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Characteristics and performance of NASICON-based CO2 sensor using Bi8Nb2O17 plus Pt as solid-reference electrode
Sensors and Actuators B 178 (2013) 163–168
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Characteristics and performance of NASICON-based CO2 sensor using Bi8 Nb2 O17 plus Pt as solid-reference electrode Heng-Yao Dang, Xing-Min Guo ∗ State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, Beijing 100083, China
a r t i c l e
i n f o
Article history: Received 15 September 2012 Received in revised form 10 December 2012 Accepted 21 December 2012 Available online 31 December 2012 Keywords: NASICON-based CO2 sensors Solid-reference electrode Thermal expansivity CO2 detection limit
a b s t r a c t The gas sensing properties of NASICON-based CO2 sensor attached with a composite of Bi8 Nb2 O17 plus small amount of Pt as a solid-reference electrode has been investigated for application of the sensor at high CO2 concentration. Compared with conventional sensor coated with Pt reference electrode, the sensor attached with solid-reference electrode was found to efficiently improve the detection limit due to the presence of solid-reference electrode prevented NASICON reacting with CO2 and water to form carbonate or bicarbonate. A good bonding interface between the solid-reference electrode and NASICON was formed as revealed by the scanning electron spectroscopy (SEM), which provided a powerful guarantee for the thermal stability of the sensor. Furthermore, the as-fabricated sensor also exhibited fast response time, small cross-sensitivity to humidity and little interference from the coexistent oxygen partial pressure at 500 ◦ C. All the present results suggest that using the composite of Bi8 Nb2 O17 plus Pt as a solid-reference electrode is a promising candidate for NASICON-based CO2 sensor. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Development of compact, inexpensive, low energy consumption and reliable solid state sensors for measuring CO2 concentrations has been in high demand for numerous applications ranging from control of combustion processes to environmental monitoring [1]. So far, many types of solid state CO2 sensors based on optical properties [2], capacitance [3], field effect transistor (FET) [4] and electrochemical potential [5] have been investigated. Among them, the electrochemical-type is more suitable than others because CO2 is not a redox gas but an acid–base active gas, which usually reacts with ions of solid rather than electrons [6]. NASICON (Na3 Zr2 Si2 PO12 : Na+ -ion conductor)-based potentiometric CO2 sensors covered with a binary carbonate Li2 CO3 –BaCO3 (1:2 in molar ratio) are one of the most popular candidates due to their excellent performance such as good sensitivity, high selectivity and fast response time [7]. However, such sensors suffer from significant interference from humidity [8,9]. Thus the long-term stability of the sensors cannot be guaranteed. The existing researches confirm that the stability of the sensor is affected not only by the electrode composition [10,11], but also by the geometry of sensor [12–14], thermal treatment history [15], and the presence of humidity [16–18]. And the alkaline metal carbonates often used as auxiliary electrode was attached on one side of NASICON disk by
∗ Corresponding author. Tel.: +86 10 62334957; fax: +86 10 62334957. E-mail address: [email protected] (X.-M. Guo). 0925-4005/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2012.12.084
mechanical press [19], leading to poor chemical stability [20] and low mechanical strength due to the thermal expansivity of auxiliary electrode layer was much different from that of NASICON electrolyte. From this point of view, our previous work demonstrated that the use of Li2 CO3 , NASICON and Pt as auxiliary electrode enhanced the electrode mechanical strength as well as expanded the amount of triple phase boundaries (TPBs), giving a good performance of the sensor [21]. Sadaoka et al. [22] reported that the degradation of the performance of the sensors due to humidity resulted from the formation of NaHCO3 and NaOH at the grain boundary of the NASICON electrolyte, due to the easy diffusion of H2 O and CO2 into NASICON. And both Lee et al. [23] and Aono and Sadaoka [24] pointed out that the following reactions might occur when the reference electrode was not sealed. 2Na+ + CO2 + 1/2O2 + 2e− = Na2 CO3
(1)
or 2Na+ + H2 O + 2CO2 + 1/2O2 + 2e− = 2NaHCO3
(2)
Thus, the Na2 O activity in NASICON may probably be altered by the formation of these new compounds at the reference electrode, which results in the electromotive force (EMF) drift of the sensors. Especially, once the Na2 CO3 formed on the surface of reference electrode, the identical electrode reaction will occur at both auxiliary electrode and reference electrode, resulting in the EMF to deviate from Nernst equation. In an attempt to overcome such a problem, a promising approach seems to introduce a new reference electrode system that
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works as an oxygen-conduction membrane and physical barrier of hindering diffusion of CO2 and water vapor [25,26]. Up to now, some oxygen ion conductors such as yttrium stabilized zirconia (YSZ) [27] and BICUVOX [28–30] have been investigated. In the present work, we propose another fast oxygen ionic conductor of Bi8 Nb2 O17 (i.e. x = 0.2 in (1 − x)Bi2 O3 ·xNb2 O5 system) for reference electrode material to upgrade the performance of the sensor. As for oxygen ion conductors, the high-temperature form of bismuth oxide ı-Bi2 O3 has a face-centered cublic (fcc) structure with an average oxygen vacancy rate of 25%, is recognized as one of the best solid state oxygen ionic conductors, and Nb2 O5 can be added by 7.6–25.0 mol% to stabilize the cubic phase down to room temperature [31]. Meng et al. [32] reported that the conductivity of Bi8 Nb2 O17 was 1.9 × 10−1 S cm−1 at 700 ◦ C, which was approximately 1–2 orders of magnitude higher than that of YSZ [33]. In addition, the thermal expansion coefficient of ı-Bi2 O3 decreases with increasing Nb2 O5 content [34]. Thus, Nb2 O5 containing could be tailored to match the thermal expansivity of NASICON electrolyte. The applicability of Bi8 Nb2 O17 plus Pt as solid-reference electrode materials for the NASICON-based potentiometric CO2 sensor was examined in this work.
