Co/CoO/Lotus Seedpod Nanoporous Carbon Composites Reduced from Co3O4 for High-Performance Microwave Absorbers

To obtain microwave-absorbing materials with high absorptivity, broad bandwidth, and lightweight properties, Co3O4/lotus seedpod carbon (Co3O4/LSC) composites were reduced under a hydrogen pressure of 3 MPa. As the reduction temperature increases, the phases of the Co3O4/LSC composites change from Co3O4 to CoO/Co. At 200 °C (named as the 3200 nanocomposite), partial Co3O4 changes to CoO, the Co3O4 completely transfers into CoO with a little Co detectable at 300 °C (3300 nanocomposite), and the concentration of Co increases more than that of the 3300 nanocomposite once temperature is raised to 400 °C (3400 nanocomposite). Moreover, the morphology of the Co3O4/LSC composites changes from irregular particles to a sintering state in the reduction process. When 3200, 3300, and 3400 nanocomposites are compared, the 3400 nanocomposite exhibits the highest permittivity and best microwave absorption performance: the 3400 nanocomposite exhibits a strong reflection loss of −60.63 dB (2.0 mm, 18.0 GHz) and an absorption bandwidth of 14.10 GHz (4.7 mm, 3.9–18.0 GHz). The hydrogenated LSC/Co3O4 nanocomposite has a strong absorption capacity and wide effective bandwidth, is lightweight, and can be used as a high-performance biomass-based microwave-absorbing material.