Pore size distribution and capacitance in microporous carbons

Modelling shows that the pore size distribution of microporous carbons determined by NLDFT only hides relatively small variations of the surface-related capacitance C/S between 0.7 and 1.3 nm. This suggests again that C/S in TEABF(4)-AN is 0.09 ± 0.01 F m(-2) for our typical carbons, including carbide-based samples.     

Double-layer capacitance of waste coffee ground activated carbons in an organic electrolyte

Using ZnCl₂ activation we prepared a series of carbon electrodes from waste coffee grounds to study the effect of mesopores on double-layer capacitance in a tetraethyl ammonium tetrafluoroborate/acetonitrile electrolyte. The activated carbon with the largest mesopore volume achieved an energy density of 34 Wh kg^?1 at low current loads, and significantly retained an energy density of 16.5 Wh kg^?1 and specific capacitance of more than 100 F g^?1 at fast charge–discharge rates (20 A g^?1). The effect of mesopores on capacitance at fast charge–discharge rates is discussed.     

Chemistry in heterocyclic ammonium fluorohydrogenate room-temperature ionic liquid

Investigation on alkali fluoride-HF system has been initiated in the 19th century. The technique is currently utilized in fluorine-chemical industry. But, the problem is that this system readily releases hazardous HF. Although organic base, e.g., amine, with HF, which is mainly applied to fluorination treatment for organic compound, reduces the HF release, the solution still requires careful handling because of limited amount of free HF. Recently family of fluorohydrogenate room-temperature ionic liquid, XF(HF)_2.3, that consists of heterocyclic ammonium cation (X⁺), F(HF)₂⁻, and F(HF)₃⁻, has gotten a lot of attentions due to the interesting physicochemical properties such as negligible vapor pressure (<7.5 × 10⁻³ Torr (=1 Pa) at 298 K), high conductivity, and low corrosiveness. This novel solvent will greatly contribute to development of fluorine chemistry. In this article, fundamental techniques and physicochemical data on the fluorohydrogenate RTIL are summarized, and molecular science in the dialkylimidazolium fluorohydrogenates leading to the understanding of the unusual properties is reviewed based on recent experimental and theoretical considerations.     

Electrodeposition of platinum nanoparticles in a room-temperature ionic liquid

The electrochemistry of the [PtCl(6)](2-)-[PtCl(4)](2-)-Pt redox system on a glassy carbon (GC) electrode in a room-temperature ionic liquid (RTIL) [i.e., N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium tetrafluoroborate (DEMEBF(4))] has been examined. The two-step four-electron reduction of [PtCl(6)](2-) to Pt, i.e., reduction of [PtCl(6)](2-) to [PtCl(4)](2-) and further reduction of [PtCl(4)](2-) to Pt, occurs separately in this RTIL in contrast to the one-step four-electron reduction of [PtCl(6)](2-) to Pt in aqueous media. The cathodic and anodic peaks corresponding to the [PtCl(6)](2-)/[PtCl(4)](2-) redox couple were observed at ca. -1.1 and 0.6 V vs a Pt wire quasi-reference electrode, respectively, while those observed at -2.8 and -0.5 V were found to correspond to the [PtCl(4)](2-)/Pt redox couple. The disproportionation reaction of the two-electron reduction product of [PtCl(6)](2-) (i.e., [PtCl(4)](2-)) to [PtCl(6)](2-) and Pt metal was also found to occur significantly. The electrodeposition of Pt nanoparticles could be carried out on a GC electrode in DEMEBF(4) containing [PtCl(6)](2-) by holding the potential at -3.5 or -2.0 V. At -3.5 V, the four-electron reduction of [PtCl(6)](2-) to Pt can take place, while at -2.0 V the two-electron reduction of [PtCl(6)](2-) to [PtCl(4)](2-) occurs. The results obtained demonstrate that the electrodeposition of Pt at -3.5 V may occur via a series of reductions of [PtCl(6)](2-) to [PtCl(4)](2-) and further [PtCl(4)](2-) to Pt and at -2.0 V via a disproportionation reaction of [PtCl(4)](2-) to [PtCl(6)](2-) and Pt. Furthermore, the deposition potential of Pt nanoparticles was found to largely influence their size and morphology as well as the relative ratio of Pt(110) and Pt(100) crystalline orientation domains. The sizes of the Pt nanoparticles prepared by holding the electrode potential at -2.0 and -3.5 V are almost the same, in the range of ca. 1-2 nm. These small nanoparticles are "grown" to form bigger particles with different morphologies: In the case of the deposition at -2.0 V, the GC electrode surface is totally, relatively compactly covered with Pt particles of relatively uniform size of ca. 10-50 nm. On the other hand, in the case of the electrodeposition at -3.5 V, small particles of ca. 50-100 nm and the grown-up particles of ca. 100-200 nm cover the GC surface irregularly and coarsely. Interestingly, the Pt nanoparticles prepared by holding the potential at -2.0 and -3.5 V are relatively enriched in Pt(100) and Pt(110) facets, respectively.     

