Large-Scale Multifunctional Electrochromic-Energy Storage Device

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Large-Scale Multifunctional Electrochromic-Energy Storage Device Based on Tungsten Trioxide Monohydrate Nanosheets and Prussian White Zhijie Bi, Xiaomin Li, Yongbo Chen, Xiaoli He, Xiaoke Xu, and Xiang Dong Gao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b08656 • Publication Date (Web): 15 Aug 2017 Downloaded from http://pubs.acs.org on August 17, 2017

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ACS Applied Materials & Interfaces

Large-Scale Multifunctional Electrochromic-Energy Storage Device Based on Tungsten Trioxide Monohydrate Nanosheets and Prussian White Zhijie Bi,a,b Xiaomin Li,*a Yongbo Chen,a,b Xiaoli He,a,b Xiaoke Xu,a,c and Xiangdong Gaoa a

State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai

Institute of Ceramics, Chinese Academy of Sciences, No. 1295 Dingxi Road, Shanghai, 200050, P.R. China b

University of Chinese Academy of Sciences, No. 19A Yuquan Road, Beijing, 100049, P.R.

China c

School of Materials Science and Engineering, Shanghai Institute of Technology, No. 100

Haiquan Road, Shanghai, 201418, P.R. China

ACS Paragon Plus Environment

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ABSTRACT

A high-performance electrochromic-energy storage device (EESD) is developed, which successfully realizes the multifunctional combination of electrochromism and energy storage by constructing tungsten trioxide monohydrate (WO3H2O) nanosheets and Prussian white (PW) film as asymmetric electrodes. The EESD presents excellent electrochromic properties of broad optical modulation (61.7%), ultrafast response speed (1.84/1.95 s) and great coloration efficiency (139.4 cm2 C−1). In particular, remarkable cyclic stability (sustaining 82.5% of its initial optical modulation after 2500 cycles as an electrochromic device, almost fully maintaining its capacitance after 1000 cycles as an energy storage device) is achieved. The EESD is also able to visually detect the energy storage level via reversible and fast color changes. Moreover, the EESD can be combined with commercial solar cells to constitute an intelligent operating system in the architectures, which would realize the adjustment of indoor sunlight and the improvement of physical comfort totally by the rational utilization of solar energy without additional electricity. Besides, a scaled-up EESD (10 × 11 cm2) is further fabricated as a prototype. Such promising EESD shows huge potential in practically serving as electrochromic smart windows and energy storage devices.

KEYWORDS tungsten trioxide, Prussian blue, multifunctional, electrochromic, supercapacitor, energy storage


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1. INTRODUCTION At present, energy saving and emission reduction are of increasing concern around the world due to the excessive resource consumption and environment pollution.1–3 In line with worldwide efforts, the investigation and exploitation of devices toward energy conversion, storage and conservation such as solar cell, lithium-ion battery, electrochemical capacitor and smart window have drawn widespread attention in the past few years.3,4 Smart windows based on electrochromic (EC) materials which adjust the interior sunlight by color variation via the insertion/extraction of small-sized ions with accompanying electrons could lower the energy consumption and improve the indoor comfort.3,5–8 Supercapacitors have been deemed as promising candidates for energy storage mainly on account of their high capacitances, good cycling stabilities and rapid charging/discharging capabilities.9,10 Pseudocapacitor, using reversible redox reactions to store energy, delivers higher capacitance than electrochemical double layer capacitor (EDLC) which is charged/discharged by adsorption/desorption of ions.11– 13

Interestingly, both electrochromic smart windows and pseudocapacitors are relied on the

reversible redox reactions of the same active materials, and they both own sandwich-like device configurations.3 Thus, it would be greatly appealing to combine electrochromism and energy storage functions into one electrochromic-energy storage device for multiple applications. Extensive studies have provoked the enthusiasm of investigators in the territory of such multifunctional devices.14–18 For instance, Tian et al.19 synthesized polyaniline (PANI) and W18O49 nanowires as intelligent supercapacitor electrode materials with energy storage level indication function. Shen et al.20 demonstrated a flexible electrochromic supercapacitor electrode based on WO3 film and Ag nanowires. Chen et al.21 fabricated NiO nanoflakes for integrated energy storage and electrochromic applications. Nevertheless, previous works chiefly centered


