RogueRose
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How can I regulate current density in electrolysis cell? (chlorate cell)
I've been hung up on how to control the current density of a chlorate cell. For example if I'm using a 5v power supply that is capable of 150A (a
750VA transformer full wave rectified w/ smoothing cap - output 5.18v).
For normal applications the circuit will draw the current it needs, but in an electrolysis cell, IDK how this would work. Some of the anodes I have
will work over a range of 30ma to 300ma / cm^2 (ideal range 80-300ma).
I was going to attempt a PbO2 anode and SS cathode at first, I do have some strips of graphite that could work (5" x 1" x 3/8") and will probably
order some Ti and MMO eventually.
So what I'm confused about is if I have an anode that is 3" x 6" = 116cm^2 which at 300ma max, the max current should be 34.8A and minimum of 9.3A (@
80ma). I would think that if I connected the transformer directly to the plates I would end up with 1293ma / cm^2.
Is there a way to limit the current without using a different PSU or adding more anode material to reach the appropriate current density?
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RedDwarf
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If you have the room to do it in your cell, you can increase the distance between the electrodes (which increase the cell resistance and therefore
reduces the cell current), otherwise you could make yourself a "simple" current limiting circuit (simple means you'd end up having to dissipate some
of the power as heat outside the cell).
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Simoski
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agreed move the cathode away from the anode.
Measure the current across the cell while moving them apart until you get the desired density.
Just because your supply can deliver 150 amps does not mean your cell will draw 150, just get the spacing right.
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markx
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Quote: Originally posted by RogueRose  | I've been hung up on how to control the current density of a chlorate cell. For example if I'm using a 5v power supply that is capable of 150A (a
750VA transformer full wave rectified w/ smoothing cap - output 5.18v).
For normal applications the circuit will draw the current it needs, but in an electrolysis cell, IDK how this would work. Some of the anodes I have
will work over a range of 30ma to 300ma / cm^2 (ideal range 80-300ma).
I was going to attempt a PbO2 anode and SS cathode at first, I do have some strips of graphite that could work (5" x 1" x 3/8") and will probably
order some Ti and MMO eventually.
So what I'm confused about is if I have an anode that is 3" x 6" = 116cm^2 which at 300ma max, the max current should be 34.8A and minimum of 9.3A (@
80ma). I would think that if I connected the transformer directly to the plates I would end up with 1293ma / cm^2.
Is there a way to limit the current without using a different PSU or adding more anode material to reach the appropriate current density?
|
To regulate current through the cell, you have to regulate the applied voltage across it. That is the most "humane" way to do it. Of course you can
also achive the same by regulating the effective resistance of the cell setup: move electrodes further apart, limit area of cathodes to the point
where they start restricting the net current, lower temperature, lower electrolyte concentration, additional resistor elements in series to the cell,
etc. But all of this is a lot of hassle....and requires mechanical intervention.....and usually generates waste in the form of heat.
I assume you are running a line frequency downstep transformer that is followed by a rectifier bridge and a cap bank as the power supply? Is so then
a"variac" before the main downstep transformer be the simplest solution for your current regulation problems.
Or you can fiddle with the dc end of the circuit and try to regulate the voltage there.
Or you can acquire a 5V 70A SMPS with variable output voltage for about 30$ and forget about the heavy iron.
Exact science is a figment of imagination.......
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phlogiston
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You can think of the cell electrically as a diode in series with a resistor.
The 'diode drop' is a fixed voltage drop that is related to the reactions taking place at the electrodes. There is a certain minimum activation energy
required to start a reaction. This voltage drop is related to that activation energy.
The (ohmic) resistance of the cell is related to the conductivity of the electrolyte. It is affected by the concentration of ions, the temperature,
the contact surface with the liquid, etc.
So, If you apply a slowly incrementing voltage across the cell, starting at 0V, you will info very little current flow at first. After you exceed the
'diode drop' voltage, you will begin to see current flow, which increases more or less linearly as you increase the voltage.
To run your cell at the desired current, it would be ideal to use a fixed-current power supply (which will regulate its output voltage to achieve the
set current).
If you only have a fixed 5V power supply, you can adjust the current by modifying the cell resistance: distance between electrodes, surface area, salt
concentration, temperature etc. (Electrode distance is the most convenient)
-----
"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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markx
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| Quote: Originally posted by phlogiston |
To run your cell at the desired current, it would be ideal to use a fixed-current power supply (which will regulate its output voltage to achieve the
set current).
If you only have a fixed 5V power supply, you can adjust the current by modifying the cell resistance: distance between electrodes, surface area, salt
concentration, temperature etc. (Electrode distance is the most convenient) |
I once also thought that a constant current power supply would be a good idea....well, it depends on how the power supply regulates the current. It
can have rather disastrous concequences under certain unfavorable conditions. If one uses a power supply that can up and downconvert the voltage to
steer the current and something goes wrong in the cell, current starts dropping, the power supply raises the voltage to compensate for the rising
resistance that it encounters. Aaaaand......in concequence the bad connection overheats and burns, the passivating anode will meet a swift end to the
point of even the Ti skeleton dissolving and the cell melting or breaking. Right before it boils dry and sets itself on fire.
Thats exactly what happened to my small perchlorate cell that I attempted to run on a constant current power supply. It was housed in a 50ml plastic
sample vial and suffered a gradual passivation of the anode during the process. As a consequence of the constant current mode the whole thing
overheated.....literally dissolved the Ti/Pt anode....boiled dry.....and melted into a blob. So it can all go to hell when the conditions turn out
right....
Exact science is a figment of imagination.......
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