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What is fundamental physical constraint which doesn't allow the best (super)capacitors rival the best batteries in energy density? Or it could change someday?
This is not a quite explanation. Basically, battery and capacitor consist of similar components: two electrodes and electrolyte or dielectric between them. Principally, in dielectric capacitors you could remove electrodes after it is charged and store energy in dielectric only since electrons will stick to it. There is even such a school experiment. But this is not very important. Supercapacitor uses electrolyte as some type of dielectric. The more important is that battery releases energy when atomic electronic shells fuse together and release energy. Capacitor stores energy in deformation of dielectric atomic electronic shells until you will overcharge it and these atomic shells will broke. But in my understanding energy of electronic shells cannot exceed energy of their breakdown. For example if you burn hydrogen you obtain some energy, but to break water molecule back to oxygen and hydrogen you need even more energy because efficiency cannot be equal to 100%. So if you use pure water as an electrolyte in supercapacitor it suppose to accumulate even more energy as water molecules deformation than you could obtain by burning hydrogen. I know that dielectrics have many important properties though.It's important to understand that capacitors work by separating charges while batteries work by chemical reactions. Because a battery doesn't separate its charges, it is able to store much more energy in a small space than a capacitor, as a battery doesn't have to worry about things like dielectric breakdown.
I do not see how this one disadvantage relates to theoretical energy density. Rather it makes its use inconvenient in comparison to battery because use need special DC to DC or DC to AC converter. But for some applications such as hybrid vehicles it is not as important since you have to use a converter all the same. From what I know you could retrieve up to 75% of energy from supercapacitor usefully by using voltage converter.A (true) capacitor will always suffer from the problem that Q = CV, i.e. the voltage will, inherently vary with its charge. That's a serious disadvantage if you want to use simple circuitry to access the stored energy.
I'm not sure in it. Chemical reactions are usually "dirty" and reduce cycle life, while electrons move freely across capacitor. Theoretically I'm not sure if falling voltage will always be a problem of any capacitor. For example there is "dielectric absorption" effect which allows you to "regain" voltage periodically. Making capacitor with giant dielectric absorption and exploiting this effect may help. But I do not insist.Storing (i.e. separating) charges chemically is really a lot smarter
Currently the best batteries available - Li-ion use principle similar to high surface area of supercapacitor, there is intercalation process in carbon and oxide occurs. So basically, Lithium atoms just swing between two materials in Li-ion battery.It's also much easier because you don't need to have a battery design with a huge surface area in a very small space for charges to accumulate on like you do in a supercapacitor.
Drakkith said:Ah, okay. I didn't realize supercaps were part electrochemical.
Increasing surface area is not only possible way to increase energy density of capacitor. Increasing dielectric permittivity is another way. It is known that metal has permittivity close to infinity. If we embed metal nanoparticles into some ceramic dielectric what kind of permittivity we could expect to have? Or we could have just metal plate embedded into ceramics.I would think a battery normally would have the greater possible power density as a capacitors electric field depends on clinging surface charges but a batteries chemical energy reaction sites extend into the atomic structures of the electrodes past the surface.
Stanley514 said:Increasing surface area is not only possible way to increase energy density of capacitor. Increasing dielectric permittivity is another way. It is known that metal has permittivity close to infinity. If we embed metal nanoparticles into some ceramic dielectric what kind of permittivity we could expect to have? Or we could have just metal plate embedded into ceramics.
Stanley514 said:Increasing surface area is not only possible way to increase energy density of capacitor. Increasing dielectric permittivity is another way. It is known that metal has permittivity close to infinity. If we embed metal nanoparticles into some ceramic dielectric what kind of permittivity we could expect to have? Or we could have just metal plate embedded into ceramics.
Could you provide formulas which prove that you cannot increase capacitance by just increasing permittivity if space between the plates stay the same?It is basically the spacing between conductors that determines the Capacitance.
I think that such material could be prepared similar to colloid. http://en.wikipedia.org/wiki/Colloid Since there will be billions of nanoparticles and average distance between vast majority of them would be proper, improper distance between just few of them may not create avalanche effect because you need whole path from one plate to another through millions of nanoparticles. And what about metal plate which is wholly embedded in ceramics? Should it not give to you giant permittivity as well as high dielectric strength?You would also be relying on very uniform spacing between the nanoparticles because the 'weakest link' would be between the closest spaced pair of particles - thereafter, I would imagine that you could get an avalanche breakdown effect.
Stanley514 said:And what about metal plate which is wholly embedded in ceramics? Should it not give to you giant permittivity as well as high dielectric strength?