  Molecular dynamics simulations were used to explain the origin and properties
of electrical double-layer capacitance in short graphene nanochannels with
width below 2 nm. The results explain the previously reported experimental
result on the non-monotonic dependence of the capacitance with the channel
width. The mechanism for the anomalous increase of the capacitance in sub-1-nm
in pore diameter is attributed here to the width-dependent radial location of
counterions in the nanochannels, and the restricted number of co-ions. Decrease
of the channel width lowers the number of co-ions and positions the counterions
closer to the channel walls. For nanochannels with width ranging from 1 to 2
nm, co-ions are allowed to enter the nanochannel, and both types of ions assume
alternating layered distributions leading to the decrease of the capacitance.
Voltage is another control parameter which allows understanding capacitance in
graphene nanochannels. As the voltage increases, due to limited space near the
charged surface, more counterions need to be located in the center of the
nanochannel resulting in further capacitance decrease.
