Investigation of the Formation of Free-Standing Graphene Nanoflakes from Ethanol in a Pilot-Scale Plasma Reactor

The interest in 2D materials has increased enormously since the discovery of graphene in 2004. The extraordinary electrical, mechanical and optical properties offer a great potential to improve a wide range of applications. However, this requires a deeper fundamental understanding of the formation of graphene and a resulting synthesis pathway that provides the necessary process conditions for efficient graphene formation.<br/>A potential synthesis route for the continuous production of few-layer graphene is the plasma-assisted gas-phase synthesis. In this work, we utilize a pilot-scale microwave plasma reactor with a frequency of 915 MHz and a maximum power of 50 kW [1] to convert ethanol into few-layer graphene to work out the influence of the carbon concentrations on the characteristics of the formed few-layer graphene. For that purpose, ethanol is evaporated and subsequently passed through an argon-hydrogen plasma. To increase the carbon concentration, the feeding rate of EtOH is stepwise adjusted from 200 g/h to 800 g/h. The produced graphene powders are collected on filter membranes and then analyzed ex-situ by different methods.<br/>Imaging measurement methods such as scanning and transmission electron microscopy are used to determine the general shape and morphology of the different graphene samples. At low carbon concentration in the plasma, typical graphene sheets are formed almost exclusively. If the carbon concentration increases due to an increased EtOH feeding rate, there is a shift towards the formation of soot-like particles between the graphene sheets.<br/>Raman spectroscopy shows peak intensity ratios of 0.85 to 0.86 for I_D/I_G ratio and 0.94 to 0.99 for I_2D/I_G ratio for all samples, indicating relatively high sp2 hybridization, low defect density as well as low number of layers. However, the results reveal that Raman spectroscopy is not able to clearly distinguish and display the fraction of soot-like particles seen on SEM and TEM images by different peak intensity ratios.<br/>A better indicator is the electrical conductivity of the material, which is in the end also a crucial factor for potential applications. Therefore, the electrical conductivity and the apparent density of the graphene powders as a function of compression force were investigated in a specially designed powder conductivity test cell by a quasi-four electrode test principle. The measurement of the electrical conductivity shows a similar trend as already suspected from the evaluation of the SEM and TEM images. A decreased carbon concentration leads to a reduced formation of soot particle-like structures and thus also to a higher electrical conductivity of the material. The highest specific conductivity of σ = 3,19 S/cm and ρ = 0.23 g/cm^-3 at 100 N cm^-2 is obtained for graphene produced with the EtOH feeding rate of 200 g/h, which is 8.6 times higher than the commercial graphene CP-0080-HP-0010 (IoLiTec Ionic Liquids Technologies GmbH, thickness 1-10ML; size 0.5-3 µm; 250 €/g, σ = 0.37 S/cm and ρ = 0.12 g/cm^-3 at 100 N cm^-2).<br/>This work demonstrates that low-defect graphene can be produced continuously in a pilot-scale plasma reactor at production rates up to 2.5 g/h. However, it is also shown that the production of graphene from ethanol with conversion rates in the range of 0.6 to 1.2 % based on the carbon mass flow rate is currently still very inefficient and cannot be easily increased by using a pilot-scale reactor. Nevertheless, this synthesis route remains one of the most promising for the production of high-quality few-layer graphene.<br/><br/>Work is performed under the scope of IGF-Project 21784 N funded by the Federal Ministry for Economic Affairs and Climate Action on the basis of a decision by the German Bundestag (IGF-Project 21784 N) and by the German Research Foundation (DFG) (SCHN 1387/1–2, 651051).<br/><br/>[1] F. Kunze et al. / Powder Technology 342 (2019) 880–886