On the Electrochemistry of Insertion Compounds and Their Applications in Electrochromism

• Main Authors: Chen, Lin-Chi, 陳林祈
• Other Authors: Ho, Kuo-Chuan
• Format: Others
• Language: zh-TW
• Published: 2001
• Online Access: http://ndltd.ncl.edu.tw/handle/10049157071771736542

博士 === 國立臺灣大學 === 化學工程學研究所 === 90 === This dissertation describes the author’s findings concerning the electrochromic (EC) thin films and systems during his PhD work. Besides the introductory (Chapter 1) and concluding (Chapter 6) chapters, four separate chapters (Chapters 2 to 5) are written as the main body of this dissertation. The first two chapters mainly deal with some universal principles and equations, derived by the author, for insertion thin-film electrodes and electrochromic devices (ECDs). The last two chapters address how the author improved a thin-film process and what novel systems he developed. Since all of the EC thin films discussed belong to the insertion compounds, this dissertation is entitled “On the Electrochemistry of Insertion Compounds and Their Applications in Electrochromism.” To present a clear picture of this dissertation, the main contents are summarized as follows. Chapter 2 illustrates the fundamental work for the insertion electrochemistry for Prussian blue (PB). According to morphology observations, the film/substrate interface was regarded as the charge-transfer plane. Then by considering a partially reduced PB film as a binary solution of PB and Everitt’s salt (ES), the kinetic pathway for the insertion processes was sketched. Upon this pathway, mathematical expressions for PITT-chronoabsorptometry, which discriminates the interferences of side reactions, were derived and employed to determine the binary diffusivities (D12) for different equilibrated potentials (E). It was observed that the D12-E curve shows a minimum diffusivity around the voltammetric peak potential and is highly consistent with those observed among lithium-inserted electrodes. To explain this common feature for the insertion electrodes, the binary-solution thermodynamics was adopted. It was demonstrated that the binary-solution model interprets the D12-E curves satisfyingly, and it was thus implied that a coupled movement of K+ and e- into a PB lattice or out of an ES lattice results in a net binary diffusion of PB and ES. Therefore, the insertion electrochemistry can be built on the binary solution model, and quantitative electrochemical analyses for insertion electrodes become more viable. Chapter 3 addresses the design and operating principles for complementary ECDs. From the observations of a complementary tungsten oxide (WO3)-polyaniline (PAni) device, it was revealed that the charge capacity ratio (CCR) of two EC layers strongly affects a device’s performances, including its two-electrode cyclic voltammogram (CV) and transmittance modulation. Through comparing the two-electrode CVs on different CCR conditions with the three-electrode CVs of WO3 and PAni, it was verified that the electrode with a less charge capacity serves as the limiting electrode. It was also observed that a CCR of unity yields the maximum optical attenuation range. By considering the limiting electrode, the design equations, describing relationships between the device’s transmittance and CCR, were derived successfully. The validity of these design equations was demonstrated in a WO3-PB ECD. Moreover, a digital simulation based on the current continuity further confirmed the argument of the limiting electrode and interpreted the two-electrode CVs very well. And the criteria for determining operating voltages were thus given. To be sure, most of the models and principles described in this chapter can be extended to other two-electrode devices. Chapter 4 states the enhanced electrodeposition of the pseudo-transparent indium hexacyanoferrate (InHCF) thin films and a new InHCF-based ECD. At first, it was discovered that an InHCF plating solution could be greatly stabilized through adding a large amount of K+ and/or H+. It was found that a 10-mM plating solution added with 1N HCl and 1N KCl could be stored as a fresh one over a week period, whereas an unmodified plating solution become useless within couples of minutes. Besides, such a cationic addition can greatly increase the charge capacity and cycle ability of InHCF films. That is, the addition of K+ and/or H+ achieves the enhanced electrodeposition for InHCF thin films. Following the enhanced electrodeposition, an ECD based on PB and InHCF, showing a blue-to-yellowish color change, was developed successfully. In this new device, PB and InHCF served as working electrode (WE) and counter electrode (CE), respectively. The KCl-saturated poly(2-acrylamido-2-methylpropane sulfonic acid) (K-PAMPS) electrolyte was made to accommodate the conduction of H+ and K+ both and to serve as the solid polymer electrolyte (SPE). Upon in situ spectroelectrochemical measurements, the device was characterized by a high coloration efficiency of ca. 103 cm2/C at 690 nm and could be operated within a very narrow voltage window (0.9 V « 0.5 V). Besides, precoloration before cell assembly can be eliminated when using InHCF. As judged from the above facts, InHCF thin films have been very promising for electrochromism since the enhanced electrodeposition was attained. Chapter 5 describes the work devoted to the innovations and inventions relating to electrochromism. Two electrochromic batteries (ECBs) and a high-voltage, solid-state TiO2 solar cell were fabricated for low-power applications. By considering the HxWO3/WO3 redox system, which has a much more negative formal potential than the Berlin green (BG)/PB system, a PB-WO3 ECB (PWECB) was made at first. In combination with the K-PAMPS electrolyte, it was found that the PWECB possesses a nominal voltage of ca. 1.35 V, which is even higher than that of a commercial nickel-metal hydride battery. It was demonstrated that such a thin-film ECB could drive a lower-power electronic device for several hours. The most remarkable matter is that the PWECB shows a green-to-blue color change during discharging, so its state of charge (SOC) can be visualized. Besides, when combining an ECB with a photovoltaic (PV) cell, it is possible to realize an all-solar-driven electronic device and may thus reduce the use of primary button cells. Following the PWECB, an InHCF-WO3 ECB (IWECB) was also fabricated successfully, based upon the enhanced InHCF electrodeposition. By using the K-PAMPS electrolyte, it was observed that the IWECB has a nominal voltage of ca. 1.24 V and exhibits a blue-to-colorless change. Hence, it offers a better visualization of SOC than the PWECB. Furthermore, the IWECB with such a high visual contrast offers a dual energy-saving function — to play both roles of a secondary battery and a solar-attenuated window at the same time. In addition to the above-mentioned ECBs, a high-voltage TiO2 solar cell was achieved by choosing InHCF as the solid-state redox species. When illuminated, the solid-state TiO2-InHCF solar cell yielded an output voltage of ca. 1.07 V. Therefore, the TiO2-InHCF solar cell was able to drive a low-power electronic device without a further series arrangement, which is almost impossible for the commercial PV cells. Moreover, the applicability of an insertion electrode for a TiO2 solar cell was demonstrated. It is expected that all of the above innovations may someday promise a better utilization of solar energy. To be certain, this dissertation will not only promote the development of the insertion electrochemistry and ECD designs but also prompt the enhanced thin-film processes and the exploration in electrochromism.