Carbon nanofibers (CNFs) have emerged as promising candidates for realizing lightweight and high-performance electromagnetic (EM) wave absorbing materials owing to their obvious merits, such as long-range conductive networks, tunable dielectric properties, and atomic-scale composition regulation. The existing challenges are how to optimize surface impedance matching through structural design and realize multifrequency response characteristics by EM synergistic effects. In this study, we propose a confined-coordination growth strategy to anchor small-sized Co nanoparticles and simultaneously introduce structural defects on the surface of CNFs to realize lightweight and superior EM wave absorption. Interestingly, these post-coordinated metal–organic framework (MOF)-derived small Co nanoparticles can balance surface impedance, strengthen interfacial polarization, and promote interfacial electric field polarization, and the sublimation of Zn species introduces structural defects to regulate the dielectric constant and trigger defect polarization. Benefiting from the combined advantages of matched impedance, long-range conductive networks, abundant dielectric‒magnetic heterointerfaces, and structural defects, the minimum reflection loss (R_L) of the CNFs reached as high as −51.0 dB, and the effective absorption bandwidth (EAB) covered the entire Ku band with a broad bandwidth of 7.33 GHz. This strategy provides integrated insight into optimizing the impedance characteristics of CNFs and manipulating the EM wave absorbing performance.

Hollow engineering is considered to be an essential subfield in promoting electromagnetic (EM) wave absorption intensity and realizing lightweight characteristics. However, the enhancement of the effective absorption bandwidth (EAB) still faces considerable challenges. Herein, hollow carbon nanocages with CoFe₂Se₄ quantum dots (HCNs@CoFe₂Se₄-QDs) with superior EM wave absorption intensity and ultra broadband EAB are produced by using tightly arranged SiO₂ spheres as hard-template materials. Specifically, the removal of SiO₂ templates inevitably results in the formation of a hollow cavity, which is favorable for optimizing impedance matching and increasing the absorption intensity. In addition, the incorporation of selenium powder effectively increases the number of heterogeneous interfaces by forming CoFe₂Se₄ quantum dots (QDs) during the pyrolysis process, leading to strengthened interfacial polarization and ultra broadband EAB. As a result, superior EM wave attenuation with a minimum reflection loss (R_L) of −67.6 dB and an EAB of 11.4 GHz is achieved with only a 20 wt% filler ratio. This design concept of hollow engineering with magnetic QDs provides inspiration for optimizing the EM wave absorption intensity and simultaneously promoting the absorption bandwidth.