Does Electrode Spacing Truly Control Capacitance and Energy Density in Graphene-Based Supercapacitors? A Molecular Simulation Perspective.

Electrode spacing is frequently considered a parameter in supercapacitor optimization, yet its intrinsic impact on capacitance and energy storage under nanoconfinement remains uncertain. In this study, classical molecular dynamics simulations were performed for graphene-based supercapacitors containing the ionic liquid [emim]-[ala] confined between planar electrodes separated by 4-12 nm. Electric potential profiles enabled the calculation of differential and total capacitances as well as stored energy densities. Mass density analyses show well-defined electric double layers (EDLs) at both electrodes. Strong overlap is observed at short separations, whereas a bulk-like central region is preserved at larger ones. Nevertheless, the total capacitance remains nearly constant (∼2.40-2.58 μF/cm2), and the differential capacitance at zero charge varies only slightly across all separations, indicating a surface-dominated storage mechanism. The integrated areal energy density up to 2.5 V is also essentially independent of spacing (∼7.4-7.7 μJ/cm2). In contrast, gravimetric and volumetric energy densities decrease with increasing separation due to mass and volume normalization effects. These results demonstrate that electrode spacing modulates EDL structure but only weakly affects intrinsic capacitive performance.