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Al3+ Electrochromic Batteries
Article
Rechargeable Aqueous Hybrid Zn2+/Al3+ Electrochromic Batteries Haizeng Li, Curtis J. Firby, Abdulhakem Y. Elezzabi [email protected] (H.L.) [email protected] (A.Y.E.)
HIGHLIGHTS An aqueous hybrid Zn2+/Al3+ electrochromic battery system is presented This hybrid system enables significantly enhanced electrochemical performance Rechargeable aqueous electrochromic batteries with high performance are demonstrated
Electrochromic technology is an excellent strategy for green buildings, as they can be reversibly colored upon application of an external voltage, which enables blocking of visible light and solar heat. We design a novel hybrid system to eliminate the energy consumed during the coloration process, while retrieving the energy consumed during bleaching. When employed in an electrochromic battery, our hybrid system endows superior electrochemical performance, including high contrast, fast response times, high capacity, and good cycling stability.
Li et al., Joule 3, 2268–2278 September 18, 2019 ª 2019 Elsevier Inc. https://doi.org/10.1016/j.joule.2019.06.021
Article
Rechargeable Aqueous Hybrid Zn2+/Al3+ Electrochromic Batteries Haizeng Li,1,2,* Curtis J. Firby,1 and Abdulhakem Y. Elezzabi1,*
SUMMARY
Context & Scale
Electrochromic batteries are a newly born technology for next-generation transparent electronics. As electrochromic materials are typically bottlenecked by limited capacity and strict selection of triggered ions, the design of high-performance electrochromic batteries remains an elusive goal. Here, we present an aqueous hybrid Zn2+/Al3+ electrochromic battery system utilizing an electrolyte with both Zn2+ and Al3+. This unique architecture yields remarkable electrochemical performance, including rapid self-coloration time (0.5 s) and switching times (3.9 and 5.1 s for the coloration and bleaching processes), a high optical contrast (88%), and a large areal capacity (185.6 mAh/m2 at 0.5 mA/cm2). Consequently, these properties, which are the highest of any reported electrochromic battery presented to date, mark our hybrid electrolyte architecture as the most promising candidate for future device integration. Furthermore, with the outstanding performance of this safe aqueous electrolyte, our hybrid Zn2+/Al3+ electrochromic battery platform is expected to be a significant catalyst for accelerating the development of electrochromic devices in the future.
Electrochromic devices exhibit great promise for many applications. However, the electrochromic devices require external voltages to trigger the color evolutions, which makes them an energy-consuming technology. Electrochromic capacitive devices, charged when colored, were proposed to recycle some consumed energy. Unfortunately, these devices bring an incompatibility between electrochromism (requiring high coloration efficiency) and energy storage (requiring high capacity). Herein, we present an inverse electrochromic device that satisfies both requirements, namely an electrochromic battery, which is charged when bleached. This electrochromic battery operates in an opposite fashion to an electrochromic capacitive device, which facilitates the ability to efficiently retrieve the consumed energy. Such unique architecture provides a new technology for next-generation electrochromic devices.
INTRODUCTION Electrochromic materials provide new opportunities for the development of transparent batteries. This is because of their tunable transmittance triggered by the reversible insertion and extraction of cations.1–3 The incorporation of electrochromic phenomena and battery functionalities in a single platform, namely an electrochromic battery, provides two possible applications for a greener world: first, establishing smart battery systems for visually displaying energy storage level in real time; and second, developing next-generation electrochromic devices which are capable of recycling the consumed energy, and thus significantly reduce the energy consumption of electrochromic devices.4 However, the smart battery system requires high capacity while the electrochromic phenomenon requires fast switching speed. The active material films in electrochromic devices are typically very thin in order to obtain a fast switching speed, which in turn, reduces the amount of stored energy for smart battery systems.5 Moreover, the utilization of monovalent ions for conventional electrochromic devices suffers a slow switching speed.6 In this regard, multivalent-cation electrolytes (e.g., Zn2+, Mg2+, or Al3+), which provide multiple charges during redox reactions, are considered to be the most attractive option in the realization of high capacity for transparent batteries and fast switching speed for next-generation electrochromic devices.6–8 Recently, Zn-ion batteries have received growing interest owing to the low cost, safety, and compatibility with a variety of aqueous electrolytes.9 Notwithstanding, a major challenge for aqueous Zn-ion electrochromic batteries (ZIEBs) has been the poor kinetics of Zn2+ cations in electrochromic materials.10 This stems from the high activation energy for interfacial
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charge-transfer and poor electrochemical activities. These factors significantly limit the capacity, switching speed, and optical contrast of the electrochromic materials utilizing Zn2+ cations. It has been reported that the use of Al3+ cations as charge carriers can bring rapid switching speeds, high optical contrast, and high-stability to WO3 electrochromic electrodes, because of its trivalence and small ionic radius (0.53 A˚).6,11 However, the incorporation of Al3+ cations in an aqueous electrochromic battery is still facing significant challenges because of the high redox potential ( 1.68 V versus standard hydrogen electrode) of Al3+/Al. In this regard, a hybrid-cation electrolyte, composed of both Zn2+ and Al3+, can effectively mitigate these disadvantages and yields a new and universal strategy to augment the electrochemical activity of electrochromic materials used in electrochromic batteries. Because of the fact that the Al3+ not only enhances the electrochromic performance in visible-light transmittance,6 but also enables high performance in near-infrared light,11 this novel approach could be utilized in all existing conventional WO3-based electrochromic device platforms. In this work, we employ the strategy of utilizing a hybrid Zn2+/Al3+-based electrolyte (1 M ZnSO4-AlCl3), and electrodeposited WO3 electrodes to fabricate novel rechargeable aqueous electrochromic batteries. The incorporation of the hybrid Zn2+/Al3+-based electrolyte leads to remarkable improvements in kinetics, capacity, and optical contrast. We show that the WO3 cathode in the hybrid Zn2+/Al3+-based electrolyte exhibits a high discharge capacity of 185.6 mAh/m2 at 0.5 mA/cm2, excellent optical contrast (88%), and fast response times (3.9 and 5.1 s for coloration and bleaching processes, respectively). Furthermore, as a proof of concept, we assembled a prototype device to demonstrate its feasibility for self-powered smart windows architecture. The prototype device, with an open circuit voltage (OCV) of 1.15 V, is able to power a 0.5 V light emitting diode (LED) for 80 min and provide a high optical contrast (77%). This novel electrochromic battery system eliminates the requirement for external biases to color the WO3 electrode and can return the energy consumed during bleaching for coloration. Thus, this architecture is more energy efficient compared to conventional electrochromic devices. Moreover, the device delivers fast charging (10 s), exhibits good cycling performance (57% contrast retention after 200 cycles), and scalability for real-world applications.
