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Cordless solid-state hydrogen sensor using proton-conductor thick film
125
Sensors and Actuators, Bl (1990) 125-129
Cordless Solid-state Hydrogen Sensor Using Proton-conductor Thick Film NORIO MIURA,
TATSURO
HARADA,
YOUICHI
SHIMIZU
and NOBORU
YAMAZOE
Department of Materials Science and Technology, Graduate School of Engineering Sciences, Kyushu University, Kasuga-shi, Fukuoka-ken 816 (Japan)
Abstract
Potentiometric as well as amperometric solidstate gas sensors using a thick film ( zz 10 pm) of proton-conductor have been developed for detecting small amounts of hydrogen in air at room temperature. The sensor elements are composed of the following electrochemical cell: reference (or counter) electrode (Au or Pt) 1 proton-conductor thick film 1sensing electrode (Pt). The thick film is formed on an alumina substrate by applying a paste of antimonic acid-polyvinyl alcohol mixture, mainly by means of a spin-coating method. The electromotive force of the potentiometric sensor varies logarithmically with H2 concentration, while the short-circuit current of the amperometric sensor is found to be proportional to H, concentration. Both give a 90% response time of about 10 s at 30 “C.
(4) The choice of materials for sensor fabrication can be less limited compared with hightemperature sensors. From these viewpoints, we have investigated new types of solid-state gas sensors that can work at room temperature. We have already reported that sensors using proton-conductors such as antimanic acid can detect small amounts of H,(CO, NH3) in air even at room temperature [l-8]. However, these investigations were mainly aimed at elucidating the basic properties of proton-conductor sensors and the sensor elements used were of rather primitive structure, based on a disc or sheet of proton-conductor. In this paper we report that proton-conductor (antimonic acid) thick films can be successfully introduced into sensor elements, which not only have an advanced structure suitable for microfabrication, but also show even better sensing performance than those elements using a disc or a sheet of proton-conductor.
1. Introduction
There are many excellent solid-state gas sensors already put into practical use, of which inflammable gas sensors using oxide semiconductors and oxygen sensors using stabilized zirconia are typical examples. Most of these gas sensors are designed to operate at elevated temperatures in order to achieve sufficient sensitivities and response rates to objective gases. These sensors must be equipped with a heater connected to external electric power sources. Compared with such sensors, the solid-state gas sensor, which can operate at room temperature, has the following advantages: (1) Room-temperature sensing seems to create interesting new application fields for gas sensors. (2) With no heater needed for the sensor element, the sensor may be operated with a small power source such as a battery. This may lead to a portable (so-called ‘cordless’) sensor. (3) The elimination of a heater simplifies the structure of the element, which is convenient for the fabrication of microsensors or integrated sensors.
2. Expesimental The proton-conductor thick film was obtained as follows. A powder mixture containing 80 wt.% of antimonic acid (Sbz05*2H,0, donated by Toagosei Chemical Ind.) and 20 wt.% of polyvinyl alcohol (PVA, degree of polymerization: 500) was mixed with water, followed by partial evaporation of water at about 80 “C. The resulting paste was applied on a porous alumina substrate (10 x 10 x 3 mm, apparent porosity: 43%) either by a screen-printing method or by a spin-coating method (rotating rate of 3500 rpm). The protonconductor fihn about 10 pm thick thus obtained was then dried at room temperature for about one day. Two types of sensor elements were fabricated, both being composed of the following electrochemical cell: reference (or counter) electrode 1 proton-conductor thick film 1 sensing electrode. A planar type had both sensing and reference electrodes on top of the proton-conductor film (see 0 Elsevier Sequoia/Printed
in The Netherlands
126
Fig. 3). The electrodes (Pt or Au) were thinly deposited by RF sputtering (Nichiden Anelva, SPF-2 IOHRF) or vacuum evaporation (Hitachi, HUS-SGB). A laminated-type element had the two electrodes sandwiching the proton-conductor film (see Fig. 6). For preparing this type of element, a reference Pt electrode was first fitted to the substrate, and after the application of the proton-conductor film, the Pt electrode was fitted on it. Gas-sensing experiments were carried out in a flow ( 100 cm3 mm-‘) of a mixture of Hz (or other reducing gases) and air. To prevent the proton-conductor film from drying, the gas was humidified by passing through water. For potentiometric sensing, the difference in potential between the sensing and reference electrodes (electromotive force) was measured by an electrometer (Advantest, TR8652) at 30 “C. For amperometric sensing, the short-circuit current flowing through the element was measured by an ammeter (Hokuto Denko, Zero Shunt Ammeter HM-101) at 30 “C.
