Effect Of Double Layer Capacitance Research Articles

CoFe2O4- mesoporous carbons derived from Eucommia ulmoides wood for supercapacitors: Comparison of two activation method and composite carbons material synthesis method

The mesoporous activated carbon (AC) was synthesized by low temperature phosphoric acid activation based on Eucommia ulmoides Oliver (EUO) wood (one-step) and EUO-charcoal (two-step). The pore structure, surface composition and graphitization degree of carbons synthesized by one-step and two-step activation were systematically investigated. The results exhibited that the specific surface area of porous carbon varies from 1264.8 m2/g (two-step activation, AC2) to 1506.7 m2/g (one-step activation, AC1), which indicated AC1 gained more well-developed mesoporous structure (the mesoporous ratio reached 75.27 %) than that of AC2. Besides, as CoFe2O4 spinel-type metal oxides nanoparticles (NPs) were anchored on ACs by hydrothermal method and chemical co-precipitation, the electrochemistry characteristics were presented in a three-electrode system. As compared with the products (ACs/CF2) prepared by chemical co-precipitation to prepared, AC1 combined with CoFe2O4 by hydrothermal method (AC1/CF1) showed higher specific capacitance of 596.26 F/g (0.005 V/s), which was profited from the synergistic effect of double-layer capacitance and pseudo-capacitance, and the capacitance retention maintained 93.7 % after 10,000 cycles of charging-discharging at 2 A/g. Therefore, EUO wood is a promising feedstock for the preparation of porous carbon, its metal compound (ACs/CFs) provided excellent performance in potential supercapacitors applications, and by the means of the comparisons for the activation method and the scheme for combination of CoFe2O4, this study generated significative guidance for the preparation of activated carbon and its composite.

Synergistic interaction between pseudocapacitive Fe3O4 nanoparticles and highly porous silicon carbide for high-performance electrodes as electrochemical supercapacitors

Composites of micro- and mesoporous SiC flakes (SiCF) and ferroferric oxide (Fe3O4), SiCF/Fe3O4, were prepared via the chemical deposition of Fe3O4 on SiCF by the chemical reduction of an Fe precursor. The SiCF/Fe3O4 electrodes were fabricated at different Fe3O4 feeding ratios to determine the optimal Fe3O4 content that can maintain a high total surface area of SiCF/Fe3O4 composites as well as cause a vigorous redox reaction, thereby maximizing the synergistic effect between the electric double-layer capacitive effects of SiCF and the pseudo-capacitive effects of Fe3O4. The SiCF/Fe3O4 electrode fabricated with a Fe3O4/SiCF feeding ratio of 1.5:1 (SiCF/Fe3O4(1.5)) exhibited the highest charge storage capacity, showing a specific capacitance of 423.2 F g−1 at a scan rate of 5 mV s−1 with a rate performance of 81.8% from 5 to 500 mV s−1 in an aqueous 1 M KOH electrolyte. The outstanding capacitive performance of the SiCF/Fe3O4(1.5) electrode could be attributed to the harmonious synergistic effect between the electric double-layer capacitive contribution of the SiCF and the pseudocapacitive contribution of the Fe3O4 nanoparticles introduced on the SiCF surface. These encouraging results demonstrate that the SiCF/Fe3O4(1.5) electrode is a promising high-performance electrode material for use in supercapacitors.

Fast and reversible redox reaction of MgCo2O4 nanoneedles on porous β-polytype silicon carbide as high performance electrodes for electrochemical supercapacitors

Abstract MgCo 2 O 4 nanoneedles were deposited onto micro and mesoporous silicon carbide flakes (SiCF) to synthesize hybrid electrode materials with high capacitive performance for use as supercapacitors. These SiCF/MgCo 2 O 4 electrodes were fabricated at different MgCo 2 O 4 feeding ratios to determine the optimal MgCo 2 O 4 amount for both total surface area coverage and a suitable redox reaction rate by maximizing the synergy between the electric double layer capacitive effects of SiCF and the Faradic reaction of MgCo 2 O 4 nanoneedles. The SiCF/MgCo 2 O 4 electrode formed at a MgCo 2 O 4 /SiCF feeding ratio of 1.8:1 (SiCF/MgCo 2 O 4 (1.8)) had a specific surface area of 1069 m 2 g −1 . This surface featured the highest specific stored charge capacity of 310.02 C g -1 at a scan rate of 5 mV s -1 with 83.2% rate performance when the scan rate was increased from 5 to 500 mV s -1 in a 1 M KOH electrolyte. The outstanding electrochemical performance of the SiCF/MgCo 2 O 4 (1.8) electrode can be attributed to the ideal electrode material design, considering both the electric double-layer capacitive contribution of SiCF and the battery-type electrochemical behavior of the MgCo 2 O 4 nanoneedles on the SiCF surface. For high capacity electrode materials, this hybrid material strategy introduces possibilities for combinations of porous silicon carbide with other battery-type binary metal oxide materials.

