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Effect of carbon dioxide on the electrical conductivity of polyethylene glycol-alkali carbonate complex film
Materials Chemistry
and Physics 42 (1995) 73-76
Materials Science Communication
Effect of carbon dioxide on the electrical conductivity of polyethylene glycol-alkali carbonate complex film Yoshiro Saikai, Yoshihiko
Sadaoka, Masanobu Matsuguchi, Hirofumi Yokouchi, Kazuhiko Tamai
Department of Applied Chemistry, Faculty of Engineering, Ehime University, 3 Bunkyocho, Matsuyama 790, Japan Received
19 December
1994;
revised4 March 1995;accepted1 April 1995
Abstract A solid film consisting of triethylene glycol, K,CO,-polyethylene glycol of high mol. wt., was prepared on a sintered alumina substrate with a pair of golden electrodes and their electrical conductivity was measured. When CO2 gas was introduced into the air, the conductivity decreased. This change was reversible. The sensitivity to COz, designated as (change in conductivity caused by CO*) /(conductivity in air), was the highest when a polyethylene glycol of mol. wt. 6000 was used. The sensing mechanism was interpreted in terms of the change in concentration of the charge carrier K+ ion by C02. Keywords: CO2 sensors; Polyethylene
glycol; Alkali carbonate;
Conductivity
1. Introduction Recently several types of CO2 sensors have been developed. Among these, the solid electrolyte type sensor consisting of an alkali carbonate and a solid electrolyte seems to be the most promising one [ l-31. However, their application is limited because the sensor has to be heated at temperatures above 400 “C. On the other hand, Egashira and co-workers [4,5] have proposed another type of sensor using a K&O,-polyethylene glycol solution supported on porous ceramics. When the solution of polyethylene glycol containing KICO, absorbs CO2 gas, the electrical resistivity increases. This type of sensor can be operated at room temperature. However, since liquid film is used in this sensor, the long-term stability is not sufficient. In order to overcome this shortcoming, in this study we have improved the sensor by solidifying the sensing layer. Prior to preparing the solidified sensor, we have studied the effect of the mol. wt. of polyethylene glycol on the electrical conductivity in the atmosphere of air or with CO1 as well as the most appropriate mol. wt. of polyethylene glycol to be used as a matrix to embed K,CO,.
Fig. 1.
Structureof the sensor.
mol. A certain amount of the solution was coated on an alumina substrate having a pair of interdigitated golden electrodes, as shown in Fig. 1. In order to prepare a sensing layer in solid state to detect CO* three kinds of solid polyethylene glycol of mol. wt. of 1000,600O and 20 000 g/mol, respectively, were also tested. They were doped with a solution comprised of a liquid polyethylene glycol and K&OS. Direct current (d.c.) was measured by applying 1% 0.1 V in air containing various concentrations of CO1.
3. Results and discussion 2. Experimental 0.1 mm01 of K,CO, was dissolved in 1 g of various liquid polyethylene glycols of mol. wt. ranging from 106 to 600 gl 0254-0584/95/$09.50 0 1995 Elsevier Science .%A. All rights reserved SSDIO254-0584(95)01579-J
When a d.c. voltage was applied to the sensor, the current became stable in a few minutes in air, the sensor was then exposed to an atmosphere containing 2% of CO*, resulting
14
Y. Saikai et al. /Materials
Chemistry and Physics 42 (199s) 73-76
in a decrease in the current. A typical response and recovery curve to alternating atmospheres of air and 2% CO1 for a sensor comprising triethylene glycol-K&JO3 are shown in Fig. 2. Similar results were obtained for other sensors consisting of various polyethylene glycol-KzC03 liquid ‘layers. In Fig. 3 the relative current changes by the exposure to COz, AZlZ( air) where AZ= Z( air)-Z( COz) versus the molecular weight of liquid polyethylene glycol is given. The result shows that triethylene glycol is the most sensitive material to detect COz.
-5.5 1 -6 z . z
-6.5 -
H
_,_ -7.5 - l3 -6 u -2
-1.8
-1.4
-1.6
-1.2
log (q-lx1 0” / Pa’ s)
Fig. 5. I vs. reciprocal of viscosity: (A) saturated with air; (0) saturated with COz.
Fig. 6. Mechanism of K+ ion migration in polyethylene glycol.
