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A novel mixed-potential type NH3 sensor based on Ag nanoparticles decorated AgNbO3 sensing electrode synthesized by demixing method
Sensors & Actuators: B. Chemical 301 (2019) 127146
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
A novel mixed-potential type NH3 sensor based on Ag nanoparticles decorated AgNbO3 sensing electrode synthesized by demixing method ⁎
Xu Li, Lei Dai , Wei Meng, Yuehua Li, Ling Wang, Zhangxing He
T
⁎
College of Chemical Engineering, North China University of Science and Technology, Tangshan 063009, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: NH3 sensors Mixed potential AgNbO3/Ag sensing electrode Demixing
Solid-electrolyte type sensors based on composite sensing electrodes (SE) have attracted extensive attention due to their excellent sensitivity and anti-interference property. Herein, we prepare AgNbO3/Ag (ANO/Ag) composite SE for the solid-electrolyte type NH3 sensor using a facile demixing method. Compared with pristine ANOSE, the sensor based on ANO/Ag-SE exhibits larger response signal, higher sensitivity and lower detection limits. What’s more, the anti-interference capability of the sensor toward NOx is effectively enhanced. The effect of the molar ratio of Ag to Nb on the performance of the sensor is also investigated. When the molar ratio of Ag:Nb is 1.05:1, the sensor exhibits largest response signals and sensitivity (−91.2 mV/decade) to NH3. In the meantime, the response signal of the sensor to a fixed NH3 concentration almost keeps constant under different relative humidity (RH) and oxygen concentrations. Thus, the demixing method will provide a new approach for the fabrication of composites SEs with high performance for gas sensors.
1. Introduction Selective catalytic reduction (SCR) using urea reduce NOx has been considered as a promising method to reduce NOx emission [1]. In order to accurately control the injected amount of urea and prevent the overflow of NH3, a sensor with high sensitivity to NH3 and excellent anti-interference property to nitrogen oxides is required for real-time monitoring [2]. Among various types of gas sensors, the solid-electrolyte type sensors can accurately detect gas content in high temperature environments [3]. Up to now, noble metals (such as Au [4,5]) and metal oxide (such as TiO2 [6], In2O3 [7], WO3 [8], V2O5 [9–11], CoWO4 [12], Cd2V2O7 [13], Ni3V2O8 [14], CeVO4 [15]) have been investigated as the SE materials for solid-electrolyte type NH3 sensors. Except for the high sensitivity of the sensor to NH3, the excellent anti-interference of the sensor against NOx is essential, especially in the SCR system. In general, the selectivity of the sensor mainly depends on the intrinsic properties of the SE. In addition, the modification of the electrode is also effectively alternative method to improve the anti-interference of the NH3 sensor against NOx [16]. Elumalai et al. improved the anti-interference of the mixed-potential type NH3 sensor to the coexistent gases using NiO/Au composite SE due to the pre-catalytic oxidation of CO, NO, and hydrocarbons by the NiO layer [4]. Wang et al. fabricated a solid-electrolyte type NH3 sensor by decorating CeVO4-SE with Au, which successfully enhanced the sensor's anti-
⁎
interference ability to NO2 [15]. According to our previously research, loading Ag on the SE can not only enhance the sensitivity of the NH3 sensor, but also effectively improve anti-interference property to NOx, which is attributed to the depression of NO2 adsorption [17]. For preparing Ag-containing electrodes or catalysts, various methods have been investigated, including impregnation methods [17], photoreduction [18,19], sol–gel method [20], electrospinning [21], co-precipitation [22] and hydrothermal methods [23]. Recently, a demixing approach to prepare two-phase composite materials by one-step high temperature synthesis procedure has attracted extensive attention [24,25]. In particular, when the materials of composite oxide is mixed with a non-stoichiometric ratio, the excess component tends to be separated with the composite oxide to form metal oxide or metal nanoparticles due to the solid solution limit of the composite oxide during preparation process [26]. The formed nanoparticles prepared by demixing method have a high level of dispersion, which is beneficial for the release of catalytic activity of the SE. Hence, AgNbO3 perovskite loaded with Ag nanoparticles was prepared by demixing method and then used as the SE for mixed-potential type NH3 sensor. The morphology and structure of the samples were characterized. The performances of the NH3 sensors with AgNbO3/Ag composite SE was investigated in detail.
Corresponding authors. E-mail addresses: [email protected] (L. Dai), [email protected] (Z. He).
https://doi.org/10.1016/j.snb.2019.127146 Received 20 March 2019; Received in revised form 5 July 2019; Accepted 14 September 2019 Available online 17 September 2019 0925-4005/ © 2019 Elsevier B.V. All rights reserved.
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2. Experimental
the nanoparticles, respectively. Fig. 2a shows the XRD patterns of ANO, ANO1.02, ANO1.05 and ANO1.1 samples. The ANO sample is indexed well in orthorhombic phase with perovskite structure (JCPDS 00-0220471) without any impurity. When the molar ratio of Ag:Nb < 1, the other two diffraction peaks appear at 2θ of 38.3° and 44.5°, respectively, which are assigned to the metallic Ag (JCPDS 04-0783). XPS results indicate that the chemical state of Ag on the surface of the ANO sample is Ag1+, because of the appearance of Ag 3d5/2 peak at 368.1 eV and Ag 3d3/2 peak at 373.9 eV (Fig. 2b). With the molar ratio of Ag:Nb increasing, the positions of Ag 3d5/2 and Ag 3d3/2 peaks shift toward higher binding energy. Because metallic silver has a higher binding energy than Ag1+, the increasing in binding energy of the Ag 3d5/2 and Ag 3d3/2 peaks indicates the increasing of the Ag0 content. Three-dimensional network-like electrolyte skeleton can strengthen the bonding between the SE and substrate accompanied by effective enlargement of the three-phase boundaries (TPBs) length [8,28,29]. Therefore, we prepared a porous layer on one side of the YSZ electrolyte, and the corresponding morphologies of the porous layer are shown in Fig. 3a and b. The porous layer closely attaches to the dense layer and its thickness is about 30 μm. Fig. 3c and d show the surface and cross-sectional micrographs of the porous layer after introducing ANO1.05-SE. It can be seen that the particles of ANO1.05-SE are irregular spheres with size among 1–5 μm. The surface of porous layer is covered by an ANO1.05 layer with a thickness about 10 μm. It is noted that the ANO1.05 layer remains porous after sintered at 900 °C, which is beneficial for the diffusion of gases to TPBs [27]. Fig. 3e–i show the element distribution of the porous layer after introducing ANO1.05-SE. A considerable number of ANO1.05 particles have invaded into the porous layer.
2.1. Preparation of sensing materials The AgNbO3 based sensing materials were prepared by demixing method. Ag2O and Nb2O5 were mixed by a stoichiometric ratio of 1:1, 1.02:1, 1.05:1 and 1.1:1, respectively, which were then calcined at 900 °C for 5 h in a 30% O2/N2 atmosphere with a heating rate of 3 °C/ min to obtain ANO, ANO1.02, ANO1.05 and ANO1.1 sensing materials, respectively. 2.2. Assembly of the sensor The bilayer YSZ electrolyte substrate with Pt electrode was prepared by the procedure reported before [27]. The platinum paste was coated on the centers of both surfaces of the porous YSZ layer and dense YSZ electrolyte pellet with a radius of 3 mm, which was used as the electrical collector and reference electrode, respectively. The platinum wires as leads were adhered to the platinum paste and then sintered at 800 °C for 1 h. The sensing materials slurry was made by the method same as the preparation YSZ slurry. After screen printing the sensing materials slurry onto the YSZ porous layer for three times and then sintered at 900 °C for 3 h, the final sensor was obtained. 2.3. Characterization of the materials and measurement of electrochemical performances The phase composition was characterized by X-ray diffraction (XRD). The microstructure and element distribution of the samples was characterized by Field emission scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS), respectively. The sensing performance of the sensor was tested in a quartz tube held in a furnace. Two-electrode system including the sensing electrode and Pt reference electrode was introduced to monitor response signal (ΔV, defined as Vsample minus Vbaseline, where Vsample is the response potential in sample gas and Vbaseline is the potential in base gas before introducing NH3) of the sensor. Three-electrode system was introduced to measure the polarization curves at a constant scan-rate of 5 mV/s using an electrochemical work station (Solartron Analytical SI 1260/ 1287). In order to avoid the polarization and accurately measure the potential of the reference electrode, two isolated Pt-layers with a radius of 3 mm were painted on the dense YSZ electrolyte pellet side as the counter and reference electrode, respectively. The direction of the oxidation current is defined as negative direction in the workstation used for the electrochemical measurement. The NH3 source was achieved by diluting standard gas (8000 ppm NH3 with N2 balance) with air and the total gas flow was controlled at 200 cm3/min. The electrochemical impedance spectroscopy (EIS) was performed from 1 MHz to 0.1 Hz with signal amplitude of 5 mV. The NO2 temperature programmed desorption (NO2-TPD) was conducted at 40–600 °C with a heating rate of 10 °C/min under He atmosphere. During the long-term stability test, the transient response of the sensor was tested every two weeks at 450 °C for four weeks. Before every test, the sensor was exposed in air at 450 °C for 24 h.
