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High-performance solid-electrolyte carbon dioxide sensor with a binary carbonate electrode
Sensors and Actuators
B, 9 (1992) 165-170
165
High-performance solid-electrolyte carbon dioxide sensor with a binary carbonate electrode* Norio Miura, Sheng Yao, Youichi Shimizu and Noboru Yamazoe Department (Received
of Materials January
Science and Technology,
3, 1992; accepted
Graduate School of Engineering Sciences, Kyushu University,
Kasuga-shi,
Fukuoka 816 (Japan)
April 20, 1992)
Abstract A compact solid-state CO* sensor based upon an Na+-conducting solid electrolyte (NASICON) can be improved drastically in response time and water vapour resistance by the use of a binary carbonate electrode of BaCO,Na,CO,. For a wide range of CO2 concentration from 4 to 400 000 ppm, the electromotive force (e.m.f.) examined at 550 “C follows a Nernst equation correspondence to a two-electron reaction of COz, with a 90% response time as short as 8 s or less. Water vapour hardly affects the sensor characteristics, in contrast to the case of a pure Na,CO, electrode. The structure and CO,-sensing mechanism of the binary carbonate electrode are also discussed.
1. Introduction
Since CO;? control is of increasing importance in various technologies, there is an ever-increasing need for inexpensive compact COz sensors with high sensitivity and selectivity. An infrared spectroscopic sensor (analyser) is commonly used for monitoring CO:! concentration. This physical CO* sensor equipped with a large optical system is accurate and stable, but it has problems with respect to price and portability. Many kinds of compact CO2 sensors using various materials, such as solid electrolytes [l-4], mixed oxides [5], zeolites [6], polymers with carbonate solution [7] and so on, have been investigated so far, although none seems to have achieved sufficient performance characteristics for practical use. Among them, those using solid electrolytes are of particular interest from the viewpoint of simplicity of the CO, detection mechanism. The sensors based on Na+-conductors, e.g., p-alumina [8] and NASICON [3,9], fitted with an Na,C03 auxiliary electrode were reported to respond well to a change in CO2 concentration, following Nernst’s equations when dry CO* was used. The time required for 90% response is reportedly several minutes at 500-
*Paper presented at the 6th International Conference on SolidState Sensors and Actuators (Transducers ‘91), San Francisco, CA, USA, June 24-28, 1991.
0925-4005/92/$5.00
700 “C. A serious problem is that they suffer from strong interference with coexistent water vapour. We report here that this interference is associated with the Na,CO, electrode, and that the use of a binary carbonate electrode of BaCO, and NazC03 brings about a solution to the problem, i.e., not only does it completely eliminate the water vapour interference but it also increases the response rate quite remarkably [lo]. A brief discussion is included on the structure of the binary carbonate electrode as well as the CO* detection mechanism of this sensor.
2. Experimental 2.1. Fabrication of sensor element A schematic drawing of the structure of the sensor element fabricated here is shown in Fig. 1. A sintered disc of a sodium ion conductor (NASICON, Na,Zr,Si2P012), 8 mm in diameter and 0.7 mm thick, ,was fixed to the end of a quartz glass tube (6 mm in diameter) with an inorganic adhesive. The reference electrode (platinum black) was applied to the surface of the disc inside the tube, on which a platinum mesh connected to a Pt wire was pressed mechanically to secure good electrical contact. The sensing electrode was fabricated by applying platinum black on the outside surface of the disc. It was then covered with a platinum mesh and finally with a carbonate layer of pure @ 1992 -
Elsevier
Sequoia.
All rights
reserved
166
C
D
2000
ppnl
CO?
2000
ppm
250
CO2
ppm
CO2
Fig. I. Structure of the CO, sensor element using Na+ conductor. A, NASICON; B, Pt-black; C, P&mesh; D, carbonate electrode (BaCO, + Na,CO,); E, Pt-wire; F, inorganic adhesive; G, quartz glass tube.
