Solid electrolyte type nitrogen monoxide gas sensor operating at intermediate temperature region

Transcription

1 Sensors and Actuators B 108 (2005) Solid electrolyte type nitrogen monoxide gas sensor operating at intermediate temperature region Isao Hasegawa, Shinji Tamura, Nobuhito Imanaka Department of Applied Chemistry, Faculty of Engineering and Handai Frontier Research Center, Osaka University, 2-1 Yamadaoka, Suita, Osaka , Japan Received 12 July 2004; received in revised form 22 November 2004; accepted 29 November 2004 Available online 18 January 2005 Abstract A new solid electrolyte type nitrogen monoxide (NO) gas sensor which can operate in the intermediate temperature region was fabricated by the combination of trivalent aluminum cation conducting (Al 0.2 Zr 0.8 ) 20/19 Nb(PO 4 ) 3 and divalent oxide anion conducting yttria stabilized zirconia (YSZ) with LiNO 3 -doped (Gd 0.9 La 0.1 ) 2 O 3 as the sensing auxiliary electrode. The present sensor shows such a practical performance of a rapid, stable, continuous, and reproducible response as low as at 523 K, and the linear relationship, which obeys the Nernst theoretical relationship, was clearly observed between the sensor EMF output and the logarithm of the NO concentration. Since the present sensor using the LiNO 3 -doped (Gd 0.9 La 0.1 ) 2 O 3 auxiliary electrode shows such a high sensing performance, it is greatly expected to be a new type of the NO gas sensing device applicable in the intermediate temperature range of around 523 K Elsevier B.V. All rights reserved. Keywords: Nitrogen monoxide; Aluminum ion conductor; Rare earth oxides; Lithium nitrate 1. Introduction Nitrogen oxides (NO x ) exhausted from automobile engines, boilers, and so on, are one of air pollutant gas species. Recently, the emission of NO x gas steadily increases and the suppression of the NO x gas emission has been requested from the environmental conservation point of view. To achieve this purpose, it is essential to develop a rapid and exact NO x gas sensing apparatus and to monitor the NO x gas concentration at every emitting site. Some instrumental apparatuses based on IR absorption and chemical luminescence have been widely utilized for the measurement of NO x gas concentration. Although these instruments can exactly detect the NO x gas concentration, they are generally too expensive and large sized, and some pre-treatments are inevitable. For the reasons described above, they are not suitable for the in situ monitoring gas sensing tool. Such disadvantages make clear Corresponding author. Tel.: ; fax: address: imanaka@chem.eng.osaka-u.ac.jp (N. Imanaka). the importance of developing NO x gas sensors which can realize in situ monitoring. Up till now, various types of compact and solid-state NO x gas sensors have been proposed to overcome these disadvantages, such as semiconductor types, [1 4] and potentiometric, and amperometric solid electrolyte types [5 7]. Among these solid-state sensors, the solid electrolyte type sensor has an advanced merit of high selectivity, because the gas sensing mechanism is based on the characteristic that only single ion species can migrate in solid electrolytes. Therefore, the solid electrolyte type sensor is expected to be the most practical candidate for the NO x gas sensing tool. Recently, we have proposed a solid electrolyte type NO x gas sensor [8] which combines two kinds of solid electrolytes of the Al 3+ ion conducting (Al 0.2 Zr 0.8 ) 20/19 Nb(PO 4 ) 3 [9] and the O 2 ion conducting yttria stabilized zirconia (YSZ) with the K + ion conducting 0.35Gd 2 O 3 0.3KNO 3 solid solution [10] as the sensing auxiliary electrode. This sensor was developed to detect NO x gas in the high temperature region, and could detect not only the individual NO x (NO or NO 2 )gas /$ see front matter 2004 Elsevier B.V. All rights reserved. doi: /j.snb

