Electrodes reaction, kinetics

The treatment may be made more detailed by supposing that the rate-determining step is actually from species O in the OHP (at potential relative to the solution) to species R similarly located. The effect is to make fi dependent on the value of 2 and hence on any changes in the electrical double layer. This type of analysis has permitted some detailed interpretations to be made of kinetic schemes for electrode reactions and also connects that subject to the general one of this chapter. 

Electrode processes are a class of heterogeneous chemical reaction that involves the transfer of charge across the interface between a solid and an adjacent solution phase, either in equilibrium or under partial or total kinetic control. A simple type of electrode reaction involves electron transfer between an inert metal electrode and an ion or molecule in solution. Oxidation of an electroactive species corresponds to the transfer of electrons from the solution phase to the electrode (anodic), whereas electron transfer in the opposite direction results in the reduction of the species (cathodic). Electron transfer is only possible when the electroactive material is within molecular distances of the electrode surface thus for a simple electrode reaction involving solution species of the fonn... 

The chaimel-flow electrode has often been employed for analytical or detection purposes as it can easily be inserted in a flow cell, but it has also found use in the investigation of the kinetics of complex electrode reactions. In addition, chaimel-flow cells are immediately compatible with spectroelectrochemical methods, such as UV/VIS and ESR spectroscopy, pennitting detection of intennediates and products of electrolytic reactions. UV-VIS and infrared measurements have, for example, been made possible by constructing the cell from optically transparent materials. 

Similarly to the response at hydrodynamic electrodes, linear and cyclic potential sweeps for simple electrode reactions will yield steady-state voltammograms with forward and reverse scans retracing one another, provided the scan rate is slow enough to maintain the steady state [28, 35, 36, 37 and 38]. The limiting current will be detemiined by the slowest step in the overall process, but if the kinetics are fast, then the current will be under diffusion control and hence obey the above equation for a disc. The slope of the wave in the absence of IR drop will, once again, depend on the degree of reversibility of the electrode process. 

Overvoltage. Overvoltage (ti. ) arises from kinetic limitations or from the inherent rate (be it slow or fast) of the electrode reaction on a given substrate. The magnitude of this value can be generally expressed in the form of the Tafel equation... 

Electrode kinetics lend themselves to treatment usiag the absolute reaction rate theory or the transition state theory (36,37). In these treatments, the path followed by the reaction proceeds by a route involving an activated complex where the element determining the reaction rate, ie, the rate limiting step, is the dissociation of the activated complex. The general electrode reaction may be described as ... 

It is not appropriate here to consider the kinetics of the various electrode reactions, which in the case of the oxygenated NaCl solution will depend upon the potentials of the electrodes, the pH of the solution, activity of chloride ions, etc. The significant points to note are that (a) an anode or cathode can support more than one electrode process and b) the sum of the rates of the partial cathodic reactions must equal the sum of the rates of the partial anodic reactions. Since there are four exchange processes (equations 1.39-1.42) there will be eight partial reactions, but if the reverse reactions are regarded as occurring at an insignificant rate then... 

V.A. Sobyanin, and V.D. Belyaev, On the nature ofNon-Faradaic catalysis on metal electrodes, Reaction Kinetics and Catalysis Letters 51(2), 373-382 (1993). 

As in chemical systems, however, the requirement that the reaction is thermodynamically favourable is not sufficient to ensure that it occurs at an appreciable rate. In consequence, since the electrode reactions of most organic compounds are irreversible, i.e. slow at the reversible potential, it is necessary to supply an overpotential, >] = E — E, in order to make the reaction proceed at a conveniently high rate. Thus, secondly, the potential of the working electrode determines the kinetics of the electron transfer process. 

Tam Tamamushi, R. Kinetic Parameters of Electrode Reactions of Metallic Compounds, London ... 

The values of exchange current density observed for different electrodes (or reactions) vary within wide limits. The higher they are (or the more readily charges cross the interface), the more readily will the equilibrium Galvani potential be established and the higher will be the stability of this potential against external effects. Electrode reactions (electrodes) for which equilibrium is readily established are called thermodynamically reversible reactions (electrodes). But low values of the exchange current indicate that the electrode reaction is slow (kinetically limited). 

