Parallel Electrode Reactions

Current flow at electrode surfaces often involves several simultaneous electrochemical reactions, which differ in character. For instance, upon cathodic polarization of an electrode in a mixed solution of lead and tin salt, lead and tin ions are discharged simultaneously, and from an acidic solution of zinc salt, zinc is deposited, and at the same time hydrogen is evolved. Upon anodic polarization of a nonconsumable electrode in chloride solution, oxygen and chlorine are evolved in parallel reactions. 

Different reactions (anodic and cathodic) can occur simultaneously at an electrode, even when there is no net current flow. In Section 2.5.1 we mentioned the example of an iron electrode in HCl + FeCl2 solution where anodic iron dissolution (2.24) and cathodic hydrogen evolution (2.25) occur simultaneously these are the reactions of spontaneous dissolution of iron not requiring a net current. 

The net (external or overall) current density at an electrode is the algebraic sum of the partial current densities of all reactions  

The current yield g is a useful parameter for the quantitative characterization of parallel reactions. This is the ratio of the partial CD, consumed in a given reaction n, to the total CD  

The principle of independent electrochemical reactions applies when several reactions occur simultaneously. It says that each reaction follows its own quantitative laws, irrespective of other reactions. At a given potential, the rates of the different reactions are not at all interrelated, and at a given CD they are merely tied together by relation (13.53). This does not mean that the reactions have no influence on each other at all. One of the reactions may produce changes in the external conditions for other reactions (e.g., in the temperature or solution pH, the amount of impurities adsorbed on the electrode). However, the form of the kinetic equation of each reaction is not affected by these changes. The principle of independent electrochemical reactions is quite general, and rarely violated (we discuss an instance of such a departure in Section 22.2). 

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This originated from a similar idea to that of the double potential step. A base potential at which all the electroactive species is electrolysed is applied, and the reverse reaction is carried out by normal pulse (Fig. 10.12). A good reason for using this technique is to diminish the problems caused by parallel electrode reactions of the initial species. 

For consecutive or parallel electrode reactions it is logical to construct circuits based on the Randles circuit, but with more components. Figure 11.16 shows a simulation of a two-step electrode reaction, with strongly adsorbed intermediate, in the absence of mass transport control. When the combinations are more complex it is indispensable to resort to digital simulation so that the values of the components in the simulation can be optimized, generally using a non-linear least squares method (complex non-linear least squares fitting). 

When there are parallel electrode reactions of the initial electroactive species or interfering species, a modification of NPV... 

Oscillatory behavior observed as periodic potential transients at constant current or periodic current transients at constant potential is found frequently when more than two parallel electrode reactions are coupled. Usually, an upper and a lower current-potential curve limit the oscillation region. These two curves represent stable states [139] according to the theory of stability of electrode states [140]. Oscillatory phenomena occurring during the oxidation of certain fuels on solid electrodes are discussed in this section. The discussion is not extended to porous electrodes because the theory of the diffusion electrode has not been developed to the point to allow an adequate description of the complex coupling of parallel electrode reactions and mass transport processes in the liquid and gaseous phase. 

There are other parallel electrochemical reactions that can occur at the electrodes within the cell, lowering the overall efficiency for CIO formation. Oxygen evolution accounts for about 1—3% loss in the current efficiency on noble metal-based electrodes in the pH range 5.5—6.5. 

Electrochemical reactors (cells, tanks) are used for the practical realization of electrolysis or the electrochemical generation of electrical energy. In developing such reactors one must take into account the purpose of the reactor as well as the special features of the reactions employed in it. Most common is the classical reactor type with plane-parallel electrodes in which positive and negative electrodes alternate and all electrodes having the same polarity are connected in parallel. Reactors in which the electrodes are concentric cylinders and convection of the liquid electrolyte can be realized by rotation of one of the electrodes are less common. In batteries, occasionally the electrodes are in the form of two long ribbons with a separator in between which are wound up as a double spiral. 

