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Solid-state amperometric CH4 sensor using, LaGaO3-based electrolyte
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Solid State Ionics 179 (2008) 1641 – 1644 www.elsevier.com/locate/ssi
Solid-state amperometric CH4 sensor using, LaGaO3-based electrolyte Zhonghe Bi a , Hiroshige Matsumoto a,b , Tatsumi Ishihara a,b,⁎ a
Department of Applied Chemistry, Faculty of Engineering, Kyushu University, 744 Motooka, Nishi-Ku, Fukuoka 819-0395, Japan b Center of Future Chemistry, Kyushu University, 744 Motooka, Nishi-Ku, Fukuoka 819-0395, Japan Received 24 October 2007; received in revised form 21 November 2007; accepted 22 November 2007
Abstract A solid-state amperometric CH4 sensor using a LaGaO3-based electrolyte was studied. For this sensor, a combination of active (anode) and inactive (cathode) electrodes for CH4 oxidation was applied. Among the various active electrode materials studied, Pd mixed with tin-doped In2O3 (ITO) was found to be the most active electrode and RuO2 the most inactive one. The sensitivities of the sensor using La0.9Sr0.1Ga0.8Mg0.2O3 (LSGM) electrolyte to CH4 were approximately 797 µA/decade at 673 K and 188 µA/decade at 873 K. The changes in oxygen pumping current upon exposure to CH4 could be attributed to the mixed potential generated by the difference in oxidation activity between the active and inactive electrodes. © 2007 Elsevier B.V. All rights reserved. Keywords: CH4 sensor; Amperometric; LaGaO3-based oxide electrolyte
1. Introduction Methane (CH4), which is the main component of city gas, is highly explosive even at low concentrations. Therefore, methane detection has become an extremely important issue with increasing urbanization. In fact, in most urban areas in Japan, kitchens are recommended to be equipped with gas leakage sensors and SnO2-based semiconductor-type sensors are at present widely used for this purpose [1–3]. However, these sensors have a high operating temperature (N 573 K) and low selectivity. In order to improve the accuracy, increased selectivity is desirable. As examples of such sensors, mixedpotential-type sensors for detecting hydrocarbons in exhaust gas from internal combustion engines have been reported [4–7]. However, this type of sensor is not suitable as a CH4 sensor due to low reproducibility and low sensitivity to CH4. Previously, we found that LaGaO3 perovskite oxide exhibits high oxide ion conductivity [8] and that solid-state amperometric sensors using ⁎ Corresponding author. Department of Applied Chemistry, Faculty of Engineering, Kyushu University, Motooka 744, Nishi-Ku, Fukuoka 819-0395, Japan. Fax: +81 92 802 2871. E-mail address: [email protected] (T. Ishihara). 0167-2738/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2007.11.018
oxygen pumping current can be used as a hydrocarbon sensor. For example, the sensor using Fe-doped LaGaO3 exhibits high selectivity to C3H6 at temperatures as low as 473 K. However, this sensor shows no response to CH4 because the reactivity of CH 4 to oxidation is extremely poor at low operating temperatures [9,10]. Recently, tin-doped In2O3 (ITO) was reported as an electrode material for mixed-potential-type gas sensors because it has relatively high conductivity [11,12]. In this study, the feasibility of solid-state amperometric sensors using a LaGaO3-based electrolyte and an ITO-mixed noble metal as an active electrode for CH4 detection at low temperatures was studied. 2. Experimental In this study, 8 mol% Y2O3-stabilized ZrO2 (Tosoh 8YSZ), 20 mol% Sm-doped CeO2 (Daiichi Kigenso), and a LaGaO3based oxide were used as electrolytes; the layer of each electrolyte was 0.4 mm thick. The LaGaO3-based oxide was prepared by conventional solid-state reaction using an appropriate molar weight ratio of La2O3 (99.99%, Wako Pure Chemicals), Ga2O3 (99.99%, High Purity Chemical Co., Ltd.), SrCO3 (99.99%, Rare Metallic Co., Ltd.), MgO (99.9%, Wako
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Fig. 1. Effects of type of electrolyte on current change against CH4 concentration at 973 K. LSGM: La0.9Sr0.1Ga0.8Mg0.2O3 LSGMC: La0.8Sr0.2Ga0.8Mg0.15 Co0.05O3. SDC: Sm0.2Ce0.8O2 YSZ: Y0.16Zr0.84O2.
