gravityzero
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Electrochemical Cells - Understanding Amperage or Current
So I've ran across a number of different electrolysis projects that I would like to try.
One concept that has me baffled is the different amounts of amperage called for.
I have one that calls for 6 Volts @ 6 Amps
I have another that calls for 24 Volts @ 50 Amps
My question is how does one regulate Amps in an Electrochemical Cell?
I thought Amperage was determined by the resistance of the materials involved.
One project uses a Diaphragm Cell. Can I just make the Diaphragm more resistant?
Maybe insert some type of resisting shunt leading to one of the electrodes?
If anyone knows what is necessary, please let me know.
I've looked around the net, but don't have any solid conclusions.
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Vomaturge
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You are right that amperage is regulated by the resistance of the materials involved. I think that for electrochemistry, you might also need a minimum
voltage to add potential energy to the chemicals. Then, the current would be equal to (volts of supply - volts needed to overcome chemical energy
barrier)/resistance of the cell. Now, how to change the current?
I would suggest designing the cell with the electrodes closer together or further apart, in order to reduce or increase its resistance. You could also
change the supply voltage, or connect another resistance (like a heating element or light bulb) in series. This will reduce the voltage that actually
makes it to the cell, but that is probably OK. I could be wrong, but I think that most electrolysis reactions work just fine with this kind of
control. A resistor used in this manner will not reduce the voltage unless some current is already flowing, so there will always be sufficient voltage
to cause some electrolysis. I would assume that those voltage and current recommendations are based on a voltage slightly higher than that needed to
complete the reactions. You can reduce them slightly if that's what it takes to get the current right. Regarding making a resistant diaphragm, I
suppose a thicker or less porous one should not let ions pass as quickly and readily.
I'm just curious, what needs 24 volts to electrolyze? Is this for a battery of electrochemical cells in series, or is it just one really tightly bound
compound that you're separating? Or did the designers of the cell just give it too high of a resistance for the current they wanted to pass, and are
using excess volts to force the current through?
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phlogiston
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The resistance of the cell is a function of the distance between the electrodes and the conductivity of the electrolyte (which mainly determined by
the concentration of ions and the temperature).
As Vomaturge said, indeed you need a minimum voltage for a reaction to occur. If you imagine a V/I curve of an electrolytical cell, starting at V=0,
I=0: as you slowly raise V, different reactions will "kick in" as you pass the minimum voltage for each reaction, each time giving a little step up in
the current. In between the steps, the current increases linearly with voltage, due to the ohmic resistance of the cell.
I'm sorry I don't have a good drawing. Googling 'amperometry' and 'voltametry' will probably yield a few illustrative graphs
In a practical sense, you can regulate the current thought the cell to a desired value by adjusting the resistance of the cell (most conventiently by
adjusting the spacing between the electrodes) or electronically, by means of a current limiting circuit.
[Edited on 29-3-2018 by phlogiston]
-----
"If a rocket goes up, who cares where it comes down, that's not my concern said Wernher von Braun" - Tom Lehrer |
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woelen
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Here you have some theoretical background and an experiment on the characteristics of an electrolysis cell:
http://woelen.homescience.net/science/chem/exps/electrolysis...
The redox potential of the combined anode and cathode reactions must be subtracted from the power supply voltage. Then there is a so-called
overpotential needed to release gas at the electrodes and finally, there is some perceived (approximately) ohmic resistance, depending on the nature
and concetration of the electrolyte, and also depending on geometry (distance between electrodes and size of electrodes).
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Fulmen
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The correct way to regulate the current is by changing electrode area. Increasing the resistance will only increase losses and temperature. Sometimes
the bath should run at elevated temperatures, in that case one can tune the resistance to provide the necessary heating. But usually the PSU is the
limiting factor in output and far more costly than a small heater.
We're not banging rocks together here. We know how to put a man back together.
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Sulaiman
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gravityzero
Reading (and understanding) woelen's page is a good start.
I think that you should give links to the documents that you are using because
24 V is a ridiculously high voltage for electrolysis - unless to keep salts molten.
Making the membrane highly resistive could be a disaster,
e.g. 50A x (24V - electrode potentials) is about 1kW dissipation in the membrane.
For efficiency it is normal to keep cell voltage as low as practical
and use external methods to control current (current density at electrodes).
The 'simplest' method is to have a variable voltage d.c. supply,
or pwm can be used if 'filtered' with inductance and capacitance.
If the d.c. voltage is fixed (e.g. battery or transformer-rectifiers-capacitors) then external resistance can be added in the form of a resistor,
filament lamp etc.
With a fixed d.c. supply the current could be controlled by adjusting geometry (larger/smaller area/spacing of electrodes) or electrolyte
conductivity,
but all of these options result in heating of the cell.
P.S. have you checked the cost of a 24V 50A power supply ?
(drat! - Fulmen types faster than me) - (or maybe more concisely)
[Edited on 29-3-2018 by Sulaiman]
CAUTION : Hobby Chemist, not Professional or even Amateur
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yobbo II
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Perhaps a repeat of what was said above but my 2 cents worth:
The whole concept of the 'voltage that it takes to make the reaction(s) occur' is coming from the pages of a book which is explaining thermodynamic
theory.
