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All sourcesNASA NTRSDOE OSTI
DOE OSTI2013

Mesoscale modeling of electric double layer capacitors with three-dimensional ordered structures

This paper presents general mathematical formulations for simulating electric double layer capacitors (EDLCs) with three-dimensional ordered structures. For the first time, a general set of boundary conditions was derived in order to account for the Stern layer without simulating it in the computational domain. These boundary conditions were valid for planar, cylindrical, and spherical electrode particles or pores. They made possible the simulations of EDLCs with complex geometries while rigorously accounting for both the Stern and diffuse layers. The model also simultaneously accounted not only for 3D electrode morphology but also for finite ion size and field-dependent electrolyte dielectric permittivity. It was used to faithfully simulate the complex structure of an EDLC electrode consisting of ordered bimodal mesoporous carbon featuring both macropores and mesopores. Areal and gravimetric capacitances were predicted based on non-solvated and solvated ion diameters. These two cases set the upper and lower bounds for the predicted capacitances. The capacitances predicted using non-solvated ion diameter were found to be in good agreement with experimental data reported in the literature. All surfaces contributed to the overall capacitance of EDLCs. The gravimetric capacitance of different bimodal carbons increased linearly with increasing specific surface area corresponding to constant areal capacitance.

30 DIRECT ENERGY CONVERSION↗
DOE OSTI2012

Fabrication of flower-like Ni{sub 3}(NO{sub 3}){sub 2}(OH){sub 4} and their electrochemical properties evaluation

Graphical abstract: Ni{sub 3}(NO{sub 3}){sub 2}(OH){sub 4} nano-flakes materials, which have a flower-like structure, were successfully synthesized by a facile solvothermal method without adding any surfactant. The as-prepared Ni{sub 3}(NO{sub 3}){sub 2}(OH){sub 4} possesses a maximum specific capacitance of 2212.5 F g{sup −1} at the current density of 5 mA, suggesting its potential application in electrode material for secondary batteries and electrochemical capacitors. Highlights: ► Flower-like Ni{sub 3}(NO{sub 3}){sub 2}(OH){sub 4} materials were fabricated in a simple method. ► High specific capacitance of 2212.5 F g{sup −1} has been achieved. ► For the first time the effects of concentration and temperature on its specific capacitance has been studied. -- Abstract: Flower-like Ni{sub 3}(NO{sub 3}){sub 2}(OH){sub 4} was successfully synthesized by a facile solvothermal method. The microstructure and surface morphology of prepared Ni{sub 3}(NO{sub 3}){sub 2}(OH){sub 4} were physically characterized by X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM) and transmission electron microscope (TEM). The electrochemical properties studies were carried out using cyclic voltammetry (CV), chronopotentiometry technology and AC impedance spectroscopy, respectively. The results indicate that the flower-like structure has a profound impact on electrode performance at high discharge capacitance. A maximum specific capacitance of 2212.5 F g{sup −1} at the current density of 5 mA could be achieved, suggesting its potential application in electrode material for secondary batteries and electrochemical capacitors. Furthermore, the effects of Ni(NO{sub 3}){sub 2}·6H{sub 2}O concentration and temperature on the microstructure and specific capacitance of prepared Ni{sub 3}(NO{sub 3}){sub 2}(OH){sub 4} have also been systematically studied. The results show that flower-like structure can be formed when the concentration is appropriate, while the temperature has just little effect on its electrochemical properties.

36 MATERIALS SCIENCE↗
DOE OSTI2015

The Majorana low-noise low-background front-end electronics

The Majorana Demonstrator will search for the neutrinoless double beta decay (ββ(0ν)) of the isotope ⁷⁶Ge with a mixed array of enriched and natural germanium detectors. In view of the next generation of tonne-scale germanium-based ββ(0ν)-decay searches, a major goal of the Majorana Demonstrator is to demonstrate a path forward to achieving a background rate at or below 1 cnt/(ROI-t-y) in the 4 keV region of interest (ROI) around the 2039-keV Q-value of the ⁷⁶Ge ββ(0ν)-decay. Such a requirement on the background level significantly constrains the design of the readout electronics, which is further driven by noise and energy resolution performances. We present here the low-noise low-background front-end electronics developed for the low-capacitance p-type point contact (P-PC) germanium detectors of the Majorana Demonstrator. This resistive-feedback front-end, specifically designed to have low mass, is fabricated on a radioassayed fused-silica substrate where the feedback resistor consists of a sputtered thin film of high purity amorphous germanium and the feedback capacitor is based on the capacitance between gold conductive traces.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2016

