Recent worldwide interest in building a decentralized, hydrogen-based energy economy has refocused attention on the solid oxide fuel cell (SOFC) as a potential source of efficient, environmentally friendly, fuel-versatile electric power. Due to its high operating temperature, the SOFC offers several potential advantages over polymer-based fuel cells, including reversible electrode reactions, low internal resistance, high tolerance to typical catalyst poisons, production of high-quality waste heat for (among other uses) reformation of hydrocarbon fuels, as well as the possibility of burning hydrocarbon fuels directly. Today, SOFCs are much closer to commercial reality than they were 20 years ago, due largely to technological advances in electrode material composition, microstructure control, thin-film ceramic fabrication, and stack and system design. These advances have led to dozens of active SOFC development programs in both stationary and mobile power and contributed to commercialization or development in a number of related technologies, including gas sensors,1 solid-state electrolysis devices,2 and iontransport membranes for gas separation and partial oxidation.3 Many reviews are available which summarize the technological advances made in SOFCs over the last 15-35 yearssreaders who are primarily interested in knowing the state-of-the art in materials, design, and fabrication (including the electrodes) are encouraged to consult these reviews.4-12 This review focuses on the factors governing SOFC cathode performancesadvances we have made over † Dedicated to Brian Steele, 1929-2003. Researcher, Entrepreneur, Consensus Seeker. 4791 Chem. Rev. 2004, 104, 4791−4843

Factors Governing Oxygen Reduction in Solid Oxide Fuel Cell Cathodes

Progress in material selection for solid oxide fuel cell technology: A review

Oxidation into CO2 is a major solution to CO abatement in air depollution treatments. The development of catalytic converters led to an extraordinary high number of publications on metal catalysts during the last fifty years. Due to the increasing price of noble metals and to remarkable progresses in oxide syntheses, catalytic oxidation of carbon monoxide over oxide catalysts has recently gained in interest, even if some oxides are known to present remarkable activity since the beginning of the 20th century. In this Review, the kinetics and mechanism of CO oxidation on single and mixed oxides are examined, alongside the catalyst structures

Catalytic Oxidation of Carbon Monoxide over Transition Metal Oxides

An overview is given on the ongoing activities in the area of the decomposition of nitrous oxide, N2O, over solid catalysts. These catalysts include metals, pure and mixed oxides, supported as well as unsupported, and zeolitic systems. The review covers aspects of the reaction mechanism and kinetics, focusing on the role of surface oxygen, the inhibition by molecular oxygen, water and other species, poisoning phenomena and practical developments.

Heterogeneous catalytic decomposition of nitrous oxide

Structure and electrical properties of La1−xSrxCo1−yFeyO3. Part 1. The system La0.8Sr0.2Co1−yFeyO3

M\"ossbauer studies combined with magnetic-susceptibility data on well-characterized LaCo${\mathrm{O}}_{3}$ in the 4.2-1200-K region show that cobalt ions exist predominantly in the low-spin ${\mathrm{Co}}^{\mathrm{III}}$ state at low temperatures which transform partially to high-spin ${\mathrm{Co}}^{3+}$ ions up to 200 K. Above 200 K, ${\mathrm{Co}}^{3+}$ and ${\mathrm{Co}}^{\mathrm{III}}$ ion pairs transform to ${\mathrm{Co}}^{\mathrm{II}}$ and ${\mathrm{Co}}^{4+}$ pairs. At high temperatures, the population of ${\mathrm{Co}}^{3+}$ decreases significantly and completely disappears at the localized-electron-collective-electron transition temperature at 1210 K. The variations of the Lamb-M\"ossbauer factor and the center shift with temperature provide valuable information on the high-spin-low-spin equilibria, as well as on the nature of the phase transitions and symmetry changes in LaCo${\mathrm{O}}_{3}$. All these changes are reflected in the transport properties of LaCo${\mathrm{O}}_{3}$. There appears to be little doubt that the first-order localized-electron-collective-electron transition in LaCo${\mathrm{O}}_{3}$ is caused essentially by the change in entropy of the $d$ electrons.

Mossbauer studies of the high-spin-low-spin equilibria and the localized-collective electron transition in LaCoO 3

Mossbauer and other studies establish that in La 1-x Sr x CoO 3 (x>0.125), ferromagnetic Sr 2+ -rich clusters coexist with paramagnetic La 3+ -rich regions in the same crystallographic phase, with the ferromagnetic component increasing with increasing x and decreasing T. The 3d holes created by Sr 2+ substitution are itinerant both above and below T C . All the experimental observations on this system can be explained on the basis of itinerant-electron ferromagnetism.

Itinerant-electron ferromagnetism in La 1−x Sr x CoO 3 : a Mössbauer study

Magnetic susceptibility measurements and Mossbauer spectra show that NdCoO3 and GdCoO3 contain predominantly low-spin CoIII ions at low temperatures which transform partially to high-spin Co3+ until a certain temperature. Beyond this temperature, there is a transfer of eg electrons from Co3+ to CoIII giving rise to di- and tetra-valent Co species followed by short-range ordering. Co3+ ions completely disappear at about 1000 K where there is a gradual transition from localized electron behaviour to collective behaviour. The cobaltates are semi-metals beyond this transition. The spin state equilibria and the electronic transition described here find support from electron transport properties, Mossbauer parameters as well as differential thermal analysis and X-ray data.

Spin state equilibria and localized versus collective d-electron behaviour in neodymium and gadolinium trioxocobaltate(III)

Rare earth cobaltite catalysts: Relation of activity to spin and valence states of cobalt

Mossbauer studies show that cobalt ions in LaCoO 3 exist predominantly in the low-spin Co III state at low temperatures and partially transform to the high-spin Co 3+ state up to 200 K. Above 200 K, Co II and Co 4+ ions are formed by the transfer of d electrons from Co 3+ to Co III ; Co 3+ ions completely disappear at the first-order localized-to-itinerant electron transition temperature.

G. Rama Rao

Papers

Mossbauer studies of the high-spin-low-spin equilibria and the localized-collective electron transition in LaCoO 3

Itinerant-electron ferromagnetism in La 1−x Sr x CoO 3 : a Mössbauer study

Spin state equilibria and localized versus collective d-electron behaviour in neodymium and gadolinium trioxocobaltate(III)

Rare earth cobaltite catalysts: Relation of activity to spin and valence states of cobalt

Localized-to-itinerant electron transitions in rare-earth cobaltates