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on the single electrodes, and studies on combined stand-alone devices simultaneously acting as electrochromic smart windows and pseudocapacitors are relatively rare. Chen et al.22 developed smart supercapacitors by depositing polyaniline onto carbon nanotube sheet as electrodes. Zhu et al.23 designed parallel-structured hybrid supercapacitors using polypyrrole or MnO2 as electrode materials. Actually, such hybrid devices are mainly served as supercapacitors with energy level indicating function, but they cannot be directly used as electrochromic devices, since these hybrid devices are divided into two independent parts which exhibit different optical transmittances. Wang et al.3 assembled a bifunctional device that combined electrochromism and energy storage in a symmetric configuration by using polyaniline as both anode and cathode. Yang et al.24 fabricated pseudocapacitive smart windows by evaporating WO3 on fluorine-doped tin oxide (FTO) glasses as electrodes. However, a rather narrow optical modulation could be found due to adopting the identical materials as both anode and cathode, since one electrode is bleached and inevitably, another electrode is colored, no matter negative or positive voltage is applied. Therefore, it still remains challenging to develop practical multifunctional devices with conspicuous optical modulation and high energy storing performance to date. Designing electrochromic-energy storage device based on two suitable asymmetric electrode materials might be a feasible strategy. As is generally known, complementary electrochromic devices (ECDs) comprised of two proper electrochromic electrodes could improve the optical modulation and coloration efficiency.25,26 Besides, based on two dissimilar active materials operating in matched potential windows, asymmetric supercapacitors (ASCs) could increase the capacitance and energy density.27,28 Choosing two appropriate materials for the positive and negative electrodes of the multifunctional device is a primary mission. Two general principles must be followed: (1) the


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two materials must concurrently possess electrochromic and capacitive functions, and (2) the materials should operate in matched potential windows in the same electrolyte to cooperate for complementary color change and synergetic energy storage. The WO3 and Prussian blue (Iron (III) hexacyanoferrate (II), PB), as electrochemical active materials, are of considerable interest owing to the low cost, large optical modulation and great coloration efficiency.29–34 Both materials possess suitable structures for the transport of Li+, which provoke interesting energy storage properties. Furthermore, when redox reactions occur with charge transfer, WO3 and PB will simultaneously undergo electrochromic processes. WO3 is a typical cathodic electrochromic material, which takes on blue color (LiαWO3) in reduction state and transparent (WO3) in oxidation state.35–37 Conversely, PB, an anodic electrochromic material, operates in opposite potential ranges compared with WO3, displaying blue color (Prussian blue, PB) in oxidation state and transparent (Prussian white, PW) in reduction state.1,38,39 Thus, a device using WO3 and PW as negative and positive electrodes in Li+ electrolyte could well realize the combination of electrochromism and energy storage. To the best of our knowledge, a stand-alone device comprised of WO3 and PW electrodes serving as both a complementary ECD and an asymmetric supercapacitor has not been reported to date. Herein, an electrochromic-energy storage device (EESD) is demonstrated, which can simultaneously act as a complementary ECD and an asymmetric supercapacitor by employing WO3H2O nanosheets and PW as negative and positive electrodes. The EESD exhibits broad optical modulation, ultrafast response time, great coloration efficiency and long cycle life. Besides, a scaled-up EESD (10 × 11 cm2) is further fabricated as a prototype. The proposed EESD is also able to visually monitor the level of stored energy by color variation. Moreover, the EESD could be combined with commercial solar cells to constitute an intelligent operating


