Enn Lust

Optimized Structure of Nanoporous Carbon-Based Double-Layer Capacitors

Electrochemical double-layer capacitors with micro-to-nanoporous electrodes based on activated carbon utilize the gross interface between carbon and the electrolyte, which fills the pores of two main classes, (i) micrometer voids between agglomerates of carbon grains and (ii) micro-to-nanometer pores inside agglomerates and grains. The bigger pores provide good transport of ions throughout the layer whereas the smaller pores generate a large interfacial area. This essentially bifunctional architecture prompted us to combine two complementary concepts in the pertinent theory of double-layer capacitors, viz. linear transmission line models and self-affine fractal models. The merit of this structure-based approach, involving also the effect of hindrance of ionic conductivity in nanopores, is that it relates basic structural characteristics to the dynamic performance. Practically, with no fitting parameters, it reproduces the main features of recently reported complex impedance data. This refers to the effect of electrode thickness, constant phase angle (CPA) and crossover between CPA and non-CPA behavior. Routes toward the optimization of the capacitor architecture for the largest possible static capacitances and rapid charging-discharging are explored. The comparison of calculated optimum capacitance values with recent experimental data reveals remarkable reserves for advanced structural design.

Electrochemical Characteristics of Carbide-Derived Carbon ∣ 1 -Ethyl-3-methylimidazolium Tetrafluoroborate Supercapacitor Cells

Supercapacitor (SC) cells based on microporous titanium carbide-derived carbon [C(TiC)] electrodes in room temperature ionic liquid (I-ethyl-3-methylimidazolium tetrafluoroborate) have been tested by cyclic voltammetry, constant current charge/discharge, and electrochemical impedance methods at temperatures from 25 to 80°C. The limits of ideal polarizability, low frequency limiting capacitance and series resistance, time constant, complex power components, time stability, and other characteristics have been calculated and discussed. The influence of temperature on the cell capacitance, characteristic time constant values, and region of ideal polarizability has been analyzed. The comparatively high energy (40 Wh kg -1 ) and power (130 kW kg -1 ) densities per active C(TiC) weight have been calculated, weakly depending on temperature, if T ≤ 59°C. A comparison of the results to the corresponding data for the C(TiC)|1 M(C 2 H 5 ) 3 CH 3 NBF 4 + acetonitrile system indicates that 1-ethyl-3-methylimidazolium tetrafluoroborate|carbide-derived carbon SC cells demonstrate lower power densities, a narrower region of ideal polarizability, and a lower constant current cyclability at higher charge/discharge rates.

Electrochemical Characteristics of Nanoporous Carbide-Derived Carbon Materials in Various Nonaqueous Electrolyte Solutions

Electrochemical characteristics for the 1 M (C2H5)3CH3NBF4 + acetonitrile (AN)jcarbide-derived carbon nanoporous (prepared from TiC, a-SiC, Mo2C, Al4C3 and B4C) interface have been established by cyclic voltammetry and electrochemical impedance spectroscopy. The gas adsorption measurements have been used for the obtaining the specific surface area, pore size distribution, nanopore volume and other characteristics, dependent on the precursor carbide used (nanopores are pores in the range of 2 nm and below – i.e. micropores according to IUPAC classification). The region of ideal polarizability, values of series and parallel capacitances, parallel resistance and capacitance and other characteristics dependent on the precursor carbide have been established. 2004 Elsevier B.V. All rights reserved.

Electric double layer structure and adsorption of cyclohexanol on single crystal cadmium, antimony and bismuth electrodes

