Mixed potential NH3 sensor based on Mg-doped lanthanum silicate oxyapatite

Sensors and Actuators B 224 (2016) 356–363

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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb

Mixed potential NH3 sensor based on Mg-doped lanthanum silicate oxyapatite Lei Dai a,b , Guixia Yang a , Huizhu Zhou a,∗ , Zhangxing He a , Yuehua Li a , Ling Wang a,b,∗∗ a b

College of Chemical Engineering, North China University of Science and Technology, Tangshan 063009, PR China Hebei Province Key Laboratory of Inorganic and Non-Metal Materials, Tangshan 063009, PR China

a r t i c l e

i n f o

Article history: Received 23 July 2015 Received in revised form 10 October 2015 Accepted 20 October 2015 Available online 23 October 2015 Keywords: NH3 sensor La10 Si5 MgO26 Sintering aid Mixed-potential Nano-structured CoWO4

a b s t r a c t A mixed-potential type NH3 sensor was fabricated using La10 Si5 MgO26 (LSMO) as electrolyte with both a dense layer and a porous layer and nano-structured CoWO4 as the sensing electrode. The dense LSMO electrolyte was prepared by solid state reaction method at 1500 ◦ C via introducing Y2 O3 as the sintering aid. The nano-structured CoWO4 powders were synthesized by hydrothermal method and then screen printed on the porous LSMO layer as the sensing electrode. The sensor exhibited well response-recovery characteristics to NH3 at 400–700 ◦ C. The response and recovery time were 11 and 13 s when the NH3 concentration was changed between 200 and 300 ppm, respectively. Good linear correlations between the V values of the sensor and the logarithm of the NH3 concentrations for 30–300 ppm were obtained. Compared with the sensor without LSMO porous layer, the sensor with LSMO porous layer exhibited larger V values and higher sensitivity due to the enhanced TPB length. The mixed-potential-model of the sensor was identified by the polarization curves in different atmospheres. The sensitivity of the sensor decreased with increasing the sintering temperature of the CoWO4 electrode, due to the growth of the CoWO4 particles. Furthermore, the present sensor also displayed small cross-sensitivities to H2 , CH4 and CO2 present in the gas mixture. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Selective catalyst reduction (SCR) systems using ammonia gas (NH3 ) as a reductant has been recognized as the most reliable solution to treat the NOx emissions from diesel engine vehicles and trucks [1,2]. In order to optimize the conversion rates of NOx and avoid the air pollution of excessive NH3 , an NH3 sensor is required to control the SCR system for maintaining NH3 slip at an acceptable level [3–5]. The non-Nernstian mixed potential type NH3 sensor based on solid electrolytes with two functional electrodes (sensing and reference electrodes) is considered to be appropriate choice for NH3 detection in the harsh environments. Up to now, the electrolytes used for NH3 sensors include yttria stabilized zirconia (YSZ) [6–10], (Al0.2 Zr0.8 )20/19 Nb(PO4 )3 [11] and Na1+x Zr2 Six P3−x O12 (NASICON) [12]. Among them, YSZ is the most widely used electrolyte for

∗ Corresponding author. Tel.: +86 315 2592170; fax: +86 315 2592170. ∗∗ Corresponding author at: North China University of Science and Technology, College of Chemical Engineering, Tangshan 063009, PR China. Tel.: +86 315 2592170; fax: +86 315 2592170. E-mail addresses: [email protected] (H. Zhou), [email protected] (L. Wang). http://dx.doi.org/10.1016/j.snb.2015.10.071 0925-4005/© 2015 Elsevier B.V. All rights reserved.

the non-Nernstian mixed potential type NH3 sensors due to its advanced merit of good chemical and mechanic stability. According to the mixed potential theory [8,9], when the sensor based on YSZ is exposed to NH3 in the presence of O2 , the electrochemical oxidation reaction of NH3 (Eq. (1)) and the reduction reaction of O2 (Eq. (2)) occur at the sensing and reference electrodes of the sensor, respectively. 2/3NH3 + O2− → 1/3N2 + H2 O + 2e− −

1/2O2 + 2e → O

2−

(1) (2)

When the rates of reactions (1) and (2) are equal, they arrive at a dynamic equilibrium, and an electric potential difference (V) is produced across two electrodes as a consequence of the coupling between the electrochemical oxidation and reduction reactions. Because the V significantly deviates from the Nernstian equation, the anomalous V is called mixed potential. The resulting V across the sensing electrode and reference electrode generally gives a logarithmic dependence on the NH3 concentration. For the mixed potential type NH3 sensor, one important role of the solid electrolyte is transferring of oxygen ion from the reference electrode to the sensing electrode where the oxidation reaction of NH3 (Eq. (1)) occurs. Therefore, it is significant to develop

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Intensity/a.u.

