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PB composite nanosheet electrodes towards energy storage smart window
Solar Energy Materials & Solar Cells 207 (2020) 110337
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Solar Energy Materials and Solar Cells journal homepage: http://www.elsevier.com/locate/solmat
A high-performance electrochromic device assembled with hexagonal WO3 and NiO/PB composite nanosheet electrodes towards energy storage smart window Jianbo Pan, Rongzong Zheng, Yi Wang, Xingke Ye, Zhongquan Wan, Chunyang Jia *, Xiaolong Weng, Jianliang Xie, Longjiang Deng State Key Laboratory of Electronic Thin Films and Integrated Devices, National Engineering Research Center of Electromagnetic Radiation Control Materials, School of Electronic Science and Engineering, University of Electronic Science and Technology of China, Chengdu, 610054, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Bi-functional device Electrochromic Energy storage Composite material WO3
Functions of electrochromism and energy exchange can be achieved by redox reactions, which indicate designing a bi-functional electrochromic energy storage device (EESD) is feasible. In this paper, a NiO/PB composite nanosheet electrode was prepared via loading Prussian blue (iron (III) hexacyanoferrate (II), PB) nanoparticles on the surface of NiO nanosheets, meanwhile, a high-performance WO3 negative electrode was also prepared via novel erythritol-assisted hydrothermal method with NH4Cl used as capping agent. The designed EESD exhibited properties of large optical modulation (67.6% at 630 nm), high coloration efficiency (109.6 cm2 C-1), fast switching speed (2.8/7.9 s for bleaching/coloration) and long cycle life (ΔT remaining was 84.1% after 4000 cycles). Meanwhile, the EESD also obtained attractive energy storage properties at a potential window from 0 to þ1.3 V. Impressively, the EESD exhibited a high areal capacitance of 11.50 mF cm-2 at current density of 0.05 mA cm-2 and the process of charging/discharging can be repeated at least 4000 cycles with a little decay. Therefore, the bi-functional EESD exhibits high promising application prospect.
1. Introduction In the 21st century, environmental and energy sources have been to the two major topics in the whole world [1,2]. In buildings, tremendous energy sources used for heating and cooling usually need to be provided for maintaining a comfortable temperature. In recent years, the elec trochromic smart window has attracted wide attention due to the function of efficiently modulating light and heat passing through win dow with a low power consumption [3,4]. More importantly, because of similar device structure to pseudo-capacitive device, a novel type of electrochromic device with both electrochromic and energy storage properties have emerged and shows promising application potential. This device is called electrochromic energy storage device (EESD) in which main components are some electrochromic active material with good pseuocapacitive performance such as WO3, MnO2 and NiO. Thus, improving the performance of active materials is the key to design high-performance EESD. WO3 is a kind of widely studied electrochromic material which ex hibits high coloring efficiency, good cycling stability, large contrast ratio
and appropriate color (between transparent and dark blue) [5,6]. Meanwhile, WO3 also plays an important role in supercapacitor due to its high pseudocapacitance [7]. For instance, Jia et al. synthesized a pancake-like mesopore WO3 and obtained high specific capacitance of 605.5 F g-1 at 0.37 A g-1 [8]. Zhu et al. reported an assembly hexagonal WO3 which achieved a high specific capacitance of 421.8 F g-1 at 0.5 A g-1 [9]. According to these previous works, WO3 exhibited great po tential in the application of EESD and has been widely studied. Gener ally, in EESD, WO3 is used as cathodic electrochromic layer (negative electrode) and other materials with complementary color are used as anodic electrochromic layer (positive electrode), WO3 and Prussian blue (iron (III) hexacyanoferrate (II), PB) [10,11], WO3 and polyaniline (PANI) [12], WO3 and MnO2 [13], for example. Some strategies, con taining design of crystal and nanostructure [14], doping [15] and compositing with other materials [16], have been used for improving electrochromic performance of WO3. In hydrothermal method, tem perature and time [17], pH value [18], or capping agents [19] are easy to control for obtaining different nanostructures such as nanowire [20], nanorod [21], nanotree [22] and nanonest [23]. Recently, our group
* Corresponding author. E-mail address: [email protected] (C. Jia). https://doi.org/10.1016/j.solmat.2019.110337 Received 15 August 2019; Received in revised form 20 November 2019; Accepted 2 December 2019 Available online 24 December 2019 0927-0248/© 2019 Elsevier B.V. All rights reserved.
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reported a novel facile hydrothermal technology applicated to directly synthesize a high-performance WO3 electrochromic film on FTO-coated glass [14]. However, the relatively poor performance can be found in electrochromic device due to mismatching of electrochromic properties between WO3 electrode and PB electrode in our previous work. For example, the lifetime of device is limited by the electrode material with worse cyclic stability. Therefore, it is meaningful to improve the per formance of positive electrode while optimizing the negative electrode in EESD. Because of a complementary color change (between transparent and blue) [24], PB was often used in assembling electrochromic device with tungsten oxide materials, such as WO3 [10,25], and W18O49 [26]. However, PB film prepared by electrodeposition method exhibited a poor cyclic stability due to closely packed nanoparticles. In addition, NiO as another anodic electrochromic material with complementary color can be easily synthesized to be nanosheets [27,28], which has more active sites for redox reaction. The method of composite material has been confirmed to be an effective means to improve performances film and combine the advantages of various materials [29,30]. There fore, designing NiO/PB composite nanosheet arrays is a promising strategy to obtain a high-performance anodic electrochromic film, in which porous structure of nanosheets can also effectively avoid the tight stacking between PB nanoparticles. Meanwhile, the NiO/PB composite nanosheet film used as positive electrode shows greater potential to assemble EESD with WO3 negative electrode. In this work, a NiO/PB composite nanosheet was successfully pre pared. Owing to the nanosheet structure of NiO, the PB nanoparticles loading on the surface of nanosheets obtained more redox reaction sites and ion buffer channels. Meanwhile, a high-performance WO3 electro chromic film was directly synthesized on FTO-coated glass by a novel erythritol-assisted hydrothermal method, which was used as negative electrode. Finally, the assembled EESD exhibited outstanding color switching property between transparent and dark blue, fast response speed, great coloration efficiency and high cyclic stability. Additionally, the EESD obtained excellent energy storage performance, which can be successfully used to drive LED (light emitting diode) and LCD (liquid crystal display). The EESD showed outstanding electrochromic and en ergy storage performances simultaneously, which indicated that the EESD can not only modulate optical transmittance but also store and release electric energy for driving some household electronics.
dissolution. Then 20 mL mixture of above solutions was transferred to a 50 mL Teflon-line stainless-steel autoclave. The FTO-coated glass sub strate (FTO-coated glass was washed with detergent, acetone, de-ionized water and anhydrous ethanol in an ultrasonic bath for 20 min per wash) was placed vertically in the autoclave. After that, the autoclave was sealed and maintained at 110 � C for 2.5 h. Finally, the obtained film was washed with de-ionized water and anhydrous ethanol, then dried in oven at 60 � C for 2 h. 2.3. Preparation of PB, NiO and NiO/PB composite films NiO film was synthesized on FTO-coated glass through hydrothermal and annealing technology according to previous work [31] with some changes. Briefly, a green transparent solution was obtained by dissolv ing the urea (3 g), K2S2O8 (0.13 g) and Ni(NO3)2�6H2O (0.5 g) in de-ionized water (40 mL). Then, 20 mL above solution was transferred into 50 mL Teflon-line stainless-steel autoclave. The FTO-coated glass substrate was placed vertically in the autoclave, and the autoclave was sealed and maintained at 90 � C for 1.5 h. After cooling down to room temperature naturally, the obtained film was washed with de-ionized water and anhydrous ethanol. Finally, the drying film was annealed at 300 � C in air for 1.5 h to obtain NiO film. PB was prepared by electrodeposition technology reported by pre vious work [10,14]. Briefly, the brown homogenous electrodeposition solution contained K3[Fe(CN)6] (0.1646 g), FeCl3 (0.0811 g), KCl (0.1864 g) and 50 mL de-ionized water. The electrodeposition process was carried out in three-electrode system using a CHI660E electro chemical workstation, and platinum sheet used as the counter-electrode, Ag/AgCl used as the reference electrode, the FTO-coating glass with and without NiO film used as working electrodes. A constant current (50 μA cm-2) was applied to working electrodes for 200 s to prepare PB and NiO/PB composite nanosheet films. After that, the obtained films were washed by de-ionized water and anhydrous ethanol for several times. Finally, drying for 6 h under 60 � C. 2.4. Assembly of EESD The NiO/PB composite nanosheet film was used as positive electrode and the WO3 film was used as negative electrode. Briefly, 3M4910VHB double-sided tape was used to partly seal the four sides of EESD and UV curing glue was used to further enhance sealability. Then, 1 M LiClO4/ PC solution was injected into the gap (about 1 mm) by syringe to be used as the liquid electrolyte. Finally, the whole EESD was sealed with UV curing glue.
