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High NH3 selectivity of NiFe2O4 sensing electrode for potentiometric sensor at elevated temperature
Analytica Chimica Acta 1089 (2019) 165e173
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Analytica Chimica Acta journal homepage: www.elsevier.com/locate/aca
High NH3 selectivity of NiFe2O4 sensing electrode for potentiometric sensor at elevated temperature Bin Yang, Chao Wang**, Ran Xiao, Hanyu Yu, Chuqi Huang, Jingxin Wang, Jinlong Xu, Hongming Liu, Feng Xia, Jianzhong Xiao* State Key Laboratory of Materials Processing and Die & Mold Technology, School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan, Hubei, 430074, PR China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
NiFe2O4 was firstly used as sensing electrode of potentiometric NH3 sensor based on the proposed novel concept. NH3 sensor based on NiFe2O4-SE exhibits good selectivity and long term stability. NiFe2O4 material has a good application prospect for NH3 detection in diesel vehicle exhaust.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 March 2019 Received in revised form 19 August 2019 Accepted 2 September 2019 Available online 3 September 2019
NiFe2O4 was synthesized using sol-gel method for sensing electrode material of YSZ based ammonia sensor. NiFe2O4-SEs sintered at 1100 C, 1150 C and 1200 C were characterized by XRD, the BET method and ESEM. By testing the NH3 response of different sensors at 650 C, it was observed that the 1150 C sintered sensor had the largest response value (104.3 mV for 320 ppm NH3) and the highest sensitivity (77.56 mV/decade), which were related to the most TPB sites and the moderate gas phase catalytic reaction. The response values of the sensor varied almost linearly with the logarithm of 20e320 ppm NH3 at 600e750 C, which was consistent with mixed-potential mechanism testified by polarization and EIS tests. When the oxygen concentration was at 7e10 vol %, its effect on the response value was within 3%. When the water vapor concentration was 3, 6 and 9 vol %, the ammonia response value was 95.1%, 92.9% and 88.7% of the values when there was no water vapor, respectively. The sensor showed very weak cross sensitivities to NOx, but non-negligible SO2 cross sensitivity. It also displayed slight signal drifts in weekly tests in eight weeks, which showed that the sensor attached with NiFe2O4-SE has a good long-term stability. © 2019 Elsevier B.V. All rights reserved.
Keywords: NH3 sensor Mixed-potential NiFe2O4-SE Elevated temperature Selectivity
1. Introduction
** Corresponding author. * Corresponding author. E-mail addresses: [email protected] (C. Wang), [email protected]. edu.cn (J. Xiao). https://doi.org/10.1016/j.aca.2019.09.006 0003-2670/© 2019 Elsevier B.V. All rights reserved.
The air we daily breathe is continuously polluted by vehicle exhaust due to rapid urban modernization. NOx (NO and NO2) exhausted mainly from diesel vehicle, are considered as harmful gases due to the serious impact on human health and environment
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[1]. Therefore, the regulations have become stricter and great efforts have been taken to reduce the emissions. Because the conventional three-way catalyst (TWC) can only remove NOx efficiently when the air-fuel ratio is controlled around 1, it is not applicable for diesel engine which works under lean-burn condition [2]. Urea-SCR is one of the most promising technologies for the removal of NOx emissions for diesel engine. In SCR system, the reductive NH3 hydrolyzed from urea reacts with NOx to produce harmless N2 and H2O under the effect of SCR catalyst [3]. However, there is a risk that ammonia escaping into the atmosphere would pollute the environment when excessive urea is injected [4]. So, in order to control the urea injection effectively, a high-performance NH3 sensor which could accurately monitor ammonia concentration after SCR catalyst is required. Over the last decades, many kinds of NH3 sensors based on different working principles have been reported [5,6]. The potentiometric type sensor is regarded as the most promising sensing device for monitoring NH3 concentration due to simple design, stable performance, wide operating temperature range and diversity selection of sensing materials. Many types of sensing materials have been investigated, such as Au [7,8], In2O3 [9,10], Mg2CuxFeO3.5þx [11], Cr2O3 [12], WO3 [13], TiO2-WO3 [14], SnO2 [15], V2O5-WO3-TiO2 [16] and CoFe2O4 [17], they all show high NH3 sensitivity, but noticeable cross sensitivity to NOx. As the main component of diesel vehicle exhaust, NOx could influence the response signal of NH3 sensor. However, few studies have recently been conducted to improve the NH3 selectivity by reducing NOx interference. So, it is eagerly desired to develop sensing material which possess high NH3 selectivity. Usually the response value of NO is significantly smaller than the one of NO2 at the same concentration [18e21]. And when the temperature is higher than 600 C, NO2 in the exhaust could reach chemical equilibrium with more than 90 vol % NO [22]. So, the response would be much smaller above 600 C even if a sensing material has a high sensitivity to NO2 below 600 C. In general, the NOx response will gradually decrease with the increase of operating temperature [23e26], so the cross-sensitivity of NOx can be minimized at a certain temperature above 600 C. Therefore, we can search a material with high NH3 sensitivity above 600 C, even if it has response to NOx, the optimum NH3 selectivity can be obtained at a certain temperature above 600 C. In general, the sensor performances stability depend strongly on the microstructure stability of sensing electrode [27], and the stable microstructure could be obtained after high temperature sintering. Ferrite electrode material could be sintered at high temperature due to their high melting point, so a ferrite sensing electrode that achieves a stable microstructure by high-temperature sintering could provide excellent long-term stability to the sensor. As a common ferrite material, NiFe2O4 caught our attention due to the high sensitivity to liquid petroleum gas [28], hydrocarbon gas [29], H2S [30], acetone [31], Cl2 [32] and NO2 [33]. Especially, the NO2 sensitivity of NiFe2O4 was found drastically decreased with the increase of operating temperature in the range of 400e500 C [33]. So, we tried to test its NH3 and NOx sensitivity above 600 C and found that the sensor attached with NiFe2O4-SE had high NH3 sensitivity and weak NOx cross sensitivity at elevated temperatures of 600e750 C. In this paper, NiFe2O4 was prepared by sol-gel method and sintered between 1100 C and 1200 C as the sensing electrode for YSZ-based potentiometric NH3 sensor. The NH3 sensing characteristics were investigated at the operating temperatures of 600e750 C, and the sample sintered at 1150 C exhibited the largest sensing response. The sensing mechanism was testified by the polarization and EIS results. The long-term stability, the effects of oxygen concentration, water vapor and other main interfering
gases on the NH3 response were studied. 2. Experiment 2.1. Synthesis and characterization of NiFe2O4 NiFe2O4 powder was prepared by the sol-gel method using nickel nitrate (Ni(NO3)2$6H2O), ferric nitrate (Fe(NO3)3$9H2O), citric acid monohydrate (C6H7O8$H2O) and ethylene glycol (C2H6O2) purchased from Sinopharm Chemical Reagent Co., Ltd, China. Stoichiometric Ni(NO3)2$6H2O and Fe(NO3)3$9H2O with the molar ratio of 1:2 were dissolved in deionized water. C2H6O2 and C6H7O8$H2O were added into the obtained solution, magnetic stirring and 80 C water bath heating were continued until a gel appeared, and then the gel was annealed at 300 C for 3 h with heating and cooling rate of 2 C/min. The annealed product was then calcined at 900 C for 6 h with heating and cooling rate of 1 C/ min. The calcined substance was then ground in an agate mortar to obtain fine powder. Part of NiFe2O4 powder was sintered at 1100 C, 1150 C and 1200 C for XRD test using the same procedure as sintering sensing electrode. The synthesized powder was stored in sealed dry container. The crystalline phase of as-synthesized NiFe2O4 at 900 C, 1100 C, 1150 C and 1200 C were characterized by X-ray diffraction (XRD) (X'Pert PRO, PANalytical B.V.). The instrument works at Cu Ka (l ¼ 1.5406 Å) radiation of 40 kV and 40 mA at room temperature. The step-scanning mode with a step length of 0.017 and scan speed of 8 /min was used to collect data. The specific surface and pore size distribution of NiFe2O4-SEs were obtained by V-Sorb 2800 (Gold App Instrument Corporation China) using BET method. 2.2. Sensor fabrication The sensor contains a 5 mol % Y2O3-doped ZrO2 (5YSZ) electrolyte, sensing and reference electrodes (SE and RE) located on both surfaces of the electrolyte. The planar electrolyte with the dimension of 10 mm 10 mm 0.3 mm was prepared by tape casting process and sintered at 1480 C for 2 h in air. The reference electrode was obtained by screen printing Pt slurry (79 wt % Pt powder, 1 wt % 8YSZ and 20 wt % organic binder) on one surface of YSZ electrolyte and then dried at 180 C for 2 h NiFe2O4 powder was mixed with 30 wt % binder (5 wt % ethocel, 94 wt % terpineol and 1 wt % span 80) as screen-printing slurry for the preparation of sensing electrode. The mixed slurry was screen printed on the other surface of YSZ, dried at 180 C for 2 h. Two Pt wires of 0.2 mm in diameter and 10 mm in length were respectively connected to both electrodes using Pt slurry. A part of both Pt wires was respectively bonded to the blank area of YSZ using high temperature ceramic adhesive. Then the sensor were sintered at 1100 C, 1150 C and 1200 C respectively for 3 h with heating and cooling rate of 2 C/ min. Pt slurry and wire were purchased from Sino-Platinum Metals Co., Ltd, China. The schematic of the potentiometric NH3 sensor is shown in Fig. 1. The surface and the cross section morphology of SE were observed by Environmental Scanning Electron Microscope (Quanta 200, FEI, Holland) at 10 kV. 2.3. Evaluation of sensing performance Sensor performance was tested in a gas flow apparatus under various gas environments controlled by mass flow controllers (MPA-80, Beijing Seven Star Electronics Company). The gas flow apparatus was connected to a quartz tube which was equipped with a furnace operated at temperatures of 600e750 C. The base gas is 10 vol % O2 with 90 vol % N2 and the sample gas is a changing
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Fig. 1. (a) Schematic of the fabricated sensor; (b) top view of the sensor; (c) bottom view of the sensor.
