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I was reading a couple of papers by Prof. B E Conway, including his book "Electrochemical Supercapacitors", where he came across this statement that the Nernstian potential of an electrochemical cell remains ideally constant during its charge/discharge.

Can anyone please explain this statement? Please try not to use any analogies with water pumps and stuff. Instead, I would highly appreciate proper chemical and mathematical equations being used wherever necessary.

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    $\begingroup$ Review the guide How to ask and Asking FAQs to prevent clarification requests, objections, downvoting or closure. $\endgroup$
    – Poutnik
    Nov 6, 2023 at 17:07
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    $\begingroup$ Supercapacitor is a different thing than a simple cell onlinelibrary.wiley.com/doi/10.1002/aenm.202003311 $\endgroup$
    – Mithoron
    Nov 6, 2023 at 17:17
  • $\begingroup$ Mithoron: The Wiley article you suggested does not even mention the word Nernst! The OP is reading a book and the book can talk about the Nernst equation! $\endgroup$
    – AChem
    Nov 6, 2023 at 17:31
  • $\begingroup$ @Mithoron I know that supercapacitors are different, but the book compared supercapacitors to batteries, where this particular statement was mentioned $\endgroup$ Nov 8, 2023 at 8:55
  • $\begingroup$ @AChem By the way, what is 'OP'? I saw it in a couple of other places, too. $\endgroup$ Nov 8, 2023 at 9:11

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According to Wikipedia, the Nernst equation is given as follows.

$$ E = E^\ominus -\dfrac{\text{R}T}{z\text{F}}\ln{Q_\text{r}} $$

Notice the dependence of cell potential $E$ on the reaction quotient $Q_\text{r}$. Thus, during charging or discharging, when $Q_\text{r}$ changes, so will the cell potential.

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    $\begingroup$ Nernst equation has nothing to do with AC or DC current. $\endgroup$
    – AChem
    Nov 6, 2023 at 17:24
  • $\begingroup$ @Achem AC current allows for the measurement of cell potential without changing the electrolytic concentrations because the reversal of current also reverses the chemical reactions that occur, preserving the reaction quotient. $\endgroup$
    – ananta
    Nov 6, 2023 at 17:27
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    $\begingroup$ No ananta. I think there is a misconception here. AC current is not used to measure cell potentials. You are confusing cell potential with conductivity measurements. The latter is indeed dependent on high frequency AC for the reasons you suggest above. $\endgroup$
    – AChem
    Nov 6, 2023 at 17:28
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    $\begingroup$ Check any university level physical chemistry textbook or Wikipedia. Search the terms conductivity measurements and potentiometry. In potentiometry the "potential" is measured with zero current drawn. Conductivity exclusively uses AC. $\endgroup$
    – AChem
    Nov 6, 2023 at 17:35
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    $\begingroup$ Of course the cell potential changes while charging/discharging; the bulk cell electrolyte concentrations change and the reaction quotient in the logarithm changes until it reaches the equilibrium constant K, where the cell potential drops until zero. $\endgroup$
    – Mäßige
    Nov 6, 2023 at 20:27

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