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Integrated electrochromism and energy storage applications based on tungsten trioxide monohydrate nanosheets by novel one-step low temperature synthesis
Solar Energy Materials and Solar Cells 183 (2018) 59–65
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Integrated electrochromism and energy storage applications based on tungsten trioxide monohydrate nanosheets by novel one-step low temperature synthesis
T
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Zhijie Bia,b, Xiaomin Lia, , Xiaoli Hea,b, Yongbo Chena,b, Xiaoke Xua,c, 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, PR China b University of Chinese Academy of Sciences, No. 19 A Yuquan Road, Beijing 100049, PR China c School of Materials Science and Engineering, Shanghai Institute of Technology, No. 100 Haiquan Road, Shanghai 201418, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Tungsten trioxide monohydrate One-step Citric acid Electrochromic Energy storage
The tungsten trioxide monohydrate (WO3·H2O) nanosheets were directly formed on fluorine-doped tin oxide (FTO) substrates without any guidance of seed layer by a novel and quite facile one-step citric acid-assisted hydrothermal method at low temperature (90 °C). The WO3·H2O nanosheets possess porous morphologies and good adhesion to the substrates, which would markedly increase the surface area of WO3·H2O and facilitate the ion diffusion during the electrochemical processes. The WO3·H2O nanosheets display superior electrochemical properties of large optical modulation (79.0%), fast switching time (tc = 10.1, tb = 6.1 s), high areal capacitance (43.30 mF cm–2) and excellent cycling stability. Furthermore, bridging electrochromic behavior with energy storage was successfully achieved. Based on the proposed WO3·H2O nanosheets, a smart energy storage electrode was demonstrated, which could monitor the level of stored energy by color changes. The results show great potential of the one-step synthesized WO3·H2O nanosheets for integrated electrochromism and energy storage applications.
1. Introduction Recently, multifunctional energy storage and conservation devices that combine novel characteristics and functions in smart and interactive modes are gradually springing up [1–4]. Supercapacitors are deemed as promising means of energy storage because of their high power density, long cycle life and fast charge/discharge capability [5–7]. Pseudocapacitor, an important type of supercapacitor, is based on the redox reactions occurred at or near the surface of active materials to store energy [8,9]. Smart windows based on electrochromic (EC) materials could adjust the interior sunlight by color variation so as to reduce the energy consumption and improve the indoor comfort. The color change for EC materials could be realized via reversible redox reactions under applied potentials, which share similar features with pseudocapacitors [10–12]. Thus, it would be greatly attractive to integrate electrochromism and energy storage functions into one electrode for combined applications of smart windows and supercapacitors whose energy storage level can be indicated in a noticeable and predictable manner. Various transition metal oxides, such as W, Ti, V and Ni oxides, have
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Corresponding author. E-mail address: [email protected] (X. Li).
https://doi.org/10.1016/j.solmat.2018.04.001 Received 20 January 2018; Received in revised form 13 March 2018; Accepted 1 April 2018 0927-0248/ © 2018 Elsevier B.V. All rights reserved.
been extensively investigated as either electrochromic or pseudocapacitive electrodes [8,9,13,14]. Among these materials, tungsten trioxide (WO3) is of great interest due to its low cost, large optical modulation and high capacitance [15]. WO3 possesses suitable structures for the insertion/extraction of Li+ ions, which induces interesting energy-storage performance. Besides, when redox reactions occur with charge transfer, WO3 would concurrently suffer electrochromic processes. WO3 is a typical cathodic electrochromic material, exhibiting blue color (LixWO3) in reduction state and transparent (WO3) in oxidation state. Hence, the obvious transmittance variations corresponding to the insertion/extraction of Li+ ions make WO3 become an ideal material to realize the integrated electrochromism and energy storage applications. Up to now, various methods have been reported for the preparation of WO3 films, such as electrodeposition, vacuum deposition, sol-gel and hydrothermal method [16–21]. Hydrothermal technique is one of the most promising approaches to synthesize WO3, since the nanostructures of WO3 can be accurately and simply controlled by altering the precursor concentration, growth temperature, time and capping agents. However, the traditional hydrothermal methods always need high reaction temperature and pressure, and the assistance of crystal seed-
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layer [13,17,18], which would not only increase the complexity of the preparation stage, but also degrade the performance of the materials due to its dense morphologies. Herein, we demonstrate a novel and quite facile one-step citric acidassisted hydrothermal method to synthesize WO3·H2O nanosheets on FTO glasses without seed-layer and post-treatment at low temperature (90 °C). The ammonium chloride (NH4Cl) was used to further control the morphology of the products. Moreover, the bifunctional combination of electrochromism and energy storage was also realized for the WO3·H2O nanosheets. To the best of our knowledge, this is the first report on the use of one-step synthesized WO3·H2O nanosheets for integrated electrochromism and energy storage functions.
substrates of the products. After stirring for 10 min, 0.5 g NH4Cl as capping agent was added into the solution to further control the morphology of the products. Afterwards, 5 M HCl solution was dropwise added into the solution to adjust the pH value to 1, assisting the formation of tungsten acid and the growth of the products. A transparent FTO glass was then transferred into the solution, which was maintained at 90 °C for 30 min at oven. Then, the FTO glass with WO3·H2O was taken out and rinsed with deionized water several times to remove any residual reagent. Finally, the FTO glass was baked at 60 °C for 1 h to remove any residual water.
2. Material and methods
The X-ray diffraction (XRD, Bruker D8 discover diffractometer) and field emission scanning electron microscope (SEM, Hitachi S-4800) were used to characterize the phase structures and morphologies. The Fourier transform infrared (FTIR) spectra were measured by a Lambda Scientific FTIR-7600 spectrometer. The electrochemical properties were investigated by a CHI660B electrochemical workstation in a threeelectrode configuration, where the sample was served as the working electrode; Ag/AgCl, Pt plate and 1 M LiClO4 in propylene carbonate (PC) were used as reference electrode, counter electrode and electrolyte, respectively. The optical properties were recorded using a Persee TU-1901 UV–vis–NIR spectrophotometer.
