Atomistic insight on the charging energetics in subnanometer pore supercapacitors

Electrodes featuring subnanometer pores are favorable to the capacitance and energy density of supercapacitors. However, there is an energy penalty to enter subnanometer pores as ions have to shed part of their solvation shell. The magnitude of such an energy penalty plays a key role in determining the accessibility and charging/discharging of these subnanometer pores. Here, we report on the atomistic simulation of Na⁺ and Cl^- ions entering a polarizable slit pore with a center-to-center width of 0.82 nm. We show that the free energy penalty for these ions to enter the pore is less than 14 kJ/mol for both Na⁺ and Cl^- ions. The surprisingly small energy penalty is caused by the van der Waals attractions between ions and pore walls, the image charge effects, the moderate (19-26%) dehydration of the ions inside the pore, and the strengthened interactions between ions and their hydration water molecules in the subnanometer pore. The results provide strong impetus for further developing nanoporous electrodes featuring subnanometer pores.