(4
3. Results and Discussion 3.1. SEiU Observation of Films Figure 1 represents SEM (Hitachi, S-510) photographs of the proton-conductor films prepared by a screen-printing method (a) as well as by a spin-coating method (b). Although the same paste was used in both methods, the resulting films were quite different in external appearance from each other, i.e., the screen-printed film had a rather rough surface with small through holes running vertically in places, while the spin-coated one had a fine uniform microstructure. Figure 2 shows a cross-sectional view of a spin-coated film formed on an alumina substrate. It is seen that the film is about 10 pm thick and stuck tight on the substrate. 3.2. Potentiometric Sensor The potentiometric sensing properties of the thick-film sensor were examined with the planartype element shown in Fig. 3. Previously we reported that Ag could be used as a reference electrode because of its inertness to dilute Hz [2]. Subsequently we found that Au was not only almost inert to H2 but also much more stable for long-term sensor operation than Ag. Thus an Au reference electrode was adopted in the present study. The 0.2 pm thick Pt sensing electrode and the 0.3 pm thick Au reference electrode had a 1 mm space between them. The responses of the element to several sample gases are shown in Fig. 4. The difference in elec-
0) Fig. 1. External appearance of the antimonic-acid thick til (SEM). (a) Screen-printed film; (b) spin-coated film.
trode potential, AEm_,++ responded quite well to H,, giving a change as large as about 200 mV to 1000 ppm Hz. The 90% response time was as short as about 10 s at 30 “C, which was ev‘en shorter than that of the previous disc-type eleme:nt
)
antimonic acid
alumina i Fig. 2. Cross-sectional view of the antimonic acid flm deposited on an alumina substrate (SEM).
127 sensing Pt electrode
(O.Zpm)
antimonic acid film /
(ca. IOvm)
alumina substrate
(-150mVldecade)
1000
Fig. 3. Structure of the potentiometric thick-film sensor (planar type).
[ 1,2]. The sensor also responded to CO (1000 ppm) in air, but it was insensitive to methane ( 15 000 ppm), propane (7000 ppm) and isobutane (1000 ppm). Figure 5 shows the variation of AE~R_Aujwith the concentration of H, and CO in air. AEcPt_Auj was found to be linear to the logarithm of H, concentration in the range 200 ppm to l%, with a slope of about - 150 mV/decade. A similar linear relationship with a slope of about -80 mV/ decade was also observed for CO in the concentration range 20- 1000 ppm. The less steep slope for CO indicates that the thick-film sensor is less sensitive to CO than to H,, as was also the case in the disc-type sensor element [7J. The gas-sensing mechanism of this sensor is described briefly below. There have been reports of thick-film H, sensors using proton-conductors such as hydrogen uranyl phosphate [9] and polyvinyl alcohol impregnated with phosphoric acid [lo], which aimed at detecting Hz, especially in inert gas. These sensors were designed to operate as an H, concentration cell and the presence of oxygen reportedly interfered severely with them. In our sensor, however, the abundant presence of oxygen is essential for steady operation. As already shown previously [ 11, the electric potential of the sensing Pt electrode on exposure to a mixture of H, + air is determined by the electro-
Time
Fig. 4. Responses of the potentiometric sensor to various sample gases at 30 “C (Hz: 1000 ppm, CO: lOOOppm, CH,: 15 000 ppm, C,Hs: 7000 ppm, i-C.,H,,: 1000 ppm).
