Electrochemical reactions, working electrodes

In the past two decades, thanks to the work of scientists in the U.K., the USSR, Japan, the USA, and other countries, solvated electrons have become an important subject of electrochemical investigations. These have been obtained in a number of solvents in many of them, they are quite stable. Their typical chemical, physical, and optical properties have been well determined. Also, the fundamental relationships for electrochemical and photoelectrochemical generation of solvated electrons have been found. As is evident from this review, solvated electrons behave as individual chemical reagents, in particular they enter into electrochemical reactions at electrodes, and have their equilibrium potential, etc. 

Although the applied potential at the working electrode determines if a faradaic current flows, the magnitude of the current is determined by the rate of the resulting oxidation or reduction reaction at the electrode surface. Two factors contribute to the rate of the electrochemical reaction the rate at which the reactants and products are transported to and from the surface of the electrode, and the rate at which electrons pass between the electrode and the reactants and products in solution. 

Because silver, gold and copper electrodes are easily activated for SERS by roughening by use of reduction-oxidation cycles, SERS has been widely applied in electrochemistry to monitor the adsorption, orientation, and reactions of molecules at those electrodes in-situ. Special cells for SERS spectroelectrochemistry have been manufactured from chemically resistant materials and with a working electrode accessible to the laser radiation. The versatility of such a cell has been demonstrated in electrochemical reactions of corrosive, moisture-sensitive materials such as oxyhalide electrolytes [4.299]. 

In both designs the catalyst-working electrode acts simultaneous as a catalyst for the catalytic reaction (e.g. C2H4 oxidation by gaseous 02) and as an electrode for the electrochemical charge transfer reaction ... 

Subsequent elegant work by Lambert and coworkers61 has shown that, while under UHV conditions the electropumped Na is indistinguishable from Na adsorbed by vacuum deposition, under electrochemical reaction conditions the electrochemically supplied Na can form surface compounds (e.g. Na nitrite/nitrate during NO reduction by CO, carbonate during NO reduction by C2FI4). These compounds (nitrates, carbonates) can be effectively decomposed via positive potential application. Furthermore the large dipole moment of Na ( 5D) dominates the UWr and O behaviour of the catalyst-electrode even when such surface compounds are formed. 

It can be seen here that for dthin electrode) / -> , the electrode will work nniformly thronghout, and the current depends only on electrochemical reaction... 

Certain three-dimensional electrodes, also known as slurry or fluidized-bed electrodes, are sometimes used as well in order to have a strongly enhanced working surface area. Electrodes of this type consist of fine particles of the electrode material (metal, oxide, carbon, or other) kept in suspension in the electrolyte solution by intense mixing or gas bubbling. A certain potential difference is applied to the system between an inert feeder elecnode and an auxiliary electrode that are immersed into the suspension. By charge transfer, the particles of electrode material constantly hitting the feeder electrode acquire its potential (fully or at least in part), so that a desired electrochemical reaction may occur at their surface. In this reaction, the particles lose their charge but reacquire it in subsequent encounters with the feeder electrode. 

The hrst working lead cell, manufactured in 1859 by a French scientist, Gaston Plante, consisted of two lead plates separated by a strip of cloth, coiled, and inserted into a jar with sulfuric acid. A surface layer of lead dioxide was produced by electrochemical reactions in the first charge cycle. Later developments led to electrodes made by pasting a mass of lead oxides and sulfuric oxide into grids of lead-antimony alloy. 

Work in this area has been conducted in many laboratories since the early 1980s. The electrodes to be used in such a double-layer capacitor should be ideally polarizable (i.e., all charges supplied should be expended), exclusively for the change of charge density in the double layer [not for any electrochemical (faradaic) reactions]. Ideal polarizability can be found in certain metal electrodes in contact with elelctrolyte solutions free of substances that could become involved in electrochemical reactions, and extends over a certain interval of electrode potentials. Beyond these limits ideal polarizability is lost, owing to the onset of reactions involving the solvent or other solution components. 

Appreciable interest was stirred by the sucessful use of nonmetallic catalysts such as oxides and organic metal complexes in electrochemical reactions. From 1968 on, work on the development of electrocatalysts on the basis of the mixed oxides of titanium and ruthenium led to the fabrication of active, low-wear electrodes for anodic chlorine evolution which under the designation dimensionally stable anodes (DSA) became a workhorse of the chlorine industry. 

The basic law of electron photoemission in solntions which links the photoemission current with the light s frequency and with electrode potential is described by Eq. (9.6) (the law of five halves). This eqnation mnst be defined somewhat more closely. As in the case of electrochemical reactions (see Section 14.2), not the fnll electrode potential E as shown in Eq. (9.6) is affecting the metal s electron work function in the solution bnt only a part E - / ) of this potential which is associated with the potential difference between the electrode and a point in the solntion jnst outside the electrode. Hence the basic law of electron photoemission into solntions should more correctly be written as... 

Figure 15.2 Schematic representation of different electrochemical cell types used in studies of electrocatalytic reactions (a) proton exchange membrane single cell, comprising a membrane electrode assembly (b) electrochemical cell with a gas diffusion electrode (c) electrochemical cell with a thin-layer working electrode (d) electrochemical cell with a model nonporous electrode. CE, counter-electrode RE, reference electrode WE, working electrode.

Principles and Characteristics Voltammetric methods are electrochemical methods which comprise several current-measuring techniques involving reduction or oxidation at a metal-solution interface. Voltammetry consists of applying a variable potential difference between a reference electrode (e.g. Ag/AgCl) and a working electrode at which an electrochemical reaction is induced (Ox + ne ----> Red). Actually, the exper-... 

Principles and Characteristics Contrary to poten-tiometric methods that operate under null conditions, other electrochemical methods impose an external energy source on the sample to induce chemical reactions that would not otherwise occur spontaneously. It is thus possible to analyse ions and organic compounds that can either be reduced or oxidised electrochemi-cally. Polarography, which is a division of voltammetry, involves partial electrolysis of the analyte at the working electrode. 

Uribe et al.117 examined the reduction of CO in liquid NH3-0.1 M KI at -50°C, using various working electrodes such as Pt, Ni, C, and Hg. The reaction of CO with electrogenerated solvated electrons produced dimeric species, which precipitated as K2C202. Electrochemical reduction of CO in an aqueous solution at porous gas-diffusion and wet-proof electrodes of Co, Ni, and Fe was carried out,178 and Cj to C3 hydrocarbons and ethylene were reported to be the products. 

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