Electrochromic device

Electrochromic materials are electroactive compounds whose visible spectra depend on the oxidation state. Possible applications are smart windows, displays, mirrors, and so on. Among the most important performance aspects in electrochromic materials, the reversibility and lifetime of the material to repeated cycles, the time of response (usually in order of seconds), the colors of the oxidized/reduced forms and the change in absorbance upon redox switching (contrast) are of interest. 

The first demonstration of a PEM with electrochromic properties was disclosed by SchlenofFand coworkers [66], using poly(butanylviologen)/ PSS films. While this film exhibited strong electrochromic response, it still required the use of an outer electrolyte solution. DeLongchamp and Hammond disclosed for the first time a solid-state device comprised of two electrochromic PEM-modified ITO electrodes separated by a 200-p,m thick poly(2-acrylamido-methane-2-propanesulfonic acid), proton-conducting PAMPS membrane (see Eigure 2.30) [196]. Both PEMs used in 

Electrode surfaces can be modified by redox polyelectrolytes via a sol-gel process, yielding random redox hydrogels or by layer-by-layer self-assembly of different redox and nonredox polyelectrolytes by alternate electrostatic adsorption from solutions containing the polyelectrolytes to produce highly organized redox-active ultrathin multilayers. 

The LbL strategy to chemically modify electrodes with redox polyelectrolyte films has become an important tool for the fabrication of devices and electrodes with important future applications in biosensors, electrochromic devices, electrocatalysts, corrosion-resistant coatings, and so on. 

The unique ability of these LbL redox multilayer systems with control of film thickness on the nanometer scale, the composition and thickness of each alternate layer separately as well as the surface charge by choice of the topmost layer brings about design and tailored properties of devices with important applications. 

Emerging new technologies such as spray deposition of alternate polyelectrolyte layers in LbL multilayers will reduce the deposition time to 1 s per layer, which will open up possibilities of real applications in electrochemical science and technology. 

Electrochromic devices (ECDs) consist of a two-electrode electrochemical cell. They include an ion-conducting liquid or solid electfolyte medium sandwiched between two electrode surfaces coated with organic or inorganic electrochromic materials, chosen for their electrical and optical properties. Their purpose is the generation of a variable-color system that can be changed in a controllable fashion for potential applications as displays, smart windows or in other technologies. 

Transparent conductor Electrochromic layer Electrolyte Ion storage layer Transparent conductor Substrate (plastic or glass) 

Information display 1 f Diffuse scattering White pigment i t 7 Absorption 

Variable reflectance mirror Specular Reflection Mirror Absorption  

Variable emittance surface A Emitting (infrared absorbing) / Non-emitting (infrared reflecting) 

By combining an electrically conductive polymer (e.g. POT) prepared by spin coating from solution with a solid polymer electrolyte and a metal oxide, a sohd state electrochromic device is constructed [713]. Substrates coated with PT can be used in electrochromic displays, in solar cells (cf Sect. 6.3), and for corrosion protection [714]. Poly(3,4-ethylenedioxythiophene-2,5-diyl), which has good electrochromic properties, (for structure cf Sect. 1.2) can used as an electrode in a solid state electrochromic cell (cf Sect. 3.4.3) [43]. PITN can be reversibly cation- and anion-doped without decomposition. This polymer, with 

Chromatic changes caused by electrochemical processes were originally described in the literature in 1876 for the product of the anodic deposition of aniline [271]. However, the electrochromism was defined as an electrochemically induced phenomenon in 1969, when Deb observed its occurrence in films of some transition metal oxides [272]. Electrochromism in polypyrrole was first reported by Diaz et al. in 1981 [273]. Electrochromism is defined as the persistent change of optical properties of a material induced by reversible redox processes. Electronic conducting polymers have been known and studied as electrochromic materials since the initial systematic studies of their electronic properties. 

Optical response of poly(aniline-N-butyl sulfonate) in contact with different polymer/electrolyte systems is reportedly a function of electrolyte composition [278]. 

Reynolds et al. [279] reported electrochromic behavior of self-doped propanesulfonated poly(3,4-propylenedioxypyrrole). The polymer has not only shown interesting electrochromic properties in the visible, but, upon doping, also exhibits a very strong absorption in the near infrared with changes in transmittance up to 97 %, extending the use of the polymer as the active layer in a visible/near infrared switchable device. Viinikanoja etal. [280] reported the electrochromism and pH-induced halochromism of self-doped poly(3-(3 -thienyloxy)propanesulfonate) multilayers. 

An electrothermal imaging device contains an array of pyroelectrical sensor elements supported by a pillar of a semiconductor like PT [160]. 

PT incorporated in a homogeneous dispersion of permanent magnetic particles linked either chemically or electrochemically to the polymer or as a dopant can be used for magnetic printing [161]. 

PoIy(bithiophene) layers are claimed for use as imaging systems for the offset printing process, because this process is based on the different wettability of printing and nonprinting areas. The neutral hydrophobic polymer can be oxidized electrochemically in an electrolyte solution (tetrabutylammonium perchlorate in acetonitrile) to form the oxidized (doped) hydrophilic form of the polymer. This reaction can be conducted reversibly for many thousand cycles changing between hydrophilic and hydrophobic behavior of the polymeric surface [162-166]. 

