| Graphene is considered as an ideal electrode material candidate for supercapacitors due to its theoretical specific surface area up to 2630 m2 g-1 and unique structure composed of individual carbon-atom layer. However, macroscopic graphene materials with a disorderedly overlapping structure constructed between graphene layers during the practical production process display a low apperant density value, which leads to a severely limited volumetric energy density when they are applied as supercapacitor electrode materials. In addition, the re-stacking phenomenon of graphene layers will also decrease the specific surface area. Therefore, the structrue design, interface assembly and pore structure control of graphene materials are the key factors to obtain electrode materials with high energy densities. In this dissertation, several graphene materials with high volumetric energy density were prepared through the interaction between the oxygen-containing functional groups on graphene oxide surface and electrolyte ions, and the physical and the electrochemical properties of these materials were characterized in detail.Activated graphene materials were produced by KOH activation of the graphene oxide materials derived from artificial graphite and natural graphite, respectively. The results showed that the crystallinity of graphite material had a significant impact on the activation process. The specific surface area(SSA) values of the activated graphene materials prepared from artificial graphite and natural graphite under the mass ratio of GO:KOH= 1:5 were 2193 and 1265 m2 g-1, respectively. Nevertheless, due to the high usage amount of KOH, the activated graphene exhibited a very low yield and low packing density. Through the electrostatic attraction between oxygen-containing groups located on the graphene oxide sheets and sodium ions contained in sodium alginate, graphene oxide sheets and alginate were interconnected closely, which were further transformed into regularly-constructed graphene/AC composites after carbonization and subsequent activation process. The experimental results reveal that the optimized composite is a nanosheet-shaped carbon material with highly porous AC particles compactly and uniformly decorated on few-layered graphene conductive scaffolds. It is this unique microstructure that endows the composite with an enhanced electronic conduction network, a hierarchical porosity and a S SA value as large as 2979 m2 g-1. The specific capacitance of the three-dimensionally porous graphene-based composite was 175 F g-1 in the ionic liquid of EMIM TFSI, corresponding to a energy density of 74.4 Wh kg-1. The volumetric energy density of this material in the ionic liquid reached 30.5 Wh L-1, which was higher than that of the activated graphene reported in literature(23 Wh L-1).In order to further improve the density of graphene-based materials, KOH solution was used as the reduction medium to reduce graphene oxide. Through the electrostatic interaction between K+ ions and oxygen-containing groups on graphene oxide layers, a highly regular and compact graphene assembly with a face-to-face laminar structure is prepared. At the same time, the irreversible aggregation of graphene layers was prohibited during the reduction process due to the seperation of K+ ions, thus a great number of ultra-micropores with width less than 0.4 nm between graphene layers formed, which realized a compression density as high as 1.58 g cm-3 and also provided high double-layer capacitance and pseudo-capacitance. Accordingly, a volumetric capacitance as high as 508 F cm-3 is achieved for the produced graphene material in the aqueous electrolyte. And the volumetric energy density is up to 30 Wh L-1. In addition, sodium sulphate solution was adopted to simply pre-treat graphene oxide, which realized the pre-intercalation of Na+ ions between graphene layers and decreasing the interlayer gas expansion pressure during the thermal reduction process. The as-prepared thermally expanded graphene materials had a higher micropore proportion compared to the product without pre-treatment, which exhibited an enhanced density. And the volumetric energy density was up to 26 Wh L-1.Self-supporting graphene film can avoid the usage of conductive additives and binders, further improving the volumetric energy density of supercapacitors. The concentrated graphene oxide suspension was tape-casted, producing graphene oxide film with good mechanical strength. However, the film cannot maintain its integrity when being hydrothermally reduced in distilled water. While the addition of KOH, Na2SO4 or H2SO4 into the hydrothermal bath led to the formation of self-supporting graphene films with integrity and flexibility. The intrinsic mechanism of this phenomenon might be the balance between the π-π attraction and electrostatic interaction among graphene layers. Furthermore, the H2O2 was used to treat graphene oxide, producing in-plane holey graphene oxide film, which was transformed into a self-supporting graphene fillm with a density of 1.14 g cm-3 after hydrothermal reduction in a H2SO4 solution. The holey graphene film displayed high specific capacitances of 179 F g-1 and 204 F cm-3 at a current density of 0.1 A g-1, as well as excellent rate capability. |
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