2.3. Characterizations
2.1. Materials Sodium tungstate dihydrate (Na2WO4·2H2O, 99.5%), lithium perchlorate (LiClO4, 99.99%) and propylene carbonate (PC, 99%) were purchased from Aladdin. Citric acid monohydrate (C6H8O7·H2O, ≥99.5%) and hydrochloric acid (HCl) were purchased from Sinopharm Chemical Reagant Co., Ltd. All the chemicals and reagents were used without further purification. Fluorine-doped tin oxide (FTO) glasses (1.5 ×2.5 cm2 in size, ~15 Ω/□) were purchased from OPV Tech Co., Ltd.
3. Results and discussion 2.2. Preparation of WO3·H2O nanosheets 3.1. Microstructural characterizations The WO3·H2O nanosheets were prepared by a quite facile citric acidassisted hydrothermal method. Briefly, 4.1231 g Na2WO4·2H2O and 2.6268 g citric acid monohydrate were uniformly dissolved into 100 ml deionized water by magnetic stirring. The citric acid acted as chelating agent, forming tungsten acid-citrate complexes with Na2WO4, which would assist the nucleation and enhance the adhesion to the FTO
Fig. 1 shows the X-ray diffraction (XRD) patterns and morphologies of the as-prepared thin film grown on FTO glass without the assist of citric acid. The obtained film exhibits orthorhombic phase of WO3·H2O (JCPDS No. 43–0679) as indicated in Fig. 1a. A quite irregular and nonuniform morphology with piled column-like and cluster-like
Fig. 1. (a) XRD pattern of the as-prepared WO3·H2O grown on FTO glass without the assist of citric acid. Top-view SEM images of WO3·H2O without the assist of citric acid in (b) low and (c) high magnifications. (d) Cross-sectional SEM image. 60
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citric acid is believed to effectively assist WO3·H2O in forming small crystalline nuclei and adhering to the FTO glasses, permitting further crystal growth. It can be seen from the high-magnification cross-sectional SEM image in Fig. 3d that the seed-like layer was firstly formed on the FTO glass at the bottom of the film, and then the microstructures were changed into two dimensional (2D) nanosheets. Thus, a plausible formation mechanism of the WO3·H2O nanosheets is put forward as a two-step growth process containing seed-layer formation and nanosheets growth, as illustrated in Fig. 3e. First, Na2WO4 reacted with citric acid to form tungsten acid-citrate complexes, which included many oxygen-containing groups [22]. The formed tungsten acid-citrate complexes were adsorbed on the surface of FTO when FTO glass was placed into the reaction solution, since the active surface of the FTO is rich in hydroxyl groups [23]. With the introduction of HCl solution, the adsorbed tungsten acid-citrate complexes were dissociated into tungsten acid precursors. Under hydrothermal condition, tungsten acid precursors were decomposed into WO3·H2O, and the FTO provided a platform for the nucleation of WO3·H2O. Gradually, a seed-layer was firstly formed on the surface of FTO glass. Subsequently, tungsten acid was further decomposed and the WO3·H2O was continually grown, forming nanosheets of the preferred orientation on the seed-layer [22,24,25]. However, for the non-citric acid assisted WO3·H2O, due to the lack of the chelation of citric acid, tungsten acid was continually decomposed into WO3·H2O, which was just piled on the surface of FTO substrates, resulting in non-uniform morphology and inferior adhesion. Fig. 3f schematically simulates the transport of Li+ ions. The 2D nanosheets could markedly increase the surface area and shorten the Li+ diffusion distance, which would be beneficial for the Li+ insertion/ extraction. The NH4Cl was added into the precursor to further control the morphology of the products. Fig. 4 shows the morphologies of the WO3·H2O grown on FTO glasses with NH4Cl. The film is also composed of sheet-like nanostructures, forming a quite rough surface (Fig. 4a and b). Different from the obtained WO3·H2O without NH4Cl (Fig. 3a and b), the nanosheet is much thinner with a decreasing thickness of 20 nm, which would further facilitate the permeation of electrolyte and shorten the Li+ diffusion paths, allowing rapid charge transport during redox process. As demonstrated before, NH4+ leads to a two-dimensionally growing tendency to form WO3 nanosheets [26]. The height of the nanosheets is about 650 nm (Fig. 4c).
Fig. 2. XRD patterns of the as-prepared WO3·H2O thin films grown on FTO glasses with and without NH4Cl in the precursor.
