Electrode Reactions mobile

A quite different approach was introduced in the early 1980s [44-46], in which a dense solid electrode is fabricated which has a composite microstructure in which particles of the reactant phase are finely dispersed within a solid, electronically conducting matrix in which the electroactive species is also mobile. There is thus a large internal reactant/mixed-conductor matrix interfacial area. The electroactive species is transported through the solid matrix to this interfacial region, where it undergoes the chemical part of the electrode reaction. Since the matrix material is also an electronic conductor, it can also act as the electrode s current collector. The electrochemical part of the reaction takes place on the outer surface of the composite electrode. 

In the case of aqueous solutions containing dissolved particles (solutes), a number of localized electron levels associated with solute particles Eirise in the mobility gap of aqueous solutions as shown in Fig. 2-34. These localized electron levels of solutes may be compared with the localized impiuity levels in semiconductors. In electrochemistry, the electron levels of the solutes of general interest are those located within the energy range from - 4 eV to - 6 eV (around the electron levels of the hydrogen and oxygen electrode reactions) in the mobility gap. 

There are several ways in which the solvent-supporting electrolyte system can influence mass transfer, the electrode reaction (electron transfer), and the chemical reactions that are coupled to the electron transfer. The diffusion of an electroactive species will be affected not only by the viscosity of the medium but also by the strength of the solute-solvent interactions that determine the size of the solvation sphere. The solvent also plays a crucial role in proton mobility water and other protic solvents produce a much higher proton mobility because of fast solvent proton exchange, a phenomenon that does not exist in aprotic organic solvents. 

The doping of a solid is similar to the enhancement of the H30+ or OH- concentration in water by adding a strong acid or base. However, while in water mobilities of dopant ions are frequently similar to those of the native defects H30+ and OH-[69, 70], dopant ions in solids (e.g. CdXg in AgCl) are almost immobile. This is also why supporting electrolytes (i.e. electrolytes with dissolved dopants that enhance the ionic conductivity, but do not influence electrochemical electrode reactions [71, 72], are unknown in solid state electrochemistry. 

Although it is not as severe in PEVD systems as in aqueous electrochemical systems in which various kinds of mobile ions are present in the electrolytes, it should be pointed out that, in the presence of reactants at the sink electrode surface, other electrochemical reactions might also take place in parallel with the desired one at the sink side. If side reactions exist, usually such parallel reactions contributions to the measured current are not easy to quantify. If it is desired to use current to monitor the reaction and product formation in PEVD, side reactions should be eliminated or at least controlled. Fortunately, only one ionic species is usually mobile in a solid electrochemical cell because of the nature of the solid electrolyte. As long as the vapor phase is properly controlled, usually one electrode reaction is predominant over a wide range of PEVD applied potentials. Virtually 100% current efficiency for product formation can be expected. 

The reproducibility of the effective mobility is governed by temperature and pH effects. An acceptable run-to-run pH reproducibility can usually be assured, at least in equipment where the effects of possible electrode reactions can be avoided. Again, temperature plays a leading role The effective mobility has a temperature coefficient of 2-3% per degree. Most buffer solutions, incidentally, have a temperature-dependent pH value, so that changing the temperature in the capillary will, for weak ions, even lead to changing degrees of dissociation and, hence, effective mobility. These effects are, by definition, different for different buffers and different sample components. 

According to the experimental data, the mobility of most ions is around 10" cm V s. These values are about 5-6 orders of magnitude smaller than the mobility of electrons and holes in a semiconductor. In order to achieve sufficient conductance in an electrochemical cell, electrolyte conductivities of o> 1(H (Q cm) , and therefore ion concentrations of c > 10 mole per liter, are required. In investigations of electrode processes it is important that the solution is made sufficiently conductive by the addition of ions which are not involved in the electrode reaction. Such a solution is usually called a supporting electrolyte . 

The conductivity of an electrolyte describes the transport of ion.s within an electrical field in the solution. Ions and molecules can also move in the solutions via diffusion. This becomes important in the electrode reactions of molecules or ions added to the supporting electrolyte. Provided that the concentration of these ions is much smaller than that of the supporting electrolyte, then the electric field does not affect the movement of the ions, i.e. these ions, as well as uncharged molecules, reach the electrodes only by diffusion. If the rate of the electrode process becomes large then the concentration of the reacting species decreases and that of the generated species increases near the electrode, which leads to a concentration profile. The corresponding diffusion process can be described by Fick s law, as will be discussed in Section 7.1.2. The diffusion process is essentially characterized by the diffusion constant D. Similarly to electrons and holes, the diffusion constant is related to the mobility by the Einstein relation (see Eq. 1.39). We have then... 

The galvanic potential differences in the electrodes of galvanic cells with oxoanionic solid electrolytes are determined by gas components which are not in reaction equilibrium with the cations, whose mobility determines the conductivity. Galvanic potential differences result according to the law of mass action for electrode reactions as follows ... 

Si stoichiometric coefficient of species i in an electrode reaction T absolute temperature (K) ut mobility of species i (cm2 mol/J s)... 

Reported studies deal with measurements of the electrode potential of zero charge Epzc [698, 699], double layer investigations [700-702] and studies of electrode reaction mechanisms [703, 704] for an overview, see also [693, 697], Numerous studies (beyond many conducted ex situ) deal with intrinsically conducting polymers, particularly photogenerated mobile charge carriers [705-707]. 

In potentiometric devices, an open-circuit voltage is measured. They also consist of an electrolyte (that is also typically made of YSZ) and two electrodes (also typically made of platinum). One of the electrodes is the working electrode (WE) - also called the sensing electrode (SE) - and the other is the reference electrode (RE). Both YSZ and Pt are suitable for high-temperature applications such as exhaust gas detection. They work as follows the electrolyte is an ionic conductor and there is a thermodynamic equilibrium between ambient oxygen, electrons in the electrodes and mobile oxygen ions. In the simplest case, the following reaction takes place in the electrodes ... 

Rate of reactions at the electrode surfaces depends on mass transfer, which mainly influences the current 1. The simplest electrode reactions are those in which the rates of all associated chemical reactions are very rapid compared to those of the mass transfer processes. If an electrode process involves only fast heterogeneous charge transfer and mobile, reversible homogeneous reactions, it implies that (1) the homogeneous reactions are at equilibrium and (2) the surface concentrations of species involved in the faradaic process are related to the electrode potential. The net rate of the electrode reaction, Vrxn, is then governed totally by the rate at which the electroactive species is brought to the surface by mass transfer, v f The reaction rate can be expressed as ... 

The selectivity of enzyme electrodes can be improved by means of another coupling principle that is capable of filtering chemical signals by eliminating interferences of the enzyme or electrode reaction caused by constituents of the sample. Compounds that interfere with signal transduction, e.g., ascorbic acid with anodic oxidation of hydrogen peroxide, can be transformed into inert products by reaction with an eHminator or anti-interference enzyme (e.g., ascorbate oxidase). Since the conversion of analyte and interferent proceed in parallel, both the eliminator and the indicator enzyme may be co-im-mobilized in one membrane. On the other hand, constituents of the sample that are at the same time intermediates of coupled enzyme reactions can be eliminated before they reach the indicator enzyme layen For this purpose several different enzyme... 

The situation is often encountered where, upon the passage of current through an electrochemical cell, only one of the mobile species is discharged at the electrodes. Examples are (a) the use of a liquid or polymeric electrolyte, where both ions are mobile, and yet where only one is able to participate in the electrode reaction and (b) a mixed conducting solid in which current is passed by electrons, but in which cations also have a significant transport number. 

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