  Electric double layer supercapacitors are promising devices for high-power
energy storage based on the reversible absorption of ions into porous,
conducting electrodes. Graphene is a particularly good candidate for the
electrode material in supercapacitors due to its high conductivity and large
surface area. In this paper we consider supercapacitor electrodes made from a
stack of graphene sheets with randomly-inserted "spacer" molecules. We show
that the large volumetric capacitances C > 100 F/cm^3 observed experimentally
can be understood as a result of collective intercalation of ions into the
graphene stack and the accompanying nonlinear screening by graphene electrons
that renormalizes the charge of the ion clusters.
