John M. Vohs

Direct oxidation of hydrocarbons in a solid-oxide fuel cell

The direct electrochemical oxidation of dry hydrocarbon fuels to generate electrical power has the potential to accelerate substantially the use of fuel cells in transportation and distributed-power applications. Most fuel-cell research has involved the use of hydrogen as the fuel, although the practical generation and storage of hydrogen remains an important technological hurdle. Methane has been successfully oxidized electrochemically, but the susceptibility to carbon formation from other hydrocarbons that may be present or poor power densities have prevented the application of this simple fuel in practical applications. Here we report the direct, electrochemical oxidation of various hydrocarbons (methane, ethane, 1-butene, n-butane and toluene) using a solid-oxide fuel cell at 973 and 1,073 K with a composite anode of copper and ceria (or samaria-doped ceria). We demonstrate that the final products of the oxidation are CO2 and water, and that reasonable power densities can be achieved. The observation that a solid-oxide fuel cell can be operated on dry hydrocarbons, including liquid fuels, without reforming, suggests that this type of fuel cell could provide an alternative to hydrogen-based fuel-cell technologies.

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 hydrocarbons

Recent developments in solid-oxide fuel cells (SOFC) that electrochemically oxidize hydrocarbon fuels to produce electrical power without first reforming them to H2 are described. First, the operating principles of SOFCs are reviewed, along with a description of state-of-the-art SOFC designs. This is followed by a discussion of the concepts and procedures used in the synthesis of direct-oxidation fuel cells with anodes based on composites of Cu, ceria, and yttria-stabilized zirconia. The discussion focuses on how heterogeneous catalysis has an important role to play in the development of SOFCs that directly oxidize hydrocarbon fuels.

Direct Oxidation of Hydrocarbons in a Solid Oxide Fuel Cell: I. Methane Oxidation

The performance of Cu cermets as anodes for the direct oxidation of in solid oxide fuel cells was examined. Mixtures of Cu and yttria‐stabilized zirconia (YSZ) were found to give similar performance to Ni‐YSZ cermets when was used as the fuel, but did not deactivate in dry . While Cu‐YSZ was essentially inert to methane, the addition of ceria to the anode gave rise to reasonable power densities and stable operation over a period of at least 3 days. Proof of direct oxidation of came from chemical analysis of the products leaving the cell. The major carbon‐containing product was , with only traces of CO observed, and there was excellent agreement between the actual cell current and that predicted by the methane conversion. These results demonstrate that direct, electrocatalytic oxidation of dry methane is possible, with reasonable performance.

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.

Direct Oxidation of Liquid Fuels in a Solid Oxide Fuel Cell

We report stable power generation. ∼0.1 W/cm 2 , form the direct electrochemical oxidation, without reforming, of toluene. n-decane, and synthetic diesel fuel at 973 K in a solid oxide fuel cell with a composite anode containing Cu and ceria. The liquid fuels were injected directly into the anode compartment and the performance for each fuel with 50% N 2 dilution was stable for a period of at least 12 h. Photographs of Cu-yttria-stabilized zirconia (YSZ) and Ni-YSZ composites exposed to 50% toluene in N 2 at 973 K for 1.5 h showed little carbon formation on the Cu-YSZ hut demonstrated that the Ni-YSZ was covered with carbon and fractured by this environment.

Fabrication of Sr-Doped LaFeO3 YSZ Composite Cathodes

Composites of yttria-stabilized zirconia (YSZ) with Sr-doped LaFeO 3 (LSF) were studied for application as high-performance cathodes for solid oxide fuel cells (SOFCs). The composites were formed by aqueous impregnation of porous YSZ with La, Sr, and Fe salts, followed by calcination at various temperatures. X-ray diffraction measurements showed that the LSF perovskite phase had formed by 1023 K and that solid-state reaction with the YSZ did not occur below approximately 1223 K. The electronic conductivity of the 40 wt % LSF-YSZ composite was maximized by calcination at 1123 K. SOFCs prepared with a 40 wt % LSF-YSZ cathode showed improved performance over SOFCs prepared with conventional LSM-YSZ cathodes at 973 K, although the performance of cells made with cathodes having lower LSF content did not perform as well. Based on measurements with a reference electrode on an electrolyte-supported cell. the impedance of the 40 wt % LSF-YSZ cathode is approximately 0.1 ohm cm 2 in air at 973 K. Finally a cathode-supported cell was fabricated from a 40 wt % LSF-YSZ cathode and shown to perform well in H 2 .

Direct in situ determination of the polarization dependence of physisorption on ferroelectric surfaces

The ability to manipulate dipole orientation in ferroelectric oxides holds promise as a method to tailor surface reactivity for specific applications. As ferroelectric domains can be patterned at the nanoscale, domain-specific surface chemistries may provide a method for fabrication of nanoscale devices. Although studies over the past 50 yr have suggested that ferroelectric domain orientation may affect the energetics of adsorption, definitive evidence is still lacking. Domain-dependent sticking coefficients are observed using temperature-programmed desorption and scanning surface potential microscopy, supported by first-principles calculations of the reaction coordinate. The first unambiguous observations of differences in the energetics of physisorption on ferroelectric domains are presented here for CH(3)OH and CO(2) on BaTiO(3) and Pb(Ti(0.52)Zr(0.48))O(3) surfaces.

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.