Equations Governing Modes of Mass Transfer

Tafel plots are composed of an anodic branch for positive overpotentials and a cathodic branch for negative overpotentials. The slope of the linear region on the Tafel plot (between 50 mV and very large overpotentials) can be used to calculate a, the transfer coefficient  

Mass transfer can be accomplished in three different ways and/or a combination of these modes (a) migration— movement of charged particles in an electric field (b) diffusion— movement of species against a concentration gradient (c) convection— movement of species induced by stirring or density gradients. The more modes of mass transfer are present concurrently, the more complicated the eqnations that describe them are. 

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