Electrochromic glazing for energy efficient buildings

Electrochromic glazing for energy efficient buildings

20

Claes Goran Granqvist Uppsala University, Sweden

20.1

Introduction

Electrochromic materials have properties that can be tuned, persistently and reversibly, by applying electrical current or voltage (Granqvist, 1995; Mortimer et al., 2015). These materials and device technologies based on them are currently under rapid development, especially as regards applications to glazingdused here as a common term for windows and glass facadesdfor energy-efficient buildings with excellent indoor comfort. The present chapter summarizes oxide-based electrochromics, especially with regard to recent advances in the field. The impetus behind much of today’s (2018) work on electrochromics can be traced to our well known environmental challenges. The amount of carbon dioxide in the earth’s atmosphere grows rapidly; it was w315 ppm in the late 1950s and presently is larger than 400 ppm as reported by the US Department of Commerce, National Oceanic and Atmospheric Administration (2018). The rising fraction of CO2 is caused by energy productiondmainly the burning of coal, oil, and gasdand leads to global warming and rising sea levels and has numerous other harmful effects (Stocker et al., 2013). It is also important to note that the global population is burgeoning and, according to the United Nations Department of Economic and Social Affairs (2015), is predicted to be 50% larger in the year 2100 than at present. This population is increasingly living in mega-cities which behave as “urban heat islands” and frequently have temperatures that are several degrees higher than those in the rural neighborhoods (Akbari et al., 2016). The energyepopulation nexus emphasizes that global energy production must be decarbonized which, in its turn, stresses that buildings must become more energy efficient. Today’s buildings are responsible for 30%e40% of the world’s use of primary energy, as emphasized by the United Nations Environmental Programme € (2009), but regional variations are large (Urge-Vorsatz et al., 2015). There are many “green” technologies, frequently with nanoattributes, that can be harnessed in the quest for better buildings (Smith and Granqvist, 2010; Ginley and Cahen, 2012; García-Martínez, 2012; Pacheco-Torgal et al., 2013a,b, 2015, 2016a,b, 2017), and energy-efficient glazing stands out as one of the most viable options. Glazing frequently allows huge amounts of energy to flow into or out from a building, and artificial cooling or heating is required to achieve a benign indoor environment. One solution to the conundrum would be to minimize the glazed area or to implement some static shading device, but these strategies diminish indoorseoutdoors contact and day-lighting, which are important features for Nanotechnology in Eco-efficient Construction. https://doi.org/10.1016/B978-0-08-102641-0.00020-7 Copyright © 2019 Elsevier Ltd. All rights reserved.

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Figure 20.1 Interior view of an electrochromic glazing with panes in fully clear and fully dark states. Further information on this type of glazing is given in Section 20.4.

human well-being. A superior solution to the energy issue is to use glazing with tunable throughput of solar energy and visible light. Such glazing is often called “smart,” “intelligent,” “adaptive,” “dynamic,” or “responsive” and makes use of “chromogenic” materials (Granqvist, 1990; Lampert and Granqvist, 1990; Lampert, 2004). Electrochromic materials are the most widely studied ones and are currently of the greatest importance for buildings (Granqvist, 2014); other members of the “chromogenic” family include thermochromics, gasochromics, and photochromics as recently surveyed by Granqvist and Niklasson (2018). Electrochromic glazing was proposed as early as in 1984 by Lampert (1984) and by Svensson and Granqvist (1984, 1985), and it had its market introduction in the 2010s. The interest in this technology, and the number of actual installations of electrochromic glazing, is growing as discussed in many review articles (Jelle et al., 2012; Granqvist, 2014; Jelle, 2015; Mardaljevic et al., 2015; Sibilio et al., 2016; Loonen et al., 2017). Fig. 20.1 shows an example of an electrochromic glazing and illustrates its most important property: the possibility to alter the optical transmittance between high and low values. The left-hand window is fully transparent, whereas the right-hand windows are fully dark; it takes some 10 min to transition between these states. The electrochromic glazing in Fig. 20.1 is based on thin surface coatings of metal oxides. Oxide-based electrochromic “smart” glazing is far from the only possible alternative (Granqvist, 2014; Jin and Overend, 2017; Casini, 2018; Granqvist and Niklasson, 2018), and other types of electrochromic devices are used for windows in cars, buses, and aircrafts as well as for variabletransparency partitioning of rooms. However, only oxide-based electrochromic materials, of the kind discussed in this chapter, appear to be in use in today’s (2018) glazing for energy efficient buildings. Before proceeding, it is important to realize that electrochromic glazing must be compatible with truly large-scale manufacturing. Flat-glass production, mainly by the float process, is forecast to reach an amazing w1010 square meters per year in

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(a) Intensity (Arb. U.)

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Figure 20.2 (a) Blackbody spectra for the indicated temperatures (vertical scales are different for the solar spectrum and the other four spectra). (b) Typical solar irradiance spectrum at the ground level during clear weather, and relative spectral sensitivity of the light-adapted eye.

2018 (Freedonia, 2014). Applying electrochromic functionality on just a tiny fraction of the world’s glazing requires production technology that can deal with square kilometers per year. Another important observation is that the market for electrochromic glazing is forecast to rise steeply during coming years (n-tech Research, 2016). Electrochromic materials and glazing, as well as other solar-energy related devices, must be characterized and quantified with regard to thermal properties, performance under solar irradiation, and visual appearance (luminous properties) (Granqvist, 1981; Smith and Granqvist, 2010). These properties are conveniently introduced from the spectra shown in Fig. 20.2. Thermal radiation is governed by blackbody curvesdwhich are shown for four temperatures in Fig. 20.1(a)d multiplied by an empirical emittance which is less than one. It is evident that thermal radiation lies at l > 2 mm at temperatures of interest for buildings, where l is the wavelength. Solar radiation impinging on the earth’s atmosphere can be approximated by blackbody radiation according to the sun’s surface temperature (5505
C) and lies in the range of 0.2 < l < 3 mm. At the ground level and during typical clear weather, this radiation is as shown in Fig. 20.2(b); the pronounced minima are due to absorption in ozone at shorter wavelengths and mainly due to absorption in water vapor at longer wavelengths. Luminous radiation for the lightadapted eye, finally, is given by the bell-shaped curve in Fig. 20.2(b) which extends over the 0.4 < l < 0.7 mm range with a peak at 0.55 mm. Only some 45% of the solar energy appears as visible light.

