As the lipophilic surfactant tetraoctylammonium bromide (TOAB) was cast from its organic solutions, needle-shape crystals formed. However, with the addition of an amphiphilic noncrystalline diblock copolymer [(poly(acrylic acid-b-styrene)] into the TOAB solutions, spherulites formed in the solid films cast from the solutions. Under preferable conditions, millimeter-sized spherulites could be obtained. It has been found that some factors such as the type of solvent, the film-forming temperature, and the ratio of polymer to surfactant can affect the spherulite formation. Small angle X-ray scattering and wide-angle X-ray diffraction investigation suggests that the formation of the spherulites and the pure TOAB crystals are organized by closely packed lamellar structure, while the addition of diblock copolymer decreased the degree of order of TOAB crystals. Dynamic light scattering study reveals that, in organic solvents such as tetrahydrofuran, TOAB formed molecular-disperse solution, while the amphiphilic copolymer chains formed micelles with or without the presence of TOAB molecules in the solution. We suppose that the morphology change of TOAB crystal is induced by the diblock polymer chains: During the solvent-evaporating film formation, the hydrophobic PS blocks of the amphiphilic copolymer resided between some of the TOAB lamellar crystallites, which might cause the splaying and branching of the surfactant crystallites during the crystal growth and eventually lead to spherulite formation. This result could provide a useful way for spherulite formation, and open interesting opportunities for adjusting the crystal morphology and/or properties of lipophilic surfactant.
Results on X-ray near edge structure (XANES) study on Sn-doped BiFeO3 (BFO) nanofibers with varying Sn concentrations of 1%, 3%, and 5% are reported. The results indicate that the oxidation state of Sn ions in the BFO structure is +4. In addition, we observe a bismuth peak (Bi M 1) at 4000 eV in the XANES spectrum, suggesting the diffusion of Bi ions onto the surface of BFO nanostructure. The diffusion is attributed to the charge compensation between donor electrons from the Sn atoms and Bi vacancies. These findings are of high relevance to surface chemistry reactions in sensing and catalytic applications.
Supercapacitor has been demonstrated as one of the most attractive electrochemical energy storage devices for practical applications, especially for electric transportation systems, because of high power density, long cycle life, impressive conversion efficiency and desirable usage safety. However, energy densities of supercapacitors are typically low comparing with lithium-ion batteries, which impedes their broad applications. The development of advanced supercapacitors mainly concerns with increasing their energy densities. Being different from conventional double layer supercapacitors, pseudocapacitive materials can deliver much higher capacitances and energy densities. For example, some metal oxides can be used as electrode materials for stable charge and discharge at high rates on the basis on of reversible surface redox reactions, resulting in desired energy densities comparable to that of lithium-ion batteries. In this work, we study an amorphous α-Nb2O5, which is prepared using a facile hydrothermal method followed by a low temperature post-annealing process. The α-Nb2O5 is evaluated as a pseudocapacitive electrode material in the LiPF6-based organic electrolyte for lithium storage performance. The α-Nb2O5 demonstrates reversible Li+ intercalation/deintercalation behavior in a voltage range of 0.1–2.5 V (vs. Li/Li+), resulting in an impressive reversible lithium storage capacity higher than 250 mAh/g at 0.2 C (1 C = 400 mA/g), together with excellent cycling stability and rate capability up to 10 C for 10000 cycles. In addition, the cyclic voltammetry (CV) profiles obtained at a low scanning rate of 0.1 mV/s show unexpected square patterns in multiple cycles even at low potentials between 1.0 and 0.1 V (vs. Li/Li+), which is distinctly different from that of crystalline orthorhombic Nb2O5 electrode material. The CV analysis also indicates apparent intercalation capacity contribution of such α-Nb2O5 electrode material during lithium ion storage. All electrochemical experiments verify typical pseudocapacitive behavior for the amorphous α-Nb2O5 material in a wide voltage range of 0.1–2.5 V (vs. Li/Li+). The special pseudocapacitive property at low potentials enables the α-Nb2O5 to be used as an attractive anode in a hybrid supercapacitor coupled with the active carbon (AC) material as cathode with a wide voltage window. As a result, the assembled α-Nb2O5//AC hybrid supercapacitor full cell can be charged to a high voltage up to 4.5 V. According to the calculated energy density of the supercapacitor based on E =1/2 CV 2, the high working voltage results in an impressive energy density of 178.5 Wh/kg for this hybrid supercapacitor cell on the basis of the mass of two active electrode materials. In addition, the full cell also shows outstanding high-rate performance, and our ongoing work will optimize associated structure to improve its cycling stability at harsh work conditions. This work offers a promising Nb2O5 material that is amorphous and showing pseudocapacitive behaviors for lithium ion storage at low potentials. Improved capacity and energy density have been achieved, together with desired cycling stability for prolonged cycles. Such an electrode material can be served as a feasible anode material for high-voltage and high-energy hybrid supercapacitor full cell when it is coupled with high-rate cathode materials.
