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Capacitive charge storage at an electrified interface investigated via direct first-principles simulations [Direct Simulation of Capacitive Charging of Graphene and Implications for Supercapacitor Design]
Understanding the impact of interfacial electric fields on electronic structure is crucial to improving the performance of materials in applications based on charged interfaces. Supercapacitors store energy directly in the strong interfacial field between a solid electrode and a liquid electrolyte; however, the complex interplay between the two is often poorly understood, particularly for emerging low-dimensional electrode materials that possess unconventional electronic structure. Typical descriptions tend to neglect the specific electrode-electrolyte interaction, approximating the intrinsic “quantum capacitance” of the electrode in terms of a fixed electronic density of states. Instead, we introduce a more accurate first-principles approach for directly simulating charge storage in model capacitors using the effective screening medium method, which implicitly accounts for the presence of the interfacial electric field. Applying this approach to graphene supercapacitor electrodes, we find that results differ significantly from the predictions of fixed-band models, leading to improved consistency with experimentally reported capacitive behavior. The differences are traced to two key factors: the inhomogeneous distribution of stored charge due to poor electronic screening and interfacial contributions from the specific interaction with the electrolyte. Lastly, our results are used to revise the conventional definition of quantum capacitance and to provide general strategies for improving electrochemical charge storage, particularly in graphene and similar low-dimensional materials.
Measurement of n-type Dry Thermally Oxidized 6H-SiC Metal-oxide Semiconductor Diodes by Quasistatic and High-Frequency Capacitance Versus Voltage and Capacitance Transient Techniques
Dry-oxidized n-type 6H-SiC metal-oxide-semiconductor capacitors are investigated using quasistatic capacitance versus voltage (C-V), high-frequency C-V, and pulsed high-frequency capacitance transient (C-t) analysis over the temperature range from 297 to 573 K. The quasistatic C - V characteristics presented are the first reported for 6H-SiC MOS capacitors, and exhibit startling nonidealities due to nonequilibrium conditions that arise from the fact that the recombination/generation process in 6H-SiC is extraordinarily slow even at the highest measurement temperature employed. The high-frequency dark C-V characteristics all showed deep depletion with no observable hysteresis. The recovery of the high-frequency capacitance from deep depletion to inversion was used to characterize the minority-carrier generation process as a function of temperature. Zerbst analysis conducted on the resulting C-t transients, which were longer than 1000 s at 573 K, showed a generation lifetime thermal activation energy of 0.49 eV.
Capacitance Measurement with a Sigma Delta Converter for 3D Electrical Capacitance Tomography
This paper will explore suitability of a newly available capacitance to digital converter for use in a 3D Electrical Capacitance Tomography system. A switch design is presented along with circuitry needed to extend the range of the capacitance to digital converter. Results are then discussed for a 15+ hour drift and noise test.
Electronic Scanning Strategies in Adaptive Electrical Capacitance Volume Tomography: Tradeoffs and Prospects
Electrical Capacitance Volume Tomography (ECVT) has been applied for imaging of multiphase flows found in industrial applications. The ill-posed nature of the image reconstruction problem in ECVT and the consequent low resolution can be alleviated by employing electronic scanning enabled by electrode segmentation and reconfiguration during data acquisition. Here we study electronic scanning strategies that mimic physical rotation and shifting of the sensor along its symmetry axis. First, we study the feasibility of electronic scanning by analyzing the capacitance transducer circuit in SPICE. Then, we simulate electronic scanning using the finite element method for different electrode shapes, for which we compare the image reconstruction results and acquisition time. We find a noticeable improvement in image resolution for the scanning cases over conventional ECVT. Lastly, among the scanning cases, we emphasize a particular electrode shape that provides the best image resolution along with the minimal amount of acquisition time.
Velocity Profiling of a Gas–Solid Fluidized Bed Using Electrical Capacitance Volume Tomography
In this study, a method of producing velocity profile maps from electrical capacitance volume tomography (ECVT) measurements by reconstructing displacement from measured changes in capacitance is developed and applied to fluidized bed systems. The mapping of the reconstruction leverages the gradient of the sensitivity distribution of the ECVT sensor to circumvent the need for image cross correlation techniques. Experimental data of both bubbling and slugging fluidized beds are collected in a cold flow model. Adaptation of the technique is discussed in detail, and velocity profiles are obtained for a range of gas flow rates. The produced velocity maps are compared against the established methods of cross correlation and against empirical correlations from the literature and are found to agree well in tracking slug and bubble velocity. The exception is when the tracked object is large relative to the ECVT sensor dimensions, a scenario that can be avoided through proper sensor design. The quantities of average velocity, momentum, and solid and gas volume fraction are derived from the image and velocity profiles. The results demonstrate and extend the power of ECVT as a measurement tool for the study and monitoring of gas–solid fluidized beds by providing a computationally cheaper alternative to 3-D cross correlation for deriving velocity profiles.
Large area, low capacitance, GaAs nanowire photodetector with a transparent Schottky collecting junction
We present experimental results on a GaAs/Indium-Tin-Oxide Schottky-like heterojunction photodetector based on a nanowire device geometry. By distributing the active detecting area over an array of nanowires, it is possible to achieve large area detection with low capacitance. Devices with bare GaAs and passivated AlGaAs/GaAs nanowires are fabricated to compare the responsivity with and without surface passivation. We are able to achieve responsivity of >0.5A/W and Signal-Noise-Ratio in excess of 7 dB for 2 V applied reverse bias with passivated nanowire devices. Capacitance-voltage measurement yields <5 nF/cm{sup 2}, which shows a strong possibility for high-speed applications with a broad area device.
