Xiangqian Shen

Magnetic carbon nanofibers containing uniformly dispersed Fe/Co/Ni nanoparticles as stable and high-performance electromagnetic wave absorbers

Carbon nanofibers with ferromagnetic metal nanoparticles (CNF–M, M = Fe, Co, and Ni) have been synthesized by carbonizing electrospun polyacrylonitrile nanofibers including metal acetylacetonate in an argon atmosphere, and their phase composition, microstructure, magnetic properties and electromagnetic (EM)-wave absorbability have been studied. The microstructure analysis shows that the in situ formed metal nanoparticles are well distributed along carbon-based nanofibers and encapsulated by ordered graphite layers. The investigation of magnetic properties and EM-wave absorbability reveals that the as-synthesized CNF–M has typical characteristics of ferromagnetic materials and exhibits excellent EM-wave absorption properties (reflection loss exceeding −20 dB) from the C-band to the Ku-band (4–18 GHz) over an absorber thickness of 1.1–5.0 mm due to the efficient complementarities of complex permeability and permittivity resulting from the magnetic metal nanoparticles and lightweight carbon, as well as the particular particle/graphite core/shell microstructures in CNF–M. Moreover, a minimum reflection loss value of −67.5, −63.1, and −61.0 dB is achieved at 16.6, 12.9, and 13.1 GHz with a matching thickness of 1.3, 1.6, and 1.7 mm for CNF–Fe, CNF–Co, and CNF–Ni, respectively. These magnetic carbon nanofibers are attractive candidates for the new type of high performance EM-wave absorbing materials.

Electrospinning preparation, characterization and magnetic properties of cobalt–nickel ferrite (Co1−xNixFe2O4) nanofibers

Uniform Co(1-)(x)Ni(x)Fe(2)O(4) (x=0.0, 0.2, 0.4, 0.6, 0.8 and 1.0) nanofibers with average diameter of 110 nm and length up to several millimeters were prepared by calcination of electrospun precursor nanofibers containing polymer and inorganic salts. The as-spun and calcined nanofibers were characterized in detail by TG-DTA, XRD, FE-SEM, TEM, SAED and VSM, respectively. The effect of composition of the nanofibers on the structure and magnetic properties were investigated. The nanofibers are formed through assembling magnetic nanoparticles with poly(vinyl pyrrolidone) as the structure-directing template. The structural characteristics and magnetic properties of the resultant nanofibers vary with chemical composition and can be tuned by adjusting the Co/Ni ratio. Both lattice parameter and particle size decrease gradually with increasing nickel concentration. The saturation magnetization and coercivity lie in the range 29.3-56.4 emu/g and 210-1255 Oe, respectively, and both show a monotonously decreasing behavior with the increase in nickel concentration. Such changes in magnetic properties can mainly be attributed to the lower magnetocrystalline anisotropy and the smaller magnetic moment of Ni(2+) ions compared to Co(2+) ions. Furthermore, the coercivity of Co-Ni ferrite nanofibers is found to be superior to that of the corresponding nanoparticle counterparts, presumably due to their large shape anisotropy. These novel one-dimensional Co-Ni ferrite magnetic nanofibers can potentially be used in micro-/nanoelectronic devices, microwave absorbers and sensing devices.

Magnetic hard/soft nanocomposite ferrite aligned hollow microfibers and remanence enhancement.

The nanocomposite SrFe(12)O(19)/Ni(0.5)Zn(0.5)Fe(2)O(4) ferrite aligned hollow microfibers with the hollow diameter to the fiber diameter estimated about 3/5 have been prepared by the gel precursor transformation process. The nanocomposite binary ferrites with different mass ratios are formed after the precursor calcined at 900°C for 2h, fabricating from SrFe(12)O(19) nanoparticles and Ni(0.5)Zn(0.5)Fe(2)O(4) nanoparticles with a uniform phase distribution. These nanocomposite ferrite microfibers show a combination of magnetic characteristics for the hard (SrFe(12)O(19)) and soft (Ni(0.5)Zn(0.5)Fe(2)O(4)) phase with an enhanced remanence owing to the exchange-coupling interactions. The aligned microfibers exhibit a shape anisotropy.

