Preparation Of POMs/MXene Composite Films And Its Application In Electrochromic Devices

Polyoxometalates(POMs)are transition metal oxides(TMOs)of d~0 or d~1 metal ions bridged by oxygen atoms.POMs have reversible redox and structural stability properties,and simultaneously undergo color changes in the process of redox,so POMs are also considered to be a promising electrochromism(EC)material.POMs have high solubility in water and organic solvents,and EC films can usually be prepared by wet film-forming methods,such as spin coating,spray coating,and electrostatic self-assembly(LBL).However,the high solubility in the POMs solvent and its poor electrical conductivity lead to the problem of poor electrical conductivity of the POMs EC films prepared by the above methods.In response to this,researchers proposed(1)composite POMs with anode conductive materials(such as polyaniline,polypyrrole);(2)immobilize POMs in carbon materials(such as activated carbon,graphene);(3)with metal Organic complex(MOC)binding and other methods.These methods for improving the conductivity of POMs films are all effective,but the color of the materials used to improve the conductivity is darker,which usually leads to the problem of increasing the conductivity and decreasing the transmittance of the film.For EC films generally used as transparent electrodes,the transmittance of the film has a huge impact on the light modulation range of the EC process and the transmittance in the faded state.Therefore,POMs EC films need to improve their electrical conductivity without sacrificing transmittance.Transition metal carbonitride(MXene)is a new type of carbon material that has emerged in recent years.It has metal-like ultra-high electrical conductivity and good capacitance properties,and has become a high-performance 2D material after graphene.The surface of MXene is negatively charged and has-OH functional group,which can be strongly combined with POMs after cationic modification.In the future,the POMs-MXene composite film with higher transparency can be obtained by controlling the film forming conditions,which provides a new idea for the preparation of POMs-based transparent composite film with high electrical conductivity.In Chapter 2,Al in titanium aluminum carbide(Ti₃AlC₂)was etched using hydrochloric acid(HCl)and lithium fluoride(LiF)to form Ti₃C₂T_x MXenes.Then,Ti₃C₂T_x MXene was compounded with P₂W₁₈ POMs by polyvinyl alcohol(PVA)solution and the POMs/PVA/MXene(PPM)EC transparent films were prepared by spin coating.Comparative experiments proved that the addition of MXene enhanced the electrochromic and charge storage properties of the films.The measured area capacitance of the PPM film was 10.86 m F cm^-2(current density 0.1 m A cm^-2),the initial transmittance was 96%,the optical contrast(?T)was 82%,and it was transparent after 17 h of coloration under open circuit.The rate is only reduced by15%,which fully demonstrates the excellent electrochemical and electrochromic properties of the PPM composite film.Then,a flexible EC film was prepared by spin coating on the ITO/PET flexible substrate,which also showed excellent performance.The areal capacitance is 11.28 m F cm^-2 and the?T is 70%at 576 nm.In Chapter 3,the PPM composite film was used as the working electrode,the MnO₂ film was electrochemically deposited on the FTO glass as the counter electrode and the charge balance layer,and the LiClO₄/PC solution was used as the electrolyte to assemble an electrochromic supercapacitor(ECCD).The tested PPM-MnO₂ device has an?T of 65%and an areal capacitance of 2.73 m F cm^-2,which provides excellent performance.Subsequently,the PMMA-Li ClO₄/PC gel electrolyte was synthesized and a solid-state device was assembled.The solid-state device has an ultrafast response time(coloring time of 0.4 s,fading time of 0.5 s)and an areal capacitance of1.25 m F cm^-2.