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High-temperature NOx sensors using zirconia solid electrolyte and zinc-family oxide sensing electrode
Solid State Ionics 152 – 153 (2002) 801 – 807 www.elsevier.com/locate/ssi
High-temperature NOx sensors using zirconia solid electrolyte and zinc-family oxide sensing electrode Serge Zhuiykov a, Takashi Ono b, Noboru Yamazoe c, Norio Miura a,* a
Advanced Science and Technology Center for Cooperative Research, Kyushu University, Kasuga, Fukuoka 816-8580, Japan b R&D Division, Riken Corporation, Kumagaya, Saitama 360-8522, Japan c Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, Kasuga, Fukuoka 816-8580, Japan Accepted 20 March 2002
Abstract Yttria-stabilized zirconia (YSZ) electrochemical sensors attached with the zinc-family oxide (ZnFe2O4 and ZnCr2O4) sensing electrode (SE) were fabricated and examined for NOx sensing properties at high temperatures. These oxide-SE-attached devices gave a linear correlation between EMF and the logarithm of NO2 (or NO) concentration from 50 to 436 ppm in the temperature range 600 – 700 jC. The sensor using the combination of ZnFe2O4 and ZnCr2O4 was found to give the highest sensitivity to NO2 in air at 700 jC among the oxide-SEs tested and reported in the literatures to date. Furthermore, addition of Pt to ZnFe2O4 was found to improve the sensing characteristics towards quick response. It was confirmed that the sensing performances to NO2 were roughly related to those to O2 for the devices tested here. D 2002 Elsevier Science B.V. All rights reserved. PACS: Gas sensors, 07.07.D Keywords: NOx sensor; Potentiometric sensor; Mixed potential; Stabilized zirconia; Spinel-type oxide
1. Introduction Solid-state NOx sensors capable of detecting NO and NO2 at high temperatures have been demanded for monitoring and controlling combustion exhausts from automobiles. These sensors are required to be able to work at temperatures higher than 500 jC [1]. Recently, several solid-state potentiometric and amperometric NOx sensors based on yttria-stabilized zirconia (YSZ) and metal-oxide-SE have been reported to date in the literatures [1– 11]. However, *
Corresponding author. Tel.: +81-92-583-8852; fax: +81-92573-8729. E-mail address: [email protected] (N. Miura).
most of these sensors are not yet confirmed to monitor NOx successfully in real car exhausts. The sensor design must be improved for this purpose. The recent development of the design of NOx sensor includes the planar structure having an inner cavity for an electrochemical oxidation of NO [12,13]. This design differs from the amperometric NOx sensors, which are also trying to measure NOx in car exhausts [14,15]. It has been also demonstrated that some of the mixedpotential type NOx sensors using oxide-SE, such as CdMn2O4 [2,3], CdCr2O4 [4,5] and WO3 [6], are able to detect NO or NO2 in oxygen-containing atmospheres. However, in spite of the promising performances, these oxide-SEs cannot be applied to in situ monitoring of NOx in car exhausts because of the loss
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of their NOx sensitivities at temperatures higher than 600 jC. Therefore, the selection of proper oxide-SE is still important for obtaining an excellent sensing performance. Quite recently, we have revealed that among various spinel-type oxides tested, zinc-family oxide sensing electrodes (SEs) are effective for detecting NOx at high temperatures [8]. This fact suggests the importance of extensive investigation of the zincfamily oxides for SE. In this paper, we focused our attention on ZnFe2O4 and ZnCr2O4 spinel-type oxides and their modifications towards the better NOx sensing characteristics. The YSZ-based devices were further subjected to detailed performance tests as well as electrochemical measurements in relation to the NOx sensing mechanism and improvement in sensing performances.
