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Electrochromic Asymmetric Supercapacitor Windows Enable Direct Determination of Energy Status by the Naked Eye Ying Zhong,† Zhisheng Chai,† Zhimin Liang,† Peng Sun,† Weiguang Xie,†,‡ Chuanxi Zhao,† and Wenjie Mai*,†,‡ †

Siyuan Laboratory, Guangzhou Key Laboratory of Vacuum Coating Technologies and New Energy Materials, Department of Physics, and ‡Guangdong Provincial Key Laboratory of Optical Fiber Sensing and Communications, Jinan University, Guangzhou, Guangdong 510632, P. R. China S Supporting Information *

ABSTRACT: Because of the popularity of smart electronics, multifunctional energy storage devices, especially electrochromic supercapacitors (SCs), have attracted tremendous research interest. Herein, a solid-state electrochromic asymmetric SC (ASC) window is designed and fabricated by introducing WO3 and polyaniline as the negative and positive electrodes, respectively. The two complementary materials contribute to the outstanding electrochemical and electrochromic performances of the fabricated device. With an operating voltage window of 1.4 V and an areal capacitance of 28.3 mF cm−2, the electrochromic devices show a high energy density of 7.7 × 10−3 mW h cm−2. Meanwhile, they exhibit an obvious and reversible color transition between light green (uncharged state) and dark blue (charged state), with an optical transmittance change between 55 and 12% at a wavelength of 633 nm. Hence, the energy storage level of the ASC is directly related to its color and can be determined by the naked eye, which means it can be incorporated with other energy cells to visual display their energy status. Particularly, a self-powered and color-indicated system is achieved by combining the smart windows with commercial solar cell panels. We believe that the novel electrochromic ASC windows will have great potential application for both smart electronics and smart buildings. KEYWORDS: SC, asymmetric, electrochromic, energy conversion and storage, self-powered

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INTRODUCTION Smart electronics such as smart phones/watches and intelligent eyeglasses have already revolutionized our everyday life. In the upcoming years, more innovative technologies will be integrated into electronics to make them even smarter. The rapid growth of smart electronics also has raised an urgent need for a smart energy storage system.1 For example, various types of wearable supercapacitors (SCs) and lithium-ion batteries have been made available for flexible electronics.2,3 Meanwhile, self-powered energy systems with integrated energy-harvesting devices have been developed to avoid frequent and inconvenient charging by using a regular household electric outlet.4,5 Thereby, designing creative energy storage devices equipped with smart functionalities is of great meaning to practical application.6,7 As is well-known, SCs show promising prospects compared with other battery technologies owing to their high power density, short charging time, and super long cycle life.8,9 As an energy storage device featuring optical function, the electrochromic SC has been intensively studied because its color transformation characteristic enables it to meet specific application demands.10 The most studied electrochromic materials mainly include transition metal oxides (e.g., WO3, NiO, V2O5, and Nb2O5) and hydroxides (Ni(OH)2),11−13 as well as organic materials such as viologen compounds and © 2017 American Chemical Society

conjugated electroactive polymers (e.g., polyaniline (PANI) and poly(3,4-ethylenedioxythiophene):polystyrene sulfonate).10,14,15 SCs based on electrochromic materials offer the superiority of realizing energy storage and color transition in one device.16,17 However, in most previous works, the research focus is only on the pure electrodes.6,18 Among these electrode materials, WO3 and PANI stand out, and considerable studies have been made because of their remarkable color-changing characteristics and outstanding pseudocapacitive performance. Up to now, only a few works about integrated stand-alone electrochromic SC devices have been reported.17,19,20 Especially, besides the electrodes, the electrolytes could perform the electrochromic function in devices, just by directly adding electrochromic materials into the electrolyte.21 Typically, WO3 shows superior pseudocapacitive performance at the negative potential. Meanwhile, PANI has also been demonstrated with excellent pseudocapacitive performance at the potential of typically −0.2 to 0.8 V. Both of them are compatible with acidic electrolytes such as H2SO4, H3PO4, and HClO4.22,23 These factors provide a possibility of electrochemical matching between WO3 and PANI.24 Actually, the Received: July 17, 2017 Accepted: September 8, 2017 Published: September 8, 2017 34085