NazC03 or a mixture of BaCO, and Na&O,. The atomic percentage of Ba in the mixture was changed from 5 at.% to 67 at.%. The carbonate layer was fixed tightly to the disc by the melting and quenching method. 2.2. Measurement of sensing properties CO,-sensing experiments were carried out in a conventional gas-flow apparatus. Sample gases (CO2 concentration: 4 ppm-40 vol.%) were prepared from pure CO2 or 1 vol.% CO;! diluted in nitrogen by mixing it with air (dry or wet) or pure oxygen. Humid air was prepared by allowing air to bubble through water at ~25 ‘C. The difference in potential between the sensing and reference electrodes (electromotive force, e.m.f.) of the sensor was measured by means of a digital electrometer (Advantest, TR 8552) mainly at 550 “C under a constant oxygen concentration of 21 vol.% and a total flow rate of 100 cm3/min. The structures of the binary carbonate system of BaCO, and Na2C03 were examined by means of X-ray diffraction (XRD) analysis (Rigaku Denki, 4011).
air (0 ppm)
alr (0 Pam CO2)
il air (a ppm)
(b)
(4
Fig. 2. Response transients to 250 and 2000 ppm CO, for the sensor element using the BaCO,-Na,CO, (Ba: 46 at.%) electrode at 550 “C: (a) dry CO,; (b) wet CO2 (2.7 kPa H,O).
a COz-containing dry air flow, the e.m.f. response increased quickly to a steady value. The steady value increased with increasing concentration of CO*. On switching back to the COz-free air, the e.m.f. quickly returned to the initial value. The observed 90% response time for the increase of CO;? concentration was as short as 8 s or less in each case for the tested concentration range. Moreover, it is found that the response transients of the sensor were hardly affected by coexistent water vapour (humidity) in the sample gas. It is observed that there is little difference between the response transients to dry COz gas shown in Fig. 2(a) and to wet CO2 gas (2.7 kPa H20) shown in Fig. 2(b). With a change in flow rate, the stationary response (e.m.f.) was unaffected. The e.m.f. changed linearly with the logarithm of CO, concentration in the whole range tested (4-400 000 ppm) at 550 “C, with a Nernstian slope of 81.6 mV/decade, as shown in Fig. 3. This value coincides well with the theoretical value CO2 concentration 16
3. Results and discussion 3.1. Sensing properties of the sensor with binary carbonate electrode Figure 2 depicts the response transients to dry and wet CO2 gas (250 and 2000 ppm) at 550 “C for the sensor element fitted with a binary carbonate electrode (BaCO, :Na2C03 = 1.7: 1.O in molar ratio, Ba content: Ba/( Ba + Na): 46 at.%). This composition corresponded to the smallest NazC03 content to be resistant to water vapour, as described below. As seen from Fig. 2(a), on switching from a synthetic dry air flow (0 ppm CO*) to
air (0 ppm)
-250
-
> E -350
-
ld
ld
/ ppm
ld
lo’
lo’
lo”
. ;
-450
-
-550
-
-650
I
1 -6
-5
-4
I
I
I
-3
-2
-1
log P(CO2)
-1
0
/ atm
Fig. 3. E.m.f. of the sensor element using the BaCO, -Na,CO, (Ba: 46 at.%) electrode as a function of CO* concentration at 550 “C.
167 TABLE I. Effects of the coexistence of CO and NO on the sensitivity (AE) to 1000 ppm COs of the sensor element using the BaCO,Na,CO, (Ba: 46 at.%) electrode at 550 “C NO (ppm)
CO @Pm)
AE (mV)
rl_...._.._.__r
gL6mV/decade
I
-500 1
100
I
I
200 1000 CO2 cont. / ppm
2000
Fig. 4. E.m.f. of the sensor element using the BaCO,-Na,CO, (Ba: 46 at.%) electrode as a function of CO, concentration at 550 “C: (a) dry CO,; (b) wet CO2 (2.7 kPa H,O).
(81.6 mV/decade at 550 “C) calculated on the assumption that the electrode reaction is a two-electron reduction per CO2 molecule. It is again noted that the presence of water vapour (2.7 kPa) had little effect on the data in the range 100-2000 ppm COz, as depicted in Fig. 4. The effects of the coexistence of CO and NO on the sensitivity (AE) of the sensor to 1000 ppm CO, were examined. As shown in Table 1, the presence of CO up to 1000 ppm had little influence on the AE values, while slight interfering effects were exerted by NO; AE deviated by z 10 mV at 100 ppm NO. These results appear to show that the present sensor has high CO2 selectivity, although interference by other gases, such as hydrocarbons, alcohols and SO,, is also to be tested.