2. Experimental 2.1. Fabrication of sensor devices A commercial half-opened YSZ tube (8 mol% Y2O3-doped, NKT) was used for fabrication of the devices. It is 300 mm in length and 5 and 8 mm in inner and outer diameters, respectively. The oxide-SE was applied on the outer surface of the YSZ tube and then sintered at 1200 jC for 2 h. The adhesion between the oxide layer obtained and the zirconia surface was reasonably good. The sintered oxide layer was about 30 Am thick. Pt paste was applied on the inner surface of the YSZ tube and then calcined at 1000 jC for 2 h to form the reference electrode (RE). The RE was always exposed to atmospheric air. The addition of Pt to the oxide (ZnCr2O4) matrix was carried out as follows. The oxide powder was loaded with Pt by mixing it with aqueous dispersion of colloidal Pt particles (Toda Kogyo, Japan, mean particle size: ca. 5 nm) for 1 h under agitation, followed by filtration, washing with deionized water, drying at 110 jC, and calcination at 1000 jC for 2 h. The loading of Pt was fixed to 0.01 wt.%. 2.2. Measurement of sensing properties Gas sensing experiments were conducted in a conventional gas-flow apparatus equipped with a furnace in the temperature range 600 – 700 jC. The sample
gases containing various concentrations of NO2 (or NO) were prepared by diluting parent dry gases with synthetic air (or N2+O2). The total flow rate of the sample gas (or the base air) was fixed at 100 cm3/min. The difference in potential (EMF) between SE and RE was monitored as a sensing signal with a digital electrometer (Advantest R8240). Both NO and NO2 concentrations were changed from 50 to 436 ppm. The ppm refers to the volume concentration of NOx in the gaseous mixture. 2.3. Characterization of sensor materials The crystal structure and surface state of various sensor materials used here were investigated by means of XRD analysis (with CuKa radiation) and SEM observation. XRD measurements were carried out with a RIGAKU X-ray diffractometer (RINT 2100 VLR/PC). SEM observation was done by using JEOL electron microscope (JSM-6340 F) operating at 3.0 or 15.0 kV. The roughness of the sample surfaces was investigated by using Color Laser 3D Profile Microscope (Keyence, VK-8500).
3. Results and discussion 3.1. Crystal structure and surface state of sensor materials XRD measurements of each of the oxide layers annealed at 1200 jC revealed that the peaks were all assigned to the franklinite phase of the ZnFe2O4 (JCPDS 22-1012) and the zincochromite phase of the ZnCr2O4 (JCPDS 22-1107), respectively. Further XRD results confirmed the excellent thermal stability of the ZnCr2O4 structure even after sintering at 1350 jC. Fig. 1 shows SEM images of the surfaces of YSZ substrate, Pt electrode, ZnFe2O4, and ZnCr2O4 layers. It is seen that the surface of YSZ substrate has no open pore and consists of zirconia grains sized from 1 to 3 Am. On the other hand, the Pt electrode was porous with an average particles size of about 1 Am. Both the ZnFe2O4 and ZnCr2O4 layers annealed at 1200 jC were also relatively porous and consisted of the polycrystalline structure with roughly uniform grain-size distribution. The average grain sizes of ZnFe2O4 and ZnCr2O4 were in the range 0.5 –1 and
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Fig. 1. SEM images of the surfaces of YSZ substrate (a), Pt electrode (b), ZnFe2O4 layer (c), and ZnCr2O4 layer (d).
0.2 –0.8 Am, respectively. This result suggests that the thermal treatment brings about the structural stability and the development of high surface-to-volume ratio of both oxide layers. The roughness of YSZ, ZnFe2O4, and ZnCr2O4 estimated with a Color Laser Microscope was about F1, F10, and F8 Am, respectively. 3.2. Sensing characteristics of the devices Each of the ZnFe 2O 4- and ZnCr 2 O 4-SE was attached to the YSZ tube and then the NOx sensing properties were examined in the temperature range 600 –700 jC. The EMF values of the devices fabricated were close to zero when NOx was absent in
the carrier gas (dry synthetic air). Therefore, the measured EMF values were regarded to the sensitivities to NO and NO2. The EMF characteristics of the devices using each of the ZnFe2O4- and ZnCr2O4-SE at 650 and 700 jC are shown in Fig. 2. As the temperature increased, the absolute EMF value at the same concentration of NO or NO2 decreased for both devices. The EMF values were almost linear to the logarithm of NO or NO2 concentration at each temperature examined, and the direction of the EMF response was positive to NO2 and negative to NO, as seen in the previous papers [2 – 7]. The EMF responses in these cases seem to be also based on mixed potential, as has been discussed before [2 –6]. The present sensors are able to detect NO or NO2 in
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within about 20% range after about 20-day operation up to 200 days examined in dry synthetic air. The air base has been quite stable during the whole test period. 3.3. Improvement of sensing properties
Fig. 2. Dependence of EMF on the logarithm of NO or NO2 concentration for the YSZ sensors using each of ZnFe2O4- and ZnCr2O4-SE at 650 and 700 jC.
