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Author: Subject: Thoughts On Anodes
bbartlog
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[*] posted on 20-6-2010 at 06:37


The theory he was referring to was this:

Quote:
A way around this is to use a second Pt-plated electrode as a cathode.


...as you would acknowledge if you weren't being deliberately obtuse.
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hissingnoise
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[*] posted on 20-6-2010 at 08:11


Well, he did say this. . .
Quote:

I have tried the platinum cathode and I can tell you that it is a very bad thing to do. I completely destroyed, (turned to powder), one of the 4 electrodes I bought, which leads me to believe that your idea of the platinum plating onto the cathode in normal operation is incorrect. It seems the plating goes from negative to positive, this is why, it seems, my platinum cathode completely dissolved in about 150 hours.

Everyone knows that Pt is resistant to everything except Aqua Regia and how many people are there who think that electroplating works by migration of metal from the cathode to the anode?



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white rabbit
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[*] posted on 20-6-2010 at 09:51




My point was simply, if the platinum is plating across, it is certainly not plating on the platinum cathode as you stated and Pt is certainly not electrochemical resistant as a cathode. So your statement that, "Pt is resistant to everything", is also incorrect.
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hissingnoise
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[*] posted on 21-6-2010 at 02:41


Quote:
also incorrect.

It's hot - I'm lazy!
Tim knows what I. . .

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white rabbit
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[*] posted on 21-6-2010 at 19:11




I will concede that platinum is indeed resistant but not impervious to electro/chemical attack. I really don't know what exactly happened to the platinum cathode but it really doesn't like to be negative in the electrolyte solution of the chlorate/ perchlorate cells.


This was my first usage of the platinum electrodes, both anode and cathode at ~70 hours at 6.5 volts / 25 amps. The anode appears to be completely unaffected and continued to look this way run after run with a stainless cathode.

Platinum Anode and  Cathode small.JPG - 98kB


This was the beginning of the cathode corrosion, and quickly continued, until after removing the electrode from solution, and leaving it exposed to the air overnight, it crumbled to dust.:(

Platinum Cathode small.JPG - 58kB
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hissingnoise
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[*] posted on 22-6-2010 at 06:32


Quote:
I really don't know what exactly happened to the platinum cathode but it really doesn't like to be negative in the electrolyte solution of the chlorate/ perchlorate cells.

White rabbit, as you know, platinum is famous for its inertness to acidic and alkaline environments.
It certainly shouldn't noticeably corrode when used as cathode or anode in a chloride cell.
Perhaps the electrode was defective in some way. . .
Quote:

This was the beginning of the cathode corrosion, and quickly continued, until after removing the electrode from solution, and leaving it exposed to the air overnight, it crumbled to dust.

If the Pt plating was not continuous enough to fully protect the electrode, corrosion and hydrogen embrittlement of the substrate metal? could possibly account for what happened. . .
It's certainly curious and at the price of the electrode, the experience must have been pretty devastating.


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[*] posted on 22-6-2010 at 15:26




You may have a good point there. That one electrode was never used as a anode. I have used three of the four electrodes so far and the two used as anodes still look like the day I first started using them. (The crusty residue washes of with water) The substrate material is niobium and is supposed to have an exceptional chemical resistance.

The price of the electrode was of no consequence, but I certainly didn't do the dance of joy.:o
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[*] posted on 22-6-2010 at 15:45


Quote: Originally posted by hissingnoise  

Everyone knows that Pt is resistant to everything except Aqua Regia and how many people are there who think that electroplating works by migration of metal from the cathode to the anode?

Quote: Originally posted by hissingnoise  

It certainly shouldn't noticeably corrode when used as cathode or anode in a chloride cell.
Perhaps the electrode was defective in some way. . .


Platinum metal may dissolve if free chlorine is liberated at the anode.

