-electron transfer reaction at a potential of 0.7 V vs. Li/Li+, which is 0.1 V lower operating potential than that of 2,6-Naph(COOLi)2 (Figure 1b). Compared to the polarization of the 2,6-Naph(COOLi)2 electrode, 4,4’-Bph(COOLi)2 electrode exhibits small and large polarization in the deep and shallow depth of discharge (DOD), respectively (Figure 1b). For further study, the internal resistances of 2,6-Naph(COOLi)2 and 4,4’-Bph(COOLi)2 electrodes were examined by using the electrochemical impedance spectroscopy using symmetric cells. The symmetric cells were constructed of two iMOF electrodes lithiated up to DOD 50% under the same condition. The results showed the charge transfer resistances of 4,4’-Bph(COOLi)2 were lower than 2,6-Naph(COOLi)2. The activation energy of the charge transfer resistance obtained from Arrenius plots was slight lower in 4,4’-Bph(COOLi)2 than in 2,6-Naph(COOLi)2 or graphite. The low internal resistance with low activation energy of 4,4’-Bph(COOLi)2electrode seems to be suitable for the negative electrode of high-power devices, and can lead a high-rate performance even at a low temperature. The asymmetric capacitors composed of the positive electrodes of active carbon (AC), and the negative electrodes of 4,4’-Bph(COOLi)2 (4,4’-Bph(COOLi)2/AC cell) or 2,6-Naph(COOLi)2 (2,6-Naph(COOLi)2/AC cell) were prepared. The 4,4’-Bph(COOLi)2/AC cell showed 56 Wh L－ 1 energy density at 1C and 2.3 kW L－ 1 power density at 60C (Figure 1d). 4,4’-Bph(COOLi)2/AC cell showed higher energy density at 60C than conventional EDLC, LIC4, LTO/AC cell, and 2,6-Naph(COOLi)2/AC cell. Interestingly, 4,4’-Bph(COOLi)2/AC cell maintained a high-rate performance at a low temperature of 0°C compared with 2,6-Naph(COOLi)2/AC cell, which could derive from the low internal resistance with the low activation energy of 4,4’-Bph(COOLi)2. The capacity retention of 4,4’-Bph(COOLi)2/AC cell was 86 % after 1000 cycles at 10C. In summary, the asymmetric capacitor with 4,4’-Bph(COOLi)2electrode can be safer than the LIC using graphite, and showed high power capability and favorable low-temperature performance, therefore our proposed capacitors will be promising for the future capacitors.

Hydrothermal express synthesis of CNT/MnO2 composite for asymmetric supercapacitor

A hydrothermal technique for the express synthesis of a composite based on carbon nanotubes (CNTs) and manganese dioxide by means of the direct reduction of potassium permanganate from an aqueous isopropanol solution is proposed in this work. The synthesis conditions (time, temperature, ratios between reagents) are optimized; the primary and secondary processes taking place in the system are considered in detail. The synthesized CNT/MnO2 composite is studied with the help of a wide range of physicochemical and electrochemical methods to assess the possibility of its use as a positively charged electrode of an asymmetric supercapacitor (SC). The assembled SC cell based on this composite and an activated carbon (AC) electrode has fairly high power typical of conventional electric double-layer capacitors (0.82 kW/kg) with a twofold increase in the energy density (12.14 W h/kg).