▲- La2SiO5

La10Si5MgO26 ▲

Fig. 1. Schematic structure of the sensor.

alternative electrolytes that can provide good oxygen ionic conductivity for the NH3 sensor. Among oxygen ionic conductors, apatite-type lanthanum silicates have drawn much attention due to their high conductivity in the intermediate temperature range (400–700 ◦ C), high oxygen ion transference number across a wide range of oxygen partial pressure, excellent long-term stability and relatively low processing cost [13–19]. In addition to the wide application in SOFC, the apatite-type lanthanum silicates also have been used as the electrolytes for electrochemical gas sensors. Takeda [20] et al prepared a concentration cell O2 sensor using apatite-type lanthanum silicate as electrolyte, which displayed excellent sensing characteristics in the operating temperature range of 300–700 ◦ C. Our group also presented the amperometric NO2 sensors based on Nb or Al doped lanthanum silicate electrolytes [21–23]. To our knowledge, there is no studies report on NH3 sensors using apatite-type lanthanum silicates as the electrolyte. Except for the electrolytes, much research also has been devoted to the sensing electrodes, which can selectively catalyze the oxidization reaction of NH3 . Various oxide materials, such as V2 O5 [6], Ni3 V2 O8 [7], tungstate [8], In2 O3 [9], NiO [24] and SiO2 /Au [25] have been exploited as the sensing electrodes in the past and showed excellent sensing performances to NH3 . Although CoWO4 has been used as the sensing electrode for YSZ-based NH3 sensors and showed good sensing performances

JCPDS 00-053-0291 20

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2θ/degree Fig. 2. XRD pattern of La10 Si5 MgO26 electrolyte sintered at 1500 ◦ C for 5 h.

at 700 ◦ C [8], further study is still needed. In this paper, a mixed potential type NH3 sensor was prepared, which was based on LSMO electrolyte with a bilayer structure (including both a dense layer and a porous layer) and nano-structured CoWO4 as the sensing electrode prepared by hydrothermal method. The sensing characteristics of the sensor were investigated at 400–700 ◦ C. 2. Experimental 2.1. Preparation and characterization of the sensor materials LSMO electrolyte was prepared by solid state reaction method. Y2 O3 was employed as a sintering aid to improve the sinterability and lower the sintering temperature. Analytically pure La2 O3 , SiO2 and MgO were mixed according to the stoichiometric ratio of La10 Si5 MgO26 , ball-milled with ethanol for 24 h and then calcined at 1300 ◦ C for 6 h. The calcined powders were mixed with 6 wt%

Fig. 3. SEM images of La10 Si5 MgO26 electrolyte sintered at 1500 ◦ C for 5 h: (A) surface and (B) cross-section without Y2 O3 aid, (C) surface and (D) cross-section with Y2 O3 aid.

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2.2. Measurement of sensing properties for the sensor Sensing performances of the NH3 sensor were tested in a gas flow apparatus with heating tube furnace in the temperature range of 400–700 ◦ C. The sample gases with different NH3 concentrations were prepared by diluting 8000 ppm NH3 parent gas by the base gas (air balance). The total flow rate of the sample gas was adjusted at 100 cm3 /min using mass-flow controllers. The V output of the sensor was monitored by the electrochemical work station. The polarization curves in the potential range of 0 to −100 mV were measured by means of potentiodynamic method at a constant scanrate of 5 mV/s in the base gas and the sample gas (100 or 200 ppm NH3 in the base gas). 3. Results and discussion 3.1. Characterization of the sensor materials The apatite-type lanthanum silicates always need high sintering temperatures (≥1650 ◦ C) to achieve full density because of its high refractory nature [26,27]. In this paper, 6 wt% Y2 O3 was employed as the sintering aid to improve the sinterability and lower the sintering temperature of LSMO. Fig. 2 showed the XRD pattern of LSMO electrolyte sintered at 1500 ◦ C for 5 h. It was seen that the main diffraction peaks of LSMO sample corresponded to the

