Wayne L. Worrell

Cu-Ni Cermet Anodes for Direct Oxidation of Methane in Solid-Oxide Fuel Cells

We have examined the use of Cu-Ni alloys as anodes for the direct oxidation of methane in solid-oxide fuel cells (SOFC) at 1073 K. Ceramic-metal (cermet) composites having alloy compositions of 0, 10, 20, 50 and 100% Ni were exposed to dry methane at 1073 K for 1.5 h to demonstrate that carbon formation is greatly suppressed on the Cu-Ni alloys compared to that of pure Ni. Increased reduction temperatures also reduced the carbon formation on the alloys. The performance of a fuel cell made with a Cu(80%)-Ni(20%) cermet was tested in dry methane for 500 h and showed a significant increase in power density with time. Impedance spectra of similar fuel cells suggest that small carbon deposits are formed with time and that the increase in performance is due to enhanced electronic conductivity in the anode. Finally, the implications of the use of metal alloys for SOFC applications are discussed.

A thermodynamic study of sodium-intercalated TaS2 and TiS2

The variation of the sodium chemical potential (μNa) with composition x in NaxTaS2 and NaxTiS2 has been measured electrochemically using propylene carbonate-based and β-alumina electrolytes. At 300°K the sodium chemical potential in NaxTaS2 varies from −63 to −25 kcal/mole as x increases from 0.003 to 0.92, respectively. Within experimental uncertainty, the compositional variation of μNa was linear. In NaxTiS2, the sodium chemical potential varies from −60 to −36 kcal/mole as x increases from 0.001 to 1.0, respectively. The compositional variation of μNa in NaxTiS2 exhibits two plateaus indicative of two-phase regions for 0.2 ≲ x ≲ 0.35 and 0.6 ≲ x ≲ 0.7. The standard free energies of intercalation for sodium-intercalated TaS2 and TiS2 are less negative than those reported for the respective lithium-intercalated compounds. The standard free energy of intercalation becomes more negative for both the sodium- and lithium-intercalated compounds as the dichalcogenide changes from TaS2 to TiS2.

A Novel Method for Preparing Anode Cermets for Solid Oxide Fuel Cells

A new method for fabrication of metal‐cermet anodes in solid‐oxide fuel cells (SOFCs) has been developed. Highly porous, yttria‐stabilized zirconia (YSZ) films were prepared using a mixture of zircon fibers (YSZp, Si‐stabilized, and <0.3% Si) and normal YSZ powders (YSZd). The films remained highly porous following calcination up to 1550°C, after which either Cu or Ni could be incorporated by impregnation with the nitrate salts. For Cu cermets, the performance increased with metal loading to at least 40% Cu. At 800°C using as the fuel and a 230 μm, YSZ electrolyte, the current‐voltage (I-V) curves for either a Cu‐ or Ni‐cermet anode formed using this new method were found to be identical to the I-V curve for a Ni cermet formed using traditional methods. Scanning electron microscopy showed that the anode films remained porous even with addition of Cu, so that additional modification was possible. Tests of this concept through the addition of ceria by impregnation with the led to an additional increase in the cell performance.

SOFCs for Direct Oxidation of Hydrocarbon Fuels with Samaria-Doped Ceria Electrolyte

Samaria-doped ceria (SDC) electrolyte-supported solid oxide fuel cells (SOFCs) with Cu-SDC and Cu-CeO 2 -SDC anode composites were fabricated. Current-voltage and impedance-spectroscopy measurements were used to characterize their performance at temperatures between 600 and 700°C. The cells demonstrated the ability to directly utilize not only hydrogen (H 2 ) but also dry butane (C 4 H 10 ) fuel. At 700°C, the maximum power density of a cell with a Cu-CeO 2 -SDC anode composite was 246 and 170 mW/cm 2 for H 2 and C 4 H 10 fuels, respectively. Impedance spectra suggested that for butane fuel, the anode resistance significantly limits the overall cell performance. It was shown that the addition of pure ceria to the anode significantly increased the catalytic activity for oxidation reactions and decreased the anode resistances.

Silicon-carbide/boron-containing coatings for the oxidation protection of graphite

Abstract Silicon carbide with its excellent oxidation resistance and chemical compatibility with carbon is an attractive coating material for the high-temperature oxidation protection of graphite and carbon-carbon composites. One of the major challenges in the development of protective SiC coatings is preventing oxidation of the graphite substrate due to crack formation in the SiC coating. A boron-containing inner layer can prevent significant oxidation through cracks in a SiC outer layer in 1 atm oxygen for up to nine days at 1500 °C. The consistency of the oxidation protection is sensitive to the quality and uniformity of the coatings.