2 I. Hasegawa et al. / Sensors and Actuators B 108 (2005) but also the total concentration of NO x (NO and NO 2 )gas at the high temperature of 723 K as intended. Besides such a sensor applicable at high temperatures, there is a brisk demand for sensors operating in the intermediate temperature region ( K) to realize development of portable type NO x gas sensors. Since the sensor previously reported cannot operate at the temperatures lower than 723 K, it is necessary to develop a new type of NO x gas sensor which can operate between 473 K and 573 K. In order to improve the conductivity of the sensing auxiliary electrode, we developed the high Li + ion conducting (1 x)/2(gd 0.9 La 0.1 ) 2 O 3 xlino 3 [11] and utilized as the sensing auxiliary electrode with Al 3+ ion and O 2 ion conductors, and the preliminary NO gas sensing characteristics are measured at K [12,13]. In this study, the optimization of the sensing auxiliary electrode was achieved, and the optimized electrode was combined with two types of solid electrolytes, (Al 0.2 Zr 0.8 ) 20/19 Nb(PO 4 ) 3 and YSZ. The sensing characteristics of the proposed new type of NO gas sensing were investigated in detail. 2. Experimental Gd 2 O 3 and La 2 O 3 powder was mixed by a ball mill method in a stoichiometric ratio and the mixed powder was heated at 1273 K for 12 h in air. The obtained (Gd 0.9 La 0.1 ) 2 O 3 solid solution was mixed with LiNO 3, pelletized, and heated at 773 K for 12 h in air. The synthesized (1 x)/2(gd 0.9 La 0.1 ) 2 O 3 xlino 3 was made into pellets after pulverizing, and sintered at 773 K for 12 h in air. The Al 3+ ion conductor (Al 0.2 Zr 0.8 ) 20/19 Nb(PO 4 ) 3 was prepared by mixing the starting materials of Al(OH) 3, ZrO 2,Nb 2 O 5, and (NH 4 ) 2 H(PO 4 ) 3 in a molar ratio of 8:32:19:114. The mixed powder was successively heated at 1273 K for 12 h, at 1473 K for 12 h, and then, at 1573 K for 12 h in air. The obtained (Al 0.2 Zr 0.8 ) 20/19 Nb(PO 4 ) 3 was pelletized and sintered at 1573 K for 12 h in air. The O 2 ion conductor (ZrO 2 ) 0.92 (Y 2 O 3 ) 0.08 (YSZ) was prepared by mixing ZrO 2 and Y 2 O 3 in a molar ratio of 92:8, and the mixed powder was heated at 1873 K for 6 h. The obtained YSZ was pelletized and sintered at 1873 K for 12 h. The sample characterization was carried out by X-ray powder diffraction with Cu K radiation (Rigaku, Multiflex 2 kw). The ac conductivity measurement was carried out using two gold electrodes as an ion-blocking electrode by a complex impedance method in the frequency range between 5 Hz and 13 MHz (Precision LCR meter 4192A, Hewlett Packard). The thermal analysis was performed using a thermal gravimetric/differential thermal analysis (TG-DTA) apparatus (Shimadzu DTG-50). After two solid electrolyte pellets of (Al 0.2 Zr 0.8 ) 20/19 - Nb(PO 4 ) 3 and (ZrO 2 ) 0.92 (Y 2 O 3 ) 0.08 were tightly fixed by inorganic adhesive agent (Asahi, Sumiceram 17-D), the (1 x)/2(gd 0.9 La 0.1 ) 2 O 3 xlino 3 solid solution electrode was put on the Al 3+ ion conducting solid electrolyte surface (Fig. 1) and heated to the operating temperature of 523 K. The Fig. 1. Cross-sectional view of the sensor element fabricated by the Al 3+ ion conducting and the O 2 ion conducting solid electrolytes with the LiNO 3 - doped (Gd 0.9 La 0.1 ) 2 O 3 auxiliary electrode. NO gas concentration was regulated by mixing the 1% NO diluted with N 2 gas with air. The total gas flow rate was kept constant at 100 ml/min, and the sensor output was monitored by an electrometer (Advantest, R8240). 3. Results and discussion Fig. 2 shows the X-ray powder diffraction patterns of (1 x)/2(gd 0.9 La 0.1 ) 2 O 3 xlino 3 (x = 0 0.4). In the region of x 0.35, the samples are in a single phase of the cubic Fig. 2. XRD patterns of (1 x)/2(gd 0.9 La 0.1 ) 2 O 3 xlino 3 (x = 0 0.4). The lattice parameter variation with the LiNO 3 content in the (1 x)/2(gd 0.9 La 0.1 ) 2 O 3 xlino 3 (Cubic phase) is also shown in the inset.

