Current flow in an electrochemical cell

Within any cell undergoing charge or discharge, one can consider at least three forms of charge transmission  

Since in the steady state, it is necessary to maintain a condition of electroneutrality in any macroscopic part of the system, the total charge flux through all cross-sections of the circuit must be the same. In particular, the rate of electron flow in the external circuit is equal to the rate of charge transfer at each electrode/electrolyte interface. 

It should be noted that the rate of charge transfer at the electrodes (i.e. the rates of the electrochemical processes) may be given directly by the reading on an ammeter inserted in the external circuit. If the current is a function of time, it is still possible to apply the above idea of flux continuity over a succession of small time intervals. It may happen that one of the various rate processes involved in charge transport in different components of a cell, is unable to maintain as high a rate as the others. 

Under these circumstances it becomes the current limiting process for the cell. 

The question now arises as to what factors are responsible for determining the rates at which the various cell processes occur. Thermodynamic arguments permit the feasibility of overall cell reactions to be predicted, but give no information on rates. To understand the latter it is necessary to consider the effects on various parts of the cell of forcing the cell voltage to assume a value different from that of the equilibrium emf. It has been shown above that in the Daniell cell at equilibrium, charge transfer across the zinc/solution interface can be described in terms of processes 

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Generally, irrespective of the technique for which they are used, electrochemical cells are constructed in a way which minimizes the resistance of the solution. The problem is particularly accentuated for those techniques which require high current flows (large-scale electrolysis and fast voltammetric techniques). When current flows in an electrochemical cell there is always an error in the potential due to the non-compensated solution resistance. The error is equal to / Rnc (see Chapter 1, Section 3). This implies that if, for example, a given potential is applied in order to initiate a cathodic process, the effective potential of the working electrode will be less negative compared to the nominally set value by a amount equal to i Rnc. Consequently, for high current values, even when Rnc is very small, the control of the potential can be critical. 

When an electric current flows in an electrochemical cell, the current is carried in solution by the movement of ions. For example, take the cell ... 

When a current flows in an electrochemical cell, by definition the system is not in equilibrium. On the other hand, if no current flows (as in an open-circuit system), then the system could be either in thermodynamic equilibrium or not. This is because although the equilibrium state can always be defined, it is not necessarily always possible to observe. In fact the time scale of the experiment may be too short to reach the equilibrium state. 

When a current I flows in an electrochemical cell, such as the one shown in Fig. 4.1, between the catalyst, or working electrode (W) and the counter electrode (C), then the potential difference Uwc deviates from its open-circuit value U c. The electrochemical cell overpotential t Wcis then defined from ... 

Molar conductivity measurements are equally applicable to both solid and liquid electrolytes. In contrast, the measurement of current flowing through an electrochemical cell on a time scale of minutes or hours while the cell is perturbed by a constant dc potential is only of value for solid solvents (Bruce and Vincent, 1987) where convection is absent. Because of the unique aspects of dc polarisation in a solid solvent this topic is treated in some detail in this chapter. Let us begin by considering a cell of the form ... 

Ohmic drop IR. A potential developed when a current I flows in an electrochemical cell. It is a consequence of the cell resistance R and is given by the product IR. It is always subtracted from the theoretical cell potential and therefore reduces that of a galvanic cell and increases the potential required to operate an electrolysis cell. 

When a current flows through an electrochemical cell in the hypothetical PEVD system proposed in section 2, the applied dc potential defines the potential difference between... 

Remember 5.1 Electrochemical reactions, which transfer charge between electrons and soluble species, are required for current to flow in an electrochemical cell. 

The similarities between ITIES and conventional electrode electrochemistry provide an arsenal of electrochemical techniques that have been previously tested in the more common electroanalytical chemistry and physical electrochemistry. To understand the similarities between ITIES and electrode electrochemistry, it is more useful to look at the differences first. Faradaic current flow through an electrochemical cell is associated with redox processes that occur at the electrode surface. The functional analog of an electrode surface in ITIES is the interface itself. However, the net current observed when the interface is polarized from an outside electric source is not a result of a redox process at the interface rather, it is an effect that is caused by an ion transport through the interface, from one phase to another. 

