| dc.description.abstract | The main objective of this thesis is to investigate electrochromic nanofilms and ionic liquid ion conductors (electrolyte) to make electrochromic devices for electrochromic smart window development. Electrochromic film and ion conductor are two main parts in the structure of an electrochromic device. Tungsten oxide (WO3) nanofilms and ionic liquid ion conductors are prepared and characterized to make electrochromic devices for further electrochromic smart windows applications.
WO3 is one of the most important and popular electrochromic materials for electrochromic smart window applications. Recent academic work has shown that nanostructured electrochromic materials have distinct advantages for high performance electrochromic device applications, including improved cycling stability, high coloration efficiency, and fast switching ability. Firstly, in this thesis, the main work relating to preparation, characterization, and application of WO3 electrochromic thin films for electrochromic smart window glass was described in Papers 1-4. Electrochromic WO3 thin films were prepared using a radio frequency (RF) sputtering method to form a transmittance switchable coating for application in electrochromic smart windows. The thickness of the WO3 film can be controlled and prepared at nanometer scale. The physical properties, including morphology and microstructures, of WO3 film samples were characterized using x-ray diffraction (XRD), scanning electron microscope (SEM), and Fourier transform infrared (FTIR) spectroscopy. The electrochromic properties of WO3 thin films were investigated using cyclic voltammetry (CV) analysis and ultraviolet-visible-near infrared (UV-VIS-NIR) spectroscopy. Experiments were performed with the aim of improving the electrochromic performance of as- prepared WO3 thin films. Effect of film thickness on the electrochromic performance of WO3 films was investigated. Subsequently, effect of the presence of oxygen on the sputtering process of WO3 films was investigated. The effect of heat treatment on WO3 coatings was also investigated.
In addition, Paper 5, Paper 6 and Paper 7 represented fundamental research work on ionic liquids (ILs). The aim of the research was to select and screen suitable ILs as ion conductors (electrolyte) for electrochromic smart window development. ILs are types of salts, which are liquid at temperatures below 100 oC. There are also ILs with liquid state at room temperature, which are called room temperature ILs. They have many advantageous properties for utilization as electrolyte in electric devices and electrochromic devices. ILs utilized in this work are an extremely safe type of electrolyte since they are not water-sensitive or oxygen-sensitive, and when used in an electrical device they are not flammable. Therefore, there is no risk of explosion or fire associated with the use of ionic liquid (IL) based electrolytes. In this thesis, chemical and physical properties, such as density, viscosity, crystallization temperature, thermal stability, ionic conductivity, and electrochemical window of ionic liquid-based electrolytes were investigated for electrochromic smart windows applications.
Paper 1 describes preparation and characterization of WO3 thin films with a thickness of 36 nm. Firstly, WO3 thin films were prepared using radio frequency (RF) sputtering method. Subsequently, electrochromic properties of as-prepared WO3 films were investigated using CV analysis and UV-VIS-NIR spectroscopy. At a scan rate of 20 mV/s, the color of the WO3 films changed at around -0.15 V and bleached at around +0.05 V. The WO3 films could be cycled for 200 times with the potential between -0.30 V and +0.30 V vs. Ag/AgCl reference electrode (3M KCl aqueous solution). The results of CV test and UV-VIS-NIR spectroscopy demonstrated that the transmittance of the WO3 film could be regulated by an adjustable external voltage (electric field).
Paper 2 reported preparation and characterization of WO3 thin films with various thickness, including 36 nm, 72 nm, 108 nm and 180 nm, using sputtering method. Effect of film thickness on the structure and physical properties of WO3 films was investigated. Bleached WO3 films were pale gray, pale blue, lemon green or brown in color. Simultaneously, colored films had a blue color with different transmittance levels. Among the WO3 film samples with various thickness of 36 nm, 72 nm, 108 nm, and 180 nm, the largest transmittance modulation, ∆T550nm, was obtained from samples with a thickness of 108 nm, which was 66% when measured using 0.5 M H2SO4 as ion conductor. This showed that the transmittance value of colored samples decreased with the increasing of film thickness. However, in the bleached samples the transmittance was not influenced significantly by the thickness of the samples. To summarize, the prepared WO3 films with various thicknesses showed various colors.