the concentration range 50 –436 ppm. It is noted that the NO2 sensitivity at 700 jC for the ZnFe2O4-SE is much higher than that for the ZnCr2O4-SE. Fig. 3(a) shows the response and recovery transients to NO2 in air for the device using the ZnCr2O4SE at 650 and 700 jC. The response of this sensor was rather quick; the 90% response and recovery times to NO2 at 700 jC were about 6 and 20 s, respectively. The responses were repeatable over the five measurements (once per week) in the temperature range 600 – 700 jC. Fast response is a very important characteristic for a practical NOx sensor. On the contrary, in spite of high sensitivity to NOx in the temperature range of 600– 700 jC, the response and the recovery of the device using the ZnFe2O4-SE were rather slow, as shown in Fig. 3(b). The 90% response time to 100 ppm NO2 in air was about 9 min even at 700 jC. The evaluation of the sensing performances of the devices using the oxide-SEs revealed that the stability of NOx sensitivities was dependent of the kind of oxide-SE. In the case of the ZnCr2O4-SE, gradual degradation was seen in NOx sensitivity. In opposite, the EMF response to NO2 for ZnFe2O4-SE showed gradual small increase rather than degradation; the EMF value to 100 ppm NO2 at 700 jC increased
In order to improve the sensing characteristics of the NOx sensor, the powder mixture of ZnFe2O4 and ZnCr2O4 (50 wt.%/50 wt.%) was used to make a modified oxide-SE attached to the YSZ tube. This sensor was also tested in the temperature range 600– 700 jC. As shown in Fig. 4(a), the NO2 sensitivity at 700 jC for the device using the combined ZnFe2O4 – ZnCr2O4-SE is increased by as much as about 50% compared with that for the ZnFe2O4-SE. This NO2 sensitivity obtained in this device is the highest value at 700 jC among those reported so far. Unfortunately, however, any improvement was not seen in response rate; the 90% response and recovery
Fig. 3. Response and recovery transients to NO2 in air for the YSZ sensors using each of the ZnCr2O4-SE (a) and the ZnFe2O4-SE (b).
S. Zhuiykov et al. / Solid State Ionics 152 – 153 (2002) 801–807
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Fig. 5. Response transients to 436 ppm NO2 at 700 jC for devices using each of the ZnFe2O4-SE and the ZnFe2O4(+Pt)-SE.
3.4. Comparison of oxygen sensing performances
Fig. 4. Dependence of EMF on the logarithm of NO2 concentration (a) and response transients to NO2 (b) at 700 jC for the device using the ZnFe2O4 – ZnCr2O4-SE.
times of the device using the ZnFe2O4 –ZnCr2O4-SE were almost same as those for the ZnFe2O4-SE, as shown in Fig. 4(b). Thus, in order to modify and increase the response rate of the device using the ZnFe2O4-SE, we examined the effect of addition of second material to the base SE material. Generally, the use of some additives to a base-sensing material is an attractive way to achieve practical improvement in the sensor characteristics. Here, we selected Pt as an additive to the oxideSE. A small amount (0.01 wt.%) of Pt was added to the ZnFe2O4 powder and then the response to NO2 of the device using the ZnFe2O4(+Pt)-SE fabricated was measured at 700 jC. As a result, the response rate of the modified device was found to be improved, while little change was observed in the NO2 sensitivity. Fig. 5 shows that the 90% response times to 436 ppm NO2 for the devices using each of the ZnFe2O4-SE and the ZnFe2O4(+Pt)-SE are about 6 and 1 min, respectively, and the NO2 sensitivity is about 52 mV in both cases.
