Interfacial ion transport in cellulose nanocrystal-based ionogels with low ionic liquid content.

Abstract

This study investigates the gel formation of cellulose nanocrystals (CNCs) in a hydrophobic ionic liquid, 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (HMIm-TFSI), with the addition of a co-solvent, dimethylformamide (DMF). CNC-based ionogels prepared with very low HMIm-TFSI contents (0.1-0.5 wt%) exhibit higher molar ionic conductivity than gels containing larger amounts of HMIm-TFSI. Application of electric fields to CNC mixtures containing 0.4 wt% HMIm-TFSI induces CNC alignment but does not enhance macroscopic ionic conductivity, indicating that conductivity is not governed by CNC orientation. The enhanced ion mobility and conductivity of CNC-HMIm-TFSI gels are attributed to structuring of HMIm-TFSI near CNC surfaces, which facilitates efficient interfacial ionic transport. Unlike the polymer-CNC networks that are capable of confining a high amount of ionic liquid (95 wt%), the CNC gels with a low HMIm-TFSI amount can order liquids at the interfacial layers and exhibit higher conductivity than the neat ionic liquid.