Ultralight Cellular Foam from Cellulose Nanofiber/Carbon Nanotube Self-Assemblies for Ultrabroad-Band Microwave Absorption

Microwave absorption materials (MAMs) with lightweight density and ultrabroad-band microwave absorption performance are urgently needed in advanced MAMs, which are still a big challenge and have been rarely achieved. Here, a new wide bandwidth absorption model was designed, which fuses the electromagnetic resonance loss ability of a periodic porous structure in the low-frequency range and the dielectric loss ability of dielectric materials in the high-frequency range. Based on this model, a lightweight porous cellulose nanofiber (CNF)/carbon nanotube (CNT) foam consisting of a cellular vertical porous architecture with the macropore diameters between 30 and 90 μm and a nanoporous architecture at a scale of 1.7-50 nm was obtained by an ice-template method using CNTs and CNFs as "building blocks". Benefiting from the unique architecture, the effective absorption bandwidth reaches 29.7 GHz, and its specific microwave absorption performance exceeds 80,000 dB·cm^-2·g^-1, which far surpasses those of the MAMs previously reported, including all CNT-based composites. Moreover, the CNF/CNT foam possesses ultralow density (9.2 mg/cm³) and strong fatigue resistance, all coming from the well-interconnected porous structure and the strong hydrogen bonds among CNF-CNF and CNF-CNT molecular chains.