500
1000
0
50
100
164
164
163
164
159
153
3.2. Sensing properties of the sensor with pure Na2C03 electrode For comparison, the sensor element fitted with a pure Na2C03 electrode as reported previously was also subjected to the same examinations. As seen in Fig. 5(a), its response transients to dry CO2 gas (250 and 1500 ppm) were quite reasonable, although the time required to achieve 90% response was about 50 s and is far longer than that observed with the binary BaCOJ-Na2C03 electrode. Moreover, the transients were found to be seriously affected by the presence of water vapour. The coexistence of 0.7 kPa HI0 extended the 90% response times for the increase in CO* concentration to ~20 min, and the steady e.m.f. values attained were far larger than those observed for dry COz, as seen from Fig. 5(b). Recovery responses were also very slow and the initial value was not recovered even in 50 min. Figure 6 shows the e.m.f. versus CO;? concentration relationship for the sensor fitted with the Na*CO, electrode. For dry CO*, the e.m.f. followed a Nernst equation with a slope of 81 mV/ decade. However, the absolute e.m.f. value decreased greatly and became less dependent on COz concentration when water vapour was present. 1500 ppm
1500 ppm
0
CO2
Cop I
after
-I (0 ppm (a)
COq12 min
air (0 ppm)
50 min
_ 8 min
b-
(b)
Fig. 5. Response transients to 250 and 1500 ppm CO, of the sensor element using the pure Na,CO, electrode at 550 “C: (a) dry CO,; (b) wet CO2 (0.7 kPa H,O).
168
-350
% .
2-45a
-4501
’ 200
I
1000
-55(
I 2000
Fig. 6. Dependence of e:m.f. of the sensor using the pure Na,CO, electrode on CO, concentration at 550°C. Water vapour pressure (kPa): (a), 0; (b), 0.4; (c), 0.7; (d), 1.8.
Thus it is obvious that the sensor element fitted with the NazC03 electrode cannot withstand operation in a humid atmosphere.
of
(reference electrode) air, Pt(NASICON (Na+ conductor) [Pt + carbonate, CO, + air( sensing electrode)
(1)
This type of CO2 sensor using only NazC03 as the auxiliary sensing electrode has been investigated by Saito and coworkers [3,9]. The sensing electrode reaction in this case is written as 2Na+ + CO2 + ( 1/2)02 + 2e- = Na,C03
1000 CO2
co2 cont. / iw
3.3. COz sensing mechanism The present sensor elements are composed the following solid-state electrochemical cell:
I
I
200
(2)
while the reference electrode reaction is 2Na+ + ( 1/2)02 + 2e- = Na,O (in NASICON)
I
2000
cont. / ppm
Fig. 7. Dependence of e.m.f. of the sensor element using the BaCO, Na,CO, (Ba: 46 at.%) electrode on CO, concentration. Operating temperature (“C): (a), 450; (b), 500; (c), 550; (d), 600.
81 mV/decade, is consistent with the theoretical value (8 1.6 mV/decade for a two-electron reaction at 550 “C). As just observed, however, the same electrode suffers from interference by water vapour. It is suspected that the interference occurs because Na,CO, tends to deteriorate to other compounds, such as NaOH, NaHCO, and NazC03*xH20, in the presence of water vapour. In the case of the binary carbonate electrode, the constant value of EC in eqn. (5) was different from that for the Na,CO, electrode. This suggests that the activity of NazC03 used in eqn. (4) should be changed correspondingly in the binary electrode. Nevertheless, the binary system was shown to give a Nernstian slope which is very close to the theoretical value for the number of electrons (n = 2.0) for the sensing electrode reaction in the temperature range 450-600 “C, as seen from Fig. 7. This means that the sensing electrode reaction, and thus the CO,-sensing mechanism, are essentially the same for both sensors.
(3) The overall chemical reaction is thus reduced to the following form: Na2C03 = Na,O + CO2
(4)
When the activities of Na2C03 and Na20 are kept constant, the e.m.f. can be expressed as E = EC+ (RT/2F) In P(C0,)
(5)
where P(C02) is the partial pressure of CO*, EC is a constant, and RT/F has the usual meaning. The observed Nernstian slope for dry CO2 at 550 “C,
3.4. Binary carbonate electrode It is inferred that the water vapour resistance of the BaC03 -NaaC03 electrode mentioned above may be ascribed to the resistance of BaC03 to hydrate formation. The solubilities of BaCO, and NazCO, in water are 0.0065 and 48.5 g at 100 “C, respectively. Such a large difference in solubility might also contribute to the water vapour resistance of the binary electrode. Figure 8 compares the XRD pattern of the binary carbonate system (Ba content: 20 and 46 at.%) with those of BaC03 and
169
1 ’
-20 0
.
20
’ 40
.
’
.
60
’
.
