Study On The Synthesis And Application Of Ultralight Multifunctional Graphene Composite Sponges

Graphene sponge, a three-dimensional（3D） graphene-based structure, exhibit light-weight, high porosity, excellent mechanical strength and flexibility, and have attracted tremendous interest in recent years. Extensive study in graphene sponge has demonstrated potential applications in energy, environmental, electronics and many other areas. Moreover, it is an effective method to put the excellent properties of graphene nanosheets into practice by assembling them into graphene sponges because 3D structures are easier to manipulate. This dissertation focuses on fabrication of graphene sponges with unique structures via different synthesis methods and the effect of interior structures of the as-prepared sponges on their properties. Graphene sponges with various structures are synthesized by introducing different functional materials into their 3D framework, and these functional materials combine with the structures enable sponges to have new applications and enhanced performance. They can not only used as supercapacitor electrodes, but also can be applied to other areas, which makes them multifunctional materials.Graphene nanoribbons are studied due to their similar structures and properties compared with graphene sheets. Here, we show that these two materials can be combined to form highly porous, ultra-low density, compressible yet elastic aerogels by a simple freeze-drying method. The as-prepared sponge can be used as efficient adsorbents and supercapacitor electrodes. The pore walls consist of stacked graphene sheets embedded with uniformly distributed thick nanoribbons unzipped from multi-walled carbon nanotubes as effective reinforcing skeletons. Owing to the large pore-size, robust and stable structure, and the nanoribbon-adhered pore walls, these hybrid aerogels show a specific capacitance of 256 F/g, which is further improved to 537 F/g by depositing controlled loading pseudo-polymers polypyrrole into the aerogels and very large adsorption capacity for a series of organic solvents and oils（100 to 350 times of aerogel weight）.Based on the above results, carbonized fibers are used as functional fillers for preparation of sponge with higher mechanical strength. Here, wasted cigarette filters composed of aligned cellulose fibers are used as the 3D template and graphene aerogels constructed by interconnected graphene nanosheets coated-carbon fibers are fabricated via a simple dip-coating method. The composite aerogels are ultralight（ρ= 7.6 mg cm-3） yet have high mechanical strength（0.07 MPa）; when used as electromagnetic wave absorber they showed a minimum reflection loss value of-30.53 d B and the bandwith of reflection loss less than^-10 d B is 4.1 GHz. Furthermore, coating polypyrrole into the composite aerogels can increase the minimum reflection loss value to-45.12 d B and mechanical strength can be improved to 0.09 MPa.Functional aerogels containing a significant amount of nitrogen-doped graphene（N-graphene） sheets grafted to carbonized cellulous fibers are also synthesized by other method. Urea was introduced into raw cotton as a molecular template as well as a nitrogen source to synthesize mushroom-like N-graphene sheets strongly attached to cotton skeletons. Synergistic effects stemming from the integration of N-graphene and carbonized cotton skeletons promise potential applications as conductive electrodes for supercapacitors, with a measured specific capacitance of 107.5 F/g in a two-electrode system. The unique structure of our sponge allows it with high porosity and flexibility. The sponge can recover to its original height after being compressed to strains up to 60%.Besides carbon materials, we also proposed a simple method for the preparation of novel structrual polypyrrole enhanced graphene sponge. Nickel foam is used as the template and polypyrrole（PPy） is electrodeposited on it subsequently. Then, a freestanding and flexible 3D polypyrrole skeleton can be obtained after removal of the template. The 3D PPy can not only used as the pressure sensor or supercapacitor electrode, but also the template for the fabrication of graphene composite sponge. Novel structural 3D PPy@G sponge is fabricated by dip-coating method, and the tensile stress of this sponge is 200 k Pa indicating good candidate for high sensitivity pressure sensor electrode. Moreover, our 3D PPy@G sponge is an ideal supercapacitor electrode, the specific capacitance is as high as 702.9 F g^-1 when tested in a three-electrode electrochemical configuration, and it still owns a high specific capacitance up to 357.5 F g^-1 when tested as supercapacitor. The specific capacitance retention of our 3D PPy@G supercapacitor is 82% even after 5000 charging-discharging cycles.