Dual-Function Electrochromic Supercapacitors Displaying Real-Time

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Applications of Polymer, Composite, and Coating Materials

Dual-Function Electrochromic Supercapacitors Displaying Real-Time Capacity in Color Tae Yong Yun, Xinlin Li, Se Hyun Kim, and Hong Chul Moon ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b15066 • Publication Date (Web): 20 Nov 2018 Downloaded from http://pubs.acs.org on November 20, 2018

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ACS Applied Materials & Interfaces

Dual-Function Electrochromic Supercapacitors Displaying Real-Time Capacity in Color Tae Yong Yun,a Xinlin Li,b Se Hyun Kimb,c,* and Hong Chul Moona,* a

Department of Chemical Engineering, University of Seoul, Seoul 02504, Republic of Korea

b

Department of Mechanical Engineering Science, Yeungnam University, Gyeongsan, North Gyeongsang 38541, Republic of Korea c

School of Chemical Engineering, Yeungnam University, Gyeongsan, North Gyeongsang 38541, Republic of Korea

*Corresponding authors: [email protected] (H.C.M), [email protected] (S.H.K.) Keywords Electrochromism, Supercapacitors, Multifunctional electronics, Electrochemical devices, Ion gels 1

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Abstract Dual-function electrochromic supercapacitors (ECSs) that indicate their real-time charge capacity in color are fabricated using tungsten trioxide (WO3) and Li-doped ion gels containing hydroquinone (HQ). The ECSs can simultaneously serve as either electrochromic devices or supercapacitors. The coloration/bleaching and charging/discharging characteristics are investigated between 0 and –1.5 V. At the optimal HQ concentration, large transmittance contrast (~91 %), high coloration efficiency (~61.9 cm2/C), high areal capacitance (~13.6 mF/cm2), and good charging/discharging cyclic stability are achieved. Flexible ECSs are fabricated on plastic substrates by exploiting the elastic characteristics of the gel electrolytes, and they exhibit good bending durability. Moreover, practical feasibility is evaluated by demonstrating the use of the ECSs as an energy storage device and a power source.

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Introduction Electrochemical electronics have found diverse applications including energy storage devices (e.g., supercapacitors1-5 and batteries6-11), sensors (e.g., electrical skins12,13), and displays [e.g., electrochemiluminescence (ECL)14-19 and electrochromic (EC)20-31 devices]. The use of typical liquid or solid electrolytes limits the range of device applications. For example, liquid electrolytes easily leak under applied strains. Hence, the realization of flexible/stretchable devices is difficult. In addition, ionic motion is severely hindered in solid electrolytes, leading to extremely low ionic conductivity even at room temperature. Therefore, their application in room-temperature electronics is limited. These issues have been addressed by the development of highly conductive and mechanically robust gel electrolytes, which are referred to as ion gels.45,46 Ion gels consisting of copolymers and ionic liquids are nonvolatile even at high temperature or under reduced pressure. Their properties are easily tunable by modifying the chemical structures of the constituents or adjusting the gel composition.45,46 Moreover, the introduction of ECL luminophores or EC chromophores into the gels yields functional ion

gels,

and their utilization extends the application spectrum to

flexible/stretchable ECL and EC displays.14-17,22-24 In addition to the fabrication of such simple single-function devices, considerable effort has been made to develop multifunctional devices. Organic light-emitting transistors that combine thin film transistors and electroluminescent devices are a representative example.4750

However, to integrate several functions, multiple layers must be deposited in a single

device via complex fabrication processes. Relatively simple dual-function electrochemical devices have also been reported, where the devices served as electrical double layer capacitors (EDLCs) or ECL devices under an applied DC or AC voltage, respectively. Nonetheless, the difference in the operational voltage range was significant; it was 0–1 V in

