Journal of The Electrochemical Society, 149 共6兲 H123-H127 共2002兲
H123
0013-4651/2002/149共6兲/H123/5/$7.00 © The Electrochemical Society, Inc.
Influence of Moisture on Potentiometric Cl2 Gas Sensor Using
a Na¿ Conducting Solid Electrolyte
Hiromichi Aono and Yoshihiko Sadaoka
Department of Materials Science and Engineering, Faculty of Engineering, Ehime University, Matsuyama,
Ehime 790-8577, Japan
The influence of moisture in a test gas containing Cl2 was investigated for a Cl2 gas sensor using a NaCl-RuO2 mixed measuring
electrode and a composite Na⫹ conductor of sodium superionic conductor, 40 wt % (Na2 O-Al2 O3 -4SiO2 ). For dried Cl2 gas, the
electron number 共n兲 for the reaction on the electrodes was 1.86 which was very close to the theoretical value n ⫽ 2.0. When water
vapor was added to the Cl2 measuring gas, the electromotive force 共emf兲 significantly decreased, since the Cl2 gas concentration
was reduced by the reaction with the H2 O gas. Based on the calibration curve observed from the relationship between emf and Cl2
concentration at 450°C under dry conditions, it is suggested that the sensor can determine parts per billion levels of Cl2 gas under
ambient conditions. Experimentally, the observed response time became faster with an increase in the flow rate and mainly
depended on the exchange time of the test gas.
© 2002 The Electrochemical Society. 关DOI: 10.1149/1.1477210兴 All rights reserved.
Manuscript submitted July 16, 2001; revised manuscript received January 23, 2002. Available electronically April 29, 2002.
Although many types of gas sensors have been investigated until
now, potentiometric gas sensors using solid electrolytes have been
reported to show rapid response, high sensitivity, and high selectivity for specific gases. In addition, their structures are more compact
than that of the other detection methods. An oxygen gas sensor using
an O2⫺ ion conductor has already been commercialized to control
the air/fuel ratio in automobile exhausts.1 Furthermore, the use of
alkali ion conductors has been demonstrated for the detection of
SOx , NOx , and CO2 . 2-7 Chlorine gas has been used in large quantities for chemical production processes. Recently, it is desired for
control and detection of chlorine and hydrogen chloride contained in
the exhaust gas from an incinerator. It seems that the use of Cl⫺
ionic conductors is suitable for the Cl2 gas sensor. The MCl2 -KCl
(M ⫽ Pb, Ba, or Sr兲 systems such as the 0.97PbCl2 -0.03KCl
and 0.97BaCl2 -0.03KCl systems have been applied as solid
electrolytes.8-13 However, the gas sensors using chlorides cannot detect Cl2 at concentrations lower than 10 ppm due to their poor sinterability and poor stability at high temperatures. On the one hand,
Ag⫹-␤-alumina and MgO-stabilized zirconia can also be used for
this application.14,15 While a high mechanical strength can be obtained for the solid electrolyte, the response of a sensor using
Ag⫹-␤-alumina is very slow even for high Cl2 gas concentrations.
In our previous study, it was reported that the Cl2 gas sensor
using a Na⫹ conductor with the composite material of polycrystalline sodium superionic conductor 共NASICON兲 and glassy
Na2 O-Al2 O3 -4SiO2 共40 wt %兲 with a NaCl-RuO2 electrode can
even detect in the low Cl2 concentration region below 10 parts per
million 共ppm兲.16,17 However, the electron number for the electrode
reaction was about n ⫽ 1.5 which did not agree with the theoretical
number of n ⫽ 2.0. We assumed that Cl2 gas directly reacted with
Na2 O on the solid electrolyte.17 Based on this assumption, we did
not examine the existence of moisture in the test gas. One of the
possibilities for a lower n value is that the Cl2 reacts with moisture
at the operating temperature of the sensor. Furthermore, humid test
gas should be used for practical application of Cl2 gas sensor. In this
study, we investigated the influence of moisture on the potentiometric Cl2 gas sensor using the Na⫹ conducting solid electrolyte.
dried powder was heated at 900°C for 2 h and then ballmilled again
for 4 h. On the other hand, sodium aluminosilicate glass,
Na2 O-Al2 O3 -4SiO2 , was prepared from reagent-grade Na2 CO3 ,
Al2 O3 , and SiO2 powders. The stoichiometric amounts were mixed
and ballmilled with methanol, then the dried powder was calcined in
air for 4 h at 900°C. Finally, the 100:40 weight ratio of NASICON
and Na2 O-Al2 O3 -4SiO2 was mixed by ballmilling for 2 h. A proper
amount of 3% PVA 共polyvinyl alcohol兲 solution was added to the
dried powders as a binder for pressing. The mixture was pressed into
a pellet at 1000 kg/cm2 and then sintered at 1050°C for 2 h.
