Ionic Liquid Vapors in Vacuum: Possibility to Derive Anodic Stabilities from DFT and UPS.

Ivar Kuusik,Rainer Pärna,Mati Kook,Vambola Kisand

doi:10.1021/acsomega.0c05369

Ivar Kuusik, Rainer Pärna + Show 2 more

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https://doi.org/10.1021/acsomega.0c05369

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Journal: ACS omega	Publication Date: Feb 15, 2021
Citations: 10	License type: CC BY 4.0

Affiliation: University of Tartu

Abstract

Ultraviolet photoelectron spectroscopy (UPS) investigations of several gas-phase ionic liquid (IL) ion pairs have been conducted. [EMIM][OTF], [PYR14][OTF], [EMIM][DCA], [PYR14][DCA], [PYR14][TCM], [PYR14][FSI], [PYR14][PF6], [S222][TFSI], [P4441][TFSI], and [EMMIM][TFSI] vapor UPS spectra are presented for the first time. The experimental low-binding-energy cutoff value (highest occupied molecular orbital, HOMO energy) of the ionic liquid ion pairs, which is of great interest, has been measured. Many studies use calculated gas-phase electronic properties to estimate the liquid-phase electrochemical stability. Hybrid density functional theory (DFT) calculations have been used to interpret the experimental data. The gas-phase photoelectron spectra in conjunction with the theoretical calculations are able to verify most HOMO energies and assign them to the cation or anion. The hybrid M06 functional is shown to offer a very good description of the ionic liquid electronic structure. In some cases, the excellent agreement between the UPS spectra and the M06 calculation validates the conformer found and constitutes as a first indirect experimental determination of ionic liquid ion-pair structure. Comparisons with recent theoretical studies are made, and implications for electrochemical applications are discussed. The new data provide a much-needed reference for future ab initio calculations and support the argument that modeling of IL cations and anions separately is incorrect.

Highlights

Ionic liquids (ILs) are generally defined as molten organic salts with a melting point below 100 °C
One important application for ionic liquids is as an electrolyte in electrochemical double-layer capacitors (EDLCs) or supercapacitors
We focused on the question of how to determine the intrinsic electrochemical stability window (EW) of the ILs and their

Summary

Introduction

Ionic liquids (ILs) are generally defined as molten organic salts with a melting point below 100 °C. ILs have attracted interest because of their uncommon physicochemical properties such as low melting temperatures, excellent solvation ability, relatively high thermal stability, low vapor pressure, nonflammability, high electrochemical stability, etc. One important application for ionic liquids is as an electrolyte in electrochemical double-layer capacitors (EDLCs) or supercapacitors. The ionic conductivity and the width of the electrochemical stability window (EW) are the most important properties. Generally the viscosity of ILs is higher and the ion conductivity is lower than in the conventional electrolytes, ILs are considered as the ideal working electrolytes for EDLCs because of their large electrochemical windows, excellent thermal stability, and negligible volatility.[1] In supercapacitors, ILs are close to commercial viability.[2]

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