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the EDLC mode and larger than peak-to-peak voltage of 5 VPP in the ECL display mode. Accordingly, each mode worked independently.17 In this work, we present energy storage devices that display their real-time charging capacity in color. These devices simultaneously function as pseudocapacitors and EC devices (ECDs). In recent years, electrochromic supercapacitors (ECSs) based on polymeric materials have been widely developed.38-42 Nonetheless, a photochemically stable inorganic material is more suitable when considering outdoor use. Thus, we selected tungsten trioxide (WO3) as the EC material. Most WO3-based ECSs require the appropriate selection of a counter anodic layer and multiple fabrication processes.32-34,36 To simplify the device configuration, we added anodic species directly into the electrolyte. Ferrocene (Fc) and Fc-derivatives, such as dimethyl Fc, are the examples of widely employed anodic species.26,27 However, electrolytes containing Fc or Fc-derivatives appeared yellowish even before coloration, which is problematic because the ideal bleached state should be as transparent and colorless as possible. To address this issue, we employed hydroquinone (HQ) as the anodic material, which does not indicate any color when dissolved in the electrolyte. The device characteristics in terms of ECD and supercapacitor performance were systematically investigated at various HQ concentrations. At the optimal HQ concentration, the device showed large transmittance contrast (~91 %), high coloration efficiency (~61.9 cm2/C), high areal capacitance (~13.6 mF/cm2), and good charging/discharging cyclic stability. Lastly, we demonstrated the practical feasibility of the device. This shows that the ECS developed in this study is one of the promising devices for multifunctional electronics.

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Experimental Section Materials All chemicals, except for lithium bis(trifluoromethylsulfonyl)imide ([Li][TFSI]) (3M Company), were purchased from Sigma-Aldrich and used without further purification unless otherwise noted. The WO3 powders,26 WO3 dispersed inks,26 and ionic liquid 1-ethy-3methylimidazolium

bis(trifluoromethylsulfonyl)imide

([EMI][TFSI])16

were

prepared

according to our previous reports. A homogeneous ion gel solution was prepared as follows: First, poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-co-HFP) and [EMI][TFSI] were completely dissolved in acetone at 50 °C. Note that we selected a 1:9:20 (PVDF-coHFP:[EMI][TFSI]:acetone) weight ratio (typical polymer mass: ~40 mg) to obtain high storage moduli (G’ > 104 Pa) and ionic conductivity (σ ~ 6.7 mS/cm) even at room temperature.24 Then, a source of lithium ions, i.e., [Li][TFSI], was added to the solution, where the weight ratio of [Li][TFSI] and [EMI][TFSI] was fixed at 1:10. In addition, we introduced HQ into the solution as an anodic material. Various weight ratios of HQ and [EMI][TFSI] (0.001, 0.005, 0.010, 0.020, and 0.030) were employed to examine the effects of HQ concentration on the device performance. We denote HQ concentration as a fraction relative to the concentration of [EMI][TFSI]. Indium tin oxide (ITO)-coated glasses (sheet resistance: 10 Ω/sq, Asahi Glass Co.) were sequentially washed with acetone (10 min), methanol (10 min), and 2-propanol (10 min) under sonication. Further cleaning with UV/ozone was conducted for 10 min before use. ITO-coated polyethylene terephthalate sheets (sheet resistance: 10 Ω/sq, Max Film) were used without additional cleaning. Device Fabrication and Characterization WO3-dispersed ink was spin coated onto the ITO-coated glass at 5000 rpm for 20 s, followed by thermal annealing at 60 °C for 10 h in vacuum.26,27 The coated WO3 layer was

