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The role of metal (Au, Pt, and Ag) electrodes in YSZ electrolyte-based impedancemetric nitric oxide (NO) sensors is investigated using impedance spectroscopy and equivalent circuit analysis. The test cell consists of a rectangular block of porous YSZ with two metal wire loop electrodes, both exposed to the same atmosphere. Of the electrode materials, only Au was sensitive to changes in NO concentration. The impedance behavior of porous Au electrodes in a slightly different configuration was compared with dense Au electrodes and was also insensitive to NO. Ag showed no sensitivity to either O 2 or NO, and the measured impedances occurred at frequencies > 10 kHz, which are typically associated with ionic conduction in YSZ. Pt and porous Au showed sensitivity to O 2 , which was quantified using power-law exponents that suggest electrochemical rate-determining mechanisms occurring at the triple phase boundary. The behavior of the dense Au suggests different rate-determining processes (e.g., diffusion or adsorption) for the O 2 reaction. Although the exact mechanism is not determined, the composition and microstructure of the metal electrode seem to alter the rate-limiting step of the interfering O 2 reaction. Impedance behavior of the O 2 reaction that is limited by processes occurring away from the triple phase boundary may be crucial for impedancemetric NO sensing.

The feasibility and benefits of using an oxygenated fuel for solid oxide fuel cell (SOFC) operation were explored using dimethyl ether (DME). A model for the flow tube chemistry was used to predict DME gas-phase decomposition kinetics and species concentration profiles for temperatures ranging from 550 to 750°C. The predictions, which were in good agreement with mass spectral measurements, showed that in the absence of a fuel cell DME decomposed primarily into equal amounts of H2, CH4, and CO above about 675°C. The same decomposition species were observed in catalytic reaction experiments at Ni-based anodes, but the mole fractions of the decomposition gases were much higher, between 550 and 650°C. Current-voltage measurements were made using SOFCs with 100 μm thick yttria-stabilized zirconia electrolyte supports, false(normalLa,normalSrfalse)false(normalCo,normalFefalse)O3 cathodes, and 50 vol % Ni-based anodes. Maximum power densities for SOFCs supplied directly with DME fuel were 0.10 and 0.21W/cm2 at 600 and 700°C, respectively. No carbon deposition was observed in cells operated at temperatures up to 700°C. © 2002 The Electrochemical Society. All rights reserved.

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