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Influence of polarization time and polarization current of Pt|YSZ-based NO sensors utilizing the pulsed polarization when applying constant charge
Sensors & Actuators: B. Chemical 290 (2019) 28–33
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Influence of polarization time and polarization current of Pt|YSZ-based NO sensors utilizing the pulsed polarization when applying constant charge
T
Nils Donkera, Anastasiya Ruchetsb, Daniela Schönauer-Kamina, Jens Zoselb, Ulrich Guthc, ⁎ Ralf Moosa, a
Department of Functional Materials, University of Bayreuth, 95440 Bayreuth, Germany Kurt-Schwabe-Institut für Mess- und Sensortechnik e.V. Meinsberg, 04736 Waldheim, Germany c Faculty of Chemistry and Food Chemistry, Dresden University of Technology, 01062 Dresden, Germany b
ARTICLE INFO
ABSTRACT
Keywords: Pulsed polarization Pt | YSZ NOx detection Exhaust gas sensor Mixed potential sensors Electrode kinetics, Dynamic sensor mode
A Pt-YSZ sensor was operated in the pulsed polarization mode. In contrast to previous studies, it was polarized with a constant charge. The polarization currents were varied. After polarization, it was self-discharged. The associated voltages during polarization and self-discharge are clearly NO-dependent in both processes. Adding NO results in higher voltages during polarization and faster discharge. In addition, it could be observed that the final values of the discharge curves after certain time depend on the NO concentration and not on the polarization current. This observation may shorten the polarization and therefore the cycle times and thus lead to a faster sensor response.
1. Introduction Nitrogen oxides emitted by combustion processes are precursors for environmental issues like smog or acid rain. To reduce these emissions, it is necessary to measure them in situ in order to optimize combustion and exhaust gas aftertreatment processes [1]. In most chemical sensor principles to detect these nitrogen oxides, a potential difference, a current, a resistance or an impedance is the measured quantity [2–5]. Potential difference measurements are often static, meaning that the sensor is in stationary equilibrium with the environment, resulting in a constant sensor voltage at a constant analyte concentration in the gas atmosphere. YSZ-based mixed potential sensors are typical examples for that kind of sensors [6]. Several electrode materials have been investigated for NOx detection [7–19] as well as for NH3 [20–24] with this sensor type. The latter is of particular interest for the application in the exhaust when applying the selective catalytic reduction of NOx by ammonia. In contrast, pulsed polarization is a dynamic sensor measuring principle, analogously to the thermo-cyclic operation of mixed potential sensors [25], metal oxide sensors [26] or like cyclic voltammetry [27]. With the pulsed polarization method, the sensor is periodically polarized by applying an additional electrical signal. Thus, as with the methods mentioned above, there is no constant measuring signal. Instead, the behavior of the subsequent discharge is measured. To obtain gas sensors, two platinum electrodes with yttria-stabilized zirconia (YSZ) as solid electrolyte in-between are required. Therefore,
⁎
potentiometric sensors like lambda probes [28–30] or planar sensors with platinum electrodes and YSZ [31,32] as solid electrolyte can be used. Interdigital electrodes are also possible, as shown recently [33,34]. In this work, a planar sensor structure was used. This is schematically shown in Fig. 1a. Fig. 1b displays the circuit used for pulsed polarization. The electric signal is usually realized by applying an additional external polarization voltage Upol for a polarization duration tpol. This voltage is applied to the sensor (DUT, device under test) by simultaneously closing the switches S1 and S4. Therefore, one electrode is positive and the other electrode is negatively polarized. The polarizing current can be measured indirectly by the shunt resistance Rshunt according to Ohm’s law. In the next step, the voltage source is disconnected from the sensor by opening all switches and the self-discharge behavior of the sensor is measured as an open circuit voltage (OCV) Udischarge for a defined duration tdischarge. This discharge is followed by another voltage pulse with reversed polarity. For that purpose, the switches S2 and S3 are closed, which also makes one electrode positive and the other electrode negative polarized, but with inverse polarity compared to the first step. A further discharge follows, for which again all switches are opened for the duration tdischarge. Such measurement cycles with alternating polarization voltages and discharge phases are repeated periodically. One measurement cycle for polarization with constant voltage is schematically shown in Fig. 2a. It could be shown in the previous works by Fischer et al. that the discharge of the sensor is strongly accelerated when NO or NO2 (NOx) are
Corresponding author. E-mail address: [email protected] (R. Moos).
https://doi.org/10.1016/j.snb.2019.03.060 Received 27 December 2018; Received in revised form 22 February 2019; Accepted 12 March 2019 Available online 14 March 2019 0925-4005/ © 2019 Elsevier B.V. All rights reserved.
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Fig. 1. a) Sensor setup: 8YSZ disk with screen-printed Pt electrodes and b) circuit which is used to apply the polarization pulses to the device under test (DUT).