80
100
Ba/(NatBa) /at% Fig. 9. Effects of water vapour on the response of the CO, sensor as correlated with Ba content in the BaCO,-Na,CO, electrodes. (CO, concentration, 390 ppm; partial pressure of water vapour in wet gas, 2.7 kPa; operation temperature, 550 “C.)
0
I
I
I
30
40
50
28 I deg.
Fig. 8. XRD patterns of various carbonates: (a) pure BaCO,; (b) BaCO,-Na,CO, (Ba: 46 at.%); (c) BaCO,-Na,CO, (Ba: 20 at.%); (d) pure Na,CO,.
Na,CO,. It is noteworthy that in the binary systems the diffraction peaks corresponding to Na,CO, are unusually weak and almost invisible at 46 at.% Ba, while the peaks for BaCO, are very strong. This means that Na2C03 is not present as a separate phase at 46 at.% Ba and such absence of free NazC03 may explain the water vapour resistivity of the sensor. In good agreement with this behaviour, the interference effect of water vapour on the e.m.f. response was found to decrease with increasing Ba content and to disappear at Ba contents above x45 at.%, as shown in Fig. 9. Further details of the binary carbonate electrodes, i.e., their microstructure and its relevance to the CO,-sensing characteristics, are now under investigation. Another merit of the use of the binary carbonate system is the lower melting point. The mixture of BaCO, and Na2C03 ( 1.7:1 .O) has a melting point of 811 “C, while the melting points of BaCO, and NazCO, are 1740 and 851 “C, respectively. TABLE 2. Sensing properties of the element using the SrCO,-Na,CO,
Therefore the binary carbonate is more easily adaptable to electrode preparation by the meltingquenching method. It is further mentioned that these effects are not unique to the combination of BaCO,, Na2C03 and NASICON. Other binary systems, such as CaCO, -NazCO, and SrC03 -Na,CO,, were also found to show similar stability in the presence of water vapour. As shown in Table 2, for example, a sensor fitted with an electrode of a binary carbonate system, SrC03 + Na*CO, (Sr content: 60 at.%), was unaffected by water vapour in the range 2501460 ppm CO* at 550 “C. It is emphasized that the binary electrodes also showed similar insensitivity to water vapour when fitted to Na-P/p”-alumina or other Na+ conductor [ 111.
4. Conclusions The use of the BaCO, -Na,CO, electrode is quite effective to upgrade a NASICON-based solid-electrolyte COZ sensor with respect to both stability in the presence of water vapour and the rate of response to CO*, although the reasons for (Sr: 60 at.%) electrode at 550 “C
E.m.f. (mV) 250 ppm Dry CO, Wet CO,a *P(H,O) = 0.7 kPa.
-457 -454
( mV/decade)
Electron number (n)
80.6 79.0
1.97 1.94
Nernstian slope 440 ppm -437 -434
865 ppm -414 -412
1460 ppm -395 -393
170
such improvements are still to be elucidated. Such upgrading of the COz-sensing characteristic is not specific to the above sensor system but is likely to be rather common to other binary carbonate electrodes.
Acknowledgements The authors are grateful to Dr Y. Sadaoka, Department of Industrial Chemistry, Ehime University, and Mr H. Futata, Yazaki Meter Co. Ltd. for their helpful discussions. We are also grateful to NGK Co. Ltd. for the donation of NASICON. This work was partially supported by a Grant-inAid for Scientific Research from the Ministry of Education, Science and Culture of Japan, and a grant from Iketani Science and Technology Foundation.
References 1 M. Gauthier and A. Chamberland, Solid-state detectors for the potentiometric determination of gaseous oxides. I. Measurement in air, J. Electrochem. Sot., 124 (1977) 1579-1583.
2 R. CBte, C. W. Bale and M. Gauthier, K&O, solid electrolyte as a CO2 probe: decomposition measurements of CaCO,, J. Electrochem. Sot., 131 (1984) 63-67. 3 T. Maruyama, S. Sasaki and Y. Saito, Potentiometric gas sensor for carbon dioxide using solid electrolytes, Solid State Ionics, 23 (1987) 107-I 12. 4 N. Imanaka, T. Kawasato and G. Adachi, A carbon dioxide gas
sensor probe based on a lithium ionic conductor, Chem. Serf., (1990) 497-500. 5 T. Ishihara, K. Kometani, M. Hashida and Y. Takita, Application of mixed oxide capacitor to the selective carbon dioxide sensor. I. Measurement of carbon dioxide sensing characteristics, J. Electrochem. Sot., 138 (1991) 173-176. 6 Y. Ishiguro, Y. Nagawa and H. Futata, A new device sensitive particularly to carbon dioxide, Proc. 2nd Int. Meet. Chemical Sensors, Bordeaux, France, July 7-10, 1986, pp. 719-722.
