**4. Conclusions**

Hypergolic ignition of furfuryl alcohol by fuming nitric acid at ambient conditions led to the fast, spontaneous, and exothermic formation of carbon nanosheets in two steps: (i) Polymerization of furfuryl alcohol to poly(furfuryl alcohol), and (ii) in-situ carbonization of the polymer by an internal temperature increment near its decomposition point. The structure and morphology of the sheets were unveiled with an arsenal of techniques, such as XRD, infrared spectroscopy (Raman/IR), UV-vis, XPS, and SEM/TEM/AFM microscopies. The released energy was directly converted into useful work by either heating acetone to boiling or spinning the Crookes radiometer. In a wider sense, the furfuryl alcohol-fuming nitric acid system could form the future basis of a more general discussion about "carbon from rocket fuel," especially considering the wealth of rocket bipropellants available today, as well as the continual progress in the field with new hypergolic fuels. Apart from its value as a new synthetic approach towards carbon materials, the present method additionally provides a way to obtain useful materials out of waste or disposed rocket propellants. Technical upgrade of the method, as well as the search of new hypergolic pairs that will provide even higher carbon yields, remains a future challenge towards safe implementation in a large scale.

**Author Contributions:** Conceptualization, experiments, and writing: A.B.B. and D.G.; formal analysis, experiments, and writing: N.C., S.T., A.B., D.M., A.A., and M.A.K. All authors have read and agreed to the published version of the manuscript.

**Funding:** A.B. acknowledges the support from the Czech Science Foundation, project GACR— EXPRO, 19-27454X. S.T. acknowledges the support from the Operational Programme Research, Development and Education – European Regional Development Fund, Project No. CZ.02.1.01/0.0/0.0/ 16\_019/0000754 of the Ministry of Education, Youth and Sports of the Czech Republic. We acknowledge support of this work by the project "National Infrastructure in Nanotechnology, Advanced Materials and Micro-/Nanoelectronics" (MIS-5002772) which was implemented under the action "Reinforcement of the Research and Innovation Infrastructure", funded by the Operational Programme "Competitiveness, Entrepreneurship and Innovation" (NSRF 2014-2020), and co-financed by Greece and the European Union (European Regional Development Fund). N.C gratefully acknowledges the IKY foundation for the financial support. This research was also co-financed by Greece and the European Union (European Social Fund- ESF) through the Operational Programme "Human Resources Development, Education and Lifelong Learning" in the context of the project "Strengthening Human Resources Research Potential via Doctorate Research" (MIS-5000432), implemented by the State Scholarships Foundation (IKY).

**Acknowledgments:** The authors greatly acknowledge Ch. Papachristodoulou for the XRD measurements.

**Conflicts of Interest:** The authors declare no conflict of interest.
