Showing papers on "Capacitance published in 2010"

Graphene-based supercapacitor with an ultrahigh energy density

Abstract: A supercapacitor with graphene-based electrodes was found to exhibit a specific energy density of 85.6 Wh/kg at room temperature and 136 Wh/kg at 80 °C (all based on the total electrode weight), measured at a current density of 1 A/g. These energy density values are comparable to that of the Ni metal hydride battery, but the supercapacitor can be charged or discharged in seconds or minutes. The key to success was the ability to make full utilization of the highest intrinsic surface capacitance and specific surface area of single-layer graphene by preparing curved graphene sheets that will not restack face-to-face. The curved morphology enables the formation of mesopores accessible to and wettable by environmentally benign ionic liquids capable of operating at a voltage >4 V.

Highly Flexible and All-Solid-State Paperlike Polymer Supercapacitors

Abstract: In recent years, much effort have been dedicated to achieve thin, lightweight and even flexible energy-storage devices for wearable electronics. Here we demonstrate a novel kind of ultrathin all-solid-state supercapacitor configuration with an extremely simple process using two slightly separated polyaniline-based electrodes well solidified in the H(2)SO(4)-polyvinyl alcohol gel electrolyte. The thickness of the entire device is much comparable to that of a piece of commercial standard A4 print paper. Under its highly flexible (twisting) state, the integrate device shows a high specific capacitance of 350 F/g for the electrode materials, well cycle stability after 1000 cycles and a leakage current of as small as 17.2 μA. Furthermore, due to its polymer-based component structure, it has a specific capacitance of as high as 31.4 F/g for the entire device, which is more than 6 times that of current high-level commercial supercapacitor products. These highly flexible and all-solid-state paperlike polymer supercapacitors may bring new design opportunities of device configuration for energy-storage devices in the future wearable electronic area.

A 10-bit 50-MS/s SAR ADC With a Monotonic Capacitor Switching Procedure

Abstract: This paper presents a low-power 10-bit 50-MS/s successive approximation register (SAR) analog-to-digital converter (ADC) that uses a monotonic capacitor switching procedure. Compared to converters that use the conventional procedure, the average switching energy and total capacitance are reduced by about 81% and 50%, respectively. In the switching procedure, the input common-mode voltage gradually converges to ground. An improved comparator diminishes the signal-dependent offset caused by the input common-mode voltage variation. The prototype was fabricated using 0.13-?m 1P8M CMOS technology. At a 1.2-V supply and 50 MS/s, the ADC achieves an SNDR of 57.0 dB and consumes 0.826 mW, resulting in a figure of merit (FOM) of 29 fJ/conversion-step. The ADC core occupies an active area of only 195 × 265 ?m2.

Facile Coating of Manganese Oxide on Tin Oxide Nanowires with High-Performance Capacitive Behavior

Abstract: In this paper, a very simple solution-based method is employed to coat amorphous MnO2 onto crystalline SnO2 nanowires grown on stainless steel substrate, which utilizes the better electronic conductivity of SnO2 nanowires as the supporting backbone to deposit MnO2 for supercapacitor electrodes. Cyclic voltammetry (CV) and galvanostatic charge/discharge methods have been carried out to study the capacitive properties of the SnO2/MnO2 composites. A specific capacitance (based on MnO2) as high as 637 F g−1 is obtained at a scan rate of 2 mV s−1 (800 F g−1 at a current density of 1 A g−1) in 1 M Na2SO4 aqueous solution. The energy density and power density measured at 50 A g−1 are 35.4 W h kg−1 and 25 kW kg−1, respectively, demonstrating the good rate capability. In addition, the SnO2/MnO2 composite electrode shows excellent long-term cyclic stability (less than 1.2% decrease of the specific capacitance is observed after 2000 CV cycles). The temperature-dependent capacitive behavior is also discussed. Such hi...

Eliminating Ground Current in a Transformerless Photovoltaic Application

Abstract: For low-power grid-connected applications, a single-phase converter can be used. In photovoltaic (PV) applications, it is possible to remove the transformer in the inverter to reduce losses, costs, and size. Galvanic connection of the grid and the dc sources in transformerless systems can introduce additional ground currents due to the ground parasitic capacitance. These currents increase conducted and radiated electromagnetic emissions, harmonics injected in the utility grid, and losses. Amplitude and spectrum of the ground current depend on the converter topology, the switching strategy, and the resonant circuit formed by the ground capacitance, the converter, the ac filter, and the grid. In this paper, the ground current in a 1.5-kW PV installation is measured under different conditions and used to build a simulation model. The installation includes a string of 16 PV panel, a full-bridge inverter, and an LCL filter. This model allows the study of the influence of the harmonics injected by the inverter on the ground current.