concentration of NH3 varying from 20 to 320 ppm with base gas. The total flow rate of the gas atmosphere is 0.5 L/min. Both electrodes of the sensor were exposed to the same gas atmosphere. The open circuit voltage difference DV between SE and RE was monitored and recorded by an electrochemical work station (VersaSTAT 3, Princeton, USA). For all measurements, the SE were connected to the positive terminal of the electrochemical work station, and the RE was connected to the negative one. The electrochemical impedance spectroscopy (EIS) was measured at 650 C by electrochemical work station in the frequency range from 0.1 Hz to 1 MHz with 10 mV exciting voltage. The polarization curves in the potential range of 150 mVe0 mV were measured by the linear voltage scanning method at a constant scan rate of 5 mV/s in the base gas and the sample gases containing 80, 160, and 320 ppm NH3. For the test of the oxygen effect, the oxygen concentration is changed step by step when the response of the sensor to 20 ppm
Fig. 3. Surface micrographs of the NiFe2O4-SE sintered at (a) 1100 C; (b) 1150 C; (c) 1200 C.
Fig. 2. The XRD patterns of the prepared NiFe2O4 powder sintered at 900 C, 1100 C, 1150 C and 1200 C.
and 320 ppm NH3 is stabilized, and the transient curve with the changed oxygen concentration is recorded by the electrochemical workstation. For the effect of water vapor, different concentrations
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of vapor is obtained by heating water at different temperatures according to the water vapor saturation pressure at a certain temperature. The sample gas was humidified with 3, 6 and 9 vol % vapor by passing through 25, 37 and 44 C water.
3. Results and discussion 3.1. Powder characterization The crystal structure of the prepared NiFe2O4 powders after sintering of 900 C, 1100 C, 1150 C and 1200 C were characterized by XRD as shown in Fig. 2. It can be found that the samples are all well crystallized. All of the diffraction peaks can be perfectly assigned to cubic NiFe2O4, which are consistent with the standard card (ICCD 00-044-1485). No others diffraction peaks are observed, confirming that the as-prepared powder is high-purity NiFe2O4, and the electrode materials sintered at 1100 C, 1150 C and 1200 C are still pure phase NiFe2O4. 3.2. Morphology of NiFe2O4-SE Fig. 3 shows surface morphology of NiFe2O4-SE sintered at 1100 C, 1150 C and 1200 C. It is observed that with the increase of sintering temperature, the grains grow up obviously with the average size of about 0.5 mm, 1.0 mm and 1.5 mm, respectively. There are many small pores in the sample sintered at 1100 C, but bigger pores at 1150 C and 1200 C. There is an obvious sintering phenomenon in 1200 C-sintered sample with some grains fusing together. Cross-sections micrograph of the samples is shown in Fig. 4. There is a tightly integrated interface between the electrode and YSZ for each sample. The NiFe2O4-SEs have a similar thickness of about 30 mm for average. 3.3. Sensing characteristics of NiFe2O4 3.3.1. NH3 sensing performance for different temperatures sintered NiFe2O4-SEs Fig. 5 shows the voltage responses of the sensors attached with 1100 C, 1150 C and 1200 C sintered NiFe2O4-SEs to different NH3
Fig. 5. Response transients curve for the sensors attached with NiFe2O4-SE sintered at 1100 C, 1150 C and 1200 C toward 20e320 ppm NH3 at 650 C.
Table 1 The specific surface area (SBET) of the electrodes sintered at different temperatures.
Fig. 4. Cross-section micrographs of the NiFe2O4-SE sintered at (a) 1100 C; (b) 1150 C; (c) 1200 C.
Electrode sintering temperature/ C
Specific surface area (SBET)/m2.g1
1100 1150 1200
1.57 1.14 0.53
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sample. On the other hand, the intensity of the electrode reaction could be directly affected by the quantity of TPB sites which are the regions where the target gas, the SE and the solid state electrolyte coexist. It can be seen from the SEM results that the 1200 C sintered sample has more obvious sintering melting phenomenon than 1150 C sintered sample, and more gas channel could be blocked by the grains melted together, the particle melting brings about less TPB sites and more difficult gas diffusion, as shown in Fig. 7, so the response signal of 1150 C sintered sample is higher than that of 1200 C sintered sample. The grain of 1100 C sintered sample is fine, so less TPB sites were formed between YSZ and NiFe2O4-SE, resulted in the lowest response signal of this sample. Since the 1150 C sintered sample has the most TPB sites and the moderate gas phase catalysis reaction, it has the largest response signal.
Fig. 6. The pore size distribution of the NiFe2O4-SEs sintered at different temperatures using the BET method.
concentrations at 650 C. For each NH3 concentration, the highest response value and response/recovery rate were always obtained from the sensor attached with 1150 C sintered NiFe2O4-SE, the lowest ones were obtained from the 1100 C sintered sample. NH3 mainly experiences the gas phase catalytic reaction during diffusing in NiFe2O4-SE and the electrochemical reaction at three phase boundary (TPB) when the sensor is exposed to NH3 and O2. The intensity of these two reactions directly affects the response signal of the sensor. The intensity of gas phase catalysis is closely related to the microstructure of NiFe2O4-SE. The specific surface area and pore size distribution of the electrodes sintered at different temperatures were obtained using the BET method. The results are shown in Table 1 and Fig. 6, respectively. It can be seen that the specific surface area and the amount of pores gradually decrease with the increase of the sintering temperature, so the contact opportunity between NH3 and NiFe2O4-SE decreases gradually and less NH3 is consumed by gas phase catalysis during diffusion. This lead to more NH3 molecules could reach the TPB to participate in the electrode reaction, so the sample sintered at 1150 C has higher response signals than the 1100 C sintered
3.3.2. NH3 sensing performance at different operating temperatures Based on its maximum NH3 response value, 1150 C sintered NiFe2O4-SE was studied in detail for NH3 sensing performances. Fig. 8 shows the voltage responses of the sensor sintered at 1150 C to different NH3 concentrations measured in the range of 600e750 C. For each NH3 concentration, the highest value was
Fig. 8. Response transient curves for the sensor attached with 1150 C sintered NiFe2O4-SE toward NH3 at different operating temperatures.
