Insight into the Camel-to-Bell Transition of Differential Capacitance in Ionic Liquid-Based Supercapacitors

Understanding ionic liquid (IL)-electrode interface is of great significance to the design of electrochemical systems. Herein, the electrical double layer (EDL) structure and charging process of the supercapacitor with ILs were investigated by engineering the cation-anion and IL-electrode interaction through molecular dynamics simulations coupled with the constant potential method. A camel-to-bell shape transition of differential capacitance was identified when the cation-anion or IL-electrode interaction decreased, manifesting the sensitive responses of EDL structure and charging mechanism to the varying interactions. Moreover, the identified camel-to-bell transition could lead to the rise of differential capacitance and energy density in the low working voltage. These findings can help guide the molecular design for high-performance IL-based supercapacitors and other electrochemical devices where the IL-electrode interfaces are crucial

Entropy driving highly selective CO2 separation in nanoconfined ionic liquids

Nowadays, the global greenhouse effect has led to the imminent development of CO2 capture, separation, and storage technologies. Hybrid membranes with nanoconfined ionic liquids (ILs) show great potential for CO2 separation, but the intrinsic mechanism is still obscure. Herein, the thermodynamical properties and solvating processes of CO2 and CH4 in ILs confined in graphene oxide were studied via performing massive molecular dynamics simulations. It was first identified that selectivity rises from 25.01 to 149.20 as the interlayer distance decreases from 3.00 to 1.50 nm, showing an ultrahigh separating selectivity. Interestingly, the solubility of CO2 in confined ILs increases by almost two orders of magnitude compared with that in bulk ILs, which is far larger than CH4 in confined ILs. The high solubility mainly originates from the fact that the confined ILs can induce the structure rearrangement and provide abundant CO2 adsorbing sites, raising the configurational entropy of CO2 in the confined ILs, and further driving the high separation selectivity of CO2 over CH4. Finally, quantitative relations between solubility, diffusion capacity, permeability, selectivity, and structural entropy of gas in confined ILs are constructed, which are meaningful for the theoretical understanding, rational design, and applications of highly efficient and low-cost separation of CO2

Insight into the Camel-to-Bell Transition of Differential Capacitance in Ionic Liquid-Based Supercapacitors

Understanding ionic liquid (IL)-electrode interface is of great significance to the design of electrochemical systems. Herein, the electrical double layer (EDL) structure and charging process of the supercapacitor with ILs were investigated by engineering the cation-anion and IL-electrode interaction through molecular dynamics simulations coupled with the constant potential method. A camel-to-bell shape transition of differential capacitance was identified when the cation-anion or IL-electrode interaction decreased, manifesting the sensitive responses of EDL structure and charging mechanism to the varying interactions. Moreover, the identified camel-to-bell transition could lead to the rise of differential capacitance and energy density in the low working voltage. These findings can help guide the molecular design for high-performance IL-based supercapacitors and other electrochemical devices where the IL-electrode interfaces are crucial

Entropy driving highly selective CO2 separation in nanoconfined ionic liquids

Nowadays, the global greenhouse effect has led to the imminent development of CO2 capture, separation, and storage technologies. Hybrid membranes with nanoconfined ionic liquids (ILs) show great potential for CO2 separation, but the intrinsic mechanism is still obscure. Herein, the thermodynamical properties and solvating processes of CO2 and CH4 in ILs confined in graphene oxide were studied via performing massive molecular dynamics simulations. It was first identified that selectivity rises from 25.01 to 149.20 as the interlayer distance decreases from 3.00 to 1.50 nm, showing an ultrahigh separating selectivity. Interestingly, the solubility of CO2 in confined ILs increases by almost two orders of magnitude compared with that in bulk ILs, which is far larger than CH4 in confined ILs. The high solubility mainly originates from the fact that the confined ILs can induce the structure rearrangement and provide abundant CO2 adsorbing sites, raising the configurational entropy of CO2 in the confined ILs, and further driving the high separation selectivity of CO2 over CH4. Finally, quantitative relations between solubility, diffusion capacity, permeability, selectivity, and structural entropy of gas in confined ILs are constructed, which are meaningful for the theoretical understanding, rational design, and applications of highly efficient and low-cost separation of CO2

Regulated interfacial stability by coordinating ionic liquids with fluorinated solvent for high voltage and safety batteries

Electrolyte is the key to improve the safety, cycle performance and temperature adaptation of lithium batteries. In this paper, we introduce a novel electrolyte that based on ionic liquids (ILs) 1-butyl-1-methylpyrrolidinium bis (trifluoromethylsulfonyl)imide (PYR14-TFSI) and fluorinated solvent 1,1,1,3,3,3-hexafluoroisopropyl methyl ether (HFPM). HFPM can significantly reduce the viscosity of PYR14-TFSI and enhance the wettability with Celgard separators. In addition, the coordination of PYR14-TFSI and HFPM promotes the local dissolution of lithium nitrate and helps to form a stable cathode/anode interface film. The electrochemical test shows that the electrochemical window of the electrolyte can reach to 5.35 V. For the Li/LiNi0.6Co0.2Mn0.2O2 half cells, the capacity retention is 94% and the Coulomb efficiency is 99.9% after 100 cycles at 0.5 C. The uniform and dense interface film is characterized and confirmed by scanning electron microscopy and X-ray photoelectron spectroscopy, etc. Furthermore, the half-cell also exhibits good thermal stability at high temperature. Considering ILs and fluoroether are non-flammable solvent, this work may pave the way for designing future generation highperformance electrolyte for lithium batteries

