| There is currently a large need for the development of solid-state potentiometric sensors for monitoring the NOX emanating from combustion processes. In an effort to better understand the phenomena behind the sensor response, sensors of the form: La2CuO4|YSZ|Pt were investigated. These sensors showed promising results.; The effect of electrode microstructure on sensor response time and sensitivity was investigated. Electrodes that have high surface/bulk ratios respond faster and are more sensitive than electrodes with low surface/bulk ratios. They are also stable at lower operating temperatures where signals are inherently larger.; The resistance of La2CuO4, a p-type semiconductor, was measured as a function of various pollutants to verify that the possibility that Fermi level changes are responsible for the potentiometric response of the sensor. The resistance of La2CuO4 and the potential of La2CuO4|YSZ|Pt sensors have similar dependencies on temperature and NOX/CO concentration. This correspondence along with temperature-programmed desorption results suggest that the adsorption/desorption of NOX/CO on/from La2CuO4 shifts the Fermi level and is measured by the electrometer as the electrode potential.; The electrochemistry at Pt and La2CuO4 electrodes in NOX environments was investigated with D.C. polarization and electrochemical impedance spectroscopy. The polarization resistance values for both electrodes decreased with increasing concentrations of NO and NO 2. Theoretical considerations and experimental approaches were refined concerning three-electrode polarization measurements in the solid-state.; Using an independent reference electrode, the sensor response was shown to be a composite of the individual electrode responses. In the case of O 2, the potentials of both electrodes change significantly; however, they do so in identical fashion and the net sensor signal is insignificant to O2. The sensor responds to NO, NO2, and CO because the Fermi level of the La2CuO4 shifts when they adsorb on it. This contribution to the sensor response is often overlooked; however, in the case of La2CuO4-based sensors it is not only a contributor to the response, it dominants the response. In other words, the sensor response is the net difference between the sums of the electrode potential contributions (heterogeneous catalysis, adsorption/desorption, and electrochemical reactions) or in other words, the "Differential Electrode Equilibria."... |
