A Dual‐Mode Electrochromic Platform Integrating Zinc Anode‐Based and Rocking‐Chair Electrochromic Devices

The complementary electrochromic device, where the optical transmittance changes upon the flow of cations back and forth between anodic and cathodic electrodes, operates in a rocking‐chair fashion if it can inherently self‐discharge. Herein, the first demonstration of a dual‐mode electrochromic platform having self‐coloring and self‐bleaching characteristics is reported, which is realized by sandwiching zinc metal within a newly‐designed Prussian blue (PB)‐WO3 rocking‐chair type electrochromic device. It is demonstrated that the redox potential differences between the zinc metal and the WO3/PB electrodes endow the self‐color‐switching of these electrodes. By employing a hybrid electrolyte of Zn2+/K+, it is further shown that the colored PB‐WO3 rocking‐chair device is capable of spontaneously bleaching when the anodic and cathodic electrodes are coupled. This dual‐mode light‐control strategy enables the electrochromic devices to exhibit four distinct optical states with the highest optical contrast of 72.6% and fast switching times (<5 s for the bleaching/coloration processes). Furthermore, the built‐in voltage of the dual‐mode electrochromic devices not only promotes energy efficiency, but also augments the bistability of the devices. It is envisioned that the broad implication of the present platform is in the development of self‐powered smart windows, colorful displays, optoelectronic switches, and optical sensors.

electrochromic materials have experienced emergence in their utilization in energy efficient "green building" windows and transparent optical-electronic devices. [7][8][9][10][11][12] The reversible light transmission is induced by electrochemical oxidation and reduction reactions of the electrochromic materials which enhances the optical absorption of the electrochromic material. Through electrochemical control from the application of voltage stimuli, dynamic optical transparency control, generated by electrochromic materials, has been realized on a variety of platforms. [13][14][15][16][17][18] Nevertheless, electrochromic technology has yet to realize widespread applications and commercialization due to its limited optical contrast, poor bistability, and unsatisfactory longevity of the material.
The electrochromic technology platform is typically realized through a device architecture comprised of an electrochromic electrode (i.e., working electrode), a counter electrode and an electrolyte. [19] Such a rudimentary electrochromic device is operated in a fashion where cations in the electrolyte are intercalated/extracted and moved back and forth between the electrochromic electrode and the counter electrode as the applied voltage is reversed. By properly matching the cathodically coloring working electrode material and the anodically coloring counter electrode material, both electrodes are able to be colored and bleached in a simultaneity, which offers an optimized optical contrast of this complementary-type electrochromic device. Many pioneer studies on this type of electrochromic device have been reported by matching different electrochromic materials, such as WO 3 and NiO, [20][21][22] or TiO 2 and NiO, [4,12] or WO 3 and KFeFe(CN) 6 (Prussian blue, PB) or its analogs (PBAs), [23][24][25] etc., which showed low operating potentials and rapid switching speed. However, the performance of the complementary-type electrochromic devices is restricted by the charge balance between the working and counter electrodes, as the different cations storage amount in working and counter electrodes will cause side reactions and thus leading to electrolyte decomposition, poor optical contrast, and low coulombic efficiency. Therefore, the careful design of both electrodes to have a matching electrochemical capacity is critical to achieve an optimized electrochromic performance.
It is well accepted that the complementary-type electrochromic devices are thin film devices possessing low capacity. [12,26,27] During the charging process, external voltage

Introduction
Electrochromic technologies offering tunable light transparency have attained considerable attention for dynamic light control applications. [1][2][3][4][5][6] Motivated by the optical properties of reversible light transmission and absorption wavelength tunability, acts as the driving force to trigger the cation intercalation into the working electrode, thus, resulting in a corresponding color switching. Conversely, during the discharging process, cations are released from the working electrode and are intercalated into the counter electrode. In this regard, such electrochromic devices are of great interest for their use in energy-efficient optoelectronic devices where the cations stored in the devices can be retrieved while simultaneously having light transparency control. However, not all of the stored cations can be selfretrieved during the discharging process, due to the inherent bistability of electrochromic materials and the low electrode redox potential difference between working and counter electrodes. [27,28] Most importantly, this class of electrochromic devices cannot be fully switched to its original color state via draining its electrical power through an external load (i.e., via powering external an electronic device). Due to the strong electrostatic interactions between embedded cations and the electrochromic material, even for devices having matching capacity, an external voltage bias is still required to switch the electrochromic device. [4] Therefore, the electrode redox potential of the working and the counter electrodes should be tailored for developing self-switching electrochromic devices. [29] To overcome the limitations (e.g., charge balance, matching electrode potential, and long-term reversibility) imposed by the complementary-type electrochromic devices, we developed a new and fundamentally novel class of electrochromic devices (i.e., zinc anode-based electrochromic devices). [30][31][32] Here, the zinc anode acts as the counter electrode, and thus facilitates self-color-switching via the redox potential gradient differences between the zinc anode and the electrochromic cathode. [17] Notwithstanding, such devices exhibit only a one-way selfswitching mechanism by discharging its voltage similar to a secondary battery. Accordingly, the exploration of reversible electrochromic devices having dual self-switching functionalities perhaps should be one of the primary tasks to be undertaken by the electrochromic community.
