*3.2. EC Performance*

To explore the potential of the prepared composite material for application as EC supercapacitor, its EC properties were investigated and compared with those of the FTO−P2W17V film. As demonstrated in Figure 4a–b, the transmittance was reduced along potentials ranging from 0 to −1.0 V. In addition, as shown in Figure 4c, the maximum transmittance modulation of the NW−P2W17V film (38.32%) was significantly higher than that of the FTO−P2W17V film (22.25%) at 580 nm, thereby indicating that the effective combination of two cathodic EC materials could indeed improve the overall performance. For the switching kinetics, the fast switching speed (i.e., the time required to achieve 90% of full modulation) for each of the two prepared films was determined, as shown in Figure 4d. Notably, FTO−P2W17V (tc = 1.49 s and tb = 1.65 s) and NW−P2W17V (tc = 1.65 s and tb = 1.64 s) films could undergo relatively rapid colouring and bleaching processes, which are important processes in the context of EC applications. Furthermore, as shown in the optical photograph presented in Figure 4e, the P2W17V-modified film turned blue, and became deeper in colour upon increasing the applied potential; this colour was attributed to the intervalence charge-transfer band (WV–O–WVI or WVI–O–WV). The transmittance showed a good linear relationship with the applied potential, indicating that the colouration state could be adjusted precisely, thereby rendering this system suitable for practical use in industry.

**Figure 4.** Visible transmittance spectrum of FTO−P2W17V (**a**) and NW−P2W17V (**b**) films at different potentials. (**c**) Visible spectra of prepared films at colored and bleached state; (**d**) Chronoamperometry measurements and corresponding in situ optical transmittance curves for FTO−P2W17V and NW−P2W17V films at 580 nm; (**e**) Plots of the transmittance value versus applied voltage for NW−P2W17V and corresponding optical images; (**f**) Coloration efficiency at 580 nm of NW−P2W17V and FTO−P2W17V films during subsequent double-potential steps (−1 V and +1 V); Cycle stability of FTO−P2W17V (**g**) and NW−P2W17V films (**h**) at 580 nm under square wave potentials of −1 V and +1 V.

The CE is a crucial factor in evaluating the correlation between the change in colour and the number of injected charges. The CE can be calculated from Equations (1) and (2) [31–33]:

$$\text{CE} = \Delta \text{OD} / (\text{Q}/\text{A}) \tag{1}$$

$$
\Delta\text{ODD}(\lambda) = \log \text{T}\_{\text{b}} / \text{T}\_{\text{c}} \tag{2}
$$

where Q is the charge density, A is the area of the composite film, and Tb and Tc are the transmittances of the film in the bleached and coloured states at a certain wavelength (λ), respectively. Figure 4f shows the variation in the optical density with respect to the extent of electric charge exchange from the electrolyte to the EC film. The CE can be obtained from the slope of the line that fits the linear region of the plot. Thus, the CE values of samples were calculated to be 116.5 cm<sup>2</sup> C−<sup>1</sup> for NW−P2W17V and 15.2 cm<sup>2</sup> C−<sup>1</sup> for FTO−P2W17V, wherein the larger value obtained for the NW−P2W17V system indicates that a large transmittance modulation can be realised through the introduction of a small amount of charge.

The electrochemical stability of a film is vital for determining its EC performance. Thus, the cycling stabilities of the FTO−P2W17V and NW−P2W17V films were tested by chronoamperometry at 580 nm over 1000 cycles. As shown in Figure 4g–h, NW−P2W17V exhibited a superior cycling stability with an initial transmittance variation of approximately 38.32%, wherein ~86% of the initial value was retained after 1000 cycles. This outstanding cycling stability should permit long-term application in real environments.

#### *3.3. Energy-Storage Performance*

The electrochemical performances of the thin films were then evaluated using CV and galvanostatic charge-discharge (GCD) tests. Figure 5a shows the CV curves of the NW−P2W17V film measured at different scan rates, wherein it can be seen that upon increasing the scan rate from 50 to 150 mV s<sup>−</sup>1, no obvious changes in shape were observed for the CV curves, although the peak potential moved slightly. The presence of characteristic symmetric reversible peaks for the NW−P2W17V film also indicate its good capacitive behaviour upon ion insertion/extraction. Furthermore, the inset of Figure 5a shows a good linear relationship between the current density and the scan rate, indicating a fast electron transfer kinetic characteristic in these redox-active materials, which therefore represents a typical surface-controlled process. Figure 5b shows the CV curves of the NW−P2W17V and FTO−P2W17V films obtained using a three-electrode system at the same scan rate in a solution of HOAc-NaAc at pH 3.5. The composite film displayed three pairs of redox peaks, which can be attributed to the redox reaction between WVI and WV, indicating a typical faradic behaviour. The redox peaks of the NW−P2W17V film have higher peak current values than those of the FTO−P2W17V film, indicating the high conductivity and low internal resistance of the NW−P2W17V film. These increased peak current values can be attributed to the influence of faradaic reactions and to hydrogen ion (H+) intercalation at the electrode/electrolyte interface.

