Fundamentals and Applications of Chemical Sensors - American

12 Amperometric Proton-Conductor Sensor for Detecting Hydrogen and Carbon Monoxide at Room Temperature

Downloaded by AUBURN UNIV on November 21, 2016 | http://pubs.acs.org Publication Date: May 29, 1986 | doi: 10.1021/bk-1986-0309.ch012

Norio Miura, Hiroshi Kato, Noboru Yamazoe, and Tetsuro Seiyama Department of Materials Science and Technology, Graduate School of Engineering Sciences, Kyushu University, Kasuga-shi, Fukuoka 816, Japan

A new type of amperometric gas sensor using a proton conductor (antimonic acid) and its sensing mechanism are proposed for detecting small amounts of H (or CO) in air at room temperature. The sensing element is composed of the following electrochemical cell : (counter electrode) air, Pt | proton conductor | Pt, sample gas (sensing electrode). The short circuit current of the cell is found to be in direct proportion to the concentration of H (or CO). It is also shown that the sensor can be modified into a simpler construction which eliminates the reference gas (air). This modified sensor is found to exhibit performances as good as that of the original one, with satisfactory stability for about two months. 2

2

In r e c e n t years, gas sensors o p e r a t i n g a t room temperature are becoming increasingly more important i n many f i e l d s . These sensors can be used as so c a l l e d " c o r d l e s s sensors", because they need no e x t e r n a l e l e c t r i c sources t o heat the sensor elements. Although e l e c t r o c h e m i c a l gas sensors which u t i l i z e l i q u i d electrolytes are a v a i l a b l e t o d e t e c t i n o r g a n i c gases, e.g., 0 , CO, C l , 2 ' * room temperature (1-3), they often have time-related problems such as leakage and c o r r o s i o n . The problems are minimized i f s o l i d e l e c t r o l y t e s are used i n place of l i q u i d electrolytes. It has been reported (4,5) that s o l i d e l e c t r o l y t e sensors using s t a b i l i z e d zirconia can detect reducible gases i n ambient atmosphere by making use of an anomalous EMF which i s unusually larger than i s expected from the Nernst equation. However, these sensors should be operated i n a temperature range above ca. 300°C mainly because the i o n i c c o n d u c t i v i t y o f s t a b i l i z e d z i r c o n i a i s too s m a l l a t lower temperatures. On the other hand, s o l i d state proton conductors such as antimonic acid (6/7}' zirconium phosphate (8), and dodecamolybdophosphoric a c i d (9) are known t o e x h i b i t r e l a t i v e l y high p r o t o n i c c o n d u c t i v i t i e s a t room temperature. We r e c e n t l y found t h a t the e l e c t r o c h e m i c a l c e l l u s i n g these proton conductors c o u l d d e t e c t H

2

S

e t c

2

0097-6156/86/0309-0203$06.00/0 © 1986 American Chemical Society

Schuetzle and Hammerle; Fundamentals and Applications of Chemical Sensors ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

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t

FUNDAMENTALS AND APPLICATIONS OF CHEMICAL SENSORS

204

s m a l l amounts of H or CO c o n t a i n e d i n a i r (10-12). T h i s s o l i d state sensor can be operated even at room temperature and belongs to a potentiometric class which u t i l i z e s electromotive force (EMF) as a sensor s i g n a l . The sensing performances of the potentiometric proton conductor sensor are shown i n F i g u r e 1. I t i s seen t h a t the 90 % response time to 2000 ppm H i n a i r at room temerature i s about 10 seconds and the EMF value of the sensor c e l l changes almost i n proportion to a logarithm of H concentration. It has been shown that the sensing electrode i s at a potential determined by electrochemical oxidation of H (or CO) and e l e c t r o c h e m i c a l r e d u c t i o n of 0 (11). Although such a p o t e n t i o m e t r i c sensor i s convenient f o r d e t e c t i n g a broad range of gas concentration, the accuracy of gas detection i s i n f e r i or to an amperometric sensor i n which the sensor signal i s i n d i r e c t proportion to the sample gas concentration. Thus we t r i e d to develop an amperometric sensor using proton conductors and found that the short c i r c u i t current of the proton conductor sensor i s proportional to the H (or CO) c o n c e n t r a t i o n i n a i r (13). We d e s c r i b e here the fabrication, performances, and the sensing mechanism of the new type of amperometric proton conductor gas sensor. 2

