Sustainable Dual-Mode Smart Windows for Energy-Efficient Buildings

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Sustainable Dual-mode Smart Windows for Energy-efficient Buildings Sílvia C Nunes, Sofia M. Saraiva, Rui F. P. Pereira, Sonia Pereira, Maria Manuela Silva, Luís D. Carlos, E. Fortunato, Rute A.S. Ferreira, Rosa Rego, and Verónica de Zea Bermudez ACS Appl. Energy Mater., Just Accepted Manuscript • Publication Date (Web): 06 Feb 2019 Downloaded from http://pubs.acs.org on February 6, 2019

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ACS Applied Energy Materials

Sustainable Dual-mode Smart Windows for EnergyEfficient Buildings Sílvia C. Nunes,*1,2 Sofia M. Saraiva,2 Rui F. P. Pereira,3 Sónia Pereira,4 Maria Manuela Silva,3 Luís D. Carlos,5 Elvira Fortunato,4 Rute A. S. Ferreira,5 Rosa Rego,2 Verónica de Zea Bermudez*2

1Chemistry

Department,

University

of

Beira

Interior,

6200-001

Covilhã,

Portugal.

[email protected]* 2Chemistry

Department and CQ-VR, University of Trás-os-Montes e Alto Douro, 5000-801 Vila

Real, Portugal. [email protected]* 3Chemistry

Department and Chemistry Center, University of Minho, 4710-057 Braga, Portugal.

4CENIMAT/I3N,

Departamento de Ciência dos Materiais, Faculdade de Ciências e Tecnologia,

2829-516 Lisboa, Portugal 5Physics

Department and CICECO - Aveiro Institute of Materials, University of Aveiro, 3810-

193 Aveiro. Portugal.

ACS Paragon Plus Environment

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KEYWORDS k-carrageenan polysaccharide, erbium triflate, NIR-emitting smart windows, energy-efficient glazing for buildings, windows daylighting control, glare reduction and heating saving

ABSTRACT Electrochromic devices (ECDs) combining visible/near infrared (NIR) transparent amorphous indium zinc oxide (a-IZO) external layers with innovative NIR emitting electrolytes composed of red seaweeds-derived kappa-carrageenan (k-Cg) polysaccharide, glycerol (Gly) and erbium triflate (ErTrif3.xH2O), are proposed as a valuable technological solution for the development of smart windows providing less heating demand, less glare and more indoors human comfort for the new generation of energy-efficient buildings. The electrolyte preparation is cheap, clean and fast. The optimized sample including 50 wt.% Gly/k-Cg and 40 wt.% ErTrif3.xH2O/k-Cg exhibits the highest ionic conductivity (1.5 × 10−4 S cm-1 at 20 ºC) and displays ultraviolet (UV)/blue and NIR emissions associated with the k-Cg based host and the Er3+ ions (4I15/2  4I13/2), respectively. The 5-layer configuration ECD tested demonstrated fast switching time (50 s), high electrochromic contrast (transmittance variations of 46/51% at 550/1000 nm), high optical density change (0.89/0.75 at 550/1000 nm), outstanding coloration efficiency (450th cycle: −15902/−13400 cm2 C−1 and +3072/+2589 cm2 C−1 at 550/1000 nm for coloration and bleaching, respectively), excellent electrochemical stability, and self-healing after mechanical damage. The ECD encompasses two voltage-operated modes: semi-bright warm (+3.0 V, transmittances of 52/61% at 550/1000 nm) and dark cold (−3.0 V, transmittances of 7/11% at 550/1000 nm).


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INTRODUCTION Buildings represent the largest energy-consuming economic sector. Half of this energy consumption corresponds to the power spent on heating, ventilation, and air-conditioning (HVAC), and artificial lighting systems.1-6 In 2010 buildings were responsible for 40% of the energy consumption and 36% of CO2 emissions in the European Union (EU).7 Space heating is the largest end-use in terms of final energy consumption accounting for two thirds of residential energy use. In the cold Scandinavian countries and in several Eastern countries, it may represent as much as 65% of the total final energy demand in buildings. However, space cooling with air conditioning has grown significantly in the last few years, especially in the Southern European countries, being responsible for the peak power in the summer period. The situation of building energy consumption in the United States of America (USA) is quite similar, HVAC and artificial lighting accounting for 30% of the total USA energy demand.8 The aforementioned figures explain why energy efficiency has become a priority target for energy policies in many countries. The EU's main legislative instruments to stimulate the improvement of the energy efficiency of buildings are the EU 2010 Energy Performance of Buildings Directive (EPBD) and the 2012 Energy Efficiency Directive.9 On November 2016 an update of EPBD was released partly to help promoting the use of smart technology in buildings. Energy-efficient windows are compulsory in a building with a sustainable use of energy. Traditional large-area glazing (windows and glass façades) guarantees indoor human comfort, good outdoors contact, and natural daylighting to buildings, but does not adapt to changing weather or changing seasons, and is not energy-efficient, enabling the inflow of too much solar energy and involving large thermal losses. Static fenestration technologies, such as blinds and


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coatings, provide some energy savings, as well as visual and thermal comfort to the building’s occupants, but do not solve entirely the problem. Blinds provide glare control, but their installation is expensive. Coatings merely block solar heat gain.10 Dynamic glazing (smart electrochromic (EC), thermochromic or photochromic windows) has great prospects in architectural window applications, allowing to regulate in real time the sunlight transmittance (daylight intake and solar energy) to internal and external changes, or simply to the occupant’s wish.2,9,11-16 Among the three categories of dynamic window technologies, EC-based ones17-20 have attracted significant attention2,17,21-26 for the fabrication of low-cost, switchable and energyperformant architectural glazing,27 because they offer, among other attributes, modulation of sunlight over a broad spectral range, controllable transmission, absorption, and/or reflectance, and open circuit memory (maintenance of the absorption state in the off state). Large-area EC glazing has been implemented worldwide in a small scale, but it is foreseen that it will occupy a niche area in the near future.28 Indeed the good performance of the current commercially available EC windows has demonstrated that this technology may reduce up to 26% of lighting energy consumption with respect to the daylighting control provided by blinds.14 The structural composition and configuration of the electrochromic devices (ECDs) influence the electrical, thermal and optical properties required for smart EC windows.29 Several technical issues must be fulfilled: electrolyte and electrodes transparency to visible light, as well as electrochemical stability within the switching voltage range, durability (30-years lifetime), high electrochromic contrast between the bleached and colored states, and aesthetic concerns (bleached state should be clear and colorless, and colored state should display neutral color). Because solar energy from the invisible solar spectrum (ultraviolet (UV) and NIR spectral