Fig. 7. Schematic explanation of TPB sites in different temperatures sintered NiFe2O4SE. (a) 1100 C; (b) 1150 C; (c) 1200 C.
Fig. 9. Voltage response versus logarithm of NH3 concentration for the sensor attached with 1150 C sintered NiFe2O4-SE at different operating temperatures.
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decade at 650 C. Therefore, the following NH3 response studies are carried out at the optimum operating temperature of 650 C.
3.3.3. Sensing mechanism When the sensor is exposed to target gas containing oxygen and ammonia, a mixed potential is formed when the oxidation reactions rate of NH3 (Eq. (1)) is equal to the reduction reaction rate of O2 (Eq. (2)) [34]. The potential difference DV between the NiFe2O4SE and Pt-RE generally gives a logarithmic dependence on the NH3 concentration. As illustrated in Fig. 9, there is a linear logarithmic relationship between the mixed potential and NH3 concentration, expressed as Eq. (3).
Fig. 10. Modified polarization curves of the sensor attached with 1150 C sintered NiFe2O4-SE recorded in base gas and samples gases containing 80, 160 and 320 ppm NH3 at 650 C.
Table 2 Comparison between the estimated and measured DV for the sensor with 1150 C sintered NiFe2O4-SE. NH3 concentration/ppm
320 ppm 160 ppm 80 ppm
DV/mV Estimated
Observed
99.7 74.5 48.8
104.3 77.3 51.6
always obtained at 650 C, and the lowest one was obtained at 750 C. It could also be seen that NH3 sensitivity decreased significantly and recovery speed became faster as operating temperature increases from 650 C. Fig. 9 displays the dependence of response values on the logarithm scale of NH3 concentration at different operating temperatures. It can be seen that in each temperature, the response value increases almost linearly with increasing NH3 concentration on a logarithmic scale. Such a linear variation is typical for the mixedpotential mechanism of gas sensor. It also can be seen that the sensor exhibits the largest sensitivity with the slope of 77.56 mV/
4NH3 þ 6O2- ¼ 2N2 þ 6H2O þ 12e
(1)
3O2 þ12e ¼ 6O2-
(2)
DV ¼ a lnPNH3þb
(3)
In order to further testify the mixed-potential model, the modified polarization curves of the sensor were measured at 650 C in base gas and sample gases (80, 160 and 320 ppm NH3), respectively, as shown in Fig. 10. The cathodic polarization curve was measured in base gas, while the anodic polarization curves were obtained by subtracting base gas value from sample gas value. The value of mixed-potential can be estimated by the intersection of the anodic and cathodic polarization curves. Table 2 lists the measured DV values and the estimated mixed-potentials for the sensor. It could be clearly seen that the estimated values are close to the measured values, indicating that the sensor follows the mixedpotential mechanism. EIS is an effective method to study electrochemical reaction kinetics and has been widely used in the field of gas sensor research [7,35,36]. The EIS of the sensor was measured between 0.1 Hz and 1 MHz at 650 C in the base gas and sample gases (20e320 ppm NH3), the Nyquist plots are shown in Fig. 11a. It could be seen that there are an inconspicuous smaller arc in high frequency and a bigger arc in the low frequency. The size of the small arc at high frequency are similar for different NH3 concentrations, whether in the base gas or the sample gas. But there is a great shrinkage toward real Z0 -axis for the large arc with increasing of NH3 concentration. A probable equivalent circuit model could be proposed for potentiometric gas sensor based on some reports, as shown in Fig. 11b. Here, Rb represents the YSZ bulk resistance, Ri represents
Fig. 11. (a) Electrochemical impedance spectroscopy of the sensor attached with 1150 C sintered NiFe2O4-SE toward different concentrations of NH3 at 650 C; (b) Nyquist plot of an ideal equivalent circuit model for YSZ-based potentiometric sensor.
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Table 3 The change rates of 20 ppm and 320 ppm NH3 response signal values with different oxygen concentrations. Oxygen concentration/vol.%
1 4 7 14
Fig. 12. The plot of the relationship between Re and the logarithm scale of NH3 concentration.
Change rate/% 20 ppm
320 ppm
49.39% 14.85% 3.21% 10.86%
21.65% 3.84% 1.72% 4.79%
concentration between 1 vol % and 14 vol %. After the NH3 response signal is stabilized at 650 C, the oxygen concentration is changed as shown in Fig. 13. The change rates for the NH3 response values at different oxygen concentrations with respect to the value at 10 vol % oxygen are summarized in Table 3. It can be seen that the effect of oxygen concentration on the signal of 320 ppm NH3 is much less than that of 20 ppm. Except for the oxygen concentration of 7 vol % and 10 vol %, oxygen concentration has a great effect on the signal, especially at 1 vol %. When oxygen concentration is between 7 and 10 vol %, the effect of oxygen on NH3 signal becomes weaker. The NH3 signal change rates of 20 ppm NH3 are 3.21% and 1.72% for 320 ppm NH3. In general, the concentration of oxygen in automobile exhaust is between 5 and 10 vol % [39], so the effect of oxygen
Fig. 13. Response of the sensor attached with 1150 C sintered NiFe2O4-SE at different oxygen concentrations for 20 ppm and 320 ppm NH3 at 650 C.
interfical resistance of the TPB, and Re represents electrochemical reaction process [13,35,37,38]. The resistance values of Rb, Ri and Re can be calculated using this mode with the Zview software. For a same sensor tested in different NH3 concentrations, the values of Rb and Ri are almost unchanged, but Re are distinguishing because of different electrochemical reaction kinetics. So, a plot of the relationship between Re value and the logarithm scale of NH3 concentration is shown in Fig. 12. It shows a good linear relationship that exists between the Re and the logarithm of NH3 concentration, which is similar to the correlation between the DV and concentration that follows the mixed-potential mechanism in Fig. 9. Therefore, the EIS results also show that the sensor conforms to the mixed-potential mechanism. 3.3.4. Interfering gases effect and long-term stability As the main component of diesel vehicle exhaust gas, oxygen concentration changes with different working conditions, so it is necessary to study the effect of oxygen concentration on NH3 response. Similarly, the concentration of NH3 in tail gas also fluctuates all the time, so low concentration of 20 ppm and high concentration of 320 ppm NH3 were investigated. In this study, the response value of the sensor was measured as a function of oxygen
Fig. 14. (a) A typical response transient plot of a water vapor resistance test for the sensor attached with 1150 C sintered NiFe2O4-SE at 650 C; (b) the average response value of the five-time water vapor resistance test results for different water vapor concentrations.
