Electrode kinetics Nernstian equilibrium

Under steady state conditions electron transfer processes with values less than about 5 X lO cm s are regarded as irreversible, i.e. when current flows the electron transfer is insufficiently fast to maintain Nernstian equilibrium at the electrode surface. In such cases kinetic data can be obtained directly from steady state current-voltage measurements analysed on the basis of the Tafel equations (see Fig. 1.4). For a cathodic reaction... 

For many electrode processes of interest, the rates of electron transfer, and of any coupled chemical reactions, are high compared with that of steady state mass transport. Therefore during any steady state experiment, Nernstian equilibrium is maintained at the electrode and no kinetic or mechanistic information may be obtained from current or potential measurements. Apart from in a few areas of study, most notably in the field of corrosion, steady state measurements are not therefore widely used by electrochemists. For the majority of electrode processes it is only possible to determine kinetic parameters if the Nernstian equilibrium is disturbed by increasing the rate of mass transport. In this way the process is forced into a mixed control region where the rates of mass transport and of the electrode reaction are comparable. The current, or potential, is then measured as a function of the rate of mass transport, and the data are, then either extrapolated or curve fitted to obtain the desired kinetic parameters. There are basically three different ways in which the rate of mass transport may be enhanced, and these are now discussed. 

In the region where no reverse peak is observed, the pure kinetic zone, it can be shown that the chemical reaction has the effect of of shifting the cathodic peak potential positive of the value for the reversible electron transfer. This is because the coupled chemical reaction reduces the concentration of Rat the surface from the value it would have had for a simple electron transfer reaction. The electrode reaction therefore has to work harder to maintain Nernstian equilibrium... 

As noted in Section 2, when the electron-transfer kinetics are slow relative to mass transport (rate determining), the process is no longer in equilibrium and does not therefore obey the Nernst equation. As a result of the departure from equilibrium, the kinetics of electron transfer at the electrode surface have to be considered when discussing the voltammetry of non-reversible systems. This is achieved by replacement of the Nernstian thermodynamic condition by a kinetic boundary condition (36). 

However, if the kinetics of charge transfer at the electrode/ electrolyte interface are so rapid that the electrochemical reactants and products stay in equilibrium at the electrode surface even though a current passes, the Nernst equation still applies to the surface concentrations. Such a process is said to be electrochemically reversible or Nernstian - sometimes written with a lower case n, a mark of distinction also accorded to the adjectives coulombic, ohmic and faradaic. 

For the first purpose we choose a chemical reaction system with some ionic species, as for example the minimal bromate reaction, for which we presented some experiments in Chap. 10. The system may be in equilibrium or in a nonequilibrium stationary state. An ion selective electrode is inserted into the chemical system and coimected to a reference electrode. The imposition of a current flow through the electrode coimection drives the chemical system (CS) away from its initial stationary state to a new stationary state of the combined chemical and electrochemical system (CCECS), analogous to driving the CS away from equilibrium in the same maimer. A potential difference is generated by the imposed current, which consists of a Nernstian term dependent on concentrations only, and a non-Nernstian term dependent on the kinetics. We shall relate the potential difference to the stochastic potential for this we need to know the ionic species present and their concentrations, but we do not need to know the reaction mechanism of the chemical sj tem, nor rate coefficients. 

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