Temperature compensation in a wireless transfer system

Abstract: Described herein are improved configurations for a resonator for wireless power transfer that includes a conductor forming one or more loops and having an inductance L, a network of capacitors, having a capacitance, C, and a desired electrical parameter, coupled to the conductor, the network having at least one capacitor of a first type with a first temperature profile of the electrical parameter, and the network having at least one capacitor of a second type with a second temperature profile of the electrical parameter.

Nernst limit in dual-gated Si-nanowire FET sensors.

Abstract: Field effect transistors (FETs) are widely used for the label-free detection of analytes in chemical and biological experiments. Here we demonstrate that the apparent sensitivity of a dual-gated silicon nanowire FET to pH can go beyond the Nernst limit of 60 mV/pH at room temperature. This result can be explained by a simple capacitance model including all gates. The consistent and reproducible results build to a great extent on the hysteresis- and leakage-free operation. The dual-gate approach can be used to enhance small signals that are typical for bio- and chemical sensing at the nanoscale.

Highly Dispersed RuO2 Nanoparticles on Carbon Nanotubes: Facile Synthesis and Enhanced Supercapacitance Performance

Abstract: RuO2/CNT nanocomposites with well-dispersed RuO2 nanoparticles (diameter <2 nm) on the carbon nanotubes’ surface, synthesized through an easy and efficient solution-based method, have been investigated for potential application in electrochemical capacitors (ECs) as electrode materials. The electrochemical results demonstrate that the supporting material of CNT can significantly promote the supercapacitance performance of RuO2. The RuO2 nanoparticles in the composite with a RuO2/CNT mass ratio of 6:7 could achieve a specific capacitance of as high as 953 F g−1. The results also demonstrate that the resulted RuO2/CNT nanocomposites are superior electrode materials for ECs with a high specific capacitance and significantly enhanced high-power and high-energy capabilities as well as improved cycling performance compared with bare RuO2. At a power density of 5000 W kg−1, the RuO2/CNT composite (RuO2/CNT = 6:7 in wt %) can still deliver an energy density of 16.8 Wh kg−1, which is about 5.8 times larger than th...

Differential capacitance of the double layer at the electrode/ionic liquids interface

Abstract: The differential capacitance of the electrical double layer at glassy carbon, platinum and gold electrodes immersed in various ionic liquids was measured using impedance spectroscopy. We discuss the influence of temperature, the composition of the ionic liquids and the electrode material on the differential capacitance/potential curves. For different systems these curves have various overall shapes, but all include several extremes and a common minimum near the open circuit potential. We attribute this minimum to the potential of zero charge (PZC). Significantly, the differential capacitance generally decreases if the applied potential is large and moving away from the PZC. This is attributed to lattice saturation [A. A. Kornyshev, J. Phys. Chem. B, 2007, 111, 5545] effects which result in a thicker double layer. The differential capacitance of the double layer grows and specific adsorption diminishes with increasing temperature. Specific adsorption of both cations and anions influences the shapes of curves close to the PZC. The general shape of differential capacitance/potential does not depend strongly on the identity of the electrode material.

Flexible Supercapacitor Based on Polyaniline Nanowires/Carbon Cloth with Both High Gravimetric and Area-Normalized Capacitance

Abstract: Flexible supercapacitor is successfully fabricated using polyaniline nanowires/carbon cloth (PANINWs/CC) nanocomposite. High gravimetric capacitance of 1079 F g−1 at a specific energy of 100.9 Wh kg−1 and a specific power of 12.1 kW kg −1 is obtained. Moreover, this approach also offers an exceptionally high area-normalized capacitance of 1.8 F cm −2 . The diffusion length of protons within the PANI-NWs is estimated to be about 60 nm by electrochemical impedance analysis, which indicates that the electrochemical performance of the electrode is not limited by the thickness of PANI-NWs. The electrochemical performance of PANI-NWS/CC remains without any deterioration, even when the cell is bent under high curvature. These results clearly present a cost-effective and simple method of fabrication of the nanostructured polymers with enormous potential in flexible energy storage device applications.

