Carbon‐Coated LiTi2(PO4)3: An Ideal Insertion Host for Lithium‐Ion and Sodium‐Ion Batteries

We report the extraordinary performance of carbon‐coated sodium super ion conductor (NASICON)‐type LiTi₂(PO₄)₃ as an ideal host matrix for reversible insertion of both Li and Na ions. The NASICON‐type compound was prepared by means of a Pechini‐type polymerizable complex method and was subsequently carbon coated. Several characterization techniques such as XRD, thermogravimetric analysis (TGA), field‐emission (FE) SEM, TEM, and Raman analysis were used to study the physicochemical properties. Both guest species underwent a two‐phase insertion mechanism during the charge/discharge process that was clearly evidenced from galvanostatic and cyclic voltammetric studies. Unlike that of Li (≈1.5 moles of Li), Na insertion exhibits better reversibility (≈1.59 moles of Na) while experiencing a slightly higher capacity fade (≈8 % higher than Li) and polarization (780 mV) than Li. However, excellent rate capability profiles were noted for Na insertion relative to its counterpart Li. Overall, the Na insertion properties were found to be superior relative to Li insertion, which makes carbon‐coated NASICON‐type LiTi₂(PO₄)₃ hosts attractive for the development of next‐generation batteries.     

Water Reduction and Dihydrogen Addition in Aqueous Conditions With ansa-Phosphinoborane

Ortho-phenylene-bridged phosphinoborane (2,6-Cl₂Ph)₂B-C₆H₄-PCy₂ 1 was synthesized in three steps from commercially available starting materials. 1 reacts with H₂ or H₂O under mild conditions to form corresponding zwitterionic phosphonium borates 1-H₂ or 1-H₂O . NMR studies revealed both reactions to be remarkably reversible. Thus, when exposed to H₂, 1-H₂O partially converts to 1-H₂ even in the presence of multiple equivalents of water in the solution. The addition of parahydrogen to 1 leads to nuclear spin hyperpolarization both in dry and hydrous solvents, confirming the dissociation of 1-H₂O to free 1 . These observations were supported by computational studies indicating that the formation of 1-H₂ and 1-H₂O from 1 are thermodynamically favored. Unexpectedly, 1-H₂O can release molecular hydrogen to form phosphine oxide 1-O . Kinetic, mechanistic, and computational (DFT) studies were used to elucidate the unique “umpolung” water reduction mechanism.     

Photochemical Hydrogen Storage with Hexaazatrinaphthylene

When irradiated with violet light, hexaazatrinaphthylene (HATN) extracts a hydrogen atom from an alcohol forming a long-living hydrogenated species. The apparent kinetic isotope effect for fluorescence decay time in deuterated methanol (1.56) indicates that the lowest singlet excited state of the molecule is a precursor for intermolecular hydrogen transfer. The photochemical hydrogenation occurs in several alcohols (methanol, ethanol, isopropanol) but not in water. Hydrogenated HATN can be detected optically by an absorption band at 1.78 eV as well as with EPR (electron paramagnetic resonance) and NMR techniques. Mass spectrometry of photoproducts reveal di-hydrogenated HATN structures along with methoxylated and methylated HATN molecules which are generated through the reaction with methoxy radicals (remnants from alcohol splitting). Experimental findings are consistent with the theoretical results which predicted that for the excited state of the HATN-solvent molecular complex, there exists a barrierless hydrogen transfer from methanol but a small barrier for the similar oxidation of water.     

Infrared and NMR Spectroscopic Fingerprints of the Asymmetric H7+O3 Complex in Solution

Infrared (IR) absorption in the 1000–3700 cm⁻¹ range and ¹H NMR spectroscopy reveal the existence of an asymmetric protonated water trimer, H₇⁺O_3, in acetonitrile. The core H₇⁺O₃ motif persists in larger protonated water clusters in acetonitrile up to at least 8 water molecules. Quantum mechanics/molecular mechanics (QM/MM) molecular dynamics (MD) simulations reveal irreversible proton transport promoted by propagating the asymmetric H₇⁺O₃ structure in solution. The QM/MM calculations allow for the successful simulation of the measured IR absorption spectra of H₇⁺O₃ in the OH stretch region, which reaffirms the assignment of the H₇⁺O₃ spectra to a hybrid-complex structure: a protonated water dimer strongly hydrogen-bonded to a third water molecule with the proton exchanging between the two possible shared-proton Zundel-like centers. The H₇⁺O₃ structure lends itself to promoting irreversible proton transport in presence of even one additional water molecule. We demonstrate how continuously evolving H₇⁺O₃ structures may support proton transport within larger water solvates.     

