Electrochemical reactions double-layer effects

The aim of mechanistic studies of chemical reactions is to determine reaction pathway(s), identifying if possible the rate-determining step (rds) and the species involved in it. This involves (1) evaluation of the reaction orders of the various participating reactants, taking into account any chemisorption effects when the process is heterogeneous (2) characterization of reaction intermediates and their adsorption behavior, and in addition in the case of electrochemical reactions, double-layer effects ... 

The electrochemical rate constants of the Zn(II)/Zn(Hg) system obtained in propylene carbonate (PC), acetonitrile (AN), and HMPA with different concentrations of tetraethylammonium perchlorate (TEAP) decreased with increasing concentration of the electrolyte and were always lower in AN than in PC solution [72]. The mechanism of Zn(II) electroreduction was proposed in PC and AN the electroreduction process proceeds in one step. In HMPA, the Zn(II) electroreduction on the mercury electrode is very slow and proceeds according to the mechanism in which a chemical reaction was followed by charge transfer in two steps (CEE). The linear dependence of logarithm of heterogeneous standard rate constant on solvent DN was observed only for values corrected for the double-layer effect. 

Refs. [i] Frumkin A (1933) Z phys Chem A 164 121 [ii] Frumkin AN (1961) Hydrogen overvoltage and adsorption phenomena, part 1, mercury. In Delahay P (ed) Advances in electrochemistry and electrochemical engineering, vol 1. Interscience, New York [iii] Frumkin AN, Petrii OA, Nikolaeva-Ferdorovich NV (1963) Electrochim Acta 8 177 [iv] Frumkin AN, Nikolaeva-Fedorovich NV, Berezina NP, Keis KhE (1975) J Electroanal Chem 58 189 [v] Fawcett WR (1998) Double layer effects in the electrode kinetics of electron and ion transfer reactions. In Lipkowski J, RossPN (eds) Electrocatalysis. Wiley-VCH, New York, p 323... 

Therefore, if the Marcus theory describes properly the effect of solvents of k, a linear correlation between In and ( op -fis ) should be observed in the experimental results. Before turning to the experimental studies, the (Sop - s ) parameter for various solvents used in electrochemical work is presented in Table 1. Inspection of these data reveals that the largest difference of the (Cop -Ss ) parameter for the listed solvents amounts to 0.263. Thus, on the basis of the Marcus theory for the outer-sphere electrode reactions, the largest change of the reaction rate for different solvents should amount to exp (const 0.263). In this estimation any double-layer effect on the rate constant was neglected. 

The broad applicability often claimed lor Eq. (j) of 12.3.7.2, therefore, needs to be tempered with an awareness of its often serious limitations . Large breakdown in the applicability of such simple relationships may result from several factors, such as nonelectrostatic contributions to the work terms, differences in between corresponding homogeneous and heterogeneous reactions, and specific solvation effects. Further measurements of electrochemical-rate parameters with due regard for double-layer effects are needed to resolve this question. 

One can derive the Butler-Volmer kinetic expressions by an alternative method based on electrochemical potentials (8, 10, 12, 19-21). Such an approach can be more convenient for more complicated cases, such as requiring the inclusion of double-layer effects or sequences of reactions in a mechanism. The first edition develops it in detail. ... 

Strength (to amplify double-layer effects) showed an increase in current as the solution was acidified, and the corresponding analysis also gave pK = 5.6. The results indicated that each of the electrochemical reactions were controlled by a common acid-base equilibrium at an electrode surface functionality. One possibility suggested was a carboxylate with the pK raised by a neighbouring hydrogen-bond acceptor. 

The apparent electrochemical reaction order is influenced by double-layer effects ... 

In the phenomenological theory, the basis of the electrical double layer effect on the elementary act of an electrochemical reaction is the application of the Br0nsted relation which connects the activation energy, Ea, of the process to the heat of the reaction (or reaction free energy) ... 

In addition to the double layer, electrochemical reactions, and diffusion effects, specific adsorption (or chemisorption) can be present in some systems. Adsorption is a process where species are chemically bound to the metal surface of an electrode due to their chemical affinity, and not due to coulombic forces based on charges difference or polarity. Adsorption and electrochemical reactions take place on the electrode... 

Mechanisms of electrochemical reactions of different systems, including transition metal complexes, were examined with a special attention paid to double layer effects and problems of generation and decay of intermediates which arise in such reactions. Electrodes modified with thin films of transition metal hexacyanoferrates and conducting polymers were investigated, also solid state electrochemistry in the absence of external supporting electrolyte were developed. Charge propagation rate in such mixed-valent solid systems and their electrocatalytic properties were studied. 

The parameter a in Equation (11.6) is positive for electrophobic reactions (5r/5O>0, A>1) and negative for electrophilic ones (3r/0Oelectrochemical promotion behaviour is frequently encountered, leading to volcano-type or inverted volcano-type behaviour. However, even then equation (11.6) is satisfied over relatively wide (0.2-0.3 eV) AO regions, so we limit the present analysis to this type of promotional kinetics. It should be remembered thatEq. (11.6), originally found as an experimental observation, can be rationalized by rigorous mathematical models which account explicitly for the electrostatic dipole interactions between the adsorbates and the backspillover-formed effective double layer, as discussed in Chapter 6. 

The central issue which has to be addressed in any comprehensive study of electrode-surface phenomena is the determination of an unambiguous correlation between interfacial composition, interfacial structure, and interfacial reactivity. This principal concern is of course identical to the goal of fundamental studies in heterogeneous catalysis at gas-solid interfaces. However, electrochemical systems are far more complicated since a full treatment of the electrode-solution interface must incorporate not only the compact (inner) layer but also the boundary (outer) layer of the electrical double-layer. The effect of the outer layer on electrode reactions has been neglected in most surface electrochemical studies but in certain situations, such as in conducting polymers and... 

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