Influence of anion composition and size on the double layer capacitance for Bi(111)|room temperature ionic liquid interface

Cyclic voltammetry and electrochemical impedance spectroscopy have been applied for investigation of electrochemically polished Bi(111) electrode in 1-ethyl-3-methyl imidazolium tris(pentafluoroethyl)trifluorophosphate (EMImFAP), 1-ethyl-3-methyl imidazolium tetracyanoborate (EMImTCB) and 1-ethyl-3-methyl imidazolium tetrafluoroborate (EMImBF₄) ionic liquids. The region of ideal polarizability, series resistance and capacitance, limiting high-frequency and low-frequency capacitances have been calculated. The lower series capacitance values at electrode potential less negative than the potential of the minimum in the capacitance vs. voltage curve for Bi(111)|EMImFAP than that for EMImBF₄, and especially for EMImTCB, have been explained by the bigger diameter of FAP^?, higher cation and anion sizes symmetry, and less expressed surface activity (i.e. lower closest approach of the FAP^? mass centre to an electrode surface) compared with BF₄^? and TCB^? anions.     

Discrete Galerkin method for Fredholm integro-differential equations with weakly singular kernels

Approximations to a solution and its derivatives of a boundary value problem of an nth order linear Fredholm integro-differential equation with weakly singular or other nonsmooth kernels are determined. These approximations are piecewise polynomial functions on special graded grids. For their finding a discrete Galerkin method and an integral equation reformulation of the boundary value problem are used. Optimal global convergence estimates are derived and an improvement of the convergence rate of the method for a special choice of parameters is obtained. To illustrate the theoretical results a collection of numerical results of a test problem is presented.     

Influence of nanoporous carbon electrode thickness on the electrochemical characteristics of a nanoporous carbon|tetraethylammonium tetrafluoroborate in acetonitrile solution interface

Electrochemical characteristics for the nanoporous carbon|Et₄NBF₄+acetonitrile interface have been studied by cyclic voltammetry and impedance spectroscopy methods. The influence of the electrolyte concentration and thickness of the nanoporous electrode material on the shape of the cyclic voltammetry and impedance curves has been established and the reasons for these phenomena are discussed. A value of zero charge potential, depending slightly on the structure and concentration of the electrolyte, the region of ideal polarizability and other characteristics have been established. The nanoporous nature of the carbon electrodes introduces a distribution of resistive and capacitive elements, giving rise to complicated electrochemical behaviour. Analysis of the complex plane plots shows that the nanoporous carbon|Et₄NBF₄+acetonitrile electrolyte interface can be simulated by an equivalent circuit, in which two parallel conduction paths in the solid and liquid phases are interconnected by the double-layer capacitance in parallel with the complex admittance of the hindered reaction of the charge transfer or of the partial charge transfer (i.e. adsorption stage limited) process. The values of the characteristic frequency depend on the electrolyte concentration and electrode potential, i.e. on the nature of the ions adsorbed at the surface of the nanoporous carbon electrode. The value of the solid state phase resistance established is independent of the thickness of the electrode material.     

Teilkörper höheren Geschlechts eines algebraischen Funktionenkörpers

Ohne Zusammenfassung     

Synthesis of DLC films by PLD from liquid target and dependence of film properties on the synthesis conditions

DLC (Diamond-like carbon films) were prepared by pulsed laser ablation of a liquid target at substrate temperatures from 18 to 600°C using 248 nm KrF excimer laser. The sp³ hybridization state carbon formation was additionally promoted by gaseous H₂O₂ flow through the reaction chamber and substrate excitation by the same laser beam. Deposited DLC films were characterised by Raman scattering spectroscopy and atomic force microscopy (AFM). Comparative AFM and Raman study shows that the increase in the content of sp³ type bonding in DLC is in correlation with the increase of the surface roughness of the samples prepared.     

Influence of electrolyte characteristics on the electrochemical parameters of electrical double layer capacitors

Electrical double layer capacitors based on ideally polarizable nanoporous carbon electrodes in propylene carbonate with the addition of different 1 M Me₃EtNBF₄, Me₂Et₂NBF₄, MeEt₃NBF₄, Et₄NBF₄, Et₃PrNBF₄ and Et₃BuNBF₄ electrolytes have been tested by cyclic voltammetry, chronoamperometry and electrochemical impedance methods. The limits of ideal polarizability, low-frequency limiting capacitance and series resistance, time constant, Ragone plots (energy density vs. power density dependencies) and other characteristics have been discussed. The influence of the electrolyte molar mass on the electrochemical characteristics of the nanoporous carbon electrode cells has been established. The applicability limits of the Srinivasan and Weidner model have been tested.     