7 Y. Shim& K. Komori and M. Egashira, Carbon dioxide sensor consisting of K,CO, -polyethylene glycol solution supported on porous ceramics. I. Measurements of carbon dioxide sensing characteristics, J. Electrochem. Sot., 136 (1989) 22562260. 8 T. Ogata,
S. Fujitsu, M. Miyayama, K. Koumoto and H. Yanagida, CO, gas sensor using B-Al,O, and metal carbonate, J.
Mater. Sci. I&t., 5 (1986) 285-286. 9 Y. Saito and T. Maruyama, Recent developments of the sensors for carbon oxides using solid electrolytes, Solid State Ionics, 28-30 (1988) 1644-1647. 10 S. Yao, Y. Shimizu, N. Miura and N. Yamazoe, Solid electrolyte
CO2 sensor using binary carbonate electrode, C/rem. Left., (1990) 2033-2036. 11 Y. Sadaoka and Y. Sakai, Humidity sensor using sintered zircon with alkali-phosphate, J. Mater. Sci., 20 (1985) 30273033.
B, 9 (1992) 165-170
165
High-performance solid-electrolyte carbon dioxide sensor with a binary carbonate electrode* Norio Miura, Sheng Yao, Youichi Shimizu and Noboru Yamazoe Department (Received
of Materials January
Science and Technology,
3, 1992; accepted
Graduate School of Engineering Sciences, Kyushu University,
Kasuga-shi,
Fukuoka 816 (Japan)
April 20, 1992)
Abstract A compact solid-state CO* sensor based upon an Na+-conducting solid electrolyte (NASICON) can be improved drastically in response time and water vapour resistance by the use of a binary carbonate electrode of BaCO,Na,CO,. For a wide range of CO2 concentration from 4 to 400 000 ppm, the electromotive force (e.m.f.) examined at 550 “C follows a Nernst equation correspondence to a two-electron reaction of COz, with a 90% response time as short as 8 s or less. Water vapour hardly affects the sensor characteristics, in contrast to the case of a pure Na,CO, electrode. The structure and CO,-sensing mechanism of the binary carbonate electrode are also discussed.
1. Introduction
Since CO;? control is of increasing importance in various technologies, there is an ever-increasing need for inexpensive compact COz sensors with high sensitivity and selectivity. An infrared spectroscopic sensor (analyser) is commonly used for monitoring CO:! concentration. This physical CO* sensor equipped with a large optical system is accurate and stable, but it has problems with respect to price and portability. Many kinds of compact CO2 sensors using various materials, such as solid electrolytes [l-4], mixed oxides [5], zeolites [6], polymers with carbonate solution [7] and so on, have been investigated so far, although none seems to have achieved sufficient performance characteristics for practical use. Among them, those using solid electrolytes are of particular interest from the viewpoint of simplicity of the CO, detection mechanism. The sensors based on Na+-conductors, e.g., p-alumina [8] and NASICON [3,9], fitted with an Na,C03 auxiliary electrode were reported to respond well to a change in CO2 concentration, following Nernst’s equations when dry CO* was used. The time required for 90% response is reportedly several minutes at 500-
*Paper presented at the 6th International Conference on SolidState Sensors and Actuators (Transducers ‘91), San Francisco, CA, USA, June 24-28, 1991.
0925-4005/92/$5.00
700 “C. A serious problem is that they suffer from strong interference with coexistent water vapour. We report here that this interference is associated with the Na,CO, electrode, and that the use of a binary carbonate electrode of BaCO, and NazC03 brings about a solution to the problem, i.e., not only does it completely eliminate the water vapour interference but it also increases the response rate quite remarkably [lo]. A brief discussion is included on the structure of the binary carbonate electrode as well as the CO* detection mechanism of this sensor.
2. Experimental 2.1. Fabrication of sensor element A schematic drawing of the structure of the sensor element fabricated here is shown in Fig. 1. A sintered disc of a sodium ion conductor (NASICON, Na,Zr,Si2P012), 8 mm in diameter and 0.7 mm thick, ,was fixed to the end of a quartz glass tube (6 mm in diameter) with an inorganic adhesive. The reference electrode (platinum black) was applied to the surface of the disc inside the tube, on which a platinum mesh connected to a Pt wire was pressed mechanically to secure good electrical contact. The sensing electrode was fabricated by applying platinum black on the outside surface of the disc. It was then covered with a platinum mesh and finally with a carbonate layer of pure @ 1992 -
Elsevier
Sequoia.