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system, which would realize the adjustment of indoor sunlight and the improvement of physical comfort totally by the rational utilization of solar energy. 2. EXPERIMENTAL 2.1. Synthesis of WO3H2O Nanosheets and PB Film Briefly, 12.5 mmol of Na2WO42H2O and equivalent amount of citric acid were uniformly dissolved into 100 ml deionized water. Afterwards, 5 M hydrochloric acid (HCl) solution was instilled in the above solution until the pH value was equal to 1. A transparent FTO glass (~15 Ω/□) was then transferred into the growth solution, which was maintained at 90 °C for 15 min. Then, the FTO substrate with WO3H2O nanosheets was fetched out and flushed by water several times. Finally, the FTO substrate was baked at 60 °C for 1 h. PB was coated on another FTO glass by electrodeposition as reported previously.1 Briefly, the electrolyte for electrodeposition involved 0.01 M K3[Fe(CN)6], 0.01 M FeCl3 and 0.05 M KCl. The electrodeposition was carried out using a CHI660B electrochemical workstation with FTO glass as working electrode, Pt plate as counter electrode and Ag/AgCl as reference electrode, under current density of 50 µA cm–2 for 400 s. Afterwards, the as-prepared PB films were flushed by water several times. Finally, the products were baked at 60 °C for 1 h. 2.2. Assembly of the Electrochromic-Energy Storage Device The PB film was pre-bleached to PW in LiClO4 propylene carbonate (PC) electrolyte at –0.2 V for 20 s before the device assembly. Then, the EESD was assembled using WO3H2O nanosheets and PW as negative and positive electrodes, 1 M LiClO4 in PC as the electrolyte and a pre-cut gasket as spacer. Finally, the device was sealed with epoxy glue.


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2.3. Characterizations The phase structures were detected by X-ray diffraction (XRD) using a Bruker D8 discover diffractometer. The compositions and morphologies were investigated via a Hitachi S-4800 scanning electron microscope (SEM) attached with an Energy-Dispersive Spectrometer (EDS). The Fourier transform infrared (FTIR) spectrum was measured by a Lambda Scientific FTIR7600 spectrometer. The electrochromic properties of the single electrodes were performed by a CHI660B electrochemical workstation with a three-electrode configuration, where WO3H2O nanosheets and PB were successively served as the working electrodes; Pt plate, Ag/AgCl and 1 M LiClO4 in PC played the roles of counter electrode, reference electrode and electrolyte. The performance measurements of EESD were tested in a two-electrode system, where WO3H2O nanosheets and PW were utilized as the two electrodes. The corresponding optical properties were recorded using an UV-Vis-NIR spectrophotometer (Persee TU-1901). 3. RESULTS AND DISCUSSION 3.1. Characterizations of WO3H2O and PB Electrodes Figure 1a displays the XRD pattern for the as-prepared tungsten oxide on the FTO substrate. After subtracting several diffraction peaks of FTO (JCPDS No. 46-1088), all peaks could be well indexed to orthorhombic phase structure of tungsten trioxide monohydrate (WO3H2O, JCPDS No. 43-0679) with the following lattice parameters of a = 5.238, b = 10.704, c = 5.12 Å. The peaks for as-prepared WO3H2O film present sharp shapes, suggesting its high crystalline quality. Figure 1b depicts the top-view morphology for WO3H2O on FTO glass. It can be seen that the WO3H2O made up of uniformly crossed nanosheets with length of 300–500 nm and thickness of 20–50 nm for each nanosheet possesses an open framework, offering enlarged


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surface area and decreased Li+ diffusion distance, which would assist charge transfer during electrochemical processes. The cross-sectional view image of WO3H2O nanosheets (inset of Figure 1b) reveals that the nanosheets of ~ 450 nm in height are arranged uniformly and perpendicularly on FTO substrate. The EDS spectrum of as-prepared PB film is shown in Figure 1c, where except the component elements of FTO glass, the peaks of C, N and Fe detected in the spectrum prove the successful formation of PB. The top-view morphology of PB film is illustrated in Figure 1d. The PB film exhibits a smooth morphology with several micro cracks on the surface, which were possibly caused by post-baking step.25 Nevertheless, the micro cracks cannot be detected by the unaided eyes. The thickness of PB film is approximately 165 nm (estimated from the inset of Figure 1d). The FTIR spectrum of the PB film is demonstrated in Figure S1. The intensive peak at 2076 cm-1 corresponds to the C≡N stretching vibration. The peaks appearing at 501 and 603 cm-1 stand for the out-of-plane and in-plane bending vibrations of Fe─C bond. The two peaks at 1616 and 3423 cm-1 represent the bending and stretching vibrations of O─H bonds, which are derived from the crystal water in the frames.


