There are 2 ways to calculate specific capacitance and one is from CV data and the other one is from galvanostatic discharge curve. Both give different values and I do not know which one is the more reliable. Any suggestion about this?
Dear Dilek, Thank you very much for asking this question.From last week, I'm also searching the answer behind it, but didn't get any clue from anywhere. Waiting for reply from experts.............
In my opinion you have to calculated it from the Constant Current Constant Voltage Methode, or at least from galvanostatic measurements.
In cyclic voltametry you never reach a thermodynamic equilibrium. You observe current maxima which correspond to an equilibrium between the reaction and the diffusion kinetics. But you change the current/peak maximum by changing the voltage sweep, because this experiment is influenced by kinetics.
The specific capacitance is a value corresponding to the energy in a system, so depending not on kinetics but on thermodynamics. Therefor you should use a constant current experiment up to a voltage boundary and then keep the voltage constant until the current gets below a threshold. From this charge you can calculate the specific capacitance.
Sascha is quite right, method based on using galvanostatic discharge curve is more accurate and reliable because specific capacitance extracted from CV curve will depend on the scan (sweep) rate. More precisely, the sweep rate should be slow enough to reach an equilibrium. But it takes time. So, if you are too impatient and apply a too fast scan rate, your results for capacitance will differ quite substantially from a more stable (thermodynamically) procedure based on galvanostatic discharge curve. Usually, fast scan rates result in lower capacitances in comparison with slow rates. To avoid this problem, there is a special renormalization procedure for CV curves based on dividing all currents by the scan rate. For more discussion on this interesting topic, check this link.
http://www.gamry.com/application-notes/testing-electrochemical-capacitors-part-1-cyclic-voltammetry-and-leakage-current/
You may read about how to calculate the specific capacitance from EIS data on our review in Chem Soc Rev (2012)
http://pubs.rsc.org/en/content/articlelanding/2012/cs/c2cs35026b
Here the link and you can access the full paper from my profile
Cyclic voltammetry is a basic characterization that helps to know the information about oxidation/reduction, reversability,... CV measure current whereas charge-discharge measure time for a fixed current. Practically any device should operate at particular current for discharge the stored charges. So charge-discharge is a reliable method to calculate specific capacitance. Specific capacitance calculated from CV can support the charge-discharge results.
If the phenomenon occurring at the electrode surface is from pure electrical double layer (EDL) that means simple charge/discharge, then you may get comparable capacitance values only if you used slow scan rates.
In our experience, we are able to compare the capacitance values of porous carbon electrodes with galvanostatic charge/discharge at 200 mA/g and cyclic voltammetry at 2 mV/s.
Additional pseudo-faradaic processes occurring with EDL process will affect the kinetics and you may not get comparable capacitance values as explained in above replies.
regards
Both the technique should give exactly similar results as both are complementary techniques, instead the voltage window as well current range should be similar in the both the techniques. Decrease in the capacitance with scan rate/discharge current also follow the same way.
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