Electrochromic Asymmetric Supercapacitor Windows Enable Direct

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Electrochromic asymmetric supercapacitor windows enable direct determination of energy status by naked eye Ying Zhong, Zhisheng Chai, Zhimin Liang, Peng Sun, Weiguang Xie, Chuanxi Zhao, and Wenjie Mai ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b10334 • Publication Date (Web): 08 Sep 2017 Downloaded from http://pubs.acs.org on September 9, 2017

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ACS Applied Materials & Interfaces

Electrochromic asymmetric supercapacitor windows enable direct determination of energy status by naked eye Ying Zhong,

‡,a

Zhisheng Chai,

‡,a

Zhimin Liang,a Peng Sun,a Weiguang Xie,a,b Chuanxi Zhaoa

and Wenjie Mai*,a,b a

Siyuan laboratory, Guangzhou Key Laboratory of Vacuum Coating Technologies and New

Energy Materials, Department of Physics, Jinan University, Guangzhou, Guangdong 510632, P.R.China b

Guangdong Provincial Key Laboratory of Optical Fiber Sensing and Communications, Jinan

University, Guangzhou, Guangdong 510632, P.R.China ‡

These authors contributed equally to this work

KEYWORDS: supercapacitor, asymmetric, electrochromic, energy conversion and storage, selfpowered

ABSTRACT:

Due to the popularity of the smart electronics, multi-functional energy storage devices, especially the electrochromic supercapacitors (SCs), have attracted tremendous research interests. Herein, a

ACS Paragon Plus Environment

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solid-state electrochromic asymmetric SC (ASC) window is designed and fabricated by introducing WO3 and polyaniline (PANI) 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 mWh 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 panel. We believe that the novel electrochromic ASC windows will have great potential application for both smart electronics and smart buildings.

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 growing of smart electronics also has raised urgent need for 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 are of great meaning to practical application.6-7


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As is well known, SC shows promising prospect compared with other battery technologies owing to its 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 since 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, Nb2O5) and hydroxides (Ni(OH)2),11-13 as well as organic materials like viologen compounds and conjugated electroactive polymers (e.g., polyaniline (PANI), PEDOT:PPS).10, 1415

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 due to their remarkable color-changing characteristic and outstanding pseudocapacitive performance. Up to now, only a few works about integrated standalone electrochromic SC devices have been reported.17,

19-20

Especially to deserve to be

mentioned, besides the electrodes, the electrolytes could perform the electrochromic function in devices, just by directly adding electrochromic materials into electrolyte.21 Typically, WO3 shows superior pseudocapacitive performance at negative potential. Meanwhile, the PANI also has been demonstrated with excellent pseudocapacitive performance at potential of typically -0.2–0.8 V. Both of them are compatible with acidic electrolytes such as H2SO4, H3PO4 and HClO4.22-23 These factors provide a possibility of an electrochemical matching between WO3 and PANI.24 Actually, the perfect matching of WO3 and PANI is also reflected in the electrochromic behaviors since the charging processes of both WO3 and PANI are accompanied by coloring processes (WO3: transparent to dark blue during cathodic


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reduction; 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 WO3PANI electrochromic ASC glass window with a wide voltage window of 1.4 V. The WO3 and PANI were grown directly on fluorine-doped tin oxide (FTO) glasses by straight forward onestep thermal evaporation and in situ electrodeposition method, respectively. The sandwich-type ASC window was fabricated by stacking the WO3 and PANI electrodes, and using H2SO4/PVA as the electrolyte and separator simultaneously. The ASC window displays a light green color in 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 self-powered 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.

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 vs. Ag/AgCl in H2SO4 electrolyte.


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Meanwhile, conducting polymer PANI would be the positive electrode, the color of which can change from light green at reduced form (-0.2 V vs. Ag/AgCl) to dark blue at 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 electrodes were prepared through an in situ electrochemical polymerization process, which was generally performed in a threeelectrode system. To fabricate the smart ASC glass window, H2SO4/PVA was used as the electrolyte and separator simultaneously. WO3 electrodes with ~200 nm, ~300 nm, and ~400 nm in thickness were prepared and tested, respectively (Figure S1 and Figure 2). It can be seen from Figure S1e that with increasing of film thickness, the areal capacitances are increased generally, for the increasing of active mass loading. However, the areal capacitance of WO3 electrode with a thickness of ~400 nm was rapidly decreased while current density increasing, showing a low rate capability. It may because of a slower transport of the electrolytic ions in a thicker film. So 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 nano-cauliflowers possess a large amount of granular voids, which are conducive for cation intercalation and extraction. The crystalline phase of the WO3 (JCPDS No. 72-0677) was confirmed by the X-ray diffraction (XRD) patterns measurement (Figure S2a). Furthermore, the chemical composition of the WO3 was further analyzed using X-


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ray photoelectron spectroscopy (XPS, 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 W6+ state (the ratio of W6+/W5+ is determined to be 7.34), indicating the predominance of W6+ in evaporated film.26 The electrochemical properties of the as-prepared WO3 electrode were studied in a threeelectrode system using 1 M H2SO4 aqueous solution as 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 have not been obvious altered even at the high scan rate. Based on 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, Galvanostatic charge/discharge (GCD) curves of the WO3 electrode at different current densities are symmetrical triangle in shape, 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, electrochromic process is accompanied with the electrochemical process. The intercalation of H+ into the electrode causes the dark blue color while the extraction of H+ return it to transparent. The optical modulation range of the electrochromic WO3 film was examined by in situ UV–visible transmission measurement. 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 vs. Ag/AgCl. As the potential changes