3 316 I. Hasegawa et al. / Sensors and Actuators B 108 (2005) Fig. 3. The compositional dependencies of the conductivity for (1 x)/2(gd 0.9 La 0.1 ) 2 O 3 xlino 3 ( ) and (Gd 0.9 La 0.1 ) 2 O 3 ( ) at 773 K. C-type rare earth oxide, but LiNO 3 appeared as a secondary phase for the sample of x = 0.4. The lattice constant estimated from the XRD pattern increases monotonously with the LiNO 3 increases (see inset in Fig. 2), and maintains a constant value at x From these results, it is clear that the (1 x)/2(gd 0.9 La 0.1 ) 2 O 3 xlino 3 forms the solid solution in the range of 0 < x Fig. 3 presents the compositional dependencies of the conductivity of the LiNO 3 -doped (Gd 0.9 La 0.1 ) 2 O 3 at 773 K. As the increase of the LiNO 3 content in the solids, the Li + ion conductivity increased by forming the solid solution and the highest conductivity was obtained for the sample at x = At x = 0.4, the conductivity decreased slightly due to the formation of LiNO 3 as the secondary phase. Fig. 4 depicts the TG analysis results for the (1 x)/2(gd 0.9 La 0.1 ) 2 O 3 xlino 3 (x = 0.35) solid solution and the mixture of LiNO 3 and (Gd 0.9 La 0.1 ) 2 O 3 in the same ratio. While the mixture sample shows a sudden weight loss by the release of water vapor at around 373 K, such a decrease related to the water vapor evaporation at ca. 373 K was not observed in the case of the (1 x)/2(gd 0.9 La 0.1 ) 2 O 3 xlino 3 (x = 0.35) solid solution. This difference also indicates that the LiNO 3 -doped (Gd 0.9 La 0.1 ) 2 O 3 solid solution is not just the mixture of LiNO 3 and (Gd 0.9 La 0.1 ) 2 O 3. From the results obtained in Figs. 2 4, it is concluded that the solid solution limit in this system is x = The (1 x)/2(gd 0.9 La 0.1 ) 2 O 3 xlino 3 (x = 0.35) solid solution, which possesses the highest conductivity, is expected to be a promising candidate for the NO gas detecting elec- Fig. 5. Temperature dependence of the Li + ion conductivity of the (1 x)/2(gd 0.9 La 0.1 ) 2 O 3 xlino 3 (x = 0.35) ( ) with the corresponding data of the K + ion conductor of 0.35Gd 2 O 3 0.3KNO 3 ( ) [10]. trode. Fig. 5 shows the temperature dependence of the Li + ion conductivity of the (1 x)/2(gd 0.9 La 0.1 ) 2 O 3 xlino 3 (x = 0.35) solid solution with the data of the K + ion conductor of the KNO 3 -doped Gd 2 O 3 (Gd 2 O 3 :KNO 3 = 0.35:0.3) solid solution which is the sensing auxiliary electrode of our previous work on the NO x gas sensor [8]. The ion conductivity of the (1 x)/2(gd 0.9 La 0.1 ) 2 O 3 xlino 3 (x = 0.35) solid solution at 523 K is about 64 times higher than that of the KNO 3 -doped Gd 2 O 3 (Gd 2 O 3 :KNO 3 = 0.35:0.3) solid at 523 K. Furthermore, the ion conductivity of the (1 x)/2(gd 0.9 La 0.1 ) 2 O 3 xlino 3 (x = 0.35) solid solution at 523 K is still higher than that of the KNO 3 doped Gd 2 O 3 (Gd 2 O 3 :KNO 3 = 0.35:0.3) at 723 K of the operating temperature in our previous paper [8]. These results explicitly indicate that the (1 x)/2(gd 0.9 La 0.1 ) 2 O 3 xlino 3 (x = 0.35) solid solution would be a promising candidate as the auxiliary electrode of the NO x sensor operating at around 523 K. The reactions were expected to occur at the detecting electrode, the interface between the sensing auxiliary electrode and the Al 3+ ion conducting