Preparation of sensor probe.—Both surfaces of the sintered pellet were polished with emery paper. RuO2 powder 共99.9%兲 as a
reference electrode was painted on one surface using methanol. The
dried mixture 共1:1兲 of RuO2 and NaCl was painted on the opposite
side as the measuring electrode using butyl acetate. This painted
disk was fixed on the top of an alumina tube with an inorganic
adhesive.
EMF measurement.—Figure 1 shows the structure of the sensor
probe and apparatus for the electromotive force 共emf兲 measurements. The prepared element was set in a quartz chamber 共chamber
volume is 20 mL兲. The 10.6 ppm Cl2 gas 共balanced with N2 兲
was diluted with artificial 共standard兲 air 共CO ⬍ 1 ppm, CO2
⬍ 2 ppm, HCl ⬍ 1 ppm, and H2 O ⬍ 10 ppm兲. To remove any
trace of water in the test gas, the artificial air and mixed gas were
cooled to ⫺70°C 共dry ice and methanol兲 and ⫺25°C 共dry ice and
25% CaCl2 solution兲, respectively. The cooling temperature of
⫺25°C for the mixed gas was higher than the boiling point of
⫺30°C for Cl2 . A wet air of 100% relative humidity 共RH兲 was made
by bubbling in water at 20°C. Wet gas was prepared by the mixing
of the 10.6 ppm Cl2 gas and the other gas. The relative humidity
共percent RH兲 was controlled by the mixing ratio of the gases. The
mixed test gas was introduced on the sensing electrode side. For the
reference side, air was introduced at the flow rate of 100 mL/min.
The sensor operated at 450°C which has been a suitable temperature
for this sensor.17 The emf of the sensor was measured with a digital
electrometer having a high internal resistance (⬎1013 ⍀).
Theoretical EMF Changes
Experimental
Preparation of a composite solid electrolyte.—For the preparation of NASICON (Na3 Zr2 Si2 PO12), stoichiometric amounts of
Na2 CO3 共99.98%兲, ZrO2 共99.9%兲, SiO2 共99.99%兲, and (NH4 ) 2 HPO4
共⬎99%兲 were mixed in an agate mortar and then heated at 900°C for
2 h in a platinum crucible. The material was then ground into a fine
powder using a ball mill for 2 h in a wet process with methanol. The
EMF vs. Cl 2 gas concentration.—For this sensor, the following
two-electron reactions are suggested to occur
Measuring electrode
Reference electrode
Total
2Na⫹ ⫹ Cl2 ⫹ 2e⫺ ⫽ 2NaCl
⫹
Na2 O ⫽ 2Na ⫹ 共 1/2兲 O2 ⫹ 2e
Na2 O ⫹ Cl2 ⫽ 2NaCl ⫹ 共 1/2兲 O2
关1兴
⫺
关2兴
关3兴
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Journal of The Electrochemical Society, 149 共6兲 H123-H127 共2002兲
H124
Figure 1. Apparatus for the emf measurement.
The emf of the cell is expected to be
emf ⫽ E ⬘o ⫹ 共 RT/nF 兲 ln关 P Cl2 a Na2 O / 共 a NaCl兲 2 共 P O2 兲 1/2兴
关4兴
where E o⬘ is a constant, R is the gas constant, T is the absolute
temperature, F is Faraday’s constant, n is the electron number for
the reaction, and P Cl2 is the concentration of Cl2 . The O2 partial
pressure on the reference electrode and the activity of Na2 O and
NaCl seems to be constant. Equation 4 can be modified to
emf ⫽ E o ⫹ 共 RT/2F 兲 ln关 P Cl2 兴
关5兴
Figure 2. Cl2 gas concentration and electron number n in humid air calculated from thermodynamic data at 450°C.
where E o is a constant value.
