The capacitance values obtained from different methods would be different. The Charge-discharge method is more accurate than others.

The capacitance values obtained from Charge-discharge (CD) are more reliable. 

The capacitance values obtained from impedance method is accurate, because capacitance is depended on frequency and time.

Sujoy, Elham. 

Recently i attended a short-course where Prof.Timothy S. Fischer gave a talk for EIS. When i asked the same question about which one being the better, he said the CV based.

Can you explain your answers with some explanations for them.

@Disha Soni: These are my results of capacitance from electrochemical. From EIS capacitance was smaller more than the other. Normally, I used CV and GPCL but this time my Prof. asked me calculate capacitance from EIS. I found an equation from internet but inductor Ls of circuit I can not define so I neglect it. So, why capacitance I calculated from EIS is so different with the other? Can I believe on this result?

José Tiago Claudino Barragan

In recent papers i have noticed that they calculate the capacitance from CD curves.

Can you please tell how exactly do you calculate the capacitance from EIS. I don't have any idea about that.

The capacitances values that you obtained are quite different due to the measurement are not realized in the same conditions.

By CV, you obtained a global capacitance for your material, and depending of the scan rate the value change (higher for low scan rate and lower for high scan rate, correlated with the diffusion phenomena at electrode interface).

By GPCL; you measure the capacitance value of your system, not only the capacitance of your material. These is why a slightly difference appears. To compare the values it could be interesting to normalize the values with the mass of your catalyst.

By EIS, you measure also the capacitance value of your catalyst. However the measure are realized in stationary method only for the applied potential. So the values normally are in accordance with the CV capacitance at low scan rate.

To obtain the capacitance by EIS with your result, it seems better to fit your measurement with an electrical equivalent circuit. The poorly semicircle is

characteristic of a capacitive loop ( Relectrolyte ( Rct Cdl) ). The means to obtained the resultas are explained more in detail in this article (Elaboration and characterization of ruthenium nano-oxides for the oxygen evolution reaction in a Proton Exchange Membrane Water Electrolyzer supplied by a solar profile).

Hi Minh Hai,

As for the question of which method is the most correct and reliable, frankly I do not have the proper answer for that. In any case, I believe both CV and galvanostatic charge/discharge curve are commonly reported together for supercapacitor papers.

As for your data and calculation themselves, I believe that the calculated capacitance are scan rate and discharge rate-dependent. As such, it is understandable that the calculated capacitance values from CV curve and charge/discharge curve are somewhat different. (however, it is interesting to see that the capacitance from discharge curve, which had a discharge time of 2.5s over 0.4V ==> ~150 mV/s, is higher than the value calculated from CV curve obtained at 50 mV/s)

For EIS calculation, I really doubt the calculation method. I suggest that you need to perform a proper equivalent circuit fitting for the data, then extract out the double layer capacitance value.

Cheers,

KC

Dear Tran Minh Hai,

I suggest  that you should fit your EIS data with one of Zview or Zsim software and extract your data. then by using the following attached file you can calculate the true capacitance from a depressed semicircle model (i.e. a constant phase element in parallel with a resistor). Also I think the differences in capacitance values achieved by EIS, CV and CD is related to basis of the techniques. In the CV and CD techniques  the DC voltage and Current was applied, however in EIS technique an AC voltage with 10 mV amplitude was added on DC voltage.

Thank you all so much, Sujoy Sarkar, Bapi Bera , Disha Soni, José Tiago Claudino Barragan ,especially Thomas Audichon, Kien-Cuong Pham, Elham Shahryari for detailed explaination. You gave me many useful information.  Capacitance values calculated from CV and GD may easier than EIS for me because electrical equivalent circuit with CPE element is not clearly understand for me. I will study more EIS as you recommended.

Thank you all scientist friends for sharing,

Sincerely.

In principle, CV and EIS should give the same capacitance, we have observed this for HUPD at Pt and other metals (in very pure solutions). However, the value of the capacitance is more directly measured using EIS. Of course, usually CPE element must be used, but it can be recalculated into capacitance, see my book.

dear Tran Minh Hai, 

for a good CEIS (measured using EIS e,g. equiv.-c model) you must set: Vdcfor Eis (at Vsource-mode) near the Vm(Imax) in CV. More accurate, right on Vo~ Vm-RImax. Then : CEIS  ~  CCV.

