Exploring Sustainable Rocket Fuels: [Imidazolyl−Amine−BH2]+‐Cation‐Based Ionic Liquids as Replacements for Toxic Hydrazine Derivatives

The application of hypergolic ionic liquids as propellant fuels is a newly emerging area in the fields of chemistry and propulsion science. Herein, a new class of [imidazolyl?amine?BH₂]⁺‐cation‐based ionic liquids, which included fuel‐rich anions, such as dicyanamide (N(CN)₂^?) and cyanoborohydride (BH₃CN^?) anions, were synthesized and characterized. As expected, all of the ionic liquids exhibited spontaneous combustion upon contact with the oxidizer 100 % HNO₃. The densities of these ionic liquids varied from 0.99–1.12 g cm^?3, and the heats of formation, predicted based on Gaussian 09 calculations, were between ?707.7 and 241.8 kJ mol^?1. Among them, the salt of compound 5 , that is, (1‐allyl‐1H‐imidazole‐3‐yl)?(trimethylamine)?dihydroboronium dicyanamide, exhibited the lowest viscosity (168 MPa s), good thermal properties (T_gT_d>130 °C), and the shortest ignition‐delay time (18 ms) with 100 % HNO₃. These ionic fuels, as “green” replacements for toxic hydrazine‐derivatives, may have potential applications as bipropellant formulations.     

Hypergolic N,N‐Dimethylhydrazinium Ionic Liquids

N,N‐Dimethylhydrazinium dicyanamide and nitrocyanamide ionic liquids (ILs) were prepared by quaterization of N,N‐dimethylhydrazine with alkyl halides followed by metathesis reactions with silver dicyanamide or silver nitrocyanamide. The key physicochemical properties, such as melting point and decomposition temperatures, density, viscosity, heat of formation, detonation pressure and velocity, and specific impulse were measured/calculated. The impact of anions and alkyl‐substituted cations on these properties is demonstrated. Droplet tests with white‐fuming nitric acid (WFNA) as an oxidizer were utilized to show that the 14 new N,N‐dimethylhydrazinium salts are hypergolic with ignition delay (ID) times ranging from 22 to 1642 ms, thereby suggesting that some may have potential as bipropellants.     

[Bis(imidazolyl)–BH2]+[Bis(triazolyl)–BH2]− Ionic Liquids with High Density and Energy Capacity

[Bis(imidazolyl)–BH₂]⁺[bis(triazolyl)–BH₂]^? and [bis(imidazolyl)–BH₂]⁺[tris(triazolyl)–BH]^? were synthesized, the cations and anions of which were functionalized with B?H groups and azoles. As B?H groups contribute to the hypergolic activity and azole groups improve the energy output, the resulting ionic liquids exhibited ignition delay times as low as 20 ms and energy outputs as high as 461.1 kJ mol^?1. In addition, densities (1.07–1.22 g cm^?3) and density‐specific impulse (≈360 s g cm^?3) values reached a relatively high level. These ionic liquids show great promise as sustainable rocket fuels.     

Size Matters! On the Way to Ionic Liquid Systems without Ion Pairing

Several, partly new, ionic liquids (ILs) containing imidazolium and ammonium cations as well as the medium‐sized [NTf₂]^? (0.230 nm³; Tf=CF₃SO₃^?) and the large [Al(hfip)₄]^? (0.581 nm³; hfip=OC(H)(CF₃)₂) anions were synthesized and characterized. Their temperature‐dependent viscosities and conductivities between 25 and 80 °C showed typical Vogel–Fulcher–Tammann (VFT) behavior. Ion‐specific self‐diffusion constants were measured at room temperature by pulsed‐gradient stimulated‐echo (PGSTE) NMR experiments. In general, self‐diffusion constants of both cations and anions in [Al(hfip)₄]^?‐based ILs were higher than in [NTf₂]^?‐based ILs. Ionicities were calculated from self‐diffusion constants and measured bulk conductivities, and showed that [Al(hfip)₄]^?‐based ILs yield higher ionicities than their [NTf₂]^? analogues, the former of which reach values of virtually 100 % in some cases.From these observations it was concluded that [Al(hfip)₄]^?‐based ILs come close to systems without any interactions, and this hypothesis is underlined with a Hirshfeld analysis. Additionally, a robust, modified Marcus theory quantitatively accounted for the differences between the two anions and yielded a minimum of the activation energy for ion movement at an anion diameter of slightly greater than 1 nm, which fits almost perfectly the size of [Al(hfip)₄]^?. Shallow Coulomb potential wells are responsible for the high mobility of ILs with such anions.     

