Sciencemadness Discussion Board - DIY Divided electrochemical cell with stirring and cooling

DIY Divided electrochemical cell with stirring and cooling

bearbot22 - 20-6-2022 at 11:00

I'm proud to present the design of an Electro-Chemical cell that has been developed and improved over a long period of time, with the aim of developing a DIY electrolysis cell that is reliable, features membrane aka diaphragm, stirrer and cooling but also is made from inexpensive and readily available parts.

Blueprint with parts list of the cell can be found in the attached file.

Wondering what to make with such a cell? Well - a lot of chemicals. Take a look in sciencemadness' Library: The Manufacture of Chemicals by Electrolysis

What follows now is a rather long "making of"- text with some hopefully useful information for anyone who wants to design or improve an electrolysis cell.

I started with a beaker-type cell because i had a beaker in my collection. On top of the beaker I put a plastic lid into which I drilled slots and holes for electrodes.

On the Internet i found a tutorial for a DIY magnetic stirrer made from an old PC: Magnets taken from a HDD glued to a CPU fan and connected to the PC power supply, and a sandwich box as enclosure. This magnetic stirrer worked really well and for a long time and the PC's power supply also provided 5V and 12V DC for electrolysis. Of course, I still needed a magnetic bar for the stirrer. I made some out of screws and nuts, wrapped with tape or shrink tubing or plastic tubes with magnets in them. But because they didn't last and weren't balanced well, I quickly opted for PTFE magnetic stirring bars.

To measure the current, I got a cheap digital multimeter from the hardware store.

Next i went for an an more flexible power supply, which I built from the transformer of an IKEA halogen lamp and an electronic adjustable AC/DC module. So I was able to adjust voltage and read the flowing current on the multimeter.

Membrane aka diaphragm was a tricky part. I tried many things: Sheep casings and gelatine ... they decomposed. A salt bridge hadn't allowed enough current to flow.
Gore-Tex, composable plastic film and Fimo haven't allowed electricity to pass at all, nor have porcelain vessels with scraped glaze, unglazed porcelain or most commercially available flower pots.

AFAIK Ceramics are fired twice to make em firm an waterproof. Suitable as a membranes, are ceramics that are fired only once such as Terracotta or Blumat water dispensers for flowers. But their thick walls allow only little current to flow.

The clay tubes i found are sold in aquarium shops in stacks of six as breeding shelter for shrimp. You 'll need to scrape the tubes off the stack and also have to grind the glazing off in- and outside. This can be done with a rough round file and a sharp screwdriver.
Similar caves are also offered with a closed bottom, but I haven't found a way to remove the inner glaze without breaking it.

If someone has the opportunity to build a test tube-shape porous ceramics that is fired only once, that would certainly be a pretty cool improvement.

Using the shrimp clay tube i needed to seal the bottom, which is not as easy as i assumed. Stuck a plastic shopping cart chip to it but no glue I know kept it. So I tried tape, stoppers, etc... nothing lasted. Even a long screw made out of nylon which i stuck through the tube to hold the bottom decomposed. It seems that during electrolysis, the inside of an anode cell turns into a corrosive hell. That's why I came up with a bracket of cable ties.

The level of anode electrolyte should be higher than the electrolyte level in the cell. I think this way osmotic pressure prevents redox reactions.

As a supposed further improvement, I bought an quite expensive fuel cell membrane. Building an anode cell for this membrane was complex because seals were needed and had to hold together with a lot of screws. This finally worked but did not allow more current to flow than the clay tube did. After the first electrolysis the (dry) membrane broke mechanically. So I decided to stick to the clay tube.

Nafion? I didn't test this. IMHO it's too expensive and difficult to obtain for a DIY cell.

For cooling, I had a double-walled vessel in mind from the beginning. First I put the beaker in a plastic cup to which I hot-glued tubes for inlet and return. I put a pot with diluted antifreeze and a small pump in a freezer (drilled holes in the Cover for tubing and power supply).
Since the beaker could move inside the cup, the stirring bar smeared off quickly or if the inlet or outlet were not properly free, it cooled too little or flooded the beaker so this was not particularly reliable.