DOI: 10.1021/acsami.7b10334 ACS Appl. Mater. Interfaces 2017, 9, 34085−34092

Research Article

ACS Applied Materials & Interfaces

electrodes were prepared through an in situ electrochemical polymerization process, which is generally performed in a three-electrode system. To fabricate the smart ASC glass window, H2SO4/PVA was used as the electrolyte and separator simultaneously. WO3 electrodes with ∼200, ∼300, and ∼400 nm thicknesses were prepared and tested (Figures S1 and 2). It can be seen from Figure S1e that with increasing of film thickness, the areal capacitances are increased generally for increasing active mass loading. However, the areal capacitance of the WO3 electrode with a thickness of ∼400 nm was rapidly decreased while current density increased, showing a low rate capability. It may be due to a slower transport of the electrolytic ions in a thicker film. Hence, taking full account of the areal capacitance and rate capability, the WO3 electrode with ∼300 nm in thickness was chosen for further study. Figure 2a shows the scanning electron microscopy (SEM) image of the WO3 negative electrode, suggesting that the cauliflower-like WO3 are uniformly grown on FTO with ∼300 nm in thickness. The WO3 nanocauliflowers possess a large amount of granular voids, which are conducive for cation intercalation and extraction. The crystalline phase of WO3 (JCPDS no. 72-0677) was confirmed by the X-ray diffraction pattern measurement (Figure S2a). Furthermore, the chemical composition of WO3 was further analyzed using X-ray photoelectron spectroscopy (Figure S2b). The W 4f spectrum exhibits four peaks, which correspond to the W6+ state (strong peaks at 35.36 and 37.50 eV) and the W5+ state (weak peaks at 34.91 and 37.04 eV). It can be inferred that the main oxidation state is the W6+ state (the ratio of W6+/W5+ is determined to be 7.34), indicating the predominance of W6+ in the evaporated film.26 The electrochemical properties of the as-prepared WO3 electrode were studied in a three-electrode system using 1 M H2SO4 aqueous solution as the electrolyte. Cyclic voltammetry (CV) curves of the WO3 electrode at different scan rates proved its superior pseudocapacitance characteristics (Figure 2b). The superior electrochemical behavior of WO3 is derived from the intercalation and extraction of H+: WO3 + xH+ + xe− ↔ HxWO3.27 The shape of the CV curves did not show any obvious alteration even at the high scan rate. On the basis of the discharge curve, the areal capacitance at a current density of 1 mA cm−2 can be calculated to be 53 mF cm−2, which is in accordance with our previous work. As exhibited in Figure 2c, the galvanostatic charge/ discharge (GCD) curves of the WO3 electrode at different current densities are symmetrical triangles, suggesting good reversibility during the charge/discharge processes. The areal capacitances based on those discharge curves are supplied in Figure S3a. CV tests were performed to measure the cycling stability of the WO3 electrode. After 4000 charge/discharge cycles, the retention of capacitance was 81% when compared with the first cycle, demonstrating the electrode’s good stability (Figure S3b). Most importantly, the electrochromic process is accompanied with the electrochemical process. The intercalation of H+ into the electrode results in the dark blue color whereas the extraction of H+ returns it to its transparent state. The optical modulation range of the electrochromic WO3 film was examined by in situ UV−visible transmission measurements. As shown in Figure 2d, the electrode is transparent with an optical transmittance of 77% at a wavelength of 633 nm at the potential of 0 V versus Ag/AgCl. As the potential changes to −0.6 V versus Ag/AgCl, its color turns to dark blue while its optical transmittance decreased to 12% at 633 nm. To further characterize the electrochromic performance of the WO3

