Cesar O. Avellaneda

Preparation of transparent CeO2–TiO2 coatings for electrochromic devices

Abstract Thin films of mixed oxides of CeO2–TiO2 with a molar ratio of Ce/Ti of 0.5 have been obtained by the sonocatalytic sol–gel method. The precursor sols consist of a mixture of Ce(NH4)2(NO3)6, titanium alkoxide (Ti(OiPr)4) and isopropanol. The films were deposited using the dip-coating technique and calcined at 450°C in an oxygen atmosphere. Xerogels were characterized by thermal analysis (DTA–TG) and X-ray diffraction. The films prepared from the precursor sols were subjected to electrochemical measurements (cyclic voltammetry and chronoamperometry), SEM and AFM. Films of CeO2–TiO2 approx. 100 nm thick present a good electrochemical response under Li+ ions insertion. The cathodic and anodic charges at a scan velocity of 50 mV/s were about 16 mC/cm2 and the process was fully reversible. The values measured up to 4500 cycles were constant indicating a good electrochemical stability of the films. The stability test and optical measurements confirm that the films can be used as ion storage (counter-electrode) in electrochromic devices.

Electrochromism in materials prepared by the sol-gel process

Electrochromism is defined as the persistent but reversible optical change (usually transmission) produced electrochemically. The preparation by the sol-gel process of thin films made of amorphous or crystalline nanoparticles of WO3, V2O5, Nb2O5, TiO2, CeO2, Fe2O3 and mixed compounds such as WO3-TiO2, CeO2-TiO2, CeO2-SnO2, have opened remarkable new opportunities for obtaining electrochromic layers exhibiting large optical transmission variation in the UV, visible or infrared range and acceptable kinetics under H+ or Li+ insertion. In this paper we give an overview of what has been recently achieved in this field, with emphasis for cathodic electrochromic coatings of Nb2O5 and TiO2 composition. Finally we stress the future developments in this fast growing field.

Electrochromic properties of Nb2O5 sol-gel coatings

Abstract Sol–gel niobium oxide coatings are promising electrochromic materials. The sols have been prepared by a sonocatalytic mixing of NbCl 5 powder, butanol and acetic acid. Thermal analysis (DTA/TG) coupled to mass spectrometry has been performed on Nb 2 O 5 precipitates to quantitatively analyse the effluents. Transparent and defect-free single and multilayers coatings have been deposited on ITO-coated glass by a dip-coating process and then calcined between 400°C and 600°C. The coatings structure change from amorphous to crystalline (TT form) and the later ones are highly textured. The films present a reversible and fast insertion/extraction kinetics for Li + ions. After insertion the amorphous coatings present a grey-brown color, while the crystalline ones are dark blue. The maximum charge density exchanged with a three-layer 200xa0nm thick coating sintered at 600°C was 16xa0mC/cm 2 with a corresponding spectral transmission change practically wavelength-independent varying from 80% to 20%. The coloring efficiency determined at λ =600xa0nm was 22xa0cm 2 /C.

Optoelectrochemical Characterization of Electrochromic Devices with Starch Based Solid Electrolytes

This paper describes the manufacturing and spectroelectrochemical characterization of all solid electrochromic devices (ECD). Both electrochromic (WO3) and storage ion (CeO2-TiO2) thin films were obtained by sol-gel method and deposited by dip-coating technique. The electrolytes were obtained by plasticization of starch with glycerol and addition of LiClO4 salt. The spectroelectrochemical measurements were performed with the complete devices as a function of the applied potential. The obtained results revealed that the color/bleaching process was reversible and the inserted/extracted charge was about 4.6 mC/cm2 for the applied potential of −2.0 V and increased up to 5.3 mC/cm2 for −2.3 V. This value was stable up to −2.5 V applied. The transmittance change at 630 nm was about 30% for 2.3 V applied and the optical density was about 0.25. The memory tests showed that the colored device bleached completely in open circuit in about 500 min. All the obtained results show that the presented devices are very good candidates to be tested as smart windows for architectural applications.

Characterization of an all sol-gel electrochromic device WO3/Ormolyte/CeO2-TiO2

Electrochemical lithium intercalation in thin films of CeO2-TiO2 and WO3, prepared by the sol-gel technique was investigated with cyclic voltammetry and spectroelectrochemical techniques in propylene carbonate solutions. A solid state system having the configuration WO3/Ormolyte/CeO2-TiO2 has been assembled. The solid electrolyte, an organically modified electrolyte (ormolyte), was prepared with different [O]/[Li] ratios. The transmittance variation of this system during a potentiostatic step from −0.7 V to 0.8 V was about 35% at 550 nm.

Caracterização de filmes finos de Nb2O5 com propriedades eletrocrômicas

The sols for thin electrochromic coatings of Nb2O5 were obtained by synthesis of the niobium butoxide from BuONa and NbCl5. The ~300nm thick films were deposited by dip-coating technique from the alkoxide solution and calcined at 560oC in O2 atmosphere during 3 hours. The particles size of niobium oxide (V) powder (~20mm) was obtained from x-ray diffraction using the Scherrer equation. The coatings were characterized by cyclic voltammetry and cronoamperommetry techniques. The spectral variation of the optical transmittance were determined in situ as a function of the cyclical potencial and memory effect. The insertion process of lithium is reversible and change the film color from transparent (T=80%) to dark blue (T=20%).

Niobia sol gel: a new material for electrochromic and photoelectric applications

Nb2O5 prepared by a sol-gel process in the form of coatings are new materials which have outstanding properties: (1) The coatings present electrochromic properties and exhibit a blue coloration under Li+ insertion with 100% reversible variation of the optical transmission in the visible and near infrared range between 80% and 20% and have a high chemical stability (tested up to 2000 cycles). (2) They are semiconductors and present a photoelectric effect when illuminating in the UV region ((lambda) < 360 nm). These films are therefore very promising for use in electrochromic devices and as electrodes for photoelectrochemical purposes and the development of nanocrystalline solar cells.

Thermodynamics and Lithium Intercalation in CeO2-TiO2 Thin Film

The kinetics and the thermodynamics of electrochemical intercalation of lithium into CeO2-TiO2 films prepared by the sol-gel process were studied by galvanostatic intermittent titration technique (GITT) as function of the depth of lithium intercalation. The open-circuit-potential versus x in Lix(CeO2-TiO2) curve consists of two straight lines with different slopes, one in the range of 0.03 ≤ x ≤ 0.09 and the other of 0.09 < x ≤ 0.15. The standard Gibbs energy for lithium intercalation was 6 kJ/mol for x = 0.09 in Lix(CeO2-TiO2) at room temperature. The chemical diffusion coefficient value, D Li + , of lithium intercalation into thin film oxide was 2.14·10−11 cm2/s at x = 0.15, and the value of the component diffusion coefficient D Li +, k was about one order of magnitude lower than the coefficient of chemical diffusion.