Irreversible electrode

To leam that accurate values of 1/2 may be obtained from the intercept of a Heyrovsky-Ilkovic graph, while the slope of such a graph indicates the extent of electrode irreversibility. 

Unless possibly they render the electrodes irreversible. 

Watson C. M., Dwyer D. J., Andie J. C., Bruce A. E., and Bruce M. R. M. (1999) Stripping analyses of mercury using gold electrodes irreversible adsorption of mercury. Anal. Chem. 71, 3181-3186. 

In this case a potassium-graphite (KCg) electrode has been used as the carbonaceous anode material. Upon anodic polarisation this electrode irreversibly deintercalates potassimn resulting in a graphite-like compound, which on subsequent cycles performs with fast kinetics of its lithium intercalation-deintercalation process [89, 90]. Accordingly, the first charging process of the battery may be written as shown in Equation 7.7 at the anode. 

Heterogeneous electron transfer reactions have been realized with more than 50 different proteins, mainly electron transferases, and also substrateconverting oxidoreductases. At bare metal electrodes irreversible adsorption accompanied by denaturation prevents a fast electron transfer to the protein molecules. Adsorption of modifiers that promote an appropriate orientation of the protein results in a facilitated direct electron transfer with different redox enzymes, for example, cytochromes and ferre-doxins. 

Gierst, Cin4tique d Approche et Reactions d"Electrodes Irreversibles, Buteneers, Liege, 1958. 

Evaluation of corrosion behavior is normally done through a function that depends on kinetic parameters depicted in Figure 3.2. Hence, the current density function for polarizing an electrode irreversibly from the corrosion potential is similar to eq. (3.8). Hence,... 

If an appreciable current flows between the electrode and the solution, thus disturbing the reversible thermodynamic equilibrium conditions, the electrode is said to be polarized and the system is then operating under irreversible conditions. 

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.28.7. 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... 

Influence of the Kinetics of Electron Transfer on the Faradaic Current The rate of mass transport is one factor influencing the current in a voltammetric experiment. The ease with which electrons are transferred between the electrode and the reactants and products in solution also affects the current. When electron transfer kinetics are fast, the redox reaction is at equilibrium, and the concentrations of reactants and products at the electrode are those specified by the Nernst equation. Such systems are considered electrochemically reversible. In other systems, when electron transfer kinetics are sufficiently slow, the concentration of reactants and products at the electrode surface, and thus the current, differ from that predicted by the Nernst equation. In this case the system is electrochemically irreversible. 

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 ... 

Overcharge Reactions. Water electrolysis during overcharge is an irreversible process. Oxygen forms at the positive electrode ... 

Examples of such irreversible species (12) include hydroxjiamine, hydroxide, and perchlorate. The electrochemistries of dichromate and thiosulfate are also irreversible. The presence of any of these agents may compromise an analysis by generating currents in excess of the analytically usehil values. This problem can be avoided if the chemical reaction is slow enough, or if the electrode can be rotated fast enough so that the reaction does not occur within the Nemst diffusion layer and therefore does not influence the current. 

Galvanic cells in which stored chemicals can be reacted on demand to produce an electric current are termed primaiy cells. The discharging reac tion is irreversible and the contents, once exhausted, must be replaced or the cell discarded. Examples are the dry cells that activate small appliances. In some galvanic cells (called secondaiy cells), however, the reaction is reversible that is, application of an elec trical potential across the electrodes in the opposite direc tion will restore the reactants to their high-enthalpy state. Examples are rechargeable batteries for household appliances, automobiles, and many industrial applications. Electrolytic cells are the reactors upon which the electrochemical process, elec troplating, and electrowinning industries are based. 

In the reductive regime, a strong, apparently irreversible, reduction peak is observed, located at -1510 mV vs. the quasi reference electrode used in this system. With in situ STM, a certain influence of the tip on the electrodeposition process was observed. The tip was therefore retracted, the electrode potential was set to -2000 mV, and after two hours the tip was reapproached. The surface topography that we obtained is presented in Figure 6.2-14. 

It should be noted that the simple Nernst equation cannot be used since the standard electrode potential is markedly temperature dependent. By means of irreversible thermodynamics equations have been computed to calculate these potentials and are in good agreement with experimentally determined results. 

The irreversible behaviour of an aluminium electrode, which readily passes a current when cathodically polarised, but almost ceases to conduct when made the anode in certain aqueous solutions, has been known for over a century. 

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