Mediated electrocatalysis, modified electrode

The most interesting reaction scheme is the electrocatalytic one. Electrocatalysis at modified electrodes is accomplished by an immobilized redox mediator, which is activated electrochemically by applying an electrical perturbation (potential or current) to the supporting electrode. As a result, the chemical or electrochemical conversion of other species located in the solution adjacent to the electrode surface (which does not occur, or occurs very slowly in the absence of the immobilized catalyst) takes place [1, 92-94]. The main advantage of this kind of electrocatalyzed reactions lies in the large number of synthetic procedures for... 

Fig. 1 Electron transfer mechanism between electrode and substrate of mediator-modified electrodes used for electrocatalysis (a) an immobilized mediator monolayer and (b) an immobilized mediator polymer layer.

Fig. 16.20 Scheme illustrating the general principle of mediated electrocatalysis at a mediator-modified electrode (a) overpotential observed for the direct oxidation of a reductant substrate SRed into an oxidized product Pqx dotted line represents the curve that would have been obtained in the absence of kinetic limitations) (b) electrochemical behavior of a redox mediator coupie (Mned/ Mqx) characterized by fast electron transfer kineties (c) electrocatalytic transformation of SRed into Pox by means of the mediator immobilized at the electrode surface... 

In this particular use of modified electrodes, i.e. electrocatalysis, the immobilized redox couple acts as an electron transfer mediator cycling between the reactive (catalyst) state and its non-catalytic state, as shown schematically in Figure 1. 

Electrocatalysis at a modified electrode is usually an electron transfer reaction, mediated by an immobilized redox couple, between the electrode and some solution substrate which proceeds at a lower overpotential than would otherwise occur at the bare electrode. This type of mediated electrocatalysis process can be represented by the scheme ... 

Mediated electrocatalysis at a polymer-modified electrode charge and mass transport processes. 

An area currently very active in electrochemical research deals with the design, fabrication and applications of chemically modified electrodes (CME s). The attractiveness of CME s stems from their potential to replace precious metals such as Pt in electrocatalysis for energy production (1-9), energy storage (10-13), electrosynthesis (14-19), electroanalysis (20-28), and other purposes (29-31). One approach has been to "immobilize", either by covalent attachment, strong adsorption or incorporation into polymeric structures, electrochemically active molecules, called mediators, which act as electron transfer bridges between the electrode surface and the solution species. It has been... 

Electrocatalysis is a type of electrosynthesis that uses surface modified electrodes, or mediators/electrocatalysts to facilitate the redox reaction. Meyer reported the design and synthesis of a chemically modified electrode that consists of a thin polymer film with covalently attached redox sites,designed to facilitate rapid electron transport for electrocatalysis. Complexes of Fe, Ru, Os, Re, and Co were synthesized in such a way that when electrochemically reduced, they reacted to form smooth electroactive polymer films that adhered well to the working electrode to form a chemically modified electrode designed for electrocatalysis. 

Description of electrocatalytic processes in such modified electrodes can be derived from the intersection between the theory of Andrieux and Saveant (1980, 1988) for mediated electrocatalysis in redox polymers and those for metal oxide electrocatalysis (Lyons et al., 1992,1994 Attard, 2001 Pleus and Schulte, 2001) and the recent models for the voltammetry of microparticles given by Lovric and Scholz (1997, 1999) and Oldham (1998) and combined by Schroder et al. (2000). 

In the fabrication of chemically modified electrodes (CMEs), one deliberately seeks, in some hopefully rational fashion, to immobilize a chemical on an electrode surface, so that the electrode thereafter displays the chemical, electrochemical, optical, and other properties of the immobilized molecule (s) (Ref. 1 and references therein). Ordinarily, electrocatalysis at a CME features a mediation of electron-transfer reactions, by the immobilized redox couple, between the electrode and some substrate that would otherwise undergo a slow electrochemical reaction at a naked electrode [1]. However, in the following, electrocatalysis will be taken in the broad sense usually attached to this term in most applications of chemically modified electrodes [2] and which can be defined as the dependence of the electrode reaction rate on the nature of the electrode material . In this section, we are not concerned about cases in which the electrode material is catalytic per se. In other words, intentional modification of the electrode composition or surface is necessary. In line with this choice, examples of electrocatal-yses triggered by deposited adatoms will be included. Selection was made of and emphasis put on examples, old and recent, that illustrate the above definitions, but the recent literature will be mostly retained. 