120

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A

650 C o

600 C

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o

550 C

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15

16

Z'/Ω 4

B 3

-1

Y2 O3 and 1 wt% polyvinyl butyral (PVB) by ball-milling for 12 h in ethanol. After drying, the mixed powders were uniaxially pressed into a cylindrical flat mold utilizing fitted stainless steel disks with a pressure of 20 MPa to obtain disk-shaped samples (13 mm in diameter and 1 mm in height), which were then sintered at 1500 ◦ C for 5 h to form the dense LSMO pellet. The LSMO porous layer on a side of above-mentioned dense LSMO electrolyte pellet was prepared by screen printing technique and then sintered at 1450 ◦ C for 5 h. The ink used in screen-printing was obtained by dispersing LSMO powder with 6 wt% sintering aid (Y2 O3 ) and 30 wt% pore-forming agent (graphite powder) into an organic dispersant (terpineol, 94 wt% and ethylcellulose, 6 wt%). CoWO4 powders were prepared by hydrothermal method via the following procedures: Co(CH3 COO)2 and Na2 WO4 were mixed in deionized water under vigorous stirring to give a 0.4 mol/L homogeneous precursor solution, which was then transferred into a Teflon-lined stainless steel autoclave, sealed and maintained at 160 ◦ C for 24 h. After the reaction was completed, the autoclave was cooled to the room temperature naturally. The products were harvested by pursuing centrifugation, washed with distilled water and ethanol three times to remove the unexpected ions, and then dried at 60 ◦ C in air for 1 h. The as-prepared CoWO4 powders were mixed with the organic dispersant (terpineol, 90 wt% and ethylcellulose, 10 wt%) according to the mass ratio of 3:7 to form the CoWO4 ink, which was screenprinted on the porous LSMO layer and then calcined at 800 ◦ C for 3 h to form the sensing electrode. Pt-layers with an area of 1 cm2 were painted on both the sensing electrode side and the back-side of the electrolyte pellet as electrical collector. Pt wires were used to make contact with two electrodes. The schematic structure of the sensor was shown in Fig. 1. The phase composition of the samples was identified by Xray diffraction (XRD, Rigaku, D/MAX2500PC) analysis under Cu-K␣ radiation with the incidence beam angle of 2◦ in the range of 10–90◦ . The microstructure of the samples was investigated by field emission scanning electron microscopy (SEM, Hitachi, S-4800). The conductivities were measured by ac impedance spectroscopy over the frequency range 0.01 Hz to 1 MHz with signal amplitude of 5 mV using the Zahner IM6e electrochemical work station.

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1 0 -1 -2 -3 -4 9

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10000/T( K ) Fig. 4. The electrical properties of La10 Si5 MgO26 electrolyte sintered at 1500 ◦ C for 5 h with Y2 O3 aid: (A) impedance spectra, (B) the dependence of total conductivity on the temperature varying from 400 to 800 ◦ C in air (inset: oxygen partial pressure dependence of the total electrical conductivity at 600 ◦ C).

standard XRD pattern of La10 Si6 O27 (JCPDS 00-053-0291) with a small amount of second phase La2 SiO5 which commonly appeared in the apatite-type lanthanum silicates [16,17]. Meanwhile, no other impurity was detected, indicating that the added Y2 O3 did not change the phase composition. Fig. 3 showed the SEM micrographs of surfaces and fractured cross-sections of the LSMO samples sintered at 1500 ◦ C for 5 h. Compared with the LSMO samples without sintering aid (Fig. 3A and B), the high densified ceramic samples were obtained at 1500 ◦ C with the help of Y2 O3 as sintering aid (Fig. 3C and D). The AC impedance spectra of sintered specimen were measured at different temperatures to calculate the conductivities of LSMO. The representative impedance spectra measured in the temperature range of 500–650 ◦ C in air were shown in Fig. 4A. According to the commonly accepted “brick layer” model, the first (high frequency) semicircle corresponds to the bulk response, and the second semicircle is attributed to the grain-boundary resistance. The tail is related to the processes occurring at the electrode interfaces. The total conductivities were 4.30 × 10−4 , 9.70 × 10−4 , 2.00 × 10−3 and 3.45 × 10−3 S/cm at 500, 550, 600 and 650 ◦ C, respectively. The relationship between the conductivities of LSMO electrolyte in air and temperatures was established (Fig. 4B), which obeyed the Arrhenius equation ( = (A/T)exp(−Ea /KT)). The activation energy of LSMO electrolyte for ionic conduction was 0.87 eV,