3.2. Influence of Ag/Nb molar ratio on sensing performances Fig. 4a–d compares the sensing properties of the sensors based on ANO-SE and ANO1.05-SE to NH3 tested at different temperatures. At 400 °C, the sensor using the single ANO-SE shows unstable and irregular response to NH3. Meanwhile, the potential can’t return to original value of baseline after removal of NH3, which indicates this sensor is unsuitable apply at this temperature. In comparison, the sensor using ANO1.05-SE exhibits more stable response/recovery characteristic, and good linear relationship between ΔV and log (PNH3) at 400 °C. At the test temperature of 450 °C, both of the sensors show stable ΔV response to NH3. Compared with the sensor with single ANO-SE, the sensor using ANO1.05-SE has improved response ΔV values. The sensitivity is increased from −40.75 mV/decade to −91.21 mV/decade and the detection limit is lowered from 100 to 25 ppm. Meantime, the both sensors reach their highest sensitivity, respectively. It is noted that the response of the sensor based on ANO1.05-SE reaches the highest value at the beginning of the transients and then decreases rapidly when the sensor is exposed to 300 or 400 ppm NH3. The reasons for such behavior are not clear at the moment. However, this behavior should be closely related to the kinetics of the sensing process [28]. The formation of stable mixed potential depends on the formation of the equilibrium of the electrochemical reactions which involves the mass transfer, adsorption, desorption and charge transfer. The higher NH3 concentration is, the more difficult the formation of the equilibrium is. Therefore, there are some fluctuations of the response plateau particularly at the high NH3 concentrations. With the test temperature increasing from 450 °C to 550 °C, the response ΔV values and sensitivity of the both sensors are decreased. The sensor based on ANO1.05-SE always exhibits better sensing performance than the sensor using ANO-SE at a fixed temperature. The influence of operating temperature on the sensing performance is related to the NH3 diffusion process through the SE to the TPB and the electrochemical reaction happening at the TPB needs certain activation energy. The electro-chemical reaction does not gain enough activation energy at below 450 °C, resulting in the increasing of the sensitivity with the increasing of temperature from 400 to 450 °C.
3. Results and discussion 3.1. Characterization of the sensing materials Fig. 1a–d show the SEM images of the as-prepared ANO, ANO1.02, ANO1.05 and ANO1.1 particles. It can be seen that ANO particles present a smooth surface. For ANO1.02 sample, some nanoparticles appear on the surface of the ANO matrix. With the molar ratio of Ag:Nb further increasing, the number of the nanoparticles is increased accompanied by the growth of size. XRD and XPS were employed to determine the phase composition of 2
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Fig. 1. (a–d) SEM images of the ANO, ANO1.02, ANO1.05 and ANO1.1 sensing materials.
Fig. 2. (a) XRD patterns of the ANO, ANO1.02, ANO1.05 and ANO1.1 sensing materials (b) XPS spectra of Ag 3d of the ANO, ANO1.02, ANO1.05 and ANO1.1 sensing materials.
of three arcs. The two arcs at high frequency represent the impedance of electrolyte and overlap each other. The arc at the low-frequency corresponds to the electrochemical reaction process at the TPBs [15]. Compared with the sensor attached with single ANO-SE, the resistance value at the intersection of the arc with the real axis is decreased dramatically for the sensor with ANO1.05-SE, indicating the electrochemical catalytic activity toward NH3 for the sensor using ANO1.05-SE is the higher [15]. On one hand, Ag nanoparticles have good catalytic activity which directly enhances the electrocatalytic activity of ANO1.05-SE. On the other hand, Ag nanoparticles can increase the electronic conductivity of the SE beneficial for electron transfer. As a result, the sensing performance of the sensor is improved. In order to investigate the effect of the molar ratio of Ag:Nb in SE on sensing performances of the sensor, the ΔV responses of the sensors
However, further increasing the temperature over 450 °C, the amount of NH3 adsorbed on the SE will decrease and the effect of heterogeneous catalysis reaction may increase at high temperature, resulting in degradation in sensing performance. Table 1 presents the sensing performance of the mixed-potential type NH3 sensors reported in other literatures. The sensor using ANO1.05-SE exhibits comparable or even higher sensitivity to NH3 compared to the sensors based on others SEs. In order to confirm the mechanism of the improving sensing performance of the sensor after loading Ag, EIS was conducted to investigate the difference in electro-catalytic activity of the sensors. Complex impedance of the sensors attached with ANO-SE or ANO1.05SE were performed in 300 ppm NH3/air at 450 °C. Corresponding results are shown in Fig. 5. The Nyquist plots of the sensors are composed 3
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Fig. 3. (a, b) SEM images of surface and cross-section of LSAO porous layer (c, d) typical morphology of surface and cross-section of LSAO porous layer after loading ANO based SE (e–i) corresponding EDS mapping of the cross-section SEM image.
the i0 value is increased from 0.030 μA/cm2 to 0.249 μA/cm2, indicating the improvement of the electrocatalytic activity. With further increasing the Ag/Nb molar ratio to 1.1, the i0 value is decreased to 0.132 μA/cm2.
with different SEs to NH3 in the NH3 concentration range of 25–400 ppm were tested at 450 °C, and the corresponding transient response curves of the sensors are shown in Fig. 6a. Upon injecting of NH3 into the testing system, the sensor using ANO1.02-SE exhibits negative ΔV response which then is recovered to the original value when NH3 is exhaustion. The ΔV response is enhanced with the increasing of NH3 concentration. When the molar ratio of Ag:Nb is 1.05:1, the sensor exhibits larger ΔV response and lower detection limit (25 ppm) compared with the sensor using ANO1.02-SE, which should be attributed to the increasing of Ag nanoparticles on the surface of ANO-SE. Further increasing the molar ratio of Ag:Nb to 1.1:1, the ΔV response values of the sensor fall down due to the dramatically growing of the Ag nanoparticles. Fig. 6b shows the dependence of ΔV values on the logarithm of NH3 concentrations for the sensors based on ANO1.02SE, ANO1.05-SE and ANO1.1-SE. All the sensors exhibit good linear dependence relationship between the ΔVs and log(PNH3). It should be noted that the sensor attached with ANO1.05-SE exhibits the highest sensitivity among the three sensors. In order to investigate the influence of Ag/Nb molar ratio on the electrocatalytic activity of sensing materials to NH3, the Tafel plots of three sensors using Ag1.02NbO3-SE, Ag1.05NbO3-SE and Ag1.1NbO3-SE were measured in 200 ppm NH3 at 450 °C, as shown in Fig. 6c. It can be seen that the Tafel plot shifts upward as the Ag/Nb molar ratio increases from 1 to 1.05, but shifts downward with further increasing the Ag/Nb molar ratio to 1.1. The exchange current density (i0) can be obtained from the extrapolation of both the anode and cathode curves to the current axis, which presents the electrocatalytic activity of the electrode [33]. When the Ag/Nb molar ratio is increased from 1 to 1.05,
3.3. The selectivity and stability of the sensor There usually are some coexistent gases in the environment where the NH3 sensors are applied. Therefore, the anti-interference properties of the sensors based on ANO-SE and ANO1.05-SE toward the coexistent gases were tested. Fig. 7a presents the response values of the two sensors to 300 ppm NH3 in the presence of other gas at 450 °C. The two sensors exhibit excellent anti-interference properties to CH4, H2 and CO2. However, for the sensor using ANO-SE, upon injection of 450 ppm NO or NO2 to sample gas, the ΔVs are reduced by 44% and 85%, respectively. In contrast, for the sensor using ANO1.05-SE, the reduction percentages of ΔVs only are 4% and 9%, indicating the cross-sensitivity of the sensor with ANO1.05-SE to nitrogen oxides are effectively suppressed. Compared the ΔVNO2/ΔVNH3 values of the sensors reported in literatures (Table1), it can be concluded that the present sensor using ANO1.05-SE has an excellent anti-interference property to NO2. NO2-TPD and AC impedance analysis were employed to investigate the reason for the improved selectivity of ANO1.05-SE. The NO2-TPD analysis results of the ANO and ANO1.05-SE samples are shown in Fig. 7b. For ANO sample, there are three peaks located at 245, 314 and 445 °C with large area, which is attributed to the decomposition of bridging nitrate species and bidentate nitrate species [34]. In comparison, the curve of ANO1.05 sample only has a small peak at 275 °C with 4
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Fig. 4. Transient response-recovery curves and sensitivities of the sensors based on ANO-SE and ANO1.05-SE at 400 °C (a), 450 °C (b), 500 °C (c) and 550 °C (d).
