Gas-Phase Synthesis of Graphene Nanoflakes in a Microwave Plasma Reactor on the Pilot Plant Scale

Research on graphene has increased significantly due to the exceptional electrical, mechanical and optical properties of this 2D material. These specific properties of graphene hold great potential in a wide range of applications. However, the successes achieved so far in practical applications have fallen short of expectations. One of the reasons for this is that the exceptional properties of high-quality graphene often cannot be successfully transferred to industrial applications. In addition, the synthesis process is a major challenge. A successful combination of high production rates of graphene with high quality (high purity, reduced number of layers, low concentration of defects and functional groups interrupting π electronic conjunction within the honeycomb structure of graphene) of such a sophisticated material has not yet been achieved. To address this challenge, we performed gas-phase synthesis of graphene on a pilot plant scale.
A 50 kW pilot-scale microwave plasma reactor with a frequency of 915 MHz is employed to convert ethanol into few-layer graphene. Initially, ethanol is evaporated and continuously fed through an argon-hydrogen plasma generated by microwave radiation. The graphene powder is collected on filter membranes and ex-situ analyzed by electron microscopy as well as Raman spectroscopy. Furthermore, the electrical conductivity σ and the apparent density ρ of the graphene powders were investigated as a function of compression force in a specially designed powder conductivity test cell by a quasi-four electrode test principle.
The influence of the precursor feeding rate, applied microwave power and residence time in the plasma on the product properties is investigated. The highest specific conductivity is observed for graphene produced with an ethanol feeding rate of 200 g/h, microwave power of 2.85 kW and a long residence time (σ = 3.19 S/cm; ρ = 0.23 g/cm³ at 100 N/cm²). The conductivity of the synthesized graphene is 8.6 times higher than that of the commercial graphene platelets CP-0080-HP-0010 (IoLiTec Ionic Liquids Technologies GmbH, thickness 1-10ML; size 0.5-3 µm; 270 USD/g), which exhibits a specific conductivity of 0.37 S/cm and the corresponding apparent density of 0.12 g/cm³ at 100 N/cm².
This work demonstrates that the synthesis of graphene, using ethanol as a precursor, has the potential to be scaled up into s continuous graphene synthesis process. The graphene could be employed in the production of functional materials, for instance as catalyst carriers in electrolyzers and fuel cells.