Engineering impedance-matched double-shells in hollow Co/carbon microspheres with gradient graphitization for high-efficiency electromagnetic wave absorption

The inherent trade-off between impedance matching and electromagnetic (EM) attenuation capability has long been a fundamental limitation in carbon-based materials, hindering further advances in their EM absorption performance. To overcome this challenge, we innovatively design hollow double-shell Co/carbon microspheres with a gradient graphitization structure, where Co₃O₄ nanoparticles pre-anchored on melamine formaldehyde microspheres can induce the formation of graphitic inner shells during high-temperature pyrolysis, and nevertheless, the outer carbon shells remain amorphous nature due to the lack of Co-relate species, ultimately resulting in a gradient graphitization from the inside out. This unique double-shell architecture combines the advantages of both gradient graphitization and hollow structure, favorable for powerful EM attenuation and impedance matching at the same time. EM analyses reveal that the outer amorphous carbon shells not only play a key role in optimizing impedance matching, but also create heterogeneous interfaces with the inner graphitic shells to enhance interfacial polarization. As a result, the as-prepared sample achieves a superior reflection loss of -62.9 dB, and its maximum effective absorption bandwidth (EAB_max) can be extended to 11.3 GHz through a rationally designed multilayer structure, significantly surpassing non-gradient counterparts. Computer simulation technology (CST) simulations further verify a remarkable radar cross-section (RCS) reduction of 22.3 dBm². This work provides an effective strategy for reconciling the conflict between impedance matching and attenuation in carbon-based materials, and also demonstrates their great potential as lightweight and broadband EM wave absorbing materials in the future.