Jan Augustynski

Metal oxide photoanodes for solar hydrogen production

The development of sustainable hydrogen production is a key target in the further facilitation of a hydrogen economy. Solar hydrogen generation through the photolytic splitting of water sensitised by semiconductor materials is attractive as it is both renewable and does not lead to problematic by-products, unlike current hydrogen sources such as natural gas. Consequently, the development of these semiconductor materials has undergone considerable research since their discovery over 30 years ago and it would seem prescient to review the more practical results of this research. Among the critical factors influencing the choice of semiconductor material for photoelectrolysis of water are the band-gap energies, flat band potentials and stability towards photocorrosion; the latter of these points directs us to focus on metal oxides. Careful design of thin films of photocatalyst material can eliminate potential routes of losses in performance, i.e., recombination at grain boundaries. Methods to overcome these problems are discussed such as coupling a photoanode for photolysis of water to a photovoltaic cell in a “tandem cell” device.

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

Abstract Superior activity of anatase TiO 2 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.

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.

ESCA study of the state of iridium and oxygen in electrochemically and thermally formed iridium oxide films

X-ray photoelectron spectroscopic measurements on the state of Ir and O species in electrochemically and thermally formed iridium oxide films are reported. The electrochemically formed films were grown to three thicknesses by means of potential cycling. The monolayer oxide film which exhibits electrochemical irreversibility in its formation and reduction already causes a change of electronic state of the Ir substrate surface atoms corresponding to formation of an Ir(OH)4 species. However, if any electrochemically reversible, thick film oxide, 100×a monolayer is formed, it behaves like bulk IrO2 powder with regard to 4f electron shell binding energies. The state of O (1s) is also different in this material from that in thinner oxide films, and contains clearly distinguishalbe ionic O2− rather than OH−, bound water, or adsorbed O2, although the latter type of species is also present. Appreciable SO42− ion incorporation or irreversible adsorption can also be detected. No difference in the oxidation state of Ir can be detected on (i.e., over ∼1.5 nm) an oxide film that has been produced electrochemically in its oxidized state or in its reduced state at the end of an anodic or cathodic sweep, respectively, prior to introduction into the electron spectrometer. This is probably due to rapid aerial oxidation of the reduced state of the oxide surface region during the process of transfer of the sample to the spectrometer in ex situ experiments. The electronic characteristics of Ir and O in a thermally formed oxide film are found to be identical with those in the electrochemically formed film only when the latter is thick but are also almost identical with those of Ir and O in chemically prepared IrO2 powder.

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.

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

Abstract The cathodic reduction of CO 2 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 (CH 4 , C 2 H 4 , C 2 H 5 OH) 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.

Correlation between the surface composition of Pt/TiO2 catalysts and their adsorption behaviour in aqueous solutions

Abstract X-ray photoelectron spectroscopy (XPS), electron microscopy and cyclic voltammetry have been used to characterize the behaviour of two differently prepared platinum catalysts supported on TiO 2 (anatase). One of these two kinds of catalysts, consisting of Pt deposits obtained by thermal hydrolytic decomposition of aqueous H 2 PtCl 6 onto Ti-supported TiO 2 films and subsequently reduced in argon at 550 °C, has been shown to exhibit a series of anomalous properties denoting the existence of strong metal-support interaction (SMSI). A substantial (0.8 eV) decrease of the Pt 4f core level binding energy, with respect to photochemically platinized, prereduced TiO 2 samples, was observed for a variety of thermally deposited Pt/TiO 2 catalysts having different Pt coverages. The occurrence of a negative binding energy shift, in spite of the significantly smaller average Pt particle size in the thermal deposits, was attributed to the appearance of a fractional negative charge on Pt particles (atoms). Interestingly, the above-mentioned chemical shift could be partially annulled by an electrochemical (anodic) oxidation of the sample and, subsequently, restored by reducing it cathodically in the same solution. In addition, the thermal Pt/TiO 2 deposits exhibited an unusual electrochemical behaviour, consisting of the non-occurrence of discrete adsorption peaks in the hydrogen region of cyclic voltammograms, the absence of the anodic formation of surface platinum oxide and the shift of the oxygen evolution to high anodic potentials. Moreover, in contrast with the well-known affinity of platinum for carbon monoxide, the SMSI Pt/TiO 2 deposits did not adsorb (in detectable amounts) CO species dissolved in aqueous solution. The direct analogy between the above features of the electrochemical behaviour and the previously described properties of the SMSI catalysts is pointed out.

Comparative impedance spectroscopy study of rutile and anatase TiO2 film electrodes

The purpose of this work was a comparative electrochemical impedance analysis of the titanium dioxide-solution interface for two kinds of thin film samples consisting respectively of anatase and rutile. These kind of films, frequently used in photoelectrochemical experiments, have recently been the subject of detailed physical studies showing important differences in electrical properties between those two forms of TiO2. The EIS measurements were carried out over a wide frequency range (from 0.03 Hz to 10 kHz), analysing the whole spectrum.

Photoelectrochemical studies pertaining to the activity of TiO2 towards photodegradation of organic compounds

Abstract Photoanodic (water and methanol oxidation) and cathodic (oxygen reduction) processes occurring at illuminated (with near - UV light) bulk and particulate anatase and rutile TiO 2 films were analysed. (Photo) current-potential curves recorded for both anodic and cathodic processes point to large differences in activity between the two forms of TiO 2 . In particular, in the presence of various organic compounds, the photocurrent onset potential of anatase tends to shift negatively (sometimes as much as 0.3 V), thus increasing drastically the difference with respect to the potential of oxygen reduction. The situation is quite different for the rutile samples, the photocurrent onset potential of which appears practically unaffected by the addition of organic species to the solution and remains close to the onset potential of oxygen reduction. The reverse reaction with respect to the photoanodic process, involving species generated by the photo-oxidation of water and OH − ions, is shown to play often an essential role in determining the photocurrent onset potential (i.e. the operating mixed potential) of the photocatalyst. This is the prevailing situation for rutile but is much less frequent for anatase. In fact, a number of species, especially among organic compounds, appear able to compete efficiently for positive holes with OH groups adsorbed at the anatase surface, leading to a virtual suppression of the reverse reaction. In such a case, the reduction of oxygen present in the solution becomes the main cathodic process. This description is consistent with the fact that, in most cases of real photodegradation processes, the anatase form of TiO 2 exhibits much greater activity than the rutile form.