Using a cylindrical 'Lock-n-Lock' container, I was able to built a much more reliable double-walled vessel. Cut a suitable hole in the plastic lid, place the beaker in it and fixed it waterproof with hot glue.

When electrolysis ran for several hours or even days, a lot of condensation water formed and finally the magnetic stirrer stood in a puddle. So I put the beaker and magnetic stirrer rihgt inside the freezer. This turned out to be a good idea and I did a lot of electrolysis successfull with this setup.
Since there already were holes in the freezer's cover, any hydrogen gas could escape.

Temperature control was done by adjusting the freezer, which was not very accurate. So I ordered a cheap 'STC1000' controller on the Internet. With this, i had the pump and freezer run only when cooling was needed.

However, the handling of the cell was still a bit fiddly, firstly because the plate with the electrical connections wasn't really fixed to the beaker and secondly the Lock'n'Lock vessel could slip on the magnetic stirrer and thus cause the stirrer to fail.

That's why I redesigned an electrolytic cell with everything fastened to a screw-on cap. For this I replaced the beaker with a cylindrical glass jar with a screw cap made of plastic and found a suitable one in a local shop with 500g of honey in it. This is the reason why I call the cell's design "Honeypot".

Maybe Honey is sold in these jars only where i live so you might look for a cylindrical glass jar with plastic screw cap filled with peanut-butter, marshmallow spread or whatever.

Cut out the lid and screwed on a sturdy disc made of acrylic glass to which stirring-motor, cathode, anode-cell and temperature sensor are attached. The attached drawing shows how its done exactly.

As a stirring drive, I initially used a cheap DIY Robotics motor with plastic gearbox, but unfortunately those fail after a few hours of continuous operation. A 20D 6V DC motor with metal gearbox plus a USB power supply seems to be much better.

As a power supply for electrolysis, I replaced the IKEA-Transformator+Multimeter- Thing with an adjustable lab power supply (2A/30W) because its short-circuit-proof and less wiring is needed.

For cooling I use a fridge with icebox. I unscrew the top cover and cut a circular hole in the insulation so i can put the cell right inside the icebox. After reassembling, this modification of the Fridge is barely visible. During electrolysis, Fridge is set to max cooling and powered via the STC-1000 controller. A soft cooling pad under the glass jar improves heat dissipation.

Hope this gets your lab a little further - BearBot22
Attachment: honeypot3.pdf (75kB)
This file has been downloaded 570 times

mysteriusbhoice - 27-6-2022 at 10:43

use ionomer cement if you want a cheap good membrane
cement and cation/anion exchange resin commonly sold in water filter store cement to ionomer cement wt ratio 1:3
you can also use plaster of paris if instead of cation exchange resin.

this mixture is mixed with water and casted across a glass container with 2 dividing walls which will produce a dam (membrane) which is highly conductive due to the imbedded ion exchange resin or plaster of paris CaSO4 which can also exchange ions.

bearbot22 - 18-7-2022 at 05:16

hi mysteriusbhoice,

the electrolyte used is 7-10% H2SO4 in water/alcohol - i'm not really sure plaster of paris aka gipsum will last.
also i read cement becomes instable when in contact with gipsum before it is totally dry.

i've never heard of ionomer cement before, so i'll give this a try.

mysteriusbhoice - 18-7-2022 at 17:23

Quote: Originally posted by bearbot22

hi mysteriusbhoice,

the electrolyte used is 7-10% H2SO4 in water/alcohol - i'm not really sure plaster of paris aka gipsum will last.
also i read cement becomes instable when in contact with gipsum before it is totally dry.

i've never heard of ionomer cement before, so i'll give this a try.

