You need to use a bi-potentiostat  with two reference electrodes to measure the potential applied to anode and cathode

Yurii V Geletii Please elaborate your answer. 

Regards

You have two compartments: anodic and cathodic.In order to measure the electrochemical potential of cathode, you need to measure the voltage between cathode and reference electrode. The same for anode. Thus, you should have 4-electrode system, which has two reference electrodes and two working electrodes. A bi-potentiostat can handle such 4-electrode configuration. E.G, see

https://books.google.com/books?id=4TBWg3dIyKQC&pg=PA376&lpg=PA376&dq=four+electrode+potentiostat&source=bl&ots=X-PWKib4I-&sig=NDFKMJGDvLwSsvNQ6YIypqSkOKw&hl=en&sa=X&ved=0ahUKEwjt47jUqJvNAhUIcz4KHc1ED884ChDoAQgxMAQ#v=onepage&q=four%20electrode%20potentiostat&f=false

Sir, Yurii V Geletii  I am highly thankful to you for your prompt response. I have solar panel of 6V in my hand, Now, I dont know how much potential it will give to cathode and how much to anode simultaneously.

Irrespective of its construction a solar cell is a two electrode device. The anode and the cathode. in electrochemical solar cells there is an electrolyte between the cathode and anode. From your question i understood that the photelectrode is the cathode and the Pt electrode is the anode. As a solar cell you need only to measure its terminal performance between the anode and the cathode. That is the I-V characteristics of the solar cell just as the normal solar cell.

In case of investigating the electrodes where one is interested in the I-V of the electrodes, one can use three electrode measurements by inserting a reference electrode in the electrolyte. This may be the standard calomel electrode SCE. Then one can measure the the anode I-V characteristics versus the reference electrode, and can measure also measure the I-V characteristics of the photo-cathode versus the reference electrode. Let us assume that one measured Ia vesrsus Va- Vr and measured Ic versus

Vr- Vc, then for a given Ia = Ic,, V of the solar cell = the summation of the two= Va-Vc.

This may lighten the problem and may resolve some of your query 

wish you success

Based on your original question your device has two compartments with Pt-electrode and electrolyte. Now you are saying: "I have solar panel of 6V in my hand." What kind of the  device do you have in your hands? Do you ask about open circuit potentials?  

In this link "http://pubs.acs.org/doi/abs/10.1021/acsami.5b04393"

Author has written that "Solar-cellpowered CO2RR was conducted for 12 h to mimic operation during a sunny day (6 h/day, 2 days). The solar cell provided −0.78 ± 0.11 V vs RHE to the cathode and +4.53 ± 0.04 V vs RHE to the Pt anode" I couldnot understood that how he determined that potential??

I have already answered you:

You have two compartments: anodic and cathodic.In order to measure the electrochemical potential of cathode, you need to measure the voltage between cathode and reference electrode. The same for anode. Thus, you should have 4-electrode system, which has two reference electrodes and two working electrodes. A bi-potentiostat can handle such 4-electrode configuration. E.G, see

https://books.google.com/books?id=4TBWg3dIyKQC&pg=PA376&lpg=PA376&dq=four+electrode+potentiostat&source=bl&ots=X-PWKib4I-&sig=NDFKMJGDvLwSsvNQ6YIypqSkOKw&hl=en&sa=X&ved=0ahUKEwjt47jUqJvNAhUIcz4KHc1ED884ChDoAQgxMAQ#v=onepage&q=four%20electrode%20potentiostat&f=false

Respected Researchers, how you determine that "The solar cell provided ?V vs RHE to the cathode and ?V vs RHE to the Pt anode? - ResearchGate. Available from: https://www.researchgate.net/post/Respected_Researchers_how_you_determine_that_The_solar_cell_provided_V_vs_RHE_to_the_cathode_and_V_vs_RHE_to_the_Pt_anode [accessed Jun 19, 2016].