2. Experimental 2.1. Materials All solvents and chemicals were of analytical grade and used without further purification. FTO-coated glass (about 10 Ω resistance, 2.2 mm thickness) was purchased from Wuhan lattice Solar Energy Technology Co., Ltd. Sodium tungstate (Na2WO4�2H2O), nickel nitrate (Ni (NO3)2�6H2O), urea, potassium persulfate (K2S2O8), potassium hy droxide (KOH), hydrochloric acid (HCl, 36%) and hydrogen peroxide (H2O2, 30%) were purchased from Chengdu Kelong Reagent Co., Ltd. Erythritol was purchased from Shanghai Yuanye Biotechnology Co., Ltd. Anhydrous propylene carbonate (PC), anhydrous lithium perchlorate (LiClO4), potassium ferricyanide (K3[Fe(CN)6]), ferric chloride (FeCl3), potassium chloride (KCl) and ammonium chloride (NH4Cl) were pur chased from Shanghai Aladdin biochemical Polytron Technologies Inc.
2.5. Characterization and calculation methods The crystal structures of films and powder were examined by X-ray diffraction using X’ Pert PRO MPD with Cu Ka radiation. The mor phologies of the samples were investigated by scanning electron mi croscope (FESEM, FEI Inspect F50). The powder for Transmission Electron Microscopy (TEM) and X-ray diffraction test was synthesized by the same method with film. The TEM images (low-resolution, highresolution) were obtained using FEI Talos-S (TEM) operated at 200 KV. Particularly, NiO/PB composite powder for TEM and Scanning Transmission Electron Microscopy (STEM) was scraped from FTOcoated glass and ultrasound-treated in anhydrous ethanol. All electro chemical measurements were carried out on an electrochemical work station (CHI660E, Shanghai Chenhua Instruments, Inc.) using a conventional three-electrode test system with platinum sheet used as the counter-electrode, Ag/AgCl used as the reference electrode and the prepared films used as working electrodes. The optical properties and kinetics of coloration and bleaching of the films were detected using a UV–vis spectrophotometer (SP60, X-Rite). The 1 M LiClO4 in PC was used as the electrolyte to test electrochemical and electrochromic properties.
2.2. Preparation of WO3 film A transparent homogeneous peroxopolytungstic acid solution was used as the tungsten source in hydrothermal process, of which prepared procedures were the same as ones in our previous work [14]. Hydro thermal precursor contained peroxopolytugstic acid solution (0.1 M, 10.5 mL), HCL (3 M, 3.5 mL), de-ionized water (17 mL), erythritol (11 g) and NH4Cl (0.45 g). Transparent precursor was obtained by ultrasonic 2
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The areal capacitance was calculated from the GCD curves according to the follow Equation (1) [10,32]:
WO3 film directly grow on the FTO-coated glass, but also used as nucleating agents to participate in crystal growth process. Therefore, under the synergistic-effect of erythritol and NH4Cl, the hex-WO3 film exhibited a porous coral-like nanostructure and TEM images display this structure more clearly in Fig. 2c. In contrast, the ort-WO3⋅0.33H2O without the addition of NH4Cl obtained a compact stacking nano structure (Fig. S1), which is adverse for ion transport in process of electrochemical reaction [14]. Additionally, as shown in Fig. 2d, the HRTEM image of coral-like nanostructure confirmed the spacing of lattice fringes was 0.381 nm. The spacing was indexed as (002) lattice plane of hex-WO3 (JCPDS no. 85–2459), which further confirmed the preferentially growth along the [001] direction was obtained. From the above results, it can be concluded that Cl- ion exhibited a similar effect with SO24 ion [19,20] to control WO3 crystal growth in hydrothermal process. And detailed discussion of formation mechanism of WO3 crystal under the synergistic-effect of polyhydroxy self-seeded and capping agents can be found in our previous work [14]. The electrochemical and electrochromic properties of hex-WO3 film were tested by a conventional three-electrode system, and 10 cycles of cyclic voltammetry (CV) were carried out between -1.0 V and þ1.0 V at a scan rate of 50 mV s-1 before measurement. Fig. 3a shows CV curve of hex-WO3 film, the large area of CV hysteresis curve and high peak currents demonstrate a good electrochemical performance. Further more, the hex-WO3 film exhibited a wide range of optical modulation after þ1.0 V and -1.0 V applied for 30 s between 330 nm and 1000 nm, respectively. In terms of appearance, the color of the hex-WO3 film changed from transparent (bleached state) to dark blue (colored state) reversibly, and the reactions in these two processes can be described as follow:
(1)
Ca ¼ IΔt / AΔV -2
where Ca is the areal capacitance (mF cm ), I is the current during discharging (mA), Δt is the discharge time (s), A is the effective area of device (cm2), and ΔV is the potential window (V). 3. Results and discussion 3.1. Overview of EESD design The EESD was successfully assembled and consisted of WO3 negative electrode, NiO/PB composite nanosheet positive electrode, and 1 M LiClO4/PC electrolyte. Herein, the hexagonal WO3 (hex-WO3) film was directly synthesized on FTO-coated glass by a novel one-step erythritolassisted hydrothermal method, where NH4Cl was used as capping agent for further controlling the trend of crystal growth efficiently. The NiO nanosheets were synthesized by hydrothermal and annealing technol ogies. Then, the PB nanoparticles were loaded on the surface of NiO nanosheets via electrodeposition to obtain NiO/PB composite nano sheets (see Fig. 1). 3.2. Performance of WO3 film Fig. 2a shows the XRD patterns of erythritol-assisted WO3 films with and without the addition of NH4Cl, in which the peaks marked with “▽” were categorized to FTO-coated glass. The erythritol-assisted WO3 film without NH4Cl can be indexed to orthorhombic WO3⋅0.33H2O (ortWO3⋅0.33H2O) on JCPDS no. 87–1023 and the XRD pattern was corre sponding to JCPDS no. 85–2459 (hex-WO3) with the addition of NH4Cl, which indicated a phase transition occurred under the influence of NH4Cl. The results of XRD characterization are very consistent with that of previous reports [14]. The reason for the phase transition could be the NHþ 4 inserted into lattice tunnels of hex-WO3, thus the hexagonal crystal system was constructed [33-35]. Meanwhile, the hex-WO3 contains trigonal cavity, four-coordinated square window and hexagonal win dow, which are conducive to the formation of orderly ion transport and better than ort-WO3⋅0.33H2O [36,37]. It is noteworthy that Cl- can generate an strong energy barrier on the faces parallel to the c-axis by adsorption, which can lead to a oriented growth of WO3 crystal along (001) face [38-40] and porous nanostructure was obtained simulta neously. Fig. 2b shows the top-view and side-view SEM images of hex-WO3 film, presenting the morphology and undulating surface. On the other hand, erythritol not only used as self-seeded agent to make
WO3 (transparent) þ xLiþ þxe- ⇌ LixWO3 (dark blue)
(2)
The switching time, defining as the time required for a device to reach 90% of full optical modulation, is used to characterize the speed of bleached and colored processes [41]. The relationship between the time and transmittance variation at 630 nm was obtained by applying step voltages of þ1.0 V and -1.0 V to hex-WO3 film with in situ measurement of transmittance simultaneously. The result is shown in Fig. 3c, the bleached time (tb) and colored time (tc) were calculated to be 3.3 s and 3.7 s, respectively. Coloration efficiency (CE), another important char acteristic parameter for evaluating the electrochromic property, is ruled as the change in optical change (ΔOD) at a specific wavelength per unit injection charge density (ΔQ) in coloring process. The calculation for mula is as follow [42]: CE ¼ ΔOD / ΔQ ¼ log (Tb / Tc) / ΔQ
Fig. 1. Schematic illustration of the process of preparing EESD. 3
(3)
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Fig. 2. (a) XRD patterns for ort-WO3 and hex-WO3 films; (b) SEM images of hex-WO3 film, while insets show high-magnification and cross-sectional images; TEM images of hex-WO3 powder: (c) low-magnification image, (d) high-resolution image (HRTEM).
Fig. 3. The hex-WO3 film: (a) CV curve (50 mV s-1), (b) UV–vis transmittance spectrum at colored and bleached states between 330 nm and 1000 nm (inserts are digital photographs at different states), (c) switching time and transmittance variation curves between colored and bleached states at 630 nm, �1.0 V, (d) variation of the optical density (ΔOD) versus charge density (Q).
