Showing papers on "Capacitance published in 2011"

Carbon-based Supercapacitors Produced by Activation of Graphene

Abstract: Supercapacitors, also called ultracapacitors or electrochemical capacitors, store electrical charge on high-surface-area conducting materials. Their widespread use is limited by their low energy storage density and relatively high effective series resistance. Using chemical activation of exfoliated graphite oxide, we synthesized a porous carbon with a Brunauer-Emmett-Teller surface area of up to 3100 square meters per gram, a high electrical conductivity, and a low oxygen and hydrogen content. This sp 2 -bonded carbon has a continuous three-dimensional network of highly curved, atom-thick walls that form primarily 0.6- to 5-nanometer-width pores. Two-electrode supercapacitor cells constructed with this carbon yielded high values of gravimetric capacitance and energy density with organic and ionic liquid electrolytes. The processes used to make this carbon are readily scalable to industrial levels.

Nanoporous metal/oxide hybrid electrodes for electrochemical supercapacitors

Abstract: Electrochemical supercapacitors can deliver high levels of electrical power and offer long operating lifetimes, but their energy storage density is too low for many important applications. Pseudocapacitive transition-metal oxides such as MnO(2) could be used to make electrodes in such supercapacitors, because they are predicted to have a high capacitance for storing electrical charge while also being inexpensive and not harmful to the environment. However, the poor conductivity of MnO(2) (10(-5)-10(-6) S cm(-1)) limits the charge/discharge rate for high-power applications. Here, we show that hybrid structures made of nanoporous gold and nanocrystalline MnO(2) have enhanced conductivity, resulting in a specific capacitance of the constituent MnO(2) (~1,145 F g(-1)) that is close to the theoretical value. The nanoporous gold allows electron transport through the MnO(2), and facilitates fast ion diffusion between the MnO(2) and the electrolytes while also acting as a double-layer capacitor. The high specific capacitances and charge/discharge rates offered by such hybrid structures make them promising candidates as electrodes in supercapacitors, combining high-energy storage densities with high levels of power delivery.

Enhancing the Supercapacitor Performance of Graphene/MnO2 Nanostructured Electrodes by Conductive Wrapping

Abstract: MnO2 is considered one of the most promising pseudocapactive materials for high-performance supercapacitors given its high theoretical specific capacitance, low-cost, environmental benignity, and natural abundance. However, MnO2 electrodes often suffer from poor electronic and ionic conductivities, resulting in their limited performance in power density and cycling. Here we developed a “conductive wrapping” method to greatly improve the supercapacitor performance of graphene/MnO2-based nanostructured electrodes. By three-dimensional (3D) conductive wrapping of graphene/MnO2 nanostructures with carbon nanotubes or conducting polymer, specific capacitance of the electrodes (considering total mass of active materials) has substantially increased by ∼20% and ∼45%, respectively, with values as high as ∼380 F/g achieved. Moreover, these ternary composite electrodes have also exhibited excellent cycling performance with >95% capacitance retention over 3000 cycles. This 3D conductive wrapping approach represents ...

Ultralayered Co3O4 for High-Performance Supercapacitor Applications

Abstract: Ultralayered Co3O4 structures with high porosity have been synthesized by a facile homogeneous precipitation process under hydrothermal conditions. The superstructures consist of well-arranged micrometer length rectangular 2D flakes with high specific surface area, pore volume, and uniform pore size distribution. The electrochemical measurements demonstrate that charge storage occurs in ultralayered Co3O4 due to reversible redox reactions. The charge–discharge study shows that the material is capable of delivering very high specific capacitance of 548 F g–1 at a current density of 8 A g–1 and retains 66% of capacitance at 32 A g–1. The charge–discharge stability measurements show excellent specific capacitance retention capability, ca. 98.5% after 2000 continuous charge–discharge cycles at high current density of 16 A g–1. The exceptional cyclic, structural, and electrochemical stability at higher current rate with ∼100% Coulombic efficiency, and very low ESR value from impedance measurements promise good...

