Kuo-Chuan Ho

Spectroelectrochemical studies of manganese phthalocyanine thin films for applications in electrochromic devices

The spectroelectrochemical properties of manganese phthalocyanine (MnPc) thin films are studied. Cyclic voltammetry (CV) and UV-vis in situ spectroelectrochemistry were done to characterize the MnPc thin films. In this study, the insertion of cations during the reduction of MnPc thin films is proposed. Two reversible redox pairs were observed at 0 and -0.9 V (vs. Ag AgCl) as judged by CVs. According to the absorption spectral data, the reaction at 0 V is related to the transformation between [Mn III Pc 2- ] and [Mn II Pc 2- ], and the reaction at -0.9 V is attributed to the transformation between [Mn II Pc 2- ] and [Mn I Pc 2- ]. The polychromic characteristics of MnPc thin films were also observed under different redox potentials and can be expressed as follows: 0.5 (light green) → -0.5 (green) → -0.9 (greenish blue) → -1.0 V (purple). The electrochromism of MnPc thin films for use in electrochromic devices (ECD) is also discussed. A complementary ECD, comprising a MnPc and polyaniline (PAni) thin film couple was constructed. This device was darkened and bleached using -1.0 and +1.0 V, respectively. The color of the device was changed from light green to green and the transmittance of the device varied from 60 to 17% at 550 nm.

Design equations for complementary electrochromic devices: application to the tungsten oxide–Prussian blue system

Abstract Complementary electrochromic devices (ECDs) have advantages in offering deeper switching over singer-layer ECDs. Therefore, it is important to gain an insight into the dominant factors that control the electrochromic performance of complementary ECDs. In this study, the design of complementary ECDs with two reversible redox electrodes operating in series is presented. By combining the electrochemical and optical properties of these two electrodes, design equations suitable for complementary ECDs are derived. As the overall extent of electrochromic reaction involved in an ECD is limited by the electrode with a lower charge capacity, the effect of charge capacity ratio on the optical attenuation range can be predicted. The required parameters in the design equations are evaluated from the fabricated cell. As an example, the tungsten oxide–Prussian blue system is discussed. The empirical prediction of the optical attenuation range is fitted well with the experimental results.

Cycling and at-rest stabilities of a complementary electrochromic device based on tungsten oxide and Prussian blue thin films

Abstract Complementary tungsten oxide (WO 3 )–Prussian blue (PB) electrochromic devices (ECDs), in combination with Li + , K + or H + -based electrolytes, have been proposed by many researchers for solar attenuation and glare reduction applications. In this study, cycling and at-rest stabilities of a solid-state ECD comprised of a WO 3 and PB thin film couple with a proton-conducting solid polymer electrolyte are discussed. The transmittance of the device varied from 71.5% to 6.0% at 550 nm. The device was darkened or bleached by the application of +1.2 V or −0.6 V, respectively. Repeated switching or cycling of the ECD over 20,000 cycles has been demonstrated, indicating a large number of switchings without great degradation or irreversible side reactions. Nevertheless, at-rest stability in the absence of cycling, at room temperature over months, has not been achieved. The cause of at-rest instability during darkened-state or bleached-state storage will be discussed. Issues related to the scale-up are also discussed.

The influence of charge capacity ratio on the performance of a complementary electrochromic system

One of the problems encountered with complementary, solid-state electrochromic devices is the development of discoloration as a result of storage or cycling. The cause of discoloration in the bleached state is the charge imbalance on one of the electrodes. This paper deals with balancing the electrode capacities in a complementary, solid-state electrochromic device so as to optimize its optical attenuation range. Complementary electrochromic system, comprising a tungsten oxide and Prussian blue (PB) thin film couple in combination with a proton-conducting, copolymer electrolyte, is studied. It has been shown experimentally that, for two complementary electrochromic layers each with an individual thickness, the charge capacities of the two electrochromic layers have to be matched in order to achieve the maximum optical attenuation range. The effect of cell size on the switching response of the complementary system is also studied.

Spectroelectrochemical Studies of Indium Hexacyanoferrate Electrodes Prepared by the Sacrificial Anode Method

Indium hexacyanoferrate (InHCF) thin films have been successfully deposited on the SnO 2 -coated surface of conductive float glass by the sacrificial anode (SA) method. It was found that InHCF films prepared in such a way possess relatively smooth surface morphology, presumably influenced by the anodic dissolution current. The films have been characterized and discussed in terms o their electro-optical properties. The compositions of as-deposited InHCF films prepared by the SA method were analyzed by atomic absorption (AA) spectroscopy and can actually be expressed as H 1- x K x InFe(CN) 6 .mH 2 O, with 0 ≤ x ≤ 0.38. Cyclic voltammetry (CV) done in various electrolytes supports the double injection/extraction mechanism during switching. It is concluded that ion permeability in an InHCF film follows the order of Na + > K + > NH 4+ > Li + . A one-electron transfer reaction is proposed or the switching of the InHCF film according to the absorption spectra. The film is shown to be electrochromic, and the redox reaction can be summarized as MInFe II (CN) 6 .mH 2 O ⇇ InFe III (CN) 6 .mH 2 O + M + + e - where M + is the metal cation in supporting electrolytes. The coloration efficiency for the InHCF film switching involving K + , measured at 410 nm, is 47.6 cm 2 /C for coloring and is 47.3 cm 2 /C for bleaching, indicating nearly ideal electrochromic behaviors.

