Hierarchically porous carbon derived from waste watermelon rind for enhanced microwave absorption

Abstract

Microwave absorbing materials are widely applied in military stealth and electromagnetic protection. Biomass-derived porous carbons have garnered considerable research interest owing to their low cost, abundant and renewable sources, tailorable architectures, and lightweight characteristics. However, achieving a synergistic combination of low filler loading, broad bandwidth, and strong absorption remains a significant challenge in existing research. In this study, waste watermelon rind was employed as a precursor, and potassium carbonate was chosen as the activating agent to fabricate heteroatom self-doped hierarchical porous carbon materials (WRP). The effects of different pyrolysis temperatures on the microstructure and microwave absorption performance of the WRP samples were systematically investigated. Experimental results demonstrate that the WRP-600 sample, prepared at 600 °C, exhibits an impressive minimum reflection loss (RLmin) of −60.08 dB and achieves an effective absorption bandwidth (EAB, for RL < −10 dB) of 7.82 GHz with an ultralow filler loading of 8 wt%. Theoretical analysis reveals that the modification of both the microstructure of the WRP-600 sample enhances its impedance matching characteristics. The outstanding microwave absorption performance is attributed to the synergistic effects of multiple loss mechanisms, including polarization loss—dominated by dipole polarization and complemented by interfacial polarization—along with conduction loss, multiple scattering/reflection, and interference loss. Furthermore, through a three-layer impedance-graded structural design, the optimal theoretical calculations demonstrated an EAB of 12.66 GHz at a total thickness of 6 mm. Subsequently, a flat panel specimen was prepared for validation, and the experimental results were in excellent agreement with the theoretical values. This study provides a valuable reference for the preparation of lightweight and high‑performance microwave absorbing materials from waste biomass, as well as for the design of broadband absorbing structures through impedance‑graded configurations.