Pure Chemicals), and the corresponding metal oxide [13,14]. In an Al2O3 mortar, the powders were carefully mixed and then fired at 1273 K for 6 h. The resulting product was pulverized and isostatically pressed into disks at 274.6 MPa in vacuum. The disks were then sintered at 1773 K for 6 h. Electrode materials were also prepared by conventional solid-state reaction. For the active electrode, metal mixed with 10 mol% ITO was prepared by the incipient wetness method using mainly metal nitrate. In the case of Pd, PdCl2 was used as the source compound. Before coating, the precursor powder was calcined at 873 K for 6 h. For the inactive electrode, commercial RuO2 paste (Tanaka Kikinzoku) was mainly used without any pretreatment. Each electrode was painted on both surfaces of a 10mm-diameter electrolyte disk by the slurry coating method and then calcined at 873 K for 30 min in air. It is also noted that the BET surface area of ITO and RuO2 is smaller than 5 m2/g. The fabricated sensor was placed in a ceramic tube (ca. 150 cc) attached to a gas flow (50 cc/min) assembly and the external electrical contacts of its active and inactive electrodes with Pt and Au meshes, respectively, were confirmed. Commercial mixtures of 21% oxygen diluted with N2, and 1% CH4 in N2 diluted with air (21% O2) were used as base and sample gases, respectively. A constant potential of 1.0 V was applied across the electrodes (the RuO2 electrode is always positive) with dc power supply (Kikusui model PA18-3A). Sensor sensitivity was defined as the change in current against a one-order-of-magnitude change in CH4 concentration. The outlet gas was analyzed with gas chromatographs (Shimadzu GC-8A) with TCD detector.
(LSGM) = La 0 . 8 Sr 0 . 2 Ga 0 . 8 Mg 0 . 1 5 Co 0 . 0 5 O 3 (LSGMC) N Sm0.2Ce0.8O2(SDC) N Y0.16Zr0.84O2 (YSZ). Although the base current was varied with thickness of electrolyte, it is noted that the base current for the sensor examined is 20, 49, 6, and 2 mA for LSGM, LSGMC, SDC, and YSZ, respectively. Therefore, the order in sensitivity could be roughly explained by the change in the base current and by the higher sensor sensitivity tend to achieve with the use of an electrolyte with higher oxide ion conductivity [13,14]. Since the highest sensitivity is achieved with La0.9Sr0.1Ga0.8Mg0.2O3 (LSGM), the properties of a sensor using LSGM as the electrolyte were studied in detail. For the sensor using LSGM, the change in current with CH4 concentration is considered to be induced by the difference in oxidation activity between the active and inactive electrodes. Thus, the electrode material was optimized. Fig. 2 shows the effects of the type of metal mixed with La-doped CeO2 on the sensitivity to CH4. The sensitivity to CH4 increases in the order RuO2 N Pd N Rh N Pt. Moreover, a sensitivity of 97 µA/decade was obtained for the sensor using the RuO2-LDC electrode. However, from the results of outlet gas analysis, the Pd-LDC electrode shows more pronounced CH4 conversion, although its sensitivity to CH4 is smaller than that of the RuO2-LDC electrode. Therefore, in this study, a Pd-based oxide cermet was studied in detail. Fig. 3 shows the effects of the type of oxide mixed with Pd on the sensitivity to CH4. The sensitivity to CH4 was markedly changed by the metal oxide mixed with Pd. Evidently, among the Pd-based active electrodes examined, ITO exhibited the largest change in current (98 µA/decade) upon exposure to CH4. The high sensitivity to CH4 of this electrode could be explained by the high conductivity of ITO; this makes it clear that Pd-ITO is the most suitable active electrode for CH4 sensors. The sensitivity to CH4 is also strongly dependent on the type of inactive electrode. Fig. 4 shows the effects of the type of inactive electrode on the CH4 sensing property. The sensitivity to CH4 increases in the order Co3O4 b Ag b Au b RuO2. It is well known that Co3O4 is highly active to oxidation reaction. Therefore, the low sensitivity of the Co3O4 electrode could be explained by the occurrence of CH4 oxidation on the active and
3. Results and discussion Fig. 1 shows the effects of the type of electrolyte on the change in current against CH4 concentration at 973 K. Note that Pd-Ce0.6La0.4O2 (LDC) (9:1 weight ratio) and Au were used as the active and inactive electrodes, respectively. The figure shows that sensor sensitivity is strongly affected by the electrolyte material; it increases in the order La0.9Sr0.1Ga0.8Mg0.2O3
Fig. 2. Effects of type of metal mixed with La-doped CeO2 on sensitivity to CH4. (Au is used for the inactive electrode).