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regions) encompasses about 60% of the whole solar energy, of which 50% belongs to the NIR region, Piccolo and Simone29 suggested performance requirements for ideal EC smart windows for different climates based on three parameters: visible solar transmittance (Tvis), solar heat gain coefficient (SHGC, ranging from 0 to 1, a measure of solar thermal energy transmitted directly or indirectly (absorbed and then transmitted inward)), and overall heat coefficient (U, the heat loss of window arisen from indoor and outdoor environment temperature difference). The ideal window for cold climate should allow radiation of any wavelength from the outside except UV radiation, and reflect all radiation from the inside.2,30 Such window should exhibit Tvis  70 %, SHGC  0.6 and U  2 W m−2 K−1 to ensure the admission of as much daylighting and heat as possible.29 In contrast, the ideal window for a hot climate is a window that should reflect NIR and UV radiations, allow visible light to enter, and be completely transparent to the NIR radiation from the inside. In this case Tvis  60 %, SHGC  0.4 and U  4 W m−2 K−1 are required to shut off as much solar radiation as possible.14,29,31 The archetypal ECDs for smart window applications are all solid state devices relying almost exclusively on oxide-based EC materials.32 A traditional ECD presents a typical sandwich multilayer configuration, often noted as glass/TCO/EC1/IC/EC2/TCO/glass, where, TCO is a transparent conducting oxide (e.g., indium tin oxide (ITO)), EC1 is an active electrode layer (e.g., cathodic tungsten oxide (WO3)), IC is an ion conducting electrolyte/separator, EC2 is an ion storage electrode (e.g., anodic nickel oxide (NiO), which exhibits complementary electrochromism), and TCO is the counter electrode (e.g., ITO). Upon application of a low voltage (usually 1-3 V) to the electrodes, reduction and ion intercalation occur in WO3, and simultaneously oxidation and ion deintercalation take place in NiO. As a consequence, both electrodes become colored and the ECD darkens (on state). The process is reversible and both


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electrodes bleach simultaneously when the voltage is inversed (off state). The magnitude of the color switch can be controlled directly by the amount of inserted charge. In traditional EC windows including TCOs that are not transparent to NIR radiation, sunlight can be finely controlled, but NIR radiation is in general semi-blocked or completely blocked. In 2011 the concept of plasmonic electrochromism was introduced in the area of smart windows for the fabrication of NIR-selective ECDs.33 Typically, localized surface plasmon resonance (LSPR) peaks in doped metal oxides fall within the near- to mid-IR region. Thus, this phenomenon allows only to selectively modulate solar heat gain without sacrificing the visible transmittance used for daylighting. In 2013 Llordés et al.13 developed innovative dual-band functionality smart windows incorporating nanocomposites composed of a “framework” of nanocrystals and a glassy material with distinct optical properties, which change when the materials are electronically charged or discharged. The nanocrystals can either block NIR light or allow it to pass through, while the glassy material can transit between a transparent state and another one that blocks visible light. Based on this approach building indoor occupants can have independent control of the sunlight and solar energy passing through a window, i.e., light inflow without heat transfer or heat transmission while blocking light. Further advances of this concept resulted in the incorporation of a cool mode (NIR light blocked), apart from the bright mode (NIR and visible admitted), and the dark mode (NIR and visible blocked).34 The cool mode enables control of 90% of NIR and 80% of the visible light from the sun and takes only minutes to switch between modes. Barawi et al.35 explored, in the same context, the unique spectro-electrochemical behavior of colloidal nanocrystals of vanadium-modified titanium oxide (VTO). An electrode layer of VTO (operating selectively in the visible region) was combined with a layer of nanocrystalline WO3


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(operating selectively in the NIR, and NIR plus visible regions) to yield an ECD modulating across four distinct optical modes upon application of low voltages: entirely transparent, blocked in the visible region, blocked in the NIR region, and blocked in the visible plus NIR regions. Very recently we introduced a completely different design concept for ECDs which allows simultaneous regulation of the solar visible and NIR radiation. This type of approach, which may be readily implemented at the industrial level, is suitable for windows of buildings situated in regions that have continental climate.36 The novelty of this system lies on two premises: (1) the replacement of the usually employed ITO by amorphous indium zinc oxide (a-IZO), a material with high electrical conductivity and high optical transparency in the visible and NIR spectral regions owing to the lower carrier concentration and higher charge mobility35, 37-40; (2) the use of a di-ureasil electrolyte doped with a protonic ionic liquid (DUPIL60), prepared by sol-gel chemistry and exhibiting high transparency and high proton conductivity. The glass/aIZO/WO3/DUPIL60/NiO/a-IZO/glass device36 tested is endowed with three voltage-actuated modes: bright hot at 0.0 V, semi-bright warm at −2.0 V, and dark cold at −2.5 V. In addition it offers very high switching efficiency and optical density modulation, good cycling stability, impressive coloration efficiency (−12538/−14818 cm2 C−1 for coloration; +2901/+3428 cm2 C−1 for bleaching at 555/1000 nm), exceptional optical memory and self-healing ability following the application of a mechanical stress. The unusual electro-optical features exhibited by this ECD were justified in terms of interfacial effects related with the structure/morphology of the three oxides employed. While IZO and WO3 were amorphous with low average roughness, NiO was polycrystalline and displayed a less homogenous topography. This implied that the electric field hitting the a-IZO/WO3 and a-IZO/NiO glass plates upon application of voltage was spread more uniformly in the former case. This effect would account for the rather distinct kinetics of the