nanostructures was observed in Fig. 1b and c. The cross-sectional SEM image in Fig. 1d further verifies the undulating morphology for WO3·H2O. Worse, the obtained film can be easily washed off by deionized water, demonstrating its inferior adhesion to FTO substrates. Fig. 2 shows the XRD patterns of the citric acid-assisted thin films grown on FTO glasses with and without NH4Cl. Both films show the same crystalline structures, and all peaks can be well indexed to the orthorhombic phase of WO3·H2O (JCPDS No. 43–0679) with the corresponding lattice constants of a = 5.238, b = 10.704 and c = 5.12 Å after subtracting several diffraction peaks of FTO (JCPDS No. 46–1088). The sharp peaks declare the high crystalline quality of the as-prepared films, which would be beneficial for the cycling stability of EC materials. The FTIR spectra of the citric acid-assisted WO3·H2O thin films in the range from 4000 to 400 cm–1 are shown in Fig. S1. The two peaks at around 3420 and 1630 cm–1 represent the stretching and bending vibrations of O─H bonds. The peak appearing at about 950 cm–1 is attributed to the stretching vibration of W═O bonds. The peak at around 660 cm–1 corresponds to the stretching vibration of W─O─W bonds. Fig. 3a and b show the morphologies of the as-prepared WO3·H2O grown on FTO glasses with citric acid, without NH4Cl under different magnifications. Compared to the non-citric acid assisted WO3·H2O with irregular morphologies, the citric acid-assisted grown WO3·H2O exhibits a porous structure made up of interconnected uniform nanosheets with lengths of about 300–500 nm and thicknesses of about 40 nm. The cross-sectional SEM image in Fig. 3c shows that the WO3·H2O nanosheets of about 650 nm in height are arranged perpendicularly and uniformly on the FTO glass. From the above comparative experiments,
3.2. Electrochemical and electrochromic performance evaluation Fig. 5a shows the cyclic voltammetry (CV) curves of WO3·H2O nanostructures with and without NH4Cl at 20 mV s–1. When negative bias is applied, W6+ ions are reduced to W5+ due to the insertion of Li+ and electrons, bringing out the formation of LixWO3·H2O and the appearance of blue color. While the bias is switched to positive direction, W5+
Fig. 3. Top-view SEM images of WO3·H2O nanosheets without NH4Cl in (a) low and (b) high magnifications. Cross-sectional SEM images of WO3·H2O nanosheets without NH4Cl in (c) low and (d) high magnifications. (e) Schematic diagram of the formation process of WO3·H2O nanosheets. (f) A model of Li+ ions transport. 61
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Fig. 4. Top-view SEM images of WO3·H2O nanosheets grown with NH4Cl in (a) low and (b) high magnifications. (c) Cross-sectional SEM image of WO3·H2O nanosheets with NH4Cl.
ions are oxidized back to W6+ which is accompanied by color loss owing to the extraction of Li+ and electrons from the host. This insertion/extraction processes of Li+ and electrons can be described as Eq. (1): WO3·H2O (transparent) + xLi+ + xe– ↔ LixWO3·H2O (blue)
diffusion and charge transfer, and more active sites for Li+ insertion/ extraction are supplied during the electrochemical process. The transmittance spectra of WO3·H2O nanostructures in colored and bleached states are shown in Fig. 5b. For the WO3·H2O nanosheets with NH4Cl, it exhibits blue color at −1.0 V with the transmittance of 8.8% at 633 nm, while the transmittance is increased to 87.8% at bleached state under + 1.0 V. A quite satisfactory optical modulation of 79.0% is successfully achieved, much higher than those of the previously reported Modoped WO3 nanowires [13], W18O49 nanowires [18], WO3
(1)
The WO3·H2O nanosheets grown with NH4Cl exhibit obvious larger current density and CV area compared to the samples without NH4Cl, suggesting that the thinner nanosheets offer an easy way for Li+
Fig. 5. (a) CV curves of WO3·H2O nanosheets with and without NH4Cl at 20 mV s−1. (b) Transmittance spectra in colored and bleached states for different samples. (c) Photographs of two samples at various states. (d) In-situ transmittance curves between colored and bleached states at 633 nm. (e) Plot of ΔOD as a function of charge density at 633 nm. (f) Cycle performance of WO3·H2O nanosheets with NH4Cl for 2000 cycles. 62
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Fig. 6. (a) CV curves at different scan rates for WO3·H2O nanosheets with NH4Cl. (b) GCD curves at different current densities. (c) GCD profile at 0.14 mA cm–2, and the corresponding in situ optical response at 633 nm. (d) Areal capacitance and optical contrast as functions of current density.
corresponding to the WO3·H2O grown without and with NH4Cl. The CE value for the WO3·H2O grown with NH4Cl is comparable to those of the previously reported MoS2/WO3 nanocomposite [29], mesoporous WO3 film [32], and Ag/WO3 film [33]. The electrochemical stability of the WO3·H2O nanosheets with NH4Cl was studied by CA measurement at 633 nm under a square wave potential switching between − 1.0 and 1.0 V for 2000 cycles. It can be seen from Fig. 5f that the WO3·H2O nanosheets sustain an optical modulation of 89.6% of the initial value after 1000 cycles, and sustain an optical modulation of 87.8% of the initial state even when subjected to 2000 cycles. The excellent cycling durability of the nanosheets is superior to those of the previously reported MoO3-WO3 thin films [30], nest-like WO3 film [31], WO3/ PEDOT: PSS composite [34], and spin-coating WO3 thin film [35], which might be attributed to the high crystalline quality and the good adhesion to the FTO substrate, resulting in slow degeneration in electrolyte solution. The electrochromic properties of the proposed WO3·H2O nanosheets are compared with those of the previously reported works, as summarized in Tab. S1. The above results demonstrate that the WO3·H2O nanosheets grown with NH4Cl exhibit an excellent electrochromic performance. Both electrochromic and capacitive behaviors of WO3·H2O are based on the redox reactions between W6+ and W5+ accompanied by the insertion/ extraction of Li+ as described in Eq. (1). Therefore, it is meaningful to develop a smart energy storage electrode using WO3·H2O nanosheets, which could monitor variations in the level of stored energy and respond to the variations via color changes. Fig. 6a shows CV curves of the WO3·H2O nanosheets with NH4Cl at various scan rates ranging from 10 to 100 mV s–1. All the CV curves show the typical electrochromic and pseudocapacitance behaviors resulting from the Faradaic redox reactions between W6+ and W5+. Fig. 6b shows the galvanostatic charge/ discharge (GCD) curves of the WO3·H2O nanosheets with NH4Cl at various current densities, with the upward lines corresponding to charging and downward lines for discharging. All profiles are almost