Z. Bi et al. / Solid State Ionics 179 (2008) 1641–1644
Fig. 3. Effects of type of oxide mixed with Pd on sensitivity to CH4.
inactive electrodes. Among the electrode combinations examined, the combination of RuO2 as the inactive electrode and PdITO as the active electrode showed the highest sensitivity to CH4. For this type of sensor, the sensitivity is as high as 527 µA/ decade. Fig. 5 shows the temperature dependences of the sensitivity to CH4 and response characteristics of the sensor upon its exposure to 1000 ppm CH4. Although the sensitivity to CH4 at 473 K was low, it increased monotonically with increasing operating temperature (data not shown). CH4 is well known to be highly stable against oxidation and hardly combusts below 500 K. In fact, the CH4 concentrations at the inlet and outlet of the measurement chamber were almost the same at 573 K. Similarly, the sensitivity to CH4 monotonically increased with increasing temperature from 573 K up to 773 K. Therefore, a higher operating temperature is more desirable from the viewpoint of sensitivity to CH4. The CH4 concentration at the outlet of the measurement chamber decreased with increasing operating temperature. Moreover, CH4 conversion monotonically increased with increasing operating temperature. At 773 K, the CH4 conversion rate was estimated to be ca. 60%. On the other hand, the temperature dependences of the 90% response and recovery characteristics of the sensor upon its
1643
Fig. 5. Temperature dependences of sensitivity and response characteristics of the sensor upon exposure to 1000 ppm CH4.
exposure to 1000 ppm CH4 are also shown in Fig. 5. The response and recovery characteristics of the sensor are poor; a rather long period (a few tens of minutes) is required for the response and recovery. Evidently, the recovery requires a longer period than the response, and at lower temperatures current occasionally showed no recovery. Since this sensor uses CH4 oxidation for detection and CH4 oxidation is not easily terminated, the recovery seems to require a longer period than the simple electronic interaction in semiconductor-type sensors. Therefore, improvements in the response and recovery characteristics are important issues for the development of CH4 sensors; these might be achieved through the use of a thinfilm electrolyte and a microelectrode. In order to confirm the detection mechanism of the sensor, open circuit potential was measured after exposure to CH4 at 773 K. The change in voltage was also observed by introducing CH4. In the experiment, the RuO2 electrode, which is nonresponsive to CH4 oxidation, was always positive. Therefore, it seems likely that mixed potential was generated in the sensor and that the applied potential of 1 V was changed with the formation of mixed potential resulting in a change in current. However, the change in current is much larger than the change in mixed potential, and the range of CH4 detection concentrations is much wider in the amperometric mode. Therefore, the amperometric mode is more suitable than the potentiometric mode for this sensor. 4. Conclusion
Fig. 4. Effects of inactive electrode on CH4 sensing property.
Although the selective detection of CH4 is highly difficult owing to the high chemical stability of CH4, it was found that the oxygen pumping current passing through a LaGaO3-based oxide markedly changes upon exposure to CH4 when Pd-ITO and RuO2 are used as the active and inactive electrodes, respectively. For all the oxide ion conductors examined, sensor sensitivity tends to increase with increasing oxide ion conductivity of the electrolyte. Although the response and recovery require a relatively long period for the sensor studied, a reproducible change in oxygen pumping current is observed
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over a wide range of CH4 concentrations. It is also noted that no response to CH4 is observed in our previous study [9,15], this sensor exhibits the much higher sensitivity to CH4. Comparing the potential response, it is seen that the current change is much larger and occurs in a wider range of CH4 concentrations. Therefore, an amperometric sensor using LSGM electrolyte and Pd-ITO and RuO2 as electrodes is highly sensitive for CH4 detection. References [1] M. Saha, A. Banerjee, A.K. Halder, J. Mondal, A. Sen, H.S. Maiti, Sens. Actuators B 79 (2001) 192–195. [2] B.K. Min, S.D. Choi, Sens. Actuators B 108 (2005) 119–124. [3] S. Bose, S. Chakraborty, B.K. Ghosh, D. Das, A. Sen, H.S. Maiti, Sens. Actuators B 105 (2005) 346–350. [4] N. Miura, T. Shiraishi, K. Shimanoe, N. Yamazoe, Electrochemi. Comm. 2 (2000) 77–80.
