Electrochemical Stability of Ionic Liquids: General Influences and Degradation Mechanisms

Vos, Nils De; Maton, Cédric; Stevens, Christian V.

doi:10.1002/celc.201402086

Cited by 160 publications

(140 citation statements)

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“…Effect of the scan rate.-It is well known that the scan rate can affect current-voltage curves and, consequently, change the electrolyte electrochemical limits determined with the J cut-off method, 5,4 but in the past this effect was not quantitatively studied. In this work, the cathodic limit of NBu 4 I was measured at scan rates of 1, 10, 100, and 1000 mV/s (see Table II).…”

Section: Resultsmentioning

confidence: 99%

“…2 There has been extensive research on modifying the structure of electrolytes to improve their electrochemical stability, and thus expanding their working range. [3][4][5][6][7] The determination of the electrochemical window of electrolytes is not well defined. Generally a current-voltage polarization curve is measured, starting at a voltage at which the electrolyte is electrochemically stable, followed by increasing or decreasing the potential to observe the anodic or cathodic decompositions, respectively.…”

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confidence: 99%

“…2 Other experimental parameters, such as the type of the working electrode and the scan rate, also affect the determined electrochemical window, although to a lesser extent. 2,5 Another disadvantage of the J cut-off method is its limited applicability to real-life devices. For this method, it is assumed that the current arises exclusively from electrolyte decomposition, ignoring non-faradaic (capacitive) current.…”

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confidence: 99%

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Unbiased Quantification of the Electrochemical Stability Limits of Electrolytes and Ionic Liquids

Mousavi

Dittmer

Wilson

et al. 2015

J. Electrochem. Soc.

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The electrochemical window is the potential range in which an electrolyte/solvent system does not get reduced or oxidized. Usually, voltammograms are measured, and the potentials at which specific current densities are reached are identified as the electrochemical limits. We measured electrochemical limits of several electrolytes-including ionic liquids-and show that this approach has disadvantages that can be overcome by an alternate approach of defining electrochemical limits. The choice of the cutoff current density, J cut-off , is arbitrary and strongly affects the determined electrochemical windows, which are strongly influenced by electrolyte mass transport. Moreover, the J cut-off method does not provide an accurate estimate of the electrochemical window at electrodes with high surface areas, where the capacitive currents are large. We propose a method that requires no definition of J cut-off . This method minimizes electrolyte mass transport effects, gives realistic electrochemical stability limits at high surface area electrodes, and is less affected by experimental parameters such as the scan rate. The method is based on linear fits of the current-voltage curve at potentials below and above the onset of electrolyte decomposition. The potential at which the two linear fits intersect is defined as the electrolyte electrochemical limit. Electrolytes and solvents are essential for the proper function of a variety of electrochemical devices, such as batteries and supercapacitors. The voltage polarization in these devices can cause electrochemical degradation of the electrolyte and solvent, resulting in device malfunction. This can be avoided by constraining the working range of the device to the electrochemical window limited by the electrolyte and solvent, i.e., the potential range in which they are chemically stable and do not get reduced or oxidized.1,2 Due to their localized charges, the inherently ionic electrolytes often have a lower electrochemical stability than neutral solvent molecules, and therefore frequently limit the device working range.2 There has been extensive research on modifying the structure of electrolytes to improve their electrochemical stability, and thus expanding their working range. [3][4][5][6][7] The determination of the electrochemical window of electrolytes is not well defined. Generally a current-voltage polarization curve is measured, starting at a voltage at which the electrolyte is electrochemically stable, followed by increasing or decreasing the potential to observe the anodic or cathodic decompositions, respectively. The potential at which a specific current density, J, is reached is defined as the cathodic or anodic limit of the electrolyte. This approach has several disadvantages, as noted by several researchers.3-5 Firstly, since the choice of the cut-off current density is arbitrary, different standards have been applied in the literature. Cut-off current densities, J cut-off , in the range of 0.01 to 5.0 mA/cm 2 were reported, 3,7-20 resulting in electrochemical ...

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Section: Resultsmentioning

confidence: 99%

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Unbiased Quantification of the Electrochemical Stability Limits of Electrolytes and Ionic Liquids

Mousavi

Dittmer

Wilson

et al. 2015

J. Electrochem. Soc.

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“…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 quaternary ammonium ions occurs via radical intermediates and involves the loss of an alkyl substituent. 4,11,[18][19][20][21] Therefore, the nature of the alkyl substituents strongly affects the electrochemical stability of these cations.…”

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confidence: 99%

“…4,[9][10][11][12][13][14][15] Reduction of quaternary ammonium ions occurs via radical intermediates and involves the loss of an alkyl substituent. 4,11,[18][19][20][21] Therefore, the nature of the alkyl substituents strongly affects the electrochemical stability of these cations.9 Quaternary ammonium ions with aromatic substituents such as benzyl groups have lower electrochemical stabilities than saturated quaternary ammonium ions.11 This effect is likely due to the higher stability of benzyl radicals as compared to alkyl radicals, making the former a better leaving group, and thus making quaternary ammonium ions with benzyl substituents more sensitive to reduction.11 Incorporation of oxygen atoms in the alkyl substituents of quaternary ammonium ions also diminishes their electrochemical stability toward reduction, likely due to the electrostatic * Electrochemical Society Active Member.z E-mail: Sadra.kashef@mpikg.mpg.de; Buhlmann@umn.edu effects of the oxygen atoms. 10,[12][13][14][15] Changing the structure of the saturated alkyl substituent (increasing the alkyl chain length, size, etc.)…”

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

Mousavi

Kashefolgheta

Stein

et al. 2015

J. Electrochem. Soc.

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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...

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Nanoconfined Ionic Liquids

Marcinkowski

Kloskowski

Namieśnik

2019

Handbook of Smart Materials in Analytical Chemistry

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Electrochemical Stability of Ionic Liquids: General Influences and Degradation Mechanisms

Cited by 160 publications

References 109 publications

Unbiased Quantification of the Electrochemical Stability Limits of Electrolytes and Ionic Liquids

Unbiased Quantification of the Electrochemical Stability Limits of Electrolytes and Ionic Liquids

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

Nanoconfined Ionic Liquids

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