A novel carbon dioxide gas sensor based on solid bielectrolyte

Sensors and Actuators B 88 (2003) 292±299

A novel carbon dioxide gas sensor based on solid bielectrolyte Ling Wang, R.V. Kumar* Department of Materials Science and Metallurgy, University of Cambridge, New Museums Site, Pembroke Street, Cambridge CB2 3QZ, UK Received 15 July 2002; received in revised form 18 September 2002; accepted 25 September 2002

Abstract A planar bielectrolyte type of CO2 gas sensor based on Nasicon and Na±b-alumina solid electrolyte pellets with BaCO3±Na2CO3 (weight ratio 44:56) as the auxiliary phase was fabricated. The sensor had good response to CO2 over a wide range covering 6 ppm to 100%. The Nasicon/Pt interface was sensitive to oxygen, while auxiliary phase applied at low temperature on the sodium rich Na±b-alumina was stable against decomposition. In this mode, both electrodes are exposed to the same test gas eliminating the need for a separate reference compartment and O2 concentration does not affect cell EMF. Interface between Nasicon and Na±b-alumina was studied by SEM and EDAX. Diffusion bonding between the two solid electrolytes results in phase change, which affects the electrical properties of electrolyte system. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Carbon dioxide; Gas sensor; Bielectrolyte; Solid electrolyte

1. Introduction In recent years, devices for continuously detecting CO2 have been in demand for environmental monitoring. Among the various methods for detecting carbon dioxide, solid-state electrochemical methods are the most promising. In particularly, all-solid potentiometric CO2 sensors based on solid-state electrolytes have drawn much attention because of their reliability, selectivity, compactness, low cost, simple structure, high sensing performances and compatibility with microelectronics. Therefore, a considerable amount of research work has been carried out for developing CO2 sensors [1±6]. Many potentiometric CO2 sensors are based on Na‡ or Li‡ conductors using solids or gaseous reference electrodes. In these examples, two conditions must be met. Firstly, the electrolyte must be dense enough to prevent gas from permeating through; secondly, good encapsulation must be realised between the sensing and the reference electrode compartment. Normally, the sensors are operated at high and frequently varying temperatures, which makes it more dif®cult to maintain a good sealing for long time. Moreover, sensor response to CO2 is found to be a function of concentration of O2, which is invariably present. In order to overcome problems of sealing and O2 interference many research works have been done which can be divided into three types. * Corresponding author. Tel.: ‡44-1223-334327; fax: 44-1223-334567. E-mail address: [email protected] (R.V. Kumar).

1. Planar-type CO2 sensors based on a single electrolyte [7± 10]. The sensors can be expressed as: O2, CO2, Pt|Na‡(Li‡) conductors| Na2CO3, Pt, CO2, O2. In this case, only a single electrolyte, Na‡ or Li‡ conductor was used. Na2O or Li2O activity on one side of electrolyte ®xed by the electrolyte composition and temperature serves as the reference electrode. The second interface containing the auxiliary phase is used as the sensing electrode. Both electrodes were exposed to same atmosphere, so O2 partial pressure is identical and therefore no O2 interference and sealing problems exist. 2. CO2 sensors with open reference electrodes [11±14]: a typical cell of the kind of sensors can be described as: O2, CO2, Au, Na2SnO3, SnO2|Na‡ conductors| Na2CO3, Au, CO2, O2. In this cell, the activity of Na2O of the reference electrode (left-hand side) is fixed by the two phases mixture. The sodium activity in Na2O is fixed by the partial pressure of O2 in the test gas. As the two phases mixture is open to the same atmosphere as the sensing electrode, they can be used only when the reference materials are chemically stable against CO2. Most compounds based on Na2O can react with CO2 at elevated temperature making the sensor stable only for short duration. 3. CO2 sensors based on interfacing two electrolytes [15±22]. As an extension of the auxiliary phase method, sensors can be developed by interfacing two or more electrolytes. For example, oxygen ion conductor (YSZ)/

0925-4005/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 4 0 0 5 ( 0 2 ) 0 0 3 7 2 - 6

