[scientizscht (OP)]

Hello

What apparatus do you need to generate electricity from two solutions of different pH?

Also, how can you calculate the energy produced?

I have not been able to figure out.

Thanks!

[Bored chemist]

Same as last time
https://www.thenakedscientists.com/forum/index.php?topic=77645.msg581986#msg581986

https://en.wikipedia.org/wiki/Standard_hydrogen_electrode
https://en.wikipedia.org/wiki/Salt_bridge

[scientizscht (OP)]

So you still need electrodes and a semipermeable membrane.

Won't the electrodes degrade with time?

[Bored chemist]

The electrodes in a typical hydrogen electrode are platinum- it's not going anywhere.
However the electrolyte will change with time.

What are you hoping to achieve?

[scientizscht (OP)]

What do you mean the electrolyte will change with time?

Also what is the equation to calculate the power of such battery?

[Bored chemist]

At the electrode there are chemical changes.
So the electrolyte will change
The calculations for the voltage are in the thread I cited earlier
The power is much more difficult to calculate.
It's going to be small, for any practical electrode size.

[scientizscht (OP)]

At the electrode there are chemical changes.
So the electrolyte will change
The calculations for the voltage are in the thread I cited earlier
The power is much more difficult to calculate.
It's going to be small, for any practical electrode size.

Why is it going to be small? Don't pH difference batteries exist?
Which factors affect the power or capacity of a two pH difference solutions and in which mathematical form they are expressed?

[chiralSPO]

If I have a single electrochemical cell, with one chamber at pH 0, and one at pH 14 the maximum voltage I could get is about 0.82 V (59 mV per pH unit at standard temp and pressure).

Subtract out the overpotentials (inefficiency at the electrode) for both reactions--let's say the electrodes are very efficient, and only waste about 200 mV each--that brings the external voltage of cell to a mere 0.42 V.

Of course, one can stack cells in series to increase the voltage, and/or in parallel to increase the available current. In this way, one can use an arbitrarily large number of arbitrarily weak cells to construct an arbitrarily large battery.

[scientizscht (OP)]

If I have a single electrochemical cell, with one chamber at pH 0, and one at pH 14 the maximum voltage I could get is about 0.82 V (59 mV per pH unit at standard temp and pressure).

Subtract out the overpotentials (inefficiency at the electrode) for both reactions--let's say the electrodes are very efficient, and only waste about 200 mV each--that brings the external voltage of cell to a mere 0.42 V.

Of course, one can stack cells in series to increase the voltage, and/or in parallel to increase the available current. In this way, one can use an arbitrarily large number of arbitrarily weak cells to construct an arbitrarily large battery.

So how do you multiply it? Do you use bigger electrode surface? Larger amount of solutions?

[Bored chemist]

Why is it going to be small?

For the same reason as last time.

So how do you multiply it?

Connect the cells in series.


You also need to consider the current.
https://en.wikipedia.org/wiki/Exchange_current_density
About  a miliamp per cm^2

I can buy sheet sof Pt foil.
A couple of the largest plates (10 by 35 cm)  they sell here
https://www.cooksongold.com/Sheet/Gw-Platinum-Sheet-0.55mm-prcode-CXA-055&add=n
will cost you about £30000
and will deliver (at best) about 350 mA at about 0.04V

So, that's about 2.1 million pounds to produce each Watt of power.

Don't pH difference batteries exist?

What do you think?

[scientizscht (OP)]

It may not be economically sensible but I want to understand the logic.

So the 10-3A/cm2 is for any pH or how does it change with different pH?
Also, if you connect electrodes in series, you multiply the voltage and the current still stays the same at 10-3A/cm2 or it will be smaller because the surface area is reduced?

[Bored chemist]

I want to understand the logic.

I'm willing to bet that you didn't follow the links  from the page I cited here, did you?

https://en.wikipedia.org/wiki/Exchange_current_density

[scientizscht (OP)]

It's not clear from that link.

[Bored chemist]

It's not clear from that link.

I didn't say it was.

the links  from the page

[scientizscht (OP)]

OK when I am asking a question I normally expect an answer from someone who knows instead of a link to a 1,000 page book.

Is the Io intrinsic exchange current the current you get or the actual I is lower or higher?

[Bored chemist]

If the answer takes 1000 pages, do you expect

someone who knows

to post it all here?
Wouldn't it make more sense to tell you about the book?

the current still stays the same at 10-3A/cm2

10^-3 A/cm²  isn't a current.
So your question makes no sense.
It's a current density (you can tell from the units) .
You haven't said anything about what the area is except that "

the surface area is reduced?

but you haven't even said why it's reduced, never mind how, or by how much.

