Electrochromic devices mechanism

Chapter 6 takes the much studied supramolecular dye-sensitized TiC>2 as an example of an operational supramolecular interfacial device. The fundamental operation of these devices are discussed, including their mechanism of operation. The application of modified semiconductor surfaces as electrochromic devices is also considered. 

For the unique properties of PBs to be exploited, PBs must be deposited properly onto a solid support. It is highly desirable to prepare mechanically robust PBs films with controlled thickness, chemical composition and crystallinity, having ion-sieving membranes and electrochromic devices in mind [6], or to create regular patterns of PB-based single molecule magnets [13],... 

Electrochromism is a phenomenon displayed by some materials reversibly changing colors. Various materials can be used to construct electrochromic devices, such as transition metal oxides, liquid crystals, photonic crystals, and polymers (Booth and Casey, 2009 Nicoletta et al., 2005 Arsenault et al., 2007 Gamier et al., 1983). Here, we will focus on the electrochromic materials based on polymers. There are several mechanisms to explain the color changes of polymer electrochromic materials like electro-induced oxidation-reduction and electrothermal chromatic transition and so on. 

Another mechanism to realize electrochromism is electrothermal chromatic transition. Electricity is much easier to control compared to heat and most of the conductive materials, including many metals and conjugated polymers, can generate heat upon pass of electric current. Electrochromic device based on the electrothermal mechanism is generally composed of chromic and electrically conducting layers. 

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. 

For graphene/CP composite films, the goal of combining the materials has been both to obtain a mechanically more robust material and to combine the attractive properties of the individual components to obtain a superior material. As discussed above, graphene/CP composite materials can be S3mthesized by a range of different methods. In this section, electropolymerization of graphene/CP composite electrode materials and the direct use of such electrodes in the field of supercapacitors and electrochromic devices will be briefly summarized. 

Generally, a rigid electrochromic device is composed of seven layers, as seen in Fig. 24.13. A mechanical support is provided by a substrate. Two electrodes provide electricity to the stmcture at least one electrode has to be transparent in order to allow the colour change to be visible. The storage film contains ions, which are labile in the electrolyte. The electrochromic layer is the colour-changing component (Fig. 24.14). 

In summary, for lithium batteries, electrochromic devices, and nickel metal hydride batteries, sol-gel processing offers new ways to prepare component materials. A sol-gel electrolyte, such as lithium silicate, can operate at modest temperatures while playing the role of a mechanical divider between the electrodes. In other cases, sol-gel processing is appropriate for large area coatings, such as tungstic oxides, where sols can be applied directly to substrates and in successive sol-gel layers. Additionally, sol-gel processing is known to lead to metastable phases, as in the case of nickel oxyhydroxides, and these metastable phases often perform better in electrochemical processes than the stable phases. 

P(TV)s have been investigated for a wide variety of versatile uses, including 3rd-order NLO applications [616], corrosion protection [617, 618] and transmissionmode electrochromic devices [619]. Heeger and Smith [204] have shown that fibers of P(TV)s drawn to ratios up to 20 1 possess enhanced conductivity (claimed up to 1200 S/cm) as well as enhanced mechanical properties, e.g. a Young s Modulus of 8 GPa and Tensile Strength of 0.5 GPa. 

Abstract This chapter details the preparation and mechanism of polymer electrolytes and their applications in electrochemical fields, such as lithium ion batteries, fuel cells, alkahne batteries, supercapacitors, solar cells, electrochromic devices and the like. Polymer electrolytes used in lithium ion batteries are divided into three categories solid, gel and composites. Recent progress made in these three categories is highlighted. Moreover, in addition to the lithium ion battery, appUcations of polymer electrolytes in other electrochemical fields and their ion conducting performance are also briefly described. 

Illustration of the mechanism of an electrochromic device (1 transparent electron conducting layers 2 ion storage layer ... 

From the electrochromic mechanism described above, it can be concluded that the polymer electrolyte must be employed in the ion conducting layer. However, what type of polymer electrolyte is adopted depends on which ion is used in an electrochromic device. Theoretically, all cations can be used in electrochromic devices. Actually, only a few kinds of cations, such as H+, Li", Na", OH, F" and the like, are appropriate. As a result, when H" is used, the ion conducting layer is based on PFSA and so on. If LF is used, the ion conducting layer is similar to that used in a lithium ion battery. And for OH , it is an alkaline polymer electrolyte. 

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