Nickel electrochromic

Inorganic oxides exhibiting electrochromism include cobalt oxide, nickel oxide, molybdenum trioxide, vanadium oxide, tungsten trioxide and their mixtures. The most important of these are those based on tungsten trioxide. 

Examples for electrochromic behavior upon electrochemical oxidation can be found among group VIII metal oxides. Thin films of transparent hydrated iridium oxide turn blue-black, whereas nickel oxide switches from pale green to brown-black, possibly due to the absorbance of Ni3+ centers [26]. The systems are much less thoroughly investigated and a detailed mechanistic explanation is not known. However, proton extraction and anion insertion have been suggested. 

In the most important series of polymers of this type, the metallotetraphenylporphyrins, a metalloporphyrin ring bears four substituted phenylene groups X, as is shown in 7.19. The metals M in the structure are typically iron, cobalt, or nickel cations, and the substituents on the phenylene groups include -NH2, -NR2, and -OH. These polymers are generally insoluble. Some have been prepared by electro-oxidative polymerizations in the form of electroactive films on electrode surfaces.79 The cobalt-metallated polymer is of particular interest since it is an electrocatalyst for the reduction of dioxygen. Films of poly(trisbipyridine)-metal complexes also have interesting electrochemical properties, in particular electrochromism and electrical conductivity.78 The closely related polymer, poly(2-vinylpyridine), also forms metal complexes, for example with copper(II) chloride.80... 

Metal oxides which undergo proton insertion reactions find extensive application in batteries and are currently being investigated as potential electrochromic materials. The properties of battery oxides, e.g. manganese dioxide [107-110] and nickel [111-114] have been extensively reviewed in the literature and will therefore not be discussed here. Rather, the properties of electrochemically grown, electrochromic oxide films will be described since this is a relatively new and interesting field. 

Figure 32 Schematic electron density-of-states diagrams for electrochromic, EC, multilayer design. The materials include ln203 Sn (ITO), nickel oxide (presiunably hydrous), tungsten oxide (also presumably hydrous) prepared so that the EC and chemically protective (PR) properties are emphasized, and an electrol)de. The Fermi energy is denoted Ep, with Epi and Ep2 pertaining to the case of an applied potential, Ucoi fiUed states are denoted by shadings. (Ref 235. Reproduced by permission of Springer Verlag)...

Nickel(II) oxide crystallizes in the NaCl structure. Thin amorphous films of it exhibit electrochromic behavior and are antiferromagnetic at Tn = 247 °C. Nickel(II) oxide films with smooth surfaces and columnar structures of preferred (100) orientation on MgO(lll), Si(lll), soda lime glass, fused silica and stainless steel can be obtained by and... 

Other materials being investigated include ferrocene with a bipyridinium salt,234 niobium oxide,235 nickel oxo-hydroxide,236 and cobalt oxohydroxide.237 The last is pale yellow in the reduced state and dark gray in the oxidized state. A typical electrolyte is lithium perchlorate in propylene carbonate. Solid electrolytes, such as a lithium salt (perchlorate, tetrafluoroborate, or triflate), in a polyepoxide238 or in a polyvinyl chloride gel in ethylene carbonate-propylene carbonate,239 lithium iodide in polyvinyl bu-tyral,240 and Naflon H (a polymeric perfluorocarbon-sulfonic acid),241 have also been tested. Some other systems use suspended particles between two panes of glass.242 When the particles are aligned by an electric field, the window becomes transparent. Combination photo-voltaic-electrochromic devices are under study.243... 

The water-insoluble, green NiO is obtained by thermal decomposition of NiC03 or Ni(N03)2 and crystallizes with the NaCl structure thin amorphous films of NiO exhibiting electrochromic behaviour (see Box 22.4) may be deposited by CVD (chemical vapour deposition, see Section 27.6) starting from [Ni(acac)2]. Nickel(II) oxide is antiferromagnetic... 

Nickel(ii) oxide, NiO, possesses electrochromic properties and has been prepared by the CVD of nickel acetylacetate (Ni(acac)2 acac 6) [58, 67, 68]. 