nanoparticles [19] and Ag nanowires/WO3 [27]. Under alternating potentials, the transmittance varies by 50.5% between the colored (20.5%) and bleached (71.0%) states for the WO3·H2O nanosheets without NH4Cl. The narrow optical modulation indicates that the thick nanosheets hinder the Li+ diffusion and reduce the utilization ratio of WO3·H2O. Fig. 5c displays the photographs of two samples at original, colored (–1.0 V) and bleached (+1.0 V) states. Fig. S2 displays the chronoamperometry (CA) curves with potential being switched between + 1.0 V and − 1.0 V for 20 s per step, and the corresponding in situ transmittance responses at 633 nm of the two samples are shown in Fig. 5d. The switching time is characterized as the time required for 90% change of the entire optical modulation [28]. For the WO3·H2O nanosheets grown with NH4Cl, the switching time is 10.1 s from the bleached to colored state, and 6.1 s for the reverse process, in which the transmittance variation reaches about 79%. The switching speed is much faster than those of the previously reported MoS2/WO3 nanocomposite [29], MoO3-WO3 thin films [30], and nest-like WO3 film [31], which could be attributed to the large surface area and short Li+ ion diffusion distance for the WO3·H2O nanosheets grown with NH4Cl. For the WO3·H2O nanosheets without NH4Cl, the coloration and bleaching times are 12.2 and 3.8 s. However, the electrochromic processes correspond to a smaller transmittance variation, which is only ~50%. Coloration efficiency (CE), which is defined as the change in optical density (OD) per unit of inserted charge (Q), i.e., CE = ΔOD/Q = log(Tb/Tc)/Q, is a vital characteristic parameter for distinguishing EC materials, where Tb and Tc denote the transmittance in bleached and colored states, Q is the amount of inserted charges per unit, which can be determined by the integral of current density vs. time (Fig. S2) [28]. A high coloration efficiency suggests a broad optical modulation with a small amount of charge insertion/extraction, which would desirably arouse long cycle life. Fig. 5e plots the relation of ΔOD with charge density at 633 nm. The CE can be accordingly evaluated from the slope of linear region of the curves, obtaining 30.6 and 42.6 cm2 C–1,
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References
symmetric at applied current densities ranging from 0.14 to 1.12 mA cm–2, demonstrating the good reversibility. Moreover, bridging electrochromic behavior with energy storage is achieved. The GCD curves at different current densities and the corresponding in situ transmittance responses at 633 nm were jointly measured, as shown in Fig. 6c and Fig. S3. When the WO3·H2O nanosheets electrode is charged to −1.0 V, the insertion of Li+ and electrons arouses the reduction of W6+ to lower valance W5+, and simultaneously the electrode takes on a blue color, reaching a fully charged state. In the reverse process, the extraction of Li+ and electrons generates the bleaching of the electrode during discharging process, and the electrode changes back to transparent when the electrode is fully discharged at 0.4 V. The broad range of optical contrast could effectively demonstrate the level of stored energy. Fig. 6d presents the areal capacitance and optical contrast as functions of charge/discharge current density. The areal capacitance is 43.30, 40.84, 38.67, 36.76 and 30.88 mF cm–2 at 0.14, 0.28, 0.42, 0.56 and 1.12 mA cm–2, respectively, which is larger than those of the previously reported WO3 nanostructures [12,27]. It is noteworthy that the WO3·H2O nanosheets retain 71.3% of the initial capacitance when the current density is 8-fold enhanced, declaring the good rate capability. The optical contrast of the WO3·H2O nanosheets between fully charged and discharged states is 77.5% at 0.14 mA cm–2 and 68.1% at 1.12 mA cm–2. The WO3·H2O nanosheets still maintain an optical contrast of 87.9% even though the current density is 8-fold enhanced, suggesting that the WO3·H2O nanosheets could exhibit a stable color change during fast charge/discharge process. The color changes from transparent to blue during charging process, and fades away during discharging process. Thus, this property of the WO3·H2O nanosheets could be utilized to visually display the energy storage level.
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4. Conclusions In summary, the WO3·H2O nanosheets were directly formed on FTO substrates without seed layer by a novel and quite facile one-step citric acid-assisted hydrothermal method at 90 °C. The NH4Cl was used to further control the morphology of the products. The WO3·H2O nanosheets present superior electrochemical properties of large optical modulation, fast response time, high areal capacitance and long cycle life. The enhanced electrochemical performance can be attributed to the porous morphologies and good adhesion to the substrates of the WO3·H2O nanosheets, which are favorable for charge transfer during the redox processes. Moreover, a smart energy storage electrode based on WO3·H2O nanosheets was also demonstrated, which could monitor the level of stored energy by color changes. The proposed WO3·H2O nanosheets are promising for combined electrochromism and energy storage applications, which might have a profound impact on our daily life in the near future. Acknowledgments This work was supported by the National Key R&D Program of China (Grant No. 2016YFA0201103), the National Natural Science Foundation of China (Grant No. 51572280) and the Foundation of the Shanghai Committee for Science and Technology (Grant No. 15JC1403600). Declarations of interest None. Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.solmat.2018.04.001. 64
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Z. Bi et al. [29] M. Rakibuddin, H. Kim, Fabrication of MoS2/WO3 nanocomposite films for enhanced electro-chromic performance, New J. Chem. 41 (2017) 15327–15333. [30] P. Jittiarporn, L. Sikong, K. Kooptarnond, W. Taweepreda, S. Stoenescu, S. Badilescu, V.-V. Truong, Electrochromic properties of MoO3-WO3 thin films prepared by a sol-gel method, in the presence of a triblock copolymer template, Surf. Coat. Technol. 327 (2017) 66–74. [31] H. Li, G. Shi, H. Wang, Q. Zhang, Y. Li, Self-seeded growth of nest-like hydrated tungsten trioxide film directly on FTO substrate for highly enhanced electrochromic performance, J. Mater. Chem. A 2 (2014) 11305–11310. [32] B.R. Koo, H.J. Ahn, Fast-switching electrochromic properties of mesoporous WO3
films with oxygen vacancy defects, Nanoscale 9 (2017) 17788–17793. [33] K. Zhou, H. Wang, S. Zhang, J. Jiu, J. Liu, Y. Zhang, H. Yan, Electrochromic modulation of near-infrared light by WO3 films deposited on silver nanowire substrates, J. Mater. Sci. 52 (2017) 12783–12794. [34] C.-W. Chang-Jian, E.-C. Cho, S.-C. Yen, B.-C. Ho, K.-C. Lee, J.-H. Huang, Y.-S. Hsiao, Facile preparation of WO3/PEDOT:PSS composite for inkjet printed electrochromic window and its performance for heat shielding, Dyes Pigm. 148 (2018) 465–473. [35] D.R. Sahu, C.-Y. Hung, S.-C. Wang, J.-L. Huang, Existence of electrochromic reversibility at the 1000th cyclic voltammetry for spin coating WO3 film, Ionics 23 (2017) 3227–3233.
