Biomimetic structural cellulose nanofiber aerogels with exceptional mechanical, flame-retardant and thermal-insulating properties

[Display omitted] •Hierarchical graphene-confined zirconium phosphate nanosheets was synthesized.•A biomimetic aerogel was prepared by the unidirectional freeze-casing technique.•The biomimetic aerogel has an ultralow thermal conductivity (18 mW·m−1·K−1).•The biomimetic aerogel shows the maximal specific Young’s modulus (104 kN·m·kg−1).•The biomimetic aerogel exhibits excellent flame-retardant properties. With the rapid increase of energy consumption, thermal-insulating materials made from abundant renewable resources are in urgent need for energy-efficient buildings, which satisfies the sustainable development of society. Cellulose nanofiber aerogels exhibit a promising prospect in thermal-insulating application, whereas still confront the inherent weakness of high flammability as well as the improvement of mechanical stiffness and thermal insulation. In the present study, inserting the growth of two-dimensional zirconium phosphate within multilayer graphene results in the formation of hierarchical graphene-confined zirconium phosphate (ZrP/RGO) nanosheets through a spatial confinement strategy. Inspired by the porous lamella-bridge architecture of Thalia dealbata stem, a unidirectional freeze-casing technique is utilized to assemble the building blocks of cellulose nanofiber and ZrP/RGO nanosheet into a biomimetic-structural aerogel which has excellent thermal-insulating, mechanical and flame-retardant properties. Compared with state-of-the-art cellulose nanofiber-based aerogels, the resulting composite aerogel perpendicular to lamellar alignments shows an ultralow thermal conductivity (18 mW·m−1·K−1), the maximal specific Young’s modulus (104 kN·m·kg−1) and high limited oxygen index (33.5) as well as very low peak heat release rate (14.1 kW/m2). Nature provides renewable resources and structural inspirations to achieve high-performance thermal insulation materials through nanoscale engineering.