A Comparison of Cu-Ceria-SDC and Au-Ceria-SDC Composites for SOFC Anodes

Solid oxide fuel cells (SOFC) with samaria-doped ceria (SDC) electrolytes were prepared with anodes made from either Cu, ceria, and SDC or Au, ceria, and SDC. These cells were tested in H 2 and n-butane fuels at 650°C. The similarity of performance (V-I) curves and impedance results for cells made with Au and Cu suggests that both metals are simply electronic conductors in the anode and that Cu does not play a catalytic role in direct-oxidation anodes made with Cu. The addition of ceria is shown to play an important role in improving anode performance, either through improved catalytic activity or mixed ion-electronic conductivity.

Alkali-metal-intercalated transition metal disulfides: A thermodynamic model

In the lithium-intercalated disulfides, LixMS2, where M = Ti, Ta, a nearly linear compositional variation of the lithium chemical potential is observed throughout the composition range 0 < x < 1.0. For most sodium-intercalated disulfides, chemical potential plateaus are observed between regions exhibiting linear variations of sodium chemical potential. Our thermodynamic model indicates that the two most important factors which determine the compositional variation of the alkali metal chemical potential are the interaction energy between intercalated alkali-metal atoms and the compositional variation of the electron chemical potential. Although these two factors determine the compositional variation of chemical potential in single-phase regions, the existence of two-phase regions in the concentration range x = 0−0.15 are influenced by the energy required to expand the interlayer gap and the configurational entropy.

An investigation of high-temperature thermodynamic properties in the Pt-Ti system

High-temperature thermodynamic properties of solid Pt-Ti alloys containing 2 to 25 at. pct titanium have been measured over the temperature range 1150 to 1400 K by a galvaniccell technique using a thoria-based electrolyte. Titanium activities exhibit large negative deviations from Raoult’s Law; at 1300 K and 20 at. pct Ti, for instance, aTi = 2.6 × 10-12. Emf results are correlated with X-ray phase data to determine accurate standard free energies of formation for the intermetallic compounds TiPt8 and TiPt3. At 1300 K, δGf0(TiPt8) = -73.8 kcal/gm atom Ti and δf0(TiPt3) = -71.3 kcal/gm atom Ti.* The high stabilities of these compounds confirm the predictions of the Engel-Brewer correlation. Platinum-titanium compounds can be formed by reaction of platinum with oxides, carbides, and nitrides of titanium.

The Oxidation of iridium-aluminum and iridium-hafnium intermetallics at temperatures above 1550C

The oxidation behavior of iridium-aluminum and iridium-hafnium intermetallics has been investigated in various oxygen pressures and at temperatutes between 1550 and 1800°C. The hafnium concentration necessary for the formation of a continuous external HfO2 scale is above 50 at.% hafnium. The aluminum concentration necessary for the formation of a continuous external Al2O3 scale is above 55 at. % aluminum, which is the aluminum-rich boundary of the IrAl intermetallic. Thus, it appears that the IrAl25 intermetallic necessary for the formation of a protective, external Al2O3 scale in the iridiumaluminum system. The activation energy for the growth of Al2O3 on iridiuim (60 at.%) aluminum intermetallic is in agreement with that determined pre viously for the NiAl intermetallic at lower temperatures. This suggests that similar process may control the Al2O3 scale growth on these two intermetallics.

High-temperature thermodynamic properties of the chromium carbides determined using the torsion-effusion technique

The pressures of carbon monoxide in equilibrium with a Cr23C6-Cr2O3-Cr mixture and with a Cr7C3-Cr2O3-Cr23C6 mixture have been measured in the temperature range 1100 to 1300 K using the torsion-effusion technique. From the equilibrium data, the following equation for ΔGof of Cr23C6 (in cal per mole) has been calculated: ΔGf° (±1200) = −77,000 - 18.3T (1150 to 1300 K) Combining the results of this study at temperatures between 1100 and 1300 K with those of Kelleyet al.,3 at temperatures between 1500 and 1720 K, the following equation for ΔGof of Cr7C3 (in cal per mole) has been determined: ΔGf° (±400) = −35,200 - 8.7T (1100 to 1720 K) ) The above equation for ΔGof of Cr7C3 has been used to re-evaluate the equilibrium data of Kelleyet al.,3 and the following equation for ΔGof of Cr3C2 (in cal per mole) has been obtained: ΔGf° (±400) = −16,400 - 4.4T (1300 to 1500 K) CHROMIUM reacts with carbon to form three carbides:1,2 Cr23C6, Cr7C3, and Cr3C2. The chromium carbides are of considerable technical importance because of their precipitation behavior in certain high-chromium steels and superalloys. A precise knowledge of their thermodynamic properties is essential for the understanding and the prediction of their chemical behavior in various environments.