It seems to me that people usually care about working electrode and the purpose of the counter electrode is to balance the current of working electrode or stabilize the potential bias between working and reference electrodes. I am interested in what is happening on the counter electrode, especially in terms of the redox reaction and capactive charging/dis-charing.
1. The current of an electro-chemical cell generally consists of faradaic current (charge/electron transfer) and non-faradaic current (charging/discharging of double layers). Does non-faradaic current on the counter electrode have to be the same as that on the working electrode? Because I was think about that the total current on both electrodes should be the same and the number of electrons transferred should also be the same.
2. The counter electrode is usually connected to the control amplifier of a potentiostat. Could anyone explain how the control amplifier is able to stabilize the bias voltage from perspective of redox reaction and electronics. E.g. how could the amplifier adjust the potential of counter electrode so that the faradaic and non-faradaic current is the same as that on working electrode
Thanks,
Dan
Dear Wu, this is an insightful question and I agree that I have not seen an explicit discussion of this idea in writing. The events occurring at the counter electrode are often ignored. However, a couple of things can be deduced from a hypothetical experiment.
(i) If oxidation is occurring at the working electrode, something is being reduced at the counter electrode since the half reduction reaction is only a hypothetical construct. By definition, a redox process requires that reduction and oxidation take place simultaneously. Since little or no current is passing through the reference electrode, there are no major redox events occurring there.
(ii) In a potential sweep experiment, the capacitive current at the counter electrode must be the same in magnitude at the counter electrode since the solution is homogeneous and the ions are distributed evenly in the solution. The rate of sweep felt by the working electrode should be the same as the counter electrode.
In order to find the answer to part (2), you could search the original work when the three electrode system was proposed and see their logic behind the 3-electrode systems for example:
Hickling, "Studies in electrode polarisation. Part IV.-The automatic control of the potential of a working electrode". Transactions of the Faraday Society, 1942,.38, 27.
Dear Dan Wu: There are various process involved among counter electrode and reference electrode, by one side and for the other, the faradaic process in the working electrode(WE). In the WE occur the reaction of interest for an electrochemist, because there, is occuring the electron transference with the electroactive specie. The target of the counter-electrode, inert nature, is to maintain the difference of voltage constant respect reference electrode, avoiding that the current do not pass for the reference electrode and can damage it. Remember the rol of support electrolite. This have as objective, to assure that the current be only diffusional. In these cases the current is large, but if we use microelectrodes, where the currents are very small, the rol of counterelectrode is not so critic or important.
For other side, the charge and discharge of double layer is a technical problem unobservable in microelectrodes.
I hope to be usefull.
Regards
Dear Wu, this is an insightful question and I agree that I have not seen an explicit discussion of this idea in writing. The events occurring at the counter electrode are often ignored. However, a couple of things can be deduced from a hypothetical experiment.
(i) If oxidation is occurring at the working electrode, something is being reduced at the counter electrode since the half reduction reaction is only a hypothetical construct. By definition, a redox process requires that reduction and oxidation take place simultaneously. Since little or no current is passing through the reference electrode, there are no major redox events occurring there.
(ii) In a potential sweep experiment, the capacitive current at the counter electrode must be the same in magnitude at the counter electrode since the solution is homogeneous and the ions are distributed evenly in the solution. The rate of sweep felt by the working electrode should be the same as the counter electrode.
In order to find the answer to part (2), you could search the original work when the three electrode system was proposed and see their logic behind the 3-electrode systems for example:
Hickling, "Studies in electrode polarisation. Part IV.-The automatic control of the potential of a working electrode". Transactions of the Faraday Society, 1942,.38, 27.
Hi Dan,
Short answer: it's complicated. Longer answer...
There are two different issues you address, one is the cell but the other is control. Taking the control issue first, the potentiostat doesn't know or care what the potential at the counter is. The potential at the working vs. the reference is being measured/controlled and whatever current is necessary to get the working electrode to it's potential comes through the counter electrode (equal magnitude opposite sign). The nature of the drive at the counter will depend on how the electronics are setup so to go into more detail there would require knowing those electronics. You mention it connected to [the output from] a control amp, but that isn't always the case. The counter can also be tied to [system] ground with the current supply actually being at the working electrode.
Now as for what is happening at the counter electrode: both the double layer capacitive charging current and the Faradaic current need to be the same as at the working electrode to preserve charge neutrality and the closed loop for current (Kirchoff's law), however, since in most [good] experimental designs the counter electrode area is > the working electrode area, the actual change in voltage necessary to get the same current tends to be smaller (note: even if it were nominally the same, depending on the nature of the Faradaic component the actual voltage change would not necessarily be the same).
The exact Faradaic reactions that take place depend on the nature of the electrolye, the counter electrode and any electroactive species in solution. If you really want to know you can swap your electrode connections and run the opposite current back through the counter and measure the resulting potential shift.
Good question Dan.
A simplified answer is: if oxidation at WE, then reduction at CE and viceversa. Since most of the times the CE is platinum, water oxidation or reduction could happen (if the oxygen is not present). In lack of other possible reactions, water is always available for being reduced or oxidized. Otherwise, it depends on your reaction on the WE, are the reaction products electrochemically active? If yes, they could participate at CE reactions.
All depends on the potential of the CE. Some potentiostat can measure and display the CE potential during your run, so you might have a look (although I am not sure how exact that is).
Good luck!
Thank you all. Your answers are very helpful.
And I have a follow-up question about the potential of counter electrode. I ran CV on ferricyanide using a commercial potentiostat and found that the potential between CE and RE is the same as that between WE and RE but had different sign. Does it make sense and why ?
"I ran CV on ferricyanide using a commercial potentiostat and found that the potential between CE and RE is the same as that between WE and RE but had different sign. Does it make sense and why?"
This is not surprising from a classical electrochemistry perspective. The signs of the cathode and the anode are just the opposite (same number of electrons are leaving or entering). Voltage being a signed quantity takes the direction of current into account.
Hi M. Farooq Wahab,
I was still a little confused. I think I understand that the current on CE and WE have to be the same and the electron transfer is the same in quantity but different direction. But given that different reactions occur on cathode and anode and their area could also be different, how could the voltage be necessarily the same?
I agree with you Dan, I am not sure that the voltage should be the same but only opposite sign. When I perform my experiments, this is not the case.
Dan, I was postulating the same potential difference (with opposite sign) with reference to a classical two electrode system. However, I believe your question of a 3 electrode configuration can be best answered by engineers who actually design potentiostats circuitry i.e. how is the potential difference maintained between the reference electrode and what happens to the voltage on the counter electrode (and how it is actually measured). There is a lot of hand-waving involved in standard textbooks as well as published literature on counter electrodes. I don't recall any systematic studies on the voltage on the counter electrode (since electrochemists tend to ignore it). See this link as well
https://www.researchgate.net/post/How_do_we_measure_the_voltage_difference_between_working_and_counter_electrode_in_a_three_electrode_setup
Did you get a chance to see what the inventor of 3 electrode configuration to say?
Hickling, "Studies in electrode polarisation. Part IV.-The automatic control of the potential of a working electrode". Transactions of the Faraday Society, 1942,.38, 27.
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