electrolyte, the interface between the two solid electrolytes, and the reference electrode are as follows. Detecting electrode: LiNO 3 (in(gd 0.9 La 0.1 ) 2 O 3 ) Li + + NO + O I 2 + e (1) Interface between the sensing electrode and the Al 3+ ion conducting electrolyte: Li (Al 0.2Zr 0.8 ) 20/19 Nb(PO 4 ) Al (Li 0.6Zr 0.8 ) 20/19 Nb(PO 4 ) 3 (2) Interface between the two solid electrolytes: Fig. 4. The TG result for the (1 x)/2(gd 0.9 La 0.1 ) 2 O 3 xlino 3 (x = 0.35) solid solution ( ) with the data of the mixture of (Gd 0.9 La 0.1 ) 2 O 3 and LiNO 3 in the same ratio (- - -). 1 3 Al O2 1 6 Al 2O 3 (3) Reference electrode: 1 4 OII 2 + e 1 2 O2 (4)

4 I. Hasegawa et al. / Sensors and Actuators B 108 (2005) (O I and O II represent the oxygen gas appearing on the surface of the sensing auxiliary electrode and the reference electrode side, respectively.) From Eqs. (1) (4), overall reaction is described as follows. LiNO (Al 0.2Zr 0.8 ) 20/19 Nb(PO 4 ) OII Al 2O (Li 0.6Zr 0.8 ) 20/19 Nb(PO 4 ) 3 + NO + O I 2 (5) Therefore, the Nernst equation derived from the Eq. (5) is described as the next equation: E = E 0 RT nf ln{(aal 2O 3 ) 1/6 (a(li 0.6 Zr 0.8 ) 20/19 Nb(PO 4 ) 3 ) 19/12 (PNO)(PO I )(alino 3 ) 1 (a(al 0.2 Zr 0.8 ) 20/19 Nb(PO 4 ) 3 ) 19/12 (PO II 2 ) 1/4 } (6) E 0 is a constant value, n = 1.00 in this case, and R, T, and F are the gas constant, absolute temperature, and the Faraday s constant, respectively. The activity of solids is constant at a fixed temperature and oxygen partial pressure is always constant at 0.21 atm. Therefore, the Eq. (6) is simplified as follows: E = C(constant) RT ln(pno) (n = 1.00) (7) nf Fig. 6 depicts a typical sensor response curve of the present sensor with the (1 x)/2(gd 0.9 La 0.1 ) 2 O 3 xlino 3 (x = 0.35) solid solution as the sensing auxiliary electrode when the NO gas concentration was changed from 200 to 2000 ppm and vice versa at 523 K. The sensor EMF outputs at the same NO gas concentration were almost equal in both increase and decrease processes. The response time defined as the time to attain a 90% of total response was within 2 min, demonstrating that the present sensor shows a rapid and reversible response for the NO gas detection. Furthermore, the response rate of the present sensor operating at 523 K is more than three times faster that of the previous sensor with Gd 2 O 3 KNO 3 as the sensing auxiliary electrode operating at 723 K. It is expected that the present sensor can respond faster by enhancing the ion conductivity of the sensing electrode. Fig. 7. Relationship between the sensor EMF output and the logarithm of the NO gas concentration in increasing ( ) and in decreasing ( ) the concentration. Fig. 7 shows the relationship between the sensor EMF outputs and the logarithm of the NO gas concentration at 523 K and the theoretical slope (n = 1.00) calculated from the Nernst equation (Eq. (7)) is also depicted as a solid line. The sensor EMF outputs decrease monotonically with the increase of the NO gas concentration and a 1:1 linear relationship was obtained between the sensor output and the logarithm of the NO gas concentration. Furthermore, the n values estimated from the observed slopes were 1.00 and 0.91 in increasing and in decreasing the NO gas concentration, respectively, which agree with the theoretical value calculated from the Nernst equation. These results explicitly mean the fact that the present sensor exactly obeys the theoretical sensing mechanism proposed in the Eqs. (1) (4). 