All rights
reserved
166
C
D
2000
ppnl
CO?
2000
ppm
250
CO2
ppm
CO2
Fig. I. Structure of the CO, sensor element using Na+ conductor. A, NASICON; B, Pt-black; C, P&mesh; D, carbonate electrode (BaCO, + Na,CO,); E, Pt-wire; F, inorganic adhesive; G, quartz glass tube.
NazC03 or a mixture of BaCO, and Na&O,. The atomic percentage of Ba in the mixture was changed from 5 at.% to 67 at.%. The carbonate layer was fixed tightly to the disc by the melting and quenching method. 2.2. Measurement of sensing properties CO,-sensing experiments were carried out in a conventional gas-flow apparatus. Sample gases (CO2 concentration: 4 ppm-40 vol.%) were prepared from pure CO2 or 1 vol.% CO;! diluted in nitrogen by mixing it with air (dry or wet) or pure oxygen. Humid air was prepared by allowing air to bubble through water at ~25 ‘C. The difference in potential between the sensing and reference electrodes (electromotive force, e.m.f.) of the sensor was measured by means of a digital electrometer (Advantest, TR 8552) mainly at 550 “C under a constant oxygen concentration of 21 vol.% and a total flow rate of 100 cm3/min. The structures of the binary carbonate system of BaCO, and Na2C03 were examined by means of X-ray diffraction (XRD) analysis (Rigaku Denki, 4011).
air (0 ppm)
alr (0 Pam CO2)
il air (a ppm)
(b)
(4
Fig. 2. Response transients to 250 and 2000 ppm CO, for the sensor element using the BaCO,-Na,CO, (Ba: 46 at.%) electrode at 550 “C: (a) dry CO,; (b) wet CO2 (2.7 kPa H,O).
a COz-containing dry air flow, the e.m.f. response increased quickly to a steady value. The steady value increased with increasing concentration of CO*. On switching back to the COz-free air, the e.m.f. quickly returned to the initial value. The observed 90% response time for the increase of CO;? concentration was as short as 8 s or less in each case for the tested concentration range. Moreover, it is found that the response transients of the sensor were hardly affected by coexistent water vapour (humidity) in the sample gas. It is observed that there is little difference between the response transients to dry COz gas shown in Fig. 2(a) and to wet CO2 gas (2.7 kPa H20) shown in Fig. 2(b). With a change in flow rate, the stationary response (e.m.f.) was unaffected. The e.m.f. changed linearly with the logarithm of CO, concentration in the whole range tested (4-400 000 ppm) at 550 “C, with a Nernstian slope of 81.6 mV/decade, as shown in Fig. 3. This value coincides well with the theoretical value CO2 concentration 16
3. Results and discussion 3.1. Sensing properties of the sensor with binary carbonate electrode Figure 2 depicts the response transients to dry and wet CO2 gas (250 and 2000 ppm) at 550 “C for the sensor element fitted with a binary carbonate electrode (BaCO, :Na2C03 = 1.7: 1.O in molar ratio, Ba content: Ba/( Ba + Na): 46 at.%). This composition corresponded to the smallest NazC03 content to be resistant to water vapour, as described below. As seen from Fig. 2(a), on switching from a synthetic dry air flow (0 ppm CO*) to
air (0 ppm)
-250
-
> E -350
-
ld
ld
/ ppm
ld
lo’
lo’
lo”
. ;
-450
-
-550
-
-650
I
1 -6
-5
-4
I
I
I
-3
-2
-1
log P(CO2)
-1
0
/ atm
Fig. 3. E.m.f. of the sensor element using the BaCO, -Na,CO, (Ba: 46 at.%) electrode as a function of CO* concentration at 550 “C.
167 TABLE I. Effects of the coexistence of CO and NO on the sensitivity (AE) to 1000 ppm COs of the sensor element using the BaCO,Na,CO, (Ba: 46 at.%) electrode at 550 “C NO (ppm)
CO @Pm)
AE (mV)
rl_...._.._.__r
gL6mV/decade
I
-500 1
100
I
I
200 1000 CO2 cont. / ppm
2000
Fig. 4. E.m.f. of the sensor element using the BaCO,-Na,CO, (Ba: 46 at.%) electrode as a function of CO, concentration at 550 “C: (a) dry CO,; (b) wet CO2 (2.7 kPa H,O).