To date, there is no report on the study of tailoring electrode redox potentials of the anodic and cathodic electrodes within a complementary-type electrochromic device. Similar to the zinc anode-based electrochromic devices, where the redox potential gradient differences between the zinc anode and the electrochromic cathode facilitate a self-color-switching behavior, by employing a redox potential gradient difference between the anodic and the cathodic electrodes, the complementary electrochromic device is able to inherently self-discharge and thus facilitating a self-color-switch. As shown in Figure 1, during the discharging process, the redox potential gradient difference between the anodic and cathodic electrodes triggers a self-color-switch of both electrodes in a simultaneity (according to the redox potentials of two electrodes, this self-color-switch process may lead to a beaching of the device (E cathodic <E anodic ) or a coloration of the device (E cathodic >E anodic )). On the other hand, during the charging process, both electrodes return to the initial optical states by applying an external potential between the two electrodes. Consequently, this type of device, where the flow of cations back and forth between anodic and cathodic electrodes can be triggered by its built-in voltage, is operated in a rocking-chair fashion (i.e., rocking-chair type electrochromic devices). Notably, this rocking-chair type electrochromic device platform is highly manipulative as it exhibits a self-color-switch. As such, by coupling the zinc anode-based electrochromic platform and the rocking-chair type electrochromic platform into a single platform, the coming age of electrochromic devices having dual self-switching functionalities is appearing on the near horizon.
Herein, we present a novel dual-mode electrochemically reversible platform enabled by sandwiching a Zn metal anode within a PB-WO 3 rocking-chair type electrochromic device, by virtue of utilizing a well-designed hybrid electrolyte system to tailor the redox potential difference between the WO 3 and the PB electrodes (Figure 2a). Such a WO 3 -Zn-PB device configuration enables independent operation of the WO 3 and the PB electrodes, thus providing additional operation flexibility (e.g., self-coloring/self-bleaching) in addition to offering precise manipulation of a single electrode. We show that the redox potential differences between the zinc anode and the WO 3 /PB electrodes endow the self-color-switching of these electrodes (Mode I shown in Figure 2b). By further altering the redox potential difference between the WO 3 and the PB electrodes through the utilization of a well-designed hybrid electrolyte system, we demonstrate a rocking-chair type electrochromic device having spontaneous self-bleaching functionality for the first time (i.e., a rocking-chair type electrochromic device, Mode II shown in Figure 2c). As such, the full utilization of the Adv. Funct. Mater. 2023, 33, 2300155 Figure 1. Schematic diagram illustrating the rocking-chair type electrochromic devices. During the discharging process, both the anodic and cathodic electrodes can self-bleach (or self-color) in a simultaneity due to the redox potential gradient difference between the two electrodes. By applying an external potential between the two electrodes, during the charging process, both electrodes return to the initial optical states.

Figure 2.
Schematic diagram illustrating the dual-mode operation processes. a) Schematic diagram illustrating the basic operation of a reversible dual-mode electrochromic device having a self-switching functionality. The WO 3 /PB electrodes can self-color and self-bleach due to the redox potential difference between the Zn metal anode and the WO 3 /PB cathodes in 1 m KCl-0.1 m ZnSO 4 . The WO 3 electrode can also self-bleach via the utilization of the redox potential difference between the WO 3 electrode and the PB electrode. Such a device configuration also enables the independent operation of either a single or a dual electrochromic electrode(s). b) Schematic diagram illustrating the Mode I operation processes. c) Schematic diagram illustrating the Mode II operation processes. www.advancedsciencenews.com

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© 2023 The Authors. Advanced Functional Materials published by Wiley-VCH GmbH redox potential differences between the three electrodes enables dual-mode operations, which is a highly promising function for electrochromic devices having dynamic light control.
