Ohmic Drop Evaluation

If the reference electrode is correctly placed in the same equipotential as the working electrode, ohmic drop in the electrolyte may be negligible, but this may not be the situation in the bulk of the electrode. Quantitative determination of the electrode resistance is difficult when using voltammetry only. It is much easier using galvanostatic cycling. 

Electron transfer kinetics may be more difficult to determine using stationary electrodes sweep voltammetry. Residual ohmic drop may interfere strongly with the determination of kinetics constants. Impedancemetry and RDEs provide useful alternatives and will be discussed in some detail. 

Carbons for Electrochemical Energy Storage and Conversion Systems 

FIGURE 1.8 Voltammogram of a carbon supercapacitor electrode. (Adapted from Toupin, M., etal., J. Power Sources, 140, 203, 2005.) 

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Figure 11. Circuit scheme illustrating the current interruption technique for the ohmic drop evaluation.

The magnitude of the ohmic drop at a microelectrode can be evaluated quite readily for case 1 from a knowledge of the specific solution resistance (obtained from conductivity measurements such as in Table 12.1) and the expressions for the voltammetric current for the specific microelectrode employed. Case 2 is also straightforward if the free concentration of ions exceeds that of the electroactive species. However, the situation is somewhat more complicated for the third class. In this case, and in case 2 for fully associated electrolyte, migration as well as diffusion can affect the observed voltammetric signals. In all three cases, the situation may be further complicated by a change in structure of the double layer. However, this is ignored for now, and is considered in the section on very small electrodes. 

A further example, which confirms the necessity of evaluating the resistivity of the medium very carefully, concerns the corrosion of rebars in reinforced concrete. In this caae the intensity of the current flowing between the anodic and cathodic zones of a macrocell depends on the resistivity of the concrete and the extent of the region involved. To determine the concrete resistivity various methods have been developed, which can be applied in the laboratory [14] as well as in the field [15]. It should be noted, however, that in the latter case most researchers have pursued the approach suggested by Wenner [16] for the evaluation of the resistivity of soils. The contribution of the ohmic drop to the electrode overvoltage cannot be neglected when the values of the corrosion rate of the rebars are appreciable, even if the current intensity is small within a given polarization potential interval, because under such conditions the interpretation of experimental results could be completely distorted. 

When the corrosion current density values are high or the exposed surface area of the working electrode is large the effect of the electric resistance of the electrolytic solution is not negligible. Yet it is not easy to find in the literature specific studies dealing with the problem of the reliability of the information contained in experimental results and of the indetermination of electrochemical parameters as a consequence of the practical impossibility of evaluating the ohmic drop correctly. 

Finally, the presence of a contribution of the ohmic drop to electrode overvoltage becomes more important when one tries to evaluate 1 using the three different versions of the three-point method [43, 44, 45]. In its simplest form, the theory of this method. 

Most corrosionists agree on the fact that corrosion current density is a very important parameter for the evaluation of the kinetics of a corrosion process and the proper choice of a metal to be used in a given environment with no prejudice to its integrity and performance. Hence it is very interesting to examine analytically the influence of the ohmic drop on the determination of the corrosion rate. In fact, this analysis makes it possible to detect a priori situations that may cause the behaviour of an electrochemical system to diverge from its ideal trend and render the use of equation (10) mandatory for a more reliable evaluation of the kinetics of the corrosion process. 

A numerical study of the influence of the ohmic drop on the evaluation of electrochemical quantities has been conducted, for example, over the AE interval [-20, 20] mV by means of the IRCOM program, which makes use of a polynomial of the sixth degree, considering some experimental polarization curves and taking the values of the electrochemical parameters obtained by the NOLI method. The examples examined have shown that the representation of experimental data by a polynomial of the sixth degree is very good and that the evaluation of the correct order of magnitude of the corrosion current density, in the presence of an ohmic contribution to the electrode potential, requires that the actual values of the Tafel slopes be known. 

The most plausible explanation for this behaviour is that the omission of the contribution of the anodic process requires more careful evaluations, especially when the values of the ohmic drop are not negligible. 

A very interesting application of the classical current interruption technique has been reported by Lorenz and Eichkom [67], who showed that by adopting the galvanostatic configuration it is possible to evaluate the importance of the ohmic drop realistically, point by point, and obtain polarization curves with a trend very close to the ideal one. In fact, it can be experimentally demonstrated that the value of R, is not constant but is influenced by the mass transfer when the current flowing in the electrolytic cell is sufficiently high. In other words, it cannot be excluded a priori that the quantity R, depends on the electrode overvoltage. 

The ohmic drop exerts a sensible influence on the evaluation of the electrochemical parameters as well as on the definition of the reaction scheme that is most suitable for describing the behaviour of a metal in a given environment. It also determines the success of many operations, such as cathodic protection by means of sacrificial anodes or impressed current and corrosion rate monitoring. 

