Dear Lasse Murtomäki ,

it is counterintuitive as you mention. But without aproper schematic, it is impossible to explain this. Have a look at this: 

I copied this image from:

https://www.gamry.com/application-notes/instrumentation/potentiostat-fundamentals/

The two electodes that have their effect on the cell are the aux and the working electrode. The ref electrode measures the effect of the applied voltage on the aux IN the solution, taking electrode polarization into account. In this way, a reference can be established as to the actual applied voltage to the solution, without the interfering production of bilayers, gasbubbles etc. Kind regards, Harrie Verhoeven.

Thanks for a prompt reply but you described how a three-electrode set-up works, and the scheme gives also that.

Dear Lasse,

In order to understand the electrode arrangement and how they are biased, one has to know that an  electrochemical cell is composed of two electrodes separated or immersed in an electrolyte. One is working electrode and the other is the counter electrode. In order to exclude the effect of the counter electrode on the electrical performance of the working electrode, one has introduced a reference electrode immersed in the electrolyte which acts as an ohmic contact  just to sense the potential of the electrolyte. The reference electrode just senses the potential of the electrolyte it does not conduct any current.  In this way, one can measure the working electrode or even the counter electrode separately.And also one can measure the whole cells from the working electrode to the counter electrode. If one measures the properties of the electrodes only this is called electrodics.

In order to measure the I-V of the electrode one drives a constant current in it till the required potential between the electrode and the reference electrode is reached this is accomplished by the potentiostat.  Since the reference electrode does no conduct any current and its function is to sense the potential of the electrolyte only, the current in the working electrode must continue flowing in the counter electrode.

If you have two working electrodes, one reference electrode and one counter electrode, you can push  different constant current in every working electrode and the total current will be passed by the counter electrode. The potential difference between  every working electrode and the reference electrode may be equal or different according to the electrode arrangement.  

This is a simple picture about the electrochemical electrode arrangement and how their i-v characteristics is measured. It agrees with your expectations.

For more information please follow the link:www.ecochemie.nl/download/Applicationnotes/Autolab_Application_Note_EC08.pdf 

Best wishes

Dear colleague,

I am fully aware of the principle of the potentiostat. Please read my question: how is the bipotentiostat set-up so that the two working electrodes can be controlled separately vs. the reference electrode. I do not believe that it goes similarly to the three-electrode set-up.

Dear colleague,:

welcome,

You have two separate working electrodes and one reference electrode and one counter electrode. Is this okay? . From the electrical point of view you can apply a separate electric source on every working electrode with the counter electrode common between the two sources as a ground. How is your connection diagram then? This is what i said.

Best wishes

Terve,

The working principle is explained in chapter 15.4.4 of the Bard and Faulkner, Electrochemical Methods, 2nd ed. 

Also, these two papers are given as references:

B. Miller, J. Electrochem. Soc. 1969 116(8): 1117-1123; doi:10.1149/1.2412229

http://jes.ecsdl.org/content/116/8/1117.full.pdf+html

D. T. Napp, David Charles. Johnson, and Stanley. Bruckenstein, Anal. Chem., 1967, 39 (4), pp 481–485, DOI: 10.1021/ac60248a017

http://pubs.acs.org/doi/pdf/10.1021/ac60248a017

If I understood correctly, one working electrode is controlled versus the reference as in a normal potentiostat, and the second working electrode is controlled against the first working electrode.

Dear colleagues: 

If the two sources are current sources then the potential on every working electrode can be controlled independently.

Best wishes

Dear Lasse Murtomäki,

sorry for having misunderstood your question. For a bipotentiostat, several options are available to control the second electrode. Not all devices on the market do support these. Therefore, it would be good to learn from you the device you are using (Brand,type,software). But the basic idea behind the rotating ring electrode is that due to the continuous mixing of the solvent at the surface of the electrode, the build up of a polarization layer and gas bubbles can be prevented. This would eliminate the need for a reference electrode since no electrode surface chemistry is interfering. The stationary electrode still needs a reference electrode. As a consequence, the rotating ring electrode would be controlled without feedback (Push mode: imposed voltage and measured current) , whereas the other electrode would be controlled by the reference electrode. The counter electrode will be close to ground potential, because it is connected to ground by the current sensing resistors needed for the feedback regulation of the reference electorde system. I hope this answer is more appropriate to your question. Kind regards, Harrie Verhoeven.

Pekka & Harrie, 

Thank you for pertinent replies. I was thinking of CHI SECM potentiostat when polarizing also the substrate.