Ultrahigh energy storage and ultrafast ion diffusion in borophene-based anodes for rechargeable metal ion batteries

Since the turn of the new century, the increasing demand for high-performance energy storage systems has generated considerable interest in rechargeable ion batteries (IBs). However, current IB technologies are not entirely satisfactory, especially the electrodes. We report here, via density functional theory calculations and first principles molecular dynamics simulations, that a borophene anode material has the fascinating properties of ultrahigh energy storage and ultrafast ion diffusion in metal (Li, Na, K, Mg, Al) IBs. Particularly for Li IBs with a borophene anode, a specific density of 3306 mA h g−1 and a high charging voltage of 1.46 V can be maintained at room temperature. Furthermore, non-ideal borophene anodes, including those with defects or oxidation and nanoribbon samples, still possess good properties for practical applications. This theoretical exploration will provide helpful guidance in searching for available or novel boron nanosheets as promising anode materials to advance commercial IB technology.

Separator modified by Ketjen black for enhanced electrochemical performance of lithium–sulfur batteries

A routine separator modified by a Ketjen black (KB) layer on the cathode side has been investigated to improve the electrochemical performances of Li–S batteries. The KB modified separator was prepared by a facile slurry coating method which offers a low-cost approach to solve the difficulties of Li–S batteries. Li–S cells assembled with this KB coated separator present excellent electrochemical performances in comparison with that of cells with a routine separator. The initial discharge capacity reaches 1318 mA h g−1 at 0.1C, and the reversible discharge capacity is maintained at 815 mA h g−1 after 100 cycles at 1C implying high capacity retention. Meanwhile, it achieves a discharge capacity of 934 mA h g−1 even at 2C demonstrating an excellent rate performance. Furthermore, electrochemical impedance spectroscopy (EIS) shows that the KB separator sample displays a lower charge transfer resistance which is beneficial for the electrochemical kinetics. The improved performance is supposed to be attributed to the porous architecture of the Ketjen black (KB) layer on the routine separator, which served as a physical barrier to block dissolved lithium polysulfides and an upper current collector to facilitate the transition of ions and electrons.

Evaluation of Ba-deficient PrBa1−xFe2O5+δ oxides as cathode materials for intermediate-temperature solid oxide fuel cells

Cobalt-free double perovskite oxides are promising cathode materials in intermediate-temperature solid oxide fuel cells, and often suffer from low activity in oxygen reduction reactions. Here, we report on Ba deficiency as an effective strategy to enhance the electrochemical performance of cobalt-free double perovskite PrBaFe2O5+δ, which is related to the formation and redistribution of oxygen vacancies in double perovskite. The effect of Ba deficiency on crystal structure, surface properties, oxygen content, electrical conductivity, thermal expansion, chemical compatibility with gadolinium-doped ceria (Gd0.1Ce0.9O1.95) electrolyte, microstructure, and electrochemical performance of PrBa1−xFe2O5+δ (x = 0.00–0.03) was evaluated systematically. Our preliminary results suggest that Ba deficiency is a feasible means to tailor the physico- and electrochemical properties of cobalt-free double perovskite, and that PrBa0.97Fe2O5+δ is a potential cathode material for intermediate-temperature solid oxide fuel cells.

N-substituted defective graphene sheets: promising electrode materials for Na-ion batteries

Using density functional theory calculations, we have investigated the adsorption of Na on pristine and N-substituted defective graphene sheets (graphitic, pyridinic, and pyrrolic structures) and explored their application in Na-ion batteries. The adsorption energy and the charge transfer of Na on the various types of sheet were calculated. The effects of N-substitution were also studied by electronic structure analysis, including the total electronic density of states, partial electron density of states, and charge density differences. The results show that electron-rich structures have a negative influence on Na binding, while electron-deficient structures are beneficial. The Na storage capacities of different sheets were evaluated by optimizing multiple Na atom adsorbed structures. We found that more Na atoms can be stored on electron-deficient sheets, making them promising for practical application as electrode materials in Na-ion batteries.

