I want to calculate specific capacitance. So first i have to find area under curve. I have used trapezium rule for that as u can see in attachment.
i have subtracted area under curve from point (-0.93,-0.35) to
(0.98,0.057) to area under curve (-0.98,-0.07) to (0.98,0.057). and i am geting area as 0.1082. and my electrode mass is 0.14 gm. Scan rate : 0.05 V/s
I am geting Sp. capacitance as 7.88 F/g, which is very low and i think i have done mistake in the process.
formula used is : Area inside loop/(mass*voltage difference*scan rate) .
can anyone help me in this..
Pankaj Rohilla,
The main problem with your provided data is that the CV was not appropriately recorded (the CV for capacitance calculation should have been recorded after a few potential cycles so that the CV becomes stable, not showing the huge initial current as shown at the beginning of your potential scan). Also, your cell or electrode or the two combined had a large resistance which had distorted the CV from a proper rectangular shape (indicated by the sloped current switch at both ends of the potential scan).
How ! Can u explain ? where i m doing mistake ? Area Calculation is my problem i think, how you calculated it.
electrode mass is 0.14 gm not 0.014 gm, can you please check the attached file.
I would prefer to use chronocoulometry or impedance spectroscopy to estimate capacitance, but I realize you may not have these available. It is most satisfying when all three agree..
I think the area value should be calculated from closed curve. In this case this area will be 0.1318. Then your Cs will be: 0.1318/(0.14*1.8*0.05) = 10.5 F/g. If you measure discharge time (GPCL) you can check this value again.
Pankaj Rohilla,
The main problem with your provided data is that the CV was not appropriately recorded (the CV for capacitance calculation should have been recorded after a few potential cycles so that the CV becomes stable, not showing the huge initial current as shown at the beginning of your potential scan). Also, your cell or electrode or the two combined had a large resistance which had distorted the CV from a proper rectangular shape (indicated by the sloped current switch at both ends of the potential scan).
Dear George,
>
What does recording after few potential implies ? I remove the battery charging connection and i connect it to potentiostat to perform EIS, CV etc. Is there something that should be done in the procedure to avoid the problems ? Any suggestions ?
I meant you should use the CV recorded after a few potential cycles. Of course, you can record every cycle in this course of continuous potential cycling, but it is the later and stable CV that you should use for capacitance calculation.
We actually published a paper recently in which the issue with the CV recorded in the first potential cycle (or voltage cycle if it was on a two electrode device) is analysed and discussed. This paper is attached for your information.
1) you should calculate the area of only closed loop delete the extra line.
2) your loop should be abovee the x-axis and then calculate the area and capacitance.
3) see my publication for details.
I prefer chronoamperometry or faradaic impedance for this purpose....
Dear Peter,
Can you send a sample calculations of capacitance form impedance data?
I have a problem.
If you have obtained an impedance plot of Z' vs. Z'', and it shows a vertical or tilted vertical portion, you can extract the Z'' data with the frequencies, f, and then plot Z'' against 1/(2*3.14*f). If the plot is linear, the slope is 1/Clf, where Clf is called the low frequency capacitance. Clf should be similar in value as the capacitance derived from the CV or GCD. The following paper shows an example of calculation of capacitance from CV and EIS.
G.A. Snook et al. Electrochemistry Communications 9 (2007) 83–88
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