D. A. Bograchev

Simulation of ion transfer under conditions of natural convection by the finite difference method

Abstract The finite difference method of calculation of non-steady-state ion transfer in electrochemical systems under the conditions of natural convection is elaborated. The method is based on the mathematical model involving the continuity equations for electrolyte species, the condition of electroneutrality, and the Navier–Stokes equations for a viscous incompressible liquid with the corresponding initial and boundary conditions. A scheme of decoupling is proposed, which provides successive calculation of the field of hydrodynamic velocities (a stream function), the distribution of electric potential, and the distribution of electrolyte species concentrations subject to the condition of electroneutrality. To enhance the efficiency of the method at large Schmidt numbers, the distribution of electrolyte species concentrations was calculated by the implicit difference scheme. The results of computational experiments are reported.

Theoretical study of the effect of electrochemical cell inclination on the limiting diffusion current

The effect of inclination of electrochemical cell with plane-parallel electrodes in the gravitational field on the limiting current of electrochemical reaction is theoretically analyzed by the example of iodine � /iodide system. The quantitative dependences of the limiting current on the angle a between the electrodes and the horizontal are obtained. It is shown that this effect is associated with the variation in the distributions of solution concentration and hydrodynamic velocity with a . It is found that the ratio between the electrode’s length and the interelectrode distance is of great importance. # 2002 Elsevier Science Ltd. All rights reserved.

Modeling of metal electrodeposition in the pores of anodic aluminum oxide

The pore filling process in the metal electrodeposition in the anodic aluminum oxide pores under the potentiostatic conditions is studied theoretically. The model takes into consideration the transfer of metal cations inside the pores and in the outer diffusion layer. The thickness of outer diffusion layer is determined by the hydrodynamic conditions. The kinetics of metal electrodeposition is described by the equation of mixed kinetics at large deviations from the equilibrium. In the quasi-steady-state approximation, the problem for the pores of different initial lengths, which are characterized by the normal distribution law, is solved analytically. The results are compared with the data obtained by the numerical method.

Carbon electrodes with high pseudocapacitance for supercapacitors

Electrochemical properties of electrodes on the basis of CH900-20 activated carbon (AC) cloth were studied in concentrated H2SO4 solutions in a wide range of potentials from −0.8 to +1 V RHE. Cyclic voltammetric curves measured in two ranges of potentials were analyzed: in the reversibility range (from 0.1 to 0.9 V) and in the deep cathodic charging range (from −0.8 to 1 V). Electric double layer (EDL) charging occurs in the reversibility range, while faradaic processes of hydrogen chemisorption and its intercalation into carbon take place in the range of negative potentials (<−0.1 V). The intercalation process is controlled by slow solid-phase hydrogen diffusion. For the first time, the maximum value of specific discharge capacity of 1560 C/g was obtained, which is much higher than the values known from the literature for carbon electrodes. On the basis of this value and Faraday’s law, it was assumed that the compound of C6H is formed in the limiting case of AC deep cathodic charging. The specific charge value grows at an increase in the concentration of H2SO4. The mechanism of double intercalation of sulfuric acid and hydrogen into the AC is suggested. The data obtained are used to develop a mathematical charging-discharge model for an AC electrode taking into account the EDL charging, chemisorption, and hydrogen intercalation.

Studies of Supercapacitor Carbon Electrodes with High Pseudocapacitance

During the last decades different new capacitor types were developed based on electrochemical processes. According to Conway [1] an electrochemical capacitor is a device in which different quasi-reversible electrochemical charging/discharging processes take place and for which the shape of the charging and discharging curves is almost linear, similarily to those in common electrostatic capacitors [1-13]. Electrochemical capacitors can be classified as film-type (dielectric), electrolytic and supercapacitors.

Development and experimental verification of a mathematical model of lithium ion battery

A model of the lithium ion battery is developed which takes into account intercalation and extraction of lithium ions in the active mass of negative and positive electrodes, the dependences of equilibrium electrode potentials on the concentration of intercalated lithium, the ion transfer in pores of electrodes and the separator, the kinetics of electrode reactions, and the electric double layer charging. As the active material for the negative electrode, UAMS graphite material is used. Lithium-nickel-cobalt oxide serves as the positive electrode. The porous structure of electrodes is studied by the method of standard contact porosimetry. Sufficiently high porosity values found for both electrodes (50% for anode and 27% for cathode) made it possible to consider the interface as regards the internal pore surface found from porosimetry data rather than as regards their external surface as in the previous studies. A comparison of calculated and experimental discharge curves demonstrates their closeness, which points to the correctness of the model. By the fitting procedure, the coefficients of solid-state diffusion of lithium ions and the rate constants for reactions on both electrodes are found.

Non-Steady-State Natural Convection in a Model Electrochemical System with Vertical and Horizontal Plane Electrodes

Non-steady-state mass-transfer processes in the electrochemical cells of rectangular cross-section with vertical and horizontal electrodes under the conditions of natural convection are simulated numerically taking into account the diffusion and convection of solely ions discharging at the electrode under limiting-current conditions. The interrelated time variations of the limiting current density at various ratios of the electrode height (length) to the interelectrode distance and the distributions of ion concentration and hydrodynamic velocity over the cell are determined.

Optimization of electrolysis in the cylindrical electrochemical cell rotating in the magnetic field

The relationships between the rotational rate of electrochemical cell, its ohmic resistance, a ratio between the radii of inner and outer cylindrical electrodes, a sum of cathodic and anodic potentials, and the mechanical energy, which is consumed for passing a prescribed current and overcoming friction forces, are analyzed. It is shown that the relationships can be used to optimize the operation of rotating electrochemical cell.

Limiting Current in Iodine–Iodide System on Vertical Electrode under Conditions of Natural Convection

The limiting current of the triiodide reduction on a vertical plane electrode in solutions containing excess potassium iodide is calculated by the Karman method for natural-convection conditions. Equations for the average and local limiting currents (which depend on the electrode height), the boundary layer thickness, and the vertical component of the fluid velocity are derived. The average limiting currents, experimentally determined as functions of concentrations of solution components and electrodes height, are compared to calculated values.

The effect of invalidity of Boussinesq approximation on the onset of natural convection in the electrochemical system

The effect of invalidity of Boussinesq approximation on the natural-convective stability of electrochemical system consisting of two plane horizontal electrodes and the binary electrolyte filling the space between the electrodes is considered. It is shown that the system stability against the rise of natural convection is higher under these conditions than under the Boussinesq approximation.