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woelen
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Passivation of MMO anode?
A friend of mine just purchased an MMO anode with titanium substrate off eBay and used it to make KClO3. He used diet salt (a mix of 1/3 part NaCl and
2/3 part KCl by weight). He had a run from a 5V power supply and let this cell run until current had dropped strongly and as a consequence the cell
cooled down and solid KClO3 crystallized. He used no additives in the solution and used a stainless steel cathode. After a few days of electrolysis he
had a lot of solid KClO3 which easily could be separated.
He disassembled the cell, cleaned the electrodes and worked up the KClO3. Now he has purchased pure KCl and wants another run, but to our horror, the
anode does not work anymore! When a concentrated solution of KCl is used, then only 0.2 A is running through the cell. Different cathodes were used,
but always the same result, just around 0.2 A of current. He tested the power suppy and the cathode and these are just fine. It really has to to with
the MMO anode.
It seems as if the anode has passivated. The anode was thoroughly cleaned and kept in clean water for some time, hoping that it would remove some
invisible layer, but this also does not work.
What is causing this problem and what can be done about it? I do not have real knowledge about this subject and could not find good info on internet.
Does anyone else have a similar experience or know of this?
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Jurre10
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Hi,
I'm the one Woelen is talking about.
He exactly writed how it went.
So we think there is an (invisable) layer on the anode,
but we both don't know how to remove it. 
Hopefully, somebody who know this problem can give us a good answer.
Jurian
[Edited on 9-1-2011 by Jurre10]
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hissingnoise
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I haven't used MMO anodes so I'm just guessing . . .
I assume you've checked and double checked the connections.
Does the Losalt possibly contain additives that could be deposited onto the anode as a polymeric insulating material which could be removed by a
solvent other than water?
A good microscope might reveal something . . .
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Jurre10
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Hi,
Thanks for your tips, but LoSalt is just 66%KCl and 33%NaCl
So i think it can't be something from, the LoSalt.
Maybe it's a bit of the glue i used to glue the anode/cathode in place?
I don't think so because i used Epoxy glue that easely holds at 100°C.
The pont is that i don't have a microscope.
There is a little bit of corrosion on the lug, but i think that won't make the connection so bad that there is just 0.23A.
Jurian
If you try your best, but you don't succeed.
Experience is what you need.
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hissingnoise
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Hmmm . . .
LoSalt - from wiki; "An iodised version is also available in the Nederlands and Middle Eastern markets".
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Jurre10
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hi,
I know, but thats a different color package and there's a little text on it hat says : iodised
So i don't have the iodised version, just naturel.
I have threats of this on 3 different forums but nobody knows whats happening?
So i'm afraid i have tobuy a new one
Jurian
If you try your best, but you don't succeed.
Experience is what you need.
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hissingnoise
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| Quote: | | So i'm afraid i have tobuy a new one |
Or save some pennies for Pt foil . . .
But don't discard your old one - keep it for further tests.
Solving the puzzle might be advantageous at some point!
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Jurre10
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Quote: Originally posted by hissingnoise  | | Quote: | | So i'm afraid i have tobuy a new one |
Or save some pennies for Pt foil . . .
But don't discard your old one - keep it for further tests.
Solving the puzzle might be advantageous at some point!
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hi,
I know, hopefully i will find it out.
but when i buy Pt, im also able to make KClO4
i just hope my mmo anode will come back to life 
Because it waas not that cheap 
Jurian
If you try your best, but you don't succeed.
Experience is what you need.
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woelen
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Testing the connection can be done easily. Just put a clamp on the titanium strip of the anode and put another clamp at an other position of the strip
and connect these through a decent 5 V lamp (e.g. lamp from a bicycle, which takes at least 500 mA). But I do not believe this is the issue, I really
think this is due to formation of some ultrathin layer, which passivates the anode. And no, having an optical microscope at hand will not help here.
Usually such layers are just nm sized and they can only be observed using extremely powerful (and expensive) microscopes, such as electron
microscopes.
| Quote: | I have threats of this on 3 different forums but nobody knows whats happening?
So i'm afraid i have tobuy a new one |
Just wait for more input from other members, this thread is just a few hours old.