Fig. 1. Schematic diagram of the structure of the sensor device using Bi8 Nb2 O17 /Pt and Pt as two different kinds of reference electrodes, respectively.
2. Experimental 2.1. Preparation of materials NASICON solid electrolyte was prepared by a sol–gel method using Na3 PO4 ·12H2 O, Si(OC2 H5 )4 and ZrO(NO2 )·2H2 O as main reagents, as described in our previous work [35]. The as-prepared NASICON precursor was compacted into a disk (8 mm in diameter and 2 mm in thickness) in a stainless steel die at a pressure of 50 kN. The disk was then sintered at different temperature in air for 2 h to obtain crystalline NASICON. And Bi8 Nb2 O17 was prepared through a conventional solid-state reaction using a mixture of Bi2 O3 and Nb2 O5 in analytical reagent. The starting oxide powders were ball-milled for 10 h in the presence of ethanol solvent. The dried mixture was pretreated at 750 ◦ C for 5 h and then, after being ground in an agate mortar, calcined finally at 850 ◦ C for 5 h to obtain the final product. The prepared powders including NASICON and Bi8 Nb2 O17 were characterized by the means of Xray diffraction (XRD, MXP21VAHF, Japan). The thermal expansivity of Bi8 Nb2 O17 also was tested by a PCY-series thermal expansion instrument made by China.
order to obtain the detection limit, the sensor was tested under high CO2 concentration in the ranges of 20–90%. The total flow rate of the mixed gas was maintained at l L/min. The effect of relative humidity (RH) was measured by allowing the sample gas to bubble through water and then passed into the testing system. The potentials of sensing electrode referring to the solid-reference electrode and Pt reference electrode respectively, were monitored under dry as well as humidity conditions at 500 ◦ C. The EMF of the sensors was measured by an electrochemical workstation (CHI1140, Chenhua Instruments Inc., China). The response and recovery time for 90% saturation were denoted by Res and Rec, respectively. In order to check the thermal stability of the sensor, the sensor temperature was changed repeatedly between 400 ◦ C and 600 ◦ C by switching off and on the electric furnace. At the same time, the potentials of the sensing electrode referring to two different reference electrodes were measured separately with time. 3. Results and discussion 3.1. Characterization of NASICON and Bi8 Nb2 O17
2.2. Fabrication of sensor The schematic diagram of the sensor is shown in Fig. 1. A paste dispersing Li2 CO3 and NASICON powder into ˛-terpineol was deposited on the NASICON sintered disk together with Pt paste as a sensing electrode. The Bi8 Nb2 O17 plus a small amount of Pt (ca. 20 wt%) as a solid-reference electrode was screen-printed on the designated region of the NASICON disk. For comparison with the conventional Pt reference electrode, Pt paste was directly printed on the same side of the NASICON disk. Thus, the potential of sensing electrode at different CO2 concentration referring two different reference electrodes can be measured simultaneously. Then the asfabricated sensor was calcined at 600 ◦ C for 2 h in air to decompose ˛-terpineol. The interface structure between NASICON electrolyte and the solid-reference electrode (or Pt reference electrode) was examined by a scanning electron microscope (SEM, JSM-6480LV, JEOL, Japan). 2.3. CO2 sensing test All electrochemical measurements were performed in a conventional gas flow apparatus equipped with an electric furnace. In
The XRD patterns of NASICON and Bi8 Nb2 O17 powders are presented in Figs. 2 and 3, respectively. As shown in Fig. 2, a broad single peak (1 1 6) ascribable to NASICON phase could be clearly observed at diffracting angle of 30–31◦ for the sample sintered at 600 ◦ C, indicating the initial formation of crystalline NASICON. After being sintered at 900 ◦ C, all the characteristic peaks are in good agreement with the literature data [36] and without any second phase. Whereas ZrO2 exists as impurity when the sample sintered at 1100 ◦ C due to the volatilization loss of sodium or phosphorus at elevated temperature [37,38]. Therefore, the sample sintered at 900 ◦ C was used for the CO2 gas sensors in the present work. In Fig. 3, the XRD patterns of the as-synthesized Bi8 Nb2 O17 indicate that all the peaks correspond the characteristic peaks of JCPDS card No. 33-210. The temperature dependences of expansion of Bi8 Nb2 O17 sample (ϕ = 8 mm, L = 18 mm) are shown in Fig. 4. The mean thermal expansion coefficient calculated from the slop is 6.9 × 10−6 K−1 , which is very close to the value of NASICON material (ca. 6.0 × 10−6 K−1 ) [39]. The interface structure between NASICON electrolyte and two kinds of reference electrodes has been investigated by SEM analysis, as shown in Fig. 5. As for Pt reference electrode (Fig. 5(a)),
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Fig. 2. XRD patterns of NASICON powders sintered at different temperatures for 2 h.