(81.6 mV/decade at 550 “C) calculated on the assumption that the electrode reaction is a two-electron reduction per CO2 molecule. It is again noted that the presence of water vapour (2.7 kPa) had little effect on the data in the range 100-2000 ppm COz, as depicted in Fig. 4. The effects of the coexistence of CO and NO on the sensitivity (AE) of the sensor to 1000 ppm CO, were examined. As shown in Table 1, the presence of CO up to 1000 ppm had little influence on the AE values, while slight interfering effects were exerted by NO; AE deviated by z 10 mV at 100 ppm NO. These results appear to show that the present sensor has high CO2 selectivity, although interference by other gases, such as hydrocarbons, alcohols and SO,, is also to be tested.
500
1000
0
50
100
164
164
163
164
159
153
3.2. Sensing properties of the sensor with pure Na2C03 electrode For comparison, the sensor element fitted with a pure Na2C03 electrode as reported previously was also subjected to the same examinations. As seen in Fig. 5(a), its response transients to dry CO2 gas (250 and 1500 ppm) were quite reasonable, although the time required to achieve 90% response was about 50 s and is far longer than that observed with the binary BaCOJ-Na2C03 electrode. Moreover, the transients were found to be seriously affected by the presence of water vapour. The coexistence of 0.7 kPa HI0 extended the 90% response times for the increase in CO* concentration to ~20 min, and the steady e.m.f. values attained were far larger than those observed for dry COz, as seen from Fig. 5(b). Recovery responses were also very slow and the initial value was not recovered even in 50 min. Figure 6 shows the e.m.f. versus CO;? concentration relationship for the sensor fitted with the Na*CO, electrode. For dry CO*, the e.m.f. followed a Nernst equation with a slope of 81 mV/ decade. However, the absolute e.m.f. value decreased greatly and became less dependent on COz concentration when water vapour was present. 1500 ppm
1500 ppm
0
CO2
Cop I
after
-I (0 ppm (a)
COq12 min
air (0 ppm)
50 min
_ 8 min
b-
(b)
Fig. 5. Response transients to 250 and 1500 ppm CO, of the sensor element using the pure Na,CO, electrode at 550 “C: (a) dry CO,; (b) wet CO2 (0.7 kPa H,O).
168
-350
% .
2-45a
-4501
’ 200
I
1000
-55(
I 2000
Fig. 6. Dependence of e:m.f. of the sensor using the pure Na,CO, electrode on CO, concentration at 550°C. Water vapour pressure (kPa): (a), 0; (b), 0.4; (c), 0.7; (d), 1.8.
Thus it is obvious that the sensor element fitted with the NazC03 electrode cannot withstand operation in a humid atmosphere.
of
(reference electrode) air, Pt(NASICON (Na+ conductor) [Pt + carbonate, CO, + air( sensing electrode)
(1)
This type of CO2 sensor using only NazC03 as the auxiliary sensing electrode has been investigated by Saito and coworkers [3,9]. The sensing electrode reaction in this case is written as 2Na+ + CO2 + ( 1/2)02 + 2e- = Na,C03
1000 CO2
co2 cont. / iw
3.3. COz sensing mechanism The present sensor elements are composed the following solid-state electrochemical cell:
I
I
200
(2)
while the reference electrode reaction is 2Na+ + ( 1/2)02 + 2e- = Na,O (in NASICON)
I
2000
cont. / ppm
Fig. 7. Dependence of e.m.f. of the sensor element using the BaCO, Na,CO, (Ba: 46 at.%) electrode on CO, concentration. Operating temperature (“C): (a), 450; (b), 500; (c), 550; (d), 600.
81 mV/decade, is consistent with the theoretical value (8 1.6 mV/decade for a two-electron reaction at 550 “C). As just observed, however, the same electrode suffers from interference by water vapour. It is suspected that the interference occurs because Na,CO, tends to deteriorate to other compounds, such as NaOH, NaHCO, and NazC03*xH20, in the presence of water vapour. In the case of the binary carbonate electrode, the constant value of EC in eqn. (5) was different from that for the Na,CO, electrode. This suggests that the activity of NazC03 used in eqn. (4) should be changed correspondingly in the binary electrode. Nevertheless, the binary system was shown to give a Nernstian slope which is very close to the theoretical value for the number of electrons (n = 2.0) for the sensing electrode reaction in the temperature range 450-600 “C, as seen from Fig. 7. This means that the sensing electrode reaction, and thus the CO,-sensing mechanism, are essentially the same for both sensors.
