Franco Decker

The electrochromic process in non-stoichiometric nickel oxide thin film electrodes

NOn-stoichiometric nickel oxide thin films are very interesting novel electrochromic electrodes for the realization of optical displays and windows of relevant technological importance. However, the exact nature of the electrochromic process in these electrodes is not yet fully clear. In this work we attempt to reach a more precise understanding of this process by examining in detail the electrochemical properties of the intercalation process of lithium ions in the nickel oxide host structure. Investigation based on cyclic voltammetry, potentiometry, impedance analysis, optical transmittance, stress measurements, scanning electron microscopy and secondary ion mass spectroscopy, suggest that the intercalation of lithium induces phase changes in the nickel oxide structure, namely a first phase related to the initial injection of lithium (with which is associated a bleaching process) and a second phase related to an excess of inserted lithium (with which is associated an opposite, darkening process).

Subband Gap Response of TiO2 and SrTiO3 Photoelectrodes

The response of and photoelectrodes to subband gap light has been explored as a function of numerous variables, including intensity and wavelength of light, applied potential, temperature, and electrolyte composition. The photoresponse is shown to be linear with light intensity, to depend on the square root of the applied potential, to occur only for , and to be independent of temperature and electrolyte composition. These results suggest a bulk excitation process involving impurity and defect‐related states in the gaps of these semiconductors. A qualitative model is presented to account for the main features of this excitation process and to provide a basis for suggesting future experiments.

A comparison of the electrochromic properties of WO3 films intercalated with H+, Li+ and Na+

The electrochromic response of W03 thin films under electrochemical insertion from nonaqueous electrolytes of one of the three different ions: protons, lithium and sodium cations is reported. In spite of the common belief that the nature of the ion does not change the electrochromic effect, we show that the sample colouring and bleaching are dependent on the intercalant ion and on the insertion rate. The facile insertion of protons is responsible for the highest optical contrast and the quickest response time of this intercalation electrode. Lithium and sodium-intercalated electrodes do not show large optical difference from the proton-intercalated tungsten trioxide unless large charging currents are used. Subtle changes in electrochromic efficiency and in the optical contrast in the red part of the spectrum can be detected; these are larger for the sodium than for the lithium intercalant. According to our analysis, the slow insertion kinetics of Na+ and the formation of a new MxWO3 (M=Li, Na) compound is responsible for most of the observed differences.

Physical and Electrochemical Analysis of an Indoor–Outdoor Ageing Test of Large‐Area Dye Solar Cell Devices

A long-term life test (3200 h) on large-area dye-sensitized cells is performed both under outdoor conditions, in the sunny Mediterranean climate in Rome (Italy), and under continuous light soaking (1 Sun, 85 °C). Different degradation rates are investigated for the outdoor samples with horizontally and vertically oriented cells (azimuth South, tilt angle 25°). Thirty identical photocells (active area=3.6 cm(2), conversion efficiencies=(4.8±0.2)%) are aged using a robust master-plate configuration. After the first 1000 h of testing in open-circuit conditions, some of the test samples are set near the maximum power point (MPP) and the life test continued further until 3200 h. A detailed analysis of the physical parameters obtained by electrochemical impedance is given together with electrolyte transmittance variation with time as a function of the ageing conditions. Faster degradation in devices working at the MPP is observed, due mainly to a progressive decrease of the triiodide concentration in the electrolyte and a likely alteration at the titania/electrolyte interface. Outdoor devices working with vertically oriented cells show clearly that the orientation of long-striped cells can affect the lifetime. The aged cells suffer an increase of recombination rate, change in the chemical capacitance, and positive shift of the titania conduction band level. A strong correlation between the increase of the electrolyte diffusion resistance and degradation phenomena is found.

