29 research outputs found
Synthesis of 6‑Substituted 6<i>H</i>‑Indolo[2,3‑<i>b</i>]quinolines from Isoindigos
A facile
approach to 6-aryl/alkyl substituted 6<i>H</i>-indoloÂ[2,3-<i>b</i>]Âquinolines from mono-<i>N</i>-substituted isoindigo
derivatives in the presence of SnCl<sub>2</sub>·2H<sub>2</sub>O in acid media is described. Pyrrole and pyridine
rings are synchronously constructed in one pot for these tetracyclic
molecules. A plausible reduction/hydrolysis/decarboxylation/cyclization/aromatization
domino reaction mechanism is proposed. Bis-<i>N</i>-substituted
isoindigo only gives the corresponding reduction product, 3,3′-bioxindole
A Green Route to a Na<sub>2</sub>FePO<sub>4</sub>F‑Based Cathode for Sodium Ion Batteries of High Rate and Long Cycling Life
Sodium ion batteries
(SIBs) are considered one of the most promising
alternatives for large-scale energy storage due largely to the abundance
and low cost of sodium. However, the lack of high-performance cathode
materials at low cost represents a major obstacle toward broad commercialization
of SIB technology. In this work, we report a green route strategy
that allows cost-effective fabrication of carbon-coated Na<sub>2</sub>FePO<sub>4</sub>F cathode for SIBs. By using vitamin C as a green
organic carbon source and environmentally friendly water-based polyacrylic
latex as the binder, we have demonstrated that the Na<sub>2</sub>FePO<sub>4</sub>F phase in the as-derived Na<sub>2</sub>FePO<sub>4</sub>F/C
electrode shows a high reversible capacity of 117 mAh g<sup>–1</sup> at a cycling rate of 0.1 C. More attractively, excellent rate capability
is achieved while retaining outstanding cycling stability (∼85%
capacity retention after 1000 charge–discharge cycles at a
rate of 4 C). Further, in operando X-ray diffraction has been used
to probe the evolution of phase structures during the charge–discharge
process, confirming the structural robustness of the Na<sub>2</sub>FePO<sub>4</sub>F/C cathode (even when charged to 4.5 V). Accordingly,
the poor initial Coulombic efficiency of some anode materials may
be compensated by extracting more sodium ions from Na<sub>2</sub>FePO<sub>4</sub>F/C cathode at higher potentials (up to 4.5 V)
Promising Proton Conductor for Intermediate-Temperature Fuel Cells: Li<sub>13.9</sub>Sr<sub>0.1</sub>Zn(GeO<sub>4</sub>)<sub>4</sub>
Commercialization
of fuel cell technologies hinges on the development
of solid electrolytes of sufficient ionic conductivity at intermediate
temperatures (200–600 °C). Here we report a novel proton
conductor derived from Li<sub>13.9</sub>Sr<sub>0.1</sub>ZnÂ(GeO<sub>4</sub>)<sub>4</sub> (LSZG), demonstrating the highest protonic conductivity
(0.034 S cm<sup>–1</sup> at 600 °C) among all known proton
conducting ceramics, which is much higher than those of several well-known
oxygen ion conducting electrolytes (e.g., ∼0.009 and 0.018
S cm<sup>–1</sup>, respectively, for zirconia- and ceria-based
oxide electrolyte at 600 °C). Interestingly, after fully replacing
the mobile Li<sup>+</sup> ions by H<sup>+</sup> through proper ion
exchange, the H<sup>+</sup> conductivity increases from 0.034 to 0.048
S cm<sup>–1</sup> at 600 °C. A simple but effective ab
initio molecular dynamics simulation study suggests a unique H<sup>+</sup>/Li<sup>+</sup> transport mechanism: the proton in LSZG moves
freely in the Li<sup>+</sup> interstitial space within the 3D Li<sup>+</sup> transport network (i.e., 4<i>c</i> and 4<i>a</i> sites, as the occupancies of the Li1 and Li2 sites are
55% and 16%, respectively). In particular, a solid oxide fuel cell
(SOFC) based on an LSZG electrolyte (∼40 μm thick) demonstrates
high open circuit voltage (∼1.1 V) and good peak power density
(377 mW cm<sup>–2</sup>) at 600 °C. The cell performance
may be further improved if the electrode–electrolyte interface
can be optimized. The new transport mechanism and excellent proton
conductivity suggest that the LSZG represents an important family
of electrolyte materials, which may be used as a proton-conducting
membrane for intermediate-temperature SOFCs and hydrogen production
or separation
Sulfonated Holey Graphene Oxide (SHGO) Filled Sulfonated Poly(ether ether ketone) Membrane: The Role of Holes in the SHGO in Improving Its Performance as Proton Exchange Membrane for Direct Methanol Fuel Cells
Sulfonated
holey graphene oxides (SHGOs) have been synthesized by the etching
of sulfonated graphene oxides with concentrated HNO<sub>3</sub> under
the assistance of ultrasonication. These SHGOs could be used as fillers
for the sulfonated aromatic polyÂ(ether ether ketone) (SPEEK) membrane.
