Schematic of the synthesis procedures of Co3O4/rGO/NF (top) and Co3O4/NF (bottom)

XRD patterns of a Co(OH)2/rGO/NF and b rGO/NF, Co3O4/NF, and Co3O4/rGO/NF. c Micro-Raman spectra of rGO/NF, Co3O4/NF, and Co3O4/rGO/NF composites

Top-view SEM images of a rGO/NF, b Co3O4/NF, and c Co3O4/rGO/NF. d N2 adsorption–desorption isotherms of Co3O4/rGO/NF and Co3O4/NF. The inset in d shows the pore size distribution of both samples

a Low- and b high-magnification TEM images, c SAED patterns of a Co3O4 nanosheet taken from Co3O4/rGO/NF. d STEM image, and elemental mapping images of e and f Co and g O

a CV curves of NF, rGO/NF, Co3O4/NF, and Co3O4/rGO/NF composites at 3 mV s-1. b Charge-discharge curves of NF, rGO/NF, Co3O4/NF, and Co3O4/rGO/NF composites at 5 mA cm-2. Charge-discharge curves of c Co3O4/NF and d Co3O4/rGO/NF at different current densities. e Cycling performance of Co3O4/rGO/NF at a current density of 7 A g-1 (the inset is a top-view SEM image of Co3O4/rGO/NF after the 3000 cycles shown in (e). f Nyquist plots of Co3O4/rGO/NF and Co3O4/NF