Hongxia Xu

Nitrogen-doped graphene nanosheets with excellent lithium storage properties

In this work, nitrogen-doped graphene nanosheets serving as lithium storage materials are presented. The nitrogen-doped graphene nanosheets were prepared by heat treatment of graphite oxide under an ammonia atmosphere at 800 degrees C for 2 h. Scanning electron microscopy, transmission electron microscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy were employed to characterize the prepared product as nitrogen-doped graphene nanosheets with a doping level of ca. 2% nitrogen, where the N binding configuration of the graphene includes 57.4% pyridinic, 35.0% pyrrolic and 7.6% graphitic N atoms. Galvanostatic charge/discharge experiments revealed that these nitrogen-doped graphene nanosheets exhibited a high reversible capacity (900 mA h g(-1) at 42 mA g(-1) (1/20 C)), excellent rate performance (250 mA h g(-1) at a current density of 2.1 A g(-1) (2.5 C)), and significantly enhanced cycling stability, which demonstrated nitrogen-doped graphene nanosheets to be a promising candidate for anode materials in high rate lithium-ion batteries.

Synthesis of Nitrogen-Doped MnO/Graphene Nanosheets Hybrid Material for Lithium Ion Batteries

Nitrogen-doped MnO/graphene nanosheets (N-MnO/GNS) hybrid material was synthesized by a simple hydrothermal method followed by ammonia annealing. The samples were systematically investigated by X-ray diffraction analysis, Raman spectroscopy, X-ray photoelectron spectroscopy, transmission electron microscopy, and atomic force microscopy. N-doped MnO (N-MnO) nanoparticles were homogenously anchored on the thin layers of N-doped GNS (N-GNS) to form an efficient electronic/ionic mixed conducting network. This nanostructured hybrid exhibited a reversible electrochemical lithium storage capacity as high as 772 mAh g(-1) at 100 mA g(-1) after 90 cycles, and an excellent rate capability of 202 mA h g(-1) at a high current density of 5 A g(-1). It is expected that N-MnO/GNS hybrid could be a promising candidate material as a high capacity anode for lithium ion batteries.

One dimensional MnO2/titanium nitride nanotube coaxial arrays for high performance electrochemical capacitive energy storage

One dimensional MnO2/titanium nitride nanotube coaxial arrays have been designed for a high performance electrochemical capacitive energy storage system based on the concept of fabricating an efficient, fast charge separation network. This nanostructured composite material was prepared by electrodepositing mesoporous MnO2 into TiN nanotube arrays (TiN NTA), which are prepared by anodization of a Ti foil substrate and subsequent nitridation using ammonia annealing. The electrodeposited mesoporous MnO2 inside the electrically conductive framework of a TiN nanotube has been found to show high specific capacitance (681.0 F g−1 at 2 A g−1), excellent rate capability (55% capacitance retention from 2 to 2000 mV s−1), and a long cycle life (3% capacitance loss after 1000 cycles). These results demonstrate that this coaxial composite nanostructure is very promising for high performance supercapacitors.

Methylamine-Gas-Induced Defect-Healing Behavior of CH3NH3PbI3 Thin Films for Perovskite Solar Cells

We report herein the discovery of methylamine (CH3NH2) induced defect-healing (MIDH) of CH3NH3PbI3 perovskite thin films based on their ultrafast (seconds), reversible chemical reaction with CH3NH2 gas at room temperature. The key to this healing behavior is the formation and spreading of an intermediate CH3NH3PbI3⋅xCH3NH2 liquid phase during this unusual perovskite-gas interaction. We demonstrate the versatility and scalability of the MIDH process, and show dramatic enhancement in the performance of perovskite solar cells (PSCs) with MIDH. This study represents a new direction in the formation of defect-free films of hybrid perovskites.

