Amperometric NOx Sensor Research Articles

A Variable-Potential Amperometric Hydrocarbon Sensor

Using the understanding of an inexpensive production NOx sensor, the operating parameters are changed to enable hydrocarbon measurement using the same sensor. A limiting-current-type amperometric hydrocarbon sensor for rich conditions (in the absence of O2) is developed in this work. To do this, an inexpensive three-chamber amperometric sensor with three separate electrochemical cells is parameterized to measure propane concentration. The sensor is tested using a controlled sensor test rig at different propane concentrations. The inputs to the sensor electrochemical cells have been modified to determine the best HC measurement parameters (HCMPs) for measuring propane at different concentrations. First, the transient performance and stability of the sensor are optimized by changing the sensor temperature, the reference cell potential, and the stabilizing cell potential at a high propane concentration (5000 ppm - balanced with nitrogen). Over the range tested, the sensor has the longest stable output duration at the temperature of 1009 K, the reference cell potential of 0.67 V and the stabilizing cell potential of 0.45 V. Using these sensor inputs for sensor temperature, reference cell potential and stabilizing cell potential, the sensor steady state behavior is studied to find the diffusion-rate-determined operating region. The sensor is shown to have a linear sensitivity to propane concentration from 0 to 3200 ppm. Finally, the sensor response time to different step changes from 0 up to 5000 ppm propane concentration are studied. It is shown that propane stepsize does not have a significant effect on the sensor response time. Consequently, using the working principles of an existing production amperometric NOx sensor and changing the sensor operating parameters, an amperometric hydrocarbon sensor that works in diffusion rate determining operating region is developed.

An electrochemical model of an amperometric NOx sensor

To help design future amperometric NOx sensors, a physics-based sensor model that includes diffusion and electrochemical submodels is developed. It is shown that NO is partly reduced in the O2 sensing chamber which affects NO concentration in the O2 sensing and in the NOx sensing chamber. Therefore, the electrochemical model is developed to simulate partial reduction of NOx on the O2 sensing electrode and reduction of NOx on the NOx sensing electrode. A transport model that simulates diffusion of the gas species through the sensor diffusion barriers and sensor chambers is coupled to the electrochemical submodels. A fully controlled sensor test-rig that provides controlled gas mixtures is employed to carry out experiments to estimate model parameters. Then, the sensor is installed on the exhaust system of a medium duty Diesel engine and then on a port injection spark ignition engine. Experiments at different engine operating conditions with different NOx concentrations from 0 to 2820 ppm have been performed to validate the model accuracy at different operating conditions. Through the validation process, the NOx sensing cell voltage is changed experimentally at different NOx concentrations to evaluate the model accuracy at different cell voltages. The model results closely match the experiments with the maximum 12% error for the NOx sensing pumping current.

Phenomenological model of a solid electrolyte NOx and O2 sensor using temperature perturbation for on-board diagnostics

Amperometric NOx sensors are increasingly used in automotive industry to meet the stringent emission measurement regulations. These sensors measure O2 and NOx concentration using two different sensing cells. In this work, a physics-based model was developed and then employed to predict the sensor output for oxygen as a function of sensor temperature and oxygen concentration. A temperature perturbation method was also developed based on the model to calibrate the sensor output with respect to oxygen concentration. The model accurately matched the experimental results for steady state and transient conditions. A two step sensor diagnostics procedure based on the sensor temperature perturbation method was then proposed. The first diagnostics step evaluates the sensor output to check if it is within the acceptable range. The second diagnosis step checks the plausibility of the sensor output based on the physics based model and temperature perturbation. A self-calibration procedure was also implemented inside the diagnostics procedure using temperature perturbation at engine-off. This self-recalibration only requires an external relative humidity measurement.

Amperometric solid electrolyte NOx sensors – The effect of temperature and diffusion mechanisms

The diffusion mechanism of a zirconia-based amperometric NOx sensor was examined by studying the effect of sensor temperature on sensor output. The structure and the exact dimensions of a production automotive NOx sensor were first measured using x-ray tomography. A simplified heat transfer model was employed and validated to estimate the sensor temperature based on sensor heater power. The sensor temperature was then changed by changing the sensor heater power to evaluate different diffusion mechanisms. Normal multi-component diffusion mechanism, Knudsen diffusion mechanism and a combination of both mechanisms (Normal and Knudsen diffusion) were evaluated at different sensor temperatures. The normal multi-component diffusion mechanism was experimentally found to be the dominant diffusion mechanism that affects the diffusive flow through the sensor diffusion barriers. A sensor model was developed based on this dominant diffusion mechanism to predict NOx concentration. Finally, the sensor model output for NOx was validated with the experiments at different Diesel engine operating conditions with different species concentrations.