Boosting the Performance of Ionic-Liquid-Based Supercapacitors with Polar Additives

Recent years have witnessed growing interests in both the fundamentals and applications of electric double layer capacitors (EDLCs), also known as supercapacitors. A number of strategies have been explored to optimize the device performance in terms of both the energy and power densities. Because the properties of electric double layers (EDL) are sensitive to ion distributions in the close vicinity of the electrode surfaces, the supercapacitor performance is sensitive to both the electrode pore structure and the electrolyte composition. In this work, we study the effects of polar additives on EDLC capacitance using the classical density functional theory within the framework of a coarse-grained model for the microscopic structure of the porous electrodes and room-temperature ionic liquids. The theoretical results indicate that a highly polar, low-molecular-weight additive is able to drastically increase the EDLC capacitance at low bulk concentration. Additionally, the additive is able to dampen the oscillatory dependence of the capacitance on the pore size, thereby boosting the performance of amorphous electrode materials. The theoretical predictions are directly testable with experiments and provide new insights into the additive effects on EDL properties.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Three-dimensional skeleton networks of graphene wrapped polyaniline nanofibers: An excellent structure for high-performance flexible solid-state supercapacitors

Thin, robust, lightweight, and flexible supercapacitors (SCs) have aroused growing attentions nowadays due to the rapid development of flexible electronics. Graphene-polyaniline (PANI) hybrids are attractive candidates for high performance SCs. In order to utilize them in real devices, it is necessary to improve the capacitance and the structure stability of PANI. Here we report a hierarchical three-dimensional structure, in which all of PANI nanofibers (NFs) are tightly wrapped inside reduced graphene oxide (rGO) nanosheet skeletons, for high-performance flexible SCs. The as-fabricated film electrodes with this unique structure showed a highest gravimetric specific capacitance of 921 F/g and volumetric capacitance of 391 F/cm 3 . The assembled solid-state SCs gave a high specific capacitance of 211 F/g (1 A/g), a high area capacitance of 0.9 F/cm 2 , and a competitive volumetric capacitance of 25.6 F/cm 3 . The SCs also exhibited outstanding rate capability (~75% retention at 20 A/g) as well as excellent cycling stability (100% retention at 10 A/g for 2000 cycles). Additionally, no structural failure and loss of performance were observed under the bending state. Lastly, this structure design paves a new avenue for engineering rGO/PANI or other similar hybrids for high performance flexible energy storage devices.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Interfacial water asymmetry at ideal electrochemical interfaces

Controlling electrochemical reactivity requires a detailed understanding of the charging behavior and thermodynamics of the electrochemical interface. Experiments can independently probe the overall charge response of the electrochemical double layer by capacitance measurements and the thermodynamics of the inner layer with potential of maximum entropy measurements. Relating these properties by computational modeling of the electrochemical interface has so far been challenging due to the low accuracy of classical molecular dynamics (MD) for capacitance and the limited time and length scales of ab initio MD. Here, we combine large ensembles of long-time-scale classical MD simulations with charge response from electronic density functional theory to predict the potential-dependent capacitance of a family of ideal aqueous electrochemical interfaces with different peak capacitances. We show that while the potential of maximum capacitance varies, this entire family exhibits an electrode charge of maximum capacitance (CMC) between -2.9 and -2.2 μC/cm 2 , regardless of the details in the electronic response. Simulated heating of the same interfaces reveals that the entropy peaks at a charge of maximum entropy (CME) of -5.1 ± 0.6 μC/cm 2 , in agreement with experimental findings for metallic electrodes. The CME and CMC both indicate asymmetric response of interfacial water that is stronger for negatively charged electrodes, while the difference between CME and CMC illustrates the richness in behavior of even the ideal electrochemical interface.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Spread Spectrum Time Domain Reflectometry and Steepest Descent Inversion to Measure Complex Impedance

In this paper, we present a method for estimating complex impedances using reflectometry and a modified steepest descent inversion algorithm. We simulate spread spectrum time domain reflectometry (SSTDR), which can measure complex impedances on energized systems for an experimental setup with resistive and capacitive loads. A parametric function, which includes both a misfit function and stabilizer function, is created. The misfit function is a least squares estimate of how close the model data matches observed data. The stabilizer function prevents the steepest descent algorithm from becoming unstable and diverging. Steepest descent iteratively identifies the model parameters that minimize the parametric function. We validate the algorithm by correctly identifying the model parameters (capacitance and resistance) associated with simulated SSTDR data, with added 3 dB white Gaussian noise. With the stabilizer function, the steepest descent algorithm estimates of the model parameters are bounded within a specified range. Furthermore, the errors for capacitance (220pF to 820pF) and resistance (50 Ω to 270 Ω) are < 10%, corresponding to a complex impedance magnitude |R +1/jωC| of 53 Ω to 510 Ω.

