Electrochromic devices polymer electrolytes

In addition to possible roles as the active electrochromic material, it should be recognized that polymers may also be used in electrochromic devices as electrolyte solution thickeners, as plasticizers, as the matrix or solid phase of gels, as solid polymer electrolytes, and even as sealants to insulate a glass imit configuration... 

Figure 8.1 Beer s law-type plot of change in optical absorbance against charge density q for the cell WO3 polymer electrolyte Prussian Blue. Reprinted from Inaba, H Iwaka, M., Nakase, K., Yasukawa, H., Seo, I. and Oyama, N., Electrochromic display device of tungsten trioxide and Prussian Blue films using polymer gel electrolyte of methacrylate , Electrochim. Acta, 40, 227-232 (1995), Copyright 1995, with permission from Elsevier Science.

A general schematic for a smart window is shown in Figure 13.10. This device is, quite literally, two chemically modified electrodes sandwiched together. In this case, the films coating the electrode surfaces are electrochromic materials. A polymer electrolyte, analogous to that used in the fuel cell discussed earlier, is sandwiched between these two electrochromic material-coated electrodes. In a recent example of this concept by Habib and Maheswari of General Motors Research Laboratories [94], the cathodic electrochromic material was a tungsten oxide and the cathodic electrochromic material was the material Prussian blue, discussed in Section II of this chapter. It seems likely that electrochromic cells will soon find their way into the commercial marketplace. 

The potential benefits of using ionic liquids as electrolytes in conducting polymer devices have been investigated by a number of authors in recent years, for applications such as actuators [8-17], supercapacitors [18-20], electrochromic devices [12, 21] and solar cells [22], with significant improvements in lifetimes and device performance reported. 

The electrolyte is probably the layer in which the maximum number of difficulties can occur. The use of a solid electrolyte, either a protonic conductor, or a lithium electrolyte is compulsory for practical applications. It can be an oxide [3], such as tantalum pentoxide, or a polymer [4]. The research field about the solid electrolytes is in fast expansion, with probable repercussions on the future of electrochromic devices. 

The principle of electrochromic devices can be exploited in tinting ordinary window glass. Very thin polymer layers embedded in a colorless Solid electrolyte and sandwiched between two layers of glass may tint a window when an electric potential is applied. The d ee of tinting can be controlled by the size of the electric potentiaL... 

Nguyen, C.A., Argun, A.A., Hammond, P.T., Lu, X., Lee, P.S., 2011. Layer-by-layer assembled solid polymer electrolyte for electrochromic devices. Chem. Mater. 23,2142-2149. 

Randin, J.P., 1982. Ion-containing polymers as semisolid electrolytes in WO 3-based electrochromic devices. J. Electrochem. Soc. 129,1215-1220. 

Flexible electrochromic devices (ECDs) are becoming increasing important for their promising applications in many areas, such as the portable and wearable electronic devices, including smart windows, functional supercapacitors, and flexible displays. Typically, an ECD consists of four parts of substrate, conductive electrode, electrochromic material, and electrolyte. Enormous efforts have been made to improve the flexibility of ECDs including utilizing flexible polymer substrates and conductive materials. 

Among the conjugated polymers, polypyrrole (PPy) is the most representative one for its easy polymerization and wide application in gas sensors, electrochromic devices and batteries. Polypyrrole can be produced in the form of powders, coatings, or films. It is intrinsically conductive, stable and can be quite easily produced also continuously. The preparation of polypyrrole by oxidation of pyrrole dates back to 1888 and by electrochemical polymerization to 1957. However, this organic p>-system attracted general interest and was foimd to be electrically conductive in 1963. Polypyrrole has a high mechanical and chemical stability and can be produced continuously as flexible film (thickness 80 mm trade name Lutamer, BASF) by electrochemical techniques. Conductive polypyrrole films are obtained directly by anodic polymerization of pyrrole in aqueous or organic electrolytes. 

Rawlicka, A., D.C. Dragunski, K.V. Guimaraes, and C.O. Avellaneda. 2004. Electrochromic devices with solid electrolytes based on natural polymers. Mol Cryst Liq Crysf 416 105-112. 

Polymer electrolytes are also sought for a variety of other applications such as sensors, electrochromic devices and photoelectrochemical cells. The ambient temperature operation of many of these requires conductivities of the same magnitude as for batteries. The need for high electrolytic conductivity stems from the fact that the rate at which the solid-state devices can be operated, for example, how fast energy from a Li battery can be drained or the colour of an electrochromic window can be switched, depends to a large extent on the mobility of ionic charge carriers, hence... 

