Abstract: As electromagnetic pollution escalates and protection demands diversify, there is an urgent requirement for versatile carbon foam materials capable of absorbing electromagnetic waves (EMWs). Furthermore, the concern about global warming and the depletion of petrochemical resources calls for facile and eco-friendly methods for the large-scale production of multi-functional and biodegradable carbon foams. Herein, cornstraw-derived carbon foams (CCFs) integrating EMW absorption, sound absorption, and heat insulation were prepared by a facile dual-template strategy. Benefiting from the dual-template effect of air bubbles and ice crystals, the obtained foam manifests an ultra-low density of 31 mg/cm³, large porosity of 0.85 and also super-broad absorption with an effective absorption bandwidth (EAB) of 7.18 GHz at 3.6 mm, even beyond most carbon-based composite foams. Moreover, abundant pores also endow the foam with good thermal insulation performance (as low as 0.041 W/(m K)) and high sound absorption coefficient (0.8 at 1250-6000 Hz), which are equivalent to commercial foams. The excellent EMW absorption performance originates from conduction loss produced by the three-dimensional (3D) interconnected network structure and also interfacial polarization and multiple scattering induced by porous structure. Additionally, the abundant closed pores in foam prevent thermal convection and thus provide good thermal-insulation performance, yet the opening pores proffer excellent sound absorption through resonance and friction absorption. This study provides new insights into the green synthesis of multi-functional microwave absorbing foam and also supplies a new thermal-insulation material for exterior walls of buildings exposed to electromagnetic environment.

Key words: Biomass-derived carbon foam, Dual-template synthesis, Microwave absorption, Heat insulation, Sound absorption