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Capacitors will never rival batteries?

  1. 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?
  2. jcsd
  3. Drakkith

    Staff: Mentor

    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.
  4. 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.
  5. sophiecentaur

    sophiecentaur 13,621
    Science Advisor
    Gold Member

    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.
    Storing (i.e. separating) charges chemically is really a lot smarter, in many ways because you are using the same energy shift for each individual 'ionic' reaction and the volts won't get any higher or lower (at least, in principle).
    However, a good capacitor has very little internal resistance, so you can expect a lot higher instantaneous current when discharging a Capacitor.
    I guess it's a little disappointing that something with the name "super" involved, would be inferior to something as ancient as a battery. But, of course, we are only a short step from "super batteries" and then there will be no contest - and no disappointment.
    I am just waiting for someone to tell us that Tesla had a design for a Super Capacitor that was suppressed by the US battery industry a hundred years ago. :tongue:
  6. 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.
    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.
  7. Drakkith

    Staff: Mentor

    The fact is that storing energy chemically is much, much easier since the atoms/molecules are neutral and don't build up an electric field that can break down the dielectric. 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.

    You are correct in that chemical reactions are "dirty", which I take to mean that you only have a small number of charge/discharge cycles available compared to a capacitor. But that has nothing to do with your original question about energy density.
  8. Also, note there are two kinds of energy density. Energy per mass, energy per volume. Depending on your application you want to maximize one or the other. (Also, energy per dollar and other measures may be relevant.)
  9. 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.
  10. sophiecentaur

    sophiecentaur 13,621
    Science Advisor
    Gold Member

    The two technologies are not actually that distinct. After all, an electrolytic capacitor only works because of the chemistry at the interface producing a very thin separation between the two electrodes.
    We could be i danger of arguing on the basis of definitions and classification rather on what is ' actually happening', which is the more interesting topic.
  11. Drakkith

    Staff: Mentor

    Ah, okay. I didn't realize supercaps were part electrochemical.
  12. Supercaps are electrochemical but they form a electrolyte charge barrier to electron/ion movement as opposed to a battery that forms a electrolyte charge path for ion movement. With identical surface reaction sizes 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.
  13. 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.
  14. Nanoscale particles and structures can also be used in batteries for full redox reactions so advances in supercapacitors could result in superbatteries with even higher energy densities.

    Battery advances:
    Pseudocapacitor advances:
  15. sophiecentaur

    sophiecentaur 13,621
    Science Advisor
    Gold Member

    I'm not sure that approach would produce a greater Capacitance for a given operating voltage as you would still be relying on the dielectric strength. It is basically the spacing between conductors that determines the Capacitance. 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.
  16. Could you provide formulas which prove that you cannot increase capacitance by just increasing permittivity if space between the plates stay the same?
    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?
  17. The free charges in the metal plate would move to cancel an electric field within the plate as it's a conductor not a insulator. This 'giant permittivity" is the imaginary part of the (complex) permittivity associated with loss of energy. For a perfect conductor it would be purely imaginary and infinite.

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