Flexible fire-safe hybrid organic–inorganic cellulose aerogels from sol–gel casting†

Abstract

The flexibility of hybrid silicon-oxide cellulose aerogels was achieved through the formation of thin, uniform silica coatings on cellulose fibres, or local regions of a classical spherical aerogel (Kistler aerogel) combined with areas of less coated cellulose fibres, making use of the flexible properties of the cellulose nanofibres. Furthermore, the inclusion of cellulose during the sol–gel formation allowed the use of traditional freeze-drying instead of CO₂ critical point drying as a method for the removal of the liquid phase. The silicon oxide morphologies revealed the possibility of fine-tuning the coating's structure by the choice of the silicon-oxide precursors. Using methyltrimethoxysilane (MTMS) resulted in the formation of classical aerogel or spherical particles, while the use of tetraethyl orthosilicate (TEOS) yielded “pearl-necklace” fibres, and the mix of (3-aminopropyl)triethoxysilane (APTES) with MTMS yielded smooth uniform coatings. The prepared coating morphologies markedly influenced the aerogel's properties (mechanical stiffness/flexibility, flame resistance and hydrophilicity). The silica coatings endured high-temperature exposure and the thermal removal of the cellulose template without substantial morphological changes was confirmed, showing the possibility to use cellulose as an effective template for the synthesis of silicon-oxide nanofibres. The possibility to selectively control aerogel properties already at the synthesis stage, using abundant and renewable materials together with the possibility of using more energy-conservative freeze-drying (rather than critical point drying), is a promising method for more sustainable aerogel preparation towards high-end commercial applications such as electrical fuel cell insulation.