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cathodic and anodic processes the extraordinarily high coloration efficiency values (in particular those of the cathodic process), and the outstanding optical memory (extremely slow bleaching after removal of the voltage). Herein, we enlarge this concept by proposing for the first time the combined use of the visible/NIR-transparent a-IZO electrode39,41 with a NIR emitting Er3+-doped polysaccharidebased electrolyte for the development of greener dynamic windows with tunable visible/NIR admission, permanent NIR emission to further reduce heat consumption indoors, and glare attenuation ability. The polymer electrolyte employed, based on a system developed by some of us recently,42 comprises the red seaweeds-derived k-carragenan (k-Cg) host biopolymer,43 glycerol (Gly), acting as a plasticizer, and ErTrif3.xH2O (Figure 1a), which exhibits multiwavelength emission from the UV/blue to the NIR spectral regions. The ECD design adopted (Figure 1b) will allow producing a new generation of dual-modulation EC windows with several advantages with respect to the currently commercialized ones. Major benefits of this smart technology include, apart from easy application by window industry, higher SHGC (less heating demand inside buildings) and ability to reduce the glare of bright sunlight (improvement of the occupant’s comfort)44 (Figure 1c). In this work, the structure, morphology, thermal properties, ionic conductivity and photoluminescence features of the new Er3+-doped electrolytes have been investigated. The produced membranes were denoted as CGxErz, where C represents k-Cg, G stands for Gly, and x and z indicate the concentrations of Gly and ErTrif3.xH2O, respectively, with respect to k-Cg. We emphasize that the use of k-Cg as host polymer is of the utmost interest. As other polysaccharides, k-Cg is a particularly attractive candidate, because it is abundant in nature, and exhibits eco-friendly and biodegradable properties.45-46 In addition, the synthesis of the k-Cg-


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based electrolytes involves a very attractive water-based cheap, clean and fast method.42,45,47 At last, we show that the optimized electrolyte employed in the glass/a-IZO/WO3/CG50Er40/NiO/aIZO/glass ECD tested endowed the device unique self-healing ability. In terms of overall performance, this ECD displays various remarkable electro-optical attributes, in particular colossal coloration efficiency, and remarkable electrochemical stability which progressively improved with cycling.

Figure 1. Chemical structures of k-Cg, Er(CF3SO3)3 and glycerol on seaweed algae (a); Schematic illustration of the ECD prototype (b) employed for the fabrication of two mode operation energy-efficient smart windows (c).


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EXPERIMENTAL SECTION Materials k-Cg (Carrageenan CG-130, Genugel, CP Kelco, 3 and 1.3 wt.% of K+ and Ca2+, respectively),43,48 erbium (III) triflate (Er(CF3SO3)3.xH2O, abbreviated here as ErTrif3.xH2O, Aldrich, 98 %), and glycerol (Gly, Sigma-Aldrich, 99 %) were used as received. High purity deionized water (H2O) was used in all experiments.

Preparation of the k-Cg-based solution The membranes, denoted as CGxErz, were prepared according to the procedure described in detail elsewhere.42 Additional details of the synthesis of the k-Cg-based ErTrif3.xH2O-doped electrolytes have been collected in Table S1. All the produced membranes were stored at 50 ºC for 2-4 days. Under these drying conditions, the colour of k-Cg does not tend to turn brown and not observed cleavage of the glycosidic linkage.49 Methods The X-ray diffraction (XRD) patterns were acquired at room temperature with a Rigaku Dmax III/C X-ray diffractometer, operating at power 40 kV/ 30 mA, over the 2 range of 10 to 70 º at 1.2 º min−1. A monochromated CuK radiation ( = 1.5418 Ǻ) was used. Differential Scanning Calorimetry (DSC) measurements were performed on a Netzsch equipment (model DSC 204). Sealed 40 µL aluminum cans contained a mass of 2-5 mg of the membranes, which were stored over phosphorous pentoxide (P2O5) for one week, were heated from 20 to 140 ºC at 10 ºC min−1. High purity nitrogen (N2) supplied with a constant flow (25 mL min−1) was used as purge gas.


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Scanning electron microscopy (SEM) images were obtained on a Hitachi S-3400N scanning electron microscope equipped with a Bruker x-flash 5010 at high vacuum operating at 20 kV. Energy dispersive spectroscopy (EDS) was also performed with an acquisition time of 30 s. The section of the membranes were coated with gold. The Fourier Transform Raman (FT-Raman) spectra were registered with a FT Raman Bruker RFS 100/S spectrometer equipped with a Nd-YAG (1064 nm, 350 mW). The spectra were collected at room temperature over the 4000-100 cm−1 range by averaging 1500 scans at a resolution of 4 cm−1. FT-Raman band envelopes analysis was performed using the iterative leastsquares curve-fitting procedure in the PeakFit software (version 4)50. The frequency, bandwidth, and intensity of the bands were changed to obtain the best fit. Band fitting was conducted using a Gaussian cross product function, using a linear baseline correction with a tolerance of 0.2%. The standard errors of the curve-fitting procedure were less than 0.0002. Ionic conductivity (i) measurements were performed from room temperature to 60 ºC, at frequencies between 65 kHz and 0.5 Hz, using an Autolab PGSTAT-12 (Eco Chemie). The ionic conductivity was determined using the relation 𝑑

[1]

𝜎𝑖 = 𝑅𝑏𝐴

where d is the thickness, Rb the bulk resistance, and A the area of the sample.. More details can be found elsewhere 42. The emission and excitation spectra were acquired on a Fluorolog3® Horiba setups described in SI. The ECD was assembled under atmospheric conditions and at room temperature using a typical sandwich multilayer configuration51-52. The active EC layers of the device, i.e., amorphous WO3 (EC1) and polycrystalline NiO (EC2), were deposited by sputtering and e-beam


