Sciencemadness Discussion Board
Not logged in [Login ]
Go To Bottom

Printable Version  
 Pages:  1  
Author: Subject: Perchlorate cell additives for Pt anode
Benignium
Hazard to Self
**




Posts: 86
Registered: 12-6-2020
Member Is Offline

Mood: Quasi-catatonic

[*] posted on 12-6-2020 at 18:24
Perchlorate cell additives for Pt anode


I'm new here. Hello! :)

Inspired originally by NurdRage's video on chlorate production, I've been dabbling with the electrolysis of K/Na chlorides. I've managed to obtain a Pt plated Ti anode for use in a perchlorate cell but, naturally, I'm worried about its longevity.

Reduction of chlorate back to chloride at the cathode resulting in constant low levels of chloride seems to be greatly detrimental when using platinum, and chromate is commonly used to prevent this. However, if I've understood correctly, there is the option to use either persulfate or fluoride instead, though I haven't managed to find further information on the use of these alternatives. I can use any one of these three, but I would like to avoid using hexavalent chromium where possible.

Furthermore, I wish to find out what kind of drop in life expectancy one might expect when using platinum to go straight from NaCl to NaClO4. This is what I'm planning as I do not require several kilos of product.

If anyone can shed some light on the subject I would greatly appreciate it.

Here you can see the electrodes I'm going to be using. The coating thickness is 2.5-3 µm.



View user's profile View All Posts By User
mackolol
National Hazard
****




Posts: 437
Registered: 26-10-2017
Location: Poland
Member Is Offline

Mood: Psychedelic

[*] posted on 13-6-2020 at 07:14


I have some information about catalysts for electrolytic chlorate production from Polish forum though I haven't ever done it yet. Albeit I plan on doing it soon.
Sodium fluoride is used when your cathode is PbO2 and it increases the current efficiency.
When you use any other cathode K2Cr2O7 is used and as you have said it prevents reducing chlorates to chlorides.
I haven't heard nothing about persulfate. I would go for chromate, but with extra care. You wouldn't want to spill the electrolyte if it didn't contain mentioned chromate either.
View user's profile View All Posts By User
Benignium
Hazard to Self
**




Posts: 86
Registered: 12-6-2020
Member Is Offline

Mood: Quasi-catatonic

[*] posted on 13-6-2020 at 11:53


Quote: Originally posted by mackolol  
Sodium fluoride is used when your cathode is PbO2 and it increases the current efficiency.
When you use any other cathode K2Cr2O7 is used and as you have said it prevents reducing chlorates to chlorides.


See, this is what I fail to understand - if it's the cathode where the reduction of chlorate to chloride happens, why does the anode material matter (please correct me if I'm wrong, but I dare assume you meant to write anode instead of cathode)? IIRC, a thin layer of chromium oxide forms on the cathode surface when chromate is used, preventing the cathode from reducing chlorate. Surely fluoride and persulfate act in some analogous manner when a titanium cathode is used? Why would these things only happen when using PbO2 but not Pt, or vice versa?

The Ebay vendor, feanor.forges, who kindly provided me with the electrodes told me he uses persulfate apparently with no problems, but he hasn't tried other additives and cannot therefore compare their performances. This, to me, is nevertheless a compelling bit of information and I am thinking of trying sodium persulfate at my own peril unless I find evidence to the contrary.

Thank you for the reply! :)


View user's profile View All Posts By User
mysteriusbhoice
National Hazard
****




Posts: 297
Registered: 27-1-2016
Member Is Offline

Mood: A little above sanity...

[*] posted on 13-6-2020 at 12:23


For perchlorate cells
the reason the additive differs is because for Pt chromate works best and I tjhink NaF also works but for PbO2 Chromate deactivates the catalyst layer by building up a deposit on the anode. so NaF is used instead.
Persulfate also works well for both materials so go for that.

as for me Fk using anodes to make perchlorates im doing will with the thermal decomposition method and for making KClO4 i mix NaClO3 + KClO3 in the melt with a ratio of 2:1 and get around 85% yield on basis of KClO3 added.
Sodium perchlorate is a bit trickier but ive also done it though the yield is at best 60% but usally 45%.

my Chlorate cell produced 1.4kg of NaClO3 in 4 days at 83% CE using CaCl2 as pH control and additive so hence I have lots of chlorate to burn!.
overall tho KClO4 has deeply dissapointed me in terms of performance as an oxidizer.
I may need to get a ball mill to make them good.