Case (c) A chemical reaction C(c) parallel to the electrode reaction has an effect only when E(c) is sufficiently fast. 

If a chemical reaction regenerates the initial substance completely or partially from the products of the electrode reaction, such case is termed a chemical reaction parallel to the electrode reaction (see Eq. 5.6.1, case c). An example of this process is the catalytic reduction of hydroxylamine in the presence of the oxalate complex of TiIV, found by A. Blazek and J. Koryta. At the electrode, the complex of tetravalent titanium is reduced to the complex of trivalent titanium, which is oxidized by the hydroxylamine during diffusion from the electrode, regenerating tetravalent titanium, which is again reduced. The electrode process obeys the equations... 

The decisive influence of the current density on the electrode reactions - including their selectivity - has been discussed in Sect. 2.3.2.1. Therefore, a uniform current density on the entire electrode area usually is indispensable. Precondition is a sufficiently constant overall cell resistance - of the electrodes, the electrolytes, and the cell separator - for every point of the electrode area. This needs satisfactory conductivities of the electrode materials and adequately dimensioned, symmetrical current feeders. For a constant electrolyte resistance, the electrodes have to be mounted parallel. This becomes more and more important... 

In voltammetric experiments, electroactive species in solution are transported to the surface of the electrodes where they undergo charge transfer processes. In the most simple of cases, electron-transfer processes behave reversibly, and diffusion in solution acts as a rate-determining step. However, in most cases, the voltammetric pattern becomes more complicated. The main reasons for causing deviations from reversible behavior include (i) a slow kinetics of interfacial electron transfer, (ii) the presence of parallel chemical reactions in the solution phase, (iii) and the occurrence of surface effects such as gas evolution and/or adsorption/desorption and/or formation/dissolution of solid deposits. Further, voltammetric curves can be distorted by uncompensated ohmic drops and capacitive effects in the cell [81-83]. 

Genera/. The central goal of fundamental electrochemical kinetics is to find out what electrons, ions, and molecules do during an electrode reaction, hr this research, one is not only concerned with the initial state (Le., the metal and the reactants in the solution next to the electrode surface before the reaction begins) and the final product of the reaction, one also has to know the intermediate species formed along the way. Thus, all practical electrode reactions (say, the electro-oxidation of methanol to C02) consist of several consecutive and/or parallel steps, each involving an intermediate radical, e.g., the adsorbed C-OH radical. I Iowcver, one finds that intermediates can be classed into two types. 

Fig. 7.53. Plots of l /lfag vs. e> (a) Electrode reaction proceeds along a single path with the formation of intermediates that do not really react further, (b) Reaction proceeds along a single path with intermediates that readily react further, (c) Intermediates are produced in a parallel reaction and do not react further, (d) Intermediates are produced in a parallel reaction, but do not react further. (Reprinted with permission from A. Damjanovic, M. A. Genshaw, and J. O M. Bockris, J. Chem. Phys. 45 4057, Fig. 1, copyright 1966, American Chemical Society.)...

A multielectron electrode reaction may also occur by a number of mechanistic routes including sequential and parallel pathways, which in complex electrokinetics may also be analysed individually in terms of elementary chemical and electrochemical steps. Figure 7 depicts plots of log j vs. Tj for (a) sequential and (b) parallel paths for multielectron transfer reactions. It is apparent that, at a given electrode potential, in... 

This section deals with transport between two parallel electrodes. The reaction at one electrode is the converse of that at the other, so that no overall change occurs to the contents of the cell. We consider only the simplest instance of this behaviour in which the anode is a metal, M, dissolved by the reaction... 

Diagnostic plots for heterogeneous catalytic electrode reactions at the RRDE have many features in common with those for simple parallel reactions [178]. This type of analysis is important in the investigation of the oxygen electrode reaction where non-electrochemical surface processes can occur. 

