Dear Nami

I think you are probing your surface condition by K3FeCN6 which is believed to have very fast electron transfer reaction. When you have covered your electrode surface by QD which is still remained unknown for me , you have made a delay on your reaction to take place. Keep in your mind that any electrode process has a specific time constant that determines the frequency range in which it appears as a peak or inflection in Bode plot. The fastest process appears in the highest frequency. The fastest process in electrochemistry is the combination of solution resistance with double layer capacitance which appears in the highest frequency(Right side of your plot).As you see the phase angle set in nearly 90 which is reasonable and correct for such a system. As the frequency lowered , the next process which is the faradic process occurs at the second highest frequency. This faradic process is an electron transfer governed by K3FeCN6. On a covered electrode ,the kinetic of  this process reduced by covering material which is not at work in bare electrode. This is why the frequency obtained on pink trend is lower than what obtained on black trend.

Hope to be helpful.

Arash Rabiei      

Dear Fatemeh Nami,

if you can show (Eis) data, then we may help.

Hi dear colleague

In what solution did you conduct your experiments?.How did you modify your electrode?

what is the nature of material that you have used for modification? What kind of reaction are you monitoring? All abovementioned parameters are effective on your impedance response. 

But in general, any factor affecting on the conductivity of surface will increase the total resistance so the left side of plot would be higher. also if the reaction that you are pursuing is kinetically unfavorable on modified electrode again you will get the same result. If possible tell me what are you doing.

Thanks

Arash Rabiei      

 Thanks, Ioannis Samaras,

Data files were sent to you.

Thanks, dear Rbiei,

All the data was recorded in K3FeC6/KCl---

The bare electrode that was shown with  the black dotted line is CPE.

After modifying with QD pink line was obtained.

I want to know how I can interpret  the Bode data in general.

Dear Nami

I think you are probing your surface condition by K3FeCN6 which is believed to have very fast electron transfer reaction. When you have covered your electrode surface by QD which is still remained unknown for me , you have made a delay on your reaction to take place. Keep in your mind that any electrode process has a specific time constant that determines the frequency range in which it appears as a peak or inflection in Bode plot. The fastest process appears in the highest frequency. The fastest process in electrochemistry is the combination of solution resistance with double layer capacitance which appears in the highest frequency(Right side of your plot).As you see the phase angle set in nearly 90 which is reasonable and correct for such a system. As the frequency lowered , the next process which is the faradic process occurs at the second highest frequency. This faradic process is an electron transfer governed by K3FeCN6. On a covered electrode ,the kinetic of  this process reduced by covering material which is not at work in bare electrode. This is why the frequency obtained on pink trend is lower than what obtained on black trend.

Hope to be helpful.

Arash Rabiei      

Dear Fatemeh Nami,

your electrode suffers, possibly, from the electrical contact of your electrode. The electrical contact forms another virtual (in parallel) electrode. If can send 2 photos:

1) photo of the electrode with the electrical contact, out from the cell and

2) photo of the electrode in the cell,

we may help more.

Hi

Please consider other plot methods in your work like -Z',Z plot

doi:10.1016/0010-938X(86)90033-8

Thank you for your suggestion.