much smaller area, which means the decreasing of formed bidentate nitrate species with higher chemical adsorption bond energy. Fig. 7c shows the Nyquist plots of the sensors exposed to 300 ppm NH3 or 300 ppm NH3 +450 ppm NO2 with air balance at 450 °C. For the sensor attached ANO-SE, the arc at low frequencies representing the electrochemical processes at the TPB is significantly decreased after 450 ppm NO2 injected in the 300 ppm NH3 sample gas, which means that the electrochemical decomposition of NO2 occurs at the TPB. In contrast, for the sensor using ANO1.05-SE, the presence of NO2 in the sample gas has no observable influence on the impedance of electrochemical processes at the TPB, indicating that NO2 does not take part in electrocatalytic reaction. Owing to the depression of adsorption and electrocatalytic activity of ANO1.05-SE to NO2, the NH3 selectivity of the sensor based on ANO1.05-SE against NO2 is effectively improved. The effect of O2 concentrations and RH on sensor response property should be considered due to changeable O2 concentrations and RH in sensor practical operation atmosphere. Fig. 8a shows the transient response of the sensor attached ANO1.05-SE to 300 ppm NH3 under different O2 concentrations. O2 concentration varying from 5 to 21 vol%
Fig. 5. Nyquist plots of the sensors based on ANO-SE and ANO1.05-SE in 300 ppm NH3 with air balanced at 450 °C.
Table 1 Comparison of the sensing performance of the present sensors with those reported in literature. Sensing electrode
Operating temperature (°C)
CNH3, ΔVNH3
CNO2, ΔVNO2
ΔVNO2/ΔVNH3
Highest sensitivity (mV/decade)
Reference
CoFe2O4 Bi0.95Ni0.05VO3.975-Ag In2O3 SnO2-Au TiO2@WO3 V2O5–WO3–TiO2/Au CeVO4 CoWO4 CdV2O7 Ni3V2O8 ANO1.05
450 550 550 650 450 550 500 600 650 650 450
320ppm, ≈80 mV 300 ppm, ≈70 mV 100ppm, ≈42 mV 100ppm, ≈64 mV 300 ppm, ≈130 mV 100 ppm, ≈80 mV 300 ppm, 40 mV 100 ppm, 18 mV 100ppm, ≈66 mV 200ppm, ≈89 mV 300 ppm, 103 mV
320ppm, ≈15 mV 300 ppm, ≈3 mV 100ppm, ≈16 mV 100ppm, ≈13 mV 300 ppm, ≈70 mV 90 ppm, ≈20 mV 300 ppm, 6 mV 100 ppm, 8 mV 100ppm, ≈6 mV 200ppm, ≈9 mV 450 ppm, 10 mV
≈19% ≈4% ≈38% ≈20% ≈54% ≈25% 15% 44% ≈9% ≈10% 10%
55 −50 −59 −57 75 −75 −55 −51 −66 −96 −91
[3] [17] [29] [30] [31] [9] [15] [32] [13] [14] This work
5
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Fig. 7. (a) Comparison of the sensing property of the sensors using ANO-SE and ANO1.05-SE to 300 ppm NH3 in the presence of CH4, H2, CO2, NO or NO2 at 450 °C; (b) NO2-TPD curves of the ANO-SE and ANO1.05-SE; (c) Comparison of Nyquist plots of the sensors based on ANO-SE and ANO1.05-SE in 300 ppm NH3 or 300 ppm NH3 and 450 ppm NO2 with air balanced at 450 °C.
Fig. 6. (a) Transient response-recovery curves at 450 °C for the sensors based on ANO1.02-SE, ANO1.05-SE and ANO1.1-SE; (b) The dependence of ΔV values on the logarithm of NH3 concentrations for the sensors based on ANO1.02-SE, ANO1.05-SE and ANO1.1-SE; (c) Tafel plots measured in 200 ppm NH3 for the sensors using ANO-SE, ANO1.02-SE, ANO1.05-SE and ANO1.1-SE at 450 °C.
measurements were conducted for the sensing performance testing of the sensor based on ANO1.05-SE to NH3 under different concentrations of O2 and RH. The standard deviations used to define the error bars are found to be within ± 1.5%. Long-term stability is another important indicator to evaluate sensing performance of the sensor. Therefore the used sensor based on ANO1.05-SE after stored for 2 weeks and 4 weeks were tested. Fig. 9 presents response signals of the sensor using ANO1.05-SE to 25–400 ppm NH3 at 450 °C. The ΔV of the sensor after stored for 2 weeks conforms well to the original ΔV at any concentration. Only a slight degradation in ΔV under high NH3 concentrations is observed for the sensor stored for 4 weeks, indicating excellent long-term stability of the sensor. The mixed-potential type NH3 sensor can be expressed a cell as follows [35].
has almost no effect on the sensing performance to NH3 of the sensor. In addition, the polarization curves of the sensor based on ANO1.05-SE in background atmosphere and 300 ppm NH3/background atmosphere were recorded with different O2 concentrations in background atmosphere, as shown in Fig. 8b. It can be seen that the potential values at the intersections of the polarization curves obtained under different O2 concentrations are very close, which is consistent with the results obtained from the transient response curves (Fig. 8a). Fig. 8c shows the transient response curves of the sensor attached ANO1.05-SE to 300 ppm NH3 under different RH. The responding values of the sensor in RH range of 0–95% show a relatively slight attenuation, which means the change of RH has almost no effect on the sensing performance to NH3 of the sensor too. Three times independent 6
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Fig. 9. ΔV values of the sensor based on ANO1.05-SE changing with time at 450 °C in experiment period of 4 weeks for a given NH3 concentration.
at the TPBs [38]. Therefore, enhancing the adsorption capacity of the SE to NH3 or accelerating the decomposition rate of NH3 at the anode is beneficial to enhance the response signals of the sensor. According to the equilibrium reaction on the two electrodes, the calculation equation of the mixed potential for the NH3 sensor can be expressed as [39]:
Emix = E0 +B∙ (ZOx ∙lnc NH3 − ZRed ∙lncO2)
(3)
Emix is the value of mixed potential of the sensor. ZOx and ZRed are reaction orders of the cathode and anode reactions. c NH3 and cO2 are concentrations of NH3 and O2 in sample gas, respectively. E0 and B are constant and their values are only related to temperature. Therefore, when the temperature and oxygen concentration are fixed, the mixed potential values of the sensor linearly depend on the logarithm of NH3 concentrations, which is in agreement with experiment result. In order to experimentally verify the mechanism of mixed-potential type sensor, the Tafel plots of the sensor based on ANO1.05-SE toward 0–300 ppm NH3 were measured at 450 °C in the potential range of 300 to −300 mV, as shown in Fig. 10. Because the catalytic decomposition of NH3 at the TPBs is an oxidation reaction, the anode current increases gradually when the NH3 concentration is increased from 0 to 300 ppm. Meanwhile, the exchange current density increases as the NH3 concentration increasing. The estimated mixed-potential can be directly obtained from the Tafel plots. Table 2 lists the corresponding estimated potential values from Tafel plots and observed potential values from transient response-recovery curves. For a fixed concentration of NH3, the potential value corresponding to the minimal current values of
Fig. 8. (a) Response–recovery curves of the sensor based on ANO1.05-SE to 300 ppm NH3 with 5%–21% O2 at 450 °C; (b) Polarization curves of the sensor attached with ANO1.05-SE recorded in 300 ppm NH3 and 5%, 10%, 15% or 21% O2;(c) Response–recovery curves of the sensor based on ANO1.05-SE to 300 ppm NH3 under 0%–95% RH at 450 °C.