Fig. 2. Sketch of a pulsed polarization cycle to illustrate the sensor principle for a) constant voltage polarization and b) constant current polarization as previous reported and applied in [28].
present. The discharge rate depends selectively on the NOx concentration [28–34]. In order to convert this accelerated discharge into a sensor signal, the sensor voltage Udischarge is evaluated at a certain time during discharge, for instance at t’ = 3 s after the end of each polarization process, in each respective cycle. These voltages serve as the sensor signal. The advantages of this method are its high NO sensitivity and selectivity [28] and its independence from a reference atmosphere [29]. Even full sensor calibration curves have been given in previous studies using constant polarization voltages. However, up to now, the physico-chemical effects on which the sensor is based have not yet been fully understood. It is disadvantageous that it takes at least one cycle to generate one sensor output value, i.e. when one output value is obtained, one has to wait for tcycle = 2(tpol + tdischarge) to obtain the next value. Only in the case of symmetrical sensors, the value for inverse polarity can also be taken due to the symmetrical sensor signal curve. This halves the time until the next valid value can be obtained. In order to improve this method, the total cycle time tcycle should be reduced to its (yet unknown) absolute minimum. Other important questions are minimum charge required for polarization as well as the reproducibility and long-term stability of the sensor voltage for each cycle. All these open points lead back to a model that allows to optimize this method and possibly also an application of the method for other sensor types. Therefore, a better insight into the sensing mechanism could further improve the good properties of sensors following the pulsed polarization technique. In particular, the role of the amount of charge during polarization is examined in this study. For this purpose, the pulsed polarization
measurements here were carried out with defined constant currents Ipol instead of a constant voltage as described in previous literature. For this, the voltage source displayed in Fig. 1b (Upol) was replaced by a current source. This makes it possible to polarize the sensor with a constant charge Qpol. Therefore, polarization current Ipol and polarization duration tpol were adapted to each other, according to Eq. (1):
Qpol = Ipol ·tpol
(1)
A schematic cycle using a constant polarization current Ipol and the corresponding voltage is shown in Fig. 2b. Here, the voltage at the sensor Usensor is measured during the entire cycle. Analogous to the polarization with constant voltage (Fig. 2a), the first part here shows the positive polarization with the charging current Ipol for the charging duration tpol by closing the switches S1 and S4 (Fig. 1b). One shaded area represents the total charge Qpol according to Eq. (1). After tpol, the current source was disconnected by opening all switches for the duration tdischarge. Negative polarization with -Ipol follows by closing switches S2 and S3 and the subsequent self-discharge by opening all switches. 2. Experimental For the subsequent measurements, a planar sensor with 8YSZ as solid electrolyte and platinum electrodes at both sides was used (Fig. 1a). The electrodes (Pt-paste, Ferro 64120410, Ferro Corporation, USA) were screen printed one after the other on an 8YSZ substrate 29
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Fig. 3. Cycles during polarizing, when previously polarized with Q = 400 μA s; a) 400 μA for 1 s b) 200 μA for 2 s, c) 100 μA for 4 s and d) 50 μA for 8 s. All measurements are performed at 400 °C in N2 with 10% O2 and 2% H2O. Additionally, steps of 50, 100 150 and 200 ppm NO were added. U5s_1_400 – U5s_8_50 represent the voltages 5 s after negative polarization. These voltages are displayed in Fig. 4 for the whole measurement. Each curve consists of 10 cycles, lying exactly on top of each other, which is an indication for the high reproducibility.
(Kerafol, Germany) and each fired for 30 min at 1300 °C peak temperature. Platinum wires were attached to the electrodes with ceramic glue for mechanical stability. A drop of platinum paste ensured the electrical contact. The sensors were then fired again after contacting both electrodes. To apply a constant current, a source meter (Keithley 2400, Keithley Instruments Inc., USA) was used as a current source and was periodically applied to the sensor via relays. For the polarity switch, relays were used as well. The circuit diagram is sketched in Fig. 1b. The sensor was polarized with 400, 200, 100, and 50 μA for 1, 2, 4, and 8 s, respectively, to obtain a total charge of 400 μA s according to Eq. (1). This total charge is approximately the charge that is transferred if the sensor is polarized with a constant voltage of 1 V for 1 s as determined in a previous measurement. This value was used to avoid decomposition of the YSZ due to too high (> 2 V) resulting polarization voltages [35]. As in previous works, 10 s were chosen as discharge time tdischarge [28–30]. Hence, each cycle (tcycle) lasted between 22 s and 36 s. This duration results from the double polarization time (2 * tpol) of 1–8 s and two times discharge time (2 * tdischarge) of 10 s. The highest voltage difference between base gas and 15 ppm NO (Usensor,base gas(t) – Usensor,NO15ppm(t)) was found at a sensor temperature of 400 °C [32]. Therefore, these measurements were conducted in a tube furnace at a sensor temperature of 400 °C. The temperature was measured with a thermocouple close to the YSZ-disc and accordingly controlled. The ceramic tube itself was purged with gases that were dosed by mass flow controllers (MFCs). In addition, water was added via an evaporator. A total flow of 1000 ml/min through the tube was adjusted. The base gas used here was 10 vol.-% O2 and 2 vol.-% H2O with N2 serving as a balance. In addition to the base gas, 50, 100, 150, and 200 vol.-ppm NO were added for 10 min each with 10 min base gas in between.