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evaporation, respectively, on a-IZO (TCO)-coated glass substrates also deposited by sputtering (see SI for details). The k-Cg-based electrolyte (IC) was sandwiched between the surface of the WO3/a-IZO-coated and the NiO/a-IZO-coated glass plates. The active surface had an area of approximately 4.84 cm2. Free space was left on each side for the electrical contacts. The optical transmittance of the ECD between 400 and 1020 nm was measured with a Gamry UV/VIS/NIR Spectro-115E spectrophotometer. The electrochromic contrast at 550 and 1000 nm was deduced from the OD values calculated from the T values in the bleached and colored states upon application of a voltage of +3.0 and −3.0 V, respectively, at 250 s intervals. The cyclic voltammetry (CV) experiments were performed using an Autolab 302 N model potentiostat/galvanostat at scans rates of 10 and 2 mV s−1. The first voltamogram was recorded from 0.0 V to +3.0 V; then to −3.0 V and then back to 0.0 V. Chronoamperometry (CA) tests were carried out using an Autolab 302 N model potentiostat/galvanostat. A voltage of −3.0 and +3.0 V was applied with a delay time of 50 s at each voltage and the current was recorded. The ECD was cycled 30 times between the colored and the bleached states. The two glass plates were then set apart. After joining the glass plates back together, the reconstructed ECD was cycled 30 plus 500 times. The cathodic and anodic charge densities were calculated through integration of the CA curves during the coloring and bleaching processes, respectively. In the set-up adopted to perform the CV and CA measurements the WO3/a-IZO substrate, the electrolyte and the NiO/a-IZO substrate acted as working electrode, reservoir of ions for insertion, and as counter/reference electrodes, respectively.


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RESULTS AND DISCUSSION Structure and Morphology The XRD patterns of the k-Cg-based membranes, non-doped and doped with ErTrif3.xH2O, are represented in Figure 2a. An intense broad and non-resolved Gaussian peak located at 20.721.1º and peaks around 15.8, 29.5 and 31.7 º are produced by all the membranes (non-doped and doped), revealing their semi-crystalline nature, with predominance of the amorphous phase. The profile of these XRD patterns is practically identical, demonstrating that the introduction of the guest salt did not have any effect on the proportion of crystallinity of the host k-Cg matrix. In all the cases the sharp Bragg reflections of the pure salt (Figure 2a, green line) are absent, meaning that the latter was efficiently dissolved the k-Cg membrane. Before analyzing the SEM data (Figure 2b) it is of interest to recall, in the light of the zipper model, a few relevant points about the gelation of k-Cg during cooling of its aqueous solutions.5356

When k-Cg is dissolved in water by heating in concentrations as low as 0.5 wt.% and in the

presence of appropriate cations, and is subsequently cooled, it forms thermoreversible hydrogels below certain temperatures. In the first stage of the gelation process the polymer chains change from random coils to helices yielding soluble clusters. In the second stage, rigid ordered double helices are formed which then aggregate into network junctions in the presence of the so-called gelling cations, such as K+ and Ca2+, which are responsible for the occurrence of intra- and intermolecular interactions, respectively.53-55 The junction zones are microcrystalline regions resulting from the association of zippers formed in turn by side-by-side association of double helices.

Consequently

the

thermoreversible

gel-sol/sol-gel

transition

associated

with

heating/cooling corresponds to the opening/closing of zippers. The crystallinity degree of k-Cg is correlated with the degree of packing of the helices.56 Recently, we concluded that the CG50Er0


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membrane contained micro-aggregates of variable shapes rich in intra- and intermolecular bridges (i.e., OSO3−  K+ O and OSO3−  Ca2+  −O3SO cross-linkages, respectively).42

Figure 2. XRD patterns (a) of the non-doped CG50Er0 (black line) and doped CG50Erz (z = 10, 20, 30 and 40) membranes and of ErTrif3.xH2O (green line). SEM images (b) of the CG50Erz membranes with z = 10% (b1), 20% (b2), 30% (b3) and 40% (b4). Scale bars = 50 m.

The SEM images of k-Cg-based ErTrif3.xH2O-doped electrolytes CG50Er10, CG50Er20, CG50Er30 and CG50Er40 demonstrate that these membranes exhibit a homogeneous texture which includes spherical micro-aggregates (Figures 2b1, 2b2, 2b3 and 2b4, respectively). In addition, it may be inferred from these images that the diameter of these aggregates decreased with the addition of more salt, whereas, in contrast, their number increased. Curiously, according to the EDS mapping images of CG50Er10, the micro-aggregates of this sample are composed of Ca2+, Er3+ and -OSO3− ions (Figures S1a1, S1a2 and S1a4 of Supporting Information, respectively). The presence of Ca2+ ions, also reported for non-doped CG50Er0 was expected.42 It is known that the gelation of k-Cg induced by Ca2+ ions occurs through the formation of intermolecular bridges


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between the cations and adjacent double k-Cg helices via electrostatic binding to two adjacent sulphate groups, i.e., OSO3−  Ca2+

−O

3SO

cross-linkages.43,57 Considering that the ionic radii

of Ca2+ and Er3+ are very close (118.0 and 106.2 pm, respectively, assuming a coordination number of 9), we may suppose that Er3+ ions bound to the k-Cg chains in a similar fashion, i.e., via [OSO3−  Er3+

 −O

3SO]

+

intermolecular cross-linkages, probably counter-balanced by the

Trif− ion. The absence of K+ ions in the micro-aggregates of the CG50Er10 membrane is, somehow, surprising, since these gelling cations existed in the commercial k-Cg employed to prepare the membranes. In k-Cg the gelation promoted by K+ ions is known to take place via the formation of intramolecular bridges: (1) a bridge formed through ionic bonding between K+ and the sulfate group of one D-galactose residue; (2) a bridge formed via an electrostatic bond between the K+ ion and the anhydro-O-3,6-ring of the other D-galactose residue, i.e., OSO3− K+





O.43,57-58 Therefore, the present result suggests that the introduction of ErTrif3.xH2O in

CG50Er0 supressed or disfavoured the formation of intramolecular interactions during the gelling process of k-Cg in the CG50Er10 membrane. The higher bonding affinity of the Er3+ ions for oxygen atoms with respect to K+ may account for this effect. As no evidences of KTrif saltingout are observed in the XRD pattern of this membrane, this would mean that in principle the K+ ions were not expelled by the matrix as a salt, but remained in it presumably “free” or weakly coordinated.

