Truls Norby

Hydrogen in oxides.

The paper reviews the history and present understanding of protons in oxides; their defect chemistry, thermodynamics, and transport. Focus is put on correlations between hydration thermodynamics and other materials properties which may help to predict proton uptake and proton conduction in oxides. Also effects of defect association and the particular problem of high grain boundary resistance in high temperature proton conductors are addressed. In the second part, a number of experimental observations attributed to the presence of hydride ions under mildly reducing conditions are discussed in relation to the unlikelihood that general thermodynamic considerations predict of finding these species under such conditions.

The promise of protonics

Fuel cells seem a good alternative to dirty and wasteful combustion. But known electrolytes — the crucial component — all have flaws. A new solid-state proton conductor suggests a solution.

Concentration and transport of protons in oxides

Abstract The defect chemistry of protons in oxides is well understood, but relationships between proton concentration and structure/composition are still rather empirical. The mechanism of proton transport has been clarified by various techniques, including simulations which now largely confer with experimental data. New proton conductors are being developed mainly among complex perovskite-related oxides with disordered or crystallographic oxygen deficiencies.

Protons in rare earth oxides

Abstract The electrical conductivity of Gd2O3, Dy2O3 and Er2O3 nominally doped with 2 mol% CaO, has been measured as a function of PH2o (3 × 10−5 − 2 × 10−2 atm) and PO2 (1–10−20 atm) in the temperature range 500–1200 °C. A defect structure with protons and oxygen vacancies compensating the acceptor dopants has been used to model the total conductivity in terms of partial conductivities. This allows determination of pre-exponential terms and enthalpies of mobilities and equilibrium constants for the formation of defects. The CaO-doped rare earth oxides are mainly protonic conductors at high PH2O and low po2 and temperature. The reaction H 2 O(g) + V o = 2H i + O o x has a negative enthalpy change, and thus oxygen vacancies are the dominant ionic charge carriers at high temperatures and protons at lower temperatures. The enthalpy for the reaction is increasingly negative with increasing molar density, which, in turn, is a measure of the enthalpy of formation of the Ln2O3 oxides and of the enthalpy of formation of oxygen vacancies.

Correlation between the characteristic green emissions and specific defects of ZnO

In this work, the correlation between the characteristic green emissions and specific defects of ZnO was investigated through a series of experiments that were designed to separate the subtle interplays among the various types of specific defects. With physical analysis and multimode Brownian oscillator modeling, the underlying mechanisms of the variant effects on green emission were revealed. The results demonstrate that the observed green emissions can be identified as two types of individual emissions, namely high energy and low energy, that are associated with specific defects and their locations. The surface modification that leads to downwards band bending was found to be responsible for the high-energy green emission. The relationship between the intensity of the low- energy green emission and the crystallographic lattice contraction indicates that oxygen vacancy is the dominant cause of such an emission that resides within the bulk of ZnO.

Redox energetics of perovskite-related oxides

The present paper focuses on the redox properties of perovskite-related oxides and presents a solution model that connects integral thermodynamic properties that are measured calorimetrically with partial thermodynamic quantities that are measured by equilibration methods. The model allows us to extract significant features of the redox energetics of non-stoichiometric oxides. It is shown that the redox behavior, e.g. the composition–partial-pressure isotherms, is independent of the stability of the non-stoichiometric oxides and as a first approximation is directly given by the relative stability of the oxidation states involved. The stability of a given oxidation state is related to the structure of the oxide, and the large difference in redox behavior between hexagonal and cubic SrMnO3xa0−xa0δ is rationalized. Whereas both the enthalpy and entropy of oxidation in general depend on temperature, a number of systems can nbe adequately described using an ideal solution approach. This implies that composition-independent enthalpic and entropic terms can be used as first approximations to describe the redox energetics of non-stoichiometric oxides. In order to illustrate the approach an overview of the redox energetics of selected La1xa0−xa0xAexMO3xa0−xa0δ phases (Ae alkaline earth, M transition metal) of interest in connection with solid oxide fuel cell and gas separation membrane applications is given.

Protonic conductivity in Ca-doped yttria

Abstract The ac conductivity of Ca-doped Y 2 O 3 has been measured as a function of doping level (0.3−10 mol% CaO), temperature (800–1200°C), partial pressure of oxygen (1–10 -20 atm) and of water vapour (3×10 −5 −2×10 −2 atm ). The ac conductivity has been delineated into contributions from protons, native ions, and electron holes. The results can be rationalized in terms of a defect structure comprising electron holes, oxygen vacancies, and protons as the positive defects compensating the negatively charged dopant ions. The conductivities of protons, native ions, and electron holes all increase with the doping level up to about 1 mol% CaO. Above this dopant concentration the conductivity levels off, and this probably reflects that the solubility limit of CaO in yttria is then exceeded in this temperature region.

Hydrogen Defects in Inorganic Solids

Abstract The defect chemistry and transport of hydrogen in ceramics, as well as relevant experimental methods, are reviewed, with examples from the literature. The treatment concentrates on behaviour of protons in oxides at high temperatures, but often in view of the more established low–temperature chemistry of protons. Other classes of compounds and other hydrogen species are discussed more briefly.

High Temperature Proton Conductors

Most of us are familiar with the use of electrolytes such as the aqueous sulphuric acid in lead accumulators and the alkaline solutions used for electrolytic production of hydrogen. Alkaline fuel cells (AFC), phosphoric acid fuel cells (PAFC) and molten cabonate fuel cells (MCFC) have been around for a few years. All these examples involve liquid electrolytes (although the liquid can be absorbed in a porous support structure). Cells with solid electrolytes may in principle be more compact and sturdy, easier and safer to operate, and less corrosive. Solid electrolytes carrying oxygen or hydrogen ions (or combinations) have particularly broad interest.

Titanium Dioxide Photocatalyst - Unresolved Problems

The present work considers the performance of TiO2-based photosensitive oxide semiconductors as photocatalysts for water purification. This paper brings together the concepts of solid state chemistry for nonstoichiometric compounds and the concepts of photocatalysis in order to discuss the reactivity between TiO2 and water including microorganisms (bacteria and viruses). The performance of TiO2 photocatalysts are considered in terms of a model of photoelectrochemical cell. The experimental data on photocatalytic removal of microorganisms from water are considered in terms of the effect of several properties, including pH, dispersion, light intensity, and temperature. It is argued that correct understanding of the performance of TiO2 photocatalysts requires recognition that properties of TiO2, which is a nonstoichiometric compound, are determined by defect disorder and the related ability to donate or accept electrons. The photocatalytic properties of TiO2 are considered in terms of the reactivity of both anodic and cathodic sites with water and the related charge transfer at the TiO2/H2O interface. It is shown that the formation of well defined photocatalysts requires knowledge of mass and charge transfer during processing and performance, respectively. The main hurdles in the development of high-performance photocatalysts are discussed.