36 research outputs found
Composite WO<sub>3</sub>/TiO<sub>2</sub> Nanostructures for High Electrochromic Activity
A composite
material consisting of TiO<sub>2</sub> nanotubes (NT) with WO<sub>3</sub> 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 TiO<sub>2</sub> made from commercially available TiO<sub>2</sub> nanoparticles
creates an interface for the TiO<sub>2</sub> 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 WO<sub>3</sub> concentration on the EC performance
were studied. The composite WO<sub>3</sub>/TiO<sub>2</sub> nanostructures
showed higher ion storage capacity, better stability, enhanced EC
contrast, and longer memory time compared with the pure WO<sub>3</sub> and TiO<sub>2</sub> materials
Reversible Hydrogen Storage by NaAlH<sub>4</sub> Confined within a Titanium-Functionalized MOF-74(Mg) Nanoreactor
We demonstrate that NaAlH<sub>4</sub> confined within the nanopores of a titanium-functionalized metalāorganic 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 Ć
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 ānanoreactorā. Confining NaAlH<sub>4</sub> within these pores alters the decomposition pathway by eliminating the stable intermediate Na<sub>3</sub>AlH<sub>6</sub> phase observed during bulk decomposition and proceeding directly to NaH, Al, and H<sub>2</sub>, in agreement with theory. The onset of hydrogen desorption for both Ti-doped and undoped nano-NaAlH<sub>4</sub>@MOF-74(Mg) is ā¼50 Ā°C, nearly 100 Ā°C lower than bulk NaAlH<sub>4</sub>. 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 H<sub>2</sub> desorption is reduced from 79.5 kJ mol<sup>ā1</sup> in bulk Ti-doped NaAlH<sub>4</sub> to 57.4 kJ mol<sup>ā1</sup> for nanoconfined NaAlH<sub>4</sub>. The structural properties of nano-NaAlH<sub>4</sub>@MOF-74(Mg) were probed using <sup>23</sup>Na and <sup>27</sup>Al solid-state MAS NMR, which indicates that the hydride is not decomposed during infiltration and that Al is present as tetrahedral AlH<sub>4</sub><sup>ĀÆ</sup> 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 Li<sub>2</sub>B<sub>12</sub>H<sub>12</sub>, Na<sub>2</sub>B<sub>12</sub>H<sub>12</sub>, and K<sub>2</sub>B<sub>12</sub>H<sub>12</sub> 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 M<sub>2</sub>B<sub>12</sub>H<sub>12</sub> salts form the corresponding metal borohydrides MBH<sub>4</sub> (M = Li, Na, K) in almost quantitative yield at 100 MPa H<sub>2</sub> and 500 Ā°C. In addition, stoichiometric mixtures of
Li<sub>2</sub>B<sub>12</sub>H<sub>12</sub> and MgH<sub>2</sub> were
found to form MgB<sub>2</sub> at 500 Ā°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 Li<sub>2</sub>B<sub>10</sub>H<sub>10</sub>
On
the basis of X-ray and neutron powder diffraction, first-principles
calculations, and neutron vibrational spectroscopy, Li<sub>2</sub>B<sub>10</sub>H<sub>10</sub> was found to exhibit atypical hexagonal
symmetry to best stabilize the ionic packing of the relatively small
Li<sup>+</sup> cations and large ellipsoidal B<sub>10</sub>H<sub>10</sub><sup>2ā</sup> anions. Moreover, differential scanning calorimetry
and neutron-elastic-scattering fixed-window scans suggested that Li<sub>2</sub>B<sub>10</sub>H<sub>10</sub>, similar to its polyhedral cousin
Li<sub>2</sub>B<sub>12</sub>H<sub>12</sub>, undergoes an orderādisorder
phase transition near 640
K. These results provide valuable structural information pertinent
to understanding the potential role that Li<sub>2</sub>B<sub>10</sub>H<sub>10</sub> plays during LiBH<sub>4</sub> dehydrogenationārehydrogenation
as well as its prospects as a superionic Li<sup>+</sup> cation conductor
Structural Behavior of Li<sub>2</sub>B<sub>10</sub>H<sub>10</sub>
On
the basis of X-ray and neutron powder diffraction, first-principles
calculations, and neutron vibrational spectroscopy, Li<sub>2</sub>B<sub>10</sub>H<sub>10</sub> was found to exhibit atypical hexagonal
symmetry to best stabilize the ionic packing of the relatively small
Li<sup>+</sup> cations and large ellipsoidal B<sub>10</sub>H<sub>10</sub><sup>2ā</sup> anions. Moreover, differential scanning calorimetry
and neutron-elastic-scattering fixed-window scans suggested that Li<sub>2</sub>B<sub>10</sub>H<sub>10</sub>, similar to its polyhedral cousin
Li<sub>2</sub>B<sub>12</sub>H<sub>12</sub>, undergoes an orderādisorder
phase transition near 640
K. These results provide valuable structural information pertinent
to understanding the potential role that Li<sub>2</sub>B<sub>10</sub>H<sub>10</sub> plays during LiBH<sub>4</sub> dehydrogenationārehydrogenation
as well as its prospects as a superionic Li<sup>+</sup> cation conductor
Structural Behavior of Li<sub>2</sub>B<sub>10</sub>H<sub>10</sub>
On
the basis of X-ray and neutron powder diffraction, first-principles
calculations, and neutron vibrational spectroscopy, Li<sub>2</sub>B<sub>10</sub>H<sub>10</sub> was found to exhibit atypical hexagonal