DO NOT use Pt to go from NaCl to NaClO4 unless you have 2x Pt electrodes and swap their potentials every few hours to prevent losses since they would plate back the Pt back and forth.

for Pt only use Pure NaClO3 as feed and purifying it for electrolysis into perchlorate needs recrystalization + additives like chromate,persulfate,fluoride.

PbO2 can also go from chloride to perchlorate and be totally fine but its not recommended since some have reported errosion and that depends on the quality of the anode you purchased.
Some types of MMO can make perchlorate due to having high IrO2 content but its a hit or miss and the efficiency is bad and its operating life will shorten.
IrO2 coated electrodes for corrosion control in concrete can make perchlorate with minimal errosion but efficiency will be SHITE like 20% or worse.
View user's profile View All Posts By User
Benignium
Hazard to Self
**




Posts: 86
Registered: 12-6-2020
Member Is Offline

Mood: Quasi-catatonic

[*] posted on 13-6-2020 at 14:04


Thank you, mysteriusbhoice. That was very informative and coincidentally exactly what I wanted to hear concerning the use of additives. It seems I will be going with sodium persulfate.

One thing I don't think I quite comprehend is what you said about starting from NaCl. Are you saying the anode gets plated with chloride and efficiency drops? If so, I'm curious as to how significant you would estimate this drop in efficiency to be?

I have to confess that I'm not all that concerned on maximizing my CE. I will be more than happy with 50% if I can achieve it. I am aware that this means I'll be wearing down the platinum for nothing, but for now, as long as I can eventually finish one batch without finishing the anode in the process, I am happy.

I hope you or someone else will let me know if the attempt is doomed to fail in this sense.

Fantastic thermal decomposition yield and chlorate cell efficiency, by the way!


View user's profile View All Posts By User
mysteriusbhoice
National Hazard
****




Posts: 297
Registered: 27-1-2016
Member Is Offline

Mood: A little above sanity...

[*] posted on 13-6-2020 at 21:28


if you are using Pt and attempting to go from chloride to perchlorate when the chloride levels get really low the platinum gets attacked according to many sources and only PbO2 suffers from additives causing problems but its an anode capable of going from chloride to perchlorate.
Efficiency of any anode in a perchlorate cell matters on the current density of the anode.
at 80ma/cm^2 its 34% at 150ma/cm^2 its 50% and at 225-250 ma/cm^2 its at 78% and thats from this 1 paper where there was this graph of current density vs current efficiency.
you can easily find it.
I ended up reading alot of stuff on how to make perchlorates only to reach the final conclusion of me just cooking my stuff since I produce a lot of chlorate.
If you are using Pt to go from chloride to perchlorate then your run maybe doomed since these platinized anodes have a really thin coating.
If you have 2x Pt electrodes and reverse their polarity every few hours then you can run from chloride to perchlorate since you will just be plating the Pt back and forth between electrode to minimize wear.
If you wanna make the best use of your Pt then you really need to first run chlorate then purify it by recrystalization which means you need to make a lot since recrystalizing NaClO3 SUCKS.

You should read from online sources like research papers concearning things related to what you are doing to get this info and not end up burning your cash for nothing.
View user's profile View All Posts By User
Benignium
Hazard to Self
**




Posts: 86
Registered: 12-6-2020
Member Is Offline

Mood: Quasi-catatonic

[*] posted on 14-6-2020 at 11:32


Well, whatever happens there is an opportunity for learning here. I appreciate your patience with me despite the fact that no one likes people who essentially ask to be spoon fed. The information you have provided is very helpful.

I will attempt going from NaCl to NaClO4, using sodium persulfate, and will post whatever results I get in this thread. This way at least in some sense I am burning my cash for science. :)


View user's profile View All Posts By User
Benignium
Hazard to Self
**




Posts: 86
Registered: 12-6-2020
Member Is Offline

Mood: Quasi-catatonic

[*] posted on 22-6-2020 at 18:39


The cell has now been running for a few hours at roughly 5A, 4.4V. I figured stirring might be beneficial so I dug my old DIY magnetic stirrer out of storage. The electrolyte volume is roughly one liter. Roughly 3 grams of sodium persulfate was added. The electrodes were adhered to the lid using RTV silicone.