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hissingnoise
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Yeah, people like dann2, or Swede, although I haven't seen posts from them for a while.
A bit of experiment seems in order though - like reversing polarity for a few minutes, or increasing voltage, current density, or both.
Dipping the anode in acid or basic solutions (H2SO4/HNO3/NH4OH) for short periods.
Carefully buffing a small area of the anode with a soft wheel and then checking the treated area for increased gas evolution.
And if all else fails, high quality graphite rods give very acceptable chlorate production rates with NaCl . . .
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metalresearcher
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Recently I tested a Ti anode which passivated shortly. By scouring with fine sandpaper I removed the passivation layer.
But I don't use it anymore for my chlorate cell. Last week I also ordered an MMO anode via ebay (probably the same as @woelen) as graphite anodes
corrode and I purchased probably the same LoSalt as @woelen from a large supermarket chain called Albert Heijn..
I don't know whether the scouring of the MMO anode is a good option as it might destroy the MMO layer.
I know that LoSalt contains very small amounts of MgCO3 and KI (iodated) but that should not disturb the cell.
@woelen: did you try a carbon rod just for test whether now it really draws 5 A ?
Now I am warned what to do when I receive my MMO anode as I have the cell ready.
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Jurre10
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Hi,
@hissingnoise : @Woelen told me Swede is verry ill 
@metalresearcher so you just removed the layer with sandpaper sounds good!
But isn't it dangerous? i think you may damage de MMO layer?
I don't have graphite rods right now so i can't try them.
Jurian
b.t.w The LoSalt on your photo is the iodised version....
If you try your best, but you don't succeed.
Experience is what you need.
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hissingnoise
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I'm sorry to hear about Swede, woelen; I hope it's not too serious and that he recovers fully soon.
I had missed his very informative posts for a while now . . .
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Jurre10
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I'm sorry to tell you
I just heard it from Woelen.
I gonna ty things like higher voltage and reversing polarity for yust few secs.
Jurian
If you try your best, but you don't succeed.
Experience is what you need.
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hissingnoise
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I'd use NaCl rather than KCl for chlorate cells even with MMOs - NaClO3's high solubility make it less convenient but it seems somewhat less corrosive
than KClO3 (possibly because of Na's smaller atomic radius).
- Jurian, I'm wondering if having two chlorides with widely differing solubilities in the cell could have had some kind of effect on the surface
oxides.
But, perhaps dann2 will chime in with a different perspective . . .
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woelen
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Yes, Swede seems to have health issues (at least that is what I know from my last contact several weeks ago). He also is very busy with his work.
Whatever is the situation at the moment, let's hope he will be back soon in good health with his invaluable input and knowledge.
Treating the anode with sandpaper is really a last resort action only to be applied when everything else fails. The MMO layer is very thin and very
little abrasive action is needed to remove that layer completely. If this is done, then a very gentle abrasive action should be used (e.g. ultrafine
CeO2 or SnO2 may be used, applied by gently rubbing with a wet piece of fabric which has some of the powder on it).
The other suggestions can be used, such as increasing the voltage. But only do that for a brief amount of time. High voltage may actually increase
passivation and more and more voltage may be needed for operating the cell.
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metalresearcher
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What about using nonpolar solvents like white spirit or acetone ?
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Jurre10
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oke guys,
Something really strange happened!
I first applied 5v and there was a little 0.15A trough the cell.
I had two things i wanted to test :
- 12V instead of 5V
- Reverse Polairity
I wanted to start, by applying 12V for 5 sec, so i did but what happened?!
The stainless steel cathode begon bubbling verry hard! Also the anode wass bubbling, but the bubbles where much smaller.
I looked to my multimeter en gues what! : 7,53A!!!
Then i stopped the cell picked up some 5V wires... just 0.13A.
So is it the anode?I don't think so!
Jurian
b.t.w i did test the power supply, it gives the right voltage,but not amperage?!
[Edited on 9-1-2011 by Jurre10]
If you try your best, but you don't succeed.
Experience is what you need.
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woelen
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It is the anode, but its overpotential has increased. Your power supply is perfectly fine, also its 5 V part.
