Deep Eutectic Solvents Based on N-Methylacetamide and a Lithium Salt as Electrolytes at Elevated Temperature for Activated Carbon-Based Supercapacitors

Abstract

This study describes the utilization of deep eutectic solvents (DESs) based on the mixture of the N-methylacetamide (MAc) with a lithium salt (LiX, with X = bis[(trifluoromethyl)sulfonyl]imide, TFSI; hexafluorophosphate, PF 6 ; or nitrate, NO 3 ) as electrolytes for carbon-based supercapacitors at 80°C. The investigated DESs were formulated by mixing a LiX with the MAc (at x Li = 0.25). All DESs show the typical eutectic characteristic with eutectic points localized in the temperature range from −85 to −52°C. Using thermal properties measured by differential scanning calorimetry (DSC), solid−liquid equilibrium phase diagrams of investigated LiX−MAc mixtures were then depicted and also compared with those predicted by using the COSMOThermX software. However, the transport properties of selected DESs (such as the conductivity (σ) and the fluidity (η −1 )) are not very interesting at ambient temperature, while by increasing the temperature up to 80°C, these properties become more favorable for electrochemical applications, as shown by the calculated Walden products: w = ση −1 (mS cm −1 Pa −1 s −1 ) (7 < w < 16 at 25°C and 513 < w < 649 at 80°C). This "superionicity" behavior of selected DESs used as electrolytes explains their good cycling ability, which was determined herein by cyclic voltammetry and galvanostic charge−discharge methods, with high capacities up to 380 F g −1 at elevated voltage and temperature, i.e., ΔE = 2.8 V and 80°C for the LiTFSI−MAc mixture at x Li = 0.25, for example. The electrochemical resistances ESR (equivalent series resistance) and EDR (equivalent diffusion resistance) evaluated using electrochemical impedance spectroscopy (EIS) measurements clearly demonstrate that according to the nature of anion, the mechanism of ions adsorption can be described by pure double-layer adsorption at the specific surface or by the insertion of desolvated ions into the ultramicropores of the activated carbon material. The insertion of lithium ions is observed by the presence of two reversible peaks in the CVs when the operating voltage exceeds 2 V. Finally, the efficiency and capacitance of symmetric AC/AC systems were then evaluated to show the imbalance carbon electrodes caused by important lithium insertion at the negative and by the saturation of the positive by anions, both mechanisms prevent in fact the system to be operational. Considering the promising properties, especially their cost, hazard, and risks of these DESs series, their introduction as safer electrolytes could represent an important challenge for the realization of environmentally friendly EDLCs operating at high temperature.

Design of Deep Eutectic Systems: A Simple Approach for Preselecting Eutectic Mixture Constituents

“…However, DESs are usually less toxic, easier to prepare, and less expensive than ILs. These advantages have led to the recent increase in applications of DESs, for example, as solvents in diverse separation methods [4][5][6][7][8], media for chemical [9][10][11][12][13][14], electrochemical [15][16][17][18][19], and biological reactions [20,21], in polymer chemistry [22][23][24], and for increasing the solubility of active pharmaceutical ingredients [25][26][27][28].…”

Gas Evolution in Activated‐Carbon‐Based Supercapacitors with Protic Deep Eutectic Solvent as Electrolyte

“…We demonstrated recently the potential use of amide‐based DESs in which a sodium or lithium salt is mixed with an amide [ N ‐methylacetamide (NMA) or formamide (FMD)] as electrolytes in electrochemical storage systems such as Li‐ion batteries and supercapacitors . Regarding the aging of these systems, it is important to investigate the impact of their decomposition at the limit of operating voltage during cycling, especially gas generated during the electrochemical reduction of amides at low potential.…”

“…We demonstrated recently the potential use of amide-based DESs in which as odium or lithium salt is mixed with an amide [N-methylacetamide (NMA) or formamide (FMD)]aselectrolytes in electrochemical storages ystemss uch as Li-ion batteries and supercapacitors. [33][34][35] Regarding the aging of these systems, it is important to investigate the impact of their decomposition at the limit of operating voltage during cycling, especially gas generated during the electrochemical reductiono fa mides at low potential. In this study,w eu sed the aforementioned technique for ad etailed investigation of aD ES formed between LiFSI and FMD (x LiFSI = 0.1 or C = 2.5 m)a se lectrolyte for an ACbased EDLC and the gas evolution during cycling.…”

Room-Temperature Molten Salts: Protic Ionic Liquids and Deep Eutectic Solvents as Media for Electrochemical Application

“…7.22a), observed by cyclic voltammetry and galvanostic charge-discharge methods, with high up to 380 F g −1 at elevated voltage and temperature, i.e., ΔE = 2.8 V and 80 °C for the Li[N(Tf) 2 ]-MAc mixture at, for example, x Li = 0.25 ( Fig. 7.22b) [144].…”

“…7.22) [144,145]. The decreasing capacitance of symmetric AC/AC systems despite good efficiency can be interpreted by the imbalanced mass of carbon electrodes caused by important lithium insertion at the negative electrode and by saturation of the positive electrode by anions; both mechanisms prevent, in fact, the system being operational.…”