View user's profile View All Posts By User
mysteriusbhoice
National Hazard
****




Posts: 297
Registered: 27-1-2016
Member Is Offline

Mood: A little above sanity...

[*] posted on 23-6-2020 at 00:14


your current density is 65ma/cm^2 assuming that electrode is the standard 2x3 inch plate electrodes.
good for chlorate but below 80ma/cm^2 for perchlorate for 30% CE overall you can run this as it is right now till you produce chlorate.
http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.824...
since you are using platinum it will be better than PbO2 but only just a little.


PS: I just got nothing better to do cuz of this pandemic..

[Edited on 23-6-2020 by mysteriusbhoice]
View user's profile View All Posts By User
Benignium
Hazard to Self
**




Posts: 86
Registered: 12-6-2020
Member Is Offline

Mood: Quasi-catatonic

[*] posted on 23-6-2020 at 07:05


Thank you for taking the time!

The anode is 2x4", or 50 cm^2, with only one platinum-plated side, so current density is 100 mA/cm^2. Cell temperature seems to have settled at 45°C.
What I gather from the paper you shared as well as a few other sources, I could reasonably expect a CE of approximately 30%, at best, while producing perchlorates. Perhaps as much as 50% during the majority of chlorate production. Frankly this would be fine. I have time.

I'm not sure what you mean by "till you produce chlorate". Seeing as I already do, do you mean while I produce chlorate or till I produce perchlorate?


View user's profile View All Posts By User
mysteriusbhoice
National Hazard
****




Posts: 297
Registered: 27-1-2016
Member Is Offline

Mood: A little above sanity...

[*] posted on 23-6-2020 at 08:57


if only one side is platinized then yea your current density is about 100ma/cm^2 which is above the 80ma/cm^2 so with that your ce would be around 30-50% for your perchlorate run.
as for your chlorate run if the cell will go to alkaline then it will be at 54% efficiency due to hypochlorite formation.
I just hope your anode survives the low chloride level right before it starts using up the chlorate since that would be the danger zone.

below is one of my yield from thermal decomposition of mixed NaClO3 + KClO3 to make KClO4.
tested with no horrible yellow puddle of "MMS" forming when mixed with HCl also very poorly soluble when compared with KClO3 and thats how you know.
tho i prolly used up around 150 grams of mixed Chlorate didnt measure it this time since yield averages around 80% when using mixed and 60% when using pure KClO3.


perchlorate.jpg - 34kB

[Edited on 23-6-2020 by mysteriusbhoice]
View user's profile View All Posts By User
Benignium
Hazard to Self
**




Posts: 86
Registered: 12-6-2020
Member Is Offline

Mood: Quasi-catatonic

[*] posted on 10-7-2020 at 21:50


So after almost exactly 7 days (=roughly 840 Ah) of operation, the cell kind of shit the bed. I monitored the cell closely for hours before anything odd occurred. Over the course of its operation the cell saw voltage creep up at a slow pace starting from 4.8V, averaging 100 millivolts of increase per day. During the last 12 or so hours the cell went from 5.4 all the way to 6.0 when finally the power supply switched over to controlled voltage and the input current dropped from 5A to 0.6A.

I was quick about turning off the power and removing the electrodes to dry. Surprisingly they looked just fine.


So, I proceeded to test the electrolyte with methylene blue which to my surprise gave me a positive for perchlorate. Or so I thought. I was thrilled but still skeptical seeing as I had only fed in under 1000 ampere hours. I also chilled down some fresh electrolyte with equal persulfate added and tested that. Positive. So obviously it has to be the sodium persulfate, right? Apparently that's exactly right. I repeated the test with fresh brine but left out the persulfate - no reaction.

Here's the presumed false positive:


I then tested the electrolyte for chloride using silver nitrate and found that there was plenty of it.


