Electrochemical activation barrier

Figure 16. Activation barrier A for the formation of a breakthrough pore in a thin surface oxide film on metal as a function of electrode potential at two different surface tensions, om, of the metal/electrolyte interface.7The solid lines indicate the values of A b against Aand the dotted lines correspond to die critical potentials for the pore formation. ACd= 1 F m-2, a = 0.01 J m-2, h = 2 x 10-9 m, a, am = 0.41 J m 2 b, am 0.21 J m 2 (From N. Sato, J. Electmchem. Soc. 129, 255, 1982, Fig. 3. Reproduced by permission of The Electrochemical Society, Inc.)...

As depicted in Scheme 1, reductive and oxidative cleavages may follow either a concerted or a stepwise mechanism. How the dynamics of concerted electron transfer/bond breaking reactions (heretofore called dissociative electron transfers) may be modeled, and particularly what the contribution is of bond breaking to the activation barrier, is the first question we will discuss (Section 2). In this area, the most numerous studies have concerned thermal heterogeneous (electrochemical) and homogeneous reactions. 

Hydrogen evolution, the other reaction studied, is a classical reaction for electrochemical kinetic studies. It was this reaction that led Tafel (24) to formulate his semi-logarithmic relation between potential and current which is named for him and that later resulted in the derivation of the equation that today is called "Butler-Volmer-equation" (25,26). The influence of the electrode potential is considered to modify the activation barrier for the charge transfer step of the reaction at the interface. This results in an exponential dependence of the reaction rate on the electrode potential, the extent of which is given by the transfer coefficient, a. 

Butler27 and Volmer28 advanced Tafel s equation by relating overpotentials to activation barriers. The quantitative relationship between current and overpotential is called the Butler-Volmer equation (eqn (32)), and is valid for electrochemical reactions that are rate limited by charge transfer. 

Providing that the interactions between the reactant and the electrode in the electrochemical transition state, and between the two reactants in the homogeneous transition state, are negligible ("weak overlap" limit), the activation barriers for reactions 10 and 11 will be closely related. 

The Chemical Meaning of an Electrochemically Irreversible Process. As a chemical consideration, the occurrence of an electrochemically irreversible process implies so large an activation barrier to the electron transfer that it is likely that (as discussed in the introductory section, Figure. 1.2) it causes breakage of the original molecular frame with formation of new species (see Chapter 7, Section 5). 

Activation Polarization Activation polarization is present when the rate of an electrochemical reaction at an electrode surface is controlled by sluggish electrode kinetics. In other words, activation polarization is directly related to the rates of electrochemical reactions. There is a close similarity between electrochemical and chemical reactions in that both involve an activation barrier that must be overcome by the reacting species. In the case of an electrochemical reaction with riact> 50-100 mV, rjact is described by the general form of the Tafel equation (see Section 2.2.4) ... 

It should be stressed at this point that in electrochemical kinetics the relative heights of the activation barriers for the individual elementary steps may not be influenced by the electrode potential in the same way. The mechanism, and thus the rate-determining step, may not be the same at different electrode potentials as ilustrated in Fig. 8. 

Amperometric sensors — A class of electrochemical sensors based on amperometry. A - diffusion-limited current is measured which is proportional to the concentration of an electrochemically active analyte. Preferred technique for - biosensors with or without immobilized enzymes (biocatalytic sensors). The diffusion layer thickness must be kept constant, either by continuous stirring or by means of an external diffusion barrier. Alternatively, micro electrodes can be... 

Charge-transfer overpotential — The essential step of an - electrode reaction is the charge (- electron or - ion) transfer across the phase boundary (- interface). In order to overcome the activation barrier related to this process and thus enhance the desirable reaction, an - overpotential is needed. It is called charge-transfer (or transfer or electron transfer) overpotential (f/ct). This overpotential is identical with the - activation overpotential. Both expressions are used in the literature [i-iv]. Refs. [i] Bard A], Faulkner LR (2001) Electrochemical methods. Wiley, New York, pp 87-124 [ii] Erdey-Gruz T (1972) Kinetics of electrode processes. Akademiai Kiadd, Budapest, pp 19-56 [Hi] Inzelt G (2002) Kinetics of electrochemical reactions. In Scholz F (ed) Electroanalytical methods. Springer, Berlin, pp 29-33 [iv] Hamann CH, Hamnett A, Viel-stich W (1998) Electrochemistry. Wiley VCH, Weinheim, p 145... 

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