perfect matching of WO3 and PANI is also reflected in the electrochromic behaviors because the charging processes of both WO3 and PANI are accompanied by coloring processes (WO3: transparent to dark blue during cathodic reduction and PANI: light green to dark blue during anodic oxidation), which will bring more interesting applications. Asymmetric SCs (ASCs) are realized by using two different electrodes with complementary potential windows, which results in an increased operation voltage of the cell system, leading to improved specific capacitance and energy density.25 Herein, we developed a solid-state WO3−PANI electrochromic ASC glass window with a wide voltage window of 1.4 V. WO3 and PANI were grown directly on fluorine-doped tin oxide (FTO) glasses by straight forward one-step thermal evaporation and in situ electrodeposition methods, respectively. The sandwich-type ASC window was fabricated by stacking the WO3 and PANI electrodes and using H2SO4/poly(vinyl alcohol) (PVA) as the electrolyte and separator simultaneously. The ASC window displays a light green color in the uncharged state and deepens into dark blue as the charge voltage increases. The superior electrochemical and electrochromic performance (wide optical modulation range, fast switching response, and great optical stability) has been demonstrated. In addition, to confirm the “smart” feature of the ASC window, we built a selfpowered and color-indicated system by introducing a commercial silicon solar cell panel. Our newly designed ASC smart windows may power electronic devices, such as mobile phones and tablet computers, and offer the striking advance to determine their energy storage level through the color of the windows just by the naked eye.

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RESULTS AND DISCUSSION In the smart electrochromic ASC, the positive electrode and negative electrode must possess matching color-changing behavior. Typically, WO3 is an n-type semiconductor that provides a suitable structure for intercalation of small H+ cations. It can be chosen as the negative electrode, which is transparent at 0 V and dark blue at −0.6 V versus Ag/AgCl in H2SO4 electrolyte. Meanwhile, the conducting polymer PANI would be the positive electrode, the color of which can change from light green at the reduced form (−0.2 V vs Ag/AgCl) to dark blue at the oxidized form (0.8 V vs Ag/AgCl) in H2SO4 electrolyte. Hence, WO3 and PANI would be a perfect pair as the electrode materials in the electrochromic ASC device. The typical process for the fabrication of electrodes and the subsequent installation of the ASC device is schematically illustrated in Figure 1. The ultrathin WO3 films were deposited on FTO glass by a thermal evaporation method as demonstrated in our previous work.17 The PANI positive

Figure 1. Schematic of the preparation process of the WO3 and PANI films on FTO glass and the fabrication of ASC windows. 34086


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Figure 2. Characterizations of the WO3 electrode. (a) SEM image of the cauliflower-like WO3 film (inset: cross-sectional SEM image of the WO3 film evaporated on an FTO substrate). (b,c) CV and GCD curves of the WO3 electrode performed in 1 M H2SO4 aqueous solution, respectively. (d) Transmittance spectra of FTO glass and the WO3 electrode under different voltage conditions, and the insets are the corresponding digital photos of the WO3 electrode.

Figure 3. Characterizations of the PANI electrode. (a) SEM image of the PANI film (inset: cross-sectional SEM image of the PANI film on an FTO substrate). (b,c) CV and GCD curves of the PANI electrode performed in 1 M H2SO4 aqueous solution, respectively. (d) Transmittance spectra of FTO glass and the PANI electrode under different voltage conditions, and the insets are the corresponding digital photos of the PANI electrode.

electrodeposited for 15 min, 0.5 h, 1 h, and 1.5 h. It can be seen that the areal capacitances of PANI electrodes are increased with longer electrodeposition times. However, the charge curve in Figure S5f shows that it is difficult to charge the PANI electrode to 0.8 V with an electrodeposition time of 1.5 h. This may be because electron and ion transmission are limited when

electrode, the transmittance spectra of FTO glass and the WO3 electrode at 0, −0.2, −0.4, and −0.6 V (vs Ag/AgCl) are measured, as shown in Figure S4. PANI electrodes with different mass loadings were prepared by controlling the electrodeposition time. Figures S5 and 3 show the CV and GCD curves of the PANI electrodes 34087


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Figure 4. The electrochemical characterizations and digital photos of the electrochromic solid-state ASC. (a) CV curves of the assembled solid-state ASC at the scan rate of 100 mV s−1 under different voltage windows. (b) GCD curves of the ASC device at different current densities. (c) Electrochemical cycle stability of the ASC device measured at a scan rate of 100 mV s−1. The inset is a digital photo of a red LED lit up by two seriesconnected ASCs. (d) Transmittance spectra of the ASC device under 0 V (bleached) and 1.4 V (colored) at the first cycle and after 5000 cycles, and the insets are digital photos of the ASC device at bleached and colored states. (e) Digital photos of the ASC window showing different colors when different voltages are applied.