Fig. 5. SEM image of the interface structure of Pt reference electrode (a) and Bi8 Nb2 O17 -Pt reference electrode (b).
Fig. 3. XRD patterns of Bi8 Nb2 O17 powders.
the thickness of Pt layer is about 30 m with porous structure. There is an obvious gap between NASICON and Pt electrode layer. This is because the thermal expansion coefficient of Pt (ca. 9.0 × 10−6 K−1 ) is much different from that of NASICON electrolyte
(ca. 6.0 × 10−6 K−1 ), leading to the thermal stress occurs [39]. To reduce the thermal stress, only in the form of very thin-film can Pt be applied on NASICON surface as a reference electrode. However, the very thin-film is hard to be fabricated by conventional screenprinting method. In contrast, the thickness of solid-reference electrode is about 50 m and firmly adhere to NASICON surface without any gap or crack as shown in Fig. 5(b). This is believed to be closely related to the low thermal expansivity of Bi8 Nb2 O17 , leading to reduce the thermal stress. The solid-reference electrode with a dense gastight morphology as physical barrier also could protect the NASICON from reacting with water vapor and CO2 [40]. 3.2. CO2 sensing properties Fig. 6 shows the EMF of sensing electrode separately referring to the solid-reference electrode and Pt reference electrode as a function of logarithm of partial pressure of CO2 at 500 ◦ C. For the solid-reference electrode, the EMF is linear to the logarithm of CO2 concentration, obeying the Nernst equation. The number of reaction electrons calculated from the slop of the correction equal to n = 2.0, in excellent agreement with the theoretical value for the following 2-electron reaction occurring at sensing electrode: Li2 CO3 = 2Li+ + CO2 + 1/2O2 + 2e−
Fig. 4. The thermal expansion curve as a function of temperature for Bi8 Nb2 O17 .
(3)
The reaction electron number (n) is 1.9 when Pt using as reference electrode. This may be caused by the slower electrode reaction as described by Satyanarayana et al. [41] due to the presence of
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Fig. 6. Dependence of the sensing electrode potential separately relative to the solid-reference electrode and Pt reference electrodes at 500 ◦ C.
gap between NASICON and Pt electrode layer (as seen in Fig. 5(a)). More seriously, the EMF significantly deviates from the straight line when the concentration exceeds 60%. In other words, the CO2 change cannot be effectively recognized by the sensor. This result proves that NaHCO3 or Na2 CO3 may be formed at the reference electrode, as mentioned in the previous section. Base on the thermodynamic calculation result, Wang and Kumar [42] demonstrated that Na2 CO3 could be formed at the reference electrode when CO2 partial pressure was higher than 104 Pa at 500 ◦ C. In addition, Kishi et al. [43] pointed out that Na+ -ion in NASICON may react with CO2 to form carbonate when Pt was directly used as the reference electrode, which caused the deviation of electrode potential. In contrast, this situation would not be occurred for the solid-reference electrode. This is because that H2 O and CO2 are cut off from contacting with NASICON by the dense solid-reference electrode layer. The obtained results clearly suggest that the solid-reference electrode can significantly improve detection limit of the sensor. In general, the EMF of sensor should be independent of coexisting oxygen partial pressure in the gas phase [44]. Fig. 7 shows the variation of EMF as a function of coexisting oxygen partial pressure at 500 ◦ C for solid-reference electrode. It is noted that the EMF maintains almost at a constant value for each fixed CO2 concentration and fluctuates within ±1 mV. The obtained
Fig. 7. Variation of EMF as a function of coexisting oxygen partial pressure at 500 ◦ C for solid-reference electrode.
