Exploring the relationship between water impurities, electrode charge density, and electric double layer structure and capacitance in carbon micropores

Abstract

Supercapacitors are becoming increasingly important in energy storage applications due to their high power density, long cycle life, and fast charging capabilities. However, to further enhance supercapacitor performance, a deeper understanding of the electric double layer (EDL) structure at the electrode-electrolyte interface is essential. This molecular dynamics (MD) study explores the structure and electrochemical behavior of 2 M 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide/dimethyl sulfoxide ([EMIM][NTf₂]/DMSO) electrolytes, with and without water impurities, confined within slit-shaped carbon micropores (widths: 0.7–1.9 nm; surface charge densities: 0 to ± 1.6 e/nm²). The findings reveal that water disrupts ion layering, reduces the counterion fraction near electrode surfaces, and alters hydrogen-bonding patterns. Disjoining pressure oscillates with pore width, with minor changes upon water addition. The disjoining pressure's parabolic dependence on surface charge matches reported data for [EMIM][NTf₂] in carbon electrodes. Water reduces disjoining pressure in moderately negative pores (σ = -0.2 to -0.8 e/nm²) but increases it in neutral and highly charged pores, reflecting hydration and adsorption changes. Calculated differential capacitance profiles exhibit camel-shaped curves, with an asymmetry between positive and negative potentials, consistent with experimental trends for [EMIM][NTf₂]/DMSO systems. The values align closely with those reported for carbon electrodes.