Synthesis and morphological characterization of the Ni-N/G. (a) Schematic illustration of the synthesis of the 3D graphene framework decorated with nickel atomic clusters (Ni-N/G). (b) XRD patterns of ZIF-8 precursors and Ni-doped ZIF-8 precursors. Representative electron microscopy images. (c) SEM, (d) TEM (inset, corresponding SAEDP), (e) HADDF-STEM, and (f) STEM images of the Ni-N/G sample and (g) corresponding EDX maps (colours) of an individual element.