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Figure 1. (a) XRD pattern of the WO3H2O nanosheets on FTO substrate. (b) Top-view SEM image of WO3H2O nanosheets (Cross-sectional view image of WO3H2O is presented in inset). (c) EDS pattern of the PB film electrodeposited on FTO substrate. (d) Top-view SEM image of PB film (Cross-sectional view image of PB is presented in inset). The electrochemical properties of WO3H2O nanosheets and PB were investigated prior to device assembly in order to verify the feasibility of our strategy as shown in Figure S2. Both materials possess electrochromic and capacitive functions, and they operate in matched potential windows. The WO3H2O is colored at negative potential and bleached at positive potential, while PB is bleached at negative potential and colored at positive potential. Thus, it would be quite feasible to adopt WO3H2O and PB as two ideal electrode materials of the electrochromic-energy storage devices, since they show opposite color changes under the same potential polarity, or in other words they show accordant color changes under the opposite potential polarity. Once the two electrode materials are assembled into a device, the colors of two electrodes would simultaneously darken or fade away under applied voltage to broaden the optical modulation and the capacitive processes can be typically accompanied by color changes. 3.2. Electrochromic and Capacitive Performance of the EESD The functioning mechanisms of the EESD are schematically illustrated in Figure 2. WO3H2O is the negative electrode, PW is the positive electrode and LiClO4 in PC is the electrolyte for the EESD (Figure 2a). As a complementary ECD, during the coloring procedure, when the positive bias applies on the PW electrode, Li+ and electrons are extracted from PW matrix (Li4FeII4[FeII(CN)6]3 (PW)→FeIII4[FeII(CN)6]3 (PB)), bringing out gradually darkening color. Meanwhile, WO3H2O is reduced to a deep blue color by acquiring electrons, coupled with the


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intercalation of Li+ forming LiαWO3H2O (WO3H2O→LiαWO3H2O) under negative bias (shown in Figure 2b). Conversely, during the bleaching procedure, LiαWO3H2O is oxidized to its initial colorless state (LiαWO3H2O→WO3H2O), while PB acquires Li+ and electrons to be reduced to colorless PW (FeIII4[FeII(CN)6]3 (PB)→Li4FeII4[FeII(CN)6]3 (PW)) (shown in Figure 2c). Similarly, when the device is used as a supercapacitor, the reactions in the charging process should be: positive electrode: Fe(II)→Fe(III), negative electrode: W(VI)→W(V), In the discharging process: positive electrode: Fe(III)→Fe(II), negative electrode: W(V)→W(VI).

Figure 2. Schematic diagrams of operating mechanisms of (a) the EESD in (b) coloring/charging and (c) bleaching/discharging processes.


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Both electrochromic and pseudocapacitive behaviors are stemmed from the reversible redox reactions between W(VI)/W(V) and Fe(II)/Fe(III), which are accompanied with the intercalation/de-intercalation of Li+ as described in Equation 1 and 2: WO3H2O + αLi+ + αe– ↔ LiαWO3H2O

(1)

Li4FeII4[FeII(CN)6]3 (PW) ↔ FeIII4[FeII(CN)6]3 (PB) + 4Li+ + 4e–

(2)

Figure 3. Electrochromic performance of the EESD. (a) Photographs of the EESD at colored (+1.2 V) and bleached (–0.6 V) states. (b) Transmittance spectra of EESD at colored and bleached states ranging from 300 to 800nm. (c) CA curve with voltage switching between +1.2 V and –0.6 V for 10 s per step, and the corresponding in situ transmittance response at 650 nm. (d) Amplified in situ transmittance response for one cycle. (e) ∆OD versus charge density monitored at 650 nm, obtaining a CE value of 139.4 cm2 C–1. (f) Cyclic stability of the EESD tested at +1.2 and –0.6 V for 2500 cycles.


