Why supercapacitor energy density is so high?

[Stanley514]

In battery we have (in ideal) case one electron per atom or three elecrons (in case of aluminum) which flow from anode to cathode.In activaded carbon there should be immensely smaller amount of free electrons than in metals.I do not have exact date for carbon, but for example in Germanium it is 2.1 x 10^12/cm-3 compared to 8.4 x 10^22/cm-3 for copper.Ten orders of magnitude smaller.I guess number of free carriers in carbon should be even lower than in Germanium.Therefore ultracap made of activated carbon shoud provide us 10 billions times smaller amount of electrons flowing from anode to cathode than battery with anode made of copper.Yet it is claimed that best ultracapacitors could rival lead-acid batteries in energ densiy.How is it possible?

 

[VCortex]

Why indeed! Anyone got a retroactive QM explanation maybe?? / bump

 

[mfb]

Capacitors are not made out of solid blocks of carbon. On surfaces (and with strong electric fields present), things are different.

Edit: Made the main point clearer

 

[VCortex]

Assuming Stanley imagined a solid lump of carbon, that answer's fine with me! I have a hard time imagining capacitors as solid chunks of stuff! :/

 

[sophiecentaur]

In a Capacitor, charges do not flow in the dielectric; instead, the molecules polarise. The flow of charge is surely in the metal of the plates towards and away from the surfaces of the plates. I don't think the two storage systems are comparable, which could account for the apparent paradox.

 

[Stanley514]

So what is carrier concentration per cm -3 in activated carbon? Could you provide any exact data?But I have doubts that even with activation carbon would have more free electrons than 1 per thousand of atoms.More likely even much less.And how many positive ions per carbon atoms have supercap to counterbalance the charge?

The flow of charge is surely in the metal of the plates towards and away from the surfaces of the plates.

In supercaps activated carbon serves as plates and "the metal".

 

[Drakkith]

So what is carrier concentration per cm -3 in activated carbon? Could you provide any exact data?But I have doubts that even with activation carbon would have more free electrons than 1 per thousand of atoms.More likely even much less.And how many positive ions per carbon atoms have supercap to counterbalance the charge?
In supercaps activated carbon serves as plates and "the metal".

The number of free electrons is irrelevant because activated carbon is the insulator, not the conductor in a supercap. The electrolyte is what conducts the current. The way activated carbon is made gives it an immense amount of surface area for such a small volume, which greatly increases the capacitance of the device.

 

[VCortex]

The number of free electrons is irrelevant because activated carbon is the insulator, not the conductor in a supercap. The electrolyte is what conducts the current. The way activated carbon is made gives it an immense amount of surface area for such a small volume, which greatly increases the capacitance of the device.

So by your definition; we have non conductive 'carbon insulators', separated by conductive (charge storing) electrolytes (whose charge depends indirectly, somehow, on the surface area of the non-conductive insulating carbon layer)..?

 

[Drakkith]

So by your definition; we have non conductive 'carbon insulators', separated by conductive (charge storing) electrolytes (whose charge depends indirectly, somehow, on the surface area of the non-conductive insulating carbon layer)..?

Hmm, I think I misread the description of the supercap.

EDLCs do not have a conventional dielectric[citation needed]. Rather than two separate plates separated by an intervening insulator, these capacitors use virtual plates that are in fact two layers of the same substrate[citation needed]. Their electrochemical properties, the so-called "electrical double layer", result in the effective separation of charge despite the vanishingly thin (on the order of nanometers) physical separation of the layers. The lack of need for a bulky layer of dielectric, and the porosity of the material used, permits the packing of plates with much larger surface area into a given volume, resulting in high capacitances in practical-sized packages.

In an electrical double layer, each layer by itself is quite conductive, but the physics at the interface where the layers are effectively in contact means that no significant current can flow between the layers[citation needed]. The double layer can withstand only a low voltage, which means that electric double-layer capacitors rated for higher voltages must be made of matched series-connected individual EDLCs, much like series-connected cells in higher-voltage batteries.

I was mistaken in that the carbon is used as the electrodes, not the insulator.

 

[VCortex]

Hmm, I think I misread the description of the supercap.

I would hazard a guess that there are several potential component designs for such devices utilising differing materials & configurations around similar principles.

I was mistaken in that the carbon is used as the electrodes, not the insulator.

Amongst other things, yes! I suppose it would be best to see how the hyperbole holds up before anyone tries to retroactively explain anything :P
Heard anecdotally that supercap energy density was somewhat comparable to lithium cells at present, would that be pertinent to the OP's question? Assuming we're not still talking about solid chunks of carbon?

 

[Stanley514]

I think there could be some fraud with those supercaps...