NH3, O2, Pt, ANO | O2− conductor | Pt, O2, NH3 The basic idea of mixed potential formation for NH3 sensor can be deduced from two electrochemical reactions (Eqs. (1) and (2)) occurred at the TPBs of the electrodes [36]. Cathode: 1/2O2 + 2e− → O2− Anode: 2/3NH3 + O
2−
→ 1/3N2 + H2O + 2e
(1) −
(2)
When the cathodic (1) and anodic (2) reactions arrive at a dynamic equilibrium in which the current flow induced by the reactions becomes zero, the anomalous potential of the SE (deviating from the Nernstian equation) is called the mixed potential [37]. The values of mixed potential are related to the oxidation rate of NH3 on the anode which involves the diffusion, adsorption and catalytic decomposition of NH3
Fig. 10. Tafel plots measured in 0–300 ppm NH3 with air balance for the sensor using ANO1.05-SE at 450 °C. 7
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Table 2 Observed potential values and estimated potential values from polarization curves and Tafel plots of the sensor based on ANO1.05-SE toward 0–300 ppm NH3 at 450 °C. NH3 concentration/ppm
0
50
100
200
300
Observed potential (mV) Estimated potential from polarization curves (mV) Estimated potential from Tafel plots (mV)
−2 −4
−27 −27
−56 −56
−89 −88
−101 −102
−4
−30
−55
−87
−105
[10] [11]
[12]
[13]
[14]
cathode and anode reactions is closed to the measured value, which indicates the mixed potential is derived from the dynamic equilibrium of the Reactions (1) and (2). The mixed-potentials also can be estimated from the intersection of the polarization curves measured in air and NH3+air, which are in close proximity to the estimated values from Tafel plots and the observed potential values. Based on above results, it can be confirmed that the present sensor using ANO1.05-SE follows mixed-potential mechanism.
[15]
[16] [17]
[18]
4. Conclusion [19]
We develop a novel mixed-potential type NH3 sensor based on Ag nanoparticles decorated ANO-SE synthesized by one-step high temperature demixing method. Highly dispersed Ag nanoparticles effectively improve the electro-catalytic activity of the ANO-SE to NH3. Compared with ANO-SE, the sensor based on ANO/Ag-SE exhibits larger response signal, higher sensitivity (increasing from −40.75 mV/ decade to −91.21 mV/decade) and lower detection limits to NH3. Meanwhile, the sensor can work at lower temperature of 400 °C with sensitivity of −53.73 mV/decade. Due to the presence of the second phase Ag particles, the adsorption of NO2 on the SE is suppressed leading to the enhancement of anti-interference capability of the sensor to NO2. Thus, the demixing method is an appropriate and effective approach to synthesize SEs of mixed-potential type sensors with excellent sensing performance. Meantime, the strategy can also be applied to prepare other sensing materials.
[20]
[21]
[22]
[23]
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[26]
[27]
Acknowledgements
[28]
The authors are grateful to financial support from Hebei Natural Science Fund for Distinguished Young Scholar (E2017209079) and National Natural Science Foundation of China (Nos. 51772097, 51672080, 51872090).
[29]
[30]
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Zhang, et al., Mixed-potential type NH3 sensor based on stabilized zirconia and Ni3V2O8 sensing electrode, Sens. Actuators B: Chem. 210 (2015) 795–802. L. Wang, W. Meng, Z. He, W. Meng, Y. Li, et al., Enhanced selective performance of mixed potential ammonia gas sensor by Au nanoparticles decorated CeVO4 sensing electrode, Sens. Actuators B: Chem. 272 (2018) 219–228. I. Lee, B. Jung, J. Park, C. Lee, J. Hwang, et al., Mixed potential NH3 sensor with LaCoO3 reference electrode, Sens. Actuators B: Chem. 176 (2013) 966–970. W. Meng, L. Wang, Y. Li, L. Dai, J. Zhu, et al., Enhanced sensing performance of mixed potential ammonia gas sensor based on Bi0.95Ni0.05VO3.975 by silver, Sens. Actuators B: Chem. 259 (2018) 668–676. H.-B. Fan, Q.-F. Ren, S.-L. Wang, Z. Jin, Y. Ding, Synthesis of the Ag/Ag3PO4/diatomite composites and their enhanced photocatalytic activity driven by visible light, J. Alloys Compd. 775 (2019) 845–852. J. Zhang, Z. Ma, Ag-Ag3VO4/AgIO3 composites with enhanced visible-light-driven catalytic activity, J. Colloid Interface Sci. 524 (2018) 16–24. Z.N. Kayani, M. Anwar, Z. Saddiqe, S. Riaz, S. Naseem, Biological and optical properties of sol-gel derived ZnO using different percentages of silver contents, Colloids Surf. B: Biointerfaces 171 (2018) 383–390. Y. Zhang, D. Deng, X. Zhu, S. Liu, Y. Zhu, et al., Electrospun bimetallic Au-Ag/ Co3O4 nanofibers for sensitive detection of hydrogen peroxide released from human cancer cells, Anal. Chim. Acta 1042 (2018) 20–28. H.B. Dias, M.I.B. Bernardi, V.S. Marangoni, A.C. de Abreu Bernardi, A.N. de Souza Rastelli, et al., Synthesis, characterization and application of Ag doped ZnO nanoparticles in a composite resin, Mater. Sci. Eng., C 96 (2019) 391–401. M. Wei, Y. Wen, D. Lei, Z. He, W. Ling, A novel electrochemical sensor for glucose detection based on Ag@ZIF-67 nanocomposite, Sens. Actuators B: Chem. 260 (2018) 852–860. J.I. Jung, D.D. Edwards, Kinetic demixing/decomposition of Ba0.5Sr0.5CoxFe1−xO3−δ(x=0.2 and 0.8), J. Eur. Ceram. Soc. 32 (2012) 3733–3743. B. Wang, B. Zydorczak, D. Poulidi, I.S. Metcalfe, K. Li, A further investigation of the kinetic demixing/decomposition of La0.6Sr0.4Co0.2Fe0.8O3−δ oxygen separation membranes, J. Membr. Sci. 369 (2011) 526–535. A. Rizea, G. Petotervas, C. Petot, M. Abrudeanu, M. Graham, et al., Transport properties of yttrium-doped zirconia-Influence of kinetic demixing, Solid State Ion. 177 (2007) 3417–3424. L. Dai, L. Ma, W. Meng, Y. Li, Z. He, et al., Impedancemetric NO2 sensor based on Pd doped perovskite oxide sensing electrode conjunction with phase angle response, Electrochim. Acta 265 (2018) 411–418. Y. Li, X. Li, Z. Tang, Z. Tang, J. Yu, et al., Hydrogen sensing of the mixed-potentialtype MnWO4/YSZ/Pt sensor, Sens. Actuators B: Chem. 206 (2015) 176–180. L. Dai, Y. Liu, W. Meng, G. Yang, H. Zhou, et al., Ammonia sensing characteristics of La10Si5MgO26-based sensors using In2O3 sensing electrode with different morphologies and CuO reference electrode, Sens. Actuators B: Chem. 228 (2016) 716–724. B. Wang, S. Yao, F. Liu, Y. Guan, X. Hao, et al., Fabrication of well-ordered porous array mounted with gold nanoparticles and enhanced sensing properties for mixed potential-type zirconia-based NH3 sensor, Sens. Actuators B: Chem. 243 (2017) 1083–1091. W. Meng, L. Dai, J. Zhu, Y. Li, W. Meng, et al., A novel mixed potential NH3 sensor based on TiO2@WO3 core–shell composite sensing electrode, Electrochim. Acta 193 (2016) 302–310. D. Quan, F. Yang, C. Yin, J. Li, S. Yang, et al., Ammonia sensors based on stabilized zirconia and CoWO4 sensing electrode, Solid State Ion. 225 (2012) 328–331. F.H. Garzon, R. Mukundan, E.L. Brosha, Solid-state mixed potential gas sensors: theory, experiments and challenges, Solid State Ion. (2000) 633–638. F. Liu, H. He, Y. Ding, C. Zhang, Effect of manganese substitution on the structure and activity of iron titanate catalyst for the selective catalytic reduction of NO with NH3, Appl. Catal. B 93 (2009) 3760–3769. X. Li, Y. Liu, L. Dai, Z. He, W. Meng, Y. Li, L. Wang, Mixed-potential type NH3 sensor based on La10Si5.5Al0.5O27 electrolyte and CuV2O6 sensing electrode, Sens. Actuators B: Chem. 294 (2019) 206–215. F. Liu, Y. Guan, M. Dai, H. Zhang, Y. Guan, et al., High performance mixed-potential type NO2 sensors based on three-dimensional TPB and Co3V2O8 sensing electrode, Sens. Actuators B: Chem. 216 (2015) 121–127. T. Ritter, J. Lattus, G. Hagen, R. Moos, A finite element model for mixed potential sensors, Sens. Actuators B: Chem. 287 (2019) 476–485. W. Chong, L. Guo, X. Ning, X. Kou, Y. Sun, et al., Enhanced nitrogen oxide sensing performance based on tin-doped tungsten oxide nanoplates by a hydrothermal method, J. Colloid Interface Sci. 512 (2017) 740–749. T. Ritter, G. Hagen, J. Lattus, R. 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Yuehua Li is now an Associate Professor in North China University of Science and Technology. Her research interest is in field of electrochemical sensors.