The electric double layer structure in aqueous and non-aqueous surface inactive electrolyte solutions and the adsorption of cyclohexanol on the electrochemically polished Sb, Bi and Cd electrodes were studied by using cyclic voltammetry, impedance and chronocoulometry. The limits of ideal polarizability, the potentials of zero charge and the values of capacity of inner layer were established. As found, the differences between the zero charge potential for various planes depend on the chemical nature and crystallographic structure of the electrode surface, as well as on the chemical nature of the solvent studied. The inner layer capacity at negative surface charges depends on the crystallographic orientation of the single crystal planes and on the electronic properties of the electrode metal. The inner layer capacity increases in the sequence of metals Sb < Bi < Hg < Cd < Zn, as the hydrophility of electrodes increases and as the contribution of metal phase decreases. The values of capacity of metal phase and the effective thickness of the thin metal surface layer were calculated by using various theoretical approximations. The effective thickness of the thin layer of metal surface increases in the sequence of metals Ga < Zn < Cd < Hg < Bi < Sb as the metallic properties of electrodes decrease. The adsorption parameters of cyclohexanol were established by using Frumkin-Damaskin adsorption theory. It was found that the adsorption characteristics depend on the chemical nature and crystallographic structure of electrode surface. The adsorption activity of cyclohexanol increases in sequence of metals Zn < Cd < Bi < Hg < Sb as the adsorption energy of solvent decreases.

Adsorption of halide anions on bismuth single crystal plane electrodes

Adsorption behaviour of Cl - , Br - and I anions on Bi single crystal plane electrodes from aqueous, methanol and 2-propanol solutions has been studied by impedance spectroscopy and chronocoulometry methods. The values of surface excess and the Gibbs energy of adsorption of halide ions on the Bi(111), Bi(001) and Bi(011) single crystal planes have been calculated using the electrode potential E and the surface charge density a as the independent electrical variables. It was found that under comparable conditions the result obtained at constant E and at constant a are coincident. The formal charge transfer coefficient (so called electrosorption valency) and the dipole moment of the dipole formed by an adsorbed halide ion and its image charge in Bi plane have been calculated. It was found that on the Bi(111) plane, where the chemical bonds are all saturated, the formal charge transfer coefficient and the dipole moment of adsorbed anion are independent of electrode potential. For more active Bi planes there is noticeable dependence of both characteristics on electrode charge and at higher positive charges the covalent bonding between Bi(011) surface atoms and I - is probable. It was concluded that the dipole formed is significantly screened by the solvent molecules and the metal electron gas, but the influence of the solvent dielectrical and other characteristics on the effective dipole moment values is comparatively small.

Influence of Mesoporous Separator Properties on the Parameters of Electrical Double-Layer Capacitor Single Cells

The specific surface area, pore size distribution function, micropore and mesopore volume, and area values have been obtained by gas adsorption/absorption method for various separator materials prepared from polypropylene, cellulose, and poly(vinylidene fluoride). Electrical double-layer capacitors (EDLCs) based on the two identical ideally polarizable nanoporous carbide-derived carbon electrodes and different mesoporous separator materials in 1 M (C 2 H 5 ) 3 CH 3 NBF 4 acetonitrile solution have been tested by cyclic voltammetry and electrochemical impedance methods. The limits of ideal polarizability, low-frequency limiting capacitance and series resistance, time constant, complex power components, and electrolyte conductivity in the separator matrix have been obtained and discussed. The model of Srinivasan and Weidner [J. Electrochem. Soc., 146, 1650 (1999)] has been modified by introducing into it the very high-frequency constant phase and charge-transfer resistance elements. The applicability limits of the modified model have been tested. Noticeable influence of separator chemical composition, thickness, specific surface area, and pore size distribution on the characteristics of the EDLC single cells have been found.

In situ hydrodynamic spectroscopy for structure characterization of porous energy storage electrodes

A primary atomic-scale effect accompanying Li-ion insertion into rechargeable battery electrodes is a significant intercalation-induced change of the unit cell volume of the crystalline material. This generates a variety of secondary multiscale dimensional changes and causes a deterioration in the energy storage performance stability. Although traditional in situ height-sensing techniques (atomic force microscopy or electrochemical dilatometry) are able to sense electrode thickness changes at a nanometre scale, they are much less informative concerning intercalation-induced changes of the porous electrode structure at a mesoscopic scale. Based on a electrochemical quartz-crystal microbalance with dissipation monitoring on multiple overtone orders, herein we introduce an in situ hydrodynamic spectroscopic method for porous electrode structure characterization. This new method will enable future developments and applications in the fields of battery and supercapacitor research, especially for diagnostics of viscoelastic properties of binders for composite electrodes and probing the micromechanical stability of their internal electrode porous structure and interfaces.

