From biomass wastes to vertically aligned graphene nanosheet arrays: A catalyst-free synthetic strategy towards high-quality graphene for electrochemical energy storage

In this contribution, the well-developed, 3D vertically aligned graphene nanosheet arrays (VAGNAs) are prepared via a simple and environmentally friendly hydrothermal carbonization approach and subsequent KOH activation process by using biomass wastes (i.e., spruce bark) as the precursor for the first time. Ascribing to the combined superior inherent properties of graphene and the special structure configuration, the as-obtained 3D VAGNAs present unique features such as 3D interconnected structure, ultrahigh specific surface area (ca. 2385 m(2)/g), large volume of hierarchical pore volume (ca. 1.68 cm(3)/g), and easily accessible open surfaces of the graphene nanosheets. As the electrode material for supercapacitor, the 3D VAGNAs exhibit a high capacitance of 398 F g(-1) at a current density of 0.5 A g(-1) and an outstanding cycling stability (96.3% capacitance retention after 10,000 cycles) in 6.0 M KOH electrolyte. Moreover, in TEABF(4)/AN electrolyte, the VAGNA-900 based symmetric supercapacitor exhibits an outstanding capacitance of 239 F g(-1) at 1 A g(-1), and a high energy density of 74.4 W h kg(-1) could be obtained at power density of 743.7 W kg(-1). Such a remarkable electrochemical performance is highly desirable for supercapacitors to compete with other energy storage devices for real applications.