Xu Li is currently a master student majoring in material science and technology in North China University of Science and Technology. His research interest is in field of electrochemical sensors.
Ling Wang received his PhD in materials physics and chemistry from University of Science and Technology Beijing in 1998. During 2001–2003, he worked as Research Associate at University of Cambridge, UK. He is now a Professor in North China University of Science and Technology. His research interest is in field of the solid electrolyte and electrochemical sensors.
Lei Dai is now a Professor with research interest in electrochemical sensors in North China University of Science and Technology. Wei Meng is now an Associate Professor in North China University of Science and Technology. Her research interest is in field of the solid electrolyte and electrochemical sensors.
Zhangxing He received his PhD in applied chemistry from Central South University in 2014, China. He is now an Associate Professor in North China University of Science and Technology. His research interest is in field of electrochemical sensors.
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Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
A novel mixed-potential type NH3 sensor based on Ag nanoparticles decorated AgNbO3 sensing electrode synthesized by demixing method ⁎
Xu Li, Lei Dai , Wei Meng, Yuehua Li, Ling Wang, Zhangxing He
T
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College of Chemical Engineering, North China University of Science and Technology, Tangshan 063009, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: NH3 sensors Mixed potential AgNbO3/Ag sensing electrode Demixing
Solid-electrolyte type sensors based on composite sensing electrodes (SE) have attracted extensive attention due to their excellent sensitivity and anti-interference property. Herein, we prepare AgNbO3/Ag (ANO/Ag) composite SE for the solid-electrolyte type NH3 sensor using a facile demixing method. Compared with pristine ANOSE, the sensor based on ANO/Ag-SE exhibits larger response signal, higher sensitivity and lower detection limits. What’s more, the anti-interference capability of the sensor toward NOx is effectively enhanced. The effect of the molar ratio of Ag to Nb on the performance of the sensor is also investigated. When the molar ratio of Ag:Nb is 1.05:1, the sensor exhibits largest response signals and sensitivity (−91.2 mV/decade) to NH3. In the meantime, the response signal of the sensor to a fixed NH3 concentration almost keeps constant under different relative humidity (RH) and oxygen concentrations. Thus, the demixing method will provide a new approach for the fabrication of composites SEs with high performance for gas sensors.
1. Introduction Selective catalytic reduction (SCR) using urea reduce NOx has been considered as a promising method to reduce NOx emission [1]. In order to accurately control the injected amount of urea and prevent the overflow of NH3, a sensor with high sensitivity to NH3 and excellent anti-interference property to nitrogen oxides is required for real-time monitoring [2]. Among various types of gas sensors, the solid-electrolyte type sensors can accurately detect gas content in high temperature environments [3]. Up to now, noble metals (such as Au [4,5]) and metal oxide (such as TiO2 [6], In2O3 [7], WO3 [8], V2O5 [9–11], CoWO4 [12], Cd2V2O7 [13], Ni3V2O8 [14], CeVO4 [15]) have been investigated as the SE materials for solid-electrolyte type NH3 sensors. Except for the high sensitivity of the sensor to NH3, the excellent anti-interference of the sensor against NOx is essential, especially in the SCR system. In general, the selectivity of the sensor mainly depends on the intrinsic properties of the SE. In addition, the modification of the electrode is also effectively alternative method to improve the anti-interference of the NH3 sensor against NOx [16]. Elumalai et al. improved the anti-interference of the mixed-potential type NH3 sensor to the coexistent gases using NiO/Au composite SE due to the pre-catalytic oxidation of CO, NO, and hydrocarbons by the NiO layer [4]. Wang et al. fabricated a solid-electrolyte type NH3 sensor by decorating CeVO4-SE with Au, which successfully enhanced the sensor's anti-
⁎
interference ability to NO2 [15]. According to our previously research, loading Ag on the SE can not only enhance the sensitivity of the NH3 sensor, but also effectively improve anti-interference property to NOx, which is attributed to the depression of NO2 adsorption [17]. For preparing Ag-containing electrodes or catalysts, various methods have been investigated, including impregnation methods [17], photoreduction [18,19], sol–gel method [20], electrospinning [21], co-precipitation [22] and hydrothermal methods [23]. Recently, a demixing approach to prepare two-phase composite materials by one-step high temperature synthesis procedure has attracted extensive attention [24,25]. In particular, when the materials of composite oxide is mixed with a non-stoichiometric ratio, the excess component tends to be separated with the composite oxide to form metal oxide or metal nanoparticles due to the solid solution limit of the composite oxide during preparation process [26]. The formed nanoparticles prepared by demixing method have a high level of dispersion, which is beneficial for the release of catalytic activity of the SE. Hence, AgNbO3 perovskite loaded with Ag nanoparticles was prepared by demixing method and then used as the SE for mixed-potential type NH3 sensor. The morphology and structure of the samples were characterized. The performances of the NH3 sensors with AgNbO3/Ag composite SE was investigated in detail.
Corresponding authors. E-mail addresses: [email protected] (L. Dai), [email protected] (Z. He).
https://doi.org/10.1016/j.snb.2019.127146 Received 20 March 2019; Received in revised form 5 July 2019; Accepted 14 September 2019 Available online 17 September 2019 0925-4005/ © 2019 Elsevier B.V. All rights reserved.
