I have designed a paper-based electrochemical cell for my research. I use carbon nanotubes as working and counter electrodes, and I use Ag/AgCl as a reference electrode. I run and record cyclic voltammograms on the device to measure the specific capacitance using 2-electrode device. I expected the CV curve to be symmetrical with respect to the zero current, since it is a supercapacitor with an internal resistance. But I keep getting inaccurate and unreasonable curves. I am not sure where the problem is, but one of the things that could be the problem is the surface areas of the electrodes. I know that the surface area of the working electrode should be smaller than that of the counter electrode, but is there any ratio between the two surface areas? And is there a relationship between the electrode surface area and the operating voltage window I may apply in the experiment? Finally, is there a specific equation I can use to estimate the corresponding current I should get at specific applied voltage when I run CV? By the way, I use phosphate buffered saline, 1×PBS, pH 7.35, as an aqueous electrolyte in the experiment.
According to the measured CV curves, the specific capacitance can be calculated using half the integrated area of the CV curve to obtain the charge (Q) and subsequently dividing the charge by the mass of the electrode (m) and width of the potential window (deltaV). This equation is
C=Q/m (deltaV)
On the other hand, according to the definition of pseudocapacitance, the specific capacitance of MnO2 can also be calculated by the formula:
Cphi=i/v
Where Cphi is the differential capacitance, i is the current density, v is the scan rate.
You may find interesting comments here:
https://www.researchgate.net/post/Why_is_the_specific_capacitance_calculated_using_cyclic_voltammetry
Don't hesitate to show your recorded CV if you insist on this case.
Motaz,
For your questions:
1) There is not a solid requirement for the surface area ratio of working electrodes to counter electrodes. As long as the counter electrode is large enough to capture almost all the current flowing from the working electrode, it should be fine.
2) I don't believe there is a relationship between electrode surface area and potential window. The potential window depends on the thermodynamically stable window of the electrolyte, the surface characteristics of the electrodes, concentrations of salts, mass loading of electrode materials etc. Most influencing factors pertain to the intrinsic properties of the electrodes and the electrolytes.
3) To estimate the current you expect to see, you can refer to the equations given by the first responder. However, I do not suggest doing so because usually the capacitance of your materials is unknown and thus doing the estimation is impossible. In addition, the magnitude of the current also depends on scan rate, mass loading of active materials and type of electrolytes. It's really hard to guess.
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