2

2


2

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2

Experimental The f i r s t sensor element examined i s represented as follows:

a i r , Pt b l a c k •counter \ UlectrodeJ

| proton conductor d i s c (

s

o

l

i

d

electrolyte)

| Pt b l a c k , sample gas /'sensing \ ^electrode/

Figure 2 shows the structure of t h i s sensor which i s s i m i l a r to that of the potentiometric sensor reported e a r l i e r (10). The only d i f ference i s that i n t h i s sensor a short c i r c u i t current between the sensing e l e c t r o d e and the counter e l e c t r o d e i s measured with an ammeter. The proton conductor, antimonic a c i d (Sb 05»2H 0), was prepared from antimony trioxide and hydrogen peroxide according to a method described elsewhere (7,14). The sample powder was mixed with 20 wt% T e f l o n powder b i n d e r (Lubron L-2, D a i k i n Ind. Co. Ltd.) and then c o l d - p r e s s e d at 4200 k g f / c m i n t o a compact d i s c 10 mm i n diameter and 1 mm i n thickness. Platinum black powder was applied on both ends of the t h i n d i s c to form a s e n s i n g - and a countere l e c t r o d e w i t h a geometric area of ca. 0.4 cm . The d i s c was then fixed to an end of a glass tube by means of Epoxy resin. E l e c t r i c a l c o n t a c t s between each e l e c t r o d e and Pt leads were made by carbon paste. The sample gas prepared by mixing small amounts of H with a i r (or N i n some cases) was passed over the s e n s i n g e l e c t r o d e at 90 cm /min, while only a i r was fed to the counter electrode at the same gas flow rate. Commercial gases without p u r i f i c a t i o n were used for a l l the experiments. The gases were humidified by passing them through water. T h i s i s necessary to prevent the proton conductor from d r y i n g . The sensor s i g n a l , a s h o r t c i r c u i t c u r r e n t of the c e l l , was measured at room temperature by means of an ammeter (Hokuto Denko Co, Ltd., Zero Shunt Ammeter HM-101). 2

2

2

2

2

2

3


205

Amperometric Proton-Conductor Sensor

12. M1URAETAL.

Results and Discussion Sensing performance for H^. Sensing performance of the amperometric sensor was examined for the detection of H i n a i r . Figure 3 shows the response curve for 2000 ppm H in a i r at room temperature. The response was s t u d i e d by changing the atmosphere of the sensing e l e c t r o d e from an a i r flow to the sample gas flow. With a i r the short c i r c u i t current between two electrodes was zero. On contact w i t h the sample gas flow, the c u r r e n t i n c r e a s e d r a p i d l y . The 90% response time was about 10 seconds and the stationary current value was 10yUA. When the a i r flow was resumed, the c u r r e n t returned to zero within about 20 seconds. F i g u r e 4 shows how the short c i r c u i t c u r r e n t depends on the concentration of H which i s d i l u t e d with a i r or N . It i s noteworthy that for H i n a i r the short c i r c u i t current i s approximately i n d i r e c t p r o p o r t i o n to the H c o n c e n t r a t i o n . As mentioned before, t h i s f a c t suggests that f o r p r a c t i c a l purpose the amperometric sensor i s more accurate than a potentiometric sensor. When H was d i l u t e d with N , the sensor exhibited a very d i f f e r e n t behavior with far greater current values and a nonlinear dependence on H concentration. In t h i s case, the c e l l i s a c t u a l l y an H -0 fuel c e l l which accounts for the greater current values. 2

2

2

2


2

2

2

2

2

2

2

Sensing mechanism. As shown p r e v i o u s l y (11), when the c i r c u i t of the c e l l i s open, the p o t e n t i a l of the sensing e l e c t r o d e i s d e t e r mined by the following reactions (1) and (2). H — 2H l / 2 0 + 2H

+

2

+

2

+ 2e + 2e

— H0 2

(1) (2)