Rapid Preparation of Silsesquioxane‐Based Ionic Liquids

Three new hybrid ionic liquids (ILs) based on cage silsesquioxane (SQ) were rapidly prepared in high yields from octa(mercaptopropyl)silsesquioxane and 1‐allyl‐3‐methylimidazolium salts (Br^?, BF₄^?, PF₆^?) through the photochemical thiol–ene reaction. These SQ‐based ILs exhibited low glass transition temperatures and good thermal stability. The unique amphiphilic nature of these hybrid ILs cause them to self‐assemble into perfect vesicles with “yolk–shell” structures, in which cages formed the “yolk” due to their aggregation and outer anions formed the “shell”.     

Synthesis,Structure and Magnetic Properties of Two Cobalt(II) Dicyanamide (dca) Complexes with Heterocyclic Nitrogen Donors Tetra(2‐pyridyl)pyrazine (tppz) and 2,4,6‐Tri(2‐pyridyl)‐1,3,5‐triazine (tptz): [Co2(tppz)(dca)4]·CH3CN and [Co(tptz)(dca)(H2O)](dca)

Two new transition metal dicyanamide complexes [Co₂(tppz)(dca)₄]·CH₃CN ( 1 ) [tppz=tetra(2‐pyridyl)pyrazine, dca=dicyanamide] and [Co(tptz)(dca)(H₂O)](dca) ( 2 ) [tptz=2,4,6‐tri(2‐pyridyl)‐1,3,5‐triazine] were synthesized and characterized by single crystal X‐ray diffraction analysis. In 1 each cobalt(II) atom is coordinated to three dca anions and one tppz molecule to form a distorted octahedral geometry, the neigbour two cobalt(II) atoms are bridged by one tppz ligand to form a dimer, then the cobalt(II) atoms in each dimer are joined together to form a ladder chain structure. In 2 the coordination geometry around the central metal is also distorted octahedral, each cobalt(II) atom is coordinated by two dca anions, one tptz molecule and one water ligand to form a cationic part, and the cationic part is linked with the free dca anions via the electrostatic attraction to give an infinite chain structure. Magnetic susceptibility measurement in the range of 2–300 K indicates that there are antiferromagnetic couplings between adjacent metal ions in 1 (T>29 K, (=?9.78 K, C=4.92 cm³·K·mol^?1) and ferromagnetic couplings in 2 (T>150 K, (=7.97 K, C=2.59 cm³·K·mol^?1) respectively.     

Effect of Dimethyl Carbonate on the Dynamic Properties and Ionicities of Ionic Liquids with [MIII(hfip)4]− (M=B,Al) Anions

Several ionic liquids (ILs) comprising [B(hfip)₄]^? [hfip=OCH(CF₃)₂] or [Al(hfip)₄]^? anions and imidazolium or ammonium cations were prepared and mixed with up to 270 mol % of dimethyl carbonate (DMC). The viscosities, conductivities, and self‐diffusion constants of these mixtures and, where possible, of the neat ILs were measured and compared with common [NTf₂]^? based ILs and their mixtures with DMC. A tremendous decrease of the viscosities and a likewise increase of the conductivities and diffusion constants can be achieved for all classes of ILs. However, the order of the conductivities is partially reversed in the diffusion data. This is probably due to the low dielectric constant of DMC and the, thus, favored ion pairing, as evidenced, for example, by the calculated ionicities. Altogether, our data show that the chemically robust, but high‐melting and more viscous [B(hfip)₄]^? ILs might be candidates for electrolytes when mixed with suitable molecular solvents.     

Coexistence of Tellurium Cations and Anions in Phosphonium-Based Ionic Liquids

Elemental tellurium readily dissolves in ionic liquids (ILs) based on tetraalkylphosphonium cations even at temperatures below 100 °C. In the case of ILs with acetate, decanoate, or dicyanamide anions, dark red to purple colored solutions form. A study combining NMR, UV-Vis and Raman spectroscopy revealed the formation of tellurium anions (Te_n)²⁻ with chain lengths up to at least n=5, which are in dynamic equilibrium with each other. Since external influences could be excluded and no evidence of an ionic liquid reaction was found, disproportionation of the tellurium is the only possible dissolution mechanism. Although the spectroscopic detection of tellurium cations in these solutions is difficult, the coexistence of tellurium cations, such as (Te₄)²⁺ and (Te₆)⁴⁺, and tellurium anions could be proven by cyclic voltammetry and electrodeposition experiments. DFT calculations indicate that electrostatic interactions with the ions of the ILs are sufficient to stabilize both types of tellurium ions in solution.     