Fig. 8. Response transients of sensor attached with two different reference electrodes to CO2 under 0% and 70% RH at 500 ◦ C: (a) Pt reference electrode and (b) Bi8 Nb2 O17 -Pt reference electrode.
results demonstrate that the solid-reference electrode successfully eliminates any coexisting oxygen interference, confirming the Bi8 Nb2 O17 is promising as a stable solid-reference electrode material for NASICON-based CO2 sensors. The response transients of sensor attached with two different reference electrodes to CO2 gas under 0% and 70% RH at 500 ◦ C are shown in Fig. 8. The response and recovery time of sensor attached with solid-reference electrode (Res: 57 s and Rec: 65 s) are a bit longer than that of sensor attached with Pt reference electrode (Res: 47 s and Rec: 60 s), but still acceptable. The reason is that open Pt reference electrode is more favorable to adsorption/desorption of gases. On the other hand, the sensor attached with solid-reference electrode exhibits a smaller cross-sensitivity to water vapor (4.5 mV) than that of Pt reference electrode (10 mV). This is because the dense solid-reference electrode acts as a natural physical barrier of humidity. These results powerfully suggest that the solid-reference electrode can effectively improve the waterresistance of sensor. 3.3. Thermal stability of the sensor In general, it is required that electrochemical sensor lifetime should last at least 2–3 years as described in the previous work [45] because the electrode or electrolyte will be used up as time passes, which is caused by the electrode reaction (oxidization or reduction). However, the sensor lifetime will be shortened by some other factors including humidity, temperature, mechanical strength of the electrode and so on. In particular, the sensor will inevitably endure the heating–cooling process in its practical application. Thus, the performance of thermal stability will affect the sensor lifetime directly. Fig. 9 shows the thermal stability of the sensor attached with two different reference electrodes. During the heat-cycle test, the temperature of sensor was switched repeatedly between 400 ◦ C and 600 ◦ C. The accurate temperature of the sensor was calibrated by K-thermocouple and is also presented in Fig. 9. As shown in Fig. 9, there is an EMF-overshooting at the initial period for each platform and thereafter the EMF becomes stable. This phenomenon can be ascribed to the heat-inertia of electric furnace as seen from the temperature curve (dash line). At the two adjacent temperature platforms of 400 ◦ C, the EMF of sensor with solid-reference electrode was located at the same potential level as shown by dotted lines. However, in the case of Pt reference electrode, the EMF at second 400 ◦ C platform did not
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Fig. 9. Behavior of EMF for the sensor attached with two different reference electrodes during the heat-cycle test in 20% CO2 .
recover to the level of the first 400 ◦ C platform (EMF: 7 mV) even when the temperature was allowed to equilibrate for 2 h. At two 600 ◦ C platforms, the EMF of the sensor continuously shifts upward with time. It can be explained that the sodium carbonate may be easily formed at elevated temperature due to the Na2 O activity in NASICON increased with the temperature as demonstrated in Ref. [42]. Additionally, the gap between Pt reference electrode and NASICON as shown in Fig. 5(a) may be expanded at elevated temperature since the thermal expansivity of Pt is great different from NASICON electrolyte. From the heat-cycle test, we can draw a conclusion that the thermal stability of the sensor attached with the solid-reference electrode is more preferable, which is believed to be closely related with the good bonding interface between the solid-reference electrode and NASICON electrolyte. 4. Conclusions In this work, a composite of Bi8 Nb2 O17 and Pt as a solidreference electrode material has been prepared for NASICON-based CO2 sensors. The EMF of the as-fabricated sensor is linear to the logarithm of CO2 concentration, obeying Nernst’s correction. Moreover, no any obvious interference from oxygen partial pressure is observed. In comparison, there is no the deserved detectability above 60% CO2 for the sensor using Pt as reference electrode, suggesting that the solid-reference electrode can significantly improve detection limit of the sensor. The solid-reference electrode also showed smaller humidity interference from water vapor than that of Pt reference electrode. And the thermal stability of the sensor attached with the solid-reference electrode is preferable as revealed from the thermal cycle test. All the obtained results suggested the composite of Bi8 Nb2 O17 plus Pt as a solid-reference electrode is suitable for NASICON-based CO2 sensor. Acknowledgements This work was supported in part by the National Natural Science Foundation of China (No. 50974012) and Program Changjiang Scholars and Innovative Research Team in University (No. 0708). References [1] J.W. Fergus, Sensing mechanism of non-equilibrium solid-electrolyte-based chemical sensors, Journal of Solid State Electrochemistry 15 (2011) 971–984. [2] R. Ulber, J.G. Frerichs, S. Beutel, Optical sensor systems for bioprocess monitoring, Analytical and Bioanalytical Chemistry 376 (2003) 342–348. [3] H.E. Endres, R. Hartinger, M. Schwaiger, G. Gmelch, M. Roth, A capacitive CO2 sensor system with suppression of the humidity interfercnce, Sensors and Acturators B: Chemistry 57 (1999) 83–87.
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Biographies Heng-Yao Dang received the B.S. degree in applied physics from the Inner Mongolia University of Science and Technology, Baitou, China, in 2007. He has been pursuing the Ph.D. degree in physical chemistry of metallurgy with the University of Science and Technology Beijing, China, since 2007. His current research interests include solid electrolyte base CO2 sensors. Xing-Min Guo received the Ph.D. degree from Northeastern University, Shenyang, China, in 1992. He is a professor with the University of Science and Technology Beijing, Beijing, China. His current research interests include iron-minerals utilization, electrochemical metallurgy, and electrochemical gas sensors.