4
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Where Tb and Tc denote the bleached and colored transmittance values at a special wavelength, respectively. As shown in Fig. 3d, with charge injection at -1.0 V, a high ΔOD value (~2.22) demonstrated that the hex-WO3 film can achieved a deep coloration. Finally, the CE value is 73.1 cm2 C-1 (by calculating the slope of the linear region of the line). In a word, the prepared hex-WO3 film with excellent electrochromic per formance exhibited great potential in the application of EESD.
synthetic route in the hydrothermal process can be described as follows [31]:
3.3. Performance comparison of anodic electrochromic films
½NiðH2 OÞ6 x ðNH3 Þx �2þ þ 2OH
¼ NiðOHÞ2 þ ð6
Fig. S2 shows the SEM images of top and cross-sectional views, it can be seen the PB film prepared by electrodeposition method has a compact morphology, which results in a poor robustness when ions are intercalating/de-intercalating from film. Therefore, the NiO/PB com posite nanosheet film was designed to improve the performance of PB film. As shown in Fig. 4a, the XRD pattern of Ni(OH)2(NiOOH)0.167 powder prepared by hydrothermal method was corresponding to JCPDS no. 89–7111, and Ni(OH)2(NiOOH)0.167 can also directly grow on FTOcoated glass. After annealing, the Ni(OH)2(NiOOH)0.167 changed into cubic NiO that could be indexed on JCPDS no. 47–1049, which proved a good crystallinity and was consistent with the previous report [31]. The SEM image of NiO in Fig. 4b exhibits the nanosheet morphology and the pore structure between the nanosheets can be clearly found. The
1:167NiðOHÞ2 þ 0:0835S2 O8 2
þ 0:167OH
COðNH2 Þ2 þ H2 O ¼ CO2 þ 2NH3
(4)
NH3 þ H2 O ¼ NH4 þ þ OH
(5)
Ni2þ þ xNH3 þ ð6
xÞH2 O ¼ ½NiðH2 OÞ6 x ðNH3 Þx �2þ
NiII ðOHÞ2 ðNiIII OOHÞ0:167 þ 0:167SO4 2
xÞH2 O þ xNH3
(6) (7)
¼
þ 0:167H2 O
(8)
Low-concentration ammonia is obtained by slow decomposition of urea at low temperatures. After that, the hydrolysis of NH3 molecules produce OH- and NHþ 4 , which results in forming a weak alkaline envi ronment. At room temperature, Ni2þ can immediately form a relative stable [Ni(H2O)6-x(NH3)x]2þ complex ions with NH3 in the weak alka line medium. With proceeding of hydrothermal process, the complex ions are decomposed and form Ni(OH)2 with OH- ions [31,43]. Since Ni (OH)2 is a typical layered double-hydroxide, ions and molecules can be present between the stacking layers along the c-axis, which is easily to
Fig. 4. (a) XRD patterns of Ni(OH)2(NiOOH)0.167 and NiO powder; (b) SEM image of NiO nanosheets; (c) SEM image of NiO/PB composite nanosheets; (d) TEM image of NiO/PB composite nanosheet; (e) STEM-HAADF (High-Angle Annular Dark Field) image at the side of NiO/PB composite nanosheet; STEM-EDS mapping images of element distribution: (f) Ni, (g) Fe; (h) The Ni and Fe overlapping image. 5
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form two-dimensional nanosheets [44]. Finally, Ni(OH)2(NiOOH)0.167 nanosheets are obtained by the oxidation of Ni(OH)2 under the effect of S2O28 ions [31], which can further anneal to form NiO nanosheets. Then, after 200 s of electrodeposition on the prepared NiO film, the PB nanoparticles were successfully loaded on the surface of NiO nano sheets uniformly which can be confirmed by comparing Fig. 4b and 4c. A clearer composite structure of single NiO/PB nanosheet can be seen in TEM image of Fig. 4d. Moreover, the uniform distribution of Ni and Fe elements in STEM mapping images at 50 nm scale is corresponding to NiO and PB, respectively, which further confirmed the NiO/PB com posite nanosheets was obtained by facile electrodeposition method. The cross-sectional SEM images of NiO and NiO/PB composite films in Fig. S3 show undulating surface, which is very different from compact PB film in Fig. S2b. It can be concluded that by the electrodeposition method with appropriate time, not only the composite material was obtained, but also the structure of hole was preserved. Comparing electrochemical performances is a visual intuitive means to confirm the superiority of NiO/PB composite nanosheets. The CV curves of NiO, PB, and NiO/PB composite films are given in Fig. 5a. It can be clearly seen that the NiO/PB composite film exhibited larger hysteresis-area and peak current, which can provide more redox reac tion charges to EESD. It is worth noting that the NiO film exhibited differential electrochemical performances in different electrolytes. The comparison of CV curves is shown in Fig. S4, evidently, a larger current density was obtained in alkaline electrolyte comparing with neutral electrolyte. Commonly, WO3 can be dissolved in alkaline solution, which makes it impossible to assemble electrochromic device with NiO using KOH/H2O as electrolyte. Consequently, the performance of NiO based electrochromic film was also improved in neutral electrolyte via constructing NiO/PB composite nanosheets. The electrochromic performances of these films are shown in Fig. 5b–e. As shown in Fig. 5b, the mark of “Area 1” showed that a similar optical property of NiO/PB composite film to NiO film at the wavelength of about 350–500 nm, meanwhile, “Area 2” showed that the similar one to PB film at about 600–800 nm. Thus, it can be concluded that NiO and PB are both contributed in the coloring process and the joint action resulted in a larger optical modulation range in NiO/PB composite film than the pure NiO and PB films. In addition, the tb and tc
of NiO/PB composite film are 3.8 s and 1.6 s (Table 1), respectively. The relatively slow switching time in composite film is possibly affected by an interface impedance between in NiO and PB, which can retard elec tron transport. The curves of optical density (ΔOD) versus charge den sity (Q) in Fig. 5d indicates CE values of PB and NiO/PB composite film are almost same and low CE value in NiO film. What’s more, as shown in Fig 5e, 4000 cycles of step chronoamperometric cyclic test were carried out and the stable peak current indicated that the cyclic stability of NiO/ PB composite film has been significantly improved. The PB film with a compact morphology can be seen in the SEM image in Fig. S2. It is easily destroyed in the process of ion insertion and extraction, thus, resulting in poor cyclic stability. Owing to the nanosheets structure of NiO sub strate, the NiO/PB composite film obtained a structure of hole, which could provide more sites for redox reaction and form a buffer effect on ion insertion and extraction simultaneously. Therefore, it is reasonable to conclude that the NiO/PB composite nanosheet film was successfully prepared with outstanding cyclic stability and higher exchanged charge density, which can match the WO3 negative electrode more effectively than NiO and PB electrodes to improve performances of EESD. 3.4. Performance of EESD The EESD was successfully assembled with the hex-WO3 film used as the negative electrode and NiO/PB composite nanosheet film used as the positive electrode. As shown in supplementary information, by applying a positive voltage, reactions (1) and (2) could occur inside the two electrodes of EESD, while a opposite voltage could cause reactions (3) and (4) to proceed, which were corresponding to colored and bleached processes. Fig. 6e and f shows these two states, the colored state of EESD presented dark blue with applied voltage of þ1.3 V, while -2.3 V could make it return to transparent. Dynamic colored and bleached processes Table 1 Switching time of anodic electrochromic films. Materials
NiO
PB
NiO/PB
Switching time tb/tc
0.8/1.9 s
0.7/1.1 s
3.8/1.6 s
Fig. 5. Electrochemical and electrochromic performances of NiO, PB and NiO/PB composite films: (a) CV curves at a scan rate of 50 mV s-1, (b) transmittance spectra at colored and bleached states between 350 nm and 1000 nm, (c) optical transmittance response at 630 nm, (d) variation of the optical density (ΔOD) versus charge density (Q), (e) peak currents of step chronoamperometric cyclic tests at �1 V for 10 s each stage. 6