Thermal behavior The DSC curves of the CG50Erz membranes in the 25-160 °C range, reproduced in Figure 3a, display an endothermic broad peak centred around 100-129 ºC assigned to the gel-sol transition.42 In the case the non-doped CG50Er0 membrane this event is centred at 126 ºC (Figure


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3a, black line), a value slightly lower than that reported earlier.42 It is important to note that in this non-doped material the presence of Gly led to the strengthening of the gel and to the improvement of its mechanical properties. This is due to the hydrogen bonded network formed between the -OH groups of Gly and the -OH and/or -C-O-C- and/or -OSO3− polar groups of kCg. A decrease of the gel-sol transition temperature (Tg-s) of CG50Er0 down to 100, 121 and 110 ºC occurred for z = 10, 20 and 40%, respectively (Figure 3a, pink, cyanide and blue lines, respectively). In the case of CG50Er30 the Tg-s remained practically unaffected (Figure 3a, red line). To explain these findings, it is useful to mention that the zipper model considers that the heat capacity of gels is a function of the number of zippers (N), the number of parallel links (N) of a single zipper, the rotational freedom (G) of a link, and the energy required to open a link.54 The slight upshift of the gel-sol transition endotherm at z = 30% indicates that in this material further aggregation of the k-Cg double helices was induced and the junction zones became slightly more heat resistant as a result of an increase of N and reduction of G. The opposite situation happened in the more dilute samples with z = 10 and 20% and in the most concentrated membrane with z = 40%. The most drastic effect arising from salt addition was observed in the case of z = 10%. This significant drop of the Tg-s of CG50Er0 observed (26 ºC) is in perfect agreement with the destruction of the intramolecular bridges via K+ ions which weakened the gel, as suggested by the above SEM and EDS mapping data, and probably led to the increase of G. All the DSC curves present an exothermic peak above 190 ºC (Figure S2 of Supporting Information), associated with thermal decomposition of the k-Cg.42 Figure 3b allows concluding that the addition of ErTrif3.xH2O to the host k-Cg-based matrix destabilized the system, shifting the onset of degradation to lower temperatures.


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Figure 3. DSC curves (a) and variation of the decomposition temperature (Td) with salt content z (b) of the non-doped (black line/symbol) and doped CG50Erz membranes and of ErTrif3.xH2O (green line). The line drawn in (b) is a guide for the eyes.


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Ionic Conductivity The ion transport is affected by factors, such as the degree of salt dissociation, salt concentration, the degree of ionic association, ion mobility, ion concentration, the dielectric constant of the host polymer, and the segmental mobility of the polymer chains.59-60 A strategy to boost the ionic conductivity of PEs is the addition of adequate organic molecules, such as Gly, which have high dielectric constant and low vapour pressure, and thus act as plasticizers.59 These plasticizing agents help to: (1) increase the amorphous phase content; (2) dissociate ionic aggregates; (3) lower the glass transition temperature.59 At 20 ºC the Nyquist plots of the CG50Erz membranes (Figure 4a) evidence two distinct groups of samples. A group of three electrolytes, including the CG50Er20, CG50Er30, and CG50Er40 membranes, which exhibit Rb values lower than 330  inset of Figure 4a and a group of composed of the CG50Er0 and CG50Er10 electrolytes which display Rb values higher than 5000 . The same conclusion can be drawn from the Arrhenius conductivity plot of the CG50Erz membranes in the 20-60 ºC temperature range (Figure 4b). This graph reveals an increase of the value of the ionic conductivity of the CG50Erz membranes with salt addition below 60 ºC. In fact, between 20 and 50 ºC the membrane with the highest conductivity is CG50Er40 (approximately 1.5 × 10−4 and 3.1 × 10−4 S cm−1 at 20 and 50 ºC, respectively) (Figure 4b, blue line). At 60 ºC the CG50Er20 sample yields, however, the highest ionic conductivity value (ca. 3.6 × 10−4 S cm-1) (Figure 4b, purple line). The ionic conductivity of CG50Er10 (Figure 4b, cyanide line) is of the same order of magnitude as that of CG50Er0 (Figure 4b, black line), although the non-doped k-Cg membrane displays slightly higher ionic conductivity in the 30-50 ºC temperature interval. It is of interest to recall at this point that CG50Er40, the electrolyte with the highest ionic conductivity below 60 ºC, exhibits a homogenous texture, where evidences of


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the formation of a significant number of small micro-aggregates due to intermolecular and intramolecular linkages were visible by SEM/EDS (Figures 2b4 and S1b of Supporting Information). In this salt-rich sample the concentration of charge carriers is clearly high. Some of the species responsible for the conductivity observed at this particular salt concentration might be42: (1) K+, Ca2+ and Er3+ cations, and Trif− anions; (2) Water protons hopping from oxygen atoms of -OSO3− to -OSO3− groups of non-cross-linked k-Cg chains; (3) Water protons hopping from oxygen atoms of OH- to OH- groups of k-Cg/Gly; (4) Water protons hopping from oxygen atoms of -C-O-C- to -C-O-C- groups of k-Cg. Proton transport in the k-Cg-based acidic membrane probably involves the dissociation of the proton from the hydrophilic sulphonic groups, followed by the transference to the water molecules of the first hydration shell.42

FT-Raman spectroscopy In an attempt to elucidate the ionic association in the CG50Erz membranes, the FT-Raman spectra were recorded. Raman spectroscopy is a very powerful tool investigate ionic association in PEs. These studies involve the analysis of ion probes, such as the Trif− ion, the vibration modes of which undergo characteristic changes (e.g., frequency shifts, splitting and/or intensity variations) upon coordination. Several species may be present in polymer electrolytes: (a) “free” or weakly bonded ions with high mobility; (b) cations bonded strongly to the host polymer and with low mobility; (c) ionic aggregates, such as contact ion pairs and ionic multiplets, with low to moderate mobility.


