Electrodes reaction

If two inert, plane, metal electrodes are placed parallel to each other in a solution that contains an electrolyte and a very small electrical potential of magnitude E is applied across them, a small current / that decreases with time will be observed to flow between them. The current will consist of the motion of positive ions to the cathode and negative ions to the anode. Initially it will obey Ohm s law, I = E/R where the resistance of the solution R is inversely proportional to the mobility of the ions present. However, after a short time, the accumulation around each electrode of ions of opposite charge, together with the depletion of ions of like charge, will produce an opposing potential in the solution, the polarization potential, which will cause the current to fall to zero as an equilibrium state is reached. 

As the potential is increased, there is a point at which no equilibrium state is reached, but instead, an appreciable steady current flows which will obey Ohm s law over a reasonable range of applied potential. The potential at which this steady current is observed is called the decomposition potential because it is accompanied by chemical reaction (electrolysis) at the electrode surfaces. These electrode reactions are quite generally the oxidation (anode) and reduction (cathode) of ionic or molecular species present in the solution. If the reactions at the electrodes are reversible, then the decomposition potential Ed is related by the Nernst equation to the free energy changes of the electrode reactions 

As an example, the electrolysis of ZiiBr2 solution will take place reversibly at a decomposition potential of about 1.3 volts to produce Zn metal at the cathode and liquid bromine, Br2 (and Brs ), at the anode according to the stoichiometric equation 

In many cases, the electrode reactions are not reversible and the decomposition potential is observed to be in excess of the thermodynamically calculated value. The excess voltage, referred to as an overvoltage, is found to vary with the nature and surface area (e.g., roughness) of the electrodes, impurities in the solution, and the actual current density passing through the solution. The relation between current density Id and overvoltage E was investigated by Tafel, who proposed the very successful empirical equation 

In the case of both reversible and irreversible electrode reactions, methods are now available for studying the steady-state and transient currents, and there has been much progress in the analysis of these currents in terms of the kinetic processes involved. 

A fuel cell is an electrochemical energy converter. Its operation is based on the following electrochemical reactions occurring simultaneously on the anode and the cathode [1]  

At the anode, hydrogen is stripped of its electrons and become protons and electrons. 

At the cathode, oxygen is oxidized, meaning that it takes the electrons and forms water. As described in Section 1.2, the HOR on Pt catalysts has a lower oxidation overpotential and a higher kinetic rate, whereas the ORR is sluggish, involving sequential and parallel steps. 

a large number of important technologies are based on or related to electrodes reactions. Besides the chlor-alkali and aluminium industries, energy conversion in batteries and fuel cells, electrodeposition, electrorefining, organic electrosynthesis, industrial and biomedical sensors, corrosion and corrosion protection, etc. are amogst those technologies. In many of them, kinetic, catalytic or specificity aspects of electrode processes are of enormous importance. 

Electrode reactions take place at the electrode—solution interface and their kinetics provide a switch between two types of electrical conductivity electronic at the electrode and ionic at the electrolyte. Unlike other heterogeneous chemical processes, they are not only thermally activated but also their rate is strongly influenced by the electrical field at the interface, the presence of solvent, and ionic species. 

The electrical current that flows through the external circuit of an electrochemical cell is a measure of the flux of electrical charge and hence the flux of material transformed in electrochemical reactions. The current measures the rate of reaction which is controlled by the electrical potential difference at the interface. 

The scope of this first chapter is to present the fundamental concepts of electrode kinetics in relation to the subsequent chapters where more specific and detailed aspects connected with electrode reactions will be treated. 

A brief synopsis will be given of those subjects covered specifically elsewhere, while a more detailed analysis is presented for those problems not covered in the present series. Reference to the kinetics of reactions in solution, covered in Vol. 2, Chap. 4 of this series, is made whenever necessary. 

The equilibrium at the interphase electrode-electrolyte, is to be considered as a dynamic situation. Indeed, there is a continuous exchange of charges between the two phases but without net current. Therefore at equilibrium oxidation and reduction reactions occur simultaneously with the same rate. 

When a potential difference is applied between the electrode and a counter electrode as represented in fig. 1.4, a current flows between these electrodes 

In order to avoid errors involved by ohmic voltage drop in the solution, the reference electrode is placed as close as possible to the working electrode. 

This overpotential is the driving force for the net current (reaction). The overpotential r is positive for oxidation reactions and negative for reduction reactions. A simplified visualization is given in fig. 1.5. 

An electrode where an oxidation reaction takes place is called ANODE, whereas an electrode with a reduction reaction is called CATHODE. 

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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... 

Cyclic voltammetry provides a simple method for investigating the reversibility of an electrode reaction (table Bl.28.1). The reversibility of a reaction closely depends upon the rate of electron transfer being sufficiently high to maintain the surface concentrations close to those demanded by the electrode potential through the Nemst equation. Therefore, when the scan rate is increased, a reversible reaction may be transfomied to an irreversible one if the rate of electron transfer is slow. For a reversible reaction at a planar electrode, the peak current density, fp, is given by... 

