The preparation processes of the WO₃ and NiO thin-film electrodes are described as follows: First, the optically transparent Sn-doped indium oxide (In₂O₃:Sn, or ITO) was prepared on glass or PET substrates by pulsed dc magnetron reactive sputtering with our homemade instrument. The ITO electrodes were cleaned ultrasonically in DIW for 1 min. WO₃ and NiO thin film were deposited onto the ITO electrodes by sputtering for 2 and 1 hr, respectively. The thickness of ITO, WO₃ and NiO thin films were 110, 70, and 60 nm, which were measured by a profilometer. The electroactive area of the WO₃ and NiO thin-film electrodes were 2.0 × 2.0 cm².Copper tape was applied to one side of the ITO as the bus bar. The WO₃ and NiO thin-film electrodes were stored in air at room temperature before device assembly. To record the spectroelectrochemical properties of the thin-film electrodes, apotentiostat/galvanostat (CHI 900B, CHI, USA) and aspectrophotometer (SolidSpec-3700, Shimadzu, Japan) were used.

Before the device assembly, both the WO₃ and NiO thin-film electrodes were pretreated with the activation process as follows: The WO₃ thin-film electrode was cycled several times in anelectrolyte using the cyclicvoltammetric (CV) method in a two-electrode electrochemical system, to ensure the function of the lithium ion insertion and extraction. The counter electrode was a platinum electrode. The electrolyte was composed of 0.1 M LiClO₄ and PC. For the NiO thin-film electrode, a solution containing 1 M NaOH and deionized water was used as the electrolyte. The NiO thin-film electrode was cycled back and forth until a stable current signal was observed. The detail of the ECD assembly is described as follows: a 0.1 ml monomer solution of the polymeric electrolyte was dropped onto the WO₃ thin-film electrode and then covering it with the NiO thin-film electrode. The UV-cureable monomer solution contained 15 wt% of ETPTA and 85 wt% of PC with 0.1 mM Fc and 1 M LiClO₄. The sandwich-like ECD was exposed under UV-irradiation for 60 s and then a quasi-solid-state ECD was formed. The ECD was allowed to cool down to the room temperature, and then was sealed with a UV- curable blocking adhesive around its four edges.

Figure 1 shows the CV analyses and the UV-v is transmittance spectra of the WO₃ thin-film electrodes during the activation processes. In Figure 1(a), a typical asymmetrical CV curve of the redox reaction of WO₃ can be observed. The redox reaction of the WO₃ can be represented by the following equations [16]:

To ensure the smooth ion insertion and extraction at the WO₃ thin film, it was scanned between −3.3 and 2 V

for 100 cycles in a two-electrode electrochemical system. When the potential was more negative than −0.5 V, the Li ion inserted into the WO₃ thin film and color changed to light blue slightly. The color of the film became deep blue at the applied potential of −3.3 V and then changed to transparent during a reverse scan. During the 100-cycle pretreatment, the current density of the electrode became stable, and it indicated that the equilibrium of the ion insertion and extraction was achieved. The transmittance difference between the bleached state and the colored state of the WO₃ thin film was performed in Figure 1(b). The transmittances at 550 nm of the fully bleached and colored state of the WO₃ thin film were 76% (at 1.5 V) and 20% (at −3.5 V), respectively.

Figure 2(a) shows the CV curves of NiO thin-film electrode during the activation processes. The electrode was cycled between −1.5 and 1.5 V in a two-electrode electrochemical system. An obvious increase of the reduction current can be observed in the first ten cycles of the CVs, which can be attributed to the redox reaction of NiO thin film, as represented by Equation (2) and (3) [16]:

During the continuous scanning of the NiO thin film in the alkaline solution, NiO transformed to Ni(OH)₂ gradually and became more electrochemically active. Thus, the reduction peak at −0.75 V became larger with the increasing cycle number. The color of the NiO thin film changed reversibly from dark brown (at 1.5 V) to transparent (at −1.5 V). From the transmittance spectra shown in Figure 2(b), a transmittance peak of NiO at 900 nm could be observed. The transmittances at 900 nm of the fully bleached and colored state of the NiO thin film were 75% (at −1.5 V) and 45% (at 1.5 V). The largest transmittance difference was 38% at 600 nm.

Figure 3 shows the schematic diagram of the quasi-solid-state ECD, and the inset is the polymeric electrolyte. The polymeric electrolyte was composed of EPTPA, LiClO₄, ferrocene and PC. After exposed to UV irradiation, the freestanding electrolyte film was obtained. The color of the polymeric electrolyte is yellowish because ferrocene was added into the electrolyte. This quasi-solid-state polymeric electrolyte made it more convenient to assemble the ECD. The CV responses of the WO₃/polymeric electrolyte/NiO ECD are shown in Figure 4. The ECD was scanned between −2.5 and 1.5 V at a scan rate of 20 mV・s⁻¹. The figure shows two sets of redox reactions, which can be distinguished by comparing to the CVs of WO₃ and NiO thin films. The first set of the redox reaction at the voltage between −2.5 and 0 V is contributed by the WO₃. When the applied voltage was more negative than −2.0 V, the color of the device changed from yellowish to blue. When the cell voltage was switched to 0.5 V, the NiO redox reaction occurred, and the color of the device changed to yellowish again. The optical properties of the ECD were also checked by using a spectrophotometer, and the results are illustrated as Figure 5. Two ECDs were investigated in this experiment to realize the effect of ferrocene in the polymeric

electrolyte. For the ECD without ferrocene, the colored state and bleached state were at −2.5 and 1.9 V. Besides, the transmittance attenuation (ΔT) at 550 nm was 22%. For the ECD with ferrocene in the electrolyte, the ΔT was 37.5%, which was 15.5% higher than that of the ECD without ferrocene. This was because that the introduction of ferrocene changed the safe operation voltage window of the ECD. When the applied voltage was higher than 1.9 V for the ECD without ferrocene, the side reaction occurred and resulted in the damage and instability of the polymeric electrolyte. However, while adding ferrocene into the electrochromic system, the bleaching voltage could be extended to 2.5 V without causing damage to the polymeric electrolyte. Therefore, the NiO thin film could be reduced completely, and thus the transmittance of the ECD could be further improved to 47.5% at 550 nm.