Sciencemadness Discussion Board

Lead dioxide anode-possibility of an unusual synthesis method

MeshPL - 20-4-2015 at 11:27

So, I want to try making chlorates. As many of you know, graphite anodes corrode quite easily when producing chlorates, so I decided to make my own PbO2 electrodes. I looked it up on the Internet, yet methods I've found were pretty complicated (and would require me to order things from many Internet Shops, what means high delivery prices). So here are my questions:

-PbO2 is usually deposited on graphite substrate. Can I use copper instead? I want to use simmilar one for the cathode as well. Size: 3mm*30mm*100mm

-Can I just plate copper with lead via electroplating in lead acetate electrolyte and lead sacrificial anode, and later oxidise it like in lead acid battery to PbO2 in 30% sulphuric acid electrolyte? Or will be the PbO2 layer too thin?

-Since K2Cr2O7 is incompatible with PbO2 anode, will addition of Mg/Ca salt serve a simmilar role? (From what I've read, it should).

violet sin - 20-4-2015 at 16:20

Did you search the forum for this info, or just ask?

j_sum1 - 20-4-2015 at 16:29

I have no experience with chlorates (although it seems to be something of a rite of passage), so take this all with a grain of salt. (heh, heh!)
I did attempt to make some graphite substrate lead dioxide electrodes one time for use with electrolysis of CuSO4 to make H2SO4. I was really disappointed with the quality of my electrodes. Coverage was poor and extremely uneven.
I then began using a piece of lead sheet as an anode for H2SO4 electrolysis and have not looked back. Over a few runs, the lead builds up a nice lead dioxide coating that is smooth and even and stable. My recommendation is to do this to make your lead dioxide electrodes for your chlorate synthesis and take the sulfuric acid as a useful byproduct.

hyfalcon - 20-4-2015 at 18:49

Sticky at the top of Techno-chemistry.

https://www.sciencemadness.org/whisper/viewthread.php?tid=24...

MeshPL - 20-4-2015 at 21:41

I have not found info on deposition of PbO2 on copper.
Quote:

I did attempt to make some graphite substrate lead dioxide electrodes one time for use with electrolysis of CuSO4 to make H2SO4. I was really disappointed with the quality of my electrodes. Coverage was poor and extremely uneven.
I then began using a piece of lead sheet as an anode for H2SO4 electrolysis and have not looked back. Over a few runs, the lead builds up a nice lead dioxide coating that is smooth and even and stable. My recommendation is to do this to make your lead dioxide electrodes for your chlorate synthesis and take the sulfuric acid as a useful byproduct.


So as you wrote standard PbO2 electrode preparation method is pretty hard to do correctly. I could try covering Pb with dioxide directly, but I'm afraid Pb is so soft, that dioxide will flake off easily. That's why I wanted to plate Cu with Pb and oxidise Pb to PbO2. Also, if I could get actual H2SO4 easily, I could use it instead of CuSO4 in the process mentioned by you. Right?

j_sum1 - 20-4-2015 at 21:54

In all honesty I think you would be better reading through a few pages of the sticky that hyfalcon mentioned rather than picking my feeble brain on this one. I can only report on my experience which is extremely limited.

But if you are asking about the structural integrity of the electrode that I have been using, I see no reason why it would flake off. The coverage is really nice and uniform.

phlogiston - 21-4-2015 at 00:41

Quote:
Can I use copper instead?


No, for two reasons:

1. The lead dioxide will oxidise the copper, forming a non-conductive layer in between. (For this reason, a layer of silver is usually plated onto lead dioxide to enable a lasting connection with a copper wire. Silver is also oxidised, but silver oxide is conductive).

2. The lead dioxide layer is never perfectly closed, and electrolyte will seep through small holes causing rapid disintegration of your copper substrate
Quote:
-Can I just plate copper with lead via electroplating in lead acetate electrolyte and lead sacrificial anode, and later oxidise it like in lead acid battery


Lead acid batteries are not made that way. https://www.youtube.com/watch?v=P7tOipB_-38
Quote:

-Since K2Cr2O7 is incompatible with PbO2 anode, will addition of Mg/Ca salt serve a simmilar role? (From what I've read, it should).


A layer of Mg and Ca hydroxides will form on the cathode which has indeed been claimed to help avoid reduction of hypochlorite/chlorite/chlorate. However, it is supposedly difficult to get a layer of the right thickness. (Too much, and you will not get much current to flow at all), and the effect may be small. If I were you I would try to get something working first, and then worry about improving the efficiency. I am also not sure if it is compatible with lead dioxide but I don't see any reason why it would not be.
A very small amount of sodium fluoride may be added to lead dioxide anode-based (per)chlorate cells to improve efficiency, but this is an extremely toxic compound and in my opinion not worth the risk.

While we are on the topic of toxicity, You seem like a pretty smart guy, and you probably want to stay that way.
Please do not underestimate the difficulty of working with lead and (especially) soluble lead compounds. Even very, very small amounts of these can affect your brain and it requires careful planning to avoid contaminating you and your house.

[Edited on 21-4-2015 by phlogiston]

[Edited on 21-4-2015 by phlogiston]

byko3y - 21-4-2015 at 01:58

PbO2 resistivity varies from 2*10^(-4) to 7*10^(-3) ohm*cm (later is the one of a usual battery). http://jes.ecsdl.org/content/94/2/42.abstract
Resistivity of lead is 2^10^(-5).
Graphene has resistivity of 1*10^(-6) ohm*cm, amphorous carbon 6*10^(-2), and same piece of graphite depending on the orientation can have resistivity from 3*10^(-4) to 3*10^(-1). https://en.wikipedia.org/wiki/Electrical_resistivity_and_con...
So, when PbO2 deposits on your graphite electrode, in case the electrode resistivity is higher then the one of PbO2 the current starts flowing mainly through the PbO2, because the oxide conducts the current to a large surface of the graphite. http://www.electronics-tutorials.ws/resistor/res_4.html

MeshPL - 21-4-2015 at 06:01

Thanks for response of all of you!