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2. Experimental
the nanoparticles, respectively. Fig. 2a shows the XRD patterns of ANO, ANO1.02, ANO1.05 and ANO1.1 samples. The ANO sample is indexed well in orthorhombic phase with perovskite structure (JCPDS 00-0220471) without any impurity. When the molar ratio of Ag:Nb < 1, the other two diffraction peaks appear at 2θ of 38.3° and 44.5°, respectively, which are assigned to the metallic Ag (JCPDS 04-0783). XPS results indicate that the chemical state of Ag on the surface of the ANO sample is Ag1+, because of the appearance of Ag 3d5/2 peak at 368.1 eV and Ag 3d3/2 peak at 373.9 eV (Fig. 2b). With the molar ratio of Ag:Nb increasing, the positions of Ag 3d5/2 and Ag 3d3/2 peaks shift toward higher binding energy. Because metallic silver has a higher binding energy than Ag1+, the increasing in binding energy of the Ag 3d5/2 and Ag 3d3/2 peaks indicates the increasing of the Ag0 content. Three-dimensional network-like electrolyte skeleton can strengthen the bonding between the SE and substrate accompanied by effective enlargement of the three-phase boundaries (TPBs) length [8,28,29]. Therefore, we prepared a porous layer on one side of the YSZ electrolyte, and the corresponding morphologies of the porous layer are shown in Fig. 3a and b. The porous layer closely attaches to the dense layer and its thickness is about 30 μm. Fig. 3c and d show the surface and cross-sectional micrographs of the porous layer after introducing ANO1.05-SE. It can be seen that the particles of ANO1.05-SE are irregular spheres with size among 1–5 μm. The surface of porous layer is covered by an ANO1.05 layer with a thickness about 10 μm. It is noted that the ANO1.05 layer remains porous after sintered at 900 °C, which is beneficial for the diffusion of gases to TPBs [27]. Fig. 3e–i show the element distribution of the porous layer after introducing ANO1.05-SE. A considerable number of ANO1.05 particles have invaded into the porous layer.
2.1. Preparation of sensing materials The AgNbO3 based sensing materials were prepared by demixing method. Ag2O and Nb2O5 were mixed by a stoichiometric ratio of 1:1, 1.02:1, 1.05:1 and 1.1:1, respectively, which were then calcined at 900 °C for 5 h in a 30% O2/N2 atmosphere with a heating rate of 3 °C/ min to obtain ANO, ANO1.02, ANO1.05 and ANO1.1 sensing materials, respectively. 2.2. Assembly of the sensor The bilayer YSZ electrolyte substrate with Pt electrode was prepared by the procedure reported before [27]. The platinum paste was coated on the centers of both surfaces of the porous YSZ layer and dense YSZ electrolyte pellet with a radius of 3 mm, which was used as the electrical collector and reference electrode, respectively. The platinum wires as leads were adhered to the platinum paste and then sintered at 800 °C for 1 h. The sensing materials slurry was made by the method same as the preparation YSZ slurry. After screen printing the sensing materials slurry onto the YSZ porous layer for three times and then sintered at 900 °C for 3 h, the final sensor was obtained. 2.3. Characterization of the materials and measurement of electrochemical performances The phase composition was characterized by X-ray diffraction (XRD). The microstructure and element distribution of the samples was characterized by Field emission scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS), respectively. The sensing performance of the sensor was tested in a quartz tube held in a furnace. Two-electrode system including the sensing electrode and Pt reference electrode was introduced to monitor response signal (ΔV, defined as Vsample minus Vbaseline, where Vsample is the response potential in sample gas and Vbaseline is the potential in base gas before introducing NH3) of the sensor. Three-electrode system was introduced to measure the polarization curves at a constant scan-rate of 5 mV/s using an electrochemical work station (Solartron Analytical SI 1260/ 1287). In order to avoid the polarization and accurately measure the potential of the reference electrode, two isolated Pt-layers with a radius of 3 mm were painted on the dense YSZ electrolyte pellet side as the counter and reference electrode, respectively. The direction of the oxidation current is defined as negative direction in the workstation used for the electrochemical measurement. The NH3 source was achieved by diluting standard gas (8000 ppm NH3 with N2 balance) with air and the total gas flow was controlled at 200 cm3/min. The electrochemical impedance spectroscopy (EIS) was performed from 1 MHz to 0.1 Hz with signal amplitude of 5 mV. The NO2 temperature programmed desorption (NO2-TPD) was conducted at 40–600 °C with a heating rate of 10 °C/min under He atmosphere. During the long-term stability test, the transient response of the sensor was tested every two weeks at 450 °C for four weeks. Before every test, the sensor was exposed in air at 450 °C for 24 h.
3.2. Influence of Ag/Nb molar ratio on sensing performances Fig. 4a–d compares the sensing properties of the sensors based on ANO-SE and ANO1.05-SE to NH3 tested at different temperatures. At 400 °C, the sensor using the single ANO-SE shows unstable and irregular response to NH3. Meanwhile, the potential can’t return to original value of baseline after removal of NH3, which indicates this sensor is unsuitable apply at this temperature. In comparison, the sensor using ANO1.05-SE exhibits more stable response/recovery characteristic, and good linear relationship between ΔV and log (PNH3) at 400 °C. At the test temperature of 450 °C, both of the sensors show stable ΔV response to NH3. Compared with the sensor with single ANO-SE, the sensor using ANO1.05-SE has improved response ΔV values. The sensitivity is increased from −40.75 mV/decade to −91.21 mV/decade and the detection limit is lowered from 100 to 25 ppm. Meantime, the both sensors reach their highest sensitivity, respectively. It is noted that the response of the sensor based on ANO1.05-SE reaches the highest value at the beginning of the transients and then decreases rapidly when the sensor is exposed to 300 or 400 ppm NH3. The reasons for such behavior are not clear at the moment. However, this behavior should be closely related to the kinetics of the sensing process [28]. The formation of stable mixed potential depends on the formation of the equilibrium of the electrochemical reactions which involves the mass transfer, adsorption, desorption and charge transfer. The higher NH3 concentration is, the more difficult the formation of the equilibrium is. Therefore, there are some fluctuations of the response plateau particularly at the high NH3 concentrations. With the test temperature increasing from 450 °C to 550 °C, the response ΔV values and sensitivity of the both sensors are decreased. The sensor based on ANO1.05-SE always exhibits better sensing performance than the sensor using ANO-SE at a fixed temperature. The influence of operating temperature on the sensing performance is related to the NH3 diffusion process through the SE to the TPB and the electrochemical reaction happening at the TPB needs certain activation energy. The electro-chemical reaction does not gain enough activation energy at below 450 °C, resulting in the increasing of the sensitivity with the increasing of temperature from 400 to 450 °C.
3. Results and discussion 3.1. Characterization of the sensing materials Fig. 1a–d show the SEM images of the as-prepared ANO, ANO1.02, ANO1.05 and ANO1.1 particles. It can be seen that ANO particles present a smooth surface. For ANO1.02 sample, some nanoparticles appear on the surface of the ANO matrix. With the molar ratio of Ag:Nb further increasing, the number of the nanoparticles is increased accompanied by the growth of size. XRD and XPS were employed to determine the phase composition of 2
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Fig. 1. (a–d) SEM images of the ANO, ANO1.02, ANO1.05 and ANO1.1 sensing materials.
Fig. 2. (a) XRD patterns of the ANO, ANO1.02, ANO1.05 and ANO1.1 sensing materials (b) XPS spectra of Ag 3d of the ANO, ANO1.02, ANO1.05 and ANO1.1 sensing materials.
of three arcs. The two arcs at high frequency represent the impedance of electrolyte and overlap each other. The arc at the low-frequency corresponds to the electrochemical reaction process at the TPBs [15]. Compared with the sensor attached with single ANO-SE, the resistance value at the intersection of the arc with the real axis is decreased dramatically for the sensor with ANO1.05-SE, indicating the electrochemical catalytic activity toward NH3 for the sensor using ANO1.05-SE is the higher [15]. On one hand, Ag nanoparticles have good catalytic activity which directly enhances the electrocatalytic activity of ANO1.05-SE. On the other hand, Ag nanoparticles can increase the electronic conductivity of the SE beneficial for electron transfer. As a result, the sensing performance of the sensor is improved. In order to investigate the effect of the molar ratio of Ag:Nb in SE on sensing performances of the sensor, the ΔV responses of the sensors
However, further increasing the temperature over 450 °C, the amount of NH3 adsorbed on the SE will decrease and the effect of heterogeneous catalysis reaction may increase at high temperature, resulting in degradation in sensing performance. Table 1 presents the sensing performance of the mixed-potential type NH3 sensors reported in other literatures. The sensor using ANO1.05-SE exhibits comparable or even higher sensitivity to NH3 compared to the sensors based on others SEs. In order to confirm the mechanism of the improving sensing performance of the sensor after loading Ag, EIS was conducted to investigate the difference in electro-catalytic activity of the sensors. Complex impedance of the sensors attached with ANO-SE or ANO1.05SE were performed in 300 ppm NH3/air at 450 °C. Corresponding results are shown in Fig. 5. The Nyquist plots of the sensors are composed 3
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Fig. 3. (a, b) SEM images of surface and cross-section of LSAO porous layer (c, d) typical morphology of surface and cross-section of LSAO porous layer after loading ANO based SE (e–i) corresponding EDS mapping of the cross-section SEM image.
