Microfluidic generation of graphene beads for supercapacitor electrode materials

Three-dimensional (3D) solid or hollow graphene beads (GBs) with an obvious crumpled surface are fabricated by using a microfluidic emulsification device and with a post-treatment process. In the emulsification process, graphene oxide (GO)/polyvinylpyrrolidone (PVP) solution is cut off by flowing dimethyl silicone oil in a T-shape microfluidic channel and forms composite droplets with a uniform size. PVP is used as the binder for GO sheets during solidification and annealing procedures. The morphologies and structures of the solidified GBs are optimized by tailoring the concentration of GO/PVP solution, the size of droplets and the way of solidification. The as-prepared GBs are employed as electrode materials for supercapacitor application in this work. As three-step pre-oxidation and calcination processes are carried out in the post-treatment process, the ring structure of PVP is retained and this leads to the coexistence of electric double-layer capacitance (EDLC) and pseudocapacitance properties. The results show that the GBs have a specific capacitance of 243 F g(-1) at a current density of 1 A g(-1), and retain 97.5% of the initial capacitance after 3000 cycles in 6 M KOH aqueous solution. Thus, the 3D crumpled GBs present a great specific capacitance value, reduce the internal electrical resistance and enhance the durability of the electrode.