Reversibility of electrode processes

Table Bl.28.1 Diagnostic tests for reversibility of electrode processes in cyclic voltaimnetry at 293 K.

Electrolytic reversal of electrode processes. If the product of one electrode in a cell is carried by diffusion or other means to the other electrode the product may be wholly, or in part restored to its original condition. An oxidized anode product is likely to be reduced if it is allowed to get to the cathode, and vice versa, The passage of current through a solution containing a mixture of ferrous and ferric chlorides is an instructive example of this effect. As has been seen, the electrode reactions are... 

Cyclic voltammetry is mainly used for studying the reversibility of electrode processes and for kinetic observations, and only sometimes for analytical purposes. The voltage cycle illustrated in Figure 16 ensures that the reaction products... 

Some period of anodic polarization of a structure is allowed at which a total effect of metal loss is not observed. This results from a certain degree of reversibility of electrode processes occurring on the metal-electrolytic environment phase interface. On the basis of laboratory and field measurements, the following criterion of corrosion hazard to an industrial structure has been assumed due to electrolytic corrosion caused by stray currents ... 

Chronoamperometry is often used for measuring the diffusion coefficient of electroactive species or the surface area of the working electrode. Analytical applications of chronoamperometry (e.g., in-vivo bioanalysis) rely on pulsing of the potential of the working electrode repetitively at fixed tune intervals. Chronoamperometry can also be applied to the study of mechanisms of electrode processes. Particularly attractive for this task are reversal double-step chronoamperometric experiments (where the second step is used to probe the fate of a species generated in the first step). 

In order to asses the analytical aspects of the rotating electrodes we must consider the convective-diffusion processes at their bottom surface, and in view of this complex matter we shall confine ourselves to the following conditions (1) as a model of electrode process we take the completely reversible equilibrium reaction ... 

Although from the thermodynamic point of view one can speak only about the reversibility of a process (cf. Section 3.1.4), in electrochemistry the term reversible electrode has come to stay. By this term we understand an electrode at which the equilibrium of a given reversible process is established with a rate satisfying the requirements of a given application. If equilibrium is established slowly between the metal and the solution, or is not established at all in the given time period, the electrode will in practice not attain a defined potential and cannot be used to measure individual thermodynamic quantities such as the reaction affinity, ion activity in solution, etc. A special case that is encountered most often is that of electrodes exhibiting a mixed potential, where the measured potential depends on the kinetics of several electrode reactions (see Section 5.8.4). 

It is evident that (once the electrochemical reversibility of the process under examination has been checked, see the next section) the experimental measurement of the peak current, ip, allows one to calculate one of the parameters appearing in the equation. For instance, if the peak current ip at a certain scan rate v is measured, knowing the area of the electrode A, the diffusion coefficient D and the concentration C of the species under study, one can compute the number of electrons n involved in the redox change. On the other... 

Whereas for reversible reactions only thermodynamic and mass-transport parameters can be determined, for quasi-reversible and irreversible reactions both kinetic and thermodynamic parameters can be measured. It should also be noted that the electrode material can affect the kinetics of electrode processes. 

Electrolysis with a dropping mercury electrode, invented by J. Heyrovsky, on which the well known polarographic method is based, enables a detailed investigation of polarization processes as well as the study of electrode processes from the point of view of kinetics, reversibility, adsorption and capacity phenomena. 

The first factor determines the tendency for dissolution to occur while the second and third, which are closely related, determine the rate of dissolution. The use of the standard electrode potentials as a measure of nobility is well known. The recognition that the exchange current density is a measure of the reversibility of a process and therefore a quantity characteristic of the reactivity of the system is more recent (13,32). As indicated by the Tafel relations, the exchange current density is a direct measure of the rate of the electrode reaction for any given value of the activation overvoltage (33). The values of iG may then be taken as a criterion for the electrochemical activity of a system. 

An alternative approach to the problem has been reported (Parker and Hammerich, 1977). The reversible potentials for a family of electrode processes (44-46) where AN refers to methyl substituted anthracenes and N A to... 

Rates of Electrode Processes.—When a metal M is inserted in a solution of its ions M(H20), the solvent being assumed for simplicity to be water, there will be a tendency for the metal to pass into solution as ions and also for the ions from the solution to discharge on to the metal in other words the two processes represented by the reversible reaction... 

Since the point of bubble evolution represents a more or less indefinite rate of discharge of hydrogen and hydroxyl ions, recent work on overvoltage has been devoted almost exclusively to measurements made at definite c.d. s it is then possible to obtain a more precise comparison of the potentials, in excess of the reversible value, which must be applied to different electrodes in order to obtain the same rate of ionic discharge in each case. The details of the methods of measurement and a discussion of the results will be given after the general problem of the mechanism of electrode processes has been considered. 

Raghava Rao et al. [89] selectively removed neutral salts contained in spent chromium tanning solutions to achieve a more efficient technique for recycling the unused chromium and process water. The electrodialysis unit contained Neosepta CL-25T and ACH-45T membranes. An application of 13-30 V to a 5 dm solution over a period of 5 - 6 h produced currents between 2 and 4 A 90% of the sodium chloride and 50% of the sodium sulfate were selectively removed with minimal transport of Cr(III) species across the membranes. Addition of EDTA to the spent liquor as well as periodic reversal of electrode polarities eliminated membrane fouling. 