As a proof of concept, we assembled a 5 cm × 5 cm WO 3 -Zn-PB (in 1 m KCl -0.1 m ZnSO 4 ) electrochromic device to demonstrate this dual-mode platform. The device exhibits four distinct optical states (fully colored, PB colored, WO 3 colored, and fully bleached states) with the highest optical contrast of 72.6% and fast switching times (<5s for the bleaching and coloration processes). The results discussed in this work are expected to advance the development of self-powered electrochromic devices and cast a new light on the design of high-performance optoelectronic devices.

Mechanism of the Dual-Mode Electrochromic Platform
To better elucidate the basic operating mechanism of this dual-mode electrochromic device platform, it is important to examine the energy level transition diagrams for WO 3 , Zn, and PB shown in Figure 3a,b. While the large redox potential difference between Zn and PB/WO 3 electrodes (≈1.12/1.06 V) is expected to serve as a driving potential to trigger the spontaneous color switching process, the similar redox potentials between WO 3 (0.30 V) [31] and PB (0.36 V) [32] brings great challenges to realize a PB-WO 3 rocking-chair type electrochromic  device having spontaneous self-color switching. Therefore, to realize a dual-mode electrochromic platform having self-coloring and self-bleaching functionalities, it is critical to alter the redox potential difference between the WO 3 and the PB electrodes. This can be accomplished by coupling the zinc anodebased electrochromic device and the aforementioned PB-WO 3 rocking-chair type electrochromic device with a well-designed electrolyte into a single platform as illustrated in Figure 2a. In such a configuration, the first operating mode (i.e., Mode I, Figure 2b) is realized by electrically connecting the Zn anode to either the WO 3 or equivalently to the PB cathode, wherein, the redox potential difference between the Zn anode and the WO 3 (or the PB) cathode allows the Zn metal to be oxidized and the WO 3 (or the PB) electrochromic layer to be reduced. During this discharging process, the Zn anode is oxidized and releases Zn 2+ and, at the same time, the K + from the KCl-ZnSO 4 electrolyte are intercalated into the WO 3 (or the PB) electrochromic cathode. This discharging process triggers a spontaneous colorswitching of the WO 3 (or the PB) electrochromic electrode where the WO 3 layer changes its color state from being transparent to a light blue color (or when the Zn anode is connected to the PB, the PB layer changes its color from dark blue to be transparent). To return to the initial color state(s), the direct electrical contact between the Zn anode and the WO 3 (or the PB) cathode is disconnected, and an external potential (1.0 V or 1.8 V) is applied between the Zn anode and the WO 3 cathode (or between the Zn anode and the PB cathode). During this charging process, the Zn 2+ are reduced and plated onto the Zn anode and the K + are de-intercalated from the WO 3 (or the PB) electrochromic cathode.
The second operating mode (i.e., Mode II, Figure 2c) requires taking advantage of the redox potential difference between the WO 3 and PB electrodes. During the discharging process, the Zn anode and the WO 3 electrode are connected, thus inducing the oxidation of the Zn anode (Zn → Zn 2+ ) and the reduction of the WO 3 electrode (due to the K + intercalation). When this process is followed by a direct electrical contact between the WO 3 and the PB electrodes, the K + are extracted from the reduced WO 3 electrode and intercalated into the PB electrode. This discharging process triggers a spontaneous color-switching process of both the WO 3 electrode (i.e., from light blue color to transparent) and the PB electrode (i.e., from dark blue color to transparent). On the other hand, during the charging process, the WO 3 -Zn-PB device returns to its initial color state. Here, the Zn 2+ are reduced and plated onto the Zn anode while the K + are de-intercalated from the PB electrode by applying an external potential of 1.8 V between the Zn anode and the PB electrode.
To realize the operating Mode II, it is important to tailor the redox potential difference between the WO 3 and PB electrodes. Therefore, it is critical to properly alter the electrolyte composition to tailor the redox potential difference between the WO 3 and the PB electrodes according to the Nernst equation (Equation (1)), [33,34] Where E cell is the cell potential of interest, E cell θ is the standard cell potential, R is the universal gas constant, T is the temperature (K), F is the Faraday constant, n is the number of electrons transferred in the cell, and a is the chemical activity for the relevant species (i.e., a red is the chemical activity of the reduced product and a ox is the chemical activity of the oxidized product).
