This capacitor is made from carbon, cement, and water.

https://news.mit.edu/2023/mit-engineers-create-supercapacitor-ancient-materials-0731

 

For creating the aluminum oxide surface, investigate "Anodizing aluminum" to find the secrets that are available outside of the internet and all the cartoons claiming to be science. You will find that aside from chemical balances, the process details are also important.
Of course, that applies to cooking and playing the violin as well.

 

if the anodizing layer is too thick wouldn't I have very small capacitance. even if it can take higher voltage?
Or can I get around that by just more surface area and having them closer together. thinner separator?

Also would leakage be reduced if both layers have anodized/oxide layer to reduce unwanted current from forming between the electrodes. does that sound right?

 

Leakage current is reduced as the dielectric layer resistance increases. Also, as the layer becomes thicker the capacitance is reduced, but the breakdown voltage rises. So it all is a number of trade-offs.

 

I see. so thats why if I form the capacitor both ways the leakage current reduces dramatically. but the capacitance is roughly halved.

 

if the anodizing layer is too thick wouldn't I have very small capacitance. even if it can take higher voltage?
Or can I get around that by just more surface area and having them closer together. thinner separator?

Also would leakage be reduced if both layers have anodized/oxide layer to reduce unwanted current from forming between the electrodes. does that sound right?

Yes, the capacitance is reduced as the oxide layer thickens, but the oxide layer is still much thinner than any physical dielectric you might use would be. For instance, the film layer of the oxide might be 0.5 µm, but a sheet of paper is typically 3 mils thick, which is 76 µm, so 150 times as thick.

 

Printer paper is completely unsuitable. it has something in it which creates a reaction that rapidly destroys and disintegrates the aluminum electrodes. drawing more and more current exponentially.
Paper towels. Napkins or toilet paper seem to be ok though. all roughly equal. Paper towel might be the easiest to use since its one layer and can be compressed pretty thin. and itsthe strongest and most tear and rip resistant.
Maybe some variety of strong thin paper towel would be good as a separator. thick enough to keep the electrodes separate but strong enough to not disintegrate rip or tear when wet.

The distance between the plates at a range of 0.5 to 2mm seems less important than surface area. as surface area around this range has more impact on the capacitance.
So more surface area is really the best way to go for a homemade capacitor. Etching the foil in some way along with more foil would be ideal.
Using an entire roll of foil with several paper towel rolls would create an absolutely monstrously large capacitance capacitor.
though because the leakage current increases with more surface area. it would need to be a very low voltage capacitor. 5-6v or so would be a good upper limit.
As well as being formed in both polarities to further reduce leakage current. So the tradeoff would be lower voltage. and you'd still only get half the capacitance
Using AC at low frequency seems to be good at forming the capacitor with lower leakage. rather than DC.
Ramping the voltage up and down with AC seems to have a counteracting effect on leakage current (Maybe it helps push out impurities into the electrolyte and keeps them from sticking to the foil.)

I think a good way to minimize leakage current would be an electrolyte bath with at least a couple inches separation. with the plates submerged in a solution. Using low frequency AC. While continuously running new fresh clean electrolyte over the electrodes the entire time. ensuring any impurities that end up in the electrolyte get removed and flushed away
One idea is on the reverse polarity is keeping the voltage lower. say 1-3v to minimize the loss in capacitance. but still be able to clean the electrodes of impurities but still keep the leakage current as low as possible

and using a very sterile and "clean of impurities"separator. Then thoroughly rinsing the separator with electrolyte solution as well.
After this. roll the capacitor up and form again at a low current.
This way all impurities should be "forced out" of the electrodes and getting the lowest leakage current possible.

 

I wasn't recommending using printer paper, merely giving it as an example of the type of thickness you are talking about when you use a physical dielectric in a DIY capacitor. If you use a physical dielectric, there's no need to use it wet.

When I was playing around with a crystal radio, I wanted to use all old-school stuff except for the diode (I used a commercial germanium diode for that). So for one capacitor I use layers of wrappers from Lindt truffles (the ones that are spherical). I got some pretty decent capacitances by layering those. The problem is that these weren't readily adjustable. So for my tuning capacitor I took a two-liter soda bottle and put tin foil on each side of it, then I could control the capacitance by filling it with varying amounts of water. It didn't work great, but it did work -- enough to demonstrate the principle.

 

For a wet electrolytic I need some kind of thin strong and micro porous clean sterile fiber/paper as the separator.
Thin porous sterile/clean and strong. no contaminants or impurities. no chemicals that can be leached out either.
A large bath for the electrodes to be washed/cleaned them while giving them an initial forming charge with AC.
Should give a decent result with low enough leakage current to function well. Also need good quality clean and pure electrolyte solution. whether that be baking soda or some other useful electrolyte solution.

I think even an option for a separator could be a really fine nylon mesh. Lots of surface area for the electrolyte to be in. with the least amount of solid material taking up space in between. and very thin. would need to be very careful to have very smooth and continuous surfaces of the electrodes and mesh so theres no places where the foil contacts each other.

 

For a wet electrolytic I need some kind of thin strong and micro porous clean sterile fiber/paper as the separator.
Thin porous sterile/clean and strong. no contaminants or impurities. no chemicals that can be leached out either.
A large bath for the electrodes to be washed/cleaned them while giving them an initial forming charge with AC.
Should give a decent result with low enough leakage current to function well. Also need good quality clean and pure electrolyte solution. whether that be baking soda or some other useful electrolyte solution.