Since the O2 adsorption – desorption behavior on the oxide-SE was found to be related to the NO2 sensitivity at high temperatures [16], we have investigated the oxygen sensing properties of the devices fabricated here. For this purpose, the EMF values of the devices were measured when oxygen concentration was changed from 1 up to 100 vol.% at 700 jC. Fig. 6 shows the Nernstian plots for the devices examined. The data for the device using pure Pt electrode were also indicated in this figure for comparison. It is seen that the ZnCr2O4-SE gives the theoretical Nernstian plots (electron number: n=4.0) as the Pt electrode does. However, the slopes of the plots for the ZnFe2O4-SE
Fig. 6. Dependence of EMF on the logarithm of O2 concentration for the YSZ devices using each of various SEs.
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Fig. 7. Response transients to O2 at 700 jC for the YSZ devices using each of various SEs.
and the ZnFe2O4(+Pt)-SE are smaller than the theoretical one (48 mV/decade) at 700 jC. Based on the above-mentioned results, we may conclude that the ZnFe2O4-based SEs are working as an irreversible oxygen electrode and these catalytic activities for the electrochemical reaction of O2 are not high enough even at 700 jC. According to the sensing mechanism based on mixed potential, lower electrochemical catalytic activity for anodic reaction of O2 will lead to higher NO2 sensitivity [16,17]. This is corresponding well to the fact that the ZnFe2O4-based SEs can give the rather high NO2 sensitivity at 700 jC. Fig. 7 shows the response transients to O2 at 700 jC for the devices tested. The response rate for the ZnCr 2 O 4 -SE was much higher than that for the ZnFe2O4-SE. The response behavior to O2 is also corresponding well to that to NO2 for the devices tested. In the case of the ZnFe2O4(+Pt)-SE, the response and recovery rates to both O2 and NO2 were higher than those for the ZnFe2O4-SE. These results suggest that the kinetic of the electrochemical reaction involving O2 at SE determines the response and recovery rates of this type NOx sensor. Further improvement of response and recovery rates of the ZnFe2O4-based device by the addition of dopants is now under investigation.
4. Conclusions The mixed-potential type NOx sensors using the zinc-family oxide-SEs were fabricated and examined for the sensing properties in the temperature range
600 – 700 jC. As a result, the device using the ZnFe2O4-SE was found to give the highest sensitivity to both NO and NO2 at 700 jC. The improvement of the sensing characteristics of this NOx sensor was possible by variation of an additional component to the ZnFe2O4-SE. This component may be another oxide, or alternatively, a small amount of a noble metal. In general, the enhancement of kinetic properties of the SE by optimizing electrochemical catalytic activity at the interface of electrolyte/electrode can extend the possibility of improvement in the sensor performance. In fact, the addition of ZnCr2O4 and a small amount of Pt to the ZnFe2O4-SE brought about improvement in the NO2 sensitivity and response rate, respectively, of the NOx sensor.
Acknowledgements This work was partially supported by the Research Development Program of University-Industry Alliance—A Matching Funds Approach—from JSPS and NEDO, as well as a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology of Japan. The authors are grateful to Dr. A. Kunimoto, Dr. Y. Gao and Dr. Y. Yan of R&D Division, Riken, for their valuable discussions. The authors are also grateful to Mrs. M. Muta of Advanced Science and Technology Center for Cooperative Research, Kyushu University for her technical assistance.
References [1] F. Menil, V. Coillard, C. Lucat, Sens. Actuators, B 67 (2000) 1. [2] N. Miura, G. Lu, N. Yamazoe, H. Kurosawa, M. Hasei, J. Electrochem. Soc. 143 (1996) L33. [3] N. Miura, H. Kurosawa, M. Hasei, G. Lu, N. Yamazoe, Solid State Ionics 86/88 (1996) 1069. [4] N. Miura, G. Lu, N. Yamazoe, Sens. Actuators, B 52 (1998) 169. [5] N. Miura, N. Yamazoe, in: H. Baltes, W. Goepel, J. Hesse (Eds.), Sensors Update, vol. 6, Wiley-VCH, Weinheim, 2000, p. 192. [6] G. Lu, N. Miura, N. Yamazoe, Ionics 4 (1998) 16. [7] S. Zhuiykov, T. Nakano, A. Kunimoto, N. Yamazoe, N. Miura, Electrochem. Commun. 3 (2001) 97. [8] S. Zhuiykov, M. Muta, T. Ono, A. Kunimoto, N. Yamazoe, N. Miura, Chem. Sens. 17 (Suppl. A) (2001) 49. [9] K.Y. Ho, M. Miyayama, H. Yanagida, J. Ceram. Soc. Jpn. 104 (1996) 995.