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results for different water vapor concentrations, as can be seen that when water concentration was 3, 6 and 9 vol %, the sensor retained 95.1%, 92.9% and 88.7% of the initial response value tested without vapor. Therefore, it does not have a significant impact on the NH3 response over the typical water vapor content of 5e6 vol % in actual automobile exhaust [7]. Besides oxygen and water vapor, the tail gas of the diesel engine mainly contains NO, NO2 and SO2, their cross sensitivities should also be considered. Fig. 15a shows the sensitivities of the sensor toward 320 ppm of each gas mentioned above, and Fig. 15b shows the response signals of 320 ppm NH3 mixed with the same concentration of SO2, NO and NO2, respectively. It demonstrates that the sensor exhibited good selectivity to NH3. Table 4 summarizes the performance comparison of potentiometric NH3 sensors using different sensing electrodes in recent years, and it further demonstrates the good NH3 sensitivity and selectivity of NiFe2O4 material. However, SO2 still shows significant cross sensitivity (1/6 of the NH3 response value) that could not be ignored. Since the implementation of the Euro 6 standard, the sulfur content of vehicle fuel in EU does not exceed 10 ppm, so the SO2 cross sensitivity of this sensor would not be considered. Actually, the long-term stability is another important factor for gas sensor. Diesel vehicle usually works intermittently, so sensors that monitor vehicle exhaust should undergo intermittent startup and durability tests. In order to simulate the actual working conditions, the long-term stability of the sensor was evaluated by a weekly test over a period of 8 weeks. At each test, the temperature of the device was raised from room temperature to 650 C and controlled at this temperature for test. The response signal of the sensor was recorded in the base gas first, and then in NH3 atmosphere by introducing 20, 40, 80, 160 and 320 ppm NH3. The device was then naturally cooled to room temperature for the next week's
Fig. 15. (a) Sensitivities of the sensor attached with 1150 C sintered NiFe2O4-SE to various gases of 320 ppm at 650 C; (b) the response signals of 320 ppm NH3 mixed with 320 ppm SO2, NO and NO2, respectively.
concentration change could be not obvious, would be within 3.21%. Water vapor is also the main component of exhaust, so it is very important to investigate the effect of vapor on the sensing performance of the NH3 sensor. The amount of water vapor in real exhausts usually varies from 2 to 11 vol % [39], so 3, 6 and 9 vol % vapor concentration were studied in this research. Fig. 14a is a typical response transient plot of a water vapor resistance test at 650 C. In the test, the NH3 concentration was fixed at 320 ppm and the transient NH3 response was recorded when different concentrations of water vapor passed. It can be seen that the addition of water vapor causes a slight decrease in sensor response possibly due to the improvement of the oxygen reduction kinetics or the deterioration of the NH3 oxidation kinetics [7]. Fig. 14b shows the average response value of the five-time water vapor resistance test
Fig. 16. Long-term stability test of sensor attached with 1150 C sintered NiFe2O4-SE.
Table 4 The comparison between this work and recent related works. Sensor structure
Operating Temp. ( C)
Sensitivity (mV/decade)
DVNO/DVNH3
DVNO2/DVNH3
Ref.
PtjYSZjPtjMg2Cu0.25FeO3.75 PtjYSZjPtjCoFe2O4 PtjYSZjAu CuOjYSZjIn2O3 AujYSZjAujVWT PtjYSZjAu þ SnO2 PtjYSZjNiFe2O4 In2O3jYSZjPtjLaCoO3
350 450 525 550 550 600 650 700
220.0 55.0 47.0 35.9 80.0 39.9 77.6 41.5
4/124 ¼ 3.2% 12/55 ¼ 21.8% 8/110 ¼ 7.0% / 5/80 ¼ 6.3% 2/45 ¼ 4.4% 8/104 ¼ 8.0% /
24/124 ¼ 19.4% 11/55 ¼ 20.0% 24/110 ¼ 22.0% 18.3/41.5 ¼ 44.0% 20/80 ¼ 25.0% 10/45 ¼ 22.2% 17/104 ¼ 16.4% 10.0%
[11] [17] [7] [9] [16] [15] This work [10]
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evaluation. The results are shown in Fig. 16. At the same time, the sensitivity for 80 ppm NH3 was investigated at 650 C during 40 h' continuous operation, the response value changed slightly with a fluctuation within 2%. It demonstrated that the sensor showed good stability, which may be related to the stable microstructure of NiFe2O4-SE after high temperature sintering. In combination with the above mentioned good NH3 selectivity and long-term stability, low water vapor and oxygen interference effect, it is believed that the NiFe2O4 reported in this study has a good application prospect in sensing electrode for potentiometric NH3 sensor. 4. Conclusions A new sensing material NiFe2O4 was prepared using sol-gel method for YSZ-based ammonia sensor. The NiFe2O4-SEs of the potentiometric type sensors were sintered at different temperatures, and their ammonia response characteristics were tested at 650 C. The 1150 C sintered sensor had the largest response value of 104.3 mV for 320 ppm NH3 and the largest sensitivity of 77.56 mV/decade. The sensing mechanism of the sensor was proved to follow the mixed-potential mechanism by polarization curve and EIS tests. The sensor had good resistance to oxygen, water vapor and NOx, and also showed good long-term stability. These good sensing performances indicate that the NiFe2O4-SE has good potential in diesel vehicle exhaust emission control system.
[12]
[13] [14]
[15]
[16] [17]
[18] [19]
[20]
[21]
[22] [23]
[24]
Declaration of competing interest [25]
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
[26]
Acknowledgments
[27]
The authors are deeply grateful to the Analytical and Testing Center of Huazhong University of Science and Technology for their hard work in XRD and SEM testing.
[28]
