Potentiostatic transients

Cahan, Nagy and Genshaw examine design criteria for an electrochemical measuring system to be used for potentiostatic transient investigation of fast electrode reactions. They emphasise the importance of co-design of the experimental cell and electronics. 

Figure 5.12. Time evolution of catalyst potential and work function for Pt/p"-Al203 during potentiostatic transients, T = 200°C, po2 = 10 10 Pa.25...

Figure 8.17. Potentiostatic transient of C2H4 oxidation on Pt/Ce02. Rate and current responses to step changes in catalyst potential, UWR) are plotted against time. T = 500°C, p02 = 5.5 kPa, PC2H4= 1 5 kPa.71 Reproduced by permission of The Electrochemical Society.

In total, the differences between Ci adsorbate stripping and COad stripping, in both potentiodynamic measurements and potentiostatic transients, can be qualitatively explained by the lower COad coverage after Ci adsorption. A quantitative comparison with a lower coverage CO adlayer reveals, however, that these coverage effects are not sufficient and that contributions from other effects, most likely related to the structure of the CO adlayer, are important for the different oxidation behavior of the respective adsorbates as well. 

Fig. 12. Steady-state anodic polarization curves (a), and potentiostatic transient curves (b), of a mild steel hemisphere in neutral Na2S04 solution. From [15].

Study of the charge-transfer processes (step 3 above), free of the effects of mass transport, is possible by the use of transient techniques. In the transient techniques the interface at equilibrium is changed from an equilibrium state to a steady state characterized by a new potential difference A(/>. Analysis of the time dependence of this transition is the basis of transient electrochemical techniques. We will discuss galvanostatic and potentiostatic transient techniques for other techniques [e.g., alternating current (ac)], the reader is referred to Refs. 50 to 55. 

Potentiostatic Transient Technique, In the potentiostatic technique the potential of the test electrode is controlled, while the current, the dependent variable, is measured as a function of time. The potential difference between the test electrode and the reference electrode is controlled by a potentiostat (Fig. 6.21). The input function, a constant potential, and the response function, i = f(t), are shown in Figure 6.22. 

Potentiostatic techniques, 787, 1115, 1118 and impurities on electrodes. 1120 potential interval measurements, 1121 p-polarized light, 802 Potentiodynamic techniques, 1423, 1438 vs. potentiostatic techniques, 1426 Potentiostatic transients, 1414 difficulties in, 1415 double layer charging, 1416 radicals in, 1416 IR drop in, 1416 Prandtl layer, 1228... 

First, it is desirable to show the quintessential potentiostatic and galvanostatic phenomena in a schematic way. Figure 7.42 is a schematic presentation of the potentiostatic transient at one chosen overpotential. The potential is applied at time t =... 

Fig. 7.42. A potentiostatic transient. The current (A-B) ascends almost vertically after being switched on, because all of it goes to charge the double layer. In B-C, the current is increasingly used in the form of electrons crossing the double layer. After C the current should decline slowly as diffusion control sets in. In reality, at solid polycrystalline electrodes, in reactions involving adsorbed intermediates, there is often some further variation of /, owing to, e.g., surface crystalline rearrangements and the effect of impurities from the solution.

Fig. 7.44. (A) The identified perfect potentiostatic transient (cf. Fig. 7.42). (B) A schematic of a more realistic potentiostatic transient as it occurs, i.e., on polycrystalline electrodes in reactions involving intermediates.

M. Fleischmann and R. Thirsk, Electrochim. Acta 2 22 (1960). Potentiostatic transient and crystal growth. 

In terms of the earlier material, this technique is nearest to the potentiostatic technique, but because here the potential is made to vary linearly with time (i.e., it is not static), the more appropriate name ispotentiodynaniic. As far as the electrode, cell, etc., are concerned, one has the same setup as with potentiostatic transients the difference is that instead of being fixed at a given value while the ip is observed, the potential is made to change at a constant rate over a chosen potential range. The range of acceptable values for the sweep rate is something to be discussed in detail later, but it may be stated now that a typical value is 10 mV s ... 

The potentiostatic transient can be divided into three time intervals. At the beginning, the first time interval, the current decays during the process of nucleation and growth. This is the double-layer charging current, iAX. In the second... 

A cell system has been designed for the potentiostatic transient investigation of fast electrode reactions.9 The main novel feature of the cell is the elimination of the classical Luggin capillary, as shown in Figure 6.4. The design provides a low-ohmic-resistance reference electrode with low stray capaitances. 

Thus, it may be concluded that potentiostatic transients can be, malyzed correctly only if the value of iR is negligible throughout the liansient or if it is dynamically compensated for in real time, during K the course of the transient. 

Typical potentiostatic transients are shown in Fig. 14K. Such data can be employed in two ways to evaluate the activation-controlled rate as a function of overpotential. We have already seen that the measured current density is related to the diffusion-limited current density by the equation... 

Fig. I4K Potentiostatic transients at high overpotentials, n = 2 D = 6x10 cn s (f = 50 niM. From Gileadi, Kirowa-Eisner and Penciner, "Interfacial Electrochemistry - An Experimental Approach" Addison Wesley, Publishers 1975, with permission.

Consider now the effect of uncompensated iR on the shape of the potentiostatic transients. This was shown in Fig. 6D. The point to remember is that although the potentiostat may put out an excellent step function - one with a rise time that is very short compared to the time of the transient measured - the actual potential applied to the interphase changes during the whole transient, as the current changes with time (cf. Section 10.2). This effect is not taken into account in the boundary conditions used to solve the diffusion equation, and the solution obtained is, therefore, not valid. The resulting error depends on the value of R, and it is very important to minimize this resis-tance, by proper cell design and by electronic iR compensation. 

As already discussed, it has been found that the initial parts of the current transients follow the i-t relation of the pre-exponential term of eq. (5.15). Values of Jv obtained from the slopes of the i vs. t plots can be used for the calculation of fi,max and ii,max needed for a comparison of the theoretical and experimental curves in normalized coordinates. Potentiostatic transients recorded at different overvoltages and normalized in the way described give a package of curves lying in the hatched region of Fig. 5.25 [5.45, 5.46]. 

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