In this study, hydrolysis agent-assisted three-dimensional (3D) flowerlike hierarchical zinc−nickel−cobalt oxide (ZNCO) nanostructured materials, which consist of two-dimensional (2D) ZNCO nanosheets anchored with one-dimensional (1D) ZNCO nanowire architectures, as binder-free electrodes for ultrahigh-rate supercapacitor applications are directly fabricated on Ni foam via a facile hydrothermal method followed by calcination. The morphologies of the as-fabricated flowerlike 3D hierarchical ZNCO nanostructures dramatically depend on the combination method (urea, hexamethylenetetramine, hexamethylenetetramine− urea) of the hydrolysis reagents used, which are denoted ZNCO-U, ZNCO-H, and ZNCO-HU, respectively. Among them, the asprepared ZNCO-HU electrode shows outstanding electrochemical performance with a higher specific capacity of 259.8 mAh g −1 at 1 A g −1 , ultrahigh capacitance retention of 83.9% at a higher current density of 50 A g −1 , and remarkable long cycle stability over 5000 cycles. The assembled asymmetric supercapacitor (ASC) device using ZNCO-HU as the cathode and N-doped graphene hydrogel (NGH) as the anode materials delivers a noticeable specific capacity of 76.5 mAh g −1 at 1 A g −1 and good rate capability of 69.65% at 10 A g −1 . The sandwiched ASC displays a superior energy density of 55.4 Wh kg −1 at a power density of 761.5 W kg −1 and excellent capacity retention of 89% up to 5000 cycles.

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Recently, as environmentally clean and sustainable energy sources, high-performance electrochemical energy storage devices, such as secondary batteries and supercapacitors have received great attention [1-3]. In particular, supercapacitors have received huge interest due to the high power density, long-term cycle stability, fast charging/discharging process, safety and low maintenance cost [4], and therefore are widely used for different applications such as consumer electronics, portable devices, electric vehicles, memory backup devices and public transportation etc [5][6][7]. In order to fulfill the growing energy density demands for next generation supercapacitors, a dramatic improvement of energy density (E = (CV 2 )/2) of supercapacitors is still one of the great challenges, which can be generally accomplished by increasing the specific capacitance (C) and operation potential window (V) [8,9]. Accordingly, the development of high-performance pseudo-capacitors and hybrid supercapacitors involves compounding the high surface area carbon materials with transition metal oxides and/or conducting polymers [10][11][12][13][14][15]. However, poor electrical conductivity (mostly due to inferior conductivity of transition metal oxides) and serious charge transfer resistance (R CT ) (due to poor interface contact/ adhesion between active materials and current collectors) have been limited to improve the performance of the supercapacitor electrodes in view of energy density, power density, cycle stability, rate capability and charge/

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In this work, we prepared network-structured carbon nanofibers using polyacrylonitrile blends (PAN150 and PAN85) with different molecular weights (150,000 and 85,000 g mol−1) as precursors through electrospinning/hot-pressing methods and stabilization/carbonization processes. The obtained PAN150/PAN85 polymer nanofibers (PNFs; PNF-73, PNF-64 and PNF-55) with different weight ratios of 70/30, 60/40 and 50/50 (w/w) provided good mechanical and electrochemical properties due to the formation of physically bonded network structures between the blended PAN nanofibers during the hot-processing/stabilization processes. The resulting carbonized PNFs (cPNFs; cPNF-73, cPNF-64, and cPNF-55) were utilized as anode materials for supercapacitor applications. cPNF-73 exhibited a good specific capacitance of 689 F g−1 at 1 A g−1 in a three-electrode set-up compared to cPNF-64 (588 F g−1 at 1 A g−1) and cPNF-55 (343 F g−1 at 1 A g−1). In addition, an asymmetric hybrid cPNF-73//NiCo2O4 supercapacitor device also showed a good specific capacitance of 428 F g−1 at 1 A g−1 compared to cPNF-64 (400 F g−1 at 1 A g−1) and cPNF-55 (315 F g−1 at 1 A g−1). The cPNF-73-based device showed a good energy density of 1.74 W h kg−1 (0.38 W kg−1) as well as an excellent cyclic stability (83%) even after 2000 continuous charge–discharge cycles at a current density of 2 A g−1.