Cathodic coloration

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)) ... 

Dnring the process of colouration in electrochromic cells by passing a charge in one direction, a colour can form in one or both of the electrodes or in the electrolyte adjacent to the electrodes. When the colour is formed by reduction at a negative electrode it is called cathodic coloration and, conversely, at the anode it is anodic coloration. Two different types of cells are shown schematically in Figure 1.31. ... 

Disordered WO3 films transform from an optically transparent to an absorbing state under ion insertion (intercalation) (cathodic coloration). The octahedral W06 units are favorable for both ion as well as electron transport. The pertinent crystal structure allows long-range diffusion through tunnels or between layers (about WO3 structures, see Section 4.2.7). Thin films show a cluster-type microstructure and a column-type macrostructure. This type of coordination leads to electronic bands, responsible for the electrochromic properties. 

An electrochromic device embodies a number of superimposed layers on a transparent substrate or between two transparent substrates, and optical transmittance is altered when an electrical potential is applied so that charge is shuttled between layers serving in the same way as anodes and cathodes in an electrical battery. One specific design with a five-layer construction shown in Figure 30 uses cathodically coloring WO3 and anodically coloring nickel oxide joined by an ion-conducting electrolytic laminate. A potential of a few volts, preferably supplied by solar cells, is applied between... 

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). 

W. Dautremont-Smith. Transition metal oxide electrochromic materials and displays A review - 1. oxides with cathodic coloration. Displays, 3(1) 3-22, January 1982. 

Chandrasekhar, E, Zay, B.J., Cai, C., Chai, Y., Lawrence, D., 2014. Matched-dual-polymer electrochromic lenses, using new cathodically coloring conducting polymers, with exceptional performance and incorporated into automated sunglasses. J. Appl. Polym. Sci. 131,547-557. 

Toxicology Irritant toxic if ingested TSCA listed HMIS Health 1, Flammability 0, Reactivity 0 Uses Pearlescent, filler, pigment for cosmetics syn. pearl polishing medium dry cell cathodes colorant for external pharmaceuticals skin protectant artificial pearls... 

Vanadium oxide is basically a cathodic ion insertion ECM (see section 8.3) and this should in principle rule it out from the class of counter-electrodes suitable for W03-based EWs. However, the colour changes upon lithiation are not too marked, i.e. from yellowish to pale blue-grey (see equation [8.5]). Therefore, the cathodic coloration of lithiated vanadium oxide, Li,V2 05, can be considered to be sufficiently weak to suggest its application as counter-electrode with WO3. Effectively, EWs of the type LiV205/W03, where the electrochromic process can be indicated as ... 

One of the most widely studied classes of polythiophenes, PEDOT, is a cathodically coloring polymer that is a dark opaque blue in its neutral form and a very transmissive light blue in its oxidized form. Sonmez s group showed that an optical variation in PEDOT films resulted in the visible region when different potentials are applied (Figure 20.12) [68],... 

Sapp et al. have shown that PEDOT can be used as a cathodically coloring polymer in dual-polymer electrochromic devices with a A.%T of 45% at 620 nm [3a]. Welsh etal. also reported that the first disubstituted derivative of PProDOTs, namely with a dimethyl group on the central carbon of the propylene... 

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). 

By combining two electrochromic materials whose neutral states are transparent in file visible, with one electrode anodically colored while the other is cathodically colored, allows fabrication of a device that can switch between highly transmissive and absorptive states. These kinds of ECDs are used as smart windows. In the case of electrochromic materials where the neutral state shows absorption in the visible, it is possible to choose as the secondary electrode a material whose neutral state absmption is conqilementaiy to fiiat of the primary electrode, and its reduced form shows file same color as t of file oxidized form of file primary electrode. In other words, file combination of the spectra of two con lementary colors leads to absorption over the whole visible spectrum turning file device black. Upon potential switching between file two redox states, the electrochromic cell will turn from black to the characteristic color of the original forms. 

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