Sciencemadness Discussion Board - Electrochemical cell math

Electrochemical cell math

Simoski - 24-2-2019 at 05:52

Hello

I'd love to understand the math of electrochemistry.
Maybe build us an online calculator or something like that.
Would someone please teach it to me or give me good resources.

I run 3 electrochemical cells, 2 potassium chlorate & 1 sodium chlorate... What measurements can I do on them with a multimeter and what math can I do on those measurements to get the current efficiency ? What Faraday laws do I need to understand to predict production in grams per hour?

8 )

Thank you in advance
Simoski

PS maybe asking what the relationship between the Faraday laws and electrochemical chlorate production is, would have been a better way of asking it.

[Edited on 24-2-2019 by Simoski]

Metacelsus - 24-2-2019 at 09:53

The net electrochemical equations for chlorate production:
At the anode:
3 H2O + Cl- + -> ClO3- + 6 +H + 6 e-
At the cathode:
6H+ + 6 e- -> 3 H2

At maximum efficiency, it takes six moles of electrons moving through the cell to produce one mole of chlorate. Faraday's constant is 96485 C/mol e- so this is equivalent to 578910 C per mole chlorate. Note that 1 C = 1 amp * 1 s

Of course, a real cell won't operate at maximum efficiency, so it will take somewhat more than this.

Simoski - 27-2-2019 at 09:19

Hi Metacelsus

Thanks for your reply.
How do I measure the voltage drop across the cell?
I mean do I measure with the power on or off?

Metacelsus - 27-2-2019 at 12:01

Measure the voltage with the power on. But you should also measure the current since that will tell you the rate of chlorate production.

Simoski - 27-2-2019 at 20:46

So with the supply on, I measure the voltage at the supply terminals.. then the voltage drop across the cell array (2 in series) is the supply voltage minus the measured voltage?

If I decouple the cell array from the supply and measure the resistance, can I then use ohms law to calculate the current? Or must I add a resistor of a know value in series and measure the voltage drop while the array is running?

Metacelsus - 27-2-2019 at 23:39

The best way to do it is with a small resistor (say, 0.1 ohm) in series. Measuring the cell resistance when it's off will be misleading since the current-voltage response will not be linear. Some multimeters also allow current measurements directly.

j_sum1 - 27-2-2019 at 23:45

Voltage supplied is largely dependent on your power supply. The resistance and hence current of a cell can vary during operation. For a feeble supply this can cause the voltage to fluctuate also but if you have a decent source you should be ok.
The alternative is to have a current-regulated power supply -- which is merely a way of adjusting the voltage to maintain a constant current.

Current is the main parameter for an electrochemical cell. It determines the rate that you make your product.

Yield = tI/(Fn)

yield is in moles
F = 96485
t is time in seconds
I is current in amps. (If you don't have a current regulated supply, you take regular measurements and figure out an average.)
n is the number of moles of electrons needed for each mole of product.

There are of course improvements you can do to make your actual yield close to the theoretical.
1. Keep current density low to preserve electrodes. This means large electrodes or long times.
2. Operate with as low as possible overpotential. This increases inefficiencies.
3. Reliable power supply. I love my current regulation.
4. Specific conditions that relate to your particular reaction. In the case of chlorates, from memory, 70°C and a touch of dichromate.

Simoski - 2-3-2019 at 13:47

Quote: Originally posted by j_sum1

Yield = tI/(Fn)

yield is in moles
F = 96485
t is time in seconds
I is current in amps. (If you don't have a current regulated supply, you take regular measurements and figure out an average.)
n is the number of moles of electrons needed for each mole of product.

Thanks J : )

This looks to be the basis of a calculator... right?

[Edited on 2-3-2019 by Simoski]

yobbo II - 5-3-2019 at 12:04

https://geocitieschloratesite.000webhostapp.com/chlorate/run...

top is https://geocitieschloratesite.000webhostapp.com/chlorate/chl...

Understand what is a mole and a mole of electricite and you done. Done obsess over voltage

woelen - 12-3-2019 at 13:42

I have written webpages about electrolysis and a powersupply:

http://woelen.homescience.net/science/chem/exps/electrolysis...
http://woelen.homescience.net/science/chem/misc/psu.html

These links may make things easier for you to understand.

The current through a cell is what matters. By keeping resistance low you can keep power efficiency high. Current efficiency is another thing than power efficiency. Current efficiency is the amount of current, which leads to the desired chemicals, relative to the total current. Current efficiency can be 100% while power efficiency is low (if all current leads to formation of the desired chemicals, but the voltage across the cells is high). A less than 100% current efficiency is due to side-reactions which lead to other products than the desired ones.

The following link covers current efficiency in a chlorate cell:

http://woelen.homescience.net/science/chem/exps/miniature_ch...

Simoski - 29-3-2019 at 21:50

Quote: Originally posted by woelen

I have written webpages about electrolysis and a powersupply:

http://woelen.homescience.net/science/chem/exps/electrolysis...
http://woelen.homescience.net/science/chem/misc/psu.html

These links may make things easier for you to understand.

The current through a cell is what matters. By keeping resistance low you can keep power efficiency high. Current efficiency is another thing than power efficiency. Current efficiency is the amount of current, which leads to the desired chemicals, relative to the total current. Current efficiency can be 100% while power efficiency is low (if all current leads to formation of the desired chemicals, but the voltage across the cells is high). A less than 100% current efficiency is due to side-reactions which lead to other products than the desired ones.

The following link covers current efficiency in a chlorate cell:

http://woelen.homescience.net/science/chem/exps/miniature_ch...

Thank you Woelen, Gentlemen!

Woelen I particularly enjoy your model of the chlorate cell as the following:

[Edited on 30-3-2019 by Simoski]

Simoski - 1-4-2019 at 06:48

Guys when I disconnect my chlorate cell from the power supply and measure the galvanic voltage ( around 3.5 volts ) is this galvanic voltage the EMF of the cell?

In the beginning of a chlorate cell run I'd imagine that this galvanic voltage is near zero. Just chloride in solution, only later when the cell has been running for a while and clo- , clo2- and clo3- ions have been created, will a galvanic voltage be present.

Any comments or thoughts?