F. Decker

Electrochemical characterization of optically passive CeVO4 counterelectrodes

Abstract Ceria–vanadia stoichiometric oxides represent a new class of optically passive counterelectrodes with favorable properties for the use as counterelectrodes in an electrochromic window. They are very transparent both in the pristine state and in the lithium-intercalated condition and have a good charge capacity (around 20 mC cm −2 ). In the present paper we report on the electrochemical properties of Ceria–Vanadia stoichiometric oxides, in liquid PC–LiClO 4 anhydrous electrolytes. The cyclic voltammograms of such electrodes show reversible reduction/oxidation peaks in two distinct regions, around 1.2–2.2 V vs. Li and 2.9–3.9 V vs. Li. GITT experiments have been performed, with the following results: Li diffusion coefficient goes from 10 −10 for pristine samples down to 10 −13 cm 2 s −1 for fully intercalated samples. The impedance of Ceria–vanadia is that of a ceramic thin film, going from a slightly p-type semiconducting state to a more insulating sample with resistivity of 0.1 GΩ cm when fully Li-intercalated. The optical properties of the samples are that of a colorless, very transparent electrode ( T >88% in the wavelength range between 450 and 740 nm). Extremely weak cathodic and anodic electrochromism can be detected in the red and in the violet part of the visible spectrum, respectively. This extremely low absorption is of no harm to the visible appearance of the samples.

Optical and electrochemical properties of cerium-zirconium mixed oxide thin films deposited by sol-gel and r.f. sputtering

Films of Ce‐Zr mixed oxide were produced by sol‐gel and r.f. sputtering. These films can be used as ‘passive’ counter-electrodes in electrochromic smart windows because they retain their full transparency in both the oxidised and reduced state. Li intercalation was accomplished electrochemically using a liquid electrolyte. Electrochemical behaviour of the samples was found to be dependent on the heat treatment (sol‐gel deposited film) and crystallite orientation (sputter deposited films). XRD analysis on sputter deposited films showed that the films are crystalline and grow following the orientation of the underlying tin doped indium oxide (ITO) film. Films of Ce‐Zr mixed oxide lacking in (111) crystallite orientation show continuous evolution of the voltammograms and reach a maximum value for the cycled charge only after a large number of cycles. The lithium diAusion coeAcient, calculated from GITT measurements, is in the range 10 ˇ12 ‐10 ˇ14 cm 2 s ˇ1 for sputter deposited films and becomes as low as 10 ˇ15 cm 2 s ˇ1 for sol‐gel deposited films. Optical constants of the thin films were calculated from reflectance and transmittance spectra. Refractive index values are in the range of 2.15‐2.30 at l=633 mn depending on the deposition method. A sharp absorption edge at about 320 nm is seen in accordance with CeO2 optical properties. # 1999 Elsevier Science Ltd. All rights reserved.

Sputter deposited cerium–vanadium oxide: optical characterization and electrochromic behavior

Vanadium pentoxide has been the subject of many works owing to its capacity to host large amounts of lithium. Many studies where V2O5 has been mixed with different transparent oxides, such as CeO2 ,Z rO 2 ,T iO 2, have been performed in order to preserve the ion-storage capacity and compensate the unwanted multicolor cathodic electrochromism. In the present work, we report on the optical properties of cerium and vanadium mixed oxides obtained by radio frequency sputtering in reactive ArO2 atmosphere. Since the electrochromic properties of thin films strongly depend on the oxidation state of the transition metal in the oxide, the role of oxygen partial pressure during the deposition is of fundamental importance. A set of samples was deposited by decreasing the oxygen partial pressure in the chamber from 1: 6t o 1:1000 of the total pressure and their optical properties were compared to those of the samples deposited in pure argon. The majority of deposited films changes coloration from yellowish to transparent upon lithium insertion, showing an anodic-like electrochromic behavior. Optical constants (refractive index and extinction coefficient) and film thickness were evaluated in the visible range from photometric measurements of both as-deposited and intercalated films.

Study of lithium diffusion in RF sputtered Nickel–Vanadium mixed oxides thin films

Abstract The kinetics of electrochemical lithium insertion inside RF sputtered Ni/V mixed oxides thin films have been investigated employing different electrochemical techniques. The AC electrochemical impedance spectra, recorded after 10 cycles, showed three steps clearly involved in the intercalation mechanism of lithium in the oxide films: (i) a charge transfer process to the electrolyte/electrode interface; (ii) a solid-state diffusion of Li; and (iii) a space limited diffusion due to the finite volume of the film. This latter portion of the impedance spectra was used to calculate the Li chemical diffusion coefficients ( D Li ). D Li values show an initial increase up to an injected charge of 30 mC cm −2 and then, upon further intercalation, a decay probably due to the limited Li motion through a decreased number of available sites. These results are in good agreement with the ones obtained by potentiostatic intermittent titration technique applied to the same electrodes. Both techniques agree in giving the same trend for D Li upon Li intercalation. The same sets of measurements were carried out after 100 and 1000 Li insertion/deinsertion cycles in order to analyze the effect of prolonged cycling on the mechanism of lithium diffusion.