This is why answering your questions takes a thousand pages.
You need to learn to ask better questions.

[chiralSPO]

Also, if you connect electrodes in series, you multiply the voltage and the current still stays the same at 10-3A/cm2 or it will be smaller because the surface area is reduced?

To a first approximation, you can add currents or voltages of electrochemical cells by connecting them in parallel or series (respectively). There is no multiplication (other than repeated addition of the cells).

To illustrate, imagine we have a cell that is capable of supplying 1 A of current at 1 V for 1 h. We can represent this with a ">". If we have six cells, we can connect them like:

>>>>>>

to get a battery capable of supplying 1 A of . current at 1 V for 1 h, or like this:

>
>
>
>
>
>

to supply 6 A at 1 V for 1 h.

or

>>>
>>>

for 2 A at 3 V for 1 h.

Or we could use each cell by itself for one hour, and have 6 h of 1 A at 1 V....

Now, I said that this is a first approximation. Electrochemical cells have internal resistance, so adding them up in series also decreases the efficiency of the whole system.

[scientizscht (OP)]

Thanks, that's helpful.

But I think there is a catch. Since the most expensive bit is the electrodes, you will not be able to produce more energy/power per cm2 of electrode whatever the array you choose, is that true? Or is there any other way?

I am puzzled by few other things:

1) Is the exchange current density, the current density that you actually get or what you get is higher or lower? There is an "electrode current density" and an "exchange current density". The Butler-Volmer equation seems to connect these two. But it is not clear if, in most cases (e.g. no extreme temperatures, concentrations etc), the electrode current density is higher or lower than the exchange current density. In other words, is the exchange current density the best you can get and in most occasions you get something less or is it the lowest you can get and in most occasions you get something better?

2) So you put one electrode in a bottle of 1M HCl and another electrode in a bottle of 0.1M HCl. In each of the bottles, I suppose the protons will react to form H2. So how exactly are you able to generate electricity out of it? My understanding so far is that the electrons of the electrode will be used to convert 2H+ to H2. So I assume that both electrodes will be positively charged at a different magnitude. If we connect them with a wire, due to the different potential, the electrons will flow from one electrode to the other. Is this true?

3) At the same time, we need a proton membrane that will allow the flow of protons in the opposite direction than the flow of electrons. Why is this needed and how do we actually connect wires to extract energy from that?

4) Is it possible to generate energy from the concentration gradient without involving any reactions but purely from the flow of protons from one bottle to the other? Without the need of electrodes to perform redox reactions? Or it's not possible?

Thanks!

[chiralSPO]

1) This will be best to look up.

2) There are many ways to do this, but essentially you will want an electrochemical half reaction that releases (or consumes) protons. Couple this half reaction with its reverse reaction, and if they are in the same pH, then there won't be any electromotive force. But if the side that is consuming protons is more acidic than the one producing them, then you can take advantage of this.

3) If you put a proton exchange membrane between solutions of different pH, protons will just move from the more acidic side to the less acidic, until they are the same--with no useful energy extracted. Better to have an anion exchange membrane and let the chloride do the walking.

4) Yes, there are ways of capturing the energy of the protons moving across the gradient without actually doing electrochemical reactions. But these are not very useful from an energy density standpoint.

[scientizscht (OP)]

2) There are many ways to do this, but essentially you will want an electrochemical half reaction that releases (or consumes) protons. Couple this half reaction with its reverse reaction, and if they are in the same pH, then there won't be any electromotive force. But if the side that is consuming protons is more acidic than the one producing them, then you can take advantage of this.

So you won't have the same platinum electrode in both of the half cells? In the more acidic, you will have a platinum electrode to consume electrons and in the other half cell you will have another electrode to do another reaction? What other electrode and what other reaction would that be? And how would I calculate the voltage and current density? I calculated the voltage for two solutions of different pH but if another reaction is involved, not sure how I can calculate it to find the power.

3) If you put a proton exchange membrane between solutions of different pH, protons will just move from the more acidic side to the less acidic, until they are the same--with no useful energy extracted. Better to have an anion exchange membrane and let the chloride do the walking.

What would be the purpose of having the chrolide moving across the membrane? How would that enable to generation of electricity?

4) Yes, there are ways of capturing the energy of the protons moving across the gradient without actually doing electrochemical reactions. But these are not very useful from an energy density standpoint.

Do you have any names for these technologies just to have a further look?

Thanks!