capacitances based on different current densities are shown in Figure S6b. The PANI electrode also shows superior stability during charge/discharge cycles, as evidenced by 94% capacitance retention after 5000 cycles (Figure S6c). The UV−visible transmission spectra exhibiting the optical modulation range of the PANI−FTO electrode are shown in Figure 3d. The PANI electrode at the leucoemeraldine state (−0.2 V vs Ag/AgCl) is light green with an optical transmittance of 74% at 633 nm, and it changes to dark blue (16% of the optical transmittance at 633 nm) while at the pernigraniline state (0.8 V vs Ag/AgCl). To further characterize the electrochromic performance of the WO3 electrode, the transmittance spectra of FTO glass and the PANI electrode at −0.2, 0, 0.2, 0.4, 0.6, and 0.8 V (vs Ag/AgCl) are tested to reveal the trend of the color change, as shown in Figure S7. The WO3 and PANI electrodes can be fabricated into a sandwiched-structure device, as shown in Figure 1. The CV curves collected from the WO3 negative electrode and the PANI positive electrode in 1 M H2SO4 electrolyte in the threeelectrode system clearly demonstrate that the operating potential windows of WO3 and PANI are −0.6 to 0 V and −0.2 to 0.8 V (Figure S8a), respectively, which suggests that the ASC device can be operated to 1.4 V. Figure 4a shows the CV curves of the solid-state ASC utilizing the H2SO4/PVA electrolyte at the scan rate of 100 mV s−1 under different voltage windows varying from 0.6 to 1.4 V. Significantly, when the operation voltage increases from 0.6 to 1.4 V, the calculated areal capacitance of the ASC increases from 9.7 to 17.4 mF cm−2, and the energy density is improved from 4.9 × 10−4 to 4.7 × 10−3 mW h cm−2, according to the equation: E = 1/2CV2. To further evaluate the electrochemical properties of the ASC, the rate-dependent CV curves at scan rates from 10 to 100 mV s−1 were recorded between 0 and 1.4 V (Figure S8b). The device shows identical quasi-rectangular CV shapes at different

more PANI are deposited on FTO glass. Hence, the PANI electrode with an electrodeposition time of 1 h was chosen for further study because of its high areal capacitance and well capacitance behavior. Figure 3a exhibits the plane view and cross-sectional view (inset image) of SEM images of the PANI positive electrode, where the thickness of the flat PANI film is determined to be ∼200 nm. To confirm the functional groups of the in situ electrochemical polymerized PANI, Fourier transform infrared (FT-IR) spectroscopy analysis was performed. The characteristic peaks in the FT-IR spectrum of PANI (Figure S6a) were present near the wave numbers of 1560 cm−1 (stretching of C−C in quinoid rings), 1475 cm−1 (stretching of C−C in benzenoid rings), 1298 cm−1 (stretching of C−N), 1240 cm−1 (stretching of C−N+), 1114 cm−1 (inplane bending of C−H on benzenoid rings), and 799 cm−1 (out-of-plane bending of C−H on benzenoid rings).28−30 The PANI film with merits of high conductivity and theoretical capacity was also examined by CV and GCD measurements in the three-electrode system. The CV curves of PANI at different scan rates are in a quasi-rectangular shape, evidencing the good pseudocapacitance behavior of PANI. Three pairs of oxidation and reduction peaks are observed on CV curves at a potential window of −0.2 to 0.8 V (Figure 3b). The dominant redox peaks at 0.05/0.32 V (100 mV s−1) can be ascribed to the redox transition of PANI between the leucoemeraldine reduced state and the emeraldine state; the extremely weak redox peaks at 0.44/0.56 V (100 mV s−1) may be attributed to overoxidation products; and the third redox peaks at 0.68/0.80 V (100 mV s−1) are originated from the exchange between the emeraldine state and the pernigraniline oxidized state.6,18 The GCD curves of the PANI electrode at different current densities exhibit relatively symmetrical triangle shapes, as shown in Figure 3c. On the basis of the discharge curve at 0.2 mA cm−2, the areal capacitance is calculated as 23.2 mF cm−2. The areal 34088


