  In the recent experiments [Chmiola et al, Science 313, 1760 (2006); Largeot
et al, J. Am. Chem. Soc. 130, 2730 (2008)] an anomalous increase of the
capacitance with a decrease of the pore size of a carbon-based porous electric
double-layer capacitor has been observed. We explain this effect by the image
forces which exponentially screen out the electrostatic interactions of ions in
the interior of a pore. Packing of ions of the same sign becomes easier and is
mainly limited by steric interactions. We call this state `superionic' and
suggest a simple model to describe it. The model reveals a possibility of a
voltage-induced first-order transition between a cation(anion)-deficient phase
and a cation(anion)-rich phase which manifests itself in a jump of capacitance
as a function of voltage.