14 SOLAR ENERGY↗
NASA NTRS1997

Microfabricated Ice-Detection Sensor

Knowledge of ice conditions on important aircraft lift and control surfaces is critical for safe operation. These conditions can be determined with conventional ice-detection sensors, but these sensors are often expensive, require elaborate installation procedures, and interrupt the airflow. A micromachined, silicon-based, flush-mounted sensor which generates no internal heat has been designed, batch fabricated, packaged, and tested. The sensor is capable of distinguishing between an ice-covered and a clean surface. It employs a bulk micromachined wafer with a 7 micrometer-thick, boron-doped, silicon diaphragm which serves as one plate of a parallel-plate capacitor. This is bonded to a second silicon wafer which contains the fixed electrodes, one to drive the diaphragm by application of a voltage, the other to measure the deflection by a change in capacitance. The diaphragm sizes ranged from 1x1 mm to 3x3 mm, and the gap between parallel-plate capacitors is 2 micrometers. A 200 V d.c. was applied to the driving electrode which caused the capacitance to increase approximately 0.6pf, from a nominal capacitance of 0.6pf, when the surface was ice free. After the sensor was cooled below the freezing point of water, the same voltage range was applied to the drive electrode. The capacitance increased by the same amount. Then a drop of water was placed over the diaphragm and allowed to freeze. This created an approximately 2mm-thick ice layer. The applied 200V d.c. produced no change in capacitance, confirming that the diaphragm was locked to the ice layer. Since the sensor uses capacitive actuation, it uses very little power and is an ideal candidate for inclusion in a wireless sensing system.

DeAnna, Russell G.↗
NASA NTRS1961

A Technique for Increasing the Sensitivity of a Solid-State Fission Probe

A small silicon p-n junction wafer, when coated with uranium 235, can be used as a compact fission probe for low power flux and power mapping. Because of the inverse relation between the magnitude of a neutron-induced fission pulse and the inherent capacitance of the detecting element (capacitance is proportional to area), the size, and hence the sensitivity, of the semiconductor detector has been limited. New developments in the field of semiconductor detectors have made it possible to fabricate large area detectors which are essentially free from the capacitance effect. However, preliminary results indicate that they are much more susceptible to radiation damage than the detectors described in this report and as such may not be suitable for flux mapping. increasing the sensitivity cannot be accomplished by simply fabricating a larger detector. It has been observed that by combining the silicon p-n junction wafers in a series configuration the capacitance effect can be bypassed, and a fission probe can be made with a resultant increase in sensitivity by a factor of ten while sustaining only a minor decrease in pulse height. Analysis further indicates that for n silicon wafers in series, if n(C(sub i)) + C(sub c)/C(sub b) less than 0.1 where C(sub i) and C(sub c) are the preamplifier input and cable capacitances, respectively, and C(sub b) is the junction capacitance of a single silicon wafer, there should be no substantial reduction in pulse height due to series circuitry.

Steinberg, Robert↗
NASA NTRS1999

Two-Phase Quality/Flow Meter

A quality and/or flow meter employs a capacitance probe assembly for measuring the dielectric constant of flow stream, particularly a two-phase flow stream including liquid and gas components.ne dielectric constant of the flow stream varies depending upon the volume ratios of its liquid and gas components, and capacitance measurements can therefore be employed to calculate the quality of the flow, which is defined as the volume ratio of liquid in the flow to the total volume ratio of gas and liquid in the flow. By using two spaced capacitance sensors, and cross-correlating the time varying capacitance values of each, the velocity of the flow stream can also be determined. A microcontroller-based processing circuit is employed to measure the capacitance of the probe sensors.The circuit employs high speed timer and counter circuits to provide a high resolution measurement of the time interval required to charge each capacitor in the probe assembly. In this manner, a high resolution, noise resistant, digital representation of each of capacitance value is obtained without the need for a high resolution A/D converter, or a high frequency oscillator circuit. One embodiment of the probe assembly employs a capacitor with two ground plates which provide symmetry to insure that accurate measurements are made thereby.

