On SuperCapacitance

A "capacitor" can be a very simple device. It's first version was called a "Leyden Jar", and was invented in 1745 (see link). In modern form, just take an ordinary glass jar and wrap the outside with aluminum foil, and also put a layer of foil on the inside of the jar. Make sure the two layers don't come any closer to each other than the thickness of the jar. Also make sure the jar itself rests upon some sort of non-conducting surface, like a wood tabletop. Then attach one wire to each layer of foil, so that you can put some energy into this capacitor, and withdraw it later.

Capacitors only store Direct Current electricity. If you hook this jar to a 9-volt battery, the battery will output some electricity that will be stored in the jar, until the voltage of the stored electricity equals the voltage of the battery. This could take several seconds. WARNING! CAPACITORS CAN BE DANGEROUS!!! You might be able to safely put your tongue across the terminals of a 9-vold battery, and feel a tingle, but if you do that to THESE wires, you could be burned or worse. The small battery is strictly limited in how fast it can deliver Amperage, the essence of a flowing electrical current; a capacitor is NOT so limited. An electrical short-circuit, such as one involving a wet tongue, is an invitation for a capacitor to dump all its stored energy AT ONCE. It's not the voltage that causes electrocution; most people can easily handle a 30,000 volt shock after scuffing shoes across a carpet on a cold dry day, and then touching a metal water pipe. It's amperage that kills, and most capacitors can easily dump dozens or hundreds of amps in an instant (and some capacitor banks are specially built to do millions of amps; see the Z-machine link). That carpet-scuffing shock involves only a fraction of an ampere; messing with Leyden jars has occasionally proved to be fatal.

Historically, the total amount of energy that a capacitor can store has actually been rather small. If this seems surprising, it is a consequence of difference between "energy" and "power". Let us consider a dam holding back a lot of water, to make this difference clear. The total energy available is represented by the amount of water behind the dam. The power of the dam is represented by the rate at which water flows from the reservior through the dam. Even if no rainfall adds water to the reservior, the dam can produce power for a considerable time (much like a battery). If the dam is made to disappear, and the whole body of water rushes through its former location at once, that is a LOT more Power in action, but the total amount of Energy released would have been the same. Power is the RATE at which energy does things, and a shorted capacitor is always equivalent to that disappeared dam. Even without a lot of total Energy available, the Power it can exhibit can be formidable.

It happens that the total Energy that a capacitor can store is directly related to the total surface area of the foil sheets. The thickness of the sheets doesn't matter; only the total area separated by an insulator matters. You can now make a MUCH MORE DANGEROUS capacitor at home this way, using two rolls of aluminum foil and two rolls of plastic wrap, all the same width, perhaps 30cm (edge view; ignore the dots):
. ----------------Layer of foil-->(unroll)
----------------Layer of plastic wrap-->(unroll)
. ----------------Layer of foil-->(unroll)
----------------Layer of plastic wrap-->(unroll)
Note how, at left, the two layers of foil are not "at the edge"; they must not be allowed a chance to contact each other. The layers also need to be offset from each other, as in this "top" view (ignore the dots):
. ______edge of first foil layer
_|_____edge of both plastic layers
||-------dotted line is edge of other foil layer
||at far left is end of plastic layers
|| . . all four layers are overlapping here in middle
||at near left is end of foil layers
||-------dotted line is edge of first foil layer
|______ edge of both plastic layers
. |_____edge of other foil layer
Now return to the first sketch above, and start rolling up the four layers, from left to right. If you are unrolling the raw materials as you roll up the assemblage, you will find that the "outside" layers must be unrolled faster than the "inside" layers. This is natural. Stop when you become afraid of just how dangerous this capacitor is becoming. Make sure that the final ends of the foil layers remain completely insulated by the plastic wrap (cut the metal before cutting the plastic!). The final thick assembled roll, hopefully with plastic on the outside, will have metal foil sticking out of both ends of the body of the roll. At each end the extended metal foil can be squished down to a minimum size; you would attach wires to those points.