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sodium ion conductor (Nasicon) junction can be used to develop a CO2 sensor. These sensors do not require separation of working and reference compartments and oxygen is self-cancelling. Because of the inherent instability of Na2CO3, it easily decomposes at high temperature, that is, Na2 CO3 ˆ Na2 O ‡ CO2 ‡ …1=2†O2. Since the activity of Na2O in Nasicon is very low, the Na2CO3 auxiliary electrode is not thermodynamically stable when melting it on Nasicon surface at high temperature (>melting point of Na2CO3 1157 K) during preparation of auxiliary electrode. Rich Na2O or Na2O2 phase may be formed on the interface, which completely changes the interfacial property. As a result, CO2 sensor response performance will be affected. If auxiliary phase is prepared at low temperature, because Na2CO3 decomposition reaction does not take place, the problem can be overcome. This has been veri®ed in our work [6]. Furthermore, if the Na2O activity in electrolyte that is in contact with Na2CO3 auxiliary phase is higher than in Nasicon and Na2CO3 auxiliary electrode is prepared at low temperature, the possibility of Na2CO3 decomposition and reaction between Na2CO3 and electrolyte is greatly minimised and the performance of sensors will be improved. According to a recent work by Vandecruys et al. [23], the Na2O activity in Na2O
5.5Al2O3 is 3  10 11 at 923 K which is four orders of magnitude higher than that in Nasicon (3  10 15 ) at same temperature. Therefore, the use of Na2O
5.5Al2O3 may result in greater stability of the interface between electrolyte and Na2CO3. On this basis, a novel CO2 sensor is designed by interfacing two Na‡ conductors, Nasicon and Na±b-alumina with Na2CO3/BaCO3 auxiliary on the side of Na±b-alumina and Pt onto Nasicon surface, as Nasicon/Pt interface is shown to be sensitive to PO2 [18]. 2. Experimental Nasicon powder was prepared using sodium carbonate, silica, zirconia and ammonium dihydrogen phosphate as starting materials. These powders were ball-milled in acetone for a total of 24 h. After the slurry was dried, the powder was calcined in an alumina crucible for 12 h at 443 K and for 6 h at 1173 K. The calcined powder was re-milled for a further 24 h. The resulting powder was pelletised (13 mm in diameter, 1 mm in thickness) and sintered at 1503 K for 24 h in air. Finally, Nasicon pellets with a nominal composition of Na3Zr2Si2PO12 were obtained. Na±b-Al2O3 was prepared by mixing Al2O3 and Na2CO3 in a molar ratio of 5.5:1. The powder mixture was ballmilled in acetone for a total of 24 h. The powder slurry was dried and then placed in an alumina crucible and calcined for 4 h at 1173 K. After being reground and dried, the powder was made into pellets (10 mm in diameter, 1 mm in thickness) and then sintered at 1923 K for 0.5 h in air. Finally,

Fig. 1. The cross-sectional view of planar-type CO2 sensor based on Nasicon interfacing Na±b-Al2O3 pellet.

Nasicon and Na±b-Al2O3 disks were attached together by diffusion bonding at 1523 K. The samples were investigated by several analytical methods. The cross-section of interface between Nasicon and Na±b-Al2O3 was analysed by SEM (JSM-5800LV) and EDAX (Noran Voyager system). The conductivity of the bielectrolyte Na±b-Al2O3 interfaced with Nasicon was measured by complex impedance spectroscopy (Schlumberger SI-1260) in the frequency range 1 Hz and 1 MHz at 573± 873 K. Fig. 1 presents the cross-sectional schematic view of CO2 sensor. After sintering the two solid electrolytes, the counter and the sensing electrodes were fabricated by painting commercial Pt paste on the two exposed surfaces of the pellets and annealed at 1173 K for 30 min. The Na2CO3/ BaCO3 (weight ratio 44:56) was used as auxiliary electrode which was deposited on the Pt-sensing electrode of Na±bAl2O3 side by painting a saturated solution of Na2CO3/ BaCO3 and drying at 373 K. The CO2 sensors were tested in controlled conditions at different CO2 concentrations, and the effect of operating temperature and partial pressure of O2 were also determined. Measurements were carried out on a gas-¯ow apparatus equipped with a heating furnace. The sensors were exposed to the ¯ow (100 cm3/min) of required sample gases. The different sample gases were prepared by diluting a parent gas (1% CO2 in Ar or pure CO2), with synthetic air or Ar using a gas blender with CO2 concentration varying from ppm levels to 100%. The sensors were investigated in 673± 873 K temperature ranges. Electromotive forces (emfs) were measured with a high impedance electrometer (>1012 O) and the data transferred to a PC-based data collecting system (Pico ADC-16). 3. Operating mechanism The electrochemical cell may be expressed as: … †CO2 ; O2 ; PtjBaCO3 =Na2 CO3 jNa b-Al2 O3 jNasiconjPt; O2 ; CO2 …‡† In the above cell, the right-hand side acts as a reference electrode and the left-hand side as a sensing electrode. Moreover, the interface between Nasicon and Na±b-alumina solid electrolytes may participate in an interfacial reaction. The following mechanism is suggested:

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sensing electrode (anodic): Na2 CO3 ˆ 2Na‡ ‰Na b-aluminaŠ ‡ CO2 ‡ …1=2†O2 ‰sensing electrodeŠ ‡ 2e reference electrode (cathodic): 2Na‡ ‰NasiconŠ ‡ …1=2†O2 ‰reference electrodeŠ ‡ 2e ˆ Na2 O‰NasiconŠ

(2)

The reaction at the interface between the two solid electrolytes: 2Na‡ ‰Na b-aluminaŠ ˆ 2Na‡ ‰NasiconŠ

(3)

So the overall reaction may be written as: Na2 CO3 ˆ 2Na2 O‰NasiconŠ ‡ CO2

(4)

The following Nernstian equation can be derived:   DG RT aNa2 O PCO2 ln Eˆ 2F 2F aNa2 CO3

(5)

where DG8 is the standard free energy of the reaction (4), aNa2 O is the activity of Na2O in the Nasicon, PCO2 is the partial pressure, aNa2 CO3 is the activity of Na2CO3 in the auxiliary and a constant at ®xed temperature and RT/F has the usual meaning. According to this equation, at constant oxygen partial pressure, sodium oxide activity, temperature and the activity of Na2CO3, the emf depends only on the carbon dioxide partial pressure. While it is important that stability of Na2CO3 with Na±bAl2O3 is enhanced, it is equally important that on the reference electrode where O2 electrode reaction is sought, no Na2CO3 is formed under any operating conditions. Nasicon is more stable than Na±b-Al2O3, and thus is more desirable for the oxygen electrode. Within the Na±b-Al2O3 compositional range, the aNa2 O in Na2O
5.5Al2O3 is higher than in Na2O
8Al2O3, and thus the former is desirable for minimising Na2CO3 dissolution at the auxiliary/electrolyte interface. According to literature [24±26], the Na2O activities in above electrolytes are expressed as: log aNa2 O…Nasicon† ˆ

12050 T

log aNa2 O…Na b-alumina† ˆ

log aNa2 O…Na b-alumina† ˆ

2:15 …Na3 Zr2 Si2 PO12 †

13703 ‡ 1:85 T 11490 0:03 T

Fig. 2. CO2 partial pressure as a function of temperature on forming Na2CO3 on surfaces of Nasicon, Na±b-Al2O3 (Na2O
8.0Al2O3) and Na±bAl2O3 (Na2O
5.5Al2O3).

(6) …Na2 O
8Al2 O3 † (7) …Na2 O
5:5Al2 O3 † (8)

The stable conditions of electrolytes in CO2 atmosphere at different temperatures can be evaluated based on the activities of Na2O in electrolytes and the standard free energies of formation for Na2O, CO2 and Na2CO3. The results are presented in Fig. 2. In this ®gure, the boundary indicates the stability region of Na2CO3, and Na2CO3 will form at the