Determination of background doping type in type-II superlattice using capacitance-voltage measurements with double mesa structure.
Abstract not provided.
Potential limits of capacitive deionization and membrane capacitive deionization for water electrolysis
Water electrolysis can delay the operation of capacitive deionization (CDI) and membrane CDI (MCDI). Here, we investigate the overpotential limits of CDI and MCDI (OP-MCDI) for water electrolysis and desalination performance. For sodium chloride concentrations of ~28,460 ppm, MCDI with mesoporous carbon electrodes could be operated at up to 2.4 V without water electrolysis, and the salt-adsorption capacity of the OP-MCDI system at 2.4 V was about two times greater than that of the MCDI at 1.2 V. This study offers an opportunity to extend the applicability of conventional CDI and MCDI systems for desalination of low-salinity solutions.
High capacitance of coarse-grained carbide derived carbon electrodes
Abstract not provided
Capacitive deionization using symmetric carbon electrode pairs
Scenarios (a)–(c) are created to study the effect of the chemical surface charge of carbon electrodes on capacitive deionization using symmetric carbon electrode pairs.
Probing the thermal Hall effect using miniature capacitive strontium titanate thermometry
The thermal Hall effect is the thermal analog of the electrical Hall effect. Rarely observed in normal metals, thermal Hall signals have been argued to be a key property for a number of strongly correlated materials, such as high temperature superconductors, correlated topological insulators, and quantum magnets. The observation of the thermal Hall effect requires precise measurement of temperature in intense magnetic fields. Particularly at low temperature, resistive thermometers have a strong dependence on field, which makes them unsuitable for this purpose. We have created capacitive thermometers which instead measure the dielectric constant of strontium titanate (SrTiO 3 ). SrTiO 3 approaches a ferroelectric transition, causing its dielectric constant to increase by a few orders of magnitude at low temperature. As a result, these thermometers are very sensitive at low temperature while having very little dependence on the applied magnetic field, making them ideal for thermal Hall measurements. Here, we demonstrate this method by making measurements of the thermal Hall effect in Bismuth in magnetic fields of up to 10 T.
Freestanding NiFe Oxyfluoride Holey Film with Ultrahigh Volumetric Capacitance for Flexible Asymmetric Supercapacitors
In this work, a freestanding NiFe oxyfluoride (NiFeOF) holey film was prepared by electrochemical deposition and anodic treatments. With the combination of good electrical conductivity and holey structure, the NiFeOF holey film offers superior electrochemical performance, due to the following reasons: (i) The residual metal alloy framework can be used as the current collector to improve electrode conductivity. Moreover, the as-prepared freestanding NiFeOF holey film can be used as a supercapacitor electrode without reliance on binders and other additives. The residual metal alloy framework and binder-free electrode effectively reduces electrode resistance, thus improving electron transport. (ii) The highly interconnected holey structure and hierarchical pore distribution provides a high specific surface area to improve electron transport, enhancing rapid ion transport and mitigating diffusion limitations throughout the holey film. (iii) The excellent mechanical characteristics facilitate flexibility and cyclability related performance. Additionally, the NiFeOF holey film presents exceptional electrochemical performance, showing that it is a promising alternative for small/micro-size electronic devices.
A combined capacitance-voltage and hard x-ray photoelectron spectroscopy characterisation of metal/Al{sub 2}O{sub 3}/In{sub 0.53}Ga{sub 0.47}As capacitor structures
Capacitance-Voltage (C-V) characterization and hard x-ray photoelectron spectroscopy (HAXPES) measurements have been used to study metal/Al{sub 2}O{sub 3}/In{sub 0.53}Ga{sub 0.47}As capacitor structures with high (Ni) and low (Al) work function metals. The HAXPES measurements observe a band bending occurring prior to metal deposition, which is attributed to a combination of fixed oxide charges and interface states of donor-type. Following metal deposition, the Fermi level positions at the Al{sub 2}O{sub 3}/In{sub 0.53}Ga{sub 0.47}As interface move towards the expected direction as observed from HAXPES measurements. The In{sub 0.53}Ga{sub 0.47}As surface Fermi level positions determined from both the C-V analysis at zero gate bias and HAXPES measurements are in reasonable agreement. The results are consistent with the presence of electrically active interface states at the Al{sub 2}O{sub 3}/In{sub 0.53}Ga{sub 0.47}As interface and suggest an interface state density increasing towards the In{sub 0.53}Ga{sub 0.47}As valence band edge.
Capacitance Behavior of Ordered Mesoporous Carbon/Fe2O3 Composites: Comparison Between 1D Cylindrical, 2D Hexagonal, and 3D Bicontinuous Mesostructures
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Current dependence of the low bias resistance of small capacitance Josephson junctions
Not Available
Sequential Capacitive Deposition of Ionic Liquids for Conformal Thin Film Coatings on Oxygen Reduction Reaction Electrocatalysts
Ionic liquid (IL)-modified carbon-supported catalysts have demonstrated significant improvements in oxygen reduction reaction (ORR) activity. However, transition of this result from the half-cell to the proton exchange membrane fuel cell (PEMFC) has been challenging. Presented here is a processing methodology that yields the formation of a thin (<2 nm) conformal IL coating on the surface of both three-dimensional electrocatalysts and bulk single crystals through the sequential capacitive deposition (SCD) of anionic and cationic components and their subsequent condensation into a hydrophobic, protic IL. In conclusion, SCD shows promise for IL incorporation into preformed PEMFC catalyst layers.