Two Models of Linear Mass Transport

In the case of non-steady-state diffusion, the diffusion layer becomes thicker with time because ions are transported from ever more distant areas. Then, the diffusion front gradually moves from the electrode surface to the bulk of the solution. Two main images are used in modern theories. According to semi-infinite diffusion model, the diffusion front is allowed to move from the electrode surface 

1) We have in mind die region outside of the double electric layer where the condition of electroneutrality is already obeyed. 

Conventional electrochemical cells have no such arrangement therefore, spontaneous convection flows, which limit the diffusion front shift, can form in them. One of the simplest models, which can evaluate this phenomenon, was proposed by Nernst. According to it, diffusion takes place in a layer of stagnant solution close to the electrode/solution interface. As mentioned earlier, the thickness of the Nernst layer increases with time until natural convection sets in, after which it remains constant. In the presence of forced convection (stirring, electrode rotation), the Nernst layer thickness depends on the intensity of convection that can be controlled, for example, by controlling the rotation speed of an RDE. Hence, the diffusion front can move only as far as to the boundary of the Nernst-type layer. Then, c x) no longer depends on time and the so-called steady state is reached. 

Despite the fact that the Nernst model is rather rough, it has been used in electrochemical kinetics up till now due to its simplicity and obviousness. It has been established by means of different methods that similar structures actually form in the conditions of natural convection. Their thickness makes up 0.01-0.03 cm however, it is rather difficult to strictly define because it depends on various factors, including the current density i. The empirical regularity const has been established experimentally under the steady-state conditions [1]. It should be mentioned that varies with the potential sweep rate v when a linear variation 

Both the semi-infinite diffusion and Nernst s steady diffusion layer concept produce the same results when the diffusion front shift is not yet large as compared to In these conditions, the semi-infinite diffusion model, which is simpler from the mathematical point of view, is to be applied. However, the Nernst model is more general because the regularities of the first model follow from it at 5j.j oo. 

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