Impedancemetric NOx Sensors Based on La0.8Sr0.2MnO3 and Au Electrodes

Abstract

NOx exhaust gas sensors for diesel powered vehicles have traditionally consisted of porous platinum (Pt) electrodes along with a dense ZrO2 based electrolyte. Although Pt is chemically and mechanically tolerant to the stringent exhaust gas environment, it is a strong catalyst for oxygen reduction, which can interfere with the sensing response to NO and NO2. Countering this behavior often adds to the complexity and cost of the NOx sensor. Recent studies have shown that dense electrodes can limit catalytic reactions driving oxygen reduction. Sensors composed of dense electrodes typically have a porous electrolyte to enable gas diffusion to NOx reaction sites. This novel architecture (i.e., dense electrodes with a porous electrolyte) seems to promote greater sensitivity to NOx, however, research in this area is still in an inchoate stage. There is particular interest in acquiring greater knowledge of the sensing behavior of non-catalytic dense electrodes as they may offer a lower cost alternative to using Pt electrodes. Perovskite metal oxide electrodes are an attractive option because of their chemical, electrical, and thermal properties. Dense gold (Au) is also studied as an alternative electrode to Pt since it does not readily promote O2 reduction and is highly stable under exhaust gas conditions. This work focuses on the potential of the perovskite, strontium-doped lanthanum manganite, and Au as electrodes for NOx sensing. The goal is to understand more about electrode reactions involving NO, NO2, O2 and H2O that contribute to NOx sensitivity and selectivity using the impedancemetric method for NOx sensing. Electrode supports for sensors were fabricated using strontium-doped lanthanum manganite powder, La0.8Sr0.2MnO3 – LSM (Inframat Advanced Materials) that was uniaxially pressed under 200 MPa and fired in 1400 °C for 4 hours to achieve a density of ~ 90%. The diameter of the pellet was 11 mm and it had a thickness of approximate 1.1 mm after firing. The electrolyte was 8 mol% Y2O3-doped ZrO2 – YSZ (Tosoh Corp.) that was made from a slurry based on standard ceramic processing methods. Two-thirds of the LSM pellet was coated with the YSZ slurry, and an Au wire was embedded within the YSZ electrolyte coating. Several LSM pellets with a YSZ coating and Au wire electrode were co-fired at 1050 °C for 1 hour resulting in LSM/YSZ/Au cells to serve as NOx sensors. Impedance measurements were collected using a Gamry Reference 600 for sensors exposed to NO and NO2 for concentrations ranging from 0 - 100 ppm at a gas flow rate of 100 sccm at 650 °C for dry and humidified (3% H2O) conditions. The oxygen concentration in the test gas environment was varied from 1 - 18%. The microstructural properties, electrochemical behavior and sensitivity of the sensors were analyzed based on scanning electron microscopy (SEM), impedance spectroscopy, equivalent circuit modeling, and oxygen partial pressure dependence. Representative results from SEM images of the surface and cross-section of the LSM/YSZ/Au sensors confirmed a dense LSM electrode microstructure with closed pores, and a porous YSZ electrolyte with a thickness of ~ 0.6 mm. Impedance data for the sensors consistently showed two separate semicircular depressed arcs indicating sensor reactions were associated with at least two distinct time constants. The impedance spectra for NO and NO2 were very similar, and the addition of H2O for humidified gas conditions did not result in a significant change in the impedance response. The change in the angular phase angle response, Δθ = θO2 – θNOx, of the sensors was more sensitive to the differences in the test gas concentrations. Here, θO2 corresponded to the phase angle response with only 10.5% O2 and N2 present; and, θNOx corresponded to the phase angle response with the addition of NO or NO2 in the gas stream. The sensitivity of the sensors was based on this Δθ calculation. Comparison of Δθ values for various operating conditions indicated the LSM/YSZ/Au sensors were slightly more sensitive to NO2 in comparison to NO, and the addition of H2O generally resulted in an increase in the NOx sensitivity by ~ 13%. Moreover, the LSM/YSZ/Au sensors demonstrated significant sensitivity to NOx for concentrations as low as 5 ppm. The sensing capability of conventional NOx sensors is limited to about 10 ppm. Other key results, based on PO2 dependence and the peak frequency behavior of the impedance as a function of O2 concentration, suggested that charge transfer and oxygen adsorption were rate limiting mechanisms impacting NOx reactions.