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Integrated electrochromism and energy storage applications based on tungsten trioxide monohydrate nanosheets by novel one-step low temperature synthesis
T
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Zhijie Bia,b, Xiaomin Lia, , Xiaoli Hea,b, Yongbo Chena,b, Xiaoke Xua,c, 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, PR China b University of Chinese Academy of Sciences, No. 19 A Yuquan Road, Beijing 100049, PR China c School of Materials Science and Engineering, Shanghai Institute of Technology, No. 100 Haiquan Road, Shanghai 201418, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Tungsten trioxide monohydrate One-step Citric acid Electrochromic Energy storage
The tungsten trioxide monohydrate (WO3·H2O) nanosheets were directly formed on fluorine-doped tin oxide (FTO) substrates without any guidance of seed layer by a novel and quite facile one-step citric acid-assisted hydrothermal method at low temperature (90 °C). The WO3·H2O nanosheets possess porous morphologies and good adhesion to the substrates, which would markedly increase the surface area of WO3·H2O and facilitate the ion diffusion during the electrochemical processes. The WO3·H2O nanosheets display superior electrochemical properties of large optical modulation (79.0%), fast switching time (tc = 10.1, tb = 6.1 s), high areal capacitance (43.30 mF cm–2) and excellent cycling stability. Furthermore, bridging electrochromic behavior with energy storage was successfully achieved. Based on the proposed WO3·H2O nanosheets, a smart energy storage electrode was demonstrated, which could monitor the level of stored energy by color changes. The results show great potential of the one-step synthesized WO3·H2O nanosheets for integrated electrochromism and energy storage applications.
1. Introduction Recently, multifunctional energy storage and conservation devices that combine novel characteristics and functions in smart and interactive modes are gradually springing up [1–4]. Supercapacitors are deemed as promising means of energy storage because of their high power density, long cycle life and fast charge/discharge capability [5–7]. Pseudocapacitor, an important type of supercapacitor, is based on the redox reactions occurred at or near the surface of active materials to store energy [8,9]. Smart windows based on electrochromic (EC) materials could adjust the interior sunlight by color variation so as to reduce the energy consumption and improve the indoor comfort. The color change for EC materials could be realized via reversible redox reactions under applied potentials, which share similar features with pseudocapacitors [10–12]. Thus, it would be greatly attractive to integrate electrochromism and energy storage functions into one electrode for combined applications of smart windows and supercapacitors whose energy storage level can be indicated in a noticeable and predictable manner. Various transition metal oxides, such as W, Ti, V and Ni oxides, have
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Corresponding author. E-mail address: [email protected] (X. Li).
https://doi.org/10.1016/j.solmat.2018.04.001 Received 20 January 2018; Received in revised form 13 March 2018; Accepted 1 April 2018 0927-0248/ © 2018 Elsevier B.V. All rights reserved.
been extensively investigated as either electrochromic or pseudocapacitive electrodes [8,9,13,14]. Among these materials, tungsten trioxide (WO3) is of great interest due to its low cost, large optical modulation and high capacitance [15]. WO3 possesses suitable structures for the insertion/extraction of Li+ ions, which induces interesting energy-storage performance. Besides, when redox reactions occur with charge transfer, WO3 would concurrently suffer electrochromic processes. WO3 is a typical cathodic electrochromic material, exhibiting blue color (LixWO3) in reduction state and transparent (WO3) in oxidation state. Hence, the obvious transmittance variations corresponding to the insertion/extraction of Li+ ions make WO3 become an ideal material to realize the integrated electrochromism and energy storage applications. Up to now, various methods have been reported for the preparation of WO3 films, such as electrodeposition, vacuum deposition, sol-gel and hydrothermal method [16–21]. Hydrothermal technique is one of the most promising approaches to synthesize WO3, since the nanostructures of WO3 can be accurately and simply controlled by altering the precursor concentration, growth temperature, time and capping agents. However, the traditional hydrothermal methods always need high reaction temperature and pressure, and the assistance of crystal seed-
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layer [13,17,18], which would not only increase the complexity of the preparation stage, but also degrade the performance of the materials due to its dense morphologies. Herein, we demonstrate a novel and quite facile one-step citric acidassisted hydrothermal method to synthesize WO3·H2O nanosheets on FTO glasses without seed-layer and post-treatment at low temperature (90 °C). The ammonium chloride (NH4Cl) was used to further control the morphology of the products. Moreover, the bifunctional combination of electrochromism and energy storage was also realized for the WO3·H2O nanosheets. To the best of our knowledge, this is the first report on the use of one-step synthesized WO3·H2O nanosheets for integrated electrochromism and energy storage functions.
substrates of the products. After stirring for 10 min, 0.5 g NH4Cl as capping agent was added into the solution to further control the morphology of the products. Afterwards, 5 M HCl solution was dropwise added into the solution to adjust the pH value to 1, assisting the formation of tungsten acid and the growth of the products. A transparent FTO glass was then transferred into the solution, which was maintained at 90 °C for 30 min at oven. Then, the FTO glass with WO3·H2O was taken out and rinsed with deionized water several times to remove any residual reagent. Finally, the FTO glass was baked at 60 °C for 1 h to remove any residual water.