4. Conclusions A new type of nitrogen monoxide gas sensor operating at 523 K was fabricated by the combination of the Al 3+ ion conducting and the O 2 ion conducting solid electrolytes with Li + ion conducting (1 x)/2(gd 0.9 La 0.1 ) 2 O 3 xlino 3 (x = 0.35) solid solution as the sensing auxiliary electrode. The present sensor shows a rapid, continuous, and reproducible response, and the linear relationship between the sensor EMF output and the logarithm of the NO gas concentration was observed with obeying the theoretical Nernst relationship. These superior characteristics provide us new type of real-time and in situ monitoring tool even at low temperature of 523 K. Acknowledgements Fig. 6. Typical sensor response curve for the NO gas concentration changing from 200 to 2000 ppm at 523 K. This work was partially supported by a Grant-in-Aid for Science Research (no ) from the Ministry of Education, Science, Sports and Culture. This work was also supported by the Iwatani Naoji Foundation.

5 318 I. Hasegawa et al. / Sensors and Actuators B 108 (2005) References [1] Y. Sadaoka, T.A. Jones, W. Göpel, Sens. Actuators B 1 (1990) 148. [2] G. Sberveglieri, S. Groppelli, P. Nelli, V. Lantto, H. Torvela, P. Romppainen, S. Leppavuori, Sens. Actuators B 1 (1990) 79. [3] N. Imanaka, S. Banno, G. Adachi, Chem. Lett. 2 (1994) 319. [4] S. Tanaka, T. Esaka, J. Mater. Res. 16 (2001) [5] S. Zhuiykov, T. Ono, N. Yamazoe, N. Miura, Solid State Ionics (2002) 801. [6] N. Imanaka, A. Oda, S. Tamura, G. Adachi, Electrochem. Solid State Lett. 5 (2002) H25. [7] M.L. Grilli, E. di Bartolomeo, E. Traversa, J. Electrochem. Soc. 148 (2001) H98. [8] A. Oda, N. Imanaka, G. Adachi, Sens. Actuators B 93 (2003) 229. [9] N. Imanaka, Y. Hasegawa, M. Yamaguchi, M. Itaya, S. Tamura, G. Adachi, Chem. Mater. 14 (2002) [10] Y.W. Kim, A. Oda, N. Imanaka, Electrochem. Commun. 5 (2003) 94. [11] N. Imanaka, S. Tamura, A. Mori, Electrochemistry 71 (2003) [12] N. Imanaka, I. Hasegawa, Sens. Lett. 1 (2003) 51. [13] I. Hasegawa, S. Tamura, N. Imanaka, Proceedings Ninth Asian Conference on Solid State Ionics, Jeju, Korea, 2004, p Biographies Isao Hasegawa was born in Tochigi Pref., Japan in He obtained his bachelor degree in applied chemistry from Osaka University in He is in master program in Osaka University. Shinji Tamura was born in Osaka, Japan in He received his BE (1997) degree in applied chemistry from Osaka University. He then obtained his ME (1999), PhD degree (2001) in materials chemistry from Osaka University. He has joined the faculty at Osaka University since 2004 and he is assistant professor. His main research fields are solid electrolytes and chemical sensors. Nobuhito Imanaka was born in Kawanishi, Hyogo, in He earned his BE (1981) and ME (1983) degrees in applied chemistry from Osaka University. He then obtained a PhD degree from Osaka University in He has joined the faculty at Osaka University since 1988 and he is professor. His main research fields include rare earths and functional materials such as solid electrolytes and chemical sensors.