Polymer-based, variable light transmission electrochromic devices have been fabricated with counter-electrodes that are either optically passive or electrochromic in the complementary mode. Two devices with pMeT, electrochemically grown by monomer oxidation on ITO-coated glass as the electrochromic electrode, having colourless counter-electrodes in both oxidized and reduced states, and operating in liquid electrolyte, such as PC-LiC104, have been developed [29, 30]. 

An all solid-state device configuration has a great advantage over a liquid one and is suitable for large-area devices. The remarkable development of polymer electrolytes is a very promising advance in the technology of the solid-state electrochromic device, which represents a very important, fertile field of research. 

On the other hand, oxyalkyl-substituted polymers are also potentially attractive for the known cation coordinating properties of their ether groups, which are expected to improve ionic transport in the polymers and, hence, to give faster electrochromic response time. The ether-group compatibility of these polymers with polyethylenoxide-based polymer electrolytes is also expected to improve the electrode-electrolyte contact in solid-state electrochromic devices. 

Finally, it again should be stressed that an all solid-state configuration represents the future for large-area devices. Thus investigations of electrochromic devices using polymer electrochromic materials and polymer electrolytes offer a very fertile and almost virgin field of research. 

Many typical electrochromic devices are composed of seven layers, as shown in Figure 20.4 [37]. One alternative to this classical model is a simplified five-layer electrochromic device, where transparent conducting layers were eliminated by Mecerreyes et al. (Figure 20.5) [37]. This type of configuration was successfully tested by using a solution of poly(ethylene oxide-lithium triflate) (PEO -I- CEsSOsLi) in tetrahydrofuran (THF) as a polymer electrolyte, while PEDOT was used as an electroactive polymer by Carpi and De Rossi [29]. 

Impedance Spectroscopy. Impedance spectroscopy has been carried out on devices with WO3 as the cathodic electrochromic layer, counter electrodes of iridium oxide, polyaniline or Prussian blue, and polymers as electrolytes (Katsube et al [1986], Friestad et al [1997]). The equivalent circuit for a whole device becomes very complicated. In the works quoted above simplified, Randles-type circuits were used for the two electrochromic layers, while the ion conductor was modeled by a pure resistance, or neglected. Extraction of device parameters from the data fitting was reported. However, it is clear that in many cases it will be difficult to distinguish the contributions from the different layers in a device, in particular if the migration impedances, ion diffusion impedances, etc. are of the same order of magnitude. When it comes to characterizing electrochromic devices, impedance spectroscopy is a very time-consuming process, since a spectrum down to low frequencies should be taken at a number of equilibrium potentials. Thus we believe that transient current measurements in many cases offer a faster alternative that sometimes allows a simple determination of diffusion coefficients. 

Figure 4.3.27. Transient current versus time response of laminated tungsten oxide-polymer electrolyte-nickel oxide electrochromic device for different directions of the ion movement. The break in the curve at 9 s is due to measurement artifacts.

Most recently Beaupre et al. developed a flexible electrochromic device using textile in 2006 [71]. The structure is made with a transparent electrode, covered with spray-coated electrochromic polymer, a gel electrolyte and finally with a conductive textile. The textile electrode is made with a textile fabric coated with copper and nickel. The other electrode is made of glass or polyester (PET) coated with ITO. Two electrochromic conductive polymers have been tested. Similar colours and colour changes are obtained for structures using two PET-ITO electrodes, or two glass-ITO electrodes, or one textile electrode with one PET-ITO electrode. The colour change is visible but slow. When a plastic electrode and a textile electrode are used, the structure is flexible. A similar structure, using a copper-coated textile cathode, was described by Zhan et al. in 2013 [72]. 

Poly(67), which is a low-gap material, has been used for the construction of a display in which the polymer acts as both anode and cathode [332]. Electrochromic devices using solid electrolytes have been prepared. One of them consists of poly(3-octylthiophene) or PP films with vanadium oxide as the counterelectrode and poly(ethyleneoxide) as the solid electrolyte [333]. However, the performances of these devices is low, reaching a maximum of 100-cycle operation, due to the instability of the polymers and the high operating temperatures. Much better results have been obtained with poly(EDOT). An electrochromic device based on this material and poly(ethylene oxide)-poly(phosphazene) as the solid electrolyte has been proposed [334] it may operate up to 1000 cycles without losses. 

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