Publisher Summary This chapter reviews the recent developments in the area of mixed ionic-electronic conducting membranes for oxygen separation, in which the membrane material is made dense—that is, free of cracks and connected-through porosity, being susceptible only for oxygen ionic and electronic transport. Emphasis is on the defect chemistry, mass transport, and the associated surface exchange kinetics. The basic elements of mixed ionic and electronic transport through dense ceramic membranes are focused. The chapter discusses mixed-conducting acceptor-doped perovskite and perovskite-related oxides and gives examples to illustrate the fundamental factors determining the oxygen fluxes through dense ceramic membranes. A key factor in the possible application of oxygen ion conducting ceramics is that, for use as solid electrolyte in fuel cells, batteries, oxygen pumps or sensors, their electronic transport number should be as low as possible. Stimulated by the search for candidate materials for electrodes in solid oxide fuel cells (SOFC) and oxygen separation membranes, researchers have explored the possibility of introducing electronic conductivity in oxygen-ion conducting fluorite-type matrices by doping with multi-valent dopants.

Chapter 10 Dense ceramic membranes for oxygen separation

Transition-metal oxides are a promising class of semiconductors for the oxidation of water, a process that underpins both photoelectrochemical water splitting and carbon dioxide reduction. However, these materials are limited by very slow charge transport. This is because, unlike conventional semiconductors, material aspects of metal oxides favor the formation of slow-moving, self-trapped charge carriers: small polarons. In this Perspective, we seek to highlight the salient features of small-polaron transport in metal oxides, offer guidelines for their experimental characterization, and examine recent transport studies of two prototypical oxide photoanodes: tungsten-doped monoclinic bismuth vanadate (W:BiVO4) and titanium-doped hematite (Ti:α-Fe2O3). Analysis shows that conduction in both materials is well-described by the adiabatic small-polaron model, with electron drift mobility (distinct from the Hall mobility) values on the order of 10–4 and 10–2 cm2 V–1 s–1, respectively. Future directions to build ...

Unravelling Small-Polaron Transport in Metal Oxide Photoelectrodes

The oxygen vacancy in WO3 has previously been implicated in the electrochromism mechanism in this material. Previous theoretical calculations on the oxygen vacancy in WO3 have not considered the full range of crystal structures adopted by the material. Here we report studies of the oxygen vacancy in seven crystal phases. The use of a very accurate tungsten plane-wave pseudopotential means that a byproduct of this study is a more detailed and complete picture of undefected WO3 than previously available. Electronic structures of the crystal phases in both undefected and defected systems have been calculated and are discussed. The band gap in WO3 is dependent upon bonding−antibonding interactions, these being dependent upon overlap in each direction. The effect of an oxygen vacancy is dependent upon the availability of both Op and Wd electrons, this being different for the various phases. A variety of behavior is predicted, which may be explained in terms of O2p−W5d mixing, including the formation of long W−...

The Oxygen Vacancy in Crystal Phases of WO3

This chapter provides an overview of major membrane concepts and materials. It discusses besides membranes made from a mixed ionic–electronic conducting, other membranes incorporating an oxygen ion conductor and presents data from oxygen permeability measurements on selected membrane materials. The rate at which oxygen permeates through a nonporous ceramic membrane is essentially controlled by two factors: the rate of solid state diffusion within the membrane and that of interfacial oxygen exchange on either side of the membrane. The basic assumption of the theory is that the lattice diffusion of oxygen or the transport of electronic charge carriers through the bulk oxide determines the rate of overall oxygen permeation. Besides the possibility of surface exchange limitations, oxygen transport through dense ceramics is necessarily influenced by the presence of high-diffusivity paths along internal surfaces such as grain boundaries.

Dense ceramic membranes for oxygen separation

The photocatalytic activities of four different commercially available iron (hydr)oxides semiconductors, i.e. hematite (alpha-Fe2O3), goethite (alpha-FeOOH), wustite (FeO) and magnetite (Fe3O4), were evaluated for bacteria inactivation at neutral pH in the absence or presence of H2O2. Our results showed that heterogeneous photocatalysis and/or photo-Fenton processes catalyzed by low concentrations of reagents (0.6 mg/L Fe3+ and 10 mg/L H2O2) under sunlight may serve as a disinfection method for waterborne bacterial pathogens. In particular, we found that, with the exception of magnetite which need H2O2 as electron acceptor, all the other semiconductor iron (hydr)oxides were photoactive under sunlight in absence of H2O2 (using only oxygen as electron acceptor). Furthermore, for all iron (hydr)oxide studied in this work, no bacterial reactivation and/or growth was observed after photo-Fenton treatment. The same antimicrobial activity was obtained for the photocatalytic semiconducting action of hematite and goethite. Additionally, a delayed disinfection effect was observed to continue in the dark for the photo-assisted wilstite-based treatment. Electron spin resonance (ESR) in combination with spin-trapping was employed to detect reactive oxygen species (ROS) involved in heterogeneous photocatalysis and/or photo-Fenton treatments mediated by iron (hydr)oxide particles. In particular, ESR confirmed that center dot OH and O-2(center dot-) radicals were the principal ROS produced under photo-assisted action of iron (hydr)oxide particles in the absence or presence of H2O2. We also found that the components of natural water (i.e. natural organic matter (NOM) and inorganic substances) did not interfere with the photocatalytic semiconducting action of hematite to bacterial inactivation. However, these components enhance the bacterial inactivation by heterogeneous photo-Fenton action of hematite. Overall our results demonstrated, for the first time, that low concentration of iron (hydr)oxides, acting both as photocatalytic semiconductors or catalysts of the heterogeneous photo-Fenton process at neutral pH, may provide a useful strategy for efficient bacterial disinfection. (C) 2014 Elsevier B.V. All rights reserved.

Marc Onillon

Papers

Influence of the elaboration and dehydration conditions of Fe(III) hydrous oxides on the characteristics of resulting α-Fe2O3 powders

Influence of structural defects on the electrical properties of α-Fe2O3 ceramics

Influence de la pression partielle d'oxygene sur la nature des defauts ponctuels dans Ca2LaFe3O8+x

Experimental study of the non stoichiometry of cesium antimonide ≈ Cs3Sb

Etude par mesures electriques des correlations entre defauts de structure et potentiel chimique de l'oxygene dans WO3 non-stoechiometrique