RESULTS To elucidate the operation of this unique electrochromic battery platform, its working mechanism is illustrated in Figure 1. The intercalation and deintercalation of the smaller Al3+ cations dominate the reaction at the WO3 cathode side with fast kinetics, high capacity, and high reversibility. In its as-prepared condition, the hybrid Zn2+/Al3+ electrochromic battery exists in a charged state where W is under its highest oxidation state (W6+). During the discharge process, Zn is stripped into the hybrid Zn2+/Al3+-based electrolyte, and Al3+ is embedded into the WO3 cathode, thus providing electrical current and leading to the coloration of the cathode. Conversely, during the charging process, Zn is plated onto Zn foil and Al3+ is extracted from the colored WO3 cathode, leading to bleaching of the cathode. 1Ultrafast
In order to implement such a novel strategy, the electrodeposited WO3 cathode is prepared by a pulsed electrodeposition process (see Figure S1 and the experimental details in the Supplemental Information), which is based on the cathodic reduction of a peroxotungstic acid precursor (Figure 2A).12 Notably, this pulsed electrodeposition approach provides the WO3 nanoparticle size needed for efficient ion diffusion, and thus leads to excellent electrochemical performance.13–15
Optics and Nanophotonics Laboratory, Department of Electrical and Computer Engineering, University of Alberta, Edmonton, Alberta T6G 2V4, Canada
2Lead
Contact
*Correspondence: [email protected] (H.L.), [email protected] (A.Y.E.) https://doi.org/10.1016/j.joule.2019.06.021
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Figure 1. Schematic of the Rechargeable Aqueous Hybrid Zn2+/Al3+ Electrochromic Battery Schematic of the working mechanism of the aqueous hybrid Zn2+/Al 3+ electrochromic battery, where Zn foil served as anode, WO 3 served as cathode, and 1 M ZnSO 4 -AlCl 3 is utilized as the electrolyte.
Scanning electron microscopy (SEM) images (Figure 2B) confirm that the uniform electrodeposited WO3 film is composed of nanoparticles. Transmission electron microscopy (TEM) and dark-field scanning transmission electron microscope (DF-STEM) images shown in Figure S2 depict the sub-2 nm size of WO3 nanoparticles and verify the composition (W and O) of electrodeposited WO3 film. Figure 2C depicts the X-ray diffraction (XRD) pattern of the electrodeposited WO3 film. Notably, the detected broad WO3 peaks, and the lack of any other apparent crystalline WO3 peaks, indicates the amorphous nature of the electrodeposited WO3.15 Moreover, the X-ray photoelectron spectroscopy (XPS) results (Figures 2D and S3) reveal that W in the film is at its highest oxidation state (W6+). The presence of W6+ indicates that the electrodeposited WO3 film can be directly used as a cathode, since the W6+ can be reduced by hosting guest ions from the electrolyte, as well as electrons generated from the Zn/electrolyte interface. The transfer of ions (Zn2+, Al3+) and electrons generate an electrical current in a similar manner to the discharge process of a typical battery, and additionally, this transfer is expected to induce a color change (defined as self-coloration) due to the reduction of W6+. The real-time evolution of this self-coloring (discharge) process in different electrolytes (e.g., 1 M ZnSO4 and 1 M ZnSO4-AlCl3) was characterized and is shown in Figure 2E. The self-coloration time (ts) is defined as the time required for the transmission to change by 90% of the optical contrast (DT). Here, we measured ts to be 0.5 s using the 1 M ZnSO4-AlCl3 electrolyte and 1.9 s using the 1 M ZnSO4 electrolyte. Electrochemical impedance spectroscopy (EIS) data, shown in Figure S4, depict lower resistances of WO3 cathode in the electrolyte containing Zn2+/Al3+, and faster kinetics. Ex situ XPS analyses of the self-colored WO3 cathodes were performed to identify the different ion-insertion reaction
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Figure 2. Electrodeposition Scheme, Morphology, Composition, and Self-Coloration of the WO3 Films (A) Schematic illustration of the electrodeposition of WO 3 on indium tin oxide (ITO) glass. (B) SEM image of the WO3 film, inset showing the high-magnified SEM. (C) XRD pattern of the as-electrodeposited WO 3 film. (D) W 4f XPS spectra of the WO3 film. (E) Real-time self-coloring (discharge) process of the WO 3 cathode tested in different electrolytes (1 M ZnSO4 and 1 M ZnSO 4 -AlCl 3 ). (F) W 4f XPS spectra of the self-colored WO3 cathode in 1 M ZnSO4 -AlCl3 .
mechanisms. Only Al was detected in the XPS results of the self-colored WO3 cathode in hybrid Zn2+/Al3+-based electrolyte (Figure S5). This points to the fact that the reduction of W6+ to both W5+ and W4+ (Figure 2F) is induced by the intercalation of Al3+. This further reveals the high electrochemical activity of the WO3 cathode toward Al3+. The XPS results of the self-colored WO3 in 1 M ZnSO4 (Figure S6) show that W6+ is reduced to W5+ because of its weak electrochemical activity toward Zn2+. In addition to the aforementioned XPS results, intercalation of the various ions in 1 M ZnSO4-AlCl3 and 1 M ZnSO4 electrolytes was further investigated with both scanning transmission electron microscopy-energy dispersive spectroscopy (STEM-EDS) mappings and time of flight-secondary ion mass spectroscopy (TOF-SIMS). These characterizations were performed to analyze the distribution of Al/Zn along the WO3 cathode. Before analysis, the WO3 cathodes were discharged under 0.1 V for 90 s in different electrolytes (e.g., 1 M ZnSO4 and 1 M ZnSO4-AlCl3). As seen in Figure 3A, after the first discharge in the 1 M ZnSO4-AlCl3 electrolyte, W, O, and Al signals exhibit the same distribution across the WO3, further confirming the Al3+ intercalation. Figure 3B clearly shows the distribution of Zn after the first discharge in 1 M ZnSO4, verifying the intercalation of Zn2+. The Al/Zn depth distributions in the initially discharged WO3 cathodes were probed via the TOF-SIMS depth profiles. The x axes of the TOF-SIMS depth profiles represents the sputtering time, which is qualitatively correlated to the distance into the WO3 film.16 The WO3/ITO interface can be defined at 1,250 s of sputtering (where thickness is 300 nm according to the thickness of WO3 film), as indicated by the peak in the Sn profile.
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Figure 3. Evidence of the Al3+ and Zn2+ Intercalations (A–D) STEM-EDS elemental mappings results for WO 3 cathodes that discharged in (A) 1 M ZnSO 4 AlCl 3 and (B) 1 M ZnSO 4 . TOF-SIMS depth profiles of WO 3 cathodes that discharged in (C) 1 M ZnSO4 -AlCl 3 and (D) 1 M ZnSO 4 . During the discharge processes, 0.1 V was applied for 90 s.
The depth profiles in Figures 3C and 3D show that the counts of Al and Zn elements are high close to the surface and gradually decreased as sputtering time increases, and the Al/Zn elements coexist with W, confirming the intercalation of Al3+/Zn2+. Notably, the distribution of Al3+ is much closer to the surface region of the WO3 film (Figure S7), in comparison to the distribution of Zn2+. This is because of the fact that the strong electrostatic interactions between Al3+ and WO3 host reduces the Al3+ diffusion depth. Moreover, the surface Al3+ intercalation region is expected to stabilize the WO3 film in the electrolyte.6,11,17 The above findings provide further proof of Al3+ and Zn2+ intercalation in WO3. The different ion-insertion reactions were evaluated by cyclic voltammetry (CV), at a scan rate of 2 mV/s in both 1 M ZnSO4-AlCl3 and 1 M ZnSO4 electrolytes. As shown in Figure 4A, there are two anodic peaks located at 0.455 V (peak 1) and 0.862 V (peak 2), without an obvious cathodic peak, suggesting a multistep deintercalation and single-step intercalation process.18 The CV curve of the WO3 cathode, measured in the ZnSO4-AlCl3 electrolyte, exhibits higher current densities (4.6 times higher at 0.1 V, 2.4 times higher at peak 1, and 5.4 times higher at peak 2), compared to that tested in ZnSO4, revealing that the WO3 cathode is more electrochemically active toward the Al3+ because of the lower activation barrier of the Al3+ intercalation.19 Thus, for the self-coloring (discharge) process, a thermodynamically downhill reaction path has been identified via the oxidization of the Zn anode and the reduction of the WO3 cathode. All of the Al3+ and Zn2+ can move toward the WO3 cathode side to reduce the WO3 cathode; however,
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Figure 4. Electrochemical and Electrochromic Performance of the WO3 Cathode in Different Electrolytes (A) CV curves of the WO 3 cathode measured in different electrolytes (1 M ZnSO 4 and 1 M ZnSO4 AlCl 3 ). (B) Galvanostatic charge and discharge curves of the WO3 cathode at 0.5 mA/cm 2 between 0.1 and 1.2 V. (C) Visible near-infrared transmittance spectra of WO 3 cathode measured at 0.1 and 1.2 V in 1 M ZnSO4 -AlCl 3 . The inset shows the corresponding photographs with 1 cm scale bars. (D) Dynamic optical transmittance measurements of WO 3 cathodes at 632.8 nm in the 0.1–1.2 V window.