Of particular importance, when investigating the cell potential of electrochromic devices, it is able to have the flexibility to modify the redox potential through the chemical activity for the relevant species within the cell. The chemical activity is related to the efficiency of gaining/losing electrons, [34] as such, the redox potential difference between the WO 3 and the PB electrodes can be tailored by selecting the types of cations used in the electrolyte solution. Figure S1a,S2a (Supporting Information) in the Supporting Information depict the cyclic voltammetry (CV) curves for the WO 3 and the PB electrodes in a 1 m ZnSO 4 solution (a typical electrolyte system used in zinc anode-based electrochromic devices) [30] measured at different scan rates. Here, the diffusion coefficient of Zn 2+ for intercalation and extraction can be estimated from the measured peak current, I p (Amps) (Equation (2)), [35] I A C Dvn p 2.69 10 5 Where n is the number of electrons transferred, A is the contact area (cm 2 ), D is the diffusion coefficient of the cation ions (cm 2 s −1 ), C is the concentration of the cation ions in the electrolyte solution (mol cm −3 ), and v is the scan rate (V s −1 ). Accordingly, the diffusion coefficients of Zn 2+ for intercalation and extraction of the WO 3 electrode are calculated to be 2.38 × 10 −11 cm 2 s −1 and 3.17 × 10 −11 cm 2 s −1 , respectively (Figure 3c); whereas for the PB electrode are 1.87 × 10 −11 cm 2 s −1 for intercalation and 5.21 × 10 −11 cm 2 s −1 for extraction as depicted in Figure 3d. Interestingly, since there is no significant difference between the diffusion coefficient values of the WO 3 and the PB electrodes, they must exhibit a very similar electrochemical activity, and, thus, the PB-WO 3 cell potential is close to their standard cell potential. This is further confirmed in a device platform, where the open circuit potential (OCP) of the PB-WO 3 cell is measured in a 1 m ZnSO 4 solution, to be 0.076 V ( Figure S3a, Supporting Information), which is nearly the same as the standard cell potential of 0.06 V shown in Figure 3a. As discussed, since the presence of the Zn 2+ is necessary for the striping/plating of the Zn anode in the zinc anode-based electrochromic devices, [27] a hybrid electrolyte is needed for the dual-mode electrochromic operation. As such, it is important to select the proper cation to intercalate and extract the PB electrode. It is well-known that the intercalation of K + into PB structures is a highly reversible and fast kinetics process due to the ease of diffusion of the K + into the open-framework atomic structure of PB lattice. [36,37] Thus, a hybrid electrolyte consisting of Zn 2+ and K + is suitable to tailor the redox potential difference between the WO 3 and the PB electrodes. Figure S1b,S2b (Supporting Information) in the Supporting Information depict the cyclic voltammetry (CV) curves for the WO 3 and the PB electrodes in a 1 m KCl-0.1 m ZnSO 4 solution measured at different scan rates. As shown in Figure 3c,d, the diffusion coefficients of K + into the WO 3 electrode are calculated to be 3.95 × 10 −10 cm 2 s −1 (for intercalation) and 4.75 × 10 −10 cm 2 s −1 (for extraction) and for the PB electrode are 3.78 × 10 −9 cm 2 s −1 (for intercalation) and 3.95 × 10 −9 cm 2 s −1 (for extraction). In our experiments, we determined that the measured diffusion coefficients of K + into the PB electrode are much higher than those diffusion coefficients into the WO 3 electrode. These results confirm that the K + is an excellent choice for the hybrid electrolyte since it can be easily extracted from the WO 3 electrode and efficiently intercalated into the PB electrode. Notably, since there is a significant difference between the diffusion coefficient values of the WO 3 and the PB electrodes, the differentiation of electrochemical activities of the two electrodes is created. As such, the redox potential difference between the WO 3 and PB electrodes is increased in the hybrid electrolyte. In order to put all our aforementioned findings into practice, a hybrid electrolyte, consisting of 1 m KCl-0.1 m ZnSO 4 , was used as the ion-conducting layer of the dual-mode electrochromic device. In this system, the OCP of the PB-WO 3 cell is found to increase from 0.076 to 0.376 V ( Figure S3b, Supporting Information). Such a relatively high OCP is critical for the self-bleaching process of the PB-WO 3 electrodes (Video S1, Supporting Information).

