L. Peter Martin

Impedancemetric NO x Sensing Using YSZ Electrolyte and YSZ ∕ Cr2O3 Composite Electrodes

An impedancemetric method for NO x sensing using an yttria-stabilized zirconia (YSZ)-based electrochemical cell is described. The sensor cell consists of a planar YSZ electrolyte and two identical YSZ/Cr 2 O 3 composite electrodes exposed to the test gas. The sensor response to a sinusoidal ac signal applied between the two electrodes is measured via two parameters calculated from the complex impedance, the modulus |Z| and phase angle Θ. While either of these parameters can be correlated to the NO x concentration in the test gas, Θ was found to provide a more robust metric than |Z|. At frequencies below approximately 100 Hz, 0 is sensitive to both the NO x and O 2 concentrations. At higher frequencies, Θ is predominantly affected by the O 2 concentration. A dual frequency measurement is demonstrated to compensate for changes in the O 2 background between 2 and 18.9%. Excellent sensor performance is obtained for NO x concentrations in the range of 8-50 ppm in background. An equivalent-circuit model was used to extract fitting parameters from the impedance spectra for a preliminary analysis of NO.-sensing mechanisms.

Impedance Characterization of a Model Au ∕ Yttria -Stabilized Zirconia ∕ Au Electrochemical Cell in Varying Oxygen and NO x Concentrations

An electrochemical cell [Au/yttria-stabilized zirconia (YSZ)/Au] serves as a model system to investigate the effect of O 2 and NO x Possible mechanisms responsible for the response are presented. Two dense Au electrodes are co-located on the same side of a dense YSZ electrolyte and are separated from the electrolyte by a porous YSZ layer, present only under the electrodes. While not completely understood, the porous layer appears to result in enhanced NO x response. Impedance data were obtained over a range of frequencies (0.1 Hz to 1 MHz), temperatures (600-700°C), and oxygen (2-18.9%) and NO x (10-100 ppm) concentrations. Spectra were fit with an equivalent circuit, and values of the circuit elements were evaluated. In the absence of NO x , the effect of O 2 on the low-frequency arc resistance could be described by a power law, and the temperature dependence by a single apparent activation energy at all O 2 concentrations. When both O 2 and NO x were present, however, the power-law exponent varied as a function of both temperature and concentration, and the apparent activation energy also showed dual dependence. Adsorption mechanisms are discussed as possibilities for the rate-limiting steps. Implications for impedancemetric NO x sensing are also discussed.

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.

Hydrogen Sensor Based on YSZ Electrolyte and Tin-Doped Indium Oxide Electrode

A solid-state electrochemical sensor has been developed for hydrogen leak detection in ambient air. The sensor uses an yttriastabilized zirconia (YSZ) electrolyte with one tin-doped indium oxide (ITO) and one Pt electrode. Excellent sensitivity, and response time ≤ 1 s, are reported for hydrogen gas in the concentration range of 0.03 to 5.5% in air. Cross-sensitivity to water vapor and CO 2 are shown to be low. The response to methane, a potentially significant source of interference for such a sensor, is significantly smaller than that for hydrogen. The sensor shows good reproducibility and was unaffected by thermal cycling over the course of this investigation (∼240 h of operation). The effects of ITO electrode thickness and thermal aging, and the sensing mechanism, are discussed. The sensor is intended for monitoring potential hydrogen leakage in vehicles powered by hydrogen fuel.

Application of tape-cast graded impedance impactors for light-gas gun experiments

Fabrication of compositionally graded structures for use as light-gas gun impactors has been demonstrated using a tape casting technique. Mixtures of metal powders in the Mg-Cu system were cast into a series of 19 tapes with uniform compositions ranging from 100% Mg to 100% Cu. The individual compositions were fabricated into monolithic pellets for characterization of microstructure, density, and sound wave velocity. Graded impactors were fabricated by stacking layers of different compositions in a sequence calculated to yield a tailored acoustic impedance profile, and were characterized by ultrasonic C-scan and white light interferometry. The graded impactors were launched into stationary Al targets using a two-stage light-gas gun, and the resulting wave profiles were measured with either VISAR or Photonic Doppler Velocimetry. For an impactor using only seven compositions ranging from Mg to Cu, the composition steps are visible in the wave profiles. An impactor utilizing the full series of 19 composition...

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.