Gypsum will last with H2SO4 as I've used it in up to 20% without issues but it will be attacked by NaOH.
Also the gypsum and cement being unstable only matters for construction applications and this membrane won't bear any load and they seem rock solid.
Anyway cation exchange resin with cement is good too and use glass container not plastic because plastic will flex.

mysteriusbhoice - 19-7-2022 at 02:23

I have a video on my channel about making various ion exchange membranes and the ionomer cement is convenient and allows for a membrane which is both not as porous but highly conductive to the particular ions of choice depending on the resin you choose.
The current flow is shown in my video and it's quite decent compared to terracotta which only really did decent past 12 volts.

sykronizer - 1-3-2023 at 23:37

the membrane is not that hard, just use the plastic separator found inside any lithium ion battery, they conduct great just make sure to not let the temp get too high otherwise the micropores will close up, less than 50 Celsius is fine.

Hexabromobenzene - 2-3-2023 at 06:39

I know about separator from li-ion battery, but separator with enough size is rare. Plastic bag is common. They made from polyethylene. Can thin bag use for electrolysis?

Sulaiman - 2-3-2023 at 08:28

I've not tried polyethylene as a membrane because I would be surprised if it works.
I've always considered polyethylene to be an impermeable electrical insulator

PS regarding pots etc. :
they have to be porous enough to allow the pores to fill with electrolyte because dry ceramics are insulators.
If very porous then too much electrolyte mixing occurs
If very fine pores then a long soak is required for the electrolyte(s) to penetrate
(the porous pots that I have add about 1 Ohm to 1M sulphuric acid or copper sulphate solutions)
Cleaning them between uses of different electrolyte can be very difficult
Metallic ions 'plating out' within the bulk of the membrane/pot can be a problem also
There are many reasons why porous pots are rarely used.
But I do like the resemblance to (or imitation of) early batteries.

[Edited on 3-3-2023 by Sulaiman]

Hexabromobenzene - 3-3-2023 at 04:30

You're right. Untreated polyethylene will be an insulator. Here, a method was proposed for making a diaphragm by impregnating a fibrous material with a polymer solution and dipping it in water. This forms a porous polymer. This can be done with polystyrene and PVC

Probably latex. It is porous in itself due to manufacturing technology. But I have no data on its chemical stability. Is it resistant to 20% alkali or 30% sulfuric acid with chlorine and oxidants sometimes? Polystyrene is resistant to all these conditions
In this case, balloons and condoms can be used as a diaphragm.

But if we do experiments with organic substances, then such a diaphragm can be destroyed. The only option for organic electrochemistry is ceramics and porous polypropylene from batteries

Improved DIY Electrolytic Cell

bearbot22 - 11-8-2023 at 06:46

I improved the cell-design. Honeypot is now a lot easier to build and to handle.
Whith a current of up to 1,5 Amperes at 8,8 Volts various electrochemical organic syntheses are possible using Honeypot, e.g. reduction of nitroalkenes to the according oximes and amines.

Diaphragm chamber

Hope you like. Here are complete instructions including cooling and stirring:

Attachment: HONEYPOT_IMPROVED.pdf (150kB)
This file has been downloaded 363 times

mysteriusbhoice - 14-8-2023 at 15:44

if the cylinder was made of ionomer cement I wonder how much higher the current can be pushed thru.

bearbot22 - 16-8-2023 at 05:26

Hi mysteriusbhoice!

Quote: Originally posted by mysteriusbhoice

use ionomer cement if you want a cheap good membrane
cement and cation/anion exchange resin commonly sold in water filter store cement to ionomer cement wt ratio 1:3
you can also use plaster of paris if instead of cation exchange resin.

this mixture is mixed with water and casted across a glass container with 2 dividing walls which will produce a dam (membrane) which is highly conductive due to the imbedded ion exchange resin or plaster of paris CaSO4 which can also exchange ions.

I tried making this kind of membrane weeks ago.
Ordered some ion exchange resin Purolite MB 400 (looks nice, like amber-golden sand).
Mixed it with cement and water in recommended ratios to cast.

