A potentiostat allows us to bias the cell equilibrium and initiate an electrochemical reaction at the WE.

Using a reference electrode, and ensuring negligible current flows through it, ensures that an accurate potential can be measured. The CE then provides a pathway for the current to flow.

You may find the following resources useful:

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

https://www.metrohm.com/en/applications/AN-EC-008

A potentiostat allows us to bias the cell equilibrium and initiate an electrochemical reaction at the WE.

Using a reference electrode, and ensuring negligible current flows through it, ensures that an accurate potential can be measured. The CE then provides a pathway for the current to flow.

You may find the following resources useful:

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

https://www.metrohm.com/en/applications/AN-EC-008

I like Marie Berthuel 's answer regarding how it works, but as for the "why" part of your question: the simple answer is "because you want to measure it". Spontaneous electrochemical reactions happen all the time, but if you want to measure how much current is due to them you have to have (some element of) control.

e.g. a bar of iron will sit and rust but if you connect an ammeter to it you will read: 0 A. Clearly electrochemistry is happening, but it's at the exchange current rate, so reduction and oxidation are exactly balanced. If you want to measure what is actually happening there you have to take control and adjust the potential, see how the current responds and then you can figure out what is happening when you would measure 0.

If you have a spontaneous reaction with isolated oxidation and reduction (e.g. battery, fuel cell) you still need some form of control to start driving things. A[n ideal] battery doesn't do anything just sitting there, it needs to have a connection between the cathode and anode, and unless you want to start a fire, that should probably be through a load of some sort.

You can certainly measure a non-zero current with an ammeter in some setups, but depending on the overall system, it is at least possible if not likely that the ammeter you connect is affecting things, but in that case you just don't know if or how.

Thanks for your responses. I found an analogy between our case and a familiar engineering case:

Assume a pipe that there is a very low flow rate of water in it b/c the slope of this pipe is very low and no flow meter can measure the flow rate. We now add a pump at the beginning of the pipe to fill the pipe with water so that we can measure the flow rate.

Everything seems OK but the problem is that this flow is not the flow that we want to measure. Similarly, the current that we make flow through the external circuit is not what we want to measure because it's not generated by our electrochemical reaction but generated by our exerted potential between WE and RE.

I want to know how much current my reaction generates not to measure a current generated by scanning voltage.

This is how I see this system, please advise if I'm wrong.

Thanks.

Pumps and plumbing is actually a common analogy to the potentiostat and electrochemistry. It does tend to breakdown when you get into details however.

The corrosion example from above is a good way to look at this: it is not possible to measure the rate when just sitting at the steady state point, you have to move the potential away: a little more positive and a little more negative. Becasue we understand how electrochemistry proceeds when the potential shifts (Butler-Volmer...which technically doesn't always hold) we can calculate how fast the reaction is proceeding in the case that we can't measure.

I don't know that this is the same as your situation. It's quite possible that you do have a reaction that can be measured at some [in service] potential. In that case the potentiostat is actually making sure that the potential is anchored and not moving over the course of the measurement. Just becuase you are controlling the potential to a set value doesn't mean you are changing it from what it would be otherwise.

Galvanic couples are a good example of this: two dissimilar metals are shorted through the potentiostat. The reference electrode in that case is not used for control but passive measurement of what that potential of the galvanic couple is relative to a control. The current is the current that [would] result from galvanic coupling of those two metals (though for the experiment they are isolated in the cell, they are shorted through the pstat, which in this case is technically a ZRA).

For a battery, the potentiostat can operate (typically as a galvanostat) as a constant load or power drain. Again, in that case, the potentiostat is in control, but the current that flows is the same as if you connected some nominal load across the battery.

If you want a more specific answer for your research you've got to provide more details about what you are looking to do.

Thanks for your reply,

I'm working on a bio-fuel cell anode and I want to know how much current per area I can get from this anode with a certain value of electron transfer efficiency from our reaction.

People doing MFC research use potentiostats all the time.