A 10-bit 50-MS/s SAR ADC With a Monotonic Capacitor Switching Procedure

Abstract: This paper presents a low-power 10-bit 50-MS/s successive approximation register (SAR) analog-to-digital converter (ADC) that uses a monotonic capacitor switching procedure. Compared to converters that use the conventional procedure, the average switching energy and total capacitance are reduced by about 81% and 50%, respectively. In the switching procedure, the input common-mode voltage gradually converges to ground. An improved comparator diminishes the signal-dependent offset caused by the input common-mode voltage variation. The prototype was fabricated using 0.13-μm 1P8M CMOS technology. At a 1.2-V supply and 50 MS/s, the ADC achieves an SNDR of 57.0 dB and consumes 0.826 mW, resulting in a figure of merit (FOM) of 29 fJ/conversion-step. The ADC core occupies an active area of only 195 × 265 μm 2 .

Interpretation of admittance, capacitance-voltage, and current-voltage signatures in Cu(In,Ga)Se2 thin film solar cells

Abstract: A series of Cu(In,Ga)Se2 (CIGS) thin film solar cells with differently prepared heterojunctions has been investigated by admittance spectroscopy, capacitance-voltage (CV) profiling, and temperature dependent current-voltage (IVT) measurements. The devices with different CdS buffer layer thicknesses, with an In2S3 buffer or with a Schottky barrier junction, all show the characteristic admittance step at shallow energies between 40 and 160 meV, which has often been referred to as the N1 defect. No correlation between the buffer layer thickness and the capacitance step is found. IVT measurements show that the dielectric relaxation frequency of charge carriers in the CdS layers is smaller than the N1-resonance frequency at low temperatures where the N1 step in admittance is observed. These results strongly contradict the common assignment of the N1 response to a donor defect at or close to the heterointerface. In contrast, an explanation for the N1 response is proposed, which relates the admittance step to a non-Ohmic back-contact acting as a second junction in the device. The model, which is substantiated with numerical device simulations, allows a unified explanation of characteristic admittance, CV, and IVT features commonly observed in CIGS solar cells.

Hybrid Supercapacitor Based on Coaxially Coated Manganese Oxide on Vertically Aligned Carbon Nanofiber Arrays

Abstract: Hybrid supercapacitor electrodes with remarkable specific capacitance have been fabricated by coaxially coating manganese oxide thin films on a vertically aligned carbon nanofiber array Ultrathin manganese oxide layers are uniformly coated around each carbon nanofiber via cathodic electrochemical deposition, likely based on water electrolysis initiated electrochemical oxidation This results in a unique core-shell nanostructure which uses the three-dimensional brush-like vertical carbon nanofiber array as the highly conductive and robust core to support a large effective surface area and provide reliable electrical connection to a thin redox active manganese oxide shell The pseudo-capacitance of 313 F/g in addition to the electrical double layer capacitance of 36 F/g is achieved by cyclic voltammetry at a scan rate of 50 mV/s and maintains at this level as the scan rate is increased up to 2000 mV/s A maximum specific capacitance of 365 F/g has been achieved with chronopotentiometry in 010 M Na2SO4 aqu

Density of States and Zero Landau Level Probed through Capacitance of Graphene

Abstract: We report capacitors in which a finite electronic compressibility of graphene dominates the electrostatics, resulting in pronounced changes in capacitance as a function of magnetic field and carrier concentration. The capacitance measurements have allowed us to accurately map the density of states D, and compare it against theoretical predictions. Landau oscillations in D are robust and zero Landau level (LL) can easily be seen at room temperature in moderate fields. The broadening of LLs is strongly affected by charge inhomogeneity that leads to zero LL being broader than other levels.

