The CPE element has two parameters, CPE-T and CPE-P.

CPE-T=Q - quasicapacitance, CPE-P=n - arc slope.

For n=1 Q=C [F]

For an (RQ) cell (R and Q connected in parallel), the capacitance can be calculated as follows.

C=(R^((1-n)/n))*(Q^(1/n))

In your opinion, what is the reason for the large difference between the capacity obtained from EIS and CV?

Hi Mehri,

Briefly, CPE can be expressed as Zcpe = 1 / (Tdl*(jw)phi). CPE-T, which means in this case Tdl, has the units of F s^(phi-1).

If you measured this in the absense of a redox reaction, then the absolute value of the double layer capacitance in F, as obtained from CV, is:

Cdl = T1/phi(1/Rs)1-1/phi

You can obtain phi in CPE-P.

Then, to compare your value of 0.0033781 with 1300 F/g, you need to calculate Cdl with the respective equations and then divide it by the grams you have.

Please, let know if your calculated values are now ok.

Dear Dmitry Galyamin

Thank you very much for your attention and response. I don't know some parameters.

T

phi

jw

Thank you for explaining more.

Dear Mehri,

There is more to it than simply answering what these parameters are. I would suggest reading the basic concepts of Electrochemical Impedance Spectroscopy (EIS) in this text:

https://www.gamry.com/application-notes/EIS/basics-of-electrochemical-impedance-spectroscopy/

As you will read, "j" represents the imaginary unit, and "w" denotes the radial frequency. A capacitance can be expressed as:

Zc = 1 / (Cjw)

This can be obtained by iteratively comparing experimental values with your equivalent circuit. However, sometimes we don't have an ideal capacitance, so we can use a Constant Phase Element (CPE) to account for the non-ideal behavior of the capacitance. As I mentioned earlier, the CPE is expressed as:

Zcpe = 1 / (T*(jw)phi)

Here, "T" is related to the capacitance, and "phi" is the dimensionless value representing non-ideality. It ranges from 0 to 1, with a value closer to 1 indicating a more "perfect" capacitance. You can then calculate the capacitance value in Farads using the previously introduced expression. For more information about CPE, you may refer to the following research paper:

Article The Origin of the CPE

Dear Dmitry Galyamin

Thank you so much for your response.