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Figure 4. Nyquist plots at 20 °C (a) and Arrhenius conductivity plot (b) of the CG50Erz membranes.

In the present section FT-Raman spectra in the region characteristic of the symmetric stretching vibration of the SO3 group (sSO3) of the CG50Erz membranes were analysed (Figure S3a of Supporting Information). Because the sSO3 band is superimposed with that due to the stretching vibration mode of the S=O group of the sulfate ester (-SO3H unit) of k-Cg, at 1063 cm-


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1,61

it was necessary to first subtract the FT-Raman spectrum of the matrix from those of the

Er3+-doped k-Cg-based membranes. The sSO3 band of the subtracted FT-Raman spectra of the CG50Erz membranes with z = 10 and 20% was resolved into three components: a sharp band at 1031 cm−1 and two weak shoulders around 1037 and 1026 cm−1 (Figure S3b of Supporting Information). In the case of the more concentrated CG50Erz membranes with z = 30 and 40% a new component emerged at 1042 cm−1 (Figure S3b of Supporting Information). In all the spectra the 1031 cm−1 event, assigned to ‘‘free’’ ions, is markedly stronger than the shoulders (Figure S3b of Supporting Information).62 The events found at 1026 and at 1037 cm−1 are tentatively associated with weakly coordinated triflate ions located in two different anionic environments.63 The band at about 1042 cm−1 is ascribed to contact ion-pairs.63-64 In conclusion, the spectroscopic analysis carried out provided evidence that, as expected, some of the charge carriers of CG50Er40 are very likely ‘‘free’’ Trif− ions or weakly coordinated species.

Photoluminescence The CG50Erz membranes display UV/blue and NIR emission ascribed to the k-Cg-based host65 and to the Er3+ 4I15/24I13/2 transition, respectively, as illustrated in Figures 5a and 5b for a selected sample. The excitation spectrum monitored around the broad-band host emission peak (420 nm) reveals two main excitation paths at 280 and 360 nm (inset in figure 5a). We note that broad band host emission in the UV/blue spectral regions is overlapped by a series of intra-4f11 self-absorptions, suggesting the presence of host-to Nd3+ radiative energy transfer (‘‘inner filter’’ effect).66


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ECD tests The success of an ECD depends on the materials’ spectral response, visual uniformity, power requirements, reversibility of charge, durability and the ability to fabricate large area devices economically.67 An ECD including the electrolyte with the highest ionic conductivity (i.e., CG50Er40) and a-IZO (resistivity of 7.97×10−4 W cm, mobility of 52.1 cm2 V−1 s−1 and carrier concentration of 1.50×1020 cm−3) was assembled (glass/a-IZO/WO3/CG50Er40/NiO/aIZO/glass). Its performance was characterized by means of the following technical parameters: switching speed (time τ required for the coloring/bleaching process), switching efficiency (optical contrast measured by the transmittance change ΔT = Tbleached-Tcolored, in %, at a given wavelength), optical density (optical modulation measured by ΔOD = −log (Tcolored/Tbleached)), and the coloration efficiency (CE = ΔOD/ΔQ, where Q in the inserted/desinserted charge density). The ECD was subject to the following steps: (1) 30 chronoamperometry (CA) cycles (± 3V, 50 s); (2) 250 s at +3 V and 250 s −2 V (Figure S4 of Supporting Information); (3) 250 s at −3 V and 250 s at –3 V (Figure S4 of Supporting Information); (4) cyclic voltammetry (CV) cycling at 10 and 5 mV s-1 (2 cycles) (± 3V) (Figure S5 of Supporting Information); (5) 1 CV cycle at 2 mV s−1 (± 3V); (6) mechanical stress; (7) 530 CA cycles (± 3V, 50 s). The Tbleached values of the ECD prior to CV and CA cycling were 52 and 61% at 550 and 1000 nm, respectively (Figure 5, orange line, Table S2 and Figure S6a of Supporting Information). Tcolored values of 7% (550 nm) and 11% (1000 nm) were measured after step 7 (Figure 5, blue line, and Table S2) in area 1 (Figure S6b of Supporting Information). T values of 45/50% and OD values of 0.89/0.75 resulted at 550/1000 nm (Table S2). It is crucial to mention that in the first cycles, voltage application did not result in homogeneous coloring and dark blue spots of variable size were


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visible in the entire area of the ECD (Figure S6b of Supporting Information). However, the colored area increased progressively with subsequent operation.

Figure 5. Emission spectra of the CG50Er40 membranes in the UV/blue (excited at 320 nm (black line) and 360 nm (orange line)) (a) and NIR (b) spectral ranges. The inset in (a) shows the excitation

spectrum

monitored

at

420

nm.

Transmission

spectra

of

the

glass/a-

IZO/WO3/CG50Er40/NiO/a-IZO/glass ECD in the 450-1020 nm range at the bleaching (+3 V, orange line, 1st cycle) and coloring (−3 V, blue line, after CV and CA cycling) states (c). Measurements were performed in area 1 of the ECD surface (Figure S6b of Supporting Information).