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could be seen in Video S1. The parameters of transmittance variation indicated an optical modulation range (ΔT) of ~67.6%. According to the Fig. 6b, the switching times of EESD were calculated to be 2.8 s and 7.9 s for coloring and bleaching, respectively. The switching speed of EESD is affected by the two electrodes, thus the reason why the bleached time is longer than the colored time in EESD could be the relatively slow oxidation-reduction reaction of NiO/PB composite nanosheet electrode in bleached process. By applying þ1.3 V for 40 s, the coloring degree of EESD in situ transmittance at 630 nm was strengthen with charge in jection, and a high CE value of 109.6 cm2 C-1 was obtained. It is worthwhile mentioning that EESD exhibited a stable electrochromic process (ΔT remaining was 84.1% after 4000 cycles), which was attributed to the high cyclic stability of hex-WO3 and NiO/PB composite nanosheet electrodes. Supplementary video related to this article can be found at https ://doi.org/10.1016/j.solmat.2019.110337. Within the normal operating voltage range of the electrochromic property, the positive voltage range from 0 to þ1.3 V was selected as energy storing potential window to study the energy storage property of EESD. As shown in Fig. 7f, the distinct redox peaks in CV curves at different scan rates ranging from 5 to 100 mV s-1 disclosed the behavior of pseudocapacitance occurs in EESD [10]. Fig. 7a shows the galvano static charge/discharge (GCD) curves that were recorded at different current densities. The similar time of charging and discharging in GCD curves indicated a good electrochemical reversibility [45,46]. According to the Equation (1), the areal capacitances of EESD were calculated to be 11.50, 10.53, 9.62, 8.45 and 6.92 mF cm-2 at current densities of 0.05, 0.1, 0.25, 0.5 and 1 mA cm-2, respectively. The relatively low areal capacitance at high charging current can be owed to the incomplete insertion/extraction of Liþ ions into/out of electrodes, especially in some inner active sites [10]. In addition, charging and discharging processes of 4000 cycles were performed at 0.5 mA cm-2 current density and the potential window was ranging from 0 to þ1.3 V. It can be found that the EESD exhibited a good cyclic stability in 2100 cycles (the areal capacitance remaining 96.2% during 2100th cycle) but relatively infe rior one (the areal capacitance remaining 82.6% during 4000th cycle) after 2100 cycles. The reason for this phenomenon can be found in the follow-up analysis. As shown in Fig. 6, it is important to note that the
voltage of -2.3 V must be applied to return to the transparent state after þ1.3 V coloring. It more clearly reflected in Fig. 7c, which shows the relationship between transmittance change and charging/discharging process at 0.05 mA cm-2. After discharging (0 V), the transmittance of the EESD is 31.7%, which is lower than the one (71.6%) at full bleaching. Therefore, the EESD is in a process of full coloring and incomplete bleaching in the potential window of 0 to þ1.3 V. With the increasing number of charging/discharge cycles between 0 V and þ1.3 V, increasingly residual Liþ ions existed in the two electrodes which resulted in reduction of areal capacitance, and they could be released by applying -2.3 V. The GCD curve of recovering (complete bleaching) EESD after 4000 cycles was test and the result is given in Fig. 7e, which demonstrated about 91.4% was retained compared with initial state. From another point, it is precisely because this incomplete redox process (0 to þ1.3 V) leads to a better cyclic stability (recovered areal capaci tance was 91.4% after 4000 cycles) than the complete one (-2.3 to þ1.3 V, ΔT remaining was 84.1% after 4000 cycles). To further confirm the practical application of the EESD in the field of energy storage, the proof-of-concept demonstration was carried out by including the EESD in the circuit. As shown in Fig. 7g, two seriesconnected EESDs were used as power supply, which could light up two parallel green LEDs after charging. Meanwhile, a demonstration of powering LCD also confirmed the good energy storage performance of the EESD. Obviously, the EESD exhibited outstanding energy storage and electrochromic performances simultaneously. Some comparisons were made and summarized in Table 2, which can further exhibit some advantages of the EESD in this work. It can be concluded that the EESD acts as a smart window to adjust color, meanwhile, the energy in dis charging process also can provide electricity to some low-power household electronics, which offers promising application to bifunction smart window. 4. Conclusions In summary, we prepared the hex-WO3 and the NiO/PB composite nanosheet films, which have been successfully used to assemble highperformance EESD. The hex-WO3 was directly synthesized on FTOcoated glass by one-step hydrothermal method, in which erythritol
Fig. 6. (a) UV–vis transmittance spectrum of EESD in colored and bleached states between 350 nm and 1000 nm; (b) Switching time and transmittance variation curves between colored and bleached states at 630 nm, applying voltages of þ1.3 V and -2.3 V; (c) Variation of the optical density (ΔOD) versus charge density (Q) of EESD; (d) Transmittance variation with 4000 cycles of coloration and bleaching at 630 nm; Digital photographs of the EESD: (e) colored state, (f) bleached state. 7
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Fig. 7. (a) GCD curves of EESD at different current densities; (b) Areal capacitances of EESD at various current densities; (c) GCD curve at 0.05 mA cm-2 with the voltage ranging from 0 to þ1.3 V and the corresponding in situ transmittance curve at 630 nm; (d) Cyclic stability of charging and discharging at 0.5 mA cm-2; (e) GCD curves at 0.5 mA cm-2 of initial, after 4000 cycles and recovering states; (f) CV curves of EESD at different scan rates; (g) Digital photographs of EESD powering LED and LCD. Table 2 Comparisons of EESD. Device structure WO3–NiO/PB WO3–MnO2 WO3-PB WO3-PANI PICA-ITO WO3 -WO3 PANI-PEDOT:PSS PRBr-PANI a
Changing Colors a
TP -dark blue TPa-dark TPa-blue TPa-dark blue Yellow-dark green TPa-blue Yellow green-dark blue TPa-blue
tc/tb [s]
Work Window [V]
Areal capacitance [mF cm-2]
Cyclic number
Ref
2.8/7.9 4.0/4.9 1.8/2.0 1.4/1.1 —— 30/30 —— ——
1.3 2 1.2 1.0 2.5 1.0 1.0 ——
11.5 19.1 5.12 —— 4.3 22 17.1 1.27
4000 2000 1000 2500 2000 —— 1000 120
This work [13] [10] [12] [47] [48] [3] [49]
TP: Transparent state.
and NH4Cl acted as self-seeded and capping agents, respectively. The NiO/PB composite nanosheets were prepared by hydrothermal, annealing and electrodeposition technologies. After electrodeposition, the surface of NiO nanosheets was uniformly covered by a layer of PB nanoparticles and reserved hole structure. Compared with the NiO and PB electrodes, the NiO/PB composite nanosheet electrode exhibited higher exchanged charge density, larger optical modulation and better cyclic stability. Finally, the assembled EESD showed excellent electro chromic properties and attractive energy storage performances at po tential window from 0 to þ1.3 V with high cyclic stability. Additionally,
the proof-of-concept demonstration of driving LED and LCD illustrated the energy storage property of EESD more clearly. The outstanding op tical modulation characteristic and energy storage property were suc cessfully implemented on a single device, which exhibited a high promising in smart equipment application for buildings and cars in future. Authors contributions section J. Pan and C. Jia conceived the idea and designed the experiments. J. 8
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Pan carried out the synthesis and material characterizations. Y. Wan and R. Zheng helped with the electrochromic tests. X. Ye helped with the energy storage tests of the EESD. Z. Wan helped with the proof-ofconcept demonstration of EESD. J. Pan and C. Jia wrote the manu script. X. Weng, J. Xie and L. Deng helped with data analysis and discussion.