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Fig. 6. SEM images of the fractured cross-section of the bilayer LSMO electrolyte (A) and CoWO4 particles after screen-printed and sintered at 800 ◦ C for 3 h (B).

Fig. 5. XRD pattern (A) and SEM image (B) of CoWO4 powders prepared by the hydrothermal method.

indicating the more excellent electrical properties of LSMO than those of YSZ in the test temperature range. Meanwhile, there was almost no change of the total conductivities at 600 ◦ C when the O2 concentration changed from 5 × 10−3 to 1.0 atm, indicating that the LSMO was pure oxygen ion conductor (inset, Fig. 4B). The mentioned above experimental results showed that the LSMO samples with Y2 O3 aid showed not only high densified structure, but also excellent electrical properties, which can provide enough high ionic conductivity for NH3 sensor applications. Fig. 5 showed XRD pattern and SEM image of the CoWO4 powders prepared by the hydrothermal method. In Fig. 5A, the diffraction data were all in the perfect agreement with the standard pattern (JCPDS 00-15-0867), demonstrating that the pure CoWO4 phase with monoclinic structure was obtained. As shown in Fig. 5B, the spherical nano-structured CoWO4 particles were observed and the particle size was about 50–100 nm. However, based on Scherrer’s equation, the crystal grains size of CoWO4 estimated from XRD result was about 20 nm. The results showed that the particles in Fig. 5B were the agglomerates of CoWO4 crystals. The LSMO with a bilayer structure including both a dense layer and a porous layer was used as the electrolyte for the NH3 sensor, which could enhance the contact between the LSMO electrolyte and CoWO4 sensing electrode, as well as increasing the length of the NH3 gas/CoWO4 sensing electrode/LSMO electrolyte triple phase boundary (TPB). Fig. 6 showed the representative cross-sectional SEM micrographs of the bilayer LSMO electrolyte with CoWO4 screen-printed on the LSMO porous layer and sintered at 800 ◦ C

for 3 h. It was seen from Fig. 6A that the CoWO4 film with a thickness of 10 ␮m had been formed on the surface of the LSMO porous layer. Meanwhile, a certain amount of CoWO4 particles had entered into the LSMO porous layer. There was no observable change of the CoWO4 particles size before (Fig. 5B) and after (Fig. 6B) screenprinted and sintered at 800 ◦ C for 3 h. Fig. 7 showed the EDS mapping of the fractured cross-section of porous layer after CoWO4 screen-printed and sintered at 800 ◦ C for 3 h. It could be seen that the elements of CoWO4 were dispersed not only on LSMO porous layer surface, but also in the LSMO porous layer. Because the CoWO4 particles adhered to the LSMO porous layer with three-dimensional structure, the area of the TPB could be greatly enhanced, which probably meant the increasing number of gas reaction sites and then improving the sensitivity of the sensor. 3.2. Sensing performances of the sensor at different temperatures This sensor could be described by the following electrochemical cell in sample gas: (+)Air + NH3 , CoWO4 /LSMO/Pt, NH3 + Air (−). When the CoWO4 sensing electrode was exposed to NH3 in air atmosphere, a non-Nernstian potential was produced on the electrodes, because reactions (1) and (2) proceeded at an equal rate. The mixed potential of the sensor is a function of the NH3 concentration, as expressed in Eq. (3) [7–9]. Emix = A + B ln CNH3

(3)

here Emix and CNH3 represent the mixed potential and NH3 concentration, respectively. A and B are the constants. The B value is negative due to the electrochemical oxidation of NH3 at the sensing

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Fig. 7. EDS mapping of the fractured cross-section of the bilayer LSMO electrolyte after screen-printing CoWO4 and sintering at 800 ◦ C for 3 h.