Fig. 13. Diagnostic plots for electrode reactions with coupled homogeneous reactions, illustrated for the RDE. (a) CE mechanism. Curve A, no effect from chemical reaction (5k = 0) curve B, effect of preceding chemical reaction (5k >0). (b) Catalytic mechanism. Curve A, in the absence of parallel chemical reaction curve B, experimental dependence predicted from eqn. (175).

At the DME, a rigorous numerical solution was obtained by Nicholson et al. using the model of the expanding plane electrode [212]. This has been followed by a number of studies on expanding-plane and expanding-sphere models together with derivation of current—potential curves [213—215]. The so-called parallel ECE mechanism, where one of the electrode reactions is a reduction and the other oxidation, has been investigated and a numerical solution obtained [216]. Numerical solutions have been found to other ECE-type mechanisms at the DME [217—219]. 

FIGURE 11 Plane parallel electrodes imbedded in insulating walls. Fluid flows from right to left, and the reactant concentration tends to decrease in the direction of fluid flow as the electrochemical reaction progresses. 

Cathodic limits on mercury. In aqueous or other protic solvents the reduction of hydronium ion or solvent generally will limit the negative potential range. The nature of some electrode reactions at highly negative potentials on mercury has been examined.63 For example, K(OH2) and Na(OH2)4 ions are reduced reversibly in aqueous solutions, but the process is accompanied by a parallel irreversible reaction due to an amalgam dissolution reaction of the alkali metal with water that produces hydrogen. 

Some commonly used batteries are shown in Table 15.5, and two are drawn schematically in Fig. 15.10. From these it can be seen that important components are the container, the anode/cathode compartment separators, current collectors to transport current from the electrode material (usually a porous, particulate paste), the electrode material itself, and the electrolyte. It should be noted that the electrode reactions can be significantly more complex than those indicated in Table 15.5, and there will probably be parallel reactions. By stacking the batteries in series, any multiple of the cell potential can be obtained. 

Isopotential lines are parallel to the electrode surfaces for what is known as the primary current distribution (no interfacial electrode polarization, or zero polarization resistance). Said another way, the solution adjacent to an electrode surface is an equipotential surface (1). This primary current distribution applies to the case of extremely fast electrochemical reactions (e.g., nonpolar-izable electrode reactions). This current distribution situation is only of interest to the corrosion engineer in cases where high current densities might be flowing (i.e., in relatively nonpolarizable cells). 

The chemical reaction mechanism of electropolymerization can be described as follows. The first step in course of the oxidative electropolymerization is the formation of cation radicals. The further fate of this highly reactive species depends on the experimental conditions (composition of the solution, temperature, potential or the rate of the potential change, galvanostatic current density, material of the electrode, state of the electrode surface, etc.). In favorable case the next step is a dimerization reaction, and then stepwise chain growth proceeds via association of radical ions (RR-route) or that of cation radical with a neutral monomer (RS-route). There might even be parallel dimerization reactions leading to different products or to the polymer of a disordered structure. The inactive ions present in the solution may play a pivotal role in the stabilization of the radical ions. Potential cycling is usually more efficient than the potentiostatic method, i.e., at least a partial reduction... 

Faraday box (cage, shield) — A grounded metallic box that houses and therefore protects the electrolytic cell (- galvanic cell) and the unshielded parts of the cables from outside electrical radiation. This box minimizes the electric - noise in the measured signal and is especially useful in the cases of very low concentrations of -> electrode-reaction substrates and of high resistance of the solution. The most popular design of it is based on a carton box covered with aluminum foil. Can be also built of wire mesh or as a series of parallel wires. 

Faraday s law — The number of moles of a substance, m, produced or consumed during an electrode process, is proportional to the electric charge passed through the electrode, q. Assuming that there are no parallel processes, q = nFm, where n and F are the number of electrons appearing in the electrode reaction equation, and the - Faraday constant, respectively. 

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