Lithium secondary batteries using modified-imidazolium room-temperature ionic liquid

Highly reversible, safe lithium secondary batteries that use imidazolium-cation-based room-temperature ionic liquid as an electrolyte and lithium metal as an anode material were realized by the molecular design. To achieve higher reduction stability, an electron-donating substituent was introduced to promote charge delocalization in the imidazolium cation of room-temperature ionic liquids.     

Capacitance response of carbons in solvent-free ionic liquid electrolytes

Ionic liquids (IL) are very promising “solvent-free” electrolytes for high-voltage double-layer supercapacitors (EDLCs) and to this purpose they are generally selected on the basis of their bulk properties, such as electrochemical stability and ion conductivity, without taking into account those of the electrified electrode-IL interface. This interface, which has yet to be well characterized, has features that notably affect electrode capacitance, and our paper for the first time highlights the importance of the molecular chemistry and structure of the ions for the double-layer capacitive response of carbonaceous electrodes in IL. The double-layer capacitive responses of negatively charged electrodes based on activated carbons and aero/cryo/xerogel carbons in two ILs featuring the same anion and different cations of almost the same size, i.e. the N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR₁₄TFSI) and 1-ethyl-3-methyl-imidazolium bis(trifluoromethanesulfonyl)imide (EMITFSI) are reported. The porosity, structure and surface chemistry of the carbons are compared to their capacitive response to evince the role played by these carbon properties and by the chemistry and structure of the IL ions in the electric double-layer.     

Anisotropic ionogels of sodium laurate in a room-temperature ionic liquid

Anisotropic thermally reversible ionogels of sodium laurate (SL) were prepared in the first discovered room-temperature ionic liquid (RTIL), ethylammonium nitrate (EAN). Polarized optical microscope images indicate that the gels are birefringent, illuminating the presence of anisotropic structures. Small-angle X-ray scattering results reveal that SL and lauric acid (LA) molecules are arranged to form lamellar structures, but no SL crystallites were confirmed by the X-ray diffraction measurements. With an increase of the SL concentration, the interlayer distance decreases. Rheological measurements indicate that the anisotropic ionogels are highly viscoelastic and the storage modulus (G') increases with an increase of the SL concentration in EAN. Electrochemical measurements indicate that the anisotropic ionogels may have potential applications in electrochemical fields. The intermolecular hydrogen bond as well as the solvatophobic interaction of SL and LA formed by a chemical reaction, CH(3)(CH(2))(10)COONa + CH(3)CH(2)NH(3)NO(3) --> CH(3)CH(2)NH(2) upward arrow + NaNO(3) downward arrow + CH(3)(CH(2))(10)COOH, can play a role in the formation of three-dimensional networks having lamellar structures which are responsible for the anisotropic ionogels. The formation of anisotropic ionogels by surfactants in RTILs could be a new phenomenon, but this is not a very classic case of organogels.     

Molecular insights into the potential and temperature dependences of the differential capacitance of a room-temperature ionic liquid at graphite electrodes

Molecular dynamics simulation studies of the structure and the differential capacitance (DC) for the ionic liquid (IL) N-methyl-N-propylpyrrolidinium bis(trifluoromethane)sulfonyl imide ([pyr(13)][TFSI]) near a graphite electrode have been performed as a function temperature and electrode potential. The IL exhibits a multilayer structure that extends 20-30 ? from the electrode surface. The composition and ion orientation in the innermost layer were found to be strongly dependent on the electrode potential. While at potentials near the potential of zero charge (PZC), both cations and anions adjacent to the surface are oriented primarily perpendicular to the surface, the counterions in first layer orient increasingly parallel to the surface with increasing electrode potential. A minimum in DC observed around -1 V(RPZC) (potential relative to the PZC) corresponds to the point of highest density of perpendicularly aligned TFSI near the electrode. Maxima in the DC observed around +1.5 and -2.5 V(RPZC) are associated with the onset of "saturation", or crowding, of the interfacial layer. The asymmetry of DC versus electrode polarity is the result of strong interactions between the fluorine of TFSI and the surface, the relatively large footprint of TFSI compared to pyr(13), and the tendency of the propyl tails of pyr(13) to remain adsorbed on the surface even at high positive potentials. Finally, an observed decreased DC and the disappearance of the minimum in DC near the PZC with increasing temperature are likely due to the increasing importance of entropic/excluded volume effects (interfacial crowding) with increasing temperature.     