**Figure 5.** (**a**) CV for the NW−P2W17V film at different scan rates (from inner to outer): 50, 70, 90, 110, 130, and 150 mV s<sup>−</sup>1. The inset shows plots of the anodic and the cathodic peak currents for C-c against scan rates; (**b**) CV for NW−P2W17V and FTO−P2W17V films at a scan rate of 50 mV/s; (**c**) Charge/discharge curves of NW−P2W17V film at various current densities; (**d**) Volumetric capacitance at various current densities of NW−P2W17V and FTO−P2W17V films; (**e**) In situ transmittance evolution at 580 nm with the charging and discharging process of the NW−P2W17V film; (**f**) Cycle performance of NW−P2W17V film measured under a current density of 0.2 mA cm<sup>−</sup>2.

The diffusion coefficient of H<sup>+</sup> ions for insertion and extraction can be estimated based on the measured peak current, Ip (A) [34,35]:

$$\mathbf{I}\_{\mathrm{P}} = 2.69 \times 10^{5} \mathrm{AC} \sqrt{Dvn^{3}} \tag{3}$$

where Ip is the peak current, A is the area of the film (cm2), *n* is the number of electrons, *D* is the diffusion coefficient of the H+ ions (cm<sup>2</sup> s<sup>−</sup>1), C is the concentration of the H+ ions in the electrolyte solution (mol cm<sup>−</sup>3), and *v* is the scan rate (V s<sup>−</sup>1). The diffusion rate of H+ in NW−P2W17V was faster than that in FTO−P2W17V. This enhanced diffusion rate for NW−P2W17V therefore accounted for the superior electrical conductivity of this material.

Owing to their fast ion intercalation/deintercalation properties and excellent cycling stabilities, we envisaged that the composite films could have great potential for use in energy-storage applications. Thus, to further evaluate the capacitive behaviours of the composite films, a series of GCD measurements were carried out at different current densities. Figure 5c shows the potential responses of the NW−P2W17V film under different currents, in addition to the dependence of the volumetric capacitance of the composite film on the current density. The GCD curves collected under different current densities are displayed in Figure 5c, which shows that the shapes of the CD profiles were essentially retained for

all the applied current ranges, demonstrating the superior charge/discharge reversibility of the sample [36]. Plateau regions are observed in the GCD curves, and the positions of the three plateaus are consistent with the CV curves, thereby indicating that the capacitance is mainly caused by the faradaic redox reaction, whereas the existence of plateaus in the curves illustrates a sound pseudocapacitive behaviour [37,38]. The calculated volumetric capacitance as a function of the current density is shown in Figure 5d. The volumetric capacitance gradually declines as the current density increases, mainly because the limited ion diffusion rate is inaccessible, and so adequate surface redox reactions of the active materials cannot be ensured at high current densities. Furthermore, the value obtained for the NW−P2W17V film was higher than that of the FTO−P2W17V film, which was ascribed to the interactions and synergistic effects between the P2W17V and TiO2 NW materials. Furthermore, the GCD curves at 0.3 mA cm−<sup>2</sup> and the corresponding in situ transmittance at 580 nm were collected and plotted in Figure 5e. During the charging process, the NW−P2W17V electrode gradually became coloured, and the decrease in transmittance was distinguishable. In contrast, the colour of the electrode was reversibly bleached during the discharge process.

The long-term cycling stability is another vital index for evaluating the properties of electrode materials [39,40]. As shown in Figure 5f, the NW−P2W17V film revealed an excellent cyclic stability with its volumetric capacitance being almost fully maintained after 1000 cycles at 0.2 mA cm−<sup>2</sup> in a voltage range of −0.5 to 0.2 V.

Subsequently, electrochemical impedance spectroscopy (EIS) was employed to investigate the inner resistances and capacitance properties of the thin films [30]. Figure 6a shows the Nyquist plots of the NW−P2W17V and FTO−P2W17V films with a frequency range of 0.01–100,000 Hz and a signal amplitude of ±5 mV. The electrode system can be described by a simple equivalent circuit (see the inset of Figure 6a), which was selected to fit the obtained impedance data for the NW−P2W17V composite film. The high-frequency part of the semicircle in the EIS spectrum indicates the speed of the electron transfer process, and the diameter is closely related to the electron transfer resistance (Rct). The Rct of the FTO−P2W17V film was significantly smaller than that of the NW−P2W17V film, indicating the lower Rct and the higher electron transfer rate of NW−P2W17V composite film. As outlined in Figure 6b, we constructed an EES device using LiClO4/PC as the electrolyte, the NW−P2W17V composite film as the negative electrode, and FTO as the positive electrode. Importantly, this EES device was capable of lighting a red LED (Figure 6c). After charging for 10 s, the device became dark blue in colour, and the system lit the red LED for a total of 20 s. These results indicate that the energy-storage states were directly reflected by the colour change. More specifically, as the charge stored inside the device increased, its colour deepened. Overall, these observations verify the potential practical application of our device in energy-storage smart windows and visual monitoring systems.

**Figure 6.** (**a**) The EIS figures of the films with different components FTO−P2W17V and NW−P2W17V film, the inset shows a simple equivalent circuit about the NW−P2W17V electrode system; (**b**) Structural diagram of the solid-state EC device architecture used in this work; (**c**) Photo of a red LED lit up and out by a solid-state EC device.
