A.A. Moya

Simulation and interpretation of electrochemical impedances using the network method

Abstract The small-signal impedance response of electrochemical cells in which the ionic transport obeys the Nernst-Planck and Poisson equations is interpreted by means of the network method. The method is general and extremely efficient; it permits treatment of multi-ion systems and imposition of conditions on both the electric potential and the electric current density through the cell. The network model of some representative electrochemical cells, including non-faradaic processes and interfacial kinetics described by a Butler-Volmer type equation, has been obtained, and the electrochemical impedance of these systems has been simulated with the aid of the PSPICE program.

Stationary electrodiffusion–adsorption processes in membranes including diffuse double layer effects: A network approach

Abstract Ion transport across membranes with surface charge due to ion adsorption, including the diffuse double layer effects, is analysed using the network simulation method. The membrane system under study is a multilayer one constituted by a membrane and two diffusion boundary layers on both sides of the membrane. The ion transport processes are described by the Nernst–Planck and Poisson equations not only in the membrane–solution interfaces, but also in the membrane bulk and in the two diffusion boundary layers. The membrane has a negative surface charge due to an anion adsorption process. The structure of the equilibrium diffuse double layers and the steady-state current–voltage characteristic have been analysed for the case of an adsorption process described by a Langmuir-type adsorption isotherm. The evolution of the electric potential difference across the membrane system in the equilibrium state of the system as a function of the bathing concentrations, have been also analysed.

A study of the transport of ions against their concentration gradient across ion-exchange membranes using the network method

The network method has been used to analyze the conditions that favour the uphill transport across ion-exchange membranes. A model for the Nernst-Planck-Poisson equations describing the ionic transport in such system is proposed, including the Donnan equilibrium relations at the membrane/solution interfaces. With this model and the electric circuit simulation program PSPICE, the transient response of the system under open circuit conditions (I=0) and the response of the system subject to an applied potential difference are simulated. The ionic concentrations and electric potential profiles, as well as the electric current density, the ionic fluxes and the charge density, have been obtained as a function of time.

A network approach to simulation of electrical properties of symmetric electrochemical cells

Abstract A network approach has been used to analyse the electrical properties of a classical symmetric electrochemical cell, i.e. a 1:1 binary electrolyte in a cell between two metal-parent-type electrodes. A network model for the Nernst-Planck and Poisson equations describing the ionic transport through the electrolyte solution and for the Butler-Volmer equations describing the interfacial kinetics in such a system has been proposed. With this model and the electric circuit simulation program pspice, the transient response of the system to a step-function-applied potential difference and the small-signal a.c. response of the system subject to a steady external voltage bias have been simulated. The study is intended mainly to emphasize the characteristics of the non-equilibrium electrical double layers at the electrode¦electrolyte solution interfaces and the overlapping effects between these space charge regions.

Steady-state, transient and small-amplitude AC responses of an electrochemical cell with immobile background charge : A network approach

Abstract The network simulation method has been used to analyse the electrical properties of an electrochemical cell containing two mobile ionic species and immobile background charge in contact with blocking electrodes. The steady-state electrical properties, the transient response of the system to a step-function applied potential difference, and the small-amplitude AC response of the cell have been obtained by simulation in a program for electric circuit analysis of the network modelling the Nernst–Planck and Poisson equations. The electric charge density and the electric potential profiles across the cell were obtained, in the steady state, as a function of the ratio of the initial value of the co-ion concentration to the concentration of immobile background charge. The electrochemical impedance of the system, and the time evolution of the surface charge density stored in the diffuse double layers at the electrode–material interfaces and of the total electric current density through the cell, were also obtained.

A study of the electrical properties of permselective membranes with symmetric, reversible interfacial processes using the network method

Abstract The electrical properties of a membrane containing a single salt M + xa0X − , with the cation being the only permeable species and the anion the blocked species at the interfaces, are analysed using the network method. The steady-state electrical properties of the system and the small-amplitude ac response of the biased system with a simultaneous dc steady-state electric current have been obtained by simulation with the PSPICE program of a previously obtained network model for a thin-layer electrochemical cell. The results obtained confirm the validity of the network approach to deal with diffusion-migration impedances of electrochemical systems with symmetric, reversible interfacial processes when a dc electric current is superimposed. Some advantages of the present approach over more classical ones are discussed.

Study of the linearity of the voltage-current relationship in symmetric and asymmetric thin-layer cells

Abstract The network method has been used to analyse the conditions of linearity of the voltage-current relationship in symmetric and asymmetric thin-layer cells with type I electrodes. A network model for the Nernst-Planck and Poisson equations describing the ionic transport through the electrolyte solution and for the Butler-Volmer equations describing the interfacial kinetics in these systems has been proposed. With this model and the electric circuit simulation program PSPICE™, a decomposition into Fourier components of the electric current-time response, which follows the application of a sinusoidal-shaped potential difference between the electrodes, is obtained. The results reveal a considerable influence of the type of contacts on the amplitude of the different harmonics.

A network approach to the simulation of electrical properties of asymmetric electrochemical cells

Abstract The network method has been used to analyse the electrical properties of a classical asymmetric electrochemical cell, i.e. a 1:1 binary electrolyte in a cell between two atomic-parent-type electrodes, the positive ion being the only electroactive species at one electrode, and the negative ion the only electroactive species at the other electrode. A network model for the Nernst-Planck and Poisson equations describing the ionic transport through the electrolyte solution and for the Butler-Volmer equations describing the interfacial kinetics in such a system has been proposed. The transient response of the system to a step-function-applied potential difference and the small-signal a.c. response of the system subject to a steady external voltage have been simulated using the electric circuit simulation program PSPICE.