In an atmosphere of H and a i r , these reactions proceed simultaneously to form a l o c a l c e l l on the sensing electrode. In t h i s s i t u a t i o n the p o t e n t i a l of the sensing e l e c t r o d e i s a mixed p o t e n t i a l (E ) where the anodic c u r r e n t i ^ j i s equal to the cathodic one i( j. The mixed potential i s determined as an intersection of both anodic and cathodic p o l a r i z a t i o n curves as shown i n Figure 5. It i s important that the anodic p o l a r i z a t i o n curve have a l i m i t i n g current region as mentioned before (11). On the counter electrode where a i r i s passed over, only reaction (2) takes place endowing the counter electrode with a potential near E . Thus, the potentiometric sensor observes the potential difference between the sensing electrode and the counter electrode E^-E^,. On the other hand, when the two e l e c trodes are e l e c t r i c a l l y connected with a lead (short c i r c u i t ) , both the sensing and the counter electrodes are forced to be at the same p o t e n t i a l ( E ) as shown i n the same f i g u r e . T h i s means t h a t the p o t e n t i a l of the sensing e l e c t r o d e s h i f t s to the d i r e c t i o n where r e a c t i o n (2) i s unfavorable, while t h a t of the counter e l e c t r o d e s h i f t s favorably for reaction (2). In order to confirm such potential s h i f t we observed the actual behavior of the sensing and counter electrode potential under both open and short c i r c u i t c o n d i t i o n s . Each p o t e n t i a l was measured a g a i n s t a s i l v e r r e f e r e n c e e l e c t r o d e which was attached to the sensor element as shown i n Figure 6. Figure 7 depicts the response curves a g a i n s t 500 ppm H i n a i r under both c o n d i t i o n s . When the c i r c u i t i s open, the change in potential occurs only at the sensing 2

M

2

c

sc

2



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Figure 1. Response curve of the potentiometric sensor to 2000 ppm H and dependence of EMF of the sensor on H concentration in a i r . 2

2

Glass

tube

Epoxy r e s i n Proton conductor 'Pt electrode Sample gas fW

2

- Pt

lead

\

o r CO air

withN

o r N2

)

Figure 2. Structure of the amperometric sensor. "Reproduced with permission from Ref. 13. Copyright 1984, Chemical Society of Japan ."

'The

1

Figure 3. Response curve of the amperometric sensor to 2000 ppm H i n a i r at room temperature. 2



12.

MIURAETAL.

Figure 4. Short H concentration "Reproduced with Chemical Society 2

207


c i r c u i t current of the amperometric sensor vs. i n a i r or N permission from Ref. 13. Copyright 1984, The of Japan ." 2

,

1

Figure 5. Schematic p o l a r i z a t i o n curves for reactions (1)and (2). "Reproduced with permission from Ref. 13. Copyright 1984, 'The Chemical Society of Japan'."



208

e l e c t r o d e . Under the short c i r c u i t c o n d i t i o n the sensing and the counter electrode are forced to s h i f t to a potential i n between as mentioned above. Moreover, i t i s noticed that the potential s h i f t i s larger for the sensing electrode. Thus, we i l l u s t r a t e schematically the sensing mechanism of the amperometric sensor i n F i g u r e 8. When the c i r c u i t i s open, r e a c tions (1) and (2) are balanced at the sensing electrode; the proton produced by the electrochemical oxidation of H w i l l be consumed at the sensing electrode by the electrochemical reduction (2). It i s important that the anodic reaction has been shown to be a d i f f u s i o n l i m i t e d process. Under the short c i r c u i t c o n d i t i o n , these two reactions are not balanced at the sensing electrode. The cathodic reaction at the sensing electrode becomes unfavorable, so that the consumption of H by r e a c t i o n (2) at the sensing e l e c t r o d e i s suppressed. The excess H thus produced on the sensing electrodes migr a t e s toward the counter e l e c t r o d e through the proton conductor membrane to be consumed by the reaction (2). This process i s accompanied by a flow of e q u i v a l e n t e l e c t r o n s as an e x t e r n a l c u r r e n t . The anodic oxidation reaction of H remains as a d i f f u s i o n - l i m i t e d process under the short c i r c u i t conditions, so that the amount of H produced by the anodic reaction i s proportional to the H concentrat i o n . T h i s e v e n t u a l l y g i v e s r i s e to an e x t e r n a l c u r r e n t roughly proportional to the H concentration i n the gas phase. 2

