Cobalt/cobalt tetroxide encapsulated within nitrogen-doped carbon polyhedral grafted onto carbon nanotubes exhibits broadband electromagnetic absorption.

To address the drawbacks of single carbon-based and cobalt-based microwave absorbing materials, including monotonous loss mechanisms, mismatched impedance matching, and limited absorption performance. Herein, a composite material comprising cobalt and cobalt tetroxide coated onto nitrogen-doped carbon polyhedral and grafted onto carbon nanotubes was synthesized using a controlled thermal oxidation strategy (Co/Co3O4@CNT/H-NPC). Synergistic optimization of phase composition and electromagnetic parameters is achieved by precisely tuning the heat-treatment temperature. Microstructural characterizations reveal that the interwoven carbon nanotube network and hollow porous carbon skeleton establish continuous conductive pathways, while abundant and stable in-situ-generated Co/Co3O4 heterointerfaces are obtained. Non-in situ XPS and synchrotron XANES/EXAFS results confirm the presence of mixed valence states of Co and CoO in the system; the unique CoCo/O coordination structure induce significant charge redistribution, whilst many polarized defect sites effectively enhance dielectric relaxation loss. Electromagnetic performance tests indicate that moderate heat treatment at 350 °C can balance dielectric loss and magnetic loss, optimize impedance matching characteristics, and prevent defects such as excessive surges in electrical conductivity and impedance mismatch caused by high-temperature oxidation. This optimized sample achieves highly efficient attenuation of electromagnetic waves through the synergistic effects of multiple gradients, including electrical conductivity loss in the carbon network, polarization loss at heterogeneous interfaces, and magnetic resonance loss in the cobalt-based components. This study elucidates the enhanced microwave-absorption mechanism of the composite system from the perspectives of atomic coordination, interfacial electronic modulation and multi-mechanism synergy. It provides feasible modulation strategies and theoretical support for the design and fabrication of high-performance carbon-based magnetic composite absorbers with coupled multiple loss mechanisms.