S. Zhuiykov et al. / Solid State Ionics 152 – 153 (2002) 801–807 [10] S. Somov, G. Reinhardt, U. Guth, W. Goepel, Sens. Actuators, B 35 – 36 (1996) 409. [11] N. Miura, G. Lu, M. Ono, N. Yamazoe, Solid State Ionics 117 (1999) 283. [12] A. Kunimoto, M. Hasei, Y. Yan, Y. Gao, T. Ono, Y. Nakanouchi, SAE Tech. Paper Series. No. 1999-01-1280 (1999). [13] M. Hasei, T. Ono, Y. Gao, Y. Yan, A. Kunimoto, SAE Tech. Paper Series. No. 2000-01-1203 (2000). [14] N. Kato, K. Nakagaki, N. Ina, SAE Tech. Paper Series. No. 960334 (1996).
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[15] N. Kato, H. Karachi, Y. Hamada, SAE Tech. Paper Series. No. 980170 (1998). [16] S. Zhuiykov, M. Muta, T. Ono, A. Kunimoto, Y. Yamazoe, N. Miura, Proc. of Transducers ’01, Munich, Springer-Verlag, Berlin, 2001, p. 1730. [17] N. Miura, S. Zhuiykov, T. Ono, M. Hasei, Y. Yamazoe, Sens. Actuators, B, 83 (2002) 222.
High-temperature NOx sensors using zirconia solid electrolyte and zinc-family oxide sensing electrode Serge Zhuiykov a, Takashi Ono b, Noboru Yamazoe c, Norio Miura a,* a
Advanced Science and Technology Center for Cooperative Research, Kyushu University, Kasuga, Fukuoka 816-8580, Japan b R&D Division, Riken Corporation, Kumagaya, Saitama 360-8522, Japan c Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, Kasuga, Fukuoka 816-8580, Japan Accepted 20 March 2002
Abstract Yttria-stabilized zirconia (YSZ) electrochemical sensors attached with the zinc-family oxide (ZnFe2O4 and ZnCr2O4) sensing electrode (SE) were fabricated and examined for NOx sensing properties at high temperatures. These oxide-SE-attached devices gave a linear correlation between EMF and the logarithm of NO2 (or NO) concentration from 50 to 436 ppm in the temperature range 600 – 700 jC. The sensor using the combination of ZnFe2O4 and ZnCr2O4 was found to give the highest sensitivity to NO2 in air at 700 jC among the oxide-SEs tested and reported in the literatures to date. Furthermore, addition of Pt to ZnFe2O4 was found to improve the sensing characteristics towards quick response. It was confirmed that the sensing performances to NO2 were roughly related to those to O2 for the devices tested here. D 2002 Elsevier Science B.V. All rights reserved. PACS: Gas sensors, 07.07.D Keywords: NOx sensor; Potentiometric sensor; Mixed potential; Stabilized zirconia; Spinel-type oxide
1. Introduction Solid-state NOx sensors capable of detecting NO and NO2 at high temperatures have been demanded for monitoring and controlling combustion exhausts from automobiles. These sensors are required to be able to work at temperatures higher than 500 jC [1]. Recently, several solid-state potentiometric and amperometric NOx sensors based on yttria-stabilized zirconia (YSZ) and metal-oxide-SE have been reported to date in the literatures [1– 11]. However, *
Corresponding author. Tel.: +81-92-583-8852; fax: +81-92573-8729. E-mail address: [email protected] (N. Miura).