2. Material and methods
The X-ray diffraction (XRD, Bruker D8 discover diffractometer) and field emission scanning electron microscope (SEM, Hitachi S-4800) were used to characterize the phase structures and morphologies. The Fourier transform infrared (FTIR) spectra were measured by a Lambda Scientific FTIR-7600 spectrometer. The electrochemical properties were investigated by a CHI660B electrochemical workstation in a threeelectrode configuration, where the sample was served as the working electrode; Ag/AgCl, Pt plate and 1 M LiClO4 in propylene carbonate (PC) were used as reference electrode, counter electrode and electrolyte, respectively. The optical properties were recorded using a Persee TU-1901 UV–vis–NIR spectrophotometer.
2.3. Characterizations
2.1. Materials Sodium tungstate dihydrate (Na2WO4·2H2O, 99.5%), lithium perchlorate (LiClO4, 99.99%) and propylene carbonate (PC, 99%) were purchased from Aladdin. Citric acid monohydrate (C6H8O7·H2O, ≥99.5%) and hydrochloric acid (HCl) were purchased from Sinopharm Chemical Reagant Co., Ltd. All the chemicals and reagents were used without further purification. Fluorine-doped tin oxide (FTO) glasses (1.5 ×2.5 cm2 in size, ~15 Ω/□) were purchased from OPV Tech Co., Ltd.
3. Results and discussion 2.2. Preparation of WO3·H2O nanosheets 3.1. Microstructural characterizations The WO3·H2O nanosheets were prepared by a quite facile citric acidassisted hydrothermal method. Briefly, 4.1231 g Na2WO4·2H2O and 2.6268 g citric acid monohydrate were uniformly dissolved into 100 ml deionized water by magnetic stirring. The citric acid acted as chelating agent, forming tungsten acid-citrate complexes with Na2WO4, which would assist the nucleation and enhance the adhesion to the FTO
Fig. 1 shows the X-ray diffraction (XRD) patterns and morphologies of the as-prepared thin film grown on FTO glass without the assist of citric acid. The obtained film exhibits orthorhombic phase of WO3·H2O (JCPDS No. 43–0679) as indicated in Fig. 1a. A quite irregular and nonuniform morphology with piled column-like and cluster-like
Fig. 1. (a) XRD pattern of the as-prepared WO3·H2O grown on FTO glass without the assist of citric acid. Top-view SEM images of WO3·H2O without the assist of citric acid in (b) low and (c) high magnifications. (d) Cross-sectional SEM image. 60
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citric acid is believed to effectively assist WO3·H2O in forming small crystalline nuclei and adhering to the FTO glasses, permitting further crystal growth. It can be seen from the high-magnification cross-sectional SEM image in Fig. 3d that the seed-like layer was firstly formed on the FTO glass at the bottom of the film, and then the microstructures were changed into two dimensional (2D) nanosheets. Thus, a plausible formation mechanism of the WO3·H2O nanosheets is put forward as a two-step growth process containing seed-layer formation and nanosheets growth, as illustrated in Fig. 3e. First, Na2WO4 reacted with citric acid to form tungsten acid-citrate complexes, which included many oxygen-containing groups [22]. The formed tungsten acid-citrate complexes were adsorbed on the surface of FTO when FTO glass was placed into the reaction solution, since the active surface of the FTO is rich in hydroxyl groups [23]. With the introduction of HCl solution, the adsorbed tungsten acid-citrate complexes were dissociated into tungsten acid precursors. Under hydrothermal condition, tungsten acid precursors were decomposed into WO3·H2O, and the FTO provided a platform for the nucleation of WO3·H2O. Gradually, a seed-layer was firstly formed on the surface of FTO glass. Subsequently, tungsten acid was further decomposed and the WO3·H2O was continually grown, forming nanosheets of the preferred orientation on the seed-layer [22,24,25]. However, for the non-citric acid assisted WO3·H2O, due to the lack of the chelation of citric acid, tungsten acid was continually decomposed into WO3·H2O, which was just piled on the surface of FTO substrates, resulting in non-uniform morphology and inferior adhesion. Fig. 3f schematically simulates the transport of Li+ ions. The 2D nanosheets could markedly increase the surface area and shorten the Li+ diffusion distance, which would be beneficial for the Li+ insertion/ extraction. The NH4Cl was added into the precursor to further control the morphology of the products. Fig. 4 shows the morphologies of the WO3·H2O grown on FTO glasses with NH4Cl. The film is also composed of sheet-like nanostructures, forming a quite rough surface (Fig. 4a and b). Different from the obtained WO3·H2O without NH4Cl (Fig. 3a and b), the nanosheet is much thinner with a decreasing thickness of 20 nm, which would further facilitate the permeation of electrolyte and shorten the Li+ diffusion paths, allowing rapid charge transport during redox process. As demonstrated before, NH4+ leads to a two-dimensionally growing tendency to form WO3 nanosheets [26]. The height of the nanosheets is about 650 nm (Fig. 4c).