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characterized using scanning electron microscopy (SEM, S-4800, Hitachi Ltd.) at an acceleration voltage of 15 kV and atomic force microscopy (AFM, Park NX10, Park Systems) in tapping mode. The resulting film exhibited a well-dispersed and smooth morphology with a root-mean-square (RMS) roughness of 8.9 nm (see Figure S1), and the thickness of the film was ~510 nm (Figure S2). Next, the prepared ion gel solution was cast onto the WO3-coated electrode and dried in vacuum. Then, another ITO-coated glass was assembled, in which double-sided tapes (88 μm) served as a spacer and an electrolyte mold (typical active area of WO3: 1 cm2). To investigate the EC and energy storage performance of the resulting devices, the WO3-coated ITO electrode and bare ITO-coated electrode were used as the working and counter electrodes, respectively. A DC voltage and square wave voltages were applied by a potentiostat (Wave Driver 10, Pine Instrument). A UV-vis spectrophotometer (V-730, Jasco) was used to record UV-vis spectra (360 to 1100 nm at a scan rate of 400 nm/min) at various applied voltages and the transient transmittance changes at a fixed wavelength (700 nm). Galvanostatic charge–discharge (GCD) measurements were conducted using a battery cycler system (WBCS3000L, WonATech).

Results and Discussion The coloration of the WO3-based ECDs occurred by reaction (1), which includes the insertion of cations (M+), such as H+, Li+, or Na+, into the WO3 layer.20 6+ 𝑊𝑂3 (bleached) + 𝑥𝑀+ + 𝑥𝑒 − → 𝑀𝑥 𝑊1−𝑥 𝑊𝑥5+ 𝑂3 (colored)

(1)

In this context, conventional ion gels composed only of copolymers and ionic liquids are not suitable for WO3-based ECDs because they contain no cation that is sufficiently small to be inserted into the WO3 layer. Therefore, Li-doped ion gels were employed. We comment that without counter anodic materials, coloration begins at a higher voltage where electrolyte 6


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decomposition occurs (see Figure S3a). The use of appropriate anodic materials is essential to realize low-voltage devices. One recent key development in WO3-based ECDs is the direct incorporation of anodic species (e.g., Fc or Fc-derivatives)26,27 into the electrolyte instead of using secondary anodic ion storage layers. As a result, device fabrication becomes considerably more convenient. However, a yellowish bleached state arising from dissolved Fc or Fc derivatives remains as an issue to be resolved (see Figure S3b) because ideally, the bleached states should be as transparent as possible without any colors. Therefore, we employed HQ as an anodic material in this work, and the ECDs with HQ-containing ion gels were highly transparent (see Figure S3c).

Figure 1. (a) Dependence of transmittance at 700 nm on HQ concentration. (b) Voltage dependence of UV-vis absorption spectra of device with HQ/[EMI][TFSI] fixed at 0.020. (c) Variation in transmittance during coloration at –1.5 V and bleaching under short-circuit conditions. (d) Optical density versus injected charge density at three HQ concentrations.

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Figure 1a shows the effects of HQ concentration on ECD performance, where five HQ weight fractions relative to [EMI][TFSI] were employed, i.e., 0.000, 0.005, 0.010, 0.020, and 0.030. As anticipated, there was no change in transmittance without HQ up to –1.5 V owing to the absence of secondary anodic materials. However, transmittance varied significantly as the applied voltage increased when even a small proportion of HQ was present. Coloration was detected at –0.5 V at the lowest HQ concentration (0.005 HQ). The color became more intense (implying lower transmittance) when the applied voltage was increased further. The transmittance contrast (ΔT) between the bleached and colored states was saturated at 0.020 HQ. The overall voltage dependence of transmittance was similar, except for color intensity, irrespective of HQ concentration. One example of full UV-vis absorption spectra at several voltages is shown for the device with 0.020 HQ (Figure 1b). A clear transition between colored and bleached states is demonstrated in Movie S1. The device response dynamics for several HQ concentrations were also examined (Figure 1c). Faster and deep coloration was observed at higher HQ concentrations. For example, the shortest coloration time (tc) (9 s) and largest ΔT (~90%) were detected at 0.020 HQ. Lower transmittance (stronger coloration) implies that more Li+ ions are inserted into the WO3 layer. Delithiation from the colored WO3 is necessary for bleaching. Thus, the device with 0.020 HQ including the largest amount of Li+ in the WO3 layer exhibited the slowest bleaching response (see Figure 1c). Figure 1d shows the variation in optical density as a function of injected charge density for three devices at different HQ concentrations, from which coloration efficiency (η) can be calculated from the slope of the linear regime. Here, η is defined as ΔOD/ΔQ or log(Tb/Tc)/ΔQ, where ΔOD and ΔQ are the changes in optical density and the amount of injected charge, respectively, and Tb and Tc are the transmittance in the bleached and colored states, respectively.20 The extracted η values were 53.7, 57.8, and 61.9 cm2/C for the ECDs with 0.005, 0.010, and 0.020 HQ, respectively. The efficiency obtained in this work was 8