Figure Bl.28.4. Cyclic voltaimnogram for a simple reversible electrode reaction in a solution containing only oxidized species.

Figure Bl.28.7. Schematic shape of steady-state voltaimnograms for reversible, quasi-reversible and irreversible electrode reactions.

In the case of an irreversible electrode reaction, the current-potential curve will display a similar shape, with... 

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. 

The effects of ultrasound-enlianced mass transport have been investigated by several authors [73, 74, 75 and 76]. Empirically, it was found that, in the presence of ultrasound, the limiting current for a simple reversible electrode reaction exhibits quasi-steady-state characteristics with intensities considerably higher in magnitude compared to the peak current of the response obtained under silent conditions. The current density can be... 

Sonoelectrochemistry has been employed in a number of fields such as in electroplating for the achievement of deposits and films of higher density and superior quality, in the deposition of conducting polymers, in the generation of highly active metal particles and in electroanalysis. Furtlienuore, the sonolysis of water to produce hydroxyl radicals can be exploited to initiate radical reactions in aqueous solutions coupled to electrode reactions. 

The combination of electrochemistry and photochemistry is a fonn of dual-activation process. Evidence for a photochemical effect in addition to an electrochemical one is nonnally seen m the fonn of photocurrent, which is extra current that flows in the presence of light [, 89 and 90]. In photoelectrochemistry, light is absorbed into the electrode (typically a semiconductor) and this can induce changes in the electrode s conduction properties, thus altering its electrochemical activity. Alternatively, the light is absorbed in solution by electroactive molecules or their reduced/oxidized products inducing photochemical reactions or modifications of the electrode reaction. In the latter case electrochemical cells (RDE or chaimel-flow cells) are constmcted to allow irradiation of the electrode area with UV/VIS light to excite species involved in electrochemical processes and thus promote fiirther reactions. 

Evans D FI 1991 Review of voltammetric methods for the study of electrode reactions Microelectrodes Theory and Applications (Nate ASI Series E vol 197) ed M I Montenegro, M A Queiros and J L Daschbach (Dordrecht Kluwer)... 

Electrode reaction rate constant k (varies) = Ij nFAY[c ... 

Potcntiomctric Biosensors Potentiometric electrodes for the analysis of molecules of biochemical importance can be constructed in a fashion similar to that used for gas-sensing electrodes. The most common class of potentiometric biosensors are the so-called enzyme electrodes, in which an enzyme is trapped or immobilized at the surface of an ion-selective electrode. Reaction of the analyte with the enzyme produces a product whose concentration is monitored by the ion-selective electrode. Potentiometric biosensors have also been designed around other biologically active species, including antibodies, bacterial particles, tissue, and hormone receptors. 

Mediator Electrochemically Generated Reagent Generator-Electrode Reaction Representative Application ... 

Studies aimed at characterizing the mechanisms of electrode reactions often make use of coulometry for determining the number of electrons involved in the reaction. To make such measurements a known amount of a pure compound is subject to a controlled-potential electrolysis. The coulombs of charge needed to complete the electrolysis are used to determine the value of n using Faraday s law (equation 11.23). 

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... 

This difference is a measure of the free-energy driving force for the development reaction. If the development mechanism is treated as an electrode reaction such that the developing silver center functions as an electrode, then the electron-transfer step is first order in the concentration of D and first order in the surface area of the developing silver center (280) (Fig. 13). Phenomenologically, the rate of formation of metallic silver is given in equation 17,... 

Fig. 2. Standard potentials of battery (a) negative electrode and (b) positive electrode reactions (13). Potentials are given in volts.

Whenever energy is transformed from one form to another, an iaefficiency of conversion occurs. Electrochemical reactions having efficiencies of 90% or greater are common. In contrast, Carnot heat engine conversions operate at about 40% efficiency. The operation of practical cells always results ia less than theoretical thermodynamic prediction for release of useful energy because of irreversible (polarization) losses of the electrode reactions. The overall electrochemical efficiency is, therefore, defined by ... 

On the electrode side of the double layer the excess charges are concentrated in the plane of the surface of the electronic conductor. On the electrolyte side of the double layer the charge distribution is quite complex. The potential drop occurs over several atomic dimensions and depends on the specific reactivity and atomic stmcture of the electrode surface and the electrolyte composition. The electrical double layer strongly influences the rate and pathway of electrode reactions. The reader is referred to several excellent discussions of the electrical double layer at the electrode—solution interface (26-28). 

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 ... 

The exchange current is directiy related to the reaction rate constant, to the activities of reactants and products, and to the potential drop across the double layer. The larger the more reversible the reaction and, hence, the lower the polarization for a given net current flow. Electrode reactions having high exchange currents are favored for use in battery apphcations. 

Fig. 5. Currrent—potential behavior of an electrode reaction based on equation 37. Tafel behavior is noted at high currents for two different values of d.

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