Fig. 2. XRD patterns of the as-prepared WO3·H2O thin films grown on FTO glasses with and without NH4Cl in the precursor.
nanostructures was observed in Fig. 1b and c. The cross-sectional SEM image in Fig. 1d further verifies the undulating morphology for WO3·H2O. Worse, the obtained film can be easily washed off by deionized water, demonstrating its inferior adhesion to FTO substrates. Fig. 2 shows the XRD patterns of the citric acid-assisted thin films grown on FTO glasses with and without NH4Cl. Both films show the same crystalline structures, and all peaks can be well indexed to the orthorhombic phase of WO3·H2O (JCPDS No. 43–0679) with the corresponding lattice constants of a = 5.238, b = 10.704 and c = 5.12 Å after subtracting several diffraction peaks of FTO (JCPDS No. 46–1088). The sharp peaks declare the high crystalline quality of the as-prepared films, which would be beneficial for the cycling stability of EC materials. The FTIR spectra of the citric acid-assisted WO3·H2O thin films in the range from 4000 to 400 cm–1 are shown in Fig. S1. The two peaks at around 3420 and 1630 cm–1 represent the stretching and bending vibrations of O─H bonds. The peak appearing at about 950 cm–1 is attributed to the stretching vibration of W═O bonds. The peak at around 660 cm–1 corresponds to the stretching vibration of W─O─W bonds. Fig. 3a and b show the morphologies of the as-prepared WO3·H2O grown on FTO glasses with citric acid, without NH4Cl under different magnifications. Compared to the non-citric acid assisted WO3·H2O with irregular morphologies, the citric acid-assisted grown WO3·H2O exhibits a porous structure made up of interconnected uniform nanosheets with lengths of about 300–500 nm and thicknesses of about 40 nm. The cross-sectional SEM image in Fig. 3c shows that the WO3·H2O nanosheets of about 650 nm in height are arranged perpendicularly and uniformly on the FTO glass. From the above comparative experiments,
3.2. Electrochemical and electrochromic performance evaluation Fig. 5a shows the cyclic voltammetry (CV) curves of WO3·H2O nanostructures with and without NH4Cl at 20 mV s–1. When negative bias is applied, W6+ ions are reduced to W5+ due to the insertion of Li+ and electrons, bringing out the formation of LixWO3·H2O and the appearance of blue color. While the bias is switched to positive direction, W5+
Fig. 3. Top-view SEM images of WO3·H2O nanosheets without NH4Cl in (a) low and (b) high magnifications. Cross-sectional SEM images of WO3·H2O nanosheets without NH4Cl in (c) low and (d) high magnifications. (e) Schematic diagram of the formation process of WO3·H2O nanosheets. (f) A model of Li+ ions transport. 61
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Fig. 4. Top-view SEM images of WO3·H2O nanosheets grown with NH4Cl in (a) low and (b) high magnifications. (c) Cross-sectional SEM image of WO3·H2O nanosheets with NH4Cl.
ions are oxidized back to W6+ which is accompanied by color loss owing to the extraction of Li+ and electrons from the host. This insertion/extraction processes of Li+ and electrons can be described as Eq. (1): WO3·H2O (transparent) + xLi+ + xe– ↔ LixWO3·H2O (blue)
diffusion and charge transfer, and more active sites for Li+ insertion/ extraction are supplied during the electrochemical process. The transmittance spectra of WO3·H2O nanostructures in colored and bleached states are shown in Fig. 5b. For the WO3·H2O nanosheets with NH4Cl, it exhibits blue color at −1.0 V with the transmittance of 8.8% at 633 nm, while the transmittance is increased to 87.8% at bleached state under + 1.0 V. A quite satisfactory optical modulation of 79.0% is successfully achieved, much higher than those of the previously reported Modoped WO3 nanowires [13], W18O49 nanowires [18], WO3
(1)
The WO3·H2O nanosheets grown with NH4Cl exhibit obvious larger current density and CV area compared to the samples without NH4Cl, suggesting that the thinner nanosheets offer an easy way for Li+
Fig. 5. (a) CV curves of WO3·H2O nanosheets with and without NH4Cl at 20 mV s−1. (b) Transmittance spectra in colored and bleached states for different samples. (c) Photographs of two samples at various states. (d) In-situ transmittance curves between colored and bleached states at 633 nm. (e) Plot of ΔOD as a function of charge density at 633 nm. (f) Cycle performance of WO3·H2O nanosheets with NH4Cl for 2000 cycles. 62
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Fig. 6. (a) CV curves at different scan rates for WO3·H2O nanosheets with NH4Cl. (b) GCD curves at different current densities. (c) GCD profile at 0.14 mA cm–2, and the corresponding in situ optical response at 633 nm. (d) Areal capacitance and optical contrast as functions of current density.