Low-density ordered mesoporous carbon−silica nanocomposites with different Fe contents have been prepared by a facile solvent-evaporation-induced self-assembly approach. Magnetic metal nanocrystallines are highly dispersed in the composites due to in situ carbothermal reduction. The optimal reflection loss calculated from the measured permittivity and permeability is −34.4 dB at 13.1 GHz. Moreover, the electromagnetic wave absorption less than −10 dB is found to exceed 5.0 GHz for an absorber thickness of 2 mm. The microwave enhancement absorption of the mesoporous C−SiO2−Fe nanocomposites is contributed to the better match between dielectric loss and magnetic loss, which originates from the high absorption by the incorporation of magnetic species as well as the multiple reflections by the ordered mesoporous structure. The mesoporous C−SiO2−Fe nanocomposites also exhibit a lower infrared emissivity in the wavelength from 8 to 14 μm than that of Fe-free powder.
Blends of high density polyethylene (HDPE) and ultra high molecular weight polyethylene (UHMWPE) were prepared by two-step processing way. Middle molecular weight polyethylene (MMWPE) as the lubricant was added into UHMWPE in the first processing step. Cocrystallization behavior of the blends was examined by DSC, XRD and light microscope (LM). A single endotherm peak in DSC curves revealed that cocrystallization probably formed between HDPE and UHMWPE. Moreover reducing heating rate didn't make the single endotherm separate. The enhancing of diffractive strength on 110 and 200 crystal face manifested that the UHMWPE molecular chains taken part in the crystallization of the blends. Reducing of the diameter of the undissolved UHMWPE particles in LM manified that the dispersion of UHMWPE in the blends obviously improved. Therefore two-step processing way could improve the dissolved degree and contribute to form the cocrystallization.
The spinel Li4Mn5O12 has been considered as a prospective 3 V cathode material for the next generation of lithium-ion batteries (LIBs) due to its high energy density and excellent cycling stability. However, the low operating voltage (∼3 V) makes Li4Mn5O12 impractical for high-energy high-power LIBs. To address this issue, Ni and Fe dual doped Li4Mn5-x-yNixFeyO12 has been prepared via a facile sol-gel method combined with post-heat-treatment. The effects of dual-cations doping on the crystal structure, morphology and electrochemical properties were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS) and galvanostatic charge/discharge analysis. As a result, Li4Mn4Ni0.5Fe0.5O12 exhibits the highest reversible specific capacity of 133 mAh/g at a specific current density of 25 mA/g after 100 cycles and exhibits a significantly improved high voltage performance with corresponding capacity of ∼80 mAh/g at an average voltage of 4.7 V vs. Li/Li+ and ∼122 mAh/g at above 4.0 V. These results indicate the dual doping of Ni and Fe can effectively improve both the operating voltage and reversible specific capacity of Li4Mn5O12 with excellent cycling stability, demonstrating a promising high-voltage cathode material for high-energy high-power LIBs.
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