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Next, the electrochromism mechanism of the WO3−PANI ASC window was investigated. Figure 5a schematically exhibits

scan rates, indicating good capacitive behavior. The GCD curves of the ASC at different current densities are shown in Figure 4b, in which the charge and discharge curves are nonlinear, indicating the presence of some redox reaction from PANI or WO3. From the GCD curves, it can be noted that the ASC achieves an areal capacitance of 28.3 mF cm−2, an energy density of 7.7 × 10−3 mW h cm−2, and an average power density of 0.13 mW cm−2 at a current density of 0.2 mA cm−2. The areal capacitances based on different current densities are supplied in Figure S8c. As listed in Table S1, the energy density is much higher than the previously reported stand-alone electrochromic SC devices, such as the WO3-based symmetric SC device (1.44 × 10−3 mW h cm−2) and the WO3-based perovskite photovoltachromic SC device (2.45 × 10−3 mW h cm−2).17,20 The long-term cycling performance of the ASC was measured using CV tests at a scan rate of 100 mV s−1 with the voltage window of 1.4 V (Figures 4c and S9). The solid-state ASC exhibits an excellent stability with only 21% degradation of the initial capacitance after 5000 cycles (Figures 4c and S9). What’s more, two assembled ASCs (1 cm × 1 cm) were connected in series and used to light up a red light-emitting diode (LED), demonstrating the great potential of ASCs (inset in Figure 4c). Our solid-state WO3−PANI ASC is not only an energy storage device, the synergetic color transition of WO3 and PANI makes it a smart window that combines energy storage and electrochromism functions. In Figure 4d, it can be seen that the optical transmittance of this WO3−PANI window is 55% at a wavelength of 633 nm in the bleached state (0 V). In this case, the window is in light green, which may be ascribed to the light green color of reduced-form PANI. When charged to 1.4 V, the window turns dark blue with an optical transmittance of 12% at a wavelength of 663 nm. It can be speculated that the WO3 negative electrode and the PANI positive electrode are both colored dark blue, which are explored and discussed in detail as below. In addition, the transmitted spectra of bleached/colored states after 5000 cycles are also provided in Figure 4d. The nearly unchanged curves demonstrate the excellent optical stability of the WO3−PANI window. One of the key parameters for electrochromic devices is the color-switching response. As a result, chronoamperometry was used to test the as-assembled WO3−PANI window under an alternating voltage of −0.5 and 1.5 V with each applied voltage that lasted for 50 s. Taking into consideration that the maximum change in transmittance spectra appeared in the green wavelength range, the corresponding in situ transmittance change at 550 nm was measured (Figure S10). The curves show that the transmittance at 550 nm changes reversibly from 14 to 42%. Typically, the coloration and bleaching time of stand-alone electrochromic devices utilizing the solid-state electrolyte are 16.3 and 33.6 s, respectively, which are comparable to other reported analogous solid-state electrochromic devices.31,32 Video S1 shows the color transition of the WO3−PANI window under CV testing at the scan rate of 100 mV s−1. It demonstrates visually distinguished color change of the window in seconds. As expected, the color change (or allochroic property) of the window can be an indicator of the energy storage level because it changes simultaneously with the voltage applied. The voltage trend from 0 to 1.4 V is shown as “light green → green → blue → dark blue” in Figure 4e. Such a color-indicating function endows the electrochromic ASCs with smart features.