Method and apparatus for determining parameters of linear motion in a surgical instrument

Abstract: A surgical instrument and method of controlling the surgical instrument are disclosed. The surgical instrument includes a housing and an elongated shaft that extends distally from the housing and defines a first longitudinal axis. The surgical instrument also includes a firing rod disposed in the elongated shaft and a drive mechanism disposed at least partially within the housing. The drive mechanism mechanically cooperates with the firing rod to move the firing rod. A motion sensor senses a change in the electric field (e.g., capacitance, impedance, or admittance) between the firing rod and the elongated shaft. The measurement unit determines a parameter of the motion of the firing rod, such as the position, speed, and direction of the firing rod, based on the sensed change in the electric field. A controller uses the measured parameter of the motion of the firing rod to control the drive mechanism.

Preparation of Highly Conductive Graphene Hydrogels for Fabricating Supercapacitors with High Rate Capability

Abstract: Graphene hydrogels prepared via hydrothermal reduction of graphene oxide dispersions (GH-Hs) were further reduced with hydrazine (Hz) or hydroiodic acid (HI) to improve their conductivities. The chemically reduced graphene hydrogels possess high conductivities of 1.3–3.2 S m–1, which are 1 order of magnitude higher than that of a GH-H (0.3 S m–1). The supercapacitor based on the Hz-reduced GH-H exhibited a high specific capacitance of 220 F g–1 at 1 A g–1, and this capacitance can be maintained for 74% as the discharging current density was increased up to 100 A g–1. Furthermore, it showed high power density and long cycle life. The high-performances of this supercapacitor make it promising for high rate charge/discharge applications.

Graphene Nanosheet/Ni2+/Al3+ Layered Double-Hydroxide Composite as a Novel Electrode for a Supercapacitor

Abstract: A hybrid chemically converted graphene nanosheet/Ni2+/Al3+ layered double-hydroxide (GNS/LDH) composite for supercapacitor material has been fabricated by a hydrothermal method. Scanning electron microscopy and transmission electron microscopy results reveal that Ni2+/Al3+ LDH platelets homogeneously grew onto the surfaces of the GNSs as spacers to keep the neighboring sheets separate. Electrochemical properties were characterized by cyclic voltammetry, galvanostatic charge/discharge measurements, and electrochemical impedance spectroscopy. The composite exhibits a maximum specific capacitance of 781.5 F/g and excellent cycle life with an increase of the specific capacitance of 38.07% after 50 cycle tests. Even after 200 cycle tests, the increase of the capacitance is 22.56% compared with the initial capacitance.

Facile synthesis of large-area manganese oxide nanorod arrays as a high-performance electrochemical supercapacitor

Abstract: Large-area manganese oxide nanorod arrays (MONRAs) and herringbones (MOHBs) were successfully synthesized on F-doped SnO2 coated glass (FTO) substrates by a simple electrochemical method. Cyclic voltammetry (CV) and galvanostatic charge/discharge measurements demonstrated that the MONRAs and MOHBs exhibited excellent specific capacitance and good cycling stability in 0.5 M Na2SO4 aqueous solution. For example, the specific capacitance of the MONRAs achieves as high as 660.7 F g−1 at a scan rate of 10 mV s−1 and 485.2 F g−1 at a current density of 3 A g−1, respectively. Furthermore, the presented method may be extended to allow similar MONRs with a specific capacitance of 583.6 F g−1 to grow on flexible Ti foil, which may have great potential application in fabricating flexible supercapacitors.

