1. Impedancemetric NOx Sensors Based On La0.8Sr0.2MnO3 and Au Electrodes
Nabamita Pal and Erica P. Murray IFM, Louisiana Tech University, Ruston, LA 71270, USA.
Introduction
In order to conform to new emission standards in US and other countries diesel vehicles need more sensitive and accurate exhaust gas sensors [1]. Traditional NOx (NO and NO2) exhaust gas sensors for diesel vehicles consist of porous platinum (Pt) electrodes along with a dense ZrO2 based electrolyte. Recent studies indicate that dense (instead of porous) electrodes can contribute to greater NOx sensitivity by limit catalytic reactions driving oxygen reduction [2]. Dense perovskite metal oxide electrodes are an attractive option because of their unique chemical, electrical, and thermal properties [3]. This work emphases the potential of the perovskite, strontium-doped lanthanum manganite (LSM), as an alternative electrode material for NOx sensing. Here the goal is to understand electrode reactions involving NO, NO2, O2 and H2O that contribute to NOx sensitivity and selectivity using the impedancemetric method for NOx sensing.
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Figure 1: Schematic diagram of the Experimental Set-up.
LSM
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YSZ
Figure 4: (a) NOx sensor electrical behavior for different concentrations of NO, (b) LSM electrode behavior for dry and wet NO2, (c) Equivalent circuit describing NOx sensor response and (d) Oxygen partial pressure dependence of LSM electrode indicating charge transfer and O2 adsorption were rate limiting.
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Methodology
Figure 2: (a) Schematic diagram (top view ) of the NOx Sensor (b) SEM image of fractured cross-section of the sensor.
Electrode supports for sensors were fabricated using these steps-
Strontium-doped lanthanum manganite powder, La0.8Sr0.2MnO3 – LSM (Inframat Advanced Materials) was uniaxially pressed under 200 MPa and fired at 1400 °C for 4 hours to achieve a density of ~ 90%.
The 8 mol% Y2O3-doped ZrO2 – YSZ (Tosoh Corp.) electrolyte was made from a slurry based on standard ceramic processing methods.
Two-third of LSM pellet was coated with YSZ slurry, an Au wire was embedded within YSZ electrolyte coating.
Several LSM pellets with YSZ coating and Au wire electrode were co-fired at 1050 °C for 1 hour resulting in LSM/YSZ/Au cells as NOx sensors.
Impedance data were collected using a Gamry Reference 600 with sensors exposed to NO and NO2 gases having concentrations range of ppm with flow rate of 100 sccm at 650 °C for dry and wet (~3% H2O) conditions. The oxygen concentration during testing was varied from %.
Conclusions
LSM appears to be a suitable electrode for NOx sensing as:
Sensor fabrication resulted in a chemically and mechanically stable interface between the LSM electrode and YSZ electrolyte.
LSM based sensors showed reasonable sensitivity to NOx even down to 5ppm.
Charge transfer and oxygen adsorption rate limiting mechanisms seemed to have limited impact on NOx reactions.
For both NO and NO2 the sensors showed a slight increase in sensitivity under wet conditions.
Results
References
[1] United States Environmental Protection Agency, 2007 Progress Report: “Vehicle and Engine Compliance Activities”, EPA-420-R , Aug 2010.
[2] Kelsey A. Stoerzinger, Wesley T. Hong, Gisele Azimi, , Livia Giordano ,Yueh-Lin Lee, . “Reactivity of Perovskites with Water: Role of Hydroxylation in Wetting and Implications for Oxygen Electrocatalysis.” J. Phys. Chem. C 119, −18512, 2015.
[3] Jie Zou, Yangong Zheng, Junliang Li, Zhongliang Zhan and Jiawen Jian, “Potentiometric NO2 Sensors Based on Thin Stabilized Zirconia Electrolytes and Asymmetric (La0.8Sr0.2)0.95MnO3 Electrodes.” Sensors, 15, ,
Figure 3: NOx sensitivity of the sensor using LSM electrode
Δθ = θO2 – θNOx, where 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.
Acknowledgement
The project is funded by the National Science Foundation under the Ceramics Division (DMR ).