3. Results and discussion The results of the measurements with constant charge, but with different polarization currents, are shown in Fig. 3. Here, the curves for the negative polarization (-Ipol) and discharge for base gas without NO and with additional 50, 100, 150, and 200 ppm NO and with different polarization currents are displayed. To analyze the stability of the sensor voltages in detail, the voltage curves for 10 cycles for each NO concentration were displayed. It can be seen that the respective individual curves are exactly on top of each other, showing a high signal reproducibility. In addition, the individual courses for different NO concentrations can be clearly distinguished from each other for all polarization currents. This applies in particular to the difference between the profiles of base gas and 50 ppm NO, whereas difference between NO concentration steps is becoming less dominant. Comparing the sensor voltages for the individual analyte gas concentrations, on the one hand, a discharge accelerated by NO can be observed for all polarization currents and times. This effect is similar to polarization with constant voltage [28,29,31,32]. Therefore, one may assume that the sensor response using the here-suggested operation mode (constant charge) is as selective as it has already been published for constant voltage polarization [36]. However, additional selectivity tests are to be conducted soon. On the other hand, a NO effect is also visible in the voltage measured during polarization with Ipol. Here, the sensor shows a stronger voltage increase when polarizing with NO than without NO. This effect is also visible for all polarization currents. In order to be able to represent all cycles, sensor voltages were also evaluated at certain points after negative polarization. These values are denoted by Ut’_tpol_Ipol, in the following. The index t’ refers to the time from the end of the last polarization. The indices tpol and Ipol denote the 30
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Fig. 4. Voltages measured 5 s after negative polarizing the sensor with Q = 400 μA s; a) 400 μA for 1 s b) 200 μA for 2 s, c) 100 μA for 4 s and d) 50 μA for 8 s. All measurements are performed at 400 °C in N2 with 10% O2 and 2% H2O. Additionally, steps of 50, 100 150 and 200 ppm NO were added.
Fig. 5. a) Comparison of the polarization/discharge curves with different polarization currents and b) the corresponding discharge curves starting at the end of polarization.
polarization duration in seconds and the polarization current in microamperes, respectively. The voltage values were taken 5 s after each negative polarization (t’ = 5 s). Since the polarization times and currents of the individual measurements differ, the voltages U5s_1_400, U5s_2_200, U5s_4_100 and U5s_8_50 are evaluated for the polarization currents 400, 200, 100, and 50 μA, respectively. The points in time when these discharge voltages were obtained are depicted in Fig. 3 by dotted
vertical lines. Thus each data point represents the voltage value at a certain time in every cycle. If the discharge behavior changes, this voltage value will also change. These voltages are shown together with the measuring duration and the number of cycles in Fig. 4. On the basis of these curves, the strong NO dependence of the discharge processes and the high stability of the cycles can be seen. Even during the measurement with a polarization time of 8 s, the sensor has run through 31
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Pt|YSZ interface is polarized, capacitive effects with long time constants could be observed. These capacities were explained with charge storage via Faradaic processes (reactions (4) and (5)) [38] or by the existence of gas reservoirs between Pt and YSZ [39]. Both explanations can be attributed to chemical effects. They are considered to be much slower compared to electronic processes at the electrodes. The high voltages of 0.23 V even after 10 s discharge in the base gas are an indication for such discharge effects with very long time constants. In this case, the NO in the gas phase would reduce the potential difference at which only these slow discharge effects occur. An explanation might be the higher exchange current density at the three-phase boundary observed in the presence of NO [40]. This increased current density across the phase boundary at a given overpotential might lead to this increased discharge. This is contrasted by the faster voltage rise during polarization by NO, which rather indicates an obstacle to current flow. Therefore, a clear explanation has not been found yet. If the voltage at the end of discharge (at t’ = 9.9 s; see inset in Fig. 5b) is plotted for all polarization currents over the NO concentration, linear curves with similar slopes are obtained (Fig. 6). On the one hand, this shows the linear sensitivity of about 215 μV/ppm NO and the independence of the sensor output voltage values from the polarization current. On the other hand, it indicates a linear dependence of this residual charge on the NO concentration, assuming that the capacitance does not change. Finally, the voltages during polarization of the sensor are considered. As already mentioned, these voltages increase faster with increasing NO content. A higher polarization voltage at constant current represents a higher resistance, i.e., an effect that counteracts the current flow. Fig. 7 shows the voltages during polarization normalized to the total charge duration tcharge (t/tcharge). It is noticeable that the NO effect occurs at the same relative time for all polarization curves. Since the sensor was polarized with constant charge Q, the same relative time represents the same transported charge according to Eq. (2):
Fig. 6. Characteristic curve of the sensor voltage at the end of the discharge process as a function of the NO concentration.
Q t = Qges tges
(2)
This indicates that the NO effect to polarization only occurs after a finite transported charge. Such behavior might be explained with the following model. YSZ has a significantly higher ionic than electronic conductivity at measuring temperature [41]. This means that the applied current flows mainly in the form of oxide ions O2− through the solid electrolyte. These oxide ions are formed by a reduction of oxygen at the negative electrode by reaction (3):
Fig. 7. Polarizing the sensor with constant total charge but different polarization currents. The time plotted relatively to the duration of the total polarization.