most of these sensors are not yet confirmed to monitor NOx successfully in real car exhausts. The sensor design must be improved for this purpose. The recent development of the design of NOx sensor includes the planar structure having an inner cavity for an electrochemical oxidation of NO [12,13]. This design differs from the amperometric NOx sensors, which are also trying to measure NOx in car exhausts [14,15]. It has been also demonstrated that some of the mixedpotential type NOx sensors using oxide-SE, such as CdMn2O4 [2,3], CdCr2O4 [4,5] and WO3 [6], are able to detect NO or NO2 in oxygen-containing atmospheres. However, in spite of the promising performances, these oxide-SEs cannot be applied to in situ monitoring of NOx in car exhausts because of the loss
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of their NOx sensitivities at temperatures higher than 600 jC. Therefore, the selection of proper oxide-SE is still important for obtaining an excellent sensing performance. Quite recently, we have revealed that among various spinel-type oxides tested, zinc-family oxide sensing electrodes (SEs) are effective for detecting NOx at high temperatures [8]. This fact suggests the importance of extensive investigation of the zincfamily oxides for SE. In this paper, we focused our attention on ZnFe2O4 and ZnCr2O4 spinel-type oxides and their modifications towards the better NOx sensing characteristics. The YSZ-based devices were further subjected to detailed performance tests as well as electrochemical measurements in relation to the NOx sensing mechanism and improvement in sensing performances.
2. Experimental 2.1. Fabrication of sensor devices A commercial half-opened YSZ tube (8 mol% Y2O3-doped, NKT) was used for fabrication of the devices. It is 300 mm in length and 5 and 8 mm in inner and outer diameters, respectively. The oxide-SE was applied on the outer surface of the YSZ tube and then sintered at 1200 jC for 2 h. The adhesion between the oxide layer obtained and the zirconia surface was reasonably good. The sintered oxide layer was about 30 Am thick. Pt paste was applied on the inner surface of the YSZ tube and then calcined at 1000 jC for 2 h to form the reference electrode (RE). The RE was always exposed to atmospheric air. The addition of Pt to the oxide (ZnCr2O4) matrix was carried out as follows. The oxide powder was loaded with Pt by mixing it with aqueous dispersion of colloidal Pt particles (Toda Kogyo, Japan, mean particle size: ca. 5 nm) for 1 h under agitation, followed by filtration, washing with deionized water, drying at 110 jC, and calcination at 1000 jC for 2 h. The loading of Pt was fixed to 0.01 wt.%. 2.2. Measurement of sensing properties Gas sensing experiments were conducted in a conventional gas-flow apparatus equipped with a furnace in the temperature range 600 – 700 jC. The sample
gases containing various concentrations of NO2 (or NO) were prepared by diluting parent dry gases with synthetic air (or N2+O2). The total flow rate of the sample gas (or the base air) was fixed at 100 cm3/min. The difference in potential (EMF) between SE and RE was monitored as a sensing signal with a digital electrometer (Advantest R8240). Both NO and NO2 concentrations were changed from 50 to 436 ppm. The ppm refers to the volume concentration of NOx in the gaseous mixture. 2.3. Characterization of sensor materials The crystal structure and surface state of various sensor materials used here were investigated by means of XRD analysis (with CuKa radiation) and SEM observation. XRD measurements were carried out with a RIGAKU X-ray diffractometer (RINT 2100 VLR/PC). SEM observation was done by using JEOL electron microscope (JSM-6340 F) operating at 3.0 or 15.0 kV. The roughness of the sample surfaces was investigated by using Color Laser 3D Profile Microscope (Keyence, VK-8500).
3. Results and discussion 3.1. Crystal structure and surface state of sensor materials XRD measurements of each of the oxide layers annealed at 1200 jC revealed that the peaks were all assigned to the franklinite phase of the ZnFe2O4 (JCPDS 22-1012) and the zincochromite phase of the ZnCr2O4 (JCPDS 22-1107), respectively. Further XRD results confirmed the excellent thermal stability of the ZnCr2O4 structure even after sintering at 1350 jC. Fig. 1 shows SEM images of the surfaces of YSZ substrate, Pt electrode, ZnFe2O4, and ZnCr2O4 layers. It is seen that the surface of YSZ substrate has no open pore and consists of zirconia grains sized from 1 to 3 Am. On the other hand, the Pt electrode was porous with an average particles size of about 1 Am. Both the ZnFe2O4 and ZnCr2O4 layers annealed at 1200 jC were also relatively porous and consisted of the polycrystalline structure with roughly uniform grain-size distribution. The average grain sizes of ZnFe2O4 and ZnCr2O4 were in the range 0.5 –1 and
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Fig. 1. SEM images of the surfaces of YSZ substrate (a), Pt electrode (b), ZnFe2O4 layer (c), and ZnCr2O4 layer (d).
