Sequential-etching assisted construction of Co3O4/N-doped carbon@CoNix yolk-shelled heterostructures with dual loss sites for highly efficient electromagnetic wave absorption

•Controlled sequential etching strategy is developed to build yolk-shell structures.•Yolk-shell structures of varied compositions allow tunable EMW absorption.•The dual loss species on the core and shell benefit synergy of loss mechanisms.•Co3O4/N-doped carbon@CoNi1.5 exhibit strong and broadband microwave absorption. The structural design and composition tuning have been very effective in fabricating hierarchical porous architectures with multiple loss mechanisms and good impedance matching for high-efficiency microwave absorption, but the design of sophisticated yolk-shell structures containing double-loss active species both in the yolk and shell has hardly been explored owing to the synthetic difficulty. To this end, we have successfully prepared novel yolk-shell Co3O4/N-doped carbon (NC)@CoNix (x = 1, 1.5, 2) composites by controlled sequential etching of ZIF-67 templates followed by direct nitridation. It was found that the sequential etching process could not only adjust the Co/Ni ratio of the shell layer in the yolk-shell structure, but also change the residual carbon content of the MOF template in the core. Due to the proper alignment of the dual-loss active Co3O4 and CoNi1.5 on the yolk and shell, the optimal impedance matching is achieved for the Co3O4/NC@CoNi1.5 composite, which exhibits better loss capability compared to the reported yolk-shell microwave adsorption materials (MAMs) with the minimum reflection loss (RLmin) value of −71.15 dB at 15.2 GHz and an effective absorption bandwidth (EAB; RLmin