the i0 value is increased from 0.030 μA/cm2 to 0.249 μA/cm2, indicating the improvement of the electrocatalytic activity. With further increasing the Ag/Nb molar ratio to 1.1, the i0 value is decreased to 0.132 μA/cm2.
with different SEs to NH3 in the NH3 concentration range of 25–400 ppm were tested at 450 °C, and the corresponding transient response curves of the sensors are shown in Fig. 6a. Upon injecting of NH3 into the testing system, the sensor using ANO1.02-SE exhibits negative ΔV response which then is recovered to the original value when NH3 is exhaustion. The ΔV response is enhanced with the increasing of NH3 concentration. When the molar ratio of Ag:Nb is 1.05:1, the sensor exhibits larger ΔV response and lower detection limit (25 ppm) compared with the sensor using ANO1.02-SE, which should be attributed to the increasing of Ag nanoparticles on the surface of ANO-SE. Further increasing the molar ratio of Ag:Nb to 1.1:1, the ΔV response values of the sensor fall down due to the dramatically growing of the Ag nanoparticles. Fig. 6b shows the dependence of ΔV values on the logarithm of NH3 concentrations for the sensors based on ANO1.02SE, ANO1.05-SE and ANO1.1-SE. All the sensors exhibit good linear dependence relationship between the ΔVs and log(PNH3). It should be noted that the sensor attached with ANO1.05-SE exhibits the highest sensitivity among the three sensors. In order to investigate the influence of Ag/Nb molar ratio on the electrocatalytic activity of sensing materials to NH3, the Tafel plots of three sensors using Ag1.02NbO3-SE, Ag1.05NbO3-SE and Ag1.1NbO3-SE were measured in 200 ppm NH3 at 450 °C, as shown in Fig. 6c. It can be seen that the Tafel plot shifts upward as the Ag/Nb molar ratio increases from 1 to 1.05, but shifts downward with further increasing the Ag/Nb molar ratio to 1.1. The exchange current density (i0) can be obtained from the extrapolation of both the anode and cathode curves to the current axis, which presents the electrocatalytic activity of the electrode [33]. When the Ag/Nb molar ratio is increased from 1 to 1.05,
3.3. The selectivity and stability of the sensor There usually are some coexistent gases in the environment where the NH3 sensors are applied. Therefore, the anti-interference properties of the sensors based on ANO-SE and ANO1.05-SE toward the coexistent gases were tested. Fig. 7a presents the response values of the two sensors to 300 ppm NH3 in the presence of other gas at 450 °C. The two sensors exhibit excellent anti-interference properties to CH4, H2 and CO2. However, for the sensor using ANO-SE, upon injection of 450 ppm NO or NO2 to sample gas, the ΔVs are reduced by 44% and 85%, respectively. In contrast, for the sensor using ANO1.05-SE, the reduction percentages of ΔVs only are 4% and 9%, indicating the cross-sensitivity of the sensor with ANO1.05-SE to nitrogen oxides are effectively suppressed. Compared the ΔVNO2/ΔVNH3 values of the sensors reported in literatures (Table1), it can be concluded that the present sensor using ANO1.05-SE has an excellent anti-interference property to NO2. NO2-TPD and AC impedance analysis were employed to investigate the reason for the improved selectivity of ANO1.05-SE. The NO2-TPD analysis results of the ANO and ANO1.05-SE samples are shown in Fig. 7b. For ANO sample, there are three peaks located at 245, 314 and 445 °C with large area, which is attributed to the decomposition of bridging nitrate species and bidentate nitrate species [34]. In comparison, the curve of ANO1.05 sample only has a small peak at 275 °C with 4
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Fig. 4. Transient response-recovery curves and sensitivities of the sensors based on ANO-SE and ANO1.05-SE at 400 °C (a), 450 °C (b), 500 °C (c) and 550 °C (d).
much smaller area, which means the decreasing of formed bidentate nitrate species with higher chemical adsorption bond energy. Fig. 7c shows the Nyquist plots of the sensors exposed to 300 ppm NH3 or 300 ppm NH3 +450 ppm NO2 with air balance at 450 °C. For the sensor attached ANO-SE, the arc at low frequencies representing the electrochemical processes at the TPB is significantly decreased after 450 ppm NO2 injected in the 300 ppm NH3 sample gas, which means that the electrochemical decomposition of NO2 occurs at the TPB. In contrast, for the sensor using ANO1.05-SE, the presence of NO2 in the sample gas has no observable influence on the impedance of electrochemical processes at the TPB, indicating that NO2 does not take part in electrocatalytic reaction. Owing to the depression of adsorption and electrocatalytic activity of ANO1.05-SE to NO2, the NH3 selectivity of the sensor based on ANO1.05-SE against NO2 is effectively improved. The effect of O2 concentrations and RH on sensor response property should be considered due to changeable O2 concentrations and RH in sensor practical operation atmosphere. Fig. 8a shows the transient response of the sensor attached ANO1.05-SE to 300 ppm NH3 under different O2 concentrations. O2 concentration varying from 5 to 21 vol%
Fig. 5. Nyquist plots of the sensors based on ANO-SE and ANO1.05-SE in 300 ppm NH3 with air balanced at 450 °C.
Table 1 Comparison of the sensing performance of the present sensors with those reported in literature. Sensing electrode
Operating temperature (°C)
CNH3, ΔVNH3
CNO2, ΔVNO2
ΔVNO2/ΔVNH3
Highest sensitivity (mV/decade)
Reference
CoFe2O4 Bi0.95Ni0.05VO3.975-Ag In2O3 SnO2-Au TiO2@WO3 V2O5–WO3–TiO2/Au CeVO4 CoWO4 CdV2O7 Ni3V2O8 ANO1.05
450 550 550 650 450 550 500 600 650 650 450
320ppm, ≈80 mV 300 ppm, ≈70 mV 100ppm, ≈42 mV 100ppm, ≈64 mV 300 ppm, ≈130 mV 100 ppm, ≈80 mV 300 ppm, 40 mV 100 ppm, 18 mV 100ppm, ≈66 mV 200ppm, ≈89 mV 300 ppm, 103 mV
320ppm, ≈15 mV 300 ppm, ≈3 mV 100ppm, ≈16 mV 100ppm, ≈13 mV 300 ppm, ≈70 mV 90 ppm, ≈20 mV 300 ppm, 6 mV 100 ppm, 8 mV 100ppm, ≈6 mV 200ppm, ≈9 mV 450 ppm, 10 mV
≈19% ≈4% ≈38% ≈20% ≈54% ≈25% 15% 44% ≈9% ≈10% 10%
55 −50 −59 −57 75 −75 −55 −51 −66 −96 −91
[3] [17] [29] [30] [31] [9] [15] [32] [13] [14] This work
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Fig. 7. (a) Comparison of the sensing property of the sensors using ANO-SE and ANO1.05-SE to 300 ppm NH3 in the presence of CH4, H2, CO2, NO or NO2 at 450 °C; (b) NO2-TPD curves of the ANO-SE and ANO1.05-SE; (c) Comparison of Nyquist plots of the sensors based on ANO-SE and ANO1.05-SE in 300 ppm NH3 or 300 ppm NH3 and 450 ppm NO2 with air balanced at 450 °C.