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Quantitative values of the luminous (lum) and solar (sol) transmittance through an electrochromic glazing, denoted Tlum and Tsol, respectively, can be computed from R dl4lum;sol ðlÞTðlÞ Tlum;sol ¼ R (20.1) dl4lum;sol ðlÞ where T(l) is the spectral transmittance, 4lum is the eye’s spectral sensitivity (Wyszecki and Stiles, 2000), and 4sol is the “air mass 1.5” solar irradiance spectrum (the sun standing 37 degrees above the horizon) (ASTM, 2008). This chapter is organized as follows: Section 20.2 discusses the energy savings potential inherent in variable-transmittance glazing technology from different perspectives and also stresses some other features on this technology. Section 20.3 then describes the most common electrochromic technologydthe one based on oxide coatings and currently used in buildings; this presentation is complemented in Section 20.4 by a case study on electrochromic foil technology applicable to glass lamination which, in the author’s opinion, is especially promising. Section 20.5 outlines some recent and ongoing endeavors to improve electrochromic glazing technology, and Section 20.6 summarizes the chapter and gives some comments. There are many reviews on electrochromic materials and glazing; the present chapter is an update and extension of recent surveys as regards the general content and references (Granqvist, 2014, 2015a,b; Granqvist et al., 2018a).

20.2

On the energy saving potential and other assets

The energy saving potential of electrochromic and other chromogenic approaches to variable-transmittance building envelopes cannot currently be evaluated in an accurate manner (Favoino et al., 2015), which is an obstacle for development. Here we approach the energy savings potential step by step by first considering a simple analog between the energy savings in electrochromic glazing and the energy generation that is possible in similarly positioned solar cells. Subsequently we review earlier work on energy modeling and on direct measurements and user-related assessments on electrochromic glazing. Finally, a brief discussion is given on electrochromic glazings’ ability to offset the need for space cooling and itsdpossibly essentialdrole for evading associated greenhouse gas emission. An intuitive and simplistic reasoning was presented several years ago by Azens and Granqvist (2003) who considered a cooled (commercial) building. The solar energy density impinging onto a window was set to be 1000 kW/m2year (the actual value is irrelevant for our argument). Roughly half of this energy density, 500 kW/ m2year, is visible light (the other part of the solar irradiation can, in principle, be removed by a static reflector for near-infrared radiation). Taking the transmittance of the electrochromic glazing to vary between 7% and 75%, which are typical numbers, the energy savings inherent in the controllability is 340 kW/m2year. But when should the window be dark or clear? Considering physical presence as the basis for the control, one may argue that 50% represents a conservative estimate of the

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fraction of the solar energy that enters a typical room when it is unoccupied. A minimum value of the energy savings is then 170 kW/m2year. The magnitude of this energy savings can be appreciated by imagining that the window is replaced by solar cells with a typical 17% efficiency; these solar cells then generate 170 kW/ m2year under the given assumptions. Hence the energy saved by the electrochromic glazing is the same as the electrical energy that could have been generated by solar cells in the same position! It should be noted that this compelling analogy presumes air conditioning powered by electricity generated with a coefficient-of-performance of unity, as is reasonable in national scenarios for electricity generation. Clearly the hand-waving argument is not sufficient for quantitative assessments of electrochromic glazing. Instead attention is now directed to a particularly ambitious effort to analyze potential energy savings in recent work by Favoino et al. (2014, 2015) who adopted an inverse performance-oriented approach to minimize the total primary energy use of buildings. This technique was applied to a number of case studies involving an office reference room with different orientations in three temperate climates with the object of evaluating and optimizing the glazing’s performance as it responded to variable boundary conditions on a monthly and daily basis. A frequency analysis was subsequently applied to the set of optimized properties in order to identify the most crucial features of the glazing. Specifically, the modeled office room (3  5  3.5 m3; glazed area 40% of 3  3.5 m2) had three climatic locations (Helsinki, London, and Rome), and the computed data for the adaptive glazing were compared with data for a comparable static reference façade complying with national standards. A variety of other parameters involving indoor comfort and dimmable artificial lighting were included in the analysis with the object of making it as realistic as possible. Data on Tlum and on thermal properties for various available glazing constructions provided inputs to the analysis. Fig. 20.3 shows total primary energy use for offices facing North, East, South, and West in Helsinki, London, and Rome; each case gives results on energy for heating, cooling, and lighting for a reference façade (R) satisfying minimum national requirements, a yearly-optimized glazed façade (Y) which represents the best possible static solution, an optimized monthly-adapted glazed façade (M), and an optimized daily-adapted glazed façade (D). The gathered information is unambiguous and shows among other things that the relative energy savings, compared to the reference case, (1) are higher for adaptive than for static glazing, (2) are larger for short (daily) than for long (monthly) modulation time, and (3) are greater for southern than northern locations especially as regards energy for cooling. The maximum decrease of the total primary energy is 17%e19% for Helsinki, 33%e34% for London, and 55%e57% for Rome. The analysis is detailed but still contains a wealth of uncertainties, and the numbers should be taken as indicative. In particular, hour-optimization is expected to lead to larger energy saving than the one shown in Fig. 20.3, and it should also be realized that the data are constrained by the performance of today’s glazing products. Nevertheless, the take-home message from the analysis is that very significant energy savings are possible with electrochromic glazing. Analyses using similar methodology have been applied also to seasonallyadapted glazing (Kasinalis et al., 2014) and to other types of high-performance glazing (Jin and Overend, 2017; Loonen et al., 2017).

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Figure 20.3 Specific total primary energy use for idealized glazing in reference rooms with the indicated geographic locations and orientations. Data are shown for heating, cooling, and lighting energy and apply to reference (R), yearly-optimized (Y), monthly-adapted (M), and daily-adapted (D) glazing. Energy savings, in percent, are stated with regard to the reference case. From Favoino, F., Overend, M., Jin, Q., 2015. The optimal thermo-optical properties and energy saving potential of adaptive glazing technologies. Applied Energy 156, 1e15.

