Double charging current

The great advantage of the RDE over other teclmiques, such as cyclic voltannnetry or potential-step, is the possibility of varying the rate of mass transport to the electrode surface over a large range and in a controlled way, without the need for rapid changes in electrode potential, which lead to double-layer charging current contributions. 

From an electroanalytical point of view, the double-layer capacitance is a nuisance resulting in the charging current, which has no analytical value. 

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... 

Figure 39. Current-time variation in nickel pitting dissolution in NaCl solution.89,91 1, double-layer charging current 2, fluctuation-diffusion current 3, minimum dissolution current 4, pit-growth current (Reprinted from M. Asanuma andR. Aogaki, Nonequilibrium fluctuation theory on pitting dissolution. II. Determination of surface coverage of nickel passive film, J. Chem. Phys. 106, 9938, 1997, Fig. 2. Copyright 1997, American Institute of Physics.)...

A charging current (non-faradaic) due to the formation of an electric double layer on the surface of the growing drop most polarographs permit a... 

Fig. 18b.5. (a) The capacitor-like metal solution interface, the double layer, (b) The equivalent circuit with solution resistance and overall double-layer capacitor, (c) Charging current transient resulting from a step-potential at... 

The Cyclic Voltammetry Experiment. Faradaic and Double-Layer Charging Currents. Ohmic Drop... 

The Faradaic current involves the passage of electrons across the electrode-solution interface. This is not the case with the double-layer charging current, ic, which arises as a consequence of the variation of the electrode potential ... 

The double-layer charging current thus tends toward a plateau equal to Cdv with a rise time equal to RuCd (Figure 1.7). On the reverse scan,... 

Convolution may also be applied to ohmic drop correction in the case where a substantial double-layer charging current is present, unlike the preceding case. It suffices first to extract the Faradaic current from the total current according to equation (1.19) [obtained from equations (1.11)]... 

The preceding derivation has assumed implicitly that the double-layer charging current is negligible in front of the Faradaic current or that it can be eliminated by a simple subtraction procedure. In cases where these conditions are not fulfilled, the following treatment will take care of the problem under the assumption that the double-layer capacitance is not affected appreciably by the Faradaic reaction but may nevertheless vary in the potential range explored. The first step of the treatment then consists of extracting the Faradaic component from the total current according to (see Section 1.3)... 

Double-layer charging current and ohmic drop are likely to interfere at high scan rates. The procedures for extracting the Faradaic component of the current and correcting the potential axis from the effect of ohmic drop described earlier (see Sections 1.3.2 and 1.4.3) should then be applied. The same is true for the double-layer effect on the electron transfer kinetics (Section 1.4.2). 

FIGURE 2.45. Equivalent circuit for the cell and instrument. WE, RE, and CE, working, reference, and counter electrodes, respectively iph, photocurrent ij/, double-layer charging current Q, double-layer differential capacitance Rc, Ru, cell compensated (by the potentiostat) and uncompensated resistances, respectively Rs, sampling resistance RP, potentiostat resistance E, potential difference imposed by the potentiostat between the reference and working electrodes Vpu, photo-potential as measured across the sampling resistor. Adapted from Figure 1 of reference 51, with permission from Elsevier. 

The agreement between simulated and experimental curves is excellent if the double-layer charging current is taken into account (Section 1.3.1). 

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