If you look at the operation of an electrolysis cell, then you can understand what happens. The redox reactions occurring in a cell needs some voltage
(around 2 volts total for both the anode and cathode reactions). The product of current and this redox voltage is the amount of energy, which is
converted to chemical energy (new more energetic compounds).
So, in theory, there would be a reaction if the applied voltage is just above the redox potential. In a real cell, however, there is more than that.
Each of the electrodes needs takes some more voltage. This voltage is the so-called overpotential needed for the reaction to occur. It depends on the
material of the electrodes, it depends on how the bubbles of gas are formed and on the concentration of the electrolyte. Finally, there is someohmic
resistance of the cell. This mainly depends on the size of the ions in solution and on the geometry of the cell.
Now let's have the following simple example:
2 V for redox potential
1 V for overpotential at anode
1 V for overpotential at cathode
ohmic resistance is 0.5 ohms
When 5 volts is applied to such a cell, then 5 - 2 - 1 - 1 = 1 volt remains for the ohmic resistance. With 0.5 ohms, a current of 2 A will flow
through the cell.
Now assume that due to some layer on the anode, the anode overpotential rises to 1.9 volts. In that case, only 5 - 2 - 1.9 - 1 = 0.1 volts remain.
With a 0.5 ohm resistance only 200 mA can run through the cell. If now you apply 12 volts, then 12 - 2 - 1.9 - 1 = 7.1 volts remain. With that current
you have 14 A through the cell.
So, what I assume here is that indeed the overpotential on the anode has increased.
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Jurre10
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Hi,
So we are back to where we begun there's a layer on the anode.
But i dont know what kind of layer and don't know how to remove it
I gonna try to reverse polarity for short times and else..:
| Quote: |
Dipping the anode in acid or basic solutions (H2SO4/HNO3/NH4OH) for short periods. |
Jurian
[Edited on 9-1-2011 by Jurre10]
If you try your best, but you don't succeed.
Experience is what you need.
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metalresearcher
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This layer is MMO, deliberately deposited on the anode before you purchased it I think.
I think @woelen is right. Applying more voltage will help. Moreover to get more chlorate ions ClO3- a higher bath temperature is better. So the
'waste' energy as heat is not actually wasted, but increases ClO3- formation.
And running the first hour on 5 volts changes the composition of the electrolyte (e.g. forming ClO3-) so continuing would require a highte voltage.
These are just my thoughts, I do not (yet) have esxperience with MMO anodes only with carbon rods in achlorate cell.
And did you add some K2Cr2O7 to improve efficiency ? That might also have some (adverse?) influence on the current through the cell.
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Jurre10
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Hi,
So i can run my cell on 12V? or not?
I do have some Potassium Dichromate, that Woelen give me.
But i didn't use it yet.
Jurian
If you try your best, but you don't succeed.
Experience is what you need.
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woelen
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No, don't run it at 12 V. You will have really excessive production of heat and strong wear of the anode. First try reversing polarity for a brief
amount of time and then use the cell in normal operation at 5 V. That might reverse the effect of formation of a thin layer. If that does not give
satisfactory results, then you can use a higher voltage, say 6 to 7 volts. This can be achieved by using a series resistor between the 12 V power
supply and the cell. A good value to start with might be 2 Ohm's (use a parallel connection of five 10 Ohm resistors, each resistor being a 10 W
model). With that value you'll have 3 A through your cell.
Applying 12 V directly to the anode simply puts too much wear on the anode and your cell will become very very hot.
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Jurre10
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Hi,
Ok, then i gonna try to reverse polarity this evening.
Hopefully, the layer will remove ...
Jurian
If you try your best, but you don't succeed.
Experience is what you need.
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Jurre10
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hi,
Started with normal polarity : 0.15A
Reversed polarity : 4.54A
again : normal :0.20A?!
Reversed : 4.73A
Again normal 0.30A
Reversed:5A
Every time of reversed was 10 seconds.
So it looks like after each time its going better!
Stopped after the 3 times.
Can i go further with this or wil i damage it?
Jurian
If you try your best, but you don't succeed.
Experience is what you need.
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