Dongfei Sun

Magnetic and electrochemical properties of CuFe2O4 hollow fibers fabricated by simple electrospinning and direct annealing

Copper ferrite (CuFe2O4) hollow fibers were fabricated by direct annealing of electrospun precursor fibers with appropriate heating rate. The crystal structure, morphology, magnetic properties and electrochemical properties of as-made CuFe2O4 hollow fibers were investigated by using X-ray diffraction, Fourier-transformed infrared spectra, scanning electron microscopy, transmission electron microscopy, vibrating sample magnetometer, and electrochemical workstation. The results show that the appropriate heating rate of 0.5 °C min−1 can result in the formation of hollow tetragonal structural CuFe2O4 fibers. Hollow fibers after annealing at high temperatures still retain the one-dimensional texture and the walls of hollow fibers consist of many nanoparticles. Magnetization results indicate that the CuFe2O4 hollow fibers have a ferromagnetic behavior and their specific saturation magnetization value increases with an increase in the annealing temperature. Moreover, the electrochemical results suggest that the capacitance characteristic of the CuFe2O4 hollow fibers is a typical pseudocapacitive capacitance. The value of the specific capacitance gradually decreases with the increase in the discharge current density.

Carbon nanofiber bridged two-dimensional titanium carbide as a superior anode for lithium-ion batteries

MXenes, a novel family of two-dimensional metal carbides, are receiving intense attention for lithium-ion batteries (LIBs) and supercapacitors because they have high volumetric capacitance exceeding all carbon materials. However, serious interlayer stacking exists in MXene particles, which greatly decreases the electrical conductivity in the bulk and hinders the accessibility of interlayers to electrolyte ions. Thus, multi-stacked MXene particles exhibit low capacitance and poor rate capability. Herein, we report an effective strategy to directly improve the electrochemical performance of multi-stacked MXene (Ti3C2Tx) particles as LIB anode materials. It was successfully realized by growing conductive “carbon nanofiber (CNF) bridges” within the gaps of each Ti3C2Tx particle as well as the outside. With the help of these CNFs, the as-prepared Ti3C2/CNF particles exhibited significantly improved reversible capacity compared with pure Ti3C2Tx particles. More remarkably, even at an ultrahigh rate of 100 C, the capacity of Ti3C2/CNF hybrid particles was just slightly lower than that of pure Ti3C2Tx particles at 1 C, and there was no capacity decay after 2900 cycles at 100 C, demonstrating excellent rate capability and superior long-term stability at the ultrahigh rate.

Synthesis of Porous δ-MnO2 Submicron Tubes as Highly Efficient Electrocatalyst for Rechargeable Li–O2 Batteries

Lithium-oxygen (Li-O2 ) batteries are receiving intense interest because of their high energy density. A new tubular δ-MnO2 material prepared by a simple hydrothermal synthesis is an efficient electrocatalyst for Li-O2 batteries. The synthesized δ-MnO2 exhibits a unique tubular structure, in which the porous walls are composed of highly dispersed ultrathin δ-MnO2 nanosheets. Such a unique structure and its intrinsic catalytic activity provide the right electrocatalyst characteristics for high-performance Li-O2 batteries. As a consequence, suppressed overpotentials-especially the oxygen evolution reaction overpotential-superior rate capability, and desirable cycle life are achieved with these submicron δ-MnO2 tubes as the electrocatalyst. Remarkably, the discharge product Li2 O2 of the Li-O2 battery exhibits a uniform nanosheet-like morphology, which indicates the critical role of the δ-MnO2 in the electrochemical process, and a mechanism is proposed to analyze the catalysis of δ-MnO2 .