But when i retrieved it from the form the result just crumbled.
Tried other ratios also but did not get anything usable.

mysteriusbhoice - 17-8-2023 at 18:09

cement takes a few days to cure completely I would let it set for a day and put it underwater for 1 week to fully cure.

khlor - 19-8-2023 at 18:22

Quote: Originally posted by bearbot22

I improved the cell-design. Honeypot is now a lot easier to build and to handle.
Whith a current of up to 1,5 Amperes at 8,8 Volts various electrochemical organic syntheses are possible using Honeypot, e.g. reduction of nitroalkenes to the according oximes and amines.

Diaphragm chamber

Hope you like. Here are complete instructions including cooling and stirring:

Very interesting project, I’ve been looking ths thread for a while.

Please allow me to give my contribution, hope it can give you a few ideas on improving your own project.

Below there are some pictures of what I can only describe as about 18 months of experimentation and hard work. This one was the latest, never used(though it is the second and early variations were used to make chlorine with some success) it has been over a year already since I last touched it before today.

The design consists of two jars with holes drilled with them for gas outlet, electrodes connections and ports for electrode feed and output, both chambers are divided by a clay/ceramic cup used as diaphragm. I used it successfully for electrodialysis as well.

I’d post the design papers but I would like to avoid having my manuscript on the net, however if you have interest I can make the drawings only.

Sulaiman - 19-8-2023 at 22:22

Quote: Originally posted by bearbot22

I'm proud to present the design of an Electro-Chemical cell ...

Congratulations !
A working finished* product.
very nice work.

* do projects ever really end ?

bearbot22 - 25-8-2023 at 08:18

Hi Khlor! Thanks a lot for adding your design here.

Quote: Originally posted by khlor

The design consists of two jars with holes drilled with them for gas outlet, electrodes connections and ports for electrode feed and output, both chambers are divided by a clay/ceramic cup used as diaphragm. I used it successfully for electrodialysis as well.

It's a different approach in designing a divided electrochemical cell, but also using household and hardware materials.
That is very inspiring for me to see!

No doubt about this being a lot of hard work - drilling holes in round glass vessels is not a beginner's job.

I specially like the concept of holding the jars and diaphragm together with springs and turnbuckles so you can adjust pressure.

Do you need additional sealing for the diaphragm?
What material is used for sealing the drilled holes?
Can you tell what current / voltage you could work with?

khlor - 2-9-2023 at 07:56

Quote: Originally posted by bearbot22

Hi Khlor! Thanks a lot for adding your design here.

Quote: Originally posted by khlor

Thank you for your words, your praise do me great honor, specially when it comes from someone doing the same thing on a semi professional level. And yes I do not have access to good tools and materials so I had to use off the shelf parts and a great deal on hard work and imagination to improvise this.
And the dilling is not as hard as one might imagine but it does take technique and practice to get it right.

It was something that came when I was in the shower, so I bout the turnbuckles and galvanized wire for the springs(yes, hand made springs)

1. Yes, I use PTFE tape(here known as plumbers tape used to seal threaded tubes) to make some sort of O ring to stay in between the mouthsof the jars and the pressure of the springs keep all in place so it does not leak, I found this design to be the best based on trial and error, so, yes... a lot of work and angry tears went into this. When working with these things, remember, PTFE is your friend!

2. I used epoxy binary compound. When it cures it looks like some ceramic, it resists well to chlorine and acidc environments, but do not let it in contact with sodium hydroxide, it will turn it into some weak rubber, I think the composite material used is aluminossilicate and sodium hydroxide will leach it, plus it loses adherence to the glass.

3. The one in the photos is a revision design built about 18 months ago, never used nor tested so I cannot answer this question honestly but, it is supped to have better current density based on the original that looks just like what you saw on the photos. With that being said I managed to squeeze about 1.5A at 5v into this. My main limitation though is electrodes, I use graphite and overtime it exfoliate, but it is cheap to replace which is good so I put up with it.