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Figure 3a shows the digital photos of EESD under colored and bleached states, respectively. The EESD exhibits typical dark-blue color at +1.2 V and transparent state at –0.6 V. The transmittance spectra of the device at different voltages in the wavelength ranging from 300 to 800 nm are depicted in Figure 3b. The EESD takes on transparent state with the transmittance of 73.8% at 650 nm under –0.6 V, while it displays dark-blue color with the transmittance declined to 12.1% at +1.2 V. The EESD presents a noticeable electrochromism with the optical modulation of 61.7%, which is quite satisfying in the electrochromic applications. Figure 3c displays the chronoamperometry (CA) curve with voltage being switched between –0.6 V and +1.2 V and corresponding in situ transmittance spectrum at 650 nm for the EESD. The switching time for coloring and bleaching, a vital parameter for electrochromic devices, is characterized as the needed time for 90% variation of the total optical modulation.6 A zoom-in view on one reversible cycle between colored and bleached states is depicted in Figure 3d. The response times for coloration and bleaching are extremely short with 1.84 s and 1.95 s, much faster than those of the reported WO3-based devices, which typically range from 3 to 10 s.40–42 The ultrafast switching speed might be resulted from the big surface area of WO3H2O nanosheets which facilitate the diffusion of Li+, and the cooperation of WO3H2O and PB. Besides, PB as a transition metal hexacyanoferrate with FeII─CN─FeIII configuration exhibits an infinite threedimensional network,32 which is beneficial to the rapid transport of Li+ since its intrinsic open features provide large micro tunnels and spaces, also contributing to the fast switching speed of EESD. Coloration efficiency (CE), another crucial criterion for electrochromic devices, is ruled as the change in optical density (OD) per unit of intercalated charge, i.e., CE = ∆OD/Q = log(Tb/Tc)/Q, where Q is the intercalated charge density, Tb and Tc denote the transmittances of the bleached and colored states.4 A high coloration efficiency value declares a broad optical


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modulation with a small amount of charge intercalation or de-intercalation, which would desirably arouse long cycle life.43 Figure 3e presents the dependence of ∆OD on charge density at 650 nm. The CE value could be accordingly extracted from slope of linear region of the line, obtaining 139.4 cm2 C–1 for the EESD, which is comparable to those of the previously reported complementary devices,25,41,42 and much larger than those of devices based on single material, either WO3 or PB.39,44–46 Figure 3f shows the electrochromic cycle life for EESD with voltage steps of +1.2 V and –0.6 V for colored and bleached states at 650 nm. The EESD sustains an optical modulation of 91.2% of the initial value after 1000 cycles, and sustains an optical modulation of 82.5% of its original state even after 2500 cycles. The excellent cycle life may be attributed to the high crystalline quality of the WO3H2O nanosheets and the good adhesion between films and substrates. The electrochromic properties of the proposed EESD are compared with those of the previously reported works, as summarized in Table S1. Figure S3 and Figure 4 show the energy storing performance of the EESD used as a supercapacitor. Figure S3 displays the cyclic voltammetry (CV) curves for the EESD at varying scan rates ranging from 10 to 100 mV s–1. All CV curves present distorted shapes, declaring that the pseudocapacitive behavior occurs in the EESD. Figure 4a depicts the galvanostatic charge/discharge (GCD) profiles recorded at diverse current densities. The coulomb efficiency (η) can be calculated according to η=td/tc, where tc and td are the charging and discharging times. The calculated coulomb efficiency for the EESD is 99.9%, 99.7%, 95.9%, 93.7%, 92.3% and 89.6% at the current densities of 0.02, 0.05, 0.1, 0.25, 0.5 and 1 mA cm–2, demonstrating its good electrochemical reversibility. From the discharge profiles, the areal capacitance is estimated to be 5.12, 4.87, 4.68, 4.35, 4.04 and 3.67 mF cm–2 at 0.02, 0.05, 0.1, 0.25, 0.5 and 1 mA cm–2 (Figure 4b), which is comparable to the previously reported supercapacitors based on