Fig. 6. (a) Transient response-recovery curves at 450 °C for the sensors based on ANO1.02-SE, ANO1.05-SE and ANO1.1-SE; (b) The dependence of ΔV values on the logarithm of NH3 concentrations for the sensors based on ANO1.02-SE, ANO1.05-SE and ANO1.1-SE; (c) Tafel plots measured in 200 ppm NH3 for the sensors using ANO-SE, ANO1.02-SE, ANO1.05-SE and ANO1.1-SE at 450 °C.
measurements were conducted for the sensing performance testing of the sensor based on ANO1.05-SE to NH3 under different concentrations of O2 and RH. The standard deviations used to define the error bars are found to be within ± 1.5%. Long-term stability is another important indicator to evaluate sensing performance of the sensor. Therefore the used sensor based on ANO1.05-SE after stored for 2 weeks and 4 weeks were tested. Fig. 9 presents response signals of the sensor using ANO1.05-SE to 25–400 ppm NH3 at 450 °C. The ΔV of the sensor after stored for 2 weeks conforms well to the original ΔV at any concentration. Only a slight degradation in ΔV under high NH3 concentrations is observed for the sensor stored for 4 weeks, indicating excellent long-term stability of the sensor. The mixed-potential type NH3 sensor can be expressed a cell as follows [35].
has almost no effect on the sensing performance to NH3 of the sensor. In addition, the polarization curves of the sensor based on ANO1.05-SE in background atmosphere and 300 ppm NH3/background atmosphere were recorded with different O2 concentrations in background atmosphere, as shown in Fig. 8b. It can be seen that the potential values at the intersections of the polarization curves obtained under different O2 concentrations are very close, which is consistent with the results obtained from the transient response curves (Fig. 8a). Fig. 8c shows the transient response curves of the sensor attached ANO1.05-SE to 300 ppm NH3 under different RH. The responding values of the sensor in RH range of 0–95% show a relatively slight attenuation, which means the change of RH has almost no effect on the sensing performance to NH3 of the sensor too. Three times independent 6
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Fig. 9. ΔV values of the sensor based on ANO1.05-SE changing with time at 450 °C in experiment period of 4 weeks for a given NH3 concentration.
at the TPBs [38]. Therefore, enhancing the adsorption capacity of the SE to NH3 or accelerating the decomposition rate of NH3 at the anode is beneficial to enhance the response signals of the sensor. According to the equilibrium reaction on the two electrodes, the calculation equation of the mixed potential for the NH3 sensor can be expressed as [39]:
Emix = E0 +B∙ (ZOx ∙lnc NH3 − ZRed ∙lncO2)
(3)
Emix is the value of mixed potential of the sensor. ZOx and ZRed are reaction orders of the cathode and anode reactions. c NH3 and cO2 are concentrations of NH3 and O2 in sample gas, respectively. E0 and B are constant and their values are only related to temperature. Therefore, when the temperature and oxygen concentration are fixed, the mixed potential values of the sensor linearly depend on the logarithm of NH3 concentrations, which is in agreement with experiment result. In order to experimentally verify the mechanism of mixed-potential type sensor, the Tafel plots of the sensor based on ANO1.05-SE toward 0–300 ppm NH3 were measured at 450 °C in the potential range of 300 to −300 mV, as shown in Fig. 10. Because the catalytic decomposition of NH3 at the TPBs is an oxidation reaction, the anode current increases gradually when the NH3 concentration is increased from 0 to 300 ppm. Meanwhile, the exchange current density increases as the NH3 concentration increasing. The estimated mixed-potential can be directly obtained from the Tafel plots. Table 2 lists the corresponding estimated potential values from Tafel plots and observed potential values from transient response-recovery curves. For a fixed concentration of NH3, the potential value corresponding to the minimal current values of
Fig. 8. (a) Response–recovery curves of the sensor based on ANO1.05-SE to 300 ppm NH3 with 5%–21% O2 at 450 °C; (b) Polarization curves of the sensor attached with ANO1.05-SE recorded in 300 ppm NH3 and 5%, 10%, 15% or 21% O2;(c) Response–recovery curves of the sensor based on ANO1.05-SE to 300 ppm NH3 under 0%–95% RH at 450 °C.
NH3, O2, Pt, ANO | O2− conductor | Pt, O2, NH3 The basic idea of mixed potential formation for NH3 sensor can be deduced from two electrochemical reactions (Eqs. (1) and (2)) occurred at the TPBs of the electrodes [36]. Cathode: 1/2O2 + 2e− → O2− Anode: 2/3NH3 + O
2−
→ 1/3N2 + H2O + 2e
(1) −
(2)
When the cathodic (1) and anodic (2) reactions arrive at a dynamic equilibrium in which the current flow induced by the reactions becomes zero, the anomalous potential of the SE (deviating from the Nernstian equation) is called the mixed potential [37]. The values of mixed potential are related to the oxidation rate of NH3 on the anode which involves the diffusion, adsorption and catalytic decomposition of NH3
Fig. 10. Tafel plots measured in 0–300 ppm NH3 with air balance for the sensor using ANO1.05-SE at 450 °C. 7
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Table 2 Observed potential values and estimated potential values from polarization curves and Tafel plots of the sensor based on ANO1.05-SE toward 0–300 ppm NH3 at 450 °C. NH3 concentration/ppm
0
50
100
200
300
Observed potential (mV) Estimated potential from polarization curves (mV) Estimated potential from Tafel plots (mV)
−2 −4
−27 −27
−56 −56
−89 −88
−101 −102
−4
−30
−55
−87
−105
[10] [11]
[12]
[13]
[14]
cathode and anode reactions is closed to the measured value, which indicates the mixed potential is derived from the dynamic equilibrium of the Reactions (1) and (2). The mixed-potentials also can be estimated from the intersection of the polarization curves measured in air and NH3+air, which are in close proximity to the estimated values from Tafel plots and the observed potential values. Based on above results, it can be confirmed that the present sensor using ANO1.05-SE follows mixed-potential mechanism.
[15]
[16] [17]
[18]
4. Conclusion [19]
We develop a novel mixed-potential type NH3 sensor based on Ag nanoparticles decorated ANO-SE synthesized by one-step high temperature demixing method. Highly dispersed Ag nanoparticles effectively improve the electro-catalytic activity of the ANO-SE to NH3. Compared with ANO-SE, the sensor based on ANO/Ag-SE exhibits larger response signal, higher sensitivity (increasing from −40.75 mV/ decade to −91.21 mV/decade) and lower detection limits to NH3. Meanwhile, the sensor can work at lower temperature of 400 °C with sensitivity of −53.73 mV/decade. Due to the presence of the second phase Ag particles, the adsorption of NO2 on the SE is suppressed leading to the enhancement of anti-interference capability of the sensor to NO2. Thus, the demixing method is an appropriate and effective approach to synthesize SEs of mixed-potential type sensors with excellent sensing performance. Meantime, the strategy can also be applied to prepare other sensing materials.
[20]
[21]
[22]
[23]
[24] [25]
[26]
[27]
Acknowledgements
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The authors are grateful to financial support from Hebei Natural Science Fund for Distinguished Young Scholar (E2017209079) and National Natural Science Foundation of China (Nos. 51772097, 51672080, 51872090).
[29]
[30]
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Yuehua Li is now an Associate Professor in North China University of Science and Technology. Her research interest is in field of electrochemical sensors.
Xu Li is currently a master student majoring in material science and technology in North China University of Science and Technology. His research interest is in field of electrochemical sensors.
Ling Wang received his PhD in materials physics and chemistry from University of Science and Technology Beijing in 1998. During 2001–2003, he worked as Research Associate at University of Cambridge, UK. He is now a Professor in North China University of Science and Technology. His research interest is in field of the solid electrolyte and electrochemical sensors.
Lei Dai is now a Professor with research interest in electrochemical sensors in North China University of Science and Technology. Wei Meng is now an Associate Professor in North China University of Science and Technology. Her research interest is in field of the solid electrolyte and electrochemical sensors.
Zhangxing He received his PhD in applied chemistry from Central South University in 2014, China. He is now an Associate Professor in North China University of Science and Technology. His research interest is in field of electrochemical sensors.
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