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similar to those of WO3-based ECDs with electrolytes directly containing anodic species (see Table S1 in the Supporting Information). The full coloration of such diffusion-controlled devices could not be achieved unless sufficient amounts of anodic species were not replenished through relatively slow diffusion, resulting in lower η. On the contrary, the ECDs based on two insoluble cathodic and anodic films, referred to as Type III ECDs, typically exhibited larger coloration efficiency exceeding 100 cm2/C.28,29,32 This behavior may be attributed to their charge-transfer controlled operation, in which the charge injection for electrochemical reactions is more efficient. Nonetheless, the optical contrast was not large owing to the inherent color of the films. Significantly modulated transmittance is an important feature for HQ-containing devices in this work. The characteristics of these ECDs are summarized in Table 1. We concluded that HQ/[EMI][TFSI] ~ 0.020 was the optimal condition, and this composition was employed for subsequent investigation. It should be noted that good long-term cyclic coloration/bleaching stability was also observed at 0.020 HQ (Figure S4 in the Supporting Information), in which the transition between colored and bleached states was clearly displayed even after 10,000 cycles. Table 1. Electrochromic characteristics of WO3-ECDs at various HQ concentrations.

a

HQ/[EMI][TFSI]a

ΔT (%)

tc (s)

tb (s)

η (cm2/C)

0.005

76.14

12

25

53.7

0.010

83.01

10

32

57.8

0.020

91.14

9

33

61.9

These values correspond to weight ratios.

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Figure 2. (a) GCD profiles and (b) change in corresponding areal capacitance of the devices at various HQ concentrations. Dependence of (c) GCD profiles and (d) corresponding areal capacitance on charging/discharging current densities for the device with 0.020 HQ.

Another remarkable function of the ECS is that it stores the charges injected for coloration 5+ as a form of colored Mx W6+ 1-x Wx O3 and benzoquinone. The charging/discharging processes

are accompanied by electrochemical reactions (i.e., reduction of WO3 and oxidation of HQ). Thus, this device is likely to be referred to as a pseudocapacitor. We recorded the GCD profiles at a current density of 0.4 mA/cm2 to investigate the feasibility of using the device as a supercapacitor and the effects of HQ contents on capacitance. Figure 2a shows the GCD curves of devices at five different HQ concentrations. Charging/discharging behavior was not detected in the device without HQ within an experimental voltage range of 0.0 to –1.5 V because no electrochemical reactions occurred. This observation is consistent with the EC behavior (see Figure 1a). At the lowest HQ concentration (0.001 HQ), the small quantity of

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HQ (the limiting reactant) hindered the overall electrochemical reactions. As a result, a large portion of unreacted WO3 remained, and the shortest discharge duration was observed. The areal capacitance (C) of the device with 0.001 HQ was determined to be 8.2 mF/cm2 from the GCD profile using equation (2).35,37 𝐼∆𝑡