Reaction between Cl 2 and H 2 O.—If the test gas contains H2 O,
the following reaction is considered
Cl2 ⫹ H2 O 2HCl ⫹ 1/2 O2
关6兴
The standard Gibbs energy ⌬G 0 , for Eq. 6, calculated using thermodynamic data is as follows18
⌬G 0 ⫽ ⫺0.0667T ⫹ 58.0585 共 kJ/mol兲
关7兴
The equilibrium constant, K, estimated by the relationship
K ⫽ exp共 ⫺⌬G 0 /RT 兲
humid 兲
2
共 P O2 兲 1/2/ 关共 P Cl2
humid 兲共 P H2O 兲兴
共 P HCl
humid 兲
⫽ 2 共 P Cl2
dry 兲
humid 兲
⫽ 4 共 P Cl2
dry 兲
2
共 P O2兲 1/2/ 关 K 共 P H2O兲兴 ⫽ x 共 P Cl2dry兲 2
关11兴
关8兴
关9兴
关10兴
( P O2) 1/2 and P H2O in the equilibrium constant seem to be constant
values for humid conditions
共 P Cl2
is 2.03 ⫻ 10⫺7 and 0.195 at 25 and 450°C, respectively. The reaction of Cl2 with H2 O vapor became a significant factor especially
when the sensor operates at high temperature.
Figure 2 shows relationship between calculated Cl2 and H2 O
equilibrated gas concentrations in ppm at an operating temperature
共450°C兲 of the present sensor.18 The theoretical electron number n
calculated is also shown as a function of H2 O concentration. The Cl2
gas concentration decreases with the increase in H2 O concentration
due to Eq. 6. When an initial Cl2 gas concentration is low, its decrement with an increase in H2 O concentration is stronger than those
for high initial Cl2 . The difference of one order of magnitude for
Cl2 gas concentration of initial dry Cl2 gas enlarges to two orders of
magnitudes for humid gas. The equilibrium constant is
K ⫽ 共 P HCl
Because Cl2 is mostly changed to HCl by the reaction with H2 O in
humid conditions, ( P HCl humid) mostly agrees with initial dry Cl2
concentration 2( P Cl2 dry)
where x is a constant value. From this relationship, Eq. 5 can be
rewritten as a one-electron reaction for humid conditions
emf ⫽ E ⬘o ⫹ 共 RT/F 兲 ln关 P Cl2dry兴
关12兴
where E ⬘o is a constant value. From this equation, the n value will
approach 1.0 for humid condition 共RH ⫽ 1 to 100%兲 when emf
plots are vs. the initial dry Cl2 gas concentration.
Results and Discussion
EMF value.—Figure 3 shows the relationship between the prescribed Cl2 concentration and emf value for the dried and humid
measuring gases. A linear relationship between the emf and the logarithm of the Cl2 concentration was obtained for all the examined
conditions. The n value was 1.50 for the test gas without drying
using the H2 O cold trap. In our previous studies, we did not use the
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Journal of The Electrochemical Society, 149 共6兲 H123-H127 共2002兲
H125
Figure 4. Relationship between initial Cl2 concentration and reduced gas
concentration calculated using the linear emf relation (n ⫽ 1.86) in Fig. 2
共flow rate is 200 mL/min兲.
Figure 3. The emf values under dried and humid conditions for the sensor
using NASICON ⫹ 40% glass at 450°C 共flow rate is 200 mL/min兲. 共䊉兲 H2 O
trapped test gas, 共⽧兲 Non-H2 O trapped test gas, and 共䉱兲 Humid test gas
共relative humidity is shown in the figure兲.
H2 O cold traps and obtained a similar n value.17 From Fig. 2, the n
value will decrease to 1.5-1.8 if gas tank contains ppm level of H2 O.
For the test gas passed through the water traps, the emf value increased and the estimated n value of 1.86 was comparable with the
theoretical value of n ⫽ 2.0. Under the humid conditions, the actual
concentration of Cl2 seems to be lower than the predicted one. If the
linear emf relationship with the Cl2 concentration for the dried condition (n ⫽ 1.86) applies as a calibration curve to estimate the reduced Cl2 gas concentration in the humid conditions, the difference
in the prescribed and actual 共most probable兲 concentration of Cl2 is
interpretable. The n values for all the humid conditions were ca. 0.9
which agreed with 1.0 as calculated in Fig. 2. It is likely that the Cl2
concentration on the measuring electrode was decreased by the Eq.