+


+

2

+

2

2

Modification of the sensor structure. The above amperometric sensor has a rather complicated construction, because the sample gas (H + air) i s separated from the reference a i r . So, we t r i e d to simplify the sensor structure as shown i n Figure 9. As proton conductor we used a thin antimonic acid membrane (mixed with Teflon powder) of 0.2 mm thickness. This membrane i s thin and porous enough to allow a p a r t of the sample gas to permeate. On the other hand, the counter Pt e l e c t r o d e was covered w i t h T e f l o n and Epoxy r e s i n i n order to avoid a d i r e c t contact with the sample gas. The modified sensor was found to exhibit the excellent response to s m a l l amounts of H i n a i r as shown i n F i g u r e 10. The 90 % r e sponse time f o r 2000 ppm H was about 10 seconds and the short c i r c u i t current depends l i n e a r l y on the H concentration in the same manner as observed i n the o r i g i n a l sensor. 2

2

2

2

Sensing mechanism of the modified sensor. The sensing mechanism i n t h i s modified sensor should be e s s e n t i a l l y the same as that of the unmodified one. I t i s noteworthy that a s t a t i o n a r y short c i r c u i t current was obtained i n spite of such sensor construction that the counter electrode was covered with Epoxy resin. Since the sensing electrode i s placed i n the same s i t u a t i o n as the unmodified sensor, t h i s f a c t i n d i c a t e s t h a t the c a t h o d i c r e a c t i o n i s allowed to take place s t a t i o n a r i l y at the counter electrode. The proton conductor membrane i s as t h i n as 0.2 mm, so t h a t the r e a c t a n t 0 and the produced H 0 w i l l permeate the membrane as shown i n F i g u r e 11. A p a r t of H w i l l n a t u r a l l y a l s o permeate through the membrane, but the transfered H w i l l be consumed by the reaction with 0 e l e c t r o chemically or c a t a l y t i c a l l y at the counter electrode. Furthermore, the r a t e of H supply to the counter e l e c t r o d e through the membrane i s r a t h e r s m a l l as compared w i t h t h a t to the 2

2

2

2

2

2


12. MIURAETAL.


209

p r o t o n c o n d u c t e r membrane

air

•H2 i n a i r

sensing electrode counter e l e c t r o d e reference electrode

(Pt)

(Pt) (Ag)


Figure 6. Configuration of the sensor attached with reference Ag electrode.

a i r ~4=

>

500ppm H

2

in air ^ — a i r

EE

600

\ CD

TD

O L_

4—'

CJ CU CD

counter

500

100

LU

0

Time Figure 7. Behaviors of respective electrode potential under open c i r c u i t and short c i r c u i t conditions.

x l —* /^Proton (Counter

electrode)x \

\

I

u "

conductor x(Sensing

2 + ?\ e

2

J electrode)

Figure 8. Sensing mechanism of the amperometric sensor. "Reproduced with permission from Ref. 13. Copyright 1984, 'The Chemical Society of Japan ." 1


210


Epoxy

resin

Proton

conductor

Sensing

electrode

(Pt)

Counter

electrode

(Pt)


Teflon Pt

lead

Ammeter

Figure 9. Structure of the modified amperometric sensor. "Reproduced with permission from Ref. 13. Copyright 1984, 'The Chemical Society of Japan ." 1

2000 p p m H 2

Figure 10. Response curve of the modified amperometric sensor to 2000 ppm H and dependence of short c i r c u i t current of the sensor on H concentration i n a i r . "Reproduced with permission from Ref. 13. Copyright 1984, 'The Chemical Society of Japan ." 2

2

1


12.

MIURAETAL.

211


sensing electrode from the gaseous bulk. Therefore, the H concent r a t i o n around the counter electrode i s considered to be lower than that around the sensing electrode. This difference i n H pressure between the two e l e c t r o d e s can account f o r the s t a t i o n a r y flow of short c i r c u i t current. 2

2

Performances f o r CO. T h i s m o d i f i e d sensor was found to be a l s o sensitive to small amounts of CO i n a i r . Figure 12 shows response curves to CO i n a i r at room temperature. Although the value of short c i r c u i t current i s rather small as compared with that for H d e t e c t i o n , the response i s s t i l l r a p i d enough. I t i s noteworthy that the current value i s also i n d i r e c t proportion to CO concentration as shown in Figure 13. The sensing mechanism i s considered to be almost the same as i n the case of H d e t e c t i o n . The anodic reaction for CO can be expressed by the following reaction. 2


2

CO + H 0 2

m» C0

2

+ 2H

+

+ 2e

It was confirmed that t h i s sensor was (15000 ppm) and propane (7000 ppm) i n a i r .