A third electrode (called a reference electrode, namely Normal Hydrogen Electrode (NHE), also Saturated Calomile Electrode (SCE) and others) is used
in an attempt to measure this (thermodynamic / theoretical) voltage in the real world and it gets close to measuring this actual voltage (that comes
from the page of a book) so long as you do not start forcing in too much current into the system. The voltage that this third electrode measures is
(just about always) higher that the book reaction (thermodynamic, theoretical) voltage due to what is described as 'over-voltage'. This third
electrode measures only the voltage associated with one of the electrodes in the cell BTW as you will connect your voltmeter either between the anode
and third electrode or the cathode and third electrode. The over-voltage is best thought as 'actual minimum voltage required in practice' to get
things happening and is related to kinetics around the electrodes and solution
interface. (One electrode at a time remember). This over-voltage or overpotential is made up of various components which we will not go into here
(look it up in Wiki if you like) and it varies with temperature, electrode (anode or cathode) current density, anode or cathode material and lots of
other things.
The voltage across the cell (now throwing the third electrode out of the cell and measuring voltage from anode terminal to cathode terminal) will
show the cell voltage, which will be the 'book' voltages + overvoltages + resistance of the electrolyte between the anode and cathode and also
connection voltage drops and (if thin) wire voltage drops + actual voltage drops in the electrodes if they are very thin or poor conductors.
The idea of FIXED voltage to 'get the reactions to happen' some from the pages of a book. If of course you do not apply this minimum (book /
thermodynamic) voltage + some over-voltage ( kinetics) you will not get any current to flow in the first place through the cell! In order to get a
sensible working current to flow you will need to up the voltage a bit more to overcome electrolyte resistance + connection resistances + wire
resistances.
When talking about what a cell needs to run in practice it is best to think in terms of the current into the cell and the area of the electrodes
(current density on electrodes).
Don't obsess over the voltage. You will of course need a certain such-and-such voltage to force in the current you want. The current which you want
will depend on surface area of electrodes. (the cell volume need to be a sensible size as well).
Stating that you need (say) 6 volts to make this or that happen is the worst possible way to describe things but it is always the figure that is
quoted unfortunately.
It best to say I ran at X (chosen) current density and the voltage that happened to be needed to do that was Y volts.
Whey someone states 'I use 20 amps at 50 volts' they may be simple quoting the power supply capability. They may have used a max. 20 amps and max. 50
volt power supply. God only knows what the actual current and voltage was. Someone may state that they ran their cell at 7 volts. The reader then
comes along a imagines that there is something very important regarding
the SEVEN VOLTS because that's what the cell was run at. YOU NEED SEVEN VOLTS. 8 volts is too high, 6 volts is too low. This is wrong. The reader then
goes out of there way to get
SEVEN volts to show across the cell by doing this, that and the other. This is not the correct way to look at things.
Get into constant current supplies. Choose a sensible current (a current density since you know what electrodes you have or will have). Put that
current through the cell and you can note the voltage (X VOLTS) if you like (just for interest sake). When telling someone the story don't just quote
the voltage and say 'you need X VOLTS to do this'. More or less will do OK. You will have them trying to 'adjust' the supply to 'achieve' exactly X
VOLTS across the cell because 'that's what you said was needed'. Better to quote current and electrode size (and perhaps voltage if you like!)
2 cents worth..........
[Edited on 29-3-2018 by yobbo II]
Attachment: CHEG320_electrochemistry lectures.pdf (1.1MB)
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gravityzero
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Thanks so much for all the helpful advice everyone! I do appreciate this community and the willingness to assist others.
I will try to find more time, in the future, to contribute what I can to help others too.
If I get some time this weekend, I will upload the patents I found regarding the different electrolysis procedures stated earlier.
Hope everyone has an enjoyable weekend.
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RawWork
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I was like you few years ago. Voltage sounded simple, but current extremely complicated. I did not know where is source of that current, how to apply
it. All adapters/chargers have two values listed: voltage and max current. Most are 5 V, 500 mA. Sometimes you can go over this current but you risk
explosion, damage, burn, death. Some have fuses inside to prevent damage, but most don't. Later I realized that current depends on resistance and
voltage. More practically on max possible current and resistance. Because voltage is constant in most cases...although even that is only relative
definition or even just opinion actually.
Today, i feel voltage is more complicated and current is simple. Why is that? Because in galvanic cells / batteries, voltage is dynamic over time, it
becomes lower the more discharged / used battery is. That may cause problem with calculations. While current won't really cause problem with
calculations, we usually calculate Q=I*t. And use that together with reduction potential aka voltage to calculate energies of galvanic cells.
My advice is avoid perfection. Calculate less, learn to control everything practically. It's enough to know when current is larger when is smaller.
And you know how to set minimum, how to set maximum... Beware of maximum, of course, as most devices will explode or require new fuse.
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yobbo II
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Most power supplies you come accros are fixed voltage.
Cells that you may play around with can be divided into two types (more or less). The type you pump power into (roughly described as electrolytic
cells) and types that power comes out of 'batteries', set them have what is called solid stuff in them or liquid. (The solid stuff is usually damp
with some liquid so they are really all liquid.)
The 'battery' types are usually described as fixed voltage and they mostly put out a fixed voltage when used properly.
The other types, we usually use them to make something are best looked upon as needing a certain current range.
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