Fabrication Of Graphene Composites For Capacitive Deionization Performance

Environmental problems, especially water crisis, have been one of the biggest and most alarming problems because of the rapid development of modern industry. Delivering safe potable water to human has been an impending challenge. However, most of the water on the earth is seawater or brackish water. Therefore, desalination of seawater and brackish water is a desirable technique to address actual water scarcity, but it is not widely used yet because of the high-power consumption required to make water drinkable. Capacitive deionization(CDI), an electrosorption process that operates by adsorbing ions in the electrical double layers(EDLs) formed at the oppositely charged electrodes under an electric field, has been recently receiving great interests in the adsorption processes due to its low energy consumption, non-polluting and environmental friendliness. However, the major issue with the application of CDI to municipal wastewater treatment lies in the scalability and affordability of CDI materials and systems. Recently, graphene and its derivatives have emerged as a key material for designing experimental water treatment strategies owing to high specific surface area and excellent conductivity. This thesis focus on the fabrication of graphene composites as high-performance electrodes for CDI. Various characterization methods were adopted to investigate their morphology and chemical structure. As working electrodes, the as-prepared graphene composites showed enhanced specific capacitance in electrochemical performance and outstanding specific electrosorptive capacity in their CDI performance. The main contents and results in this thesis are listed as follows:1. Graphene-chitosan-Mn3O4(Gr-Cs-Mn3O4) composites are prepared by a green method, where Gr-Cs hydrogels are firstly prepared from the self-assembly of chitosan with graphene oxide(GO) nanosheets, then Gr-Cs-Mn3O4 composites are obtained by oxidizing Mn(II) ions which are adsorbed by Gr-Cs hydrogels. The synergistic effect of the high capacitance of Mn3O4 coating and the porosity left by chitosan offers Gr-Cs-Mn3O4 hybrids a superior electrochemical performance with good cycling stability and low inner resistance. The resultant Gr-Cs-Mn3O4 composites exhibit a hierarchical porous structure with a specific surface area of 240 m2/g, excellent specific capacity of 190 F/g and outstanding specific electrosorptive capacity of 12.7 mg/g, much higher than those of pristine reduced graphene oxide electrodes.2. Three-dimensional Mn-doped reduced graphene oxide-polypyrrole(RGO-PPy-Mn) composites are facilely prepared by a two-step hydrothermal process. Pyrrole(Py) monomers are polymerized to PPy by KMnO4 in the presence of graphene oxide(GO), then MnO2 is obtained simultaneously after redox reaction between GO and KMnO4. Enhanced electrochemical capacity endow RGO-PPy-Mn composite electrodes with outstanding specific electrosorptive capacity of 18.4 mg/g. The key to the high performance of this hybrid is the addition of KMnO4:(1) The conductivity of RGO-PPy-Mn increased due to the the high conductivity of PPy and the bridge function of RGO for the electronic hopping between PPy chains.(2) MnO2 nanoparticles had the potential to enhance the energy density through optimizing the ionic diffusion in graphene sheets, which can enhance the overall capacitance of the RGO-PPy-Mn composites.(3) PPy served as nanostructured scaffolds for electron transporting between MnO2 and graphene, which provided an efficient strategy for overcoming the low conductivity of MnO2.3. Three-dimensional reduced graphene oxide-melamine formaldehyde composites(3D RGO-MF) are prepared by carbonization of GO-MF composites which are prepared through the electrostatic attraction between GO and MF nanoparticles. The obtained composites exhibit a hierarchical porous structure with a specific surface area of 352 m2/g and abundant nitrogen doping of 10.86%. Thus, they have an enhanced specific capacity of 76.8 F/g, which is much larger than that of pristine RGO electrode(23 F/g). The excellent electrochemical capacity with low inner resistance endows the 3D RGO-MF electrodes with outstanding specific electrosorptive capacity of 21.93 mg/g.4. Mesoporous graphene electrodes are facilely fabricated via a simple thermal treatment of graphene oxide as well as site-localized etching of Fe3O4 nanoparticles(E-Gr-Fe3O4). The satisfied electrochemical stability should be attributed to larger surfaces and more active edges sites of pore-hierarchical structure provided by E-Gr-Fe3O4 network. Therefore, the E-Gr-Fe3O4 electrode with a better cyclability and stability would greatly facilitate the electrode-electrolyte interaction, and hence have a better CDI performance. The classical Langmuir and Freundlich models are used to fit the curve of E-Gr-Fe3O4 electrosorption isotherm, and the results show that Freundlich isotherm can better fit the electrosorption curves E-Gr-Fe3O4 based CDI system.5. A novel nitrogen-doped reduced graphene sponge composite(NRGS) was fabricated based on a melamine sponge-template to prevent the restack of graphene sheets. The anion-exchange polymer layered graphene composites(A-NRGS) were fabricated by coating the surface of NRGS electrode with cross-linked quaternized poly(vinyl alcohol)(C-qPVA). Additionally, the minimized co-ions effect and enhanced wettability of A-NRGS composites favors the diffusion of the electrolyte from the electrolyte to electrode. Therefore, A-NRGS composites have excellent electrochemical capacity(100 F/g) and membrane capacitive deionization(CDI) performance(12 mg/g), much higher than both of RGO(50 F/g, 6 mg/g) and NRGS electrodes without anion-exchange membrane.