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Bernd Wicklein1 Judith Arranz1 Ursula Fillat2 Andraz Kocjan3 Manoella S. Cavalcante4 1 Pilar Aranda1 Eduardo Ruiz-Hitzky1

1, Materials Science Institute of Madrid, Madrid, , Spain
2, National Institute for Agricultural and Food Research and Technology (INIA-CIFOR), Madrid, , Spain
3, Jozef Stefan Institute, Ljubljana, , Slovenia
4, Federal University of Pará, Para, , Brazil

Nanofibers of natural origin are versatile building blocks and precursors for diverse hybrid and nanocomposite materials. Their structural and chemical diversity allows for combining with organic or inorganic compounds that convey functional properties such as magnetic, catalytic, optical or electrical properties.
Here, we present recent work on the extraction of cellulose nanofibers (CNF) from olive tree pruning, an important agricultural residue in the Mediterranean region. Such cellulose nanofibers are semi-flexible, 2 nm thick fibrils with extraordinary mechanical and optical properties. We show how such cellulose nanofibers can constitute super-insulating, flame-retardant foams by combination with earth-abundant resources like sepiolite nanoclay. The foams display 50 % lower thermal conductivity values as compared to commercial polymer insulation foams, while showing high flame retardancy without the need for halogenated additives. These results can pave the way for the development of sustainable and eco-friendly thermal insulation materials.
On the other hand, the optical transparency and mechanical strength of cellulose nanofibers is ideal for the design of paper-based, opto-electronic devices. We demonstrate how CNF can substitute indium tin oxide coated glass for the construction of electro-chromic films based on self-assembly with V2O5 nanofibers.
Nanocellulose can not only be isolated from plants but also from gluconacetobacter bacteria as re-growing, extracellular cellulose hydrogel. These hydrogels are easily impregnated with metal salts, which upon precipitation and carbothermal reduction render sponge-like carbon-metal hybrid materials. Following this strategy we prepared carbon-Ni and carbon-Fe hybrids with attractive functional properties.

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