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Figure 6 allows inferring that the ECD displays excellent cycling stability. This behaviour was evaluated before (Figures 6a/6d) and after (Figures 6b-e,c-f) applying a mechanical stress. The latter effect consisted of tearing apart, on purpose, the a-IZO/NiO- and the electrolyte/WO3/a-IZO-coated glass plates after ca. 30 CA cycles between the bleached and coloured states. Figure6b/6e and 6c/6f confirm that the performance of the reconstructed ECD submitted to further cycling did not differ essentially from that of the original ECD (Figures 6a/6d). This unusual feature is tentatively correlated with the rupture/reformation of the extensive hydrogen-bonded network present in CG50Er40 that resulted from the pressure exerted to separate the plates apart. Equally interesting are the coloration efficiency values calculated at the 450th cycle at 550/1000 nm which may be considered outstanding: −15902/−13400 cm2 C−1 for coloration (Qin = -0.05597 mC cm-2) and +3072/+2589 cm2 C−1 for bleaching (Qout = + 0.2897 mC cm-2). Normally, the ECD concept introduced in this work would enable, apart from continuous NIR emission, high solar visible and NIR admission in the bleached state, i.e., a bright hot mode, as reported previously.36 However, because the transmittance exhibited by CG50Er40 in the visible and NIR spectral regions is not high (slightly higher than 50/60% at 550/1000 nm) (Figure 5c, orange line), the present ECD lacks a truly bright hot mode, offering a twomodulation operation encompassing a semi-bright warm mode (+3.0 V, visible and NIR light admitted) and dark cold mode (−3.0 V, visible and NIR light blocked). At this stage it is useful to compare

the

performance

of

this

ECD

with

that

of

the

analogue

glass/a-

IZO/WO3/DUPIL60/NiO/a-IZO/glass ECD introduced very recently which only differs in the nature of the electrolyte.36 In the latter device the bright hot mode (0.0 V) corresponds to considerably higher transmittances (70%/83% at 555/1000 nm), whereas the semi-bright warm


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mode (−2.0 V) corresponds to considerably lower transmittances (37%/35% at 555/1000 nm).36 The results obtained here were, however, expected. The Tbleached values reported in the literature for the visible spectral region of ECDs incorporating electrolytes made from polysaccharides are of the same order of magnitude of the present ones 68% for agar-based electrolytes,68-69 58% for chitosan-based electrolytes70 and 55-60% for starch-based electrolytes.71

Figure 6. Variation of the current density (a-c) and of the inserted (-Qin, closed symbols)/deinserted

(Qout,

open

symbols)

charge

density

(d-f)

of

the

glass/a-

IZO/WO3/CG50Er40/NiO/a-IZO/glass ECD during CA with voltage steps of −3 V and +3 V at every 50 s. Red symbols - before the applied mechanical stress, blue symbols - after the applied mechanical stress.

The use of the CG50Er40 electrolyte instead of an electrolyte with higher transparency both in the visible and NIR spectral regions was deliberate. The goal was to create an ECD (and


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ultimately a window) providing additional comfort to the buildings occupants, without jeopardizing the access to daylight and outdoor views. In particular we wanted to address glare effects which are often associated with high-transmission windows, especially on sunny days in rooms with inappropriate orientation.44 In short, globally the present glass/a-IZO/WO3/CG50Er40/NiO/a-IZO/glass ECD and the glass/a-IZO/WO3/DUPIL60/NiO/a-IZO/glass ECD proposed earlier36 resemble closely in terms of CE magnitude and self-healing. In practice, the combined use of the NIR emitting CG50Er40 electrolyte and a-IZO electrode resulted in four major advantages: glare attenuation, more important heat consumption reduction, superior electrochemical stability, and the use of a greener electrolyte synthesized in a cleaner and faster way.

CONCLUSIONS A new ECD approach is introduced in this work that implies the incorporation of a NIR-emitting polysaccharide-based electrolyte membrane and the use of visible/NIR transparent high electrical mobility a-IZO TCO instead of the usual ITO. Novel green electrolytes based on k-Cg, ErTrif3.H2O and Gly were prepared using a water-based cheap and straightforward synthesis process. The optimized CG50Er40 membrane exhibits 1.5 × 10−4 S cm−1 at 20 ºC and presents UV/blue and NIR emission associated with the k-Cg based host and the Er3+ ions (4I15/2  4I13/2), respectively. The prototype ECD tested demonstrated fast switching time, high switching efficiency, high optical density modulation, extraordinarily high coloration efficiency, excellent electrochemical stability, and self-healing behavior following mechanical stress. This ECD provides a dual voltage-controlled visible/NIR light blocking function: semi-bright warm mode


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(+3.0 V, NIR and visible light admitted) and dark cold mode (−3.0 V, visible and NIR light blocked). Major advantages of the present ECD design are the attenuation of glare and the significant reduction of indoors heat consumption. The electrolytes proposed here and the use of a-IZO TCOs produced at room temperature which is compatible with low cost flexible substrates, open exciting opportunities for the development of sustainable zero-energy smart windows to cope with the energy challenges in the building sector. Improvements of the electrolytes are on-going. Judicious mixtures of various carrageenans (k-Cg, iota-Cg and lambda-Cg) are being employed to enhance the mechanical properties and dimensional stability above 50 ºC. The erbium salt was employed here as a model compound. Electrolytes incorporating erbium complexes exhibiting high-very high quantum efficiency are now being used to yield more efficient NIR emitting systems. These works have also been extended to other lanthanide ions. Visible/NIR transparent TCOs other than a-IZO are also being used.

ASSOCIATED CONTENT Supporting Information. SEM images, DSC curves, FT-Raman and curve-fitting the in sSO3 region, Transmission spectra, Cyclic voltammograms, Photographs of the bleached (a) and colored (b) states, Relevant details of the synthesis and Optical parameters.

AUTHOR INFORMATION Corresponding Author *[email protected] and [email protected]


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Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT This work was supported by FEDER, through COMPETE and Fundação para a Ciência e a Tecnologia (FCT) (FCOMP-01-0124-FEDER-037271, Pest-OE/QUI/UI0616/2014 and and UID/CTM/50025/2013), project LUMECD (POCI-01-0145-FEDER-016884 and PTDC/CTMNAN/0956/2014), project UniRCell (Ref. SAICTPAC/0032/2015, POCI-01-0145-FEDER016422), and by the Portuguese National NMR Network (RNRMN). Sílvia C. Nunes acknowledges FCT for grants (Post-PhD Fellowships of UniRCell and LUMECD projects). Rui F. P. Pereira acknowledges FCT for SFRH/BPD/87759/2012 grant. The authors thank CPKelco (U.S.A.) for providing the k-carrageenan sample.

