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

The electronic structure of the perovskite La1−xSrxCoO3 has been obtained as a function of Sr substitution and volume from a series of generalized-gradient-corrected, full-potential, spin-density-functional band-structure calculations. The energetics of different spin configurations are estimated using the fixed-spin-moment (FSM) method. From the total energy versus spin magnetic moment curve for LaCoO3 the ground state is found to be nonmagnetic with the Co ions in a low-spin (LS) state, a result that is consistent with the experimental observations. Somewhat higher in energy, we find an intermediate-spin (IS) state with spin moment ∼1.2μB/%f.u. From the anomalous temperature dependent susceptibility along with the observation of an IS state we predict metamagnetism in LaCoO3 originating from an LS-to-IS transition. The IS state is found to be metallic and the high-spin (HS) state of LaCoO3 is predicted to be a half-metallic ferromagnet. With increasing temperature, which is simulated by a corresponding ...

Itinerant metamagnetism and possible spin transition in LaCoO3 by temperature/hole doping

The electronic structure of the perovskite La$_{1-x}$Sr$_x$CoO$_3$ has been obtained as a function of Sr substitution and volume from a series of generalized-gradient-corrected, full-potential, spin-density-functional band structure calculations. The energetics of different spin configurations are estimated using the fixed-spin-moment (FSM) method. From the total energy vs spin magnetic moment curve for LaCoO$_3$ the ground state is found to be nonmagnetic with the Co ions in a low-spin (LS) state, a result that is consistent with the experimental observations. Somewhat higher in energy, we find an intermediate-spin (IS) state with spin moment $\sim$1.2 $\mu_{B}$/f.u. 
From the anomalous temperature dependent susceptibility along with the observation of an IS state we predict metamagnetism in LaCoO$_3$ originating from an LS-to-IS transition. The IS state is found to be metallic and the high-spin (HS) state of LaCoO$_3$ is predicted to be a half-metallic ferromagnet. With increasing temperature, which is simulated by a corresponding change of the lattice parameters we have observed the disappearence of the metamagnetic solution that is associated with the IS state. The FSM calculations on La$_{1-x}$Sr$_x$CoO$_3$ suggest that the hole doping stabilizes the IS state and the calculated magnetic moments are in good agreement with the corresponding experimental values. Our calculations show that the HS state cannot be stabilized by temperature or hole doping since the HS state is significantly higher in energy than the LS or IS state. Hence the spin-state transition in LaCoO$_3$ by temperature/hole doping is from an LS to an IS spin state and the present work rules out the other possibilities reported in the literature.

https://arxiv.org/pdf/cond-mat/0104308.pdf

Itinerant metamagnetism and possible spin transition in LaCoO$_3$ by temperature/hole doping

59 Co and 139 La NMR measurements have been performed to study the microscopic magnetic properties closely related with the spin state of a trivalent cobalt ion in LaCoO 3 . The temperature-independent 59 Co and 139 La Knight shifts below ∼ 30 K clearly revealed the presence of a nonmagnetic ground state at low temperatures. We calculated the temperature dependence of the magnetic susceptibility χ and the 59 Co Knight shift 59 K in a single ion model with the low-spin 1 A 1 ground state above which the high-spin 5 T 2 state is located. Then, both the trigonal crystal-field and spin-orbit interactions are taken into account. From this analysis, we infer the presence of an antiferromagnetic exchange interaction between the high-spin states. The characteristic temperature dependence of the 59 Co hyperfine coupling constant is also discussed.

Spin State and Hyperfine Interaction in LaCoO3 : NMR and Magnetic Susceptibility Studies

The itinerant electron ferromagnetism in La1-xSrxCoO3 (0.1 or=0.3). Electrical resistivity studies show that itinerant electron behaviour with a positive temperature coefficient of resistance at low temperatures is obtained for x >or=0.2. Metallization and long-range ferromagnetic order thus seems to be associated with different origins. Chemical systematics obtained from the changes in lattice parameters show that the expansion of LaCoO3 on the substitution of La by Sr stabilizes the paramagnetic states of Co ions which are in predominantly diamagnetic low-spin state in LaCoO3. The core S=1 spins of trivalent Co seems to be coupled ferromagnetically by the itinerant electron in a manner similar to the Zener double-exchange mechanism.

P N Santhosh

Papers

On the origin of itinerant electron behaviour and long-range ferromagnetic order in La1-xSrxCoO3