Moerk, J. Steven↗
NASA NTRS2008

High-Voltage, Asymmetric-Waveform Generator

The shapes of waveforms generated by commercially available analytical separation devices, such as some types of mass spectrometers and differential mobility spectrometers are, in general, inadequate and result in resolution degradation in output spectra. A waveform generator was designed that would be able to circumvent these shortcomings. It is capable of generating an asymmetric waveform, having a peak amplitude as large as 2 kV and frequency of several megahertz, which can be applied to a capacitive load. In the original intended application, the capacitive load would consist of the drift plates in a differential-mobility spectrometer. The main advantage to be gained by developing the proposed generator is that the shape of the waveform is made nearly optimum for various analytical devices requiring asymmetric-waveform such as differential-mobility spectrometers. In addition, this waveform generator could easily be adjusted to modify the waveform in accordance with changed operational requirements for differential-mobility spectrometers. The capacitive nature of the load is an important consideration in the design of the proposed waveform generator. For example, the design provision for shaping the output waveform is based partly on the principle that (1) the potential (V) on a capacitor is given by V=q/C, where C is the capacitance and q is the charge stored in the capacitor; and, hence (2) the rate of increase or decrease of the potential is similarly proportional to the charging or discharging current. The proposed waveform generator would comprise four functional blocks: a sine-wave generator, a buffer, a voltage shifter, and a high-voltage switch (see Figure 1). The sine-wave generator would include a pair of operational amplifiers in a feedback configuration, the parameters of which would be chosen to obtain a sinusoidal timing signal of the desired frequency. The buffer would introduce a slight delay (approximately equal to 20 ns) but would otherwise leave the fundamental timing signal unchanged. The buffered timing signal would be fed as input to the level shifter. The output of the level shifter would serve as a timing and control signal for the high-voltage switch, causing the switch to alternately be (1) opened, allowing the capacitive load to be charged from a high-voltage DC power supply; then (2) closed to discharge the capacitive load to ground. Hence, the output waveform would closely approximate a series of exponential charging and discharging curves (see Figure 2).

Beegle, Luther W.↗
NASA NTRS2016

Evaluation of Methods for Molecular Dynamics Simulation of Ionic Liquid Electric Double Layers

We investigate how systematically increasing the accuracy of various molecular dynamics modeling techniques influences the structure and capacitance of ionic liquid electric double layers (EDLs). The techniques probed concern long-range electrostatic interactions, electrode charging (constant charge versus constant potential conditions), and electrolyte polarizability. Our simulations are performed on a quasi-two-dimensional, or slab-like, model capacitor, which is composed of a polarizable ionic liquid electrolyte, [EMIM][BF4], interfaced between two graphite electrodes. To ensure an accurate representation of EDL differential capacitance, we derive new fluctuation formulas that resolve the differential capacitance as a function of electrode charge or electrode potential. The magnitude of differential capacitance shows sensitivity to different long-range electrostatic summation techniques, while the shape of differential capacitance is affected by charging technique and the polarizability of the electrolyte. For long-range summation techniques, errors in magnitude can be mitigated by employing two-dimensional or corrected three dimensional electrostatic summations, which lead to electric fields that conform to those of a classical electrostatic parallel plate capacitor. With respect to charging, the changes in shape are a result of ions in the Stern layer (i.e. ions at the electrode surface) having a higher electrostatic affinity to constant potential electrodes than to constant charge electrodes. For electrolyte polarizability, shape changes originate from induced dipoles that soften the interaction of Stern layer ions with the electrode. The softening is traced to ion correlations vertical to the electrode surface that induce dipoles that oppose double layer formation. In general, our analysis indicates an accuracy dependent differential capacitance profile that transitions from the characteristic camel shape with coarser representations to a more diffuse profile with finer representations.