Optimization of the Electrochromic Electrodes
It is important to further show that the inclusion of the K + into the electrolyte makes the WO 3 and PB electrodes more electrochemically active, compared to a pure Zn 2+ electrolyte.  Figure 3e,f, the Zn 2+ intercalation/extraction in pure Zn 2+ electrolyte is a relatively slow kinetic process (i.e., smaller enclosed area of CV), compared to the K + intercalation/extraction process in hybrid K + -Zn 2+ electrolyte. This is attributed to the fact that the multivalent Zn 2+ cations lead to a large lattice distortion, due to the strong electrostatic interactions, hence, resulting in a drastic depreciation in current densities during successive cycling ( Figure S4a, Supporting Information). [37,38] In contrast, for the PB electrode, a pair of well-defined redox peaks (at 1.1 and 1.5 V, Figure 3f) is observed in the hybrid electrolyte. These peaks are attributed to the reduction/oxidation processes of low-spin Fe II /Fe III coupling to the C atoms via the intercalation/extraction of K + . [37] Remarkably, the redox peaks of the PB film in the hybrid electrolyte remain almost unchanged during cycling ( Figure S4b, Supporting Information), suggesting a highly reversible redox reaction is taking place during cycling.
The redox reactions induce color variations of both the WO 3 and the PB electrodes. As presented in the insets in Figure 3e, the WO 3 electrode exhibits a transparent color in its oxidized state and a blue color via the intercalation of the K + . On the other hand, the PB electrode exhibits a blue color in its oxidized state and a transparent color when intercalated with the K + (insets in Figure 3f). The color behaviors of PB and WO 3 electrodes are consistent with the classification of anodic and cathodic electrochromic materials, respectively. [39] The areal capacities of the PB and the WO 3 electrodes when intercalated with the Zn 2+ and the hybrid K + /Zn 2+ are evaluated and compared in Figure 3g. Both the PB and the WO 3 electrodes show relatively high capacities of 147 and 92 mAh m −2 , respectively in the K + /Zn 2+ hybrid electrolyte compared to the capacities (i.e., 56 mAh m −2 (for PB) and 47 mAh m −2 (for WO 3 )) measured in the pure Zn 2+ electrolyte. The 2.6 times increase in the PB electrode capacity and the 1.9 times increase in the WO 3 electrode capacity further confirm the higher activity of the K + intercalation/extraction process. Notably, since the capacity of the PB electrode is slightly higher than that of the WO 3 electrode, fully self-bleaching of the reduced WO 3 electrode can be easily achieved by electrically coupling the WO 3 and the PB electrodes. As shown in Figure 3h,i, the optical transmittances of the PB-WO 3 rocking-chair type device for the two voltage values of −1 and 0 V are demonstrated. Clearly, a relatively high 47.8% optical contrast is observed when using the hybrid 1 m KCl-0.1 m ZnSO 4 electrolyte in comparison to the 20.3% achieved with the 1 m ZnSO 4 electrolyte ( Figure S5, Supporting Information). Although such a PB-WO 3 rocking-chair device enables self-bleaching of the WO 3 and PB electrodes due to the redox potential gradient difference, this moderate optical contrast (47.8%) of the PB-WO 3 device is not satisfactory for high-performance dynamic light control applications. A major source of this limitation is due to the insufficient charging/ discharging of the PB electrode, where the charge imbalance between two electrodes (i.e., different capacities) impedes the device from achieving its optimal performance. Nonetheless, a fully bleaching and coloration of the PB electrode can be realized by increasing the capacity of the WO 3 electrode (by increasing the thickness) or decreasing the capacity of the PB electrode (by decreasing the thickness), and thus, matching the charge balance of both electrodes. In this way, the optical contrast of the PB-WO 3 device is supposed to increase but cannot achieve its maximum value. To achieve the maximum optical contrast of the device, a single electrochromic electrode must express its maximum optical contrast; however, the optical contrast of a single electrochromic electrode is also determined by the thickness of the electrode film, as such, critical choice of the film thickness is required. In spite of that, the optimum film thickness for maximum optical contrast may not be the same as the thickness needed for the charge balance. To overcome such a limitation, the zinc anode is sandwiched between the PB and the WO 3 electrodes to offer independent coloration/ bleaching of the PB and the WO 3 electrodes to maximize the optical contrast of each electrode.