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Many otherdless elaboratedanalyses of the performance of electrochromic glazing of various types have been reported, as discussed later. Energy savings, with accompanying effects on CO2 emission abatement and downsizing of air coolers, have been obtained by computer simulation on buildings with and without electrochromic glazing and sometimes with consideration of different geographic locations (Gugliermetti and Bisegna, 2003; Porta-Gandara et al., 2003; Jonsson and Roos, 2010a; Dussault et al., 2012; Sbar et al., 2012; Tavares et al., 2014, 2015), and measurements on analogously equipped test rooms or modules have been reported as well (Aleo et al., 2001; Klems, 2001; Lee et al., 2006a,b; Assimakopoulos et al., 2007; Piccolo, 2010; Pittaluga, 2015); specialized studies have considered the roles of control strategies for the electrochromic glazing (Sullivan et al., 1994; Assimakopoulos et al., 2004, 2007; Jonsson and Roos, 2010b), exterior shading devices (Aldawoud, 2013), lighting energy savings in split-pane windows in which the various panes can be individually tuned (Fernandes et al., 2013), and “dual-band” electrochromic glazing allowing individual control of Tlum and of the transmittance of infrared solar radiation (DeForest et al., 2015, 2017; Baldassarri et al., 2016). Data from this large number of studies are in line with results in Fig. 20.3, but detailed comparisons are not possible. Furthermore, the boundary conditions of the various studies are frequently too different to allow their outcomes to be compared in a meaningful way. Nevertheless, the bottom line is that very substantial energy saving can be accomplished with electrochromic glazing. In addition, life-cycle assessments and eco-efficiency analyses are favorable for electrochromic glazing (Papaefthemiou et al., 2006, 2009; Syrrakou et al., 2006; Baldassarri et al., 2016; Pierucci et al., 2018). Energy efficiency is only one of the assets of electrochromic glazing technology, and another very important advantage regards improved interior comfort for the buildings’ users, specifically with regard to lowered thermal stress, superior daylighting, and glare reduction (Moeck et al., 1998; Lee and DiBartholomeo, 2002; Clear et al., 2006; Zinzi, 2006; Lee and Tavil, 2007; Piccolo et al., 2009; Piccolo and Simone, 2009, 2015; Ajaji and André, 2015). Conducive environments lead to job satisfaction whichdin a wide sense of the termdis energy efficient, although quantification is very hard. Another aspect of electrochromic glazing regards financial benefits for the operators of buildings equipped with an energy-efficient technology (Eichholtz et al., 2010). A comprehensive plan for reversing global warming was presented recently by Hawken (2017) and is of interest in the present context. This “drawdown” plan embraces 100 solutions in all fields and ranks them by importance. The highestranking solution by 2050 with regard to total atmospheric CO2-equivalent reduction is “refrigeration,” specifically the phasing out of hydrofluorocarbons used as chemical refrigerants (Reese, 2018). Clearly, increased deployment of electrochromic glazing lowers the need to install air coolers and thereby affects the eventual hydrofluorocarbon emission favorably. The quantitative effect is hard to assess, and it has been pointed out that the “drawdown” plan is not analytically rigorous (Duchin, 2017), but the example highlights that new energy-efficient technology in the buildings sector may have unexpected, and in this case highly benign, consequences.

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20.3 20.3.1

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Operating principles and materials Generic device design

)

Tr a

ss

la

(G

r El ec Tr troc an h sp rom ar en ic fil m tc Gl on as du s cto

Electrochromic device

ns lo par n (o s en r e tor t c le a g o n ct e du ro fi ch lm cto r ro m ic f ilm Io n ) (e co lec nd tro uc lyt tor e)

Fig. 20.4 introduces a generic design of an electrochromic device for glazing applications (Granqvist, 1995, 2015a, 2000) and depicts five superimposed layers either on a single substrate or positioned between two substrates in a laminate configuration. Glass is most commonly used as a substrate material and is obviously well suited for glazing in buildings, but polymers is an alternative and thin, flexible foils of polyethylene terephthalate (PET) permit fabrication of devices by a roll-to-roll technology as outlined in Section 20.4 (Azens et al., 2003, 2005; Granqvist, 2015b; Granqvist et al., 2018b). The middle part of an electrochromic device is a conductor for ions and an insulator for electrons; it can be a layer of a polymer electrolyte or a transparent thin film. The ions should be small so they are able to move easily under an electric field; the most common alternatives are protons (hydrogen ions, Hþ) and lithium ions (Liþ). This ion conductor is in contact with an electrochromic thin film, which conducts both ions and electrons. Tungsten oxide is the foremost example; it became the first widely known electrochromic material (Deb, 1973) and, as far as is known, is used in all of today’s (2018) electrochromic glazing (Granqvist, 2014; Thummavichai et al., 2017). The other side of the centrally positioned ion conductor is in contact with an ion storage film, which is able to conduct both ions and electrons. Preferably, the ion storage film exhibits electrochromism in a sense that is complementary to that of the first electrochromic film, as elaborated and exemplified later. The entire three-layer construction is located between thin-film transparent electrical conductors, which can be of many types and employ doped wide-bandgap semiconductors, coinage metals, carbon nanotubes or graphene, metal nanowires, some organic materials,

Ions

Figure 20.4 Generic electrochromic device design. Arrows illustrate the movement of positive ions in an electrical field.

