Zirconia-Based Electrochemical Sensor Research Articles

A review of zirconia oxygen, NOx, and mixed potential gas sensors – History and current trends

Zirconia-based electrochemical sensors have revolutionized oxygen monitoring and combustion control. These robust, solid-state devices are found in virtually every internal combustion engine today providing feedback information for tight air to fuel ratio control thus minimizing unwanted emissions and improving engine efficiency. Variations on the sensor design and operational mode enable the measurement of nitrogen oxides to part per million levels, providing control information for exhaust after-treatment systems. Emerging designs include the development of "gas analyzers on a chip" capable of monitoring oxygen, carbon monoxide, nitrogen oxides, ammonia and hydrocarbons. • Zirconia based electrochemical gas sensors can detect wide variety of pollutants. • Potentiometric, amperometric, impedance-based, and pulsed polarization methods are reviewed. • Modeling of sensor behavior can predict sensor response. • Machine learning techniques can be applied for sensor signal decoding.

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

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

Dynamometer Testing of Planar Mixed-Potential Sensors

Recently strengthened federal regulations are driving the reduction of pollutants emitted from heavy-duty diesel engines. The use of selective catalyst reduction (SCR) and exhaust gas recirculation (EGR) systems combined with regenerative particulate traps have reduced NOx and particulate matter (PM) tailpipe emissions from heavy-duty diesel vehicles by 95%.1 Despite these recent advances, there is a lack of low cost exhaust gas sensor technology to monitor tailpipe emissions and to control and maintain optimized SCR and EGR system efficiencies2. Derivatives of the hugely successful robust, zirconia-based electrochemical sensors currently used for gasoline engine control and OBD in conjunction with three-way catalytic converters, has attracted considerable interest in diesel applications. However, the implementation of the diesel analogue to OBD has proved to be more complicated.Mixed-potential sensors fabricated via well-established commercial manufacturing methods present a promising avenue to enable the widespread utilization of NOx sensing technology. The Materials Synthesis and Integrated Devices group at LANL has worked in collaboration with Electro-Science Laboratories (ESL, King of Prussia, PA) to fabricate planar mixed-potential sensors via the readily scalable, cost-effective high temperature co-fired ceramic (HTCC) technology already employed in the manufacturing of planar O2 λ-sensors.The two sides of the planar, self-heated, tape cast sensor prototype are shown in Figure 1. A Pt-heater with independent leads is printed on the backside of the ceramic substrate. The sensing component consists of dense La1-xSrxCrO3 (LSCrO)- working and Pt-counter electrodes coated with a porous YSZ electrolyte layer. The voltage response of a sensor operated at 490˚C to 100 ppm of varying analyte gases is shown in Figure 2. Operated at open circuit, this combination of electrode materials serves as a sensor for reducing gases, such as hydrocarbons and ammonia. However, the sensor can be transformed to a total NOx sensor by appropriate tuning of the current bias and operating temperature3. The performance of the planar sensors under engine-out conditions was recently evaluated at the Oak Ridge National Laboratory National Transportation Research Center. The sensor was mounted in a quartz tube with the Pt heater resistance set to correspond to an operating temperature of 490˚C. The sensor was placed in the exhaust gas downstream of an orifice regulating the flow to 1000ccm and upstream of the FTIR used to analyze the exhaust gas components. A California Instruments Flame-Ionized Detector (FID) was also used to analyze total hydrocarbon content (THC) immediately upstream of the orifice. Figure 3 shows the sensor response during varying engine operating conditions. The sensor response while operating in hydrocarbon sensing mode (0 bias) during engine start-up is shown in Figure 3a. The sensor response closely tracks the THC measured by the FID, while the THC measured via FTIR lags the response of both the FID and the sensor. Figure 3b shows the sensor response while operating in NOx sensing mode (+0.2 μA bias) while the engine was held at 1600 rpm and EGR was cycled on and off. The sensor response qualitatively tracks the NO concentration in the exhaust stream, with the sensor response becoming more negative with increasing NO, in agreement with the biased response shown in Figure 2. In this work, we will discuss the performance of the planar, mixed-potential sensors under engine-out conditions and compare the dynamometer performance with laboratory measurements.

Development of a robust electrochemical oxygen sensor

Zirconia-based electrochemical sensors are useful tools for the measurement of rapidly fluctuating oxygen levels in fluidized-bed combustors. A drawback of these sensors is the cumbersome associated equipment needed to provide a reference air supply. The use of a sealed metal/metal oxide-referenced probe offers the prospect of miniaturization of the technology and the potential for wider application. A robust sensor has been developed, which has a viable operating life for use as a diagnostic tool in the mapping of oxygen and fuel distributions in a fluidized-bed combustor. This paper presents an outline of the development programme, describes the sensor design and characteristics of the output signal.