Ohmic distortion

Whereas large charging current contributions are found only under fairly demanding conditions (low analyte concentrations, very high v), ohmic distortions (iR losses) are almost always present. A full chapter in this book is devoted to... 

Ohmic effects render Epc more negative, Epa more positive, AEp and 8Ep larger, and X smaller than the true values. Since experimental approaches to elimination of iRu errors are not foolproof (see Chap. 7), the presence of ohmic distortions should be tested by measurements on a Nernstian couple such as ferrocene/ferrocenium under conditions identical to those used to probe the test compound. In principle, errors in the measured CV parameters for a test compound can be eliminated by referencing its responses to those of the Nernstian standard. Note that this approach is accurate only if the current level of the standard, rather than its concentration, is equal to that of the test compound, since the diffusion coefficients of the two species may appreciably differ. 

Returning to the three-electrode setup, it could seem that no ohmic drop would affect the measurement of the potential difference between the working and reference electrodes, since there is practically no current flow between both electrodes. However, this is not totally true. The reference electrode is located at a given distance from the working electrode surface, and, as a result of this separation, the potential difference measured contains a part of the ohmic drop in the solution which is called residual ohmic drop, IRU (with I being the current and Ru the uncompensated resistance). For more details concerning the minimization of the ohmic distortion of the current-potential response, see Sects. 1.8 and 5.4. 

Coles BA, Compton RG, Brett CMA, Brett AMCFO (1995) Ohmic distortion of current-potential curves at wall-jet electrodes. J Electroanal Chem 381 99-104... 

Temptation has been strong, and not always resisted, to approach total compensation by damping the oscillations out with an appropriately placed capacitance so as to reach the ideal situation of total compensation with unconditional stability. In fact, the cure is worse than the disease. The additional response time accompanying introduction of the damping capacitance will indeed distort the Faradaic current in a more severe and undecipherable manner than does ohmic drop. 

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. 

In voltammetric experiments, electroactive species in solution are transported to the surface of the electrodes where they undergo charge transfer processes. In the most simple of cases, electron-transfer processes behave reversibly, and diffusion in solution acts as a rate-determining step. However, in most cases, the voltammetric pattern becomes more complicated. The main reasons for causing deviations from reversible behavior include (i) a slow kinetics of interfacial electron transfer, (ii) the presence of parallel chemical reactions in the solution phase, (iii) and the occurrence of surface effects such as gas evolution and/or adsorption/desorption and/or formation/dissolution of solid deposits. Further, voltammetric curves can be distorted by uncompensated ohmic drops and capacitive effects in the cell [81-83]. 

A major advantage of microelectrodes is that electrochemical data is less distorted by ohmic drop than when recorded with electrodes of conventional size... 

Gas bubbles can also deactivate the electrode surface by forming a physical curtain [173,174], While bubble movement may enhance in situ mass transfer resulting in an increase of the current [175-178], the blocking effect of bubbles can distort measured parameters by introducing a sizable ohmic drop [179-183]. Bubble effects... 

One of the main disadvantages of voltammetric techniques like CV is the distortion caused by the combination of the double-layer charging process with the ohmic drop, related to the uncompensated resistance of the solution, Ru (see Sect. 1.9). This distortion can be very significant for macroelectrodes. 

As can be seen in this figure, the combined effect of ohmic drop and double-layer capacitance is much more serious in the case of CV. The increase of the scan rate (and therefore of the current) causes a shift of the peak potentials which is 50 mV for the direct peak in the case of the CV with v = 100 V s 1 with respect to a situation with Ru = 0 (this shift can be erroneously attributed to a non-reversible character of the charge transfer process see Sect. 5.3.1). Under the same conditions the shift in the peak potential observed in SCV is 25 mV. Concerning the increase of the current observed, in the case of CV the peak current has a value 26 % higher than that in the absence of the charging current for v = 100 Vs 1, whereas in SCV this increase is 11 %. In view of these results, it is evident that these undesirable effects in the current are much less severe in the case of multipulse techniques, due to the discrete nature of the recorded current. The CV response can be greatly distorted by the charging and double-layer contributions (see the CV response for v = 500 V s-1) and their minimization is advisable where possible. 

In order to avoid the distortion caused by these two effects, the usual approach is to compensate the resistance Ru by a positive feedback loop (this is imperative in systems like plasticized membranes for which the uncompensated resistance can be of the order of megaohms [32-34]). Another possibility is to use microelectrodes, for which a decrease in the measured current is obtained which minimizes the ohmic drop and charging current distortion (see Sects. 2.7 and 5.4.1). 

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]). 

Different IVR for PPX films with PbO and Pd nanoparticles can be probably caused by the difference in the work functions for PbO and Pd. Besides, nanoparticles PbO and Pd most likely have a various surface, and a field around charged nanoparticle depends on surface distortions. It is believed that unlike M nanoparticles of Pd, SC nanoparticles PbO resulted from Pb oxidation contains essential surface defects, for example, bulges where the density of a charge increases. Such defects cause local amplification of electric field in a interspace between the charged and neutral particles of a film. As a result, the effect of field on barrier of electron tunneling increases leading to deviation IVR from classical ohmic dependence. 

Ohmic drop distortion — Distortion of an electrochemical response caused by uncompensated ohmic resistance (see - IRU (ohmicpotential) drop). 

At first glance, the asymmetry of the Cjif versus curve corresponding to the permselective isotherm is large enough to identify the type of carrier limitation. However, on a practical basis these curves are significantly distorted by slow interfacial kinetics and Ohmic potential drops, in addition to differences in the carriers interactions in the host. 

It is assumed that the capacity measured, C, is not distorted due to the leakage effect at the interface, a finite value of the ohmic resistance of the electrode and electrolyte, etc. A correct allowance for these obstacles is an individual problem, which is usually solved by using an equivalent electrical circuit of an electrode where the quantity in question, Csc, appears explicitly. Several measurement techniques and methods of processing experimental data have been suggested to find the equivalent circuit and its elements (see, e.g.. Ref. 40). 

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