Boosting Microwave Absorption via Hollow CoFe/CoFe₂O₄ Nanoparticles Anchored on Lotus-Leaf-Derived Biochar

Biochar composites are promising for electromagnetic wave absorption due to their renewability and low cost, but performance optimization remains a challenge. Here, hollow CoFe/CoFe₂O₄ composite nanoparticles, in which the CoFe and CoFe₂O₄ phases coexist, are grown in situ on lotus-leaf-derived porous carbon via a Kirkendall-type process during heat treatment. The optimized composite (pyrolyzed at 800 °C) exhibits a saturation magnetization of 69.20 emu/g and a coercivity of 482 Oe; a calculated minimum reflection loss (RL_min) of -71.4 dB is obtained at a matching thickness of 2.70 mm, with an effective absorption bandwidth (EAB) of 5.68 GHz at 1.70 mm, covering most of the Ku band. The enhanced microwave absorption performance is attributed to the synergy of dielectric and magnetic losses, polarization relaxations, and improved impedance matching, together with multiple scattering within the hollow structure. In addition, the exchange coupling between the FeCo and CoFe₂O₄ may contribute to magnetic dissipation, while hollow cavities may promote internal reflections for further energy attenuation. This work provides a biomass-derived route to high-performance biochar-based absorbers competitive with synthetic counterparts.