Electrochromic materials, anodic

This cell contains an anodic electrochromic material associated with WO3. 

Anodic Electrochromic Materials. The most commonly used anodic electrochromic materials are nickel oxide (Svensson and Granqvist [1986]) and iridium oxide (Gottesfeld et al. [1978]). They switch from a transparent state to a colored one upon extraction of protons. Charge-balancing electrons are simultaneously extracted from the valence band. The films are probably a mixture of oxide and hydroxide components in the bleached state, since there needs to exist a reservoir of protons in the films. Due to the high cost of iridium, the use of nickel oxide is favored for large scale appfications. Recently, a class of mixed nickel oxides with enhanced modulation between the transparent and the colored state have been discovered (Avendano et al. [2003]). Intercalation of Li into ifickel oxide films has been attempted, but the optical properties are not modulated very much (Decker et al. [1992]). The mechanism of optical absorption is not known in detail. However, in... 

Counter Electrode Materials. The anodic electrochromic materials and ion storage materials have not been so widely studied as the cathodic electrochromic materials discussed above. In general, the main features of the impedance spectra are similar to those shown above. The impedance response of nickel oxide films with... 

As the equilibria above show, WO3 is a cathodic electrochromic material, i.e. the darkening of the electrochromic fihn occurs at the cathode in the ceU. In contrast, I1O2 is an anodic electrochromic material, and the reversible colour change of the electrochromic device depends on proton rather than fithium ion migration ... 

Miscellaneous. Iridium dioxide, like RUO2, is useful as an electrode material for dimensionally stable anodes (DSA) (189). SoHd-state pH sensors employing Ir02 electrode material are considered promising for measuring pH of geochemical fluids in nuclear waste repository sites (190). Thin films (qv) ofIr02 ate stable electrochromic materials (191). 

If an anodically colored electrochromic material (e.g., Ir02) is used as one electrode in the device in Eig. 33.1fi and a cathodically colored (e.g., WO3) is used as the other electrode, a much larger change in transmission per charge supplied can be seen compared to the case when only one electrode is electrochromic. Also, the use of an intercalation material as the counter electrode may be advantageous for the device shown in Eig. 33.1a, as it can minimize undesired reactions on the counter electrode. 

Since PB and WO32 are respectively anodically and cathodically coloring electrochromic materials, they can be used together in a single device121-125 so that their electrochromic reactions are complementary (Equation (15)) ... 

Electrochromic displays, 6 572t, 582—583 Electrochromic foil device, laminated polyester- based, 23 22 Electrochromic materials, 6 571-587 anodically colored inorganic films, 6 579-580... 

An electrochromic cell is schematically formed by three main layers electrochromic material/ion conductor (electrolyte)/ion storage layer (counter electrode). Since the different oxides can colour either in anodic (NiO) or in cathodic (WO3) polarization, it is interesting to make the counter electrode also an active material, and to associate electrochromic materials with cathodic coloration as well as anodic coloration. In practice, there are seven successive layers, as shown in Figure 14.1(a). 

Electrochromic displays are simple electrochemical cells with an electrochromic material as one of the electrodes. A potential applied between the anode and cathode, typically less than 5 V, produces the desired color change. Once switched, the current between the anode and cathode diminishes. Most electrochromic displays have a memory (stay switched even if no potential is applied as long as the electronic connection between anode and cathode is broken), many on the order of hours. Thus, they are ideal for low-power applications. Unfortunately, many printed electrochromic devices are relatively slow. Speeding up the device requires a liquid electrolyte, which is difficult to manufacture and encapsulate in an inexpensive device. 

The mechanism is, however, more complicated than simply the anion X entering the polymer during anodic polarization and leaving it upon the cathodic one, as cation motion can also be involved [2]. Conducting polymers are thus a class of ion-insertion electrochromic materials that act during an electrochromic process as mixed electronic and ionic conductors. 

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. 

A new bipropylenedioxythiophene, poly(spiroBiProDOT), has been reported with dual cathodically and anodically coloring properties, displaying three different colors in the oxidized, neutral and reduced states [61, 73]. (l-Phenylethyl)-2,5-di(2-thienyl)-17f-pyrrole [P(PETPy)] was used as the anodically coloring material and PEDOT as the cathodically coloring electrochromic material for dual-type ECDs [4]. 

Dual-type polymer electrochromic devices based on copolymers of 2-benzyl-5,12-dihydro-27f-pyrrolo [3, 4 2,3] [1, 4]dioxocino[6,7-6]quinoxaline (DPOQ) and 5,12-dihydrothieno[3, 4 2,3] [1, 4]dithiocino [6,7- >]quinoxaline (DTTQ) with bithiophene were developed. P (DPOQ-co-BT) and P(DTTQ-co-BT) were used as the anodically coloring and PEDOT as the cathodically coloring electrochromic materials [81]. Each device performed with a favorable switching time, optical contrast, open-circuit memory and stability. 

The bandgap of PEDOT ( g = 1.6-1.7 eV)itselfis 0.5 eV lower than polythiophene, which results in an absorbance maximiun in the NIR region. Compared to other substituted polythiophenes, these materials exhibit excellent stability in the doped state which is associated with high conductivity. Doped PEDOT is almost transparent in the visible region (with a sky-blue tint) and the neutral polymer is deep blue. Because PEDOT and its alkyl derivatives are cathodically-coloring electrochromic materials, they are suitable for use with anodically-coloring conducting polymers in the construction of dual polymer ECDs (63). 

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