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Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements We are grateful to the National Natural Science Foundation of China (Grant nos. 51773027, 21572030) and National Key R@D Program of China (No. 2017YFB0702802) for financial support. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.solmat.2019.110337. References [1] S.F. Tie, C.W. Tan, A review of energy sources and energy management system in electric vehicles, Renew. Sustain. Energy Rev. 20 (2013) 82–102. [2] J. Wang, L. Zhang, L. Yu, Z. Jiao, H. Xie, X.W. Lou, X.W. Sun, A bi-functional device for self-powered electrochromic window and self-rechargeable transparent battery applications, Nat. Commun. 5 (2014) 4921. [3] K. Wang, H. Wu, Y. Meng, Y. Zhang, Z. Wei, Integrated energy storage and electrochromic function in one flexible device: an energy storage smart window, Energy Environ. Sci. 5 (2012) 8384–8389. [4] S. Zhang, S. Cao, T. Zhang, A. Fisher, J.Y. Lee, Al3þ intercalation/de-intercalationenabled dual-band electrochromic smart windows with a high optical modulation, quick response and long cycle life, Energy Environ. Sci. 11 (2018) 2884–2892. [5] D. Ma, H. Wang, Q. Zhang, Y. Li, Self-weaving WO3 nanoflake films with greatly enhanced electrochromic performance, J. Mater. Chem. 22 (2012) 16633–16639. [6] S.H. Lee, H.M. Cheong, C.E. Tracy, A. Mascarenhas, A.W. Czanderna, S.K. Deb, Electrochromic coloration efficiency of a-WO3-y thin films as a function of oxygen deficiency, Appl. Phys. Lett. 75 (1999) 1541–1543. [7] D. Sarkar, S. Mukherjee, S. Pal, D.D. Sarma, A. Shukla, Hexagonal WO3 nanorods as ambipolar electrode material in asymmetric WO3//WO3/MnO2 supercapacitor, J. Electrochem. Soc. 165 (2018) A2108–A2114. [8] J. Jia, X. Liu, R. Mi, N. Liu, Z. Xiong, L. Yuan, C. Wang, G. Sheng, L. Cao, X. Zhou, X. Liu, Self-assembled pancake-like hexagonal tungsten oxide with ordered mesopores for supercapacitors, J. Mater. Chem. A 6 (2018) 15330–15339. [9] M. Zhu, W. Meng, Y. Huang, Y. Huang, C. Zhi, Proton-insertion-enhanced pseudocapacitance based on the assembly structure of tungsten oxide, ACS Appl. Mater. Interfaces 6 (2014) 18901–18910. [10] Z. Bi, X. Li, Y. Chen, X. He, X. Xu, X. Gao, Large-scale multifunctional electrochromic-energy storage device based on tungsten trioxide monohydrate nanosheets and prussian white, ACS Appl. Mater. Interfaces 9 (2017) 29872–29880. [11] S. Xie, Z. Bi, Y. Chen, X. He, X. Guo, X. Gao, X. Li, Electrodeposited Mo-doped WO3 film with large optical modulation and high areal capacitance toward electrochromic energy-storage applications, Appl. Surf. Sci. 459 (2018) 774–781. [12] W.Q. Wang, X.L. Wang, X.H. Xia, Z.J. Yao, Y. Zhong, J.P. Tu, Enhanced electrochromic and energy storage performance in mesoporous WO3 film and its application in a bi-functional smart window, Nanoscale 10 (2018) 8162–8169. [13] S. Xie, Y. Chen, Z. Bi, S. Jia, X. Guo, X. Gao, X. Li, Energy storage smart window with transparent-to-dark electrochromic behavior and improved pseudocapacitive performance, Chem. Eng. J. 370 (2019) 1459–1466. [14] J. Pan, Y. Wang, R. Zheng, M. Wang, Z. Wan, C. Jia, X. Weng, J. Xie, L. Deng, Directly grown high-performance WO3 films by a novel one-step hydrothermal method with significantly improved stability for electrochromic applications, J. Mater. Chem. A 7 (2019) 13956–13967. [15] J. Zhou, Y. Wei, G. Luo, J. Zheng, C. Xu, Electrochromic properties of vertically aligned Ni-doped WO3 nanostructure films and their application in complementary electrochromic devices, J. Mater. Chem. C 4 (2016) 1613–1622. [16] G.F. Cai, D. Zhou, Q.Q. Xiong, J.H. Zhang, X.L. Wang, C.D. Gu, J.P. Tu, Efficient electrochromic materials based on TiO2@WO3 core/shell nanorod arrays, Sol. Energy Mater. Sol. Cells 117 (2013) 231–238. [17] Z. Gu, T. Zhai, B. Gao, X. Sheng, Y. Wang, H. Fu, Y. Ma, J. Yao, Controllable assembly of WO3 nanorods/nanowires into hierarchical nanostructures, J. Phys. Chem. B 110 (2006) 23829–23836.
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A high-performance electrochromic device assembled with hexagonal WO3 and NiO/PB composite nanosheet electrodes towards energy storage smart window Jianbo Pan, Rongzong Zheng, Yi Wang, Xingke Ye, Zhongquan Wan, Chunyang Jia *, Xiaolong Weng, Jianliang Xie, Longjiang Deng State Key Laboratory of Electronic Thin Films and Integrated Devices, National Engineering Research Center of Electromagnetic Radiation Control Materials, School of Electronic Science and Engineering, University of Electronic Science and Technology of China, Chengdu, 610054, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Bi-functional device Electrochromic Energy storage Composite material WO3
Functions of electrochromism and energy exchange can be achieved by redox reactions, which indicate designing a bi-functional electrochromic energy storage device (EESD) is feasible. In this paper, a NiO/PB composite nanosheet electrode was prepared via loading Prussian blue (iron (III) hexacyanoferrate (II), PB) nanoparticles on the surface of NiO nanosheets, meanwhile, a high-performance WO3 negative electrode was also prepared via novel erythritol-assisted hydrothermal method with NH4Cl used as capping agent. The designed EESD exhibited properties of large optical modulation (67.6% at 630 nm), high coloration efficiency (109.6 cm2 C-1), fast switching speed (2.8/7.9 s for bleaching/coloration) and long cycle life (ΔT remaining was 84.1% after 4000 cycles). Meanwhile, the EESD also obtained attractive energy storage properties at a potential window from 0 to þ1.3 V. Impressively, the EESD exhibited a high areal capacitance of 11.50 mF cm-2 at current density of 0.05 mA cm-2 and the process of charging/discharging can be repeated at least 4000 cycles with a little decay. Therefore, the bi-functional EESD exhibits high promising application prospect.
1. Introduction In the 21st century, environmental and energy sources have been to the two major topics in the whole world [1,2]. In buildings, tremendous energy sources used for heating and cooling usually need to be provided for maintaining a comfortable temperature. In recent years, the elec trochromic smart window has attracted wide attention due to the function of efficiently modulating light and heat passing through win dow with a low power consumption [3,4]. More importantly, because of similar device structure to pseudo-capacitive device, a novel type of electrochromic device with both electrochromic and energy storage properties have emerged and shows promising application potential. This device is called electrochromic energy storage device (EESD) in which main components are some electrochromic active material with good pseuocapacitive performance such as WO3, MnO2 and NiO. Thus, improving the performance of active materials is the key to design high-performance EESD. WO3 is a kind of widely studied electrochromic material which ex hibits high coloring efficiency, good cycling stability, large contrast ratio
and appropriate color (between transparent and dark blue) [5,6]. Meanwhile, WO3 also plays an important role in supercapacitor due to its high pseudocapacitance [7]. For instance, Jia et al. synthesized a pancake-like mesopore WO3 and obtained high specific capacitance of 605.5 F g-1 at 0.37 A g-1 [8]. Zhu et al. reported an assembly hexagonal WO3 which achieved a high specific capacitance of 421.8 F g-1 at 0.5 A g-1 [9]. According to these previous works, WO3 exhibited great po tential in the application of EESD and has been widely studied. Gener ally, in EESD, WO3 is used as cathodic electrochromic layer (negative electrode) and other materials with complementary color are used as anodic electrochromic layer (positive electrode), WO3 and Prussian blue (iron (III) hexacyanoferrate (II), PB) [10,11], WO3 and polyaniline (PANI) [12], WO3 and MnO2 [13], for example. Some strategies, con taining design of crystal and nanostructure [14], doping [15] and compositing with other materials [16], have been used for improving electrochromic performance of WO3. In hydrothermal method, tem perature and time [17], pH value [18], or capping agents [19] are easy to control for obtaining different nanostructures such as nanowire [20], nanorod [21], nanotree [22] and nanonest [23]. Recently, our group
* Corresponding author. E-mail address: [email protected] (C. Jia). https://doi.org/10.1016/j.solmat.2019.110337 Received 15 August 2019; Received in revised form 20 November 2019; Accepted 2 December 2019 Available online 24 December 2019 0927-0248/© 2019 Elsevier B.V. All rights reserved.
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reported a novel facile hydrothermal technology applicated to directly synthesize a high-performance WO3 electrochromic film on FTO-coated glass [14]. However, the relatively poor performance can be found in electrochromic device due to mismatching of electrochromic properties between WO3 electrode and PB electrode in our previous work. For example, the lifetime of device is limited by the electrode material with worse cyclic stability. Therefore, it is meaningful to improve the per formance of positive electrode while optimizing the negative electrode in EESD. Because of a complementary color change (between transparent and blue) [24], PB was often used in assembling electrochromic device with tungsten oxide materials, such as WO3 [10,25], and W18O49 [26]. However, PB film prepared by electrodeposition method exhibited a poor cyclic stability due to closely packed nanoparticles. In addition, NiO as another anodic electrochromic material with complementary color can be easily synthesized to be nanosheets [27,28], which has more active sites for redox reaction. The method of composite material has been confirmed to be an effective means to improve performances film and combine the advantages of various materials [29,30]. There fore, designing NiO/PB composite nanosheet arrays is a promising strategy to obtain a high-performance anodic electrochromic film, in which porous structure of nanosheets can also effectively avoid the tight stacking between PB nanoparticles. Meanwhile, the NiO/PB composite nanosheet film used as positive electrode shows greater potential to assemble EESD with WO3 negative electrode. In this work, a NiO/PB composite nanosheet was successfully pre pared. Owing to the nanosheet structure of NiO, the PB nanoparticles loading on the surface of nanosheets obtained more redox reaction sites and ion buffer channels. Meanwhile, a high-performance WO3 electro chromic film was directly synthesized on FTO-coated glass by a novel erythritol-assisted hydrothermal method, which was used as negative electrode. Finally, the assembled EESD exhibited outstanding color switching property between transparent and dark blue, fast response speed, great coloration efficiency and high cyclic stability. Additionally, the EESD obtained excellent energy storage performance, which can be successfully used to drive LED (light emitting diode) and LCD (liquid crystal display). The EESD showed outstanding electrochromic and en ergy storage performances simultaneously, which indicated that the EESD can not only modulate optical transmittance but also store and release electric energy for driving some household electronics.
