Enhanced Electromagnetic Energy Conversion in an Entropy‐driven Dual‐magnetic System for Superior Electromagnetic Wave Absorption

Abstract Thermochemical conversion is a highly effective method for upgrading organic solid wastes into high‐value materials, contributing to carbon neutrality and peak, carbon emission goals. It also serves as a pathway to develop energy‐efficient electromagnetic wave absorbing (EMWA) materials. In this study, fish skin to successfully in situ nitrify Prussian Blue into Fe 3 N under external thermal driving condition, resulting in high saturation magnetization is utilized. The resulting Fe 3 N@C demonstrates outstanding EMWA property, achieving a minimum reflection loss of −71.3 dB. Furthermore, by introducing cellulose nanofiber, a portion of the iron nitride is transformed into iron carbide, resulting in Fe 3 C/Fe 3 N@C. This composite exhibits enhanced EMWA properties owing to wider local charge redistribution and stronger electronic interactions, achieving an effective absorption bandwidth ( EAB ) of 6.64 GHz. Electromagnetic simulations and first‐principles calculations further elucidate the EMWA mechanism, and the maximum reduction value of the radar‐cross section reached 37.34 dB·m 2 . The design of multilayer gradient metamaterials demonstrated an ultra‐broadband EAB of 11.78 GHz. This paper presents an efficient strategy for atomic‐level biomass waste utilization to prepare Fe 3 N, provides novel insights into the conversion between metal nitrides and metal carbides, and offers a promising direction for the development of advanced EMWA materials.