Composite WO₃/TiO₂ Nanostructures for High Electrochromic Activity

A composite material consisting of TiO₂ nanotubes (NT) with WO₃ electrodeposited on its surface has been fabricated, detached from its Ti substrate, and attached to a fluorine-doped tin oxide (FTO) film on glass for application to electrochromic (EC) reactions. Several adhesion layers were tested, finding that a paste of TiO₂ made from commercially available TiO₂ nanoparticles creates an interface for the TiO₂ NT film to attach to the FTO glass, which is conductive and does not cause solution-phase ions in an electrolyte to bind irreversibly with the material. The effect of NT length and WO₃ concentration on the EC performance were studied. The composite WO₃/TiO₂ nanostructures showed higher ion storage capacity, better stability, enhanced EC contrast, and longer memory time compared with the pure WO₃ and TiO₂ materials

Reversible Hydrogen Storage by NaAlH₄ Confined within a Titanium-Functionalized MOF-74(Mg) Nanoreactor

We demonstrate that NaAlH₄ confined within the nanopores of a titanium-functionalized metal–organic framework (MOF) template MOF-74(Mg) can reversibly store hydrogen with minimal loss of capacity. Hydride-infiltrated samples were synthesized by melt infiltration, achieving loadings up to 21 wt %. MOF-74(Mg) possesses one-dimensional, 12 Å channels lined with Mg atoms having open coordination sites, which can serve as sites for Ti catalyst stabilization. MOF-74(Mg) is stable under repeated hydrogen desorption and hydride regeneration cycles, allowing it to serve as a “nanoreactor”. Confining NaAlH₄ within these pores alters the decomposition pathway by eliminating the stable intermediate Na₃AlH₆ phase observed during bulk decomposition and proceeding directly to NaH, Al, and H₂, in agreement with theory. The onset of hydrogen desorption for both Ti-doped and undoped nano-NaAlH₄@MOF-74(Mg) is ∼50 °C, nearly 100 °C lower than bulk NaAlH₄. However, the presence of titanium is not necessary for this increase in desorption kinetics but enables rehydriding to be almost fully reversible. Isothermal kinetic studies indicate that the activation energy for H₂ desorption is reduced from 79.5 kJ mol^–1 in bulk Ti-doped NaAlH₄ to 57.4 kJ mol^–1 for nanoconfined NaAlH₄. The structural properties of nano-NaAlH₄@MOF-74(Mg) were probed using ²³Na and ²⁷Al solid-state MAS NMR, which indicates that the hydride is not decomposed during infiltration and that Al is present as tetrahedral AlH₄^¯ anions prior to desorption and as Al metal after desorption. Because of the highly ordered MOF structure and monodisperse pore dimensions, our results allow key template features to be identified to ensure reversible, low-temperature hydrogen storage

Understanding and Mitigating the Effects of Stable Dodecahydro-<i>closo</i>-dodecaborate Intermediates on Hydrogen-Storage Reactions

Alkali metal borohydrides can reversibly store hydrogen; however, the materials display poor cyclability, oftentimes linked to the occurrence of stable <i>closo</i>-polyborate intermediate species. In an effort to understand the role of such intermediates on the hydrogen storage properties of metal borohydrides, several alkali metal dodecahydro-<i>closo</i>-dodecaborate salts were isolated in anhydrous form and characterized by diffraction and spectroscopic techniques. Mixtures of Li₂B₁₂H₁₂, Na₂B₁₂H₁₂, and K₂B₁₂H₁₂ with the corresponding alkali metal hydrides were subjected to hydrogenation conditions known to favor partial or full reversibility in metal borohydrides. The stoichiometric mixtures of MH and M₂B₁₂H₁₂ salts form the corresponding metal borohydrides MBH₄ (M = Li, Na, K) in almost quantitative yield at 100 MPa H₂ and 500 °C. In addition, stoichiometric mixtures of Li₂B₁₂H₁₂ and MgH₂ were found to form MgB₂ at 500 °C and above upon desorption in vacuum. The two destabilization strategies outlined above suggest that metal polyhydro-<i>closo</i>-polyborate species can be converted into the corresponding metal borohydrides or borides, albeit under rather harsh conditions of hydrogen pressure and temperature