electrode [6,28]. Therefore, the potentials are shifted in a negative direction during NH3 exposure. In order to determine the role of the LSMO porous layer for the sensing performance, the responses of the sensors with and without LSMO porous layer were examined by stepwise changing the NH3 concentrations with air as the base gas. Fig. 8A showed the V variation of the sensor with successive changes of NH3 concentration in the range of 30–300 ppm at 600 ◦ C. Before the introduction of NH3 gas into the gas stream, the measured difference between the potentials of CoWO4 and Pt electrodes resided at an equilibrium baseline near ∼0 mV. When NH3 gas was introduced into the gas stream, the negative deviation of the measured potential difference should be due to the presence of NH3 gas. The response of the sensor was found to be relatively stable, reproducible and fast at 600 ◦ C. For the sensor with the LSMO porous layer, the response time (t90+ ) defined as the time required by the sensor to attain 90% of the stable V was found to be approximately 11 s when the NH3 concentration was changed from 200 to 300 ppm, whereas the recovery time (t90− ) defined as the time required by the sensor to attain within 10% of the initial V value was found to be approximately 13 s when the NH3 concentration was decreased from 300 to 200 ppm. Fig. 8B showed the dependence of the sensor response (V) on the NH3 concentration for the sensor with and without

the LSMO porous layer at 600 ◦ C, respectively. The V values for two sensors were almost proportional to the logarithm of the NH3 concentrations, which agreed with the mixed potential mechanism that has been verified in the prior literatures [7,8]. Compared with the sensor without LSMO porous layer, the sensor with LSMO porous layer exhibited more negative response V values and much higher sensitivity, which was mostly probably due to the enhanced TPB length when the LSMO porous layer was used as the scaffold for the preparation of CoWO4 sensing electrode. The good response/recovery of the sensor with LSMO porous layer was also observed at 400, 500 and 700 ◦ C (shown in Fig. 9), while the negative deviations of the measured Vs were weaken gradually with decreasing the temperature. Fig. 10 showed the relationships between the measured Vs and NH3 concentrations at different temperatures. The sensitivities were −72.18, −51.14, −35.96 and −9.29 mV/decade at 400, 500, 600 and 700 ◦ C, respectively. The sensitivity decreased with increasing the temperature, which might be related to the gradually decreased adsorption of NH3 on the sensing electrode. Compared with the NH3 sensor reported in previous literatures, the present sensor displayed comparable or even better performances at 400–600 ◦ C [6,10,12]. Although the sensitivity of the sensor was relatively lower, well response-recovery characteristics and good linear correlations

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Fig. 9. V response and recovery transients to NH3 for the sensor at 400–700 ◦ C.

Fig. 8. (A) The sensor transient response to successive change of NH3 concentration and (B) Dependence of the sensor response (V) on the NH3 concentration for the sensor with and without LSMO porous layer at 600 ◦ C (Y represents V value; X represents logarithm of NH3 concentration).

between the V values and the logarithm of the NH3 concentrations were obtained at 700 ◦ C. In order to further testify the mixed-potential-model, the measurements of polarization (current–voltage) curves of the sensor were performed in the potential range of 0 to −100 mV at a scanrate of 5 mV/s. The cathodic polarization curves of the sensor were measured in air, while the anodic polarization curves were measured in 100 or 200 ppm NH3 sample gas, respectively. Fig. 11 showed the obtained original cathodic and the modified anodic polarization curves which were obtained by subtracting the current values in the base gas (air) from that in sample gas (100 or 200 ppm NH3 in air) [7]. The mixed-potential could be estimated from the intersection of the anodic and cathodic polarization curves. Table 1 listed the estimated and measured V values for the sensor. It was clearly seen that the estimated potential differences were very close to the experimentally observed V values, indicating that the sensor followed the mixed-potential model [7,29,30]. 3.3. Influence of the sintering temperature of CoWO4 sensing electrode The microstructure of the sensing electrode plays an important role to sensor performance, which is closely related with the sintering temperature. In this study, the CoWO4 sensing electrodes

Fig. 10. Dependence of the sensor response (V) on the NH3 concentration for the sensor at 400–700 ◦ C.