0.2 –0.8 Am, respectively. This result suggests that the thermal treatment brings about the structural stability and the development of high surface-to-volume ratio of both oxide layers. The roughness of YSZ, ZnFe2O4, and ZnCr2O4 estimated with a Color Laser Microscope was about F1, F10, and F8 Am, respectively. 3.2. Sensing characteristics of the devices Each of the ZnFe 2O 4- and ZnCr 2 O 4-SE was attached to the YSZ tube and then the NOx sensing properties were examined in the temperature range 600 –700 jC. The EMF values of the devices fabricated were close to zero when NOx was absent in
the carrier gas (dry synthetic air). Therefore, the measured EMF values were regarded to the sensitivities to NO and NO2. The EMF characteristics of the devices using each of the ZnFe2O4- and ZnCr2O4-SE at 650 and 700 jC are shown in Fig. 2. As the temperature increased, the absolute EMF value at the same concentration of NO or NO2 decreased for both devices. The EMF values were almost linear to the logarithm of NO or NO2 concentration at each temperature examined, and the direction of the EMF response was positive to NO2 and negative to NO, as seen in the previous papers [2 – 7]. The EMF responses in these cases seem to be also based on mixed potential, as has been discussed before [2 –6]. The present sensors are able to detect NO or NO2 in
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within about 20% range after about 20-day operation up to 200 days examined in dry synthetic air. The air base has been quite stable during the whole test period. 3.3. Improvement of sensing properties
Fig. 2. Dependence of EMF on the logarithm of NO or NO2 concentration for the YSZ sensors using each of ZnFe2O4- and ZnCr2O4-SE at 650 and 700 jC.
the concentration range 50 –436 ppm. It is noted that the NO2 sensitivity at 700 jC for the ZnFe2O4-SE is much higher than that for the ZnCr2O4-SE. Fig. 3(a) shows the response and recovery transients to NO2 in air for the device using the ZnCr2O4SE at 650 and 700 jC. The response of this sensor was rather quick; the 90% response and recovery times to NO2 at 700 jC were about 6 and 20 s, respectively. The responses were repeatable over the five measurements (once per week) in the temperature range 600 – 700 jC. Fast response is a very important characteristic for a practical NOx sensor. On the contrary, in spite of high sensitivity to NOx in the temperature range of 600– 700 jC, the response and the recovery of the device using the ZnFe2O4-SE were rather slow, as shown in Fig. 3(b). The 90% response time to 100 ppm NO2 in air was about 9 min even at 700 jC. The evaluation of the sensing performances of the devices using the oxide-SEs revealed that the stability of NOx sensitivities was dependent of the kind of oxide-SE. In the case of the ZnCr2O4-SE, gradual degradation was seen in NOx sensitivity. In opposite, the EMF response to NO2 for ZnFe2O4-SE showed gradual small increase rather than degradation; the EMF value to 100 ppm NO2 at 700 jC increased
In order to improve the sensing characteristics of the NOx sensor, the powder mixture of ZnFe2O4 and ZnCr2O4 (50 wt.%/50 wt.%) was used to make a modified oxide-SE attached to the YSZ tube. This sensor was also tested in the temperature range 600– 700 jC. As shown in Fig. 4(a), the NO2 sensitivity at 700 jC for the device using the combined ZnFe2O4 – ZnCr2O4-SE is increased by as much as about 50% compared with that for the ZnFe2O4-SE. This NO2 sensitivity obtained in this device is the highest value at 700 jC among those reported so far. Unfortunately, however, any improvement was not seen in response rate; the 90% response and recovery
Fig. 3. Response and recovery transients to NO2 in air for the YSZ sensors using each of the ZnCr2O4-SE (a) and the ZnFe2O4-SE (b).
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Fig. 5. Response transients to 436 ppm NO2 at 700 jC for devices using each of the ZnFe2O4-SE and the ZnFe2O4(+Pt)-SE.