the Al3+ takes precedence to intercalate into the WO3 because of the lower activation barrier of the Al3+ compared to that of the Zn2+. This is consistent with the XPS result (Figure S5) where only the Al3+ was intercalated into the WO3 when using the ZnSO4-AlCl3 electrolyte. Figure 4B depicts the galvanostatic charge/discharge curves of WO3 cathodes tested in the two electrolytes at 0.5 mA/cm2. A stable and high discharge capacity of 185.6 mAh/m2 is achieved in the hybrid electrolyte. It should be noted that this discharge capacity is more than six times the discharge capacity achieved in 1 M ZnSO4 electrolyte (30.4 mAh/m2) and higher than the capacity of W18O49 films tested in 1 M H2SO4 (111 mAh/m2 at 0.16 mA/cm2).20 The initial Coulombic efficiency at 0.5 mA/cm2 of the hybrid system is 89%, which is higher than previously reported for the WO3-based batteries.21,22 The initial round-trip energy efficiency (50% at 0.5 mA/cm2), arising from the round-trip voltage hysteresis, is comparable to that of ZnFe2O4 for Li-ion batteries.23 The Al3+ deintercalation energy is higher than the intercalation energy, which is attributed to the easy intercalation of Al3+ into the amorphous WO3 surface and the strong electrostatic interactions between the Al3+ and the WO3 host. The 50% energy efficiency implies that half of the energy consumed for decoloration can be retrieved. The reaction mechanisms of the WO3 cathode in different electrolytes was further investigated in a Zn2+/Al3+-absent electrolyte (DI water adjusted to the pH value of 3 via dropwise addition of a mixture of 1 M H2SO4 and 3M HCl). As shown in Figure S8, the WO3 cathode only exhibits a low capacity of 8.8 mAh/m2, confirming that
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Al3+ cations dominate the reaction at cathode side in the hybrid electrolyte. Remarkably, even at 1mA/cm2, the WO3 cathode in a hybrid electrolyte still exhibits a stable discharge capacity of 133.8 mAh/m2 (Figure S9A), as well as rapid, stable, and reversible optical transmittance modulation during the galvanostatic discharge and charge process (Figure S9B). This further supports the high rate capabilities of WO3 in our hybrid electrolyte system, compared to the WO3 cathode in 1 M ZnSO4 (Figures S9A and S9C). Figure 4C shows the change in optical transmittance of the WO3 cathode measured in the hybrid electrolyte. The WO3 cathode exhibits DT of 88% at 632.8 nm (without background correction), which is higher than that measured in 1 M ZnSO4 (82%, Figure S10) and higher than previous reports.2,14,20,24–26 Notably, the WO3 cathode in hybrid Zn2+/ Al3+-based electrolyte exhibits strong light absorption in the green and red regions of the visible spectrum (86% at 550 nm), compared to the WO3 cathode in 1 M ZnSO4 (72.9% at 550 nm, Figure S10). This is due to the reduction of W6+ to W4+ in the hybrid system.11 Dynamic optical transmittance measurements at 632.8 nm were performed to evaluate the kinetics under different ion-insertion reaction mechanisms. Figure 4D shows the reversible electrochromic switching behaviors of the WO3 cathode in the two electrolytes under consideration. Clearly, faster switching times (3.9 and 5.1 s for coloration and bleaching, respectively) is achieved from the hybrid ZnSO4-AlCl3 electrolyte, compared to the 1 M ZnSO4 (6.9 and 6.6 s for coloration and bleaching, respectively). The switching times of 3.9 s for coloration and 5.1 s for bleaching with DT 88% at 632.8 nm make the WO3 cathode in a hybrid electrolyte system the most promising candidate among all the previously reported electrochromic materials.27–29 Furthermore, even though the optical contrast, the response times, and the capacity of the electrochromic battery may differ when utilizing solid electrolyte for real applications, these parameters are the highest of any reported electrochromic battery utilized liquid electrolyte to date.30,31 The high capacity indicates that the hybrid system can be potentially used for smart battery systems with a visual indication of energy storage level, while the fast switching speed along with high DT indicates the potential applications for next-generation electrochromic devices that are capable of recycling the consumed energy. In addition, the WO3 cathode in the hybrid electrolyte system shows significantly improved cycle stability compared to the 1 M ZnSO4. As shown in Figure S11, the WO3 cathode in the hybrid electrolyte system retains 92% of its initial DT after 2,500 CV cycles, while the WO3 cathode in 1 M ZnSO4 shows no activity after 1,000 CV cycles. The significantly enhanced cycling stability in the hybrid system is due to the strong electrostatic forces and small ionic radius of Al3+. Notably, the fast-reversible switching behavior in the hybrid electrolyte reveals that the WO3 cathode can be recharged by an external voltage in a few seconds, and thus this system can be utilized for self-powered coloration and rechargeable electrochromic batteries. A prototype device was demonstrated to illustrate the potential of this novel Zn2+/Al3+ electrochromic battery chemistry for practical applications. Figure 5A shows the schematic configuration of the prototype Zn2+/Al3+ hybrid electrochromic battery, where the anode (Zn foil) is sandwiched between two WO3 cathodes, and 1 M ZnSO4-AlCl3 is used as the electrolyte. The optical transmittance spectra of the prototype device are shown in Figure 5B, where the as-assembled device exhibits a high transparency (79% at 632.8 nm). An OCV of 1.15 V enables the electrochromic battery to light an LED (0.5 V regulated) for 80 min until depleted (colored) with a DT of 77% at 632.8 nm (Figures 5B and 5C). The depleted/colored device can be quickly charged via applying an external voltage (1.2 V). Figure 5D shows the charging process of the depleted/colored
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Figure 5. Configuration and Electrochromic Performance of a Prototype Device (A) Schematic configuration of the prototype hybrid Zn 2+/Al 3+ electrochromic battery, where the arrows indicate current flow, I is the total current, while the I 1 and I 2 are the currents contributed by each WO 3 cathode. (B) Visible near-infrared transmittance spectrum of the prototype device measured under different states. The inset shows the OCV of the device. (C) Photographs of a 0.5 V LED powered by the prototype device. (D) Charging process of the discharged device via an external voltage (1.2 V). (E) Real-time transmittance spectra of prototype device at 632.8 nm in the 0.1–1.2 V window.
