Impedancemetric NOx Sensors Based on LSM and LSM-Au Sensing Electrodes

Abstract

An ideal NOx sensing electrode for diesel exhaust gas systems should possess high sensitivity and selectivity to NOx (i.e., NO and NO2) and low cross-sensitivity to interfering exhaust gases. In addition, it should be possible to fabricate the electrode readily at low cost. Gold (Au) has been rigorously studied as a sensing electrode for Y2O3-doped ZrO2 – YSZ electrolyte based sensors and found to demonstrate high NOx sensitivity and low water cross-sensitivity. However, the low melting temperature of Au and thermal expansion mismatch with YSZ limits the feasibility of Au as a sensing electrode. The perovskite, strontium-doped lanthanum manganite, La0.8Sr0.2MnO3 – LSM is an attractive alternative to Au from a manufacturing perspective as tolerates the high firing processing steps for accompanying sensor components and its thermal expansion coefficient is more compatible with YSZ. In addition, LSM has demonstrated low cross-sensitivity to interfering gases, such as NH3. Yet managing water cross-sensitivity remains challenging for LSM based sensors. In this work a detailed study was carried out on LSM and LSM-Au composite sensing electrodes in order to explore the combined benefits of LSM and Au. The NOx response of LSM and LSM-Au supported sensors was evaluated with respect to sensitivity, selectivity, accuracy, and response rate using the impedancemetric method. In addition, the frequency dependence of the sensors was studied to better understand the role of the operating frequency on the electrochemical behavior of the sensors. LSM and LSM-Au composite sensing electrodes were fabricated using standard ceramic processing methods. Electrode supports containing LSM-Au were made using powder mixtures of 90 wt% LSM (Inframat Advanced Materials) and 10 wt% Au (Alfa Aesar) that was uniaxially pressed at 200 MPa and fired at 1400 °C for 5 hours to achieve a density of ~ 95%. The electrolyte was 8 mol% YSZ (Tosoh Corp.) that was made into a slurry. A portion of the LSM-Au pellet was dip coated with the YSZ slurry. Several LSM-Au pellets with YSZ coating were co-fired at 1000 °C for 1 hour. The counter electrode was coated onto the porous YSZ electrolyte using a slurry made from LSM-Au powder, and the pellets were fired again at 1000 °C for 1 hour resulting in LSM-Audense/YSZ/LSM-Auporous cells to serve as NOx sensors. The sensors with pure LSM SE were made following the same procedure as LSM-Au based sensors. Frequency dependent impedance measurements were collected using a Gamry Reference 600 for sensors exposed to various gases at a flow rate of 500 sccm and operated at 575 °C under dry and humidified conditions. The oxygen concentration in the test gas environment was varied from 5 - 18%. The microstructural properties were analyzed based on scanning electron microscopy (SEM), X-ray powder diffraction (XRD), Mercury Intrusion Porosimetry (MIP) and Archimedes method. The electrochemical behavior of the sensors in terms of frequency dependence, response time (τ90), exhaust gas cross-sensitivity, and oxygen partial pressure dependence was determined from impedance spectroscopy data. The impedance studies indicated the LSM-Au based NOx sensors demonstrated about 25% greater sensitivity to NOx for concentrations between 0–50 ppm, in comparison to LSM based sensors. As the gas concentration increased, NOx sensitivity for the LSM based sensors became more similar to the response for LSM-Au based sensors. Sensor sensitivity was based on the angular phase component of the impedance that varies with frequency. It was found that NOx sensors with an LSM-Au sensing electrode were sensitive to NOx over a wider frequency range from 1 - 250 Hz, in comparison to LSM based sensors where the frequency range was 1 - 125 Hz for NOx sensitivity. Cross-sensitivity studies indicated that sensors composed of LSM and LSM-Au electrodes were mostly impacted by interference of H2O and O2. However, the addition of Au in LSM significantly reduced the impact of water cross-sensitivity. Data indicated water cross-sensitivity was approximately 58% lower for sensors with LSM-Au electrodes, in comparison to those with LSM electrodes. However, increasing O2 in the gas stream caused water sensitivity to become more similar for both electrodes. This indicated oxygen cross-sensitivity was slightly higher for sensors with LSM-Au electrodes versus LSM electrodes. Measurements for the response time (τ90) indicated LSM-Au based sensors were slower than those supported with LSM, possibly due to oxygen inference. Overall, sensors containing the LSM-Au composite electrode demonstrated improved NOx sensing capabilities, particularly at lower NOx concentrations, and lower water cross-sensitivity in comparison to those with an LSM electrode. Improving response time and reducing oxygen dependence may be possible by varying the amount of Au and LSM within the sensing electrode.