Haskins, Justin B.↗
DOE OSTI2019

Few-Layer Graphene Functionalized by Carbon Nanotubes and MnO{sub 2} Nanoparticles for High-Performance Supercapacitors

Few-layer graphene (FLG) exfoliated from graphite is an attractive electrode material due to its high conductivity and high chemical stability. But usually limited by their specific surface area (SSA), its capacitance needs to be improved when used in supercapacitors. In this study, FLG has been prepared by combining electrochemical exfoliation and thermal expansion. It has a SSA of 148 m{sup 2}/g and delivers a capacitance of 55 F/g at a discharge current density of 1 A/g. To increase its capacitance, carbon nanotubes (CNTs) and MnO{sub 2} nanoparticles are functionalized onto FLG surface separately. Both modification could achieve a composite electrode with enhanced capacitance. The CNT/FLG and MnO{sub 2}/FLG electrodes show a specific capacitance of 106 and 336 F/g at the current density of 1 A/g, respectively, while the former exhibits a much better rate performance and cycling stability than the latter. This work demonstrates an optional way to improve the performance of FLG-based supercapacitors.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Synthesis of polyaniline/SnO{sub 2} nanocomposite and its improved electrochemical performance

Highlights: • Supercapacitors based on PANI/SnO{sub 2} nanocomposite were reported. • Supercapacitors using this material gave a specific capacitance of 501 F g{sup −1}. • Capacitance retention was 85.8% over 2000 charge–discharge cycles. - Abstract: SnO{sub 2} was prepared by the hydrothermal route. Aniline monomer was polymerized in the suspension of SnO{sub 2} to form inorganic–organic nanocomposite materials, in which SnO{sub 2} particles were embedded within polyaniline (PANI). Morphological and structural characterizations of the prepared samples were carried out using scanning electron microscope; transmission electron microscope; power X-ray diffraction and Fourier transform infrared spectroscopy. Their electrochemical properties were also investigated using cyclic voltammetry, galvanostatic charge–discharge and electrochemical impedance spectroscopy. The as-prepared nanocomposites had excellent properties in the capacitance, and its specific capacitance was up to 501 F g{sup −1} with a specific energy density of 66.8 W h kg{sup −1} at a power density of 960.6 W kg{sup −1}. The device fulfilled the requirement of long durability necessary for an energy storage system, since after 2000 galvanostatic charge–discharge cycles, retention of 85.8% in capacitance was observed. These results indicated that the PANI/SnO{sub 2} had a synergistic effect of the complementary properties of both components.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Facile synthesis of nitrogen-doped porous carbon as robust electrode for supercapacitors

Highlights: • An efficient and facile method is developed to prepare porous N doped carbon. • A high areal capacitance of 682.8 mF cm{sup −2} is achieved by this electrode. • This electrode also exhibit outstanding cycling preformance. - Abstract: Porous carbon nanomaterials are highly attactive as electrode materials for supercapacitors (SCs), but the relatively low capacitance as a result of their intrinsic electric double-layer mechanism severely hinders their practical applications. In addition, the complicated preparation processes of most porous carbon nanomaterials previously reported is less impressive for large-scale application. In this work, we report an efficient and simple method to synthesize porous N doped carbon (PNC) on flexible carbon cloth as high-performance electrode for SCs. Benefiting from the unique porous architecture and high N dopant content, the as-fabricated PNC electrode delivers excellent capacitive performance. A high areal capacitance of 682.8 mF cm{sup −2} at 2 mA cm{sup −2} and outstanding cycling performance (less than 4% capacitance fading after 10,000 cycles) is achieved by this PNC electrode. This work opens up new opportunities in developing advanced carbon-based electrode for energy storage.

36 MATERIALS SCIENCE↗
DOE OSTI2018

α-Fe2O3/rGO nanospindles as electrode materials for supercapacitors with long cycle life

Highlights: • Spindle-like α-Fe{sub 2}O{sub 3} and reduced graphene (rGO) composites have been synthesized by a simple hydrothermal method. • The synthesized samples exhibit excellent electrochemical performance with high specific capacitance. • The ASCs exhibit outstanding capacitance performance with maximum energy density. - Abstract: Spindle-like α-Fe{sub 2}O{sub 3} and reduced graphene (rGO) composites have been synthesized by a simple hydrothermal method. The synthesized samples exhibit excellent electrochemical performance with high specific capacitance and good cycling life of about 87.5% reserved after 10,000 cycles. A solid-state asymmetric supercapacitor (ASC) with 1.5 V Voltage has been assembled using α-Fe{sub 2}O{sub 3}/rGO composite anode and the NiCo{sub 2}O{sub 4} nanowires cathode. The ASCs exhibit outstanding capacitance performance with maximum energy density of 73 Wh kg{sup −1} at the power density of 1357 W kg{sup −1}, and more than 219% capacitance retention after 10,000 cycles.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2018

Ruthenium oxide nanostring clusters anchored Graphene oxide nanocomposites for high-performance supercapacitors application