Typical electro-optical cells consist of (i) an electrically conductive indium tin oxide (ITO) glass as electrode, (ii) a thin film of the electrochromic polymer in the electrolyte (e.g. lithium perchlorate in acetonitrile), and (iii) hold by a spacer a nickel gauze on a back glass plate [179]. 

Poly(isothianaphthene) films on semitransparent gold electrodes can retain about 75% of their initial optical density after 6 x 10 cycles [180]. The conditions of the electrosynthesis are important for the electrochromic properties under many repeated redox cycles [181]. A comparison of different materials brought the result that 3-methylPT is a better material for electrochromic applications than 

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Tetrathiafulvalene (TTE) has also been used in electrochromic devices. TTE-based polymers spin-coated onto transparent electrode surfaces form stable thin films with reproducible electrochromic properties (100). The slow response of these devices has been attributed to the rate of ion movement through the polymer matrix. 

The added electron is delocalized on the monovalent radical ion to which it is reduced (3). There is no general agreement on the molecular representation of the reduced stmcture. Various other viologen compounds have been mentioned (9,12). Even a polymeric electrochromic device (15) has been made, though the penalty for polymerization is a loss in device speed. Methylviologen dichloride [1910-42-5] was dissolved in hydrated... 

Progress in the development of solid electrolytes is also being achieved from advances in several other fields of technology such as fuel and electrolysis cells, thermoelectric converters, electrochromic devices, and sensors for many chemical and physical quantities. 

Conducting polymers have found applications in a wide variety of areas,44 45 and many more have been proposed. From an electrochemical perspective, the most important applications46 appear to be in batteries and supercapacitors 47,48 electroanalysis and sensors49-51 electrocatalysis,12,1, 52 display and electrochromic devices,46 and electromechanical actuators.53... 

At present, intercalation compounds are used widely in various electrochemical devices (batteries, fuel cells, electrochromic devices, etc.). At the same time, many fundamental problems in this field do not yet have an explanation (e.g., the influence of ion solvation, the influence of defects in the host structure and/or in the host stoichiometry on the kinetic and thermodynamic properties of intercalation compounds). Optimization of the host stoichiometry of high-voltage intercalation compounds into oxide host materials is of prime importance for their practical application. Intercalation processes into organic polymer host materials are discussed in Chapter 26. 

Metal Oxides Tungsten trioxide, undoubtedly the most widely studied electrochromic material, is used in several types of commercial electrochromic devices. 

Electrolytes for Electrochromic Devices Liquids are generally used as electrolytes in electrochemical research, but they are not well suited for practical devices (such as electrochromic displays, fuel cells, etc.) because of problems with evaporation and leakage. For this reason, solid electrolytes with single-ion conductivity are commonly used (e.g., Nafion membranes with proton conductivity. In contrast to fuel cells in electrochromic devices, current densities are much lower, so for the latter application, a high conductivity value is not a necessary requirement for the electrolyte. 

Bohnke, O., Applications of proton condnctors in electrochromic devices, in Proton Conductors Solids, Membranes and Gels—Materials and Devices, P. Colomban, Ed., Cambridge University Press, New York, 1992. 

Leventis, N. Electrochromic devices. In McGraw-Hill Encyclopedia of science and Technology, 8th ed. McGraw-Hill New York, 1997 Vol. 6, pp 153-156. 

The discussion of Brouwer diagrams in this and the previous chapter make it clear that nonstoichiometric solids have an ionic and electronic component to the defect structure. In many solids one or the other of these dominates conductivity, so that materials can be loosely classified as insulators and ionic conductors or semiconductors with electronic conductivity. However, from a device point of view, especially for applications in fuel cells, batteries, electrochromic devices, and membranes for gas separation or hydrocarbon oxidation, there is considerable interest in materials in which the ionic and electronic contributions to the total conductivity are roughly equal. 

Monk, P. Mortimer, R. Rosseinsky, D., Electrochromism and Electrochromic Devices. Cambridge University Press 2007. 

Sheng, K. Bai, H. Sun, Y. Li, C. Shi, G., Layer-by-layer assembly of graphene/polyaniline multilayer films and their application for electrochromic devices. Polymer 2011. 

Electronically conducting polymers also have an important role to play as electrodes in electrochromic devices. This is described in Chapter 9. 

Figure 2.30 Scheme of a solid electrochromic device. The device can operate with two electrochromic films as shown or with an electrochromic and an ion-storage film. 

For choosing the right way of preparation of PCM modified electrodes, the decisive question is the intention of preparation. There are two principally different goals (1) The electrodes ought to be applied for electrocatalysis, for electrochromic devices and the like, and (2) modified electrodes are prepared to study the electrochemistry of the compounds. For the first goal, there are two principally different approaches (a) the preparation of films on electrode surfaces, and (b) the incorporation of the PCM into a matrix, for example, a mixture of graphite and a binder, leading to composite electrodes. 

Electrochromic devices The strong differences in the spectra of the PCMs in their various oxidation states, the ease... 

Electrochromic devices work in the opposite sense to an electrochemical cell instead of harnessing the chemical reaction between the electrodes to give an electric current, an electric current is applied to the cell, causing the movement of ions through the electrolyte and creating a coloured compound in one of the electrodes. When an electric current is applied to the following device (Figure 5.23) ... 

FIGURE 5.23 (a) Diagram of an electrochromic device (b) electrochromic office windows. 

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