XAS and electrochemical characterization of lithium intercalated V2O5 xerogels

Abstract The use of sol-gel processes in the preparation of cathode materials is of growing interest because of their ease and flexibility. The electrochemical properties, e.g. the rate of lithium intercalation, appear to depend on the morphology of the thin-film vanadium oxide xerogels that can be changed by modifying the preparation. In this context, in order to extend the study to bulk materials, xerogel powder samples with surface areas in the range 2–5 m 2 /g have been prepared from pure vanadium pentoxide hydrogels, or in the form of composites, from carbon powder added to hydrogels. The electrochemical properties have been correlated with the morphological and structural changes induced by the presence of carbon using X-ray and XAS spectroscopy.

Stress and electrochromism induced by Li insertion in crystalline and amorphous V2O5 thin film electrodes

Vanadium pentoxide thin films produced by reactive dc magnetron sputtering have different and peculiar electrochemical behavior in LiClO4-propylene carbonate solutions depending on the sputtering conditions which determine the film crystal structure. Deep charge-discharge cycles have been performed under constant current density and the thin film electrode potential, the mechanical stress and the optical transmittance have been followed in situ simultaneously. Close correlation has been noticed between the stress changes in the crystalline film and the electrode potential steps which are typical of the lithium insertion mechanisms in V2O5 cathode materials used in secondary lithium batteries. The irreversibility of the electrochemical processes and the amorphisation of the as-grown crystalline films have been described in comparison with the behavior of the V2O5 film electrodes which are amorphous from growth.

The reduction of molecular oxygen at single crystal rutile electrodes

The reduction of molecular oxygen was investigated on single crystal rutile electrodes. Cyclic voltammetry and a rotating ring-disk electrode were employed to elucidate the mechanism and energetics of the reaction. It was found that oxygen reduction proceeds through a surface species at potentials positive of the flatband potential in the electrolytes used. It was also demonstrated that the majority of the reaction proceeds via a 4-electron pathway while a small percentage of the oxygen is reduced by two electrons to hydrogen peroxide.

H Insertion and Electrochromism in NiO x Thin Films

Nonstoichiometric, anhydrous thin films were produced by sputtering. They can be bleached by means of hydrogen insertion equally fast and efficiently either in an electrochemical cell or in a gas chamber, as demonstrated in this work by using a Pd covered thin film electrode. The electrode mechanical stress and transmittance changes during H insertion and extraction, measured in situ, showed an increase in the compressive stress in the cathodic (bleaching) cycle and a relaxation of the system in the anodic (coloring) process. These experiments demonstrate that the aqueous solvent plays no major role on the coloring‐bleaching process of thin films.

Copper protection by self-assembled monolayers of aromatic thiols in alkaline solutions

Copper corrosion in alkaline solutions is inhibited by the formation of self-assembled monolayers of aromatic thiols, made of either benzenethiol or 2-naphthalenethiol or 4-acetamidothiophenol. Electrochemical experiments, based on voltammetry and impedance spectroscopy, point out the much lower reactivity of copper surfaces towards oxidation, when covered by compact adlayers of the above molecules bonded through the S atom. The peculiar shape and peak position in the voltammetric reduction of residual oxides grown on modified metal surfaces suggest that they are due to Cu(I) suboxides, probably grown on reactive metal defects. XPS experiments have confirmed that the aromatic adlayers are still covering most of the Cu surface even after 1 h immersion in 0.5 M NaOH. The main changes in Auger and XP spectra indicate the formation of much less Cu(2)O in the protected samples than in the corresponding bare Cu aged in NaOH. From the experimental data the presence of defective copper oxides on modified Cu has been deduced.

A comparison of the electrochromic behavior and the mechanical properties of WO sub 3 and NiO sub x thin film electrodes

The charge-discharge cycles of WO{sub 3} and NiO{sub x} thin film electrodes in Li{sup +} propylene carbonate electrolyte have been followed in situ with a laser beam system that allows the simultaneous measurement of film transmittance and stress. Quantitative relationships have been established between film thickness, absorbance variation, mechanical compression, and charge inserted. In this paper, the applications to electrochromic devices are discussed.