Try rice paper sold at smoke shops.

 

Is rice paper porous? Also isn't it on the more fragile side?
I thought it was coated in something. Similar to baking sheet paper or something?
I would be buying it on amazon if I were to get something.

 

I wonder if I should get some extra heavy duty foil thats slightly thicker and more resiliant and less prone to "crinkling up" and having a mind of its own.
That should still be thin enough and strong enough to use.

 

It would likely improve the durability of whatever capacitor you made.
Now I am wondering if you have measured the leakage current of any of these capacitors. That is, the current that continues to flow after the time constant is satisfied and the voltage has stopped rising. And I am wondering about how polarized the capacitors are. That has been a puzzle to me, as to what the mechanism of the polarization actually is. What sort of capacitance would you get if you substituted a layer of that clear plastic wrap instead of the paper towel and the electrolyte? Mechanically it would be fragile, but if it were not punctured it would be quite a capacitor.

 

With the wet electrolyte solution and either paper towels. napkin. or toilet paper as the porous separator. The leakage current varies from some dozens of microamps at lower voltage (15-50uA). about 5v
up to a significan't significant portion of a milliamp at 16v. so quite a lot of leakage at higher voltage.
Its still enough capacitance to make a spark if you charge it up with a 0.5k to 1k resistor then short the terminals. So anywhere from 100 to 500uF at 16v

Clear plastic wrap would be pretty useless for low voltage high capacitance.
Probably not much better than stacking layers of foil and printer paper then pressing it down with a few books stacked on top of each other or something.
it would be fine with high voltage low capacitance. say 100-300v maybe. but it would be more dangerous too. and not safe.

I actually did try to make one out of sheets of paper and foil. dry. with no liquid. it worked but there was still enough leakage for it to discharge itself quite quickly. it was a 4-layer stack with the terminal tabs folded so its like an alternating layer sandwich.
It was fine up to 32v the highest i tested. but still no spark off it. and it self discharged faster than I could hook an LED up to it with a resistor. not as impressiveas the electrolytic wet foil paper towel capacitor.

I think possibly the idea of washing/rinsing all the materials might help in reducing leakage current. for an electrolytic design.
Run AC at low frequency from -1.5v to rated voltage 16v or so. through the capacitor while rinsing in a bath of distilled water and baking soda. then rinse again with distilled water to rinse away the old solution.
Rinse separator as well with baking soda and distilled water and squeeze out several times. rinse again with distilled water. squeeze out the excess again.
Then put all the rinsed and cleaned items together in a new container with new fresh electrolyte. to maybe get a reduction in leakage current.
At least this is my idea for a possibility.

 

Keep in mind that electrodes & solution might be making a battery rather than a capacitor...

I'm wondering if acetic acid might work (dilute?), otherwise known as vinegar.

 

I am still wondering about the choice of baking soda, and a weak solution at that, as an anodizing material. And I also wonder about how polarized the capacitors actually are. In commercially available electrolytic capacitors the reverse poolarity leakage current is much greater than the forward leakage current. How different is it in the experimental capacitors?

 

Reversing the polarity just forms the capacitor in reverse. So the leakage current is great until its formed. and then it drops. So I guess the leakage current would reduce due to having an oxide layer on both electrodes. Ive tried it and that seems to be roughly what happens. it doesn't prevent you from forming it both ways like a bipolar capacitor.

Also im pretty sure vinegar would just dissolve the aluminum into nothing after a while. maybe dissolve the oxide layer.
The capacitors are polarized if you only form them in one polarity. but its pretty easy to form them both ways (bipolar capacitor)

Unlike commerically made polarized capacitors which you can't form backwards even at extremely tiny small currents. theres probably a few reasons why it doesn't work.
(They just act like a bad diode or zener diode and draw higher and higher currents until they become damaged or turn into a bad resistor)

 

Some years ago I helped a bit to Polish firm into dentistry PEO process. Thus I checked a tens of materials in hundreds of electrolytes at several PEO regimes - and if firm was asking from me only evaluate the pore (cavernes) characterization let the human flesh may grow into bubbles, I measured as well the electrode (size of postal mark) against electrolyte. If most of PEO-ed metalls gave something alike 100-200 pF, some gave even 500 microfarads - just caverns multiplied capacitance over 2 million fold!! Yet dont believe it are capable to stand any large voltages or be longlasting.

 

Some years ago I helped a bit to Polish firm into dentistry PEO process. Thus I checked a tens of materials in hundreds of electrolytes at several PEO regimes - and if firm was asking from me only evaluate the pore (cavernes) characterization let the human flesh may grow into bubbles, I measured as well the electrode (size of postal mark) against electrolyte. If most of PEO-ed metalls gave something alike 100-200 pF, some gave even 500 microfarads - just caverns multiplied capacitance over 2 million fold!! Yet dont believe it are capable to stand any large voltages or be longlasting.

The presently available "super capacitors" use some scheme with carbon to achieve a much greater surface area in the same volume as a regular capacitor.

 

RE: MrBill2 - its very probable, the supercaps use one or another system applying the PEO process. PEO is just the electrolysis in salts solution, BUT the voltage is 300-600 Volts while amperage is 0.5-10 Amps per square inch. Then millions of small sparks are dancing over the surface until its ready.