Effect of water on local structure and dynamics in a protic ionic liquid based electrolyte.

Ionic liquids 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. Here we report on the influence of the addition of water to a protic ionic liquid to form a hybrid electrolyte suggested for supercapacitor applications. Our 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 IR spectroscopy we show that water is inserted in the ionic matrix both as single molecules and in small aggregates. Water molecules hydrogen bond to the available proton on the ionic liquid cation and effectively separate 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. We reveal a local diffusive-type process that correlates well with macroscopic ion transport, e.g., 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.