Electrodes material

The electrode material plays an important, although little understood role for the outcome or organic electrosyntheses. Innumerable reports (see e.g. Swann, 1956) bear witness to much painstaking work on electrode preparation and pretreatment, sometimes to an extent that one despairs of ever getting any order in this vast, amorphous body of know-how. Just to take one example, it has been reported that the temperature at which a certain solid electrode was cast had a marked effect upon product distribution (Swann et al., 1966)  

The choice of electrode material has been considered most critical for cathodic processes, which is possibly a reflection of the fact that there really are lots of cathode materials from which to choose. Due 

Anodic reactions at Pt have been claimed to be dependent upon the surface state of the platinum. The Kolbe reaction is perhaps the best known case (for a review, see Conway and Vijh, 1967) for which a change in the surface composition has been held responsible and indeed necessary for the reaction to occur. Thus, at a low potential, 0-8 V, acetate in aqueous solution is completely oxidized to carbon dioxide and water on pure platinum sites (i.e. we have in effect a fuel cell electrode). On raising the potential, PtO and adsorbed oxygen begin to cover the surface and oxygen evolution takes place in the range between 1-2- 1-8 V. A further increase in the 

Surface Composition of Platinum Anodes in Aqueous Solution 

To continue with the Kolbe reaction, it has been shown that carbon anodes strongly favour the carbonium ion pathway (Koehl, 1964) at least for simple alkanecarboxylic acids. Also, for phenyl-acetic acid and 1-methylcyclohexylacetic acid the same tendency towards carbonium ion formation on carbon anodes was observed, the phenomenon being explained as due to the presence of paramagnetic centres in carbon. These would bind the initially formed radicals, impede their desorption and hence promote the formation of carbonium ions via a second electron transfer (Ross and Finkelstein, 1969). However, cases of Kolbe oxidations in which no dependence on anode material was noticeable have been found more recently (Brennan and Brettle, 1973 Eberson and Nilsson, 1968a Sato et al., 1968). Actually, the nature of the carbon material determines the yield of products formed via the radical versus carbonium ion pathway (Brennan and Brettle, 1973). Yields of the 

The material of the electrode is of decisive importance for electrochemical work. 

Mercury has the longest history as an electrode for analytical purposes. Its large cathodic range of about -2.0 V vs. the saturated calomel electrode (SCE) is 

The main advantage of these solid electrodes is their large anodic range, up to about +1.0 V vs. SCE. The number of electrochemical oxidations, especially of 

it is important to remember that the products, reaction mechanism and the rate of the process may depend on the history and pretreatment of the electrode and that, indeed, the activity of the electrode may change during the timescale of a preparative electrolysis. Certainly, the mechanism and products may depend on the solution conditions and the electrode potential, purely because of the effect of these parameters on the state of the electrode surface. 

Before considering the role of the electrode material in detail, there is one further factor which should be pointed out. The product of an electrode process may be dependent on the timescale of the contact between the electroactive species and the electrode surface, particularly when a chemical reaction is sandwiched between two electron transfers in the overall process. This was first realized when it was found that i-B curves and reaction products at a dropping mercury electrode were not always the same as those at a mercury pool electrode (Zuman, 1967a). For example, the reduction of p-diacetylbenzene at a mercury pool was found to be a four-electron process, giving rise to the dialcohol, while at a dropping mercury electrode the product was formed by a two-electron process where only one keto group was reduced (Kargin ef al., 1966). These facts were interpreted in terms of the mechanism 

under the hydrodynamic conditions prevailing at high rotation rates, the one-electron product is removed more rapidly by convection than by the chemical reaction, while at slow rotation speeds the chemical reaction and further electron transfer predominates. The form of the electrode and the hydrodynamic conditions prevailing in the electrolysis solution are therefore parameters which require eontrolling but which give additional flexibility in the design of syntheses. 