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(61) Fujishima, M.; Matsuo, Y.; Takatori, H.; Uchida, K. Proton-conductive acid–base complex consisting of κ-carrageenan and 2-mercaptoimidazole. Electrochemistry Communications 2008, 10, 1482-1485. (62) Wendsjo, A.; Lindgren, J.; Thomas, J. O.; Farrington, G. C. The Effect of Temperature and Concentration on the Local Environment in the System M(Cf3so3)(2)Peo(N) for M=Ni, Zn and Pb. Solid State Ionics 1992, 53, 1077-1082. (63) de Zea Bermudez, V.; Ostrovskii, D.; Lavoryk, S.; Cristina Goncalves, M.; Carlos, L. D. Urethane cross-linked poly(oxyethylene)/siliceous nanohybrids doped with Eu3+ ions Part 2. Ionic association. Phys Chem Chem Phys 2004, 6, 649-658. (64) Huang, W.; Frech, R.; Wheeler, R. A. Molecular structures and normal vibrations of trifluoromethane sulfonate (CF3SO3-) and its lithium ion pairs and aggregates. The Journal of Physical Chemistry 1994, 98, 100-110. (65) Fateixa, S.; Carvalho, R. S.; Daniel-da-Silva, A. L.; Nogueira, H. I. S.; Trindade, T. Luminescent Carrageenan Hydrogels Containing Lanthanopolyoxometalates. European Journal of Inorganic Chemistry 2017, 2017, 4976-4981. (66) Gonçalves, M. C.; Silva, N. J. O.; de Zea Bermudez, V.; Sá Ferreira, R. A.; Carlos, L. D.; Dahmouche, K.; Santilli, C. V.; Ostrovskii, D.; Correia Vilela, I. C.; Craievich, A. F. Local Structure and Near-Infrared Emission Features of Neodymium-Based Amine Functionalized Organic/Inorganic Hybrids. The Journal of Physical Chemistry B 2005, 109, 20093-20104. (67) Granqvist, C. G. Oxide electrochromics: An introduction to devices and materials. Sol Energ Mat Sol C 2012, 99, 1-13. (68) Pawlicka, A.; Grote, J. G.; Kajzar, F.; Silva, M. M.; Rau, I. Agar and DNA Bio-Membranes for Electrochromic Devices Applications. Non-Linear Optics and Quantum Optics 2013, 45, 113-129. (69) Raphael, E.; Avellaneda, C. O.; Aegerter, M. A.; Silva, M. M.; Pawlicka, A. Agar-Based Gel Electrolyte for Electrochromic Device Application. Mol Cryst Liq Cryst 2012, 554, 264-272. (70) Alves, R.; Sentanin, F.; Sabadini, R.; Fernandes, M.; Bermudez, V.; Pawlicka, A.; Silva, M. Samarium (III) triflate-doped chitosan electrolyte for solid state electrochromic devices. Electrochimica Acta, 2018; 267, 51-62. (71) Pawlicka, A.; Dragunski, D.; Guimarães, K.; Avellaneda, C. Electrochromic Devices with Solid Electrolytes based on Natural Polymers. Mol Cryst Liq Cryst 2004, 416, 105-112.


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Page 34 of 41

SYNOPSIS

High-performance electrochromic devices incorporating visible/near-infrared transparent amorphous indium zinc oxide with near-infrared emitting kappa-carrageenan-based electrolytes doped with erbium triflate, are proposed for application in smart windows of energy-efficient buildings


34

Page 35 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41




a

29.5 31.7

15.8 18.3

21.1

z 40 30 20 10 0

10

b1

b2

b3

b4

20.7

b

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41

Page 36 of 41

20

30

40

2 (º)

50

60

70


Page 37 of 41

exo

a

200

z

-1

Heat flux (mW.g )

40 30 20

endo

10

0

20

40

60

80

100

120

140

160

T(ºC) 220

b 210 200

Td (ｺºC)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41


190 180 170 160

0

10

20

30

40

z (%)



Page 38 of 41

80000

a

Z'' ()

60000

40000

99 Hz

65 kHz 750 500

20000 250 0

0 0

10000

20000

0

30000

200

40000

400

600

800

50000

60000

Z' ()

T (ºC) 60

50

40

30

20

z

b

0 10 20 30 40

10-3

-1  (Scm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41

10-4

10-5

10-6

3,0

3,1

3,2

-1

3,3

3,4

1000/T (K )


Page 39 of 41

b

250 4

4

300

350

400

4

I15/2 F7/2 4

4

I15/2 G11/2

4

4

I15/2 I13/2

Intensity (a.u.)

Intensity (a.u.)

a

2

I15/2 H11/2

340 390 440 490 540 590 640

1480 1500 1520 1540 1560 1580 1600

Wavelength (nm)

Wavelength (nm)

c Transmittance (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41


70

Near-infrared

Visible

60 50 40 30

51 %

46 %

20 10 0 450

500

550

600

650

700

750

800

Wavelength (nm)


850

900

950 1000


-2 Current density (mA cm )

0.09

a

b

0.03 0.00 -0.03 500

1000

1500

2000

2500

30000

500

1000

1500

2000

2500

30000

5000

Time (s)

Time (s) -2

c

0.06

0

Charge density (mA cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41

Page 40 of 41

d

0.40

10000

40000 Time (s)

50000

e

f

0.30 0.20 0.10 0.00

0

5

10

15

Cycle

20

25

30 0

5

10

15

Cycle

20

25


30

0

100

200

Cycle

300

400

500

Dark cold

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41


Semi-bright warm

Page 41 of 41


Sustainable Dual-Mode Smart Windows for Energy-Efficient Buildings

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