电化学动力学Chaper 3 - Mass transfer.ppt

Chapter 3 MASS TRANSPORT AT THE ELECTRODE/ELECTROLYTE INTERFACES Mass transfer Mass transfer : the movement of species in the media (liquid, gas or solid). Rate of mass transfer: the flux of species i, Ji (mol s-1 cm-2) --- the amount of material passing through a unit area within an interval of unit time The modes of mass transfer Diffusion ------ Movement of a charged species driven by a gradient (梯度)of chemical potential (i.e., concentration gradient). The mass transfer in electrochemical systems In an electrochemical cell, various kinds of mass transfer may function, Ji = JiD + JiM + JiC . Usually, some measures may be taken so that only one kind of mass transfer contribute predominantly and others are negligible. In a solution, the natural convection is the predominant mass transfer mode and it maintains the concentrations of all species uniform. But the velocity of the fluid decreases sharply upon approaching a solid surface. Thus, within a very thin layer near electrode surface, the convection can be neglected. The diffusion and migration become predominant. -------Nernst boundary layer If excess of inert electrolyte（惰性电解质） (supporting electrolyte) is added to the solution and its concentration is much larger than the electro-active species, the electric field and the migration rate in the solution can be greatly reduced. Thus, the diffusion will be the only predominant mass transfer mode within the thin layer near electrode surface ---- diffusion layer. The changes on the local concentration of solution species due to mass transfer Fick’s second law of diffusion: Transient and Steady-state mass transfer processes Transient process --- Upon imposing an electrochemical polarization (e.g., potential or current perturbation) to an electrode system, the occurrence of interfacial reactions would render the concentration of species in the vicinity（邻近） of electrode different from the bulk solution. A concentration gradient then arises and in the mean time d