New Generation High Performance Lithium-Ion Capacitor Laminate Cells

In recent years considerable research has been focused on the development of high energy density EC capacitors. Among all the energy storage systems that have been investigated and developed in the last few years, Lithium-ion Capacitors (LICs) have emerged to be one of the most promising because LICs achieve higher energy density than conventional Electric Double-Layer Capacitors (EDLCs), and better power performance than Li-ion batteries (LIBs) as well being capable of long cycle life which is shown in Fig. 1. LICs contain a pre-lithiated LIB anode electrode and an EDLC cathode electrode. Previously, we have reported a LIC with activated carbon (AC) cathode and hard carbon (HC)/stabilized lithium metal powder (SLMP) anode electrodes with high energy density, high power density and long cycle life1-7. From all the study before, we have achieved a high energy density LIC pouch cell4 with an energy density as high as 30 Wh kg-1 but only with a usable specific power of about 1.0 kW kg-1 and a maximum specific power of 2.0 kW kg-1. Therefore in this paper, we wish to report new generation high performance LIC laminate cells with higher power density and longer cycle life by sacrificing the energy density of the cells. The low temperature performance of the LIC laminate cells is also included in this paper. It can be concluded from the following data figures (from Fig. 2 to Fig. 4) that the specific energy and energy density as high as 20 Wh kg-1 and 38 Wh L-1 have been achieved, respectively. The laminate cells obtained a maximum specific power of 7.8 kW kg-1, respectively. The developed LICs can operate in a temperature as wide as from -40 degree C to +70 degree C. After 50,000 cycles, the LIC laminate cells still have 92% of the initial discharge capacity.

New Generation High Performance Lithium-Ion Capacitor Laminate Cells

In recent years considerable research has been focused on the development of high energy density EC capacitors. Among all the energy storage systems that have been investigated and developed in the last few years, Lithium-ion Capacitors (LICs) have emerged to be one of the most promising because LICs achieve higher energy density than conventional Electric Double-Layer Capacitors (EDLCs), and better power performance than Li-ion batteries (LIBs) as well being capable of long cycle life which is shown in Fig. 1. LICs contain a pre-lithiated LIB anode electrode and an EDLC cathode electrode. Previously, we have reported a LIC with activated carbon (AC) cathode and hard carbon (HC)/stabilized lithium metal powder (SLMP) anode electrodes with high energy density, high power density and long cycle life1-7. From all the study before, we have achieved a high energy density LIC pouch cell4 with an energy density as high as 30 Wh kg-1 but only with a usable specific power of about 1.0 kW kg-1 and a maximum specific power of 2.0 kW kg-1. Therefore in this paper, we wish to report new generation high performance LIC laminate cells with higher power density and longer cycle life by sacrificing the energy density of the cells. The low temperature performance of the LIC laminate cells is also included in this paper. It can be concluded from the following data figures (from Fig. 2 to Fig. 4) that the specific energy and energy density as high as 20 Wh kg-1 and 38 Wh L-1 have been achieved, respectively. The laminate cells obtained a maximum specific power of 7.8 kW kg-1, respectively. The developed LICs can operate in a temperature as wide as from -40 degree C to +70 degree C. After 50,000 cycles, the LIC laminate cells still have 92% of the initial discharge capacity.

Effect of Anodic Deposition Parameters on Electrochemical Behavior and Microstructure of Mn-Ni Oxide As a Pseudocapacitive Electrode