Modeling of Effect of Double-Layer Capacitance and Failure of Lead-Acid Batteries in HRPSoC Application

Lead-acid batteries fail faster in partial state-of-charge start-stop technology than in SLI application. Accumulation of lead sulfate on negative electrode’s surface has been identified as the cause. It is also known that life can be enhanced by increasing capacitance of negative electrode. A bench-marking test cycle is used to explain these observations through a one-dimensional model. It is shown that, at the large discharge current densities used, faradaic reactions in the electrodes are spatially inhomogeneous, and charging is unable to reverse its effects. Consequently, lead sulfate deposit is larger on electrode’s surface than at its center. Model uses a rate expression for charging modified to include diffusion of Pb2 +, and predicts that sulfate continues to accumulate with cycling. A portion of electrode becomes inactive when volume fraction of sulfate reaches a critical value there. Battery fails when inactive area becomes large. It is shown that double-layer capacitance suppresses the non-uniformity in the faradaic reaction and alters the pattern of accumulation of sulfate. Negative electrode does not benefit from this since its capacitance is low. Sulfate accumulates in positive electrode also, but does not reach critical levels since positive electrode’s capacitance is large. This explains the life enhancing effect of capacitance.

Green synthesis and electrochemical characterization of rGO–CuO nanocomposites for supercapacitor applications

Reduced graphene oxide (rGO) were prepared from graphene oxide (GO) by using piperine as a green reducing agent extracted from Piper nigrum. The obtained rGO had few defects and lacked connectivity between the layers. To overcome these defects, copper oxide (CuO) nanoparticles were synthesized ultrasonically and nanocomposites of rGO–CuO were prepared. The conductivities of the rGO, CuO and rGO–CuO nanocomposites were determined by AC impedance spectroscopy in different electrolytes. Morphology, composition and electronic structure of CuO, rGO and rGO–CuO nanocomposites were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photon spectroscopy (XPS) and electrochemical techniques. Transmission electron microscopy (TEM) images portrait CuO as a fish caught in the net of rGO layers. The rGO–CuO nanocomposite exhibiting lower resistance and higher capacitance was used in fabrication of supercapacitor electrodes. The specific capacitance of the fabricated supercapacitor was found to be 137 Fg −1. The supercapacitor performance of the nanocomposite electrode is attributed to the synergistic effect of double-layer capacitance of rGO and redox capacitance of CuO nanoparticles.

Investigation of the Electrical Characteristics on Measurement Frequency of a Thin-Film Ceramic Humidity Sensor

Efforts have been made by the researchers to enhance the sensitivity of a humidity sensor by selecting and fabricating different nanostructures thin films and optimizing their pore morphology. However, other characteristics, such as nonlinearity, hysteresis, reproducibility, response and recovery times, and selectivity, are equally important for real time measurement, monitoring, and controlling of humidity. It is reported that a low signal frequency is essential for enhanced sensitivity, but the effect of the signal frequency on other characteristics is rarely investigated. We have investigated the role of the signal frequency on the response characteristics of a nanostructure ceramic thin-film interdigitated electrode humidity sensor for the first time. Our studies show that although the sensitivity is high at low frequency, the performance of the sensor is highly nonlinear and frequency dependent. At high frequency above 100 kHz, the sensitivity is reduced, but other characteristics of the sensors are improved significantly. Improved characteristics are due to the reduced effect of double layer capacitance that plays a significant role for extreme sensitivity at low frequency. The effect of double layer capacitance is rarely considered for evaluating the response characteristics in the literatures. Results show that at 200-kHz frequency, nonlinearity and hysteresis are 9.39% and 16.77%, while at 100 Hz, these values are 5588% and 70.8%, respectively. Improve response characteristics will reduce the complexity of signal conditioning, particularly nonlinearity and hysteresis, the compensation of which is essential for almost all types of humidity sensors.

Probing the electrochemical properties of TiO2/graphene composite by cyclic voltammetry and impedance spectroscopy

Abstract This work describes the role of graphene in charge transport and diffusion mechanism at TiO2/graphene electrode–electrolyte interface. To explore the mechanism, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were used. The CV results depict that TiO2 and TiO2/graphene electrodes behave differently in terms of charge transport and ion adsorption under the steady state conditions. The performance of TiO2 electrode–electrolyte interface is mainly limited by the charge transport and pseudo-capacitive effects while the response of TiO2/graphene electrode–electrolyte interface is mainly dominated by the double layer capacitive effects. The EIS measurement leads to the direct determination of broad range of parameters, i.e. series resistance, charge transport, rate capability and ion diffusion. The experimental results and their analysis will have a significant impact on understanding the role of graphene in the electrochemical response of an electrode–electrolyte interface.