device. The optical transmittance of the device can be easily recovered to its original state in 10 s (the blue curve in Figure 5B) with the OCV recovered to 1.17 V (Figure S12A). The dynamic transmittance at 632.8 nm, shown in Figure 5E, further confirms the fast kinetics and high reversibility of the prototype electrochromic battery. The switching times are measured to be 5.7 s for coloration and 10.3 s for bleaching, which is slightly decayed compared to that of single WO3 cathode (Figure 4D). This decay is attributed to the high electrode resistance as the size of WO3 cathodes increased in our prototype device.32,33 Remarkably, our hybrid Zn2+/Al3+ electrochromic battery also exhibits a good cycling stability reaching 57% of its initial optical contrast after 200 cycles (Figure S12B). Such a feature is a marked improvement compared to previous reports.2,14 The prototype hybrid Zn2+/Al3+ electrochromic battery exhibits a high areal capacity of 126.3 mAh/m2 at a high current density of 1 mA/cm2 (Figure S13), indicating its high rate capability. In addition, a large-area device (10 3 8 cm) was constructed to demonstrate the scalability of the hybrid Zn2+/Al3+ electrochromic battery (Figure S14). The results are shown to be consistent with the performance of the device shown in Figure 5.
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Table 1. Comparison of Current State-of-the-Art Electrochromic Films and Devices Reference
Materials
Film or Device
Coloration
Bleaching
DT
Response Time (s)
Capacity
Size (cm2)
SelfPowered
Gu et al.36
PolyoxometalatesW18O49
Film
1 V versus Ag+/Ag
0.4 V versus Ag+/Ag
30% at 500 nma
Coloration: 26 Bleaching: 86
—
*
No
Xu et al.37
WO3
Film
0.8 V versus Ag+/Ag
0.8 V versus Ag+/Ag
60% at 630 nma
Coloration: 10.9 Bleaching: 12.2
—
*
No
Zhao et al.31
W18O49
Device
Discharge
Addition of H2O2
50% at 650 nma
—
429 mAh/g at 2 V for 60 s
—
Yes
Zhang et al.11
WO3-x
Film
-0.9 V versus Ag+/Ag
0.7 V versus Ag+/Ag
93.2% at 633 nma
Coloration: 16 Bleaching: 13
—
*
No
1V
73% at 650 nm
—
—
2.8
1 V versus Ag+/Ag
49.6% at 632.8 nm
—
41.9 mAh/g at 100 mA/g
*
Device Li et al.38
MoO3W0.71Mo0.29O3
Film
No
2.5 V
2.5 V
41.9% at 632.8 nm
—
2.33 mAh/m2 at 0.05 mA/cm2
64
0.7 V
1V
73% at 633 nm
Coloration: 12.7 Bleaching: 15.8
25 mAh/m2 at 0.3 mA/cm2
12.3
No
3.5 V versus Li+/Li
89.1% at 550 nma
Coloration: 52.6 Bleaching: 9.5
183 mAh/g at 200 mA/g
*
No
1V
72.7% at 550 nm
—
127.8 mAh/m2 at 0.06 mA/cm2
3.75
WO3-PEDOT
Device
Cao et al.40
Ta-TiO2
Film Device
WO3
-1 V versus Ag+/Ag
Device Cai et al.39
this work
4V
1.5 V versus Li+/Li 3.5 V
Film
0.1 V versus Zn2+/Zn
1.2 V versus Zn2+/Zn
88% at 632.8 nm
Coloration: 3.9 Bleaching: 5.1
185.6 mAh/m2 at 0.5 mA/cm2
*
Device
Discharge
1.2 V
77% at 632.8 nm
Coloration: 5.7 Bleaching: 10.3
126.3 mAh/m2 at 1 mA/cm2
13.6
Yes
The unit of capacity in this work is mAh/m2, because the weight of active material in an electrochromic film is negligible. *The performance of the film is usually tested in a cuvette, and the size of the film is <3 cm2. a The optical contrast, DT, value is obtained after optical correction via subtracting the transmittance loss of substrate.
DISCUSSION To date, no common protocol has been defined to compare the electrochromic performance being reported by different groups, because of the implementation of a diverse array of test procedures. Obviously, the performance (i.e., response time and optical contrast) of single film tested in a three-electrode configuration is significantly better than that of the device. As well, electrochromic films and devices exhibiting smaller optical contrast will possess faster response times. Furthermore, the film/device size exerts a significant effect on both the response times and capacity (i.e., larger devices typically possess slower response times and lower capacity). Several key performance metrics are summarized in Table 1 to clearly compare the present work with current, state-of art electrochromic films and devices. Clearly, our hybrid system provides higher optical contrast, faster switching times, and a large capacity at a high current density while maintaining a large device area, and thus is the most promising candidate among all the previously reported electrochromic films or devices. In summary, a novel aqueous electrochromic battery architecture, with a hybrid Zn2+/Al3+-based electrolyte, has been demonstrated for the first time. Utilizing a hybrid electrolyte provides a universal strategy to enhance the electrochemical performance of electrochromic materials for electrochromic batteries. Furthermore, electrochromic batteries eliminate the requirement of the ion storage layer demanded by conventional electrochromic devices, which significantly simplifies their fabrication and construction. Most importantly, energy is not consumed to color the
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electrochromic battery, while they simultaneously provide energy to power external electronics. Our architecture demonstrates rapid self-coloration time (0.5 s) and switching times (3.9 s/5.1 s for coloration/bleaching state), a high optical contrast (88%), and a large areal capacity (185.6 mAh/m2 at 0.5 mA/cm2). These properties mark a significant improvement over previously reported electrochromic materials, making our hybrid electrolyte framework the most promising candidate for electrochromic batteries. These functions facilitate new opportunities for the development of next-generation electrochromic devices. In their early stages of development, electrochromic batteries represent a possible alternative to electrochromic windows and transparent batteries that merits further research and development. The identification of their real-world applications needs further investigation. The round-trip energy efficiency is a key metric to evaluate the consumed energy in an electrochromic battery. Unlike the crystal WO3, whose intercalation sites are well-known and are located at the vacant sites between the WO6 octahedral units,34 the intercalation sites in amorphous WO3 have not been identified.35 The debate, regarding whether the insertion mechanism in the amorphous WO3 electrode is through the intercalation of bare ions or through the solvated or hydrated ions (see Supplemental Experimental Procedures), still needs to be resolved.
EXPERIMENTAL PROCEDURES Full details of the experimental procedures are described in the Supplemental Information.
SUPPLEMENTAL INFORMATION Supplemental Information can be found online at https://doi.org/10.1016/j.joule. 2019.06.021.
ACKNOWLEDGMENTS This work is supported by the Natural Sciences and Engineering Research Council of Canada (grant file no. CRDPJ 509210-17) and All Weather Windows Ltd.
AUTHOR CONTRIBUTIONS All authors conceived the idea, designed the experiments, and analyzed the data. H.L. performed the synthesis of the materials. H.L. performed the film deposition and electrochemical performance testing, and C.J.F. carried out the microscopy characterizations. H.L. wrote the manuscript. All authors contributed to editing the manuscript. A.Y.E. supervised the project.