Hybrid supercapacitors, or briefly pseudocapacitors, are an emerging class of energy storage devices, with the capability of providing both high power and energy density. In contrast to conventional electric double layer capacitors, which accumulate charge mostly through an electrostatic mechanism, pseudocapacitors utilize the pseudocapacitance arising from reversible Faradic reactions occurring at the electrode surface [1]. Although various metal oxides have been investigated as active material for pseudocapacitors, greater attention has been given to manganese oxides. In recent years, also Mn-based binary oxides have been studied for pseudocapacitance, in particular, mixed oxides comprising Ni or Co as the other component [2-4] In the present work, Mn-Ni oxide thin films were deposited potentiodynamically at scan rate of 100 mVs-1, pH=7 and room temperature, on a stainless steel substrate. Undoubtedly, a survey of the literature shows that a systematic study on the effect of operating parameters on the properties of the mixed oxide thin films is still lacking. Accordingly, this work was undertaken with the aim of exploring the processing space for these oxides studying in particular the effects of the electrolyte Ni to Mn molar ratio, the deposition peak potential and the deposited mass on chemical composition, microstructure and capacitance behavior of Mn-Ni oxide thin films. The binary oxide deposits were characterized by energy dispersive X-ray spectroscopy (EDX) and scanning electron microscopy (SEM). The electrochemical behavior of the Mn-Ni oxide electrodes was studied by cyclic voltammetry (CV) at 5, 20, 50 and 100 mVs-1 and electrochemical impedance spectroscopy (EIS) in 1 M Na2SO4electrolyte. Expectedly, with increasing the Ni to Mn molar ratio in solution (from 0.1 to 10), the molar fraction of Ni in the oxide film was found to increase, resulting, on the one hand, in smoothing of the surface morphology and, on the other hand, in a change of the specific capacitance through a maximum of 145 Fg-1 at approximately 12% at Ni fraction and CV scan rate of 20 mVs-1. With further increase of the Ni fraction in the oxide to about 19% at, the specific capacitance started to decrease gradually. Changing the peak potential in the potentiodynamic deposition from +1.2 to +1.6 V vs. Ag/AgCl, the CV curves revealed a box-like shape for all values of the peak potential, while the capacitance showed a decreasing trend. EIS results revealed that charge transfer resistance increased with increasing peak potential and the lowest Rct was found for sample prepared at peak potential of 1.2 V vs. Ag/AgCl. The mass load per unit area of the film was controlled by the number of cycles. Finally, specific capacitance of 196 Fg-1 was obtained at scan rate of 5 mVs-1 for the Mn-Ni oxide film deposited potentiodynamically (5 cycles, 100 mVs-1, pH=7 and RT) in solution with Ni to Mn molar ratio of 2. At the high scan rate of 100 mVs-1, the specific capacitance was 135 Fg-1 showing good retention of the capacitance at high rate.

A Metal-Free Organic Aqueous Capacitor Using Quinonic Compounds Couple

Redox-capacitors are electrochemical energy storage devices using the electric double layer capacity and pseudo-capacity derived from redox reactions. Because of its higher energy density than conventional electric double layer capacitors, higher power density than batteries and long cycle life, redox-capacitor is a promising candidate for grid-scale stationary energy storage device.We demonstrated the aqueous redox-capacitors using inexpensive quinonic and hydroquinonic organic compounds as redox active materials, which facilitate charge/discharge by proton insertion/extraction in aqueous electrolyte. These quinonic/hydroquinonic compounds are supported in nanoporous carbon. This capacitor exhibited high energy density (＞20 Wh kg-1) and high power density (＞5 A g-1) without the degradation after 10000 charge/discharge cycles [Sci. Rep. 4, 3591 (2014).].In this study, we investigated the dependence of loading rate of quinonic compound to carbon materials for further enhancement of energy density. It was found that quinonic compound, which is highly loaded in carbon (>70 wt%), can be utilized by adding a slight amount of carbon with high electrical conductivity. Finally, the high energy density of approximately 30 Wh kg-1 was achieved in the metal-free organic aqueous capacitor using quinonic compounds couple.

