Jaco Visser

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.

Effect of electrode material and design on sensitivity and selectivity for high temperature impedancemetric NOx sensors

Solid-state electrochemical sensors using two different sensing electrode compositions, gold and strontium-doped lanthanum manganite (LSM), were evaluated for gas-phase sensing of NO x (NO and NO 2 ) using an impedancemetric technique. An asymmetric cell design utilizing porous yttria-stabilized zirconia (YSZ) electrolyte exposed both electrodes to the test gas (i.e., no reference gas). Sensitivity to less than 5 ppm NO and response/recovery times (10-90%) less than 10 s were demonstrated. Using an LSM sensing electrode, a virtual identical sensitivity toward NO and NO 2 was obtained, indicating that the equilibrium gas concentration was measured by the sensing electrode. In contrast, for cells employing a gold sensing electrode, the NO x sensitivity varied depending on the cell design: Increasing the amount of porous YSZ electrolyte on the sensor surface produced higher NO 2 sensitivity compared to NO. To achieve comparable sensitivity for both NO and NO 2 , the cell with the LSM sensing electrode required operation at a lower temperature (575°C) than the cell with the gold sensing electrode (650°C). The role of surface reactions is proposed to explain the differences in NO and NO 2 selectivity using the two different electrode materials.

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.

Dielectric spectroscopy for measuring the composition of gasoline/water/ethanol mixtures

Dielectric spectroscopy is applied to investigate the feasibility of biofuel composition measurement. Measurements were made on the pure components (ethanol, gasoline and water) and mixtures thereof. Preliminary measurement results indicate that the components have distinguishable spectral dielectric behavior in the ranges 10-100 kHz and 200 MHz-2GHz. A simple linear-mixing algorithm is presented for calculating the composition from two different parameters of the measured data. Measurements show that ethanol concentration is relatively easy to determine by RF impedance measurements of the permittivity of the mixture, provided that the baseline is compensated for the water content. Unambiguous measurement of water content of different types of fuel at low frequencies is still hampered by the salinity of water and the presence of additives (static dissipaters) that lower the conductivity of gasoline.

Combining impedance spectroscopy with optical absorption spectroscopy in the UV for biofuel composition measurement

A capacitive probe is generally used in a flex-fuel engine for measuring the ethanol content in biofuel. However, the water content in biofuel of high ethanol content cannot be disregarded or considered constant and the full composition measurement of ethanol, gasoline and water in biofuel is required. Electrical impedance spectroscopy with a customized capacitive probe operating in the 10 kHz to 1 MHz frequency range is combined with optical absorption spectroscopy in the UV spectral range between 230 and 300 nm for a full composition measurement. This approach is experimentally validated using actual fuels and the results demonstrate that electrical impedance spectroscopy when supplemented with optical impedance spectroscopy can be used to fully determine the composition of the biofuel and applied for a more effective engine management. A concept for a low-cost combined measurement system in the fuel line is presented.