DECLARATION OF INTERESTS The authors declare no competing interests. Received: January 2, 2019 Revised: March 18, 2019 Accepted: June 18, 2019 Published: July 22, 2019
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Rechargeable Aqueous Hybrid Zn2+/Al3+ Electrochromic Batteries Haizeng Li, Curtis J. Firby, Abdulhakem Y. Elezzabi [email protected] (H.L.) [email protected] (A.Y.E.)
HIGHLIGHTS An aqueous hybrid Zn2+/Al3+ electrochromic battery system is presented This hybrid system enables significantly enhanced electrochemical performance Rechargeable aqueous electrochromic batteries with high performance are demonstrated
Electrochromic technology is an excellent strategy for green buildings, as they can be reversibly colored upon application of an external voltage, which enables blocking of visible light and solar heat. We design a novel hybrid system to eliminate the energy consumed during the coloration process, while retrieving the energy consumed during bleaching. When employed in an electrochromic battery, our hybrid system endows superior electrochemical performance, including high contrast, fast response times, high capacity, and good cycling stability.
Li et al., Joule 3, 2268–2278 September 18, 2019 ª 2019 Elsevier Inc. https://doi.org/10.1016/j.joule.2019.06.021
Article
Rechargeable Aqueous Hybrid Zn2+/Al3+ Electrochromic Batteries Haizeng Li,1,2,* Curtis J. Firby,1 and Abdulhakem Y. Elezzabi1,*
SUMMARY
Context & Scale
Electrochromic batteries are a newly born technology for next-generation transparent electronics. As electrochromic materials are typically bottlenecked by limited capacity and strict selection of triggered ions, the design of high-performance electrochromic batteries remains an elusive goal. Here, we present an aqueous hybrid Zn2+/Al3+ electrochromic battery system utilizing an electrolyte with both Zn2+ and Al3+. This unique architecture yields remarkable electrochemical performance, including rapid self-coloration time (0.5 s) and switching times (3.9 and 5.1 s for the coloration and bleaching processes), a high optical contrast (88%), and a large areal capacity (185.6 mAh/m2 at 0.5 mA/cm2). Consequently, these properties, which are the highest of any reported electrochromic battery presented to date, mark our hybrid electrolyte architecture as the most promising candidate for future device integration. Furthermore, with the outstanding performance of this safe aqueous electrolyte, our hybrid Zn2+/Al3+ electrochromic battery platform is expected to be a significant catalyst for accelerating the development of electrochromic devices in the future.
Electrochromic devices exhibit great promise for many applications. However, the electrochromic devices require external voltages to trigger the color evolutions, which makes them an energy-consuming technology. Electrochromic capacitive devices, charged when colored, were proposed to recycle some consumed energy. Unfortunately, these devices bring an incompatibility between electrochromism (requiring high coloration efficiency) and energy storage (requiring high capacity). Herein, we present an inverse electrochromic device that satisfies both requirements, namely an electrochromic battery, which is charged when bleached. This electrochromic battery operates in an opposite fashion to an electrochromic capacitive device, which facilitates the ability to efficiently retrieve the consumed energy. Such unique architecture provides a new technology for next-generation electrochromic devices.
INTRODUCTION Electrochromic materials provide new opportunities for the development of transparent batteries. This is because of their tunable transmittance triggered by the reversible insertion and extraction of cations.1–3 The incorporation of electrochromic phenomena and battery functionalities in a single platform, namely an electrochromic battery, provides two possible applications for a greener world: first, establishing smart battery systems for visually displaying energy storage level in real time; and second, developing next-generation electrochromic devices which are capable of recycling the consumed energy, and thus significantly reduce the energy consumption of electrochromic devices.4 However, the smart battery system requires high capacity while the electrochromic phenomenon requires fast switching speed. The active material films in electrochromic devices are typically very thin in order to obtain a fast switching speed, which in turn, reduces the amount of stored energy for smart battery systems.5 Moreover, the utilization of monovalent ions for conventional electrochromic devices suffers a slow switching speed.6 In this regard, multivalent-cation electrolytes (e.g., Zn2+, Mg2+, or Al3+), which provide multiple charges during redox reactions, are considered to be the most attractive option in the realization of high capacity for transparent batteries and fast switching speed for next-generation electrochromic devices.6–8 Recently, Zn-ion batteries have received growing interest owing to the low cost, safety, and compatibility with a variety of aqueous electrolytes.9 Notwithstanding, a major challenge for aqueous Zn-ion electrochromic batteries (ZIEBs) has been the poor kinetics of Zn2+ cations in electrochromic materials.10 This stems from the high activation energy for interfacial
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charge-transfer and poor electrochemical activities. These factors significantly limit the capacity, switching speed, and optical contrast of the electrochromic materials utilizing Zn2+ cations. It has been reported that the use of Al3+ cations as charge carriers can bring rapid switching speeds, high optical contrast, and high-stability to WO3 electrochromic electrodes, because of its trivalence and small ionic radius (0.53 A˚).6,11 However, the incorporation of Al3+ cations in an aqueous electrochromic battery is still facing significant challenges because of the high redox potential ( 1.68 V versus standard hydrogen electrode) of Al3+/Al. In this regard, a hybrid-cation electrolyte, composed of both Zn2+ and Al3+, can effectively mitigate these disadvantages and yields a new and universal strategy to augment the electrochemical activity of electrochromic materials used in electrochromic batteries. Because of the fact that the Al3+ not only enhances the electrochromic performance in visible-light transmittance,6 but also enables high performance in near-infrared light,11 this novel approach could be utilized in all existing conventional WO3-based electrochromic device platforms. In this work, we employ the strategy of utilizing a hybrid Zn2+/Al3+-based electrolyte (1 M ZnSO4-AlCl3), and electrodeposited WO3 electrodes to fabricate novel rechargeable aqueous electrochromic batteries. The incorporation of the hybrid Zn2+/Al3+-based electrolyte leads to remarkable improvements in kinetics, capacity, and optical contrast. We show that the WO3 cathode in the hybrid Zn2+/Al3+-based electrolyte exhibits a high discharge capacity of 185.6 mAh/m2 at 0.5 mA/cm2, excellent optical contrast (88%), and fast response times (3.9 and 5.1 s for coloration and bleaching processes, respectively). Furthermore, as a proof of concept, we assembled a prototype device to demonstrate its feasibility for self-powered smart windows architecture. The prototype device, with an open circuit voltage (OCV) of 1.15 V, is able to power a 0.5 V light emitting diode (LED) for 80 min and provide a high optical contrast (77%). This novel electrochromic battery system eliminates the requirement for external biases to color the WO3 electrode and can return the energy consumed during bleaching for coloration. Thus, this architecture is more energy efficient compared to conventional electrochromic devices. Moreover, the device delivers fast charging (10 s), exhibits good cycling performance (57% contrast retention after 200 cycles), and scalability for real-world applications.