1
0.5
b
100
50
0
time I min
Fig. 2. Response and recovery curve to alternating air and 2% CO2 atmospheres at 35 “C, 0% RH, d.c. = 1 V: (a) introducing CO*; (h) cutting off coz.
OB I 0.5 z 3
0.4 0.3 0.2
-li\
0.1 0
200
400
600
800
MolecularWeight Fig. 3. AI/I(air)
vs. mol. wt. of polyethylene glycol. AI=I(air)-I(C0,).
KI C02+H,0tLH+
80 2 e
60-
z 5
40-
In order to elucidate the factors that affect the conduction in polyethylene glycol-K,C03, the viscosity was measured for pure polyethylene glycols and the solutions containing K&O3 which were bubbled with either air or 2% COz prior to the viscosity measurements. The results are shown in Fig. 4. The viscosity of the polyethylene glycols containing K&O3 was higher than that of pure polyethylene glycol. It was also confirmed that CO2 affects little the viscosity. Fig. 5 gives the logarithm of the current versus the logarithm of the reciprocal of the viscosity. This Figure shows that the conductivity depends on the viscosity of the liquid. The mechanism of the change in conductivity caused by the exposure to COz can be interpreted as follows. CO, dissolves in water which is present in polyethylene glycol in a small amount, generating C03*- as described in Eq. ( 1). On the other hand, KzCOS dissociates in polyethylene glycol into K+ and COJ2-, see Eq. (2). It is believed that these dissociated K+ ions easily migrate from site to site in the polyethylene glycol solution as illustrated in Fig. 6. Eq. (3) can be derived from Eqs. ( 1) and (2). According to this Eq., when the pressure of CO2 increases, the concentration of K+ must decrease resulting in a decrease in the current.
[K+] =
20 0
200
400
600
800
Molecular Weight
Fig. 4. Viscosity vs. mol. wt. of polyethylene glycol: (0) polyethylene glycol; (A) polyethylene glycol-K&O3 in air; ( q) polyethylene glycolK,COs in 2% CO*.
K2 -2H+
+CO,*-
(1)
K4
K3
K2C03 e
01
+HCO,*-
K+ + KC03*K&[K2C03]
K,bU-Nl
_ [H+l2 ”
@co,>
2K+ +CO,*-
(2)
1
(3)
“*
Rb2C03 and Cs2C03 were also used instead of K,CO,. Fig. 7 compares the relative change in current AZ/Z( air) for the three kinds of alkali carbonate having cations of different sizes which act as charge carriers. It was found that the sensitivity is in the order of K2C03 < Rb2C03 < CSCO~.
15
Y. Saikai et al. /Materials Chemistry and Physics 42 (1995) 73-76
0.45 I 1.3
1.4
1.6
1.5
1.7
ionic radius I A
Fig. 7. Effect of size of K+, Rb+ and Csf cations on Al/l(air).
01
10
0
20
30
40
CO,% 0.5
p’:::-: W
0.4
=" ;j
0.3
0.2
this Figure. Cs2C03 is more sensitive than K2C03 as expected from the preceding Figure. In order to prepare a sensor with the solid-state sensing layer, three kinds of polyethylene glycols with higher mol. wt. 1000, 6000, 20 000, respectively, were used. A solid polyethylene glycol was mixed with a solution of triethylene glycol containing K&O3 under heating. The melted mixture was put on the same substrate as illustrated in Fig. 1 and then cooled to form the solid-state sensing layer. The response and recovery curve between air and 2% CO* atmosphere is shown in Fig. 9. The current decreases when the sensor is exposed to CO,. This change in conductivity is reversible. It was found that mol. wt. 60000 polyethylene glycol is the most adequate material for the solid-state sensing layer. In Fig. 10, AZlZ( air) is plotted against the CO* concentration. As shown in this Figure, the sensor in the solid state does not demonstrate any sensitivity in the low CO, concentration range below 0.5%. But above 1% CO2 concentration, AZlZ( air) for the solid sensor becomes larger than that of the sensor comprising a liquid layer. This may be due to the higher water content in the solid layer than in the liquid layer. Rb,CO, and Cs,CO, were also used instead of K,COs. As shown in Fig. 11, AZlZ( air) depends on the ionic radius of the alkali cation of the carbonate doped in polyethylene gly~01. The sensitivity with Cs2C03 is higher than that with K&Os. The result is similar to that in the liquid layer. Consequently, the mechanism of the change in conductivity with CO1 is similar in both the solid and liquid layer.
