Capacity Of Electrical Double Layer Research Articles

Characteristics of activated carbon prepared from waste PET by carbon dioxide activation

Poly(ethylene)terephthalate (PET) waste was subjected to carbonization in a nitrogen stream at 1098K. The coke was then activated in a carbon dioxide stream under various conditions: temperatures of 1173, 1198 and 1213K as well as process times of 4, 5, 6 and 8h. The activated carbons were characterized using various methods: the structure from Raman and XRD measurements, the porosity from low temperature nitrogen adsorption, the surface properties from cyclic voltammetry and the hydrogen storage capacity from the low temperature adsorption isotherm of H2. The results demonstrated the importance of the temperature and the duration of the process. Higher temperatures result in the etching of graphitic domains of better crystallinity. A relatively small increase in activation time at the highest temperature used yielded a significant increase in the degree of burn-off and porous structure development. The microporosity of these carbons is similar to that of commercial activated carbons. They also have a similar capacity to adsorb water pollutants (e.g. 4-chlorophenol). The PET carbon sample with maximum burn-off exhibited higher values of the microporous structure parameters and the electric double layer capacity in electrolyte solution than the other three samples. The same sample exhibits a sufficient hydrogen storage capacity, which after optimization of the activation conditions should yield an effective storage material. This confirms the possibility of producing activated carbon from waste PET with satisfactory properties by the simple processes of carbonization and activation. The activated carbons obtained have potential use as water pollutant adsorbents, low-cost materials for hydrogen storage and electrode materials in supercapacitors or fuel cells.

Synthesis of carbon nanotube/Ni nanocomposite film by electrophoresis and electroless deposition without Pd pretreatment

Here we report the fabrication of carbon nanotubes (CNTs)/Ni nanocomposite film by electrophoretic deposition (EPD) and subsequent electroless deposition. The present technique requires no pretreatment with expensive Pd species for activation of the surface of CNTs. The EPD bath was modified with NiSO4·6H2O to form Ni clusters on the surface of CNTs. As a result, a thin Ni layer (6nm thickness) was fabricated on CNTs, forming a CNT/Ni nanocomposite film with a thickness of 5–10μm. The fabricated CNT/Ni film had nanoporous structure and exhibited electric double layer capacity and catalytic activities for decoloration of methyl orange.

Cyclic voltammetric and FTIR studies of powdered carbon electrodes in the electrosorption of 4-chlorophenols from aqueous electrolytes

Several types of carbon materials (activated carbon, carbon black, multiwalled carbon nanotubes) differing in porosity and surface chemistry were used to prepare powdered electrodes. Activated carbon (Norit R3-ex) was demineralized and modified by oxidation with conc. HNO3, heat treatment in NH3 at 900°C or heat treatment in argon at 1800°C. Carbon black (Vulcan XC72) was flushed with an organic solvent, while the MWCNTs were functionalized to the hydroxyl and carboxyl forms. Nitrogen adsorption isotherms were used to characterize the pore structure of these materials. Their surface chemistry was assessed using thermogravimetry (TG), elemental analysis, FTIR, EDS and XPS. The ability to adsorb (isotherms) 4-chlorophenol (4-CP) in aqueous solution was determined. Cyclovoltammetric (CV) measurements of powdered carbon electrodes were carried out for blank electrolyte solution (0.1M Na2SO4) and with different concentrations of 4-CP. Changes in the electric double layer capacity and other electrochemical parameters were estimated from the CV curves. The dependence of the electrochemical behavior of a powdered carbon bed on porosity and surface chemistry is analyzed and discussed. The electrochemical properties were related to chlorophenol adsorption ability and FTIR spectral analysis of the adsorption layer.

