Electrical Double Layer Structure in Ionic Liquids and Its Importance for Supercapacitor, Battery, Sensing, and Lubrication Applications

Abstract

Ionic liquids (ILs) have become highly popular solvents over the last two decades in a range of fields, especially in electrochemistry. Their intrinsic properties include high chemical and thermal stability, wide electrochemical windows, good conductivity, high polarity, tunability, and good solvation properties, making them ideal as solvents for different electrochemical applications. At charged surfaces such as electrodes, an electrical double layer (EDL) forms when exposed to a fluid. IL ions form denser EDL structures compared to conventional solvent/electrolyte systems, which can cause differences in the behavior for electrochemical applications. This Perspective discusses some recent work (over the last three years) where the structure of the EDL in ILs has been examined and found to influence the behavior of supercapacitors, batteries, sensors, and lubrication systems that employ IL solvents. More fundamental work is expected to continue in this area, which will inform the design of solvents for use in these applications and beyond.

Atomic force microscopy probing interactions and microstructures of ionic liquids at solid surfaces

“…46 All these observed microstructures for the interfacial ILs, due to the variation of IL-solid interactions, are highly important in phenomena like capacitance and nanoscale friction. 21,47,48 A surface-active IL electrolyte, containing amphiphilic structures, can exhibit an atypical bilayer ion packing, which is different from conventional alternating layers. This is because the strong van der Waals interactions enhance the charge storage performance.…”

Ethanol on Graphite: Ordered Structures and Delicate Balance of Interfacial and Intermolecular Forces

“…It is known that structures at interfaces where different materials and/or molecules meet have fundamental impacts on materials properties and behavior, interfacial phenomena, and even device performance in practical applications. − For small molecules, especially those capable of forming hydrogen bonds such as water, methanol, ethanol, and so on, knowledge of their interfacial structures has particular importance due to their broad existence as common solvents and in fuel cells, catalysis, macromolecular systems, and interstellar matter. − Given the relatively large strength and directional nature among intermolecular forces, hydrogen bonds play an important role in the organization of assembly structures, which may be in competition or sometimes cooperation with substrate–molecule interactions. Hence, to reveal interfacial structural details as a result of a delicate balance of all forces involved, it is crucial to use structure-probing techniques with suitable surface sensitivity, for example, scanning probe microscopy, , reflection–absorption infrared spectroscopy, − and reflection diffraction methods. − In recent years, reflection high-energy electron diffraction (RHEED) has been used to elucidate unanticipated ordered assemblies and unusual phase-transformation behavior of small molecules physisorbed or chemisorbed on different surfaces. − The technique’s strengths of a (sub)­nanometer thick penetration depth at grazing incidence angles and resulting surface sensitivity make noncontacting RHEED highly suitable for temperature- and thickness-dependent studies of interfacial structures and phase transitions of molecular systems, beside its typical use during thin-film fabrications in materials science.…”

Knowledge and Technology Used in Capacitive Deionization of Water

“…Much research has been conducted on the use of electrochemical capabilities to absorb ions and transfer them between an electrolyte medium and a charged surface, which led to the introduction of the Electrical Double Layers theory. This concept has been widely defined and used in electrochemical devices such as batteries, supercapacitors, electrical appliances, and deionization systems [ 115 , 116 ].…”