For thermodynamic reasons, an electrochemical reaction can occur only within a dehnite region of potentials a cathodic reaction at electrode potentials more negative, an anodic reaction at potentials more positive than the equilibrium potential of that reaction. This condition only implies a possibility that the electrode reaction will occur in the corresponding region of potentials it provides no indication of whether the reaction will actually occur, and if so, what its rate will be. The answers are provided not by thermodynamics but by electrochemical kinetics. 

The specific rate of an electrode reaction depends not only on electrode polarization but also on tfie reactant concentrations. Changes in reactant concentrations affect not only reaction rates but also the values of equilibrium potentials. To differentiate both these influences, kinetic equations are generally used (especially at high values of polarization), relating the current density not with the value of polarization AE but with the potential of the electrode E ... 

Different electrode reactions will occur independently, and their kinetic coefficients are unrelated. But for the forward and reverse process of a given reaction, such a correlation should exist, since at the equilibrium potential the corresponding partial current densities assume equal values. 

The standard rate constant kP characterizes the rates of both the forward and reverse processes. Its value is independent of the reference electrode selected, in contrast to what holds true for the values of k and and it is also independent of the component concentrations, in contrast to what holds true for the exchange CD. Therefore, this constant is an unambiguous characteristic of the kinetic properties exhibited by a given electrode reaction. 

In those cases where i. (region A in Eig. 6.6), the real current density i essentially coincides with the kinetic current density i 4, and the electrode reaction is controlled kinetically. When 4 ik (region C), we practically have i 4, and the reaction is diffusion controiled. When 4 and 4 have comparable values, the electrode operates under mixed control (region B). The relative valnes of these current densities depend on the kinetic parameters and on the potential. 

In the present chapter we consider the electrochemical methods used to measure OCV and electrode potentials and to study the kinetics of electrode reactions. These methods are also described in great detail in the book by Bard and Eaulkner (2001). 

At mercury and graphite electrodes the kinetics of reactions (15.21) and (15.22) can be studied separately (in different regions of potential). It follows from the experimental data (Fig. 15.6) that in acidic solutions the slope b 0.12 V. The reaction rate is proportional to the oxygen partial pressure (its solution concentration). At a given current density the electrode potential is independent of solution pH because of the shift of equilibrium potential, the electrode s polarization decreases by 0.06 V when the pH is raised by a unit. These data indicate that the rate-determining step is addition of the first electron to the oxygen molecule ... 

Many papers have been published regarding HTSCs used as inert, nonconsumable electrodes for kinetic and mechanistic studies of various electrode reactions occurring at them. Most of these studies were performed at room temperature when the materials were not in their state of superconductivity. Unfortunately, to date a given reaction has rarely been studied at similar temperatures just above and below r , that is, at temperatures where the same material is once in its normal state and once in its superconducting state. The electronic stracture of materials differs sharply between these two states, and quantitative studies under these conditions might provide valuable information as to the mechanism of the elementary act of charge transfer from the electrode to a reacting species, and vice versa. 

Another result of the cold-fusion epopee that was positive for electrochemistry are the advances in the experimental investigation and interpretation of isotope effects in electrochemical kinetics. Additional smdies of isotope effects were conducted in the protium-deuterium-tritium system, which had received a great deal of attention previously now these effects have become an even more powerful tool for work directed at determining the mechanisms of electrode reactions, including work at the molecular level. Strong procedural advances have been possible not only in electrochemistry but also in the other areas. 

Frumkin, A. N., V. S. Bagotsky, Z. A. lofa, and B. N. Kabanov. Kinetics of Electrode Reactions [in Russian], Moscow University PubUshers, Moscow, 1952. 

Hence the picture of the cathodic and anodic waves obtainable for a completely reversible redox couple by means of the RDE corresponds fully with that in Fig. 3.9 the value of i, i.e., the height of the sigmoidal waves, is linearly proportional to to1/2 and to C (see eqn. 3.89 and the Levich constant). If for a well chosen combination of C and E a plot of i against co1/2 deviates from a straight line passing through the origin, then in the kinetics of the electrode reaction we have to deal only with a rapid electron transfer (cf., Fig. 3.10) or even with a slow electron transfer (cf., Fig. 3.11), in which latter instance the transfer coefficient a plays an appreciable role (cf., eqns. 3.17 and 3.18). 

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). 

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