Construction And Performance Study Of Polyoxotungstate-based Electrochromic/Energy Storage Bifunctional Electrode Materials

With the rapid development of the global economy,energy and environmental issues are becoming increasingly prominent,and there is an urgent need to develop new energy sources that are efficient,clean and sustainable,as well as new technologies associated with energy conversion and storage.Energy materials and energy storage and energy saving devices have become a current research hotspot.Electrochromic energy storage devices are capable of supplying power externally and also change color in real time and intuitively with their own energy storage state to reflect the energy storage state,which has high application value in the field of electrochromic smart windows,wearable devices and flexible displays.Polyoxometalates（POMs）are metal-oxygen clusters consisting of high-valent former transition metals bonded by bridging oxygen atoms.POMs have good redox and proton conductivity,multi-electron storage and transfer capability,and are a pseudocapacitive material with both negative potential window and high specific capacitance.At the same time,POMs are also a cathodic electrochromic material with diverse structures and easily tunable properties.Therefore,electrochromic energy storage bifunctional materials and devices based on POMs have great research value and application prospects,and there are few reports in this area at home and abroad.In this paper,a series of Wells-Dawson structured polymetallic tungstates were selected to construct three electrochromic/energy storage bifunctional composite film materials based on POMs and titanium dioxide nanowires（TiO₂NW）using a combination of hydrothermal synthesis and layer-by-layer self-assembly（LbL）techniques.Explore the microstructure design and directed synthesis mechanism of composite materials;explore the electrochemical mechanism of electrochromic and energy storage of materials,and coordinat the performance requirements of electrochromic and energy storage of materials.Studying the enhanced mechanism of response speed,optical modulation range,energy storage performance and cyclic stability of functional materials can lay a theoretical and experimental foundation for the preparation of multifunctional electrochromic-energy storage devices.The main contents are as follows:1.NW-P₂W₁₇ nanocomposite film based on monolacunary polyoxotungtate cluster K₁₀P₂W₁₇O₆₁ and TiO₂ nanowires was fabricated.XPS and EDS illustrate that the composite film consists of P₂W₁₇ and TiO₂NW;SEM and TEM show that the FTO glass substrate was covered with TiO₂NW grown vertically and POMs nanoparticles are deposited on the nanowires,and the thickness of NW-P₂W₁₇ is about 560 nm.Compared to FTO-P₂W₁₇thin films,the nanocomposite films showed outstanding electrochromic properties of greater transmittance modulation（33.5%）,higher coloration efficiency(150.34 cm² C^-1 at 600 nm)and cyclic stability（electrochromic and energy storage can both reach 1000 cycles）.At a current density of 0.14 m A cm^-2,the volumetric capacitance of NW-P₂W₁₇ reaches 172.3 F cm^-3 with a working voltage window of 1.77 V.2.NW-(P₂W₁₇)₂Ce nanocomposite film based on monocerium substituted sandwich-type polyoxotungstate cluster(P₂W₁₇)₂Ce and TiO₂ nanowires was fabricated.NW-(P₂W₁₇)₂Ce has a microstructure similar to that of NW-P₂W_17.Compared to FTO-(P₂W₁₇)₂Ce thin films,the nanocomposite films showed outstanding electrochromic properties of greater transmittance modulation（46.66%）,higher coloration efficiency(84.02 cm² C^-1 at 580 nm)and cyclic stability（electrochromic and energy storage can both reach 1000 cycles）.At a current density of 0.2 m A cm^-2,the volumetric capacitance of NW-(P₂W₁₇)₂Ce reaches 195 F cm^-3 with a working voltage window of 1.1 V.3.NW-P₂W₁₇V nanocomposite film based on one vanadium atoms substituted polyoxotungstate cluster P₂W₁₇V and TiO₂ nanowires was fabricated.Compared to FTO-P₂W₁₇V thin films,the nanocomposite films showed outstanding electrochromic properties of greater transmittance modulation（38.32%）,higher coloration efficiency(116.51 cm² C^-1 at 580 nm)and cyclic stability（electrochromic and energy storage can both reach 1000 cycles）.At a current density of 0.2 m A cm^-2,the volumetric capacitance of NW-P₂W₁₇V reaches 297.1 F cm^-3 with a working voltage window of 1.1 V.4.An electrochromic energy storage device was assembled with NW-P₂W₁₇V composite film modified electrode as the negative electrode,FTO glass as the positive electrode,and LiClO₄/PC as the solid electrolyte.During the charging process,the device turned its color from colorless to a dark blue,enabling visualization of the energy storage state;and the system lightened the red LED light for 20 s.The above results show that all nanocomposite films have significantly enhanced electrochromic and energy storage properties compared to individual POMs films.These excellent electrochemical properties are from the unique three-dimensional structure of TiO₂NW,which allows the POMs to avoid stack and aggregation during the layer-by-layer self-assembly,improving the specific surface area of the nanocomposite film material,increasing the number of active sites during the redox reaction,shortening the ion diffusion path,and accelerating the insertion/withdrawal rate of electrolyte ions,the electrochemical performance of the POMs material was improved;In addition,both POMs and TiO₂NW are cathodic electrochromic materials,which can produce synergistic effects and lead to significantly improved electrochromic performance.This study provides a valuable theoretical and experimental basis for the development of POMs-based electrochromic energy storage multifunctional materials and devices.