Current-potential data

Robinson D, Walsh FC (1991) The performance of a 500 Amp rotating cylinder electrode reactor, Part 1. Current-potential data and single pass studies, Hydrometallurgy 26 93 Chem Abstr 114 (1991) 194767w... 

A strategy for handling the ohmic drop problem that combines satisfactory accuracy with minimal tedium therefore consists of using the positive feedback compensation as much as possible and, when necessary, correcting the residual ohmic drop by the approximate procedures discussed above. A more general and more rigorous treatment of ohmic drop may be devised with the help of convolutive treatments of the current-potential data, discussed in the next section. 

Secondly, the surface of the drop is constantly being renewed, therefore meaning that its surface is always clean. Problems caused by contamination are more readily controlled than with a conventional solid electrode, thus leading to reproducible current-potential data. 

Wfe can be satisfied that the variables we have (e.g. A, D, etc., as above) are accurate if they allow us to model a system precisely. Vie say that the variables fit the data. Figure 10.1 shows a cyclic voltammogram (CV), with superimposed on this current-potential data simulated with the DigiSim package (as described in more detail below). The fit between theory and experiment is seen to be good, so we say that the derived variables fit the data. 

Take a mean value of 80 (i.e., 0.83 eV). Numerical calculations show that T] < 0.2 V is the condition up to which 9.38 yields the experimental version of Tafel s law (of course, the value depends on the Fs chosen and the allowed T, for the applicability of 9.38 will be roughly halved at the lower limit and doubled at the higher one. In any case, this harmonic approximation, which is involved in the Weiss—Marcus theory, cannot be applied to the experimental current-potential data, which in reality extend over 0.2 V and even 1.0 V (for hydrogen and oxygen evolution). 

Convolution potential sweep voltammetry (CPSV) refers to the mathematical transformation of LSV current—potential data resulting in curves with shapes like conventional polarograms which are suitable for logarithmic analysis. The method was first proposed for the study of electrode kinetics by Imbeaux and Saveant [74] but is equivalent in all respects to a semi-integral technique reported earlier by Oldham [75— 77]. A very readable description of the method has been presented by Bard and Faulkner [21]. 

For the outer-sphere Co(NH3)63+ reduction, the SERS and current-potential data are closely compatible in that the SERS intensities drop sharply at potentials towards the top of the voltammetric wave where the overall interfacial reactant concentration must decrease to zero. Some discrepancies between the SERS and electrochemical data were seen for the inner-sphere Cr(NH3)sBr2 and Cr(NH3)sNCS2+ reductions, in that the SERS intensities decrease sharply to zero at potentials closer to the foot of the voltammetric wave. This indicates that the inner-sphere reactant bound to SERS-active sites is reduced at significantly lower overpotentials than is the preponderant adsorbate. (15)This suggests that SERS-active surface sites might display unusual electrocatalytic activity in some cases. 

The marked irreversibility of the oxygen evolution and reduction reactions in aqueous solutions has imposed severe limitations on the mechanistic information which can be obtained for both reactions. In general, at the current densities normally employed for kinetic studies, the current-potential data are insensitive to the back reaction, which normally occurs early on in the multi-step reaction sequence. Further, the reduction and oxidation processes are usually studied only at widely separated potentials. Thus, the surface conditions, whether in the case of metals or bulk oxides, probably differ sufficiently such that the reduction and oxidation pathways may not be complementary. The situation is complicated further by the large number of possible pathways for both reactions. 

A more simple analysis of LSV waves can give essentially the same information as CPSV and NPSV. Analysis of theoretical current-potential data for Nemstian and purely kinetic waves revealed that a nearly linear region... 

Uniformity of the current density over the whole area of the electrode is important for the interpretation of current-potential data. We recall that the measured quantities are the total current and the potential at a certain point in solution, where the reference electrode (or the tip of the Luggin capillary) is located. From this we can calculate the average current density, but not its local value. As for the potential, we have already noted that unless the cell is properly designed, the potential measured may be grossly in error if the reference electrode is located at a point where the local current density deviates significantly from its average value, as shown in Fig. 6C. 

Determination of the transfer coefficient from current-potential data was discussed earlier (Eq. 13b). The concentration dependence of the reaction rate can be easily obtained with simple order reactions at constant potential... 

To study the kinetics of the catalytic process, Tafel representations from the current-potential data in the rising portion of the voltammetric curve can be used (Doherty and Vos, 1992). Under several conditions, plots of log i vs. E lead to straight lines whose slope (TSL) satisfies (Lyons et al., 1994) ... 

Figure 7.22 (A) Plots of overall electron-transfer number and (B) percentage H2O2 produced as a function of electrode potential at three different concentrations of 02-saturated KOH solution. Calculations based on current-potential data collected at rotation rate of 1600 rpm. Reprinted with permission from Ref. 66.

The type of analysis described above requires that both halves of the redox couple are stable and available (a known finite concentration of each must be present in solution to define an equilibrium potential), and that the equilibrium potential can either be measured, or calculated from the Nernst equation (i.e. the standard potential is known). This is often not the case, e.g. particularly in organic electrochemistry, one half of the redox couple may be unstable. However, Tafel type plots may still prove useful. Current-potential data are analysed using the Tafel equation in the fonn... 

FIGURE 5.12 Current-potential data for Cu-catalyzed Si microwire array photoelectrodes in contact with the Me2Fc °-CH30H redox system (a) under 100 mW cm- of AM 1.5 G illumination and (b) with varying amounts of Me2FcBp4. (From Santori, E.A. et al.. Energy Environ. ScL, 5,6867, 2012.)... 

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