Modification des polymères conducteurs avec de petites particules métalliques; propriétés des films de polypyrrole et de polyaniline platines

The properties of two π-conjugated conducting polymers, polypyrrole, and polyaniline, modified with small amounts of Pt, have been investigated. Both polymers were prepared by electrochemical (cyclic voltammetric) polymerization in the form of thin films (less than 1 μm for polypyrrole, ca. 50-μm thick for polyaniline). It is shown that incorporation, via electrodeposition, of small amount of dispersed Pt particles, inside the polymer film, leads to radical change of its properties. Thus, the polypyrrole film electrode containing ca. 200 μg · cm-2 of Pt exhibits remarkably stable electrocatalytic activity towards anodic oxidation of an important fuel cell reactant – CH3OH. In contrast with the bulk Pt metal or the Pt dispersed on other supports, a polypyrrole/Pt composite does apparently not undergo poisoning, even in the course of prolonged oxidation runs. We show also that the incorporation of Pt microparticles, into several tens of μm thick polyniline films, results in a large enhancement of their redox switching rate between isolating and conducting states and vice versa

The role of the surface intermediates in the photoelectrochemical behaviour of anatase and rutile TiO₂

Superior activity of anatase TiO2 towards photodegradation of a series of organic compounds is interpreted in terms of differences in the surface behaviour of anatase and rutile. Very stable surface peroxides are shown to be formed at the anatase during photo-oxidation reactions but to be absent from the rutile surface

Photoelectrochemistry of nanostructured semiconductors : the case of anatase TiO₂

During recent 10 years, the interest in characterization of colloidal semiconductor films, focused initially on the observation of the quantum size effect, extended to more general aspects of their photoelectrochemical behavior. These include, in particular, the mechanism of charge separation in such networks of nanoparticles and the way by which charge carriers are transported across the films. In this chapter, an apparently anomalous photoelectrochemical behavior of titanium dioxide films composed of ca. 30 nm in diameter or larger nanoparticles is delineated. It is shown, among others, that nanoporous rutile Ti0₂ films exhibit much lower activity toward photooxidation of small organic molecules than the films composed of anatase nanoparticles of similar size. The maximum of the spectral photoresponse of the anatase Ti0₂ films, with thicknesses ranging from less than 1 μm to ea. 45 μm, is located close to 300 nm corresponding to a short (few tens nm) penetration depth of the incident light. In addition, the nanoporous anatase films with thicknesses exceeding by 2-3 orders of magnitude the actual light penetration depth preserve excellent photocurrent-voltage characteristics. This is also the case for the nonannealed anatase films formed by electrophoretic deposition. The unusual behavior of the nanocrystalline anatase films is considered in terms of the self-doping induced by the passage of the photocurrent through the film, leading to the Mott transition in the shallow donor level of anatase. This results in a sharp rise in the electrical conductivity of the film. The self-doping mechanism is proposed to account equally for the electron transport through the dye-sensitized nanostructured anatase electrodes used in liquid-junction photovoltaic cells

Tandem cell for water cleavage by visible light

A tandem cell or photoelectrochemical system for the cleavage of water to hydrogen and oxygen by visible light has two superimposed photocells, both cells being connected electrically. The photoactive material in the top cell is a semiconducting oxide placed in contact with an aqueous solution. This semiconducting oxide absorbs the blue and green part of the solar emission spectrum of a light source or light sources and generates with the energy collected oxygen and protons from water. The not absorbed yellow and red light transmits the top cell and enters a second photocell, the bottom cell, which is mounted, in the direction of the light behind, preferably directly behind the top cell. The bottom cell includes a dye-sensitized mesoporous photovoltaic film. The bottom cell converts the yellow, red and near infrared portion of the sunlight to drive the reduction of the protons, which are produced in the top cell during the photo catalytic water oxidation process, to hydrogen

Long-term activation of the copper cathode in the course of CO₂ reduction

The cathodic reduction of CO2 at copper electrodes in aqueous solutions of bicarbonates was investigated with special attention directed towards the changes in the current efficiency of the principal products (CH4, C2H4, C2H5OH) as a function of the electrolysis duration. A severe deactivation of the copper cathode, starting usually 20–30 min after the beginning of the electrolysis run, is shown to be little affected by the pre-electrolysis of the solution as well as by the nature of the electrode pretreatment. A periodic anodic activation procedure is described which, when applied to the copper cathode, allows high hydrocarbon yields to be maintained over prolonged electrolysis runs

Photon-driven reduction reactions on silver

Irradiation of a polycrystalline silver electrode with near-u.v./visible light in solutions containing either dissolved carbon dioxide or nitrate ions produces strong enhancement of the cathodic current. Maximum photocurrent efficiency is observed for the photon energies of about 3.5 eV characteristic of surface plasmons on silver. In the presence of carbon dioxide in solution, the electrode illumination not only increases the rate of CO2 to CO reduction but also shifts the onset of the CO production by about 0.5 V to less cathodic potentials

Influence of incorporated metal particles on switching of polyaniline films

In contrast with the cyclic voltarnmograms, the ac impedance spectra of polyaniline films are shown to be strongly affected by the presence of relatively small amounts of Pt microparticles. The main effect observed is a large increase in the rate of redox switching of such doped films

Effect of the surface roughness on the spectral distribution of photoemission current at the silver/solution contact

The use of an aqueous electrolyte containing dissolved CO2 enabled revealing of an unusually intense photoemission from a bulk silver electrode. The observed cathodic photocurrent exhibits a sharp peak at ∼360 nm corresponding to the energy of surface plasmons in silver. The amount and the spectral distribution of the photocurrent are strongly affected by the extent of roughness of the silver surface. In particular, an electrochemical roughening procedure, analogous to that leading to the surface enhancement of Raman scattering (SERS) at silver, causes appearance of a long tail on the low‐energy side of the photoemission spectra. In consequence, significant photocurrents, corresponding to a quantum yield of ∼7×10−3, are observed for the deliberately roughened silver electrodes illuminated with the wavelength of 514 nm, most frequently used in SERS experiments. It is postulated that this intense photoemission, discovered in the presence of CO2 in the solution, equally takes place in the absence of scavengers. However, in the latter case no significant photocurrents can be detected as most of the photoemitted electrons (solvated or not) return back to the electrode