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Characteristics and performance of NASICON-based CO2 sensor using Bi8 Nb2 O17 plus Pt as solid-reference electrode Heng-Yao Dang, Xing-Min Guo ∗ State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, Beijing 100083, China
a r t i c l e
i n f o
Article history: Received 15 September 2012 Received in revised form 10 December 2012 Accepted 21 December 2012 Available online 31 December 2012 Keywords: NASICON-based CO2 sensors Solid-reference electrode Thermal expansivity CO2 detection limit
a b s t r a c t The gas sensing properties of NASICON-based CO2 sensor attached with a composite of Bi8 Nb2 O17 plus small amount of Pt as a solid-reference electrode has been investigated for application of the sensor at high CO2 concentration. Compared with conventional sensor coated with Pt reference electrode, the sensor attached with solid-reference electrode was found to efficiently improve the detection limit due to the presence of solid-reference electrode prevented NASICON reacting with CO2 and water to form carbonate or bicarbonate. A good bonding interface between the solid-reference electrode and NASICON was formed as revealed by the scanning electron spectroscopy (SEM), which provided a powerful guarantee for the thermal stability of the sensor. Furthermore, the as-fabricated sensor also exhibited fast response time, small cross-sensitivity to humidity and little interference from the coexistent oxygen partial pressure at 500 ◦ C. All the present results suggest that using the composite of Bi8 Nb2 O17 plus Pt as a solid-reference electrode is a promising candidate for NASICON-based CO2 sensor. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Development of compact, inexpensive, low energy consumption and reliable solid state sensors for measuring CO2 concentrations has been in high demand for numerous applications ranging from control of combustion processes to environmental monitoring [1]. So far, many types of solid state CO2 sensors based on optical properties [2], capacitance [3], field effect transistor (FET) [4] and electrochemical potential [5] have been investigated. Among them, the electrochemical-type is more suitable than others because CO2 is not a redox gas but an acid–base active gas, which usually reacts with ions of solid rather than electrons [6]. NASICON (Na3 Zr2 Si2 PO12 : Na+ -ion conductor)-based potentiometric CO2 sensors covered with a binary carbonate Li2 CO3 –BaCO3 (1:2 in molar ratio) are one of the most popular candidates due to their excellent performance such as good sensitivity, high selectivity and fast response time [7]. However, such sensors suffer from significant interference from humidity [8,9]. Thus the long-term stability of the sensors cannot be guaranteed. The existing researches confirm that the stability of the sensor is affected not only by the electrode composition [10,11], but also by the geometry of sensor [12–14], thermal treatment history [15], and the presence of humidity [16–18]. And the alkaline metal carbonates often used as auxiliary electrode was attached on one side of NASICON disk by
∗ Corresponding author. Tel.: +86 10 62334957; fax: +86 10 62334957. E-mail address: [email protected] (X.-M. Guo). 0925-4005/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2012.12.084
mechanical press [19], leading to poor chemical stability [20] and low mechanical strength due to the thermal expansivity of auxiliary electrode layer was much different from that of NASICON electrolyte. From this point of view, our previous work demonstrated that the use of Li2 CO3 , NASICON and Pt as auxiliary electrode enhanced the electrode mechanical strength as well as expanded the amount of triple phase boundaries (TPBs), giving a good performance of the sensor [21]. Sadaoka et al. [22] reported that the degradation of the performance of the sensors due to humidity resulted from the formation of NaHCO3 and NaOH at the grain boundary of the NASICON electrolyte, due to the easy diffusion of H2 O and CO2 into NASICON. And both Lee et al. [23] and Aono and Sadaoka [24] pointed out that the following reactions might occur when the reference electrode was not sealed. 2Na+ + CO2 + 1/2O2 + 2e− = Na2 CO3
(1)
or 2Na+ + H2 O + 2CO2 + 1/2O2 + 2e− = 2NaHCO3
(2)
Thus, the Na2 O activity in NASICON may probably be altered by the formation of these new compounds at the reference electrode, which results in the electromotive force (EMF) drift of the sensors. Especially, once the Na2 CO3 formed on the surface of reference electrode, the identical electrode reaction will occur at both auxiliary electrode and reference electrode, resulting in the EMF to deviate from Nernst equation. In an attempt to overcome such a problem, a promising approach seems to introduce a new reference electrode system that
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works as an oxygen-conduction membrane and physical barrier of hindering diffusion of CO2 and water vapor [25,26]. Up to now, some oxygen ion conductors such as yttrium stabilized zirconia (YSZ) [27] and BICUVOX [28–30] have been investigated. In the present work, we propose another fast oxygen ionic conductor of Bi8 Nb2 O17 (i.e. x = 0.2 in (1 − x)Bi2 O3 ·xNb2 O5 system) for reference electrode material to upgrade the performance of the sensor. As for oxygen ion conductors, the high-temperature form of bismuth oxide ı-Bi2 O3 has a face-centered cublic (fcc) structure with an average oxygen vacancy rate of 25%, is recognized as one of the best solid state oxygen ionic conductors, and Nb2 O5 can be added by 7.6–25.0 mol% to stabilize the cubic phase down to room temperature [31]. Meng et al. [32] reported that the conductivity of Bi8 Nb2 O17 was 1.9 × 10−1 S cm−1 at 700 ◦ C, which was approximately 1–2 orders of magnitude higher than that of YSZ [33]. In addition, the thermal expansion coefficient of ı-Bi2 O3 decreases with increasing Nb2 O5 content [34]. Thus, Nb2 O5 containing could be tailored to match the thermal expansivity of NASICON electrolyte. The applicability of Bi8 Nb2 O17 plus Pt as solid-reference electrode materials for the NASICON-based potentiometric CO2 sensor was examined in this work.