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for YSZ-based mixed-potential type NH3 sensors, Ceram. Int. 42 (2016) 9363e9370. Xishuang Liang, et al., New type of ammonia/toluene sensor combining NASICON with a couple of oxide electrodes, Sens. Actuators B Chem. 150 (2010) 355e359. Jingjing Zhang, et al., Mixed-potential NH3 sensor based on Ce0.8Gd0.2O1.9 solid electrolyte, Sens. Actuators B Chem. 249 (2017) 76e82. Weiwei Meng, et al., A novel mixed potential NH3 sensor based on TiO2@ WO3 coreeshell composite sensing electrode, Electrochim. Acta 193 (2016) 302e310. Tomoaki Sato, Hiroshi Ikeda, Norio Miura, Mixed-potential type zirconiabased NH3 sensor using SnO2-disk sensing-electrode attached with sputtered Au, ECS Electrochem. Lett. 3 (2014) B13eB15. €nauer, et al., Selective mixed potential ammonia exhaust gas Daniela Scho sensor, Sens. Actuators B Chem. 140 (2009) 585e590. Yu Yuan, et al., Effects of CoFe2O4 electrode microstructure on the sensing properties for mixed potential NH3 sensor, Sens. Actuators B Chem. 239 (2017) 462e466. Quan Diao, et al., Mixed-potential-type NO2 sensor using stabilized zirconia and Cr2O3-WO3 nanocomposites, Sens. Actuators B Chem. 180 (2013) 90e95. Geyu Lu, Norio Miura, Noboru Yamazoe, High-temperature sensors for NO and NO2 based on stabilized zirconia and spinel-type oxide electrodes, J. Mater. Chem. A. 7 (1997) 1445e1449. Norio Miura, Geyu Lu, Noboru Yamazoe, High-temperature potentiometric/ amperometric NOx sensors combining stabilized zirconia with mixed-metal oxide electrode, Sens. Actuators B Chem. 52 (1998) 169e178. Norio Miura, et al., Mixed potential type sensor using stabilized zirconia and ZnFe2O4 sensing electrode for NOx detection at high temperature, Sens. Actuators B Chem. 83 (2002) 222e229. Serge Zhuiykov, in: Electrochemistry of Zirconia Gas Sensors, first ed., CRC Press, Boca Raton, 2007. Ying Chen, Jian-Zhong Xiao, Synthesis of composite La1.67Sr0.33NiO4-YSZ for a potentiometric NOx sensor by microwave-assisted complex-gel auto-combustion, Ceram. Int. 39 (2013) 9599e9603. Ho Truong Giang, et al., High sensitivity and selectivity of mixed potential sensor based on Pt/YSZ/SmFeO3 to NO2 gas, Sens. Actuators B Chem. 183 (2013) 550e555. Lihong Zhou, et al., Sensing properties of YSZ-based NOx sensors with doubleperovskite (La0.8Sr0.2)2FeNiO6-d sensing electrodes, Ceram. Int. 40 (2014) 9257e9263. Lihong Zhou, et al., The effects of sintering temperature of (La0.8Sr0.2)2FeMnO6-d on the NO2 sensing property for YSZ-based potentiometric sensor, Sens. Actuators B Chem. 206 (2015) 311e318. Ivan Romanytsia, et al., Application of advanced morphology Au-X (X¼YSZ, ZrO2) composites as sensing electrode for solid state mixed-potential exhaust NOx sensor, Sens. Actuators B Chem. 207 (2015) 391e397. L. Satyanarayana, K. Madhusudan Reddy, Sunkara V. Manorama, Nanosized spinel NiFe2O4: a novel material for the detection of liquefied petroleum gas in air, Mater. Chem. Phys. 82 (2003) 21e26. Chu Xiangfeng, Dongli Jiang, Chenmou Zheng, The preparation and gassensing properties of NiFe2O4 nanocubes and nanorods, Sens. Actuators B Chem. 123 (2007) 793e797. Yan-Li Liu, et al., Hydrogen sulfide sensing properties of NiFe2O4 nanopowder doped with noble metals, Sens. Actuators B Chem. 102 (2004) 148e154. N. Rezlescu, et al., Semiconducting gas sensor for acetone based on the fine grained nickel ferrite, Sens. Actuators B Chem. 114 (2006) 427e432. R.B. Kamble, V.L. Mathe, Nanocrystalline nickel ferrite thick film as an efficient gas sensor at room temperature, Sens. Actuators B Chem. 131 (2008) 205e209. Jinlong Xu, et al., Superior sensitive NiFe2O4 electrode for mixed-potential NO2 sensor, Ceram. Int. 45 (2019) 2962e2967. Jeffrey W. Fergus, Sensing mechanism of non-equilibrium solid-electrolytebased chemical sensors, J. Solid State Electrochem. 15 (2011) 971e984. Chao Wang, et al., Effect of MgO doping on the BiVO4 sensing electrode performance for YSZ-based potentiometric ammonia sensor, Solid State Electron. 147 (2018) 19e25. Eric R. Macam, Bryan M. Blackburn, Eric D. Wachsman, Effect of La2CuO4 electrode area on potentiometric NOx sensor response and its implications on sensing mechanism, Sens. Actuators B Chem. 158 (2011) 304e312. Norio Miura, Mitsunobu Nakatou, Serge Zhuiykov, Impedancemetric gas sensor based on zirconia solid electrolyte and oxide sensing electrode for detecting total NOx at high temperature, Sensor, Actuat. B-Chem. 93 (2003) 221e228. Jonathan Michael Rheaume, Solid State Electrochemical Sensors for Nitrogen Oxide (NOx) Detection in Lean Exhaust Gases, Doctoral dissertation, UC Berkeley, 2010. Perumal Elumalai, et al., Stabilized zirconia-based sensor attached with NiO/ Au sensing electrode aiming for highly selective detection of ammonia in automobile exhausts, Electrochem. Solid St. 11 (2008) J79eJ81.
Contents lists available at ScienceDirect
Analytica Chimica Acta journal homepage: www.elsevier.com/locate/aca
High NH3 selectivity of NiFe2O4 sensing electrode for potentiometric sensor at elevated temperature Bin Yang, Chao Wang**, Ran Xiao, Hanyu Yu, Chuqi Huang, Jingxin Wang, Jinlong Xu, Hongming Liu, Feng Xia, Jianzhong Xiao* State Key Laboratory of Materials Processing and Die & Mold Technology, School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan, Hubei, 430074, PR China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
NiFe2O4 was firstly used as sensing electrode of potentiometric NH3 sensor based on the proposed novel concept. NH3 sensor based on NiFe2O4-SE exhibits good selectivity and long term stability. NiFe2O4 material has a good application prospect for NH3 detection in diesel vehicle exhaust.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 March 2019 Received in revised form 19 August 2019 Accepted 2 September 2019 Available online 3 September 2019
NiFe2O4 was synthesized using sol-gel method for sensing electrode material of YSZ based ammonia sensor. NiFe2O4-SEs sintered at 1100 C, 1150 C and 1200 C were characterized by XRD, the BET method and ESEM. By testing the NH3 response of different sensors at 650 C, it was observed that the 1150 C sintered sensor had the largest response value (104.3 mV for 320 ppm NH3) and the highest sensitivity (77.56 mV/decade), which were related to the most TPB sites and the moderate gas phase catalytic reaction. The response values of the sensor varied almost linearly with the logarithm of 20e320 ppm NH3 at 600e750 C, which was consistent with mixed-potential mechanism testified by polarization and EIS tests. When the oxygen concentration was at 7e10 vol %, its effect on the response value was within 3%. When the water vapor concentration was 3, 6 and 9 vol %, the ammonia response value was 95.1%, 92.9% and 88.7% of the values when there was no water vapor, respectively. The sensor showed very weak cross sensitivities to NOx, but non-negligible SO2 cross sensitivity. It also displayed slight signal drifts in weekly tests in eight weeks, which showed that the sensor attached with NiFe2O4-SE has a good long-term stability. © 2019 Elsevier B.V. All rights reserved.
Keywords: NH3 sensor Mixed-potential NiFe2O4-SE Elevated temperature Selectivity
1. Introduction
** Corresponding author. * Corresponding author. E-mail addresses: [email protected] (C. Wang), [email protected]. edu.cn (J. Xiao). https://doi.org/10.1016/j.aca.2019.09.006 0003-2670/© 2019 Elsevier B.V. All rights reserved.
