Extensive efforts to develop a solid-oxide fuel cell for transportation, the bottoming cycle of a power plant, and distributed generation of electric energy are motivated by a need for greater fuel efficiency and reduced air pollution. Barriers to the introduction of hydrogen as the fuel have stimulated interest in developing an anode material that can be used with natural gas under operating temperatures 650 degrees C < T < 1000 degrees C. Here we report identification of the double perovskites Sr2Mg(1-x)MnxMoO(6-delta) that meet the requirements for long-term stability with tolerance to sulfur and show a superior single-cell performance in hydrogen and methane.
Transport and magnetic properties of polycrystalline La 2 CoMnO 6Ϫ␦ , 0р␦р0.05, have revealed the existence of two distinguishable monoclinic ferromagnetic phases separated by a two-phase domain 0.02р␦ р0.05 and a pseudotetragonal (c/aϽ&) phase with ␦ у0.05 that was prepared at 600°C. A nearly oxygenstoichiometric sample having a magnetization M (5 K, 50 kOe)ϭ5.78 B /f.u. with a Curie temperature T c Ϸ226 K is identified as atomically ordered La 2 Co 2ϩ Mn 4ϩ O 6 containing about 1.8% antiferromagnetic spins at antisites. This ferromagnetic phase is an n-type polaronic conductor that progressively traps mobile electrons at the oxygen vacancies that introduced them on lowering the temperature. Although the x-ray-diffraction pattern can be indexed in orthorhombic ͑Pbnm͒ or monoclinic ( P2 1 /n) symmetry with Ϸ90°, atomic order identifies the space group as P2 1 /n. A second monoclinic, ferromagnetic phase with T c Ͻ150 K and ␦ Ϸ0.05 has a large, positive thermoelectric power that increases progressively with decreasing temperature. Quenching a ␦ Ϸ0.02 sample from 1350°C into liquid N 2 gave a single phase with T c ϭ134 K and ␦ Ϸ0.05. A sample with ␦ у0.05 that was synthesized at 600°C was pseudotetragonal (c/aϽ&) and had a paramagnetic Weiss constant ϽT c Ϸ225 K as well as a significantly smaller magnetization, but its magnetization curve M (T) showed no evidence of spin-glass behavior; its large, positive thermoelectric power was characteristic of polaronic conduction without trapping of mobile charge carriers at lower temperatures. Interpretation of the two phases with ␦Ϸ0.05 is based on the hypothesis that introduction of high-spin Mn 3ϩ by the oxygen vacancies creates around it additional Mn 3ϩ and intermediate-spin Co 3ϩ at neighboring sites; the resulting gain in elastic energy from cooperative, dynamic Jahn-Teller deformations at these ions must be sufficient to overcome the cost of about 0.2 eV for the electron transfer from a Co 2ϩ ion to a Mn 4ϩ ion.
Electrodes F 3000Double Perovskites as Anode Materials for Solid-Oxide Fuel Cells. -The double perovskites Sr2Mg1-xMnxMoO6-δ as anode materials for solid-oxide fuel cells meet the requirements for long-term stability with tolerance to sulfur and show a superior single-cell performance in hydrogen and methane. Preliminary results indicate that optimization of the chemistry and the morphology of these double perovskites can provide an anode material for a solid-oxide fuel cell that operates on natural gas. -(HUANG, Y.-H.; DASS, R. I.; XING, Z.-L.; GOODENOUGH, J. B.; Sci. (Washington, D. C., USA) 312 (2006) 5771, 254-257; Tex. Mater. Inst., Univ. Tex., Austin, TX 78712, USA; Eng.) -W. Pewestorf 28-016
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