Development of a conductivity microsensor considering electric double layer capacity

► Gold microelectrode array sensor conductivity sensor has been developed. ► Sensor functionality was examined by the impedance spectroscopy . ► Data was interpreted through the equivalent circuit. ► Scale-down behavior was deduced from measured data. ► Strong non-ideal polarization occurred and is discussed. An electrochemical sensor of a novel design has been developed for the electric sensing of conductive particles in a solution. System is made of PMMA plastic (plexiglass) by means of UV lithography and galvanic electroplating to assure a low manufacturing cost. Our sensor consists of a gold microelectrode array embedded in plexiglass. Sensor design allows the simple integration of control electronics. Sensor characteristics were investigated by the impedance spectroscopy and its functionality examined by the conductivity measurement. The effect of ion concentration on the impedance spectra was studied. Three electrode fields with different characteristic electrode dimension of 250, 100 and 50 μm were manufactured and scale-down effects were verified. The impedance responses of the sensor are interpreted through an equivalent RC circuit. Capacitive dispersion of the electric double layer (EDL) impedance was discovered and the substitution by the constant phase element (CPE) is discussed.

Influence of high-temperature treatment of granular activated carbon on its structure and electrochemical behavior in aqueous electrolyte solution

Activated carbon Norit R3-ex (demineralized) was annealed at various temperatures (950–2700 °C) in an argon atmosphere. The changes of the porosity of the products were characterized on the basis of N2 adsorption isotherms (at 77 K). The texture of the samples was investigated by x-ray diffraction, Raman spectroscopy, and scanning electron microscopy. The presence of surface oxygen (Fourier transform infrared) and its content in the surface layer (from energy dispersive spectroscopy) were determined. The electrical resistivity of powdered samples was measured. Cyclovoltammetry of carbon (powdered electrodes) were carried out and the electrical double-layer capacitances were estimated from the cyclic voltammetry curves. Heat treatment increased the degree of crystallization of the samples, which was correlated with changes in their conductivity. A rapid drop in porosity (at 1800–2100 °C) took place in parallel with a decrease in the electrical double layer capacity. The presence of surface oxygen as a result of oxygen chemisorption on freshly annealed carbon samples was confirmed using several methods.

The Rate-Determining Step of Electroadsorption Processes into Nanoporous Carbon Electrodes Related to Water Desalination

Using potential perturbation techniques, we investigated the kinetics of electroadsorption processes of ions with different dimensions and different concentrations into activated carbon electrodes with various porous structures. We found that the ratio between the pore size and the ion size is of great significance in terms of electroadsorption kinetics. Microporous carbon fiber electrodes exhibited a lower charging rate as compared with mesoporous aerogel carbon. The use of activated aerogel electrodes with a fractal structure composed of both micro- and mesopores, was found to be the best for effective electroadsorption due to both high electrical double layer capacity obtained and high rates of electroadsorption. We also explored the kinetics of electroadsorption processes into molecular sieve carbon electrodes, demonstrating that, despite the hindered adsorption processes into these electrodes, it is possible to obtain effective deionization by using them for selective water desalination processes.

Room-Temperature Ionic Liquids: Excluded Volume and Ion Polarizability Effects in the Electrical Double-Layer Structure and Capacitance

We study structures of room-temperature ionic liquids at electrified interfaces and the corresponding electrical double-layer capacities using a self-consistent mean-field theory. Ionic liquids are modeled as segmented dendrimers and the effective dielectric constant is calculated from the local distribution of ions to accommodate the excluded volume and the local dielectric screening effects. The resulting camel-shaped capacitance curve is further analyzed in terms of the thickness of alternating layers and the polarization of ions at electrified interfaces.

導電性高分子を用いた電解液蓄電による新規蓄電デバイス

We investigated the charge storage ability of electrolytes containing polyaniline and polyaniline-like molecules, and indicate the possibility of increasing the energy density of electric double layer capacitors (EDLC) by applying this electrolyte. Cyclic voltammetry and galvanostatic charge/discharge measurements show that polyaniline and most of the analogous compounds can be used as reversible charge storage materials in propylenecarbonate (PC) and/or γ-butyrolactone (GBL) which are widely used as electrolytes of EDLCs and batteries. Especially N,N′-diphenylphenylenediamine (NNDP) is highly soluble in GBL and PC, and is able to store large amount of electric charge by its two-step redox reaction. With an NNDP/GBL electrolyte solution, about 30% redox capacity is added to the electric double layer capacity of an activated carbon electrode. The shuttle effect with the NNDP cation is effectively suppressed by using an ion-exchange membrane as separator. Although further improvement in solubility and n-type charge storage materials are needed to effectively increase the energy density, we expect that our new system can double the amount of energy density of current EDLCs.

