Abstract
Humid hydrophobic ionic liquids—widely used as electrolytes—have narrowed electrochemical windows due to the involvement of water, absorbed on the electrode surface, in electrolysis. In this work, we performed molecular dynamics simulations to explore effects of adding Li salt in humid ionic liquids on the water adsorbed on the electrode surface. Results reveal that most of the water molecules are pushed away from both cathode and anode, by adding salt. The water remaining on the electrode is almost bound with Li+, having significantly lowered activity. The Li+-bonding and re-arrangement of the surface-adsorbed water both facilitate the inhibition of water electrolysis, and thus prevent the reduction of electrochemical windows of humid hydrophobic ionic liquids. This finding is testified by cyclic voltammetry measurements where salt-in-humid ionic liquids exhibit enlarged electrochemical windows. Our work provides the underlying mechanism and a simple but practical approach for protection of humid ionic liquids from electrochemical performance degradation.
Highlights
Humid hydrophobic ionic liquids—widely used as electrolytes—have narrowed electrochemical windows due to the involvement of water, absorbed on the electrode surface, in electrolysis
We have investigated the effect of adding salt in humid hydrophobic ionic liquids (ILs) on distributions of ion and water at the electrode surface
Combining molecular dynamics (MD) simulations, density functional theory (DFT) calculations and cyclic voltammetry (CV) measurements, we found that the electrochemical stability window of salt-in-humid ILs is expanded, compared with the humid one
Summary
Humid hydrophobic ionic liquids—widely used as electrolytes—have narrowed electrochemical windows due to the involvement of water, absorbed on the electrode surface, in electrolysis. Room-temperature ionic liquids (ILs), with unique properties, including excellent thermal stability, nonflammability, and especially wide electrochemical windows, are an emerging class of candidates for EES devices[5,6,7,8,9], such as supercapacitors[8,9], batteries[10], and solar cells[11]. Owing to their hygroscopic nature, ILs can spontaneously adsorb water from the humid environment, regardless of their hydrophilicity or hydrophobicity[12]. Much of current research has demonstrated that the water electrosorption on the electrode is governed by the working voltage and the association of water molecules with their neighbors (including surface charges, electrode materials, and IL ions)[14,17,18]
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