On the key role of electrolyte-electrode van der Waals interactions in the simulation of ionic liquids-based supercapacitors

By assembling a tetradentate N-donor ligand tetra(pyridine-4-yl)benzene-1,4-diamine (TBD) with a copper ion, three novel Cu(I)-based coordination frameworks, bearing one-, two-, and three-dimensional (3D) architectures, namely, [Cu2Br2(TBD)·H2O] (1), [Cu2Br2(TBD)] (2), and [CuI(TBD)] (3), have been synthesized and characterized. Owing to the different coordination modes of [Cu2Br2] dinuclear units, compounds 1 and 2 display distinct structures with double-stranded chains for 1 and 4-connected layers for 2. Compound 3 presents a (3,4)-connected 3D network, which is fabricated from neutral Cu-TBD layers with the bridging organic ligands. Solid-state photoluminescence test implies that the three compounds exhibit bright yellow emissions with the luminescence lifetime at a microsecond scale. Interestingly, through the alternation of the ratios with compound 3 and blue phosphor, tunable luminescence emissions from yellow to white light have been achieved. Moreover, the luminescence sensing ability toward various metal cations was examined on 1. The compound could be competent to serve as a selective fluorescent probe for the recognition of Fe3+ despite the existence of other interferences. 

Assembly of One-, Two-, and Three-Dimensional Cu(I)-Based Coordination Frameworks for Tunable White Light Emission and Effective Fluorescence Sensing

Graphene carriers can improve the properties of ionic liquids and enhance their CO2 capture performance. Density functional theory calculations were employed to evaluate the feasibility of utilizing graphene as a carrier material to enhance the CO2 adsorption capacity of hydroxypyridine-based ionic liquids (ILs). The results indicated that BN co-doped fluorinated graphene represented a promising carrier material capable of weakening the interaction between cations and anions while enhancing the charges of O and N in the anionic species of ILs, thereby increasing the adsorption capacity of ionic liquids for CO2. Furthermore, the microstructure and properties of the graphene-ILs composites were explored. Doping atoms in graphene were found to induce alterations in the distribution of surface electrostatic potential, influencing the adsorption sites of ionic liquids and their properties. Compared to hydrogen-terminated graphene, fluorinated graphene was a more stable carrier material due to its higher binding energy with hydroxypyridine-based ILs. The higher binding energy was not derived from the direct interaction between doped atoms and ILs, but rather from the impact of doped atoms on the electronic structure of graphene. In methanol solvent, the adsorption of ILs on various graphene surfaces was a spontaneous exothermic process, indicating the feasibility of material preparation. 

Enhancing CO2 adsorption capacity of hydroxypyridine-based ionic liquids using fluorinated graphene as carrier Material: A density functional theory study

Electric double layers (EDLs) play a key role in the electrochemical and energy storage of supercapacitors. It is important to understand the structure and properties of EDLs. In this work, quantum chemical calculations and molecular dynamics (MD) simulations are used to study the microstructure of EDLs of four different substituents of imidazolium-based bis(trifluoromethylsulfonyl)imide ionic liquids (ILs) on the Au(111) surface. It is shown that the particle interactions influence the different arrangements of the anion and cation. More alkyl substitutions and longer alkyl chains result in a higher ELUMO and thus a stronger interaction energy between cations and electrodes. Strong interactions produce linear patterns of anions/cations on the electrode and a maximum value of differential capacitance near PZC, whereas weak interactions generate worm-like patterns of anions/cations on Au(111) and a minimum value of differential capacitance near the PZC. We hold the opinion that the alkyl substitution has more effects on the EDLs. Our analysis provides a new perspective on EDLs structures at the atomic and molecular level. This study provides a good basis and guidance for further understanding the interface phenomena and characteristics of ionic liquids in electrochemical and energy device applications. 

Theoretical Insight into the Imidazolium-Based Ionic Liquid Interface Structure and Differential Capacitance on Au(111): Effects of the Cationic Substituent Group.

Ionic liquids have drawn great interest as electrolytes for energy storage applications in which they form characteristic electrical double layers at electrode interfaces. For ionic liquids at carbon electrodes, their...

