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

. 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]

. (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]

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

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

. 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]

. 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]

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

. 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]