Scott A. Barnett

A direct-methane fuel cell with a ceria-based anode

Fuel cells constitute an attractive power-generation technology that converts chemical energy directly and with high efficiency into electricity while causing little pollution. Most fuel cells require hydrogen as the fuel, but viable near-term applications will need to use the more readily available hydrocarbons, such as methane. Present-day demonstration power plants and planned fuel-cell electric vehicles therefore include a reformer that converts hydrocarbon fuel into hydrogen. Operating fuel cells directly on hydrocarbons would obviously eliminate the need for such a reformer and improve efficiency. In the case of polymer-electrolyte fuel cells, which have been studied for vehicle applications, the direct use of methanol fuel has been reported, but resulted in fuel permeating the electrolyte,. Solid oxide fuel cells — promising candidates for stationary power generation — can also use hydrocarbon fuel directly to generate energy, but this mode of operation resulted in either carbon deposition at high temperatures or poor power output at low operating temperatures. Here we report the direct electrochemical oxidation of methane in solid oxide fuel cells that generate power densities upto 0.37 W cm−2 at 650 °C. This performance is comparable to that of fuel cells using hydrogen, and is achieved by using ceria-containing anodes and low operating temperatures to avoid carbon deposition. We expect that the incorporation of more advanced cathodes would further improve the performance of our cells, making this solid oxide fuel cell a promising candidate for practical and efficient fuel-cell applications.

Growth of single‐crystal TiN/VN strained‐layer superlattices with extremely high mechanical hardness

Single‐crystal TiN/VN strained‐layer superlattices (SLS’s) with layer thicknesses lTiN =lVN =λ/2 (where λ is the period of the superlattice) ranging from 0.75 to 16 nm have been grown on MgO(100 ) substrates by reactive magnetron sputtering. Cross‐sectional transmission electron microscopy (TEM) and x‐ray diffraction examinations showed that the films were single crystals exhibiting coherent interfaces and several orders of superlattice reflections. There was no evidence in either plan‐view or cross‐sectional TEM analyses of misfit interfacial dislocation arrays. The primary defects observed were dislocation loops with a diameter of 8–10 nm extending through several layers and small defects with a diameter of 1–2 nm that were confined within single layers. Microindentation hardness values H, measured as a function of λ in films with a total thickness of 2.5 μm, increased from 2035±280 kg mm−2 for Ti0.5V0.5N alloys (i.e., λ=0) to reach a maximum of 5560±1000 kg mm−2 at λ=5.2 nm and then decreased rapidly t...

Electrochemical performance of (La,Sr)(Co,Fe)O3–(Ce,Gd)O3 composite cathodes

Abstract La0.6Sr0.4Co0.2Fe0.8O3 (LSCF) and LSCF–Ce0.8Gd0.2O3 (GDC) composite cathodes on YSZ electrolytes were studied for potential applications in low-temperature solid oxide fuel cells (SOFCs). Impedance spectroscopy measurements were performed in oxygen partial pressures ranging from 10−3 to 1 atm over a temperature range from 500 to 750 °C. The LSCF electrodes yielded low-current interfacial resistance values that were a factor of 10 lower than for (La,Sr)MnO3 cathodes. The addition of 50 vol.% GDC to LSCF resulted in an additional factor of ≈10 decrease in the polarization resistance. Polarization resistance values for the LSCF–GDC cathodes were as low as 0.01 Ω cm2 at 750 °C and 0.33 Ω cm2 at 600 °C.

A thermally self-sustained micro solid-oxide fuel-cell stack with high power density

High energy efficiency and energy density, together with rapid refuelling capability, render fuel cells highly attractive for portable power generation. Accordingly, polymer-electrolyte direct-methanol fuel cells are of increasing interest as possible alternatives to Li ion batteries. However, such fuel cells face several design challenges and cannot operate with hydrocarbon fuels of higher energy density. Solid-oxide fuel cells (SOFCs) enable direct use of higher hydrocarbons, but have not been seriously considered for portable applications because of thermal management difficulties at small scales, slow start-up and poor thermal cyclability. Here we demonstrate a thermally self-sustaining micro-SOFC stack with high power output and rapid start-up by using single chamber operation on propane fuel. The catalytic oxidation reactions supply sufficient thermal energy to maintain the fuel cells at 500–600 °C. A power output of ∼350 mW (at 1.0 V) was obtained from a device with a total cathode area of only 1.42 cm2.

Model of superlattice yield stress and hardness enhancements

A model is presented that explains the yield stress and hardness enhancements that have been observed in superlattice thin films. The stress required for dislocations to glide across layers with different shear moduli was calculated using an expression that accounts for core effects and all interfaces in trapezoidal or sawtooth composition modulations. The predicted strength/hardness enhancement increased with increasing superlattice period Λ, before reaching a saturation value that depended on interface widths. A second mechanism, where dislocations glide within individual layers, was important at large Λ and gave a decrease in strength/hardness with increasing Λ. The combination of these two mechanisms gives a strength/hardness maximum versus Λ in good quantitative agreement with experimental results for nitride and metal superlattices. The results indicate that superlattice strength/hardness depends strongly on interface widths and the difference in shear moduli of the two components for Λ values below the maximum, and on the average shear modulus for larger Λ.

