Vohs, John M

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Now showing 1 - 10 of 72
  • Publication
    Interaction of Platinum Films with the (0001#) and (0001) Surfaces of ZnO
    (1994) Petrie, W. T; Vohs, John M
    In this investigation, the growth, structure, and electronic properties of Pt films on the polar surfaces of ZnO were examined using high-resolution electron energy-loss spectroscopy (HREELS) and low-energhy, electron diffraction (LEED). The growth mode of vapor-deposited Pt films on ZnO(0001#) and ZnO(0001) at 300 K was found to be nearly layer-by-layer. The surfaces of Pt films produced in this manner exhibited hexagonal symmetry and were stable up to 600 K. At higher temperatures, the Pt agglomerated into particles which remained oriented with respect to the ZnO substrate. HREELS results indicate that there are only weak interactions at the Pt/ZnO(0001#) interface, while charge transfer and Schottky barrier formation occures at the Pt/ZnO(0001) interface.
  • Publication
    Inexpensive Ultrahigh Vacuum Heatable/Coolable xyz-Rotary Motion Sample Manipulator
    (1995) Peterson, Susan L; Schulz, Kirk H; Schulz, Carl A; Vohs, John M
    A simple design for a heatable, coolable, rotable sample manipulator, suitable for ultrahigh vacuum (UHV) applications, is described. Highlights of the design include using a combination power/thermocouple feedthrough for heating, cooling, and temperature measurement; and the use of primarily "off-the-shelf" components available from most UHV components vendors. The described manipulator is capable of sample cooling to ~100 K, sample heating to above 900 K, while maintaining 360o of rotary motion, ~1 in. of x and y motion, and 2 in. of z motion. The apparatus can be assembled for approximately $5500 (all new parts) and uses about 3 l of liquid N2 per day. © 1995 American Institute of Physics.
  • Publication
    A Study of Carbon Formation and Prevention in Hydrocarbon-Fueled SOFC
    (2006-04-21) Kim, Taeyoon; Liu, G.; Boaro, Marta; Vohs, John M; Lee, S.-I.; Gorte, Raymond J; Al-Madhi, O. H; Dabbousi, B. O.
    The formation and removal of the carbonaceous deposits formed by n-butane and liquid hydrocarbons, such as n-decane and proprietary light and heavy naphthas, between 973 and 1073 K on YSZ and ceria-YSZ, has been studied to determine conditions for stable operation of direct-utilization SOFC. First, it is shown that deactivation of SOFC with Cu-ceria-YSZ anodes operating on undiluted n-decane, a mixture of 80% n-decane and 20% toluene, or light naphtha at temperatures above 973 K is due to filling of the pores with polyaromatic compounds formed by gas-phase, free-radical reactions. Formation of these compounds occurs at a negligible rate below 973 K but increases rapidly above this temperature. The rate of formation also depends on the residence time of the fuel in the anode compartment. Because steam does not participate in the gas-phase reactions, carbonaceous deposits could form even at a H2O:C ratio of 1.5, a value greater than the stability threshold predicted by thermodynamic calculations. Temperature-programmed-oxidation (TPO) measurements with 20% H2O in He demonstrated that carbon deposits formed in pure YSZ were unreactive below 1073 K, while deposits formed on ceria-YSZ could be removed at temperatures as low as 923 K. Based on these results, we discuss strategies for avoiding carbon formation during the operation of direct-utilization anodes on oil-based liquid fuels.
  • Publication
    A Comparison of Molten Sn and Bi for Solid Oxide Fuel Cell Anodes
    (2010-01-19) Jayakumar, Abhimanyu; Vohs, John M; Lee,; Gorte, Raymond J; Hornés,
    Molten Sn and Bi were examined at 973 and 1073 K for use as anodes in solid oxide fuel cells with yttria-stabilized zirconia (YSZ) electrolytes. Cells were operated under "battery" conditions, with dry He flow in the anode compartment, to characterize the electrochemical oxidation of the metals at the YSZ interface. For both metals, the open-circuit voltages (OCVs) were close to that expected based on their oxidation thermodynamics, ~0.93 V for Sn and ~0.48 V for Bi. With Sn, the cell performance degraded rapidly after the transfer of approximately 0.5-1.5 C/cm2 of charge due to the formation of a SnO2 layer at the YSZ interface. At 973 K, the anode impedance at OCV for freshly reduced Sn was approximately 3 Ω cm2 but this increased to well over 250 Ω cm2 after the transfer of 1.6 C/cm2 of charge. Following the transfer of 8.2 C/cm2 at 1073 K, the formation of a 10 µm thick SnO2 layer was confirmed by scanning electron microscopy. With Bi, the OCV anode impedance at 973 K was less than 0.25 Ω cm2 and remained constant until essentially all of the Bi had been oxidized to Bi2O3. Some implications of these results for direct carbon fuel cells are discussed.
