• Sample with Semicircle: This sample likely has good charge transfer kinetics, and the semicircle diameter can be used to calculate Rct.
• Sample with Linear Behavior: This sample may be dominated by diffusion processes or have poor charge transfer kinetics. Extend the frequency range or optimize the electrode material to improve charge transfer.

Dear Khoiria Nur Atika Putri ,

in addition to Maher Jabeen :

a) protect[1] (better) the electrode's (Ohmic) electrical contact from the electrolyte.

b) reduce the applied AC potential (0.01 V).

c) What is the applied DC potential(s)[2].

1. [Khoiria Nur Atika Putri] Partial quote "...what possibly did I do wrong?..."

2. OCV ?

Change your applied potentials

Keep electrodes distance the same for all samples

Thank you so much Maher Jabeen for the insights.

Ioannis Samaras Thank you so much for the guidance,

a) I've made sure that the electrical contact is well protected from touching the electrolyte.

b) I'll try it! thanks

c) The applied DC potential was 0.01 V, what does it possibly affect the measurement?

Thank you so much Siraj Ullah , as suggested I'll try reducing the AC potential.

The electrodes distance were always ensured to be constant since I use a closed cell with supported electrodes channels...

Dear Khoiria Nur Atika Putri ,

the electrochemical cell polarization based on the DC (and the 'very high' AC) potential VDC affects the 'thickness'[1] of the microscopic ECDL capacitance[1,2]. Commonly, It ('thickness') ranges[1] from 0.1 to 10 nm.

So, start (EIS) with a(ny) natural (unmodified 'thickness', unblended) ECDL, e.g. VDC[2]=OCV, and a ('lost') distinguished semicircle will be ... (re)observed, again.

1. https://en.wikipedia.org/wiki/File:EDLC-simplified-principle.png in https://en.wikipedia.org/wiki/Double-layer_capacitance

The "thickness" of a charged layer in the metallic electrode, i.e., the average extension perpendicular to the surface, is about 0.1 nm, and mainly depends on the electron density because the atoms in solid electrodes are stationary. In the electrolyte, the thickness depends on the size of the solvent molecules and of the movement and concentration of ions in the solvent. It ranges from 0.1 to 10 nm as described by the Debye length. The sum of the thicknesses is the total thickness of a double layer.

2. From the answer of Khoiria Nur Atika Putri "c) The applied DC potential was 0.01 V, what does it possibly affect the measurement?"