Morphology and crystallinity-controlled synthesis of MnO2 hierarchical nanostructures and their application in lithium ion batteries

We demonstrate a novel strategy for the well-controlled preparation of MnO2 hierarchical nanostructures, i.e. MnO2 submicron fibers composed of nanosheets or nanorods with different crystalline phases (δ- and α-MnO2), by integrating electrospinning, hydrothermal synthesis and subsequent heat-treatment. Specifically, δ-MnO2 submicron fibers with different surface morphologies are synthesized via a template-assisted hydrothermal process and by using electrospun nanofibers (Polyacrylonitrile (PAN), oxidized PAN and carbonized PAN nanofibers); after that, α-MnO2 nanostructures composed of different nanorod-like nanostructures are prepared by the heat-treatment of the corresponding δ-MnO2 nanostructures at 700 °C in air. The effects of the reaction parameters (i.e. different templates and growth conditions) on the morphology and crystal phase are investigated in detail. The results demonstrate that the microstructures and structural conversion of the MnO2 nanostructures are closely correlated with the initial templates. Furthermore, the potential application of the α-MnO2 hierarchical nanostructures as the anode for a lithium ion battery is studied, and the results show that the pine-like α-MnO2 exhibits a stable capacity up to 380 mA h g−1 after 150 cycles.

Enhanced field emission properties from aligned graphenes fabricated on micro-hole patterned stainless steel

The graphene emitters on micro-hole patterned stainless steel (SUS304) were prepared using electrophoresis method. The field emission property of three-dimensional graphene emitters was enhanced remarkably compared to that of graphene on flat substrates. The turn-on and threshold fields of the patterned emitter were, respectively, 4.8 and 5.6 V μm−1 lower than those of graphene on flat SUS304 (turn on field is 5.6 V μm−1). The micro-hole patterned cathode provides 10 times higher current density due to vertical aligned sharp edges of graphene in micro holes, and this design may open a potential way to layered-nanomertial-based cold cathodes.

Comparison between metal ion and polyelectrolyte functionalization for electrophoretic deposition of graphene nanosheet films

Graphene nanosheets (GNSs) were modified either by metal magnesium (Mg) ions or a polyelectrolyte, polydiallyldimethylammonium chloride (PDDA), to produce positive charges on them. The two materials were used to produce GNS films with different surface morphologies. They were deposited on single-crystal silicon substrates using simple direct current electrophoretic deposition method. The Mg ion-modified GNS film has a relatively rough surface with some GNSs almost perpendicular to the surface, whereas the PDDA-modified GNS film has a relatively smooth surface with most GNSs parallel to the substrate. This difference is attributed to the different interactions of Mg ions and PDDA molecules with GNSs. Because of the favorable surface morphology, the Mg ion-modified GNS film displays better field emission compared with the PDDA-modified GNS film. Biocompatibility tests indicate that the rougher Mg ion-modified GNS film is more beneficial to cell adhesion and growth.

Fabrication of Zn2TiO4 and TiN nanofibers by pyrolysis of electrospun precursor fibers

In this paper, Zn2TiO4 and TiN nanofibers were prepared by a facile combination of electrospinning followed by thermolysis under different atmospheres. An N,N-dimethyl formamide (DMF)–ethanol solution of poly(vinylpyrrolidone) (PVP), Ti(IV)–isopropoxide (Ti(OC4H9)4) and zinc acetate (ZnAc) was used as the electrospinning precursor and the electrospun nanofibers were calcined at 600 °C in air to generate Zn2TiO4 nanofibers. Subsequently, a conversion from Zn2TiO4 nanofibers to TiN nanofibers was realized through a heat treatment at 1000 °C under an ammonia atmosphere. Investigations using field-emission scanning electron and transmission electron microscopes (FESEM and TEM) revealed the nano-scale fibrous morphology of the obtained products. Energy-dispersive spectroscopy (EDS) and X-ray diffraction (XRD) studies confirmed the chemical composition and crystalline structure of the Zn2TiO4 and TiN nanofibers, respectively. The electrochemical properties of the as-prepared TiN nanofibers indicated that the capacitance characteristic of the TiN phase was the electric double-layer capacitance. The value of the specific capacitance gradually decreased with the increase of discharge current density.