corresponding to the WO3·H2O grown without and with NH4Cl. The CE value for the WO3·H2O grown with NH4Cl is comparable to those of the previously reported MoS2/WO3 nanocomposite [29], mesoporous WO3 film [32], and Ag/WO3 film [33]. The electrochemical stability of the WO3·H2O nanosheets with NH4Cl was studied by CA measurement at 633 nm under a square wave potential switching between − 1.0 and 1.0 V for 2000 cycles. It can be seen from Fig. 5f that the WO3·H2O nanosheets sustain an optical modulation of 89.6% of the initial value after 1000 cycles, and sustain an optical modulation of 87.8% of the initial state even when subjected to 2000 cycles. The excellent cycling durability of the nanosheets is superior to those of the previously reported MoO3-WO3 thin films [30], nest-like WO3 film [31], WO3/ PEDOT: PSS composite [34], and spin-coating WO3 thin film [35], which might be attributed to the high crystalline quality and the good adhesion to the FTO substrate, resulting in slow degeneration in electrolyte solution. The electrochromic properties of the proposed WO3·H2O nanosheets are compared with those of the previously reported works, as summarized in Tab. S1. The above results demonstrate that the WO3·H2O nanosheets grown with NH4Cl exhibit an excellent electrochromic performance. Both electrochromic and capacitive behaviors of WO3·H2O are based on the redox reactions between W6+ and W5+ accompanied by the insertion/ extraction of Li+ as described in Eq. (1). Therefore, it is meaningful to develop a smart energy storage electrode using WO3·H2O nanosheets, which could monitor variations in the level of stored energy and respond to the variations via color changes. Fig. 6a shows CV curves of the WO3·H2O nanosheets with NH4Cl at various scan rates ranging from 10 to 100 mV s–1. All the CV curves show the typical electrochromic and pseudocapacitance behaviors resulting from the Faradaic redox reactions between W6+ and W5+. Fig. 6b shows the galvanostatic charge/ discharge (GCD) curves of the WO3·H2O nanosheets with NH4Cl at various current densities, with the upward lines corresponding to charging and downward lines for discharging. All profiles are almost
nanoparticles [19] and Ag nanowires/WO3 [27]. Under alternating potentials, the transmittance varies by 50.5% between the colored (20.5%) and bleached (71.0%) states for the WO3·H2O nanosheets without NH4Cl. The narrow optical modulation indicates that the thick nanosheets hinder the Li+ diffusion and reduce the utilization ratio of WO3·H2O. Fig. 5c displays the photographs of two samples at original, colored (–1.0 V) and bleached (+1.0 V) states. Fig. S2 displays the chronoamperometry (CA) curves with potential being switched between + 1.0 V and − 1.0 V for 20 s per step, and the corresponding in situ transmittance responses at 633 nm of the two samples are shown in Fig. 5d. The switching time is characterized as the time required for 90% change of the entire optical modulation [28]. For the WO3·H2O nanosheets grown with NH4Cl, the switching time is 10.1 s from the bleached to colored state, and 6.1 s for the reverse process, in which the transmittance variation reaches about 79%. The switching speed is much faster than those of the previously reported MoS2/WO3 nanocomposite [29], MoO3-WO3 thin films [30], and nest-like WO3 film [31], which could be attributed to the large surface area and short Li+ ion diffusion distance for the WO3·H2O nanosheets grown with NH4Cl. For the WO3·H2O nanosheets without NH4Cl, the coloration and bleaching times are 12.2 and 3.8 s. However, the electrochromic processes correspond to a smaller transmittance variation, which is only ~50%. Coloration efficiency (CE), which is defined as the change in optical density (OD) per unit of inserted charge (Q), i.e., CE = ΔOD/Q = log(Tb/Tc)/Q, is a vital characteristic parameter for distinguishing EC materials, where Tb and Tc denote the transmittance in bleached and colored states, Q is the amount of inserted charges per unit, which can be determined by the integral of current density vs. time (Fig. S2) [28]. A high coloration efficiency suggests a broad optical modulation with a small amount of charge insertion/extraction, which would desirably arouse long cycle life. Fig. 5e plots the relation of ΔOD with charge density at 633 nm. The CE can be accordingly evaluated from the slope of linear region of the curves, obtaining 30.6 and 42.6 cm2 C–1,
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References
symmetric at applied current densities ranging from 0.14 to 1.12 mA cm–2, demonstrating the good reversibility. Moreover, bridging electrochromic behavior with energy storage is achieved. The GCD curves at different current densities and the corresponding in situ transmittance responses at 633 nm were jointly measured, as shown in Fig. 6c and Fig. S3. When the WO3·H2O nanosheets electrode is charged to −1.0 V, the insertion of Li+ and electrons arouses the reduction of W6+ to lower valance W5+, and simultaneously the electrode takes on a blue color, reaching a fully charged state. In the reverse process, the extraction of Li+ and electrons generates the bleaching of the electrode during discharging process, and the electrode changes back to transparent when the electrode is fully discharged at 0.4 V. The broad range of optical contrast could effectively demonstrate the level of stored energy. Fig. 6d presents the areal capacitance and optical contrast as functions of charge/discharge current density. The areal capacitance is 43.30, 40.84, 38.67, 36.76 and 30.88 mF cm–2 at 0.14, 0.28, 0.42, 0.56 and 1.12 mA cm–2, respectively, which is larger than those of the previously reported WO3 nanostructures [12,27]. It is noteworthy that the WO3·H2O nanosheets retain 71.3% of the initial capacitance when the current density is 8-fold enhanced, declaring the good rate capability. The optical contrast of the WO3·H2O nanosheets between fully charged and discharged states is 77.5% at 0.14 mA cm–2 and 68.1% at 1.12 mA cm–2. The WO3·H2O nanosheets still maintain an optical contrast of 87.9% even though the current density is 8-fold enhanced, suggesting that the WO3·H2O nanosheets could exhibit a stable color change during fast charge/discharge process. The color changes from transparent to blue during charging process, and fades away during discharging process. Thus, this property of the WO3·H2O nanosheets could be utilized to visually display the energy storage level.
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4. Conclusions In summary, the WO3·H2O nanosheets were directly formed on FTO substrates without seed layer by a novel and quite facile one-step citric acid-assisted hydrothermal method at 90 °C. The NH4Cl was used to further control the morphology of the products. The WO3·H2O nanosheets present superior electrochemical properties of large optical modulation, fast response time, high areal capacitance and long cycle life. The enhanced electrochemical performance can be attributed to the porous morphologies and good adhesion to the substrates of the WO3·H2O nanosheets, which are favorable for charge transfer during the redox processes. Moreover, a smart energy storage electrode based on WO3·H2O nanosheets was also demonstrated, which could monitor the level of stored energy by color changes. The proposed WO3·H2O nanosheets are promising for combined electrochromism and energy storage applications, which might have a profound impact on our daily life in the near future. Acknowledgments This work was supported by the National Key R&D Program of China (Grant No. 2016YFA0201103), the National Natural Science Foundation of China (Grant No. 51572280) and the Foundation of the Shanghai Committee for Science and Technology (Grant No. 15JC1403600). Declarations of interest None. Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.solmat.2018.04.001. 64
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