Nonlinear Diffusion Behavior for the Prussian Blue Electrode: I. Variable Diffusivity Revealed by Potentiostatic Intermittent Titration Technique-Chronoabsorptometry

Nonlmear diffusion (NLD) behavior for the Prussian blue (PB) electrode was revealed using an in situ method combining the adxantages of the potentiostatic intermittent titration technique (PITT) and chronoabsorptometry, namely, the PITT-chronoabsorptomeiry method. By applying the PITT-chronoabsorptometry method, the disruptive influence of attack by dissolved O 2 was climinated and the diffusivities were precisely determined. Prior to the diffusivity determination, the charge-transfer plane and diffusion species involved in the PB-Everitts salt (ES) redox process were proposed based on the morphological open structures of PB and the binary-solution model. This model suggests that a partially reduced PB film could be viewed as a binary solid solution of PB and ES. The diffusivity measured for the PB-ES redox system is a function of the equilibrium potential. The potential corresponding to the minimum diffusivity is close to the peak potentials obtained from the cyclie voltammogram. This characteristic is very similar to a common diffusivity-potential dependence found in lithium-inserted electrodes and implies that such NLD behavior could be a common behavior for other insertion compounds. Moreover, this work not only provides the opportunity to furiher understand the PB electrochromic process but also proposes a robust in situ approach for determining diffusivity at other electrochromic electrodes.

Nonlinear Diffusion Behavior for the Prussian Blue Electrode. II. Interpretation of Variable Diffusivity during the Insertion/Extraction Processes

The apparent diffusivity (D 12 ) has been measured at the Prussian blue (PB) electrode. It was shown that D 12 varies with the applied potential (E) and there exists a minimum around the voltammetric peak potential. By applying the binary-solution model, which considers a partially reduced PB film as a binary mixture of PB and Everitts salt (ES), the peak-shaped D 12 -E behavior was successfully explained. According to the model, the concave-up D 12 -E behavior is attributed to a corresponding concave-down excess energy-potential characteristic, which results in the maximum excess free energy and suppresses the random mixing of PB and ES. In addition, it is deduced that the 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. To be sure, the binary-solution model provides a common foundation for other insertion electrodes, as long as their redox behaviors are analogous to that of PB.

Influence of charge capacity ratio on the optical attenuation of a tungsten oxide-polyaniline electrochromic device

Hybrid inorganic/organic complementary electrochromic device (ECD), comprising a tungsten oxide (WO3) and polyaniline (PAni) thin film couple in combination with a proton- conducting polymer electrolyte, is studied. Spectroelectrochemical measurements were done to characterize both the electrochromic thin films and the devices. The safe operating voltage of the device can be determined from these data and ranging from +1.0 V to -0.3 V. The primary motivation of the present investigation is to examine the connection between the optical attenuation range and the charge capacity ratio of WO3 relative to PAni. It was found, experimentally, that the performance of the complementary ECD is limited by the electrode with the least charge capacity. Furthermore, it has been verified experimentally that, for two complementary electrochromic layers each with an individual thickness, the charge capacities of the two electrochromic layers have to be matched in order to achieve the maximum optical attenuation range. That is, the principle in obtaining the maximum optical attenuation in a complementary ECD is to equalize the charge capacity on both electrochromic layers.

Optical Attenuation for Complementary Electrochromic Devices: Application to the Tungsten Oxide-Polyaniline System

The optical attenuation of complementary electrochromic devices (ECDs) with two reversible electrodes operating in tandem is presented. The transmittances of electrochromic thin film electrodes are related to the electrochemical and optical properties of each coated electrochromic (EC) layer. By combining the electrochemical and optical properties of these two electrodes, a general variable transmittance function suitable for describing the switching characteristics of complementary ECDs is developed. To test the validity of the model, hybrid inorganic/organic complementary ECD, comprising a tungsten oxide (WO3) and polyanilinc (PAni) thin film couple in combination with a proton-conducting polymer electrolyte, is studied. Spectroelectrochemical measurements were performed in order to obtain the coloration efficiencies of both WOJ and PAni thin films. The measured optical attenuation for the ECD is in good agreement with the model prediction using the parameters obtained from the individual thin film measurements. Furthermore, it has been verified, both theoretically and experimentally, that the charge capacities of the two EC layers have to be matched in order to achieve the maximum optical attenuation.