The spatial variation on the electrode of current density i is often referred to as the current distribution. Since the current density is related to reactirai rate through Faraday s law, the current distribution is thus a manner of expressing the variation of reaction rate within an electrochemical cell. As for traditional chemical reactors, nonuniformities in reaction rate may be anticipated if the fluid flow is inadequate to prevent concentration gradients. However, electrical field effects also influence the current distribution in an electrochemical cell, and thus reaction rates can be nonuniform even if perfect mixing is achieved in the reactor. Electrochemical cells of course have two electrodes, and sometimes optimizing a current distribution of one electrode is more important than the other. Depending on the proximity of the two electrodes, the current distributions of the electrodes may or may not influence each other. 

Two types of current may flow in an electrochemical cell, faradaic and non-faradaic. All currents that are aeated by the reduction and/or oxidation of chemical species in the cell are termed faradaic currents. Faradaic currents may be described by the following equation ... 

Processes at Electrodes. Different processes take part in current flow through an electrochemical cell. Among them are the following which are essential ... 

As it has been pointed out in the Preface the reference electrode allows the control of the potential of a working electrode or the measurement of the potential of an indicator electrode relative to that reference electrode. The rate, the product, and the product distribution of electrode reactions depend oti the electrode potential. A knowledge of the electrode potential is of utmost importance in order to design any electrochemical device or to carry out any meaningful measurement. When current flows through an electrochemical cell the potential of one of flie electrodes should remain practically constant—it is the reference electrode—in order to have a well-defined value for the electrode potential of the electrode under investigation or to control its potential. An ideally non-polarizable electrode or an electrode the behavior of which is close to it may serve as a reference electrode. The choice and the construction of the reference electrode depend on the experimental or technical conditions, among others on the current applied, the nature and composition of the electrolyte (e.g., aqueous solution, nonaqueous solution, melts), and temperature. 

Residual Current Even in the absence of analyte, a small current inevitably flows through an electrochemical cell. This current, which is called the residual current, consists of two components a faradaic current due to the oxidation or reduction of trace impurities, and the charging current. Methods for discriminating between the faradaic current due to the analyte and the residual current are discussed later in this chapter. 

Electrochemical cells are familiar—a flashlight operates on current drawn from electrochemical cells called dry cells, and automobiles are started with the aid of a battery, a set of electrochemical cells in tandem. The last time you changed the dry cells in a flashlight because the old ones were dead, did you wonder what had happened inside those cells Why does electric current flow from a new dry cell but not from one that has been used many hours We shall see that this is an important question in chemistry. By studying the chemical reactions that occur in an electrochemical cell we discover a basis for predicting whether equilibrium in a chemical reaction fa-... 

Thus, in an electrochemical cell the electrolyte has a small but finite resistance, Rt, resulting in a potential drop, Fd, between the working and reference electrodes. From Ohm s law, VA = /Rc, where / is the current flowing across the working electrode/electrolyte interface. As a result of this resistance, the measured potential Vm is related to the real potential, Vrt by ... 

The overall rate of an electrochemical reaction is measured by the current flow through the cell. In order to make valid comparisons between different electrode systems, this current is expressed as cunent density,/, the current per unit area of electrode surface. Tire current density that can be achieved in an electrochemical cell is dependent on many factors. The rate constant of the initial electron transfer step depends on the working electrode potential, Tlie concentration of the substrate maintained at the electrode surface depends on the diffusion coefficient, which is temperature dependent, and the thickness of the diffusion layer, which depends on the stirring rate. Under experimental conditions, current density is dependent on substrate concentration, stirring rate, temperature and electrode potential. 

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