1) Potentiostatic EIS measures the impedance by applying a sinusoidal voltage to the sample and measuring the current.

2) Galvanostatic EIS (GEIS) involves the application of an AC current and the measurement of the potential. It (GEIS) is commonly used in battery and fuel cell studies. GEIS is often recommended for corrosion samples with an unstable OCP. It (GEIS) is a perfectly valid technique for EIS, but it must be used carefully to obtain valid results. It is possible for the sample to encounter "high voltage levels" that threaten the integrity of the experiment.

Dear Vidyanand,

I think Faradaic Impedance is when you are doing the measurement with the application of some biased potential to trigger the Faradaic reaction and simultaneously measuring the Impedance.

Non-Faradaic Impedance is the measurement done without application of any biased potential or at OCV.

Thank you Santhosh..The answer makes some sense...

1) Potentiostatic EIS measures the impedance by applying a sinusoidal voltage to the sample and measuring the current.

2) Galvanostatic EIS (GEIS) involves the application of an AC current and the measurement of the potential. It (GEIS) is commonly used in battery and fuel cell studies. GEIS is often recommended for corrosion samples with an unstable OCP. It (GEIS) is a perfectly valid technique for EIS, but it must be used carefully to obtain valid results. It is possible for the sample to encounter "high voltage levels" that threaten the integrity of the experiment.

Dear Vidyanand , what I realize so far,

redox reaction always takes place in the interface of solid/liquid.

for Faradaic process you can easily measure the potential, as the electrode is conducting in nature. a simple multimeter can be used to measure OCP(open circuit potential).

I think for Non-faradaic process, the electrode is insulating in nature like oxide with high band gap. In that case, you can not easily measure electrode potential. But you can not stop the interfacial redox reaction. this time we can still measure the electrode potential by charge-voltage curve ..measurement is not straight forward.

Non faradaic process, you can think about displacement current D instead of current in conductor.

Double layer model i think can be applicable to both of the process, it visualize solid-liquid interface in perspective of charge distribution.

I may refine my answer in the next time. hope it helps, waiting for your comment and suggestion

Strictly speaking, a Faradaic process is one that involves electron transfer through an interface, such as "Pt electrode/aqueous solution of CuSO4", and "graphite/polypyrrole", causing a redox reaction. In the former, electron transfer causes a chemical change or conversion in the liquid state between Cu(II) and Cu(I), whilst in the latter, electron transfer leads to oxidation state change in the solid state.

The phrase "electrode surface" does not give a clear description because the surface can be in contact with air. Also, as in the example of graphite/polypyrrole, when the electrode surface is coated with an active material, it would be confusing to simply say surface.

It is inappropriate to use only if there is a semicircle on the EIS plot to judge if an electrode process is Faradaic or non-Faradaic. This is because a semicircle can also appear on the EIS plot of a porous carbon electrode in a liquid electrolyte, resulting from ion transfer crossing the "porous carbon/liquid electrolyte" interface.

In other words, one cannot use EIS to differentiate if an electrode process is Faradaic or non-Faradaic. As a matter of fact, none of the known electrochemical methods, if used alone, is able to differentiate between Faradaic and non-Faradaic processes.

Atanu Das Thanks a lot for responding to this question, which was in fact posted around three years back, and reigniting the scope for further discussion and new insights.

I agree to your point that the potential at which the Faradiac process occurs can be easily determined, but by using a three-electrode assembly using proper reference electrode preferably in a potentiostat (still I don't prefer the multimeter approach, and I doubt how accurate this method would be, even if it is possible!!!).

Also, your comment regarding the non-Faradaic process also needs some clarification I believe. As my understanding, a double layer model is a simple (yet complex) model visualizing the electrode|electrolyte interface. Whenever, a redox reaction occurs by means of electron transfer within this double layer region (by the transfer of electron between the electrode and any component in the electrolyte at the inner Helmoltz-plane/region), it should be considered as a faradaic process. Any other processes occurring at the electrode|electrolyte interface (e.g. adsorption of ions as in the case of carbon, again at the inner-Helmoltz plane/region) is a non-Faradaic process.

I guess it is very nicely explained by Prof. George Zheng Chen in his comment and in one of the deleted comments above.

Thanks a lot..

Hi Vidyanand,

The classification of Impedance measurement into Faradaic and non-Faradaic depends on the system under analysis. If the system has redox species undergoing charge -transfer at the interface and shows Faradaic resistance, it can be classified as Faradaic EIS.

Faradaic resistance is the sum of charge-transfer resistance and mass-transfer resistance (Warburg impedance), so the systems which show mass-transfer resistance without charge-transfer at the interface, can still be categorized as Faradaic EIS (See Electrochemical Methods, Bard and Faulkner, Chapter 10). A purely non-Faradaic response is observed in the dielectric Capacitor (free from mass-transfer and redox reactions) where the polarization of dielectric occurs in the presence of an electric field. The Nyquist plot for ideal capacitor in parallel with a resistor shows only a semi-circle. Thus, observation of a semi-circle cannot be a criterion for the classification as pointed out by Professor Chen also. EIS is like a black box, and one must have some prior knowledge of the system to make meaningful conclusions.

Also, I think, making measurements at OCV or under a bias potential may not be a criterion for the classification. For instance, charge-transfer features have been observed in the EIS measurement of fuel cells and batteries at OCV.