Doctor of Philosophy
Intelligent Polymer Research Institute
Chartarrayawadee, Widsanusan, Novel graphene composite materials, Doctor of Philosophy thesis, Intelligent Polymer Research Institute, University of Wollongong, 2011. http://ro.uow.edu.au/theses/3508
Graphene is another form of carbon material which has many outstanding properties such as large surface area, high Young’s modulus, high charge carrier mobility, good thermal conductivity and excellent transmittance properties. Graphene can be synthesized by different methods and can be readily prepared in both solid powder and colloidal aqueous dispersion. Furthermore, graphene as a carbon material has potential to form composites with other materials such as metal nanoparticles, conducting polymer and other forms of carbon material such as carbon nanotubes. As part of this research, graphene was prepared from graphene oxide as a colloidal aqueous dispersion; keeping in mind its facile synthesis and versatility of use. Electrophoretic deposition (EPD) was employed through all research work as it is a superior method to produce graphene/graphene composite electrodes from graphene/graphene composite colloidal aqueous dispersions.
The investigation of graphene composite materials began with the combination of graphene with platinum nanoparticles. The preparation of graphene:platinum composites electrodes was on ITO coated glass designed for the eventual applications of electrochemical catalysis in I-/I3- for dye-sensitized solar cells (DSSC) and in sulfuric acid for hydrogen generation. The fabrication of these electrodes were achieved by two methods: (i) Electrophoretic deposition of platinum nanoparticles on to preformed electrophoretically deposited graphene (ITO/G/Pt electrodes), and (ii) electrophoretic deposition from a colloidal dispersion of a nanocomposite of graphene and platinum (ITO/G:Pt electrodes). Fabrication by both methods shows that platinum nanoparticles can be deposited on graphene sheets without the assistance of a linker or any other chemicals. Both types of fabricated electrodes show electrochemical catalysis behaviour in I-/I3- for dye-sensitized solar cells (DSSC) and electrochemical catalysis behaviour in sulfuric acid for hydrogen generation from acid. The electrochemical active surface area (ECSA) of platinum was determined to compare both fabrication methods. The nanocomposite electrodes (ITO/G:Pt) show a superior electrochemical active surface area (ECSA) than the layered electrodes (ITO/G/Pt) due to the smaller size of the platinum nanoparticles that were compositely formed with the graphene.
The second investigation was carried out with graphene and Poly(3,4-ethylenedioxythiophene:polystyrene sulfonic acid (PEDOT:PSS). The aim of this electrode fabrication was to produce transparent graphene electrodes by an electrophoretic deposition on a non-conducting substrate (PVDF membrane) followed by printing of PEDOT layers on the graphene for electrochromics (PVDF/graphene/PEDOT electrodes). For electrochromics, the fabricated PVDF/graphene/PEDOT electrodes were rendered transparent by soaking with ionic liquid. The ionic liquid with smaller ions and lower viscosity was shown to be advantageous for the redox processes of PEDOT. The number of printed PEDOT layers and graphene electrophoretic deposition period were optimized to achieve the best electrochromic performances. The electrochromic performances (contrast and switching time) of PVDF/graphene/PEDOT electrodes were measured and compared with those of printed PEDOT electrodes without preformed graphene layers. PVDF/graphene/PEDOT electrodes show shorter switching times than printed PEDOT electrodes without graphene. This suggests that the graphene layers promoted more efficient PEDOT electrochemical oxidation and reduction resulting in switching times that were faster.
The final investigation was on the combination of graphene with multi-walled carbon nanotubes (MWNTs) to serve as electrochemical supercapacitors material. The preparation of graphene and multi-walled carbon nanotubes composite materials was readily achieved by the sonication of graphene oxide (GO) with MWNTs to obtain graphene oxide and multi-walled carbon nanotubes composites (GO:MWNTs). The fabrication of graphene and multi-walled carbon nanotubes composite electrodes on glassy carbon substrates was achieved by two methods: (i) electrochemical reduction of electrophoretically deposited GO:MWNTs on glassy carbon electrodes, and (ii) electrophoretic deposition of chemically reduced GO:MWNTs composites using L-ascorbic acid (LAA). Composites of graphene with MWNTs show superior electrochemical capacitance than graphene alone; as MWNTs serve as conducting wires which help to promote charge transfer in the system. However, LAA reduced GO and GO:MWNTs electrodes show higher electrochemical capacitance than GO and GO:MWNTs electrodes prepared by electrochemical reduction respectively. This can be explained by the presence of pseudo capacitance from remaining oxygen functional groups left after being reduced by LAA. Furthermore, LAA is also advantageous as a mild chemical reducing agent that can maintain the interaction between GO and MWNTs.
In conclusion, a range of graphene composites with other materials have been successfully investigated and show promise for use in catalytic applications for DSSC and hydrogen generation from acid, in electrochromics and in electrochemical supercapacitors.