*Article* **Vanadium-Substituted Dawson-Type Polyoxometalate–TiO2 Nanowire Composite Film as Advanced Cathode Material for Bifunctional Electrochromic Energy-Storage Devices**

**Yu Fu, Yanyan Yang \*, Dongxue Chu, Zefeng Liu, Lili Zhou, Xiaoyang Yu and Xiaoshu Qu \***

College of Chemical and Pharmaceutical Engineering, Jilin Institute of Chemical Technology, Jilin 132022, China; fuyu1996211@163.com (Y.F.); chudongxue1997@163.com (D.C.); lzf122321@163.com (Z.L.); z13069189739@163.com (L.Z.); yangyangyu@jlict.edu.cn (X.Y.) **\*** Correspondence: yyy200409@163.com (Y.Y.); xiaoshuqu@jlict.edu.cn (X.Q.)

**Abstract:** Polyoxometalates (POMs) demonstrate potential for application in the development of integrated smart energy devices based on bifunctional electrochromic (EC) optical modulation and electrochemical energy storage. Herein, a nanocomposite thin film composed of a vanadiumsubstituted Dawson-type POM, i.e., K7[P2W17VO62]·18H2O, and TiO2 nanowires were constructed via the combination of hydrothermal and layer-by-layer self-assembly methods. Through scanning electron microscopy and energy-dispersive spectroscopy characterisations, it was found that the TiO2 nanowire substrate acts as a skeleton to adsorb POM nanoparticles, thereby avoiding the aggregation or stacking of POM particles. The unique three-dimensional core−shell structures of these nanocomposites with high specific surface areas increases the number of active sites during the reaction process and shortens the ion diffusion pathway, thereby improving the electrochemical activities and electrical conductivities. Compared with pure POM thin films, the composite films showed improved EC properties with a significant optical contrast (38.32% at 580 nm), a short response time (1.65 and 1.64 s for colouring and bleaching, respectively), an excellent colouration efficiency (116.5 cm<sup>2</sup> C<sup>−</sup>1), and satisfactory energy-storage properties (volumetric capacitance = 297.1 F cm−<sup>3</sup> at 0.2 mA cm<sup>−</sup>2). Finally, a solid-state electrochromic energy-storage (EES) device was fabricated using the composite film as the cathode. After charging, the constructed device was able to light up a single light-emitting diode for 20 s. These results highlight the promising features of POM-based EES devices and demonstrate their potential for use in a wide range of applications, such as smart windows, military camouflage, sensors, and intelligent systems.

**Keywords:** polyoxometalate; TiO2 nanowire; composite film; bifunctional electrochromic energy storage
