underground
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Cell design
I have just bought an MMO anode for chlorates and Pt for perchlorates. The MMO is 3" x 10" and the Pt is 2" x3". How much the volume of the cell and
the current must be for minimum anode wear ?
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markx
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Quote: Originally posted by underground  | | I have just bought an MMO anode for chlorates and Pt for perchlorates. The MMO is 3" x 10" and the Pt is 2" x3". How much the volume of the cell and
the current must be for minimum anode wear ? |
Regarding anode wear usually less of a current density shall yield a longer life, but one may see a drop in current efficiency requiring longer run
times for the same amount of product. So it's a fickle balance at best.
I would say as a rule of thumb one can assume about 150-200mA/cm2 for a relatively safe operation when no specific specs are available for
the MMO on purchase. It is not the current that causes anode passivation on Ti substrates, but rather the polarisation of the anode. As in how much of
a charge deficit is caused at the anode due to electrons being drawn off from it by the power supply vs. how much can be replenished by the
electrochemical reactions occurring at the anode, yielding a net potential difference. This is not only influenced by current density, but also by the
voltage of the power supply, the concentration of the electrolyte regarding specific ions and the temperature of the cell just to name a few. The
first two parameters determine how much charge is drawn off and the rest determine how much can be replenished. The higher the concentration of
electrolyte, the more probable it is to find a suitable ion to take part in the reaction at the anode and give off charge for replenishing the ones
that are drawn off by power supply. The higher the electrolyte temperature, the faster ions migrate to anode and again replenish the charge deficit
efficiently.
If these conditions are not met the anode charge deficit shall become large and thus it polarises to a higher potential which makes it possible for
the "inert" components of the anode itself to give off charge and participate in electrochemical processes, forming oxides that passivate the surface
or dissolve into electrolyte as bulk.
Reagarding Pt wear, it is not a simple process and there are very numerous factors involved. As a rule of thumb again one can assume about
150-200mA/cm2 if not stated otherwise, but to reach a minimum wear rate in chlorate/perchlorate systems is not a simple task. It requires
monitoring and keeping a certain ratio between the products, so a lot of work, not just a "fire and forget" type of process. But taking your time,
keeping the current density in safe limits and not aiming for full conversion are probably the best approaches one can take in an amateur setting to
get the most life out of a Pt anode. Also I've found it is not required to raise the cell voltage above 5V for perchlorate conversion. It will run
nicely at low voltages in the range of 4,2-4,5V, even though the literature mostly states 5,5-6,5V as the usual range. It might be viable in case of
solid Pt anodes, but a Ti substrate one shall be subject to increased risk of passivation at such high cell voltages. Especially at the end of
conversion when the chlorate content has dropped and thus the anode polarises more strongly towards the potentials that enable titanium to form a
nonconductiong oxide barrier.
Exact science is a figment of imagination.......
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underground
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Mark thanks a lot for your reply. So the 3"x10" MMO anode how much current do i have to supply for 200mA/cm2 ?
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markx
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If my calculation is within reason, then 75-80A would amount to this particular current density for a 30in2 anode, assuming both sides of
the anode are evenly active. This means cathodes facing towards both sides of the anode or surrounding it in an even manner. If a single cathode is
facing only one side of anode then the other side shall see a much lower current density. Less so if your anode is the mesh type.
But for simplicity sake I would suggest to stay around 30-35A of current. It will be sufficent to produce more chlorate than you can reasonably use.
And one can run a conveniently sized cell (5-7l capacity) without external cooling. Also the power supply, current leads, connectors and electrode
stems do not have to be monstrously huge to avoid overheating and burning of connections.
Exact science is a figment of imagination.......
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underground
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I go t a modified computer power supply that it says that produce 20A so i will stay there. Also i saw from NurdRage that he says that we actually
need a small cathode for small surface area to avoid the side reaction of chlo3 with H, reducing efficiency.
[Edited on 9-5-2019 by underground]
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