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... 

To investigate the effect of the nanostructure on the electrochromic performance of NiO, transparent devices were assembled from nontemplated and DG-structured films with a FTO counter electrode and a 1M KOH(aq) electrolyte, see Fig. 6.9a. The active electrode material used was limited in area to 0.95mm. During the nickel electroplating process limiting the deposition area improve the control and quality of the deposit. 

Nickel oxide can be deposited on an ITO-coated glass by anodic electrochemical deposition [19] or by sputtering followed by hydration in basic electrolytes [20]. The electrochromic chracteristics of the films vary according to the preparation procedure the films produced by sputtering... 

The reasons for this difference are not clear they may be tentatively explained assuming that the electrochromic process requires for the original sputtered NiO film a gradual but progressive hydration to reach a NiO Hy form of unknown stoichiometry. However, differences in the basic structure of the two types of nickel oxide cannot be excluded. 

It has been shown recently [22,23] that sputtered non-stoichiometric nickel oxide, NiO films, if treated under particular conditions, which mainly require activation in water-free aprotic electrolytes, undergo a reversible electrochromic process involving the insertion of lithium ions. 

This proposed mechanism of the electrochromic process in nickel oxide electrodes is further supported by measurements of the expansion of the oxide host matrix during the activation process, obtained by determining the mechanical stress induced by ion intercalation [25,26]. For this experiment, galvanostatic polarization (using very low current densities, i.e. of a few pA cm", to avoid undesired side reactions like metal electrodeposition)... 

As expected from equation [8.13], the lithiated nickel oxide electrode becomes transparent during the cathodic cycle (Li" insertion) and coloured in the following anodic cycle (Li" extraction). It is interesting to note that the electrochromic efficiency of lithiated nickel oxide is 0.04mCcm" at 633 nm (25), i.e. of the same order as that of tungsten oxide. 

In conclusion, lithiated nickel oxide is a very convenient electrochromic materal in its optimized concentration range which extends up to l(X)mCcm" pm" while further lithiation may induce an opposite behaviour. However, the optimized range is largely sufficient to assure proper electrochromic behaviour and this places LiyNiO among the most promising anodic ECMs. 

The choice of a suitable counter-electrode for a successful EW is not easy since only a few compounds fulfil the desired operational requirements which call for an uncommon combination of electrochemical and optical properties. The most promising, and, thus far, the mostly used materials are indium tin oxide, nickel oxide, iridium oxide and cobalt oxide among the inorganic ECMs, and polyaniline (PANI) among the organic ECMs. The electrochromic properties of indium tin oxide and PANI have been described in Chapter 7. Therefore, here attention will be mainly focused on transition metal oxide counter-electrodes. 

Figure 8.12 illustrates the cyclic voltammetry and the transmittance of the window. The feasibility of the electrochromic system is confirmed by the excellent peak definition and low peak separation, as well as by the large transmittance variation upon cycling. One problem associated with the use of lithiated nickel oxide counter-electrodes remains the low diffusion of the Li" ions (calculated to be of the order of 10 cm s [24]), which may lead to long response times. However, this problem may be alleviated by optimizing the morphology of the electrode. 

Lampert and coworkers [36] have used a modified amorphous PEO-LiCFaSOs electrolyte for the realization of WO3 laminated windows using several types of counter-electrodes, such as niobium oxide, nickel oxide and a new class of solid redox polymerization electrodes [63]. These latter electrodes have an advantage over inorganic layers in that they can be tailored to the electrochromic material and ion specifically. Figure 8.18 illustrates the optical transmittance of a EW made of WOa/modified a-PEO/ion storage polymer [63]. 

Scheme 11 Synthesis of electrochromic gelators 45 and 46 (a) 188 °C, Cu powder, K2CO3, 18-crown-6 (b) 160 °C, mesitylene, Cu powder (c) H2/Raney nickel (d) THF, stearoyl chloride, EtaN...

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... 

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