Modulating water cluster formation by the hydrophilicity of mixed ionic liquids

Abstract

Ionic liquids (ILs) have been well known as favorable electrolytes in electrical double layer capacitors (EDLCs) for their unique chemical and thermal properties. However, their stable electrochemical window (EW) will be limited by the adsorption of water. Meanwhile, the state of water molecules in ILs can be modulated by their hydrophilicity, which is adjustable through combining various types of cations and anions. In this work, molecular dynamics simulations were carried out to investigate the behavior and distribution of water molecules in two kinds of ILs 1-butyl-3-methyl-imidazolium bis(trifluoromethylsulfonyl)imide ([BMIM][TFSI]) and 1-butyl-3-methyl-imidazolium acetate ([BMIM][OAc]), as well as their mixture in different ratios. We found that water molecules tend to aggregate in the hydrophobic [BMIM][TFSI] at a molar concentration as low as 0.1. The introduction of hydrophilic anion of OAc profoundly influences the water aggregation because of their strong interactions with water. It is demonstrated that mixing 20% mol OAc can efficiently stabilize water molecules and maintain the relatively good diffusivity of the hydrophobic IL. The results in this work provide not only valuable information to understand the mechanism of water aggregation in mixing ILs, but also a simple strategy to tune the properties of ILs and improve their performance in EDLCs.