Factor of interfacial potential for the surface plasmon-polariton resonance sensor response

In this work, we investigate how the application of an external potential difference to the sensitive gold-electrolyte interface influences the optical response of a sensor based on the surface plasmon-polariton resonance (SPPR). The SPPR resonant angle shift was registered for an aqueous solution of sulfuric acid as an electrolyte at different potential sweep rates. From the measurements of SPPR curves versus the applied voltage, the potential of zero charge of the gold electrode in the electrolyte solution was estimated. To explain the external voltage influence on the SPPR sensor response, a theoretical model was used that takes into account three factors: free electron concentration change in the space-charge layer (SCL) in the surface layer of gold, dependence of the capacity of electrical double layer at the interface on the voltage, and gold film surface roughness.

Developing Ion Electroadsorption Stereoselectivity, by Pore Size Adjustment with Chemical Vapor Deposition onto Active Carbon Fiber Electrodes. Case of Ca2+/Na+ Separation in Water Capacitive Desalination

By using carbon chemical vapor deposition (CVD) onto active porous carbon fibers, we were able to reduce the pore size of the active carbon to obtain selective carbon molecular sieves (CMS) of variable properties. The pore size of these carbons was characterized by the adsorption of molecules of different dimensions (molecular probe adsorption) from the gas phase. By manipulating the several experimental parameters of the CVD process, the pore size of the CMS could be finely tuned to fall between sizes of alkaline earth cations (Ca2+; Mg2+, 6−7 A in diameter), and the much smaller size of hydrated monovalent cations (4 A in diameter). This was proven by voltammetric measurements of the pore-tailored carbon electrodes in the corresponding salt solutions, which also provided a measure of the differential capacity of the electrical double layer (EDL) vs the potential (e.g., at potentials negative to the potential of the zero charge, the EDL capacity was very small for CaCl2 and MgCl2 solutions, while it was ...

Influence of the electrolyte composition on the kinetics of the oxygen evolution reaction and ozone production processes

The influence of electrolyte composition (pure sulphuric or perchloric acid and their mixtures) on the kinetics of the oxygen evolution reaction (OER) and the electrochemical ozone production (EOP) on beta-PbO2 electrodes was investigated by electrochemical impedance spectroscopy and quasi-stationary polarisation curves. Analysis of the impedance spectra revealed a dependency of the electric double layer capacity and the adsorption pseudocapacitance, due to reaction intermediates, on electrolyte composition. The pseudocapacitance is a function of electrode potential and electrolyte composition, indicating the surface coverage by the reaction intermediates depends on these parameters. Tafel slope data support primary water discharge as rate determining step of the OER and EOP electrode processes. EOP current efficiency data (phiEOP) show pure electrolytes lead to maximum O3 production when compared to mixed electrolytes. According to the theoretical analysis of the electrode mechanism, this behaviour is a consequence of the reduction in surface concentration of the active centres leading to EOP caused by anion adsorption.

Ion Sieving Effects in the Electrical Double Layer of Porous Carbon Electrodes: Estimating Effective Ion Size in Electrolytic Solutions

