Using impedance spectroscopy the electrical response of sensors with various porous Y-stabilized ZrO₂ (YSZ) microstructures was measured for gas concentrations containing 0–100 ppm NO with 10.5%O₂ at temperatures ranging from 600–700 °C. The impedance response increased substantially as the sensor porosity increased from 46%–50%. Activation energies calculated based on data from the impedance measurements increased in magnitude (97.4–104.9 kJ/mol for 100 ppm NO) with respect to increasing YSZ porosity. Analysis of the oxygen partial pressure dependence of the sensors suggested that dissociative adsorption was the dominant rate limiting. The PWC/DNP theory level was used to investigate the gas-phase energy barrier of the 2NO+O₂→2NO₂ reaction on a 56-atom YSZ/Au model cluster using Density Functional Theory and Linear Synchronous Transit/Quadratic Synchronous Transit calculations. The reaction path shows oxygen surface reactions that begin with NO association with adsorbed O₂ on a Zr surface site, followed by O₂ dissociative adsorption, atomic oxygen diffusion, and further NO₂ formation. The free energy barrier was calculated to be 181.7 kJ/mol at PWC/DNP. A qualitative comparison with the extrapolated data at 62% ± 2% porosity representing the YSZ model cluster indicates that the calculated barriers are in reasonable agreement with experiments, especially when the RPBE functional is used.

Composite NO_x sensors were fabricated by combining partially and fully stabilized yttria-doped zirconia with alumina forming a composite electrolyte, Y₂O₃-ZrO₂-Al₂O₃, and strontium-doped lanthanum manganese oxide mixed with gold to form the composite sensing electrode, La_0.8 Sr_0.2MnO₃-Au. A surface chemistry analysis of the composite sensor was conducted to interpret defects and the structural phases present at the Y₂O₃-ZrO₂-Al₂O₃ electrolyte, as well as the charge conduction mechanism at the LaSrMnO₃-Au electrode surface. Based on the surface chemistry analysis, ionic and electronic transport properties, and microstructural features of sensor components, the working principle was described for NO_x sensing at the composite sensor. The role of the composite materials on the NO_x sensing response, cross-sensitivity to O₂, H₂O, CO, CO₂, and CH₄, and the response/recovery rates relative to sensor accuracy were characterized by operating the composite ${\mathrm{NO}}_{\mathrm{x}}$ sensors via the impedimetric method. The composite sensors were operated at temperatures ranging from 575 to 675 °C in dry and humidified gas environments with NO and NO₂ concentrations varying from 0 to 100 ppm, where the balance gas was N₂. It was found that the microstructure of the composite NO_x sensor electrolyte and sensing electrode had a significant effect on interfacial reactions at the triple phase boundary, as well as the density of active sites for oxygen reactions. Overall, the composite NO_x sensor microstructure enabled a high NO_x sensing response, along with low cross-sensitivity to O₂, CO, CO₂, and CH₄, and promoted NO detection down to 2 ppm.

The influence of electrode configuration on the impedancemetric response of nitric oxide (NO) gas sensors was investigated for solid electrochemical cells [Au/yttria-stabilized zirconia (YSZ)/Au)]. Fabrication of the sensors was carried out at 1050 °C in order to establish a porous YSZ electrolyte that enabled gas diffusion. Two electrode configurations were studied where Au wire electrodes were either embedded within or wrapped around the YSZ electrolyte. The electrical response of the sensors was collected via impedance spectroscopy under various operating conditions where gas concentrations ranged from 0 to 100 ppm NO and 1%–18% O₂ at temperatures varying from 600 to 700 °C. Gas diffusion appeared to be a rate-limiting mechanism in sensors where the electrode configuration resulted in longer diffusion pathways. The temperature dependence of the NO sensors studied was independent of the electrode configuration. Analysis of the impedance data, along with equivalent circuit modeling indicated the electrode configuration of the sensor effected gas and ionic transport pathways, capacitance behavior, and NO sensitivity.

Managing H2O Cross‐Sensitivity Using Composite Electrolyte NOx Sensors | InTechOpen, Published on: 2017-05-31. Authors: Erica Perry Murray, Khawlah Kharashi and Kehinde Adedeji