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WO3 materials.23,47 It can be found that the areal capacitance gradually declines as current density increases, attributing to some inner active sites that become unavailable for the charge storage, restricting the movement of electroactive ions under large current density.30 Additionally, it is worth noting that the EESD maintains 71.7% of the initial areal capacitance even when the current density is 50-fold enhanced, declaring its good rate capability. Furthermore, the EESD reveals good cyclic stability with its areal capacitance almost fully maintained after 1000 cycles at 0.25 mA cm–2, as shown in Figure 4c.

Figure 4. Capacitive evaluation of the EESD. (a) GCD profiles of the device at varying current densities: 1, 0.5, 0.25, 0.1, 0.05 and 0.02 mA cm–2. (b) Areal capacitances of EESD at varying current densities. (c) Cyclic stability measured at 0.25 mA cm–2 for 1000 cycles. The superior electrochemical performance is not only observed on the small-sized EESD, but also maintained when the device is scaled up (10 × 11 cm2 in size). Digital photographs of the large-scale EESD in colored and bleached states are displayed in Figure 5a and b. A fine optical modulation of 53.6% at 650 nm is achieved for the large EESD as depicted in Figure 5c. It can be calculated from the in situ optical response in Figure 5d that the response times of the large device are increased to 5.2 and 5.6 s for the coloration and bleaching processes, which are also satisfying for the electrochromic devices. Figure 5e displays the GCD curves for the large EESD recorded at 0.02, 0.05 and 0.1 mA cm–2, and the corresponding areal capacitances are 2.58, 2.2


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and 1.97 mF cm–2, respectively (Figure S4). The declining performance of the large-scale EESD may be attributed to the inferior uniformity of the active materials on large FTO substrates compared with the small-sized EESD.

Figure 5. Electrochemical performance of the large-scale EESD. Photographs of the large-scale EESD at (a) colored and (b) bleached states. (c) Transmittance spectra of large-scale device at colored and bleached states under +1.2 V and –0.6 V at 300–800nm. (d) In situ transmittance responses of the large device for 20 s per step at 650 nm. (e) GCD curves of the large device at varying current densities: 0.1, 0.05 and 0.02 mA cm–2. 3.3. Energy Level Indicating Function of the EESD


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Figure 6. Energy level indicating function of the EESD. (a) Photographs of the device under various energy storage levels, 1.2, 0.6 and 0 V. (b) The corresponding transmittance spectra under fully charged (1.2 V) and discharged (0 V) states at a charge/discharge current density of 0.02 mA cm–2. GCD profiles at varying current densities of (c) 0.02 and (d) 1 mA cm–2 with the voltage ranging from 0 to 1.2 V, and the corresponding in situ transmittance responses measured at 650 nm for the EESD. (e) Optical contrast of the EESD with respect to the charge/discharge current density. We have separately investigated the electrochromic and capacitive performance of the EESD as the above mentioned. Interestingly, when the redox reactions occur with charge transfer in