Abnormal Enhanced Free Ions of Ionic Liquids Confined in Carbon Nanochannels

Revealing the structure and behavior of confined ionic liquids (ILs) is essential for their applications in green chemical processes. Here, we explore the electroconductivity (sigma) and ionic correlation of imidazole ILs confined in graphene nanochannels via joint molecular dynamics simulation and theoretical analysis. The ideal and actual s of ILs are first calculated, showing a growing tendency and up to the bulk value as the nanochannel size ranges from 1 to 10 nm. To account for the ionic correlation, the ionicity was determined by the ratio of the actual to ideal s, reflecting the average fraction of free ions in the confined ILs. Amazingly, the ionicity of all three ILs shows an abnormal changing tendency, which first increases and reaches the maximum at 2 nm and then decreases to the bulk value. The conformational analysis, pair dissociating energy, and residence time are further obtained, proving that the abnormal enhanced ionicity should be attributed to the structure reconstruction of ILs near the graphene wall. The analytical model of ionicity herein can guide the rational design of efficient IL-based nanoporous electrodes and solid catalysts

Ionophobic nanopores enhancing the capacitance and charging dynamics in supercapacitors with ionic liquids

Nano-porous electrodes combined with ionic liquids (ILs) are widely favored to promote the energy density of supercapacitors. However, this is always accompanied by the reduced power density, especially considering the high viscosity and large steric hindrance of ILs. Here, we use computational simulations coupled with the equivalent circuit model to quantify the charging process and overall performance of the IL-based supercapacitor. We find that the ions in the electrode with ionophobic pores display a new charging mechanism under a threshold potential, namely co-ion adsorption, which has been ignored before. This identified abnormal charging process can not only efficiently enhance the differential capacitance but also remarkably speed up the charging dynamics. Meanwhile, the pores with various sizes and lengths in the electrode demonstrate the same tendency, reflecting the relative universality of the collaborative enhancement of the co-ion adsorption process. Furthermore, the quantitative relation between charging time/capacitance and electric voltage/ionophobic properties is further obtained to evaluate the critical conditions for synergistically improving the energy density and power density of supercapacitors. These findings may advance the understanding of charging mechanisms in porous electrodes and manifest that the ionophobicity is one important factor in the rational design of supercapacitors with ILs or other electrochemical devices in the field of chemical engineering

Manipulating mechanism of the electrokinetic flow of ionic liquids confined in silica nanochannel

The electrokinetic flow of ionic liquids (ILs) is widely used in electrochemical engineering applications. Herein, flow behavior and controlling mechanism of ILs driven by electrical field were investigated via molecular dynamics simulations. During the flow process, the cations prefer to allocate at the interface in a perpendicular configuration, while the anions tend to distribute randomly. In the interface region, the perpendicular cations and accumulated ILs would significantly enhance the local viscosity of ILs, which dominates the velocity distribution and total flux of the electrokinetic flow of confined ILs. Compared with the pressure and surface charging density, the nanochannel size would induce a larger impact on the average velocity, total flux, and streaming current of the electrokinetic flow of confined ILs. These quantitative results on the electrokinetic flow are crucial for the rational design of ILs-based devices or other chemical engineering processes. (c) 2022 Elsevier Ltd. All rights reserved

Manipulating mechanism of the electrokinetic flow of ionic liquids confined in silica nanochannel

The electrokinetic flow of ionic liquids (ILs) is widely used in electrochemical engineering applications. Herein, flow behavior and controlling mechanism of ILs driven by electrical field were investigated via molecular dynamics simulations. During the flow process, the cations prefer to allocate at the interface in a perpendicular configuration, while the anions tend to distribute randomly. In the interface region, the perpendicular cations and accumulated ILs would significantly enhance the local viscosity of ILs, which dominates the velocity distribution and total flux of the electrokinetic flow of confined ILs. Compared with the pressure and surface charging density, the nanochannel size would induce a larger impact on the average velocity, total flux, and streaming current of the electrokinetic flow of confined ILs. These quantitative results on the electrokinetic flow are crucial for the rational design of ILs-based devices or other chemical engineering processes. (c) 2022 Elsevier Ltd. All rights reserved

Topological engineering of two-dimensional ionic liquid islands for high structural stability and CO2 adsorption selectivity

Ionic liquids (ILs) as green solvents and catalysts are highly attractive in the field of chemistry and chemical engineering. Their interfacial assembly structure and function are still far less well understood. Herein, we use coupling first-principles and molecular dynamics simulations to resolve the structure, properties, and function of ILs deposited on the graphite surface. Four different subunits driven by hydrogen bonds are identified first, and can assemble into close-packed and sparsely arranged annular 2D IL islands (2DIIs). Meanwhile, we found that the formation energy and HOMO-LUMO gap decrease exponentially as the island size increases via simulating a series of 2DIIs with different topological features. However, once the size is beyond the critical value, both the structural stability and electrical structure converge. Furthermore, the island edges are found to be dominant adsorption sites for CO2 and better than other pure metal surfaces, showing an ultrahigh adsorption selectivity (up to 99.7%) for CO2 compared with CH4, CO, or N-2. Such quantitative structure-function relations of 2DIIs are meaningful for engineering ILs to efficiently promote their applications, such as the capture and conversion of CO2