The role of porosity on impedancemetric NOx sensing is discussed for sensors composed of a porous yttria-stabilized zirconia (YSZ) electrolyte and dense Au electrodes. NOx sensors considered here were fabricated at firing temperatures of 950–1200°C, which established a range of electrolyte microstructures where the porosity ranged from approximately 50% to 44%. Analysis of the electrical response of the NOx sensors indicated that sensors fired at 1050°C resulting in an electrolyte porosity of 46% demonstrated higher NOx sensitivity based on the operating conditions studied. The impedance of the sensors demonstrated a strong dependence on the electrolyte porosity. The activation energy of the sensors, which ranged from 109.2 to 81.1 kJ/mol, decreased with decreasing electrolyte porosity. Sensors with an electrolyte porosity ≥46% were limited by dissociated adsorption, whereas gas diffusion was rate limiting for sensors with an electrolyte porosity <46%. The impedancemetric response of the porous sensors to NO concentrations ≤10 ppm was distinguishable at operating frequencies as high as 40 Hz, thereby suggesting rapid sensing capabilities. Overall, the microstructure of the sensors composed of a YSZ electrolyte with 46% porosity promoted a strong, rapid, and highly sensitive response to NOx.
Part of the book: Progresses in Chemical Sensor