dissolution. Then 20 mL mixture of above solutions was transferred to a 50 mL Teflon-line stainless-steel autoclave. The FTO-coated glass sub strate (FTO-coated glass was washed with detergent, acetone, de-ionized water and anhydrous ethanol in an ultrasonic bath for 20 min per wash) was placed vertically in the autoclave. After that, the autoclave was sealed and maintained at 110 � C for 2.5 h. Finally, the obtained film was washed with de-ionized water and anhydrous ethanol, then dried in oven at 60 � C for 2 h. 2.3. Preparation of PB, NiO and NiO/PB composite films NiO film was synthesized on FTO-coated glass through hydrothermal and annealing technology according to previous work [31] with some changes. Briefly, a green transparent solution was obtained by dissolv ing the urea (3 g), K2S2O8 (0.13 g) and Ni(NO3)2�6H2O (0.5 g) in de-ionized water (40 mL). Then, 20 mL above solution was transferred into 50 mL Teflon-line stainless-steel autoclave. The FTO-coated glass substrate was placed vertically in the autoclave, and the autoclave was sealed and maintained at 90 � C for 1.5 h. After cooling down to room temperature naturally, the obtained film was washed with de-ionized water and anhydrous ethanol. Finally, the drying film was annealed at 300 � C in air for 1.5 h to obtain NiO film. PB was prepared by electrodeposition technology reported by pre vious work [10,14]. Briefly, the brown homogenous electrodeposition solution contained K3[Fe(CN)6] (0.1646 g), FeCl3 (0.0811 g), KCl (0.1864 g) and 50 mL de-ionized water. The electrodeposition process was carried out in three-electrode system using a CHI660E electro chemical workstation, and platinum sheet used as the counter-electrode, Ag/AgCl used as the reference electrode, the FTO-coating glass with and without NiO film used as working electrodes. A constant current (50 μA cm-2) was applied to working electrodes for 200 s to prepare PB and NiO/PB composite nanosheet films. After that, the obtained films were washed by de-ionized water and anhydrous ethanol for several times. Finally, drying for 6 h under 60 � C. 2.4. Assembly of EESD The NiO/PB composite nanosheet film was used as positive electrode and the WO3 film was used as negative electrode. Briefly, 3M4910VHB double-sided tape was used to partly seal the four sides of EESD and UV curing glue was used to further enhance sealability. Then, 1 M LiClO4/ PC solution was injected into the gap (about 1 mm) by syringe to be used as the liquid electrolyte. Finally, the whole EESD was sealed with UV curing glue.
2. Experimental 2.1. Materials All solvents and chemicals were of analytical grade and used without further purification. FTO-coated glass (about 10 Ω resistance, 2.2 mm thickness) was purchased from Wuhan lattice Solar Energy Technology Co., Ltd. Sodium tungstate (Na2WO4�2H2O), nickel nitrate (Ni (NO3)2�6H2O), urea, potassium persulfate (K2S2O8), potassium hy droxide (KOH), hydrochloric acid (HCl, 36%) and hydrogen peroxide (H2O2, 30%) were purchased from Chengdu Kelong Reagent Co., Ltd. Erythritol was purchased from Shanghai Yuanye Biotechnology Co., Ltd. Anhydrous propylene carbonate (PC), anhydrous lithium perchlorate (LiClO4), potassium ferricyanide (K3[Fe(CN)6]), ferric chloride (FeCl3), potassium chloride (KCl) and ammonium chloride (NH4Cl) were pur chased from Shanghai Aladdin biochemical Polytron Technologies Inc.
2.5. Characterization and calculation methods The crystal structures of films and powder were examined by X-ray diffraction using X’ Pert PRO MPD with Cu Ka radiation. The mor phologies of the samples were investigated by scanning electron mi croscope (FESEM, FEI Inspect F50). The powder for Transmission Electron Microscopy (TEM) and X-ray diffraction test was synthesized by the same method with film. The TEM images (low-resolution, highresolution) were obtained using FEI Talos-S (TEM) operated at 200 KV. Particularly, NiO/PB composite powder for TEM and Scanning Transmission Electron Microscopy (STEM) was scraped from FTOcoated glass and ultrasound-treated in anhydrous ethanol. All electro chemical measurements were carried out on an electrochemical work station (CHI660E, Shanghai Chenhua Instruments, Inc.) using a conventional three-electrode test system with platinum sheet used as the counter-electrode, Ag/AgCl used as the reference electrode and the prepared films used as working electrodes. The optical properties and kinetics of coloration and bleaching of the films were detected using a UV–vis spectrophotometer (SP60, X-Rite). The 1 M LiClO4 in PC was used as the electrolyte to test electrochemical and electrochromic properties.
2.2. Preparation of WO3 film A transparent homogeneous peroxopolytungstic acid solution was used as the tungsten source in hydrothermal process, of which prepared procedures were the same as ones in our previous work [14]. Hydro thermal precursor contained peroxopolytugstic acid solution (0.1 M, 10.5 mL), HCL (3 M, 3.5 mL), de-ionized water (17 mL), erythritol (11 g) and NH4Cl (0.45 g). Transparent precursor was obtained by ultrasonic 2
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The areal capacitance was calculated from the GCD curves according to the follow Equation (1) [10,32]:
WO3 film directly grow on the FTO-coated glass, but also used as nucleating agents to participate in crystal growth process. Therefore, under the synergistic-effect of erythritol and NH4Cl, the hex-WO3 film exhibited a porous coral-like nanostructure and TEM images display this structure more clearly in Fig. 2c. In contrast, the ort-WO3⋅0.33H2O without the addition of NH4Cl obtained a compact stacking nano structure (Fig. S1), which is adverse for ion transport in process of electrochemical reaction [14]. Additionally, as shown in Fig. 2d, the HRTEM image of coral-like nanostructure confirmed the spacing of lattice fringes was 0.381 nm. The spacing was indexed as (002) lattice plane of hex-WO3 (JCPDS no. 85–2459), which further confirmed the preferentially growth along the [001] direction was obtained. From the above results, it can be concluded that Cl- ion exhibited a similar effect with SO24 ion [19,20] to control WO3 crystal growth in hydrothermal process. And detailed discussion of formation mechanism of WO3 crystal under the synergistic-effect of polyhydroxy self-seeded and capping agents can be found in our previous work [14]. The electrochemical and electrochromic properties of hex-WO3 film were tested by a conventional three-electrode system, and 10 cycles of cyclic voltammetry (CV) were carried out between -1.0 V and þ1.0 V at a scan rate of 50 mV s-1 before measurement. Fig. 3a shows CV curve of hex-WO3 film, the large area of CV hysteresis curve and high peak currents demonstrate a good electrochemical performance. Further more, the hex-WO3 film exhibited a wide range of optical modulation after þ1.0 V and -1.0 V applied for 30 s between 330 nm and 1000 nm, respectively. In terms of appearance, the color of the hex-WO3 film changed from transparent (bleached state) to dark blue (colored state) reversibly, and the reactions in these two processes can be described as follow:
(1)
Ca ¼ IΔt / AΔV -2
where Ca is the areal capacitance (mF cm ), I is the current during discharging (mA), Δt is the discharge time (s), A is the effective area of device (cm2), and ΔV is the potential window (V). 3. Results and discussion 3.1. Overview of EESD design The EESD was successfully assembled and consisted of WO3 negative electrode, NiO/PB composite nanosheet positive electrode, and 1 M LiClO4/PC electrolyte. Herein, the hexagonal WO3 (hex-WO3) film was directly synthesized on FTO-coated glass by a novel one-step erythritolassisted hydrothermal method, where NH4Cl was used as capping agent for further controlling the trend of crystal growth efficiently. The NiO nanosheets were synthesized by hydrothermal and annealing technol ogies. Then, the PB nanoparticles were loaded on the surface of NiO nanosheets via electrodeposition to obtain NiO/PB composite nano sheets (see Fig. 1). 3.2. Performance of WO3 film Fig. 2a shows the XRD patterns of erythritol-assisted WO3 films with and without the addition of NH4Cl, in which the peaks marked with “▽” were categorized to FTO-coated glass. The erythritol-assisted WO3 film without NH4Cl can be indexed to orthorhombic WO3⋅0.33H2O (ortWO3⋅0.33H2O) on JCPDS no. 87–1023 and the XRD pattern was corre sponding to JCPDS no. 85–2459 (hex-WO3) with the addition of NH4Cl, which indicated a phase transition occurred under the influence of NH4Cl. The results of XRD characterization are very consistent with that of previous reports [14]. The reason for the phase transition could be the NHþ 4 inserted into lattice tunnels of hex-WO3, thus the hexagonal crystal system was constructed [33-35]. Meanwhile, the hex-WO3 contains trigonal cavity, four-coordinated square window and hexagonal win dow, which are conducive to the formation of orderly ion transport and better than ort-WO3⋅0.33H2O [36,37]. It is noteworthy that Cl- can generate an strong energy barrier on the faces parallel to the c-axis by adsorption, which can lead to a oriented growth of WO3 crystal along (001) face [38-40] and porous nanostructure was obtained simulta neously. Fig. 2b shows the top-view and side-view SEM images of hex-WO3 film, presenting the morphology and undulating surface. On the other hand, erythritol not only used as self-seeded agent to make
WO3 (transparent) þ xLiþ þxe- ⇌ LixWO3 (dark blue)
(2)
The switching time, defining as the time required for a device to reach 90% of full optical modulation, is used to characterize the speed of bleached and colored processes [41]. The relationship between the time and transmittance variation at 630 nm was obtained by applying step voltages of þ1.0 V and -1.0 V to hex-WO3 film with in situ measurement of transmittance simultaneously. The result is shown in Fig. 3c, the bleached time (tb) and colored time (tc) were calculated to be 3.3 s and 3.7 s, respectively. Coloration efficiency (CE), another important char acteristic parameter for evaluating the electrochromic property, is ruled as the change in optical change (ΔOD) at a specific wavelength per unit injection charge density (ΔQ) in coloring process. The calculation for mula is as follow [42]: CE ¼ ΔOD / ΔQ ¼ log (Tb / Tc) / ΔQ
Fig. 1. Schematic illustration of the process of preparing EESD. 3
(3)
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Fig. 2. (a) XRD patterns for ort-WO3 and hex-WO3 films; (b) SEM images of hex-WO3 film, while insets show high-magnification and cross-sectional images; TEM images of hex-WO3 powder: (c) low-magnification image, (d) high-resolution image (HRTEM).