The air we daily breathe is continuously polluted by vehicle exhaust due to rapid urban modernization. NOx (NO and NO2) exhausted mainly from diesel vehicle, are considered as harmful gases due to the serious impact on human health and environment
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[1]. Therefore, the regulations have become stricter and great efforts have been taken to reduce the emissions. Because the conventional three-way catalyst (TWC) can only remove NOx efficiently when the air-fuel ratio is controlled around 1, it is not applicable for diesel engine which works under lean-burn condition [2]. Urea-SCR is one of the most promising technologies for the removal of NOx emissions for diesel engine. In SCR system, the reductive NH3 hydrolyzed from urea reacts with NOx to produce harmless N2 and H2O under the effect of SCR catalyst [3]. However, there is a risk that ammonia escaping into the atmosphere would pollute the environment when excessive urea is injected [4]. So, in order to control the urea injection effectively, a high-performance NH3 sensor which could accurately monitor ammonia concentration after SCR catalyst is required. Over the last decades, many kinds of NH3 sensors based on different working principles have been reported [5,6]. The potentiometric type sensor is regarded as the most promising sensing device for monitoring NH3 concentration due to simple design, stable performance, wide operating temperature range and diversity selection of sensing materials. Many types of sensing materials have been investigated, such as Au [7,8], In2O3 [9,10], Mg2CuxFeO3.5þx [11], Cr2O3 [12], WO3 [13], TiO2-WO3 [14], SnO2 [15], V2O5-WO3-TiO2 [16] and CoFe2O4 [17], they all show high NH3 sensitivity, but noticeable cross sensitivity to NOx. As the main component of diesel vehicle exhaust, NOx could influence the response signal of NH3 sensor. However, few studies have recently been conducted to improve the NH3 selectivity by reducing NOx interference. So, it is eagerly desired to develop sensing material which possess high NH3 selectivity. Usually the response value of NO is significantly smaller than the one of NO2 at the same concentration [18e21]. And when the temperature is higher than 600 C, NO2 in the exhaust could reach chemical equilibrium with more than 90 vol % NO [22]. So, the response would be much smaller above 600 C even if a sensing material has a high sensitivity to NO2 below 600 C. In general, the NOx response will gradually decrease with the increase of operating temperature [23e26], so the cross-sensitivity of NOx can be minimized at a certain temperature above 600 C. Therefore, we can search a material with high NH3 sensitivity above 600 C, even if it has response to NOx, the optimum NH3 selectivity can be obtained at a certain temperature above 600 C. In general, the sensor performances stability depend strongly on the microstructure stability of sensing electrode [27], and the stable microstructure could be obtained after high temperature sintering. Ferrite electrode material could be sintered at high temperature due to their high melting point, so a ferrite sensing electrode that achieves a stable microstructure by high-temperature sintering could provide excellent long-term stability to the sensor. As a common ferrite material, NiFe2O4 caught our attention due to the high sensitivity to liquid petroleum gas [28], hydrocarbon gas [29], H2S [30], acetone [31], Cl2 [32] and NO2 [33]. Especially, the NO2 sensitivity of NiFe2O4 was found drastically decreased with the increase of operating temperature in the range of 400e500 C [33]. So, we tried to test its NH3 and NOx sensitivity above 600 C and found that the sensor attached with NiFe2O4-SE had high NH3 sensitivity and weak NOx cross sensitivity at elevated temperatures of 600e750 C. In this paper, NiFe2O4 was prepared by sol-gel method and sintered between 1100 C and 1200 C as the sensing electrode for YSZ-based potentiometric NH3 sensor. The NH3 sensing characteristics were investigated at the operating temperatures of 600e750 C, and the sample sintered at 1150 C exhibited the largest sensing response. The sensing mechanism was testified by the polarization and EIS results. The long-term stability, the effects of oxygen concentration, water vapor and other main interfering
gases on the NH3 response were studied. 2. Experiment 2.1. Synthesis and characterization of NiFe2O4 NiFe2O4 powder was prepared by the sol-gel method using nickel nitrate (Ni(NO3)2$6H2O), ferric nitrate (Fe(NO3)3$9H2O), citric acid monohydrate (C6H7O8$H2O) and ethylene glycol (C2H6O2) purchased from Sinopharm Chemical Reagent Co., Ltd, China. Stoichiometric Ni(NO3)2$6H2O and Fe(NO3)3$9H2O with the molar ratio of 1:2 were dissolved in deionized water. C2H6O2 and C6H7O8$H2O were added into the obtained solution, magnetic stirring and 80 C water bath heating were continued until a gel appeared, and then the gel was annealed at 300 C for 3 h with heating and cooling rate of 2 C/min. The annealed product was then calcined at 900 C for 6 h with heating and cooling rate of 1 C/ min. The calcined substance was then ground in an agate mortar to obtain fine powder. Part of NiFe2O4 powder was sintered at 1100 C, 1150 C and 1200 C for XRD test using the same procedure as sintering sensing electrode. The synthesized powder was stored in sealed dry container. The crystalline phase of as-synthesized NiFe2O4 at 900 C, 1100 C, 1150 C and 1200 C were characterized by X-ray diffraction (XRD) (X'Pert PRO, PANalytical B.V.). The instrument works at Cu Ka (l ¼ 1.5406 Å) radiation of 40 kV and 40 mA at room temperature. The step-scanning mode with a step length of 0.017 and scan speed of 8 /min was used to collect data. The specific surface and pore size distribution of NiFe2O4-SEs were obtained by V-Sorb 2800 (Gold App Instrument Corporation China) using BET method. 2.2. Sensor fabrication The sensor contains a 5 mol % Y2O3-doped ZrO2 (5YSZ) electrolyte, sensing and reference electrodes (SE and RE) located on both surfaces of the electrolyte. The planar electrolyte with the dimension of 10 mm 10 mm 0.3 mm was prepared by tape casting process and sintered at 1480 C for 2 h in air. The reference electrode was obtained by screen printing Pt slurry (79 wt % Pt powder, 1 wt % 8YSZ and 20 wt % organic binder) on one surface of YSZ electrolyte and then dried at 180 C for 2 h NiFe2O4 powder was mixed with 30 wt % binder (5 wt % ethocel, 94 wt % terpineol and 1 wt % span 80) as screen-printing slurry for the preparation of sensing electrode. The mixed slurry was screen printed on the other surface of YSZ, dried at 180 C for 2 h. Two Pt wires of 0.2 mm in diameter and 10 mm in length were respectively connected to both electrodes using Pt slurry. A part of both Pt wires was respectively bonded to the blank area of YSZ using high temperature ceramic adhesive. Then the sensor were sintered at 1100 C, 1150 C and 1200 C respectively for 3 h with heating and cooling rate of 2 C/ min. Pt slurry and wire were purchased from Sino-Platinum Metals Co., Ltd, China. The schematic of the potentiometric NH3 sensor is shown in Fig. 1. The surface and the cross section morphology of SE were observed by Environmental Scanning Electron Microscope (Quanta 200, FEI, Holland) at 10 kV. 2.3. Evaluation of sensing performance Sensor performance was tested in a gas flow apparatus under various gas environments controlled by mass flow controllers (MPA-80, Beijing Seven Star Electronics Company). The gas flow apparatus was connected to a quartz tube which was equipped with a furnace operated at temperatures of 600e750 C. The base gas is 10 vol % O2 with 90 vol % N2 and the sample gas is a changing
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Fig. 1. (a) Schematic of the fabricated sensor; (b) top view of the sensor; (c) bottom view of the sensor.
concentration of NH3 varying from 20 to 320 ppm with base gas. The total flow rate of the gas atmosphere is 0.5 L/min. Both electrodes of the sensor were exposed to the same gas atmosphere. The open circuit voltage difference DV between SE and RE was monitored and recorded by an electrochemical work station (VersaSTAT 3, Princeton, USA). For all measurements, the SE were connected to the positive terminal of the electrochemical work station, and the RE was connected to the negative one. The electrochemical impedance spectroscopy (EIS) was measured at 650 C by electrochemical work station in the frequency range from 0.1 Hz to 1 MHz with 10 mV exciting voltage. The polarization curves in the potential range of 150 mVe0 mV were measured by the linear voltage scanning method at a constant scan rate of 5 mV/s in the base gas and the sample gases containing 80, 160, and 320 ppm NH3. For the test of the oxygen effect, the oxygen concentration is changed step by step when the response of the sensor to 20 ppm
Fig. 3. Surface micrographs of the NiFe2O4-SE sintered at (a) 1100 C; (b) 1150 C; (c) 1200 C.
Fig. 2. The XRD patterns of the prepared NiFe2O4 powder sintered at 900 C, 1100 C, 1150 C and 1200 C.
and 320 ppm NH3 is stabilized, and the transient curve with the changed oxygen concentration is recorded by the electrochemical workstation. For the effect of water vapor, different concentrations
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of vapor is obtained by heating water at different temperatures according to the water vapor saturation pressure at a certain temperature. The sample gas was humidified with 3, 6 and 9 vol % vapor by passing through 25, 37 and 44 C water.
3. Results and discussion 3.1. Powder characterization The crystal structure of the prepared NiFe2O4 powders after sintering of 900 C, 1100 C, 1150 C and 1200 C were characterized by XRD as shown in Fig. 2. It can be found that the samples are all well crystallized. All of the diffraction peaks can be perfectly assigned to cubic NiFe2O4, which are consistent with the standard card (ICCD 00-044-1485). No others diffraction peaks are observed, confirming that the as-prepared powder is high-purity NiFe2O4, and the electrode materials sintered at 1100 C, 1150 C and 1200 C are still pure phase NiFe2O4. 3.2. Morphology of NiFe2O4-SE Fig. 3 shows surface morphology of NiFe2O4-SE sintered at 1100 C, 1150 C and 1200 C. It is observed that with the increase of sintering temperature, the grains grow up obviously with the average size of about 0.5 mm, 1.0 mm and 1.5 mm, respectively. There are many small pores in the sample sintered at 1100 C, but bigger pores at 1150 C and 1200 C. There is an obvious sintering phenomenon in 1200 C-sintered sample with some grains fusing together. Cross-sections micrograph of the samples is shown in Fig. 4. There is a tightly integrated interface between the electrode and YSZ for each sample. The NiFe2O4-SEs have a similar thickness of about 30 mm for average. 3.3. Sensing characteristics of NiFe2O4 3.3.1. NH3 sensing performance for different temperatures sintered NiFe2O4-SEs Fig. 5 shows the voltage responses of the sensors attached with 1100 C, 1150 C and 1200 C sintered NiFe2O4-SEs to different NH3
Fig. 5. Response transients curve for the sensors attached with NiFe2O4-SE sintered at 1100 C, 1150 C and 1200 C toward 20e320 ppm NH3 at 650 C.
