I have a bank of ultracapacitors which were commercially made. Most of them are made by Maxwell Technologies, however, I have a few from a company called Ioxus as well. I have them configured in series/parallel such that I have about 600 Watt hours of storage between a high voltage of 60V and a low voltage of 32V. I generally consider the bank to be a buffer as 600 Watt Hours does not go very far.
The nice thing about capacitors is that they are essentially maintenance free. They do not suffer damage if discharged to zero volts, I am not sure about reverse polarity, although in testing they seem to have a diode internally or something that limits reverse polarity to about a half a volt. They can dump an incredible amount of power, in my setup, limited only by the cable and connectors and of course the fuses. They could put out far more instantaneous power than I could imagine using at this point, all be it for only a short time.
They seem to have a higher self discharge per watt hour of storage than batteries, but not to the point that they are not effective at providing power overnight. They do require some circuitry to ensure that they do not get into an over voltage condition which is reported to damage them. Because of the series arrangement they have some vulnerability to this if not protected. I have yet to experience detectable damage, although in testing and temporary set ups I have over charged a few.
The downside is that to if you wanted to use them as primary storage, they are way too expensive. I think that new, each 3000 Farad, 2.7V capacitor would be about $50 US. In my setup I use 96 of them per 178 Watt Hours of usable energy storage.
If only they, or something that could function similarly, could be had at a much lower price. The commercial units are optimized to provide bursts of power for functions such as regenerative braking in vehicles and other such high amperage usages. I don't need the ability to put out the massive power that these things can put out, I would only be drawing it at a few thousand watts max at present.
The first electric double layer capacitors that I saw were the button cell units of about a farad at 5.5 volts. They have activated carbon electrodes separated by an ion permeable membrane and generally used a sulfuric acid solution as the electrolyte.
Sulfuric acid tends to be rather corrosive to most metals. I recalled that Edison chose an electrolyte of potassium hydroxide for his batteries on the basis that it would preserve the battery materials better than an acid.
A little checking revealed that there are a few companies making alkaline ultracapacitors, KA Power is one, and they seem to be marketed in packages of about 30 Watt hours and optimized again for high current, and in this case utilized to help truck engines start, especially in cold weather. They also appear to use asymmetrical electrodes to get more current out of them.
I have done a bit of electroplating in the past as well as built a small unit to generate hydrogen by electrolysis. I recall that stainless steels can generally withstand a few volts before they pit and corrode due to galvanic corrosion when used in electrolytic cells.
So, I got to thinking, why not coat some stainless steel mesh with activated carbon and separate them with a cloth that won't break down in alkaline solution and see what happens. I searched online to see if anyone else had done this and did notice one fellow had posted a video online along similar lines using a tic tac box and some plaster mesh, which I don't think was stainless. He reported a 1.2 Volt 400 farad capacitor as his result.
Encouraged I drafted my son to help out and we ordered some stuff and set off to the store. We bought the following items:
Activated Carbon
This is activated carbon sold for aquarium filters, probably not the perfect activated carbon for this project, but it should still yield significant surface area per unit of mass.
Stainless Steel Mesh
It is of a size that would be suitable for a 100 micron sieve, We probably could have used larger wire size but it was what I found on E-bay in 316 grade.
Sodium Hydroxide
Sold as drain cleaner, this was obtained from my friend Paul down at Chiarugi hardware on the end of my block.
Binding Agent
We used Epoxy as a binding agent as I am thinking that if I mix enough carbon in it will make a conductive paste that we can coat with more activated carbon. We wanted a longer hardening time, but the five minute epoxy was all we found stocked.
Container for a small experiment
We used PVC pipe as it is relatively inert and readily available at the hardware store. We got one inch size as it seemed about the right size for a small experiment that might fail anyway.
End caps
PVC end caps for the one inch pipe
PVC cleaner
wanted to make a good joint so we got cleaner, as near as I can tell this stuff is mostly acetone.
PVC Glue
PVC glue
So, we started off cutting three inch long pieces of the three inch pipe.
Then we cleaned one end and glued end caps onto one end of each piece.
Next we got a blender (hope we don't get in trouble with a certain someone for what we did with it)