if, i am getting specific capacitance 151 F/g at 1 A/g in GCD then what will be the appropriate scan rate in CV to get same specific capacitance at potential window 0.9 V for both? How EIS depends on specific capacitance?
The capacitance values derived from the CV and GCD can be comparable when the constant current of the linear GCD is the same as or comparable to the average current on the rectangular CV. It is a common practice to record CVs at a range of different scan rates, and then applying the average current of each CV in the GCD experiment.
EIS results can be used for capacitance analysis according to the equation of Z'' = 1/(2*31.4*f*C). In practice, Z'' (imaginary impedance) is plotted against 1/f (reciprocal of the frequency) and the slope of the linear portion on the plot at the low frequency end is used to derive the capacitance.
CV and GCD both measure the dynamic response of an electrode (material) to a large potential or current disturbance, which means the electrode remains at a unsteady or dynamic state. However, with a very small disturbance (5 - 10 mV), EIS studies the electrode under a very steady state. Because there is only one steady state at a given constant potential, but many unsteady states, EIS only produces one capacitance value, but CV and GCD can produce decreasing capacitance when increasing the CV scan rate or GCD current.
However, if the electrode is disturbed within the capacitive potential range, CV at a sufficiently low scan rate and GCD at a sufficiently low current should produce very comparable capacitance values as that from EIS.
Hello Pravin Wadekar,
You can't predict such aspects and you don't have to correlate them as well. Actually cyclic voltammetry (CV), charge-discharge (CD), and electrochemical impedance spectroscopy (EIS) all give the same electrochemical characteristics at different point of view. For CV, current was measured against fixed potential window and scan rate. On the other hand, time is measured against fixed potential and current density for CD. And EIS gives frequency dependent impedance/capacitance values.
I suggest use CV for finding the optimum potential window. Then operate CD in same potential window with different current density. In supercapacitor study, CD is more reliable. So, focus mainly the CD-derived capacitance values like other researchers.
In spite of that, if you really want to get your CV, CD, and EIS derived capacitance values near to each other, try to use high scan rates and current densities as supercapacitor is related to fast charge-discharge time. To have high capacitance value from EIS, you should calculate capacitance value at minimum frequency. you can also apply VDC near Vmax (from CV) further to have greater capacitance. In your case, VDC = Vmax = 0.9 V.
Specific capacitance calculated from EIS depends on frequency. Specific capacitance increases whether decreasing the frequency. please go the my publications for clear insights. Here I'm attaching a few.
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The capacitance values derived from the CV and GCD can be comparable when the constant current of the linear GCD is the same as or comparable to the average current on the rectangular CV. It is a common practice to record CVs at a range of different scan rates, and then applying the average current of each CV in the GCD experiment.
EIS results can be used for capacitance analysis according to the equation of Z'' = 1/(2*31.4*f*C). In practice, Z'' (imaginary impedance) is plotted against 1/f (reciprocal of the frequency) and the slope of the linear portion on the plot at the low frequency end is used to derive the capacitance.
CV and GCD both measure the dynamic response of an electrode (material) to a large potential or current disturbance, which means the electrode remains at a unsteady or dynamic state. However, with a very small disturbance (5 - 10 mV), EIS studies the electrode under a very steady state. Because there is only one steady state at a given constant potential, but many unsteady states, EIS only produces one capacitance value, but CV and GCD can produce decreasing capacitance when increasing the CV scan rate or GCD current.
However, if the electrode is disturbed within the capacitive potential range, CV at a sufficiently low scan rate and GCD at a sufficiently low current should produce very comparable capacitance values as that from EIS.
Also, EIS (VDC~Vmax, from CV) is the finest (backup[1]) method. It outputs a respectable, model based, steady state[2], capacitance value, that is near the higher creditable[3] capacitance value(s) produced either from the CV(s), or from the GCD(s) method.
1. Very useful to include EIS, also[3], even as a secondary (and concluding[3]) method for capacitance.
2. iDC (VDC) ~ 0, for VDC(time-->oo)=Vmax ; in practice : timeoo~1hour.
3. As a materials' characterization exemplary method for SC-electrodes.
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