  • Publication
    Influence of Composition and Cu Impregnation Method on the Performance of Cu/CeO2/YSZ SOFC Anodes
    (2006-03-09) Jung, Sukwon; Lu, Chun; He, Hongpeng; Gorte, Raymond J; Ahn, Kipyung; Vohs, John M
    In this study, we report on how different impregnation procedures affect the distribution and morphology of the Cu component in Cu/CeO2/YSZ composite anodes and how this affects anode performance. Two different methods for Cu addition to the porous YSZ anode were investigated: impregnation using aqueous solutions of Cu(NO3)2 and impregnation using aqueous solutions of Cu(NO3)2 plus urea. The latter method produced a homogeneous distribution of Cu throughout the anode while the former resulted in a higher concentration of Cu near the exposed surface relative to that in the bulk. Studies of the thermal stability of the deposited copper layers, and the influence of the Cu distribution on cell performance when operating with humidified H2 as the fuel are also presented.
  • Publication
    The Stability of LSF-YSZ Electrodes Prepared by Infiltration
    (2007-03-13) Wang, Wensheng; Vohs, John M; Gross, Michael D; Gorte, Raymond J
    Composite electrodes were prepared by adding 40 wt % La0.8Sr0.2FeO3 (LSF) into porous yttria-stabilized zirconia (YSZ) and their performance was studied as a function of time and calcination temperature. X-ray diffraction (XRD) patterns of the LSF-YSZ composites indicated an expanded lattice parameter after calcination above 1523 K, suggesting that Zr reacted with the LSF to form a Zr-doped perovskite; but XRD provided no evidence for reaction between LSF and YSZ after calcination at 1373 K or after operation for 1000 h at 973 K and 700 h at 1073 K. A composite of 40 wt % La0.8Sr0.2Fe0.9Zr0.1O3 in YSZ showed reasonable performance at 973 K, with an area-specific resistance (ASR) of 0.22 Ω cm2. Based on symmetric-cell measurements, electrodes calcined at 1123 K showed an initial ASR of 0.13 Ω cm2 at 973 K but this increased linearly with time to 0.55 Ω cm2 after 2500 h at 973 K. However, the ASR depended strongly on current density, decreasing dramatically under both anodic and cathodic polarization. Electrodes calcined at 1373 K showed an ASR of 2.5 Ω cm2 at 973 K but this value also decreased dramatically under polarization. Scanning electron microcopy images demonstrate that aging at 973 K and calcination at 1373 K cause significant sintering of the LSF. It is therefore suggested that deactivation is caused by morphological changes, rather than solid-state reactions, with a dense layer of LSF forming over the YSZ substrate.
  • Publication
    Modeling Impedance Response of SOFC Cathodes Prepared by Infiltration
    (2011-03-21) Bidrawn, Fred; Küngas, Rainer; Vohs, John M; Gorte, Raymond J
    A mathematical model has been developed to understand the performance of electrodes prepared by infiltration of La0.8Sr0.2FeO3 (LSF) and La0.8Sr0.2MnO3 (LSM) into yttria-stabilized zirconia (YSZ). The model calculates the resistances for the case where perovskite-coated, YSZ fins extend from the electrolyte. Two rate-limiting cases are considered: oxygen ion diffusion through the perovskite film or reactive adsorption of O2 at the perovskite surface. Adsorption is treated as a reaction between gas-phase O2 and oxygen vacancies, using equilibrium data. With the exception of the sticking probability, all parameters in the model are experimentally determined. Resistances and capacitances are calculated for LSF-YSZ and there is good agreement with experimental values at 973 K, assuming adsorption is rate limiting, with a sticking probability between 10-3 and 10-4 on vacancy sites. According to the model, perovskite ionic conductivity does not limit performance so long as it is above ~10-7 S/cm. However, the structure of the YSZ scaffold, the ionic conductivity of the scaffold, and the slope of the perovskite redox isotherm significantly impact electrode impedance. Finally, it is shown that characteristic frequencies of the electrode cannot be used to distinguish when diffusion or adsorption is rate-limiting.