𝐶 = 𝑆∆𝑉

(2)

where I, Δt, S, and ΔV are the discharging current (A), discharging time (s), surface area of the active materials (cm2), and scanned potential window (V), respectively. Longer Δt was observed at higher HQ concentration, and the C value of the device was also larger (see Figure 2a and 2b). When we plotted the calculated C as a function of HQ concentration, C was saturated at 0.020 HQ, similar to transmittance contrast (Figure 2b). Therefore, we concluded that 0.020 HQ is the optimal concentration for using the device as an ECD and a supercapacitor. In

addition,

we

investigated

the

dependence

of

device

performance

on

charging/discharging current density (Figure 2c). As current density increased, areal capacitance decreased. For example, the C value was ~13.6 mF/cm2 at 0.4 mA/cm2 and ~9.6 mF/cm2 (~30% lower) at 1.5 mA/cm2 (Figure 2d). The discharging (and bleaching) of the device in this work includes the extraction of Li+ doped in the colored WO3 layer, which is governed by the relatively slow diffusive mass transport of Li ions.32 Thus, the stored charges could not be efficiently extracted, and lower capacitance was estimated at high current density. The capacitive performance of the device was compared to those of previously reported ECSs,32-35,37 in which two-terminal WO3-based devices were selected for fairness (see Table 2). Even though the direct comparison was still unfair owing to the use of different electrolytes and their composition, we realized that the areal capacitance of our ECS was

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comparable to the others. In addition, another important metric, i.e., ΔT, was significantly higher for the ECSs based on HQ-containing gel electrolytes (this work).

Table 2. Summary of performance of WO3-based ECSs. ref

Electrode

Electrolyte

Working Voltage (V)

ΔT (%)

Areal Capacitance (mF/cm2)

Current Density (mA/cm2)

(32)

FTO

1 M LiClO4 in propylene carbonate

–1.2

~50

5.2

0.02

(33)

FTO

H2SO4 + poly(vinyl alcohol) gel

–1.4

~43

28.3

0.2

(34)

FTO

H2SO4 (aq)

–0.9

~76

12.8

0.4

(35)

FTO

0.5 M H2SO4 (aq)

–2.5

~68

5.3

0.05

(37)

FTO

1 M LiClO4 in propylene carbonate

–1.4

~33

11.8

0.1

this work

ITO

Li+ & HQ containing ion gel (PVDF-co-HFP + [EMI][TFSI])

–1.5

~91

13.6

0.4

Understanding the temperature dependence of capacitive properties is important while considering the practical use of ECSs. Therefore, we recorded additional GCD profiles at 0 and 40 °C and plotted them with the profile obtained at 25 °C (Figure 3a). The current density for charging/discharging was 0.4 mA/cm2. The charging/discharging process required more time at higher temperature, which implies larger capacitance. The increase in areal capacitance can be explained by the more effective insertion/extraction of Li+ ions into the WO3 layer based on larger diffusivity at higher temperature.37 Moreover, the enhanced diffusivity allows for operation at higher current density. For example, an areal capacitance of ~14.1 mF/cm2 was obtained under the highest experimental current density (1.5 mA/cm2 ) at 40 °C, which is ~64 % larger than that obtained at 0 °C upon discharging at the same current density (Figure 3b). 12


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Figure 3. (a) GCD profiles at three representative temperatures, i.e., 0, 25, and 40 °C, at a current density of 0.4 mA/cm2. (b) Dependence of areal capacitance on current density.

The charging/discharging cyclic stability of the device was evaluated. Figure 4 shows the changes in the capacitance retention of the device during the test. Surprisingly, only a few studies have examined the cyclic charging/discharging stability of two-terminal ECSs based on WO3 to date. The capacitance retention of the device that utilized H2SO4 (aq) was halved after only 500 cycles at a charging voltage of –2.5 V.35 This rapid deterioration presents a limitation of aqueous electrolytes that can be easily electrolyzed. Another device based on a propylene carbonate (PC) electrolyte including LiClO4 was stably operated, but the test was conducted only up to 1,000 cycles.32 Approximately 84% of the initial areal capacitance was maintained for the ECS in this work even after 10,000 consecutive charging/discharging

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cycles at a current density of 0.4 mA/cm2. This indicates highly stable operating characteristics (Figure 4).

Figure 4. Cyclic stability of ECS at a current density of 0.4 mA/cm2. Inset displays the first 10 and the last 10 GCD profiles of the ECS.