Structural Behavior of Li₂B₁₀H₁₀

On the basis of X-ray and neutron powder diffraction, first-principles calculations, and neutron vibrational spectroscopy, Li₂B₁₀H₁₀ was found to exhibit atypical hexagonal symmetry to best stabilize the ionic packing of the relatively small Li⁺ cations and large ellipsoidal B₁₀H₁₀^2– anions. Moreover, differential scanning calorimetry and neutron-elastic-scattering fixed-window scans suggested that Li₂B₁₀H₁₀, similar to its polyhedral cousin Li₂B₁₂H₁₂, undergoes an order–disorder phase transition near 640 K. These results provide valuable structural information pertinent to understanding the potential role that Li₂B₁₀H₁₀ plays during LiBH₄ dehydrogenation–rehydrogenation as well as its prospects as a superionic Li⁺ cation conductor

Structural Behavior of Li₂B₁₀H₁₀

On the basis of X-ray and neutron powder diffraction, first-principles calculations, and neutron vibrational spectroscopy, Li₂B₁₀H₁₀ was found to exhibit atypical hexagonal symmetry to best stabilize the ionic packing of the relatively small Li⁺ cations and large ellipsoidal B₁₀H₁₀^2– anions. Moreover, differential scanning calorimetry and neutron-elastic-scattering fixed-window scans suggested that Li₂B₁₀H₁₀, similar to its polyhedral cousin Li₂B₁₂H₁₂, undergoes an order–disorder phase transition near 640 K. These results provide valuable structural information pertinent to understanding the potential role that Li₂B₁₀H₁₀ plays during LiBH₄ dehydrogenation–rehydrogenation as well as its prospects as a superionic Li⁺ cation conductor

Structural Behavior of Li₂B₁₀H₁₀

On the basis of X-ray and neutron powder diffraction, first-principles calculations, and neutron vibrational spectroscopy, Li₂B₁₀H₁₀ was found to exhibit atypical hexagonal symmetry to best stabilize the ionic packing of the relatively small Li⁺ cations and large ellipsoidal B₁₀H₁₀^2– anions. Moreover, differential scanning calorimetry and neutron-elastic-scattering fixed-window scans suggested that Li₂B₁₀H₁₀, similar to its polyhedral cousin Li₂B₁₂H₁₂, undergoes an order–disorder phase transition near 640 K. These results provide valuable structural information pertinent to understanding the potential role that Li₂B₁₀H₁₀ plays during LiBH₄ dehydrogenation–rehydrogenation as well as its prospects as a superionic Li⁺ cation conductor

Anion Reorientations in the Superionic Conducting Phase of Na₂B₁₂H₁₂

Quasielastic neutron scattering (QENS) methods were used to characterize the reorientational dynamics of the dodecahydro-<i>closo</i>-dodecaborate (B₁₂H₁₂^2–) anions in the high-temperature, superionic conducting phase of Na₂B₁₂H₁₂. The icosahedral anions in this disordered cubic phase were found to undergo rapid reorientational motions, on the order of 10¹¹ jumps s^–1 above 530 K, consistent with previous NMR measurements and neutron elastic-scattering fixed-window scans. QENS measurements as a function of the neutron momentum transfer suggest a reorientational mechanism dominated by small-angle jumps around a single axis. The results show a relatively low activation energy for reorientation of 259 meV (25 kJ mol^–1)

Stable Interface Formation between TiS₂ and LiBH₄ in Bulk-Type All-Solid-State Lithium Batteries