The maximum voltage that a capacitor can store is related to the quality of the insulator between its conductors. Plastic wrap is pretty thin stuff, and probably can't be expected to withstand much more than 12 volts. IF IT CAN (and I'm not saying it will), then you could hook it up to an ordinary 12-volt car battery, and charge this capacitor. Don't be surprised if you get a big spark when you make the connection! BE AWARE that hydrogen gas that naturally occurs in the vicinity of lead-acid car batteries can explode if a big spark happens. (Have I made it clear: THIS IS DANGEROUS!!! ?) If the plastic wrap can't withstand the 12 volts, then you will see an even bigger bang when the capacitor self-destructs. If the plastic CAN withstand the 12 volts, then THE CHARGED CAPACITOR WILL BE QUITE DEADLY IF MISHANDLED. All by itself, a car battery can put out a dangerous amount of amperes, and all car batteries have warnings about it. But that is nothing compared to the flood of amperes this capacitor can release, when its two wires are shorted together. YOU HAVE BEEN WARNED!!!

However awesome that particular capacitor may seem, IT IS STILL INADEQUATE for the purpose of storing a decent total amount of energy, such as you might want to use to power your air-conditioning system on a hot night. That is, if you had a method of gathering solar energy during the day, you would want to gather and store some extra for use at night. Batteries are seldom more than 75% efficient at storing and releasing energy; capacitors are almost 100% efficient. Therefore the search goes on, for ever-higher-capacity capacitors. Current versions are called "supercapacitors", or even "ultracapacitors".

They've almost reached the point where you can power your cell phone with one for a decent amount of time (remember, the huge power surge I've talking about before involved an electrical short-circuit; when a proper electrical "load" is used, a capacitor works like a battery --just not as long-lasting, until very recently). A cell-phone supercapacitor would be cool because it can be charged at maximum rate (equal to a short-circuit discharge rate), and be fully charged in seconds. It also will be good for many thousands or even millions of charge-discharge cycles; capacitors have no moving parts (or even moving chemicals like batteries do) to wear out. MOST of the entire chemical-battery industry will be doomed when supercapacitors become capacious enough (likely within 5 years); plan your investments accordingly!

And now it is almost time for the Idea this post is really about, for which all the preceding was background information. As mentioned, the amount of energy that a capacitor can store is directly related to the total surface area of its conductors. It also is related to the ability of its insulating layers to withstand voltage. The more voltage a fully-charged capacitor can handle, the longer it takes to "drop" to a useless level, when powering something.

So, we want to constuct our Ideal Capacitor out of layers such that its conductors are as thin as possible, and its insulators are as resistive as possible.

For the first, I recommend "graphene". This is the stuff, rolled up, of which carbon nanotubes are made. It is one layer of a chunk of graphite. It is one atom thick, a completely 2-dimensional array of hexagons, and an excellent electrical conductor. See link.

For the insulating layer, I want to suggest something that I'm not sure is known to exist, but OUGHT to be able to exist. The chemical compound "boron nitride" should be able to form a 2-dimensional hexagonal lattice identical to graphene. I have a link that mentions "a repeating array of hexagons in 3-dimensional space", but that's not quite what we need here. PERHAPS the 3-dimensionality of the array is only minor, something like this:
B< (the odd symbols represent chemical bond angles)
__>N (ignore the underscores)
B<
__>N
I've portrayed a vertical zigzag; a horizontal zigzag would have all the borons on one side and all the nitrogens on the other side of the imaginary 2-dimensional plane between them. That will be good enough!

Anyway, the boron-nitride chemical bond is a very strong bond, and such bonds always make excellent electrical insulators. Therefore we should seek to construct our Ideal Capacitor out of two layers of graphene, and two layers of boron nitride, rolled up very much as I described earlier for two layers each of aluminum foil and plastic wrap.