Ref: Kvantitativ Analyse ("Quantitative Analysis"), Katrine Seip Førland, Tapir Academic Press (1984)
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[*] posted on 19-7-2010 at 12:39
BDD Makes Perchlorate


I recently came across the following interesting abstract. I've been loosely following the development of BDD anodes for a while now, looking for experimental confirmation that they will in fact make perchlorate. The results stated here seem to verify it, and it sounds promising, IMO.

The other important question, still unanswered AFAIK, is: How well would a BDD anode hold up in a perchlorate cell?

In any case, it seems to me that it might now be worth buying that small BDD slab available from e6 corp. to do some testing. The main issue there is: How do you make an electrical connection to it?

Electrochimica Acta
Volume 54, Issue 7, 28 February 2009, Pages 2102-2107
ELECTROCHEMISTRY FOR A HEALTHY PLANETENVIRONMENTAL ANALYTICAL AND ENGINEERING ASPECTS Selection of papers from the 6th ISE

Spring Meeting 16-19 March 2008, Foz do Iguacu, Brazil

doi:10.1016/j.electacta.2008.09.040
Copyright © 2008 Elsevier Ltd All rights reserved

The occurrence of perchlorate during drinking water electrolysis using BDD anodes

M.E. Henry Bergmann, Johanna Rollina and Tatiana Iourtchouka
Anhalt University, FB 6/7, Bernburger Str. 55, 06366 Koethen, Germany
Received 30 April 2008; revised 15 September 2008; accepted 17 September 2008. Available online 27 September 2008.

Abstract
Electrochemical studies were carried out to estimate the risks of perchlorate formation in drinking water disinfected by
direct electrolysis. Boron Doped Diamond (BDD) anodes were used in laboratory and commercially available cells at 20 °C. The current density was changed between 50 and 500 A m−2. For comparison, other anode materials such as platinum and mixed oxide were also tested. It was found that BDD anodes have a thousandfold higher perchlorate formation potential compared with the other electrode materials that were tested. In long-term discontinuous experiments all the chloride finally reacted to form
perchlorate. The same result was obtained when probable oxychlorine intermediates (OCl, ClO2, ClO3) were electrolysed in synthetic waters in the ppm range of concentrations. The tendency to form perchlorate was confirmed when the flow rate of drinking water was varied between 100 and 300 L h−1 and the temperature increased to 30 °C. In a continuous flow mode of operation a higher chloride concentration in the water resulted in a lower perchlorate formation. This can be explained by
reaction competition of species near and on the anode surface for experiments both with synthetic and local drinking waters. It is concluded that the use of electrodes producing highly reactive species must be more carefully controlled in hygienically and environmentally oriented applications.
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[*] posted on 19-7-2010 at 12:46


Hello JP,

Good to see you posting again.
I have not been doing much lately in the Chlorate/Perchlorate line.

The BDD sounds like a great Anode.

Dann2
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jpsmith123
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[*] posted on 23-7-2010 at 20:33


Hi Dann2,

I've been tinkering with electrical stuff and putzing around with a mini-lathe, but seeing the above abstract has re-ignited my interest in BDD anodes.

I was following this thread for a while, but now it seems most of the participants have moved on to other things.

Anyway, I'm going to start seriously looking again to see if I can buy some niobium/BDD anode material somewhere at a reasonable price. I'll post here if I have any luck.
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[*] posted on 24-7-2010 at 05:57


Quote: Originally posted by jpsmith123  
The main issue there is: How do you make an electrical connection to it?
Think about using vacuum deposition. Given that the technology was invented by Thomas Edison (yes, really, it surprised me), it's certainly achievable in a home lab, particularly now that refrigeration service pumps are so common.
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[*] posted on 24-7-2010 at 06:39


Being that the diamond is doped with boron, it's an electrical conductor (although not a very good one according to the specs), so it might be possible to metallize it by simple electroplating, for that matter, and then something could be soldered or brazed to it.