Note: my chathode uses galvanizedl wire, not the best material I am gonna change it, I am open to sucgestions. I thought of iron or copper wire.

Edit:
Aditional note, this design is basically the product of my need to collect the gases, in the middle tubes of each jar as you can see, hoses would be connected to allow for chlorine or whatever gas comes at the anode and hydrogen on the cathode to be collected, true I could make it simpler and not care about electrolytes but I decided to go all the way and score on the sodium hydroxide I know I can always find a use later. You can later check my thread on HCl production plant here on technochemistry at the time of the experiments this specific design was the one that allowed to make many grams of chlorine gas.

Edit2: typos correction
[Edited on 2-9-2023 by khlor]

[Edited on 2-9-2023 by khlor]

DXFlatline - 18-10-2023 at 05:57

Ceramic pots for electrolysis membranes are widely available commercially. The correct term is "porous cup", I bought mine from Amazon.

khlor - 28-10-2023 at 11:57

I wish I knew this way back. Feel so dumb now.

DXFlatline - 19-11-2023 at 08:08

I've started making one based on bearbot's designs:

It's not finished yet needs a stiring paddle and the STG-1000 probe putting in glass. I've got a unique method for cooling but I've not tested it yet and I need to make a potentiostat circuit.

bearbot22 - 4-1-2024 at 07:26

This looks very nice!

Quote: Originally posted by DXFlatline

I've got a unique method for cooling but I've not tested it yet and I need to make a potentiostat circuit.

What method of cooling is it?
Potentiostat.. whow!.. this would also require a reference electrode!?
Seems you're up for a more sophisticated cell.

I'd love to hear about your plan, DXFlatline, please keep us updated.

Hexabromobenzene - 28-3-2024 at 14:50

mysteriusbhoice
I found a new method of manufacturing diaphragm for electrolysis. It looks like your method, but does not needs solvents. This diaphragm is flexible and chemically resistant. For the manufacture of diaphragm, you will need foam LDPE for packaging. You need to dip it in boiling water. In hot water, the foam softens and bubbles of gas come out of it forms micro holes. Then the foam WHILE IT HOT is rolled up with a rolling pin to create a denser structure . You need of thick foam at least 1 cm thick. More thin foam diaphragms noticeably pass water. Thicker LDPE Foam is usually carried out by welding a few less subtle ones. Look for them. You can also use the LDPE foam for construction

dicyanin - 4-4-2024 at 10:55

Beautiful work :cool:

I am tempted to build one according to your design. Is the resistance across the shrimp breeding tube diaphragm workable? I've used flowerpots succesfully in the past, but resistance is usually high even after soaking in dilute sulfuric acid, things heat up fast and especially the anode suffers from this. The other downside of flowerpots is their shape & size and the fact that the setup is open to the atmosphere releasing corrosive vapors.

I have always wondered about Tyvek, which is semipermeable high density polyethylene, it allows for gas/vapour exchange but is waterproof, so I'd assume it would let current pass but not the solvent? Tyvek is said to be quite resistant against acid and base.

Or perhaps these cylindrical shaped corundum lab crucibles?

You can buy Nafion perfluorosulfonic acid-PTFE membrane from China but even the cheaper knockoffs are $20 for a 3 x 3 cm size film. I've also seen H-type divided cells for sale but they aren't cheap either, up to $200 for 2x 100ml size with a clamp and Nafion diaphragm included.

To prepare a lead cathode, lead can be finicky regarding surface purity and give inconsistent results. It is best to pre-treat them in a divided cell with 10% aq. H₂SO₄ or Na₂SO₄ electrolyte, use the cathode-to-be as anode (and another piece of lead as cathode) and run a current through it until it is covered with a thick chocolate-brown layer of lead dioxide. Then switch polarity and run a current until all oxide is reduced, leaving a gray layer of spongy lead. In this way even roofing sheet grade lead can be used as a reliable high overvoltage cathode for electroreductions with no other preparation that a light sanding and an acetone rub to clean the surface before pre-treatment.

