Electrolyte porosity based on Archimedes method.
NOx sensors composed of partially stabilized zirconia (PSZ), fully stabilized zirconia (FSZ), and PSZ–FSZ composite electrolytes were investigated using impedance spectroscopy under dry and humidified gas conditions. The impedance data were used to interpret the electrochemical behavior of the various sensors as the water concentration in the gas stream varied. The sensors were operated in the presence of 0–100 ppm NO with 1–18% O2 and 3–10% H2O with N2 as the balance gas. The operating temperature of the sensors ranged from 600 to 700°C. The impedance response for sensors containing ≥ 50 vol% PSZ slightly decreased under humidified gas conditions, in comparison to dry gas conditions; whereas, a significant increase in impedance occurred for sensor largely containing FSZ. This indicated water cross-sensitivity was substantial at FSZ-based sensors. The microstructural properties, NOx sensitivity, oxygen partial pressure and temperature dependence, as well as the response time of the sensors composed of the various electrolytes were characterized in order to interpret the electrochemical response with respect to water cross‐sensitivity. Analysis of the data indicated that sensors composed of a PSZ–FSZ composite electrolyte with 50 vol% PSZ were more suitable for detecting NOx while limiting water cross‐sensitivity.
- NOx sensors
- porous zirconia
- impedancemetric gas sensing
- partially stabilized zirconia
Advancements in diesel engine technology and emissions regulations in various countries are driving the need for NO
Cross‐sensitivity in NO
Recently, the authors found composite electrolyte NO
Standard ceramic processing techniques were used to fabricate NO
The microstructure and morphology of the electrolytes were analyzed using scanning electron microscopy (SEM) and Archimedes method. The electrochemical behavior of the sensors was characterized using a Gamry Reference 600 to perform impedance spectroscopy. Impedance measurements were performed for sensors operating over a temperature range of 600–700°C where the concentration of NO and NO2 was varied from 0 to 100 ppm in O2 concentrations of 1–18% with N2 as the balance. Data were collected for dry and humidified (3–10% H2O) environments using a standard gas handling system with mass flow controllers that maintained a flow rate of 100 standard cubic centimeters per minute (sccm). The Gamry instrument was configured to apply a signal amplitude of 50 mV over an operating frequency range of 1 Hz–1 MHz. Measurements were collected in triplicate to insure the data were consistent and stable. Equivalent circuit modeling using Gamry EIS300 software was used to acquire a detailed understanding of the electrochemical behavior of the sensors.
3. Morphology and microstructure
The microstructure of the porous electrolyte can impact gas sensor reactions. Studies have found the porosity of the electrolyte effects gas transport to the triple phase boundary (TPB) [3, 7]. The TPB is the location where the electrolyte, electrode, and gas phase are in contact. The TPB is important as NO
|PSZ||75 PSZ–25 FSZ||50 PSZ–50 FSZ||25 PSZ–75 FSZ||FSZ||PSZ coarse|
4. Electrochemical response
Impedance spectroscopy is a powerful and commonly used technique for interpreting the electrical response of electrochemical devices and systems . The impedance is an AC measurement that describes the opposition to current flow due to resistance, inductance, and capacitance effects. The impedance, varies with frequency,
where the angular frequency, . In addition, the real, and imaginary, impedance typically produce a semicircular shaped response or arc that is presented in the complex plane. In practice, more than one arc is common due to various reactions occurring at different rates. The impedance measurements for the NO
The addition of water to the gas stream caused the low frequency impedance arc to slightly decrease for sensors with a PSZ, 75 PSZ–25 FSZ, and 50 PSZ–50 FSZ electrolyte. Quite the opposite behavior occurred for sensors containing a FSZ and 25 PSZ–75 FSZ electrolyte. The change in the impedance due to water cross‐sensitivity is shown for sensors composed of PSZ, FSZ, and 50 PSZ–50 FSZ in Figure 3a–c. In other studies, analysis of water adsorption experiments at oxide surfaces have found that molecular water strongly adsorbs onto the surface of Y2O3 and surface reactions result in the formation of hydroxyl groups [10, 11]. Furthermore, computational studies indicate dissociation of water molecules is a mechanism for the formation of hydroxyl species at Y2O3–ZrO2 surfaces; and, interfacial reactions between the oxide and hydroxyl groups can enhance oxygen ion conductivity . However, it is also possible for adsorbed water molecules and hydroxyl species to block oxygen adsorption sites along the electrolyte/electrode interface and subsequently hinder interfacial reactions with oxygen . In the present study, the Y2O3 content of the sensor electrolyte was approximately 4.7 and 8 mol% for PSZ and FSZ supported sensors, respectively. It is possible that the higher Y2O3 content of FSZ allowed greater adsorption of molecular water and hydroxyl groups to take place at sensors containing an FSZ based electrolyte, in comparison to sensors with ≥50 vol% PSZ. This would decrease the available sites for NO and O2 adsorption. For such as case, triple‐phase‐boundary reactions requiring NO and O2 would be limited, thereby, causing the sensor impedance to increase in the presence of water, as shown in Figure 3b. The slight decrease in impedance for PSZ sensors likely occurred due to enhanced oxygen ion conductivity. The lower Y2O3 content of PSZ possibly resulted in less molecular water coverage such that sufficient adsorption of oxygen was able to take place. The adsorbed oxygen along with resulting hydroxyl species participated in reactions that produced oxygen ions, which enabled triple‐phase‐boundary reactions to more readily proceed at PSZ‐based sensors.
