Minimizing the electrosorption of water from humid ionic liquids on electrodes

Abstract

In supercapacitors based on ionic liquid electrolytes, small amounts of absorbed water could potentially reduce the electrochemical window of electrolytes and cause performance degradation. The same would take place if ionic liquids are used as solvents for electrocatalysis involving the dissolved molecular species. In this work, we carry out molecular dynamics simulations, with gold and carbon electrodes in typical ionic liquids, hydrophobic and hydrophilic, to study electrosorption of water. We investigate the effects of hydrophobicity/hydrophilicity of ionic liquids and electrodes on interfacial distribution of ions and electrosorbed water. Results reveal that using hydrophilic ionic liquids would help to keep water molecules away from the negatively charged electrodes, even at large electrode polarizations. This conclusion is supported by electrochemical cyclic voltammetry measurements on gold and carbon electrodes in contact with humid ionic liquids. Thereby, our findings suggest potential mechanisms for protection of electrodes from water electrosorption.

Atomic Force Spectroscopy on Ionic Liquids

“…In particular, in ionic liquids used as electrolytes in fuel cells, water is generated as the product of the reaction between hydrogen and oxygen and can thus alter the reactions at the electrode surfaces [41]. Even small amounts of water can modify the interface structure of an ionic liquid under applied electric potential [108]. On the interface between butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl) amide ([BMP][TFSA], also referred to as [Py 1,4 ][TFSI]) and Au(111), Zhong et al [109], using AFS, observed that upon a water increase from 30 ppm to 90 ppm, the stiffness of the first interface layers decreased significantly while the thickness of the layers increased.…”

Adding salt to expand voltage window of humid ionic liquids

“…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 . It has been reported that due to the strong interaction with anions, water molecules are excluded from the negatively charged electrode with hydrophilic ILs 18 .…”

“…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 . It has been reported that due to the strong interaction with anions, water molecules are excluded from the negatively charged electrode with hydrophilic ILs 18 . However, it is still an unaddressed issue that, for hydrophobic ILs that have been widely used as electrolytes 19,20 , they become humid when exposed to the atmosphere 12 , and unfortunately, their water much favors adsorption on both negatively and positively charged electrodes 15,16,18 .…”

Ion adsorption-induced reversible polarization switching of a van der Waals layered ferroelectric

“…In a broader perspective, the attraction phenomena between likecharged objects have been extensively studied in many systems such as macroions ranging from colloids, polymers to DNA solutions 53 . In the case of ionic liquids, both molecular dynamic simulations 54 and experimental evidences 55 have revealed a cation-rich interface between ionic liquid and positively charged surface because of the speci c adsorption of cations in ionic liquid. These reports give us a hint that the EDL structures would be determined not only by long-range electrostatic forces, but also by some short-range interactions.…”