(3) The overall chemical reaction is thus reduced to the following form: Na2C03 = Na,O + CO2
(4)
When the activities of Na2C03 and Na20 are kept constant, the e.m.f. can be expressed as E = EC+ (RT/2F) In P(C0,)
(5)
where P(C02) is the partial pressure of CO*, EC is a constant, and RT/F has the usual meaning. The observed Nernstian slope for dry CO2 at 550 “C,
3.4. Binary carbonate electrode It is inferred that the water vapour resistance of the BaC03 -NaaC03 electrode mentioned above may be ascribed to the resistance of BaC03 to hydrate formation. The solubilities of BaCO, and NazCO, in water are 0.0065 and 48.5 g at 100 “C, respectively. Such a large difference in solubility might also contribute to the water vapour resistance of the binary electrode. Figure 8 compares the XRD pattern of the binary carbonate system (Ba content: 20 and 46 at.%) with those of BaC03 and
169
1 ’
-20 0
.
20
’ 40
.
’
.
60
’
.
80
100
Ba/(NatBa) /at% Fig. 9. Effects of water vapour on the response of the CO, sensor as correlated with Ba content in the BaCO,-Na,CO, electrodes. (CO, concentration, 390 ppm; partial pressure of water vapour in wet gas, 2.7 kPa; operation temperature, 550 “C.)
0
I
I
I
30
40
50
28 I deg.
Fig. 8. XRD patterns of various carbonates: (a) pure BaCO,; (b) BaCO,-Na,CO, (Ba: 46 at.%); (c) BaCO,-Na,CO, (Ba: 20 at.%); (d) pure Na,CO,.
Na,CO,. It is noteworthy that in the binary systems the diffraction peaks corresponding to Na,CO, are unusually weak and almost invisible at 46 at.% Ba, while the peaks for BaCO, are very strong. This means that Na2C03 is not present as a separate phase at 46 at.% Ba and such absence of free NazC03 may explain the water vapour resistivity of the sensor. In good agreement with this behaviour, the interference effect of water vapour on the e.m.f. response was found to decrease with increasing Ba content and to disappear at Ba contents above x45 at.%, as shown in Fig. 9. Further details of the binary carbonate electrodes, i.e., their microstructure and its relevance to the CO,-sensing characteristics, are now under investigation. Another merit of the use of the binary carbonate system is the lower melting point. The mixture of BaCO, and Na2C03 ( 1.7:1 .O) has a melting point of 811 “C, while the melting points of BaCO, and NazCO, are 1740 and 851 “C, respectively. TABLE 2. Sensing properties of the element using the SrCO,-Na,CO,
Therefore the binary carbonate is more easily adaptable to electrode preparation by the meltingquenching method. It is further mentioned that these effects are not unique to the combination of BaCO,, Na2C03 and NASICON. Other binary systems, such as CaCO, -NazCO, and SrC03 -Na,CO,, were also found to show similar stability in the presence of water vapour. As shown in Table 2, for example, a sensor fitted with an electrode of a binary carbonate system, SrC03 + Na*CO, (Sr content: 60 at.%), was unaffected by water vapour in the range 2501460 ppm CO* at 550 “C. It is emphasized that the binary electrodes also showed similar insensitivity to water vapour when fitted to Na-P/p”-alumina or other Na+ conductor [ 111.
4. Conclusions The use of the BaCO, -Na,CO, electrode is quite effective to upgrade a NASICON-based solid-electrolyte COZ sensor with respect to both stability in the presence of water vapour and the rate of response to CO*, although the reasons for (Sr: 60 at.%) electrode at 550 “C
E.m.f. (mV) 250 ppm Dry CO, Wet CO,a *P(H,O) = 0.7 kPa.
-457 -454
( mV/decade)
Electron number (n)
80.6 79.0
1.97 1.94
Nernstian slope 440 ppm -437 -434
865 ppm -414 -412
1460 ppm -395 -393
170
such improvements are still to be elucidated. Such upgrading of the COz-sensing characteristic is not specific to the above sensor system but is likely to be rather common to other binary carbonate electrodes.
Acknowledgements The authors are grateful to Dr Y. Sadaoka, Department of Industrial Chemistry, Ehime University, and Mr H. Futata, Yazaki Meter Co. Ltd. for their helpful discussions. We are also grateful to NGK Co. Ltd. for the donation of NASICON. This work was partially supported by a Grant-inAid for Scientific Research from the Ministry of Education, Science and Culture of Japan, and a grant from Iketani Science and Technology Foundation.
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