I also tried to run the cell with fresh electrolyte. Barely any current flowed until I manually turned the voltage up to as much as 9.3V. I had to do this twice because minutes after the first test something constricted the flow of current again. I don't believe that I damaged the electrodes much since the voltage increase lasted for a split second before voltage normalized to below 6V and the current rose to 5A.

The voltage kept creeping and after about 10 hours I realized what must be happening. The insulation near where the negative wire was crimped to the alligator clip had hardened and even begun melting. The cathode connection was hot enough to cause a burn. So, while I re-saturated the electrolyte with sodium chloride (figured I might as well) I made some adjustments. This is v2.1, where I added the flat pieces of metal to help with efficiency and heat distribution. I can now touch any part of the connections without any discomfort.


I powered up the cell and everything is fine again. The voltage is 4.7, the lowest it's been so far. The road to perchlorates continues.




[Edited on 11-7-2020 by Benignium]

[Edited on 11-7-2020 by Benignium]
View user's profile View All Posts By User
mysteriusbhoice
National Hazard
****




Posts: 297
Registered: 27-1-2016
Member Is Offline

Mood: A little above sanity...

[*] posted on 11-7-2020 at 00:02


yea you need to make sure your connections are SOLID as they heat up like in the case of my cell where the cathode connection heats up to 154 Celsius at the top of it.
I had to create a copper buss bar just for that connection.
I would suggest using Binder clips those black triangular ones instead of aligator clips because they bite down hard which is good and I use them in my setup which runs 25-30 amps of current at 5v.
View user's profile View All Posts By User
Benignium
Hazard to Self
**




Posts: 86
Registered: 12-6-2020
Member Is Offline

Mood: Quasi-catatonic

[*] posted on 11-7-2020 at 00:05


Thanks! I'll definitely shop around for some of those for v3. :)
View user's profile View All Posts By User
markx
International Hazard
*****




Posts: 620
Registered: 7-8-2003
Location: Northern kingdom
Member Is Offline

Mood: Very Jolly

[*] posted on 17-7-2020 at 01:36


Connections to Ti electrodes tend to passivate, especially if the contact surface is negligible like when one uses an alligator clip or just wraps the wires around the electrode stems. It is a constant problem with quickly improvised current leads. But even solid connections secured by bolts tend to passivate on Ti over time. So it makes sense to move and clean them regularily. On my large cell I have the Ti electrode stems drilled, tapped and the current leads connected to them by stainless bolts, but even these connections passivate from time to time.



Exact science is a figment of imagination.......
View user's profile View All Posts By User
B(a)P
International Hazard
*****




Posts: 614
Registered: 29-9-2019
Member Is Offline

Mood: Confined

[*] posted on 17-7-2020 at 13:14


For my chlorate cell I run 8 gauge wire from my transformer to copper tabs (soldered on) about 15 mm by 60 mm. for each electrode I have a second copper tab of the same size. The copper tabs have holes drilled in each end of them and I bolt them to either side of the top of the electrode. They need cleaning every 150 h or so to minimise the resistance at the connection.
View user's profile View All Posts By User
mysteriusbhoice
National Hazard
****




Posts: 297
Registered: 27-1-2016
Member Is Offline

Mood: A little above sanity...

[*] posted on 18-7-2020 at 00:27


Quote: Originally posted by markx  
Connections to Ti electrodes tend to passivate, especially if the contact surface is negligible like when one uses an alligator clip or just wraps the wires around the electrode stems. It is a constant problem with quickly improvised current leads. But even solid connections secured by bolts tend to passivate on Ti over time. So it makes sense to move and clean them regularily. On my large cell I have the Ti electrode stems drilled, tapped and the current leads connected to them by stainless bolts, but even these connections passivate from time to time.

I wonder if copper plating the Ti leads will help prevent passivation
View user's profile View All Posts By User
Benignium
Hazard to Self
**




Posts: 86
Registered: 12-6-2020
Member Is Offline

Mood: Quasi-catatonic

[*] posted on 27-7-2020 at 18:08


22 days and 20 hours (548 h, 2640 Ah) have passed with the cell being operational, and it's time for some results. As my previous MMO chlorate cell had an abysmal CE of around 20%, I thought it only reasonable to grossly underestimate the new cell design despite clear improvements like the use of additives and a significantly higher current density.