The electrode is often considered to be inert and its role is simply to act as a source or a sink for electrons, depending on its potential. It is clear, however, that the rate of many electrode processes, and indeed their products, is dependent on the material from which the electrode is constructed. 

Indeed a detailed kinetic investigation of the reduction of alkyl iodides at a lead electrode in dimethylformamide shows that the process is complex and involves catalysis by a lead alkyl species at the surface (Fleischmann et al., 1971a). The catalytic cycle proposed was 

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In recent years, advances in experimental capabilities have fueled a great deal of activity in the study of the electrified solid-liquid interface. This has been the subject of a recent workshop and review article [145] discussing structural characterization, interfacial dynamics and electrode materials. The field of surface chemistry has also received significant attention due to many surface-sensitive means to interrogate the molecular processes occurring at the electrode surface. Reviews by Hubbard [146, 147] and others [148] detail the progress. In this and the following section, we present only a brief summary of selected aspects of this field. 

If two redox electrodes both use an inert electrode material such as platinum, tlie cell EMF can be written down iimnediately. Thus, for the hydrogen/chlorine fiiel cell, which we represent by the cell Fl2(g) Pt FICl(m) Pt Cl2(g) and for which it is clear that the cathodic reaction is the reduction of CI2 as considered in section... 

Some values for and (3 for electrochemical reactions of importance are given in table A2.4.6, and it can be seen that the exchange currents can be extremely dependent on the electrode material, particularly for more complex processes such as hydrogen oxidation. Many modem electrochemical studies are concerned with understanding the origin of tiiese differences in electrode perfomiance. 

Berendsen, H.J.C. Van Gunsteren, W.F. Molecular dynamics with constraints, in The Physics of Superionic Conductors and Electrode Materials ed. J.W. Perram, NATO ASI Series B 92 (1983) 221-240 (Plenum, New York). 

Ions impacting onto the cathode during a discharge cause secondary electrons and other charged and neutral species from the electrode material to be ejected. Some of these other particles derived... 

Electromagnetic flow meters ate avadable with various liner and electrode materials. Liner and electrode selection is governed by the corrosion characteristics of the Hquid. Eor corrosive chemicals, fluoropolymer or ceramic liners and noble metal electrodes are commonly used polyurethane or mbber and stainless steel electrodes are often used for abrasive slurries. Some fluids tend to form an insulating coating on the electrodes introducing errors or loss of signal. To overcome this problem, specially shaped electrodes are avadable that extend into the flow stream and tend to self-clean. In another approach, the electrodes are periodically vibrated at ultrasonic frequencies. 

Molten Carbonate Fuel Cell. The electrolyte ia the MCFC is usually a combiaation of alkah (Li, Na, K) carbonates retaiaed ia a ceramic matrix of LiA102 particles. The fuel cell operates at 600 to 700°C where the alkah carbonates form a highly conductive molten salt and carbonate ions provide ionic conduction. At the operating temperatures ia MCFCs, Ni-based materials containing chromium (anode) and nickel oxide (cathode) can function as electrode materials, and noble metals are not required. 

Ethylene glycol can be produced by an electrohydrodimerization of formaldehyde (16). The process has a number of variables necessary for optimum current efficiency including pH, electrolyte, temperature, methanol concentration, electrode materials, and cell design. Other methods include production of valuable oxidized materials at the electrochemical cell s anode simultaneous with formation of glycol at the cathode (17). The compound formed at the anode maybe used for commercial value direcdy, or coupled as an oxidant in a separate process. 

Polysilicon. Polysihcon is used as the gate electrode material in MOS devices, as a conducting material for multilevel metallization, and as contact material for devices having shallow junctions. It is prepared by pyrolyzing silane, SiH, at 575—650°C in a low pressure reactor. The temperature of the process affects the properties of the final film. Higher process temperatures increase the deposition rate, but degrade the uniformity of the layer. Lower temperatures may improve the uniformity, but reduce the throughput to an impractical level. 

