  Electric capacitors are commonly used in electronic circuits for short-term
storage of small amounts of energy. It is desirable however to use capacitors
to store much larger energy amounts to replace rechargeable batteries.
Unfortunately, the existing capacitors cannot store a sufficient energy to be
able to replace common electrochemical energy storage systems. Here we examine
energy storage capabilities of graphene nanocapacitors, which are try-layers
involving an Al film, Al2O3 dielectric layer, and a single layer of carbon
atoms, i.e., graphene. This is a purely electronic capacitor and therefore it
can function in a wide temperature interval. The capacitor shows a high
dielectric breakdown electric field strength, of the order of 1000 kV/mm (i.e.,
1GV/m), which is much larger than the table value of the Al2O3 dielectric
strength. The corresponding energy density is 10 to 100 times larger than the
energy density of a common electrolytic capacitor. Moreover, we discover that
the amount of charge stored in the dielectric layer can be equal or can even
exceed the amount of charge stored on the capacitor plates. The dielectric
discharge current follows a power-law time dependence. We suggest a model to
explain this behavior.