Comparison Study of Various Anode Materials for Li-Ion Capacitors

Lithium-ion Capacitors (LICs) have attracted much attention as a new power source due to its high energy density compared to conventional electric double-layer capacitors (EDLCs), and its high power density compared to batteries as well as having the long cycle life. Recently, some research work1-2 in our group has been done on LICs. In this abstract, various carbon material anode electrode including hard carbon, soft carbon and graphite were studied and compared for the Li-ion Capacitors application.To compare the hard carbon, soft carbon and graphite, three Li-ion capacitor full cells with these carbon electrodes were assembled to compare the power and cycle performance. Figure 1 presents the voltage profiles of the LIC full cells with various carbon electrodes. It can be seen that the swing potential of hard carbon and soft carbon anode electrodes is in a wider range than graphite electrodes. The swing potential range of hard carbon and soft carbon negative electrodes is 0.2-0.4 and 0.4-0.7 V vs. Li/Li+, respectively; which are above 0 V vs. Li/Li+ and can suppress the formation of lithium dendrites in anode to a large extent. On the other side, the potential plateau of graphite anode electrode is near 0 V vs. Li/Li+, which increases the possibility for lithium dendrites to form and short the LIC cells.The LIC full cells with various carbon electrodes were also tested in Figure 2. This was done in order to compare the energy, power and cycle performance. It can be obviously observed from Figure 2(a) that LIC full cell with hard carbon electrode displays both higher energy density and higher power density than the cells with soft carbon and graphite. For the cycle performance as shown in Figure 2(b), after 1000 cycles, the LIC cell with hard carbon can retain up to 100% while the LIC cells with soft carbon and graphite only retain up to 80% and 78% of the initial discharge energy density, respectively. Based on the evaluation of the electrochemical properties including energy, power and cycle performance of the various carbon electrodes, it is concluded that hard carbon is the best option for negative electrode, as it displays a higher energy density, power density and longer cycle life implemented in two-electrode Li-ion capacitor systems.

Surface engineering of self-assembled TiO2 nanotube arrays: A practical route towards energy storage applications

Abstract Here, we present a facile route to establish and control a net negative charge on the surface of TiO 2 nanotube as an enabling platform to achieve extraordinary capacitive behavior compared to conventional electric double layer capacitors. The engineering of the surface electrical properties of TiO 2 nanotube resulted in an overall improvement in electrochemical performance which is beneficial in a range of applications including wearable light weight electronic devices and energy storage applications. The maximum aerial capacitance of 1.6 mF cm −2 obtained here along with good capacitance retention of 97% after 1000 cycles demonstrate promising properties of these structures for applications in energy storage area.

Voltage characteristics and capacitance balancing for Li4Ti5O12/activated carbon hybrid capacitors

Asymmetric hybrid capacitors comprising an activated carbon (AC) positive electrode and a Li 4 Ti 5 O 12 (LTO) negative electrode can achieve much higher energy density than conventional electric double layer capacitors. However, when operated at 3 V, the positive electrode develops an over-potential, leading to gas generation. Here, we investigated the feasibility of realizing a stable potential at the positive electrode using capacitance balancing. The voltage characteristics of the positive and negative electrodes were measured using a three-electrode system. The linear relationship between the potential and capacitance of the AC electrode was used to balance the capacitances of the two electrodes. Thus, stable operation and enhanced specific capacity were obtained at an operation voltage of 3 V.

A highly ordered titania nanotube array as a supercapacitor electrode

We report the synthesis of self-organized titania nanotubes and nanocrystalline titania powders employing an alternative and novel approach. Integrating these nanostructures in a binder-free working electrode improved the capacitance up to 911 μF cm(-2), which is around one to two orders of magnitude higher than the conventional electric double layer capacitors.

電力用蓄電装置の基礎的研究

Storage batteries or pumped hydraulic power stations have been used to store electric energy. However, there are problems with storage batteries, e.g., long charging time, limited cycle life, low Coulomb efficiency, and inaccurate residual power meters. Because of these problems, practical power applications have not been achieved. To solve these problems, a complex system of capacitors and electronic circuits called the ECS (Energy Capacitor System) is proposed. The capacitor part of the ECS is a group of electric double-layer capacitors of increased energy density. The circuits part is built around switching regulators. Although the capacitors alone are not capable of delivering stable output, the accompanying circuits compensate for various characteristics throughout the charge and discharge cycle. This combination of capacitors and electronic circuits increases the energy density by about 16 times that of the conventional electric double-layer capacitors, close to the level of lead-acid batteries but with lower output resistance and better voltage stability. The ECS inherently features short charging time, very long cycle life, and good Coulomb efficiency. The cells are completely sealed and made from materials which are not harmful to the environment.