Nano-architectured Co(OH)2 electrodes constructed using an easily-manipulated electrochemical protocol for high-performance energy storage applications

Abstract: A simple, low-cost, and efficient electrochemical strategy, which includes the co-deposition of a Ni–Cu layer, selective etching of Cu from the film (leaving nano-porous Ni), and electrodeposition of Co(OH)2 nano-whiskers on the obtained Ni substrate, is used to construct a nano-structured electrode. This process can be conducted on many conductive surfaces, which can be cheap, flexible, and wearable, and can be integrated into advanced mobile micro-power systems. Due to its unique nano-architecture, the prepared Co(OH)2 electrode shows exceptional energy storage performance as compared to that of the conventional version of the electrode. The optimum specific capacitance obtained in this study, evaluated using cyclic voltammetry (CV), was as high as 2800 F/g. When the CV scan rate was increased from 5 to 200 mV/s, only a 4% decay in the capacitance was found, indicating excellent high-power capability. These characteristics make the nano-structured Co(OH)2 electrode a promising candidate for supercapacitor applications.

A Fully-Integrated Switched-Capacitor Step-Down DC-DC Converter With Digital Capacitance Modulation in 45 nm CMOS

Abstract: Implementing efficient and cost-effective power regulation schemes for battery-powered mixed-signal SoCs is a key focus in integrated circuit design. This paper presents a fully-integrated switched-capacitor DC-DC converter in 45 nm digital CMOS technology. The proposed implementation uses digital capacitance modulation instead of traditional PFM and PWM control methods to maintain regulation against load current changes. This technique preserves constant frequency switching while also scaling switching and bottom-plate losses with changes in load current. Therefore, high efficiency can be achieved across different load current levels while maintaining a predictable switching noise behavior. The converter occupies only 0.16 mm2, and operates from 1.8 V input. It delivers a programmable sub-1 V power supply with efficiency as high as 69% and load current between 100 μA and 8 mA. Measurement results confirm the theoretical basis of the proposed design.

Tunneling Field-Effect Transistor: Capacitance Components and Modeling

Abstract: We report a numerical simulation study of gate capacitance components in a tunneling field-effect transistor (TFET), showing key differences in the partitioning of gate capacitance between the source and drain as compared with a MOSFET. A compact model for TFET capacitance components, including parasitic and inversion capacitances, was built and calibrated with computer-aided design data. This model should be useful for further investigation of performance of circuits containing TFETs. The dependence of gate-drain capacitance Cgd on drain design and gate length was further investigated for reduction of switching delay in TFETs.

Metal-Ferroelectric-Meta-Oxide-semiconductor field effect transistor with sub-60mV/decade subthreshold swing and internal voltage amplification

Abstract: This work reports the first complete experimental demonstration and investigation of subthreshold swing, SS, smaller than 60 mV/decade, at room temperature, due to internal voltage amplification in FETs with a Metal-Ferroelectric-Metal-Oxide gate stack. The investigated p-type MOS transistor is a dedicated test structure to explore the negative capacitance effect by probing the internal voltage between the P(VDF-TrFE) and SiO 2 dielectric layers of the gate stack. We find that the region of internal surface potential amplification, dψ S /dV g >1, corresponds to an S-shape of the polarization versus ferroelectric voltage (associated with negative capacitance). In Fe-FETs the internal voltage amplification could significantly lower their SS, even without reaching sub-60mV/dec values. SS min as low as 46 to 58 mV/decade and average swings, SS avg , as small as 51 to 59 mV/dec are observed for the first time in a minor loop hysteretic characteristics of Fe-FETs.

Asymmetric supercapacitors based on stabilized α -Ni(OH) 2 and activated carbon

Abstract: In this work, stabilized Al-substituted α-Ni(OH)2 materials were successfully synthesized by a chemical coprecipitation method. The experimental results showed that the 7.5% Al-substituted α-Ni(OH)2 materials exhibited high specific capacitance (2.08 × 103 F/g) and excellent rate capability due to the high stability of Al-substituted α-Ni(OH)2 structures in alkaline media, suggesting its potential application in electrode material for supercapacitors. To enhance energy density, an asymmetric type pseudo/electric double-layer capacitor was considered where α-Ni(OH)2 materials and activated carbon act as the positive and negative electrodes, respectively. Values for the maximum specific capacitance of 127 F/g and specific energy of 42 W·h/kg were demonstrated for a cell voltage between 0.4 and 1.6 V. By using the α-Ni(OH)2 electrode, the asymmetric supercapacitor exhibited high energy density and stable power characteristics. The hybrid supercapacitor also exhibited a good electrochemical stability with 82% of the initial capacitance over consecutive 1,000 cycle numbers.