A flicker-free electrolytic capacitor-less ac-dc LED driver

Abstract: The electrolytic capacitor is the key component that limits the operating lifetime of LED drivers. If an ac-dc LED driver with power factor correction (PFC) control is allowed to output a pulsating current for driving the LEDs, the electrolytic capacitor will no longer be required. However, this pulsating current will introduce light flicker that varies at twice the power line frequency. In this paper, a configuration of flicker-free electrolytic capacitor-less single-phase ac-dc driver for LED lighting is proposed. The configuration comprises an electrolytic capacitor-less PFC converter and a bidirectional converter, which serves to absorb the ac component of the pulsating current of the PFC converter, leaving only a dc component to drive the LEDs. The output filter capacitor of the bidirectional converter is intentionally designed to have a large voltage ripple, thus its capacitance can be greatly reduced. Consequently, film capacitors can be used instead of electrolytic capacitors, leading to the realization of a flicker-free ac-dc LED driver that has a long lifetime. The proposed solution is generally applicable to all single-phase PFC converters. A prototype with 48-V, 0.7-A output is constructed and tested. Experimental results are presented to verify the effectiveness of the flick-free electrolytic capacitor-less ac-dc LED driver.

Supercapacitor Capacitance Exhibits Oscillatory Behavior as a Function of Nanopore Size

Abstract: Supercapacitors composed of slit-shaped micropores ranging in size from 0.67 to 1.8 nm in a room-temperature ionic liquid were studied to investigate the dependence of capacitance (C) on the pore size (d) using molecular dynamics simulations. The capacitance versus pore size (i.e., the C–d curve) was found to exhibit two peaks located at 0.7 and 1.4 nm, respectively. Specifically, as the pore shrinks from 1.0 to 0.7 nm, the capacitance of the micropore increases anomalously, in good agreement with experimental observations. We report herein that the second peak within 1.0 to 1.8 nm is a new feature of the C–d curve. Furthermore, by analogy to the wave interference, we demonstrate that the interference of two electrical double layers near each slit wall does not only explain the entire C–d curve, including the anomalous character, but also predicts the oscillatory behavior of C–d curve beyond 1.8 nm.

Ferroelectric negative capacitance MOSFET: Capacitance tuning & antiferroelectric operation

Abstract: A design methodology of ferroelectric (FE) negative capacitance FETs (NCFETs) based on the concept of capacitance matching is presented. A new mode of NCFET operation, called the “antiferroelectric mode” is proposed, which, besides achieving sub-60mV/dec subthreshold swing, can significantly boost the on-current in exchange for a nominal hysteresis. Design considerations for different device parameters (FE thickness, EOT, source/drain overlap & gate length) are explored. It is suggested that relative improvement in device performance due to FE negative capacitance becomes more significant in very short channel length devices because of the increased drain-to-channel coupling.

Superior Capacitance of Functionalized Graphene

Abstract: We report the superior capacitance of functionalized graphene prepared by controlled reduction of graphene oxide (GO). In a solvothermal method, GO dispersed in dimethylformamide (DMF) was thermally treated at a moderate temperature (150 °C), which allows a fine control of the density of functionalities. Surface functionalities on graphene would enable a high pseudocapacitance, good wetting property, and acceptable electric conductivity. A specific capacitance up to 276 F/g was achieved based on functionalized graphene at a discharge current of 0.1 A/g in a 1 M H2SO4 electrolyte, which is much higher than the benchmark material. The excellent performance of the functionalized graphene signifies the importance of controlling the surface chemistry of graphene-based materials.

Compact-designed supercapacitors using free-standing single-walled carbon nanotube films

Abstract: We reported the realization of assembling compact-designed supercapacitors using large-scaled free-standing and flexible single-walled carbon nanotube (SWCNT) films as both anode and cathode. A prototype of the processing procedures was developed to obtain the uniform spreading of the SWCNT films onto the separators serving as both electrodes and charge collectors without metallic current collectors, leading to a simplified and lightweight architecture. The area of SWCNT film on a separator can be scaled up and its thickness can be extended. High energy and power densities (43.7 Wh kg−1 and 197.3 kW kg−1, respectively) were achieved from the prepared SWCNT film-based compact-designed supercapacitors with small equivalent series resistance. The specific capacitance of this kind of compact-designed SWCNT film supercapacitor is about 35 F g−1. These results clearly show the potential application of free-standing SWCNT film in compact-designed supercapacitor with enhanced performance and significantly improved energy and power densities.