6. In addition, we measured the influence of the HCl gas concentration and O2 gas concentration for the dried and humid measuring
gases to confirm Eq. 6. Although the humid HCl gas hardly influenced emf value, it was slightly increased by the increase in HCl gas
concentration in dry condition. We also confirmed that emf was
influenced by O2 gas concentration for HCl containing dry condition. This emf response would ascribe that Cl2 gas was produced
from HCl and O2 gas using the opposite direction of the Eq. 6 for
dry conditions. Figure 4 shows the relationship between the prescribed and the interpreted Cl2 gas concentration in the humid conditions. The Cl2 gas concentration was strongly reduced by the existence of moisture. With the change in the dried test gas from 1
ppm Cl2 to standard air, the emf decreased with time and, finally, a
steady value of the emf was 0.035 V 共indicating 0.86 ppb of Cl2 兲.
Such a lower value suggests that the sensor may be detectable for
ppb levels of Cl2 under ambient conditions. Using these interpreted
results, the equilibrium constant, K, in Eq. 6 was estimated and the
correlation between the equilibrium constant and the prescribed Cl2
concentration is shown in Figure 5. In this figure, we plotted the
calculated ( P HCl) 2 ( P O2) 1/2/( P Cl2P H2O) values as the K. While the K
value had a tendency to decrease with an increase in the prescribed
Cl2 concentration, the mean value 共and standard deviation兲 of the
equilibrium constant was about 0.015 which is lower than the ther-
modynamically interpreted value (K ⫽ 0.195). The lower value
suggests that the equilibrium state in Eq. 6 was not achieved within
this period.
Response behavior.—Figure 6 shows the emf responses when
the prescribed Cl2 concentration was changed between 1.1 and 4.2
ppm under the gas flow rate of 200 mL/min. The relationship between the 70% response time 共R兲 and reciprocal of the flow rate
(1/F) for the dried test gas is shown in Fig. 7. This R-1/F relation is
very useful for examining the rate-determining reaction phenomenon on the measuring electrode.13 When the response time is only
limited by the gas exchange time in the test chamber 共ca. 80 cm3兲,
Figure 5. The relationship between initial Cl2 gas concentration and the
estimated K values for Eq. 6 共flow rate is 200 mL/min兲.
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H126
Journal of The Electrochemical Society, 149 共6兲 H123-H127 共2002兲
Figure 6. The emf responses for dried and humid measuring gases at 450°C
共flow rate is 200 mL/min兲.
the response time is proportional to the reciprocal flow rate and is
zero for 1/F ⫽ 0 共i.e., F ⫽ ⬁兲. Although the observed results contained some scattering, the response time depended on the flow rate
and the extrapolated R values to 1/F ⫽ 0 was close to zero for the
examined concentrations. This means that the reaction on the measuring electrode was very fast and the response behavior was mainly
determined by the gas exchange time in the test chamber. The 70%
response time for the lower concentrations is slower than that for the
high Cl2 gas concentrations. In the low Cl2 gas concentration region,
the achievement of a steady state would be retarded by the adsorption of Cl2 on the Teflon gas lines at room temperature. Figure 8
shows the R-1/F relationship when the concentration changed from
1.1 to 2.1 ppm under the dried and humid conditions. Although the
Cl2 gas concentration should be strongly reduced by the reaction
with H2 O for humid conditions at high temperature as shown in Fig.
4, the response time was hardly influenced by the humidity and the
extrapolated value for 1/F ⫽ 0 was essentially zero.
Figure 7. The relationship between the 70% response time and the reciprocal flow rate for dried conditions at 450°C. The concentration changes are
indicated in ppm.
Figure 9. The emf values in 3.7 ppm Cl2 gas under dry and humid (RH
⫽ 40%) conditions at 450°C.
Figure 8. The relationship between the 70% response time and the reciprocal flow rate under dried and humid conditions at 450°C when 1.1 ppm Cl2
gas changed to 2.1 ppm.