Fig. 1. Schematic diagram of the structure of the sensor device using Bi8 Nb2 O17 /Pt and Pt as two different kinds of reference electrodes, respectively.
2. Experimental 2.1. Preparation of materials NASICON solid electrolyte was prepared by a sol–gel method using Na3 PO4 ·12H2 O, Si(OC2 H5 )4 and ZrO(NO2 )·2H2 O as main reagents, as described in our previous work [35]. The as-prepared NASICON precursor was compacted into a disk (8 mm in diameter and 2 mm in thickness) in a stainless steel die at a pressure of 50 kN. The disk was then sintered at different temperature in air for 2 h to obtain crystalline NASICON. And Bi8 Nb2 O17 was prepared through a conventional solid-state reaction using a mixture of Bi2 O3 and Nb2 O5 in analytical reagent. The starting oxide powders were ball-milled for 10 h in the presence of ethanol solvent. The dried mixture was pretreated at 750 ◦ C for 5 h and then, after being ground in an agate mortar, calcined finally at 850 ◦ C for 5 h to obtain the final product. The prepared powders including NASICON and Bi8 Nb2 O17 were characterized by the means of Xray diffraction (XRD, MXP21VAHF, Japan). The thermal expansivity of Bi8 Nb2 O17 also was tested by a PCY-series thermal expansion instrument made by China.
order to obtain the detection limit, the sensor was tested under high CO2 concentration in the ranges of 20–90%. The total flow rate of the mixed gas was maintained at l L/min. The effect of relative humidity (RH) was measured by allowing the sample gas to bubble through water and then passed into the testing system. The potentials of sensing electrode referring to the solid-reference electrode and Pt reference electrode respectively, were monitored under dry as well as humidity conditions at 500 ◦ C. The EMF of the sensors was measured by an electrochemical workstation (CHI1140, Chenhua Instruments Inc., China). The response and recovery time for 90% saturation were denoted by Res and Rec, respectively. In order to check the thermal stability of the sensor, the sensor temperature was changed repeatedly between 400 ◦ C and 600 ◦ C by switching off and on the electric furnace. At the same time, the potentials of the sensing electrode referring to two different reference electrodes were measured separately with time. 3. Results and discussion 3.1. Characterization of NASICON and Bi8 Nb2 O17
2.2. Fabrication of sensor The schematic diagram of the sensor is shown in Fig. 1. A paste dispersing Li2 CO3 and NASICON powder into ˛-terpineol was deposited on the NASICON sintered disk together with Pt paste as a sensing electrode. The Bi8 Nb2 O17 plus a small amount of Pt (ca. 20 wt%) as a solid-reference electrode was screen-printed on the designated region of the NASICON disk. For comparison with the conventional Pt reference electrode, Pt paste was directly printed on the same side of the NASICON disk. Thus, the potential of sensing electrode at different CO2 concentration referring two different reference electrodes can be measured simultaneously. Then the asfabricated sensor was calcined at 600 ◦ C for 2 h in air to decompose ˛-terpineol. The interface structure between NASICON electrolyte and the solid-reference electrode (or Pt reference electrode) was examined by a scanning electron microscope (SEM, JSM-6480LV, JEOL, Japan). 2.3. CO2 sensing test All electrochemical measurements were performed in a conventional gas flow apparatus equipped with an electric furnace. In
The XRD patterns of NASICON and Bi8 Nb2 O17 powders are presented in Figs. 2 and 3, respectively. As shown in Fig. 2, a broad single peak (1 1 6) ascribable to NASICON phase could be clearly observed at diffracting angle of 30–31◦ for the sample sintered at 600 ◦ C, indicating the initial formation of crystalline NASICON. After being sintered at 900 ◦ C, all the characteristic peaks are in good agreement with the literature data [36] and without any second phase. Whereas ZrO2 exists as impurity when the sample sintered at 1100 ◦ C due to the volatilization loss of sodium or phosphorus at elevated temperature [37,38]. Therefore, the sample sintered at 900 ◦ C was used for the CO2 gas sensors in the present work. In Fig. 3, the XRD patterns of the as-synthesized Bi8 Nb2 O17 indicate that all the peaks correspond the characteristic peaks of JCPDS card No. 33-210. The temperature dependences of expansion of Bi8 Nb2 O17 sample (ϕ = 8 mm, L = 18 mm) are shown in Fig. 4. The mean thermal expansion coefficient calculated from the slop is 6.9 × 10−6 K−1 , which is very close to the value of NASICON material (ca. 6.0 × 10−6 K−1 ) [39]. The interface structure between NASICON electrolyte and two kinds of reference electrodes has been investigated by SEM analysis, as shown in Fig. 5. As for Pt reference electrode (Fig. 5(a)),
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Fig. 2. XRD patterns of NASICON powders sintered at different temperatures for 2 h.
