Preparation And Exploration Of NiO And WO₃ Based Electrochromic Films And Devices

Electrochromic material is a kind of smart material,which can persistently and reversibly change its color under external electrical stimulation, and posses certain optical modulation at the same time. Electrochromic device can also be obtained by assembling electrochromic material, which is also the core layer in the device, corresponding electrolyte and counter electrode together into a sandwich structure. Electrochromic devices have many potential applications in various fields, such as exterior windows for architectures (or so-called smart window), displays and glasses with changeable colors. However, there are still many shortcoming of electrochromic materials, such as the high preparing cost, insufficient optical modulation, low response speed and poor cycling stability, which in turn limit large-area commercialization of electrochromic devices.WO3, which exhibits excellent optical modulation in the near-infrared region, is a kind of promising electrochromic material for potential applications in green energy-saving smart window. In addition, NiO, which has been studied for a long time, has great prospects as a complementary counter electrode for WO3. In this dissertation, the researches were mainly focused on WO3 and NiO-based electrochromic materials prepared by low-cost chemical solution process, and to develop methods, such as ions doping, surface modification and nanostructures, to enhance their electrochromic properties and cycling stability. The influences of species and assembling states of the materials on electrochromic devices were also studied to optimize properties of the devices. And the detailed contents include information as follows:To improve the properties of NiO counter electrode, NiO-based films doped with Li or/and Ti ions were prepared by sol-gel method. Li or/and Ti ions mono-/bi-doping can greatly improve the optical modulation and cycling stability of NiO-based films, especially for Li-Ti-NiO films which exhibit excellent cycling stability for over 13,000 cycles, more than tens times higher than that of pure NiO. Moreover, Li-Ti-NiO films were further modified by three types of surfactants:cathodic ones of SDS, anionic ones of CTAB and non-ionic ones of PEG-1000. The addition of three type of surfactants all accelerated the response speed of Li-Ti-NiO films.In order to enhance the properties of WO3 electrochromic films, we combined nanostructure modification with ions doping by using hydrothermal process. Different WO3 nanostructures, such as nanowires, nanorods/branches and nanoflowers, were obtained by changing the doping content of Ni ions. Vertically aligned one dimensional (1D) WO3 nanorods structure facilitated the ions insertion/extraction process during the electrochromic reactions by providing more free spaces and larger reactive surface areas. The modification can also reduce the internal resistance of WO3-based films, and enhance its optical modulation and cycling stability. The optimized Ni-WO3 nanostructure film obtained large optical modulation in the near-infrared region for about 85%, and good cycling stability for about 5,500 cycles.Electrochromic devices were assembled by using NiO-based film as the counter electrode, and organic polymer of thiophene derivative, amorphous WO3 and crystal Ni-WO3 films as electrochromic layer respectively. The influences of electrochromic material, counter electrode, electrolyte and the assembling state of the counter electrode on the electrochromic properties and lifetime of the device were studied, in order to optimize the assembling parameters and improve the properties of electrochromic device.In this dissertation, we obtained different kinds of electrochromic devices with good optical modulation in the visible, near-infrared and both these regions respectively, through the properties enhancements of WO3 and NiO-based electrochromic materials and the optimization of assembling parameters of the devices. Electrochromic device with rapid color change within 1 s, high optical modulation for about 56% and excellent stability over 1,000,000 cycles were also achieved, which has good practical value. We also explored the preparation of electrochromic materials and devices with larger size, and acquired 10 cm×10 cm sized device with uniform colors. These researches could provide important references for future studies on enhancing the properties and stability of electrochromic materials and devices.