[5] J.H. Lee, B.K. Kim, K.Y. Lee, H.I. Kim, K.W. Han, Sens. Actuators B 59 (1999) 9–15. [6] W. Göpel, G. Reinhardt, M. Rösch, Solid State Ionics 136–137 (2000) 519–531. [7] E.L. Brosha, R. Mukundan, D.R. Brown, F.H. Garzon, Sens. Actuators B 87 (2003) 47–57. [8] T. Ishihara, H. Matsuda, Y. Takita, J. Am. Chem. Soc. 116 (1994) 3801–3803. [9] A. Dutta, T. Ishihara, H. Nishiguchi, Y. Takita, J. Electrochem. Soc. 151 (2004) H122–H127. [10] A. Dutta, H. Nishiguchi, Y. Takita, T. Ishihara, Sens. Actuators 108 (2005) 368–373. [11] X.G. Li, W. Xiong, G.M. Kale, Electrochem. Solid-State Lett. 8 (2005) H27–H30. [12] L.P. Martin, R.S. Glass, J. Electrochem. Soc. 152 (2005) H143–H147. [13] T. Ishihara, T. Shibayama, S. Ishikawa, K. Hosoi, H. Nishiguchi, Y. Takita, J. Eur. Ceram. Soc. 24 (2004) 1329–1335. [14] T. Ishihara, H. Furutani, M. Honda, T. Yamada, T. Shibayama, T. Akbay, N. Sakai, H. Yokokawa, Y. Takita, Chem. Mater. 11 (1999) 2081–2088. [15] T. Ishihara, M. Fukuyama, A. Dutta, K. Kabemura, H. Nishiguchi, Y. Takita, J. Electrochem. Soc. 150 (2003) H241–H245.
Solid State Ionics 179 (2008) 1641 – 1644 www.elsevier.com/locate/ssi
Solid-state amperometric CH4 sensor using, LaGaO3-based electrolyte Zhonghe Bi a , Hiroshige Matsumoto a,b , Tatsumi Ishihara a,b,⁎ a
Department of Applied Chemistry, Faculty of Engineering, Kyushu University, 744 Motooka, Nishi-Ku, Fukuoka 819-0395, Japan b Center of Future Chemistry, Kyushu University, 744 Motooka, Nishi-Ku, Fukuoka 819-0395, Japan Received 24 October 2007; received in revised form 21 November 2007; accepted 22 November 2007
Abstract A solid-state amperometric CH4 sensor using a LaGaO3-based electrolyte was studied. For this sensor, a combination of active (anode) and inactive (cathode) electrodes for CH4 oxidation was applied. Among the various active electrode materials studied, Pd mixed with tin-doped In2O3 (ITO) was found to be the most active electrode and RuO2 the most inactive one. The sensitivities of the sensor using La0.9Sr0.1Ga0.8Mg0.2O3 (LSGM) electrolyte to CH4 were approximately 797 µA/decade at 673 K and 188 µA/decade at 873 K. The changes in oxygen pumping current upon exposure to CH4 could be attributed to the mixed potential generated by the difference in oxidation activity between the active and inactive electrodes. © 2007 Elsevier B.V. All rights reserved. Keywords: CH4 sensor; Amperometric; LaGaO3-based oxide electrolyte
1. Introduction Methane (CH4), which is the main component of city gas, is highly explosive even at low concentrations. Therefore, methane detection has become an extremely important issue with increasing urbanization. In fact, in most urban areas in Japan, kitchens are recommended to be equipped with gas leakage sensors and SnO2-based semiconductor-type sensors are at present widely used for this purpose [1–3]. However, these sensors have a high operating temperature (N 573 K) and low selectivity. In order to improve the accuracy, increased selectivity is desirable. As examples of such sensors, mixedpotential-type sensors for detecting hydrocarbons in exhaust gas from internal combustion engines have been reported [4–7]. However, this type of sensor is not suitable as a CH4 sensor due to low reproducibility and low sensitivity to CH4. Previously, we found that LaGaO3 perovskite oxide exhibits high oxide ion conductivity [8] and that solid-state amperometric sensors using ⁎ Corresponding author. Department of Applied Chemistry, Faculty of Engineering, Kyushu University, Motooka 744, Nishi-Ku, Fukuoka 819-0395, Japan. Fax: +81 92 802 2871. E-mail address: [email protected] (T. Ishihara). 0167-2738/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2007.11.018
