**4. Conclusions**

In this work, a suitably designed nanocomposite film composed of vanadium-substituted Dawson-type POMs were fabricated on a TiO2 nanowire array substrate. Compared with the dense packing structure, the core—shell nano structure exhibited enhanced EC and electrochemical properties with significant optical contrast (38.32% at 580 nm), short response time (1.65 and 1.64 s for colouring and bleaching, respectively), and satisfactory volumetric capacitance (297.1 F cm−<sup>3</sup> at 0.2 mA cm<sup>−</sup>2), which mainly originate from the unique three-dimensional structure of a nanocomposite with low tortuosity and a high specific surface area. TiO2 NW not only provided a transparent substrate with greater adhesion, but it also shortened the electrons/ions diffusion pathway, resulting in uniform and fast reaction kinetic characteristics. A solid-state EES device was fabricated using the composite film as the cathode. In terms of its potential practical applications, the developed device was demonstrated to light up a red LED, and the energy-storage state of the device was easily monitored by observing its change in colour, so as to achieve the purpose of real-time monitoring, and avert the damage caused by overcharging and over-discharging to the supercapacitor. These results therefore confirm the promising features of POM-based EES devices and demonstrate their potential for use in a wide range of multifunctional supercapacitors, such as self-charging supercapacitors, smart energy storage windows, and electrochromic supercapacitors.

**Supplementary Materials:** The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/molecules27134291/s1, Figure S1: The IR spectra of K7[P2W17VO62]·18H2O; Figure S2: The UV-vis spectra of K7[P2W17VO62]·18H2O; Figure S3: CV curve of K7[P2W17VO62]·18H2O in HOAc-NaOAc solution (pH = 3.5); Figure S4: The SEM images of FTO−P2W17V (inset: the cross-sectional images of prepared films); Figure S5: The SEM images of TiO2 NW (inset: the cross-sectional images of prepared films); Figure S6: 2D AFM images of (a) FTO−P2W17V and 3D AFM images of (b) FTO−P2W17V; Figure S7: High-resolution XPS spectra for C1s (a), V2p (b) and P2p (c) of FTO−P2W17 film; Figure S8: EDS elemental mapping patterns of P in the NW−P2W17V film.

**Author Contributions:** Conceptualization, X.Q. and Y.Y.; methodology, X.Q. and Y.Y.; validation, Y.F., D.C. and X.Q.; formal analysis, D.C. and L.Z.; investigation, L.Z., Z.L. and X.Y.; resources, X.Q. and Y.Y.; data curation, Y.F. and X.Q.; writing—original draft preparation, Y.F.; writing—review and editing, X.Q.; funding acquisition, X.Q. and X.Y. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was financially supported by the National Natural Science Foundation of China (22071080, 21902058) and the Natural Science Foundation of Jilin Province (YDZJ202101ZYTS175).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Data are contained within the article and supplementary materials.

**Conflicts of Interest:** The authors declare no conflict of interest.

**Sample Availability:** Samples of the compounds are available from the authors.

#### **References**