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or hybrids of two or more of the these possibilities (Granqvist, 2007a, 2014; Ginley et al., 2010; Barquinha et al., 2012). Interfacial layers may be able to enhance the electrochromic functionality (Lin et al., 2015a,b). When a potential is applied between the two transparent electrical conductors, ion transport can occur between the electrochromic film and the ion storage film, and the charge of these ions is balanced by electrons inserted into or removed from the electrochromic film and the ion storage film via the transparent electrical conductors. These electrons are the actual cause of the changes of the optical transmittance of the electrochromic films, as further discussed later. Reversing the potential or, for appropriate combinations of materials, short-circuiting regains the original optical performance. Electrochromic devices usually exhibit open-circuit memory, which means that the change of the transmittance can be stopped at any intermediate level between two endpoints. Consequently, electricity is required only to change the optical properties, which leads to energy efficient operation of the electrochromic glazing. The open-circuit memory is dependent on sufficiently low electrical conductivity of the central ion-conducting layer, which is easier to reach by use of a polymer electrolyte, with a typical thickness of several micrometers, than with a normally much thinner inorganic ion-conducting film. The voltage needed to induce ion transport in the electrochromic device is typically only a few volts DC, which permits easy power supply for example by integrated solar cells (Lampert, 2003; Bogati et al., 2017). The electrochromic device is obviously similar to an electrical battery with visible electrical charging state. This analogy can be valuabledalbeit it is seldom benefitted from to a sufficient extentdand electrochromic devices and electrical batteries often share the same characteristic features, assets, and limitations. For example, both types of devices degrade rapidly by overcharging but they can also exhibit “self-repair,” and rejuvenation of degraded electrochromic oxide films has been demonstrated recently under both galvanostatic and potentiostatic treatment, as discussed in more detail later (Wen et al., 2015a, 2016a). Furthermore, batteries as well as electrochromic devices are unable to alter their properties very rapidly, and an electrochromic glazing on the scale of square meters may need tens of minutes to go from fully dark to fully transparent. As noted previously, it is advantageous to use a thin-film counter electrode with optical properties that are complementary to those of the primary electrochromic thin film. Such complementarity can be achieved since there are electrochromic oxides of two important kinds: those darkening upon ion insertion and referred to as “cathodic” and other ones that become dark under ion expulsion and called “anodic” (the batterytype nomenclature should be noted). Oxides of W, Mo, Ti, and Nb are cathodic, whereas oxides of Ni and Ir are anodic (Granqvist, 1995, 2014), and an intermediate situation is at hand for V2O5 which shows anodic and cathodic traits in different wavelength ranges (Talledo and Granqvist, 1995; Lykissa et al., 2014; Vernardou, 2017). Oxides of Cr, Mn, Fe, Co, Cu, Rh, and Ta can show some electrochromism but do not attain a fully bleached state and/or exhibit weak electrochromism and therefore are not of major interest for practical applications. Mixed electrochromic oxides can have properties better than their components. Devices including W-oxide-based and Ni-oxide-based thin films are particularly interesting (Niklasson and Granqvist, 2007) and are used in several types

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of electrochromic glazing (Granqvist, 2014), including the one discussed in Section 20.4. Moving electrical charge from Ni oxide to W oxide makes both of these films turn dark, and both films regain their transparency when the charge is returned. Furthermore, the optical absorption of W oxide and Ni oxide is complementary and can lead to a rather neutral color in an electrochromic glazing, which is usually a desirable feature.

20.3.2

The key role of nanostructure

The nanostructure is important for all electrochromic materials and involves several length scales. These aspects are given a brief presentation later with a special focus on W oxide, which has been studied in considerable detail. It is first noted that the majority of the electrochromic oxides are comprised of octahedral-like structural units arranged with different degrees of corner-sharing and edge-sharing (Granqvist, 1995). This structure is favorable, and the spaces between the octahedra are large enough to permit some transport of small ions. Looking at stoichiometric WO3, the simplest atomic arrangement consists of corner-sharing octahedra, each having a centrally positioned tungsten atom and six surrounding oxygen atoms. This cubic structure is an oversimplification, however, and a tetragonal structure applies to bulk-like WO3 at normal temperature and pressure. The latter structure is better with regard to ion transport than the cubic one since the inter-octahedral distances are greater. Hexagonal structures form readily in thin films, nanorods, and nanowires of W oxide (Granqvist, 2014), and this structure is still better for ion transport. Empirical data on nanostructures in W oxide thin films have been reported many times, and especially clear information is found for evaporated W oxide films studied by X-ray scattering (Nanba and Yasui, 1989). Cluster-type structures were evident and were based on hexagonal-like entities which became larger and more interconnected in films deposited onto substrates with increasing temperature. These findings are consistent with the observation that trimeric W3O9 and larger molecules tend to be formed upon gas-phase synthesis (Maleknia et al., 1991; Kluge et al., 2017) and also with computed lowest-energy structures of (WO3)q clusters (Sai et al., 2012). Electrochromic films are often designated “amorphous,” but some local order exists even when the films are deposited onto substrates at room temperature. Some ion transport can be accomplished in the structures discussed. However, it is important to create electrochromic films with clear porosity in order to promote facile ion mobility. Many thin-film deposition techniques may be able to produce nanoporous films, though with greater or smaller ease. For the case of sputter deposition, which is often used to manufacture electrochromic glazing, it is adequate to consider “zone diagrams” describing characteristic structural properties as a function of deposition parameters such as the pressure in the sputter plasma and the substrate temperature in comparison with the melting point of the deposited species (Thornton, 1977; Petrov et al., 2003; Anders, 2010). Higher-than-normal pressure in the sputter plasma, along with low substrate temperature, is conducive and yields films with “zone 1” structure signified by columnar features. Still larger porosity can be created

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by oblique-angle depositiondcharacterized by a large angle between the direction of the incident deposition species and the substrate’s surface normal (Le Bellac et al., 1995; Hawkeye et al., 2014; Barranco et al., 2016)das demonstrated for W-oxide-based electrochromic films (Gil-Rostra et al., 2012). Rapid changes of the optical properties are often reported for small electrochromic coatings with “zone 1” type and are ascribed to the particular nanostructure (Giannuzzi et al., 2015; Kim et al., 2015). A treatment on the appropriate nanostructures of electrochromic thin films must also include the role of the films’ surfaces, and recent research has emphasized the significance of surface features including deposition-dependent exposed crystal facets for anodically coloring electrochromic Ni oxide in Li-ion-conducting electrolytes (Wen et al., 2015b).

20.3.3 Optical properties The origin of the optical absorption in electrochromic oxides has been of keen interest and widely studied for many years. The topic is complicated for several different reasons and nothing but a simplistic picture, although one capturing the most basic features, is given later. Complications are related to the absence of well-defined crystalline structure, to oxygen deficiency, and to incorporation of mobile ions, hydroxyl groups, and water molecules. Despite these difficulties, a detailed picture has emerged for W oxide (Hjelm et al., 1996; de Wijs and de Groot, 1999; Bondarenko et al., 2015; Triana et al., 2015, 2017; Hamdi et al., 2016; Wang et al., 2018), whereas much less is known for Ni oxide and other electrochromic oxides. Insertion/extraction of protons (Hþ) and electrons (ee), and associated coloring and bleaching, in electrochromic WO3 can be represented by a very simplified electrochemical reaction according to ½WO3 þ Hþ þ e

 bleached

4½HWO3

 colored

(20.2)

Here Hþ can be replaced by Liþ or some other small ion (Granqvist, 1995). A fully reversible reaction can only be obtained if the reaction is partial (Berggren and Niklasson, 2006; Berggren et al., 2007; Wen et al., 2015a), which means that the optically absorbing material is HxWO3 with x < 0.5. The corresponding reaction for Ni oxide is ½NiðOHÞ2

 bleached

  4 NiOOH þ Hþ þ e colored

(20.3)

