Synthesis of nanomaterials and design of nanostructures for energy storage application
Dam, Duc Tai
Date of Issue2014
School of Chemical and Biomedical Engineering
This thesis aims to develop nanostructures/nanomaterials by various approaches and investigate their potential applications in the field of energy storage. Synthesis of porous nanomaterials with large active surface area and suitable pore size distribution has drawn considerable attention from scientific society owing to theirs great potential application and the fields of catalysis, biosensor, energy storage and conversion. The thesis mainly focuses on energy storage system (i.e. supercapacitor) and comprises of 3 major parts. The first part (chapter 1) provides background to common energy storage system of supercapacitor. In this part, the author would also like to elaborate on different type of active materials used in fabrication of supercapacitor electrode. Transition metal oxides/hydroxides such as Co(OH)2, NiO and MnO2 has captured considerable attention from scientists and has been investigated in many studies. This first part of thesis also highlights the major problems associated with this particular system. Those limitations include narrow operating potential window, inadequate cycle life and high electrode resistance. More important, some potential approaches to overcome addressed issues are also proposed. The second part (chapters 2, 3 and 4) of this thesis focuses on the synthesis of composite materials of ITO nanowire and different mesoporous metal oxides/hydroxides such as Co(OH)2, NiO and MnO2. Beside kinetics of oxidation/reduction reactions, electron transport and electrolyte diffusion are two key processes which are crucial in performance of supercapacitor electrode. Therefore, as an attempt to enhance above processes, ITO nanowire array is incorporated into electrode design. ITO nanowire is chosen as structure-directing agent due to its high conductivity which would address issue of high electrode resistance. On top of that, the introduction of ITO microarray enhances stability of composite electrode, resulting in larger operating window and prolonged cycle life. Two main techniques are utilized for preparation of those composite nanostructures. Conventional chemical vapor deposition (CVD) is carried out to grow ITO nanowire array on titanium substrate and potentiostatic electrodeposition is performed to coat mesoporous metal oxides/hydroxides along ITO nanowires. Another highlight in coating process is the advantage of using hexagonal lyotropic liquid crystalline (LLC) phase as soft template to generate mesoporous structures of different morphologies. This approach results in enlarged surface area and enhanced specific capacitance. It is worth noting that even electrodeposition technique is employ to coat all three active materials along ITO nanowire, three different morphologies are achieved. Field emission scanning electron microscopy (FESEM) allows us to confirm the morphologies of Co(OH)2, NiO and MnO2 are nanosheets, nanoflakes and nanofiber, respectively. It is experimentally and scientifically proven that open spaces between ITO nanowires allow better diffusion of electrolyte species into nanosheet network, resulting in lower electrode internal resistance, more effective electrolyte diffusion and enhanced supercapacitor performance. Consequently, Co(OH)2/ITO, NiO/ITO and MnO2/ITO systems exhibit superior electrochemical performances as compared to Co(OH)2, NiO and MnO2 thin films, respectively. The specific capacitance values of α-Co(OH)2/ITO, NiO/ITO, MnO2/ITO electrodes are calculated to be 1128, 1025 and 821 F g-1, respectively. In a nutshell, his electrode engineering technique can be applied to other transition metal oxides/hydroxides to construct hierarchical structures for a large variety of applications. The third part of this thesis (chapter 5) is designed to open up new synthetic strategy so as to further improve electrochemical behavior of existing heterostructure. In near future, we intend to develop systematic and rational techniques to fabricate stable three-dimensional (3D) microstructures of nano-building blocks. Those are highly desirable to achieve enhanced electron transport, improved ionic diffusion, excellent rate capability and great stability.