RESULTS To elucidate the operation of this unique electrochromic battery platform, its working mechanism is illustrated in Figure 1. The intercalation and deintercalation of the smaller Al3+ cations dominate the reaction at the WO3 cathode side with fast kinetics, high capacity, and high reversibility. In its as-prepared condition, the hybrid Zn2+/Al3+ electrochromic battery exists in a charged state where W is under its highest oxidation state (W6+). During the discharge process, Zn is stripped into the hybrid Zn2+/Al3+-based electrolyte, and Al3+ is embedded into the WO3 cathode, thus providing electrical current and leading to the coloration of the cathode. Conversely, during the charging process, Zn is plated onto Zn foil and Al3+ is extracted from the colored WO3 cathode, leading to bleaching of the cathode. 1Ultrafast
In order to implement such a novel strategy, the electrodeposited WO3 cathode is prepared by a pulsed electrodeposition process (see Figure S1 and the experimental details in the Supplemental Information), which is based on the cathodic reduction of a peroxotungstic acid precursor (Figure 2A).12 Notably, this pulsed electrodeposition approach provides the WO3 nanoparticle size needed for efficient ion diffusion, and thus leads to excellent electrochemical performance.13–15
Optics and Nanophotonics Laboratory, Department of Electrical and Computer Engineering, University of Alberta, Edmonton, Alberta T6G 2V4, Canada
2Lead
Contact
*Correspondence: [email protected] (H.L.), [email protected] (A.Y.E.) https://doi.org/10.1016/j.joule.2019.06.021
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Figure 1. Schematic of the Rechargeable Aqueous Hybrid Zn2+/Al3+ Electrochromic Battery Schematic of the working mechanism of the aqueous hybrid Zn2+/Al 3+ electrochromic battery, where Zn foil served as anode, WO 3 served as cathode, and 1 M ZnSO 4 -AlCl 3 is utilized as the electrolyte.
Scanning electron microscopy (SEM) images (Figure 2B) confirm that the uniform electrodeposited WO3 film is composed of nanoparticles. Transmission electron microscopy (TEM) and dark-field scanning transmission electron microscope (DF-STEM) images shown in Figure S2 depict the sub-2 nm size of WO3 nanoparticles and verify the composition (W and O) of electrodeposited WO3 film. Figure 2C depicts the X-ray diffraction (XRD) pattern of the electrodeposited WO3 film. Notably, the detected broad WO3 peaks, and the lack of any other apparent crystalline WO3 peaks, indicates the amorphous nature of the electrodeposited WO3.15 Moreover, the X-ray photoelectron spectroscopy (XPS) results (Figures 2D and S3) reveal that W in the film is at its highest oxidation state (W6+). The presence of W6+ indicates that the electrodeposited WO3 film can be directly used as a cathode, since the W6+ can be reduced by hosting guest ions from the electrolyte, as well as electrons generated from the Zn/electrolyte interface. The transfer of ions (Zn2+, Al3+) and electrons generate an electrical current in a similar manner to the discharge process of a typical battery, and additionally, this transfer is expected to induce a color change (defined as self-coloration) due to the reduction of W6+. The real-time evolution of this self-coloring (discharge) process in different electrolytes (e.g., 1 M ZnSO4 and 1 M ZnSO4-AlCl3) was characterized and is shown in Figure 2E. The self-coloration time (ts) is defined as the time required for the transmission to change by 90% of the optical contrast (DT). Here, we measured ts to be 0.5 s using the 1 M ZnSO4-AlCl3 electrolyte and 1.9 s using the 1 M ZnSO4 electrolyte. Electrochemical impedance spectroscopy (EIS) data, shown in Figure S4, depict lower resistances of WO3 cathode in the electrolyte containing Zn2+/Al3+, and faster kinetics. Ex situ XPS analyses of the self-colored WO3 cathodes were performed to identify the different ion-insertion reaction
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Figure 2. Electrodeposition Scheme, Morphology, Composition, and Self-Coloration of the WO3 Films (A) Schematic illustration of the electrodeposition of WO 3 on indium tin oxide (ITO) glass. (B) SEM image of the WO3 film, inset showing the high-magnified SEM. (C) XRD pattern of the as-electrodeposited WO 3 film. (D) W 4f XPS spectra of the WO3 film. (E) Real-time self-coloring (discharge) process of the WO 3 cathode tested in different electrolytes (1 M ZnSO4 and 1 M ZnSO 4 -AlCl 3 ). (F) W 4f XPS spectra of the self-colored WO3 cathode in 1 M ZnSO4 -AlCl3 .
mechanisms. Only Al was detected in the XPS results of the self-colored WO3 cathode in hybrid Zn2+/Al3+-based electrolyte (Figure S5). This points to the fact that the reduction of W6+ to both W5+ and W4+ (Figure 2F) is induced by the intercalation of Al3+. This further reveals the high electrochemical activity of the WO3 cathode toward Al3+. The XPS results of the self-colored WO3 in 1 M ZnSO4 (Figure S6) show that W6+ is reduced to W5+ because of its weak electrochemical activity toward Zn2+. In addition to the aforementioned XPS results, intercalation of the various ions in 1 M ZnSO4-AlCl3 and 1 M ZnSO4 electrolytes was further investigated with both scanning transmission electron microscopy-energy dispersive spectroscopy (STEM-EDS) mappings and time of flight-secondary ion mass spectroscopy (TOF-SIMS). These characterizations were performed to analyze the distribution of Al/Zn along the WO3 cathode. Before analysis, the WO3 cathodes were discharged under 0.1 V for 90 s in different electrolytes (e.g., 1 M ZnSO4 and 1 M ZnSO4-AlCl3). As seen in Figure 3A, after the first discharge in the 1 M ZnSO4-AlCl3 electrolyte, W, O, and Al signals exhibit the same distribution across the WO3, further confirming the Al3+ intercalation. Figure 3B clearly shows the distribution of Zn after the first discharge in 1 M ZnSO4, verifying the intercalation of Zn2+. The Al/Zn depth distributions in the initially discharged WO3 cathodes were probed via the TOF-SIMS depth profiles. The x axes of the TOF-SIMS depth profiles represents the sputtering time, which is qualitatively correlated to the distance into the WO3 film.16 The WO3/ITO interface can be defined at 1,250 s of sputtering (where thickness is 300 nm according to the thickness of WO3 film), as indicated by the peak in the Sn profile.
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Figure 3. Evidence of the Al3+ and Zn2+ Intercalations (A–D) STEM-EDS elemental mappings results for WO 3 cathodes that discharged in (A) 1 M ZnSO 4 AlCl 3 and (B) 1 M ZnSO 4 . TOF-SIMS depth profiles of WO 3 cathodes that discharged in (C) 1 M ZnSO4 -AlCl 3 and (D) 1 M ZnSO 4 . During the discharge processes, 0.1 V was applied for 90 s.
The depth profiles in Figures 3C and 3D show that the counts of Al and Zn elements are high close to the surface and gradually decreased as sputtering time increases, and the Al/Zn elements coexist with W, confirming the intercalation of Al3+/Zn2+. Notably, the distribution of Al3+ is much closer to the surface region of the WO3 film (Figure S7), in comparison to the distribution of Zn2+. This is because of the fact that the strong electrostatic interactions between Al3+ and WO3 host reduces the Al3+ diffusion depth. Moreover, the surface Al3+ intercalation region is expected to stabilize the WO3 film in the electrolyte.6,11,17 The above findings provide further proof of Al3+ and Zn2+ intercalation in WO3. The different ion-insertion reactions were evaluated by cyclic voltammetry (CV), at a scan rate of 2 mV/s in both 1 M ZnSO4-AlCl3 and 1 M ZnSO4 electrolytes. As shown in Figure 4A, there are two anodic peaks located at 0.455 V (peak 1) and 0.862 V (peak 2), without an obvious cathodic peak, suggesting a multistep deintercalation and single-step intercalation process.18 The CV curve of the WO3 cathode, measured in the ZnSO4-AlCl3 electrolyte, exhibits higher current densities (4.6 times higher at 0.1 V, 2.4 times higher at peak 1, and 5.4 times higher at peak 2), compared to that tested in ZnSO4, revealing that the WO3 cathode is more electrochemically active toward the Al3+ because of the lower activation barrier of the Al3+ intercalation.19 Thus, for the self-coloring (discharge) process, a thermodynamically downhill reaction path has been identified via the oxidization of the Zn anode and the reduction of the WO3 cathode. All of the Al3+ and Zn2+ can move toward the WO3 cathode side to reduce the WO3 cathode; however,
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Figure 4. Electrochemical and Electrochromic Performance of the WO3 Cathode in Different Electrolytes (A) CV curves of the WO 3 cathode measured in different electrolytes (1 M ZnSO 4 and 1 M ZnSO4 AlCl 3 ). (B) Galvanostatic charge and discharge curves of the WO3 cathode at 0.5 mA/cm 2 between 0.1 and 1.2 V. (C) Visible near-infrared transmittance spectra of WO 3 cathode measured at 0.1 and 1.2 V in 1 M ZnSO4 -AlCl 3 . The inset shows the corresponding photographs with 1 cm scale bars. (D) Dynamic optical transmittance measurements of WO 3 cathodes at 632.8 nm in the 0.1–1.2 V window.