Excess molar volumes of binary mixtures (an ionic liquid + water): A review

This review covers recent developments in the area of excess molar volumes for mixtures of {ILs (1) + H₂O (2)} where ILs refers to ionic liquids involving cations: imidazolium, pyridinium, pyrrolidinium, piperidinium, morpholinium and ammonium groups; and anions: tetraborate, triflate, hydrogensulphate, methylsulphate, ethylsulphate, thiocyanate, dicyanamide, octanate, acetate, nitrate, chloride, bromide, and iodine. The excess molar volumes of aqueous ILs were found to cover a wide range of values for the different ILs (ranging from −1.7 cm³ · mol⁻¹ to 1.2 cm³ · mol⁻¹). The excess molar volumes increased with increasing temperature for all systems studied in this review. The magnitude and in some cases the sign of the excess molar volumes for all the aqueous ILs mixtures, apart from the ammonium ILs, were very dependent on temperature. This was particularly important in the dilute IL concentration region. It was found that the sign and magnitude of the excess molar volumes of aqueous ILs (for ILs with hydrophobic cations), was more dependent on the nature of the anion than on the cation.     

DFT Study of the Geometrical and Electronic Structures of Geminal Dicationic Ionic Liquids 1,3‐Bis[3‐methylimidazolium‐1‐yl]hexane Halides

In this work, the geometrical and electronic properties of the mono cationic ionic liquid 1‐hexyl‐3‐methylimidazolium halides ([C₆(mim)]⁺_X^?, X=Cl, Br and I) and dicationic ionic liquid 1,3‐bis[3‐methylimidazolium‐1‐yl]hexane halides ([C₆(mim)₂X₂], X=Cl, Br and I) were studied using the density functional theory (DFT). The most stable conformer of these two types ionic liquids (IL) are determined and compared with each other. Results show that in the most stable conformers, in both monocationic ILs and dicationic ILs, the Cl^? and Br^? anions prefer to locate almost in the plane of the imidazolium ring whereas the I^? anion prefers nearly vertical location respect to the imidazolium ring plan. Comparison of hydrogen bonding and ionic interactions in these two types of ionic liquids reveals that these ionic liquids can be formed hydrogen bond by Cl^? and Br^? anion. The calculated thermodynamic functions show that the interaction of cation — anion pair in the dicationic ionic liquids are more than monocationic ionic liquids and these interactions decrease with increasing the halide anion atomic weight.     

Coupling of Bifunctional CoMn‐Layered Double Hydroxide@Graphitic C3N4 Nanohybrids towards Efficient Photoelectrochemical Overall Water Splitting

The development of durable, low‐cost, and efficient photo‐/electrolysis for the oxygen and hydrogen evolution reactions (OER and HER) is important to fulfill increasing energy requirements. Herein, highly efficient and active photo‐/electrochemical catalysts, that is, CoMn‐LDH@g‐C₃N₄ hybrids, have been synthesized successfully through a facile in situ co‐precipitation method at room temperature. The CoMn‐LDH@g‐C₃N₄ composite exhibits an obvious OER electrocatalytic performance with a current density of 40 mA cm^?2 at an overpotential of 350 mV for water oxidation, which is 2.5 times higher than pure CoMn‐LDH nanosheets. For HER, CoMn‐LDH@g‐C₃N₄ (η₅₀=?448 mV) requires a potential close to Pt/C (η₅₀=?416 mV) to reach a current density of 50 mA cm². Furthermore, under visible‐light irradiation, the photocurrent density of the CoMn‐LDH@g‐C₃N₄ composite is 0.227 mA cm^?2, which is 2.1 and 3.8 time higher than pristine CoMn‐LDH (0.108 mA cm^?2) and g‐C₃N₄ (0.061 mA cm^?2), respectively. The CoMn‐LDH@g‐C₃N₄ composite delivers a current density of 10 mA cm^?2 at 1.56 V and 100 mA cm^?2 at 1.82 V for the overall water‐splitting reaction. Therefore, this work establishes the first example of pure CoMn‐LDH and CoMn‐LDH@g‐C₃N₄ hybrids as electrochemical and photoelectrochemical water‐splitting systems for both OER and HER, which may open a pathway to develop and explore other LDH and g‐C₃N₄ nanosheets as efficient catalysts for renewable energy applications.     

