Hi there:

  This is an interesting problem. Generally, if your electrode is porous (or alternately has a finite resistance such as that of the electrolyte in series with it) the capacitance should decrease with scan rate (the functional form of the decay is roughly exponential however the exact form of decay depends on the geometry). So, at your lowest scan rate you should, in principle, observe your highest capacitance. If the trend is reversed, then you may want to take a look at your cell design. I have a few questions here:

1) Is your counter electrode area sufficiently large (larger than working)? Generally, the potentiostat, forces a current through the counter electrode to balance out the current at the working. However, if your working electrode is ideally polarized in the potential windown in which you're working, the role of the counter electrode is difficult to understand.  I would suggest placing a very large counter electrode.

2) Have you tried cross checking your CV values with those obtained via Galvanostatic (constant current charge discharge, capacitance should decrease with increasing current density) method and Impedance methods? (In the impedance method, the capacitance should decrease with increasing frequency)

3) Also, how did you calculate your capacitance from CV? I'd suggest integrating your CV curve and divide by the potential window width (if the CV is roughly rectangular that is.

Hope this helps. As the problem may not lie with your chosen material/system and could be something having to do with cell and electrode design, do share your outcome.

-RN

Check whether you are applying a smooth linear scan or a staircase in your CV. In staircase mode, the capacitive current is also a function of the staircase step size in addition to the scan rate.

A pseudocapacitor governed by Faradaic reactions at the electrode materials.  The large redox current peaks are present.the specific capacitances of the amorphous materials with increasing scan rate, the specific capacitance decreases gradually.

Thank you for kind reply

I used Pt  plates as counter electrode, So the surface are of counter is so larger than working( GCE 0.28 cm2). And I got  capacitance via Galvanostatic, the capacitance increased with increasing current density and remain constant in higher current density. I found this same results in high scan rate. I calculated capacitance with integrating  CV curve and divide by the potential window width.

Regards

Simply, the capacitance decreased with increasing the scan rate, as at lower scan rates the electrolyte ions have sufficient time to penetrate the pores of the material, while at higher scan rates only it  accumulates on the outer surface

Dear Gommaa

Thank you for your concentration and kind reply. Generally, The capacitance decresed with increasing the scan rate based on your above mechanism, But for My case the capacitance increase with increasing the scan rate and will remain constant.

Please find the attached file

Regards

A very interesting reference about this behavior.

Lang, X.; Hirata, A.; Fujita, T.; Chen, M., Nanoporous metal/oxide hybrid electrodes for electrochemical supercapacitors. Nature Nanotechnology 2011, 6 (4), 232-236.

Interesting phenomenon! Was this result published? Can I have the article title? Thanks.