The obtained SHGO-incorporated SPEEK membrane has a uniform and dense
structure, exhibiting higher performance as proton exchange membranes
(PEMs), for instance, higher proton conductivity, lower activation
energy for proton conduction, and comparable methanol permeability,
as compared to Nafion 112. The sulfonated graphitic structure of the
SHGOs is believed to be one of the crucial factors resulting in the
higher performance of the SPEEK/SHGO membrane, since it could increase
the local density of the −SO<sub>3</sub>H groups in the membrane
and induce a strong interfacial interaction between SHGO and the SPEEK
matrix, which improve the proton conductivity and lower the swelling
ratio of the membrane, respectively. Additionally, the proton conductivity
of the membrane could be further enhanced by the presence of the holes
in the graphitic planes of the SHGOs, since it provides an additional
channel for transport of the protons. When used, direct methanol
fuel cell with the SPEEK/SHGO membrane is found to exhibit much higher
performance than that with Nafion 112, suggesting potential use of
the SPEEK/SHGO membrane as the PEMs
Synthesis of Pyrido-Fused Quinazolinone Derivatives via Copper-Catalyzed Domino Reaction
A simple and efficient synthesis
of 11<i>H</i>-pyridoÂ[2,1-<i>b</i>]Âquinazolin-11-ones
by CuÂ(OAc)<sub>2</sub>·H<sub>2</sub>O-catalyzed reaction of easily
available substituted isatins and
2-bromopyridine derivatives has been developed. The reaction involves
C–N/C–C bond cleavage and two C–N bond formations
in a one-pot operation. This methodology is complementary to previously
reported synthetic procedures, and two plausible reaction mechanisms
are discussed
Nickel–Cobalt Hydroxide Nanosheets Coated on NiCo<sub>2</sub>O<sub>4</sub> Nanowires Grown on Carbon Fiber Paper for High-Performance Pseudocapacitors
A series
of flexible nanocomposite electrodes were fabricated by
facile electro-deposition of cobalt and nickel double hydroxide (DH)
nanosheets on porous NiCo<sub>2</sub>O<sub>4</sub> nanowires grown
radially on carbon fiber paper (CFP) for high capacity, high energy,
and power density supercapacitors. Among different stoichiometries
of Co<sub><i>x</i></sub>Ni<sub>1–<i>x</i></sub>DH nanosheets studied, Co<sub>0.67</sub>Ni<sub>0.33</sub> DHs/NiCo<sub>2</sub>O<sub>4</sub>/CFP hybrid nanoarchitecture showed the best
cycling stability while maintaining high capacitance of ∼1.64
F/cm<sup>2</sup> at 2 mA/cm<sup>2</sup>. This hybrid composite electrode
also exhibited excellent rate capability; the areal capacitance decreased
less than 33% as the current density was increased from 2 to 90 mA/cm<sup>2</sup>, offering excellent specific energy density (∼33 Wh/kg)
and power density (∼41.25 kW/kg) at high cycling rates (up
to150 mA/cm<sup>2</sup>)
Hybrid Composite Ni(OH)<sub>2</sub>@NiCo<sub>2</sub>O<sub>4</sub> Grown on Carbon Fiber Paper for High-Performance Supercapacitors
We
have successfully fabricated and tested the electrochemical performance
of supercapacitor electrodes consisting of NiÂ(OH)<sub>2</sub> nanosheets
coated on NiCo<sub>2</sub>O<sub>4</sub> nanosheets grown on carbon
fiber paper (CFP) current collectors. When the NiCo<sub>2</sub>O<sub>4</sub> nanosheets are replaced by Co<sub>3</sub>O<sub>4</sub> nanosheets,
however, the energy and power density as well as the rate capability
of the electrodes are significantly reduced, most likely due to the
lower conductivity of Co<sub>3</sub>O<sub>4</sub> than that of NiCo<sub>2</sub>O<sub>4.</sub> The 3D hybrid composite NiÂ(OH)<sub>2</sub>/NiCo<sub>2</sub>O<sub>4</sub>/CFP electrodes demonstrate a high areal capacitance
of 5.2 F/cm<sup>2</sup> at a cycling current density of 2 mA/cm<sup>2</sup>, with a capacitance retention of 79% as the cycling current
density was increased from 2 to 50 mA/cm<sup>2</sup>. The remarkable
performance of these hybrid composite electrodes implies that supercapacitors
based on them have potential for many practical applications