more than 150 cycles. Nevertheless, the discharge behavior remains stable, both with and without NO addition to the base gas. The small difference in the discharge voltages between the individual NO steps and the large jump between base gas and 50 ppm NO leads to the conclusion that this method may be suitable to detect even lower NO concentrations. This will be tested in further measurements. Fig. 5a shows the course of the positive polarization and discharge voltages if the sensors were polarized with the same charge but various polarization currents. Here, it is visible that the discharge curves at the end of the discharge are almost equal. This is highlighted in Fig. 5b, where only the discharge curves are compared, now with a corrected time t’ that begins as soon as the discharge starts. It visualizes that all voltage curves converge to an apparent final value for the same analyte concentration in the gas. Therefore, the final value during discharge appears to depend only on the surrounding NO concentration and not on the voltage at the beginning of the discharge that differs for each individual polarization current. The voltage after being polarized with 400 μA starts at 1.5 V, which is significantly higher than the 0.7 V after polarization with 50 μA. Nevertheless, the discharge voltages after 10 s of discharge yield the same value in the respective gas atmospheres. This means, the same charge causes the same sensor voltage at the end of discharge, depending only on NO concentration. This behavior might be explained by the pseudo-capacitive behavior of the Pt/YSZ transition mentioned in the literature [37,38]. If the
O2 + 4e
2 O2
(3)
and will be oxidized to oxygen at the positive electrode by reaction (4):
2 O2
O 2 + 4e
(4)
In addition to oxygen incorporation and removal reaction, it is believed that platinum can be oxidized or reduced at the electrodes to form platinum oxides PtOx [37,38] by reaction (5):
Pt + xO 2
PtOx + 2xe
(5)
Especially for the reaction of platinum, it must be assumed that only previously formed platinum oxides will be reduced and platinum oxides will be formed in the range of monolayers. This would therefore be a process limited to a certain charge, which could represent the course at the beginning of polarization. In this case, the NO would inhibit the reduction (3) or oxidation (4) of the oxygen. However, the processes taking place here are not yet known and will be further investigated, preferably with the possibility to separate different processes from each other to get more insight in the contributing processes and the sensing mechanism. 32
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4. Conclusion
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The measurements show that it is possible to realize YSZ-based pulsed polarization sensors not only by polarizing with a constant voltage, but also with a constant charge. The NO influence is observed both during polarization of the sensor and during discharge. In both cases, the responses are stable. It has also been found out that the discharge voltages reached a final value in the same gas atmosphere. This effect is independent of the voltage at the beginning of the discharge. The value of these voltages at the end of the discharge is linearly dependent on the NO concentration. It also shows that the NO effect during polarization only occurs after a certain transported charge. This is attributed to a finite but yet unidentified process. Since this is only a first attempt with this mode of operation, further investigations to understand and improve the sensor principle will follow. A very first model approach for polarization with constant voltage was given by Fischer et al. [36]. Here, the increased discharge time was related to the local NO/NO2 equilibrium at the electrodes. It was assumed that a thermodynamic equilibrium is established at the electrodes and approximately 50% NO and 50% NO2 are present. This equilibrium is changed during polarization to an oxidizing and a reducing side. At the reducing side, NO2 can be reduced and thus additional oxygen is supplied, while NO may consume oxygen at the oxidizing side. According to this model, this enables the oxygen gradient to degrade faster after polarization and thus leads to faster depolarization. Further investigations will concern the oxygen dependence of the sensor response. Since YSZ is known as an oxide ion conductor and is already used as an oxygen sensor in the form of a lambda probe, corresponding influences are to be expected. Moreover, the effects of other interfering gases as well as a discriminability between NO and NO2 and the lower detection limit of this method shall be investigated. The questions of the minimum possible cycle duration and the minimum necessary polarization charge are also of interest to minimize the measurement duration. Acknowledgement We thank the German Research Foundation (DFG) for financial support under grants MO 1060/30-1 and ZO 139/3-1. References [1] N. Docquier, S. Candel, Combustion control and sensors: a review, Prog. Energy Combust. Sci. 28 (2002) 107–150. [2] J.W. Fergus, Materials for high temperature electrochemical NOx gas sensors, Sens. Actuators B 121 (2007) 652–663. [3] S. Zhuiykov, N. Miura, Development of zirconia-based potentiometric NOx sensors for automotive and energy industries in the early 21st century: What are the prospects for sensors? Sens. Actuators B 121 (2007) 639–651. [4] A. Afzal, N. Cioffi, L. Sabbatini, L. Torsi, NOx sensors based on semiconducting metal oxide nanostructures: Progress and perspectives, Sens. Actuators B 171-172 (2012) 25–42. [5] F. Ménil, V. Coillard, C. Lucat, Critical review of nitrogen monoxide sensors for exhaust gases of lean burn engines, Sens. Actuators B 67 (2000) 1–23. [6] N. Miura, T. Sato, S.A. Anggraini, H. Ikeda, S. Zhuiykov, A review of mixed-potential type zirconia-based gas sensors, Ionics 20 (2014) 901–925. [7] N. Miura, G. Lu, N. Yamazoe, High-temperature potentiometric/amperometric NOx sensors combining stabilized zirconia with mixed-metal oxide electrode, Sens. Actuators B 52 (1998) 169–178. [8] S. Zhuiykov, T. Ono, N. Yamazoe, N. Miura, High-temperature NOx sensors using zirconia solid electrolyte and zinc-family oxide sensing electrode, Solid State Ionics 152-153 (2002) 801–807. [9] N. Miura, K. Akisada, J. Wang, S. Zhuiykov, T. Ono, Mixed-potential-type NOx sensor based on YSZ and zinc oxide sensing electrode, Ionics 10 (2004) 1–9. [10] P. Elumalai, J. Wang, S. Zhuiykov, D. Terada, M. Hasei, N. Miura, Sensing Characteristics of YSZ-Based Mixed-Potential-Type Planar NOx Sensors Using NiO Sensing Electrodes Sintered at Different Temperatures, J. Electrochem. Soc. 152 (2005) H95–H101. [11] X. Liang, S. Yang, J. Li, H. Zhang, Q. Diao, W. Zhao, G. Lu, Mixed-potential-type zirconia-based NO2 sensor with high-performance three-phase boundary, Sens. Actuators B 158 (2011) 1–8.
