Electron Transport in Electrodes Modified With Synthetic Clays Containing Electrochemically Active Transition Metal Sites
Gilles Villemure, 2002. "Electron Transport in Electrodes Modified With Synthetic Clays Containing Electrochemically Active Transition Metal Sites", Electrochemical Properties of Clays, Alanah Fitch
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Clay-modified electrodes (CMEs) are one type of chemically modified electrode. They are prepared by depositing thin films of clays on conductive substrates (Macha and Fitch 1998; Baker and Senaratne 1993; Bard and Mallouk 1992; Fitch 1990). The aim is to make use of the physical and chemical properties of the clay coatings to control the electron transfer processes occurring at the electrode solution interface.
Clay minerals have many desirable properties as electrode surface modifiers: high thermal and chemical stabilities, well defined layered structures with large surface area, wide adsorption capabilities and potential as catalysts and/or catalyst supports (Newman and Brown 1987). CMEs have been used in selective analysis (Zen et al. 1996a; Zen and Chen 1997), in catalysis (Oyama and Anson 1986; Ouyang and Wang 1998) or as support matrices for catalysts (Ghosh et al. 1984; Gobi and Ramaraj 1998), in the fabrication of electrochemical (Rong and Mallouk 1993) and photoelectrochemical devices (Gobi and Ramaraj 1994; Shyu and Wang 1997) etc. CMEs are also useful devices for the study of mass transport in clay films. For example, Fitch and coworkers (Stein and Fitch 1996) have discussed the use of CMEs for the study of the diffusion of pollutants through clay beds. Since the clay films used in CMEs are very thin, transport of probe species through the films occurs on a
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Electrochemical Properties of Clays
The articles in this text have been assembled to give readers an idea of the large number of research possibilities associated with combining electrochemical methods with clay research. The first article is introductory in nature. It describes the common features of the diffuse double layer that intrigue clay chemists and electrochemists alike. Based on this common understanding it develops the flux equations that underpin all electrochemical experiments and ends by describing experimental details that the beginning electrochemists should be aware of when attempting to apply electrochemical methods to clays. The group of Fitch gives extensive review to applications of clay-modified electrode work in understanding flux of materials in clay films. At the workshop Dr. Janet Osteryoung presented the use of electrochemical methods, such as microelectrode cyclic voltammetry, in elucidating the charge of colloids. Dr. Osteryoung, at the time head of the NSF division of Chemistry, was unable to write an article, however the technique is extensively reviewed in the first article. The technique relies upon the difference in diffusion coefficient of a cation distributed between the diffuse double layer of a colloid and the bulk solution can be used to determine the distance between charges on the colloid. The first article also attempts to set the clay-modified electrode context for each of the other submissions in this volume: Yamagishi, Manias, Amonette, and Villemure. The use of clay-modified electrodes is detailed, particularly for the information it can give on diffusion in composite materials (see the article of Prof. Aki Yamagishi, now of the University of Tokyo). One of the problems with clay-modified electrodes has been in the structure of the films obtained and the thickness of those films. Yamagishi presents results using Langmuir Blodgett clay film formation using surfactants. Those clays films are then used in clay-modified electrodes to determine intrinsic electroactivity of very thin clay films. Along the same lines recent researchers are interested in the flux of both electroactive and electroinactive materials in nanocomposites of clays. One exciting area is that of polymer composites which are designed to give high sodium or potassium flux and good stability at relatively high temperatures. These membranes can be used in developing the next generation of fuel cells. The conductivity of such films exposes an interesting theoretical and experimental problem. E. Manias, A.Z. Panagiotopoulos, D.B. Zax and E.P. Giannelis of the Departments of Materials Science and Engineering, Chemical Engineering, and Chemistry and Chemical Biology, Cornell University review some of the theoretical work which attempts to elucidate the mechanisms by which high sodium flux occurs in these nanocomposites. The introductory article by Susan Macha, Scott Baker, and Alanah Fitch of Loyola University Chicago covers some of the experimental electrochemical techniques used to determine the conductivities of these clay nanocomposite films. Another exciting area of application of electrochemistry to problems in clay chemistry is in the area of the direct reduction and oxidation of redox metals in clay crystals. This work is of practical interest because of its potential application to remediation of contaminated military sites. It is known that clay interceptor beds that are reduced with dithionite serve to control contaminant plumes. James E. Amonette of Pacific Northwest Labs gives a review of the reactivity of iron in clay minerals. This review is followed by an article by Prof. Gilles Villemure of University of New Brunswick, Canada, which shows some of the most successful electrochemical experiments involving direct reduction of metals in clay lattices.