symmetry to best stabilize the ionic packing of the relatively small
Li<sup>+</sup> cations and large ellipsoidal B<sub>10</sub>H<sub>10</sub><sup>2ā</sup> anions. Moreover, differential scanning calorimetry
and neutron-elastic-scattering fixed-window scans suggested that Li<sub>2</sub>B<sub>10</sub>H<sub>10</sub>, similar to its polyhedral cousin
Li<sub>2</sub>B<sub>12</sub>H<sub>12</sub>, undergoes an orderādisorder
phase transition near 640
K. These results provide valuable structural information pertinent
to understanding the potential role that Li<sub>2</sub>B<sub>10</sub>H<sub>10</sub> plays during LiBH<sub>4</sub> dehydrogenationārehydrogenation
as well as its prospects as a superionic Li<sup>+</sup> cation conductor
Anion Reorientations in the Superionic Conducting Phase of Na<sub>2</sub>B<sub>12</sub>H<sub>12</sub>
Quasielastic neutron scattering (QENS)
methods were used to characterize
the reorientational dynamics of the dodecahydro-<i>closo</i>-dodecaborate (B<sub>12</sub>H<sub>12</sub><sup>2ā</sup>)
anions in the high-temperature, superionic conducting phase of Na<sub>2</sub>B<sub>12</sub>H<sub>12</sub>. The icosahedral anions in this
disordered cubic phase were found to undergo rapid reorientational
motions, on the order of 10<sup>11</sup> jumps s<sup>ā1</sup> 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<sup>ā1</sup>)
Stable Interface Formation between TiS<sub>2</sub> and LiBH<sub>4</sub> in Bulk-Type All-Solid-State Lithium Batteries
In this study, we assembled a bulk-type
all-solid-state battery
comprised of a TiS<sub>2</sub> positive electrode, LiBH<sub>4</sub> electrolyte, and Li negative electrode. Our battery retained high
capacity over 300 dischargeācharge cycles when operated at
393 K and 0.2 C. The second discharge capacity was as high as 205
mAh g<sup>ā1</sup>, corresponding to a TiS<sub>2</sub> utilization
ratio of 85%. The 300th discharge capacity remained as high as 180
mAh g<sup>ā1</sup> with nearly 100% Coulombic efficiency from
the second cycle. Negligible impact of the exposure of LiBH<sub>4</sub> 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 TiS<sub>2</sub>/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 LiBH<sub>4</sub> occurred at the TiS<sub>2</sub> surface at the
battery operating temperature of 393 K and/or during the initial charge.
During this oxidation reaction of LiBH<sub>4</sub> with hydrogen (H<sub>2</sub>) release just beneath the TiS<sub>2</sub> surface, a third
phase, likely including Li<sub>2</sub>B<sub>12</sub>H<sub>12</sub>, precipitated at the interface between LiBH<sub>4</sub> and TiS<sub>2</sub>. Li<sub>2</sub>B<sub>12</sub>H<sub>12</sub> has a lithium
ionic conductivity of logĀ(Ļ / S cm<sup>ā1</sup>) = ā4.4,
charge transfer reactivity with Li electrodes, and superior oxidative
stability to LiBH<sub>4</sub>, and thereby can act as a stable interface
that enables numerous dischargeācharge 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 Li<sub>2</sub>B<sub>12</sub>H<sub>12</sub>, which are thermodynamically available
in the ternary LiāBāH system
Impact of Ionic Liquid Pretreatment Conditions on Cellulose Crystalline Structure Using 1āEthyl-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, 3ā20 wt % microcrystalline cellulose
(Avicel) solutions were treated with 1-ethyl-3-methylimidazolium acetate
([C<sub>2</sub>mim]Ā[OAc]) and a mixture of [C<sub>2</sub>mim]Ā[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 [C<sub>2</sub>mim]Ā[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)
OrderāDisorder Transitions and Superionic Conductivity in the Sodium <i>nido</i>-Undeca(carba)borates
The
salt compounds NaB<sub>11</sub>H<sub>14</sub>, Na-7-CB<sub>10</sub>H<sub>13</sub>, Li-7-CB<sub>10</sub>H<sub>13</sub>, Na-7,8-C<sub>2</sub>B<sub>9</sub>H<sub>12</sub>, and Na-7,9-C<sub>2</sub>B<sub>9</sub>H<sub>12</sub> all contain geometrically similar, monocharged, <i>nido</i>-undecaĀ(carba)Āborate anions (i.e., truncated icosohedral-shaped
clusters constructed of only 11 instead of 12 {BāH} + {CāH}
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>-polyĀ(carba)Āborate
cousins. All exhibited orderādisorder phase transitions somewhere
between room temperature and 375 K. All disordered phases appear to
possess highly reorientationally mobile anions (> ā¼10<sup>10</sup> jumps s<sup>ā1</sup> above 300 K) and cation-vacancy-rich,
close-packed or body-center-cubic-packed structures [like previously
investigated <i>closo</i>-polyĀ(carba)Āborates]. 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-CB<sub>9</sub>H<sub>10</sub><sup>ā</sup> and CB<sub>11</sub>H<sub>12</sub><sup>ā</sup> <i>closo</i>-carbaborate anions. This study significantly
expands the known toolkit of solid-state, polyĀ(carba)Āborate-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