So, for a fast measurement of C start with CCV that is usually quite OK. However, you must, also, shift to CEIS, for detailed electrodes' optimization processes.

as  i feel both Capacitance calculated by two methods should not agree-- there will be pseudo in CV. EIS is near accurate but in presence of CTR it is erroneous; I measured in 1973 capacitance of mercury / aq.Kcl  by constructing  a transformer bridge. pl see my ppt and my double layer set up in BHU India in 1973

whatever method you use to calculate by EIS will be the same as we use the same source of values.

Capacitance calculated at high sweep rate will have adsorption contribution

[email protected]

About CFE pl see a book on electrical engg

@prof V.S Muralidharan : Woah, your ppt is really really useful for me. Thank you so so much ^^ I will read it carefully. And thank you for giving me your email address. Hope all best things will come to you.

@Ioannis Samaras: Thank you so much , professor. Your comment help me know more information about these electrochemical measurements a lot. It is really useful for my research. I sincerely respect your help.

By the way, could you recommend some reference books for understanding clear? All best wishes for you :)

@Andrzej Lasia" Thank you so much for your comment, professor. I calculated capacitance from CV, GD discharge time and EIS but capacitance from EIS was really smaller (~ micro Fara) than from CV and GD (~mili Fara). Therefore, I wonder may be I was wrong when calculating. Could you give me you book file or the name of this book to find? wishes you all the best.

Sincerely.

dear Tran Minh Hai,

send us the frequency range (and other details, also : Vdc, ...,  [[email protected]]) for your capacitance from EIS.

Without the impedance data and details of the modeling it is impossible to judge your results.

To calculate the capacitance value you can use the formula, Csp= I/S*m formula

I=Current density , S is the scan rate and m is the mass of the electrode

@Vasanth Raj : I know this fomular but I think we need change it to integral of IdV for using CV curve in other to calculate capacitance. i use Csp= I/S*m , only one point of current ( I) can be used.

By the way, I think if you use current density you don't need mass unit.

Sincerely,

@Ioannis Samaras, Andrzej Lasia :  dear professors, the  frequency range 0.3MHz- 100 mHz and Vdc 20 mV. I  kept standard as set up for EIS. I has just changed frequency range. This is the origin file of three measurement I used to calculate capacitance. Please help me see it. Thank you so much!

For CV I would chose one potential and calculate |Delta I| and estimate capacitance: C=|Delta I|/(2*v) and repeat this at different potentials. If you integrate you will get integral, nor differential capacitance. EIS gives differential capacitance.

In constant current charging dV/dt depends on time (and potential) so you will get different values at different potentials.

It is not a good approach to calculate capaxitance from one frequency at the maximum if imaginary part. Do full approximation/modeling of your impedances and estimate the capacitance. Probably, you will have to use CPE and Brug's formula (see my book).

dear Tran Minh Hai,

a SC are like a coin. One side is the common Electrolytic capacitor (Double-layer capacitances and other classic contributions in C). This is the side that you "observe" in EIS. You are implementing an approx. formula (in your ppt), and you are focusing only at the high Frequency F1 (~570Hz) e.g. the "big" semicircle.  However, the other one, 2nd-side of the coin is the SUPER (-Capacitor), e.g. the "small" 2nd semicircle that you "defocus". So, if you use (ok, the same formula, but) at F2, i.e. at about 3 to 4 orders lower Frequency F2 (~ ** 10-1Hz), then (at the other semicircle) you may solve the main problem. This is the reason for the apparent-nominal huge differences in C between the EIS and the other methods. Certainly, the main problem is not "the methods", but the implementation in EIS.

PS1: A recommendation : remake (or include in) EIS at a higher Vdc~100mVm, also.

PS2: A theoretical discussion about the differential capacitance :  there are some minor errors (not like 3 to 4 orders), in the certain cases, when the system is NOT quasi-linear. We might return to this, later on, in case you have a highly non-linear system.

Fine

Hi

How should I calculate capacitance, charge transfer resistance and solution resistance for EIS data after fitting to an equivalent circuit?

I am using Zman software for data fitting and obtained the attached results. this is the first time doing EIS so dont know exactly how to predict the obtained parameters and circuit.

Zman file is also attached herewith.

thanks..

The modeling seems to be incorrect. Look, e.g. at a small value(!) of R2 or Qa3>1. You did not show the standard deviations of the parameters.

This is an example of overfitting. The model should be simplified and the importance of the parameters verified using sequential F and t statistical tests (see my book on EIS).

Besides, the impedance data are in a binary format readible only by your software, Show ASCII impedance results: freq, Z', Z". You have to read more before starting your data modeling.

Minh-Hai Tran, In my opinion, using EIS results to calculate the capacitance can be an accurate technique. To calculate capacitance from the EIS data, using ZView software would be helpful.