Kejun Zhang

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.

Mesoporous NiCo2O4 nanoflakes as electrocatalysts for rechargeable Li–O2 batteries

Herein, we report the facile synthesis of mesoporous NiCo(2)O(4) nanoflakes and their application in nonaqueous Li-O(2) batteries as cathode catalysts. The assembled Li-O(2) batteries presented lower overpotentials and enhanced cyclability, which should be attributed to the superior electrocatalytic activity and the mesoporous nanostructure of NiCo(2)O(4).

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).

Molybdenum nitride/n-doped carbon nanospheres for lithium-o2 battery cathode electrocatalyst.

Molybdenum nitride/N-doped carbon nanospheres (MoN/N-C) are synthesized by hydrothermal method followed by ammonia annealing. The as-prepared MoN/N-C nanospheres manifest considerable electrocatalytic activity toward oxygen reduction reaction in nonaqueous electrolytes because of its nanostructure and the synergetic effect between MoN and N-C. Furthermore, the MoN/N-C nanospheres are explored as cathode catalyst for Li-O2 batteries with tetra-(ethylene glycol) dimethyl ether as the electrolyte. The assembled batteries deliver alleviated overpotentials and improved battery lifespan, and their excellent performances should be attributed to the unique hierarchical structure and high fraction of surface active sites of cathode catalyst.

A hybrid material of vanadium nitride and nitrogen-doped graphene for lithium storage

Vanadium nitride and nitrogen-doped graphene nanosheet (G) hybrid materials were prepared by a facile sol–gel method combined with a thermal treatment at 800 °C under ammonia atmosphere. It was found that VN nanoparticles adhered to the surface of nitrogen-doped graphene nanosheets and/or were embedded in the graphene layers of the hybrid material (VN-G). This nanostructured material promises an efficient electronic and ionic conducting network, which exhibits dramatically increased specific capacities after rate capability test in comparison to the original value under the same current density. The most probable explanations for these distinct characteristics are deduced from observations by advanced transmission electron microscopy together with X-ray diffraction and electron energy-loss spectroscopy, which illustrate a gradual activation of nitride during lithiation/delithiation processes, owing to slow kinetics of VN reaction with lithium. The electrochemical results demonstrate that the weight ratio of VN to G has a significant effect on the performance and related kinetics of the materials.

In situ synthesis of a graphene/titanium nitride hybrid material with highly improved performance for lithium storage

A graphene/titanium nitride (G/TiN) hybrid as an anode material of lithium ion batteries is prepared by a simple in situhydrolysis method combined with ammonia annealing. TiN nanoparticles as obtained are ∼5 nm in size and homogeneously anchored on G. The G/TiN hybrid anode delivers a reversible capacity as high as 646 mA h g−1 at 20 mA g−1 and exhibits an enhanced initial coulombic efficiency of 75%, much higher than that of pure graphene (G: 52%) in the first cycle. The capacity retention is as much as 86% after 200 cycles. At a current density of 2000 mA g−1, the hybrid anode still retains 325 mA h g−1 while that of G is only 98 mA h g−1. It is demonstrated that the G/TiN hybrids display a superior electrochemical performance owing to the highly efficient mixed (electron and Li+) conducting network. The internal defects between G layers induced by nitrogen-doping in G/TiN may improve reversible Li storage, whereas the catalytic sites on the surface of G related to the decomposition of the electrolyte may be occupied by TiN, leading to a decreased irreversible capacity. Moreover, the formation of Li3N in the interface is beneficial to interface transport, which is confirmed by aberration-corrected scanning transmission electron microscopy. The anchoring of TiN nanoparticles on G is promising prospect for energy storage applications in high performance lithium-ion batteries.

Transition-metal nitride nanoparticles embedded in N-doped reduced graphene oxide: superior synergistic electrocatalytic materials for the counter electrodes of dye-sensitized solar cells

Transition-metal nitride TMN (MoN, TiN, VN) nanoparticle–N-doped reduced graphene oxide (NG) hybrid materials are presented as alternative low-cost platinum-free counter electrodes for dye-sensitized solar cells (DSCs). A high concentration of active catalytic sites and an efficient electronic/ionic mixed conducting network generated by the nanostructure afford promising synergistic effects on the electrocatalytic characteristics for triiodide reduction. On the basis of these advantages, these nanostructured hybrid based cells deliver significantly enhanced photovoltaic performances. The efficiencies of devices employing VN–NG, TiN–NG and MoN–NG are 6.279%, 7.498% and 7.913% respectively, which are comparable with that of Pt devices (7.858%). Such a design strategy is facile, cost effective and versatile, thus it may be extended to other inexpensive platinum-free counter electrode materials.

Oxygen-enriched carbon material for catalyzing oxygen reduction towards hybrid electrolyte Li-air battery

Graphene oxide, with sufficient oxygen-containing groups, is integrated with electronically conductive carbon nanotubes to be explored as an efficient metal-free catalyst for the oxygen reduction reaction. Preliminary theoretical calculations with the density functional theory method indicate that the existence of graphene oxide is favorable for the adsorption and subsequent four-electron reduction reactions of O2. Furthermore, this oxygen-enriched hybrid material was tested as a cathode in aprotic/aqueous hybrid electrolyte Li-air batteries. The hybrid material exhibited a very low overpotential (the voltage gap at 0.1 mA cm−2 is only 0.17 V) and better electrocatalytic performance owing to abundant oxygen containing groups and its excellent electroconductivity. These experimental and theoretical demonstrations should provide an important mechanistic insight into carbon-based metal-free catalysts in fuel cells and metal-air battery applications. We believe that the demonstrations shown in this paper provide a promising strategy to investigate highly efficient metal-free catalysts for advanced energy devices.

1D coaxial platinum/titanium nitride nanotube arrays with enhanced electrocatalytic activity for the oxygen reduction reaction: towards Li-air batteries.

CAT ON A HOT TIN SUPPORT: Coaxial Pt/TiN nanotube arrays are used to achieve a superior electrocatalytic activity of platinum towards the oxygen reduction reaction (ORR). Compared to a commercial Pt/C catalyst, the Pt/TiN NTA materials delivers a higher mass activity and specific activity for the ORR. Hence, these materials are useful as cathodes for hybrid electrolyte Li-air batteries, as demonstrated.