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# ì „ê¸°í™”í•™7

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1 Mass Transfer by Migration and Diffusion § Consider an overall electrode reaction (O + n e « R), composed of a series of steps that cause the conversion the dissolved oxidized species (O) to a reduced form (R) in solution. § In general, the current (electrode reaction) is governed by the rates of the processes such as 1) Mass transfer (e.g., of O from the bulk solution to the electrode surface) 2) Electron transfer at the electrode surface 3) Chemical reactions preceding or following the electron transfer 4) Other surface reactions, such as adsorption and desorption Factors Affecting Electrode Reaction Rate

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2 Fig. Pathway of a general electrode reaction Fig § The rate constants for some of these processes (e.g., electron transfer at the electrode surface) depend on the applied potential. § When a steady state current is obtained, the rates of all reaction in a series are the same. § The magnitude of this current is limited by the inherent sluggishness of one or more reactions called rate-determining steps . § A fast reaction step is characterized by a small resistance (or impedance), while a slow step is represented by a high resistance.
3 § In this section, we discuss the general partial differential equations governing mass transfer. These will be used frequently in derivation of equations appropriate to different electrochemical techniques. § Mass transfer, that is, the movement of material from one location in solution to another, arises either from differences in electrical (migration) or chemical potential (diffusion) at the two locations or from movement of a volume element of solution (convection). General Mass Transfer Equation ※ The modes of mass transfer are : 1) Migration : Movement of a charged body under the influence of an electric field (a gradient of electric potential) 2) Diffusion : Movement of a species under the influence of a gradient of chemical potential (a concentration gradient) 3) Convection : Stirring or hydrodynamic transport. Generally fluid flow occurs because of natural convection caused by density gradients and forced convection.

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4 Mass transfer to an electrode is governed by the Nernst-Planck equation, written for one-dimensional mass transfer along the x-axis as where J j (x) is the flux of species j (mol/s cm
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ì „ê¸°í™”í•™7

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