Cation effect on the electrochemical reduction of polyoxometalates in room temperature ionic liquids†
Juliette I. Phillips A , Shinya Azuma A B , Junqiao Lee
A , Tadaharu Ueda C D and Debbie S. Silvester A *
+ Author Affiliations
- Author Affiliations
A School of Molecular and Life Sciences, Curtin University, GPO Box U1987, Perth, WA 6845, Australia.
B Graduate School of Integrated Arts and Sciences, Kochi University, Kochi 780-8520, Japan.
C Department of Marine Resource Science, Faculty of Agriculture and Marine Sciences, Kochi University, Nankoku 783-8520, Japan.
D Centre for Advanced Marine Core Research, Kochi University, Nankoku 783-8502, Japan.
Australian Journal of Chemistry 75(11) 865-876 https://doi.org/10.1071/CH22140
Submitted: 20 June 2022 Accepted: 22 September 2022 Published: 22 November 2022
© 2022 The Author(s) (or their employer(s)). Published by CSIRO Publishing. This is an open access article distributed under the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License (CC BY-NC-ND)
Abstract
Polyoxometalates (POMs) are compounds that undergo multiple successive one-electron redox transitions, making them convenient model reactants to study ion solvation effects. Room temperature ionic liquids (RTILs) are solvents made entirely of ions, and are expected to have interactions with the highly negatively charged POM reduction products. In this work, 12 RTILs with a range of different anions ([FSI]−=bis(fluorosulfonyl)imide, [TFSI]−=bis(trifluoromethylsulfonyl)imide, [BETI]−=bis(pentafluoroethylsulfonyl)imide, [BF4]−, [PF6]−) and cations (imidazolium, pyrrolidinium, sulfonium, ammonium, phosphonium) were employed as solvents to study the kinetics and thermodynamics of [S2W18O62]4− reduction, to shed light on solvation effects and ion-pairing effects caused by different RTIL structures. Up to six reversible reduction processes (producing highly negatively charged [S2W18O62]10−) were observed. For the RTILs that showed multiple processes, a clear trend in both the thermodynamics (inferred from the reduction peak potentials) and kinetics (inferred from the peak-to-peak separation) was observed, in the order: imidazolium < sulfonium ≈ ammonium < pyrrolidinium < phosphonium, supporting strong interactions of the negatively charged POM reduction products with the cation. Two related POMs, [P2W18O62]6− and [PW12O40]3−, were also studied in the optimum RTIL found for [S2W18O62]4− ([C2mim][FSI]=1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide), revealing fast kinetics and asymmetric peaks for [PW12O40]3−. This work demonstrates the importance of understanding the solvation effects of RTIL ions for highly charged electrogenerated products, allowing tuning of the RTIL structure to achieve the optimum kinetics and thermodynamics for an electrochemical process.
Keywords: cation, cyclic voltammetry, electrochemistry, ionic liquids, ion-pairing, polyoxometalates, reduction, solvation.
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