The only thing I would add to your design, although it is optional, is an extra hole for a reference electrode over the catholyte. I've seen Chinese calomel and silver-silver chloride reference electrodes sold for about $30 a piece. This allows for fine-tuning the required voltage over the cathode for a specific reaction, minimizing loss as heat and competing hydrogen evolution.

Quote: Originally posted by Hexabromobenzene

I know about separator from li-ion battery, but separator with enough size is rare. Plastic bag is common. They made from polyethylene. Can thin bag use for electrolysis?

Back in the day I've used the separator envelopes inside car batteries, which separate the lead electrodes from the lead dioxide wafer electrodes. They consist of microporous polyethylene, and they work well as diaphragm, but in my experience they tend to be leaky and easily damaged. Also I don't want to go through the hardship of opening an old car battery again, probably contaminated myself with enough lead for one lifetime during that ordeal.
I don't think a plastic bag will work. The porous PE separators resemble paper more than plastic bags in how they feel and how easily they tear.

Here's some reading material on the manufacture of microporous battery separators.

[Edited on 4-4-2024 by dicyanin]

Hexabromobenzene - 8-4-2024 at 13:30

See my post above. I made a porous material of LDPE packaging foam. If it works as a diaphragram, it will be a very cheap durable and persistent material

Mateo_swe - 6-5-2024 at 14:56

Maybe try a pre-made membrane, then you have something to compare against with your DIY membranes.
They dont have to be very expensive, there are different ones and brands but they usually are somewhere like 25 to 30$ for a 10x10cm piece.
https://www.fuelcellstore.com/fuel-cell-components/membranes

Maybe find out if there are any pottery hobbyists where you live and ask if they can make a non-glazed clay divided cell for you with the divider made pretty thin.
Its good to glaze the top 1-2cm otherwise the fluids might creep over by capillary forces.
Or if you have some pottery skills yourself and access to a oven.
Just dont choose a ceramic type clay as these have closed pores and thus wont work.

bearbot22 - 22-6-2024 at 08:43

Quote: Originally posted by dicyanin

Beautiful work :cool:

Quote: Originally posted by Hexabromobenzene

I know about separator from li-ion battery, but separator with enough size is rare. Plastic bag is common. They made from polyethylene. Can thin bag use for electrolysis?

I my case [i regular do organic reductions to amines using H2SO4(7%) in H2O/IPA mix as electrolyte] 1,5 Amperes with ~7 Volts are possible with the shrimp clay tube. Could be more Volts and Amps but i'm limited by cooling capabilities, because the run needs to be kept below 15°C.

As far as i remember Tyvek was one of my first tries as a membrane. It's a good electric insulator and no liquid i know of would permeate it. Did not work for me.

I didn't try corundum as they were quite expensive from local Lab supply

I totally agree on cathode surface to be a crucial factor, therefore i use them only once and prepare them with care.
mine are cut out of rolled lead sheets from roofing supply which his supposed to be 99.9% pure.
The suggested oxidize-reduce pretretment was used by me in the beginning.
Actual i get best results with this pretreatment, done right before starting the run:

cut to size piece of lead sheet is rolled on a clean hardened glass plate both sides. sanded wet both sides, cleaning the glass plate very time it's turned over. Finally polished to a mirror-like finish using a heavy brass (polished) tube. No polishing paste, no acetone wipe.
Right after polishing the cathode is put in diluted sulfuric acid and coated with zinc using a circular anode taken from a zinc-carbon battery (20mA / 0,8V / 20min).

Reference electrode would be a next step but this would also include more sophisticated power supply etc... would love to read from anyone to challenge this.

Catslovechemistytoo - 11-8-2024 at 07:13

Quote: Originally posted by bearbot22

I my case [i regular do organic reductions to amines using H2SO4(7%) in H2O/IPA mix as electrolyte] 1,5 Amperes with ~7 Volts are possible with the shrimp clay tube. Could be more Volts and Amps but i'm limited by cooling capabilities, because the run needs to be kept below 15°C.

