Uncompensated electrolyte resistance

Real systems deviate frequently from ideal behavior due to large uncompensated electrolyte resistance, slow kinetics of electron transfer, and site-to-site interactions (Ilangovan and Chandrasekara Pillai, 1997). Such deviations from Nernstian behavior can be expressed by an interaction term, r, which can be estimated from the variation of peak potential with potential scan rate ... 

Figure 3. 7 vs. co 1/2 plots for the impedance data from Figure 2. The uncompensated electrolyte resistance of 150 (l is subtracted.

In fact, the behavior of an aluminum electrode in aqueous solution is more complex and thus the equivalent circuit shown in Fig. 6 is used to describe it. It consists of an RC combination where C = Cox is the above discussed capacitance of the oxide and R = i ox results from the remaining ionic conductivity of the oxide. The oxide/electrolyte interface is represented by an RC combination in series to the first one where C = Ch is the capacitance of the Helmholtz layer and R = Rqt is the charge-transfer resistance of the interface. Finally a serial resistance 2 EL for the (uncompensated) electrolyte resistance is introduced. In case of Al,... 

The second factor is associated with the fact that all electrolyte solutions exhibit finite resistance to the flow of current. Thus, the potential that is measured (Em eas) between the working and reference electrodes consists of two contributors, the real thermodynamic potential Cereal) and that arising from uncompensated solution resistance (IRU)... 

Consider the situation under potentiostatic conditions. Here, the potential control takes care that the sum of the potential drop across the double layer, DL, and through the electrolyte up to the position of the RE (and possibly additional external series resistances) is constant, i.e. that U = DL + I Rn or / = (U - DL)/Rn. Rn is the sum of the uncompensated cell resistance and possible external resistances and I the total current through the cell. Hence, a perturbation of a state on the NDR branch towards larger values of Dl causes, on the one hand, a decrease of the faradaic current If, and, on the other hand, a decrease of the current through the electrolyte, I. The charge balance through the cell, which can be readily obtained from the general equivalent circuit of an electrochemical cell (Fig. 8), tells us whether the fluctuation is enhanced or decays ... 

The effect of uncompensated IR drop on corrosion rate determination using polarization resistance measurements was discussed in depth by Mansfeld [1-3]. He showed that in electrochemical measurements of the polarization resistance the experimental value Rp is the sum of the true value Rp and the uncompensated ohmic resistance R which is essentially the electrolyte resistance but can also contain the resistance of surface films. 

The analysis of LSV and CV curves (today, usually computer assisted) requires electrochemical experiments free from artifacts to provide an accurate faradaic response [112]. The influence of the uncompensated solution resistance, R, (see Eq. (13)) can be greatly reduced by the use of a powerful potentiostat with fast output. However, in the common case of undercompensation of the solution resistance (e.g., in solutions with low concentration of the supporting electrolyte) the effect is the increase of the cathodic and the corresponding anodic peak separation in a manner that could be mistaken for an apparent slow rate electron transfer or the quasi-reversible regime (potentials are shifted to more negative/positive directions). 

The commonly used pretreatment protocols for activating solid electrodes are reviewed in this chapter. Specifically, the pietreatment of carbon, metal, and semiconductor electrodes (thin conducting oxides) is discussed. Details of how the different electrode materials are produced, how the particular pretreatment works, and what effect it has on electron-transfer kinetics and voltammetric background current are given, since these factors determine the electroanalytical utility of an electrode. Issues associated with cell design and electrode placement (Chapter 2), solvent and electrolyte purity (Chapter 3), and uncompensated ohmic resistance (Chapter 1) are discussed elsewhere in this book. This... 

Another important question is the importance of correcting for the uncompensated Ohmic resistance in experiments of the ORR. This problem is not new, but it is often disregarded. It was elaborated by van der Vliet et al. [68] who showed that if one omits to correct measured current for IR drop erroneous values of specific activity and E a is obtained. This is a consequence of the fact that no matter how close Lugin capillary is to the working electrode, it is difficult to reduce electrolyte resistance below 10 Q. For measured ORR current of approx. 1 mA this would shift electrode potential by 10 mV. For ORR Tafel slope of 120 mV dec", this potential shift induces approx. 25% underestimated ORR activity (expressed as A,esa or 7k,mass a d corrected for mass transfer) and also incorrect value of Tafel slope. For these purposes, positive feed-back scheme is proposed [52]. 

FIG. 7 Simplified equivalent circuit for charge-transfer processes at externally biased ITIES. The parallel arrangement of double layer capacitance (Cdi), impedance of base electrolyte transfer (Zj,) and electron-transfer impedance (Zf) is coupled in series with the uncompensated resistance (R ) between the reference electrodes. (Reprinted from Ref. 74 with permission from Elsevier Science.)... 

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