Paper 3 described preparation and characterization of WO3 thin films with thickness of 72 nm using RF sputtering method. In addition, the effect of oxygen during the sputtering process on WO3 film formation and effect of heat treatment on WO3 film were investigated. As shown in Paper 3, Sample D was prepared as follows. Firstly, a WO3 thin film was coated on the surface of a conductive indium tin oxide (ITO) glass using RF sputtering under O2 and argon atmosphere (volume ratio of O2 and argon is ca. 1:4). Subsequently, the obtained WO3 thin film was treated with heat flow. In the end, Sample D was obtained. The results showed that the coating of the film on the surface of Sample D was a crystalline WO3 film. Sample D had a transmittance value of 50.9%. It also had the best aging ability compared with the other films that were prepared. Sample D had the largest transmittance modulation value among the four samples. The results also showed that electrochromic performance of the WO3 sample films were improved in the presence of O2 during the RF sputtering process. Moreover, heat treatment might result in a transition of crystalline phase from amorphous to monoclinic of the sputtered WO3 thin films. Furthermore, aging durability showed a large improvement after the crystal transition. The crystal transition also resulted in an increased transmittance modulation value of WO3 thin film samples.
Paper 4 presented the research work on electrochromic materials (ECMs) and electrochromic windows (ECWs). In order to show dynamic and flexible solar radiation ability, ECWs can be characterized by several different solar radiation glazing factors as follows: ultraviolet solar transmittance (Tuv), visible solar transmittance (Tvis), solar transmittance (Tsol), solar material protection factor (SMPF), solar skin protection factor (SSPF), external visible solar reflectance (Rvis,ext), internal visible solar reflectance (Rvis,int), solar reflectance (Rsol), solar absorbance (Asol), emissivity (e), solar factor (SF), and color rendering factor (CRF). Comparison of these important solar quantities for various ECM and ECW combinations and configurations enable the selection of the most appropriate ECM/ECW for specific electrochromic smart window and building applications.
Paper 5 presented the investigations of thermal properties of ILs, including thermal conductivity and thermal diffusivity. The thermal properties were crucial and important to utilize ILs as ion conductors in electrochromic smart windows. Thermal conductivity of water and some pure ILs, including BmimBF4, BmimPF6, OmimCl, BmimFeCl4, and OmimFeCl4, was measured. It was found that thermal conductivity measurements of ILs using the hot disk method had high accuracy compared with the thermal conductivity measurement values of water, BmimBF4, and BmimPF6 as reported in the literature. Therefore, the hot disk method can be utilized for thermal conductivity measurement of ILs. In addition, the thermal diffusivity of pure ILs, including BmimBF4, BmimPF6, BmimFeCl4, OmimCl, and OmimFeCl4, was measured. The results showed that ILs resulted in less energy loss than water in energy storage. This also demonstrated that ILs have better performance than water based ion conductors in electrochromic smart windows due to less energy loss and consumption.
Paper 6 investigated density, viscosity, heat capacity, decomposition temperature, ionic conductivity, and electrochemical window of IL based electrolytes (ion conductor). These physical and chemical properties of ILs are important when ILs are applied as electrolytes (ion conductor) in making electrochromic smart window and electric devices. Initially, density of ILs was measured using a density meter. Thereafter, viscosity of various ILs was investigated using a rheological method. In addition, crystalline temperature of various ILs samples was investigated using differential scanning calorimetry (DSC). Moreover, decomposition temperature of ILs was investigated using thermogravimetric analysis (TGA). Furthermore, ionic conductivity and electrochemical window of ILs samples were measured using electrochemical instruments.
Paper 7 presented a pure ionic liquid of 1-butyl-3-methylimidazolium tetrachloroferrate (BmimFeCl4), which was synthesized and utilized as electrolyte (ion conductor) in this work. Chemical structure, physical and thermal stability properties, including density, viscosity, melting point and decomposition temperature, of BmimFeC14 were investigated. In addition, electrochemical properties of the prepared ILs, including electrochemical window and ionic conductivity, were investigated. Moreover, BmimFeCl4 is used as an electrolyte in an electric device, iron-ion battery, to investigate the performance of ILs ion conductor for reversible redox couple transformation in electrochemical reaction. The results showed that ILs have good electrochemical properties and can be further applied as ion conductors in electrochromic smart windows. The reason why BmimFeCl4 was utilized in this paper is that: (1) Easy to synthesize; (2) Not water or oxygen sensitive; (3) Low cost; (4) Non-flammable; (5) Good thermal, physical and chemical properties, including low melting point, high decomposition temperature, and low viscosity. BmimFeCl4 is not a perfect ion conductor for electrochromic devices because of the color of BmimFeC14. However, Due to the good properties of ILs, other transparent ILs with color will be further employed to produce electrochromic glass for electrochromic smart windows applications. | en_US |
