Diffusion nonsteady-state current

Regarding the nonsteady-state current density (/d,o) induced by the diffusion process at any time shown in Figure 2.9, it can be expressed as Eqn (2.52) after a modification from Eqn (2.32) ... 

The Laplace transfonnation method of solving nonsteady-state diffusion problems was briefly treated in Chapter 4. Thus, one can study all sorts of problems by using various types of current or potential stimuli (as in researches using transients see Section 7.7) and analyzing how transport in solution influences the response of the system. For example, a sinusoidally varying current, density can be used with... 

As the electrolysis proceeds, there is a progressive depletion of the Ox species at the interface of the test electrode (cathode). The depletion extends farther and farther away into the solution as the electrolysis proceeds. Thus, during this nonsteady-state electrolysis, the concentration of the reactant Ox is a function of the distance x from the electrode (cathode) and the time /, [Ox] =f(x,t). Concurrently, concentration of the reaction product Red increases with time. For simplicity, the concentrations will be used instead of activities. Weber (1) and Sand (2) solved the differential equation expressing Fick s diffusion law (see Chapter 18) and obtained a function expressing the variation of the concentration of reactant Ox and product Red on switching on a constant current. Figure 6.10 shows this variation for the reactant. 

The well-known property of those probes is that the limiting diffusion current is proportional to rmder steady-state conditions. For use in electrochemical engineering, an increasing interest is now focused on the nonsteady behavior of those small electrodes under conditions of fluctuating velocity gradient y(f). 

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