In-situ growth of ultrathin Ni6MnO8 nanosheets on nickel foam as a binder-free positive electrode for asymmetric supercapacitor: Effects of alkaline aqueous and redox additive electrolytes

In this paper, the growth of ultrathin Ni6MnO8 nanosheet arrays on nickel foam via a simple hydrothermal reaction and calcination process without the introduction of other nickel sources has reported. The reaction between nickel ions on the substrate and manganese ions in a solution developed a good contact between the active material and the substrate. To improve capacitance performance of the active material, two strategies including the use of redox active additive electrolyte (K3Fe(CN)6 + KOH) together with different aqueous electrolytes (KOH, NaOH, LiOH) were followed. The electrode was characterized by X-ray diffraction technique (XRD), Fourier transforms infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), Reflection electron energy loss spectroscopy (REELS), Brunauer, Emmett, and Teller (BET), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX). The as-synthesized Ni6MnO8 nanosheet in LiOH electrolyte exhibited a maximum specific capacitance of 824 F g− 1, which is superior to that in KOH and NaOH at 1.0 A g− 1. Afterward, the fabricated asymmetric device exhibited good electrochemical performance and cyclic stability of about 79% at 1.5 mA after 1500 charge/discharge cycles as well as a volumetric energy density of 54 mWh cm− 3 at a volumetric power density of 0.41 mW cm− 3.