Nonfaradaic processes charging current

The charging of the double layer is responsible for the background (residual) current known as the charging current, which limits die detectability of controlled-potential techniques. Such a charging process is nonfaradaic because electrons are not transferred across the electrode-solution interface. It occurs when a potential is applied across the double layer, or when die electrode area or capacitances are changing. Note that the current is the tune derivative of die charge. Hence, when such processes occur, a residual current flows based on die differential equation... 

The relation between E and t is S-shaped (curve 2 in Fig. 12.10). In the initial part we see the nonfaradaic charging current. The faradaic process starts when certain values of potential are attained, and a typical potential arrest arises in the curve. When zero reactant concentration is approached, the potential again moves strongly in the negative direction (toward potentials where a new electrode reaction will start, e.g., cathodic hydrogen evolution). It thus becomes possible to determine the transition time fiinj precisely. Knowing this time, we can use Eq. (11.9) to find the reactant s bulk concentration or, when the concentration is known, its diffusion coefficient. 

Residual currents, also referred to as background currents, are the sum of faradaic and nonfaradaic currents that arise from the solvent/electrolyte blank. Faradaic processes from impurities may be practically eliminated by the careful experimentalist, but the nonfaradaic currents associated with charging of the electrode double layer (Chap. 2) are inherent to the nature of a potential sweep experiment. Equation 23.5 describes the relationship between this charging current icc, the double-layer capacitance Cdl, the electrode area A, and the scan rate v ... 

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. 

Under some conditions an electrode may be in a potential region where charge-transfer reactions do not occur because they are either thermodynamically or kinet-ically unfavorable. However, such processes as adsorption can occur, and the structure of the electrode-solution interface can change, causing transitory changes in current and/or potential. These processes are called nonfaradaic processes. 

Faradme Processes. Consider an ideally polarized electrode only nonfaradaic processes occur, no charges cross the interface, and no continuous current can flow. Upon addition of a substance that can be oxidized or reduced at the particular potential difference, current now flows—the electrode is depolarizedy and the substance responsible is called a depolarizer. 

Transport processes of this type are called nonfaradaic transport. The nonfaradaic transport considered here is a steady-state process, in contrast to nonfaradaic currents mentioned previously that were due, for example, to charging of the electric double layer. Electrokinetic processes are of great practical significance, as discussed in Section 31.3. 

As the field of electrochemical kinetics may be relatively unfamiliar to some readers, it is important to realize that the rate of an electrochemical process is the current. In transient techniques such as cyclic and pulse voltammetry, the current typically consists of a nonfaradaic component derived from capacitive charging of the ionic medium near the electrode and a faradaic component that corresponds to electron transfer between the electrode and the reactant. In a steady-state technique such as rotating-disk voltammetry the current is purely faradaic. The faradaic current is often limited by the rate of diffusion of the reactant to the electrode, but it is also possible that electron transfer between the electrode and the molecules at the surface is the slow step. In this latter case one can define the rate constant as ... 

For electrolyses involving time scales shorter than about 500 /is, the diffusion layer is of the same order as S, and the absorbance is sensitive to the evolving concentration profile of R (6, 46, 47). The resulting optical transients can be useful for characterizing rather fast electrochemical processes, which are otherwise complicated severely by nonfaradaic contributions to current and charge functions. Theoretical absorbance transients can be computed from (17.1.13), once the diffusion-kinetic equations defining the concentration profile of R have been solved, either analytically or by numeric methods such as digital simulation. 

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