I think the main problem is that the metal-to-diamond joint will be exposed to the environment of the chlorate/perchlorate cell, so most common materials would probably not last very long.

I remember seeing a patent from 50 or 60 years ago wherein diamond powder was metallized and attached to the metal core of a grinding wheel using a paste of TiH2. The assembly was then fired to 400 or 500 degrees C in a furnace.

That's probably something like what you'd have to do to make a reliable connection to it, i.e., a direct diamond to valve metal bond.

In any case, another disadvantage of trying to use bulk BDD slab material available from e6, is that the conductivity of the material is rather low, so, depending to a certain extent on how you connect to it and use it, the current you could put through it would probably be limited to a few amps.

Quote: Originally posted by watson.fawkes  
Quote: Originally posted by jpsmith123  
The main issue there is: How do you make an electrical connection to it?
Think about using vacuum deposition. Given that the technology was invented by Thomas Edison (yes, really, it surprised me), it's certainly achievable in a home lab, particularly now that refrigeration service pumps are so common.


[Edited on 24-7-2010 by jpsmith123]
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dann2
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[*] posted on 28-7-2010 at 12:51


Hello Folks,

Have a look at:

http://www.advoxi.com/products

These things may be available in the dumpsters soon. (like the MMO)

Still getting around to the SnO2/Bismuth doped Anode to see if it's any good. Perhaps next month.

Dann2

[Edited on 28-7-2010 by dann2]

diagram04.jpg - 23kB
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jpsmith123
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[*] posted on 28-7-2010 at 20:27


Thanks for that link Dann2.

I'm going to email them and find out what's available and at what price.
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[*] posted on 3-8-2010 at 14:20


Unfortunately, I found out that they won't sell just BDD anodes, but only the whole electrolytic cell. Even more unfortunate is the price...they want over $5000 US dollars for a small cell for swimming pools (if I've converted the currency correctly).

So it looks like that company is out. Rats. We need to keep looking I guess.
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[*] posted on 3-8-2010 at 15:49


Thats bad news!

If you Google

homemade boron doped diamond

some interesting looking links appear. I did not read them yet though. They seem to suggest that it may be possible to do it in an Amateur setting.


Also De Beers BDD Electrodes are distributed by Windsor Scientific, Slough, UK

http://www.windsorscientific.co.uk/index.php?main_id=92&...

240 pounds sterling for a disk 3mm in diameter. (ouch). The damm thing is hardly much bigger that a good sized full stop!

If it made an everlasting Perchlorate Anode though.........

Perhaps someone who is a bit flush with cash could stand into the breech.

Cheers,
Dann2 (holding on tightly to his hard earned cash :D )



[Edited on 4-8-2010 by dann2]
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[*] posted on 3-8-2010 at 17:40


Buy the platinum anode and you're done. It saves 100's of hours of your precious time looking for alternatives.
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[*] posted on 4-8-2010 at 09:40


No doubt platinum is great anode material, but it seems BDD has the potential to be much better: more efficient at making perchlorate, more robust and longer lasting, and hopefully, someday, significantly cheaper.

BDD may even be able to do things that, AFAIK, other anode materials can't, e.g., directly convert HCl onto HClO4.

I thought by now somebody would be doing something with "poly(hydridocarbyne)", i.e., applying diamond films to substrates by thermolysis of a precursor solution, similar to how MMO anodes are made.

I thought the patent holders (the processes to make the poly(hydridocarbyne) are patented) would have an interest in developing applications like this for their invention. Apparently not.
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[*] posted on 20-8-2010 at 22:44


Well it seems it'll be awhile before BDD anodes compete with Pt or PbO2, for our purposes at least.

The best price I've got so far is 960 Euro for a BDD over Nb plate anode of dimensions about 7.5"x3.25"x0.1" with a 5 micron BDD coating on both sides.

The recommended current density for this anode is 100 ma/cm^2.

The manufacturer says it is suitable for perchlorate electrosynthesis, which again confirms what we've already concluded.

