Graphene is one of the most attractive materials because of its outstanding properties. Here we report a straightforward and environmentally friendly process, in which the few-layer graphene is continuously prepared in gas phase by one-step pyrolysis of methane by alternative-current arc plasma under substrate free and atmospheric conditions. The graphene is generated in high purity with the yield of more than 2 g/h. No further operations such as. Graphene is one of the most attractive materials because of its outstanding properties. Here we report a straightforward and environmentally friendly process, in which the few-layer graphene is continuously prepared in gas phase by one-step pyrolysis of methane by alternative-current arc plasma under substrate free and atmospheric conditions. The graphene is generated in high purity with the yield of more than 2 g/h. No further operations such as centrifugation, purification, sonication, and drying are needed. The synthesized graphene powder mainly consists of 1–3 layered flakes. The electrochemical performance of the graphene as the cathode conductive filler for LiFePO4 lithium ion batteries is investigated. The most effective electron transporting network in lithium ion batteries is obtained under 2% of graphene, together with 1% of carbon black. Moreover, the addition of graphene increases the specific capacity of the cathode and shows a good rate performance.••Few-layer graphenePlasma pyrolysisLithium iron phosphateElectrochemical performanceCarbon nanomaterials have a privileged position in academia and industry due to their superior physical and chemical properties. Historically, the synthesis process of carbon black (CB) is closely related to the plasma process. Plasma technology provides a reasonable alternative to flame aerosol technology due to the direct cracking of a hydrocarbon without oxygen by an external supply of electric energy, as the reaction of CnHm→n C + 0.5 m H2,,. There is no polluting emissions such as CO2, CO, SO2, NOx and other advantages like high conversion rate of hydrocarbon, energy effective, and process simplicity. Therefore, the plasma technology has been widely used for applications in different fields,,.The first demonstrated production of CB with plasma process is dated back to 1920 by Rose. In the plasma reaction system, the gaseous hydrocarbon fluid was pyrolyzed by two electrodes in a cylindrical chamber. Since then, much research has been done on the study of producing industrial quantities of CB by plasma technology.In the 1990s, engineering firm Kvaerner developed a direct-current (DC) discharge plasma technology for industrial production of CB and hydrogen. The main technical challenge is to use hydrogen byproducts directly as plasm. 2.1. MaterialsThe Methane (99.99%), hydrogen (99.99%) and argon (99.99%) were purchased from Xinhang Industrial Gases Co. Ltd. (Fuzhou, China). AC high voltage power source was supplied by Jianglin Electric Technology Co. Ltd. (open circuit voltage 20 kV, Jiaxing, China), and graphite ring electrode was purchased from Jiekai Carbon Co., Ltd. (Haimen, China).2.2. Apparatus for continuous preparation of grapheneFig. 1 shows the schematic diagram of the discharge device for continuous preparation of graphene. The discharge device consists of the following four main components:•(1)A gas supply system with respective pressure relief valves and rotameters. Argon as the carrier gas can be easily ignited to form a stable plasma arc, which assist the pyrolysis of methane. The argon, hydrogen and methane flow from respective rotameters which were regulated carefully, mixed together and flowed into the quartz tube.•(2)A high-voltage AC power supply system with maximum power of 10 kW. This system is composed of a voltage regulator, an inverter as well as a high-frequency and high-voltage transformer. High-voltage wire was used to connect with those electrical appli. 3.1. Yields of solid products under different carrier gas flowFig. 2 shows the yield of nanosized carbonaceous materials with different carrier gas flow (the methane flow of 2400 mL/min). When the carrier gas flow is low (8000 mL/min), it is difficult to rotate the arc, resulting in high concentration of plasma and high temperature of the arc. Excessive temperature causes the radicals from the methane pyrolysis to reform gaseous rather than solid products. As the carrier gas flow increases above 12,000 mL/min, the arc begins to rotate, and the methane/argon ratio was more suitable for pyrolysis. The optimal ratio occurs when the argon flow rate is 16,660 mL/min, and the maximum yield can reach 2.1 g/h. But the yield will decrease as the gas velocity increases further, because the high carrier velocity shortens the residence time of methane in the plasma, and the significant decrease in plasma density and temperature leads to the failure of methane decomposition. The experimental results are consistent with previous studies.3.2. Effect of H2/CH4 proportions on graphene morphology and purityAccording to the previous research carbon black can be obtained by plasma pyrolysis of methane, and the graphene can be further prepared by ad.
