**1. Introduction**

With the continuing development of sustainable resources, devices for energy storage and conversion, such as solar cells, supercapacitors, and electrochromic (EC) devices, have attracted increasing attention [1–3]. EC devices are known to change colour via charge insertion/extraction or reversible redox reactions driven by an external electric field [4,5]. Simultaneously, the ion intercalation/deintercalation steps taking place during the reversible redox reactions of the EC process can also generate a pseudocapacitive behaviour [6,7], thereby resulting in EC devices and supercapacitors having similar working mechanisms and device structures. Based on this principle, one can envisage that these two functions could be integrated into a single electrochromic energy-storage (EES) device using the same material. As such, several EES devices have been widely explored. For example, Feng et al. [8] utilised exfoliated graphene/V2O5 as the active material of a microsupercapacitor to judge its charge-discharge state via the observed colour. In addition, Xue et al. [9] synthesised a smart EC supercapacitor device using a porous co-doped NiO

**Citation:** Fu, Y.; Yang, Y.; Chu, D.; Liu, Z.; Zhou, L.; Yu, X.; Qu, X. Vanadium-Substituted Dawson-Type Polyoxometalate–TiO2 Nanowire Composite Film as Advanced Cathode Material for Bifunctional Electrochromic Energy-Storage Devices. *Molecules* **2022**, *27*, 4291. https://doi.org/10.3390/ molecules27134291

Academic Editor: Xiaobing Cui

Received: 3 June 2022 Accepted: 1 July 2022 Published: 4 July 2022

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film as the positive electrode. This device exhibited a high specific capacitance, high energy density, and good cycle stability. After charging, these two devices were able to light up light-emitting diodes (LEDs).

Among the various EES materials reported to date, polyoxometalates (POMs) demonstrate a multi-electron reaction specificity during the electrochemical redox process, which contributes to their chromatic transitions and high-efficiency energy-storage performances [10–12]. As an example, Ma et al. [13] synthesised a POMs-based supramolecular crystalline material, namely, H3PMoVI 12O40·(BPE)2.5·3H2O (BPE = 1,2-Bis(4-pyridyl)ethylene), via a one-step hydrothermal method. The compound had a high specific capacitance (i.e., 137.5 F g−<sup>1</sup> at 2Ag<sup>−</sup>1) and good cycle stability (i.e., 92.0% after 1000 cycles) than parent H3PMoVI 12O40. This work provided an alternative method for improving the performance of POMs-based capacitor electrode materials. In addition, Wang et al. [14] reported a high-performance PW12-based EC device, wherein the optical contrast of the optimised device containing an I <sup>−</sup>/I3<sup>−</sup> redox couple in the electrolyte reached 59.4%. The PW12-EC device also showed a fast response time for bleaching and colouration. However, POM materials tend to aggregate or stack to form dense structures, which can hinder ion diffusion and affect their electrochemical properties. To overcome this issue, the incorporation of POMs into nanostructures or composite materials has been investigated to increase their surface areas [15]. For example, L et al. [16] prepared graphene oxide/W18O49 nanorod (rGO-WNd) composites through the high-temperature thermal reduction of ammonium tungstate and graphene oxide (GO). Compared with the cycle stability, capacitance, and EC properties of the pure WNd film, the corresponding properties of the Rgo-WNd composite film were significantly enhanced. This could be attributed to a higher degree of ion diffusion and the acceleration of charge transfer after the addition of rGO. As a result, the response times of such materials are improved.

Titanium dioxide is recognised as a promising candidate for EC and energy-storage applications owing to its excellent electrochemical stability, optical modulation, reversibility, and mass transport properties, as well as the fact that it enhances contact with the electrolyte and improves the resulting reaction kinetics [17]. In recent years, various TiO2 nanostructures, such as nanorods, nanotubes, and nanowires, have received attention as excellent composite materials because of their large specific surface areas and orderly structures. For example, Khanna et al. [18] fabricated a TiO2@NiTi system for use as an electrode in energy-storage applications, and this material produced a specific capacitance of ~1 F g<sup>−</sup>1. This result reveals that their system is a promising material for energy-storage applications. In addition, Ji et al. [19] designed and fabricated a novel bilayer composite with an excellent energy-storage performance by combining an aligned TiO2 nanoarray (TNA) and random TiO2 nanowires (TiO2 NWs) with a poly(vinylidene fluoride) (PVDF) matrix. A superior discharge energy density of 16.13 J cm−<sup>3</sup> was obtained for the 5 vol% TiO2 NW/TNA-PVDF composite, which was 2.0 times higher than that of the pure PVDF matrix (8.23 J cm<sup>−</sup>3). Furthermore, Lv et al. [20] synthesised TiO2 nanotube membrane electrodes that exhibited excellent EC performances, combining a high colouration contrast with a transmittance of 65% in the visible spectrum, in addition to a good cycle stability (88.2% for initial optical modulation after 1000 cycles). Zhang et al. [21] reported a novel EC device based on polyaniline nanofibers wrapped with antimony-doped tin oxide/TiO2 nanorods (ATO/TiO2@PANI film) as an EC electrode material. Compared with the pure PANI film, the EC device based on ATO/TiO2@PANI film shows better electrochromic performance.

Based on the above considerations, our group previously designed a series of POM-based EC thin film materials [22,23]. In 2020, we reported the first dual-function electrochromic-energy storage material based on POMs and TiO2 nanowires [24]. However, the response time of the film is long, and its capacitive performance is relatively low. As we know, the structure and composition of POMs have a great influence on their electrochemical activity; therefore, the electrochromic-energy storage properties could be adjusted easily by changing the type of POMs. In general, the lacunary and substituted Dawson

structures can show enhanced electrochromic performances [25]. Thus, in the current study, to improve the performances of these materials, we chose vanadium-substituted Dawsontype polyoxotungstate K7[P2W17VO62]·18H2O (P2W17V) and TiO2 nanowires to fabricate a nanocomposite thin film via hydrothermal and layer-by-layer (LbL) self-assembly methods. The microstructure of TiO2 is regulated by a hydrothermal treatment, allowing its nanowire array to be employed as the substrate for the composite film. The synergistic effects of the TiO2 NWs and the POMs could improve the EC properties of the composite film. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS) are used to investigate the surface morphology, structure, and chemical properties of the obtained nanocomposite film. Finally, the EC and energy-storage properties of the composite film are compared with those of the pure P2W17V-modified fluorine-doped tin oxide (FTO) film.
