Advisor: Dr. Erica P. Murray. Institute for Micromanufacturing (IFM), Louisiana Tech University, Ruston, LA 71272. NO x. Sensors using Au/Pt Electrodes ...
NOx Sensors using Au/Pt Electrodes Nabamita Pal Advisor: Dr. Erica P. Murray Institute for Micromanufacturing (IFM), Louisiana Tech University, Ruston, LA 71272
Introduction Conventional NOx sensors typically have porous electrodes and dense electrolyte.
In this novel approach for NOx sensors with dense electrodes and porous electrolyte were used. Dense electrodes can enable greater NOx sensitivity as they are less prone to heterogeneous catalysis. Advancements in diesel engine technology is resulting in lower emissions. NOx detection at the single parts per million level is possible with dense NOx sensing electrodes. Conventional NOx sensor
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Research Motivation Platinum (Pt) electrodes are commonly used in NOx sensors as the material tolerates the stringent exhaust gas environment. However, Pt is also a strong catalyst for O2 reduction, which can interfere with accurately detecting NOx at concentrations below 10 ppm.
Gold (Au) is a promising alternative electrode, but its melting temperature (i.e., 1060 °C) limits manufacturing options for a viable commercial NOx sensor. To address this manufacturing issue, Au/Pt electrodes are under consideration. Dense Au/Pt electrodes are expected to extend the processing temperature tolerance of the electrode, but may also limited reaction sites along the triple phase boundary (TBP) and compromise NOx sensitivity. The aim of this research project was to explore the feasibility of dense Au/Pt electrodes for NOx sensing.
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Experimental: Sensor Fabrication Materials Electrolyte: Porous zirconia based ceramic pellet Electrode: Pt and Au wires
Fabrication Zirconia based pellets were made by using standard ceramic processing methods Au/Pt electrode was made by wrapping Pt and Au wires together. Electrodes embedded into the electrolyte were co-fired at 1050°C for 1 hour in air.
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Characterization Impedance Spectroscopy was used to evaluate electrical response of the NOx sensors.
Schematic diagram of the NOx Sensors with Au/Pt sensing electrode
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Experimental: Testing Apparatus
The characterization conditions for the sensors was as follows:
A Gamry Reference 600 impedance analyzer was used to measure the electrical response of the sensors and the signal amplitude was 100 mV.
Schematic diagram of the experimental set-up.
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Result : Electrical Response -24
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C with Au sensing electrode 625 625°C with Au sensing electrode
625°C with Au/Pt Au/Pt sensing electrode electrode C with 625 625°C with Au/Ptsensing sensing electrode
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-20
10.5% O2 + N2
10.5% O2 + N2
Z''(k)
Z''(k)
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10.5% O2 + N2+ 100 ppm NO
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10.5% O2 + N2+ 100 ppm NO
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HFA -12
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LFA -4
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(a)
(b)
0
0 0
5
10
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25
30
35
0
5
Z'(k)
10
15
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Z'(k)
In the electrical response in Figures a and b show two distinct arcs. The larger arc is High Frequency Arc (HFA) and the smaller arc is Low Frequency Arc (LFA). The LFA (interfacial resistance) is very important in this study because it describes the NOx sensing response. The LFA was larger for the Au/Pt electrode in comparison to the pure Au electrode, which indicated some interference with interfacial reactions at the Au/Pt and zirconia interface. In Figure a, the distortion of the LFA suggested a diffusion mechanism. 7
Result : NO Sensitivity 1.4
-2.0
10.5% O2 + N2+ NO
10.5% O2 + N2+ 100 ppm NO 1.2
-2.5
Au sensing electrode
Temp. = 600C f = 25Hz
Au sensing electrode
Au/Pt sensing electrode
Au/Pt sensing electrode
1.0 -3.0 m = .011 0.0013
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f = 25Hz
-3.5
0.6 -4.0
0.4
m = .012 0.0015
-4.5
0.2
-5.0
0.0
600C
625C
650C
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0
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60
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100 110
[NO] ppm
Sensitivity plot for the NOx sensor where, Δθ = θO2 – θNOx, 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 in the gas stream.
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Result : Oxygen partial pressure dependence (Rate limiting mechanism) 0.9
Au sensing electrode Temp. = 600C
10.5% O2 + N2
0.8
log RLFA( cm2)
10.5% O2 + N2+ 100 ppm NO 0.7 m = - 0.33
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0.5 m = - 0.25
0.4
0.3 -2.2
-2.0
-1.8
-1.6
-1.4
-1.2
-1.0
-0.8
-0.6
log Po2 (atm)
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Conclusions
NOx sensors based on the Au/Pt sensing electrode demonstrated similar sensitivity to that observed for Au sensing electrodes. Thus, Pt had a negligible effect on the sensing behavior.
The temperature dependence of the Au/Pt sensing electrodes was also very similar to that of Au as the sensitivity decreased with operating temperature.
The preliminary oxygen partial pressure data suggested the rate limiting mechanism involved charge transfer. Additional data will be collected to determine the rate limiting mechanism for sensors using a Au/Pt electrode.
Overall, the data indicated the Au/Pt sensing electrode is a promising alternative to Au sensing electrodes.
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Acknowledgement This work is supported by the BoR / NSF / EPSCoR through the seed funding under CIMM.