Capacity of graphene anode in ionic liquid electrolyte

The graphene anode was investigated in an ionic liquid electrolyte (0.7 M lithium bis(trifluoromethanesulfonyl)imide (LiNTf₂)) in room temperature ionic liquid (N-methyl-N-propylpyrrolidinium bis(trifluoromethanesulfonyl)imide (MPPyrNTf₂)). SEM and TEM images suggested that the electrochemical intercalation/deintercalation process in the ionic liquid electrolyte without vinylene carbonate (VC) leads to small changes on the surface of graphene particles. However, a similar process in the presence of VC results in the formation of a coating (SEI—solid electrolyte interface) on the graphene surface. During charging/discharging tests, the graphene electrode working together with the 0.7 M LiNTf₂ in MPPyrNTf₂ electrolyte lost its capacity, during cycling and stabilizes at ca. 200 mAh g^?1 after 20 cycles. The addition of VC to the electrolyte (0.7 M LiNTf₂ in MPPyrNTf₂?+?10 wt.% VC) considerably increases the anode capacity. Electrodes were tested at different current regimes: ranging between 50 and 1,000 mA g^?1. The capacity of the anode, working at a low current regime of 50 mA g^?1, was ca. 1,250 mAh g^?1, while the current of 500 mA g^?1 resulted in capacity of 350 mAh g^?1. Coulombic efficiency was stable and close to 95 % during ca. 250 cycles. The exchange current density, obtained from impedance spectroscopy, was 1.3?×?10^?7 A cm^?2 (at 298 K). The effect of the anode capacity decrease with increasing current rate was interpreted as the result of kinetic limits of the electrode operation.     

Phase transition and conductive acceleration of phosphonium-cation-based room-temperature ionic liquid

An unusual ionic conduction phenomenon related to the phase transition of a novel phosphonium-cation-based room-temperature ionic liquid (RTIL) is reported; we found that in the phase change upon cooling, a clear increase in ionic conductivity was seen as the temperature was lowered, which differs from widely known conventional RTILs; clearly, our finding of abnormality of the correlation between temperature change and ionic conduction is the first observation in the electrolyte field.     

Reversed-phase liquid chromatographic method for the determination of selected room-temperature ionic liquid cations

The separation of selected 1-alkyl- and 1-aryl-3-methylimidazolium-based room temperature ionic liquid cations has been performed using reversed-phase high-performance liquid chromatography with electrospray ionization mass detection. The RP-HPLC method development started with the selection of a column taking into account especially the resolution of low molecular congeners of the selected group. Mobile phase composition was optimized for peak resolution, sensitivity and high reproducibility of retention values. The results of the method development were applied to the determination of exemplary ionic liquid species present in the medium used in cytotoxicity studies.     

Vibrational energy relaxation of a diatomic molecule in a room-temperature ionic liquid

Vibrational energy relaxation (VER) dynamics of a diatomic solute in ionic liquid 1-ethyl-3-methylimidazolium hexafluorophosphate (EMI(+)PF(6) (-)) are studied via equilibrium and nonequilibrium molecular dynamics simulations. The time scale for VER is found to decrease markedly with the increasing solute dipole moment, consonant with many previous studies in polar solvents. A detailed analysis of nonequilibrium results shows that for a dipolar solute, dissipation of an excess solute vibrational energy occurs almost exclusively via the Lennard-Jones interactions between the solute and solvent, while an oscillatory energy exchange between the two is mainly controlled by their electrostatic interactions. Regardless of the anharmonicity of the solute vibrational potential, VER becomes accelerated as the initial vibrational energy increases. This is attributed primarily to the enhancement in variations of the solvent force on the solute bond, induced by large-amplitude solute vibrations. One interesting finding is that if a time variable scaled with the initial excitation energy is employed, dissipation dynamics of the excess vibrational energy of the dipolar solute tend to show a universal behavior irrespective of its initial vibrational state. Comparison with water and acetonitrile shows that overall characteristics of VER in EMI(+)PF(6) (-) are similar to those in acetonitrile, while relaxation in water is much faster than the two. It is also found that the Landau-Teller theory predictions for VER time scale obtained via equilibrium simulations of the solvent force autocorrelation function are in reasonable agreement with the nonequilibrium results.     

Adsorption of phytosterol ethoxylates on silica in an aprotic room-temperature ionic liquid

The adsorption of a nonionic surfactant at a silica/room-temperature ionic liquid interface has been characterized on the basis of analytical data obtained through a combination of surface force measurements, in situ soft-contact atomic force microscope (AFM) images, and quartz crystal microbalance with dissipation monitoring (QCM-D) data. The surfactant employed in this study is a kind of phytosterol ethoxylate (BPS-20), and the ionic liquid selected here is aprotic 1-ethyl-3-methyl-imidazolium bis(trifluoromethylsulfonyl)imide (EmimTFSI). This ionic liquid spontaneously forms solvation layers on silica, being composed of an Emim(+) cation layer and EmimTFSI ion pair layers. The addition of BPS-20 disrupts these solvation layers and suggests a surfactant layer adsorbed at the interface. This is the first report demonstrating the adsorption of nonionic surfactants at the solid/aprotic ionic liquid interface.     

