Preparation And Electrochromic Properties Of WO₃-NiO Solid State Devices With High Charge Capacity

Electrochromism is a kind of phenomenon which has reversible changes in optical properties on the surface of the materials.Electrochromic（EC）materials can be reversibly changed through the extraction/insertion of ions by applying an external potential.The common electrochromic materials include transition metal oxides,conductive polymers,organic small molecules and complexes,which are widely used in smart windows of energy efficient buildings,spacecraft thermal control coatings,and antiglare rearview mirrors.This dissertation is oriented to the requirements of smart windows for energy efficient buildings.In order to solve the mismatch of charge capacity between the electrochromic layers and ion storage layers,the EC properties of Ni O films in different electrolyte are investigated.In this dissertation,the electron beam evaporation method is used to prepare Ni O films.These Ni O films are cycled in different electrolyte,such as Al（Cl O₄）₃-PC,Li Cl O₄-PC,Na Cl O₄PC,TMAH-PC,[BMIM]Cl-PC solution,in order to figure out the influences of different cations(Al³⁺,Li⁺,Na⁺)and anions（OH^-,Cl^-）on the electrochromic properties of Ni O films,such as charge storage capacity,optical modulation and response speed are investigated.Ni O films have similar charge capacities in Al³⁺,Li⁺and Na⁺electrolytes,and the maximum coulomb efficiency（49%）,optical modulation（15.9%）and the fastest response speed can be obtained in Li⁺electrolyte.The charge capacity of Ni O film in the process of coloration is 3.65m C/cm²,which is 50%higher than that of Ni O film in Li⁺electrolyte.According to the above results,the electrochromic reaction process of Ni O film in different electrolytes can be described.The electrochromic reaction process of Ni O films in Al³⁺,Li⁺,Na⁺and Cl^-electrolytes is the result of the interaction of the shallow insertion of electrolyte ions and the surface reaction,while it is only the surface reaction process of Ni O films in OH^-electrolytes.It is proved that the influence of electrolyte ions on the electrochromic properties of Ni O films is related to the ion radius and the ion valence state.The composite electrolyte of[BMIM]Cl and Li Cl O₄is composed.When the[BMIM]Cl content is 15 wt.%,the best EC performances of Ni O films can be obtained.The coloration charge capacity is 12.4 m C/cm² and the optical modulation at visible region is 48%.The optical modulation of electrochromic devices（ECDs）using composite electrolyte is 54.8%.Furthermore,in order to solve the problem of charge capacity mismatch in WO₃-Ni O devices,a new electrochemical reaction is introduced on the Ni O films by the anion-assisted method by adding 2 wt.%～8 wt.%of H₂O to Li⁺electrolyte,or pretreating Ni O film using alkali solution.The electrochromic performances of Ni O films in the electrolyte with different content of water are investigated.High charge capacity（7 times higher than that in anhydrous electrolyte）and high optical modulation（59.5%at 511 nm）of Ni O films can be obtained.Due to the low content of OH^-in the electrolyte and the formation of Ni OOH fixed on the Ni O film,the WO₃films in the device can be stable without corrosion.A solid-state electrochromic device with the structure of ITO/WO₃/electrolyte/Ni O/ITO is assembled,which has a large optical modulation of 62%and a long-life stability up to 10,000 cycles without attenuation.The influence of cycling temperature and applied voltage on the charge capacity and EC performances of WO₃,Ni O films,and WO₃-Ni O devices are investigated in this dissertation.For WO₃ and Ni O films,the increase of cycling temperature and applying voltage can improve the charge capacity and response speed of them.The influence of cycling temperature on the bleached state transmittance of WO₃ films is more significant,while the influence of applying voltage of WO₃ films is on the colored state transmittance.With the increase of cycling temperature,the charge capacity of ECDs gradually increases,which is similar to the charge capacity of Ni O film,but much less than that of WO₃ film.At different temperatures,the diffusion coefficient of Li⁺in the device follows the Arrhenius relation.With the increase of cycling temperature,the diffusion coefficient of Li⁺in the devices are increased gradually.The effect of applied voltage on the device charge capacity is also significant.At the same temperature,the charge capacity of ECDs is gradually increased with the increase of applied voltage.The cycling temperature has significant effects on the EC performance.At the temperature of-20°C,the optical modulation of ECDs is attenuated rapidly to 2%.While the cycling temperature rises to 50°C,the performances of the ECDs are recovered.According to the diffusion-controlled reaction model,the electrode potential required for electrochromic reaction at high temperature will be much lower,so that Li⁺is easier to overcome the detrapping energy barrier and escape from the deeply trapped electrochromic films at high temperature,so as to achieve the purpose of performance recovery of the electrochromic film and device.