Alternating current electrode processes

In Chapter 2, we approached alternating-current electrode polarization impedance from the phenomenological point of view, which parallels the historical development of this subject. Before we embark upon descriptions of electrochemical cells, ion-specific electrodes, and potentiometric techniques, it is necessary to discuss some of the electrochemical processes that occur at the interface between a solid electrode surface and a contacting electrolyte. 

Conductance of a solution is a measure of its ionic composition. When potentials are applied to a pair of electrodes, electrical charge can be carried through solutions by the ions and redox processes at the electrode surfaces. Direct currents will result in concentration polarization at the electrodes and may result in a significant change in the composition of the solution if allowed to exist for a significant amount of time. Conductance measurements are therefore made using alternating currents to avoid the polarization effects and reduce the effect of redox processes if they are reversible. 

The physical approach uses alternating current (ac-) dielectrophoresis to separate metallic and semiconducting SWCNTs in a single step without the need for chemical modifications [101]. The difference in dielectric constant between the two types of SWCNTs results in an opposite movement along an electric field gradient between two electrodes. This leads to the deposition of metallic nanotubes on the microelectrode array, while semiconducting CNTs remain in the solution and are flushed out of the system. Drawbacks of this separation technique are the formation of mixed bundles of CNTs due to insufficient dispersion and difficulties in up-scaling the process [102]. 

The DC arc process has several advantages over an alternating current (AC) arc furnace. An AC arc furnace requires three electrodes during operation, while a DC furnace only needs one. The DC system is considered utility friendly because it does not introduce a flicker into the utility system. In addition, EPI states that DC arc systems have lower electrode consumption, energy costs, and noise levels than AC systems. 

Lynntech, Inc. s (Lynntech s), electrokinetic remediation of contaminated soil technology is an in situ soil decontamination method that uses an electric current to transport soil contaminants. According to Lynntech, this technology uses both direct current (DC) and alternating current (AC) electrokinetic techniques (dielectrophoresis) to decontaminate soil containing heavy metals and organic contaminants. A non homogeneous electric field is applied between electrodes positioned in the soil. The field induces electrokinetic processes that cause the controlled, horizontal, and/or vertical removal of contaminants from soils of variable hydraulic permeabilities and moisture contents. 

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. 

Alternating-current and frequency effects. With an AC rather than a DC voltage applied to the electrodes, the processes above reverse themselves with the period of the alternating voltage. But each process proceeds at a different rate (with a characteristic relaxation time) so that their relative contributions to energy dissipation vary with frequency. As the frequency is increased concentration-polarization can be reduced or eliminated, particularly if the electrode reaction is reversible (fast electron transfer in both directions). 

Anode passivation and sludge deposition on the electrodes can limit the process. Alternating current and promotion of high turbulence prevent this problem, thus increasing the time required for the electrode replacement. 

The use of a potential-step technique such as cyclic staircase voltammetry represents a simple alternative to Ichise s method (j0 of obtaining information on both adsorption and electron transfer kinetics. The current decay immediately after a step is primarily capacitive while current at later times is almost totally due to electron transfer reactions. Thus, by measuring the current at several times during each step and by changing the scan rate, information on both the kinetics of the electrode process and the differential capacity can be obtained with a single sweep. 

Other techniques for the study of electrode processes include, for example, potential sweep, alternating current (ac), and rotating electrodes [15]. 

An alternative method of presenting the current-potential curves for electroless metal deposition is the Evans diagram. In this method, the sign of the current density is suppressed. Figure 22 shows a general Evans diagram with current-potential functions i = f(E) for the individual electrode processes, Eqs (43 and 44). According to this presentation of the mixed-potential theory, the current-potential curves for individual processes, ic = iu = f(E) and ia = = f(E), intersect. The... 

---