oxygen pumping current can be used as a hydrocarbon sensor. For example, the sensor using Fe-doped LaGaO3 exhibits high selectivity to C3H6 at temperatures as low as 473 K. However, this sensor shows no response to CH4 because the reactivity of CH 4 to oxidation is extremely poor at low operating temperatures [9,10]. Recently, tin-doped In2O3 (ITO) was reported as an electrode material for mixed-potential-type gas sensors because it has relatively high conductivity [11,12]. In this study, the feasibility of solid-state amperometric sensors using a LaGaO3-based electrolyte and an ITO-mixed noble metal as an active electrode for CH4 detection at low temperatures was studied. 2. Experimental In this study, 8 mol% Y2O3-stabilized ZrO2 (Tosoh 8YSZ), 20 mol% Sm-doped CeO2 (Daiichi Kigenso), and a LaGaO3based oxide were used as electrolytes; the layer of each electrolyte was 0.4 mm thick. The LaGaO3-based oxide was prepared by conventional solid-state reaction using an appropriate molar weight ratio of La2O3 (99.99%, Wako Pure Chemicals), Ga2O3 (99.99%, High Purity Chemical Co., Ltd.), SrCO3 (99.99%, Rare Metallic Co., Ltd.), MgO (99.9%, Wako
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Fig. 1. Effects of type of electrolyte on current change against CH4 concentration at 973 K. LSGM: La0.9Sr0.1Ga0.8Mg0.2O3 LSGMC: La0.8Sr0.2Ga0.8Mg0.15 Co0.05O3. SDC: Sm0.2Ce0.8O2 YSZ: Y0.16Zr0.84O2.
Pure Chemicals), and the corresponding metal oxide [13,14]. In an Al2O3 mortar, the powders were carefully mixed and then fired at 1273 K for 6 h. The resulting product was pulverized and isostatically pressed into disks at 274.6 MPa in vacuum. The disks were then sintered at 1773 K for 6 h. Electrode materials were also prepared by conventional solid-state reaction. For the active electrode, metal mixed with 10 mol% ITO was prepared by the incipient wetness method using mainly metal nitrate. In the case of Pd, PdCl2 was used as the source compound. Before coating, the precursor powder was calcined at 873 K for 6 h. For the inactive electrode, commercial RuO2 paste (Tanaka Kikinzoku) was mainly used without any pretreatment. Each electrode was painted on both surfaces of a 10mm-diameter electrolyte disk by the slurry coating method and then calcined at 873 K for 30 min in air. It is also noted that the BET surface area of ITO and RuO2 is smaller than 5 m2/g. The fabricated sensor was placed in a ceramic tube (ca. 150 cc) attached to a gas flow (50 cc/min) assembly and the external electrical contacts of its active and inactive electrodes with Pt and Au meshes, respectively, were confirmed. Commercial mixtures of 21% oxygen diluted with N2, and 1% CH4 in N2 diluted with air (21% O2) were used as base and sample gases, respectively. A constant potential of 1.0 V was applied across the electrodes (the RuO2 electrode is always positive) with dc power supply (Kikusui model PA18-3A). Sensor sensitivity was defined as the change in current against a one-order-of-magnitude change in CH4 concentration. The outlet gas was analyzed with gas chromatographs (Shimadzu GC-8A) with TCD detector.