Fig. 3. The hex-WO3 film: (a) CV curve (50 mV s-1), (b) UV–vis transmittance spectrum at colored and bleached states between 330 nm and 1000 nm (inserts are digital photographs at different states), (c) switching time and transmittance variation curves between colored and bleached states at 630 nm, �1.0 V, (d) variation of the optical density (ΔOD) versus charge density (Q).
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Where Tb and Tc denote the bleached and colored transmittance values at a special wavelength, respectively. As shown in Fig. 3d, with charge injection at -1.0 V, a high ΔOD value (~2.22) demonstrated that the hex-WO3 film can achieved a deep coloration. Finally, the CE value is 73.1 cm2 C-1 (by calculating the slope of the linear region of the line). In a word, the prepared hex-WO3 film with excellent electrochromic per formance exhibited great potential in the application of EESD.
synthetic route in the hydrothermal process can be described as follows [31]:
3.3. Performance comparison of anodic electrochromic films
½NiðH2 OÞ6 x ðNH3 Þx �2þ þ 2OH
¼ NiðOHÞ2 þ ð6
Fig. S2 shows the SEM images of top and cross-sectional views, it can be seen the PB film prepared by electrodeposition method has a compact morphology, which results in a poor robustness when ions are intercalating/de-intercalating from film. Therefore, the NiO/PB com posite nanosheet film was designed to improve the performance of PB film. As shown in Fig. 4a, the XRD pattern of Ni(OH)2(NiOOH)0.167 powder prepared by hydrothermal method was corresponding to JCPDS no. 89–7111, and Ni(OH)2(NiOOH)0.167 can also directly grow on FTOcoated glass. After annealing, the Ni(OH)2(NiOOH)0.167 changed into cubic NiO that could be indexed on JCPDS no. 47–1049, which proved a good crystallinity and was consistent with the previous report [31]. The SEM image of NiO in Fig. 4b exhibits the nanosheet morphology and the pore structure between the nanosheets can be clearly found. The
1:167NiðOHÞ2 þ 0:0835S2 O8 2
þ 0:167OH
COðNH2 Þ2 þ H2 O ¼ CO2 þ 2NH3
(4)
NH3 þ H2 O ¼ NH4 þ þ OH
(5)
Ni2þ þ xNH3 þ ð6
xÞH2 O ¼ ½NiðH2 OÞ6 x ðNH3 Þx �2þ
NiII ðOHÞ2 ðNiIII OOHÞ0:167 þ 0:167SO4 2
xÞH2 O þ xNH3
(6) (7)
¼
þ 0:167H2 O
(8)
Low-concentration ammonia is obtained by slow decomposition of urea at low temperatures. After that, the hydrolysis of NH3 molecules produce OH- and NHþ 4 , which results in forming a weak alkaline envi ronment. At room temperature, Ni2þ can immediately form a relative stable [Ni(H2O)6-x(NH3)x]2þ complex ions with NH3 in the weak alka line medium. With proceeding of hydrothermal process, the complex ions are decomposed and form Ni(OH)2 with OH- ions [31,43]. Since Ni (OH)2 is a typical layered double-hydroxide, ions and molecules can be present between the stacking layers along the c-axis, which is easily to
Fig. 4. (a) XRD patterns of Ni(OH)2(NiOOH)0.167 and NiO powder; (b) SEM image of NiO nanosheets; (c) SEM image of NiO/PB composite nanosheets; (d) TEM image of NiO/PB composite nanosheet; (e) STEM-HAADF (High-Angle Annular Dark Field) image at the side of NiO/PB composite nanosheet; STEM-EDS mapping images of element distribution: (f) Ni, (g) Fe; (h) The Ni and Fe overlapping image. 5
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form two-dimensional nanosheets [44]. Finally, Ni(OH)2(NiOOH)0.167 nanosheets are obtained by the oxidation of Ni(OH)2 under the effect of S2O28 ions [31], which can further anneal to form NiO nanosheets. Then, after 200 s of electrodeposition on the prepared NiO film, the PB nanoparticles were successfully loaded on the surface of NiO nano sheets uniformly which can be confirmed by comparing Fig. 4b and 4c. A clearer composite structure of single NiO/PB nanosheet can be seen in TEM image of Fig. 4d. Moreover, the uniform distribution of Ni and Fe elements in STEM mapping images at 50 nm scale is corresponding to NiO and PB, respectively, which further confirmed the NiO/PB com posite nanosheets was obtained by facile electrodeposition method. The cross-sectional SEM images of NiO and NiO/PB composite films in Fig. S3 show undulating surface, which is very different from compact PB film in Fig. S2b. It can be concluded that by the electrodeposition method with appropriate time, not only the composite material was obtained, but also the structure of hole was preserved. Comparing electrochemical performances is a visual intuitive means to confirm the superiority of NiO/PB composite nanosheets. The CV curves of NiO, PB, and NiO/PB composite films are given in Fig. 5a. It can be clearly seen that the NiO/PB composite film exhibited larger hysteresis-area and peak current, which can provide more redox reac tion charges to EESD. It is worth noting that the NiO film exhibited differential electrochemical performances in different electrolytes. The comparison of CV curves is shown in Fig. S4, evidently, a larger current density was obtained in alkaline electrolyte comparing with neutral electrolyte. Commonly, WO3 can be dissolved in alkaline solution, which makes it impossible to assemble electrochromic device with NiO using KOH/H2O as electrolyte. Consequently, the performance of NiO based electrochromic film was also improved in neutral electrolyte via constructing NiO/PB composite nanosheets. The electrochromic performances of these films are shown in Fig. 5b–e. As shown in Fig. 5b, the mark of “Area 1” showed that a similar optical property of NiO/PB composite film to NiO film at the wavelength of about 350–500 nm, meanwhile, “Area 2” showed that the similar one to PB film at about 600–800 nm. Thus, it can be concluded that NiO and PB are both contributed in the coloring process and the joint action resulted in a larger optical modulation range in NiO/PB composite film than the pure NiO and PB films. In addition, the tb and tc
of NiO/PB composite film are 3.8 s and 1.6 s (Table 1), respectively. The relatively slow switching time in composite film is possibly affected by an interface impedance between in NiO and PB, which can retard elec tron transport. The curves of optical density (ΔOD) versus charge den sity (Q) in Fig. 5d indicates CE values of PB and NiO/PB composite film are almost same and low CE value in NiO film. What’s more, as shown in Fig 5e, 4000 cycles of step chronoamperometric cyclic test were carried out and the stable peak current indicated that the cyclic stability of NiO/ PB composite film has been significantly improved. The PB film with a compact morphology can be seen in the SEM image in Fig. S2. It is easily destroyed in the process of ion insertion and extraction, thus, resulting in poor cyclic stability. Owing to the nanosheets structure of NiO sub strate, the NiO/PB composite film obtained a structure of hole, which could provide more sites for redox reaction and form a buffer effect on ion insertion and extraction simultaneously. Therefore, it is reasonable to conclude that the NiO/PB composite nanosheet film was successfully prepared with outstanding cyclic stability and higher exchanged charge density, which can match the WO3 negative electrode more effectively than NiO and PB electrodes to improve performances of EESD. 3.4. Performance of EESD The EESD was successfully assembled with the hex-WO3 film used as the negative electrode and NiO/PB composite nanosheet film used as the positive electrode. As shown in supplementary information, by applying a positive voltage, reactions (1) and (2) could occur inside the two electrodes of EESD, while a opposite voltage could cause reactions (3) and (4) to proceed, which were corresponding to colored and bleached processes. Fig. 6e and f shows these two states, the colored state of EESD presented dark blue with applied voltage of þ1.3 V, while -2.3 V could make it return to transparent. Dynamic colored and bleached processes Table 1 Switching time of anodic electrochromic films. Materials
NiO
PB
NiO/PB
Switching time tb/tc
0.8/1.9 s
0.7/1.1 s
3.8/1.6 s
Fig. 5. Electrochemical and electrochromic performances of NiO, PB and NiO/PB composite films: (a) CV curves at a scan rate of 50 mV s-1, (b) transmittance spectra at colored and bleached states between 350 nm and 1000 nm, (c) optical transmittance response at 630 nm, (d) variation of the optical density (ΔOD) versus charge density (Q), (e) peak currents of step chronoamperometric cyclic tests at �1 V for 10 s each stage. 6
