Fig. 1 SEM images of NiCo2O4 microsphere prepared by hydrothermal method: (a) before and (b, c) after calcination with (d) XRD patterns of NiCo2O4 powders[34]

Fig. 2 SEM images of flower-like nickel-cobalt oxides prepared with initial Ni/Co molar ratio of (a) 1 : 0, (b) 7 : 3, (c) 5 : 5, (d) 3 : 7 and (e, f) 0 : 1[43]

Fig. 3 (a, b) SEM images of flower-shaped NiCo2O4 prepared by microwave-assisted approach, (c) schematic illustration of the morphological evolution of flower-shaped microsphere precursors[46]

Fig. 4 (a, b) TEM images of the N-CNT/NiCo2O4 nanosheets, (c) schematic diagram of the fabrication process of N-CNT/ NiCo2O4[57]

Fig. 5 Illustration of the fabrication process of Co3O4@NiCo2O4[60]

Fig. 6 SEM images of NiCo2O4 nanosheets prepared by electrodeposition: (a, b) before and (c, d) after calcination, and (e) schematic illustration of the transportation of species on porous NiCo2O4 nanosheets[71]

Fig. 7 SEM images of the NiCo2O4 precursor NWAs/carbon textiles composite: (a) low and (b) high magnification, SEM images of the NiCo2O4 NWAs/carbon textiles composite: (c) low and (d) high magnification, and (e) schematic illustration of the formation of NiCo2O4 NWAs/carbon textiles composite[81]

Fig. 8 (a, b) SEM images of the NiCo2O4 nanotubes and (c) schematic illustration of preparation of NiCo2O4 nanofibers, nanotubes and nanobelts[86]

Fig. 9 Representative TEM images of the samples obtained after reaction for (a) 4 h, (b) 6 h, (c) 10 h, (d) 12 h and (e) their corresponding schematic illustration of the formation process for hierarchical NiCo2O4 tetragonal microtubes[87]