Miscellaneous. Iridium dioxide, like RUO2, is useful as an electrode material for dimensionally stable anodes (DSA) (189). SoHd-state pH sensors employing Ir02 electrode material are considered promising for measuring pH of geochemical fluids in nuclear waste repository sites (190). Thin films (qv) ofIr02 ate stable electrochromic materials (191). 

The electrolyte thus formed can conduct electric current by the movement of ions under the influence of an electric field. A cell using an electrolyte as a conductor and a positive and a negative electrode is called an electrolysis cell. If a direct-current voltage is appHed to a cell having inert electrode material such as platinum, the hydrogen ions (cations) migrate to the cathode where they first accept an electron and then form molecular hydrogen. The ions... 

Sodium tungstate is used in the manufacture of heteropolyacid color lakes, which are used in printing inks, plants, waxes, glasses, and textiles. It is also used as a fuel-ceU electrode material and in cigarette filters. Other uses include the manufacture of tungsten-based catalysts, for fireproofing of textiles, and as an analytical reagent for the deterrnination of uric acid. 

Electrically, the electrical double layer may be viewed as a capacitor with the charges separated by a distance of the order of molecular dimensions. The measured capacitance ranges from about two to several hundred microfarads per square centimeter depending on the stmcture of the double layer, the potential, and the composition of the electrode materials. Figure 4 illustrates the behavior of the capacitance and potential for a mercury electrode where the double layer capacitance is about 16 p.F/cm when cations occupy the OHP and about 38 p.F/cm when anions occupy the IHP. The behavior of other electrode materials is judged to be similar. 

The substance named first represents the positive electrode the substance named second is the negative electrode. In all cases except for air(oxygen) systems, the active electrode material is the oxide or the hydroxide of the named species. 

In electrode kinetics a relationship is sought between the current density and the composition of the electrolyte, surface overpotential, and the electrode material. This microscopic description of the double layer indicates how stmcture and chemistry affect the rate of charge-transfer reactions. Generally in electrode kinetics the double layer is regarded as part of the interface, and a macroscopic relationship is sought. For the general reaction... 

The exchange current density, depends on temperature, the composition of the electrolyte adjacent to the electrode, and the electrode material. The exchange current density is a measure of the kinetic resistance. High values of correspond to fast or reversible kinetics. The three parameters, a, a. ... 

Design possibilities for electrolytic cells are numerous, and the design chosen for a particular electrochemical process depends on factors such as the need to separate anode and cathode reactants or products, the concentrations of feedstocks, desired subsequent chemical reactions of electrolysis products, transport of electroactive species to electrode surfaces, and electrode materials and shapes. Cells may be arranged in series and/or parallel circuits. Some cell design possibiUties for electrolytic cells are... 

Electrode materials and shapes may have a profound effect on cell designs. Anode materials encountered ia electrochemical processes are... 

Electrodes. At least three factors need to be considered ia electrode selection as the technical development of an electroorganic reaction moves from the laboratory cell to the commercial system. First is the selection of the lowest cost form of the conductive material that both produces the desired electrode reactions and possesses stmctural iategrity. Second is the preservation of the active life of the electrodes. The final factor is the conductivity of the electrode material within the context of cell design. An ia-depth discussion of electrode materials for electroorganic synthesis as well as a detailed discussion of the influence of electrode materials on reaction path (electrocatalysis) are available (25,26). A general account of electrodes for iadustrial processes is also available (27). 

Selection. The widely used cathode materials iaclude Hg, Pb, Al, Zn, Ni, Fe, Cu, Sn, Cd, and C. Because of mechanical iaconvenience, mercury is not an attractive electrode material for large cells. The preferred material is lead or an amalgam. Because Pb is soft and has a tendency to deform, however, it presents some mechanical problems. The problems can be overcome by hot dip or electroplating on steel, copper, or other rigid base material. 

A range of gaskets, electrode materials, and electrode geometries is available... 

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