Potential faradaic current response

The faradaic current response to a single potential step... 

The Faradaic current response to a potential perturbation can be expressed as ... 

In Fig. II. 1.12, cyclic voltammograms incorporating both IR drop and capacitance effects are shown. Effects for the ideal case of a potential independent working electrode capacitance give rise to an additional non-Faradaic current (Fig. II. 1.12b) that has the effect of adding a current, /capacitance = Cw x v, to both the forward and backward Faradaic current responses. Tbe capacitance, Cw, is composed of several components, e.g. double layer, diffuse layer, and stray capacitance, with the latter becoming relatively more important for small electrodes [61]. On the other hand, the presence of uncompensated resistance causes a deviation of the applied potential from the ideal value by the term R x /, where R denotes the uncompensated resistance and I the current. In Fig. II. 1.12, the shift of the peak potential, and indeed the entire curve due to the resistance, can clearly be seen. If the value of Ru is known (or can be estimated from the shape of the electrochemically reversible... 

The Faradaic current responses for Pt/C electrodes at different constant potentials in the course of glyoxylic acid adsorption, upon the exchange of 0.5 M H2SO4 solution to 0.1 M glyoxylic acid + 0.5 M H2SO4 solution, are shown in Fig. 28a. The CO2 formation was also simultaneously monitored by mass spectrometry at m/z = 44 in the course of adsorption (Fig. 28b). 

One must keep in mind that modern electrochemical instrumentation compensates for the potential drop i (Rn + Rnc) through the use of appropriate circuitry (positive feedback compensation). This adds a supplementary potential to the input potential of the potentiostat (equal to the ohmic drop of the potential), which is generated by taking a fraction of the faradaic current that passes through the electrochemical cell, such that in favourable cases there will be no error in the control of the potential. However, such circuitry can give rise to problems of reliability in the electrochemical response on occasions when an overcompensation is produced. 

As stated in Sect. 5.2.3.4, there is always a potential difference generated by the flow of faradaic current I through an electrochemical cell, which is related to the uncompensated resistance of the whole cell (Ru). This potential drop (equal to IRU) can greatly distort the voltammetric response. At microelectrodes, the ohmic drop of potential decreases strongly compared to macroelectrodes. The resistances for a disc or spherical microelectrode of radius rd or rs are given by (see Sect. 1.9 and references [43, 48-50]). 

Another more sophisticated approach is to make a Fourier Transform analysis of the response in the way proposed by Bond et al. [84, 85]. In this case, the perturbation is a continuous function of time (a ramped square wave waveform) which combines a dc potential ramp with a square wave of potential that can be described as a combination of sinusoidal functions. Under these conditions, the faradaic contribution to the response generates even harmonics only (i.e., the non-faradaic current goes exclusively through odd harmonics). Thus, the analysis of the even harmonics will provide excellent faradaic-to-non-faradaic current ratios. 

Figure 5.4 Current response for a 12.5 pm platinum microelectrode modified with a [Os(bpy)2 py(p3p)]2+ monolayer following a potential step where the overpotential rj was —100 mV the supporting electrolyte is 0.1 M TBABF4 in acetonitrile. The inset shows ln[fp(f)] versus f plots for the Faradaic reaction when using a 12.5 pm (top) and 5 pm (bottom) radius platinum microelectrode. Reprinted with permission from R. J. Forster, Inorg. Chem., 35, 3394 (1996). Copyright (1996) American Chemical Society...

The study of the variation of the current response with time under potentiostatic control is chronoamperometry. In Section 5.4 the current resulting from a potential step from a value of the potential where there is no electrode reaction to one corresponding to the mass-transport-limited current was calculated for the simple system O + ne-— R, where only O or only R is initially present. This current is the faradaic current, If, since it is due only to a faradaic electrode process (only electron transfer). For a planar electrode it is expressed by the Cottrell equation4... 

Second harmonic — Any nonlinear oscillating system produces higher harmonic oscillations. The second harmonic is the response having twice the frequency of the basic oscillation. The - current response of a faradaic electrode reaction (- faradaic reaction) to perturbations of the - electrode - potential is generally nonlinear, and thus higher harmonic oscillations of the - alternating current (AC) are produced in - AC voltammetry. Since the -> capacitive current is a much more linear function of the electrode potential, the capacitive contribution to higher harmonic currents are rather small which allows a desirable discrimination of theses currents in electro-analytical applications. 

If the potential on the semiconductor has imposed on it an a.c. component, then the effect is to change the accumulated charge in the depletion layer according to eqn. (38). Assuming that no faradaic current is passing, i.e. the semiconductor is deep in depletion, the capacitive response of the semiconductor layer may be approximated as... 

Fig. 13G Current response to a triangular potential sweep, plotted on an X-Y recorder, (a) ideally polarizable interphase (b) real interphase, with finite faradaic current.

FIGURE 19.6 Simultaneously measured faradaic (a) and mass spectrometric (b) current response on the copper electrode potential in a thin-layer flow-through DEMS cell. Solution contained (mol L-1) CuS04 (un-hydrous)—0.008 EDTA—0.01 H2CO (solid lines) or D2CO (dotted lines)—0.02 solvent H20 (A) or D20 (B). pH = 13 (adjusted by adding NaOH or NaOD, respectively) temperature 25°C. Potential sweep rate 5 mV s Electrolyte flow rate 30 p,L s-1. Circles indicate open-circuit conditions (filled—H2 evolution, open—D2 evolution). (From Jusys, Z. and Vaskelis, A., Electrochim. Acta, 42, 449, 1997.)... 

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