Study of adsorption of tetraethylammonium ions on Bi single crystal planes from solutions in ethanol

The adsorption of tetraethylammonium (TEA⁺) ions on the (001) and planes of the bismuth single crystal from solutions in ethanol has been investigated by impedance measurement method. The experimental data were obtained in 0.02 M mixed-electrolyte solutions and the calculations performed with electrode potential as the independent electrical variable. The Gibbs energy of adsorption of TEA⁺ ions has been calculated using the simple virial adsorption isotherm, and it was found that the adsorption of TEA⁺ cations is weaker than the adsorption of halide anions. The electrosorption valency evaluated has a nearly constant value in the potential region studied. It was concluded that the formed effective surface dipole is significantly screened by the solvent molecules and the metal electron gas. The analysis of the impedance spectra was performed by fitting the experimental data to the various equivalent circuits. It was found that the behaviour of TEA⁺ ions at Bi(hkl)∣ethanol interface can be described with the equivalent circuit, corresponding to the classical Frumkin–Melik-Gaikazyan model. The results obtained indicate that only weak interaction between TEA⁺ ions and bismuth surface takes place, and there is no remarkable partial charge transfer from the adsorbed ions to the Bi surface atoms.     

Electrochemical characteristics of Ce0.8Gd0.2O1.9|La0.6Sr0.4CoO3-δ + Ce0.8Gd0.2O1.9 half-cell

The analysis of the medium temperature half-cell Ce_0.8Gd_0.2O_1.9|70 wt% La_0.6Sr_0.4CoO_3- (LSCO) + 30 wt % Ce_0.8Gd_0.2O_1.9 (CGO) has been made by electrochemical impedance, cyclic voltammetry and chronoamperometry. The shape of complex impedance plots depends on temperature and cathodic polarisation of the electrode. Nyquist (Z, Z-) plots were fitted by equivalent circuit taking into account the electrolyte properties (at very high frequencies), charge transfer process at grain boundaries (at high frequencies), and medium and low frequency O₂ reduction process at the cathode surface and inside the porous cathode material. Two different time constants have been obtained for the cathode process, i.e. for electroreduction of oxygen. It was found that the addition of CGO into the cathode material (LSCO) only somewhat decreases the surface catalytic activity but the noticeably higher low-frequency resistance (i.e. mainly diffusion-like mass transfer resistance R_D) values at lower temperatures have been calculated. It was found that the mainly bulk diffusion-limited process at T773 K deviates toward the kinetically mixed process (diffusion + charge transfer) with increasing temperature.     

Comparison of carbon aerogel and carbide-derived carbon as electrode materials for non-aqueous supercapacitors with high performance

Two porous carbon materials, one synthesised by pyrolysis of an organic aerogel prepared using sol–gel method and the other synthesised from molybdenum carbide by high temperature chlorination method, were tested as supercapacitor electrode materials in a non-aqueous tetraalkylammonium salt-based electrolyte. The gravimetric capacitance values calculated for the carbon aerogel (CAG)-based system were almost two times smaller (~55?F?g^?1) compared to carbide-derived carbon (C(Mo₂C))-based system (~125?F?g^?1). However, due to the very wide region of ideal polarizability, 3.6?V for C(Mo₂C) and 3.8?V for CAG-based test cells, very high energy densities up to 63?Wh?kg^?1 (34?Wh?dm^?3) and power densities up to 757?kW?kg^?1 (314?kW?dm^?3) were estimated for these systems, respectively. CAG-based system shows very short characteristic charge/discharge time constant values (0.05?s).     

Piecewise polynomial collocation for linear boundary value problems of fractional differential equations

We consider a class of boundary value problems for linear multi-term fractional differential equations which involve Caputo-type fractional derivatives. Using an integral equation reformulation of the boundary value problem, some regularity properties of the exact solution are derived. Based on these properties, the numerical solution of boundary value problems by piecewise polynomial collocation methods is discussed. In particular, we study the attainable order of convergence of proposed algorithms and show how the convergence rate depends on the choice of the grid and collocation points. Theoretical results are verified by two numerical examples.