Versatile Assembly of Metal–Phenolic Network Foams Enabled by Tannin–Cellulose Nanofibers

Metal–phenolic network (MPN) foams are prepared using colloidal suspensions of tannin‐containing cellulose nanofibers (CNFs) that are ice‐templated and thawed in ethanolic media in the presence of metal nitrates. The MPN facilitates the formation of solid foams by air drying, given the strength and self‐supporting nature of the obtained tannin–cellulose nanohybrid structures. The porous characteristics and (dry and wet) compression strength of the foams are rationalized by the development of secondary, cohesive metal‐phenolic layers combined with a hydrogen bonding network involving the CNF. The shrinkage of the MPN foams is as low as 6% for samples prepared with 2.5–10% tannic acid (or condensed tannin at 2.5%) with respect to CNF content. The strength of the MPN foams reaches a maximum at 10% tannic acid (using Fe(III) ions), equivalent to a compressive strength 70% higher than that produced with tannin‐free CNF foams. Overall, a straightforward framework is introduced to synthesize MPN foams whose physical and mechanical properties are tailored by the presence of tannins as well as the metal ion species that enable the metal–phenolic networking. Depending on the metal ion, the foams are amenable to modification according to the desired application. Metal–phenolic network foams are prepared using tannin–cellulose nanofibers as a structured background. The method relies on ice‐templating the nanofibers to promote the metal–phenolic networking in situ, during thawing in ethanol solutions of metal nitrates. This leads to strong and homogenous lightweight foams, whose properties are tailorable by adjusting the tannin type and content, as well as the metal ion.