Effect of Interface Layer Capacitance on Polydimethylsiloxane in Electrowetting-on-Dielectric Actuation

Electrowetting is an effective way to manipulate small volume of liquid in microfluidic applications. It has been sophisticatedly used in the fields of Lab-on-a-Chip (LoC) devices, optics, biomedical applications, and electronic paper (e-paper). Generally, Young-Lippmann (Y-L) equation is used to relate the mechanical and electrical force involved in electrowetting-on-dielectric (EWOD) based actuation. And the general trend is to neglect the effect of double layer capacitance formed at the metal-liquid interface considering the Debye-length to be in the order of nanometer. But, at electrode-electrolyte-insulator interface, the effect of interface layer capacitance becomes significant and often leads to the mismatch between the experimental observation and theoretical result. In this work, the surface behaviour of polydimethylsiloxane (PDMS) for EWOD application is studied experimentally and a term “k” has been introduced in the Y-L equation to match the theoretical and experimental result. Effect of interface layer capacitance has been observed in contact angle versus applied voltage experiment with different pH buffer solution. The introduction of “k” term takes care of the interface layer capacitance which can not be neglected and plays a vital role when the applied electric potential is high.

Transient dynamic and modeling parameter sensitivity analysis of 1D solid oxide fuel cell model

In this paper, a multiphysics solid oxide fuel cell (SOFC) dynamic model is developed by using a one dimensional (1D) modeling approach. The dynamic effects of double layer capacitance on the electrochemical domain and the dynamic effect of thermal capacity on thermal domain are thoroughly considered. The 1D approach allows the model to predict the non-uniform distributions of current density, gas pressure and temperature in SOFC during its operation. The developed model has been experimentally validated, under different conditions of temperature and gas pressure. Based on the proposed model, the explicit time constant expressions for different dynamic phenomena in SOFC have been given and discussed in detail. A parameters sensitivity study has also been performed and discussed by using statistical Multi Parameter Sensitivity Analysis (MPSA) method, in order to investigate the impact of parameters on the modeling accuracy.

Electrolytic Characteristics of Tetramethylammonium Compounds and Performance of Electric Double-Layer Capacitors Evaluated by Using Transmission-Line Model

The use of oxalatoborate ions, difluoro(oxalato)borate (DFOB−) and bis(oxalato)borate (BOB−), improved the solubility of the tetramethylammonium compounds in propylene carbonate (PC) and increased the capacitances of electric double-layer capacitors (EDLCs). Ionic conductivities of tetramethylammonium difluoro(oxalato)borate (TMADFOB) solutions in PC were slightly lower than that of tetraethylammonium tetrafluoroborate (TEABF4) solutions. However, the conductivities were higher than those of tetramethylammonium bis(oxalato)borate (TMABOB) and tetraethylammonium bis(oxalato)borate (TEABOB) solutions. We have used a transmission-line network of finite length as the equivalent circuit of the average pore of an activated carbon electrode. The equation we derived has enough flexibility to show the effect of double-layer capacitance on the length and shape of a single pore. We evaluated the performance of EDLCs by comparing the theoretical and the experimental discharge curves. The TMADFOB solution in PC afforded the highest capacitance.

A Transient Fuel Cell Model to Simulate HTPEM Fuel Cell Impedance Spectra

This paper presents a spatially resolved transient fuel cell model applied to the simulation of high temperature PEM fuel cell impedance spectra. The model is developed using a 2D finite volume method approach. The model is resolved along the channel and across the membrane. The model considers diffusion of cathode gas species in gas diffusion layers and catalyst layer, transport of protons in the membrane and the catalyst layers, and double layer capacitive effects in the catalyst layers. The model has been fitted simultaneously to a polarization curve and to an impedance spectrum recorded in the laboratory. A simultaneous fit to both curves is not achieved. In order to investigate the effects of the fitting parameters on the simulation results, a parameter variation study is carried out. It is concluded that some of the fitting parameters assume values which are not realistic. In order to remedy this, phenomena neglected in this version of the model must be incorporated in future versions.

