Abstract
Recent experiments have shown that the capacitance of sub-nanometer pores
increases anomalously as the pore width decreases, thereby opening a new avenue for developing
supercapacitors with enhanced energy density. However, this behavior is still subject to some
controversy since its physical origins are not well understood. Using atomistic simulations, we
show that the capacitance of slit-shaped nanopores in contact with room-temperature ionic
liquids exhibits a U-shaped scaling behavior in pores with width from 0.75 to 1.26 nm. The left
branch of the capacitance scaling curve directly corresponds to the anomalous capacitance increase
and thus reproduces the experimental observations. The right branch of the curve indirectly
agrees with experimental findings that so far have received little attention. The overall U-shaped
scaling behavior provides insights on the origins of the difficulty in experimentally observing the
pore-width dependent capacitance. We establish a theoretical framework for understanding the
capacitance of electrical double layers in nanopores and provide mechanistic details into the
origins of the observed scaling behavior. The framework highlights the critical role of “ion
solvation” in controlling pore capacitance and the importance of choosing anion/cation couples
carefully for optimal energy storage in a given pore system.
