Electrochemical study of 3D graphene composites and the creation of ultralight 3D SiC
Thesis or dissertation
University of Exeter
This research fabricated and tested various graphene-related 1D, 2D and 3D materials. We described how using specifically designed graphene foam (GF) as templates can transform its unique structures and excellent properties to new materials. Graphene, GF, Polypyrrole (PPY), Polyaniline (PANI), PPY-GF, PANI-GF, SiC foam, SiC nanowires and SiC nanoflakes will be described in this thesis. The chemical vapour deposition method was used to produce graphene and GFs. PPY-GF, PPY, PANI and PANI-GF were prepared by both chemical and electrochemical (Chronopotentiometry) methods. SiC foams were produced by a low-cost carbothermal reduction of SiO with GF, and then the SiC nanoflakes were separated from SiC nanowires and purified via a multistep sonication process. The synthesised materials were characterised by a variety of techniques such as SEM, EDX, XRD, TEM, Raman, AFM and TGA. The electrochemical properties of the materials were measured in a three electrode cell using cyclic voltammetry (CV), galvanostatic charge-discharge and A.C impedance spectroscopy techniques. The mechanical properties of the GF and SiC foams were investigated by conducting compression tests under in-situ SEM imaging. The as-produced graphene in this research was few layer graphene with layer number varies from 2 to 15. The GFs was found to be extremely light weight with an average density value of 5 mg cm-3. Using GF as electrode materials for supercapacitors, we obtained 100% capacity retention after 10,000 of charge-discharge cycles. The PPY-GF composite electrode exhibited an outstanding specific capacitance of 660 Fg-1, which is superior to the performance of most of the existing PPY-CNT, PPY-graphite and PPY-Graphene electrodes reported to date. In contrast to the PPY which shows a big structure degradation and a 30% capacity loss after only hundreds of CV cycles, the PPY-GF composite showed nearly 100% capacity retention after 6,000 cycles of charge-discharge. Our post-test characterisations showed no structural loss for the GF and PPY-GF. The excellent pseudocapacitive performance of the electrodes was found to be related to three key parameters: the open porosity feature of the GF which provides short pathways for ion diffusion and charge transportation, the dual charge storage mode in the composite, and the excellent mechanical properties of the GF. Due to its high flexibility and void spaces, the GF played successfully the role as a holder and stabilizer for the electroactive materials in protecting them from any structural degradation during the repeated ion intercalation-de-intercalation processes. In the SiC project, we have successfully created extremely light-weighted SiC foams with a density range of 9-20 mg cm-3, with various shapes, by using the GF as templates. These foams are the lightest reported SiC structures, and they consist of hollow trusses made from 2D SiC and long 1D SiC nanowires growing from the trusses, edges and defect sites. The 1D SiC nanowires, being confirmed as 3C-structure, appeared in a variety of shapes and sizes and are highly flexible; the 2D SiC is hexagonal, and upon breakup the resulting 2D nanoflakes have an average size of 2 µm and a thickness value of 2-3 nm which is 5-9 layers of SiC. They, to the best of our knowledge, are probably the thinnest and largest reported SiC flakes. Ultimately, in this research we have successfully produced two extremely lightweight and simultaneously strong foams: the GF and SiC foam. We have explored the GFs by efficiently addressing a key issue in the cycle life of energy storage devices, by creating an ideal architecture of such 3D GF-based electrodes. We have developed a completely novel 3D SiC structure made from continuously linked 2D layered SiC reinforced with 1D SiC nanowires. In-situ compression studies have revealed that both the GF and SiC foams can recover significantly, up to 85% in the case of GF, after compression strain exceeding 70%. The SiC foam did not experience any dramatic failure under the compression loads, as do in conventional ceramics. Compared with most existing lightweight foams of similar density, the present 3D SiC exhibited superior compression strengths and an significantly enhanced strength-to-weight ratio.
University of Exeter
PhD in Engineering