Electrocatalysis modified electrodes

The term modified electrodes encompasses a broad variety of electrode materials obtained by attaching a monomolecular layer of a specific compound on the surface of a conducting solid [338]. In electrocatalysis, modified electrodes are common in the field of oxygen reduction, where carbon materials can be modified for example by attaching layers of macrocycles. Modified electrodes are very common in the field of molecular or supramolecular electrochemistry, especially in the organic area. 

In the first part of the present review, new techniques of preparation of modified electrodes and their electrochemical properties are presented. The second part is devoted to applications based on electrochemical reactions of solute species at modified electrodes. Special focus is given to the general requirements for the use of modified electrodes in synthetic and analytical organic electrochemistry. The subject has been reviewed several times Besides the latest general review by Murray a number of more recent overview articles have specialized on certain aspects macro-molecular electronics theoretical aspects of electrocatalysis organic applicationssensor electrodes and applications in biological and medicinal chemistry. 

Electrocatalysis for Fuel Cells at Enzyme-Modified Electrodes... 

ELECTROCATALYSIS FOR FUEL CELLS AT ENZYME-MODIFIED ELECTRODES... 

Figure 17.7 Electrocatalysis of O2 reduction by Pycnoporus cinnabarinus laccase on a 2-aminoanthracene-modified pyrolytic graphite edge (PGE) electrode and an unmodified PGE electrode at 25 °C in sodium citrate buffer (200 mM, pH 4). Red curves were recorded immediately after spotting laccase solution onto the electrode, while black curves were recorded after exchanging the electrochemical cell solution for enzyme-fiiee buffer solution. Insets show the long-term percentage change in limiting current (at 0.44 V vs. SHE) for electrocatalytic O2 reduction by laccase on an unmodified PGE electrode ( ) or a 2-aminoanthracene modified electrode ( ) after storage at 4 °C, and a cartoon representation of the probable route for electron transfer through the anthracene (shown in blue) to the blue Cu center of laccase. Reproduced by permission of The Royal Society of Chemistry fi om Blanford et al., 2007. (See color insert.)...

Electrocatalysis in oxidation has apparently first been shown for ascorbic acid oxidation by Prussian blue [60] and later by nickel hexacyanoferrate [61]. More valuable for analytical applications was the discovery in the early 1990s of the oxidation of sulfite [62] and thiosulfate [18, 63] at nickel [62, 63] and also ferric, indium, and cobalt [18] hexacyanoferrates. More recently electrocatalytic activity in thiosulfate oxidation was shown also for zinc [23] hexacyanoferrate. Prussian blue-modified electrodes allowed sulfite determination in wine products [64], which is important for the wine industry. 

F.H. Wu, G.C. Zhao, X.W. Wei, and Z.S. Yang, Electrocatalysis of tryptophan at multi-walled carbon nanotube modified electrode. Microchim. Acta 144, 243-247 (2004). 

G.C. Zhao, L. Zhang, X.W. Wei, Z.S. Yang, Myoglobin on multi-walled carbon nanotubes modified electrode direct electrochemistry and electrocatalysis. Electrochem. Commun. 5, 825—829 (2003). 

We will discuss here applications of polyelectrolyte-modified electrodes, with particular emphasis on layer-by-layer self-assembled redox polyelectrolyte multilayers. The method offers a series of advantages over traditional technologies to construct integrated electrochemical devices with technological applications in biosensors, electrochromic, electrocatalysis, corrosion prevention, nanofiltration, fuel-cell membranes, and so on. 

In addition, such redox-active organometallic dendrimers are interesting materials with which to modify electrode surfaces. Applications of these dendrimer modified electrodes in the fields of amperometric and potentiometric biosensors, molecular recognition, as well as in electrocatalysis and photoelectrochemistry, clearly represent interesting areas of future research. 

PossibiKties of electrocatalysis of reactions at electrodes are among the powerful incentives for the electrochemical study of POMs. Interesting results were obtained both in electrocatalytic reductions and oxidations, provided the appropriate form of the POM is used. Two recent reviews devoted to the electrochemical properties of polyoxometalates as electrocatalysts are available [8, 9]. The second one focuses more specifically on electrocatalysis on modified electrodes. In the present text, attention will be drawn specially to the basic principles that could be considered to govern most of solution processes. The principles will be illustrated by several recent experimental results, even though earlier achievements will also be described briefly. 

Strictly speaking, electrocatalysis applies to the dependence of the electrode reaction rate on the nature of the electrode material [152]. In the following, this term will be used in a broader sense and will be admitted to include the possibility that the catalyst be homogeneously dissolved in the electrolyte solution as well as the case where the catalyst is attached to the electrode surface. A short chapter on the electrocatalysis of inorganic chemicals by chemically modified electrodes can also be found in Vol. 10 of this Encyclopedia [9]. 

B. Keita, L. Nadjo, Electrocatalysis hy Chemically Modified Electrodes of the Main Inorganic Chemicals, Encyclopedia of Electrochemistry,... 

For choosing the right way of preparation of PCM modified electrodes, the decisive question is the intention of preparation. There are two principally different goals (1) The electrodes ought to be applied for electrocatalysis, for electrochromic devices and the like, and (2) modified electrodes are prepared to study the electrochemistry of the compounds. For the first goal, there are two principally different approaches (a) the preparation of films on electrode surfaces, and (b) the incorporation of the PCM into a matrix, for example, a mixture of graphite and a binder, leading to composite electrodes. 

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. 

Research into the use of modified electrodes for electrocatalysis continues to be a very active and promising area of research, as it seems very probable that electrocatalysis via chemically modified electrodes can be achieved for a large number of reactions. 

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. 

This brief review attempts to summarize the salient features of chemically modified electrodes, and, of necessity, does not address many of the theoretical and practical concepts in any real detail. It is clear, however, that this field will continue to grow rapidly in the future to provide electrodes for a variety of purposes including electrocatalysis, electrochromic displays, surface corrosion protection, electrosynthesis, photosensitization, and selective chemical concentration and analysis. But before many of these applications are realized, numerous unanswered questions concerning surface orientation, bonding, electron-transfer processes, mass-transport phenomena and non-ideal redox behavior must be addressed. This is a very challenging area of research, and the potential for important contributions, both fundamental and applied, is extremely high. 

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