Table 1 The specific surface area (SBET) of the electrodes sintered at different temperatures.
Fig. 4. Cross-section micrographs of the NiFe2O4-SE sintered at (a) 1100 C; (b) 1150 C; (c) 1200 C.
Electrode sintering temperature/ C
Specific surface area (SBET)/m2.g1
1100 1150 1200
1.57 1.14 0.53
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sample. On the other hand, the intensity of the electrode reaction could be directly affected by the quantity of TPB sites which are the regions where the target gas, the SE and the solid state electrolyte coexist. It can be seen from the SEM results that the 1200 C sintered sample has more obvious sintering melting phenomenon than 1150 C sintered sample, and more gas channel could be blocked by the grains melted together, the particle melting brings about less TPB sites and more difficult gas diffusion, as shown in Fig. 7, so the response signal of 1150 C sintered sample is higher than that of 1200 C sintered sample. The grain of 1100 C sintered sample is fine, so less TPB sites were formed between YSZ and NiFe2O4-SE, resulted in the lowest response signal of this sample. Since the 1150 C sintered sample has the most TPB sites and the moderate gas phase catalysis reaction, it has the largest response signal.
Fig. 6. The pore size distribution of the NiFe2O4-SEs sintered at different temperatures using the BET method.
concentrations at 650 C. For each NH3 concentration, the highest response value and response/recovery rate were always obtained from the sensor attached with 1150 C sintered NiFe2O4-SE, the lowest ones were obtained from the 1100 C sintered sample. NH3 mainly experiences the gas phase catalytic reaction during diffusing in NiFe2O4-SE and the electrochemical reaction at three phase boundary (TPB) when the sensor is exposed to NH3 and O2. The intensity of these two reactions directly affects the response signal of the sensor. The intensity of gas phase catalysis is closely related to the microstructure of NiFe2O4-SE. The specific surface area and pore size distribution of the electrodes sintered at different temperatures were obtained using the BET method. The results are shown in Table 1 and Fig. 6, respectively. It can be seen that the specific surface area and the amount of pores gradually decrease with the increase of the sintering temperature, so the contact opportunity between NH3 and NiFe2O4-SE decreases gradually and less NH3 is consumed by gas phase catalysis during diffusion. This lead to more NH3 molecules could reach the TPB to participate in the electrode reaction, so the sample sintered at 1150 C has higher response signals than the 1100 C sintered
3.3.2. NH3 sensing performance at different operating temperatures Based on its maximum NH3 response value, 1150 C sintered NiFe2O4-SE was studied in detail for NH3 sensing performances. Fig. 8 shows the voltage responses of the sensor sintered at 1150 C to different NH3 concentrations measured in the range of 600e750 C. For each NH3 concentration, the highest value was
Fig. 8. Response transient curves for the sensor attached with 1150 C sintered NiFe2O4-SE toward NH3 at different operating temperatures.
Fig. 7. Schematic explanation of TPB sites in different temperatures sintered NiFe2O4SE. (a) 1100 C; (b) 1150 C; (c) 1200 C.
Fig. 9. Voltage response versus logarithm of NH3 concentration for the sensor attached with 1150 C sintered NiFe2O4-SE at different operating temperatures.
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decade at 650 C. Therefore, the following NH3 response studies are carried out at the optimum operating temperature of 650 C.
3.3.3. Sensing mechanism When the sensor is exposed to target gas containing oxygen and ammonia, a mixed potential is formed when the oxidation reactions rate of NH3 (Eq. (1)) is equal to the reduction reaction rate of O2 (Eq. (2)) [34]. The potential difference DV between the NiFe2O4SE and Pt-RE generally gives a logarithmic dependence on the NH3 concentration. As illustrated in Fig. 9, there is a linear logarithmic relationship between the mixed potential and NH3 concentration, expressed as Eq. (3).
Fig. 10. Modified polarization curves of the sensor attached with 1150 C sintered NiFe2O4-SE recorded in base gas and samples gases containing 80, 160 and 320 ppm NH3 at 650 C.
Table 2 Comparison between the estimated and measured DV for the sensor with 1150 C sintered NiFe2O4-SE. NH3 concentration/ppm
320 ppm 160 ppm 80 ppm
DV/mV Estimated
Observed
99.7 74.5 48.8
104.3 77.3 51.6
always obtained at 650 C, and the lowest one was obtained at 750 C. It could also be seen that NH3 sensitivity decreased significantly and recovery speed became faster as operating temperature increases from 650 C. Fig. 9 displays the dependence of response values on the logarithm scale of NH3 concentration at different operating temperatures. It can be seen that in each temperature, the response value increases almost linearly with increasing NH3 concentration on a logarithmic scale. Such a linear variation is typical for the mixedpotential mechanism of gas sensor. It also can be seen that the sensor exhibits the largest sensitivity with the slope of 77.56 mV/
4NH3 þ 6O2- ¼ 2N2 þ 6H2O þ 12e
(1)
3O2 þ12e ¼ 6O2-
(2)
DV ¼ a lnPNH3þb
(3)
In order to further testify the mixed-potential model, the modified polarization curves of the sensor were measured at 650 C in base gas and sample gases (80, 160 and 320 ppm NH3), respectively, as shown in Fig. 10. The cathodic polarization curve was measured in base gas, while the anodic polarization curves were obtained by subtracting base gas value from sample gas value. The value of mixed-potential can be estimated by the intersection of the anodic and cathodic polarization curves. Table 2 lists the measured DV values and the estimated mixed-potentials for the sensor. It could be clearly seen that the estimated values are close to the measured values, indicating that the sensor follows the mixedpotential mechanism. EIS is an effective method to study electrochemical reaction kinetics and has been widely used in the field of gas sensor research [7,35,36]. The EIS of the sensor was measured between 0.1 Hz and 1 MHz at 650 C in the base gas and sample gases (20e320 ppm NH3), the Nyquist plots are shown in Fig. 11a. It could be seen that there are an inconspicuous smaller arc in high frequency and a bigger arc in the low frequency. The size of the small arc at high frequency are similar for different NH3 concentrations, whether in the base gas or the sample gas. But there is a great shrinkage toward real Z0 -axis for the large arc with increasing of NH3 concentration. A probable equivalent circuit model could be proposed for potentiometric gas sensor based on some reports, as shown in Fig. 11b. Here, Rb represents the YSZ bulk resistance, Ri represents
Fig. 11. (a) Electrochemical impedance spectroscopy of the sensor attached with 1150 C sintered NiFe2O4-SE toward different concentrations of NH3 at 650 C; (b) Nyquist plot of an ideal equivalent circuit model for YSZ-based potentiometric sensor.
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Table 3 The change rates of 20 ppm and 320 ppm NH3 response signal values with different oxygen concentrations. Oxygen concentration/vol.%
1 4 7 14
Fig. 12. The plot of the relationship between Re and the logarithm scale of NH3 concentration.
