Electrochemical processing

Kirk-Othmer Encyclopedia of Chemical Technology (4th Edition) 

Electrochemical systems convert chemical and electrical energy through charge-transfer reactions. These reactions occur at the interface between two phases. Consequendy, an electrochemical ceU contains multiple phases, and surface phenomena are important. Electrochemical processes are sometimes divided into two categories electrolytic, where energy is supplied to the system, eg, the electrolysis of water and the production of aluminum and galvanic, where electrical energy is obtained from the system, eg, batteries (qv) and fuel cells (qv). 

The industrial economy depends heavily on electrochemical processes. Electrochemical systems have inherent advantages such as ambient temperature operation, easily controlled reaction rates, and minimal environmental impact (qv). Electrosynthesis is used in a number of commercial processes. Batteries and fuel cells, used for the interconversion and storage of energy, are not limited by the Carnot efficiency of thermal devices. Corrosion, another electrochemical process, is estimated to cost hundreds of millions of dollars aimuaUy in the United States alone (see Corrosion and CORROSION control). Electrochemical systems can be described using the fundamental principles of thermodynamics, kinetics, and transport phenomena. 

Determining the cell potential requites knowledge of the thermodynamic and transport properties of the system. The analysis of the thermodynamics of electrochemical systems is analogous to that of neutral systems. Eor ionic species, however, the electrochemical potential replaces the chemical potential (1). 

The electrochemical potential, )T, of a species is a function of the electrical state as well as temperature, pressure, and composition is the absolute activity, which can be broken down into three parts as shown. Eor an electrolyte. A, which dissociates into cations and v anions, the chemical potential of the electrolyte can be expressed by 

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The molecular-level observation of electrochemical processes is another unique application of STM [53, 54]. There are a number of experimental difficulties involved in perfonning electrochemistry with a STM tip and substrate, although many of these have been essentially overcome in the last few years. 

The combination of electrochemistry and photochemistry is a fonn of dual-activation process. Evidence for a photochemical effect in addition to an electrochemical one is nonnally seen m the fonn of photocurrent, which is extra current that flows in the presence of light [, 89 and 90]. In photoelectrochemistry, light is absorbed into the electrode (typically a semiconductor) and this can induce changes in the electrode s conduction properties, thus altering its electrochemical activity. Alternatively, the light is absorbed in solution by electroactive molecules or their reduced/oxidized products inducing photochemical reactions or modifications of the electrode reaction. In the latter case electrochemical cells (RDE or chaimel-flow cells) are constmcted to allow irradiation of the electrode area with UV/VIS light to excite species involved in electrochemical processes and thus promote fiirther reactions. 

Amongst other spectroscopic teclmiques which have successfiilly been employed in situ in electrochemical investigations are ESR, which is used to investigate electrochemical processes involving paramagnetic molecules, Raman spectroscopy and ellipsometry. 

Electrochemical processes can become diffusion controlled if the fonnation of the activated complex is fast compared with the diffusion of the reacting anion to the surface or dissolving cations from the surface. In aqueous... 

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