R.J. Gorte

A comparative study of water-gas-shift reaction over ceria supported metallic catalysts

Abstract Water-gas-shift (WGS) reaction rates have been measured on Pd/ceria, Ni/ceria, Fe/ceria, Co/ceria, ceria, and Pd/silica. Pd/ceria exhibited much higher activities than either ceria alone or Pd/silica, demonstrating a cooperative effect between Pd and ceria. Pd/ceria and Ni/ceria showed essentially the same activities and were much more active than either Co/ceria or Fe/ceria. Reaction orders on Pd/ceria were approximately zeroth-order in CO, half-order in H 2 O, inverse-half-order in CO 2 and inverse-first-order in H 2 . Diffuse-reflectance FTIR measurements on Pd/ceria indicate the ceria exists in a reduced state under WGS conditions and is covered by carbonate species that are removed only by reoxidation of ceria The implications of these results for improved WGS catalysis are discussed.

Anodes for Direct Oxidation of Dry Hydrocarbons in a Solid-Oxide Fuel Cell

The manufacture of fuel cells that can operate directly on various hydrocarbon fuels, without the need for reforming, has the potential of greatly speeding the application of fuel cells for transportation and distributed-power applications. This paper will briefly review the literature in this area and describe recent developments in solid-oxide fuel cells (SOFCs) that demonstrate that direct-oxidation fuel cells are possible with Cu-based anodes. A new method for synthesizing thin-electrolyte, anode-supported cells is described that is based on tape casting with graphite pore formers (see Figure), followed by impregnation with aqueous solutions of Cu(NO3)2 and Ce(NO3)3. The performance of model SOFCs for direct conversion of n-butane and methane is shown. Finally, future developments that are needed for this technology to be commercialized are discussed.

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.

Novel SOFC anodes for the direct electrochemical oxidation of hydrocarbon

This paper describes recent developments in solid-oxide fuel cells (SOFC) that use Cu-based cermets as the anode for direct oxidation of hydrocarbon fuels, including liquids such as gasoline, to generate electrical power without the need for first reforming that fuel to H2. Cu–YSZ cermets were found to be stable in hydrocarbon environments, but exhibited low performance for direct oxidation. Reasonable power densities could only be achieved with the addition of a catalytic oxide, like ceria, with the Cu cermet. Electrochemical oxidation studies demonstrated that the initial products for reaction depend on the catalytic oxide. Finally, the effect of sulfur impurities in the fuel is discussed.

Role of Hydrocarbon Deposits in the Enhanced Performance of Direct-Oxidation SOFCs

We have examined the changes that occur in the performance of solid oxide fuel cells (SOFCs) with Cu-ceria-yttria-stabilized zirconia anodes at 973 K following exposure to various hydrocarbon fuels, including methane, propane, n-butane, n-decane. and toluene. For cells with Cu contents of 20 wt % or less, large increases were observed in the power densities for operation in H 2 after the anode had been exposed to any of the hydrocarbons except methane. The increased performance is completely reversible upon oxidation of the anode and subsequent reduction in H 2 . The enhancement decreases with increasing Cu content. implying that the deposits improve the connectivity of the metallic phase in the anode. Impedance spectra taken on cells before and after exposure to hydrocarbon fuels confirm that the conductivity of the anode improves after exposure. Temperature-programmed oxidation and weight changes were used to show that the deposits that enhance performance correspond to ∼ 1 wt % of the anode and are probably not graphitic. Measurements of the open-circuit voltages in hydrocarbon fuels suggest that equilibrium is established with partial oxidation products and that the chemical structure of the deposits change upon current flow. Finally, the implications of these results for operation of SOFC on hydrocarbons without added steam and with low copper contents are discussed.

Impedance Spectroscopy for the Characterization of Cu-Ceria-YSZ Anodes for SOFCs

The performance of electrodes in direct-utilization, solid oxide fuel cells (SOFCs) has been studied on anode-supported and electrolyte-supported cells using impedance spectroscopy, coupled with calculations of the potential distribution in the electrolyte. The cells in these studies were composed of a Cu-ceria-yttria-stabilized zirconia (YSZ) anode, a YSZ electrolyte, and a Sr-doped LaMnO 3 (LSM)-YSZ cathode and were operated at 983 K using both H 2 and n-butane as fuel. Both calculations and experiments show that three-electrode measurements on anode-supported electrolytes, with the reference electrode opposite the anode, provide no additional information over two-electrode measurements and cannot be used to estimate the performance of individual electrodes. Three-electrode measurements were able to estimate anode and cathode performance on thick, electrolyte-supported cells, with symmetric placement of the working electrodes. However, both experiments and calculations demonstrate that differences in the kinetics of the two electrodes make perfect separation of anode and cathode processes difficult. The cathode performance of LSM-YSZ in these experiments was described by a single arc in the Cole-Cole plot, with a frequency of 2 kHz and a resistance of 0.4 Ω cm 2 . The performance of the anode in H 2 was also characterized by a single arc, with a frequency of 4 Hz and a resistance of 0.8 Ω cm 2 . While anode performance in H 2 is only weakly dependent on current density, nonlinear processes are observed with n-butane, so that the area-specific resistances depend strongly on the current density.

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.

SOFC Anodes Based on Infiltration of La0.3Sr0.7TiO3

Composites formed by infiltration of 45 wt % La0.3Sr0.7TiO3 LST into 65% porous yttria-stabilized zirconia YSZ were examined for application as solid oxide fuel cell SOFC anodes. Although LST does not react with YSZ, the structure of the LST deposits was strongly affected by the calcination temperature. At 1373 K, the LST formed loosely packed, 0.1 m particles that filled the YSZ pores. The conductivity of this composite depended strongly on the pretreatment conditions but was greater than 0.4 S/cm after heating to 1173 K in humidified 3% H2O H2. Following calcination at 1573 K, the LST had sintered significantly, decreasing the conductivity of the composite by a factor of approximately 5. The addition of a catalyst was critical for achieving reasonable electrochemical performance, with the addition of 0.5 wt % Pd and 5 wt % ceria increasing the power density of otherwise identical cells from less than 20 to 780 mW/cm2 for operation in humidified 3% H2O H2 at 1073 K. Electrodes prepared from LST deposits calcined at 1373 K were found to exhibit a much better performance than those prepared from LST deposits calcined at 1573 K, demonstrating that the structure of the composite is critical for achieving high 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.

Steam reforming of n-butane on Pd/ceria

We have examined the steam reforming of n-butane on ceria, 1 wt% Pd/ceria, 1 wt% Pd/alumina, and 15 wt% Ni/silica between 573 and 873 K, with H2O : C ratios between 1.0 and 2.0. No stable rates could be observed on Ni/silica due to rapid coking under these conditions. While rates were stable on the other catalysts, Pd/ceria showed a much higher activity than either Pd/alumina or ceria individually. Of additional interest, CO2 : CO ratios were much higher on Pd/ceria and approached equilibrium. The reaction order for n-butane on Pd/ceria was 0.15. For H2O, reaction order changed from 0.6 to zero at the stoichiometric, n-butane : H2O ratio. It is suggested that the high activity of Pd/ceria for this reaction is due to a dual-function mechanism, in which ceria can be oxidized by H2O and then supply oxygen to the Pd.