Certain microporous carbons have pores of molecular dimensions, the size of which can be adjusted over a wide range to fit the dimensions of a large variety of molecules. With knowing the dimensions of these molecules, size-calibrated porous carbons can be produced. In this work, we took advantage of the electronic conductivity of carbon in using it as an electrode in electrolytic solutions. By changing the electrode potential, we could induce electroadsorption and electrodesorption of ions of different dimensions into pore-calibrated carbons, thus enabling us to estimate effective ion sizes. A few fundamental questions such as whether ions accommodated in the electrodes' pores are in a solvated state, which is important for novel electrochemical energy storage devices and for microbiological systems, could be addressed. Analysis of the results indicates that all the cations that are employed are electroadsorbed in the electrode pores in hydrated states. Compared with the monovalent cations, the bivalent cations exhibit dimensions that make them almost twice as big. Monovalent anions are basically adsorbed in a nonhydrated state. The doubly charged sulfate anion is adsorbed in the hydrated state. The nitrate, a multiatom planar anion, has a smaller effective dimension than the monoatomic halogen anions, in accordance with the observation that the pores in many activated carbons are slit-shaped. The analysis of the results enabled us to establish a scale of ionic effective dimensions in aqueous media, as presented below (MTBE stands for methyl tert-butyl ether): cations, [3.62 Å, N2] < Cs+ < K+ < Na+ < Li+ < [4.21 Å, CF4] < [5.05 Å, SF6] < [5.8 Å, MTBE] < Ba2+, Ca2+, and Mg2+; anions, [3.62 Å, N2] < NO3- < Cl- < F- < Br- < [4.21 Å, CF4] < ClO4- < [5.05 Å, SF6] < [5.8 Å, MTBE] < SO2-. We discovered that as long as the pore size is considerably greater than the ion size, the electric double-layer capacity at moderate concentrations is independent of both the size and charge of the ions in solutions.

Electrochemical modelling of chloride migration in cement based materials: Part I: Theoretical basis at microscopic scale

A quantification of electrical double layer influence on chlorides electrodiffusion process in concrete is presented. The modelling is carried out at a microscopic scale of a concrete capillary. The study, which is based on electrical magnitudes engendered by transfers (density of current, potential, space charge density and conductivity), shows the generation of an induced electrical field opposed to the external field imposed. Simulations of the distribution of these parameters through a pore with variable section have shown that when the pores are small, more the opposition of the induced field to the electrodiffusion flux is efficient. Furthermore, the study reveals the existence of two time scales: – τ D characterizes the transfer process, it depends mainly on the nature of the chemical species present in pore solution, – τ c corresponds to the charge establishing time at the interface capillary wall-pore solution. It depends mainly on the electrical double layer capacity.

Carbon Electrodes for Double‐Layer Capacitors I. Relations Between Ion and Pore Dimensions

We characterized activated carbon electrodes for electrical double‐layer capacitor (EDLC) systems. High‐surface‐area carbons were prepared by carbonization of cotton cloth at elevated temperatures (up to 1050°C), followed by activation at 900°C by oxidation with during different time periods. Specific surface areas and characteristic pore sizes obtained from gas adsorption isotherms were correlated with those obtained from ion electroadsorption at the electrical double layer. Electrolytes studied included aqueous LiCl, NaCl, and KCl solutions and nonaqueous propylene carbonate solutions with and salts. We found clear evidence that the porous carbons thus formed exhibit ion sieving properties, and that increasing activation time systematically increases the average pore sizes of these carbons. The electric double layer (EDL) capacity of these samples (calculated from voltammetric measurements) depends strongly on the adsorption interaction of the ions in the pores, and hence the relationship between the average pore size and the effective ion size determines the specific EDL capacitance of these samples. The following order of dimension of adsorbed species was found, based on the ion sieving of the various synthesized carbons of different average pore size © 2000 The Electrochemical Society. All rights reserved.

Adsorption Activity of Cu, Au and Ni Solid Electrodes in Molten Carbonates

The surface energy, the electric double layer capacity and the "estance´´ (the charge derivative of the surface tension) were analyzed as a function of the electric potential across copper, gold and nickel solid electrodes in the eutectic melt 62 mol% Li

Macrokinetics of polyaniline based electrode: effects of porous structure, microkinetics, diffusion, and electrical double layer

