17 th International Conference on Environmental Science and Technology Athens, Greece, 1 to 4 September 2021 CEST2021_00765 CO 2 Utilization for Preparation of Carbon Nanostructures GIANNAKOPOULOU T. * , PLAKANTONAKI N., VAGENAS M., PAPAILIAS I., BOUKOS N., TODOROVA N. and TRAPALIS C. Institute of Nanoscience and Nanotechnology, NCSR “Demokritos”, Patriarhou Grigoriou & Neapoleos, 15341 Attiki, Greece *corresponding author: e-mail: t.giannakopoulou@inn.demikritos.gr Abstract The CO2 conversion to valuable materia ls is a challenging way to manage pollution problems. In the present work, CO2 was used as a feedstock to synthesize solid carbon nanostructures by employing a simple magnesiothermic reduction reaction under constant ga s flow in a tube furnace. It was shown that the CO2 reduction at 675 o C led to the simultaneous formation of different nanocarbon morphologies including gra phene , tubular and spherical carbon nanostructures. The synthesized material was characterized using XRD analysis, Raman spectroscopy, SEM and TEM microscopies which demonstrated its good crystallinity and morphological diversity. Electrochemical tests were performed to evaluate specific capacitance a nd cycling stability of the prepared sample. The calculated values of ~ 325 F/g at scan rate 0.1 V/s revealed that the obtained nanostructures can be used as effective functional material for supercapactior applications. Keywords: CO2 conversion, nanocarbons, metallothermic reduction, Mg 1. Introduction Being one of the major greenhouse gases, carbon dioxide (CO2) is responsible for the global warming due to its constantly increasing emissions from human a ctivities. Many efforts have been made to reduce the CO2 emissions as well as to capture, store or convert the CO2 into value- added materials (Khoo 2015). The CO2 conversion into solid nanocarbons such as graphene, carbon nanotubes (CNTs) or nanofibers (CNFs) is a promising way to address the mentioned issue. Several conversion approaches like plasma methods, electrochemical reduction using molten salts and metallothermic reduction can be referenced (Li (2021), Liu 2020). The metallothermic reduction reactions typically use reactive alkali or alkaline earth metals to reduce CO2 to carbons (Dabrowska 2012). The earliest experiments were performed in closed-type batch reactors with solid (dry ice), supercritical or gaseous CO2 and a few reductants (i.e., Li, K, Na, Ca, etc.). The reactions required high pressures and resulted mainly in graphene-structured carbons. Recently, the controlled synthesis of nanocarbons with specific morphologies including mesoporous graphene, CNTs and hollow carbon nanoboxes was realized under constant CO2 flow at atmospheric pressure through changing the reaction temperature (Zhang 2013). The variation in morphology was associated with different mechanisms of the nanocarbons growth over solid, liquid or gaseous Mg. Recent studies (Xing 2015, Xing 2017, Baik 2020) used mixed reductants and showed that the addition of several metals like Zn, Cu or Ni to Mg influences the porosity, graphitization degree or nanostructure of the prepared graphenic carbons. In the present paper, we investigate the reduction of gaseous CO 2 using Mg reducing agent only and demonstrate the formation of graphene as well as tubular and spherical nanocarbons at 675 o C that is lower than the temperatures applied in the previous studies. Furthermore, the plethora of the obtained carbon nanostructures results in a supercapacitor electrode material with a highly conductive interconnected network that contributes to its superior performance. 2. Experimental 2.1. Nanostructures preparation In a typical experiment, 0.5 g of Mg powder (Alfa Aesar, 99.8%, 325 mesh) was placed in alumina crucible and heated in tube furnace to 675 o C, which is a slightly higher temperature than the melting point of Mg (~ 650 o C). Heating rate of 10 o C/min under Ar flow a t 50 mL/min was applied. After reaching the desired temperature, Ar was switched to CO2 that was flowing at 20 mL/min for 60 min. Then, CO2 was turned off and the sample was lef t to cool down to the room temperature under 50 mL/min Ar flow. After the reaction, the black product was collected and stirred in 3.0 M HCl solution at ~70 o C for 3 h to remove the unreacted Mg and the produced impurities (MgO). Finally, the mixture was washed with deionized water several times until the supernatant exhibits pH ~ 7 and dryed in furnace at 60 o C. 2.2 Structural and morphological characterization The X-ray diffraction (XRD) pattern of the prepared material was taken on a Siemens D500 diffractometer with a CuKα radiation source in a Bragg-Bretano geometry. The scanning velocity was 0.03 o /3s. The Raman spectra were measured using an inVia Renishaw Raman