3.4. Comparison of oxygen sensing performances
Fig. 4. Dependence of EMF on the logarithm of NO2 concentration (a) and response transients to NO2 (b) at 700 jC for the device using the ZnFe2O4 – ZnCr2O4-SE.
times of the device using the ZnFe2O4 –ZnCr2O4-SE were almost same as those for the ZnFe2O4-SE, as shown in Fig. 4(b). Thus, in order to modify and increase the response rate of the device using the ZnFe2O4-SE, we examined the effect of addition of second material to the base SE material. Generally, the use of some additives to a base-sensing material is an attractive way to achieve practical improvement in the sensor characteristics. Here, we selected Pt as an additive to the oxideSE. A small amount (0.01 wt.%) of Pt was added to the ZnFe2O4 powder and then the response to NO2 of the device using the ZnFe2O4(+Pt)-SE fabricated was measured at 700 jC. As a result, the response rate of the modified device was found to be improved, while little change was observed in the NO2 sensitivity. Fig. 5 shows that the 90% response times to 436 ppm NO2 for the devices using each of the ZnFe2O4-SE and the ZnFe2O4(+Pt)-SE are about 6 and 1 min, respectively, and the NO2 sensitivity is about 52 mV in both cases.
Since the O2 adsorption – desorption behavior on the oxide-SE was found to be related to the NO2 sensitivity at high temperatures [16], we have investigated the oxygen sensing properties of the devices fabricated here. For this purpose, the EMF values of the devices were measured when oxygen concentration was changed from 1 up to 100 vol.% at 700 jC. Fig. 6 shows the Nernstian plots for the devices examined. The data for the device using pure Pt electrode were also indicated in this figure for comparison. It is seen that the ZnCr2O4-SE gives the theoretical Nernstian plots (electron number: n=4.0) as the Pt electrode does. However, the slopes of the plots for the ZnFe2O4-SE
Fig. 6. Dependence of EMF on the logarithm of O2 concentration for the YSZ devices using each of various SEs.
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Fig. 7. Response transients to O2 at 700 jC for the YSZ devices using each of various SEs.
and the ZnFe2O4(+Pt)-SE are smaller than the theoretical one (48 mV/decade) at 700 jC. Based on the above-mentioned results, we may conclude that the ZnFe2O4-based SEs are working as an irreversible oxygen electrode and these catalytic activities for the electrochemical reaction of O2 are not high enough even at 700 jC. According to the sensing mechanism based on mixed potential, lower electrochemical catalytic activity for anodic reaction of O2 will lead to higher NO2 sensitivity [16,17]. This is corresponding well to the fact that the ZnFe2O4-based SEs can give the rather high NO2 sensitivity at 700 jC. Fig. 7 shows the response transients to O2 at 700 jC for the devices tested. The response rate for the ZnCr 2 O 4 -SE was much higher than that for the ZnFe2O4-SE. The response behavior to O2 is also corresponding well to that to NO2 for the devices tested. In the case of the ZnFe2O4(+Pt)-SE, the response and recovery rates to both O2 and NO2 were higher than those for the ZnFe2O4-SE. These results suggest that the kinetic of the electrochemical reaction involving O2 at SE determines the response and recovery rates of this type NOx sensor. Further improvement of response and recovery rates of the ZnFe2O4-based device by the addition of dopants is now under investigation.
4. Conclusions The mixed-potential type NOx sensors using the zinc-family oxide-SEs were fabricated and examined for the sensing properties in the temperature range
600 – 700 jC. As a result, the device using the ZnFe2O4-SE was found to give the highest sensitivity to both NO and NO2 at 700 jC. The improvement of the sensing characteristics of this NOx sensor was possible by variation of an additional component to the ZnFe2O4-SE. This component may be another oxide, or alternatively, a small amount of a noble metal. In general, the enhancement of kinetic properties of the SE by optimizing electrochemical catalytic activity at the interface of electrolyte/electrode can extend the possibility of improvement in the sensor performance. In fact, the addition of ZnCr2O4 and a small amount of Pt to the ZnFe2O4-SE brought about improvement in the NO2 sensitivity and response rate, respectively, of the NOx sensor.
Acknowledgements This work was partially supported by the Research Development Program of University-Industry Alliance—A Matching Funds Approach—from JSPS and NEDO, as well as a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology of Japan. The authors are grateful to Dr. A. Kunimoto, Dr. Y. Gao and Dr. Y. Yan of R&D Division, Riken, for their valuable discussions. The authors are also grateful to Mrs. M. Muta of Advanced Science and Technology Center for Cooperative Research, Kyushu University for her technical assistance.
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