Mediated electrocatalysis, modified

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. 

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). 

Kalimuthu et al. reported an electrochemical study of EbDH where the enzyme was immobilized on a 5-(4 -pyridinyl)-l,3,4-oxadiazole-2-thiol modified gold electrode and trapped under a membrane. No direct electrochemistry of EbDH was observed, but in the presence of ferrocenium methanol as an effective artificial electron acceptor mediated electrocatalysis of EbDH was demonstrated with its native substrate ethylbenzene (Figure 5.32A) and also the related substrate p-ethylphenol (Figure 5.32B). The catalytic system was modelled by electrochemical simulation across a range of sweep rates and concentrations of substrate and mediator. 

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...

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... 

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". ...

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. 

One of the most fruitful trends in the comprehension and control of electrochemical reaction kinetics and electrocatalysis has been the development of modified electrodes to achieve redox mediators of solution processes. This strategy is based on the electrochemical activation (through the application of an electrical perturbation to the electrode) of different sites at a modified surface. As a result of this activation, the oxidation or the reduction 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) can take place4 [40, 69, 70]. 

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. 

It is often necessary to apply potentials in excess of that required by thermodynamics to drive a particular electrochemical reaction when the kinetics of a redox reaction are slow at a bare electrode. This overpotential can be lowered by accelerating the rate of electrochemical reaction by modifying the electrode surface. For surface-immobilized redox sites, electrocatalysis is accomplished by shuttling electrons by repeated cycling between the catalytic and precatalytic state between the electrode and the electroactive substrate this process is known as mediated electrocataly sis. ... 

Poly(l-vinylimidazole)i2-[Os-(4,4 -dimethyl-2,2-bipyridyl)2Cl2] and poly(vi-nylpyridine)-[Os-(Ai,Ai -methylated-2,2 -biimidazole)3] were reported for their efficient capability of mediating electrons transfer between bacterial cells to electrodes. With S. oneidensis, the osmium redox polymer modified anode showed a 4-fold increase in current generation and a significant decrease in the start-up time for electrocatalysis. Using an anode modified with electropolymerized polypyrrole, a dramatie improvement in energy output was noticed in the MFCs. MFCs with a polypyrrole/ anthraquinone-2,6-disulfonic disodium salt (PPy/AQDS)-modified anode... 

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