Fig. 4. (a) Scheme of the synthesis a yolk–shell structure ZnCo₂O₄ material, (b) Ragone plots of AACSDs (modified from ref. 85, copyright permission from the Royal Society of Chemistry); (c) illustration for the preparation process and (d) cycling stability of core–shell-like NiO@Co₃O₄–NiO FTN/Ni foam (modified from ref. 88, copyright permission from Elsevier).