As I understood, you are doing experiment in non aqueous medium. So please be sure that your cell contains no water,  either from cleaning or non pure solvent. Also, are you using reference electrode designed for non aqueous measurements?

Did you polish/clean/sonicated the electrode before using? If you have another electrode, use both and compare the results, 

 As I understood, you are doing experiment in non aqueous medium. So please be sure that your cell contains no water,  either from cleaning or non pure solvent. Also, are you using reference electrode designed for non aqueous measurements?

I think that there is something decomposed at that potential, maybe trace water in your electrolyte or something on your GCE. 

Do you observe this peak with other electrodes such platine? If the EC signal is still there I agree with people that attribute the peak to some impurities that your solution may contain.

as Alaa A. El-Shafei.

Also, does your cell permit humidity and air pollution from the environment ?

I agree that water in the electrolyte salt, in the solvent, or entering the cell from the lab atmosphere can be a problem.  You will also want to be sure the solvent is of the highest grade.

Maria asked the question but did not respond to any comment. Why we have to spend our time to answer her question? Maria, where are you?

I think the  peak is due to water oxidation reaction which mean that your system is not dry and most probably it is due to using Ag/AgCl reference electrode that contains water. I would recommend you to use Ag/AgNO3 in Acetonitrile or DCM as a reference electrode.

Many thanks to all for your answers.

Dear Dr Geletii, am sorry for my late response. I was away these days and am now back in the lab. many thanks for your input. I greatly appreciate it. We did polish/sonicate the electrodes (counter and working) but I will repeat it now am back. Just to be sure that this part is indeed ok and I will have a go again, although, I did check with other working electrodes and  the peak is still there. So probably is not from working or counter electrode? 

Dear Wong thanks for your answer. I don't think that is due to that coz I run in the same window the same electrolyte/solvent system in the past and it was not there this peak.

Dear Prof El-Shafei, many thanks. Yes my expt is in an organic solvent. I didn't try a non-aqueous reference electrode. I will have a go with the Ag/AgNO3 in MeCN. Dear Hussein thanks for suggesting it, also.

Of course I thank all of you that I did not respond personally, your input is very important also. I will have a go these days in the lab based on the suggestions you gave me and I will let you know.  

Pt working electrode should catalyze water oxidation, if your solvent is wet. Oxidation of water on glassy carbon requires a high overpotential. Comparing CVs on Pt- and GC electrodes you can prove whether water is responsible for the high current

Valuable info, many thanks. I will have a go and let you know. 

wish you luck and you are welcome at any time

Dear Dr Geletii, 

I ran the blank with GC (green line) and Pt (blue line) working electrode and what is observed is that the peak at ca. 1.8-2 V has a lower intensity with the Pt working electrode. 

Dear Maria, 

It looks like you are close to the solvent oxidation potential. Recently, in our laboratory we run CV in DCM. On Monday I can check what we had. As far as I remember, we saw something similar to yours

Dear Maria

From the results you got after using two different electrodes (GC and Pt), it seems that the peak is not due to water oxidation reaction since, as Prof . Geletii mentioned, water oxidation reaction is much easier on Pt compared to GC electrode and so it was expected to get higher current in case of Pt.  

Therefore this peak might be due to Oxygen Reduction Reaction (ORR), so i would like you to try doing deoxygenation (using Ar)  of your electrolyte before the experiment and do the experiment under Ar Atm. to check what will happen.

Good Luck

Dear Hussein, 

Many thanks for your answer.

I always deoxygenate the solution (electrolyte + solvent) and I am running under Ar atmosphere. There is a peak at ca. -1.2 V due to the presence of oxygen and I am purging the solution until this is gone..

I also ran with the non-aqueous reference electrode (Ag/AgNO3 in MeCN) and the result is the same.

I think, the only thing remained to check is the electrolyte. I will recrystallized it again from acetone and see.. It might pick up something from the atmosphere.

Dear Maria

Okay That's good to try to recrystalize your electrolyte nBu4NPF6 .

Also you can have a look on this article, where they report the electrochemical windows of some electrolytes (including nBu4NPF6)  in DCM and Acetonitrile.

http://onlinelibrary.wiley.com/doi/10.1002/cphc.200500361/epdf

Kind Regards

Great, many thanks!

Maria, I've just look at CV in DCM on GC electrode recorded in our lab. It looks very similar to yours. You do not have any reasons to worry about. This is the solvent limit.

Maria, the CV recorded is normal, the form of CV differ according to type of solvent and pH .

Dear Prof Geletii many thanks for looking it up! 

The reason I worried is because in the past I did manage to get a blank (same solvent and electrolyte) where  this "peak" was not so intense (blue line).

Maria, 

this is not a "peak," but the solvent oxidation. The increase of current at E > 1.7 V is very steep. You can't measure the height of this "peak." You should worry about the potential you could reach in a given solvent. Commonly, 10-20 mV difference from run to run  is within an experimental error 

Many thanks, now is clear

also you have to care about the potential window as you changed your reference electrode

wrll, there are two parameter important, 

First , the type of working electrode are using and second type of solution use. Now in this case, if your solution is aqueous and when you upload a potential greater than 1.8 V , you begin to see the discharge of solvent , in other words , the discharge of oxygen. 

in non aqueous solvent needs inert gases to discharge the oxygen