were sintered at 800 and 900 ◦ C for 3 h after screen-printed on the LSMO porous layer, respectively. And then the responses of the sensor were examined by stepwise change in the NH3 concentration from 30 ppm up to 300 ppm at 600 ◦ C, as shown in Fig. 12. The two sensors exhibited a rapid change in the V upon injecting NH3 and the subsequent recovery to an original level after removal of the NH3 . There was almost no baseline shift in the sensor signal, implying the good reversibility of the sensor. The particle size of the CoWO4 sintered at 900 ◦ C was of micron size and much larger than that sintered at 800 ◦ C (inset, Fig. 12), which would lead to the decrease of the catalysis activity and the TPB length for the anodic reaction (1). Fig. 13 showed the dependence of Vs on the NH3 concentrations at 600 ◦ C for the sensors with the CoWO4 sensing electrode sintered at 800 and 900 ◦ C, respectively. Compared with the sensor with CoWO4 sintered at 800 ◦ C, the sensor with CoWO4 prepared at 900 ◦ C displayed much lower V values and sensitivity (−9.19 mV/decade). 3.4. Selectivity to coexistent gases The cross-sensitivities of the sensor to other gases were examined at 600 ◦ C. As shown in Fig. 14, the positive or negative Vs were obtained according to the reducing and oxidation gases. However, the deviation directions of the measured V to CO2 (oxidation gas) was negative, which could be attributed to the higher activity of the reference electrode to CO2 than that of the sensing electrode

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Fig. 12. Response transients to different concentrations of NH3 at 600 ◦ C for the sensor with CoWO4 sensing electrode sintered at 800 and 900 ◦ C.

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Potential/mV Fig. 11. Polarization curves recorded in air, 100 and 200 ppm NH3 at 500 and 600 ◦ C for the sensor.

[31,32]. It was clearly seen from Fig. 14 that the response V value to 100 ppm NH3 was −17.6 mV at 600 ◦ C, while the response V values to 100 ppm H2 , CH4 and CO2 were less than −1.5 mV, indicating a low cross-sensitivity to these gases for the sensor. Meanwhile, it should be noted that the response V value of the sensor to NO2 was a relatively high and positive value, which was resulted from the equilibrium between the electron transfer rates of the NO2 reduction reaction (Eq. (4)) and oxygen ion oxidation reaction (Eq. (5)) when the sensor was exposed to both NO2 and O2 gases [33]. NO2 + 2e− = NO + O2− 2−

O

Fig. 13. Dependence of the sensor response (V) on the NH3 concentration at 600 ◦ C for the sensor with CoWO4 sensing electrode sintered at 800 and 900 ◦ C.

(4)

−

= 1/2O2 + 2e

(5)

The response V value of the sensor to 100 ppm NH3 changed from −17.6 to −9.76 mV due to the introduction of 100 ppm NO2 , as shown in inset of Fig. 14. So the anti-interference of NO2 to the sensor needs to be improved in the future work. Table 1 Comparison between the estimated and measured V for the sensor. Temperature (◦ C)

NH3 concentration (ppm)

V (mV) Estimated

Observed

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−17.58 −29.30

Fig. 14. The sensor response to 100 ppm NH3 , H2 , CH4 , CO2 or NO2 in air at 600 ◦ C (inset is a comparison of the sensor response to 100 ppm NH3 and 100 ppm NO2 + 100 ppm NH3 ).

