In impedance (from EIS Nyquist plots), it is possible to measure charge transfer resistance and ohmic resistance. But, how can measure the diffusion resistance from from EIS Nyquist plots?
I will be thankful if anybody could help me….
Regards
Hi,
It is only possible to infer the effect of diffusion from EIS plots indirectly through its effect on Kinetics. If the first semi-circle is Rs-solution resistance, the second projected semi-circle is the kinetics, the linear straight line behavior on the complex plane plot represents diffusion domination if the second semi-circle is not complete, and a straight line exists. If you would like to actually obtain diffusion coefficient or other parameters. Look up Levich equation and using a rotation Disk electrode (RDE). It is easy to run an experiment at various rotation rates, and measure the limiting current at a fixed potential of your choice. Then a plot of Limiting or peak current (if in a CV) plotted against the rotation rate (radians/sec) should be linear and provide a slope related to the diffusion coefficient of the species that is being electrodeposited/interacted with the electrode. The relationships will involve either the levich equation. There are many such techniques to measure the various diffusion, kinetic rate parameters.
When diffusion effects completely dominate the electrochemical reaction mechanism, the impedance is called the Warburg Impedance. From this, one can calculate the diffusion-controlled resistance.
Diffusion can also result in the formation of a Warburg impedance. The frequency of the potential perturbation determines the impedance. Because diffusing reactants don't have to travel very far at high frequencies, the Warburg impedance is small. The reactants must diffuse more at low frequencies, increasing the Warburg-impedance.
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