Influence of surface nanostructure-induced innermost ion structuring on capacitance of carbon/ionic liquid double layers

The capacitance of carbon electrodes can be enhanced by tuning nanoscale roughness of the electrode surface, which facilitates separation between cations and anions within the electrical double layer. As the extent of ion separation depends on the ion correlation of bulk electrolytes, it is expected that the intrinsic electrical properties of ionic liquids (ILs) will also influence the double-layer structure and capacitance at nanostructured electrode interfaces. In this work, we use constant voltage molecular dynamics simulations to investigate the differential capacitance profiles for three different ionic liquids at model graphene electrode with subnanometer scale surface roughness. We focus on the ionic liquids composed of 1-butyl-3-methylimidazolium (BMIm+) cations and bistriflimide (TFSI–), triflate (OTf–), or tetrafluoroborate (BF4–) anions. Surprisingly, we find that [BMIm+][TFSI–] exhibits significantly enhanced differential capacitance at high positive potential compared to [BMIm+][BF4–] and [BMIm+][OTf–] with smaller anions. We demonstrate that this unexpected capacitance trend is due to nonpolar interactions between trifluoromethyl groups of TFSI– anions that promote TFSI–/TFSI– pairing at the electrode interface. These anion pairs lead to significantly reduced BMIm+ density in the [BMIm+][TFSI–] double layer compared to the other two ionic liquids. By contrast, no anion pairing occurs for [BMIm+][BF4–] and [BMIm+][OTf–] at the same voltage regime, so nonpolar alkyl groups of BMIm+ cations still predominantly accumulate at the nanostructured electrode interface, resulting in only slight enhancement of capacitance due to the surface roughness effect. This physical insight is important for controlling IL interfacial structure to enhance the capacitance of nanostructured electrodes. 

Importance of Anion–Anion Pairing for Capacitance of Carbon/Ionic Liquid Interfaces

Molecular lanthanide phosphonates [Ln2 (H3 tpmm)2 (H2 O)6 ] ⋅ xH2 O (Ln=Eu, EuP; Ln=Tb, TbP) were synthesized. Single-crystal X-ray diffraction confirmed that EuP has a sandwich-like dinuclear structure, in which the Eu(III) center adopts a {EuO8 } distorted dodecahedral geometry. XRPD patterns prove that TbP and EuP are isomorphous and isostructural. EuP and TbP are highly thermally stable approaching 450 °C and exhibit red- and green-light emissions from the characteristic 4 f-4 f transition of the Eu3+ and Tb3+ , respectively. Interestingly, luminescence modulation is achieved for the chemically mixed Eu/Tb phosphonate analogues, c-Eux Tb2 -x P (x=1.5, 1, 0.5), and physically mixed Eu/Tb phosphonate materials, p-yEuP : zTbP (y : z=3 : 1, 1 : 1, 1 : 3), with varying the excitation wavelength. Of particular note, near-white-light emission is also achieved for c-EuTbP, p-EuP : TbP, and p-EuP : 3TbP when excited at 365 nm. Therefore, these dinuclear molecular lanthanide phosphonates emitting excitation wavelength and Eu3+  : Tb3+ ratio dependent luminescence might be potential candidates for color-tunable luminescence materials and white-light-emitting materials. On the other hand, the bright green-light emission makes TbP to be an excellent reusable luminescence sensor for selective detection of Fe3+ with Stern-Volmer quenching constant (KSV ) of 9.66×103  M-1 and detection limit (DL) of 0.42 μM through absorption competition caused luminescence quenching effect.

Excitation-Wavelength-Dependent Luminescence of Chemically and Physically Mixed Europium and Terbium Phosphonates: Color-Tunable Luminescence, Near-White-Light Emission, and Selective Fe3+ Detection.

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Importance of Anion–Anion Pairing for Capacitance of Carbon/Ionic Liquid Interfaces

Excitation-Wavelength-Dependent Luminescence of Chemically and Physically Mixed Europium and Terbium Phosphonates: Color-Tunable Luminescence, Near-White-Light Emission, and Selective Fe3+ Detection.