(3)

insensitive to methane

Long-term s t a b i l i t y . As for a p r a c t i c a l use, the long-term s t a b i l i ty i s one of the important factors. Figure 14 shows the results of a long-term s t a b i l i t y test for the modified sensor at room temperature. Except f o r the beginning of the t e s t p e r i o d , the short c i r cuit current to 1.3 v o l % H i n a i r was stable for about two months. The anomalously large current at the beginning has not been understood well yet. 2

Conclusions The results of our work may be summarized as follows: 1) A new type of amperometric sensor using a proton conductor could detect small amounts of H or CO i n a i r at room temperature. 2) The short c i r c u i t c u r r e n t of the sensor c e l l was i n d i r e c t proportion to the sample gas concentration. 3) The possible sensing mechanism of t h i s sensor was proposed. 4) The sensor c o u l d be m o d i f i e d i n t o a s i m p l e r c o n s t r u c t i o n where the reference gas (air) was no longer necessary. 5) The modified sensor was stable for about two months. 2

Acknowledgments This work was p a r t i a l l y supported by the Grant-in-Aid for Developmental S c i e n t i f i c Research (No. 60850151) from the M i n i s t r y of Education, Science and Culture, Japan.



212


Figure 11. Sensing mechanism of the modified amperometric sensor.

Figure 12. Response curves of the modified amperometric sensor to CO.



MIURA ET AL.


CO C o n c e n t r a t i o n /

ppm

Figure 13. Short c i r c u i t current of the modified amperometric sensor vs. CO concentration i n a i r .

Operation

time /

day

F i g u r e 14. Long-term s t a b i l i t y of the m o d i f i e d amperometric sensor a t room temperature.


214


Literature Cited Dietz, H. ; Haecker, W. ; Jahnke, H. "Advances in Electrochemistry and Electrochemical Engineering " Gerischer H. ; Tobias, C., Ed. ; John Wiley & Sons: New York, 1977, Vol. X, p.1. 2. Belanger, G. Anal. Chem. 1974, 46, 1576. 3. Isobe, M. Keisoku Gijustu 1976, 4, 43. 4. Shimizu, F. ; Yamazoe, N. ; Seiyama, T. Chem. Lett. 1978, 229. 5. Okamoto, H. ; Obayashi, H. ; Kudo, T. Solid State Ionics 1980, 1, 319. 6. England, W. A. ; Cross, M. G. ; Hammett, A. ; Wiseman, P. J. ; Goodenough, J. B. Solid State Ionics 1980, 1, 231. 7. Ozawa, Y. ; Miura, N. ; Yamazoe, N. ; Seiyama, T. Nippon Kagaku Kaishi 1983, 488. 8. Jerus, P. ; Clearfield, A. Solid State Ionics 1982, 6, 79. 9. Nakamura, O. ; Kodama, T. ; Ogino, I. ; Miyake, Y. Chem. Lett. 1979, 17. 10. Miura, N. ; Kato, H. ; Yamazoe, N. ; Seiyama, T. Denki Kagaku 1982, 50, 858. 11. Miura, N. ; Kato, H. ; Yamazoe, N. ; Seiyama, T. " Proc. of the Int. Meeting on Chemcal Sensors " Seiyama, T. ; Fueki, K.; Shiokawa, J. ; Suzuki, S., Ed., Kodansha/Elsevier, Tokyo, 1983; p.233. 12. Miura, N. ; Kato, H. ; Yamazoe, N. ; Seiyama, T. Chem. Lett. 1983, 1573. 13. Miura, N. ; Kato, H. ; Ozawa, Y. ; Yamazoe, N. ; Seiyama, T. Chem. Lett. 1984, 1905. 14. Miura, N. ; Tokunaga, K. ; Harada, T. ; Yamazoe, N. ; Seiyama, T. Denki Kagaku 1978, 46, 113. Downloaded by AUBURN UNIV on November 21, 2016 | http://pubs.acs.org Publication Date: May 29, 1986 | doi: 10.1021/bk-1986-0309.ch012

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