the Al3+ takes precedence to intercalate into the WO3 because of the lower activation barrier of the Al3+ compared to that of the Zn2+. This is consistent with the XPS result (Figure S5) where only the Al3+ was intercalated into the WO3 when using the ZnSO4-AlCl3 electrolyte. Figure 4B depicts the galvanostatic charge/discharge curves of WO3 cathodes tested in the two electrolytes at 0.5 mA/cm2. A stable and high discharge capacity of 185.6 mAh/m2 is achieved in the hybrid electrolyte. It should be noted that this discharge capacity is more than six times the discharge capacity achieved in 1 M ZnSO4 electrolyte (30.4 mAh/m2) and higher than the capacity of W18O49 films tested in 1 M H2SO4 (111 mAh/m2 at 0.16 mA/cm2).20 The initial Coulombic efficiency at 0.5 mA/cm2 of the hybrid system is 89%, which is higher than previously reported for the WO3-based batteries.21,22 The initial round-trip energy efficiency (50% at 0.5 mA/cm2), arising from the round-trip voltage hysteresis, is comparable to that of ZnFe2O4 for Li-ion batteries.23 The Al3+ deintercalation energy is higher than the intercalation energy, which is attributed to the easy intercalation of Al3+ into the amorphous WO3 surface and the strong electrostatic interactions between the Al3+ and the WO3 host. The 50% energy efficiency implies that half of the energy consumed for decoloration can be retrieved. The reaction mechanisms of the WO3 cathode in different electrolytes was further investigated in a Zn2+/Al3+-absent electrolyte (DI water adjusted to the pH value of 3 via dropwise addition of a mixture of 1 M H2SO4 and 3M HCl). As shown in Figure S8, the WO3 cathode only exhibits a low capacity of 8.8 mAh/m2, confirming that
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Al3+ cations dominate the reaction at cathode side in the hybrid electrolyte. Remarkably, even at 1mA/cm2, the WO3 cathode in a hybrid electrolyte still exhibits a stable discharge capacity of 133.8 mAh/m2 (Figure S9A), as well as rapid, stable, and reversible optical transmittance modulation during the galvanostatic discharge and charge process (Figure S9B). This further supports the high rate capabilities of WO3 in our hybrid electrolyte system, compared to the WO3 cathode in 1 M ZnSO4 (Figures S9A and S9C). Figure 4C shows the change in optical transmittance of the WO3 cathode measured in the hybrid electrolyte. The WO3 cathode exhibits DT of 88% at 632.8 nm (without background correction), which is higher than that measured in 1 M ZnSO4 (82%, Figure S10) and higher than previous reports.2,14,20,24–26 Notably, the WO3 cathode in hybrid Zn2+/ Al3+-based electrolyte exhibits strong light absorption in the green and red regions of the visible spectrum (86% at 550 nm), compared to the WO3 cathode in 1 M ZnSO4 (72.9% at 550 nm, Figure S10). This is due to the reduction of W6+ to W4+ in the hybrid system.11 Dynamic optical transmittance measurements at 632.8 nm were performed to evaluate the kinetics under different ion-insertion reaction mechanisms. Figure 4D shows the reversible electrochromic switching behaviors of the WO3 cathode in the two electrolytes under consideration. Clearly, faster switching times (3.9 and 5.1 s for coloration and bleaching, respectively) is achieved from the hybrid ZnSO4-AlCl3 electrolyte, compared to the 1 M ZnSO4 (6.9 and 6.6 s for coloration and bleaching, respectively). The switching times of 3.9 s for coloration and 5.1 s for bleaching with DT 88% at 632.8 nm make the WO3 cathode in a hybrid electrolyte system the most promising candidate among all the previously reported electrochromic materials.27–29 Furthermore, even though the optical contrast, the response times, and the capacity of the electrochromic battery may differ when utilizing solid electrolyte for real applications, these parameters are the highest of any reported electrochromic battery utilized liquid electrolyte to date.30,31 The high capacity indicates that the hybrid system can be potentially used for smart battery systems with a visual indication of energy storage level, while the fast switching speed along with high DT indicates the potential applications for next-generation electrochromic devices that are capable of recycling the consumed energy. In addition, the WO3 cathode in the hybrid electrolyte system shows significantly improved cycle stability compared to the 1 M ZnSO4. As shown in Figure S11, the WO3 cathode in the hybrid electrolyte system retains 92% of its initial DT after 2,500 CV cycles, while the WO3 cathode in 1 M ZnSO4 shows no activity after 1,000 CV cycles. The significantly enhanced cycling stability in the hybrid system is due to the strong electrostatic forces and small ionic radius of Al3+. Notably, the fast-reversible switching behavior in the hybrid electrolyte reveals that the WO3 cathode can be recharged by an external voltage in a few seconds, and thus this system can be utilized for self-powered coloration and rechargeable electrochromic batteries. A prototype device was demonstrated to illustrate the potential of this novel Zn2+/Al3+ electrochromic battery chemistry for practical applications. Figure 5A shows the schematic configuration of the prototype Zn2+/Al3+ hybrid electrochromic battery, where the anode (Zn foil) is sandwiched between two WO3 cathodes, and 1 M ZnSO4-AlCl3 is used as the electrolyte. The optical transmittance spectra of the prototype device are shown in Figure 5B, where the as-assembled device exhibits a high transparency (79% at 632.8 nm). An OCV of 1.15 V enables the electrochromic battery to light an LED (0.5 V regulated) for 80 min until depleted (colored) with a DT of 77% at 632.8 nm (Figures 5B and 5C). The depleted/colored device can be quickly charged via applying an external voltage (1.2 V). Figure 5D shows the charging process of the depleted/colored
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Figure 5. Configuration and Electrochromic Performance of a Prototype Device (A) Schematic configuration of the prototype hybrid Zn 2+/Al 3+ electrochromic battery, where the arrows indicate current flow, I is the total current, while the I 1 and I 2 are the currents contributed by each WO 3 cathode. (B) Visible near-infrared transmittance spectrum of the prototype device measured under different states. The inset shows the OCV of the device. (C) Photographs of a 0.5 V LED powered by the prototype device. (D) Charging process of the discharged device via an external voltage (1.2 V). (E) Real-time transmittance spectra of prototype device at 632.8 nm in the 0.1–1.2 V window.
