Achieving large scale synthesis of high quality graphene is a critical step to exploit the practical application of graphene in a variety of fields. Researchers at State Key Laboratory of Molecular Engineering of Polymers, Department of Macromolecular Science, Fudan University, China have reported a environmental friendly and ultrafast catalytic microwave method in which a small amount of graphite flake served as the catalyst to promote microwave exfoliation and reduction of graphene oxide (GO).
Microwave irradiation of GO powder leads to the formation of microwave exfoliated graphene oxide (MEGO), which usually occurs in ~15 min. This reaction is triggered in presence of a small amount of graphite powder (< 1 mg) and the exfoliation process is completed within 5 s with the formation of a large volume of catalytic microwave exfoliated graphite oxide (CMEGO) (Fig. 1). The graphite flakes with highly extended π-system efficiently absorb the microwave as a susceptor and convert the energy to activate the nearby gas molecules while the microwave plasma generates a local ultrahigh energy environment.
Fig. 1 Schematic of the synthesis of CMEGO
The exfoliation and reduction of GO in the ultrahigh energy environment created by catalytic microwave plasma leads to a more complete removal of oxygen functional groups and yields CMEGO with a lower lattice defects, higher specific surface area (886 m2/g), large C/O ratio (19.4), good electrical conductivity (53180 S/m) as well as excellent solvent dispersability and processability. The morphology of CMEGO indicates that it is thoroughly exfoliated as evidenced by the formation of smooth, thinner and transparent graphene sheets with a weak lattice distortion (Figs. 2(a) and 2(b)). HR-TEM images show that the CMEGO has a more regular lattice and fewer graphene layers (Figs. 2(c) and 2(d)).
Fig. 2 (a, b) SEM; and (c, d) HR-TEM images of CMEGO
Use of CMEGO as an anode material in lithium-ion batteries has enabled very high reversible capacities of 2260 mAh/g and 469 mAh/g at a charge/discharge rate of 0.1 A/g and 30 A/g, respectively, and an outstanding capacity retention of 91.4% after 1000 cycles at 5.0 A/g. Similarly, use of CMEGO as an anode material in sodium-ion batteries offered very high reversible capacities of 424 mAh/g and 218 mAh/g at 0.1 A/g and 30 A/g, respectively, and a stable capacity retention of 85.7% after 1000 cycles at 5.0 A/g.
The catalytic microwave irradiation strategy employed for the large-scale synthesis of high quality graphene is promising for applications in energy storage and conversion.
T.S.N. Sankara Narayanan
For more information, the reader may kindly refer: Runze Liu, A Catalytic Microwave Process for Superfast Preparation of High-Quality Reduced Graphene Oxide, Angew. Chem. Int. Ed. 10.1002/anie.201708714
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