Hollow Metal–Organic-Framework-Derived Magnetic Carbon Nanocage-Based Phase Change Composites for Energy Storage, Photothermal Conversion, and Microwave Absorption

Rapid advancements in integrated multifunctional electronics have intensified the demand for high-performance composite phase change materials (PCMs) that combine thermal management, solar–thermal conversion, and microwave absorption. In this study, a hierarchical porous Fe–N–C carrier was prepared via a green two-step approach using biomass-derived tannic acid etching, followed by controlled carbonization. n-Docosane (n-22) was dual-encapsulated by Fe–N–C and an additional silica shell for effective PCM composite n-22@Fe–N–C@SiO2. The resulting material exhibits a high latent heat of 130.7 J/g with minimal permeability (4.31%), an outstanding solar–thermal conversion efficiency of 89.18%, and superior microwave absorption performance, achieving a minimum reflection loss of −35.10 dB at 5.45 GHz and an ultrawide absorption bandwidth of 7.16 GHz, effectively covering the entire X band. The graphitized carbon framework and uniformly dispersed Fe3O4 nanoparticles synergistically reduce interfacial thermal resistance, enhance photon capture and phonon transport, and notably increase the conductivity loss and magnetic loss for microwave absorption, endowing n-22@Fe–N–C@SiO2 as a promising candidate for integrated thermal management and electromagnetic protection for both electronics and wearable radiation-proof textiles.