Figure 5. The electrochromic mechanism and the corresponding digital photos. (a) Schematic illustrating the kinetic features of ions in the device during the charge/discharge process. (b) Digital photos of the WO3 and PANI electrodes in a two-electrode system with 1 M H2SO4 aqueous solution as the electrolyte during the charge/discharge process.

the kinetic features of ions in the ASC device during the charge/discharge processes. As mentioned above, the redox reactions (WO3 + xH+ + xe− ↔ HxWO3) occur during the charge/discharge process on the WO3 electrode. During the charge process, H+ cations are intercalated into the WO3 lattice with the flow of electrons. The change in the electronic band structure brings about the optical transmittance change of the WO3 film (transparent → dark blue). Conversely, the H+ cations are extracted from the WO3 lattice as the color turns transparent.33,34 With regard to the PANI electrode, from −0.2 to 0.8 V (vs Ag/AgCl), it successively displays the following color in different states: light green in the leucoemeraldine state, green in the emeraldine state, and dark blue in the pernigraniline state.35−37 When uncharged, it remains in the leucoemeraldine state. During the charging process, the PANI electrode is p-doped with anions (SO42−).38−40 Hence, the leucoemeraldine state PANI is transformed into the emeraldine state and then further oxidized to the pernigraniline state gradually. On the contrary, the pernigraniline state PANI is reduced during the discharging process. Figure 5b shows a twoelectrode system of WO3 and PANI with 1 M H2SO4 aqueous solution as the electrolyte. While the WO3 negative electrode displays color transition during cathodic reduction (with intercalation of H+ cations), the PANI positive electrode displays color transition during anodic oxidation (with pdoping by SO42− anions). The overall process is demonstrated as follows 2n WO3 transparent

+ PANI + nx H 2SO4 ↔ 2nHxWO3 light green 2nx +

+ (PANI

)(SO4 )nx

dark blue

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2−

dark blue

(0 < x < 1)

(1)


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Figure 6. Overview of the self-powered system. (a) Self-powered and color-indicated system consisting of a commercial silicon solar panel, two pieces of WO3−PANI ASC window, a red LED, and a 2000 Ω resistor. (b) Photovoltaic performance of the solar cell panel and the electrochemical performance of the series-connected ASC windows. (c) Voltage and corresponding photocurrent as a function of time for light-charging of the selfpowered system. (d) Digital photos of the ASC window in the self-powered system when (1) uncharged (light green); (2) fully charged (dark blue); (3) powering the red LED; and (4) fully discharged (back to light green again).

several typical states are shown in Figure 6d. In Figure 6, the windows are light green when uncharged. Then, they were connected to the working solar cell panel and gradually turned dark blue as shown in Figure 6. As the energy storage level can be determined by the color, after the windows turned dark blue, they were disconnected from the solar cell panel and connected to the parallel-connected LED and resistor. As shown in Figure 6, the red LED lit up and then gradually faded. Because the LED requires a minimum working voltage, the smart windows could not be fully discharged and remained light blue after the LED was disconnected. Therefore, a 2000 Ω resistor was introduced in parallel to the LED, and it can fully discharge the smart windows. More details are explained in the Supporting Information and Figure S12. Herein, a smart system combining electrochromism and solar energy harvesting and storage has been achieved, the color of which is almost transparent when uncharged and turns dark blue when fully charged by the solar cell panel, as schematically shown in Figure 7a. It may have a wide range of applications, such as smart sunglasses and energy-saving windows for buildings, vehicles, and airplanes. For instance, as shown in Figure 7b, we proposed a novel concept of smart sunglasses. The smart sunglasses are composed of two WO3−PANI

