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1、Chapter 3Mass Transport at Electrode/Electrolyte InterfacesnMass transport : the movement of species in the media (liquid, gas or solid).nRate of mass transport: 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 Mass transp
2、ortnDiffusion - Movement of a species driven by a gradient of chemical potential (i.e., concentration gradient).The modes of mass transportn nMigrationMigration - -Movement of a species driven by a gradient Movement of a species driven by a gradient of electric potential (i.e., electric field).of el
3、ectric potential (i.e., electric field).n nConvection Convection -Movement of a species with the fluid flow.Movement of a species with the fluid flow. Natural convectionNatural convection caused by density gradient caused by density gradient Forced convectionForced convection caused by stirring or o
4、ther hydrodynamic means caused by stirring or other hydrodynamic meansThe mass transport in electrochemical systems nIn an electrochemical cell, various kinds of mass transport may function simultaneously, Ji = JiD + JiM + JiC . nUsually, some measures may be taken so that only one kind of mass tran
5、sport contribute predominantly and others are negligible. In a solution, the natural convection is the predominant mass transport 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 thi
6、n layer near electrode surface, the convection can be neglected. The diffusion and migration become predominant. -Nernst boundary layer If excess of inert electrolyte (supporting electrolyte) is added to the solution and its concentration is much larger than the electro-active species, the electric
7、field and the migration rate in the solution can be greatly reduced. Thus, the diffusion will be the only predominant mass transport mode within the thin layer near electrode surface - diffusion layer.The changes on the local concentration of solution species due to mass transportFicks second law of
8、 diffusion:Ji1Ji2The relationship between the current density and the mass transport rate ORneThe net rate of interfacial reaction equals to the flux of reactant O toward (or product R away from) the electrode surface at x = 0ObRbJOJRIn the case that only diffusion is consideredTransient and Steady-
9、state mass transport processesnTransient process - The combined processes of the development of concentration gradient due to interfacial reactions and the diffusion to eliminate the concentration would make the concentration under dynamic changing and a concentration profile gradually extends towar
10、d the bulk solution. nSteady-state process - Due to natural (or forced ) convection, the concentration gradient would not extend unlimitedly toward the bulk. A steady-state profile of concentration will established finally.Semi-empirical treatment of Steady-state mass transportDiffusion zoneConvecti
11、on zonel - the thickness of the diffusion layermass transport coefficientORneA stagnant layer of thickness A stagnant layer of thickness exists in the exists in the vicinity the vicinity the electrode surface, while theelectrode surface, while thenatural or forced convection maintains the natural or
12、 forced convection maintains the concentration uniformly equal to its bulk concentration uniformly equal to its bulk value beyond value beyond x x = = i id dDiffusion zonecicibcisibThe assumption of “Nernst boundary layer”Convection is necessary for mass transport to reach steady-stateSteady-state m
13、ass transport in real electrochemical cells due to natural and forced convectionNavier Stokes Equation - the newtons first law (F = ma) for a fluid:maForce due to pressure gradientForce due to friction Natural convection due to density gradient n nThe velocity parallel to solid surface The velocity
14、parallel to solid surface surfsurf - fluid surface layer, the distance beyond which the velocity of fluid is almost uniform, e.g., vy 0.99 u0 ;Within surf , The profiles of fluid velocity pf laminar flow near a solid surfacen nThe velocity perpendicular to The velocity perpendicular to solid surface
15、 solid surface The effective diffusion layer and its thicknessn nThe concentration profile The concentration profile n nThe convection-diffusion equation The convection-diffusion equation Solving this equation together with Navier Stokes equation gives the concentration profiles: ci(x)ieffThe limiti
16、ng diffusion current densityO + neRkckaAs the reaction rate at electrode/electrolyte is high enough so that any amount of reactant O delivered to the reaction plane by mass transport can be immediately reacted off, the concentration of O is almost zero, the current will reach a limiting level,iL - limiting diffusion current densityThe strategies