Highlights: • GO-RuO{sub 2} nanocomposites were synthesized by simple hydrothermal method. • GO-RuO{sub 2} nanocomposites were characterized by XRD, SEM, TEM and EDS. • GO-RuO{sub 2} nanostring composite has shown a maximum specific capacitance of 859 Fg{sup −1} at the scan rate of 5 mVs{sup −1}. • GO-RuO{sub 2} nanostring composite electrode exhibits tremendous cyclic stability. - Abstract: Here, we report simple hydrothermal method for the preparation of Graphene oxide (GO) is embellished with nanostring clusters of ruthenium oxide (GO-RuO{sub 2}). The structure of the nanocomposite material was ascertained using X-ray diffraction (XRD), Raman spectroscopy, Scanning electron spectroscopy (SEM), Energy dispersive spectroscopy (EDS) and Transmission spectroscopy (TEM). The supercapacitor performance of the nanocomposites was investigated in acidic electrolyte, 1.0 M H{sub 2}SO{sub 4}, in a three-electrode setup. GO-RuO{sub 2} nanostring composite has shown a maximum specific capacitance of 859 Fg{sup −1} at the scan rate of 5 mVs{sup −1} using Cyclic voltammetry (CV) and by using Chronopotentiometry (CP) the specific capacitance towered up to 512 Fg{sup −1} at a current density of 2.5 Ag{sup −1}. Further, the material also possesses a high energy density of 246 Wh kg{sup -1}with a maximum power density of 1028 W kg{sup -1}. GO-RuO{sub 2} nanostring composite electrode exhibits tremendous cyclic stability with the specific capacitance retention of nearly 99% of the initial capacitance.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2014

Supercapacitors specialities - Materials review

The electrode material is a key component for supercapacitor cell performance. As it is known, performance comparison of commercial available batteries and supercapacitors reveals significantly lower energy storage capability for supercapacitor devices. The energy density of commercial supercapacitor cells is limited to 10 Wh/kg whereas that of common lead acid batteries reaches 35-40 Wh/kg. For lithium ion batteries a value higher than 100 Wh/kg is easily available. Nevertheless, supercapacitors also known as ultracapacitors or electrochemical capacitors have other advantages in comparison with batteries. As a consequence, many efforts have been made in the last years to increase the storage energy density of electrochemical capacitors. A lot of results from published work (research and review papers, patents and reports) are available at this time. The purpose of this review is a presentation of the progress to date for the use of new materials and approaches for supercapacitor electrodes, with focus on the energy storage capability for practical applications. Many reported results refer to nanostructured carbon based materials and the related composites, used for the manufacture of experimental electrodes. A specific capacitance and a specific energy are seldom revealed as the main result of the performed investigation. Thus for nanoprous (activated) carbon based electrodes a specific capacitance up to 200-220 F/g is mentioned for organic electrolyte, whereas for aqueous electrolyte, the value is limited to 400-500 F/g. Significant contribution to specific capacitance is possible from fast faradaic reactions at the electrode-electrolyte interface in addition to the electric double layer effect. The corresponding energy density is limited to 30-50 Wh/kg for organic electrolyte and to 12-17 Wh/kg for aqueous electrolyte. However such performance indicators are given only for the carbon material used in electrodes. For a supercapacitor cell, where two electrodes and also other materials for cell assembling and packaging are used, the above mentioned values have to be divided by a factor higher than four. As a consequence, the specific energy of a prototype cell, hardly could exceed 10 Wh/kg because of difficulties with the existing manufacturing technology. Graphene based materials and carbon nanotubes and different composites have been used in many experiments reported in the last years. Nevertheless in spite of the outstanding properties of these materials, significant increase of the specific capacitance or of the specific energy in comparison with activated or nanoporous carbon is not achieved. Use of redox materials as metal oxides or conducting polymers in combination with different nanostructured carbon materials (nanocomposite electrodes) has been found to contribute to further increase of the specific capacitance or of the specific energy. Nevertheless, few results are reported for practical cells with such materials. Many results are reported only for a three electrode system and significant difference is possible when the electrode is used in a practical supercapacitor cell. Further improvement in the electrode manufacture and more experiments with supercapacitor cells with the known electrochemical storage materials are required. Device prototypes and commercial products with an energy density towards 15-20 Wh/kg could be realized. These may be a milestone for further supercapacitor device research and development, to narrow the storage energy gap between batteries and supercapacitors.

25 ENERGY STORAGE↗