Convergence Condition of Explicit Finite Element Method for Heat Transfer Problem

The convergence condition of the explicit difference method for the heat transfer problem is aiready obtained. On the other hand, if the problem is formulated by using the weighted residual method for spatial axis, we have no tool to estimate the critical timestep width. In this paper, the estimation method is theoretically presented, and its propriety is examined through a number of numerical experiments

Structural characteristics of alkylimidazolium-based salts containing fluoroanions

An overview of recent structural studies on alkylimidazolium-based salts containing fluoroanions is presented. Alkylimidazolium cations have been most extensively used for syntheses of ionic liquids (room temperature molten salts) because they usually exhibit low melting points, low viscosities and high conductivities. This review mainly focuses on structures of alkylimidazolium-based salts combined with a fluorocomplex anion ((FH)nF⁻, BF₄⁻, PF₆⁻, AsF₆⁻, SbF₆⁻, NbF₆⁻, TaF₆⁻), N(SO₂CF3)₂⁻ (TFSI⁻) and OSO₂CF₃⁻. The first part describes crystal structures of these salts and the second part describes computational, spectroscopic and diffraction studies on their liquid structures. Related studies on ionic liquids of non-alkylimidazolium cation and non-fluoroanion are also briefly summarized

Room-Temperature Ionic Liquids with High Conductivities and Wide Electrochemical Windows

Room-temperature ionic liquids (RTILs), N-alkyl-N-methylpyrrolidinium (RMPyr⁺) and N-alkyl-N-methylpiperidinium (RMPip⁺) fluorohydrogenates formulated by RMPyr( HF )₂.₃F and RMPip( HF )₂.₃F ( R = ethyl, propyl, and butyl), have been synthesized by the reactions of the corresponding chlorides and anhydrous hydrogen fluoride. These RTILs exhibit relatively low viscosities (11.5-34.4 cP), high conductivities (12.3-74.6 mS cm⁻¹), and wide electrochemical widows around 5 V using glassy carbon electrodes. The fluorohydrogenate salts of symmetric cations, N, N-dimethylpyrrolidinium (DMPyr⁺) and N, N-dimethylpiperidinium (DMPip⁺) obtained by evacuation at room temperature are solids, exhibiting the composition of DMPyrF-2HF and DMPipF-2HF

Electrolytes toward High-Voltage Na3V2(PO4)2F3 Positive Electrode Durable against Temperature Variation

High power and energy density, long cyclability, and tolerance for wide temperature (seasonal and daily operational temperature differences) must be considered to construct large‐scale sodium secondary batteries. In this regard, Na₃V₂(PO₄)₂F₃ (NVPF) has become a subject of interest as a high‐performance positive electrode material owing to its high energy density. However, the high operating voltage of NVPF causes continuous decomposition of electrolytes during cycles, resulting in significant capacity fading and low Coulombic efficiency. In this study, the electrochemical performance of the NVPF electrode in organic solvent electrolytes with and without additives and an ionic liquid is investigated at high voltage regimes over a wide temperature range (−20 °C to 90 °C). The results reveal that the performance of organic electrolytes is still insufficient even with additives, and the ionic liquid electrolyte demonstrates high electrochemical stability and cyclability with NVPF electrodes over a temperature range from −20 °C to 90 °C, achieving stable cycling over 500 cycles. The detailed electrochemical analysis combined with X‐ray photoelectron and energy dispersive X‐ray spectroscopy indicates that a sturdy cathode electrolyte interphase layer around the electrode protects it from capacity fading at high voltage and elevated temperature, resulting in high Coulombic efficiency

Physical and Electrochemical Properties of 1-ethyl-3-methylimidazolium Ionic Liquids of Mixed Anions, (FH)ₙF⁻, BF₄⁻, and N(SO₂CF₃)₂⁻

Physical and electrochemical properties of 1-ethyl-3-methylimidazolium ionic liquids of mixed anions, (FH)₂.₃F⁻, BF₄⁻, and N(SO₃)₂, have been investigated. Molar volume shows almost linear behavior, whereas molar conductivity is decreased by mixing for the systems involving (FH)₂.₃F⁻ due to the enhancement of ion association in spite of the decrease in viscosity. The currents at the anode and cathode limits in the cyclic voltammogram of EMIm(FH)₂.₃F decreases with decrease in the molar ratio of (FH)ₙF⁻, suggesting the involvement of (FH)ₙF⁻ for both electrode reactions. Electrochemical stability of the BF₄-TFSA mixture is unchanged by mixing

Electrochemical Synthesis of Graphite-Tetrafluoroaluminate Intercalation Compounds

Graphite tetrafluoroaluminate intercalation compounds (AlF₄-GICs) have been prepared by electrochemical oxidation of a natural graphite electrode in a 1.0 M nitromethane solution of tetraethylammmonium tetrafluoroaluminate ([TEA][AlF₄]). Galvanostatic electrolysis suggests that the intercalation reaction occurs above 0.8 V vs. Ag⁺/Ag. Powder X-ray diffraction measurements of the AlF₄-GIC obtained by potentiostatic electrolysis reveal that the most AlF₄-rich phase is the stage-3 GIC with a gallery height of 0.79 nm. This gallery height agrees with the theoretical value calculated from the size of AlF₄⁻ that locates its two-fold axis perpendicular to the graphite layers. Co-intercalation of the solvent is suggested by the composition of the stage-3 GIC (C₅₅AlF₄) and is confirmed by release of the solvent above 350 K during thermogravimetric analysis. Although the AlF₄-GIC shows the higher air stability than those of the GICs with typical inorganic complex anions, it slowly decomposes into GICs at higher stages after exposure to the air over 1000 h. Increase of gallery height was observed during this period, which possibly results from reorientation of AlF₄− between the layers. The thermodynamic stability of AlF₄-GIC is evaluated based on a Born-Harber cycle