It is, therefore, very important for a corrosionist to be able to adopt experimental and theoretical techniques that will enable him to evaluate the discrepancies between actual response and ideal trend and assist him in the study of particular processes. The presence of a ohmic drop may, in fact, result in the introduction of systematic errors in... 

Chapter 6 Evaluation and Compensation of Ohmic Drop L. Clerbois and F. P. IJsseling... 

In practice a combination of these methods is often used i.e., a good cell design to minimise the ohmic drop, instrumental compensation of the greater part of the remaining error and, finally, removal of the last part by evaluation, calculation and correction of the experimental data. 

The evaluation of ohmic drop with alternating current at high frequencies is based on the electric equivalent of the metal-solution interface of Fig. 6.12, where represents the electric solution resistance between working and reference electrodes. 

Finally, it is possible in principle to evaluate and/or calculate the ohmic drop, and subsequently, to correct the experimental data for its influence after the polarization measurements have been performed. 

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. 

The sum of ohmic drops across each conducting medium represents a part of the overall voltage between the terminals in an eiectrochemical cell. The precise size of that share depends on the system in question. The aim here is to evaluate these terms by focusing on simpie cases. 

Finally, it is worth mentioning that LaCroix et al [28] reported in 1989 ultrafast electrical response data on a timescale of 100 ps for pANI film (ca. 0.2 pm thickness) in 2 m sulphuric acid using an ultramicroelectrode (ca. 0.2 mm area) with appropriate correction for uncompensated ohmic drop. However, the optical response data are reported in arbitrary units and thus they are difficult to evaluate with a view to practical uses. In general, it would seem that 100 ps are not a viable reference timescale for electrochromic response, at least for conventional electrochromic devices. 

A knowledge of the conductivity of the electrolyte solutions, diaphragms and ion-exchange membranes is essential to calculate the ohmic drop in a cell, and thus to evaluate the dissipative energy losses in cell operation. Some definitions pertinent to this topic are noted helow. 

Remark The ohmic drop has been evaluated by switching off the polarization. Then, the R.I. component vanishes immediately, whereas the overvoltage component decreases more slowly... 

To characterize potentiomebic probes, one must evaluate their response function, selectivity coefficient, response time, and ohmic drop. The response function of a potentiometric probe is a calibration curve of the measured membrane potential with the log of the concentration (or activity) of the primary ion must be acquired. To use concentration values, the activity coefficients must be known and many of than are tabulated. Another option is to use (when possible) dilute solutions such that the activity coefficients maybe neglected. 

Small galvanostatic transients superimposed to the longer pulse permit determination of the ohmic drop AUq in front of the electrode in close connection to the changes of its surface environment with time (Strehblow and Wenners, 1977). AUq is small in the plateau region and increases when the potential step is observed at r = T (Fig. 1-37). It has been interpreted as the ohmic drop within the salt layer. The major part of the potential step AE is interpreted as the potential drop across a poreless part of the salt film at the electrode surface. A discussion based on the evaluation of these galvanostatic transients leads to a bilayer... 

To evaluate the fuel cell performance in detail, the ohmic drop and the cathode overpotential (rj ) were determined and are shown in Fig. 16.8. The ohmic drop was measured using an in situ current interruption method during the H -O PEFC operation and rj was calculated using the equation ... 

The current interrupt technique is the most widely used method of ohmic drop and ohmic resistance evaluation of various electrochemical systems including fuel cells. The principle behind the current interrupt method is the performance of the voltage response of the fuel cell for a given step change of current flow. 

Another parameter essential for quantitative applications of micropipettes is the internal ohmic resistance, R. It is largely determined by the solution resistance inside the narrow shaft of the pipette, and can be minimized by producing short (patch-type) pipettes. The micropipette resistance has been evaluated from AC impedance measurements. Beattie et al. measured the resistance of micropipettes filled with aqueous KCl solutions (0.01, 0.1, and 1 M) [18b]. The value obtained for a 3.5/am-radius pipette was within the range from 10 to 10 As expected, the tip resistance was inversely proportional to the concentration of KCl in the filling solution. In ref. 18b, the effect of pipette radius on the tip resistance was evaluated using a constant concentration of KCl. The pipette resistance varied inversely with the tip radius. The iR drop was found to be 4.5-8 mV for the pipette radii of 0.6 to 19/rm when 10 mM KCl was used. 

The term (o — o,c) represents the potential drop through the electrolyte. As defined here, the numerical evaluation of the potential drop requires accounting for the variation in electrolyte conductivity within the diffusion layer. An alternative approach is to define the Ohmic potential drop as being that calculated using Laplace s equation with a uniform solution conductivity. In this case, an additional term is required to accotmt for the influence of the conductivity variation within the diffusion layer on the measured potential. This is incorporated into a concentration overpotential, discussed in Section 5.7.3. 

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