It seems that I will need to make lead dioxide electrodes the usuall way. Or try plating lead with PbO2 via oxidation.

What concerns lead-acid batteries, I was talking about the older methods, and refering only to Pb plating, not Cu under it. Nevertheless, It would be just better to try covering Pb with PbO2 like in older batteries, without copper under it, since both Pb and PbO2 may not be hermetical. (How old were this batteries? The first ones)

MeshPL - 21-4-2015 at 06:39

What concerns alkali earth metals in preparation of chlorates: www.isca.in › 9.ISCA-RJCS-2012-100.pdf

It would seem that even extreme concentrations of alkali earth metal salt would not be bad since this paper mentions solutions of their salts without NaCl/KCL. Nevertheless, It will not harm to try. I will buy HCl anyway and MgCaCO3 is a good fertilizer so it will not be wasted. I'll just combine them (carefully) and get magnesium and calcium chloride.

phlogiston - 21-4-2015 at 07:18

As you might expect, there are very good reasons why certain materials are used as the substrate, such as graphite, platinum and MMO-coated titanium.

A lead substrate will not work.

There is an extensive body of literature on this topic and some really excellent threads that you really should read before continuing.
Yes, it is a lot to read.
However, if you don't you will just be wasting your time replicating experiments that will not work.
Thanks to the people that have been kind enough to document and describe their experiments and results here, you can use it to your benefit and avoid the route that leads to failure.

[Edited on 21-4-2015 by phlogiston]

MeshPL - 21-4-2015 at 09:07

I'm not talking about plating lead substrate. I want to oxidise lead substrate to make a layer of PbO2 form, what should work fine. It is how the oldest batteries were made, and from what I've seen it SHOULD work. Simply:
Pb + H2SO4 -electrolysis-> PbSO4 + H2
PbSO4 + 2H2O-electrolysis-> PbO2 + H2SO4 + H2

MeshPL - 21-4-2015 at 11:28

Ok, after a bit of internet research, I can conclude, that Pb coating with PbO2 described in previous post does work, but obtained electrodes corrode somewhat easily. They are fine for some applications though.

Source is available in Polish only: http://www.vmc.org.pl/index.php/twoje-laboratorium/item/238-...

papaya - 21-4-2015 at 12:34

Since I'm very interested in these type of subjects for a long time I want to share some of my thoughts.
It's known that it's not impossible to prepare durable PbO2 anode coatings on substrates like titanium, niobium, tantalum or even less "noble" graphite that work properly for per/chlorate manufacturing(some had success at to do this at home!), since these types of substrate can resist corrosion due to very passive oxide layer formation. It's also known that it's possible to grow PbO2 layer straight on a Pb metal base via electrolysis in H2SO4 and the anode prepared this way falls apart if it's used in chlorate cells. Where is the difference between these two? Perhaps because the PbO2 coating on Pb/PbO2 anode is very porous and the electrolyte reaches the metal very easily. But why it's so porous? Perhaps because in the process of H2SO4 electrolysis gases (oxygen) are given off from anode due to SIDE reaction (actually water decomposition).

Now my thought is to try to find out a way how to avoid this gas formation.. maybe conduct electrolysis at lowest possible voltages, or maybe you know some additives that can increase oxygen evolution overpotential to inhibit this side reaction, or maybe both?

phlogiston - 21-4-2015 at 14:40

It is of course possible to form a layer of PbO<sub>2</sub> on lead in this way.
But the resulting electrode is useless for (per)chlorate cells.
Feel free to try it, of course, just don't be too disappointed when the anode quickly disintegrates.

MeshPL - 21-4-2015 at 23:17

Just a question about graphite substrate lead dioxide anodes: can I use acetates instead of nitrates? This would make the process 2 times cheaper for me.

[Edited on 22-4-2015 by MeshPL]

Mesa - 22-4-2015 at 09:24

Which acetate are you referring to? Lead(II) acetate or Lead(IV) acetate.

I had the idea a while back to use conductive glass as a substrate for PbO2 coated electrodes but thermal decomposition of the PbO2 would make it unfeasable for what I planned.

[Edited on 22-4-2015 by Mesa]

MeshPL - 22-4-2015 at 12:02

Lead (II) acetate.
Lead dioxide electrodes are usually made from nitrates of copper and lead. I'd rather use acetates (I can make myself), than nitrates. I wonder, if acetates are suitable for that.

Also, can inert electrodes be made from SnO2 (non-stechiometric) or SiC? Not that I could make ones from the later.

Is tin dioxide actually conductive? If it was it could be used as a substrate plating for lead dioxide. It could not be oxidised by PbO2 (already oxidised), and would not oxidise substrate (at least copper substrate) as well.

[Edited on 22-4-2015 by MeshPL]

[Edited on 23-4-2015 by MeshPL]

jock88 - 26-4-2015 at 13:20


Some reading here

http://oxidizing.typhoonguitars.com/chlorate/anodes.html


Also see this post

http://www.sciencemadness.org/talk/viewthread.php?tid=1425
post of 10 - 02 - 2004

I wonder how he managed to get the lead dioxide coating so smooth and consistent?

[Edited on 26-4-2015 by jock88]