This reaction is believed to occur on hydrous grain boundaries (Avenda~no et al., 2005, 2009). If Liþ is the mobile ion, the reaction is surface-dominated (Wen et al., 2015b). Most electrochromic oxides consist of octahedra-like structural units, which make it possible to formulate a schematic model for cathodic and anodic electrochromism based on the electronic band structure (Granqvist, 1995). The oxides have oxygen 2p bands that are well separated from the metal’s d band, and the octahedral symmetry gives rise to a splitting of this latter band into subbands that are conventionally designated eg and t2g (Goodenough, 1971). Stoichiometric WO3, with cathodic coloration,

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has a full O2p band and an empty d band, and these bands are separated by an energy gap that is wide enough to yield optical transmittance in a thin film. Introducing ions and charge balancing electrons leads to partial filling of the d band (together with optical absorption as discussed shortly). Anodically coloring electrochromic oxides, on the other hand, have unoccupied t2g states, and introduction of ions and electrons may fill these states to the top of the band so that the material exhibits a gap between the eg and t2g subbands. A thin film of such a material then becomes transparent, provided that the band gap is large enough. Electrochromic V2O5 represents a somewhat different case with both cathodic and anodic characteristics. Now the deviation from octahedral coordination is sufficiently large that the d band exhibits a narrow split-off part within the band gap. Insertion of ions and electrons into V2O5 is able to fill this narrow band, which leads to a widening of the optical band gap. This feature of the band structure is important for the electrochromism in V2O5 (Talledo and Granqvist, 1995; Lykissa et al., 2014) and also for band gap widening under photoinjection of hydrogen into V2O5 (Gavrilyuk et al., 2011). The detailed mechanism for the optical absorption is considered next, again for the case of W oxide. When ions and electrons are introduced the electrons become localized on tungsten sites and some W6þ sites turn into W5þ sites. By absorbing a photon, the inserted electrons can acquire sufficient energy to jump to an adjacent site. Transfer between two sites, called i and j, can be represented in a schematic way as (Schirmer et al., 1977; Granqvist, 1995; Ederth et al., 2004) Wi5þ þ Wj6þ þ photon/Wi6þ þ Wj5þ

(20.4)

More specifically, the electrons are thought to enter localized states lying 0.1e0.2 eV below the conduction band. The atoms are displaced so that a potential well is formed, and strong electronephonon interaction produces polarons with a linear extent of 0.5e0.6 nm (Niklasson and Granqvist, 2007). More elaborate descriptions have been given (Bondarenko et al., 2015), and the role of W4þ has been emphasized in some work (Zhang et al., 1997; Darmawi et al., 2015). Polaron-induced optical absorption in electrochromic W oxide gives a broad band centered at l z 0.85 mm and reaching into the wavelength range for visible light so that the color is blue in transmission. The spectral absorption can be theoretically understood in considerable detail (Reik and Heese, 1967; Bryksin, 1982; Triana et al., 2015).

20.3.4

Some comments on mixed electrochromic oxides

Mixed oxides can have better electrochromic properties than pure oxides. One reason for this is that additional optical transitions are possible and can yield a mixing of polaron-induced optical absorption bands. Much research has been devoted to oxides based on WeTi, WeV, WeNi, WeNb, WeMo, WeTa, MoeTi, MoeV, MoeNb, MoeCe, TieV, TieZr, IreSn, IreTa, NieAl, NieTi, NieV, NieFe, and many others (Granqvist, 2014; Lin et al., 2015c). Complexation between WO3 and an organic material gives other possibilities (Zhou et al., 2016a). Recent studies have

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been devoted to WeTi oxide (Arvizu et al., 2014) and NieIr oxide (Wen et al., 2015c, 2016b) and it has been demonstrated that small amounts of Ti or Ir can lead to significantly improved electrochemical durability. Still better electrochromic properties can be reached with ternary compositions, and the results of a comprehensive investigation by Arvizu et al. (2017) were recently reported for thin films of W1exeyTixMoyO3 with x < 0.2 and y < 0.2. The well-known ability of Ti addition to produce electrochemical durability in WO3 (Hashimoto and Matsuoka, 1991, 1992; Arvizu et al., 2014) was combined with the also wellestablished ability of Mo addition to add color neutrality to WO3 (Faughnan and Crandall, 1977; Lin et al., 2013a; Arvizu et al., 2016a; Li et al., 2016). Fig. 20.5 reports chromaticity coordinates for several oxide films with Ti contents of w10 at% and shows that approximate color neutrality can be reached (Arvizu et al., 2017). Electrochemical durability was lowered at large Mo contents and it was suitable to keep the amount of Mo below w6%. A complementary study has been reported for W1exeyNixTiyO3 films by Morales-Luna et al. (2016). Complex oxides of anodically coloring electrochromic oxides have been studied recently as well, and interesting data have been reported for oxide films of NieWeLi (Gillaspie et al., 2010), NieAleLi (Lin et al., 2013b), NieZreLi (Lin et al., 2013c), NieFeeC (Lin et al., 2015c), NieTieLi (Zhou et al., 2015), and NieLiPON (Cha et al., 2013).

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Flexible electrochromic foils: a case study

The generic electrochromic device construction in Fig. 20.4 can be varied in several ways, one of which is discussed later. The design of present interest is shown in Fig. 20.6 and comprises (1) a 0.175 mm thick foil of PET coated with a transparent electrical conducting In2O3:Sn film and a film of electrochromic W oxide, (2) another 0.175 mm thick foil of PET coated with a transparent electrical conducting In2O3:Sn film and a film based on electrochromic Ni oxide, and (3) a polymer electrolyte joining the two electrochromic oxides (Granqvist, 2015b). This foil can be employed in different ways; it can be attached onto the surface of a glass pane, it can be suspended between two glass panes in glazing thereby effectively changing the glazing from double-glazed to triple-glazed without significantly increasing the weight, and it can be used as a laminate between glass panes as illustrated in the left-hand panel of Fig. 20.6. This laminate construction can provide additional functionality to an electrochromic glazing and give spall shielding, burglar protection, sound damping, etc. An example of a laminate electrochromic glazing is given in Fig. 20.1. The optical transmittance modulation of a foil-based electrochromic glazing can be understood from Fig. 20.7 (Granqvist et al., 2010). The upper panel reports T(l) for luminous radiation after charge exchange to the indicated levels and indicates that the transmittance can be changed between widely separated extrema. The middle and lower panels in Fig. 20.7 show optical modulation of the W-oxide-based and Ni-oxide-based parts subsequent to disassembly of the electrochromic foil. It is clearly seen that the oxide films show some optical complementarity with the W-oxide-based

