Hi Oussama Khiter

I can't say with certainty, but my guess is that you are using a digital potentiostat with an incorrect data acquisition. Digital potentiostats use digital staircase rather than an analog ramp. CVs using analog ramps produce the characteristic box shape for capacitors. For a digital staircase, to get that shape the acquisition must integrate across the entire step (surface mode) rather than just sampling the current near the end. Many systems either do not have that option or you have not enabled it in the software.

Without a surface mode, digital staircase CVs effectively filter out most/all of the capacitive information, as well as any Faradaic current due to adsorbed species (hence, "surface" mode). This is good for a lot of electrochemical testing but bad if that contains the data you need. You may also be able drive an analog scan through the potentiostat, but that tends to come with added cost and does not produce better results (if done properly, the results are the same).

This is a typical distorted CV curve, often referred to as an oval-shaped curve, and it arises due to either sluggish electron transport or slow/insufficient ion diffusion. There are several potential reasons for this behavior:

1. Poor electrode contact during fabrication, leading to increased contact resistance contributing to the overall impedance.

2. Overloading the electrodes with excessive metal oxide, which significantly reduces their overall electrical conductivity.

3. Lack of porosity in the electrode material, resulting in sluggish ion diffusion throughout the electrode.

If you observe rectangular-shaped CV curves at much lower scan rates compared to the one you've presented, it rules out the third possibility (lack of porosity). Consider conducting Electrochemical Impedance Spectroscopy (EIS) tests to determine which type of resistance predominates in the system: the combined series resistance, the charge-transfer resistance, or the diffusion resistance. You can refer to this article as well: https://www.sciencedirect.com/science/article/abs/pii/S0272884223020679?via%3Dihub

Dear Oussama Khiter ,

try a new[1] CV (set), using a reduced-V range about [-0.2V, 0.9V].

1. Also, apply/add and the lower V-rate: 1 mV/s

Dear Ioannis Samaras ,

thank you again, i will do the measurements again considering these recommendations.

Dear Jacob Ketter ,

thank you very much , i am using potentiate from CH instruments, previous results with the same device were quite good but this one was different that's why i think the problem is in the test itself or the electrode material

Dear Prashant Choudhary ,

thank you very much, yes possibly it's the third reason because i am using flyash-activaetd carbon and its BET measurements showed low specific surface area, i will do the measurements again at lower scan rates.