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Influence of polarization time and polarization current of Pt|YSZ-based NO sensors utilizing the pulsed polarization when applying constant charge
T
Nils Donkera, Anastasiya Ruchetsb, Daniela Schönauer-Kamina, Jens Zoselb, Ulrich Guthc, ⁎ Ralf Moosa, a
Department of Functional Materials, University of Bayreuth, 95440 Bayreuth, Germany Kurt-Schwabe-Institut für Mess- und Sensortechnik e.V. Meinsberg, 04736 Waldheim, Germany c Faculty of Chemistry and Food Chemistry, Dresden University of Technology, 01062 Dresden, Germany b
ARTICLE INFO
ABSTRACT
Keywords: Pulsed polarization Pt | YSZ NOx detection Exhaust gas sensor Mixed potential sensors Electrode kinetics, Dynamic sensor mode
A Pt-YSZ sensor was operated in the pulsed polarization mode. In contrast to previous studies, it was polarized with a constant charge. The polarization currents were varied. After polarization, it was self-discharged. The associated voltages during polarization and self-discharge are clearly NO-dependent in both processes. Adding NO results in higher voltages during polarization and faster discharge. In addition, it could be observed that the final values of the discharge curves after certain time depend on the NO concentration and not on the polarization current. This observation may shorten the polarization and therefore the cycle times and thus lead to a faster sensor response.
1. Introduction Nitrogen oxides emitted by combustion processes are precursors for environmental issues like smog or acid rain. To reduce these emissions, it is necessary to measure them in situ in order to optimize combustion and exhaust gas aftertreatment processes [1]. In most chemical sensor principles to detect these nitrogen oxides, a potential difference, a current, a resistance or an impedance is the measured quantity [2–5]. Potential difference measurements are often static, meaning that the sensor is in stationary equilibrium with the environment, resulting in a constant sensor voltage at a constant analyte concentration in the gas atmosphere. YSZ-based mixed potential sensors are typical examples for that kind of sensors [6]. Several electrode materials have been investigated for NOx detection [7–19] as well as for NH3 [20–24] with this sensor type. The latter is of particular interest for the application in the exhaust when applying the selective catalytic reduction of NOx by ammonia. In contrast, pulsed polarization is a dynamic sensor measuring principle, analogously to the thermo-cyclic operation of mixed potential sensors [25], metal oxide sensors [26] or like cyclic voltammetry [27]. With the pulsed polarization method, the sensor is periodically polarized by applying an additional electrical signal. Thus, as with the methods mentioned above, there is no constant measuring signal. Instead, the behavior of the subsequent discharge is measured. To obtain gas sensors, two platinum electrodes with yttria-stabilized zirconia (YSZ) as solid electrolyte in-between are required. Therefore,
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potentiometric sensors like lambda probes [28–30] or planar sensors with platinum electrodes and YSZ [31,32] as solid electrolyte can be used. Interdigital electrodes are also possible, as shown recently [33,34]. In this work, a planar sensor structure was used. This is schematically shown in Fig. 1a. Fig. 1b displays the circuit used for pulsed polarization. The electric signal is usually realized by applying an additional external polarization voltage Upol for a polarization duration tpol. This voltage is applied to the sensor (DUT, device under test) by simultaneously closing the switches S1 and S4. Therefore, one electrode is positive and the other electrode is negatively polarized. The polarizing current can be measured indirectly by the shunt resistance Rshunt according to Ohm’s law. In the next step, the voltage source is disconnected from the sensor by opening all switches and the self-discharge behavior of the sensor is measured as an open circuit voltage (OCV) Udischarge for a defined duration tdischarge. This discharge is followed by another voltage pulse with reversed polarity. For that purpose, the switches S2 and S3 are closed, which also makes one electrode positive and the other electrode negative polarized, but with inverse polarity compared to the first step. A further discharge follows, for which again all switches are opened for the duration tdischarge. Such measurement cycles with alternating polarization voltages and discharge phases are repeated periodically. One measurement cycle for polarization with constant voltage is schematically shown in Fig. 2a. It could be shown in the previous works by Fischer et al. that the discharge of the sensor is strongly accelerated when NO or NO2 (NOx) are
Corresponding author. E-mail address: [email protected] (R. Moos).
https://doi.org/10.1016/j.snb.2019.03.060 Received 27 December 2018; Received in revised form 22 February 2019; Accepted 12 March 2019 Available online 14 March 2019 0925-4005/ © 2019 Elsevier B.V. All rights reserved.
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Fig. 1. a) Sensor setup: 8YSZ disk with screen-printed Pt electrodes and b) circuit which is used to apply the polarization pulses to the device under test (DUT).