Monitoring intracellular pH fluctuation with an excited-state intramolecular proton transfer-based ratiometric fluorescent sensor

Intracellular pH is a key parameter related to various biological and pathological processes. In this study, a ratiometric pH fluorescent sensor ABTT was developed harnessing the amino-type excited-state intramolecular proton transfer (ESIPT) process. Relying on whether the ESIPT proceeds normally or not, ABTT exhibited the yellow fluorescence in acidic media, or cyan fluorescence in basic condition. According to the variation, ABTT behaved as a promising sensor which possessed fast and reversible response to pH change without interference from the biological substances, and exported a steady ratiometric signal (I₄₇₈/I₅₄₆). Moreover, due to the ESIPT effect, large Stokes shift and high quantum yield were also exhibited in ABTT. Furthermore, ABTT was applied for monitoring the pH changes in living cells and visualizing the pH fluctuations under oxidative stress successfully. These results elucidated great potential of ABTT in understanding pH-dependent physiological and pathological processes.     

Pourbaix diagrams in weakly coupled systems: a case study involving acridinol and phenanthridinol pseudobases

The thermodynamics of proton‐coupled electron transfer (PCET) in weakly coupled organic pseudobases was investigated using 2,7‐dimethyl‐9‐hydroxy‐9‐phenyl‐10‐tolyl‐9,10‐dihydroacridine (AcrOH) and 6‐phenylphenanthridinol (PheOH) as model compounds. Pourbaix diagrams for two model compounds were constructed using the oxidation potentials and the pK_a values obtained, respectively, from cyclic voltammetry and photometric titrations. Our comparative study reveals the importance of having the redox active –N center closer to –OH functionality on the thermodynamics of PCET process: PheOH exhibits a wider range of pH values (pH = 2.8 to 13.3) in which both the alcohol and the corresponding alkoxy radical are expected to coexist in solution. This result indicates that a concerted mechanism is more likely to be discovered in pseudobases analogous to PheOH. The thermochemical data also indicate that the concerted PCET mechanism cannot be achieved if water is used as the proton acceptor: assuming the pK_a of hydronium ions as ?1.7, the PCET involving PheOH or AcrOH as proton/electron donors and water as the proton acceptor is expected to follow the stepwise ET/PT mechanism. Copyright © 2015 John Wiley & Sons, Ltd.     

Numerical study for the electrified instability of viscoelastic cylindrical dielectric fluid film surrounded by a conducting gas

The linear electrohydrodynamic cylindrical instability of annular Walters B^′${\mathrm{B}}^{\prime }$ viscoelastic dielectric fluid layer surrounded by a conducting gas in the presence of radial electric field is investigated. The obtained dispersion relation is found to be complicated and cannot be treated theoretically easily. Two limiting cases of interest are investigated, when the inertia is dominant, and when both the kinematic viscosity and viscoelasticity are high, and the corresponding new stability conditions are obtained for both cases. We solve the eigenvalue problem numerically using the continuation method which gives better results than the classical non-linear solvers such as Newton and Secant methods. It is found that the applied radial electric field has a dual role on the stability of the considered system, depending of the chosen wavenumbers range. Both the kinematic viscoelasticity and liquid depth are found to have stabilizing effects, while both the kinematic viscosity and surface tension have destabilizing effects on the considered system. The stability or instability breaks down for critical wavenumber values at which the growth rate vanishes. The behaviors of both the maximum growth rate and the corresponding dominant wavenumber are discussed in detail corresponding to the effect of all physical parameters. Finally a comparison between the results obtained here for Walters B^′${\mathrm{B}}^{\prime }$ viscoelastic fluids, and those obtained here too if the fluid is replaced by a Rivlin–Ericksen viscoelastic one is achieved. The limiting cases of absence of electric field and/or kinematic viscoelasticity are also investigated in detail.