The simultaneous and reversible color transition during the charge and discharge processes is clearly demonstrated in Figure 5b and Video S2. To further explore the smart function of our WO3−PANI ASC window, we have put this into practical application. As shown in Figure 6, a self-powered and color-indicated system consisting of a commercial silicon solar cell panel (effective area: 8.4 cm2), two pieces of WO3−PANI ASC window, a red LED, and a 2000 Ω resistor was designed, and a series of measurements were performed. The picture and the corresponding equivalent circuit of this system are shown in Figures 6a and S11a, respectively. The photovoltaic performance of the solar cell panel and the electrochemical performance of the series-connected ASC windows were measured. The photocurrent−voltage (I−V) curve of the solar cell panel is presented in Figure 6b, which indicates that the short-circuit photocurrent (Isc) and the open-circuit voltage (Voc) is 42 mA and 3.2 V, respectively. The CV curve of the series-connected ASC windows with the voltage window of 3.2 V at 100 mV s−1 is also shown in the bottom of Figure 6b. Therefore, it is easy to charge the two series-connected ASC to the targeted voltage (Voc of the solar cell panel). In Figure 6c, the temporal charging profile obtained by stimulated solar light demonstrates the dynamic changes in both the photocurrent and voltage with that of the charging time. At the beginning, the voltage and photocurrent of the system are both 0. When the light turns on, the photocurrent rapidly reaches the maximum current of 11 mA and then gradually decreases to 1.2 mA in 60 s. Meanwhile, the voltage exhibits a plateau near the Voc of the solar cell panel after 40 s under illumination. To demonstrate the self-powered function of this system, emphasizing its capability to harvest solar energy and deliver energy in darkness, the light-charge and galvanostatical dark-discharge of the system were performed. As shown in Figure S11b, the voltage−time curves indicate that the SC was fully charged by the solar cell panel in 40 s under illumination and was discharged to 0 V in 9.5, 53, and 88 s at currents of 5, 1, and 0.5 mA, respectively. Two smart windows were charged by the solar cell panel and then used to power up a red LED. The pictures of the system in

Figure 7. Potential applications of the self-powered system. (a) Schematic illustrating the self-powered system is almost transparent when uncharged, and dark blue when fully charged. (b) Concept of a new kind of smart sunglasses consisting of two WO3−PANI windows, a solar cell panel, and Bluetooth headsets. 34090


ACS Applied Materials & Interfaces windows, a solar cell panel and Bluetooth headsets. As normal sunglasses, it is almost transparent in general status. While under the sunlight condition, the solar cell panel starts to generate electricity, which can charge the ASC windows to adjust their color to dark blue and power the Bluetooth headsets. Moreover, the Bluetooth headsets can be powered by the ASC windows even in the dark. Hence, the smart sunglasses can work in all weather conditions, overcoming one of the major drawbacks of solar energy.

CONCLUSIONS In this work, a unique matching of WO3 and PANI in both the electrochemical and electrochromic properties was discovered. A solid-state electrochromic ASC smart window based on WO3 and PANI, which provides a wide voltage window of 1.4 V, was designed and fabricated. The optical transmittance of the smart window decreases from 55 to 12% at a wavelength of 633 nm when a voltage of 1.4 V is applied. This sharp color contrast remains steady even after 5000 cycles. As an energy storage device, the ASC exhibits an areal capacitance of 28.3 mF cm−2 and a high energy density of 7.7 × 10−3 mW h cm−2 with an excellent cycle stability. To expand the application of the smart window, a self-powered and color-indicated system was built by integrating our ASC glass window with a commercial silicon solar cell panel. Such a smart system can not only harvest and store solar energy but can also directly determine its energy storage level through the color transition of the window. It can potentially be used for smart sunglasses and energy-saving windows for buildings, vehicles, and airplanes.

ACKNOWLEDGMENTS

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REFERENCES

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EXPERIMENTAL SECTION

Experimental Section is available in the Supporting Information.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b10334. Preparation of the WO3 negative electrode; preparation of the polyaniline (PANI) positive electrode; fabrication of the solid state WO3-PANI smart windows; characterization of the morphologies, crystal forms and nanostructure of the samples; and calculation methods (PDF) Color transition of the WO3-PANI window device under CV testing (MPG) Color transition of the WO3 and PANI electrodes in 1 M H2SO4 during the charge/discharge process (MPG)

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Y.Z. thanks Jinan University’s Scientific Research Creativeness Cultivation Project for Outstanding Undergraduates Recommended for Postgraduate Study. We are grateful for the financial supports from the National Natural Science Foundation of China (grant 21376104, 61604061 and 51772135), the Natural Science Foundation of Guangdong Province, China (grants 2014A030306010 and 2014A030310302), and the Science and Technology Planning Project of Guangdong Province, China (grant 2016B020244002).

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Research Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Weiguang Xie: 0000-0002-3706-6359 Wenjie Mai: 0000-0003-4363-2799 Author Contributions

Y.Z. and Z.C. contributed equally to this work. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest. 34091


Research Article

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