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film displaying cathodic coloration mainly at longer wavelengths within the luminous spectrum whereas the Ni-oxide-based film has anodic coloration particularly at shorter wavelengths. Fig. 20.8 reports optical modulation during repeated coloration and bleaching of a 240 cm2 area electrochromic foil (Granqvist et al., 2010). The upper panel indicates mid-luminous (l ¼ 0.55 mm) transmittance for two subsequent cycles set to give transmittance modulation DT equal to 55%. Half of the transmittance interval (from point 1 to 2) took w10 s, 90% of this range (from point 1 to 3) took w20 s, and the full span (from point 1 to 4) took w30 s. Slower cycling could give larger values of DT. The lower panel of Fig. 20.8 shows the evolution of the maximum and minimum transmittance during a number of colorebleach cycles. The optical modulation range stayed practically unaltered during thousands of cycles.

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Figure 20.8 Mid-luminous transmittance for an electrochromic foil of the type shown in Fig. 20.6 set to give a transmittance modulation DT equal to 55%. The upper panel reports transmittance for two subsequent colorebleach cycles (numbers 1e4 refer to the main text) and the lower panel indicates the evolution of the maximum and minimum transmittance during repeated colorebleach cycles. From Granqvist, C.G., Green, S., Niklasson, G.A., Mlyuka, N.R., von Kræmer, S., Georén, P., 2010. Advances in chromogenic materials and devices. Thin Solid Films 518, 3046e3053.

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Electrochromic foils for glazing will be exposed to ultraviolet irradiation, which can lead to a photochromic effect on top of the electrochromism (Zhang et al., 2008; Granqvist et al., 2008). The photochromism can be removed by coating the W oxide film with a protective layer such as Ta2O5 (Wang et al., 2016a). Photochromism has been observed also in Ni-oxide based films (Miyazaki et al., 2016). Colorebleach dynamics is of obvious interest for electrochromic glazing. For small surfaces, the time for transitioning from dark to fully transparent, or vice versa, can be as small as some seconds, but large glazing is slower since more electrical charge must be introduced or extracted via electrical contacts (known as “bus bars”) at one or, preferably, several of the glazing’s edges. Electrochromic glazing may display an “iris effect,” and the color change is faster at the edge than in the center. However, this often unwanted feature may be counteracted through the use of appropriate electrical drive circuitry (Degerman Engfeldt et al., 2011). The pertinent optical modulation range depends on the application of the electrochromic glazing. It is often desirable to use glazing with high bleached-state transmittance, and antireflection coatings can then be employed as long as they do not give rise to too much light scattering (“haze”) (Granqvist et al., 2008; Jonsson et al., 2010). However, if glare control is most important, it is possible to lower the colored-state transmittance radically by use of superimposed electrochromic foils. For example, if the transmittance is 10% in the dark state for an electrochromic foil, then two superimposed foils lead to a transmittance as small as w1%. Long-term durability is another critical parameter, which is discussed later. Electrochromic foil such as the one in Fig. 20.6 can be produced by roll-to-roll web coating (Granqvist, 2012; Granqvist et al., 2018b), which is a very well-established technology for combining low cost with high-productivity (Bishop, 2015a,b); this thin-film deposition process can be aligned with continuous lamination of W-oxidecoated and Ni-oxide-coated PET foils using a polymer electrolyte. The final products are then large flexible sheets appropriate for glass lamination, which can be cut to any desired size and shape (even round), and “bus bars” can be applied. Hence the manufacturing of the electrochromic foil can be separated from the site for glazing production, which leads to advantages with regard to logistics.

20.5

Towards superior electrochromic glazing: some recent results

20.5.1 Improved durability and rejuvenation of electrochromic thin films by electrochemical treatment The service lifetime of electrochromic glazing must be counted in decades. Today’s technology seems capable of achieving this, but improved durability nevertheless is of much interest since it will permit larger and faster modulation of the optical transmittance. Durability issues have been studied since the very beginning of electrochromic technology. Early work was focused on etching of W oxide films in strongly acidic electrolytes (Randin, 1978; Arnoldussen, 1981), but this problem could be

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avoided by the use Li-ion-conducting polymer electrolytes (Andersson et al., 1989; Passerini et al., 1989) which became available as outcomes of battery-related research and development. Later durability-related research has often considered entire electrochromic devices, and results can be found in many publications (Badding et al., 1997; Czanderna et al., 1999; Lampert et al., 1999; Nagai et al., 1999; Sbar et al., 1999; Tracy et al., 1999; Lee et al., 2001; Kubo et al., 2003). Recent advances toward device longevity are considered below, and it is shown that electrochemical pretreatment of electrochromic films in liquid electrolytes can significantly improve their durability and that electrochemical posttreatment in liquid electrolytes can rejuvenate degraded films. Some recent endeavors toward lifetime prediction are mentioned as well. One very recently discovered possibility to boost the electrochemical cycling durability of electrochromic WO3 films, reported by Arvizu et al. (2018), employed electrochemical pretreatment, specifically immersion of the films in an electrolyte of lithium perchlorate (LiClO4) in propylene carbonate (PC) followed by the application of a voltage of 6 V against a conventional Li/Liþ counter electrode for several hours (the polarity of the voltage was set so as to maximize optical transparency). The associated current density displayed an initial sharp decrease followed by a distinct broad peak and a final slow decrease, whereas the optical transmittance remained high and almost unchanged throughout the electrochemical treatment. Electrochemical cycling and concurrent optical transmittance measurements at a mid-luminous wavelength were performed on as-deposited and electrochemically pretreated WO3 films; data are reported in Fig. 20.9. The voltage applied to the electrochromic film was swept repeatedly between two endpoints while the current density was measured. The voltage range was chosen to be 1.5e4.0 V and the sweep rate was 20 mV/s; this range is significantly wider than in most prior work on the electrochromic properties of WO3 immersed in LiClO4ePC and was used to enable durability studies within short time spans. The areas encircled by consecutive voltage sweeps in the positive and negative directions, signifying charge capacity, decline very rapidly upon extended cycling for the as-deposited film (Fig. 20.9(a)), whereas the decrease is seen to be insignificant for the electrochemically pretreated film (Fig. 20.9(b)). Analogous results for the optical transmittance show a dramatic decrease of the optical modulation range for the as-deposited film (Fig. 20.9(c)) while the pretreated film maintained its modulation range with almost no change (Fig. 20.9(d)). The reason for the intriguing durability enhancement by electrochemical pretreatment is not known at present. Also recently, although prior to the discovery of the electrochemical pretreatment technique for achieving enhanced electrochromic properties, it was found by Wen et al. (2015a) that electrochromic films that had been degraded by excessive voltage cycling in electrolytes of LiClO4ePC could be rejuvenated and recover their initial properties by electrochemical treatment in the same kind of electrolyte. The original work was performed on WO3 films subjected to galvanostatic posttreatment in which a constant current with a density of 105 A/cm2 was passed through the film during several hours in the “bleaching direction.” The degradationerejuvenation cycle could be repeated several times with only some minor loss of efficiency (Wen et al., 2015d), and analogous galvanostatic rejuvenation was later demonstrated

