Current-potential curves usefulness

Fig. 12. Rotating (45 Hz) ruthenium dioxide/titanium dioxide electrode (35%w/w ruthenium dioxide) in 1.0 M NaCl solution, (a) Standard rate constant-potential curve assuming a constant Tafel slope of 70mV. Dcl = 5 x 10 6cm s-1, Dc = 7 x 10 6cm s-1, E° = 1050mV SCE, and R = 0.8 ohm cm2. (b) Standard rate constant-potential curve assuming a constant Tafel slope of 40mV. DC1 = 5 x 10 8cm s 1, Z)ct2 = 7 x 10 8cm s 1, E° = 1050mV SCE, and R = 0.8ohm cm2. (c) Common experimental and calculated current-potential curve using the parameters of Fig. 12(b). (d) Double layer capacity curve.

Recording ORR Current—Potential Curves Using RDE Technique 193... 

Exchange reactions between bulk and adsorbed substances can be studied by on-line mass spectroscopy and isotope labeling. In this section the results on the interaction of methanol and carbon monoxide in solution with adsorbed methanol and carbon monoxide on platinum are reported [72], A flow cell for on-line MS measurements (Fig. 1.2) was used. 13C-labeled methanol was absorbed until the Pt surface became saturated. After solution exchange with base electrolyte a potential scan was applied. Parallel to the current-potential curve the mass intensity-potential for 13C02 was monitored. Both curves are given in Fig. 3.1a,b. A second scan was always taken to check the absence of bulk substances. 

The basic theory of mass transfer to a RHSE is similar to that of a RDE. In laminar flow, the limiting current densities on both electrodes are proportional to the square-root of rotational speed they differ only in the numerical values of a proportional constant in the mass transfer equations. Thus, the methods of application of a RHSE for electrochemical studies are identical to those of the RDE. The basic procedure involves a potential sweep measurement to determine a series of current density vs. electrode potential curves at various rotational speeds. The portion of the curves in the limiting current regime where the current is independent of the potential, may be used to determine the diffusivity or concentration of a diffusing ion in the electrolyte. The current-potential curves below the limiting current potentials are used for evaluating kinetic information of the electrode reaction. 

The first catalysts reported for the electroreduction of C02 were metallophthalocyanines (M-Pc).126 In aqueous solutions of tetraalkylammonium salts, current-potential curves at a cobalt phthalocyanine (Co-Pc)-coated graphite electrode showed a reduction current peak whose height was proportional to the C02 concentration and to the square root of the potential sweep rate at a given C02 concentration. On electrolysis, oxalic acid and glycolic acid were detected, but formic acid was not. Mn and Pd phthalocyanines were inactive, while Cu and Fe phthalocyanines were slightly active. At the potentials used for C02 reduction, M-Pc catalysts would be in their dinegative state, and the occupied dz2 orbital of the metal ion in the metallophthalocyanine was suggested to play an important role in the catalytic activity. 

In cyclic voltammetry, simple relationships similar to equations (1.15) may also be derived from the current-potential curves thanks to convolutive manipulations of the raw data using the function 1 /s/nt, which is characteristic of transient linear and semi-infinite diffusion.24,25 Indeed, as... 

Figure 4.14b and c illustrate the possibility of using convolution (Section 1.3.2) to transform all the voltammograms, whether they are plateau- or peakshaped, into a plateau-shaped wave. Measuring the height of this plateau allows determination of the kinetic constant, showing that this does not necessarily require that the raw current-potential curve be plateau-shaped. The standard potential, FpQ, may also be determined this way. 

The scan rate of the potential is usually in the range from 0.020 V s-1 to 100 V s"1. Until a few years ago the resulting current-potential curves were recorded with a normal X-Y recorder up to a scan rate of about 0.5 V s-1 for higher scan rates it was necessary to use an oscilloscope. Recently, however, the use of personal computers interfaced with the electrochemical apparatus has overcome most recording problems. 