As I understand it, BDD does slowly wear away, supposedly as OH attacks and damages it where there are defects in the crystal structure, or something like that. But it will nevertheless last a long time, maybe 10,000 hours or more of continuous use under harsh conditions.

I had hoped that, by now, conductive (boron doped) diamond powder would be commercially available...alas it isn't, as far as I know. Which is unfortunate as it seems to me that an electrophoretically coated platinum or MMO anode would be the easiest way to make a BDD anode at home.

Another method might be thermolysis of "poly(hydridocarbyne), with the addition of some boron compound as a dopant.
And I'm really surprised that nothing seems to be happening with this stuff.

Lastly, it may be possible to rig up a homemade cvd apparatus and make BDD at home, but I certainly don't have the resources to even try right now.

Luckily, it's been shown possible to make high quality BDD using alcohol vapor instead of methane and hydrogen gasses, and it's been shown that boric acid can be added to the alcohol as a boron dopant.

Also, luckily, the vacuum requirements are lax, so a simple water aspirator might suffice instead of a rotary vane vacuum pump.

The two remaining technical hardships would be (1) keeping the substrate temperature between 700 and 900 degrees C (or something like that); and (2) creating the plasma with which to dissociate the vapor into the active species.

It's possible that a dielectric barrier discharge can be used instead of a hot filament or a microwave (or RF) plasma, but I couldn't find any specific examples in the literature where a DBD was used (although I didn't look very thoroughly). If DBD will suffice for this purpose, I can see the apparatus being greatly simplified...with the major PITA being finding the optimum combination of pressure, discharge power, electrode spacing, etc.
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[*] posted on 24-8-2010 at 04:22


Quote: Originally posted by jpsmith123  

The best price I've got so far is 960 Euro for a BDD over Nb plate anode of dimensions about 7.5"x3.25"x0.1" with a 5 micron BDD coating on both sides.

The recommended current density for this anode is 100 ma/cm^2.


Thats not too far off from the price of Pt.

Dann2
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jpsmith123
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[*] posted on 25-8-2010 at 11:08


I think e6 has convinced me that, with regard to presently available commercial technology, bulk BDD, rather than BDD coated substrates, is the way to go for diamond anodes. (Although I think it's possible someday that thick, electrophoretically applied coatings over the right substrate might be an exception).

Apparently e6 experimented with both BDD coatings and bulk material, and decided the latter was best. It seems that the coatings can develop pinholes which ultimately result in delamination of the coating. This is apparently why manufacturers of coated anodes recommend limiting the current density to 100 mA/cm^2, whereas the bulk BDD anodes can run from 0.5 to 1.0 Amp/cm^2.

Their literature predicts a life expectancy of >45,000 hours, when running at 0.5 A/cm^2.

A piece of BDD that's 20 x 20 x 0.6 mm would cost around $200 USD.
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[*] posted on 25-8-2010 at 12:51


Anyone in the UK have a moment to talk to these people?

Advanced Oxidation Limited
Unit 1
Homelands Business Centre
Burrington
Umberleigh
North Devon
EX37 9JG
United Kingdom

Telephone:
Alan +44 (0)7811 541339
Paul +44 (0)7702 707228

They say they sell wastewater treatment units containing BDD anodes including ones for spa/hot tub/pools.
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[*] posted on 8-9-2010 at 12:03


Where can I get platinum metal sheet or wire ?
Jewelry supply shops like cooksongold.com sell only jewelry grade Pt which is Pt alloyed with 5% Cu which is unsuitable for anodes as the copper is dissolved (AFAIK).
On Ebay I cannot find pure platinum for the 'normal' day price of $1600/oz or only slightly above which is 1600/0.031= EUR 51200/kg only for MUCH higher.
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[*] posted on 8-9-2010 at 12:11


The biggest suppliers of PGMs are Johnson Matthey and Degussa - check them or their subsidiaries out first!

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