Fig. 2. Sketch of a pulsed polarization cycle to illustrate the sensor principle for a) constant voltage polarization and b) constant current polarization as previous reported and applied in [28].
present. The discharge rate depends selectively on the NOx concentration [28–34]. In order to convert this accelerated discharge into a sensor signal, the sensor voltage Udischarge is evaluated at a certain time during discharge, for instance at t’ = 3 s after the end of each polarization process, in each respective cycle. These voltages serve as the sensor signal. The advantages of this method are its high NO sensitivity and selectivity [28] and its independence from a reference atmosphere [29]. Even full sensor calibration curves have been given in previous studies using constant polarization voltages. However, up to now, the physico-chemical effects on which the sensor is based have not yet been fully understood. It is disadvantageous that it takes at least one cycle to generate one sensor output value, i.e. when one output value is obtained, one has to wait for tcycle = 2(tpol + tdischarge) to obtain the next value. Only in the case of symmetrical sensors, the value for inverse polarity can also be taken due to the symmetrical sensor signal curve. This halves the time until the next valid value can be obtained. In order to improve this method, the total cycle time tcycle should be reduced to its (yet unknown) absolute minimum. Other important questions are minimum charge required for polarization as well as the reproducibility and long-term stability of the sensor voltage for each cycle. All these open points lead back to a model that allows to optimize this method and possibly also an application of the method for other sensor types. Therefore, a better insight into the sensing mechanism could further improve the good properties of sensors following the pulsed polarization technique. In particular, the role of the amount of charge during polarization is examined in this study. For this purpose, the pulsed polarization
measurements here were carried out with defined constant currents Ipol instead of a constant voltage as described in previous literature. For this, the voltage source displayed in Fig. 1b (Upol) was replaced by a current source. This makes it possible to polarize the sensor with a constant charge Qpol. Therefore, polarization current Ipol and polarization duration tpol were adapted to each other, according to Eq. (1):
Qpol = Ipol ·tpol
(1)
A schematic cycle using a constant polarization current Ipol and the corresponding voltage is shown in Fig. 2b. Here, the voltage at the sensor Usensor is measured during the entire cycle. Analogous to the polarization with constant voltage (Fig. 2a), the first part here shows the positive polarization with the charging current Ipol for the charging duration tpol by closing the switches S1 and S4 (Fig. 1b). One shaded area represents the total charge Qpol according to Eq. (1). After tpol, the current source was disconnected by opening all switches for the duration tdischarge. Negative polarization with -Ipol follows by closing switches S2 and S3 and the subsequent self-discharge by opening all switches. 2. Experimental For the subsequent measurements, a planar sensor with 8YSZ as solid electrolyte and platinum electrodes at both sides was used (Fig. 1a). The electrodes (Pt-paste, Ferro 64120410, Ferro Corporation, USA) were screen printed one after the other on an 8YSZ substrate 29
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Fig. 3. Cycles during polarizing, when previously polarized with Q = 400 μA s; a) 400 μA for 1 s b) 200 μA for 2 s, c) 100 μA for 4 s and d) 50 μA for 8 s. All measurements are performed at 400 °C in N2 with 10% O2 and 2% H2O. Additionally, steps of 50, 100 150 and 200 ppm NO were added. U5s_1_400 – U5s_8_50 represent the voltages 5 s after negative polarization. These voltages are displayed in Fig. 4 for the whole measurement. Each curve consists of 10 cycles, lying exactly on top of each other, which is an indication for the high reproducibility.
(Kerafol, Germany) and each fired for 30 min at 1300 °C peak temperature. Platinum wires were attached to the electrodes with ceramic glue for mechanical stability. A drop of platinum paste ensured the electrical contact. The sensors were then fired again after contacting both electrodes. To apply a constant current, a source meter (Keithley 2400, Keithley Instruments Inc., USA) was used as a current source and was periodically applied to the sensor via relays. For the polarity switch, relays were used as well. The circuit diagram is sketched in Fig. 1b. The sensor was polarized with 400, 200, 100, and 50 μA for 1, 2, 4, and 8 s, respectively, to obtain a total charge of 400 μA s according to Eq. (1). This total charge is approximately the charge that is transferred if the sensor is polarized with a constant voltage of 1 V for 1 s as determined in a previous measurement. This value was used to avoid decomposition of the YSZ due to too high (> 2 V) resulting polarization voltages [35]. As in previous works, 10 s were chosen as discharge time tdischarge [28–30]. Hence, each cycle (tcycle) lasted between 22 s and 36 s. This duration results from the double polarization time (2 * tpol) of 1–8 s and two times discharge time (2 * tdischarge) of 10 s. The highest voltage difference between base gas and 15 ppm NO (Usensor,base gas(t) – Usensor,NO15ppm(t)) was found at a sensor temperature of 400 °C [32]. Therefore, these measurements were conducted in a tube furnace at a sensor temperature of 400 °C. The temperature was measured with a thermocouple close to the YSZ-disc and accordingly controlled. The ceramic tube itself was purged with gases that were dosed by mass flow controllers (MFCs). In addition, water was added via an evaporator. A total flow of 1000 ml/min through the tube was adjusted. The base gas used here was 10 vol.-% O2 and 2 vol.-% H2O with N2 serving as a balance. In addition to the base gas, 50, 100, 150, and 200 vol.-ppm NO were added for 10 min each with 10 min base gas in between.
