Ronald I. Dass

Bio: Ronald I. Dass is an academic researcher from University of Texas at Austin. The author has contributed to research in topics: Ferromagnetism & Magnetoresistance. The author has an hindex of 11, co-authored 14 publications receiving 1847 citations.

Double Perovskites as Anode Materials for Solid-Oxide Fuel Cells

Abstract: 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.

Multiple magnetic phases of La 2 CoMnO 6 − δ ( 0 δ 0 . 0 5 )

Abstract: Transport and magnetic properties of polycrystalline ${\mathrm{La}}_{2}{\mathrm{CoMnO}}_{6\ensuremath{-}\ensuremath{\delta}},$ $0l~\ensuremath{\delta}l~0.05,$ have revealed the existence of two distinguishable monoclinic ferromagnetic phases separated by a two-phase domain $0.02l~\ensuremath{\delta}l~0.05$ and a pseudotetragonal $(c/al\sqrt{2})$ phase with $\ensuremath{\delta}g~0.05$ that was prepared at 600 \ifmmode^\circ\else\textdegree\fi{}C. A nearly oxygen-stoichiometric sample having a magnetization $M(5\mathrm{K},50\mathrm{kOe})=5.78{\ensuremath{\mu}}_{B}/\mathrm{f}.\mathrm{u}.$ with a Curie temperature ${T}_{c}\ensuremath{\approx}226\mathrm{K}$ is identified as atomically ordered ${\mathrm{La}}_{2}{\mathrm{Co}}^{2+}{\mathrm{Mn}}^{4+}{\mathrm{O}}_{6}$ 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 (Pbnm) or monoclinic ${(P2}_{1}/n)$ symmetry with $\ensuremath{\beta}\ensuremath{\approx}90\ifmmode^\circ\else\textdegree\fi{},$ atomic order identifies the space group as ${P2}_{1}/n.$ A second monoclinic, ferromagnetic phase with ${T}_{c}l150\mathrm{K}$ and $\ensuremath{\delta}\ensuremath{\approx}0.05$ has a large, positive thermoelectric power that increases progressively with decreasing temperature. Quenching a $\ensuremath{\delta}\ensuremath{\approx}0.02$ sample from 1350 \ifmmode^\circ\else\textdegree\fi{}C into liquid ${\mathrm{N}}_{2}$ gave a single phase with ${T}_{c}=134\mathrm{K}$ and $\ensuremath{\delta}\ensuremath{\approx}0.05.$ A sample with $\ensuremath{\delta}g~0.05$ that was synthesized at 600 \ifmmode^\circ\else\textdegree\fi{}C was pseudotetragonal $(c/al\sqrt{2})$ and had a paramagnetic Weiss constant $\ensuremath{\theta}l{T}_{c}\ensuremath{\approx}225\mathrm{K}$ 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 $\ensuremath{\delta}\ensuremath{\approx}0.05$ is based on the hypothesis that introduction of high-spin ${\mathrm{Mn}}^{3+}$ by the oxygen vacancies creates around it additional ${\mathrm{Mn}}^{3+}$ and intermediate-spin ${\mathrm{Co}}^{3+}$ 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 ${\mathrm{Co}}^{2+}\mathrm{ion}$ to a ${\mathrm{Mn}}^{4+}\mathrm{ion}.$

Synthesis and Characterization of Sr2MgMoO6 − δ An Anode Material for the Solid Oxide Fuel Cell

Abstract: The double-perovskite Sr 2 MgMoO 6-δ (SMMO) was investigated as an anode material of a solid oxide fuel cell. Via a synthetic method based on thermal decomposition of metal complexes with ethylenediaminetetraacetic acid as the complexant, phase-pure SMMO was readily obtained. Oxygen vacancies are introduced by reduction with 5% H 2 at 800°C. With a 300 μm thick La 0.8 Sr 0.2 Ga 0.83 Mg 0.17 O 2.815 disk as the electrolyte and SrCo 0.8 Fe 0.2 O 3-δ as the cathode, the SMMO anode showed power densities of 0.84 W/cm 2 in H 2 and 0.44 W/cm 2 in CH 4 at 800°C. Moreover, it performed stably on power cycling and tolerated sulfur and moisture well. Only 1% degradation in the output was observed in H 2 containing 5 parts per million (ppm) H 2 S and 16% degradation in H 2 containing 50 ppm H 2 S compared with the output in pure H 2 . Thermogravimetric analysis showed a drop in mass at around 750°C in the atmospheres of both air and 5% H 2 , indicative of the formation of oxygen vacancies. The mean thermal expansion coefficient was a = 12.7 X 10 -6 K -1 at the operating temperatures. The conductivity strongly depended on the atmosphere, and the electronic activation energies were E a = 0.084 eV in H 2 and 0.126 eV in CH 4 . Our results show that SMMO is a potential anode material for operation with natural gas.