Experimental evidence of ferroelectric negative capacitance in nanoscale heterostructures

Abstract: We report a proof-of-concept demonstration of negative capacitance effect in a nanoscale ferroelectric-dielectric heterostructure. In a bilayer of ferroelectric Pb(Zr0.2Ti0.8)O3 and dielectric SrTiO3, the composite capacitance was observed to be larger than the constituent SrTiO3 capacitance, indicating an effective negative capacitance of the constituent Pb(Zr0.2Ti0.8)O3 layer. Temperature is shown to be an effective tuning parameter for the ferroelectric negative capacitance and the degree of capacitance enhancement in the heterostructure. Landau’s mean field theory based calculations show qualitative agreement with observed effects. This work underpins the possibility that by replacing gate oxides by ferroelectrics in nanoscale transistors, the sub threshold slope can be lowered below the classical limit (60 mV/decade).

Oscillation of capacitance inside nanopores.

Abstract: Porous carbons of high surface area are promising as cost-effective electrode materials for supercapacitors. Although great attention has been given to the anomalous increase of the capacitance as the pore size approaches the ionic dimensions, there remains a lack of full comprehension of the size dependence of the capacitance in nanopores. Here we predict from a classical density functional theory that the capacitance of an ionic-liquid electrolyte inside a nanopore oscillates with a decaying envelope as the pore size increases. The oscillatory behavior can be attributed to the interference of the overlapping electric double layers (EDLs); namely, the maxima in capacitance appear when superposition of the two EDLs is most constructive. The theoretical prediction agrees well with the experiment when the pore size is less than twice the ionic diameter. Confirmation of the entire oscillatory spectrum invites future experiments with a precise control of the pore size from micro- to mesoscales.

Enhancement of the capacitance in TiO2 nanotubes through controlled introduction of oxygen vacancies

Abstract: The many applications of high energy storage devices have forged an increasing interest in research areas related to electrochemical capacitors. Here, in this work, we present a facile method for the fabrication of self-organized titania nanotubes grown by anodic oxidation of titanium foil with different subsequent heat-treatment regimes for use as binder-free working electrodes in supercapacitor applications. The capacitance of these highly ordered titania nanotubes, when exposed to a reductive atmosphere during annealing, was determined to be well above 900 µF cm−2, confirming that the capacitance contribution was pseudocapacitive in nature. The behaviour of oxygen depleted titania in the anatase to rutile (A → R) phase transformation and also in electrochemical charge storage has been studied in detail. It was found that upon the reduction of Ti4+ to Ti3+, with oxygen depletion of the structure, the A → R phase transformation was promoted. In addition, the fabricated electrodes showed highly reversible charge–discharge stability.

Superionic state in double-layer capacitors with nanoporous electrodes

Abstract: In recent experiments (Chmiola et al 2006 Science 313 1760; Largeot et al 2008 J. Am. Chem. Soc. 130 2730) an anomalous increase of the capacitance with a decrease of the pore size of a carbon-based porous electric double-layer capacitor has been observed. We explain this effect by image forces which exponentially screen out the electrostatic interactions of ions in the interior of a pore. Packing of ions of the same sign becomes easier and is mainly limited by steric interactions. We call this state 'superionic' and suggest a simple model to describe it. The model reveals the possibility of a voltage-induced first order transition between a cation(anion)-deficient phase and a cation(anion)-rich phase which manifests itself in a jump of capacitance as a function of voltage.

Three-dimensionally arrayed and mutually connected 1.2-nm nanopores for high-performance electric double layer capacitor.

Abstract: Zeolite-templated carbon is a promising candidate as an electrode material for constructing an electric double layer capacitor with both high-power and high-energy densities, due to its three-dimensionally arrayed and mutually connected 1.2-nm nanopores. This carbon exhibits both very high gravimetric (140-190 F g(-1)) and volumetric (75-83 F cm(-3)) capacitances in an organic electrolyte solution. Moreover, such a high capacitance can be well retained even at a very high current up to 20 A g(-1). This extraordinary high performance is attributed to the unique pore structure.