Steady-State Kinetics, There are two electrochemical methods for determination of the steady-state rate of an electrochemical reaction at the mixed potential. In the first method (the intercept method) the rate is determined as the current coordinate of the intersection of the high overpotential polarization curves for the partial cathodic and anodic processes, measured from the rest potential. In the second method (the low-overpotential method) the rate is determined from the low-overpotential polarization data for partial cathodic and anodic processes, measured from the mixed potential. The first method was illustrated in Figures 8.3 and 8.4. The second method is discussed briefly here. Typical current—potential curves in the vicinity of the mixed potential for the electroless copper deposition (average of six trials) are shown in Figure 8.13. The rate of deposition may be calculated from these curves using the Le Roy equation (29,30) ... 

Fig. 11 Cyclic current-potential curve forAu(lOO) in 0.1 M H2SO4 solution beginning of polarization —0.2 V (versus SCE) using a freshly prepared reconstructed surface. Scan rate ...

In many electrochemical techniques, we measure current-potential curves for electrode reactions and obtain useful information by analyzing them. In other techniques, although we do not actually measure current-potential curves, the current-potential relations at the electrodes are the basis of the techniques. Thus, in this section, we briefly discuss current-potential relations at the electrode. 

Polarography and Voltammetry Both methods are the same in that current-potential curves are measured. According to the IUPAC recommendation, the tenn polarography is used when the indicator electrode is a liquid electrode whose surface is periodically or continuously renewed, like a dropping or streaming mercury electrode. When the indicator electrode is some other electrode, the term voltammetry is used. However, there is some confusion in the use of these terms. 

Voltammetry is a term used to include all the methods that measure current-potential curves (voltammograms) at small indicator electrodes other than the DME [6], There are various types of voltammetric indicator electrodes, but disk electrodes, as in Fig. 5.17, are popular. The materials used for disk electrodes are platinum, gold, graphite, glassy carbon (GC), boron-doped diamond8, carbon paste, etc. and they can be modified in various ways. For electrode materials other than mercury, the potential windows are much wider on the positive side than for mercury. However, electrodes of stationary mercury-drop, mercury-film, and mercury-pool are also... 

A rotating disk electrode (RDE) [7] is used to study electrode reactions, because the mass transfer to and from the electrode can be treated theoretically by hydrodynamics. At the RDE, the solution flows toward the electrode surface as shown in Fig. 5.22, bringing the substances dissolved in it. The current-potential curve at the RDE is S-shaped and has a potential-independent limiting current region, as in Fig. 5.6. The limiting current (A) is expressed by Eq. (5.33), if it is controlled by mass transfer ... 

As described in Chapter 8, current-potential curves in polarography and voltammetry are useful for obtaining mechanistic information on electrode reactions. However, for complicated electrode processes, the information obtained from the current-potential curves is not conclusive enough. In order to get more conclusive information, it is desirable to confirm the reaction products and/or intermediates by some other technique. In this chapter, we focus our discussion on such techniques. We deal with electrolytic and coulometric techniques in Section 9.1 and the combinations of electrochemical and non-electrochemical techniques in Section 9.2. 

If we measure a residual current-potential curve by adding an appropriate supporting electrolyte to the purified solvent, we can detect and determine the electroactive impurities contained in the solution. In Fig. 10.2, the peroxide fonned after the purification of HMPA was detected by polarography. Polarography and voltammetry are also used to determine the applicable potential ranges and how they are influenced by impurities (see Fig. 10.1). These methods are the most straightforward for testing solvents to be used in electrochemical measurements. 

Numerical calculations by Nicholson [26] provide a basis for the study of heterogeneous charge transfer using CV. Theoretical data indicate that both the shape of the waves and AEp depend upon a number of factors including a, k°, Ex and v. The current potential curves were derived in terms of a and a function t//, related to A by... 

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