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>-indolo­[2,3-<i>b</i>]­quinolines from mono-<i>N</i>-substituted isoindigo derivatives in the presence of SnCl₂·2H₂O 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 Na₂FePO₄F‑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 Na₂FePO₄F 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 Na₂FePO₄F phase in the as-derived Na₂FePO₄F/C electrode shows a high reversible capacity of 117 mAh g^–1 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 Na₂FePO₄F/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 Na₂FePO₄F/C cathode at higher potentials (up to 4.5 V)

Promising Proton Conductor for Intermediate-Temperature Fuel Cells: Li_13.9Sr_0.1Zn(GeO₄)₄

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_13.9Sr_0.1Zn­(GeO₄)₄ (LSZG), demonstrating the highest protonic conductivity (0.034 S cm^–1 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^–1, respectively, for zirconia- and ceria-based oxide electrolyte at 600 °C). Interestingly, after fully replacing the mobile Li⁺ ions by H⁺ through proper ion exchange, the H⁺ conductivity increases from 0.034 to 0.048 S cm^–1 at 600 °C. A simple but effective ab initio molecular dynamics simulation study suggests a unique H⁺/Li⁺ transport mechanism: the proton in LSZG moves freely in the Li⁺ interstitial space within the 3D Li⁺ 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^–2) 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 HNO₃ under the assistance of ultrasonication. These SHGOs could be used as fillers for the sulfonated aromatic poly­(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 −SO₃H 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>-pyrido­[2,1-<i>b</i>]­quinazolin-11-ones by Cu­(OAc)₂·H₂O-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 NiCo₂O₄ 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 NiCo₂O₄ nanowires grown radially on carbon fiber paper (CFP) for high capacity, high energy, and power density supercapacitors. Among different stoichiometries of Co_<i>x</i>Ni_1–<i>x</i>DH nanosheets studied, Co_0.67Ni_0.33 DHs/NiCo₂O₄/CFP hybrid nanoarchitecture showed the best cycling stability while maintaining high capacitance of ∼1.64 F/cm² at 2 mA/cm². 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/cm², offering excellent specific energy density (∼33 Wh/kg) and power density (∼41.25 kW/kg) at high cycling rates (up to150 mA/cm²)

Hybrid Composite Ni(OH)₂@NiCo₂O₄ Grown on Carbon Fiber Paper for High-Performance Supercapacitors

We have successfully fabricated and tested the electrochemical performance of supercapacitor electrodes consisting of Ni­(OH)₂ nanosheets coated on NiCo₂O₄ nanosheets grown on carbon fiber paper (CFP) current collectors. When the NiCo₂O₄ nanosheets are replaced by Co₃O₄ 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 Co₃O₄ than that of NiCo₂O_4. The 3D hybrid composite Ni­(OH)₂/NiCo₂O₄/CFP electrodes demonstrate a high areal capacitance of 5.2 F/cm² at a cycling current density of 2 mA/cm², with a capacitance retention of 79% as the cycling current density was increased from 2 to 50 mA/cm². 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_0.7Sr_0.3CoO_3‑δ (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‑Nb₂O₅

While T-Nb₂O₅ 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-Nb₂O₅ 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-Nb₂O₅ 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-Nb₂O₅ 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-Nb₂O₅ and other important intercalation-type Li-ion battery materials reveals the key structural features that lead to the exceptionally fast kinetics of T-Nb₂O₅ 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 MnO₂ Electrode Using <i>in Operando</i> Raman Spectroscopy

While manganese oxide (MnO₂) 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 MnO₂ 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⁺, Na⁺, and K⁺) 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 MnO₂. Also, theoretical calculations of phonon energy associated with the models of interlayer cation-incorporated MnO₂ 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 MnO₂ electrode are correlated quantitatively with the amount of charge stored in the MnO₂ 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 MnO₂ electrode