I totally agree on cathode surface to be a crucial factor, therefore i use them only once and prepare them with care.
mine are cut out of rolled lead sheets from roofing supply which his supposed to be 99.9% pure.
The suggested oxidize-reduce pretretment was used by me in the beginning.
Actual i get best results with this pretreatment, done right before starting the run:

cut to size piece of lead sheet is rolled on a clean hardened glass plate both sides. sanded wet both sides, cleaning the glass plate very time it's turned over. Finally polished to a mirror-like finish using a heavy brass (polished) tube. No polishing paste, no acetone wipe.
Right after polishing the cathode is put in diluted sulfuric acid and coated with zinc using a circular anode taken from a zinc-carbon battery (20mA / 0,8V / 20min).

Reference electrode would be a next step but this would also include more sophisticated power supply etc... would love to read from anyone to challenge this.

Interesting. But how are you using zinc coating under acidic conditions? Or are you using a different electrolyte in that case? I don't understand your logic of doing this, zinc and lead have nearly similar overpotentials, why coat it with zinc?
And in the case of organic reductions to amines, what typical yields do you get? I wonder if electrochemistry can substitute NaBH4 or Na(AcO)3BH in reducing imines.

I've read about another way to prepare lead cathode:
A lead electrode is put into 20% sulfuric acid and anodized (another lead electrode is used as cathode). The current is set to 0.08-0.1 A/cm^2, and electrolysis is done for 5 minutes. The oxidised lead electrode is washed with water, then the polarity of the cell is changed and the oxides are reduced to lead sponge. This is repeated for 3 times and in the end of the process the lead electrode is anodized once again. (Source: Tafel J. Ber., 1900, Vol. 33, Page 2224) Edit: just realised another person already wrote about this here.

[Edited on 11-8-2024 by Catslovechemistytoo]

bearbot22 - 13-8-2024 at 02:34

Quote: Originally posted by Catslovechemistytoo

Zinc is used on cathode-side, so it will not oxidise, even in acidic conditions.
I read about Zinc plating increases reducing potential for aromatic nitroolefins in a reference and my experiments confirm this. Unfortunately i can't find the ref. anymore. Electrochemical organic reductions are influenced by all aspects of a cathode, not just overpotential.

>And in the case of organic reductions to amines, what typical yields do you get? I wonder if electrochemistry can substitute NaBH4 or Na(AcO)3BH in reducing
>imines.

I get about 50 to 60% of theoretical yield, maybe more. Some % of product is lost because solvents used for extraction are not optimal.
In theory it's possible to use electrolytical reduction instead of reducing agents. Electrical current is most powerful reductor in chemistry. You can check electrochemistry_of_organic_compounds in Sciencemadness' Library.

>I've read about another way to prepare lead cathode:
>A lead electrode is put into 20% sulfuric acid and anodized (another lead electrode is used as cathode). The current is set to 0.08-0.1 A/cm^2, and
>electrolysis is done for 5 minutes. The oxidised lead electrode is washed with water, then the polarity of the cell is changed and the oxides are reduced to
>lead sponge. This is repeated for 3 times and in the end of the process the lead electrode is anodized once again. (Source: Tafel J. Ber., 1900, Vol. 33,
>Page 2224) Edit: just realised another person already wrote about this here.

The polished cathode surface is beneficial in my reductions for physical reasons, i guess. Hydrogen bubbles are separating faster from a smooth surface. A spongy cathode is probably blocked to a certain level by Hydrogen Gas sticking to it.
I'm aware H2 bubbling could be avoided by using lower voltages, but I need to raise voltage to pump enough current through the cell.

Catslovechemistytoo - 16-8-2024 at 06:07

Quote: Originally posted by bearbot22

I get about 50 to 60% of theoretical yield, maybe more. Some % of product is lost because solvents used for extraction are not optimal.
In theory it's possible to use electrolytical reduction instead of reducing agents. Electrical current is most powerful reductor in chemistry. You can check electrochemistry_of_organic_compounds in Sciencemadness' Library.