3. Results and discussion The results of the measurements with constant charge, but with different polarization currents, are shown in Fig. 3. Here, the curves for the negative polarization (-Ipol) and discharge for base gas without NO and with additional 50, 100, 150, and 200 ppm NO and with different polarization currents are displayed. To analyze the stability of the sensor voltages in detail, the voltage curves for 10 cycles for each NO concentration were displayed. It can be seen that the respective individual curves are exactly on top of each other, showing a high signal reproducibility. In addition, the individual courses for different NO concentrations can be clearly distinguished from each other for all polarization currents. This applies in particular to the difference between the profiles of base gas and 50 ppm NO, whereas difference between NO concentration steps is becoming less dominant. Comparing the sensor voltages for the individual analyte gas concentrations, on the one hand, a discharge accelerated by NO can be observed for all polarization currents and times. This effect is similar to polarization with constant voltage [28,29,31,32]. Therefore, one may assume that the sensor response using the here-suggested operation mode (constant charge) is as selective as it has already been published for constant voltage polarization [36]. However, additional selectivity tests are to be conducted soon. On the other hand, a NO effect is also visible in the voltage measured during polarization with Ipol. Here, the sensor shows a stronger voltage increase when polarizing with NO than without NO. This effect is also visible for all polarization currents. In order to be able to represent all cycles, sensor voltages were also evaluated at certain points after negative polarization. These values are denoted by Ut’_tpol_Ipol, in the following. The index t’ refers to the time from the end of the last polarization. The indices tpol and Ipol denote the 30
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Fig. 4. Voltages measured 5 s after negative polarizing the sensor with Q = 400 μA s; a) 400 μA for 1 s b) 200 μA for 2 s, c) 100 μA for 4 s and d) 50 μA for 8 s. All measurements are performed at 400 °C in N2 with 10% O2 and 2% H2O. Additionally, steps of 50, 100 150 and 200 ppm NO were added.
Fig. 5. a) Comparison of the polarization/discharge curves with different polarization currents and b) the corresponding discharge curves starting at the end of polarization.
polarization duration in seconds and the polarization current in microamperes, respectively. The voltage values were taken 5 s after each negative polarization (t’ = 5 s). Since the polarization times and currents of the individual measurements differ, the voltages U5s_1_400, U5s_2_200, U5s_4_100 and U5s_8_50 are evaluated for the polarization currents 400, 200, 100, and 50 μA, respectively. The points in time when these discharge voltages were obtained are depicted in Fig. 3 by dotted
vertical lines. Thus each data point represents the voltage value at a certain time in every cycle. If the discharge behavior changes, this voltage value will also change. These voltages are shown together with the measuring duration and the number of cycles in Fig. 4. On the basis of these curves, the strong NO dependence of the discharge processes and the high stability of the cycles can be seen. Even during the measurement with a polarization time of 8 s, the sensor has run through 31
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Pt|YSZ interface is polarized, capacitive effects with long time constants could be observed. These capacities were explained with charge storage via Faradaic processes (reactions (4) and (5)) [38] or by the existence of gas reservoirs between Pt and YSZ [39]. Both explanations can be attributed to chemical effects. They are considered to be much slower compared to electronic processes at the electrodes. The high voltages of 0.23 V even after 10 s discharge in the base gas are an indication for such discharge effects with very long time constants. In this case, the NO in the gas phase would reduce the potential difference at which only these slow discharge effects occur. An explanation might be the higher exchange current density at the three-phase boundary observed in the presence of NO [40]. This increased current density across the phase boundary at a given overpotential might lead to this increased discharge. This is contrasted by the faster voltage rise during polarization by NO, which rather indicates an obstacle to current flow. Therefore, a clear explanation has not been found yet. If the voltage at the end of discharge (at t’ = 9.9 s; see inset in Fig. 5b) is plotted for all polarization currents over the NO concentration, linear curves with similar slopes are obtained (Fig. 6). On the one hand, this shows the linear sensitivity of about 215 μV/ppm NO and the independence of the sensor output voltage values from the polarization current. On the other hand, it indicates a linear dependence of this residual charge on the NO concentration, assuming that the capacitance does not change. Finally, the voltages during polarization of the sensor are considered. As already mentioned, these voltages increase faster with increasing NO content. A higher polarization voltage at constant current represents a higher resistance, i.e., an effect that counteracts the current flow. Fig. 7 shows the voltages during polarization normalized to the total charge duration tcharge (t/tcharge). It is noticeable that the NO effect occurs at the same relative time for all polarization curves. Since the sensor was polarized with constant charge Q, the same relative time represents the same transported charge according to Eq. (2):
Fig. 6. Characteristic curve of the sensor voltage at the end of the discharge process as a function of the NO concentration.
Q t = Qges tges
(2)
This indicates that the NO effect to polarization only occurs after a finite transported charge. Such behavior might be explained with the following model. YSZ has a significantly higher ionic than electronic conductivity at measuring temperature [41]. This means that the applied current flows mainly in the form of oxide ions O2− through the solid electrolyte. These oxide ions are formed by a reduction of oxygen at the negative electrode by reaction (3):
Fig. 7. Polarizing the sensor with constant total charge but different polarization currents. The time plotted relatively to the duration of the total polarization.