Fig. 5. SEM image of the interface structure of Pt reference electrode (a) and Bi8 Nb2 O17 -Pt reference electrode (b).
Fig. 3. XRD patterns of Bi8 Nb2 O17 powders.
the thickness of Pt layer is about 30 m with porous structure. There is an obvious gap between NASICON and Pt electrode layer. This is because the thermal expansion coefficient of Pt (ca. 9.0 × 10−6 K−1 ) is much different from that of NASICON electrolyte
(ca. 6.0 × 10−6 K−1 ), leading to the thermal stress occurs [39]. To reduce the thermal stress, only in the form of very thin-film can Pt be applied on NASICON surface as a reference electrode. However, the very thin-film is hard to be fabricated by conventional screenprinting method. In contrast, the thickness of solid-reference electrode is about 50 m and firmly adhere to NASICON surface without any gap or crack as shown in Fig. 5(b). This is believed to be closely related to the low thermal expansivity of Bi8 Nb2 O17 , leading to reduce the thermal stress. The solid-reference electrode with a dense gastight morphology as physical barrier also could protect the NASICON from reacting with water vapor and CO2 [40]. 3.2. CO2 sensing properties Fig. 6 shows the EMF of sensing electrode separately referring to the solid-reference electrode and Pt reference electrode as a function of logarithm of partial pressure of CO2 at 500 ◦ C. For the solid-reference electrode, the EMF is linear to the logarithm of CO2 concentration, obeying the Nernst equation. The number of reaction electrons calculated from the slop of the correction equal to n = 2.0, in excellent agreement with the theoretical value for the following 2-electron reaction occurring at sensing electrode: Li2 CO3 = 2Li+ + CO2 + 1/2O2 + 2e−
Fig. 4. The thermal expansion curve as a function of temperature for Bi8 Nb2 O17 .
(3)
The reaction electron number (n) is 1.9 when Pt using as reference electrode. This may be caused by the slower electrode reaction as described by Satyanarayana et al. [41] due to the presence of
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Fig. 6. Dependence of the sensing electrode potential separately relative to the solid-reference electrode and Pt reference electrodes at 500 ◦ C.
gap between NASICON and Pt electrode layer (as seen in Fig. 5(a)). More seriously, the EMF significantly deviates from the straight line when the concentration exceeds 60%. In other words, the CO2 change cannot be effectively recognized by the sensor. This result proves that NaHCO3 or Na2 CO3 may be formed at the reference electrode, as mentioned in the previous section. Base on the thermodynamic calculation result, Wang and Kumar [42] demonstrated that Na2 CO3 could be formed at the reference electrode when CO2 partial pressure was higher than 104 Pa at 500 ◦ C. In addition, Kishi et al. [43] pointed out that Na+ -ion in NASICON may react with CO2 to form carbonate when Pt was directly used as the reference electrode, which caused the deviation of electrode potential. In contrast, this situation would not be occurred for the solid-reference electrode. This is because that H2 O and CO2 are cut off from contacting with NASICON by the dense solid-reference electrode layer. The obtained results clearly suggest that the solid-reference electrode can significantly improve detection limit of the sensor. In general, the EMF of sensor should be independent of coexisting oxygen partial pressure in the gas phase [44]. Fig. 7 shows the variation of EMF as a function of coexisting oxygen partial pressure at 500 ◦ C for solid-reference electrode. It is noted that the EMF maintains almost at a constant value for each fixed CO2 concentration and fluctuates within ±1 mV. The obtained
Fig. 7. Variation of EMF as a function of coexisting oxygen partial pressure at 500 ◦ C for solid-reference electrode.