Anyway, at this point if I had a cell efficiency of 50% I would have used something like 110% of the electricity required to fully convert 1000 mL of saturated NaCl solution. The electrolyte was no longer as opaque from dispersed bubbles and the color of hypochlorite ions had disappeared some time ago, so I took a 10 mL sample for testing. The voltage had been staying comfortably at around 5 volts for a long time.

First, a 1 mL sample was taken to qualitatively test for perchlorate. This sample was diluted and acidified with a drop of ~36% H2SO4. The sample was boiled and 5 milligrams in total of potassium metabisulfite was added which, based on sulfur dioxide production, was an excess. An equivalent volume of hot saturated KCl solution was added and an immediate precipitate was observed. The precipitate was washed with room temperature DH2O, which was decanted off and discarded. The washing was repeated four times and the a sample was taken from the fourth washing. Perchlorate was identified from the sample using methylene blue.



Something to note is that when I got a false positive due to persulfate earlier, that only happened with chilled electrolyte samples. At room temperature the normal methylene blue color prevailed. This time even at RT there was an instant reaction, and the color wouldn't fade away as it did with the persulfate. The clumps of precipitate were much more defined, with no fine enough precipitate to even partially give the solution a purple/violet appearance.

Next, a more quantitative test was carried out on the remaining 9 mL of electrolyte. The sample was treated with sulfuric acid and potassium metabisulfite followed by the addition of hot saturated KCl solution. The precipitate was vacuum filtered and washed thoroughly twice using 2x25 mL RT DH2O. The fluffy, crystalline precipitate was dried and weighed. 8050 milligrams of dry precipitate was obtained.



At this point I powered off the cell and inspected the electrodes. The anode looks pretty rough. The silicone had also visibly corroded.



The electrolyte was no longer perfectly clear and contained silvery solid in suspension as clouds and flakes. When compacted the solid stuff had the consistency of watery jelly. Filtering it out was a pain in the anus. The solution is now crystal clear, but I'm fairly certain there are still some colloidal particles in it. It has a shimmering quality similar to a solution of silver nitrate shortly after dropping in a piece of copper. I hope that the majority of however much platinum it contained can be recovered eventually by burning the filter papers and dumping the remains in some aqua regia.



As a final test to further examine the purity I took 100 mg of the assumed KClO4, mixed it thoroughly with 100 mg of sugar in a test tube and added a few drops of concentrated H2SO4. The mixture did not ignite, which should mean it's relatively free of chlorates. I will now process the rest of the electrolyte in the same manner.



In many ways (at least one) this project has been a success. Certainly not wallet friendly at roughly 8.5 €cents/gram, assuming that the anode is ruined and cannot withstand another run. But happiness is the number two priority and I sure am happy. :)


I will attempt another batch from chloride and I will share the results here. I will also confirm the yield as soon as I've processed the perchlorate.


View user's profile View All Posts By User
markx
International Hazard
*****




Posts: 620
Registered: 7-8-2003
Location: Northern kingdom
Member Is Offline

Mood: Very Jolly

[*] posted on 28-7-2020 at 01:44


Good job! And congratulations on a successfull conversion run! :)

The fluffy percipitate is well known to me. I trust it is a combination of titanium and platinum compounds that from a gel like slime in the electrolyte. It is near to impossible to filter out effectively and the best action that I've found to deal with it is "pure classic": just do nothing....
It shall settle at the bottom of the cell eventually (afer several days) and decanting or carefully siphoning the clear solution on top of it makes for a complete separation. The small amount of contaminated electrolyte left can be filtered or discarded or turned into a contaminated batch of perchlorate. Anyways the losses are not considerable with such an approach.
The anode may look rather spent, but it shall remain fuctional as long as it conducts evenly over the surface (gas evolution) and the cell voltage does not rise. I've managed to squeeze 2-3 full conversion runs out of DIY Pt anodes before they give up the ghost. More product than I can reasonably consume in years of activity, so it is quite an effective method. Especially if one does not resort to buying the anodes, but makes them in home workshop starting from chloroplatinic acid.

