Abstract
Quaternary ammonium ions, NR4+, are among the most electrochemically stable organic cations. Because of their wide electrochemical windows, they are frequently used in batteries and electrochemical capacitors. Improving the electrochemical stability and expanding the electrochemical windows of quaternary ammonium ion is highly desired. In this work, we investigated the electrochemical stability of quaternary ammonium ions and showed that the chain length, type (primary vs. secondary), size, and steric hindrance of the saturated alkyl substituents have only a very small effect (less than 150 mV) on their electrochemical stability toward reduction. To provide a molecular understanding of substituent effects on electrochemical stability, quantum calculations were performed employing density functional theory, and it was shown that the structure of saturated aliphatic alkyl substituents has only minimal effects on the electronic environment around the positive nitrogen center and the LUMO energy level of quaternary ammonium cations. Moreover, a linear correlation between the cathodic limit and the LUMO energy levels of the NR4+, N-butylpyridinium, and 1-ethyl-3-methylimidazolium ions was found, suggesting that electrochemical stabilities of new cations may be computationally predicted on the basis of LUMO energies of these systems.
Highlights
With the increasing global demand for energy, improving existing energy storage technologies is as critical as developing more efficient methods for energy production
Several reports have indicated that increasing the chain length or size of alkyl substituents in quaternary ammonium ions improves the electrochemical stability of these ions.[2,9,10,11,12,22,23]
Reduction of quaternary ammonium cations occurs by quantum tunneling of an electron from an occupied state at the Fermi energy level of the electrode to the lowest unoccupied molecular orbital (LUMO) of the cation.[23,45]
Summary
With the increasing global demand for energy, improving existing energy storage technologies is as critical as developing more efficient methods for energy production. Based on Equations 1 and 2, a few models were suggested for the prediction of electrochemical windows of electrolytes (based on ab initio calculations, molecular dynamics, and density functional theory).[53,54,56,57] Equations 1 and 2 imply that the LUMO and HOMO energy levels of molecules have a linear correlation with the corresponding standard reduction and oxidation potentials, respectively.
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