Martine Ulmann

Enhanced Visible Light Conversion Efficiency Using Nanocrystalline WO3 Films

Drastically improved photooxidation efficiencies,when compared to bulk WO 3 , are obtained with nanocrystalline WO 3 films (see Figure) deposited from a colloidal solution of tungstic acid. Saturation of the photocurrent, which occurs even for moderate potential differences, indicates a completely different mechanism of charge separation. The electrodes should be highly effective for photoelectrolysis of water by sunlight.

Photoelectrochemical Oxidation of Water at Transparent Ferric Oxide Film Electrodes

Abstract Thin film Fe 2 O 3 photoanodes deposited onto conducting glass substrates were employed for the light-induced splitting of water. The effects of the employed precursor (inorganic or co-ordination compounds), dopants (Ti(IV) and Al(III)) and preparation conditions upon photoelectrochemical characteristics of the films are discussed. A photoelectrolysis cell including an arrangement of three thin film Fe 2 O 3 photoanodes placed one behind another is shown to improve considerably the light harvesting efficiency.

Electrocatalytic oxidation of methanol on platinum microparticles in polypyrrole

The oxidation of methanol on gold electrodes modified with polypyrrole and platinum is reported. These electrodes were characterized by cyclic voltammetry and by 12h polarizations in methanol solutions. They were found to give higher currents and lower rates of drift than electrodes of platinum and platinized gold. The effect of varying the amount of platinum deposited is also discussed.

Electrochemical incorporation of lithium into palladium from aprotic electrolytes

Among numerous questions raised by the recent paper of Fleischmann et al. [l], describing their heavy water electrolysis experiments, an important one concerns the specific influence of the cation of LiOD electrolyte on the behaviour of a palladium cathode. In fact. the reported observations of an excess heat generation in the latter experiments seem to be closely associated with the presence of Li+ cations in the electrolyte [2]. One of the typical features of the prolonged electrolyses of a 0.1 M LiOD + D20 solution between palladium cathodes and platinum anodes. mentioned by Fleischmann et al. [l], was the build-up of high cathodic overvoltages. This leads to an important question: what is the critical value of the electrode potential which would allow the incorporation of lithium into the PdD, cathode [3]? Lithium has been shown to alloy electrochemically, at ambient temperature. with a number of metals including the three noble metals. gold. silver and platinum [4]. In these electrolysis experiments, performed in a 1 M LiClO, + propylene carbonate solution, the formation of lithium alloys at Au, Ag and Pt cathodes was observed to start at potentials slightly more positive than ca. 0.4 V with respect to the Li+/Li electrode. The aim of the present study was to establish in which potential range the Lit cations undergo reduction at a palladium cathode and whether such a reaction is

Highly selective photo-oxidation reactions at nanocrystalline TiO2 film electrodes

Photocurrent–voltage characteristic of the junction formed between a porous nanocrystalline TiO2 film and an electrolyte is shown to be governed by the kinetics of the interfacial hole transfer to the oxidizable species in the solution.

The involvement of surface-bonded intermediates in the competitive photo-oxidation of methanol and hydroxyl ions at a TiO2 electrode

Abstract The addition of methanol to a 0.1 M aqueous NaOH solution results in a significant shift of the photocurrent onset of an n-type TiO 2 electrode towards negative potentials. The photo-oxidation of this alcohol, acting as an efficient hole scavenger, at potentials close to the apparent onset of the photocurrent, is shown to occur without mediation of the surface peroxo species involved in the photo-oxidation of water at TiO 2 .

Production of hydrogen in non oxygen-evolving systems: co-produced hydrogen as a bonus in the photodegradation of organic pollutants and hydrogen sulfide

This report was prepared as part of the documentation of Annex 10 (Photoproduction of Hydrogen) of the IEA Hydrogen Agreement. Subtask A of this Annex concerned photo-electrochemical hydrogen production, with an emphasis on direct water splitting. However, studies of non oxygen-evolving systems were also included in view of their interesting potential for combined hydrogen production and waste degradation. Annex 10 was operative from 1 March 1995 until 1 October 1998. One of the collaborative projects involved scientists from the Universities of Geneva and Bern, and the Federal Institute of Technology in Laussane, Switzerland. A device consisting of a photoelectrochemical cell (PEC) with a WO{sub 3} photoanode connected in series with a so-called Grazel cell (a dye sensitized liquid junction photovoltaic cell) was developed and studied in this project. Part of these studies concerned the combination of hydrogen production with degradation of organic pollutants, as described in Chapter 3 of this report. For completeness, a review of the state of the art of organic waste treatment is included in Chapter 2. Most of the work at the University of Geneva, under the supervision of Prof. J. Augustynski, was focused on the development and testing of efficient WO{sub 3} photoanodes for the photoelectrochemical degradation of organic waste solutions. Two types of WO{sub 3} anodes were developed: non transparent bulk photoanodes and non-particle-based transparent film photoanodes. Both types were tested for degradation and proved to be very efficient in dilute solutions. For instance, a solar-to-chemical energy conversion efficiency of 9% was obtained by operating the device in a 0.01M solution of methanol (as compared to about 4% obtained for direct water splitting with the same device). These organic compounds are oxidized to CO{sub 2} by the photocurrent produced by the photoanode. The advantages of this procedure over conventional electrolytic degradation are that much (an order of magnitude) less energy is required and that sunlight can be used directly. In the case of photoproduction of hydrogen, as compared to water splitting, feeding the anodic compartment of the PEC with an organic pollutant, instead of the usual supporting electrolyte, will bring about a substantial increase of the photocurrent at a given illumination. Thus, the replacement of the photo-oxidation of water by the photodegradation of organic waste will be accompanied by a gain in solar-to-chemical conversion efficiency and hence by a decrease in the cost of the photoproduced hydrogen. Taking into account the benefits and possible revenues obtainable by the waste degradation, this would seem to be a promising approach to the photoproduction of hydrogen. Hydrogen sulfide (H{sub 2}S) is another waste effluent requiring extensive treatment, especially in petroleum refineries. The so-called Claus process is normally used to convert the H{sub 2}S to elemental sulfur. A sulfur recovery process developed at the Florida Solar Energy Center is described briefly in Chapter 4 by Dr. C. Linkous as a typical example of the photoproduction of hydrogen in a non oxygen-evolving system. The encouraging results obtained in these investigations of photoelectrochemical hydrogen production combined with organic waste degradation, have prompted a decision to continue the work under the new IEA Hydrogen Agreement Annex 14, Photoelectrolytic Hydrogen Production.