(LSGM) = La 0 . 8 Sr 0 . 2 Ga 0 . 8 Mg 0 . 1 5 Co 0 . 0 5 O 3 (LSGMC) N Sm0.2Ce0.8O2(SDC) N Y0.16Zr0.84O2 (YSZ). Although the base current was varied with thickness of electrolyte, it is noted that the base current for the sensor examined is 20, 49, 6, and 2 mA for LSGM, LSGMC, SDC, and YSZ, respectively. Therefore, the order in sensitivity could be roughly explained by the change in the base current and by the higher sensor sensitivity tend to achieve with the use of an electrolyte with higher oxide ion conductivity [13,14]. Since the highest sensitivity is achieved with La0.9Sr0.1Ga0.8Mg0.2O3 (LSGM), the properties of a sensor using LSGM as the electrolyte were studied in detail. For the sensor using LSGM, the change in current with CH4 concentration is considered to be induced by the difference in oxidation activity between the active and inactive electrodes. Thus, the electrode material was optimized. Fig. 2 shows the effects of the type of metal mixed with La-doped CeO2 on the sensitivity to CH4. The sensitivity to CH4 increases in the order RuO2 N Pd N Rh N Pt. Moreover, a sensitivity of 97 µA/decade was obtained for the sensor using the RuO2-LDC electrode. However, from the results of outlet gas analysis, the Pd-LDC electrode shows more pronounced CH4 conversion, although its sensitivity to CH4 is smaller than that of the RuO2-LDC electrode. Therefore, in this study, a Pd-based oxide cermet was studied in detail. Fig. 3 shows the effects of the type of oxide mixed with Pd on the sensitivity to CH4. The sensitivity to CH4 was markedly changed by the metal oxide mixed with Pd. Evidently, among the Pd-based active electrodes examined, ITO exhibited the largest change in current (98 µA/decade) upon exposure to CH4. The high sensitivity to CH4 of this electrode could be explained by the high conductivity of ITO; this makes it clear that Pd-ITO is the most suitable active electrode for CH4 sensors. The sensitivity to CH4 is also strongly dependent on the type of inactive electrode. Fig. 4 shows the effects of the type of inactive electrode on the CH4 sensing property. The sensitivity to CH4 increases in the order Co3O4 b Ag b Au b RuO2. It is well known that Co3O4 is highly active to oxidation reaction. Therefore, the low sensitivity of the Co3O4 electrode could be explained by the occurrence of CH4 oxidation on the active and
3. Results and discussion Fig. 1 shows the effects of the type of electrolyte on the change in current against CH4 concentration at 973 K. Note that Pd-Ce0.6La0.4O2 (LDC) (9:1 weight ratio) and Au were used as the active and inactive electrodes, respectively. The figure shows that sensor sensitivity is strongly affected by the electrolyte material; it increases in the order La0.9Sr0.1Ga0.8Mg0.2O3
Fig. 2. Effects of type of metal mixed with La-doped CeO2 on sensitivity to CH4. (Au is used for the inactive electrode).
Z. Bi et al. / Solid State Ionics 179 (2008) 1641–1644
Fig. 3. Effects of type of oxide mixed with Pd on sensitivity to CH4.
inactive electrodes. Among the electrode combinations examined, the combination of RuO2 as the inactive electrode and PdITO as the active electrode showed the highest sensitivity to CH4. For this type of sensor, the sensitivity is as high as 527 µA/ decade. Fig. 5 shows the temperature dependences of the sensitivity to CH4 and response characteristics of the sensor upon its exposure to 1000 ppm CH4. Although the sensitivity to CH4 at 473 K was low, it increased monotonically with increasing operating temperature (data not shown). CH4 is well known to be highly stable against oxidation and hardly combusts below 500 K. In fact, the CH4 concentrations at the inlet and outlet of the measurement chamber were almost the same at 573 K. Similarly, the sensitivity to CH4 monotonically increased with increasing temperature from 573 K up to 773 K. Therefore, a higher operating temperature is more desirable from the viewpoint of sensitivity to CH4. The CH4 concentration at the outlet of the measurement chamber decreased with increasing operating temperature. Moreover, CH4 conversion monotonically increased with increasing operating temperature. At 773 K, the CH4 conversion rate was estimated to be ca. 60%. On the other hand, the temperature dependences of the 90% response and recovery characteristics of the sensor upon its
1643
Fig. 5. Temperature dependences of sensitivity and response characteristics of the sensor upon exposure to 1000 ppm CH4.