Ultrafine Molybdenum Carbide Nanoparticles Composited with Carbon as a Highly Active Hydrogen‐Evolution Electrocatalyst

The replacement of platinum with non‐precious‐metal electrocatalysts with high efficiency and superior stability for the hydrogen‐evolution reaction (HER) remains a great challenge. Herein, we report the one‐step synthesis of uniform, ultrafine molybdenum carbide (Mo₂C) nanoparticles (NPs) within a carbon matrix from inexpensive starting materials (dicyanamide and ammonium molybdate). The optimized catalyst consisting of Mo₂C NPs with sizes lower than 3 nm encapsulated by ultrathin graphene shells (ca. 1–3 layers) showed superior HER activity in acidic media, with a very low onset potential of ?6 mV, a small Tafel slope of 41 mV dec^?1, and a large exchange current density of 0.179 mA cm^?2, as well as good stability during operation for 12 h. These excellent properties are similar to those of state‐of‐the‐art 20 % Pt/C and make the catalyst one of the most active acid‐stable electrocatalysts ever reported for HER.     

Novel Cyclic Sulfonium‐Based Ionic Liquids: Synthesis,Characterization, and Physicochemical Properties

A new series of ionic liquids composed of three cyclic sulfonium cations and four anions has been synthesized and characterized. Their physicochemical properties, including their spectroscopic characteristics, ion cluster behavior, surface properties, phase transitions, thermal stability, density, viscosity, refractive index, tribological properties, ion conductivity, and electrochemical window have been comprehensively studied. Eight of these salts are liquids at room temperature, at which some salts based on [NO₃]^? and [NTf₂]^? ions exhibit organic plastic crystal behaviors, and all the saccharin‐based salts display relatively high refractive indices (1.442–1.594). In addition, some ionic liquids with the [NTf₂]^? ion exhibit peculiar spectroscopic characteristics in FTIR and UV/Vis regions, whilst those salts based on the [DCA]^? ion show lower viscosities (34.2–62.6 mPa s at 20 °C) and much higher conductivities (7.6–17.6 mS cm^?1 at 20 °C) than most traditional 1,3‐dialkylimidazolium salts.     

Absolute Brønsted Acidities and pH Scales in Ionic Liquids

Although receiving large interest over the last years, some fundamental aspects of Brønsted acidity in ionic liquids (ILs) have up to now been insufficiently highlighted. In this work, standard states, activity, and activity coefficient definitions for IL solvent systems were developed from general thermodynamic considerations and then extended to a general mixed solvent standard state. By using the bromide/bromoaluminate systems as representative ILs, formulae for thermodynamically consistent pH scales for ILs with simple (Br^?) and complex ([Al_nBr_3n+1]^?) anions were derived on the basis of the chemical potential of the proton. Supported by quantum chemical [ccsd(t)/MP2/DFT/COSMO‐RS] calculations, Gibbs solvation energies of the proton were calculated, which allowed the ILs to be ranked in absolute acidity, that is, pH_abs or μ_abs(H⁺, IL), and additionally allowed their acidity to be compared with molecular Brønsted acid systems. It was shown that bromoaluminate ILs are suited for reaching superacidic conditions. The complexity of autoprotolysis processes in C₆MIM⁺[AlBr₄]^? (C₆MIM=1‐hexyl‐3‐methylimidazolium) with or without the addition of basic (i.e. Br^?) or acidic (AlBr₃ and/or HBr) solutes was examined in detail by model calculations, and they indicated a large thermodynamic influence of small deviations from the exact stoichiometric composition.     

Alkyl Ammonium Cation Stabilized Biocidal Polyiodides with Adaptable High Density and Low Pressure

The effective application of biocidal species requires building the active moiety into a molecular back bone that can be delivered and decomposed on demand under conditions of low pressure and prolonged high‐temperature detonation. The goal is to destroy storage facilities and their contents while utilizing the biocidal products arising from the released energy to destroy any remaining harmful airborne agents. Decomposition of carefully selected iodine‐rich compounds can produce large amounts of the very active biocides, hydroiodic acid (HI) and iodine (I₂). Polyiodide anions, namely, I₃^?, I₅^?, which are excellent sources of such biocides, can be stabilized through interactions with large, symmetric cations, such as alkyl ammonium salts. We have designed and synthesized suitable compounds of adaptable high density up to 3.33 g cm^?3 that are low‐pressure polyiodides with various alkyl ammonium cations, deliverable iodine contents of which range between 58.0–90.9 %.     

