ELECTRODE REACTIONS IN ION TRANSFER

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 electrodes used in conventional polarography and voltammetry are electronic conductors such as metals, carbons or semiconductors. In an electrode reaction, an electron transfer occurs at the electrode/solution interface. Recently, however, it has become possible to measure both ion transfer and electron transfer at the interface between two immiscible electrolyte solutions (ITIES) by means of polarography and voltammetry [16]. Typical examples of the immiscible liquid-liquid interface are water/nitrobenzene (NB) and water/l,2-dichloroethane (DCE). 

An interesting study [52] of the protonation kinetics and equilibrium of radical cations and dications of three carotenoid derivatives involved cyclic voltammetry, rotating-disk electrolysis, and in situ controlled-potential electrochemical generation of the radical cations. Controlled-potential electrolysis in the EPR cavity was used to identify the electrode reactions in the cyclic volt-ammograms at which radical ions were generated. The concentrations of the radicals were determined from the EPR amplitudes, and the buildup and decay were used to estimate lifetimes of the species. To accomplish the correlation between the cyclic voltammetry and the formation of radical species, the relative current from cyclic voltammetry and the normalized EPR signal amplitude were plotted against potential. Electron transfer rates and the reaction mechanisms, EE or ECE, were determined from the electrochemical measurements. This study shows how nicely the various measurement techniques complement each other. 

The combination of chemistry and electricity is best known in the form of electrochemistry, in which chemical reactions take place in a solution in contact with electrodes that together constitute an electrical circuit. Electrochemistry involves the transfer of electrons between an electrode and the electrolyte or species in solution. It has been in use for the storage of electrical energy (in a galvanic cell or battery), the generation of electrical energy (in fuel cells), the analysis of species in solution (in pH glass electrodes or in ion-selective electrodes), or the synthesis of species from solution (in electrolysis cells). 

Such electron transfers between ions and electrodes result in chemical changes (changes in the valence or oxidation state of the ions), i.e., in electrodic reactions. When ions receive electrons from the electrode, they are said to be electronated, or to undergo reduction when ions donate electrons to the electrodes, they are said to be deelectronated, or to undergo oxidation. 

This presentation deals with ion-transfer and electron-transfer types of reactions. The electrode reactions in mixtures of water with solvents of lower Lewis basicity will be discussed first, followed by the presentation of such reactions in mixtures of water with solvents of higher donor properties. 

The investigations carried out on the rate of ion-transfer electrode reactions in mixtures of water with solvents with a higher donor number than water, such as HMPA, DMF, or DMSO, revealed that the rate constant was decreased when the concentration of the organic solvent in the mixture increased [221-223, 225, 226, 228, 229, 231, 233, 275], but no minimum was found. The shape of this decrease depends on the mixed solvent and to some extent also on the nature of the reaction under study. 

In general, all the reactions that involve transfer of electrons and/or ions in the electrode are called electrode reactions in electrochemistry. Any electrode reaction has its electron energy level, which we call the Fermi level of the reaction. In fact, the Fermi level of an electrode reaction is equivalent to what we call the equilibrium electrode potential of the reaction. 

In ion transfer reactions the transfer of an ion or proton from the solution to the surface of an electrode is one elementary step. It is often accompanied by either total discharge (e.g., deposition of a metal ion on a metal electrode of the same composition) or partial discharge (e.g., adsorption of halide ions see also below). While for outer sphere electron transfer the reaction coordinate describes the solvent reorganization, the reaction coordinate for ion transfer reactions is associated with the motion of the ion. The rate-determining step in an ion transfer reaction is often the adsorption step of the ion on the electrode, which involves the penetration of the barrier formed by the adsorbed solvent (see, e.g.. Ref. 2 and section 5.2 for a discussion). 

Electrochemical microsystem technology can be scaled down from macroscopic science to micro and further to nanoscale through EMST to ENT [1]. In ENT, electrochemistry involves in the production process to realize nanoproducts and systems which must have reproducible capability. The size of the products and systems must be in the submicron range. It considers electrochemical process for nanostructures formation by deposition, dissolution and modification. Electrochemical reactions combining ion transfer reactions (ITR) and electron transfer reactions (ETR) as applicable in EMST are also applied in ENT. Molecular motions play an important role in ENT as compared with EMST. Hence, mechanical driven system has to be changed to piezo-driven system to achieve nanoscale motions in ENT. Due to the molecular dimension of ENT, quantum effects are always present which is not important in the case of EMST. The double layer acts as an interface phenomenon between electrode and electrolyte in EMST, however, double layer in the order of few nanometers even in dilute electrolyte interferes with the nanostmcture in ENT. 

The majority of electrode processes take place via a number of consecutive (and/or simultaneous), respectively, competitive steps. Even such apparently simple processes as the electrodeposition of univalent ions consist of at least two steps, viz. neutralization and incorporation into the crystal lattice. If the electrode reaction involves the transfer of more than one electron it usually occurs in two steps. Complications may arise from preceding and subsequent chemical reactions, adsorption and desorption, etc. The rate will be determined by the step with the smallest rate constant (rate-determining, hindered or slowest step). 

We have considered above the Butler-Volmer equation for the relationship between current density and potential under the situation when transport of ions in solution makes little or no difference to the rate of an electrode reaction. In order to considered the situation in which transport does control the flow we shall adopt a correspondingly simple counterassumption electron transfer at the interface no longer has control of the electrode reaction. 

Transport processes are involved when a current is passed through a fuel cell. Ions and neutral species that participate in the electrochemical reactions at the anode or cathode have to be transported to the respective electrode surfaces. In Section 1.3.2, we introduced the charge transfer kinetics-controlled electrode reactions in... 

With the help of such cell constructions, as described by Figures 2.6 and 2.7, assisted by calculations on the basis of thermodynamic data where measurements were not possible, the electromotive series of electrode reactions could be determined for ion transfer and electron transfer reactions relative to a particular reference electrode. In order to get some idea of the factors which control the position of an electrode reaction in such a scheme, an ion transfer reaction of type (2.23) and a redox reaction of type (2.27) shall be analyzed. 

Equation (2.41) shows again that the Galvani potential difference of an electrode in contact with a redox electrolyte depends on the chemical potential of the electrons in the metal used as the electrode. This difference does not, however, appear in the measured cell voltage vs. a reference electrode, because it is compensated at the external connection to the metal of the reference electrode by the contact potential between the two metals as explained in Figure 2.5 and Equations (2.35) and (2.36). The consequence for the two cycles of ion transfer (type 2.23) and redox reactions (type 2.27) is that the chemical potentials of the electrons in the individual electrodes have to be replaced by the chemical potential of the electrons in the metal of the reference electrode. The relative position of an electrode reaction in the electromotive series is therefore exclusively controlled by the siun of all the other chemical potentials in these cycles. 

Electrode reactions involve charge transfer as a fundamental step, wherein a neutral species is converted into an ion, or an ion is converted into a neutral species. Both reactions thus involve electron transfer. At the cathode, the charge transfer reaction involves the conversion of an oxygen molecule into oxide ions. The electrodes in solid state electrochemical devices may either be purely electronic conductors, or may exhibit both ionic and electronic conductivity (the so-called mixed ionic electronic conduction, MIEC). In addition, the electrodes may be either single phase or composite, two-phase. For the purposes of illustration, in what follows we will examine the overall cathode reaction in a system with a single phase, purely electronically conducting electrode. 

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