0.1 0
0.8 ,
0.6
0.25
0.5
0.75
r" a
1
CO,%
Fig.8. (a) Al/l(air)
I
vs. CO* concentration;
0.4
0.2
(b) details in the low concen-
tration range. 0 0
0.5
1
1.5
2
CO,%
Fig. 10. AllI( air) vs. CO, concentration (0) liquid state sensor.
for the ( W) solid-state sensor and
a
0
50
100
time I min
Fig. 9. Response and recovery curve for the solid-state sensor to alternating air and 2% CO. atmospheres: (a) introducing CO,; (b) cutting off CO*.
In Fig. 8, AZlZ(air) is plotted as a function of the CO2 concentration. The sensitivity increases rapidly in the range of low concentrations of COz up to about 1% as shown in
1 ’ K' 0.45 u 1.3
I
1.4
1.5
1.6
1.7
ionic radius IA Fig. 11. Al/l(air)
solid-state sensors.
vs. ionic radius of the K’, Rb+ and Cs+ cations for the
76
Y. Saikai
et al. /Materials
Chemistry and Physics 42 (1995) 73-76
4. Conclusions
References [ 11 R. C&e, C.W. Bale and M. Gauthier, J. Electrochem. Sot.. I31 (1984)
Among the solutions of K&O3 in polyethylene glycol of various mol. wt., the triethylene glycol solution showed the highest sensitivity to CO,. It was found that C&O3 is more sensitive than K,C03. The solid-state sensing layer was prepared by mixing triethylene glycol, alkali carbonate and a high mol. wt. polyethylene glycol (PEG6000).
63. [2] T. Maruyama, X.-Y. Ye and Y. Saito, Solid State Ionics, 23 (1987) 113. [3] N. Miura, S. Yao, Y. Shimizu and N. Yamazoe, J. Electrochem. Sot., 139 (1992) 1384. [4] Y. Shimizu, K. Komori and M. Egashira, J. Electrochem. Sot., 136 (1989) 2256. [5] X.-Q. Wu, Y. Shimizu and M. Egashira, J. Electrochem. Sot., 136 (1989) 2892.
and Physics 42 (1995) 73-76
Materials Science Communication
Effect of carbon dioxide on the electrical conductivity of polyethylene glycol-alkali carbonate complex film Yoshiro Saikai, Yoshihiko
Sadaoka, Masanobu Matsuguchi, Hirofumi Yokouchi, Kazuhiko Tamai
Department of Applied Chemistry, Faculty of Engineering, Ehime University, 3 Bunkyocho, Matsuyama 790, Japan Received
19 December
1994;
revised4 March 1995;accepted1 April 1995
Abstract A solid film consisting of triethylene glycol, K,CO,-polyethylene glycol of high mol. wt., was prepared on a sintered alumina substrate with a pair of golden electrodes and their electrical conductivity was measured. When CO2 gas was introduced into the air, the conductivity decreased. This change was reversible. The sensitivity to COz, designated as (change in conductivity caused by CO*) /(conductivity in air), was the highest when a polyethylene glycol of mol. wt. 6000 was used. The sensing mechanism was interpreted in terms of the change in concentration of the charge carrier K+ ion by C02. Keywords: CO2 sensors; Polyethylene
glycol; Alkali carbonate;
Conductivity
1. Introduction Recently several types of CO2 sensors have been developed. Among these, the solid electrolyte type sensor consisting of an alkali carbonate and a solid electrolyte seems to be the most promising one [ l-31. However, their application is limited because the sensor has to be heated at temperatures above 400 “C. On the other hand, Egashira and co-workers [4,5] have proposed another type of sensor using a K&O,-polyethylene glycol solution supported on porous ceramics. When the solution of polyethylene glycol containing KICO, absorbs CO2 gas, the electrical resistivity increases. This type of sensor can be operated at room temperature. However, since liquid film is used in this sensor, the long-term stability is not sufficient. In order to overcome this shortcoming, in this study we have improved the sensor by solidifying the sensing layer. Prior to preparing the solidified sensor, we have studied the effect of the mol. wt. of polyethylene glycol on the electrical conductivity in the atmosphere of air or with CO1 as well as the most appropriate mol. wt. of polyethylene glycol to be used as a matrix to embed K,CO,.