When we simultaneously consider the two functions of the devices (namely, ECD and supercapacitor), the corresponding device can visually indicate its real-time capacity by the intensity of the blue color. Transient transmittance and cell voltage profiles are shown in Figure 5a. Transmittance decreased with coloration at a current density of 0.4 mA/cm2, and the device was simultaneously charged up to –1.5 V. The device color was observed at four points during charging (see 1 to 4 designated in Figure 5a). The device was transparent at point 1 (0.0 V, fully discharged state). As charging proceeded, the blue color became more intense (Figure 5b). When we switched the device to the discharge mode, cell voltage and transmittance returned to the original state (Figure 5a). Further, the blue color faded, and the device eventually became transparent again (Figure 5c). Similar device behaviors were

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observed at higher current charging/discharging densities, but the transmittance level of the colored states was not sufficiently low (see Figure S5).

Figure 5. (a) GCD profiles at 0.4 mA/cm2 within a potential range of 0 to –1.5 V and corresponding in-situ transmittance change measured at 700 nm. Photographs of the device at eight points designated in (a) during (b) charging and (c) discharging.

As we employed rubbery gel electrolytes that do not leak even under strains, flexible ECSs were successfully demonstrated on plastic substrates. The bending durability of the device was evaluated in terms of the changes in areal capacitance (C/Co) and transmittance contrast (ΔT/ΔTo), where two bending radii (R) of 1.25 (Figure S6a) and 0.7 cm (Figure S6b) were applied. The corresponding strains (ε) were calculated to be 0.5 and 0.9% using the equation ε = D/2R, where D (~ 125 μm) is the thickness of the substrates.51 To investigate bending 15



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stability, we measured the changes in areal capacitance and transmittance contrast (C/Co and ΔT/ΔTo) as a function of the number of times the device was bent. Even after bending 2,000 times at 0.5% strain, ~90% of the initial Co and ~78% ΔTo were well maintained (Figure S6a). Similar good bending stability was observed when the device was subjected to larger strain (ε ~ 0.9%) (Figure S6b).

Figure 6. (a) Circuit diagram of the ECS. Photographs of ECSs in energy storage mode: (b) before charging and (c) after charging, in which only ECSs were connected with an external power supply. Photographs of ECSs in power source mode: (d) during turn-on of LED (discharging) and (e) after full discharge. For this application, four ECSs (dimensions: 10 mm × 10 mm each) are connected in series.

As a proof-of-concept demonstration, ECSs were included in the circuit depicted in Figure 6a. An ECS works as an energy storage device or an electrical power source when the circuit 16


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is connected to switch 1 or 2, respectively. The ECS was transparent before discharging (Figure 6b). As the device was charged, it became blue, indicating the charged state (Figure 6c). The bulb turned on when we switched the device to the power source mode and connected it to a white light-emitting diode (Figure 6d). Finally, the device returned to the original transparent state when it was fully discharged (Figure 6e). These results successfully show the two modes of the ECSs in this work. Conclusions In this work, functional ECSs were fabricated using a WO3 film as an EC layer. In contrast to previously reported ECDs including Fc or Fc-derivatives, high transparency was realized by employing HQ as an anodic species. We systematically examined the EC and energy storage performance at various HQ concentrations. As a result, a value of 0.020 for the weight ratio of HQ/[EMI][TFSI] was determined to be optimal. The optimized ECS exhibited large transmittance contrast (~91%), high coloration efficiency (~61.9 cm2/C), and high areal capacitance (~13.6 mF/cm2). Moreover, the ECSs showed good charging/discharging cyclic and bending stability. The practical application of the ECS was demonstrated, in which the device served as either an energy storage device or electrical power source. Simultaneously, the device displayed its real-time capacity in color. These results imply that the ECSs in this work have tremendous potential for application in smart electronics and multifunctional windows for buildings and cars in future.

Acknowledgments This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (NRF-2016R1C1B2006296), and Human Resources Program in the Transportation Specialized Lighting Core Technology Development (No. N0001364) granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea

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Dual-Function Electrochromic Supercapacitors Displaying Real-Time

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