Oxygen surface exchange kinetics of erbia-stabilized bismuth oxide

The surface oxygen exchange kinetics of bismuth\ud oxide stabilized with 25 mol% erbia (BE25) has been studied\ud in the temperature and pO2 ranges 773–1,023 K and 0.1–\ud 0.95 atm, respectively, using pulse-response 18O–16O isotope\ud exchange measurements. The results indicate that BE25\ud exhibits a comparatively high exchange rate, which is rate\ud determined by the dissociative adsorption of oxygen. Defect\ud chemical considerations and the observed pO2\ud 1=2 dependence\ud of the rate of dissociative oxygen adsorption suggest\ud electron transfer to intermediate superoxide ions as the rate\ud determining step in surface oxygen exchange on BE2

Oxygen permeation modelling of perovskites

A point defect model was used to describe the oxygen nonstoichiometry of the perovskites La0.75Sr0.25CrO3, La0.9Sr0.1FeO3, La0.9Sr0.1CoO3 and La0.8Sr0.2MnO3 as a function of the oxygen partial pressure. Form the oxygen vacancy concentration predicte by the point defect model, the ionic conductivity was calculated assuming a vacancy diffusion mechanism. The ionic conductivity was combined with the Wagner model for the oxidation of metals to yield an analytical expression for the oxygen permeation current density as a function of the oxygen partial pressure gradient. A linear boundary condition was used to show the effect of a limiting oxygen exchange rate at the surface

A single-phase gadolinium-doped ceria cathode for highly efficient CO₂ electrolysis

High-temperature solid-oxide CO2 electrolysers enable high-efficiency conversion of electrical energy to valuable fuels and chemicals and as such facilitate a sustainable-energy technology. Conventional cermet-based fuel electrodes used in such solid-oxide cells (SOCs) like nickel-yttria-stabilized zirconia (Ni-YSZ) suffer from morphological degradation and destructive carbon deposition. In recent years, there has been an increasing interest in employing single-phase ceria-based fuel electrodes, which are known to exhibit excellent carbon deposition resistance. Under sufficiently reducing conditions, doped ceria (substituted with trivalent cations such as samarium or gadolinium to generate mobile oxygen vacancies) becomes a mixed ionic-electronic conductor, showing appreciable electronic conductivity. Here, we show for the first time stable high performance in CO2 electrolysis using a ceria-based SOC. The single full cell incorporating a 10 mol% gadolinium-doped ceria (GCO) fuel electrode delivers a current density as high as 1.51 A cm−2 at 800 °C during pure CO2 electrolysis, which is the best electrode performance reported to date among all-ceramic cathode materials. We demonstrate that the electrode performance in CO2 electrolysis is linked with the electronic conductivity, and hence, with the electronic charge carrier concentration in GCO. The results of the present work pave the way for development of robust, nickel-free SOCs for direct CO2 electrolysis.</p

Oxygen separation using mixed ionic-electronic conducting perovskite membranes: Present and prospects

Among novel technologies under development as cost-effective alternatives to conventional oxygen production methods, mixed ionic?electronic conducting ceramic membranes offer great promise. These oxygen-transport membranes selectively separate oxygen from an air supply, or other source, at elevated temperature (700°-1000°C) under an oxygen chemical potential gradient. Besides direct use in schemes for oxygen production, the membranes have attracted much interest for the conversion of natural gas into synthesis gas (CO + H2). A discussion on the oxygen transport properties, oxygen stoichiometry, and phase stability of SrCo0.8Fe0.2O3-d and Ba0.5Sr0.5Co0.8Fe0.2O3-d, and related compositions covers the influence of CO2 adsorption on the kinetics of surface oxygen exchange, chemical expansion, and the phenomenon of kinetic demixing; and prospects to use these materials as oxygen transport membranes. This is an abstract of a paper presented at the 2006 AIChE National Meeting (San Francisco, CA 11/12-17/2006)

Dense ceramic membranes for methane conversion

Dense ceramic membranes made from mixed oxygen-ionic and electronic conducting perovskite-related oxides allow separation of oxygen from an air supply at elevated temperatures (>700 °C). By combining air separation and catalytic partial oxidation of methane to syngas into a ceramic membrane reactor, this technology is expected to significantly reduce the capital costs of conversion of natural gas to liquid added-value products. The present survey is mainly concerned with the material properties that govern the performance of the mixed-conducting membranes in real operating conditions and highlights significant developments in the field

Interpretation of the Gerischer impedance in solid state ionics

The Gerischer impedance in its most elementary form, ZG(ω)=Z0(k+jω)−1/2, has been observed in the frequency response of mixed conducting solid electrolyte systems. This simple transfer function can be derived directly from Fick's second law by including a reaction term. The Gerischer impedance was observed in the electrode dispersion of heavily Tb-doped yttria-stabilised zirconia (YSZ) ceramics. The mixed conducting samples were provided with ionically blocking gold electrodes. The occurrence of a Gerischer impedance in the Tb–YSZ system is tentatively modelled with the formation of immobile complexes of oxygen vacancies and trivalent cations. The presented results indicate that for mixed conducting materials it is not allowed to represent the electronic and ionic conduction path as independent components in an equivalent circuit

Solid state aspects of oxidation catalysis

The main subject of this review is the consideration of catalytic oxidation reactions, which are greatly influenced by solid state effects in the catalyst material. Emphasis is laid upon the correlation between the presence of mobile ionic defects, together with the associated ionic conductivity, and the catalytic performance. Both total and selective oxidation reactions and oxidative conversion reactions are considered. Well-known examples of such behaviour include oxidative methane conversion with lanthanide oxides, carbon monoxide oxidation on fluorite type catalysts, selective olefin oxidation using vanadia based catalysts, etc. Furthermore, because oxygen exchange between gas and solid is always part of the oxidation process, this is considered too.\ud \ud The discussion of the application of the solid oxides under consideration to practically important oxidation processes, together with the influence thereon of their solid state properties, forms a major part of this review. Computational modelling and simulation of catalyst structure and behaviour is also considered.\ud \ud Special attention is given to the potentialities offered by using ionic and mixed conducting oxides either as the electrode material in a solid electrolyte fuel cell (SOFC) or as a separating, dense membrane in a ceramic membrane reactor. The use of porous membranes in such reactors is also taken into consideration. On the one hand these may be used to study the above relationship between catalytic behaviour and solid state properties, on the other hand to obtain a reactor configuration allowing better use of reactants and/or catalysts. Besides the controlled supply of (or removal) of oxygen to (or from) the side where the catalyst and the reactants are located, a promising feature of both experimental approaches is that the oxygen flux may alter the relative presence of different oxygen species (O2,O,O2−,O22−,O3−,O−, etc.) on the catalyst surface. In this way species are provided having a strong influence on the selectivity for partial oxidation reactions and oxidative conversion reactions.\u