In this study, we assembled a bulk-type all-solid-state battery comprised of a TiS₂ positive electrode, LiBH₄ electrolyte, and Li negative electrode. Our battery retained high capacity over 300 discharge–charge cycles when operated at 393 K and 0.2 C. The second discharge capacity was as high as 205 mAh g^–1, corresponding to a TiS₂ utilization ratio of 85%. The 300th discharge capacity remained as high as 180 mAh g^–1 with nearly 100% Coulombic efficiency from the second cycle. Negligible impact of the exposure of LiBH₄ to atmospheric-pressure oxygen on battery cycle life was also confirmed. To investigate the origin of the cycle durability for this bulk-type all-solid-state TiS₂/Li battery, electrochemical measurements, thermogravimetry coupled with gas composition analysis, powder X-ray diffraction measurements, and first-principles molecular dynamics simulations were carried out. Chemical and/or electrochemical oxidation of LiBH₄ occurred at the TiS₂ surface at the battery operating temperature of 393 K and/or during the initial charge. During this oxidation reaction of LiBH₄ with hydrogen (H₂) release just beneath the TiS₂ surface, a third phase, likely including Li₂B₁₂H₁₂, precipitated at the interface between LiBH₄ and TiS₂. Li₂B₁₂H₁₂ has a lithium ionic conductivity of log­(σ / S cm^–1) = −4.4, charge transfer reactivity with Li electrodes, and superior oxidative stability to LiBH₄, and thereby can act as a stable interface that enables numerous discharge–charge cycles. Our results strongly suggest that the creation of such a stable interfacial layer is due to the propensity of forming highly stable, hydrogen-deficient polyhydro-<i>closo</i>-polyborates such as Li₂B₁₂H₁₂, which are thermodynamically available in the ternary Li–B–H system

Impact of Ionic Liquid Pretreatment Conditions on Cellulose Crystalline Structure Using 1‑Ethyl-3-methylimidazolium Acetate

Ionic liquids (ILs) have been shown to affect cellulose crystalline structure in lignocellulosic biomass during pretreatment. A systematic investigation of the swelling and dissolution processes associated with IL pretreatment is needed to better understand cellulose structural transformation. In this work, 3–20 wt % microcrystalline cellulose (Avicel) solutions were treated with 1-ethyl-3-methylimidazolium acetate ([C₂mim]­[OAc]) and a mixture of [C₂mim]­[OAc] with the nonsolvent dimethyl sulfoxide (DMSO) at different temperatures. The dissolution process was slowed by decreasing the temperature and increasing cellulose loading, and was further retarded by addition of DMSO, enabling in-depth examination of the intermediate stages of dissolution. Results show that the cellulose I lattice expands and distorts prior to full dissolution in [C₂mim]­[OAc] and that upon precipitation the former structure leads to a less ordered intermediate structure, whereas fully dissolved cellulose leads to a mixture of cellulose II and amorphous cellulose. Enzymatic hydrolysis was more rapid for the intermediate structure (crystallinity = 0.34) than for cellulose II (crystallinity = 0.54)

Order–Disorder Transitions and Superionic Conductivity in the Sodium <i>nido</i>-Undeca(carba)borates

The salt compounds NaB₁₁H₁₄, Na-7-CB₁₀H₁₃, Li-7-CB₁₀H₁₃, Na-7,8-C₂B₉H₁₂, and Na-7,9-C₂B₉H₁₂ all contain geometrically similar, monocharged, <i>nido</i>-undeca­(carba)­borate anions (i.e., truncated icosohedral-shaped clusters constructed of only 11 instead of 12 {B–H} + {C–H} vertices and an additional number of compensating bridging and/or terminal H atoms). We used first-principles calculations, X-ray powder diffraction, differential scanning calorimetry, neutron vibrational spectroscopy, neutron elastic-scattering fixed-window scans, quasielastic neutron scattering, and electrochemical impedance measurements to investigate their structures, bonding potentials, phase-transition behaviors, anion orientational mobilities, and ionic conductivities compared to those of their <i>closo</i>-poly­(carba)­borate cousins. All exhibited order–disorder phase transitions somewhere between room temperature and 375 K. All disordered phases appear to possess highly reorientationally mobile anions (> ∼10¹⁰ jumps s^–1 above 300 K) and cation-vacancy-rich, close-packed or body-center-cubic-packed structures [like previously investigated <i>closo</i>-poly­(carba)­borates]. Moreover, all disordered phases display superionic conductivities but with generally somewhat lower values compared to those for the related sodium and lithium salts with similar monocharged 1-CB₉H₁₀^– and CB₁₁H₁₂^– <i>closo</i>-carbaborate anions. This study significantly expands the known toolkit of solid-state, poly­(carba)­borate-based salts capable of superionic conductivities and provides valuable insights into the effect of crystal lattice, unit cell volume, number of carbon atoms incorporated into the anion, and charge polarization on ionic conductivity