Ohmic drop, cyclic voltammogram

Once the cell resistance, Ru, or the residual resistance ARu, is known, another possible strategy to handling ohmic drop problems consists of introducing ohmic drop and double-layer charging into the theoretical treatment of the cyclic voltammograms.19 The following relationships, obtained from the equivalent circuits in Figure 1.5, may be used for this purpose. 

FIGURE 1.11. Convolution of the cyclic voltammetric current with the function I j Jnt, characteristic of transient linear and semi-infinite diffusion. Application to the correction of ohmic drop, a —, Nernstian voltammogram distorted by ohmic drop , ideal Nernstian voltammogram. b Convoluted current vs. the applied potential, E. c Correction of the potential scale, d Logarithmic analysis. 

Effect of Ohmic Drop and Double-Layer Charging on Nernstian Cyclic Voltammograms... 

The entire cyclic voltammogram is no longer reversible according to the definition we have attached to this term so far. In other words, the symmetry and translation operations as in Figures 1.4 and 6.1 do no longer allow the superposition of the reverse and forward trace. It also appears that the midpoint between the anodic and cathodic peak potentials does not exactly coincide with the standard potential. The gap between the two potentials increases with the extent of the ohmic drop as illustrated in Figure 6.2 for typical conditions, which thus provides an estimate of the error that would result if the two potentials were regarded as equal. 

In order to confirm this behavior, the cyclic voltammograms obtained at a planar electrode in CV and SCV (for A = 5 mV) for a Nemstian charge transfer process at different values of the scan rate are shown in Fig. 5.11. The effect of the ohmic drop and charging current has been considered by including an uncompensated resistance Ru = 0.1 K 2 and a double-layer capacitance Cdi = 20pFcm 2. 

When the electrochemical properties of some materials are analyzed, the timescale of the phenomena involved requires the use of ultrafast voltammetry. Microelectrodes play an essential role for recording voltammograms at scan rates of megavolts-per-seconds, reaching nanoseconds timescales for which the perturbation is short enough, so it propagates only over a very small zone close to the electrode and the diffusion field can be considered almost planar. In these conditions, the current and the interfacial capacitance are proportional to the electrode area, whereas the ohmic drop and the cell time constant decrease linearly with the electrode characteristic dimension. For Cyclic Voltammetry, these can be written in terms of the dimensionless parameters yu and 6 given by... 

The popularity of the cychc voltammetry (CV) technique has led to its extensive study and numerous simple criteria are available for immediate anal-j sis of electrochemical systems from the shape, position and time-behaviour of the experimental voltammograms [1, 2], For example, a quick inspection of the cyclic voltammograms offers information about the diffusive or adsorptive nature of the electrode process, its kinetic and thermodynamic parameters, as well as the existence and characteristics of coupled homogeneous chemical reactions [2]. This electrochemical method is also very useful for the evaluation of the magnitude of imdesirable effects such as those derived from ohmic drop or double-layer capacitance. Accordingly, cyclic voltammetry is frequently used for the analysis of electroactive species and surfaces, and for the determination of reaction mechanisms and rate constants. 

In order to improve the detection of short-lived intermediates, the potential step or chronoamperometric experiment can be replaced by a cyclic voltammet-ric experiment, which involves applying a triangular potential ramp. With a fast UVA is spectrometer, e.g. a diode array system, additional UVWis/NIR spectroscopic information as a function of the potential can be recorded simultaneously to the voltammetric data. However, recording cyclic voltammograms with the simple cell shown in Fig. II.6.4 is complicated by the presence of ohmic drop in the solution phase, which is amplified by poor cell design. In this kind of cell, the peak-to-peak separation in cyclic voltammograms of a reversible redox couple may increase by several hundreds of millivolts. Voltammetric data (and simultaneously recorded spectroscopic data) are therefore very difficult to interpret quantitatively. 

Figure 7 Background-subtracted experimental (solid lines) and simulated (open circles) cyclic voltammograms for a solution of ferrocene (10 mmol I h in acetonitrile containing 0.6 mol I TEAR. Sweep rates are 100, 200, and 500 kV s and the gold microelectrode radius is 5 pm. Simulations allow for ohmic drop and RC constant and k° is 3.1 cms k (Reproduced with the permission of the American Chemical Society from Analytical Chemistry GO (1988) 305.

In acetonitrile solutions, the cyclic voltammograms for PVF in LiC104 electrolyte are nearly Nernstian in character. Epeak (anodic) is approximately equal to Epeak (cathodic), and the peaks are almost symmetrical. Thinner films approach the ideal behaviour most closely. As film thickness is increased (at a fixed scan rate), a larger ohmic drop in the film causes separation of the anodic and cathodic peaks (AEpgak) and the peaks become asymmetric. Figure 2.4 shows a typical cyclic voltammogram for the PVF/LiC104/acetonitrile system. 

Figure 4. On line ohmic drop compensated cyclic voltammetry of 2,5-di-(p-anisole)pyrylium perchlorate (3), 5 mM, in acetonitrile, 0.6 M NBU4BF4, at a 5 /xm radius gold disk ultramicroelectrode and a scan rate of 153 kV.s 20 C. (a) in the absence or (b) in the presence of 1. (c) Background subtracted voltammogram (b - a).

Fig. 1. Cyclic voltammogram of 0.01 M NaBr in water and 0.01 M tetrabutylammonium tetraphenylborate in nitrobenzene. Scan rate 0.1 Vs. Ohmic potential drop compensation adjusted to 1.35 kQ [21]...

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