Electrocatalysis of electrode reactions at macromolecular layers involves the direct participation of the polymer material. Instead of a direct electron transfer between the Fermi level of the metallic electrode and the redox-active substance in solution (which is the classical electrocatalytic situation), the electron transfer is mediated by the surface-immobilized film. Furthermore the overpotential at which a given substrate reaction occurs at an appreciable rate can be appreciably lowered at a polymer-modified electrode compared to that obtained at an uncoated electrode. Consequently the electroactive polymer layer plays a central role. 

Fundamental contributions to the theory of mediated electrocatalysis at polymer-modified electrodes have been made by a number of researchers, most notably Andrieux and coworkers,... 

Mediated Electrocatalysis at Polymer-Modified Electrodes The Steady-State Response... 

Table 2.3 gives results of a calculation carried out by Albery and Hillman that show optimum layer thickness values for optimal mediated electrocatalysis. We can perform some very simple calculations to obtain quantitative estimates of the catalytic advantage of using a polymer-modified electrode with results from Table 2.3. Albery and Hillman assume that typically for efficient mediation k ME must be ca. 10 cms Furthermore the rate constant for the mediated process at the polymer-coated electrode Atme must be greater than that for the direct unmediated process k. We see from Table 2.3 that when W 1, the optimum case is LSk, with k ME = kKXEbo and L 3Xe- When V = 1, L Xo Xl and k ME = kK XoXi) b(i- Furthermore when F 1, the optimum...

Because gold displays very weak chemisorbing properties, the activated chemisorption model of electrocatalysis is assumed to be inapplicable in the case of this metal in aqueous media. The alternative, which is well established in the chemically modified electrode [48] and redox sensor [49] area, is the interfacial cyclic redox mediator model which, in the case of gold in aqueous media, is sometimes referred to as the incipient hydrous oxide/adatom mediator (IHOAM) [18,33] model. In the case of the Group 11 metals the mediator systems are unusual in that their redox transitions involve couples with nonequilibrium (or metastable) reduced and oxidized states. 

Figure 17.4 Cartoon representation of strategies for studying and exploiting enzymes on electrodes that have been used in electrocatalysis for fuel cells, (a) Attachment or physisorption of an enzyme on an electrode such that redox centers in the protein are in direct electronic contact with the surface, (b) Specific attachment of an enzyme to an electrode modified with a substrate, cofactor, or analog that contacts the protein close to a redox center. Examples include attachment of the modifier via a conductive linker, (c) Entrapment of an enzyme within a polymer containing redox mediator molecules that transfer electrons to/from centers in the protein, (d) Attachment of an enzyme onto carbon nanotubes prepared on an electrode, giving a large surface area conducting network with direct electron transfer to each enzyme molecule.

Figure 6.7 illustrates the voltammetric response of the third-generation SOD-based 02 biosensors with Cu, Zn-SOD confined onto cystein-modified Au electrode as an example. The presence of 02" in solution essentially increases both the cathodic and anodic peak currents of the SOD compared with its absence [150], Such a redox response was not observed at the bare Au or cysteine-modified Au electrodes in the presence of 02". The observed increase in the anodic and cathodic current response of the Cu, Zn-SOD/cysteine-modified Au electrode in the presence of 02 can be considered to result from the oxidation and reduction of 02, respectively, which are effectively mediated by the SOD confined on the electrode as shown in Scheme 3. Such a bi-directional electromediation (electrocatalysis) by the SOD/cysteine-modified Au electrode is essentially based on the inherent specificity of SOD for the dismutation of 02", i.e. SOD catalyzes both the reduction of 02 to H202 and the oxidation to 02 via a redox cycle of its Cu (II/I) complex moiety as well as the direct electron transfer of SOD realized at the cysteine-modified Au electrode. Thus, this coupling between the electrode and enzyme reactions of SOD could facilitate the development of the third-generation biosensor for 02". ...

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