Theory of large-amplitude sinusoidal voltammetry for reversible redox reactions

Abstract Sinusoidal voltammetry, where the voltage input is a pure sine wave without any dc ramp, is an effective technique for interrogating electrochemical systems containing reversible redox reactions. In particular, it eliminates the need for baseline subtraction, since purely faradaic information can be obtained from the higher harmonics free from the effects of double layer capacitance. In this article we derive a novel analytical solution for the long-time current response, i.e. as the current approaches a periodic state, using excitation amplitudes of any magnitude. We define a new protocol for deducing the underlying parameters of the electrochemical system, such as bulk concentrations, diffusion coefficients, the half-wave potential, and the double-layer capacitance (if it is linear), from the amplitudes and phases of the different harmonics, and the dc part of the current response. In addition, we show how the Hann window can be used in conjunction with the FFT to derive clean harmonic information from a current signal containing large linear capacitive interference, where leakage in the power spectrum from the fundamental harmonic contaminates the higher harmonics.

Large-amplitude ac voltammetry: Theory for reversible redox reactions in the “slow scan limit approximation”

Abstract Analytical solutions for the current response of an ac voltammetric experiment on a reversible redox system have traditionally relied on two approximations: the “slow scan limit approximation” and small ac potential amplitudes. There has been no rigorous analytical investigation of the limits of validity of the solutions derived under the first assumption, and the second assumption only allows for small currents, which restricts the applicability of the method. In this article, we derive a novel analytical solution for the current response, valid for ac potential excitations of any magnitude. We establish rigorous estimates of the error induced by the slow scan limit approximation and discuss in detail how this should influence the choice of experimental parameters. The effects of double-layer capacitance are generally large under the slow scan limit approximation and can cause difficulties in isolating the higher harmonics due to spectral leakage in the power spectrum of the FFT. We demonstrate how the Hann window can be used to overcome this problem in the case of linear capacitance. Finally we show how parameters describing the electrochemical system (including linear capacitance) can be deduced from features of the time-dependent harmonic envelopes and the dc part of the current response, without the need for baseline subtraction.

Shrinking core discharge model for the negative electrode of a lead-acid battery

This article presents a shrinking core model for the discharge of porous lead particle at the negative electrode of a lead-acid battery by considering the reaction in a separate particle of the solid matrix. The model relates the shrinking unreacted lead core with the maximum amount of active material that can be reacted before termination of the discharge process due to poorly soluble low-conducting product. The developed model equations also incorporate the effect of double-layer capacitance and a dissolution–precipitation mechanism on the discharge process. The expressions for evaluating concentration and potential distributions as functions of time and distance are presented in three different models of a porous lead particle. For the simple case, equations are solved to achieve analytical expressions and where the coupled potential and concentration gradients with dissolution–precipitation mechanism are taken into account; the numerical method of lines is utilized to study the discharge behavior. The simulation outcomes are in good agreement when compared with the experimental data for discharge.

Experimental Strategy for Characterization of Essential Dynamical Variables in Oscillatory Systems: Effect of Double-Layer Capacitance on the Stability of Electrochemical Oscillators

An experimentally accessible algorithm for changing the time scale associated with a dynamical variable is proposed. In general, a differential controller can be applied to (a) identify the essential species in oscillatory systems and (b) explore their role in the feedback loops. Here, we report on classifying electrochemical oscillators by changing the time scale over which the electrode potential varies; the type of different electrochemical oscillators is identified based on whether the controlled modification of pseudo-capacitance induces or suppresses current oscillations.

High-frequency generation of electrochemiluminescence at microelectrodes

High-frequency generation of electrochemiluminescence (ECL) from ion-annihilation reactions has been investigated at microelectrodes. Well-defined, stable, reproducible luminescence curves were obtained when a multicycle square wave was applied to a microelectrode in the presence of either 9,10-diphenylanthracene, 9,10-dimethylanthracene, or 9-phenylanthracene. ECL efficiencies of ca. 5%, 2%, and 2% were obtained for these compounds, respectively, using ruthenium(II) tris(bipyridine) as a standard. Due to the reduced effects of double-layer capacitance and ohmic drop, the microelectrode can be pulsed at a more rapid rate than electrodes of conventional size. The electrochemical-time constant, which was directly evaluated from the luminescence data, determined the upper frequency limit

On porous electrodes in electrolyte solutions: I. Capacitance effects

On the basis of a model, describing the pores of a porous electrode as essentially circular cylindrical channels of uniform diameter and of semi-infinite length, a unified and generally applicable treatment is given for the individual or combined effects of double layer capacitance and faradaic process in a.c. measurements. Furthermore explicit expressions are derived for the faradaic process when it consists of diffusion coupled with a fast or a first-order slow electrode reaction. The difficulties in the mathematical treatment of potentiostatic or galvanostatic pulse techniques are briefly discussed.