Binary metal oxides with three-dimensional (3D) superstructure have been regarded as desirable electrode materials for the supercapacitor due to the combination of the improved electrical conductivity and effective porous structure. 3D hierarchical flower-shaped nickel cobaltite (NiCo2O4) microspheres have been fabricated by a rapid and template-free microwave-assisted heating (MAH) reflux approach followed by pyrolysis of the as-prepared precursors. The flower-shaped NiCo2O4 microspheres, composed of ultrathin nanopetals with thickness of about 15 nm, are endowed with large specific surface area (148.5 m(2) g(-1)) and a narrow pore size distribution (5-10 nm). The as-fabricated porous flower-shaped NiCo2O4 microspheres as electrode materials for supercapacitor exhibited high specific capacitance of 1006 F g(-1) at 1 A g(-1), enhanced rate capability, and excellent electrochemical stability with 93.2% retention after 1000 continuous charge-discharge (CD) cycles even at a high current density of 8 A g(-1). The desirable integrated performance enables it to be a promising electrode material for the electrochemical supercapacitor (EC).

A 3D-nanonet structured cobalt-basic-carbonate precursor has been obtained by a facile, low cost and eco-friendly route under ambient temperature and pressure. After calcination in air, the as-prepared precursor was converted to a 3D-nanonet hollow structured Co3O4 with its original frame structure almost preserved. Encouragingly, by alternating experimental parameters (Table S1 in the Supporting Information ), such as concentration of the starting reagents and calcination temperature, we got the optimized condition for the final product with desirable electrochemical performance (Figure S1 in the Supporting Information ). The pseudocapacitive properties of the obtained Co3O4 were evaluated by cyclic voltammetry (CV), galvanostatic charge-discharge measurement and electrochemical impedance spectroscopy in 6.0 M KOH solution. At different scan rates of 5, 10, 20, and 30 mV s(-1), the corresponding specific capacitances were 820, 755, 693, and 656 F g(-1), respectively. The material also exhibited superior charge-discharge stability and maintained 90.2% of its initial capacitance after 1000 continuous charge-discharge cycles at a current density of 5 A g(-1). From a broad view, our research and the outstanding results not only present a feasible access to nanostructured Co3O4 but also remind us of paying more attention to the simple synthetic methods without complex processes and sophisticated instruments.

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Flexible functional devices based on two dimensional (2D) materials are extremely suitable for malleable, portable and sustainable applications.