Effectively absorbing and converting electromagnetic (EM) energy for reutilization has an important implication in the fields of energy and environment. Herein, multi-walled carbon nanotubes (MWCNTs) are confinedly implanted into flower-like cobaltosic oxide (Co3O4) to tailor the conductive network and polarization genes. It controllably tunes the EM property and optimizes the low-frequency microwave absorption. The minimum reflection loss reaches −61.4 dB at 3.44 GHz. A novel self-powered device is constructed to achieve the effective conversion from waste EM energy to useful electric energy. Energy conversion process inside flower-like Co3O4-MWCNT architecture is investigated, and the contribution of dielectric loss and magnetic loss to energy conversion is exhibited. This work provides a promising approach in combating EM pollution and alleviating energy problem. Display omitted •m-MWCNT is implanted into flower-like Co3O4 to tailor microstructure, tuning EM property and low-frequency absorption.•Energy conversion mechanism is dissected deeply, and the contribution of dielectric loss and magnetic loss is shown.•A novel self-powered device is constructed to harvest and convert waste EM energy to useful electric energy.