Non-Faradaic Energy Storage by Room Temperature Ionic Liquids in Nanoporous Electrodes

The enhancement of non-Faradaic charge and energy density stored by ionic electrolytes in nanostructured electrodes is an intriguing issue of great practical importance for energy storage in electric double layer capacitors. On the basis of extensive molecular dynamics simulations of various carbon-based nanoporous electrodes and room temperature ionic liquid (RTIL) electrolytes, we identify atomistic mechanisms and correlations between electrode/electrolyte structures that lead to capacitance enhancement. In the symmetric electrode setup with nanopores having atomically smooth walls, most RTILs showed up to 50% capacitance increase compared to infinitely wide pore. Extensive simulations using asymmetric electrodes and pores with atomically rough surfaces demonstrated that tuning of electrode nanostructure could lead to further substantial capacitance enhancement. Therefore, the capacitance in nanoporous electrodes can be increased due to a combination of two effects: (i) the screening of ionic interactions by nanopore walls upon electrolyte nanoconfinement, and (ii) the optimization of nanopore structure (volume, surface roughness) to take into account the asymmetry between cation and anion chemical structures.