Blackening in Zirconia-Based Electrochemical Oxygen Sensor at High Pumping Potentials

Abstract

Zirconia-based electrochemical sensors have been used on vehicles for many years to measure the oxygen concentration in the automotive exhaust. The most common type is a single electrochemical cell based on oxygen-ion conducting zirconia. This sensor operates as a simple Nernst cell where the open circuit voltage of this cell changes with variations in the oxygen partial pressure (pO2) existing at the electrodes. The reference electrode is maintained at a constant pO2 (air), and the measurement electrode is exposed to the exhaust gas. The emf generated by this Nernst cell gives a measurement of the concentration of oxygen in the exhaust gas. All gasoline vehicles currently utilize an oxygen sensor to control their emission systems. Although successfully used for stoichiometric air-to-fuel (A/F) control for many years, these Nernst-type oxygen sensors are not useful for applications away from stoichiometry because of their low sensitivity. The Nernst cell has a logarithmic dependence of the emf on oxygen partial pressure. Double zirconia-cell sensors have been developed which have a more linear oxygen dependence and therefore a wider range of operation. These “universal exhaust gas oxygen” (UEGO) sensors are applicable to A/F measurement and control from very rich to very lean A/F mixtures. These double-cell sensors are based on oxygen pumping, which involves the transfer of oxygen from one side of a zirconia cell to the other by passing an electric current through the device. Most new vehicles, as well as many diesel powertrains, employ these UEGO sensors to meet the ever-stricter emission requirements mandated by law. A novel method has been developed to use this double-cell oxygen-pumping device (UEGO sensor) to measu­re the concentration of other ‘oxygen containing molecules’ in the exhaust gas, e.g. H2O, CO2, SO2, and NOx. In order to mea­sure these oxygen containing species, a voltage that is greater than the dissociation potential for a particular measurement species is applied across the cell. The dissociated species contributes oxygen that can now be measured by the sensor. The output of the cell provides a measure of the oxygen generated from the decomposition of the measurement gas, and thus a measure of its concentration. By applying differing dissociation potentials across the cell, the output then provides a measurement of the amount of oxygen containing species with dissociation potential less than the applied potential. As an example, for a simple binary mixture of air and water, the oxygen concentration alone can be determined by first measuring the output of the sensor with a low potential applied. Then the measurement of the concentration of oxygen plus water can be measured by subsequently applying a voltage larger than the dissociation potential of water. The water concentration can be determined by subtracting the signal due to oxygen from the signal due to the combination of the two constituents. In principle, these measurements can be achieved without any modification to the existing sensor hardware. A simple modification to the electronic control circuitry that enables variable pumping voltages is all that is required to perform this type of measurement. Figure 1 shows the sensitivity of a commercially available UEGO sensor for the dissociation of H2O as a function of dissociation potential. A typical operating value for the sensing voltage (Vs) for oxygen measurement is 450 mV. The plot below was generated from data measured in the lab at various water concentrations in a carrier gas of dry air. From the plot it is evident that water dissociation is occurring at potentials greater than 800 mV. Additionally, the output begins to saturate at a potential approaching 1100 mV, suggesting that full dissociation of the water has been achieved. As the pumping voltage increases, there is the possibility to negatively impact the function of the UEGO sensor. Blackening of the zirconia element is a potentially irreversible form of element degradation. Blackening occurs when oxygen is removed from the zirconia lattice. Figure 2 shows the I-V curves for the pumping cell of a commercial UEGO sensor. At high pumping voltages, the pumping current begins to rise exponentially. In this overpotential region, oxygen is being removed from the zirconia and contributing to the pumping current. As the potential is increased further, zirconia in the lattice is reduced to zirconium metal, and electronic conductivity becomes more prevalent. The resulting reduction of oxygen from the lattice causes distortion in the lattice and the resulting stress can cause cracking of the element. Techniques to detect early onset of blackening, potential methods for blackening protection, and approaches to reverse blackening via re-oxidation will also be presented. Figure 1