Accurate Simulations of Electric Double Layer Capacitance of Ultramicroelectrodes

Abstract: This paper aims to develop rigorous and accurate numerical tools for simulating electric double layers formed near ultramicroelectrodes. It also aims to assess the validity of existing models and to identify the dominant physical phenomena that must be accounted for. The electric double layer capacitance was numerically predicted for spherical ultramicroelectrodes of various radii in aqueous electrolyte. The model accounted for the Stern and diffuse layers, the finite size of ions, and the dependency of the electrolyte dielectric permittivity on the local electric field. This study reveals that models reported in the literature suffer from severe limitations. First, it demonstrates that the electrolyte field-dependent dielectric permittivity significantly affects the predicted Stern layer and total specific capacitances and must be accounted for. The finite ion size and the Stern layer also need to be considered in simulating electric double layers under high concentrations and surface potential. This stu...

A superionic state in nano-porous double-layer capacitors: insights from Monte Carlo simulations

Abstract: Recently observed anomalous properties of ionic-liquid-based nanoporous supercapacitors [C. Largot et al., J. Am. Chem. Soc., 2008, 130, 2730-2731] have attracted much attention. Here we present Monte Carlo simulations of a model ionic liquid in slit-like metallic nanopores. We show that exponential screening of the electrostatic interactions of ions inside a pore, as well as the image-charge attraction of ions to the pore surface, lead to the 'anomalous' increase of the capacitance with decreasing the pore width. The simulation results are in good agreement with the experimental data. The capacitance as a function of voltage is almost constant for low voltages and vanishes above a certain threshold voltage. For very narrow pores, these two regions are separated by a peak. With increase of the pore size the peak turns into a bump and disappears for wide pores. This effect, related to a specific character of the voltage-induced filling of nanopores with counterions at high densities, is yet to be verified experimentally.

Synthesis of electrochemically-reduced graphene oxide film with controllable size and thickness and its usein supercapacitor

Abstract: An electrochemical synthesis method of reducing graphene oxide (GO) under constant potential is reported. Electrochemical technique offers control over reaction parameters such as the applied voltage, electrical current and reduction time; whereas the desired size and thickness of the film can be pre-determined by controlling the amount of precur- sor GO deposited on the electrode with defined shape and surface area. This synthesis technique produces high quality electrochemically reduced GO (ERGO) film with control- lable size and thickness. Electrochemical symmetrical supercapacitors based on ERGO films achieved a specific capacitance of 128 F/g with an energy density of 17.8 Wh/kg operating within a potential window of 1.0 V in 1.0 M NaNO3. The supercapacitor was shown to be stable, retaining ca. 86% of the original specific capacitance after 3500 charge–discharge cycles. The results indicate that this simple synthesis technique for providing graphene-like materials has great potential in various applications such as energy storage.

Capacitive power transfer for contactless charging

Abstract: The simplicity and low cost of capacitive interfaces makes them very attractive for wireless charging stations. Major benefits include low electromagnetic radiation and the amenability of combined power and data transfer over the same interface. We present a capacitive power transfer circuit using series resonance that enables efficient high frequency, moderate voltage operation through soft-switching. An included analysis predicts fundamental limitations on the maximum achievable efficiency for a given amount of coupling capacitance and is used to find the optimum circuit component values and operating point. Automatic tuning loops ensure the circuit operates at the optimum frequency and maximum efficiency over a wide range of coupling capacitance and load conditions. An example interface achieves near 80% efficiency at 3.7 W with only 63pF of coupling capacitance. An automatic tuning loop adjusts the frequency from 4.2 MHz down to 4MHz to allow for 25% variation in the nominal coupling capacitance. The duty cycle is also automatically adjusted to maintain over 70% efficiency for light loads down to 0.3 W.