[Edited on 28-7-2020 by markx]




Exact science is a figment of imagination.......
View user's profile View All Posts By User
Benignium
Hazard to Self
**




Posts: 86
Registered: 12-6-2020
Member Is Offline

Mood: Quasi-catatonic

[*] posted on 29-7-2020 at 08:47


Thank you markx! I was too impatient to let the gunk settle, and thankfully regular qualitative filter paper worked adequately well. I really hope the anode is good for more perchlorate. Based on cell voltage and gas evolution it just might be. I'm starting over with fresh electrolyte tonight. Whatever happens, at this point I already have more than I will probably ever need. I might have to get into rocketry! :)




[Edited on 29-7-2020 by Benignium]
View user's profile View All Posts By User
mysteriusbhoice
National Hazard
****




Posts: 297
Registered: 27-1-2016
Member Is Offline

Mood: A little above sanity...

[*] posted on 29-7-2020 at 11:36


I wonder how much chloroplatinic acid costs and with how much its needed and the platinum price being half that of gold.
how cheaply can you make DIY platinized electrodes.
View user's profile View All Posts By User
markx
International Hazard
*****




Posts: 620
Registered: 7-8-2003
Location: Northern kingdom
Member Is Offline

Mood: Very Jolly

[*] posted on 29-7-2020 at 14:27


Quote: Originally posted by mysteriusbhoice  
I wonder how much chloroplatinic acid costs and with how much its needed and the platinum price being half that of gold.
how cheaply can you make DIY platinized electrodes.


Chloroplatinic acid itself does cost an unreasonable price (150€/gram), but a few grams of platinum (30€/gram) turned into chloroplatinic acid via aqua regia is probably a lifetime worth of stock for preparing reasonably durable anodes. Well, even when resorting to buying the acid itself, the cost of a reasonably sized anode shall be marginal compared to the ready made product.




Exact science is a figment of imagination.......
View user's profile View All Posts By User
Benignium
Hazard to Self
**




Posts: 86
Registered: 12-6-2020
Member Is Offline

Mood: Quasi-catatonic

[*] posted on 30-7-2020 at 19:36


The potassium perchlorate has now dried enough to be weighed. There is 1027 grams. Wowzers!

The cell has been running with fresh electrolyte now for 30 hours and voltage has been very stable at 4.5 volts. If the anode can handle the whole run I will be collecting the more useful sodium perchlorate.

@markx, do you have pictures of your anode preparation that you can share?


View user's profile View All Posts By User
markx
International Hazard
*****




Posts: 620
Registered: 7-8-2003
Location: Northern kingdom
Member Is Offline

Mood: Very Jolly

[*] posted on 31-7-2020 at 03:51


Quote: Originally posted by Benignium  


@markx, do you have pictures of your anode preparation that you can share?




Yep....take a look around in this topic, it is mainly centered around the Pt anode development:

http://www.sciencemadness.org/talk/viewthread.php?tid=144788




Exact science is a figment of imagination.......
View user's profile View All Posts By User
B(a)P
International Hazard
*****




Posts: 614
Registered: 29-9-2019
Member Is Offline

Mood: Confined

[*] posted on 24-9-2020 at 19:04


I recently acquired some barium chloride with the view to convert it via electrolysis first to chlorate then to perchlorate.
I have a cell with a platinised titanium anode and titanium cathode.
Without doing a whole lot of research I jumper into the setup like I usually would for the sodium or potassium salts.
To aid with efficiency I usually add 1 g of potassium dichromate along with the chloride when I am first setting up. I failed to look into what might happen when dichromate is added to a saturated solution of barium chloride. I didn't realise how insoluble barium dichromate was and I immediately got a fine yellow precipitate in my previously clear solution.
Given barium dichromate is only soluble at 0.3 mg/L per 100 mL water will I have enough free dichromate to prevent the reduction of chlorate at the cathode?
Has anyone else tried the electrolysis of barium chloride? If so what additives did you use?
This paper explores pH, temp and current density, but does not mention any additives, they used an MMO anode. They also used a rotating cathode which is quite interesting. https://krc.cecri.res.in/ro_2006/044-2006.pdf
Any assistance would be greatly appreciated!

View user's profile View All Posts By User
 Pages:  1  

  Go To Top