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to -0.6 V vs. 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 WO3 electrode, transmittance spectra of FTO glass and WO3 electrode at 0 V, -0.2 V, -0.4 V and -0.6 V (vs. Ag/AgCl) are measured, as shown in Figure S4. PANI electrode with different mass loading were prepared by controlling the electrodeposition time. Figure S5 and Figure 3 show the CV and GCD curves of the PANI electrodes 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 time. 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. That maybe because electron and ion transmission transport are limited when more PANI are deposited on FTO glass. So the PANI electrode with an electrodeposition time of 1 h was chosen for further study due to its high areal capacitance and well capacitance behavior. Figure 3a exhibits the plane view and cross-sectional view (inset image) SEM images of the PANI positive electrode, where the thickness of flat PANI film is determined to ~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) present near 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 (in-plane 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 quasi-rectangular shape, evidencing the PANI’s good


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pseudocapacitance behavior. Three pairs of oxidation and reduction peaks are observed on CV curves at potential window of -0.2 V 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 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 emeraldine state and pernigraniline oxidized state.6, 18 GCD curves of the PANI electrode at different current densities exhibit the relatively symmetrical triangle shape, as shown in Figure 3c. Based on the discharge curve at 0.2 mA cm-2, the areal capacitance is calculated as 23.2 mF cm-2. The areal capacitances based on other current density are supplied 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 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 pernigraniline state (0.8 V vs. Ag/AgCl). To further characterize the electrochromic performance of WO3 electrode, transmittance spectra of FTO glass and PANI electrode at -0.2 V, 0 V, 0.2 V, 0.4 V, 0.6 V and 0.8 V (vs. Ag/AgCl) are tested to reveal the trend of 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. CV curves collected from the WO3 negative electrode and PANI positive electrode in 1 M H2SO4 electrolyte in the three-electrode system clearly demonstrate that the operating potential windows of WO3 and PANI are -0.6–0 V and -0.2–0.8 V (Figure S8a), respectively,


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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 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 mWh 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 to 1.4 V (Figure S8b). The device shows identical quasi-rectangular CV shape at different scan rates, indicating good capacitive behavior. GCD curves of the ASC at different current densities are shown in Figure 4b, in which the charge and discharge curves are nonlinear, indicting the presence of some redox reaction from PANI or WO3. From GCD curve, the ASC achieves an areal capacitance of 28.3 mF cm-2, an energy density of 7.7×10-3 mWh 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 other current density are supplied in Figure S8c.As listed in Table S1, the energy density is much higher than previous reported stand-alone electrochromic SC devices, such as WO3-based symmetric SC device (1.44×10-3 mWh cm-2) and WO3-based perovskite photovoltachromic SC device (2.45×10-3 mWh 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 (Figure 4c and S5). The solid-state ASC exhibits excellent stability with only 21% degradation of the initial capacitance after 5000 cycles (Figure 4c and Figure 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).


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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 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 to dark blue with optical transmittance of 12% at a wavelength of 663 nm. It can be speculated that WO3 negative electrode and PANI positive electrode are both colored to dark blue, which are explored and discussed in details 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 WO3PANI window. One of the key parameters for electrochromic device is the coloration switching response. As a result, chronoamperometry was used to test the as-assembled WO3-PANI window under an alternating voltage of -0.5 V and 1.5 V with each applied voltage 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 the transmittance at 550 nm changes reversibly from 14% to 42%. Typically, the coloration and bleaching time of stand-alone electrochromic devices utilizing solid-state electrolyte are 16.3 s and 33.6 s, respectively, which are comparable to other reported analogous solid-state electrochromic devices.31-32 The 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


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level because it changes simultaneously with the voltage applied. The trend of “light green → green → blue → dark blue” was demonstrated in Figure 4e as voltage increased from 0 to 1.4 V. Such color-indicating function endows the electrochromic ASCs with smart features. Next, the electrochromism mechanism of the WO3-PANI ASC window was investigated. Figure 5a schematically exhibits 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 electron flows. The change of electronic band structure brings about the optical transmittance change of WO3 film (transparent → dark blue). And conversely, H+ cations are extracted from WO3 lattice as the color turns to 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 leucoemeraldine state, green in emeraldine state and dark blue in pernigraniline state.35-37 When uncharged, it remains in leucoemeraldine state. During the charging process, the PANI is p-doped with anions (SO42).38-40 Hence, the leucoemeraldine state PANI is transformed into emeraldine state and then further oxidized into pernigraniline state gradually. On the contrary, the pernigraniline state PANI is reduced during the discharging process. Figure 5b shows a two-electrode system of WO3 and PANI with 1 M H2SO4 aqueous solution as the electrolyte. While the WO3 negative electrode colors during cathodic reduction (with intercalation of H+ cations), the PANI positive electrode colors during anodic oxidation (with p-doping by SO42- anions). The overall process is demonstrated as follows: 2nWO + PANI + nxH SO ↔ 2nH WO + PANI SO

(0

Electrochromic Asymmetric Supercapacitor Windows Enable Direct

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