That is a challenging question which I had to deal with when synthesizing aluminum hydride electrochemically. To solve it, I have used my potentiostat to measure both the working and counter electrode potentials during an experiment. Then, I could go back and hypothesize the counter electrode reactions. I have had the most luck doing this with a Bio-Logic potentiostat, but I am sure that other potentiostats have the option as well.

Good suggestion, I tried this in an aqueous electrolyte real quick (see attached pictures). It is evident that there is a H2 or O2 evolution reaction at the counter electrode. This makes sense because I see bubbles form on the platinum during positive or negative current.

Next I will do the same experiment on a non-aqueous 3-electrode cell. However, I will still have trouble identifying what kind of reaction is going on because I don't know the formal potentials for many of the possible irreversible redox reactions that could happen.

In the end, it won't matter too much because I will replace the platinum counter electrode with a partially discharged intercalation material to act as a ion sink/source, that way the electrolyte composition does not change. I am just curious what is actually happening at the platinum.

Thanks!

Have you got some electrochemical results (e.g. CV and GCD) of this three electrode system? During the test, have you observed some gas generation on the counter electrode? In my opinion, the electrolyte ions would accumulate on the surface of the counter electrode first to balance the charge. Then there might be decomposition of the solvent (PC).

As Jin mentioned a electric double layer (EDLC) of charges will be formed at the interface of Pt and electrolyte solution. To counter the charges consumed on the cathode side the charges will keep accumulating on the interface. Since Pt is inert to balance the charge the potential on the counter will rapidly change to accommodate the additional charges. If the overpotential for electrolyte reduction is crossed then the reduction reaction dominates the EDLC. You can do a visible examination of the Pt surface after reaction for any evidence of electrolyte reduction.

It is better to use Na metal or some standard redox materials that has fixed redox potential.

I have some more data to share. This time I did a non-aqueous 3-electrode test in 1M NaClO4 using platinum as my counter electrode, at the same time I measured the potential between platinum and the reference electrode (0.01M AgNO3 in AN/PC, ~0.3V vs SHE). Before doing CV, I did a quick galvanostatic cycle at 20uA for 2 minutes. I think this is an easy method at finding the electrochemical window of your electrolyte; however, the electrolyte oxidation potential was actually a bit higher when I did run CV, but maybe because the overpotential was higher at higher currents.

In the attached picture look at the plot of CE voltage vs RE during positive current. The voltage begins to gradually decrease. I am not sure, but I was wondering if this would represent the EDLC. At around -3V the CE voltage reaches a plateau, this should be Na metal formation, or something similar, because I also observe precipitation on the platinum electrode (look at picture 2, does that look like Na metal?). The potential makes sense because Na/Na+ is around -2.7 vs SHE, and my Ag/Ag+ RE is about 0.3 V vs SHE.

When the current switches from positive to negative, the CE voltage does not immediately switch to positive potential like in the aqueous system. I believe this is because some of the Na metal that has good contact with the platinum is able to oxidize back into Na+. Notice in the CV data that this also occurs when switching from positive to negative current, but does not occur as much when switching from negative to positive current. This could be because the electrolyte decomposition product is not as reversible, or because it is a gas and floats away. The same explanation works for the aqueous system and why we don't see the same phenomenon.

During negative current, oxidation of the electrolyte occurs because I can see gas evolution. Either the PC solvent is breaking down into H2 or O2 gas, or perhaps ClO4- ions are breaking down into O2 or Cl2 gas.

Let me know what you think.