Current-potential curves cathode

Fig. 2-3 Potential scheme (a) and circuit (b) for measuring a current-potential curve in cathodic polarization (explanation in the text).

Fig. 21-6 The dependence of the passivation process on the shape of the cathodic partial current potential curve (a) Anodic partial current potential curve, (b) cathodic partial current-potential curve without local cathode rest potential (c) cathodic partial current potential curve with local cathode rest potential I7j p.

It must be emphasised that in evaluating the limiting cathode potential to be applied in the separation of two given metals, simple calculation of the equilbrium potentials from the Nernst Equation is insufficient due account must be taken of any overpotential effects. If we carry out, for each metal, the procedure described in Section 12.2 for determination of decomposition potentials, but include a reference electrode (calomel electrode) in the circuit, then we can ascertain the value of the cathode potential for each current setting and plot the current-potential curves. Schematic current-cathode potential... 

FIGURE 1-7 Current-potential curve for the system O + ne - " R, assuming that electron-transfer is rate limiting, C0 = CR, and a = 0.5. Hie dotted lines show the cathodic ((.) and anodic ( ) components. 

FIGURE 1-8 Tafel plots for the cathodic and anodic branches of the current-potential curve. 

Experiments by Freund and Spiro/ with the ferricyanide-iodide system showed that the additivity principle held within experimental error for both the catalytic rate and potential when the platinum disk had been anodically preconditioned, but not when it had been preconditioned cathodically. In the latter case the catalytic rate was ca 25% less than the value predicted from adding the current-potential curves of reactions (15) and (16). This difference in behavior was traced to the fact that iodide ions chemisorb only on reduced platinum surfaces. Small amounts of adsorbed iodide were found to decrease the currents of cathodic Fe(CN)6 voltam-mograms over a wide potential range. The presence of the iodine couple (16) therefore affected the electrochemical behavior of the hexacyanofer-rate (II, III) couple (15). 

This equation describes the cathodic current-potential curve (polarization curve or voltammogram) at steady state when the rate of the process is simultaneously controlled by the rate of the transport and of the electrode reaction. This equation leads to the following conclusions ... 

Fig. 1. Current-potential curves for a generalized electroless deposition reaction. The dashed line indicates the curve for the complete electroless solution. The partial anodic and cathodic currents are represented by ia and ic, respectively. Adapted from ref. 28. 

During the forward (cathodic) scan, the current-potential curve is thus expressed in normalized form as... 

The normalized current-potential curves are thus a function of the two parameters A and oc. An example corresponding to a = 0.5 is shown in Figure 1.19. Decreasing the parameter A as a result of a decrease in the rate constant and/or an increase in scan rate triggers a shift of the cathodic potential toward negative values and of the anodic potential in the reverse direction, thus increasing the irreversibility of the cyclic voltammetric response. When complete irreversibility is reached (i.e., when there is no anodic current underneath the cathodic current, and vice versa), a limiting situation is reached, characterized by... 

Thin-film ideal or Nemstian behavior is the starting point to explain the voltammetric behavior of polyelectrolyte-modified electrodes. This condition is fulfilled when (i) the timescale of the experiment is slower than the characteristic timescale for charge transport (fjD pp, with Ithe film thickness) in the film, that is all redox within the film are in electrochemical equibbrium at any time, (ii) the activity of redox sites is equal to their concentration and (iii) all couples have the same redox potential. For these conditions, anodic and cathodic current-potential waves are mirror images (zero peak splitting) and current is proportional to the scan rate [121]. Under this regime, there exists an analytical expression for the current-potential curve ... 

Interaction Between Partial Reactions. The original mixed-p)otential theory assumes that the two partial reactions are independent of each other (1). In some cases this is a valid assumption, as was shown earlier in this chapter. However, it was shown later that the partial reactions are not always independent of each other. For example, Schoenberg (13) has shown that the methylene glycol anion (the formaldehyde in an alkaline solution), the reducing agent in electroless copper deposition, enters the first coordination sphere of the copper tartrate complex and thus influences the rate of the cathodic partial reaction. Ohno and Haruyama (37) showed the presence of interference in partial reactions for electroless deposition of Cu, Co, and Ni in terms of current-potential curves. 

Figure 8.10. Effect of NaCN on the current-potential curves for reduction of Cu in an electroless solution at 25°C containing 0.05 M CUSO4, 0.075 M EDTP [1,1,1, -(ethylenedinitrilo)tetra-2-propanoll, and 5.8mL/L of HCHO. The Pt cathode (0.442 cm ) was rotated at 100rpm the scan rate is lOmV/s. (From Ref. 45, with permission from the American Electroplaters and Surface Finishers Society.)...

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) ... 

Potential sweep relations consist of current-potential curves in which the potential is varied in a regular manner and the corresponding current is recorded. Cyclic voltammetry is this experiment but with the potential sweep—and the resulting current—plotted for the cathodic — anodic direction, followed by a sweep in the anodic —> cathodic direction. Several such diagrams are shown in the text. 

AgCl electrode) and the cathodic current due to the reduction of hydrogen ion begins to flow at about -1.1 V. Between the two potential limits, only a small current (residual current) flows. In curve 2, there is an S-shaped step due to the reduction of Cd2+, i.e. Cd2++2e +Hg <=t Cd(Hg). In DC polarography, the current-potential curve for the electrode reaction is usually S-shaped and is called a polaro-graphic wave. 

The impurity gives a signal that disturbs the measuring system. An example is shown in Fig. 10.1 [10], The residual current-potential curves were obtained with a platinum electrode in propylene carbonate (PC) containing various concentrations of water. Because water is amphiprotic, its cathodic reduction and anodic oxidation are easier than those of PC, which is aprotic and protophobic. Thus, the potential window is much narrower in the presence of water than in its absence. Complete removal of water is essential for measuring electrode reactions at very negative or positive potentials. 

Fig. 10.1 Effect of water on the residual current-potential curves at a platinum electrode in PC [10]. (a) Cathodic side in 0.1 M Bu4NCI04. Water concentration curve 1,...

The rate of oxygen reduction is the controlling factor in the overall rate in Fig. 13(b). The measurable current—potential curve results from the balance of the anodic and cathodic processes as indicated by the broken line. The corrosion potential, EM, is also shown. 

Figure 9 shows a cyclic voltammogram (CV) of hexacyanoferrate(III) (Fe(III)) in water observed at a gold microelectrode (8 fim wide x 33 fim long x 0.2 fim thick). A cathodic current at 200 mV corresponds to reduction of Fe(III) to Fe(II). At a micrometer-sized electrode, mass transfer of a solute in water to the electrode proceeds very efficiently owing to hemispherical diffusion of the solute. This is proved by a characteristic sigmoidal current-potential curve in the CV, different from a peak current observed at a millimeter-size electrode (linear diffusion) [32,64]. Using... 

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