Electrochemical Stability of Quaternary Ammonium Cations: An Experimental and Computational Study

Abstract

Quaternary ammonium ions, NR 4 + , 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 NR 4 + , 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. With the increasing global demand for energy, improving existing energy storage technologies is as critical as developing more efficient methods for energy production. For one important class of energy storage technologies, namely electrical energy storage, the energy density of a device is usually limited by the voltage window in which the device can function. Therefore, expanding the working voltage range of such devices is highly desired. The electrochemical decomposition of electrolytes, which are commonly used in energy storage devices such as electrochemical capacitors, often limits the useful voltage window 1 because these devices can function properly only within the electrochemical window of their electrolyte. Modifying the structure of electrolytes and improving their electrochemical stabilities has been the focus of numerous studies to date. [2][3][4][5][6][7][8][9][10][11][12][13][14][15] The electrochemical window of an electrolyte is the voltage range in which the electrolyte is chemically stable and does not get reduced or oxidized as a result of the applied potential. 16 The upper end of the electrochemical window is usually limited by the oxidation of the anion, and the lower end of the electrochemical window is determined by the reduction of the cation. 1 One of the most electrochemically stable classes of organic cations comprises the quaternary ammonium ions, NR 4 + . These are highly inert toward reduction, offer wide electrochemical windows, and are, therefore, frequently used in batteries and capacitors. 2,5,6,17 Much research has been devoted to improving the electrochemical stability of quaternary ammonium ions. 4,[9][10][11][12][13][14][15] Reduction of qua...

Electrodeposition of Zinc from Alkaline Electrolytes Containing Quaternary Ammonium Salts and Ionomers: Impact of Cathodic‐Anodic Cycling Conditions

“…In the authors intend to improve the reliability of the traditional approach of estimating the reaction potential of an organic by inspecting the potential at which some representative c.d. value is reached.…”

“…This approach is inspired by – two papers concentrating on the quantitative assessment of cathodic QAS reactivity ‐, but, on the one hand it expands it to take also adsorption into account, and on the other hand it improves on statistical consistency, avoiding some drawbacks that are intrinsic to the literature method. Access to adsorption information is enabled by the concurrent HER, that was not occurring in, where organic electrolytes with a high cathodic reactivity threshold were considered.…”

“…In order to inhibit the dendrite growth, cationic surfactants are preferred instead, because they adsorb more easily on the negatively charged cathode. Among them, quaternary ammonium salts (QAS) with long aliphatic chains have been widely investigated, because of their electrochemical stability in alkaline medium at room temperature and their effectiveness in the dendrite suppression . Electrochemical and morphological characterizations were carried out in the majority of literature studies.…”

Electrochemical synthesis of copper(i) acetylides via simultaneous copper ion and catalytic base electrogeneration for use in click chemistry

“…11 By comparison to most probases, QAS are more resistant to electrochemical reduction; the effects of chain length, branching and steric hindrance having very little effect on both their stability towards reduction and the electronic environment around their cationic nitrogen centres. 12 Such features are generally desirable for their use as 'inert' electrolytes. However, there are benets to employing such salts as both the background electrolytes and as probases in electrochemical systems, namely, the increased sustainability incurred from omitting any additional base or probase reagents.…”

Synthesis and investigation of a tris-cyclometalated iridium complex bearing a single quarternary ammonium group

“…Similar reductive decomposition of quarternary ammonium salts has been observed in electrochemical studies. [27,28] Subsequent one electron transfer from the benzylic radical to the iridium center would lead to restoring of oxidation state +3 on the iridium center and formation of an electrophilic benzylic cation that could be involved in a reaction with various nucleopilic species, which could be water present in the DMSO solvent in this case. It should also be noted that the reactions were performed in the presence of oxygen, which is the likely reason for the observation of oxidation of the initial product from containing a hydroxy methyl group to the formyl group in [Ir(ppy)2fppy] (SI Figure 8).…”