Application of TiO2 to amperometric NOx sensors based on NASICON

The work focused on improvement of the current response to NO and the reduction of response time of both NO and NO2. By adding a layer of TiO2 catalyst layer, the shortcoming of NO insensitivity of traditional NASICON based amperometric NO2 sensor was solved. TiO2 was known as an n-type semiconductor and a high-performance NO oxidation catalyst. We separately applied different types of TiO2 (rutile and Degussa P25) on the device and compared the test results with the traditional type without a catalyst layer. The results showed that the TiO2 attached device exhibited a significant improvement to NO response, and P25 was better than rutile TiO2 on account of the higher activity of anatase TiO2. In addition, TiO2 also played a role of a semiconductor electrode, resulting in the increase of response to NO2. The response time of NO and NO2 was separately shrunk from 153s, 70s to 77s, 43s owing to the P25 layer. The current responses were found to be barely affected by the coexistence of CO2.

A Stable Sensing-Electrode Material in Reducing Atmosphere at High Temperature for Zirconia-Based Amperometric NOx Sensor

A Stable Sensing-Electrode Material in Reducing Atmosphere at High Temperature for Zirconia-Based Amperometric NOx Sensor

The Amperometric NO<sub>x</sub> Sensors’ Electrode Polarization and its Effect on the Offset

Amperometric NOx sensors have been developed for the applications of on-board vehicle exhaust sensing for diesel powertrains to meet the strict government emission regulations in North America and Europe. The sensing mechanism of these sensors to NOx was studied along with several noise factors that affect the NOx output. It was found that cross sensitivity to oxygen may result in an increased offset. The results from this study indicate that polarization of one or more of the electrodes is a major factor affecting the magnitude of the offset.

Effect of Sr addition to La-based perovskite-type oxide as an electrode material for zirconia-based amperometric-type NOx sensor

The sensing characteristics of an amperometric NOx sensor with yttria-stabilized zirconia and a La-based perovskite-type oxide sensing electrode were examined. La1-xSrxMO3 (M = Co, Mn, x = 0, 0.2, 0.4, 0.6) were synthesized using the spray pyrolysis method. The NO2 response of a sensor fabricated with La1-xSrxMnO3 increased with increasing amount of Sr, and the trend was in contrast to that of the sensors fabricated with La1-xSrxCoO3. LaSr0.2MnO3 was determined to be the most appropriate material for the sensor in terms of a high NO2 response and low O2 current. In order to discuss the effects of Sr addition to La-based perovskite oxides for the sensor, NO adsorption and desorption properties of oxides and the relationship between the oxidation number of the B-site cation and Sr substitution were examined using temperature-programmed desorption and X-ray photoelectron spectroscopy, respectively.

Effects of Sr Addition to La-Based Perovskite Sensing-Electrode on YSZ-Based Amperometric-Type NOx Sensor

Sensing characteristics of amperometric NOx sensor using yttria-stabilized zirconia (YSZ) and La-based perovskite-type oxide sensing-electrode (SE) was examined. La0.8Sr0.2MO3(M = Co, Mn, Fe) and LaMnO3 was synthesized by means of spray pyrolysis method. The sensor attached with La0.8Sr0.2MnO3 as a SE showed the highest occupancy rate of NO2 response in total current as well as low current response to CO, C3H6, NH3 at 600°C in O2 (21%). The response was increased with increasing NO2 concentration in the examined range between 50 to 800 ppm. The comparison of sensing property between LaMnO3-SE and La0.8Sr0.2MnO3-SE showed the substitution Sr for La improved the NO2 response for the sensors.

Amperometric-type NOx sensor based on YSZ electrolyte and La-based perovskite-type oxide sensing electrode

At present, lean-burn type engines or diesel engines for passenger cars have been developed and installed into automobiles to improve fuel efficiency. In these cases, NOx storage catalysts or selective catalytic reduction (SCR) systems are also installed for reducing NOx emission. In order to control these systems more efficiently, NOx sensor which responds selectively, quickly, and quantitatively are necessary. Here we focused on the NO direct decomposition function of La-based perovskite oxides. We applied them to a sensing electrode for the amperometric NOx sensor using yttria-stabilized zirconia (YSZ). La0.8Sr0.2MO3 (M = Co, Mn, Fe, Ni) powders were synthesized by means of spray pyrolysis method. Among four kinds of oxides studied, the sensor using La0.8Sr0.2MnO3-SE showed the highest sensitivity to NO2 as well as low base current even in the presence of excess O2 (21 vol%). The sensor also exhibited excellent NO2 selectivity at 500°C. The sensitivity was increased with an increase of NO2 concentration in an examined range between 50 to 800 ppm, and did not change in the examined O2 concentration range (5-21 vol%).

The Effect of Electrode Polarization on the Offset for Amperometric NOx Sensors

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