Metall-Pseudohalogenide. 27. Zur Struktur der Dicyanamide zweiwertiger 3d-Metalle M(N(CN)2)2

Metal Pseudohalides. 27. Structures of Dicyanamides of Bivalent 3d Metals M(N(CN)₂)₂ On the basis of infrared investigations in the region of 200–2 400 cm^?1, of electronic spectra and X-ray studies the structures of the dicyanamides of bivalent 3d metals M(N(CN)₂)₂ (M = Mn, Fe, Co, Ni, Cu, Zn) are described. The compounds may be attributed to two different structure types with the anions acting as tri- or bi-dentate bridging ligands, cobalt and manganese dicyanamide are forming structural isomers with pseudooctahedral (α-forms) or pseudotetrahedral (β-forms) microsymmetry. The polymer-tetrahedral type of the compounds, which is also including Zn(N(CN)₂)₂ seems to contain a hitherto undescribed bonding mode of bridging dicyanamide groups.     

Boronium‐Cation‐Based Ionic Liquids as Hypergolic Fluids

Two series of boronium‐cation‐based ionic liquids were prepared and fully characterized by ¹H, ¹³C, and ¹¹B NMR and infrared spectroscopy, differential scanning calorimetry (DSC), and elemental analysis. The structure of bis(1‐methyl‐1H‐imidazole‐3‐yl)dihydroboronium dicyanoborohydride ( 5 a ) was determined by single‐crystal X‐ray diffraction. The densities of these ionic liquids range from 1.05 to 1.28 g cm^?3, and the heats of formation, predicted on the basis of Gaussian 03 calculations, fall between ?164.6 and 430.5 kJ mol^?1. Compound 5 b , bis(1‐allyl‐1H‐imidazole‐3‐yl)dihydroboronium dicyanoborohydride, exhibits the lowest viscosity (35 mPa s) and shortest ignition‐delay time (14 ms) in combination with 100 % HNO₃.     

Graphitic Carbon Nitride (g‐C3N4): An Interface Enabler for Solid‐State Lithium Metal Batteries

Solid‐state Li metal batteries (SSLMBs) have attracted considerable interests due to their promising energy density as well as high safety. However, the realization of a well‐matched Li metal/solid‐state electrolyte (SSE) interface remains challenging. Herein, we report g‐C₃N₄ as a new interface enabler. We discover that introducing g‐C₃N₄ into Li metal can not only convert the Li metal/garnet‐type SSE interface from point contact to intimate contact but also greatly enhance the capability to suppress the dendritic Li formation because of the greatly enhanced viscosity, decreased surface tension of molten Li, and the in situ formation of Li₃N at the interface. Thus, the resulting Li‐C₃N₄|SSE|Li‐C₃N₄ symmetric cell gives a significantly low interfacial resistance of 11 Ω cm² and a high critical current density (CCD) of 1500 μA cm^?2. In contrast, the same symmetric cell configuration with pristine Li metal electrodes has a much larger interfacial resistance (428 Ω cm²) and a much lower CCD (50 μA cm^?2).     

From Polyhalides to Polypseudohalides: Chemistry Based on Cyanogen Bromide

Pseudohalogens are defined as molecular entities that resemble the halogens in their chemistry. While our understanding of polyhalogen chemistry has increased over the last years, research on polypseudohalogen compounds is lacking. The pseudohalogen BrCN possesses a highly pronounced σ‐hole at the bromine side of the molecule, inducing strong halogen bonding. This allows the synthesis and characterization of new polypseudohalogen anions, as shown by the single‐crystal X‐ray diffraction of [PNP][Br(BrCN)] and [PNP][Br(BrCN)₃]. Both the nearly linear anion [Br(BrCN)]^? and the distorted pyramidal anion [Br(BrCN)₃]^? were characterized by Raman spectroscopy and quantum‐chemical calculations. The behavior of the polypseudohalogen compounds in solution and as room‐temperature ionic liquids (RT‐ILs) using the [NBu₄]⁺ cation was studied by ¹³C and ¹⁵N NMR spectroscopy. These types of ILs are capable of dissolving elemental gold and offer themselves as promising compounds in metal recycling.     

Electrodeposition of Three‐Dimensional Porous Platinum Film on Removable Polyaniline Template for High‐Performance Electroanalysis

Three‐dimensional porous platinum (Pt_por) films are prepared based on Pt electrodeposition on polyaniline (PANI) modified electrodes followed by selective dissolution of PANI with HNO₃. Electrochemical quartz crystal microbalance data suggest that the PANI‐H₂PtCl₆ interaction involves redox and coordination reactions, depending on the working potential. The Pt_por shows better electrocatalytic performance than the Pt/PANI and conventionally electrodeposited Pt. The Pt_por modified glassy carbon electrode (GCE) can electrocatalyze the oxidation of H₂O₂ with a sensitivity of 414 µA cm^?2 mM^?1 and a detection limit of 9 nM, and the chitosan‐glucose oxidase/Pt_por/GCE can sense glucose with a sensitivity of 93.4 µA cm^?2 mM^?1.