more than 150 cycles. Nevertheless, the discharge behavior remains stable, both with and without NO addition to the base gas. The small difference in the discharge voltages between the individual NO steps and the large jump between base gas and 50 ppm NO leads to the conclusion that this method may be suitable to detect even lower NO concentrations. This will be tested in further measurements. Fig. 5a shows the course of the positive polarization and discharge voltages if the sensors were polarized with the same charge but various polarization currents. Here, it is visible that the discharge curves at the end of the discharge are almost equal. This is highlighted in Fig. 5b, where only the discharge curves are compared, now with a corrected time t’ that begins as soon as the discharge starts. It visualizes that all voltage curves converge to an apparent final value for the same analyte concentration in the gas. Therefore, the final value during discharge appears to depend only on the surrounding NO concentration and not on the voltage at the beginning of the discharge that differs for each individual polarization current. The voltage after being polarized with 400 μA starts at 1.5 V, which is significantly higher than the 0.7 V after polarization with 50 μA. Nevertheless, the discharge voltages after 10 s of discharge yield the same value in the respective gas atmospheres. This means, the same charge causes the same sensor voltage at the end of discharge, depending only on NO concentration. This behavior might be explained by the pseudo-capacitive behavior of the Pt/YSZ transition mentioned in the literature [37,38]. If the
O2 + 4e
2 O2
(3)
and will be oxidized to oxygen at the positive electrode by reaction (4):
2 O2
O 2 + 4e
(4)
In addition to oxygen incorporation and removal reaction, it is believed that platinum can be oxidized or reduced at the electrodes to form platinum oxides PtOx [37,38] by reaction (5):
Pt + xO 2
PtOx + 2xe
(5)
Especially for the reaction of platinum, it must be assumed that only previously formed platinum oxides will be reduced and platinum oxides will be formed in the range of monolayers. This would therefore be a process limited to a certain charge, which could represent the course at the beginning of polarization. In this case, the NO would inhibit the reduction (3) or oxidation (4) of the oxygen. However, the processes taking place here are not yet known and will be further investigated, preferably with the possibility to separate different processes from each other to get more insight in the contributing processes and the sensing mechanism. 32
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4. Conclusion
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The measurements show that it is possible to realize YSZ-based pulsed polarization sensors not only by polarizing with a constant voltage, but also with a constant charge. The NO influence is observed both during polarization of the sensor and during discharge. In both cases, the responses are stable. It has also been found out that the discharge voltages reached a final value in the same gas atmosphere. This effect is independent of the voltage at the beginning of the discharge. The value of these voltages at the end of the discharge is linearly dependent on the NO concentration. It also shows that the NO effect during polarization only occurs after a certain transported charge. This is attributed to a finite but yet unidentified process. Since this is only a first attempt with this mode of operation, further investigations to understand and improve the sensor principle will follow. A very first model approach for polarization with constant voltage was given by Fischer et al. [36]. Here, the increased discharge time was related to the local NO/NO2 equilibrium at the electrodes. It was assumed that a thermodynamic equilibrium is established at the electrodes and approximately 50% NO and 50% NO2 are present. This equilibrium is changed during polarization to an oxidizing and a reducing side. At the reducing side, NO2 can be reduced and thus additional oxygen is supplied, while NO may consume oxygen at the oxidizing side. According to this model, this enables the oxygen gradient to degrade faster after polarization and thus leads to faster depolarization. Further investigations will concern the oxygen dependence of the sensor response. Since YSZ is known as an oxide ion conductor and is already used as an oxygen sensor in the form of a lambda probe, corresponding influences are to be expected. Moreover, the effects of other interfering gases as well as a discriminability between NO and NO2 and the lower detection limit of this method shall be investigated. The questions of the minimum possible cycle duration and the minimum necessary polarization charge are also of interest to minimize the measurement duration. Acknowledgement We thank the German Research Foundation (DFG) for financial support under grants MO 1060/30-1 and ZO 139/3-1. References [1] N. Docquier, S. Candel, Combustion control and sensors: a review, Prog. Energy Combust. Sci. 28 (2002) 107–150. [2] J.W. Fergus, Materials for high temperature electrochemical NOx gas sensors, Sens. Actuators B 121 (2007) 652–663. [3] S. Zhuiykov, N. Miura, Development of zirconia-based potentiometric NOx sensors for automotive and energy industries in the early 21st century: What are the prospects for sensors? Sens. Actuators B 121 (2007) 639–651. [4] A. Afzal, N. Cioffi, L. Sabbatini, L. Torsi, NOx sensors based on semiconducting metal oxide nanostructures: Progress and perspectives, Sens. Actuators B 171-172 (2012) 25–42. [5] F. Ménil, V. Coillard, C. Lucat, Critical review of nitrogen monoxide sensors for exhaust gases of lean burn engines, Sens. Actuators B 67 (2000) 1–23. [6] N. Miura, T. Sato, S.A. Anggraini, H. Ikeda, S. Zhuiykov, A review of mixed-potential type zirconia-based gas sensors, Ionics 20 (2014) 901–925. [7] N. Miura, G. Lu, N. Yamazoe, High-temperature potentiometric/amperometric NOx sensors combining stabilized zirconia with mixed-metal oxide electrode, Sens. Actuators B 52 (1998) 169–178. [8] S. Zhuiykov, T. Ono, N. Yamazoe, N. Miura, High-temperature NOx sensors using zirconia solid electrolyte and zinc-family oxide sensing electrode, Solid State Ionics 152-153 (2002) 801–807. [9] N. Miura, K. Akisada, J. Wang, S. Zhuiykov, T. Ono, Mixed-potential-type NOx sensor based on YSZ and zinc oxide sensing electrode, Ionics 10 (2004) 1–9. [10] P. Elumalai, J. Wang, S. Zhuiykov, D. Terada, M. Hasei, N. Miura, Sensing Characteristics of YSZ-Based Mixed-Potential-Type Planar NOx Sensors Using NiO Sensing Electrodes Sintered at Different Temperatures, J. Electrochem. Soc. 152 (2005) H95–H101. [11] X. Liang, S. Yang, J. Li, H. Zhang, Q. Diao, W. Zhao, G. Lu, Mixed-potential-type zirconia-based NO2 sensor with high-performance three-phase boundary, Sens. Actuators B 158 (2011) 1–8.
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