I assume that you're talking about certain nitroolefins here that were reduced, right?
Electrochemical reduction is probably the way to reduce these compounds in bulk, and probably the cheapest way also. Although I'm not sure if we're allowed to talk about them on this forum.

Would you mind running an experiment on your cell to reduce imines of aromatic amines (or bulkier amines in general, like pyrrolidine) and simple carbonyls? I've read that although simple amines (like methylamine, ethylamine) can form imines and then easily reduced in an electrolytic cell - higher amines and aromatic amines tend to form many side-products and give poor yields or do not work at all.
For example, you can try to reduce the imines of benzylamine and acetone, or aniline and acetone - in case of benzylamine and acetone, for example, you could form the imine before you reduce it. As an electrolyte, you can use potassium acetate in IPA/ethanol. I'm not sure about the pH of reaction in this case, maybe a buffer of H3PO4/KOH can be used, or KOH can be added as reaction progresses, to keep pH around 10-11?
I would try this myself, but I'm having problems with bulding my cell.

bearbot22 - 20-8-2024 at 08:31

@Catslovechemistytoo
Even if I want to, i can't run your experiment. My clay membranes are soaked in acid and benzylamine or aniline aren't available.

But if you tell me about your cell building problems, maybe I could try to help you with that.

Divided Cell using "Blumat" - very simple

bearbot22 - 5-10-2024 at 10:46

The construction of an electrolytic cell with membrane (diaphragm) is particularly simple with a “Blumat Easy” watering cone.

These modern Ollas consist of a hollow clay cone and a plastic adapter for PET bottles and are ideally suited as an anode cell.
Together with a container, electrodes, crocodile clips and a lid with a matching slot and hole, the cell is almost complete.
and may be put on a mag stirrer if needed. [see picture]
Unfortunately, electrical resistance of the porous but thick clay layer is quite high.
For many electrolysis processes, the required current can only be achieved using high voltage, an this will heat the cell rapidly.
However, the resistance can be reduced by making the clay layer thinner. To do this, just rub the cone over a piece of coarse sandpaper
laid out on your workbench and continue turning the Blumat in small steps.
The thinner the wall, the lower the electrical resistance. The picture shows that quite a lot of material can be removed.
In this test, the upper diameter was reduced from 22 mm to 17 mm and the sanded Blumat was about 1 cm shorter overall.
From the original 23.5 grams, 14.6 grams were left after sanding.[see picture]

An electrolysis test was done using this setup:
Nickel sheet as cathode, carbon rod as anode and 7% sulfuric acid as electrolytes. Blumat cones were soaked with electrolyte.

Current flow:
a) Without membrane 100mA at 2V, 250mA at 3V, 1200mA at 4V
b) With original Blumat: 0mA at 2V, 50mA at 3V, 110mA at 4V
c) With sanded Blumat: 30mA at 2V, 130mA at 3V, 350mA at 4V

According to simplified calculations, the original Blumat increases the electrical resistance by 33 ohms or more, depending on the voltage applied,
and by 8 to 11 ohms in the sanded version.
The clay tube used in the “Honeypot” [see older posts] which has a 3.5 times larger surface adds 2.8 ohms at 4V in a similar setup.

For a drip test, I filled the Blumat to the brim (approx. 17ml) with water. Overnight, 1 ml leaked from the original cone and 12 ml from the sanded cone.
(Clay tube: 17 of 32ml)
Dripping means electrolyte and ions may wander through the diaphragm both ways. To deal with this, here are two ideas:

- Fill the Cell to a level higher than the surrounding electrolyte and keep it higher during electrolysis by adding electrolyte as needed.

- Use a gel-electrolyte inside the cell. Xanthan (2g/liter) works well in diluted sulfuric acid with almost no increase in resistance.
But it might be difficult to find a suitable gelling agent for your electrolysis if chlorine .... is involved.