CONFINEMENT EFFECTS on the TRANSPORT PROPERTIES of ROOM TEMPERATURE IONIC LIQUID BASED ELECTROLYTES in SUPERCAPACITORS

Abstract

The study of transport properties in supercapacitors is important for optimizing their power density. Properties that control charging kinetics in supercapacitors include ion self-diffusion coefficients and ionic conductivity which are significantly different at electrode/electrolyte interfaces compared to their bulk counterparts. Here, we use molecular dynamics simulations to elucidate the effect of electrolyte composition and confinement on ion self-diffusion coefficients and ionic conductivity in mixtures of room temperature ionic liquids with organic solvents. Our results reveal that ion diffusion significantly slows down at the electrode/electrolyte interface compared to the bulk. In particular, diffusion coefficients in the dilute regime are found to be several orders of magnitude smaller than their bulk counterparts. This effect is more pronounced when a potential difference is applied between the electrodes. We show that the ionic conductivity of the electrolytes also significantly diminishes due to confinement effects manifested by the decrease in the coordination numbers of ions. Our findings depict that electrolyte composition also plays an important role in ion dynamics: Introduction of an organic solvent to a concentrated ionic solution significantly increases both bulk and interfacial diffusion coefficients while it leads to a maximum in ionic conductivity at intermediate dilution levels. These results reveal that optimizing the solvent content of an ionic liquid-based electrolyte can potentially boost the power density of supercapacitors.

Keywords

• energy storage
• supercapacitors
• molecular dynamics
• diffusivity
• conductivity

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