Photoelectrochemical behavior of LiCoO2 membrane electrode

Abstract We studied the photoresponse of a membrane electrode of stoichiometric LiCoO 2 used in lithium ion batteries. The electrode LiCoO 2 in contact with dimethyl carbonate, propylene carbonate and LiClO 4 presents the behavior of a p-semiconductor/electrolyte junction. The deinsertion of lithium of the LiCoO 2 suppresses the photoresponse at 3.9 V versus the lithium potential. The reappearance of the photoresponse after re-intercalation suggests that the alterations in the electronic structure of the Li x CoO 2 electrode probed by the photoelectrochemical experiments during deinsertion are reversible until x =0.5. The direct band gap energy of the LiCoO 2 stoichiometry is 2.5 eV before and after lithium reinsertion. The experiments showed also that a small photo-intercalation current is observed in this cathodic material.

Sputter-deposited cerium vanadium mixed oxide as counter-electrode for electrochromic devices

Abstract Oxide films deposited by reactive radio frequency sputtering of target made of mixed CeO 2 and V 2 O 5 (1:1) powders show an anodic electrochromic character upon lithium insertion. As-deposited films look yellowish and become fully transparent for an inserted charge of 30 mC/cm 2 of Li ions. A total charge of 60 mC/cm 2 has been reversible cycled in the 1 to 5 V versus Li range in an electrochemical cell without any significant degradation. Following these preliminary results, films of Ce–V mixed oxide made by r.f. sputtering are suitable for working in tandem with tungsten oxide in a practical electrochromic device.

Fe-containing CeVO4 films as Li intercalation transparent counter-electrodes

Iron containing CeVO4 films were prepared using the sol‐gel method. The crystalline structure of powders and films with Fe:Ce:V ratios of 0.1:1:1, 0.3:1:1 and 0.5:1:1 were investigated by X-ray diffraction (XRD) and infrared (IR) spectroscopy. XRD revealed the predominance of a CeVO4-W (wakefieldite) crystalline phase with a small amount of monoclinic CeVO4 ,C eO 2 and Fe2O3. Ex situ IR absorbance spectra of charged and discharged films show the changes in intensity of the V‐O stretching mode at 770 cm 1 . Cyclic voltammetry showed that Li intercalation occurs in three steps with total capacities of 22 mC cm 2 (Fe:Ce0.1), 32 mC cm 2 (Fe:Ce0.3) and 37 mC cm 2 (Fe:Ce 0.5). The increase in the overall charge capacity with increasing Fe content is accompanied by a loss of capacity with cycling. The lithium diffusion coefficient, calculated by the galvanostatic intermittent titration method (GITT), the potentiostatic intermittent titration method (PITT) and electrochemical impedance spectroscopy (EIS), ranges from 10 12 to 10 14 cm 2 s 1 . UV‐vis in situ transmittance spectra showed that these films are highly transparent with a photopic transmittance TVis 0.85 for intercalated and deintercalated states. This suggests that Fe:Ce:V oxide films are good candidates for optically passive counter-electrodes in electrochromic (EC) devices.

Electrochemical and optical characterization of RF-sputtered thin films of vanadium-nickel mixed oxides

The optical and electrochemical properties of thin films of a nickel vanadium mixed oxide have been investigated as a new Li + intercalation compound, to be used as an optically passive counterelectrode in electrochromic switching devices. The thin film samples were prepared by radio frequency (RF) sputtering under different preparation conditions, using targets with different Ni V ratios and a reactive (20% O 2 ) or, alternatively, non-reactive (pure Ar) sputtering atmosphere. The optical properties of as-grown and Li + inserted extracted electrodes were determined by computer fitting of the UV Vis NIR transmittance and specular reflectance spectra. The electrochemical measurements of Li + insertion and extraction consisted of cyclic voltammetry, chronopotentiometry and potentiostatic intermittent titration technique. The samples showed interesting electrochemical and optical properties, a good electrochemical reversibility and ion-storage capacity exceeding 40 mC cm -2 , lasting for several hundreds cycles at a current of 50 μA cm - 2 . The electrodes displayed an almost passive electrochromic behavior during the electrochemical experiments.

Characterization of electrodeposited TiO2 films

Stress variations of semiconducting n-TiO 2 films prepared by anodic oxidation in concentrated alkaline solution are studied. The relationship between stress and photoelectrochemical response is investigated. The presence of sodium ions in the reticular lattice is found to be responsible for the particular behaviour of the oxide. Results are compared with TiO 2 prepared by thermal oxidation in air

Photoelectrochemical response and photoconductivity of poly(3-methylthiophene)

Abstract We report on the photoelectrochemical response and photoconductivity of poly(3-methylthiophene), PMeT, films deposited on different substrates. The frequency dependency of the modulated photoconductivity of PMeT (ρ=3.6×103 Ω cm) is sublinear indicating that the measured photoconductivity is not due to a thermal effect caused by sample heating. An interesting feature was the larger backwall photoconductivity (with the laser beam reaching the film from the substrate side), when compared to photoconductivity in the normal, frontwall illumination. This result is an indication for heterogeneity in the deposited film. The photoelectrochemical response of PMeT films in contact with the electrolyte was studied by spectral response and by the time dependence of the photoelectrochemical response under irradiation with the Ar+ laser light (close to the maximum of the spectral photoresponse), with light intensity varying from 10−5 W cm−2 to 30 mW cm−2. From that experiment we can infer that charge trapping occurs mainly in the polymer surface in contact with the electrolyte. Another experiment was performed with two different light beams, one from a lamp plus monochromator, chopped and at low intensity and another from an Ar+ laser, continuous and at high intensity. When the laser beam incides from the electrolyte side, i.e. when light is absorbed in the space-charge layer, we observe a strong drop in the photocurrent. We suppose that the same space-charge layer trapping phenomenon is responsible for the drop in the photocurrent.