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WO3H2O (W(VI)/W(V)) and PB (Fe(III)/Fe(II)), the EESD will simultaneously store energy and change color reversibly. Thus, the functions of energy storage and electrochromism could be simultaneously realized. Figure 6a shows the photographs of the device under different energy storage states. When EESD is charged to 1.2 V, the device presents a dark-blue state, suggesting the transformation of WO3H2O to LiαWO3H2O and Li4FeII4[FeII(CN)6]3 (PW) to FeIII4[FeII(CN)6]3 (PB). During the discharging process, the color of the device gradually fades away. When the device is discharged to 0 V, it exhibits a quasi-transparent color, denoting the formation of WO3H2O and Li4FeII4[FeII(CN)6]3. Figure 6b illustrates the transmittance spectra of the EESD under the fully charged (1.2 V) and discharged (0 V) states at a charge/discharge current density of 0.02 mA cm–2. The device exhibits an optical contrast of ~45% between the charged (1.2 V) and discharged (0 V) states at 650 nm, much broader than that of the reported symmetric device adopting polyaniline nanowires as electrodes.3 The transmittance variation accordant with different amount of stored charges makes the EESD intelligently displaying the energy storage level via various colors. The GCD profiles at different current densities between 0 and 1.2 V and the corresponding in situ transmittance responses at 650 nm were measured (Figure 6c, d and Figure S5). The color of the EESD transforms from quasi-transparent to darkblue during the charging procedure, while the color gradually disappears during the discharging procedure. Therefore, this feature of our multifunctional EESD could be employed to visually indicate the level of energy storing and reliably adjust the transmission of the sunlight during charge/discharge processes. Figure 6e presents the optical contrast versus the charge/discharge current density. The optical contrast of EESD between fully charged and discharged states is 45.0% and 32.3% at 0.02 and 1 mA cm–2, respectively. The device still maintains an optical


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contrast of 71.8% even when the current density is 50 times increased, suggesting that our EESD exhibits a stable color change during fast charge/discharge processes. 3.4. Integrated the EESD with Silicon-Based Solar Cells

Figure 7. Integrated the EESD with silicon-based solar cells. (a) The circuit diagram of the smart operating system. (b) Solar cell is charging for EESD. (c) One EESD can independently drive an LCD screen. (d) Two EESDs in series can lighten a red LED. Furthermore, we have proposed to integrate our EESD with commercial silicon-based solar cells to constitute an intelligent operating system in the architectures, considering that the charge/discharge process of the device is accompanied by reversible color change. Figure 7a schematically depicts the circuit diagram of the smart system, in which the EESD plays a role of “smart window”. When the sunlight is powerful, the “smart window” is charged utilizing a commercial silicon-based solar cell, and the color for the “smart window” would simultaneously


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deepen to block off the strong sunlight. When the sunlight is weak, after cutting off the contact between the smart window and solar cell, the stored energy in the smart window can be used to drive the interior electric equipment and the color for the smart window will again lighten allowing the sunlight to enter. In this way, our EESD could adjust the interior sunlight and improve the indoor comfort totally by the rational utilization of solar energy, thus sharply reducing the electric energy consumption. A silicon solar cell is utilized to convert solar energy into electric energy in order to charge our EESD as shown in Figure 7b. After charging by solar cell, one EESD can individually power an LCD (Figure 7c) and two EESDs in series can lighten a red LED (Figure 7d). By combining our newly designed EESD with solar cells, a smart system in the buildings has been implemented, which can effectively convert, store and use solar energy, and should have a great practical application on our daily life in the future. 4. CONCLUSIONS In summary, a high-performance electrochromic-energy storage device (EESD) was developed, which was structured by WO3H2O nanosheets and PW film. Our EESD exhibits broad optical modulation, ultrafast response time, great coloration efficiency and long cycle life. Besides, a scaled-up EESD (10 × 11 cm2) was further fabricated as a prototype. The proposed EESD is also able to visually monitor the level of stored energy by color variation. Moreover, the EESD could be combined with commercial solar cells to constitute an intelligent operating system, which would realize the adjustment of indoor sunlight and the improvement of physical comfort totally by the rational utilization of solar energy. Such promising EESD could be practically served as multifunctional smart windows for buildings, roofs and vehicles in the near future.


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ASSOCIATED CONTENT Supporting Information The calculation of areal capacitance, the material selection rules, the FTIR spectrum of PB film, the electrochromic and capacitive properties of the as-prepared WO3H2O and PB electrodes, the summarized comparison of electrochromic properties with the previously reported works, the CV curves of the EESD at varying scan rates, the areal capacitances of the large-scale EESD, the GCD profiles at varying current densities and the corresponding in situ optical responses of the EESD. The Supporting Information is available free of charge on the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was financially supported by the National Key R&D Program of China (Grant No. 2016YFA0201103). REFERENCES (1)

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Large-Scale Multifunctional Electrochromic-Energy Storage Device

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