Fig. 8. Response transients of sensor attached with two different reference electrodes to CO2 under 0% and 70% RH at 500 ◦ C: (a) Pt reference electrode and (b) Bi8 Nb2 O17 -Pt reference electrode.
results demonstrate that the solid-reference electrode successfully eliminates any coexisting oxygen interference, confirming the Bi8 Nb2 O17 is promising as a stable solid-reference electrode material for NASICON-based CO2 sensors. The response transients of sensor attached with two different reference electrodes to CO2 gas under 0% and 70% RH at 500 ◦ C are shown in Fig. 8. The response and recovery time of sensor attached with solid-reference electrode (Res: 57 s and Rec: 65 s) are a bit longer than that of sensor attached with Pt reference electrode (Res: 47 s and Rec: 60 s), but still acceptable. The reason is that open Pt reference electrode is more favorable to adsorption/desorption of gases. On the other hand, the sensor attached with solid-reference electrode exhibits a smaller cross-sensitivity to water vapor (4.5 mV) than that of Pt reference electrode (10 mV). This is because the dense solid-reference electrode acts as a natural physical barrier of humidity. These results powerfully suggest that the solid-reference electrode can effectively improve the waterresistance of sensor. 3.3. Thermal stability of the sensor In general, it is required that electrochemical sensor lifetime should last at least 2–3 years as described in the previous work [45] because the electrode or electrolyte will be used up as time passes, which is caused by the electrode reaction (oxidization or reduction). However, the sensor lifetime will be shortened by some other factors including humidity, temperature, mechanical strength of the electrode and so on. In particular, the sensor will inevitably endure the heating–cooling process in its practical application. Thus, the performance of thermal stability will affect the sensor lifetime directly. Fig. 9 shows the thermal stability of the sensor attached with two different reference electrodes. During the heat-cycle test, the temperature of sensor was switched repeatedly between 400 ◦ C and 600 ◦ C. The accurate temperature of the sensor was calibrated by K-thermocouple and is also presented in Fig. 9. As shown in Fig. 9, there is an EMF-overshooting at the initial period for each platform and thereafter the EMF becomes stable. This phenomenon can be ascribed to the heat-inertia of electric furnace as seen from the temperature curve (dash line). At the two adjacent temperature platforms of 400 ◦ C, the EMF of sensor with solid-reference electrode was located at the same potential level as shown by dotted lines. However, in the case of Pt reference electrode, the EMF at second 400 ◦ C platform did not
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Fig. 9. Behavior of EMF for the sensor attached with two different reference electrodes during the heat-cycle test in 20% CO2 .
recover to the level of the first 400 ◦ C platform (EMF: 7 mV) even when the temperature was allowed to equilibrate for 2 h. At two 600 ◦ C platforms, the EMF of the sensor continuously shifts upward with time. It can be explained that the sodium carbonate may be easily formed at elevated temperature due to the Na2 O activity in NASICON increased with the temperature as demonstrated in Ref. [42]. Additionally, the gap between Pt reference electrode and NASICON as shown in Fig. 5(a) may be expanded at elevated temperature since the thermal expansivity of Pt is great different from NASICON electrolyte. From the heat-cycle test, we can draw a conclusion that the thermal stability of the sensor attached with the solid-reference electrode is more preferable, which is believed to be closely related with the good bonding interface between the solid-reference electrode and NASICON electrolyte. 4. Conclusions In this work, a composite of Bi8 Nb2 O17 and Pt as a solidreference electrode material has been prepared for NASICON-based CO2 sensors. The EMF of the as-fabricated sensor is linear to the logarithm of CO2 concentration, obeying Nernst’s correction. Moreover, no any obvious interference from oxygen partial pressure is observed. In comparison, there is no the deserved detectability above 60% CO2 for the sensor using Pt as reference electrode, suggesting that the solid-reference electrode can significantly improve detection limit of the sensor. The solid-reference electrode also showed smaller humidity interference from water vapor than that of Pt reference electrode. And the thermal stability of the sensor attached with the solid-reference electrode is preferable as revealed from the thermal cycle test. All the obtained results suggested the composite of Bi8 Nb2 O17 plus Pt as a solid-reference electrode is suitable for NASICON-based CO2 sensor. Acknowledgements This work was supported in part by the National Natural Science Foundation of China (No. 50974012) and Program Changjiang Scholars and Innovative Research Team in University (No. 0708). References [1] J.W. Fergus, Sensing mechanism of non-equilibrium solid-electrolyte-based chemical sensors, Journal of Solid State Electrochemistry 15 (2011) 971–984. [2] R. Ulber, J.G. Frerichs, S. Beutel, Optical sensor systems for bioprocess monitoring, Analytical and Bioanalytical Chemistry 376 (2003) 342–348. [3] H.E. Endres, R. Hartinger, M. Schwaiger, G. Gmelch, M. Roth, A capacitive CO2 sensor system with suppression of the humidity interfercnce, Sensors and Acturators B: Chemistry 57 (1999) 83–87.
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Biographies Heng-Yao Dang received the B.S. degree in applied physics from the Inner Mongolia University of Science and Technology, Baitou, China, in 2007. He has been pursuing the Ph.D. degree in physical chemistry of metallurgy with the University of Science and Technology Beijing, China, since 2007. His current research interests include solid electrolyte base CO2 sensors. Xing-Min Guo received the Ph.D. degree from Northeastern University, Shenyang, China, in 1992. He is a professor with the University of Science and Technology Beijing, Beijing, China. His current research interests include iron-minerals utilization, electrochemical metallurgy, and electrochemical gas sensors.