Rotating current transients

At a double electrode, such as the rotating ring—disc electrode, a potential step at the disc will produce a ring current transient, the form of which is affected only by Faradaic current components at the disc. This fact can be very useful in separating Faradaic and non-Faradaic processes. 

Fig. 8. Typical results for the rotation speed step experiment. The values of t are found from the current transient using eqn. (8). These are plotted against t according to eqn. (7). A value of D can then be found from the gradient.

Although a major advantage of rotating disk electrode techniques, compared to stationary electrode methods, is the ability to make measurements at steady state without the need to consider the time of electrolysis, the observation of current transients at the disk or ring following a potential step can sometimes be of use in understanding an electrochemical system. For example the adsorption of a component. 

A good test of the protective properties of self-assembled monolayer films is to investigate to what extent these layers can inhibit the activation of the inclusions. This can be done by cathodic potential jumps and monitoring the current transients. Figure 12 shows such transients for an ummodified surface at difieient rotation velocities. 

Figure 12 Current transients for AlMgl samples after potential jumps from the OCP to -800 mV for different rotation velocities.

As the field of electrochemical kinetics may be relatively unfamiliar to some readers, it is important to realize that the rate of an electrochemical process is the current. In transient techniques such as cyclic and pulse voltammetry, the current typically consists of a nonfaradaic component derived from capacitive charging of the ionic medium near the electrode and a faradaic component that corresponds to electron transfer between the electrode and the reactant. In a steady-state technique such as rotating-disk voltammetry the current is purely faradaic. The faradaic current is often limited by the rate of diffusion of the reactant to the electrode, but it is also possible that electron transfer between the electrode and the molecules at the surface is the slow step. In this latter case one can define the rate constant as ... 

In this section we will demonstrate the transient response of an inductor circuit with a switch that is normally closed. The initial condition of the inductor will not be specified by an IC= line in the circuit. Instead, the initial condition will be determined by PSpice from the initial state of the circuit before the switch changes position. If you wish to specify the initial condition of the inductor, it is specified in the same way as the initial condition of a capacitor. For an inductor, the direction for positive current is into the dotted terminal, as shown in Figure 6.1. The dot is always shown on the inductor graphic. The graphic should be rotated to obtain the desired direction of positive current flow. 

Study of the charge transfer processes (step 3 above), free from 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. The analysis of the time dependence of this transition is a basis of transient electrochemical techniques. We will discuss galvanostatic and potentiostatic transient techniques. For other techniques [e.g., alternating current (ac) and rotating electrodes], the reader is referred to references in the Further Reading list. 

Another example for the HMRRD electrode is given in Fig. 9 for Fe in alkaline solutions [12, 27]. The square wave modulation of the rotation frequency co causes the simultaneous oscillation of the analytical ring currents. They are caused by species of the bulk solution. Additional spikes refer to corrosion products dissolved at the Fe disc. This is a consequence of the change of the Nemst diffusion layer due to the changes of co. This pumping effect leads to transient analytical ring currents. Besides qualitative information, also quantitative information on soluble corrosion products may be obtained. The size of the spikes is proportional to the dissolution rate at the disc, as has been shown by a close relation of experimental results and calculations [28-30]. As seen in Fig. 7, soluble Fe(II) species are formed in the po-... 

Experimental values of (ihv)J(ihv) are larger than predicted theoretical values at high t, which is attributed to the deposition of particles on the electrode surface. As the electrode is stationary and there exists no convection to sweep the deposited particles away from the electrode surface, this residual current persists after illumination is stopped, distorting the form of the observed light-off current-time transient. Consequently, theoretical analysis of the transient was not attempted. The rate constant k may be obtained from (//, ) and the rotation speed dependence of the photocurrent. From equations (9.101) and (9.76), it can be seen that ... 

This result does not stand in isolation. Peter et al. [150] have recently reported that transient data on p-GaP, using the theory of section 5, also show evidence that the potential distribution is far from the ideal portrayed in Sect. 3, and recent rotating ring disc studies by Kelly and Notten [190] have also lent credence to this idea. If these results are indeed correct, it is evident that much of the current theory of semiconductors will have to be applied with more critical care than has been done so far. It is clear that much remains to be done. 

In the non-steady state, changes of stoichiometry in the bulk or at the oxide surface can be detected by comparison of transient total and partial ionic currents [32], Because of the stability of the surface charge at oxide electrodes at a given pH, oxidation of oxide surface cations under applied potential would produce simultaneous injection of protons into the solution or uptake of hydroxide ions by the surface, resulting in ionic transient currents [10]. It has also been observed that, after the applied potential is removed from the oxide electrode, the surface composition equilibrates slowly with the electrolyte, and proton (or hydroxide ion) fluxes across the Helmholtz layer can be detected with the rotating ring disk electrode in the potentiometric-pH mode [47]. This pseudo-capacitive process would also result in a drift of the electrode potential, but its interpretation may be difficult if the relative relaxation of the potential distribution in the oxide space charge and across the Helmholtz double layer is not known [48]. 

Newman2°° 2° modeled the transient response of a disk electrode to step changes in current. The solution to Laplace s equation was performed using a transformation to rotational elliptic coordinates and a series expansion in terms of Lengendre polynomials. Antohi and Scherson expanded the solution to the transient problem by expanding the number of terms used in the series expansion. ... 

Two of the electrochemical techniques used in protein film voltammetry are shown in Fig. 4-3. In cyclic voltammetry the electrode potential is swept in a linear manner back and forth between two limits. The rate at which the potential is scanned defines the time scale of the experiment and this can be varied from < 1 mV s to > 1000 V s . This is a very large dynamic range, and it is possible to carry out both steady-state and transient experiments on the same sample of enzyme. " Cyclic voltammetry is important because it provides the big picture and produces a signal that links the reaction or active site of interest to a particular potential. In chronoamperometry, the current is monitored at a constant potential following a perturbation such as a step to this potential or addition of a substrate. This experiment is important because it separates the potential and time dependencies of a response. In both types of experiment, it is usually important to be able to rotate the electrode in order to control transport of the substrate and product to and from the enzyme film. 

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