(La,Sr)MnO3-(Ce,Gd)O2-x composite cathodes for solid oxide fuel cells

Abstract Impedance spectroscopy was used to measure the oxygen reaction kinetics of composite cathodes consisting of La0.8Sr0.2MnO3 (LSM) and Ce0.8Gd0.2O2−x (GDC) on yttria-stabilized zirconia (YSZ) and GDC electrolytes. The LSM–GDC electrodes are analogous to the widely used LSM–YSZ cathodes but have potential advantages since LSM is non-reactive with GDC and the ionic conductivity of GDC is higher than that of YSZ. LSM–GDC cathodes with 0–60 wt.% GDC were characterized over a temperature range of 600–750°C at oxygen partial pressures between 10−3 and 1 atm. Using YSZ electrolytes, the low-current interfacial resistance was smallest for 50 wt.% GDC, where the value was 0.49 Ω cm2 at 750°C. This is two to three times lower than that for LSM–YSZ composite cathodes on YSZ with similar structure. The majority of the LSM–GDC cathodes appeared to be limited by adsorption, as well as oxygen ion transfer into the electrolyte.

Growth, structure, and microhardness of epitaxial TiN/NbN superlattices

Epitaxial TiN/NbN superlattices with wavelengths/ranging from 1.6 to 450 nm have been grown by reactive magnetron sputtering on MgO(100). Cross-sectional transmission electron microscopy (XTEM) studies showed well-defined superlattice layers. Voided low-angle boundaries, aligned perpendicular to the film plane, were also present. High-resolution images showed misfit dislocations for Λ = 9.4 nm, but not Λ = 4.6 nm. Up to ninth-order superlattice reflections were observed in diffraction, indicating that the interfaces were relatively sharp. Analysis of the first-order x-ray superlattice reflection intensities indicated that the composition modulation amplitude increased and the coherency strains decreased for Λ increased from 2 to 10 nm. Vickers microhardness H was found to increase rapidly with increasing Λ, from 1700 kg/mm 2 for a TiN–NbN alloy (i.e., Λ = 0) to a maximum of 4900 kg/mm 2 at Λ = 4.6 nm. H decreased gradually for further increases in Λ above 4.6 nm, to H = 2500 kg/mm 2 at Λ = 450 nm. The hardness results are compared with theories for strengthening of multilayers.

Elastic constants of single‐crystal transition‐metal nitride films measured by line‐focus acoustic microscopy

The elastic constants of single‐crystal NbN, VN, and TiN films were determined from surface acoustic wave (SAW) dispersion curves obtained by the use of an acoustic microscope with a line‐focus beam. Measurements were carried out for single‐crystal nitride films grown on the (001) plane of single‐crystal cubic‐symmetric MgO substrates. The phase velocities measured as functions of the angle of propagation display the expected anisotropy. Dispersion curves of SAWs propagating along the symmetry axes were obtained by measuring the wave velocities for various film thicknesses and frequencies. Using a modified simplex method, an inversion of the SAW dispersion data yielded the elastic constants of cubic symmetry, namely c11, c12, and c44. The Rayleigh surface wave velocities calculated from the determined elastic constants and known mass densities agree well with a result measured by Brillouin scattering spectroscopy reported elsewhere.

Structure and Strength of Multilayers

Nanometer-scale multilayer materials exhibit a wealth of interesting structural and mechanical property behaviors. Physical-vapor-deposition technology allows almost unlimited freedom to choose among elements, alloys, and Compounds as layering constituents and to design and produce materials with compositional and structural periodicities approaching the atomic Scale. These materials have tremendous interface area density, approaching 10 6 mm/mm 3 , so that a Square centimeter area of a one-micron-thick multilayer film with a bilayer period of 2 nm has an interface area of roughly 1,000 cm 2 . Hence interfacial effects can dominate multilayer structure and properties leading to unusually large strains and frequently stabilization of metastable structures. The atomic-scale layering of different materials also leads to very high hardnesses and good wear resistance. These materials are a test-bed for examination of the fundamental aspects of phase stability and for exploring mechanical strengthening mechanisms. They are also becoming increasingly interesting for applications such as hard coatings, x-ray optical elements, in microelectromechanical Systems (MEMS), and in magnetic recording media and heads. In this article, we review some of the interesting structures and mechanical properties that have been observed in nanometer-scale artificial multilayer structures. Superlattice thin films are readily deposited by vapor-phase techniques such as sputter deposition, evaporation, and chemical vapor deposition, as well as by electrochemical deposition. Superlattice deposition Systems are similar to conventional film deposition Systems, except for the provision to modulate the fluxes and thereby produce alternating super-lattice layers.

Effect of LSM-YSZ cathode on thin-electrolyte solid oxide fuel cell performance

The effect of cathode composition, processing and structure on the performance of medium-temperature (600–800 °C) solid oxide fuel cells (SOFCs) is described. The cathodes and physical supports for the SOFCs were two-phase mixtures of (La1 − xSrx)1 − yMnO3 (LSM) and Yttria-stabilized Zirconia (YSZ), the electrolytes were < 10 μm thick YSZ, and the anodes were Ni-YSZ with Y-doped CeO2 interfacial layers. It was found that the cathode overpotential was the primary factor limiting cell power densities during operation with air as the oxidant and humidified hydrogen as the fuel. Increasing the YSZ volume fraction in LSM-YSZ cathodes from 0 to 60% reduced the low-current area-specific resistance of the cells (in air and humidified hydrogen) from ~ 3.3 to 0.7 Ω cm2. The use of LSM with y = 0.1 suppressed the formation of zirconate phases during cathode sintering. Optimal cathode porosity was ≈ 40%. Decreasing the cathode porosity below ≈ 30% resulted in a mass transport limitation at high current densities due to the small pore size (< 0.5 μm) and large cathode thickness (≈ 1 mm). The maximum power densities measured in humidified H2 and air ranged from ~110 mW cm−2 at 600 to 470 mW cm−2 at 800 °C.