Change rate/% 20 ppm
320 ppm
49.39% 14.85% 3.21% 10.86%
21.65% 3.84% 1.72% 4.79%
concentration between 1 vol % and 14 vol %. After the NH3 response signal is stabilized at 650 C, the oxygen concentration is changed as shown in Fig. 13. The change rates for the NH3 response values at different oxygen concentrations with respect to the value at 10 vol % oxygen are summarized in Table 3. It can be seen that the effect of oxygen concentration on the signal of 320 ppm NH3 is much less than that of 20 ppm. Except for the oxygen concentration of 7 vol % and 10 vol %, oxygen concentration has a great effect on the signal, especially at 1 vol %. When oxygen concentration is between 7 and 10 vol %, the effect of oxygen on NH3 signal becomes weaker. The NH3 signal change rates of 20 ppm NH3 are 3.21% and 1.72% for 320 ppm NH3. In general, the concentration of oxygen in automobile exhaust is between 5 and 10 vol % [39], so the effect of oxygen
Fig. 13. Response of the sensor attached with 1150 C sintered NiFe2O4-SE at different oxygen concentrations for 20 ppm and 320 ppm NH3 at 650 C.
interfical resistance of the TPB, and Re represents electrochemical reaction process [13,35,37,38]. The resistance values of Rb, Ri and Re can be calculated using this mode with the Zview software. For a same sensor tested in different NH3 concentrations, the values of Rb and Ri are almost unchanged, but Re are distinguishing because of different electrochemical reaction kinetics. So, a plot of the relationship between Re value and the logarithm scale of NH3 concentration is shown in Fig. 12. It shows a good linear relationship that exists between the Re and the logarithm of NH3 concentration, which is similar to the correlation between the DV and concentration that follows the mixed-potential mechanism in Fig. 9. Therefore, the EIS results also show that the sensor conforms to the mixed-potential mechanism. 3.3.4. Interfering gases effect and long-term stability As the main component of diesel vehicle exhaust gas, oxygen concentration changes with different working conditions, so it is necessary to study the effect of oxygen concentration on NH3 response. Similarly, the concentration of NH3 in tail gas also fluctuates all the time, so low concentration of 20 ppm and high concentration of 320 ppm NH3 were investigated. In this study, the response value of the sensor was measured as a function of oxygen
Fig. 14. (a) A typical response transient plot of a water vapor resistance test for the sensor attached with 1150 C sintered NiFe2O4-SE at 650 C; (b) the average response value of the five-time water vapor resistance test results for different water vapor concentrations.
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results for different water vapor concentrations, as can be seen that when water concentration was 3, 6 and 9 vol %, the sensor retained 95.1%, 92.9% and 88.7% of the initial response value tested without vapor. Therefore, it does not have a significant impact on the NH3 response over the typical water vapor content of 5e6 vol % in actual automobile exhaust [7]. Besides oxygen and water vapor, the tail gas of the diesel engine mainly contains NO, NO2 and SO2, their cross sensitivities should also be considered. Fig. 15a shows the sensitivities of the sensor toward 320 ppm of each gas mentioned above, and Fig. 15b shows the response signals of 320 ppm NH3 mixed with the same concentration of SO2, NO and NO2, respectively. It demonstrates that the sensor exhibited good selectivity to NH3. Table 4 summarizes the performance comparison of potentiometric NH3 sensors using different sensing electrodes in recent years, and it further demonstrates the good NH3 sensitivity and selectivity of NiFe2O4 material. However, SO2 still shows significant cross sensitivity (1/6 of the NH3 response value) that could not be ignored. Since the implementation of the Euro 6 standard, the sulfur content of vehicle fuel in EU does not exceed 10 ppm, so the SO2 cross sensitivity of this sensor would not be considered. Actually, the long-term stability is another important factor for gas sensor. Diesel vehicle usually works intermittently, so sensors that monitor vehicle exhaust should undergo intermittent startup and durability tests. In order to simulate the actual working conditions, the long-term stability of the sensor was evaluated by a weekly test over a period of 8 weeks. At each test, the temperature of the device was raised from room temperature to 650 C and controlled at this temperature for test. The response signal of the sensor was recorded in the base gas first, and then in NH3 atmosphere by introducing 20, 40, 80, 160 and 320 ppm NH3. The device was then naturally cooled to room temperature for the next week's
Fig. 15. (a) Sensitivities of the sensor attached with 1150 C sintered NiFe2O4-SE to various gases of 320 ppm at 650 C; (b) the response signals of 320 ppm NH3 mixed with 320 ppm SO2, NO and NO2, respectively.
concentration change could be not obvious, would be within 3.21%. Water vapor is also the main component of exhaust, so it is very important to investigate the effect of vapor on the sensing performance of the NH3 sensor. The amount of water vapor in real exhausts usually varies from 2 to 11 vol % [39], so 3, 6 and 9 vol % vapor concentration were studied in this research. Fig. 14a is a typical response transient plot of a water vapor resistance test at 650 C. In the test, the NH3 concentration was fixed at 320 ppm and the transient NH3 response was recorded when different concentrations of water vapor passed. It can be seen that the addition of water vapor causes a slight decrease in sensor response possibly due to the improvement of the oxygen reduction kinetics or the deterioration of the NH3 oxidation kinetics [7]. Fig. 14b shows the average response value of the five-time water vapor resistance test
Fig. 16. Long-term stability test of sensor attached with 1150 C sintered NiFe2O4-SE.
Table 4 The comparison between this work and recent related works. Sensor structure
Operating Temp. ( C)
Sensitivity (mV/decade)
DVNO/DVNH3
DVNO2/DVNH3
Ref.
PtjYSZjPtjMg2Cu0.25FeO3.75 PtjYSZjPtjCoFe2O4 PtjYSZjAu CuOjYSZjIn2O3 AujYSZjAujVWT PtjYSZjAu þ SnO2 PtjYSZjNiFe2O4 In2O3jYSZjPtjLaCoO3
350 450 525 550 550 600 650 700
220.0 55.0 47.0 35.9 80.0 39.9 77.6 41.5
4/124 ¼ 3.2% 12/55 ¼ 21.8% 8/110 ¼ 7.0% / 5/80 ¼ 6.3% 2/45 ¼ 4.4% 8/104 ¼ 8.0% /
24/124 ¼ 19.4% 11/55 ¼ 20.0% 24/110 ¼ 22.0% 18.3/41.5 ¼ 44.0% 20/80 ¼ 25.0% 10/45 ¼ 22.2% 17/104 ¼ 16.4% 10.0%
[11] [17] [7] [9] [16] [15] This work [10]
B. Yang et al. / Analytica Chimica Acta 1089 (2019) 165e173
evaluation. The results are shown in Fig. 16. At the same time, the sensitivity for 80 ppm NH3 was investigated at 650 C during 40 h' continuous operation, the response value changed slightly with a fluctuation within 2%. It demonstrated that the sensor showed good stability, which may be related to the stable microstructure of NiFe2O4-SE after high temperature sintering. In combination with the above mentioned good NH3 selectivity and long-term stability, low water vapor and oxygen interference effect, it is believed that the NiFe2O4 reported in this study has a good application prospect in sensing electrode for potentiometric NH3 sensor. 4. Conclusions A new sensing material NiFe2O4 was prepared using sol-gel method for YSZ-based ammonia sensor. The NiFe2O4-SEs of the potentiometric type sensors were sintered at different temperatures, and their ammonia response characteristics were tested at 650 C. The 1150 C sintered sensor had the largest response value of 104.3 mV for 320 ppm NH3 and the largest sensitivity of 77.56 mV/decade. The sensing mechanism of the sensor was proved to follow the mixed-potential mechanism by polarization curve and EIS tests. The sensor had good resistance to oxygen, water vapor and NOx, and also showed good long-term stability. These good sensing performances indicate that the NiFe2O4-SE has good potential in diesel vehicle exhaust emission control system.
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[13] [14]
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[16] [17]
[18] [19]
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[22] [23]
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Declaration of competing interest [25]
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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Acknowledgments
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The authors are deeply grateful to the Analytical and Testing Center of Huazhong University of Science and Technology for their hard work in XRD and SEM testing.
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