Effect of Water on Local Structure and Dynamics in a Protic Ionic Liquid-Based Electrolyte

Ionic liquids (ILs) are promising candidates for electrolytes for next-generation energy storage and conversion systems. However, a high viscosity of the IL, hampering the ion transport, has led to strategies based on the dilution of the IL with a low-viscosity solvent. Herein, the influence of the addition of water to a protic IL to form a hybrid electrolyte suggested for supercapacitor applications is reported. The experiments directly test predictions from previous molecular dynamics simulations on this and other protic IL/water hybrid electrolytes. From small-angle X-ray scattering and infrared spectroscopy, it is shown that water is inserted in the ionic matrix both as single molecules and in small aggregates. Water molecules hydrogen bonds to the available proton on the IL cation and effectively separates the ion pairs, resulting in an increase in the charge correlation distance. The change in the local structure is also reflected in the local dynamics probed by neutron spin-echo spectroscopy. A local diffusive-type process is revealed that correlates well with macroscopic ion transport, for example, the ionic conductivity. The results from neutron scattering also infer that the different local environments created by the addition of water have a relatively short lifetime.