A number of electrode processes involve an initial step of molecular dissociative adsorption at the electrode metal surface. Such reactions have important technological significance in the fields of fuel-cell and gas-battery development. For the cases of simple reactions involving, for example, H2 or Cl 2, these steps are the reverse of the final molecule-producing step in the corresponding gas evolution process. Examples are as follows ... 

Cyclic voltammetry measurements revealed that in the ease of DCNDBQT, different CV eurves were obtained depending on the solvent used. In dichlo-romethane the CV exhibited two reversible oxidation and one irreversible reduction processes (not shown). The irreversibility of the reduction process can be explained by the conditions of recording the electrochemical measurements (scan rate, electrolyte, electrode material), which play an important role on the reversibility of the processes [24]. 

Metal deposition and dissolution (34) In the electrodeposition of solid metals such as silver and zinc, the cation is transported across the electrochemical interface to sites on the electrode surface (Figure 6-4). The positive charge of the cation is offset by electrons from the metal, and the adsorbed species becomes an adatom. These species have surface mobility and migrate along the electrode surface to an imperfection such as a step dislocation, where they enter into the crystal lattice. In the absence of sufficient step dislocations to accommodate the rate of deposition, the adatom surface concentration increases until two- or three-dimensional nucleation occurs. The rate of such nucleation and surface migration strongly influences the morphology of the electrocrystalhzation process. The reverse of this process is involved with electrodissolution of crystalline electro-deposits. 

The formation and dissociation of S-S bonds in poly(tricyanuric acid), which is proposed as electrode material for lithium batteries, has been studied [590,591]. The reversibility of the process essential for the use of this material in a secondary battery could be established. Further studies of battery materials have been reported [592, 593]. X-ray absorption near edge structure spectroscopy has been successfully employed in studies of inhibiting species in passive films and the adjacent electrolyte solutions. 

Comparison of the half-wave potentials of the oxidizable and reducible forms of the redox reactants with the potentiometrically determined value of the standard redox potential is the simplest method for proof of reversibility and agreement between these two sets of data provides strong support for reversibility of the process being studied. However, in many polarographic reactions, it is found that one form of the conjugate redox pair is not sufficiently stable to be prepared or to permit determination of E% potentiometrically. In some of these cases, auxiliary methods can be used, particularly where the reactive species is relatively stable when formed at the surface of the mercury electrode. Such methods include the commutator method, an anodic stripping technique which has recently been reviewed by Barendrecht, and oscil-lopolarographic and cyclic voltammetric techniques. 

The first wave corresponds to a one-electron reversible process as suggested by the Tomes criterion, i.e., IE3/4-E1/4I = 60 mV. The diffusion coefficient, determined by the limiting current and for n=l, was 1.4x10 cm s in agreement with the value obtained in cyclic voltammetry for scan rates above 300 mV/s, when the process is reversible. Furthermore, the Ei/2 values were the same ( 5 mV) when different electrode radii were used (5, 12.5 and 25 xm) confirming the reversibility of the process (5). 

Since the 1960s, cyclic voltanunetry has been the most widely used technique for studies of electrode processes with coupled chentical reactions. The theory was developed for numerous mechanisms involving different combinations of reversible, quasi-reversible, and irreversible heterogeneous ET and homogeneous steps. Because of space limitations, we will only consider two well-studied examples—(i.e., first-order reversible reaction preceding reversible ET) and E Ci (i.e., reversible ET followed by a first-order irreversible reaction)—to illnstrate general principles of the coupled kinetics measurement. A detailed discussion of other mechanisms can be found in Chapter 12 of reference (1) and references cited therein, including a seminal publication by Nicholson and Shain (19). 

Scanning tunneling microscope (STM) has become a popular tool for imaging electrode surface in situ with atomic resolution and has been proved to be a very useful aid to understand the fundamentals of electrode processes (5). This technique has, however, some limitation. Because STM uses tunneling current as a probe, only the surface of conducting materials can be imaged. Thus, STM measurement of semiconductor electrode is not possible under reverse bias because of the existence of... 

Two features of this definition are worth noting. One is that EPH is defined as the heat of a reversible reaction, which essentially eliminates the various uncertainties arising from the irreversible factors such as overvoltage. Joule heat, thermal conductivity, concentration gradient and forced transfer of various particles like ions and electrons in electrical field, and makes the physical quantity more definite and comparable. This indicates that EPH is a characteristic measure of a cell reaction, because the term 8 (AG)/8T) p is an amoimt independent on reaction process, and only related to changes in the function of state. That is to say, EPH is determined only by the initial and the final states of the substances taking part in the reaction that occurs on the electrode-electrolyte interfaces, although other heats due to irreversible factors are accompanied. EPH is, unlike the heat of dissipation (Joule heat and the heats due to irreversibility of electrode processes and transfer processes), one of the fundamental characteristics of the electrode process. 

Serna C, Lopez-Tenes M, GonztQez J (2001) Reversible multistep electrode processes. Consideration of the bulk presence of intermediate species and of the values of the diffusion coefficients in voltammetry. Electrochim Acta 46 2699-2709... 

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