To determine optimum operating voltages for real-world applications of the dual-mode electrochromic devices, the electrochromic performance of the WO 3 and the PB electrodes at various voltage activations are investigated and presented in Figure 4. Figure 4a,b depicts the optical transmittances of the WO 3 and PB electrodes in a 1 m KCl -0.1 m ZnSO 4 electrolyte, respectively. The WO 3 electrode is transparent when it is charged to 1.0 V and colors blue when discharged at 0.1 V. It is worth noting that the maximum optical transmittance change (defined as the transmittance difference between the charged and discharged states [16] ) of the WO 3 electrode, is 71.5% (Figure 4a, without subtracting the transmittance loss of an ITO/glass substrate), which is higher than those previously reported values. [40,41] On the contrary, the PB electrode is blue colored when it is charged to 1.6 V and bleached when it is discharged to 0.8 V. The optical transmittance change of the PB electrode is 76.8% (Figure 4b), which is higher than those of previous reports. [42,43] While high optical contrast is highly sought after in electrochromic devices, it is same important to have fast color switching times. To highlight the fast-switching times of the device, the switching times between the different optical states (colored and bleached) were determined from transmittance changes at a specific wavelength (i.e., 632.8 nm) in real time. Figure 4c shows a coloration time, t c , of 3.2 s at 0.1 V and a bleaching time, t b , of 2.6 s at 1.0 V for the WO 3 electrode. As a comparison, at a midpoint voltage value of 0.5 V, t c = 5.5 s, which is slower than the coloration time of 3.2 s at 0.1 V. The voltage dependency of t c is attributed to the initial charging voltage of the electrode. The low discharging voltage accelerates the ion intercalation process, thus resulting in a shorter switching time. [44] Similar to the WO 3 electrode, the PB electrode also exhibits a relatively fast bleaching time t b = 3.6 s at 0.8 V, compared to 6.7 s at 1.0 V, while the coloration time t c = 4.3 s at 1.6 V (Figure 4d). Interestingly, the switching t c and t b times in the hybrid KCl-ZnSO 4  faster than the switching times (t c = 8.5 and t b = 7.8 for the WO 3 electrode, and t c = 8.2 and t b = 7.9 for the PB electrode) in the pure 1 m ZnSO 4 electrolyte system ( Figure S6,S7, Supporting Information). These faster switching times further confirm that the K + intercalation/extraction process is more electrochemically active, compared to that of Zn 2+ . Along with the high optical modulations and the fastswitching times exhibited by the K + /Zn 2+ electrolyte system, the coloration efficiency (CE), defined as the change in optical density (ΔOD) per unit of charge intercalated into the electrochromic layer at a particular wavelength, [40] is calculated to be 101.6 cm 2 C −1 for the WO 3 electrode in such a hybrid electrolyte system (Figure 4e). This CE value is higher than that measured in a pure 1 m ZnSO 4 (i.e., 73.6 cm 2 C −1 , Figure S8, Supporting Information), owing to the fast intercalation of K + . Furthermore, the PB electrode exhibits a CE value of 129.9 cm 2 C −1 (Figure 4f) in the hybrid electrolyte system, which is also higher than that measured in a pure 1 m ZnSO 4 (i.e., 77.4 cm 2 C −1 , Figure S9, Supporting Information).

Operation of Dual-Mode Electrochromic Devices
Together, the present findings confirm the superior electrochromic performance of the PB and WO 3 electrodes when used with a hybrid electrolyte. The next step is to investigate the device performance of the dual-mode electrochromic platform. Here, a 5 cm × 5 cm WO 3 -Zn-PB electrochromic device is assembled as depicted in Figure 2a and Figure 5a. As previously discussed, the redox potential difference (built-in voltage, 1.467 V between Zn anode and PB cathode, Figure S10, Supporting Information) allows the device to switch its color while simultaneously supplying electrical power to light an LED, as shown in Figure 5a (and Video S2, Supporting Information). The demonstrated self-color-switching feature points out the fact that this electrochromic device platform is energy efficient and operation flexible.