device. The optical transmittance of the device can be easily recovered to its original state in 10 s (the blue curve in Figure 5B) with the OCV recovered to 1.17 V (Figure S12A). The dynamic transmittance at 632.8 nm, shown in Figure 5E, further confirms the fast kinetics and high reversibility of the prototype electrochromic battery. The switching times are measured to be 5.7 s for coloration and 10.3 s for bleaching, which is slightly decayed compared to that of single WO3 cathode (Figure 4D). This decay is attributed to the high electrode resistance as the size of WO3 cathodes increased in our prototype device.32,33 Remarkably, our hybrid Zn2+/Al3+ electrochromic battery also exhibits a good cycling stability reaching 57% of its initial optical contrast after 200 cycles (Figure S12B). Such a feature is a marked improvement compared to previous reports.2,14 The prototype hybrid Zn2+/Al3+ electrochromic battery exhibits a high areal capacity of 126.3 mAh/m2 at a high current density of 1 mA/cm2 (Figure S13), indicating its high rate capability. In addition, a large-area device (10 3 8 cm) was constructed to demonstrate the scalability of the hybrid Zn2+/Al3+ electrochromic battery (Figure S14). The results are shown to be consistent with the performance of the device shown in Figure 5.
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Table 1. Comparison of Current State-of-the-Art Electrochromic Films and Devices Reference
Materials
Film or Device
Coloration
Bleaching
DT
Response Time (s)
Capacity
Size (cm2)
SelfPowered
Gu et al.36
PolyoxometalatesW18O49
Film
1 V versus Ag+/Ag
0.4 V versus Ag+/Ag
30% at 500 nma
Coloration: 26 Bleaching: 86
—
*
No
Xu et al.37
WO3
Film
0.8 V versus Ag+/Ag
0.8 V versus Ag+/Ag
60% at 630 nma
Coloration: 10.9 Bleaching: 12.2
—
*
No
Zhao et al.31
W18O49
Device
Discharge
Addition of H2O2
50% at 650 nma
—
429 mAh/g at 2 V for 60 s
—
Yes
Zhang et al.11
WO3-x
Film
-0.9 V versus Ag+/Ag
0.7 V versus Ag+/Ag
93.2% at 633 nma
Coloration: 16 Bleaching: 13
—
*
No
1V
73% at 650 nm
—
—
2.8
1 V versus Ag+/Ag
49.6% at 632.8 nm
—
41.9 mAh/g at 100 mA/g
*
Device Li et al.38
MoO3W0.71Mo0.29O3
Film
No
2.5 V
2.5 V
41.9% at 632.8 nm
—
2.33 mAh/m2 at 0.05 mA/cm2
64
0.7 V
1V
73% at 633 nm
Coloration: 12.7 Bleaching: 15.8
25 mAh/m2 at 0.3 mA/cm2
12.3
No
3.5 V versus Li+/Li
89.1% at 550 nma
Coloration: 52.6 Bleaching: 9.5
183 mAh/g at 200 mA/g
*
No
1V
72.7% at 550 nm
—
127.8 mAh/m2 at 0.06 mA/cm2
3.75
WO3-PEDOT
Device
Cao et al.40
Ta-TiO2
Film Device
WO3
-1 V versus Ag+/Ag
Device Cai et al.39
this work
4V
1.5 V versus Li+/Li 3.5 V
Film
0.1 V versus Zn2+/Zn
1.2 V versus Zn2+/Zn
88% at 632.8 nm
Coloration: 3.9 Bleaching: 5.1
185.6 mAh/m2 at 0.5 mA/cm2
*
Device
Discharge
1.2 V
77% at 632.8 nm
Coloration: 5.7 Bleaching: 10.3
126.3 mAh/m2 at 1 mA/cm2
13.6
Yes
The unit of capacity in this work is mAh/m2, because the weight of active material in an electrochromic film is negligible. *The performance of the film is usually tested in a cuvette, and the size of the film is <3 cm2. a The optical contrast, DT, value is obtained after optical correction via subtracting the transmittance loss of substrate.
DISCUSSION To date, no common protocol has been defined to compare the electrochromic performance being reported by different groups, because of the implementation of a diverse array of test procedures. Obviously, the performance (i.e., response time and optical contrast) of single film tested in a three-electrode configuration is significantly better than that of the device. As well, electrochromic films and devices exhibiting smaller optical contrast will possess faster response times. Furthermore, the film/device size exerts a significant effect on both the response times and capacity (i.e., larger devices typically possess slower response times and lower capacity). Several key performance metrics are summarized in Table 1 to clearly compare the present work with current, state-of art electrochromic films and devices. Clearly, our hybrid system provides higher optical contrast, faster switching times, and a large capacity at a high current density while maintaining a large device area, and thus is the most promising candidate among all the previously reported electrochromic films or devices. In summary, a novel aqueous electrochromic battery architecture, with a hybrid Zn2+/Al3+-based electrolyte, has been demonstrated for the first time. Utilizing a hybrid electrolyte provides a universal strategy to enhance the electrochemical performance of electrochromic materials for electrochromic batteries. Furthermore, electrochromic batteries eliminate the requirement of the ion storage layer demanded by conventional electrochromic devices, which significantly simplifies their fabrication and construction. Most importantly, energy is not consumed to color the
2276 Joule 3, 2268–2278, September 18, 2019
electrochromic battery, while they simultaneously provide energy to power external electronics. Our architecture demonstrates rapid self-coloration time (0.5 s) and switching times (3.9 s/5.1 s for coloration/bleaching state), a high optical contrast (88%), and a large areal capacity (185.6 mAh/m2 at 0.5 mA/cm2). These properties mark a significant improvement over previously reported electrochromic materials, making our hybrid electrolyte framework the most promising candidate for electrochromic batteries. These functions facilitate new opportunities for the development of next-generation electrochromic devices. In their early stages of development, electrochromic batteries represent a possible alternative to electrochromic windows and transparent batteries that merits further research and development. The identification of their real-world applications needs further investigation. The round-trip energy efficiency is a key metric to evaluate the consumed energy in an electrochromic battery. Unlike the crystal WO3, whose intercalation sites are well-known and are located at the vacant sites between the WO6 octahedral units,34 the intercalation sites in amorphous WO3 have not been identified.35 The debate, regarding whether the insertion mechanism in the amorphous WO3 electrode is through the intercalation of bare ions or through the solvated or hydrated ions (see Supplemental Experimental Procedures), still needs to be resolved.
EXPERIMENTAL PROCEDURES Full details of the experimental procedures are described in the Supplemental Information.
SUPPLEMENTAL INFORMATION Supplemental Information can be found online at https://doi.org/10.1016/j.joule. 2019.06.021.
ACKNOWLEDGMENTS This work is supported by the Natural Sciences and Engineering Research Council of Canada (grant file no. CRDPJ 509210-17) and All Weather Windows Ltd.
AUTHOR CONTRIBUTIONS All authors conceived the idea, designed the experiments, and analyzed the data. H.L. performed the synthesis of the materials. H.L. performed the film deposition and electrochemical performance testing, and C.J.F. carried out the microscopy characterizations. H.L. wrote the manuscript. All authors contributed to editing the manuscript. A.Y.E. supervised the project.
DECLARATION OF INTERESTS The authors declare no competing interests. Received: January 2, 2019 Revised: March 18, 2019 Accepted: June 18, 2019 Published: July 22, 2019
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