Highly Efficient Layer-by-Layer-Assisted Infiltration for High-Performance and Cost-Effective Fabrication of Nanoelectrodes
We
present a novel cathode fabrication technique for improved performance
and production efficiency of SOFCs based on an infiltration method
assisted by layer-by-layer (LbL) assembly of polyelectrolytes. Preparation
of the electrode with LbL-assisted infiltration leads to a 6.5-fold
reduction in the electrode fabrication time while providing uniform
and small formation of Pr<sub>0.7</sub>Sr<sub>0.3</sub>CoO<sub>3‑δ</sub> (PSC) particles on the electrode. The increased surface area by
24.5% and number of active sites of the prepared electrode exhibits
superior electrochemical performance up to 36.1% while preserving
the electrical properties of the electrode. Because of its versatility
and tenability, the LbL-assisted infiltration process may become a
new route for fabrication of composite electrodes for other energy
storage and conversion devices
Unraveling the Nature of Anomalously Fast Energy Storage in T‑Nb<sub>2</sub>O<sub>5</sub>
While
T-Nb<sub>2</sub>O<sub>5</sub> has been frequently reported
to display an exceptionally fast rate of Li-ion storage (similar to
a capacitor), the detailed mechanism of the energy storage process
is yet to be unraveled. Here we report our findings in probing the
nature of the ultrafast Li-ion storage in T-Nb<sub>2</sub>O<sub>5</sub> using both experimental and computational approaches. Experimentally,
we used <i>in operando</i> Raman spectroscopy performed
on a well-designed model cell to systematically characterize the dynamic
evolution of vibrational band groups of T-Nb<sub>2</sub>O<sub>5</sub> upon insertion and extraction of Li ions during repeated cycling.
Theoretically, our model shows that Li ions are located at the loosely
packed 4g atomic layers and prefer to form bridging coordination with
the oxygens in the densely packed 4h atomic layers. The atomic arrangement
of T-Nb<sub>2</sub>O<sub>5</sub> determines the unique Li-ion diffusion
path topologies, which allow direct Li-ion transport between bridging
sites with very low steric hindrance. The proposed model was validated
by computational and experimental vibrational analyses. A comprehensive
comparison between T-Nb<sub>2</sub>O<sub>5</sub> and other important
intercalation-type Li-ion battery materials reveals the key structural
features that lead to the exceptionally fast kinetics of T-Nb<sub>2</sub>O<sub>5</sub> and the cruciality of atomic arrangements for
designing a new generation of Li-ion conduction and storage materials
Probing the Charge Storage Mechanism of a Pseudocapacitive MnO<sub>2</sub> Electrode Using <i>in Operando</i> Raman Spectroscopy
While manganese oxide (MnO<sub>2</sub>) has been extensively studied
as an electrode material for pseudocapacitors, a clear understanding
of its charge storage mechanism is still lacking. Here we report our
findings in probing the structural changes of a thin-film model MnO<sub>2</sub> electrode during cycling using <i>in operando</i> Raman spectroscopy. The spectral features (e.g., band position,
intensity, and width) are correlated quantitatively with the size
(Li<sup>+</sup>, Na<sup>+</sup>, and K<sup>+</sup>) of cations in
different electrolytes and with the degree of discharge to gain better
understanding of the cation-incorporation mechanism into the interlayers
of pseudocapacitive MnO<sub>2</sub>. Also, theoretical calculations
of phonon energy associated with the models of interlayer cation-incorporated
MnO<sub>2</sub> agree with the experimental observations of cation-size
effect on the positions of Raman bands. Furthermore, the cation-size
effects on spectral features at different potentials of MnO<sub>2</sub> electrode are correlated quantitatively with the amount of charge
stored in the MnO<sub>2</sub> electrode. The understanding of the
structural changes associated with charge storage gained from Raman
spectroscopy provides valuable insights into the cation-size effects
on the electrochemical performances of the MnO<sub>2</sub> electrode