Faradaic processes electrochemical cells

The second meaning of the word circuit is related to electrochemical impedance spectroscopy. A key point in this spectroscopy is the fact that any -> electrochemical cell can be represented by an equivalent electrical circuit that consists of electronic (resistances, capacitances, and inductances) and mathematical components. The equivalent circuit is a model that more or less correctly reflects the reality of the cell examined. At minimum, the equivalent circuit should contain a capacitor of - capacity Ca representing the -> double layer, the - impedance of the faradaic process Zf, and the uncompensated - resistance Ru (see -> IRU potential drop). The electronic components in the equivalent circuit can be arranged in series (series circuit) and parallel (parallel circuit). An equivalent circuit representing an electrochemical - half-cell or an -> electrode and an uncomplicated electrode process (-> Randles circuit) is shown below. Ic and If in the figure are the -> capacitive current and the -+ faradaic current, respectively. 

The two major classes of voltammetric technique 4 Evaluation of reaction mechanisms 6 General concepts of voltammetry 6 Electrodes roles and experimental considerations 8 The overall electrochemical cell experimental considerations 12 Presentation of voltammetric data 14 Faradaic and non-Faradaic currents 15 Electrode processes 17 Electron transfer 22 Homogeneous chemical kinetics 22 Electrochemical and chemical reversibility 25 Cyclic voltammetry 27 A basic description 27 Simple electron-transfer processes 29 Mechanistic examples 35... 

A faradaic current in an electrochemical cell is the current that results from an oxidation/reduction process. A nonfaradaic current is a charging current that results because the mercury drop is expanding and must be charged to the electrode potential. The charging of the double layer is similar to charging a capacitor. 

Faradaic current An electric current produced by oxidation/ reduction processes in an electrochemical cell. 

In an electrochemical cell, the conductivity is inversely related to the resistance in the electrolyte/test medium. The presence of certain chemical or ionic species may affect the resistance of the electrochemical cell. This change in resistance or conductivity can then be used to quantify the amount of the analyte presented. Molar and equivalent conductivities are commonly used to express the conductivity in an electrochemical cell. The conductivity measurement can be made relatively straightforward, using a DC mode or with a potential or current excitation. However, any faradaic or change transfer process occurring at the electrode surface will affect the conductivity measurement in an electrochemical cell. Furthermore, conductivity measurement in general does not provide sufficient specificity or sensitivity to quantify the analyte. This limits the use of conductivity of an electrochemical cell for sensing applications. 

A voltammetric sensor is characterized by the current and potential relationship of an electrochemical cell. Voltammetric sensor utilizes the concentration effect on the current-potential relationship. This relationship depends on the rate by which the reactant (commonly the sensing species) is brought to the electrode surface (mass transfer) and the kinetics of the faradaic or charge transfer reaction at the electrode surface. In an electrochemical reaction, the interdependence between the reaction kinetics and the mass transfer processes establishes the concentration of the sensing species at the electrode surface relative to its bulk concentration and, hence, the rate of the faradaic process. This provides a basis for the operation of the voltammetric sensor. 

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. 

With modern computerized frequency-analysis instrumentation and software, it is possible to acquire impedance data on cells and extract the values for all components of the circuit models of Figure 2.>7, This type of analysis, w hich is called electrochemical impedance spectroscopy, reveals the nature t>f the faradaic processes and often aids in the investigation of the mechanisms of electron-transfer reactions. In the section that follows, we explore the processes at the electrode-solution interface that give rise to the faradaic impedance. 

Measurement of the electric conductivity of an electrochemical cell can be the basis for an electrochemical sensor. This differs from an electrical (physical) measurement, for the electrochemical sensor measures the conductivity change of the system in the presence of a given solute concentration. This solute is often the sensing species of interest. Electrochemical sensors may also involve measuring capacita-tive impedance resulting from the polarization of the electrodes and/or the faradaic or charge transfer processes. 

Any electrochemical cell can be represented in terms of an equivalent electrical circuit that comprises a combination of resistances, capacitances or inductances as well as mathematical components. At least the circuit should contain the doublelayer capacity, the impedance of the faradaic or non-faradaic process and the high-frequency resistance. The equivalent circuit has the character of a model, which more or less precisely reflects the reality. The equivalent circuit should not involve too many elements because then the standard errors of the corresponding parameters become too large (see Sect. II.5.7), and the model considered has to be assessed as not determined, i.e. it is not valid. 

The simple / uGai model of the electrochemical cell provides a challenging control situation. The presence of dif-fusional faradaic current reduces the reactance of the working electrode interface by adding a parallel noncapaci-tive current path across However, some electrode processes can transiently increase the reactance of the interface, thus decreasing the control loop stability. For example, potential-dependent adsorption or desorption of ions at the interface or passivation/depassivation phenomena can destabilize an otherwise... 

It was shown that an electrochemical cell could be described with a simple equivalent circuit (Fig. 4a) containing the electrolyte resistance, the double-layer capacitance, and the impedance of the faradaic process. Previously, the faradaic process was described in terms of a simple charge-transfer resistance (Sect. 2.6.2.3) neglecting the diffusion of electroactive species. 

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