Thin-film diffusion electrode

Types of Electrochromic Materials. Electrochromic materials are of three basic types (2). In a given electrolyte solution, type I materials are soluble in both the reduced and oxidized (redox) states. Type II materials are soluble in one redox state, but form a solid film on the surface of an electrode following electron transfer. Electrochromic polymers are examples of type III materials, where both redox states are solids, generally studied as thin films on electrode surfaces. For types II and III, once the redox state has been switched, no further charge injection is needed to retain the new electrochromic state, and such systems are said to have optical memory . In contrast, for type I electrochromic materials, diffusion of the soluble electrochemically-generated product material away from the electrode occurs and it is necessary to keep current flowing until the whole solution has been electrolyzed. 

The enzyme can be immobilized on the electrode by several techniques (53). The simplest method, first used in 1962, is to trap an enzyme solution between the electrode surface and a semipermeable membrane. Another technique is to immobilize the enzyme in a polymer gel such as polyacrylamide which is coated on the electrode surface. Very thin-membrane films can be obtained by electropolymerization techniques (49,54,55) using polypyrrole, polyindole, or polyphenylenediamine films, among others. These thin films (qv) offer the advantage of improved diffusion of substrate and product that... 

Conventional electronic devices are made on silicon wafers. The fabrication of a silicon MISFET starts with the diffusion (or implantation) of the source and drain, followed by the growing of the insulating layer, usually thermally grown silicon oxide, and ends with the deposition of the metal electrodes. In TFTs, the semiconductor is not a bulk material, but a thin film, so that the device presents an inverted architecture. It is built on an appropriate substrate and the deposition of the semiconductor constitutes the last step of the process. TFT structures can be divided into two families (Fig. 14-12). In coplanar devices, all layers are on the same side of the semiconductor. Conversely, in staggered structures gate and source-drain stand on opposing sides of the semiconductor layer. 

Electropolymerization is also an attractive method for the preparation of modified electrodes. In this case it is necessary that the forming film is conductive or permeable for supporting electrolyte and substrates. Film formation of nonelectroactive polymers can proceed until diffusion of electroactive species to the electrode surface becomes negligible. Thus, a variety of nonconducting thin films have been obtained by electrochemical oxidation of aromatic phenols and amines Some of these polymers have ligand properties and can be made electroactive by subsequent inincorporation of transition metal ions... 

With these solid-oxide electrolytes, designed to operate in relatively 02-rich feed (e.g. air), gas-diffusion electrodes with their enhanced contact area, are not necessary, and electrode materials can be applied directly onto the electrolyte surfaces in thin films. 

The use of thin-film cathodes for battery application usually results in a better performance due to shorter diffusion path of intercalated cation through solid matrix. The thin film electrodes are used in manufacturing of rolled type batteries and thin film cells. 

Mainly, three approaches have been used to immobilize the enzyme on transducer or electrode surface, single layer, bilayer, and sandwich configurations [69, 98], In some studies enzymes are covalently linked with sol-gel thin films [99], Sol-gel thin films are highly convenient for fast, large, and homogeneous electron transfer [17]. With an increase in gel thickness the signal decays and diffusion of analytes to biomolecule active site becomes difficult eventually these factors lead to poor response. By employing thin films various biosensors such as optical and electrochemical biosensors have been reported. 

Thin-layer cell design is based on reduction of diffusion path length the mobile phase is directed along the working electrode surface as a thin film of liquid (see Figure 3-1). 

There is a complication, though a thin (c. cm) layer of solution exists between the electrode and the bulk solution that is relatively immobile. This forms because of the inherent viscous drag of the solution as it moves over the solid electrode. We call this thin film of immobile liquid the diffusion layer, where the latter has a thickness S. The thickness of the layer depends on the rotation speed according to the following ... 

There are two main types of thin-film catalyst layers catalyst-coated gas diffusion electrode (CCGDL), in which the CL is directly coated on a gas diffusion layer or microporous layer, and catalyst-coated membrane, in which the CL is directly coated on the proton exchange membrane. In the following sections, these catalyst layers will be further classified according to their composition and structure. 

The effects of mercury film electrode morphology in the anodic stripping SWV of electrochemically reversible and quasi-reversible processes were investigated experimentally [47-51], Mercury electroplated onto solid electrodes can take the form of either a uniform thin film or an assembly of microdroplets, which depends on the substrate [51 ]. At low sqtrare-wave frequencies the relationship between the net peak crrrrent and the frequency can be described by the theory developed for the thin-film electrode because the diffusion layers at the snrface of microdroplets are overlapped and the mass transfer can be approximated by the planar diffusion model [47,48],... 

However, at the highest frequencies the diffusion layers are muchthiimer, the microdroplets tend to behave as independent hemispherical microelectrodes and the response differs significantly from the theory of the thin-film electrode [49]. 

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