Intra- versus intergranular low-field magnetoresistance of Sr2FeMoO6 thin films

Abstract: Thin films of (001)-oriented Sr2FeMoO6 have been epitaxially deposited on LaAlO3 and SrTiO3 (001) substrates. Comparison of their transport and magnetic properties with those of polycrystalline ceramic samples shows a metallic versus semiconductor temperature dependence and a saturation magnetization Ms at 10 K of 3.2 μB/f.u. in the film as against 3.0 for a tetragonal polycrystalline sample. However, the Curie temperature TC≈389 K is reduced from 415 K found for the tetragonal ceramic, which lowers Ms at 300 K in the thin films to 2.0 μB/f.u. compared to 2.2 μB/f.u. in the ceramics. A Wheatstone bridge arrangement straddling a bicrystal boundary has been used to verify that spin-dependent electron transfer through a grain boundary is responsible for the low-field magnetoresistance found in polycrystalline samples below TC.

Lowering the Temperature of Solid Oxide Fuel Cells

Abstract: Fuel cells are uniquely capable of overcoming combustion efficiency limitations (e.g., the Carnot cycle). However, the linking of fuel cells (an energy conversion device) and hydrogen (an energy carrier) has emphasized investment in proton-exchange membrane fuel cells as part of a larger hydrogen economy and thus relegated fuel cells to a future technology. In contrast, solid oxide fuel cells are capable of operating on conventional fuels (as well as hydrogen) today. The main issue for solid oxide fuel cells is high operating temperature (about 800°C) and the resulting materials and cost limitations and operating complexities (e.g., thermal cycling). Recent solid oxide fuel cells results have demonstrated extremely high power densities of about 2 watts per square centimeter at 650°C along with flexible fueling, thus enabling higher efficiency within the current fuel infrastructure. Newly developed, high-conductivity electrolytes and nanostructured electrode designs provide a path for further performance improvement at much lower temperatures, down to ~350°C, thus providing opportunity to transform the way we convert and store energy.

Materials for Solid Oxide Fuel Cells

Abstract: Solid oxide fuel cells (SOFCs) have the promise to improve energy efficiency and to provide society with a clean energy producing technology. The high temperature of operation (500−1000 °C) enables the solid oxide fuel cell to operate with existing fossil fuels and to be efficiently coupled with turbines to give very high efficiency conversion of fuels to electricity. Solid oxide fuel cells are complex electrochemical devices that contain three basic components, a porous anode, an electrolyte membrane, and a porous cathode. In this short review, a survey of the types and properties of materials that have been considered for each of these components is presented with an emphasis on the requirements for operation at intermediate temperature (500−800 °C). Some directions for future research are discussed.

Enhanced Sulfur and Coking Tolerance of a Mixed Ion Conductor for SOFCs: BaZr0.1Ce0.7Y0.2–xYbxO3–δ

Abstract: The anode materials that have been developed for solid oxide fuel cells (SOFCs) are vulnerable to deactivation by carbon buildup (coking) from hydrocarbon fuels or by sulfur contamination (poisoning). We report on a mixed ion conductor, BaZr(0.1)Ce(0.7)Y(0.2-)(x)Yb(x)O(3-delta), that allows rapid transport of both protons and oxide ion vacancies. It exhibits high ionic conductivity at relatively low temperatures (500 degrees to 700 degrees C). Its ability to resist deactivation by sulfur and coking appears linked to the mixed conductor's enhanced catalytic activity for sulfur oxidation and hydrocarbon cracking and reforming, as well as enhanced water adsorption capability.