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could be seen in Video S1. The parameters of transmittance variation indicated an optical modulation range (ΔT) of ~67.6%. According to the Fig. 6b, the switching times of EESD were calculated to be 2.8 s and 7.9 s for coloring and bleaching, respectively. The switching speed of EESD is affected by the two electrodes, thus the reason why the bleached time is longer than the colored time in EESD could be the relatively slow oxidation-reduction reaction of NiO/PB composite nanosheet electrode in bleached process. By applying þ1.3 V for 40 s, the coloring degree of EESD in situ transmittance at 630 nm was strengthen with charge in jection, and a high CE value of 109.6 cm2 C-1 was obtained. It is worthwhile mentioning that EESD exhibited a stable electrochromic process (ΔT remaining was 84.1% after 4000 cycles), which was attributed to the high cyclic stability of hex-WO3 and NiO/PB composite nanosheet electrodes. Supplementary video related to this article can be found at https ://doi.org/10.1016/j.solmat.2019.110337. Within the normal operating voltage range of the electrochromic property, the positive voltage range from 0 to þ1.3 V was selected as energy storing potential window to study the energy storage property of EESD. As shown in Fig. 7f, the distinct redox peaks in CV curves at different scan rates ranging from 5 to 100 mV s-1 disclosed the behavior of pseudocapacitance occurs in EESD [10]. Fig. 7a shows the galvano static charge/discharge (GCD) curves that were recorded at different current densities. The similar time of charging and discharging in GCD curves indicated a good electrochemical reversibility [45,46]. According to the Equation (1), the areal capacitances of EESD were calculated to be 11.50, 10.53, 9.62, 8.45 and 6.92 mF cm-2 at current densities of 0.05, 0.1, 0.25, 0.5 and 1 mA cm-2, respectively. The relatively low areal capacitance at high charging current can be owed to the incomplete insertion/extraction of Liþ ions into/out of electrodes, especially in some inner active sites [10]. In addition, charging and discharging processes of 4000 cycles were performed at 0.5 mA cm-2 current density and the potential window was ranging from 0 to þ1.3 V. It can be found that the EESD exhibited a good cyclic stability in 2100 cycles (the areal capacitance remaining 96.2% during 2100th cycle) but relatively infe rior one (the areal capacitance remaining 82.6% during 4000th cycle) after 2100 cycles. The reason for this phenomenon can be found in the follow-up analysis. As shown in Fig. 6, it is important to note that the
voltage of -2.3 V must be applied to return to the transparent state after þ1.3 V coloring. It more clearly reflected in Fig. 7c, which shows the relationship between transmittance change and charging/discharging process at 0.05 mA cm-2. After discharging (0 V), the transmittance of the EESD is 31.7%, which is lower than the one (71.6%) at full bleaching. Therefore, the EESD is in a process of full coloring and incomplete bleaching in the potential window of 0 to þ1.3 V. With the increasing number of charging/discharge cycles between 0 V and þ1.3 V, increasingly residual Liþ ions existed in the two electrodes which resulted in reduction of areal capacitance, and they could be released by applying -2.3 V. The GCD curve of recovering (complete bleaching) EESD after 4000 cycles was test and the result is given in Fig. 7e, which demonstrated about 91.4% was retained compared with initial state. From another point, it is precisely because this incomplete redox process (0 to þ1.3 V) leads to a better cyclic stability (recovered areal capaci tance was 91.4% after 4000 cycles) than the complete one (-2.3 to þ1.3 V, ΔT remaining was 84.1% after 4000 cycles). To further confirm the practical application of the EESD in the field of energy storage, the proof-of-concept demonstration was carried out by including the EESD in the circuit. As shown in Fig. 7g, two seriesconnected EESDs were used as power supply, which could light up two parallel green LEDs after charging. Meanwhile, a demonstration of powering LCD also confirmed the good energy storage performance of the EESD. Obviously, the EESD exhibited outstanding energy storage and electrochromic performances simultaneously. Some comparisons were made and summarized in Table 2, which can further exhibit some advantages of the EESD in this work. It can be concluded that the EESD acts as a smart window to adjust color, meanwhile, the energy in dis charging process also can provide electricity to some low-power household electronics, which offers promising application to bifunction smart window. 4. Conclusions In summary, we prepared the hex-WO3 and the NiO/PB composite nanosheet films, which have been successfully used to assemble highperformance EESD. The hex-WO3 was directly synthesized on FTOcoated glass by one-step hydrothermal method, in which erythritol
Fig. 6. (a) UV–vis transmittance spectrum of EESD in colored and bleached states between 350 nm and 1000 nm; (b) Switching time and transmittance variation curves between colored and bleached states at 630 nm, applying voltages of þ1.3 V and -2.3 V; (c) Variation of the optical density (ΔOD) versus charge density (Q) of EESD; (d) Transmittance variation with 4000 cycles of coloration and bleaching at 630 nm; Digital photographs of the EESD: (e) colored state, (f) bleached state. 7
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Fig. 7. (a) GCD curves of EESD at different current densities; (b) Areal capacitances of EESD at various current densities; (c) GCD curve at 0.05 mA cm-2 with the voltage ranging from 0 to þ1.3 V and the corresponding in situ transmittance curve at 630 nm; (d) Cyclic stability of charging and discharging at 0.5 mA cm-2; (e) GCD curves at 0.5 mA cm-2 of initial, after 4000 cycles and recovering states; (f) CV curves of EESD at different scan rates; (g) Digital photographs of EESD powering LED and LCD. Table 2 Comparisons of EESD. Device structure WO3–NiO/PB WO3–MnO2 WO3-PB WO3-PANI PICA-ITO WO3 -WO3 PANI-PEDOT:PSS PRBr-PANI a
Changing Colors a
TP -dark blue TPa-dark TPa-blue TPa-dark blue Yellow-dark green TPa-blue Yellow green-dark blue TPa-blue
tc/tb [s]
Work Window [V]
Areal capacitance [mF cm-2]
Cyclic number
Ref
2.8/7.9 4.0/4.9 1.8/2.0 1.4/1.1 —— 30/30 —— ——
1.3 2 1.2 1.0 2.5 1.0 1.0 ——
11.5 19.1 5.12 —— 4.3 22 17.1 1.27
4000 2000 1000 2500 2000 —— 1000 120
This work [13] [10] [12] [47] [48] [3] [49]
TP: Transparent state.
and NH4Cl acted as self-seeded and capping agents, respectively. The NiO/PB composite nanosheets were prepared by hydrothermal, annealing and electrodeposition technologies. After electrodeposition, the surface of NiO nanosheets was uniformly covered by a layer of PB nanoparticles and reserved hole structure. Compared with the NiO and PB electrodes, the NiO/PB composite nanosheet electrode exhibited higher exchanged charge density, larger optical modulation and better cyclic stability. Finally, the assembled EESD showed excellent electro chromic properties and attractive energy storage performances at po tential window from 0 to þ1.3 V with high cyclic stability. Additionally,
the proof-of-concept demonstration of driving LED and LCD illustrated the energy storage property of EESD more clearly. The outstanding op tical modulation characteristic and energy storage property were suc cessfully implemented on a single device, which exhibited a high promising in smart equipment application for buildings and cars in future. Authors contributions section J. Pan and C. Jia conceived the idea and designed the experiments. J. 8
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Pan carried out the synthesis and material characterizations. Y. Wan and R. Zheng helped with the electrochromic tests. X. Ye helped with the energy storage tests of the EESD. Z. Wan helped with the proof-ofconcept demonstration of EESD. J. Pan and C. Jia wrote the manu script. X. Weng, J. Xie and L. Deng helped with data analysis and discussion.
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