Briggs White

Investigation of La2CuO4 ∕ YSZ ∕ Pt Potentiometric NO x Sensors with Electrochemical Impedance Spectroscopy

La 2 CuO 4 (LCO) powders were used to fabricate electrodes for solid-state potentiometric NO x sensors [LCO|yttria-stabilized zirconia (YSZ)|Pt], and symmetrical Pt (Pt|YSZ|Pt) and La 2 CuO 4 (LCO|YSZ|LCO) cells. The impedances of the sensors and symmetrical cells were measured. Three phenomena were observed in the impedance spectra: O 2 ion conduction in the YSZ electrolyte, surface conduction on the La 2 CuO 4 electrode, and the electrochemical reaction of O 2 at the electrode/electrolyte/gas interface. The impedance associated with O 2 reduction/oxidation decreased in the presence of O 2 , NO, and NO 2 , in contrast to the potentiometric response which increased in NO, decreased in NO 2 , and was unaffected by changes in O 2 concentration. The impedance associated with surface conduction on the La 2 CuO 4 electrode was affected by NO and NO 2 in much the same way as the previously reported potentiometric sensor response (AOCP) in terms of temperature and concentration dependencies providing evidence for an adsorption mechanism for the potentiometric response.

Non-Nernstian Planar Sensors Based on YSZ with an Nb2O5 Electrode: Discussion on Sensing Mechanism

Solid electrolyte based sensors have been widely studied for the detection of CO/HC and NOx at high temperature. Nevertheless, the discussion about the sensing mechanism of non-nernstian electrochemical gas sensors, with a semiconducting sensing electrode, is still open. In this work, a study of the influence of the metallic electrode under the semiconducting metal oxide Nb2O5 on the sensor response was performed. Planar sensors based on tape-cast YSZ layers were fabricated. Two Pt or Au finger electrodes were deposited in a parallel arrangement on a single side of YSZ. One of these electrodes was covered with Nb2O5 thick film. In order to better understand the sensing mechanism, the electrical measurements were correlated with catalytic measurements performed on both sensing and reference electrodes.

A Theoretical Framework for Prediction of Solid State Potentiometric Gas Sensor Behavior.

A 3 electrode, Luggins Probe configuration was employed in making D. C. polarization and open-circuit potential (OCP) measurements on Platinum and La2CuO4 electrodes in atmospheres containing O2, NO, NO2, CO and CO2 at 500{degree sign}C. Each electrode potential, referenced to an isolated embedded reference electrode, changed when exposed to the aforementioned gases and the differences summed to equal the sensor signals previously seen when both electrodes were exposed to the same environment. Both the La2CuO4 and Pt electrode potentials went down when exposed to reducing gases (NO and CO) and went up when exposed to oxidizing gases (O2 and NO2). The Pt responses (DOCP) were larger than those for La2CuO4 for all gases. La2CuO4|YSZ|Pt sensors responded positively to NO and CO, negatively to NO2 and negligibly to O2. The D.C. polarization behavior for the Pt and La2CuO4 electrodes were contrasted and related to their individual gas responses (DOCPs).