Investigation Of Cobalt Ions Doped Tungsten Oxide Electrochromic Films And Exploration Of Various Counter Electrodes In Electrochromic Devices

Electrochromism(EC)refers to a phenomenon that materials can be reversibly changed in optical properties by applying a suitable potential difference.When thin films composed by these materials undergo an electric field assisted migration of ions and electrons,optical modulation such as transmittance,absorbance,reflectance and emittance could be achieved with their variation in oxidation or reduction states.A typical electrochrmic device(ECD)consists of two transparent substrates that sandwich five functional layers,including one transparent conductive layer,an electrochromic(EC)layer,an electrolyte layer,an ions storage layer and another transparent conductive layer.Wherein,the EC layer acts as the significant role and dominates the EC performance of the device.Among all the EC materials,WO3 and polythiophene derivatives have been recognized as the most significant electrochromic materials and been widely studied.In this thesis,we focus on trying to further improving the electrochromic performance of WO3 films and developing unconventional counter electrodes in electrochromic devices.WO3 usually exists as crystalline and armorphous states due to their preparation process.It has been generally believed that the amorphous WO3 films are more capable of optical modulation than crystalline one.For ECDs,the counter electrodes(CEs)always play a vital role,and they will determine the performance of the device.Traditional ions storage layer in generally,are nickel oxide(NiO),Prussian blue(PB),vanadium pentoxide(V2O5),but these CEs based ECD exhibit relative insufficient transmittance modulation and long-term cycle stability,thus will limit their further practical applications.Hence,this thesis focuses on crystalline WO3 films and new types CEs.First,Co ions doped WO3 films are introduced with modified morphology and enhanced electrochromic(EC)properties by seed free hydrothermal method.Effects of Co dopant on the structure and electrochromic performance of the WO3 films are systematically investigated.It is found that compared with the pure WO3 film,Co ions doped WO3 films present significant variation in surface morphology,and also exhibit much higher transmittance modulation(75.4%at 680 nm and 76.7%at 890 nm),higher CE value(65.7 cm2 C-1)and better long-term stability(over 4000 cycles).The explanation of the enhanced EC performance is mainly attributed to the vertically aligned nanostructure of the films,which provided larger specific surface area,larger number of accessible intercalation sites and rapid ions diffusion.Thus,these Co-doped WO3 nano wire arrays films may offer a promising way for developing materials with high EC performance.Second,a PECD consisted of carbon quantum dots(CQDs)sensitized TiO2 as photoanode,poly(3,4-(2,2-dimethylpropylenedioxy)thiophene)(PProDOT-Me2)film as electrochromic(EC)electrode,Br-/Br3-with Li+as electrolyte was fabricated,which shows advantages of highly transparency for bleached state when compared with conventional photoelectrochromic device.It exhibits a reversible optical modulation under constant illumination(100 mW cm-2)by simply altering its circuit states between on and off.A high optical window of 54.6%with coloration time of 4.9 s,and bleaching time of 5.2 s could be achieved at 580 nm.Additionally,stable switching for transmittance modulation after 400 cycles demonstrates its good cycle durability,exhibiting great potential in developing energy-saving applications.Third,Transparent TiO2 film was employed as the counter electrode(CE)for the first time in ferrocene/ferricinium(Fc/Fc+)mediated electrochromic device(ECD),offering electro-catalytic effect towards Fc+ reduction,and the device achieved high transmittance modulation of 69.8%at 700 nm and over 60 thousands cycle stability.This results distinctly outperform the traditional complementary ECD based on generally used NiO,PB and V2O5 CE.The electro-catalytic activity was verified systematically by cyclic voltammetry(CV),electrochemical impendence spectroscopy(EIS),and Tafel polarization measurements,revealing that the TiO2 CE exhibited better catalytic activity and fostering rate of Fc+reduction compared with bare FTO substrate.Thus,this work expands the list of potential CE,and may help to exploit the way of developing new generation ECD and its commercial application.