Bixiao Wang

Highly conductive carbon–CoO hybrid nanostructure arrays with enhanced electrochemical performance for asymmetric supercapacitors

In this work, we report the synthesis of hybrid nanowire arrays by growing highly conductive carbon onto rough CoO nanowire arrays on 3D nickel foam. The CoO@C nanostructure arrays (CCNAs) are obtained via a hydrothermal method, followed by controlling the annealing and carbon deposition process at a relatively low temperature in the chemical vapor deposition (CVD) stage. In the carbon shell, apart from partial amorphous carbon, crystalline carbon was observed via TEM. With deposited carbon, the electrical conductivity and capacitance behaviors are dramatically promoted. The growth mechanism is proposed by TEM and XPS analyses, which firstly indicates that CoO could catalyze the decomposition of C2H2 at the low temperature of 427 °C in a reduction and catalytic process. The obtained CCNAs with a more hydrophilic surface and low resistance are tested as the working electrodes of supercapacitors, which lead to an ultrahigh specific capacitance of 3282.2 F g−1 approaching to the theoretical value. Good rate capability and 96.9% capacitance retention after 10 000 cycles suggest that such hybrid electrode possesses a great potential application. After assembling it as the positive electrode and activated carbon as the negative electrode, the aqueous asymmetric supercapacitor demonstrates an energy density value up to ∼58.9 W h kg−1 which is the highest value achieved among the Co-based supercapacitors.

Asymmetric Supercapacitor Based on Porous N-doped Carbon Derived from Pomelo Peel and NiO Arrays

A three dimensional (3D) porous framework-like N-doped carbon (PFNC) with a high specific surface area was successfully fabricated through ammonia doping and graphitization based on pomelo peel. The obtained PFNC exhibits an enhanced specific capacitance (260 F g(-1) at 1 A g(-1)) and superior cycling performance (capacitance retention of 84.2% after 10000 cycles at 10 A g(-1)) on account of numerous voids and pores which supply sufficient pathways for ion diffusion during cycling. Furthermore, a fabricated asymmetric PFNC//PFN device based on PFNC and porous flake-like NiO (PFN) arrays achieves a specific capacitance of 88.8 F g(-1) at 0.4 A g(-1) and an energy density of 27.75 Wh kg(-1) at a power density of 300 W kg(-1) and still retains 44 F g(-1) at 10 A g(-1) and 13.75 Wh kg(-1) at power density of 7500 W kg(-1). It is important that the device is able to supply two light-emitting diodes for 25 min, which demonstrates great application potential.

Rapid synthesis of CuO nanoribbons and nanoflowers from the same reaction system, and a comparison of their supercapacitor performance

One-dimensional CuO nanoribbons and three-dimensional CuO nanoflowers were synthesized via a facile, rapid, low-temperature, one-pot water bath method, in which the synthesis was performed in Cu(CH3COO)2/NaOH and aqueous/ethanol systems at 70 °C for 15 min. Control over the shape and dimensionality of the well-defined CuO single crystals was achieved simply by varying the order of addition of the reactive materials. X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and selected area electron diffraction were used to characterize the products. The formation mechanism in the in situ, rapid reaction was investigated. In Brunauer-Emmett-Teller and thermogravimetry measurements, the nanoribbons exhibited a higher specific surface area and higher adsorption capabilities than the nanoflowers. Using cyclic voltammetry, chronopotentiometry and EIS measurement for supercapacitance, it was shown that the nanoflower electrodes had better performance than the nanoribbon electrodes, however, the nanoribbon/C electrodes had better performance than the nanoflower/C electrodes at lower current density, but were worse at higher current density.

Controlled growth of NiMoO4·H2O nanoflake and nanowire arrays on Ni foam for superior performance of asymmetric supercapacitors

A facile hydrothermal method is developed for fabrication of large-scale NiMoO4·H2O arrays with robust adhesion on Ni foam. Importantly, the morphology of NiMoO4·H2O can be easily controlled to be nanoflake (H-NF) or nanowire (H-NW) arrays by using NH4F as additive. The obtained nanoflake morphology delivers better electrochemical activity than that of nanowire. The electrochemical performance of anhydrous NiMoO4 arrays obtained by annealing the NiMoO4·H2O has also been investigated for comparison. It is believed that the presence of the structural water of NiMoO4 enhances the capacitive performance by making it a good ionic conductor. Furthermore, an asymmetric supercapacitor (ASC) is constructed using the as-prepared NiMoO4·H2O nanoflake arrays as the positive electrode and activated carbon (AC) as the negative electrode. The optimized ASC with an extended operating voltage range of 0–1.6 V displays excellent electrochemical performance with a high energy density of 53.8 W h kg−1 at a power density of 239 W kg−1 in addition to superior rate capability. Moreover, the H-NF//AC ASC device exhibits remarkable cycling stability with 73.4% specific capacitance retention after 4000 cycles. Our result shows that this unique NiMoO4·H2O nanoflake array is promising for electrochemical energy applications.

Designing 3D interconnected continuous nanoporous Co/CoO core-shell nanostructure electrodes for a high-performance pseudocapacitor

A high-performance supercapacitor electrode is designed and fabricated with the 3D interconnected continuous nanoporous Co/CoO core-shell hybrid nanostructure grown on nickel foam. The Co/CoO core-shell hybrid nanostructures are obtained via a hydrothermal method, followed by high-temperature annealing in hydrogen atmosphere, and finally placed in air at 50 °C for 1 h. The Co/CoO core-shell nanostructure assembled by a conductive metal-core and a CoO shell, brings low resistance, high specific capacitance of 5.632 F cm-2 and good capability stability (81.5% capacitance retention after 6000 cycles). An asymmetric supercapacitor device built by the Co/CoO (positive electrode) and activated carbon (negative electrode) can deliver a working voltage of 1.7 V and display a high energy density of 0.002 67 Wh cm-2 at a power density of 0.001 62 W cm-2, which is far superior to that of a supercapacitor at a similar power density.