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4. Conclusions We have successfully developed a mixed-potential type NH3 gas sensor based on Mg-doped lanthanum silicate oxyapatite solid electrolyte and nano-structured CoWO4 sensing electrode. The sinterability of the LSMO was effectively improved by the introduction of Y2 O3 sintering aid. The CoWO4 sensing electrode with the particle size of 50–100 nm was successfully prepared via hydrothermal synthesis method. The fabricated sensor showed fast response/recovery to NH3 and good reproducibility. The sensitivities were −72.18, −51.14, −35.96 and −9.29 mV/decade at 400, 500, 600 and 700 ◦ C, respectively. The porous layer of LSMO electrolyte obviously improved sensing performance of the sensor compared with the sensor without LSMO porous layer due to the enhanced TPB length. The sensor with the CoWO4 sintered at 800 ◦ C displayed better performances than that of 900 ◦ C, indicating that the microstructure of the sensing electrode played an important role on the sensitivity of the sensor. The sensor also showed a low cross-sensitivity to H2 , CH4 and CO2 except for NO2 . Acknowledgements The authors are grateful to financial support from National Natural Science Foundation of China (51272067, 51472073, 51201058), Iron-steel United Foundation of Hebei Province of China (E2014209009) and the Education Department of Hebei Province of China (Z2015140). References [1] B. Guan, Reggie Zhan, H. Lin, Z. Huang, Review of state of the art technologies of selective catalytic reduction of NOx from diesel engine exhaust, Appl. Therm. Eng. 66 (2014) 395–414. [2] J.H. Li, H.Z. Chang, L. Ma, J.M. Hao, R.T. Yang, Low-temperature selective catalytic reduction of NOx with NH3 over metal oxide and zeolite catalysts—a review, Catal. Today 175 (2011) 147–156. [3] M. Koebel, M. Elsener, M. Kleemann, Urea-SCR: a promising technique to reduce NOx emissions from automotive diesel engines, Catal. Today 59 (2000) 335–345. [4] P. Forzatti, Present status and perspectives in de-NOx SCR catalysis, Appl. Catal., A: Gen. 222 (2001) 221–236. [5] R. Moos, D. Schönauer-Kamin, Review: recent developments in the field of automotive exhaust gas ammonia sensing, Sens. Lett. 6 (2008) 821–825. [6] D. Schönauer-Kamin, M. Fleischer, R. Moos, Influence of the V2 O5 content of the catalyst layer of a non-Nernstian NH3 sensor, Solid State Ionics 262 (2014) 270–273. [7] F.M. Liu, R.Z. Sun, Y.H. Guan, X.Y. Cheng, H. Zhang, Y.Z. Guan, et al., Mixed-potential type NH3 sensor based on stabilized zirconia and Ni3 V2 O8 sensing electrode, Sens. Actuators, B: Chem. 210 (2015) 795–802. [8] Q. Diao, F.S. Yang, C.G. Yin, J.G. Li, S.Q. Yang, X.S. Liang, et al., Ammonia sensors based on stabilized zirconia and CoWO4 sensing electrode, Solid State Ionics 225 (2012) 328–331. [9] I. Lee, B. Jung, J. Park, C. Lee, J. Hwang, C.O. Park, Mixed potential NH3 sensor with LaCoO3 reference electrode, Sens. Actuators, B: Chem. 176 (2013) 966–970. [10] V.V. Plashnitsa, P. Elumalai, Y. Fujio, T. Kawaguchi, N. Miura, Spontaneous gradual accumulation of hexagonally-aligned nano-silica on gold nanoparticles embedded in stabilized zirconia: a pathway from catalytic to NH3-sensing performance, Nanoscale 3 (2011) 2286–2293. [11] T. Nagai, S. Tamura, N. Imanaka, Solid electrolyte type ammonia gas sensor based on trivalent aluminum ion conducting solids, Sens. Actuators, B: Chem. 147 (2010) 735–740. [12] X.S. Liang, G.Y. Lu, T.G. Zhong, F.M. Liu, B.F. Quan, New type of ammonia/toluene sensor combining NASICON with a couple of oxide electrodes, Sens. Actuators, B: Chem. 150 (2010) 355–359. [13] E.D. Wachsman, K.T. Lee, Lowering the temperature of solid oxide fuel cells, Science 334 (2011) 935–939. [14] P.R. Slater, J.E.H. Sansom, J.R. Tolchard, Development of apatite-type oxide ion conductors, Chem. Rec. 4 (2004) 373–384. [15] X.G. Cao, S.P. Jiang, Effect of Sr and Al or Fe co-doping on the sinterability and conductivity of lanthanum silicate oxyapatite electrolytes for solid oxide fuel cells, Int. J. Hydrogen Energy 39 (2014) 19093–19101.

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Biographies Lei Dai is now an Associate Professor with research interest in electrochemical sensors in North China University of Science and Technology. Guixia Yang is currently a master student majoring in material science and technology in North China University of Science and Technology. Her research interest is in field of electrochemical sensors. Huizhu Zhou 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 is now an Associate Professor in North China University of Science and Technology. His research interest is in field of electrochemical sensors. Yuehua Li is now an Associate Professor in North China University of Science and Technology. Her 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.