Three-dimensional (3D) hierarchical nanostructures have been demonstrated as one of the most ideal electrode materials in energy storage systems due to the synergistic combination of the advantages of both nanostructures and microstructures. In this study, the honeycomb-like mesoporous NiO microspheres as promising cathode materials for supercapacitors have been achieved using a hydrothermal reaction, followed by an annealing process. The electrochemical tests demonstrate the highest specific capacitance of 1250 F g(-1) at 1 A g(-1). Even at 5 A g(-1), a specific capacitance of 945 F g(-1) with 88.4% retention after 3500 cycles was obtained. In addition, the 3D porous graphene (reduced graphene oxide, rGO) has been prepared as an anode material for supercapacitors, which displays a good capacitance performance of 302 F g(-1) at 1 A g(-1). An asymmetric supercapacitor has been successfully fabricated based on the honeycomb-like NiO and rGO. The asymmetric supercapacitor achieves a remarkable performance with a specific capacitance of 74.4 F g(-1), an energy density of 23.25 Wh kg(-1), and a power density of 9.3 kW kg(-1), which is able to light up a light-emitting diode.

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Keywords:b-Ni(OH) 2 nanosheets hybrid supercapacitor multilayer nickel foam A B S T R A C T Ultrafine b-Ni(OH) 2 nanosheets with an average diameter of about 14.5 nm have been fabricated by immersing Na(HCO 3 ) 2 nanoparticles into 6 M KOH solution. The multilayer nickel foam system was introduced to improve the cycling stability of ultrafine b-Ni(OH) 2 nanosheets without using acetylene black. The results demonstrated that the specific discharge capacity of the b-Ni(OH) 2 electrode was up to 524.5C g À1 at 5 A g À1 and retained 83.8% after 1000 cycles, which was better than those of other b-Ni (OH) 2 electrodes obtained using acetylene black as conductors (retained 59.1% after 1000 cycles). Even after 5000 cycles, the specific capacity also maintained 80.8%. In addition, a hybrid supercapacitor was successfully fabricated based on ultrafine b-Ni(OH) 2 nanosheets and active carbon, the corresponding energy density and power density at 0.1 A g À1 were calculated to be 20.45 Wh kg À1 and 75 W kg À1 , respectively. What's more, the hybrid supercapacitor could light up a set of low-voltage light-emitting diodes.

3D transition metal oxides, especially constructed from the interconnected nanowires directly grown on conductive current collectors, are considered to be the most promising electrode material candidates for advanced supercapacitors because 3D network could simultaneously enhance the mechanical and electrochemical performance. The work about design, fabrication, and characterization of 3D gully-network Co3O4 nanowire arrays directly grown on Ni foam using a facile hydrothermal procedure followed by calcination treatment will be introduced. When evaluated as a binder-free battery-type electrode for supercapacitor, a high specific capacity of 582.8 C g−1 at a current density of 1 A g−1, a desirable rate capability with capacity retention about 84.8% at 20 A g−1, and an outstanding cycle performance of 93.1% capacity retention after 25,000 cycles can be achieved. More remarkably, an energy density of 33.8 W h kg−1 at a power density of 224 W kg−1 and wonderful cycling stability with 74% capacity retention after 10,000 cycles can be delivered based on the hybrid-supercapacitor with the as-prepared Co3O4 nanowire arrays as a positive electrode and active carbon as negative electrode. All the unexceptionable supercapacitive behaviors illustrates that our unique 3D gully-network structure Co3O4 nanowire arrays hold a great promise for constructing high-performance energy storage devices.

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Well-shaped Mn3O4 tetragonal bipyramids with a high reversible capacity of 822.3 mA h g−1 are synthesized by a simple hydrothermal method without any surfactants or coordination compounds. The formation mechanism of the Mn3O4 tetragonal bipyramids is discussed from the view of crystallography.

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