region above each of the line. Under normal operating conditions activity of Na2O in Nasicon is too low to form Na2CO3. Since the cell is an open cell, it is also important that the Na±bAl2O3/Nasicon interface along the edges are not subject to the formation of Na2CO3 which may interfere with the interfacial equilibrium between the two electrolytes. In this respect, there is a possibility of formation of Na2CO3 on Na±b-Al2O3 (Na2O/Al2O3 ratio 1:5.5) when CO2 partial is higher than 105 Pa or operating temperature is lower than 398 K. Usually, the operating temperature is higher than 623 K and CO2 partial pressure detected is <105 Pa (1 atm) during sensing, so it is reasonable to assume that Na±b-Al2O3 on the interfacial region will not react with CO2 during the operation. 4. Results and discussion 4.1. Structural characterisation Interface between Nasicon and Na±b-Al2O3 was studied by SEM and EDAX. Fig. 3 shows the SEM and EDAX results of the cross-section of a Nasicon±Na±b-Al2O3 bielectrolyte pellet. SEM photo clearly indicates that Nasicon and Na±b-Al2O3 pellets are tightly bonded to each other. Na±b-Al2O3 remained as a single phase, while the Nasicon consists of both white and grey phases. From EDAX analysis, it is deduced that the white phase is nearly pure ZrO2, while the grey phase alumina-enriched, but ZrO2 depleted Nasicon. Thus, during sintering, alumina can readily diffuse from Na±b-Al2O3 into Nasicon phase by displacing and precipitating some ZrO2 as a secondary phase. As shown in the maps of element distribution, no elements from the Nasicon phase are found to enter the Na±b-Al2O3 phase. One consequence of this reaction is that the effective conductivity of the combined electrolyte is lower and the activation energy higher than either Nasicon or Na±b-Al2O3. The relationship between total conductivity of pellet and

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Fig. 3. SEM image of cross-section interface and elements distribution of Nasicon interfacing Na±b-Al2O3 composite pellet.

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Fig. 4. The conductivity of Nasicon interfacing Na±b-Al2O3 composite pellet as a function of temperature.

temperature is shown in Fig. 4. For example, at 850 K, the value of conductivity for the combined system is 2  10 3 S/cm, which is at least two orders of magnitude lower than either Nasicon or Na±b-Al2O3, and the activation energy at 0.42 eV is considerably higher than the values of 0.24 eV for Nasicon and 0.15 eV for Na±b-Al2O3. These results suggest that the electrical property is dominated by the interface between the two electrolytes, which by implication has a low conductivity. 4.2. Sensing performance Fig. 5 shows the typical responses of emf to stepwise increase in the CO2 concentration from 0 to 1,000,000 ppm (100%) in temperature range from 673 to 873 K. When CO2 concentration was changed, emfs reached a steady-state value in 50±70 s. The response was rapid and continuous. Fig. 6 shows the relationship between response time and temperature. The relationship between response time and temperature is useful for expressing the reaction rate on the electrode. The response time consists of both reaction time and the exchange time for the test gas. The former depends on the temperature and CO2 concentration and the latter on volume of test chamber and ¯ow rate of test gas. On ®xing other conditions, increasing temperature can only shorten time of electrode reaction. From Fig. 6, the response time de®ned as the time to obtain a 90% response ®rstly becomes shorter with temperature increasing, but when temperature is higher than 860 K, response time does not change any more and may reach the time required for gas exchange. The typical response time is 50±70 s in the operating temperature when CO2 was varied from 973 to 1886 ppm. The recovery on switching off CO2 was somewhat slower. For a given CO2 sensor, the variation of the sensor EMF with the CO2 content change in the gas mixture at 760, 809, and 859 K were presented in Fig. 7. The variation of EMF with the logarithm of the partial pressure of CO2 is found to be linear at each temperature. The slopes of the calibration curves obtained from the measured EMF are 79.7, 86.5

Fig. 5. Response transients of the sensors to various CO2 concentrations at different temperatures: (a) CO2 concentration 6 ppmÐ1% at 909 K; (b) CO2 concentration 600 ppmÐ100% at 818 K.

and 89.9 mV per decade at 760, 809 and 859 K, respectively. The corresponding number of transferred electrons calculated from these slopes are 1.9, 1.86 and 1.9 and correspond closely to a two-electron exchange reactions as suggested. Detection range is an important property of a sensor which was investigated experimentally by covering a wide range of CO2 concentrations from 6 ppm to 100%. A good linear relationship was obtained in the entire range (Fig. 8).

Fig. 6. The relationship between response time and temperature on CO2 content varying from 973 to 1886 ppm.

