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I'm trying to measure capacitance of a cell I'm creating. It's composed of two copper-sheets with nano-material coated on one side, sandwiched with a membrane and wetted with $\ce{KOH}$. All of the three electrode setups I've found on the internet are for solution based voltamettry, but this cell is not in a solution. The setup that my labmates had yesterday had the reference electrode, $\ce{Ag|AgCl}$ setting in a solution, and not connected to the cell, which doesn't make any sense to me.

  • What reference electrode should be used to do this kind of measurement, and what should the schematic look like for our setup?
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The reason reference electrodes are needed in 3 electrode solution cells is that you're trying to control the interfacial potential of the working electrode (basically from the surface to a point very near the electrode in solution). The wiki's kinda lousy, but the diagram explains essentially what the potentiostat is doing: It's controlling the counter electrode (the op-amp output), to set the potential from the working electrode (ground) to a point just into the solution and the reference electrode is used to read the potential and provide feedback to the amplifier to correct the output. This way, any extra solution resistance between the counter and reference electrodes doesn't reduce the interfacial potential (known as iR drop in electrochemical circles) because you're measuring the potential near the surface. In other words, we're interested in applying a potential across $Z_2$ precisely, but we don't care about what's happening with $Z_1$.

potentiostat schematic

Now, since you have a capacitor, what you're trying to do is control the potential of one plate versus the other plate, so you want the point where you're getting feedback to be one plate, and the other plate is ground—one plate is the "reference electrode" and the other is the "working electrode". Your capacitor is $Z2$ and there's no solution, so there's no $Z1$. (In some cases, it still makes sense to use a separate cable for the reference input, but it's probably fine to just short them together at the inputs) Many potentiostats will work with the counter electrode and reference electrode inputs shorted together (which is how one typically connects a 2 electrode cell), but a resistor between them can help prevent it from oscillating by increasing the time constant of the system.

You're not really using a reference electrode in this case because you don't care about measuring the applied potential versus some known potential, just in controlling the potential across the capacitor. In a cell in which we're interested in studying redox reactions and such, it's important to know how the potentials relate to one another and to compensate for the various potentials that arise when electrodes are dipped in electrolyte solutions.

If you're really interested in the details of potentiostats, Bank has a good primer with lots of details.

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  • $\begingroup$ Our setup is basically two copper plates with a membrane between the two. Would we attach the RE and the CE to one plate and the WE to another? I get that you said a resister would be necessary, but where would the resister be attached? Can you describe what the oscillating would look like? $\endgroup$
    – John Snow
    Jun 17, 2015 at 21:26
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    $\begingroup$ Yes, the RE and CE go together and WE input to the other. If it needs a resistor, then that can go between the RE and CE, with the RE only attached to the plate. (it probably doesn't need to be that big, depending on the capacitance) Most modern potentiostats are built to tolerate a lot of capacitance, so it may not be needed. There is an example of a marginally potentiostat on page 10 of the link above. If it's completely unstable, you basically just see a sine wave (though if you're not sampling fast enough, you may not see it; if you get weird behaviour, try adding the resistor). $\endgroup$
    – Michael DM Dryden
    Jun 17, 2015 at 21:51
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Well, I'm not an electrical engineer by any means, but I do have some experience with this. Basically, the potentiostat is an operational amplifier. The wikipedia page for potentiostat shows the schematic of a test cell as a series of impedances.

enter image description here

In order to test your capacitor, simply place your capacitor in for $Z_2$, and a resistor in for $Z_1$. Unfortunately, I can't be much help in selecting a resistance value, but it will depend on your potentiostat and the capacitance of your device.

enter image description here

In my tests, a resistor wasn't necessary at all, so first try connecting the counter and reference to one plate of the capacitor and the working electrode to the other plate.

If it turns out the resistive impedance is necessary, ask an electrical engineer for help, and use "op amp" in your explanations, because they won't know what a potentiostat is (most likely).

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  • $\begingroup$ All of the electrodes I've looked at look like they need to be in-solution. Is there a reference electrode, that I could use for solid state? $\endgroup$
    – John Snow
    Jun 17, 2015 at 18:23
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    $\begingroup$ You don't need an actual reference electrode, just clip the lead for the reference to the capacitor. $\endgroup$
    – buckminst
    Jun 17, 2015 at 19:01

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