The standard porosimetry method (SPM) was used to study in situ real porous structures of poly(aniline) (PAn) swollen in electrolytes containing γ-butyrolactone (GBL). The effects of swelling, the degree of doping, the nature of counterions, and type the of polymerization (chemical or electrochemical) on porous PAn structure were studied. All the PAn forms impregnated with GBL had high porosities of several dozen per cent, high specific surface areas of 80–400 m 2 cm −3, pore radii of 1–100 nm, and average fibril radii of 2–10 nm. Electrochemically synthesized PAn showed better electrochemical characteristics than chemically synthesized PAn. The model of the intercalated porous electrode was used to characterize the discharge curves. Comparison of calculated and experimental curves enabled determination of essential microscopic characteristics: specific exchange current density ( i 0) of real PAn/electrolyte interphase doping–dedoping, specific capacity ( C S) of electrical double layer (EDL) of this interface, and diffusion coefficient ( D) of counterions in the polymer. These characteristics depend upon the nature of counterions and the method of polymerization (chemical or electrochemical). Small D values of 8×10 −19–1×10 −17 cm 2 s −1 result from the solid-phase nature of diffusion; i 0=1×10 −15–1×10 −12 A cm −2, and C S=0.6–5.0 μF cm −2. Very low D, i 0 and C S are compensated by very high S resulting in better electrochemical characteristics. Thin electrodes (<1 mm) showed very small ohmic losses, while the rate of electrochemical doping–dedoping was usually limited by solid-phase diffusion and discharge. The effects of structure, mode, and microscopic parameters on discharge curves were analyzed.

Theory of the Transient Potential Response of Ion-Selective Electrodes Based on the Polarizable Oil/ Water Interface

The theory of the transient potential response of potentiometric ion-selective electrodes (J. Electroanal. Chem. Interfacial Electrochem., 378, 215 (1994)) is further elucidated. General solutions of the transient potential vs. time curves for a stepwise change in the activity (concentration) of a potential-determining ion in an external solution were numerically obtained for different cases concerning their ion-transfer kinetics. Also, the response time, defined as the time which is taken by the potential of an ion-selective electrode to change from its initial value to within a given limit of the final value, was numerically obtained, and is discussed in terms of the ion concentrations, the kinetic parameters of the ion-transfer reaction, and the electrical double layer capacity of the ion-selective electrode interface. Furthermore, the theory is extended so as to include the transient potential response of neutral-carrier-based ion-selective electrodes. The theory predicts the effect of the concentration of a neutral-carrier, which depends on the reaction mechanism of the carrier-assisted ion transfer across the interface.

On the contribution of the metal to the electrical double-layer capacity: Ab initio model calculations on lithium clusters

Results obtained earlier on the electronic structure of three-dimensional lithium clusters are reconsidered in the light of a physically more reasonable concept for the contribution by the metal, C M, to the differential double-layer capacity. The plane specificity of C M, is investigated at the pzc for a metal/vacuum interface as a first approximation, and the results are compared with jellium model predictions.

The electrical double layer of carbon and graphite electrodes: Part IV. dependence of carbon electrode dimensions and electrical capacity on pH

When the aqueous medium of a high surface carbon electrode is changed from neutral to acidic at potentiostatic conditions, the electrode contracts, consumes a significant amount of negative electric charge and its electric double-layer capacity increases mainly at the more positive potential range. The minimum in the curve of electrode length vs. potential which for a reversible system coincides with the potential of zero charge, is shifted negatively upon acidification, and the slope of the electrode length changes with respect to potential ( LD) does not show the expected linear dependence on electric charge as it does in the case of a neutral solution. The effects are partially reversed when the solution is neutralized again. It is suggested that a single mechanism, the generalized surface quinone-hydroquinone (Q-HQ)-type transition that is catalysed in acidic medium is responsible for these phenomena. Thus, charge consumption is due to the HQ to Q conversion and the increase in the electric double-layer capacity results from the reversible couple. Electrode contraction and the loss of the linear LD-Q e relation are due to the increase of the surface tension by the electron added to the conjugated system close to the interface. The shift in the length minimum arises from the decrease in surface oxide dipole because of the change of some keto- to enol-type surface groups. Monitoring the dimensional changes of the electrode may be employed for detecting the addition of π bonds to the graphitic conjugated system in the course of pH or potential changes.

The electrical doublelayer of carbon and graphite electrodes: Part I. Dependence on electrolyte type and concentration

The electrical double layer capacity per unit of the true surface area of graphitized carbon cloth and carbon black was investigated at various conditions. It was found independent of the type and valency of cations and anions as long as these are not specifically adsorbed. It was also independent of concentration and did not show the typical deep minimum at low concentrations. The only feature observed in the capacity-potential curve is a flat minimum. In order to interpret the results, surface heterogeneity and the existence of localized π-electron states at the outer graphitic layer of the electrode had to be assumed.