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Figure 20.9 Time-dependent evolution of cyclic voltammograms (panels a and b) and corresponding optical transmittance at a wavelength of 0.55 mm (panels c and d) for a w0.3 mm thick WO3 film in as-deposited state (panels a and c) and after electrochemical pretreatment in an electrolyte of LiClO4ePC (panels b and d). Arrows indicate voltage sweep direction. From Arvizu, M.A., Qu, H.-Y., Niklasson, G.A., Granqvist, C.G., 2018. Electrochemical pretreatment of electrochromic WO3 films gives greatly improved cycling durability. Thin Solid Films 653, 1e3.

also for other cathodically coloring electrochromic materials such as TiO2 (Wen et al., 2016c) and MoO3 (Arvizu et al., 2016b). Rejuvenation of degraded WO3 films can be performed also by potentiostatic posttreatment (Wen et al., 2016a) and a specific experiment, discussed later, was performed on films degraded in the same principle manner as in Fig. 20.9. Data from this rejuvenation experiment are shown in Fig. 20.10. Specifically, a WO3 film was voltage-cycled 10 times in the range of 2.0e4.0 V, followed by a 10-min “resting” period, after which the sample was subjected to 20 harsh voltage cycles in the range of 1.5e4.0 V. The mid-luminous optical transmittance modulation then decreased rapidly, which clearly is consistent with results in Fig. 20.9. A voltage of 5.55 V was subsequently applied in the “bleaching direction,” which led to a current whose density declined rapidly at the beginning, then displayed a distinct peak, and finally dropped to a low value. The optical transmittance increased slightly when the voltage was applied, then remained fairly constant, and finally grew to the same magnitude as for the uncycled film. At the end of the experiment, voltage cycling was resumed in the

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2.0e4.0 V range, and it was verified that the initial electrochromic performance was recovered, i.e., the WO3 film had been rejuvenated as regards its electrochromic performance. Rejuvenation was possible only when the voltage was higher than w5 V and happened faster when the voltage was increased. Degradation of the electrochromic properties upon extended voltage cycling can be assigned to irreversible trapping of Li ions, following notions that “amorphous” WO3 can be described as a network of connected sites with low inter-site barriers, and permitting fast ion transport throughout the host material, and other sites with larger energy barriers which are able to trap diffusing ions (Bisquert, 2002, 2003, 2008; Bisquert and Vikhrenko, 2002). In this description, rejuvenation is connected with the release of the trapped ions under electrochemical treatment. Conclusive evidence in favor of the model was obtained by studying the Li content, including its depth profile, by advanced analytic techniques such as time-of-flight elastic recoil detection analysis (Arvizu et al., 2015) and time-of-flight secondary ion mass spectroscopy (Baloukas et al., 2017). Electrochemical rejuvenation can be accomplished also in anodically coloring electrochromic Ni oxide films immersed in LiClO4ePC; this phenomenon is associated with ion accumulation and ion release involving Li as well as Cl (Qu et al., 2017). Lifetime prediction is of obvious interest for electrochromic devices, irrespectively of strategies for electrochemical pre or posttreatment of films. Recent work in this area has been reported for electrochromic Ni-oxide-based (Wen et al., 2014, 2016d) and W-oxide-based films (Wen et al., 2017). The decline of the charge capacity during voltage cycling (cf. Fig. 20.9) could be described by a power-law or, alternatively, a stretched-exponential expression. The underlying models for degradation may be connected with dispersive chemical kinetics (Plonka, 2001) involving a variety of diffusion-limited reactions that are currently poorly understood.

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20.5.2 Electrolyte functionalization Foil-based electrochromic glazing, such as the one discussed above, embodies a layer of a polymer electrolyte which can be functionalized by nanoparticles. Recent research by Bayrak Pehlivan et al. (2012a, b) along these lines employed a “model electrolyte” of polyethyleneimineelithium bis(trifluoromethylsulfonyl) (PEIeLiTFSI), but analogous functionalization can be implemented for the electrolyte of the electrochromic foil in Fig. 20.6 (Bayrak Pehlivan et al., 2014). One interesting possibility is to use nanoparticles of a transparent electrical conductor such as In2O3:Sn, which makes it possible to create near-infrared plasmon-based absorption and lower Tsol without appreciably altering Tlum (Bayrak Pehlivan et al., 2012b), which may be important for electrochromic glazing especially in warm climates. Fig. 20.11 reports T(l) and demonstrates that intense near-infrared absorption emerges for increasing amounts of nanoparticles. For the case of 7 wt% of In2O3:Sn, the electrochromic foil is characterized by Tlum ¼ 83.3% and Tsol ¼ 56.3%, while the electrolyte remains practically haze-free. The measured results on T(l) can be reconciled with calculations, as demonstrated by the symbols in Fig. 20.11, which are based on quantitative theoretical understanding of In2O3:Sn (Hamberg and Granqvist, 1986) and on detailed descriptions (“effective-medium models”) for the optical properties of dilute suspensions of nanoparticles (Niklasson et al., 1981).