exposure to 1000 ppm CH4 are also shown in Fig. 5. The response and recovery characteristics of the sensor are poor; a rather long period (a few tens of minutes) is required for the response and recovery. Evidently, the recovery requires a longer period than the response, and at lower temperatures current occasionally showed no recovery. Since this sensor uses CH4 oxidation for detection and CH4 oxidation is not easily terminated, the recovery seems to require a longer period than the simple electronic interaction in semiconductor-type sensors. Therefore, improvements in the response and recovery characteristics are important issues for the development of CH4 sensors; these might be achieved through the use of a thinfilm electrolyte and a microelectrode. In order to confirm the detection mechanism of the sensor, open circuit potential was measured after exposure to CH4 at 773 K. The change in voltage was also observed by introducing CH4. In the experiment, the RuO2 electrode, which is nonresponsive to CH4 oxidation, was always positive. Therefore, it seems likely that mixed potential was generated in the sensor and that the applied potential of 1 V was changed with the formation of mixed potential resulting in a change in current. However, the change in current is much larger than the change in mixed potential, and the range of CH4 detection concentrations is much wider in the amperometric mode. Therefore, the amperometric mode is more suitable than the potentiometric mode for this sensor. 4. Conclusion
Fig. 4. Effects of inactive electrode on CH4 sensing property.
Although the selective detection of CH4 is highly difficult owing to the high chemical stability of CH4, it was found that the oxygen pumping current passing through a LaGaO3-based oxide markedly changes upon exposure to CH4 when Pd-ITO and RuO2 are used as the active and inactive electrodes, respectively. For all the oxide ion conductors examined, sensor sensitivity tends to increase with increasing oxide ion conductivity of the electrolyte. Although the response and recovery require a relatively long period for the sensor studied, a reproducible change in oxygen pumping current is observed
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over a wide range of CH4 concentrations. It is also noted that no response to CH4 is observed in our previous study [9,15], this sensor exhibits the much higher sensitivity to CH4. Comparing the potential response, it is seen that the current change is much larger and occurs in a wider range of CH4 concentrations. Therefore, an amperometric sensor using LSGM electrolyte and Pd-ITO and RuO2 as electrodes is highly sensitive for CH4 detection. References [1] M. Saha, A. Banerjee, A.K. Halder, J. Mondal, A. Sen, H.S. Maiti, Sens. Actuators B 79 (2001) 192–195. [2] B.K. Min, S.D. Choi, Sens. Actuators B 108 (2005) 119–124. [3] S. Bose, S. Chakraborty, B.K. Ghosh, D. Das, A. Sen, H.S. Maiti, Sens. Actuators B 105 (2005) 346–350. [4] N. Miura, T. Shiraishi, K. Shimanoe, N. Yamazoe, Electrochemi. Comm. 2 (2000) 77–80.
[5] J.H. Lee, B.K. Kim, K.Y. Lee, H.I. Kim, K.W. Han, Sens. Actuators B 59 (1999) 9–15. [6] W. Göpel, G. Reinhardt, M. Rösch, Solid State Ionics 136–137 (2000) 519–531. [7] E.L. Brosha, R. Mukundan, D.R. Brown, F.H. Garzon, Sens. Actuators B 87 (2003) 47–57. [8] T. Ishihara, H. Matsuda, Y. Takita, J. Am. Chem. Soc. 116 (1994) 3801–3803. [9] A. Dutta, T. Ishihara, H. Nishiguchi, Y. Takita, J. Electrochem. Soc. 151 (2004) H122–H127. [10] A. Dutta, H. Nishiguchi, Y. Takita, T. Ishihara, Sens. Actuators 108 (2005) 368–373. [11] X.G. Li, W. Xiong, G.M. Kale, Electrochem. Solid-State Lett. 8 (2005) H27–H30. [12] L.P. Martin, R.S. Glass, J. Electrochem. Soc. 152 (2005) H143–H147. [13] T. Ishihara, T. Shibayama, S. Ishikawa, K. Hosoi, H. Nishiguchi, Y. Takita, J. Eur. Ceram. Soc. 24 (2004) 1329–1335. [14] T. Ishihara, H. Furutani, M. Honda, T. Yamada, T. Shibayama, T. Akbay, N. Sakai, H. Yokokawa, Y. Takita, Chem. Mater. 11 (1999) 2081–2088. [15] T. Ishihara, M. Fukuyama, A. Dutta, K. Kabemura, H. Nishiguchi, Y. Takita, J. Electrochem. Soc. 150 (2003) H241–H245.