# Why does the Nernstian cell potential remain ideally constant during charge or discharge of an electrochemical cell?

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.

• Review the guide How to ask and Asking FAQs to prevent clarification requests, objections, downvoting or closure. Nov 6, 2023 at 17:07
• Supercapacitor is a different thing than a simple cell onlinelibrary.wiley.com/doi/10.1002/aenm.202003311 Nov 6, 2023 at 17:17
• 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! Nov 6, 2023 at 17:31
• @Mithoron I know that supercapacitors are different, but the book compared supercapacitors to batteries, where this particular statement was mentioned Nov 8, 2023 at 8:55
• @AChem By the way, what is 'OP'? I saw it in a couple of other places, too. Nov 8, 2023 at 9:11

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