Spectroelectrochemical identification of a pentavalent uranyl tetrachloro complex in room-temperature ionic liquid

Reduction of U(VI)O(2)Cl(4)(2-) in a mixture of 1-ethyl-3-methylimidazolium tetrafluoroborate and its chloride at E°' = -0.996 V vs Fc/Fc(+) and 298 K affords U(V)O(2)Cl(4)(3-), which is kinetically stable and exhibits typical character of U(V) in the UV-vis-NIR absorption spectrum.     

Physical and electrochemical characterization of activated carbons with high mesoporous ratio for supercapacitors based on ionic liquid as the electrolyte

A series of activated carbons with high mesoporous ratio were prepared by KOH reactivation based on activated carbon as the precursor. As the KOH/AC mass ratio was increased to 4:1, the mesoporous ratio increases from 60% to 76%, and the average pore size from 2.23 to 3.14?nm. Moreover, the specific capacitance for the activated carbon in ionic liquid 1-ethyl-3-methylmidazolium tetrafluoroborate ([EMIm]BF₄) can reach the maximum value of 189?F?g^?1 (8.0???F?cm^?2). In addition, the decrease of specific capacitance for activated carbons by KOH reactivation with current density increase shows two regimes, suggesting that activated carbons with high mesoporous ratio are much fit for charge?Cdischarge at larger current density.     

Supercapacitor based on graphene and ionic liquid electrolyte

A new kind of supercapacitor by using chemical reduced graphene (CRG) as electrode material and ionic liquid with addition of acetonitrile as electrolyte is assembled and investigated. CRG materials with high surface area are prepared by chemical reduction of graphene oxide. The capacitive properties of the supercapacitor composed of the CRG and ionic liquid electrolyte are studied by electrical impedance spectroscopy, cyclic voltammetry and galvanostatic charge–discharge. With the combined advantages of graphene and ionic liquid, the supercapacitor shows perfect performance. The supercapacitor possesses wide cell voltage and good stability. The specific capacitance, energy density, and specific power density of the present supercapacitor are 132?Fg^??, 143.7?Wh?kg^??, and 2.8?kW?kg^??, respectively. The results demonstrate the potential application of electrical energy storage devices with high performance based on this new kind of supercapacitor.     

Lyotropic liquid crystalline phases formed by phyosterol ethoxylates in room-temperature ionic liquids

The phase behaviors of four phytosterol ethoxylates surfactants (BPS-n, n = 5, 10, 20, and 30) with different oxyethylene units in room temperature ionic liquid, 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim]BF₄), have been studied. The polarized optical microscopy and small-angle X-ray scattering techniques are used to characterize the phase structures of these binary systems at 25 °C. The structure and ordering of the liquid crystalline (LC) phases in such BPS-n/[Bmim]BF₄ systems are found to be influenced by BPS-n concentration and the temperature. Due to the bulky and rigid cholesterol group, the phytosterol ethoxylates surfactants exhibit different properties and interaction mechanism from the conventional C_nEO_m type nonionic surfactant systems. The rheological measurements indicate a highly viscoelastic nature of these lyotropic LC phases and disclose a lamellar phase characteristic with a rather strong rigidity at high surfactant concentrations. The control experiment with Brij 97(polyoxyethylene (10) oleyl ether)/[Bmim]BF₄ system and the FTIR measurements help to recognize that the solvophobic interaction combining with the hydrogen bonding are the main driving forces for the LC phases formation.     

Supercapacitor based on graphene and ionic liquid electrolyte

A new kind of supercapacitor by using chemical reduced graphene (CRG) as electrode material and ionic liquid with addition of acetonitrile as electrolyte is assembled and investigated. CRG materials with high surface area are prepared by chemical reduction of graphene oxide. The capacitive properties of the supercapacitor composed of the CRG and ionic liquid electrolyte are studied by electrical impedance spectroscopy, cyclic voltammetry and galvanostatic charge–discharge. With the combined advantages of graphene and ionic liquid, the supercapacitor shows perfect performance. The supercapacitor possesses wide cell voltage and good stability. The specific capacitance, energy density, and specific power density of the present supercapacitor are 132 Fg⁻¹, 143.7 Wh kg⁻¹, and 2.8 kW kg⁻¹, respectively. The results demonstrate the potential application of electrical energy storage devices with high performance based on this new kind of supercapacitor.