Wet-State Densification of Ionically Cross-Linked Nanocellulose Gels for Producing Stiff Porous Structures

Cellulose nanofiber (CNF) aerogels can offer both transparency and thermal insulation but typically suffer from mechanical weakness. Here, we report a wet-state densification strategy to overcome this limitation. Flowable CNF dispersions were converted to self-supporting hydrogels via inter-CNF cross-linking with zirconium ions (Zr⁴⁺), followed by uniaxial compression and supercritical drying to produce aerogels with bulk densities of 25-95 mg cm^-3. The compression induced a uniform anisotropic layered structure while maintaining mesopores of 10-60 nm and specific surface areas of 380-440 m² g^-1. The aerogels with the highest density of 95 mg cm^-3 attained an outstanding tensile stiffness (E_t ∼ 122 MPa), while their light transmittances and thermal conductivity remained approximately 75% at 600 nm and 21 mW m^-1 K^-1, respectively, even after densification. This wet-state densification of CNF hydrogels delivers specific aerogels that combine the excellent features of mechanical robustness, optical transparency, and thermal superinsulation.