Fig. 1.
Structureof the sensor.
mol. A certain amount of the solution was coated on an alumina substrate having a pair of interdigitated golden electrodes, as shown in Fig. 1. In order to prepare a sensing layer in solid state to detect CO* three kinds of solid polyethylene glycol of mol. wt. of 1000,600O and 20 000 g/mol, respectively, were also tested. They were doped with a solution comprised of a liquid polyethylene glycol and K&OS. Direct current (d.c.) was measured by applying 1% 0.1 V in air containing various concentrations of CO1.
3. Results and discussion 2. Experimental 0.1 mm01 of K,CO, was dissolved in 1 g of various liquid polyethylene glycols of mol. wt. ranging from 106 to 600 gl 0254-0584/95/$09.50 0 1995 Elsevier Science .%A. All rights reserved SSDIO254-0584(95)01579-J
When a d.c. voltage was applied to the sensor, the current became stable in a few minutes in air, the sensor was then exposed to an atmosphere containing 2% of CO*, resulting
14
Y. Saikai et al. /Materials
Chemistry and Physics 42 (199s) 73-76
in a decrease in the current. A typical response and recovery curve to alternating atmospheres of air and 2% CO1 for a sensor comprising triethylene glycol-K&JO3 are shown in Fig. 2. Similar results were obtained for other sensors consisting of various polyethylene glycol-KzC03 liquid ‘layers. In Fig. 3 the relative current changes by the exposure to COz, AZlZ( air) where AZ= Z( air)-Z( COz) versus the molecular weight of liquid polyethylene glycol is given. The result shows that triethylene glycol is the most sensitive material to detect COz.
-5.5 1 -6 z . z
-6.5 -
H
_,_ -7.5 - l3 -6 u -2
-1.8
-1.4
-1.6
-1.2
log (q-lx1 0” / Pa’ s)
Fig. 5. I vs. reciprocal of viscosity: (A) saturated with air; (0) saturated with COz.
Fig. 6. Mechanism of K+ ion migration in polyethylene glycol.
1
0.5
b
100
50
0
time I min
Fig. 2. Response and recovery curve to alternating air and 2% CO2 atmospheres at 35 “C, 0% RH, d.c. = 1 V: (a) introducing CO*; (h) cutting off coz.
OB I 0.5 z 3
0.4 0.3 0.2
-li\
0.1 0
200
400
600
800
MolecularWeight Fig. 3. AI/I(air)
vs. mol. wt. of polyethylene glycol. AI=I(air)-I(C0,).
KI C02+H,0tLH+
80 2 e
60-
z 5
40-
In order to elucidate the factors that affect the conduction in polyethylene glycol-K,C03, the viscosity was measured for pure polyethylene glycols and the solutions containing K&O3 which were bubbled with either air or 2% COz prior to the viscosity measurements. The results are shown in Fig. 4. The viscosity of the polyethylene glycols containing K&O3 was higher than that of pure polyethylene glycol. It was also confirmed that CO2 affects little the viscosity. Fig. 5 gives the logarithm of the current versus the logarithm of the reciprocal of the viscosity. This Figure shows that the conductivity depends on the viscosity of the liquid. The mechanism of the change in conductivity caused by the exposure to COz can be interpreted as follows. CO, dissolves in water which is present in polyethylene glycol in a small amount, generating C03*- as described in Eq. ( 1). On the other hand, KzCOS dissociates in polyethylene glycol into K+ and COJ2-, see Eq. (2). It is believed that these dissociated K+ ions easily migrate from site to site in the polyethylene glycol solution as illustrated in Fig. 6. Eq. (3) can be derived from Eqs. ( 1) and (2). According to this Eq., when the pressure of CO2 increases, the concentration of K+ must decrease resulting in a decrease in the current.
[K+] =
20 0
200
400
600
800
Molecular Weight
Fig. 4. Viscosity vs. mol. wt. of polyethylene glycol: (0) polyethylene glycol; (A) polyethylene glycol-K&O3 in air; ( q) polyethylene glycolK,COs in 2% CO*.
