Fabrication of micro-supercapacitors made of graphene-based hybrid films

Abstract

Micro-supercapacitors, which are one type of the miniaturized electrochemical capacitors, are considered as a new class of post micro-power sources due to their superior electrochemical performances such as permanent cycle-life and ultrahigh power density. However, they typically exhibit the low volumetric energy density, and consequently they are still limited for uses in micro-energy storages. For challenging this problem, in the first part of this thesis, it will be focused on fabricating graphene-based hybrid films with various shapes of nanoparticles and nanosheets as electrodes for application in micro-supercapacitors. After that, in the second part, device architectures of micro-supercapacitors will be developed for high performance micro-supercapacitors such as ultrahigh energy and power densities. At last, at the end of each part, combining synthetic graphene-based hybrid film electrodes with developed device architectures will be introduced for achieving high performance micro-supercapacitors. In the chapter 1, spherical $SnO_2$ nanoparticles with a diameter below 5 nm are uniformly decorated on GO sheets via a two-step method such as a chemical redox reaction and post-thermal annealing process, resulting in the homogeneous $SnO_2$/GO hybrids and their mechanically robust films. Combination with a novel device fabrication process of micro-laser patterning which lead to an ultrahigh volumetric capacitance, the hybrid film based micro-supercapacitor exhibits high energy density, remarkable rate capability, and excellent cycle stability. In chapter 2, compact structured hybrid films of $MnO_x$/GO that consisted of the multi-valence $MnO_x$ are successfully created via a two-step process involving the simple chemical redox reaction and post-thermal annealing with the form of multi-layered structure. In the hybrid film, GO sheets can act as either the growth template of the $MnO_x$ nanosheets or the mechanical supporter for the production of compact hybrid film. Due to our synthetic method and device fabrication process, the hybrid film exhibits the high packing density of 2.4 g/$cm^3$, and the hybrid film based micro-supercapacitor reveals the enhanced electrochemical performances such as remarkable ultrahigh volumetric capacitance, and semi-permanent cycle life. In chapter 3, by introducing a GO sheet as a phase stabilizer and a mechanical supporter, $MoS_2$/GO hybrid films consisting of a high concentration of distorted metallic 1T-$MoS_2$ are successfully fabricated via a post-annealing process in air with the form of restacked film. After the annealing process, the $MoS_2$ nanosheets undergo direct binding with the oxygen functional groups of GO, maintaining their highly conducting 1T-phase and leading to distorted 1T-$MoS_2$ or 1T'-$MoS_2$. A hybrid film-based micro-supercapacitor reveals significantly enhanced electrochemical properties, such as an ultrahigh volumetric power density, superior rate capability, and semi-permanent cycle life. In chapter 4, scalable heterostructures of BN/GO hybrid films are produced by a simple solution-based method using an electrostatic interaction assembly of hydroxyl functionalized BN and amine-functionalized graphene. This synthetic method has diverse advantages such as versatility, low cost, and easily applicable to other layered materials. The hybrid films prepared by this fabrication method tend to be stacked alternately and a compact structured freestanding film. Due to the function of BN as a mechanical supporter and spacer of GO sheets, the optimized hybrid film based supercapacitor exhibits enhanced electrochemical performances such as a high volumetric capacitance, superior rate capability, and permanent cycle life. Furthermore, a hybrid film based flexible device reveals superior bending property even after 1000 times bending cycles. In chapter 5, using a micro-laser patterning technique, volumetric capacitance of reduced graphene oxide (RGO) film is optimized and maximized by fully utilizing graphene characteristics such as tunable functionality, chemical stability, and superior electrical performance. Establishing the maximum number of edge planes of graphene formed by micro-laser patterning is a critical strategy for realizing an ultrahigh volumetric energy density, which results from edge planes of graphene storing significantly more charges compared with its basal planes. After that, a KOH electrolyte is facilitated to induce either pseudo-capacitance or electrochemical double-layer capacitance through synergetic collaboration with defect-rich RGO electrodes. Based on electrochemical results, the volumetric stack capacitance is approximately 10 times higher compared with those previously reported micro-supercapacitors. In chapter 6, through the introduction of metal sputtering deposition, the Ni layer of a current collector with a thickness of below 1 μm is created on one side of a partially reduced RGO film as a model system. This technique can create good electrical contact between the RGO film and current collector, which retains its film form with a high packing density (~1.31 g/$cm^3$). From the electrochemical characterizations, the optimized Ni-sputtered RGO film exhibits enhanced electrochemical properties such as superior rate performance and semi-permanent cycle life than neat RGO film.