L. Wang, R.V. Kumar / Sensors and Actuators B 88 (2003) 292±299

Fig. 7. The emf of the CO2 sensors as a function of CO2 concentration at 760, 809 and 859 K.

At CO2 content <6 ppm, emf did not reach stable value indicating a slow reaction at very low concentration. In order to examine reproducibility, two sensors were tested simultaneously in identical conditions. The results of the two sensors are presented in Fig. 9. If Eq. (5) is rewritten as:

297

Fig. 9. The experimental results from two different sensors at identical experimental conditions.

and the slopes of 86.5 and 85.9 mV per decade agree well with theoretical Nernstian slope of 80.4 mV per decade at 810 K. However, there is a relatively large difference in

the value of A, which can be ascribed to an unknown potential difference arising from a junction potential due to reaction (3), which represents migration of Na‡ from one electrolyte to the other. The additional potential difference at the interface can constrain further migration of Na‡ ions, and it is not clear whether it can be controlled from one sensor to another. Although sensors have excellent Nernstian-type response, calibration of sensors are necessary for practical applications. For a given CO2 sensor, the temperature effect on emf also was tested in ®xed CO2 contents and the results are shown in Fig. 10. As expected by Eq. (5), the relationship between temperature and emf is linear for each ®xed value of CO2 content. According to detection mechanism, although oxygen is involved in the electrode reactions, the PO2 pressure term does not appear in the overall reaction. The emf output variation with the oxygen content in the mixture gas where

Fig. 8. The detection range of sensor.

Fig. 10. Sensor signal as a function of temperature for different CO2 partial pressures.

emf ˆ A ‡ B log PCO2 where Aˆ

DG 2F

2:303RT aNa2 O log 2F aNa2 CO3

and

Bˆ

2:303RT 2F

The following calibration equations can be obtained: emf ˆ 777:1

86:5 log PCO2 …mV† for sensor 1

and emf ˆ 734:4

85:9 log PCO2 …mV† for sensor 2

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precipitating some ZrO2 from Nasicon. As the result, conductivity of the combined Nasicon±Na±b-Al2O3 composite pellet is lower than that of Nasicon or Na±b-Al2O3 pellet. A compact CO2 sensor based on combining both the sodium ion conductors Nasicon and Na±b-Al2O3 was prepared. The planar device shows fairly good CO2-sensing properties. The response is rapid, continuous, stable and reproducible. The sensor follows Nernstian type of behaviour in the range from 6 ppm to 100% CO2 concentration and is independent of PO2 in the gas. Acknowledgements Fig. 11. The emf output variation (760 K) with the oxygen partial pressure at two different CO2 concentrations (512 and 4814 ppm).

The ®nancial support of EPSRC for this project is gratefully acknowledged. References

Fig. 12. The comparison of sensor signal output on Ar and air as diluent.

the CO2 concentration is ®xed at 512 and 4814 ppm is presented in Fig. 11. During the variation of the oxygen content, the emf output always maintains the same value, showing that any interference from oxygen was not observed. In order to further support this, the sensing behaviour of a given sensor to CO2 gas was checked under the conditions that CO2 contents vary from 500 ppm to 100% using either pure Ar or air as diluents. The results are shown in Fig. 12. Although O2 partial pressure in the two diluent gases is quite different, the emf results are really identical. Hence, the sensor can be used in an environment, where the oxygen concentration varies with time. 5. Conclusions Nasicon is interfaced with Na±b-Al2O3 sintering. SEM and EDAX analysis shows bonds well to Na±b-Al2O3 pellet, helped of alumina from Na±b-Al2O3 to Nasicon

by diffusion that Nasicon by diffusion resulting in

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Biographies L. Wang obtained his MSc from the University of Hebei (1990) and PhD from the Beijing University of Science and Technology in China (1999). He is a research associate at the University of Cambridge. His interests are sensors, electrolytes and new materials. R.V. Kumar has a degree in BTech (Hons) from IIT, Bombay (1978) and a PhD from McMaster University, Canada (1983). He is a senior lecturer at the University of Cambridge, His current interests include the field of solid-state ionics, sensors and materials development for process and pollution control.