CoNi-decorated carbon microtubes from kapok fibers for electromagnetic wave absorption

Biomass-derived carbon materials have attracted considerable attention for electromagnetic wave (EMW) absorption owing to their sustainability, low cost, and porous structures. However, their practical application is often hindered by poor impedance matching and a single loss mechanism. In this study, a hierarchical heterostructure is constructed by introducing magnetic components to enhance the synergy between dielectric and magnetic losses while optimizing impedance matching. In particular, natural kapok fibers are utilized as a precursor to prepare hollow carbon microtubes (CMTs) via carbonization, and CoNi nanoparticles are controllably grown on the surface of these CMTs using a hydrothermal method, successfully fabricating CoNi/CMT composites. The incorporation of CoNi nanoparticles substantially enhances interfacial and dipole polarization as well as promotes natural and exchange resonance, thereby synergistically improving magnetic loss capabilities and establishing an integrated electromagnetic dissipation mechanism. The CoNi/CMT-4 sample exhibits outstanding EMW absorption performance, i.e., a minimum reflection loss (RL) of −50.36 dB and an effective absorption bandwidth of 5.04 GHz. Furthermore, within a thickness range of 1–5 mm, the material maintains RL values below −10 dB across the entire 4–18 GHz frequency range, demonstrating exceptional broadband EMW absorption performance and strong absorption characteristics. This study provides a new design strategy for developing high-performance biomass-derived carbon-based EMW absorbers.