  • Publication
    Modification of LSF-YSZ Composite Cathodes by Atomic Layer Deposition
    (2017-01-01) Rahmanipour, M.; Cheng, Yuan; Onn, Tzia M; Vohs, John M; Donazzi, A.; Gorte, Raymond J
    composite, Solid-Oxide-Fuel-Cell (SOFC) electrodes of La0.8Sr0.2FeO3 (LSF) and yttria-stabilized zirconia (YSZ) were prepared by infiltration methods and then modified by Atomic Layer Deposition (ALD) of ZrO2, La2O3, Fe2O3, or La2O3-Fe2O3 codeposited films of different thicknesses to determine the effect of surface composition on cathode performance. Film growth rates for ALD performed using vacuum procedures at 573 K for Fe2O3 and 523 K for ZrO2 and La2O3 were determined to be 0.024 nm ZrO2/cycle, 0.019 nm La2O3/cycle, and 0.018 nm Fe2O3/cycle. For ZrO2 and Fe2O3, impedance spectra on symmetric cells at 873 K indicated that polarization resistances increased with coverage in a manner suggesting simple blocking of O2 adsorption sites. With La2O3, the polarization resistance decreased with small numbers of ALD cycles before again increasing at higher coverages. When La2O3 and Fe2O3 were co-deposited, the polarization resistances remained low at high film coverages, implying that O2 adsorption sites were formed on the co-deposited films. The implications fo these results for future SOFC electrode development are discussed.
  • Publication
    Polarization-Induced Hysteresis in CuCo-Doped Rare Earth Vanadates SOFC Anodes
    (2012-09-14) Adijanto, Lawrence; Gorte, Raymond J; Padmanabhan, Venu Balaji; Vohs, John M
    The physical and electrochemical properties of strontium substituted cerium vandates in which a portion of the cerium cations have been substituted with transition metals (Ce0.8Sr0.1Cu0.05TM0.05VO4−0.5x, TM = Ni or Co) were investigated and their suitability for use in solid oxide fuel cell (SOFC) anodes was assessed. Upon reduction at elevated temperature, Cu and Co or Cu and Ni were exsolved from the electronically conductive Ce1−xSrxVO4 lattice to produce Cu-Ni and Cu-Co catalytic nanoparticles. The Ce0.8Sr0.1Cu0.05Co0.05VO3 appears to have high activity and relatively high hydrocarbon tolerance, suggesting that intimate contact between the exsolved Cu and Co and that the majority of the Co nanoparticles must be at least partially coated with the Cu. The electrochemical performance when used in anodes operating on hydrogen has been characterized, and the results demonstrate the exsolution of both metals from the host lattice; but observed dynamic changes in the structure of the resulting metal nanoparticles as a function of SOFC operating conditions complicate their use in SOFC anodes.
  • Publication
    Growth and Structure of Pd Films on ZnO(0001)
    (2006-10-27) Bera, Parthasarathi; Vohs, John M
    The growth and structure of Pd films on Zn(O0001) were investigated using high resolution electron energy loss spectroscopy, x-ray photoelectron spectroscopy, and low energy electron diffraction. Vapor deposited Pd films at 300 K were found to follow a two-dimensional (2D) island growth mode, in which 2D metal islands are formed up to a critical coverage at which point growth occurs primarily in a layer-by-layer fashion on top of the islands. Heating to only 350 K was found to be sufficient to induce partial agglomeration of Pd films into three-dimensional particles. In addition to causing further agglomeration into particles, heating to 700 K resulted in partial reduction of the ZnO surface and the formation of a PdZn alloy.