Adsorption of pyridine on the (111), (001) and (001) faces of bismuth

The adsorption of pyridine (PY) on Bi(111), Bi(001) and Bi(011) single crystal electrodes has been studied in 0.1 M aqueous NaF solution for concentrations of PY ranging from 1 to 200 mM. In the range of potentials explored, adsorption maximum coverage of the surface and the desorption of PY at E = −1.8 V (SCE) have been observed. At the negative potentials and polarization close to the potential of zero charge, PY molecules at bismuth single crystal planes assume a tilted orientation, with the hydrocarbon ring facing the metal. The saturation coverage Λmax and the limiting capacity C1 decrease, and the shift of zero charge potential EN, due to the displacement of surface water by a monolayer of PY, rises in the sequence of planes Bi(011) < Bi(001) < Bi(111) as the vertical component of the orientation of the PY molecules increases. Adsorption isotherms, values of the attraction constant a and standard Gibbs energy of adsorption ΔGads0 have been determined. As in the case of other organic compounds studied, the activity of Bi planes increases in the sequence of planes Bi(111) < Bi(001) < Bi(011). The partial change transfer from PY to Bi electrodes increases in the same direction of planes. The attractive interaction between the adsorbed molecules in the sequence of planes Bi(011) < Bi(111) < Bi(001) as the superficial density of atoms decreases. The absolute value of the Gibbs energy of adsorption of Py increases in the sequence of electrodes Ag Hg>Bi(001)>Bi(011).

Influence of surface pretreatment of bismuth and cadmium electrodes to the electric double layer and adsorption characteristics of organic compounds

Abstract Influence of the surface structure of electrodes on the electric double layer properties, as well as on the adsorption characteristics of various organic compounds has been investigated by cyclic voltammetry, impedance and electron microscopic methods at variously prepared Bi, Sb and Cd electrodes. The systematic trends of the influence of surface roughness and energetic inhomogeneity on the electric double layer characteristics (value of differential capacity, potential of diffuse minimum, Parsons-Zobel factor, inner layer capacity, roughness function) have been established. The dependencies of the shape of adsorption-desorption maxima of organic compounds and other adsorption parameters (values of attraction interaction, adsorption equilibrium constant, shape of adsorption isotherm) on the electrode surface structure have been investigated. The Debye-length dependent roughness function has been calculated. The approximate values of the linear parameter of homogeneous regions, which prevail at the surface of polycrystalline electrodes have been calculated using various theoretical models. The established values of linear parameters of homogeneous regions have been compared with the characteristics obtained from the electron microscopic studies.

Influence of charge density and electrolyte concentration on the electrical double layer characteristics at rough cadmium electrodes

Abstract A new effective diffuse layer thickness dependent surface roughness model, based on the non-linear Poisson–Boltzmann theory, has been used for the interpretation of impedance data for rough cadmium electrodes. The so-called electrochemical roughness function values at various electrode potentials and surface charge densities have been calculated. It was found that the roughness function values, the inverse value of Parsons–Zobel plot, as well as the fitting coefficient in the Valette–Hamelin approach, obtained from the electrochemical impedance data, are effective (seeming) parameters and do not characterise the real geometrical structure of electrode surface. However, they characterise the deviation of real experimental system from ideally smooth (flat) surface. In the region of moderate rational electrode potentials 0≤| E |≤0.2 V and surface charge densities (1≤| a |≤4 μC cm −2 ) the roughness function rises with the decrease of the effective thickness of diffuse layer. At higher electrode potentials and surface charge densities the surface roughness function levels of to the constant value, i.e. to the so-called geometrical surface roughness value. The new effective diffuse layer thickness dependent surface roughness theory needs future corrections taking into account the role of crystallographic inhomogeneity of the geometrically rough electrode surface, i.e. the dependence of the local surface charge density on the crystallographic structure of energetically homogeneous regions exposed on the macro-polycrystalline electrode surface.