K2 -2H+
+CO,*-
(1)
K4
K3
K2C03 e
01
+HCO,*-
K+ + KC03*K&[K2C03]
K,bU-Nl
_ [H+l2 ”
@co,>
2K+ +CO,*-
(2)
1
(3)
“*
Rb2C03 and Cs2C03 were also used instead of K,CO,. Fig. 7 compares the relative change in current AZ/Z( air) for the three kinds of alkali carbonate having cations of different sizes which act as charge carriers. It was found that the sensitivity is in the order of K2C03 < Rb2C03 < CSCO~.
15
Y. Saikai et al. /Materials Chemistry and Physics 42 (1995) 73-76
0.45 I 1.3
1.4
1.6
1.5
1.7
ionic radius I A
Fig. 7. Effect of size of K+, Rb+ and Csf cations on Al/l(air).
01
10
0
20
30
40
CO,% 0.5
p’:::-: W
0.4
=" ;j
0.3
0.2
this Figure. Cs2C03 is more sensitive than K2C03 as expected from the preceding Figure. In order to prepare a sensor with the solid-state sensing layer, three kinds of polyethylene glycols with higher mol. wt. 1000, 6000, 20 000, respectively, were used. A solid polyethylene glycol was mixed with a solution of triethylene glycol containing K&O3 under heating. The melted mixture was put on the same substrate as illustrated in Fig. 1 and then cooled to form the solid-state sensing layer. The response and recovery curve between air and 2% CO* atmosphere is shown in Fig. 9. The current decreases when the sensor is exposed to CO,. This change in conductivity is reversible. It was found that mol. wt. 60000 polyethylene glycol is the most adequate material for the solid-state sensing layer. In Fig. 10, AZlZ( air) is plotted against the CO* concentration. As shown in this Figure, the sensor in the solid state does not demonstrate any sensitivity in the low CO, concentration range below 0.5%. But above 1% CO2 concentration, AZlZ( air) for the solid sensor becomes larger than that of the sensor comprising a liquid layer. This may be due to the higher water content in the solid layer than in the liquid layer. Rb,CO, and Cs,CO, were also used instead of K,COs. As shown in Fig. 11, AZlZ( air) depends on the ionic radius of the alkali cation of the carbonate doped in polyethylene gly~01. The sensitivity with Cs2C03 is higher than that with K&Os. The result is similar to that in the liquid layer. Consequently, the mechanism of the change in conductivity with CO1 is similar in both the solid and liquid layer.
0.1 0
0.8 ,
0.6
0.25
0.5
0.75
r" a
1
CO,%
Fig.8. (a) Al/l(air)
I
vs. CO* concentration;
0.4
0.2
(b) details in the low concen-
tration range. 0 0
0.5
1
1.5
2
CO,%
Fig. 10. AllI( air) vs. CO, concentration (0) liquid state sensor.
for the ( W) solid-state sensor and
a
0
50
100
time I min
Fig. 9. Response and recovery curve for the solid-state sensor to alternating air and 2% CO. atmospheres: (a) introducing CO,; (b) cutting off CO*.
In Fig. 8, AZlZ(air) is plotted as a function of the CO2 concentration. The sensitivity increases rapidly in the range of low concentrations of COz up to about 1% as shown in
1 ’ K' 0.45 u 1.3
I
1.4
1.5
1.6
1.7
ionic radius IA Fig. 11. Al/l(air)
solid-state sensors.
vs. ionic radius of the K’, Rb+ and Cs+ cations for the
76
Y. Saikai
et al. /Materials
Chemistry and Physics 42 (1995) 73-76
4. Conclusions
References [ 11 R. C&e, C.W. Bale and M. Gauthier, J. Electrochem. Sot.. I31 (1984)
Among the solutions of K&O3 in polyethylene glycol of various mol. wt., the triethylene glycol solution showed the highest sensitivity to CO,. It was found that C&O3 is more sensitive than K,C03. The solid-state sensing layer was prepared by mixing triethylene glycol, alkali carbonate and a high mol. wt. polyethylene glycol (PEG6000).
63. [2] T. Maruyama, X.-Y. Ye and Y. Saito, Solid State Ionics, 23 (1987) 113. [3] N. Miura, S. Yao, Y. Shimizu and N. Yamazoe, J. Electrochem. Sot., 139 (1992) 1384. [4] Y. Shimizu, K. Komori and M. Egashira, J. Electrochem. Sot., 136 (1989) 2256. [5] X.-Q. Wu, Y. Shimizu and M. Egashira, J. Electrochem. Sot., 136 (1989) 2892.