Effects of reduced graphene on crystallization behavior, thermal conductivity and tribological properties of poly(vinylidene fluoride)

The reduced graphene/poly(vinylidene fluoride) nanocomposite films were prepared by the solution casting-thermal reduction process using graphene oxide and poly(vinylidene fluoride) resin. The results show that with the presence of reduced graphene nano sheets in the nanocomposite, the structure of poly(vinylidene fluoride) tends to transform from α- to β-phase and the β-phase fraction and its crystallinity are largely affected by the reduced graphene content. The thermal conductivity of poly(vinylidene fluoride) can be effectively improved by the reduced graphene nano sheets introduction and when the reduced graphene content increased to about 10.0 wt%, it is about two times higher than the pure poly(vinylidene fluoride), due to the excellent thermal conductivity of reduced graphene and formation of thermal conductive networks. With comparison of the pure poly(vinylidene fluoride), the reduced graphene/poly(vinylidene fluoride) nanocomposite films show a better tribological property at a low reduced graphene content (below 4.0 wt%), mainly owing to the lubricant and heat spreading effects of reduced graphene nano sheets. The friction coefficient and wear rate for 0.75 wt% reduced graphene/poly(vinylidene fluoride) nanocomposite film compared to the pure poly(vinylidene fluoride) are improved by 41.2% and 49.4%, respectively.

Fabrication and Characterization of Heterostructural CoFe2O4/ Pb(Zr0.52Ti0.48)O3 Nanofibers by Electrospinning

Heterostructural CoFe2O4/Pb(Zr0.52Ti 0.48)O3 composite nanofibers with diameters about 100 nm were prepared by electrospinning. The thermal decomposition process, structure and morphology of the precursor composite fibers and the calcined CoFe 2O4/Pb(Zr0.52Ti0.48)O3 nanofibers were investigated by thermogravimetric and differential scanning calorimetry (TG-DSC), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and high-resolution TEM (HRTEM). It is found that just the spinel CoFe2O4 (CFO) and perovskite Pb(Zr0.52Ti 0.48)O3 (PZT) phases coexist in the composite CFO/PZT nanofibers obtained at calcination temperature of 950°C. The grain sizes of CFO and PZT increase with the calcination temperature whilst the grain growth process would be limited due to the separation effects for these two phases. When the grain sizes of CFO and PZT in the nanofiber reach about the size range of the nanofiber diameter, these grains are alternatively distributed along the nanofiber length direction and the well-defined heterostructure is formed between these nanograins of CFO and PZT.

Three-dimensional Li3V2(PO4)3/C nanowire and nanofiber hybrid membrane as a self-standing, binder-free cathode for lithium ion batteries

A three-dimensional (3D) mace-like Li3V2(PO4)3/C nanowire and nanofiber hybrid membrane was fabricated by using Ni nanoparticles as a catalyst and a modified electrospinning method followed by a hot-pressing heat treatment. The results indicate that the prepared membrane is composed of Li3V2(PO4)3 nanowires and Li3V2(PO4)3/C composite fibers. This hybrid membrane has a high specific surface area of 227.56 m2 g−1 with a combination of micropores and mesopores, which can be used as self-standing, binder-free cathodes for lithium ion batteries. This hybrid membrane electrode exhibits good rate performance and cyclic stability in the voltage range of 3.0–4.8 V with an initial discharge capacity of 115.3 mA h g−1 at 5C and 108.6 mA h g−1 at 10C and a capacity retention of 81.4% and 75.1% respectively after 500 cycles, 78.8% and 70.3% after 1000 cycles. The coulombic efficiencies both at 5C and 10C maintain about 100% during cycling. This excellent electrochemical performance may be attributed to the unique 3D long-range conductive networks and mace-like fiber structure, which favorably improve the reaction kinetics of Li3V2(PO4)3.