Diffusivity and Structure of Room Temperature Ionic Liquid in Various Organic Solvents

Area of Science:

• Electrochemistry
• Materials Science
• Physical Chemistry

Background:

• Room-temperature ionic liquids (RTILs) are promising electrolytes for electric double layer capacitors (EDLCs) due to their wide potential window and stability.
• Low ion diffusivity in neat RTILs limits EDLC power density, necessitating mixtures with organic solvents.

Purpose of the Study:

• To investigate the diffusivity of ions in RTILs mixed with various organic solvents.
• To compare experimental diffusion data with molecular dynamics (MD) simulations.
• To understand the factors influencing ion transport and dissociation in RTIL-solvent mixtures.

Main Methods:

• Pulsed-field gradient nuclear magnetic resonance (PFG-NMR) was used to measure cation and anion diffusivity.
• Ionic conductivity and molar conductivity were determined to estimate ion dissociation.
• Hydrodynamic radii of ions were calculated.

Main Results:

• Ion diffusivity in RTIL-solvent mixtures generally followed solvent properties and agreed well with MD predictions.
• MD simulations slightly underestimated diffusivity in solvents with long-chain molecules.
• Ion coordination suggests approximately one solvent molecule per ion, with limited solvent-ion interactions.

Conclusions:

• The viscosity of the organic solvent is the primary factor governing ion diffusivity in RTIL mixtures.
• Understanding these interactions is key to optimizing RTIL-based electrolytes for high-performance EDLCs.