Nanostructured Polymer-Derived Ceramic Aerogels for Environmental and Energy Applications

As technologies grow towards more demanding and complex applications, energy and environmental sectors carry a relevant fraction of the ecological burden for the next generations to come. Renewable energies, efficient industrial processes and clever management of environmental resources must be taken into serious account to reduce as much as possible our footprint. In this scenario, material science aims at finding solution for the most disparate scientific issues, these including the synthesis of multifunctional and performing materials able to fill the gap with innovative and green technologies. Among others, ceramic materials have shown a growing flexibility towards green and functional applications also thanks to the Polymer-Derived Ceramic route (PDC). This pathway to ceramic materials has the advantage of controlling their composition at the molecular level via chemical reactions, thus permitting to obtain complex ceramic systems with particular functionalities after controlled pyrolysis of preceramic polymers. Besides, Polymer-Derived Ceramic routes can be implemented with many processing solutions, such as the outgrowing fields of 3D printing and ceramic matrix composites. Among these, porous and cellular ceramics can be synthesized via PDC routes as well, laying the foundations for new, cheap and functional sorbents and scaffolds. In such a scenario, ceramic aerogels are ultraporous materials possessing high surface areas, low density and pore size distributions that easily reach few nanometers while maintaining rather high porosities, generally above 90%. Hence, when compared with other porous ceramics, aerogels present outperforming microstructural features that make them intriguing for applications in the energy and environmental fields, and when the PDC route is combined with aerogels processing, the outcome is something new. This thesis deals exactly with these two concepts, in the framework of innovative energy storage and environmental pollution mitigation. As a matter of fact, this work offers novel synthesis pathways for PDC ceramic aerogels belonging to the Si-C-N-O system, where their chemistry and microstructure have been tuned to serve for the abovementioned applications. Particular attention has been devoted to Si-N, Si-C-N, Si-C and Si-O-C aerogels, characterizing their thermal evolution when changing polymeric precursors and synthesis parameters. On this point, perhydropolysilazane (PHPS) and Durazane® 1800 were employed as precursors for silicon nitride and carbonitride aerogels, SPR-036 and polymethylhydrosiloxane (PHMS) as precursors for SiOC, and SMP-10 polycarbosilane as starting point for producing SiC aerogels. Our interest in making of these aerogels lies both on the urge of understanding the principles behind their synthesis steps, how each of them has an impact on the final aerogel chemistry, microstructure and thermal stability, and given this, how processing parameters can be exploited for novel applications of such peculiar materials. Overall, this thesis offers a paper collection of these novel aerogels featuring applications such as thermal and thermochemical energy storage, thermal insulation, electrochemistry and polluted water management, where we demonstrate the versatility and the potential of PDC routes towards the synthesis of ultraporous functional ceramics.