As mentioned previously, the microstructure of the sensor electrolyte affects the number of particles and reaction sites along the triple‐phase‐boundary where NO
5. Equivalent circuit analysis
To further interpret the electrochemical response of the sensors equivalent circuit analysis was carried out. The impedance arcs in Figure 3 were simulated using an equivalent circuit model consisting of resistors, R1 and R2, and constant phase elements, CPE1 and CPE2. Figure 4 shows the equivalent circuit model that was used to analyze the impedance data for each of the sensors in greater depth. The components R1 and CPE1 described the resistance and nonideal capacitance behavior of the electrolyte. Components R2 and CPE2 corresponded to the interfacial resistance and nonideal capacitance behavior at the electrodes. The constants
The behavior of R2 and CPE2 are useful for gaining insight about electrode reactions that influence sensing behavior. It was found that the interfacial resistance, R2, decreased as the operating temperature of the sensor increase. This was expected as interfacial reactions, such as charge transfer, are able to proceed more readily at higher temperatures. The relationship between the capacitance, C, and constant phase element, CPE2 is described by the following equation:
6. Sensor sensitivity
The angular phase component of the impedance, θ, is often more responsive the changes in NO
The change in the angular phase, Δθ, is given by:
where θO2 is the baseline response when 10.5% O2 is present with N2; and, θNO corresponds to the response when a specific amount of NO gas is added. Figure 6 shows Δθ with respect to NO and water concentration for the PSZ, FSZ, and 50 PSZ–50 FSZ composite‐based sensors at an operating frequency of 40 Hz at 650°C. Data was collected at 40 Hz as the maximum sensitivity was achieved at this operating frequency. The slope of the data, Δθ/Δ[NO], is defined as the sensor sensitivity in units of degrees/ppm NO. The highest sensitivity was achieved during dry gas conditions for the various sensors. As the water concentration was increased in the gas stream the sensitivity of the sensors decreased. However, the 50 PSZ–50 FSZ composite based sensors demonstrated the greatest tolerance to changes in the gas humidity as the sensitivity decreased very slightly. The sensors with an FSZ electrolyte had a higher sensitivity than sensors composed of PSZ, but the sensitivity of the FSZ based sensors decreased most significantly with increasing water in the gas stream. Although the FSZ electrolyte sensors provide higher sensitivity, the 50 PSZ–50 FSZ electrolyte contributes to a more reliable sensor.
In application the temperature of the sensor can fluctuate due to changes driving conditions. Thus, a stable NO
7. Rate limiting mechanisms and activation energies
The gases traveling through the porous electrolyte and reacting at the triple‐phase‐boundary undergo various reaction steps including adsorption, dissociation, diffusion, charge transfer, and oxygen ion transport (not necessarily in that order). The porous microstructure creates specific pathways for gas and ionic transport. The electrolyte material and microstructure can impact how readily gas transport and related reactions occur. A common approach to interpreting rate‐limiting mechanisms that impact gas transport and associated reactions is to evaluate the oxygen partial pressure (PO2) dependence, which is described by the following power law relationship:
The power law exponent,
|Power law exponent, m|
|NO (ppm)||PSZ||50 PSZ–50 FSZ||FSZ|
The activation energy associated with the various sensors also aids interpretation of how readily sensor reactions are able to proceed. The activation energy, Ea, of the sensors for operating temperatures ranging from 600 to 700°C was determined using the Arrhenius equation the given below:
where RLFA is the diameter of the low frequency impedance arc, R represents the ideal gas constant,
|Activation energy (eV)|
|NO (PPM)||PSZ||75 PSZ–25 FSZ||50 PSZ–50 FSZ||25 PSZ–75 FSZ||FSZ|
8. Sensor response time and stability
Time‐based measurements for the angular phase response, θ, were collected for sensors with a PSZ, FSZ, and composite PSZ–FSZ electrolyte. Figure 8 shows the typical time‐based response for sensors with a 50 PSZ–50 FSZ electrolyte as the NO composition was varied over 0–100 ppm. The dry and humidified data overlap as the 50 PSZ–50 FSZ electrolyte enables the sensor to be less prone to water cross‐sensitivity. A baseline shift was observed for the time‐based data collected for sensors composed the other electrolytes studied. The response time for each sensor was evaluated based on the τ90 response, which is the time required for the sensor to achieve 90% of the steady state response after a step change in gas concentration. Table 4 shows the τ90 response for the various sensors. The sensors containing single‐phase PSZ were the slowest as τ90 = 16 s, which possibly relates to the lower ionic conductivity of the electrolyte, in comparison to single‐phase FSZ‐based sensors where τ90 = 5 s. As seen in Table 4, the sensor response time decreased resulting in a faster sensor as the FSZ concentration increased.
|Time constant – (s)|
|Sensor electrolyte||PSZ||75 PSZ–25 FSZ||50 PSZ–50 FSZ||25 PSZ–75 FSZ||FSZ|
The stability of a 50 PSZ–50 FSZ based sensors was evaluated for over 150 hours at 650°C and an operating frequency of 40 Hz with 3% water present. Figure 9 shows sensor drift was negligible for both the baseline response where θ remained about −17.25°, and the sensor response with humidified gas where θ remained nearly constant at about −16.25°. This data suggests that the 50 PSZ–50 FSZ electrolyte is capable of providing a very stable sensing response. Further evaluation is necessary to determine the extent of the 50 PSZ–50 FSZ based sensor stability over longer time intervals.
Impedance spectroscopy was used to interpret the electrochemical response of NO
The authors thank Mr. Robert Novak and Dr. Jaco Visser of Ford Motor Company for providing meaningful discussions that contributed to this work. Funding for this work was provided by the National Science Foundation under the Ceramics Program (DMR‐1410670).
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