Graphene oxide nanosheets/multi-walled carbon nanotubes hybrid as an excellent electrocatalytic material towards VO2+/VO2+ redox couples for vanadium redox flow batteries

A graphene oxide nanosheets/multi-walled carbon nanotubes (GO/MWCNTs) hybrid with excellent electrocatalytic redox reversibility towards VO2+/VO2+ redox couples for vanadium redox flow batteries (VRFB) has been prepared by an electrostatic spray technique after efficient ultrasonic treatment. The structures and electrochemical properties of GO/MWCNTs are investigated by transmission electron microscopy, scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, Raman spectroscopy and cyclic voltammetry. GO/MWCNTs are shown to be cross-linked and form an electrocatalytic hybrid with an effective mixed conducting network, leading to efficiently fast ion and electron transport characteristics. Compared with the pure GO nanosheets and MWCNTs, GO/MWCNTs deliver a much better electrocatalytic redox reversibility towards the positive VO2+/VO2+ couple, especially for the reduction from VO2+ to VO2+. The excellent experimental results demonstrate that the newly developed hybrid material holds great promise in the application of VRFB.

Mesoporous Coaxial Titanium Nitride-Vanadium Nitride Fibers of Core–shell Structures for High-Performance Supercapacitors

In this study, titanium nitride-vanadium nitride fibers of core-shell structures were prepared by the coaxial electrospinning, and subsequently annealed in the ammonia for supercapacitor applications. These core-shell (TiN-VN) fibers incorporated mesoporous structure into high electronic conducting transition nitride hybrids, which combined higher specific capacitance of VN and better rate capability of TiN. These hybrids exhibited higher specific capacitance (2 mV s(-1), 247.5 F g(-1)) and better rate capability (50 mV s(-1), 160.8 F g(-1)), which promise a good candidate for high-performance supercapacitors. It was also revealed by electrochemical impedance spectroscopy (EIS) and X-ray photoelectron spectroscopy (XPS) characterization that the minor capacitance fade originated from the surface oxidation of VN and TiN.

Facile Preparation of Mesoporous Titanium Nitride Microspheres for Electrochemical Energy Storage

In this study, mesoporous TiN spheres with tunable diameter have been fabricated via a facile template-free strategy. Under ammonia atmosphere, mesoporous TiO₂ spheres are directly converted into mesoporous TiN spheres with the addition of cyanamide to retain the original morphology. The electrochemical performance of the resultant mesoporous TiN spheres demonstrates that this material can be a promising electrode material for nonaqueous supercapacitors with high energy densities.

Molybdenum nitride based hybrid cathode for rechargeable lithium-O2 batteries

Molybdenum nitride/nitrogen-doped graphene nanosheets (MoN/NGS) are synthesized and used as an alternative O(2) electrode for Li-O(2) batteries. In comparison with electrocatalysts proposed previously, this hybrid cathode exhibits a high discharge potential (around 3.1 V) and a considerable specific capacity (1490 mA h g(-1), based on carbon + electrocatalyst).

Nanostructured Titanium Nitride/PEDOT:PSS Composite Films As Counter Electrodes of Dye-Sensitized Solar Cells

The composite films of titanium nitride in conjunction with polystyrenesulfonate-doped poly (3,4-ethylene-dioxythiophene) (PEDOT:PSS) were prepared by a simple mechanical mixture of TiN and PEDOT:PSS under ultrasonication, which was demonstrated to deliver an effectively combined network of both high electrical conductivity and superior electrocatalytic activity. The composite films have been explored as an alternative for the counter electrodes of dye-sensitized solar cells. It was manifested that these nanostructured TiN-PEDOT:PSS composite films displayed excellent performance comparable to Pt-FTO counter electrode due to the combined network endowing more favorable and efficient interfacial active sites. Among them, the energy conversion efficiency of the cell with TiN(P)-PEDOT:PSS as counter electrode reached 7.06%, which was superior to 6.57% of the cell with Pt-FTO counter electrode under the same experimental conditions.

NiCo2S4 sub-micron spheres: an efficient non-precious metal bifunctional electrocatalyst

Urchin-like NiCo2S4 sub-micron spheres integrated with nano-sized and micro-sized structures, which were synthesized via a facile one-pot method, deliver efficient electrocatalytic activities for oxygen reduction and evolution reactions. The excellent electrocatalytic property of NiCo2S4 sub-micron spheres is originated from their unique urchin-like microstructure, composition and d-electronic configurations of the transition metal ions.