NiCo₂O₄ nanoneedle-nanosheet hybrid structure on CC substrate for high-performance flexible supercapacitors

In this paper, three different morphologies of NiCo₂O₄ nanoarrays, namely, nanosheet (S-NiCo₂O₄), nanoneedle (N-NiCo₂O₄) and their hybrid nanostructure (M-NiCo₂O₄) were successfully prepared on flexible carbon cloth (CC) by a hydrothermal method. The structures, surface morphologies and compositions of the samples were respectively characterized by XRD, SEM, EDS, and XPS, and the effect of NiCo₂O₄ morphologies on the electrochemical performances was systematically investigated. It is found that the M-NiCo₂O₄/CC electrode demonstrates the best supercapacitive performance among the three kinds of samples, typically its specific capacitance is 1347.4 F/g at 1 A/g (1509.1 F/cm² at 1 mA/cm²), much higher than that of S-NiCo₂O₄ (938.4 F/g at 1 A/g) and N-NiCo₂O₄ (1022.4 F/g at 1 A/g). Meanwhile, the M-NiCo₂O₄/CC sample exhibits excellent rate capability (81.6%, from 1 A/g to 15 A/g) and cycling stability (92.4% retention after 10,000 cycles). In addition, an flexible solid-state asymmetric supercapacitor (ASC) with M-NiCo₂O₄/CC as positive electrode manifests great capacity retention (94% after 5000 cycles) and outstanding energy density of 41.7 Wh/kg at the power density of 750 W/kg. Moreover, the charge-discharge time shows no significant change after 2000 bends, demonstrating its application potential in the field of flexible SCs. The excellent performance is attributed to the unique hybrid porous structure of one-dimensional (1D) nanoneedles and two-dimensional (2D) nanosheets.