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Conclusions and perspectives

Electrochromic glazing has progressed dramatically since its beginnings more than three decades ago (Lampert, 1984; Svensson and Granqvist, 1984, 1985) and is usually based on thin films of W oxide (first reported by Deb (1973)) and Ni oxide (with a stable electrochromism first reported by Svensson and Granqvist (1986)). This kind of glazing is presently (2018) produced by at least four companies and is implemented in buildings, primarily in Europe and the USA, where it provides energy efficiency, indoor comfort, and other amenities (ChromoGenics, 2018; EControl-Glass, 2018; SageGlass, 2018; ViewGlass, 2018). Low-cost manufacturing and long-term durability are obvious requirements for successful products and, as emphasized in Section 20.4, web-coating can be deployed to create light-weight and rugged electrochromic devices comprised of large sheets or even on a roll suitable for glass lamination and incorporation in glazing. Reactive DC magnetron sputtering seems to be the deposition technology of choice. However, alternative thin-film techniques, for example involving solegel deposition (Li et al., 2015) or inkjet printing (Wojcik et al., 2015), may be put to good use. It should be noted that electrochromism does not have to rely on rare-earth elements with questionable availability and significant ecological footprint (Alonso et al., 2012), which is in marked contrast with the situation for many other purportedly “green” technologies. Recent work has shown that the electrolyte in the electrochromic device can be an eco-friendly biohybrid (Fernandes et al., 2015, 2017), which supports the notion of electrochromic technology being benign from an environmental perspective. In the future, it is likely that multifunctional electrochromic glazing will be developed, and this topic attracts a great deal of interest (Cai et al., 2016; Huang et al., 2016). For example, it is possible to combine electrochromism with energy generation (Bella et al., 2016; Cannavale et al., 2016), energy storage (Yang et al., 2014, 2016; Cai et al., 2015; Shen et al., 2016; Zhou et al., 2016b), or light-emission (Luo et al., 2017). Another kind of multifunctionality employs “dual-band” electrochromic devices which are able to modulate luminous light and near-infrared solar radiation separately (Garcia et al., 2011; Llordés et al., 2013, 2016; Runnerstrom et al., 2014; Williams et al., 2014; Wang et al., 2016b). Yet another interesting possibility for electrochromic glazing is to invoke thermochromic nanoparticles based on VO2 in the electrolyte, which gives options to automatically tune Tsol in accordance with dynamic heating and cooling requirements (Li et al., 2010, 2012; 2014; Ji et al., 2016; Granqvist and Niklasson, 2017). It should be noted that polymer-integrated VO2 nanoparticles have been demonstrated recently via high-throughput roll-to-roll fabrication (Kim et al., 2018). Furthermore, it is possible to have photocatalytic remediation of indoor air (Granqvist et al., 2007b; Stefanov et al., 2017) in combination with electrochromic glazing; the temperature increase caused by optical absorption in a darkened device can contribute significantly to the efficiency of the air purification. This Chapter has been focused on glass-based products, but this is not the only possibility for web-coated electrochromic devices. Other options may be found in

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membrane architecture, which is often used in sports stadiums, function halls, etc. (Koch, 2004; LeCuyer, 2008). The membranes of interest are based on transparent or translucent ethylene tetrafluoroethylene (ETFE) with well-documented durability even under intense solar irradiation. Coating the ETFE with a transparent electrical conductor is a critical step for making electrochromic devices, but advances in thin-film deposition (Fahlteich et al., 2015) as well as on sub-second heat treatment of thin films on polymer substrates (Skorupa and Schmidt, 2014; Weller and Jungh€ahnel, 2015) indicate that the technical challenges can be overcome. Hence, flexible web-coated electrochromic membranes are interesting possibilities for future innovative architecture.

References Ajaji, Y., André, P., 2015. Thermal comfort in an office building equipped with smart electrochromic glazing: an experimental study. Energy Procedia 78, 2464e2469. Akbari, H., Cartalis, C., Kolokotsa, D., Muscio, A., Pisello, A.L., Rossi, F., Santamouris, M., Synnefa, A., Wong, N.H., Zinzi, M., 2016. Local climate change and urban heat island mitigation techniques: the state of the art. Journal of Civil Engineering and Management 22 (1), 1e16. Aldawoud, A., 2013. Conventional fixed shading devices in comparison to an electrochromic glazing system in hot, dry climate. Energy and Buildings 59, 104e110. Aleo, F., Pennisi, A., Scalia, S., Simone, F., 2001. Optical and energetic performances of an electrochromic window tested in a “PASSYS” cell. Electrochimica Acta 46, 2243e2249. Alonso, E., Sherman, A.M., Wallington, T.J., Everson, M.P., Field, F.R., Roth, R., Kirchain, R.E., 2012. Evaluating rare earth element availability: a case with revolutionary demand on clean technologies. Environmental Science and Technology 46, 3406e3414. Anders, A., 2010. A structure zone diagram including plasma-based deposition and ion etching. Thin Solid Films 518, 4087e4090. Andersson, A.M., Granqvist, C.G., Stevens, J.R., 1989. Electrochromic LixWO3/polymer laminate/LiyV2O5 devices: toward all-solid-state smart window. Applied Optics 28, 3295e3302. Arnoldussen, T.C., 1981. A model for electrochromic tungstic oxide microstructure and degradation. Journal of the Electrochemical Society 128, 117e123. Arvizu, M.A., Triana, C.A., Stefanov, B.I., Granqvist, C.G., Niklasson, G.A., 2014. Electrochromism in sputter-deposited WeTi oxide films: durability enhancement due to Ti. Solar Energy Materials and Solar Cells 125, 184e189. Arvizu, M.A., Wen, R.-T., Primetzhofer, D., Klemberg-Sapieha, J.E., Martinu, L., Niklasson, Granqvist, C.G., 2015. Galvanostatic ion detrapping rejuvenates oxide thin films. ACS Applied Materials and Interfaces 7, 26387e26390. Arvizu, M.A., Granqvist, C.G., Niklasson, G.A., 2016a. Electrochromism in sputter deposited W1eyMoyO3 thin films. Journal of Physics: Conference Series 682, 012005/1e012005/6. Arvizu, M.A., Granqvist, C.G., Niklasson, G.A., 2016b. Rejuvenation of degraded electrochromic MoO3 thin films made by DC magnetron sputtering: preliminary results. Journal of Physics: Conference Series 764, 012009/1e012009/6. Arvizu, M.A., Niklasson, G.A., Granqvist, C.G., 2017. Electrochromic W1exeyTixMoyO3 thin films made by sputter deposition: large optical modulation, good cycling durability, and approximate color neutrality. Chemistry of Materials 29, 2246e2253.

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