Abstract
Electrochromic (EC) materials, which are able to change their optical properties reversibly upon charge insertion/extraction, have received high attention due to their unique characteristics in the past decades. Among inorganic EC materials, nickel oxide is considered to be a good anodic candidate because it has low material cost and an excellent contrast. Usually, nickel oxide is used as a counter-electrode in complementary EC devices assembled with a cathodic EC electrode, such as tungsten oxide. Coloration of inorganic EC devices depends upon ion-intercalation into the transition metal oxide film. Therefore, the switching speeds of coloring/bleaching are limited by ion transporting into a solid metal oxide film. To obtain sufficient optical contrast, large charge (ion/electron) insertion and extraction must extend to the bulk of the EC film. Consequently, the switching time of such devices is typically in the order of tens seconds to minutes, even for small area devices. On the contrary, a fast-switching device may result in insufficient optical contrast because of less ion-intercalation. In this study, transparent ITO nano-particles are sprayed on ITO coated glass substrates to form a porous conducting network. And then nickel oxide films are deposited onto this substrate by two methods. The porous structure provides large active surface area for charge insertion and extraction. This nano-structure leads to large surface area for depositing EC materials. Electrolyte can penetrate through the porous nano-structured film. When an electrochemical redox reaction carried out, ion intercalation and charge compensation could happen rapidly at or near the electrode/electrolyte interface. This dissertation contents two parts: Part I: A nano-structured nickel oxide (NSNO) based EC electrode was successfully produced by a spray pyrolysis technique (SPT) onto the porous ITO nano-particle layer. In this case, the improvements in the switching speed and transmittance contrast were not obvious. It is because the NSNO film prepared by SPT only covers the top surface of the ITO nano-particle layer. With this porous structure, however, the NSNO based EC electrodes showed great improvement in the cycling durability. Part II: In the present work, the nickel oxide film is prepared using an electro-deposition method followed by a thermal oxidation process. It is intended to form a nano-composite nickel oxide (NNO) layer. From the elemental maps and TEM images, we ensure that the nickel oxide completely covers the whole surface of ITO nano-particle layer and forms the core-shell structure with ITO nano-particles instead of just covering top surface of the ITO nano-particle layer. High porosity in the NNO layer offers large active surface area for redox reaction. Electrochromic electrodes fabricated with the NNO layers produce high transmittance variation (66.3% at a wavelength of 552 nm), fast switching speed (coloring: 3.8 sec, bleaching: 3.0 sec) and good durability (over 2000 cycles without degradation), which are much better than those of ones made with the traditional nickel oxide films. These good EC properties promote the potential application of the NNO structure for EC devices.