Hybrid Electrodes Based on Zn–Ni–Co Ternary Oxide Nanowires and Nanosheets for Ultra-High-Rate Asymmetric Supercapacitors

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 as-prepared 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.