Erica Perry Murray

Solid Oxide Fuel Cells Utilizing Dimethyl Ether Fuel

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 H 2 , CH 4 , 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, (La,Sr)(Co,Fe)O 3 cathodes, and 50 vol % Ni-based anodes. Maximum power densities for SOFCs supplied directly with DME fuel were 0.10 and 0.21 W/cm 2 at 600 and 700°C, respectively. No carbon deposition was observed in cells operated at temperatures up to 700°C.

Effect of Electrode Composition and Microstructure on Impedancemetric Nitric Oxide Sensors based on YSZ Electrolyte

The role of metal (Au, Pt, and Ag) electrodes in yttria-stabilized zirconia (YSZ) electrolyte-based impedancemetric nitric oxide (NO) sensors is investigated using impedance spectroscopy and equivalent circuit analysis. Focus on the metal/porous YSZ interface is based on previous studies using a symmetric cell (metal/YSZ porous /YSZ dense /YSZ porous /metal) and attempts to further elucidate the important processes responsible for sensing. The current test cell consists of a rectangular slab of porous YSZ with two metal-wire loop electrodes (metal/YSZ porous /metal), 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. 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.

Direct Solid Oxide Fuel Cell Operation Using Isooctane

Solid oxide fuel cells (SOFCs) consisting of an 8 mol % Y-stabilized ZrO 2 (YSZ) electrolyte, La 0.8 Sr0 .2 MnO 3 -YSZ (LSM-YSZ) cathode, and Ni-YSZ anode support were operated directly using isooctane. The electrical performance of SOFCs operating with 5.3 vol % C 8 H 18 in N 2 was measured over a temperature range of 600-775°C. Open-circuit voltages achieved slightly over 1 V. The maximum power densities attained were 0.10 and 0.24 W/cm 2 at 650 and 700°C, respectively. Mass spectroscopy analysis showed C 8 H 18 thermally decomposed largely into ethylene and methane, which suggested SOFC fuel reactions primarily, involved these gas species.

Aging Studies of Sr-Doped LaCrO3 ∕ YSZ ∕ Pt Cells for an Electrochemical NO x Sensor

The stability and NO x sensing performance of electrochemical cells of the structure Sr-doped LaCrO 3-δ (LSC)/yttria-stabilized zirconia (YSZ)/Pt are being investigated for use in NO x aftertreatment systems in diesel vehicles. Among the requirements for NO x sensor materials in these systems are stability and long lifetime (up to 10 years) in the exhaust environment. In this study, cell aging effects were explored following extended exposure to a test environment of 10% O 2 at operating temperatures of 600-700°C. The data show that aging results in changes in particle morphology, chemical composition, and interfacial structure. Impedance spectroscopy indicates an initial increase in the cell resistance during the early stages of aging, which is correlated principally to densification of the Pt electrode. Also, X-ray photoelectron spectroscopy indicates formation of SrZrO 2 solid-state reaction product in the LSC, a process which is of finite duration. Subsequently, the overall cell resistance decreases with aging time, due in part to roughening of YSZ-LSC interface, which improves interface adherence and enhances charge transfer kinetics at the gas phase/YSZ/LSC triple-phase boundary. This study constitutes a first step in the development of a basic understanding of aging phenomena in solid-state electrochemical systems with applications not only to sensors, but also to fuel cells, membranes, and electrolyzers.

Investigating the Stability and Accuracy of the Phase Response for NOx Sensing 5% Mg-modified LaCrO3

Impedance spectroscopy measurements were carried out on LaCr{sub 0.95}Mg{sub 0.05}O{sub 3} (LCM) asymmetric interdigitated electrodes supported on fully stabilized 8-mol% Y{sub 2}O{sub 3}-stabilized ZrO{sub 2} (YSZ) electrolytes. Experiments were carried out using 0-50 ppm NO{sub x}, 5-15% O{sub 2} with N{sub 2} as the balance, over temperatures ranging from 600-700 C. AC measurements taken at a constant frequency between 1-100 Hz indicated the phase response of the sensor was less sensitive to fluctuations in the O{sub 2} concentration and the baseline drift was limited. Specific frequencies were observed where the sensor response was essentially temperature independent.

Managing H2O Cross‐Sensitivity Using Composite Electrolyte NOx Sensors

NOx sensors composed of partially stabilized zirconia (PSZ), fully stabilized zirconia (FSZ), and PSZ–FSZ composite electrolytes were investigated using impedance spectroscopy under dry and humidified gas conditions. The impedance data were used to interpret the electrochemical behavior of the various sensors as the water concentration in the gas stream varied. The sensors were operated in the presence of 0–100 ppm NO with 1–18% O2 and 3–10% H2O with N2 as the balance gas. The operating temperature of the sensors ranged from 600 to 700 C. The impedance response for sensors containing ≥ 50 vol% PSZ slightly decreased under humidified gas conditions, in comparison to dry gas conditions; whereas, a significant increase in impedance occurred for sensor largely containing FSZ. This indicated water cross-sensitivity was substantial at FSZbased sensors. The microstructural properties, NO x sensitivity, oxygen partial pressure and temperature dependence, as well as the response time of the sensors composed of the various electrolytes were characterized in order to interpret the electrochemical response with respect to water cross-sensitivity. Analysis of the data indicated that sensors composed of a PSZ–FSZ composite electrolyte with 50 vol% PSZ were more suitable for detecting NO x while limiting water cross-sensitivity.