The optical transmittance spectra of the device, taken at different color states (fully colored, fully bleached, PB colored, and WO 3 colored), are shown in Figure 5b. At a wavelength of 632.8 nm, the maximum optical modulation of 72.6% (without subtracting the transmittance loss of the ITO-glass substrates) is higher than most of the electrochromic devices reported to date. [11,[45][46][47][48] As a key parameter when considering the suitability of the device for practical applications, fast switching between the different color states is highly desirable. Figure 5c,d shows the dynamic at 632.8 nm optical transmittance of the device along with the color switching of the WO 3 and PB electrodes. The response times, defined as the time required to achieve 90% of the maximum optical contrast, [30] are measured to be 4.0 s for coloration (at 0.1 V) and 3.8 s for bleaching (at 1.0 V) along with the color switching of the WO 3 electrode (Figure 5c; Video S3, Supporting Information). Comparable response times of 4.4 s for coloration (at 1.6 V) and 3.6 s for bleaching (at 0.8 V) are obtained, along with the color switching of the PB electrode (Figure 5d). These response times are shorter than those previously reported electrochromic devices. [12,49,50] Our results cast a new light on the novelty of the dual-mode architecture by not only demonstrating reduced energy consumption through retrieving the consumed energy along with the spontaneous color switching (self-color change and powering of external electronics), but also enabling independent operation of the top and bottom electrochromic electrodes, thus offering precise manipulation of a single electrode (i.e., to reach its maximum contrast). Figure 6 depicts the reversible color switching processes between four different color states (fully colored, fully bleached, PB colored and WO 3 colored) of the dual-mode electrochromic devices. These color states are realized either via powering a 0.5 V regulated LED or by directly connecting the colored WO 3 and PB electrodes for the discharging process, or through applying an external voltage for the charging process. For example, the device displaying a PB-colored state at the initial state (a combination of colored PB electrode and bleached WO 3 electrode), can be self-colored (via discharging) to a fully colored state via connecting the Zn and WO 3 electrodes to power a 0.5 V regulated LED. Hence after, the fully colored device is able to self-bleach itself to a fully transparent state via connecting the PB-WO 3 and the Zn-PB subsequently. Through applying an external voltage of 1.8 V between the PB and Zn electrodes, the device switches to the initial PB-colored state. Interestingly, as such a device configuration enables independent operation of the WO 3 and PB electrodes, the PB-colored device can self-bleach to a fully bleached state via directly connecting the Zn anode and the PB electrode (Video S2, Supporting Information). Although the choice of WO 3 and PB results in a blue color of the device, the design of PB analogs (e.g., cobalt hexacyanoferrate) is considered a promising platform for electrochromic devices having color neutrality. [51] Notably, the operation processes shown in Figure 6 are highly flexible and effective as their color change (discharging process) can be triggered by powering external electronics. Even though the external biases are still required for operations, such an approach is more effective and reliable for realworld electrochromic devices, compared to the previous work based on redox potential gradients, [32] thus offering a promising platform for multifunctional electrochromic devices.
Along with the exciting self-coloration and self-bleaching characteristics of the dual-mode electrochromic devices, an additional advantage offered is the enhanced device bistability, featuring a zero-energy consumption while maintaining a colored or colorless state. Such an attribute is a key function of an electrochromic device. [52] Figure 7a; Figure S11a (Supporting Information) in the Supporting Information depict the optical transmittance change of the WO 3 and PB electrodes, respectively, during 420 s under the open circuit conditions (off voltage after the operation). At a fully discharged state, the optical transmittance contrast is increased by 18.8% for the WO 3 electrode and decreased by 16.2% for the PB electrode, while showing no apparent change for both electrodes at the fully charged state. This self-charging characteristic is a common phenomenon for inorganic electrochromic materials as the intercalated guest ions (e.g., Zn 2+ , K + ) can be spontaneously extracted from the materials and diffuse into the electrolyte. [18,47] As such, a conventional complementary type electrochromic device cannot maintain its colored/colorless state after turning off the power supply for a long time. [53] On the other hand, the zinc anodebased electrochromic device platform exhibits a spontaneous color-switching behavior (i.e., discharging property) due to the high value of redox potential difference between the zinc metal anode and the electrochromic cathode. In other words, the extracted guest Zn 2+ or K + ions can be spontaneously re-intercalated into the electrochromic electrode by simply connecting the Zn anode with the electrochromic cathode (Figure 7b; Figure S11b, Supporting Information). Thus, the zinc anodebased electrochromic device platform is able to firmly maintain a fully discharged state without the need for external power input and it can also power an external electronic device. To further shed light on the device bistability, the open circuit potentials (OCP) between the Zn anode and the WO 3 /PB cathodes were measured and displayed in Figure 7c; Figure S12 (Supporting Information) in the Supporting Information, respectively. As shown in Figure 7c, in a bleached state (WO 3 charged state), the high OCP value between the Zn anode and the WO 3 cathode provides the driving potential that activates the oxidation of Zn (i.e., stripping of Zn into the electrolyte) and the reduction of the WO 3 electrode (i.e., intercalation of K + into WO 3 ), and through this color-switching mechanism, electrical energy is spontaneously supplied. On the other hand, in the colored state (WO 3 discharged state), the OCP value gradually increases with time. It is realized by the intercalated guest ions (e.g., K + ) that are spontaneously extracted from the WO 3 electrode and diffuse into the electrolyte, thus increasing OCP. By connecting the Zn anode with the electrochromic cathode, the dual-mode device is able to maintain its discharged color state and also supply electrical energy to an external load (Figure 7b; Figure S11b, Supporting Information). While the aforementioned results are remarkable for electrochromic devices, they warrant further investigation of the cycling durability of this device. As depicted in Figure 7d; Figure S13 (Supporting Information) in the Supporting Information, the capacity of the PB electrode (in a device platform) remained nearly constant for 1 000 cycles, while the WO 3 electrode maintains 50.5% of its initial capacity after 1 000 cycles.

Conclusion
In this article, for the first time, a dual-mode electrochromic device having both self-coloring and self-bleaching operations is demonstrated via coupling the zinc anode-based electrochromic platform into a rocking-chair type electrochromic device. It is demonstrated that the zinc anode-based electrochromic platform induces a self-color-switching behavior, reduces the energy consumption during operation, enables the independent operation of a single electrochromic electrode, also augments the bistability of the devices. By employing a hybrid electrolyte system of Zn 2+ /K + , the colored PB-WO 3 rocking-chair type device is shown to be capable of spontaneously bleaching by utilizing the redox potential difference between the anodic and cathodic electrodes. Such a dual-mode light-control strategy enables the electrochromic devices to possess four distinct optical states exhibiting a high optical contrast of 72.6% and fast switching times (<5 s for the bleaching/ coloration processes). The advances detailed in this article are expected to accelerate future dynamic light control technology for self-powered smart windows, colorful displays, spatial light modulators, optical sensors, and optoelectronic devices.
Preparation of the WO 3 Electrode: The W powder (1.8 g) was added to 60 mL of H 2 O 2 solution (30%) and stirred for 12 h to form a yellow peroxotungstic acid colloid. Next, the sediments were filtered to obtain a clear colloid. The clear colloid was electrodeposited onto a cleaned ITO glass substrate at −0.3 V for 180 s to obtain a WO 3 electrode. The thickness of the WO 3 film was measured to be 300 nm.
Preparation of the PB Electrode: The K 3 [Fe(CN) 6 ] (10 mM), FeCl 3 ·6H 2 O (10 mM), and KCl (50 mM) were dissolved in distilled water under stirring. Next, the electrodeposition was performed at a constant current density (−0.05 mA cm −2 , 240 s) onto a cleaned ITO glass substrate to obtain a PB electrode. The thickness of the PB film was measured to be 300 nm.
Assembly of the PB-Zn-WO 3 Electrochromic Device: The PB-Zn-WO 3 electrochromic device (5 cm × 5 cm) was constructed by sandwiching a thin Zn square frame between a PB electrode and a WO 3 electrode. The effective area of the prototype device was 9 cm 2 . The 1 m KCl-0.1 m ZnSO 4 solution was used as the electrolyte. For better comparison, 1 m ZnSO 4 solution was also used as the electrolyte for the devices.
Electrochemical Measurements and Materials Characterizations: All the electrodeposition, electrochemical and electrochromic measurements were carried out using a Zahner electrochemical workstation (Zennium CIMPS-1). The electrochemical measurements were performed using a three-electrode configuration. The dynamic of 632.8 nm optical transmittance of the electrodes/devices was conducted by directing a helium-neon laser beam onto or through the samples and onto a photodiode and the dynamic data were collected using a storage oscilloscope. The spectroscopic measurements for optical transmittance spectra were conducted using an Ocean Optics USB4000 spectrometer, without subtracting the transmission loss due to the substrate.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.