The CV shows a wave prior to solvent oxidation. That is, something is getting oxidized at the Pt electrode. If PBS contains hloride ions, chlorine may be formed but that will be a gas. What it could do a potentials larger than 1 is to oxidize any organic molecules... Tipically white. 

You dont say what you have in solution. Which ions? which phases? PBS and what else? 

I believe your problem is when the electrode acts as anode 

1. It is better that replace Pt with Au as counter electrode.

2. What is your reference electrode material exactly?

Hey,

Since you have done many cycles of CV, I think some ions from the electrolyte ( or something of similar that you use) got reduced on to the Pt. You should be able to see the reduction peak from the CV.

Something was oxidized or reduced and precipitated on the electrode. During your CV, when you do pass the most current? If its during oxidation, something is reduced at the Pt electrode. It will pass the same amount of opposite current, but may move the potential anywhere to find that current. Is there something in your buffer mix that is easily reduced? It might be a metal ion. 

Hi, thanks a lot for all the answers: 

1.) To clarify, both RE and CE are Pt;

2. As I far as I know, the ions in normal PBS should not be reduced in the scanned potential window;

3.) The solution was not purged with inert gas. Does this caused big problems ? 

4.) When I try to use 0.5M H2SO4 to clean, it usually gives a quite stable  profile shown in attached picture. It is a bit deviated from the typical CV response for Au in H2SO4. 

It is much appreciated if anyone could provide some insights based on your experiences!

Maybe your electrode is affected by PBS adsorption phenomenon.

Does it happen in any potential range?

I have some workmates that use PBS in tribocorrosion studies and I haven't heard anything about this kind of problems.

From the CV you could see that some reduction happening at the potential of around 0.45 V . You could find out whether this potential matches with reduction potential of PBS. You could try purging with inert gas to see any difference are there or try to avoid the reduction region if possible. It's also advisable to check the reference electrode potential against a standard. Hope this helps.

The CV shows a wave prior to solvent oxidation. That is, something is getting oxidized at the Pt electrode. If PBS contains hloride ions, chlorine may be formed but that will be a gas. What it could do a potentials larger than 1 is to oxidize any organic molecules... Tipically white. 

You dont say what you have in solution. Which ions? which phases? PBS and what else? 

I believe your problem is when the electrode acts as anode