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Journal of The Electrochemical Society, 149 共6兲 H123-H127 共2002兲
As shown in Fig. 5, the K values were lower than that estimated
from the thermodynamic data. If the test gas was maintained long
enough at the working temperature, the observed K value may be
comparable to the theoretical value of 0.195. Experimentally, the
test gas was gradually heated from room temperature to the working
temperature 共450°C兲, so that the Cl2 concentration at the point of the
sensor decreases with a decrease in the flow rate. Figure 9 shows the
emf changes with the flow rate. The emf value for the dry gas was
only slightly decreased with a decrease in the flow rate. However,
the emf for the humid gas was more significantly decreased with a
decrease in the flow rate. The calculated K values were 0.0091 and
0.026 for 200 and 50 mL/min at 40% RH, respectively. The flow
rate dependencies convinced us that the K value approached to the
theoretical value with the decrease in the gas flow rate for humid
condition. The reason for the low K value ascribed to that the test
gas 共Cl2 and H2 O兲 did not react enough at the working temperature
because of rapid gas flow.
Conclusion
The emf response was examined for the potentiometric Cl2 gas
sensor using the NaCl-RuO2 mixed measuring electrode and
the composite Na⫹ conductor of NASICON 40 wt %
(Na2 O-Al2 O3 -4SiO2 ) under dry and humid conditions. Experimentally, for the dried test gas containing Cl2 , the n value was 1.86,
which was comparable with the theoretical value of n ⫽ 2.0. At
450°C, the low emf for air without Cl2 suggested that the sensor
may be able to detect for ppb levels of Cl2 under ambient conditions. It is expected that the sensor is useful to detect ppb levels Cl2
in the exhaust gas from an incinerator.
H127
Acknowledgments
The present work was supported by Grants-in-Aid from The
Ministry of Education, Science and Culture of Japan 共no.
09750925兲 共H. Aono兲 and by The Mazda Foundation 共no. 98KK087兲 共H. Aono兲.
Ehime University assisted in meeting the publication costs of this article.
References
1. W. J. Fleming, J. Electrochem. Soc., 124, 21 共1977兲.
2. R. Cote, C. W. Bale, and M. Gauthier, J. Electrochem. Soc., 131, 63 共1984兲.
3. Y. Saito, T. Maruyama, and S. Sasaki, Rep. Res. Lab. Eng. Mater., Tokyo Inst.
Technol., 9, 17 共1984兲.
4. N. Imanaka, Y. Yamaguchi, G. Adachi, and J. Shiokawa, J. Electrochem. Soc., 134,
725 共1987兲.
5. N. Miura, S. Yao, Y. Shimizu, and N. Yamazoe, J. Electrochem. Soc., 139, 1384
共1992兲.
6. Y. Sadaoka, Y. Sakai, and T. Manabe, J. Mater. Chem., 2, 945 共1995兲.
7. H. Aono, H. Supriyatono, and Y. Sadaoka, J. Electrochem. Soc., 145, 2981 共1998兲.
8. Y. Niizeki, S. Itabashi, Y. Onodera, and O. Takagi, Denki Kagaku oyobi Kogyo
Butsuri Kagaku, 54, 948 共1986兲.
9. H. Aono, A. Yamabayashi, E. Sugimoto, Y. Mori, and Y. Sadaoka, Chem. Lett.,
1996, 689.
10. Y. Niizeki and S. Shibata, J. Electrochem. Soc., 145, 2445 共1998兲.
11. A. Pelloux, P. Fabry, and P. Durante, Sens. Actuators B, 7, 245 共1985兲.
12. H. Aono, E. Sugimoto, Y. Mori, and Y. Okajima, J. Electrochem. Soc., 140, 3199
共1993兲.
13. H. Aono, A. Yamabayashi, E. Sugimoto, Y. Mori, and Y. Sadaoka, Sens. Actuators
B, 40, 7 共1997兲.
14. G. Hötzel and W. Weppner, Solid State Ionics, 18-19, 1223 共1986兲.
15. Y. Yan, N. Miura, and N. Yamazoe, Sens. Actuators B, 24, 287 共1995兲.
16. H. Aono and Y. Sadaoka, Chem. Lett., 2000, 34.
17. H. Aono and Y. Sadaoka, J. Electrochem. Soc., 147, 4363 共2000兲.
18. I. Barin, Thermochemical Data of Pure Substances, VCH, Weinheim 共1993兲.
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