Concentration
1Ocxxl
/ ppm
Fig. 5. Dependence of the potentiometric response on gas concentration at 30 “C.
chemical hydrogen oxidation (1) and the electrochemical oxygen reduction (2) : H2 +2H+
+ 2e-
(1)
( 1/2)02 + 2H+ + 2e- +H,O
(2) These reactions proceed simultaneously to form a local cell and the electrode potential eventually reaches a steady value, a mixed potential (EM), when the corresponding anodic and cathodic currents become equal in magnitude. In other words, the mixed potential is given by the intersection of the anodic- and cathodic-polarization curves. Under the particular condition that the concentration of Hz (C,,) is far smaller than that of 02, the anodic polarization curve will be dominated by a limiting current region where the anodic current is proportional to C,, , while the cathodic one will follow a Tafel-like equation. The intersecting potential, EM, will be given in the form EM = a + b log C,,
(a, b are constants)
(3) This equation verifies the linear dependence of AECPt_Aujon the logarithm of H2 concentration shown in Fig. 5. The same explanation also holds for CO sensing when eqn. (1) is replaced by the reaction CO+HzO+C0,+2H+
+2e-
(4)
3.3. Amperometric Sensor Although a potentiometric sensor covers a broad range of gas concentrations, its accuracy of gas detection is usually inferior to that of an amperometric sensor, which gives a signal output in direct proportion to the sample gas concentrations. We have already reported that the disc-type sensor elements fabricated from a mixture of antimanic acid and a Teflon binder can operate amperometrically under the short-circuit condition [3,4]. It was found in this study that an amperometric sensor element could be fabricated successfully by using a proton-conductor thick film. The fabricated sensor element had a laminatedtype structure, as shown in Fig. 6, in order to
128 antimonlc acid film (ca.~o~~) sensing Pt electrode
\
\
r
Ag reference
electrode
(0.2pm) antimonic acid film
T alumina /
(4
cdunter Pt electrode (0.2 pm 1
alumina
substrate
(3mm)
5cQl
Fig. 6. Structureof the amperometricthick-filmsensor(laminated type). minimize the electric resistance of ionic conduction between the electrodes. Only the spin-coated proton-conductor film, having no through holes, was applicable to this type of structure. The H, sensing performance of the laminated element is shown in Fig. 7. The short-circuit current (output signal) was found to increase linearly with the H, concentration in air up to 6000 ppm. The 90% response time to 1000 ppm Hzwas again as short as about 10 s at 30 “C. It is thus clear that, with such an excellent performance, the laminated element is quite promising as an amperometric H2 sensor element. Although the output signal to a given H2 concentration is still small compared with that of the previous disc-type element, it could be intensified through optimization of the sensor design and fabrication techniques. To clarify the sensing mechanism of the above element, one should know the behaviour of each Pt electrode in the sample gas. For this purpose, the element was fitted with an additional Ag electrode on the outer surface of the proton-conductor film, as shown in Fig. 8(a). Under the open-circuit condition, the electric potential of each Pt electrode referring to the Ag electrode was measured in varying concentrations of Hz in air. As shown in Fig. 8(b), each electrode potential had a linear dependence on the logarithm of H2 concentration, while the outer electrode was slightly more dependent than the inner one. The two electrodes
0
2ooo
Loo0
H, concentration
6M I ppm
Fig. 7. (a) Dependenceof the amperometricresponseon H, concentrationin air and (b) a response transient of the shortcircuit current at 30 “C.
substrate
inner electrode
t ‘“k d
outerelectrode
01 100
500 1000
Hz concentration
5000
I
ppm
@I
Fig. 8. (a) Laminated-type element with an additional Ag electrode; (b) behaviour of electric potential of Pt electrodes with reference to Ag in varying concentrations of H, in air.
differed by 1OO- 150 mV in electric potential in the examined H, concentration range, and such a large potential difference would be a driving force for the current flow under the short-circuit condition. It was found that the output current of the laminated element decreased drastically to almost insignificant values when the outer Pt electrode was covered with another spin-coated film of antimonic acid. This’ implies that the spin-coated film was sufficiently tight to gas permeation. The same drastic decrease in output current also resulted when part of the alumina substrate of the element was covered with epoxy resin. This indicates that the gas transport necessary for the inner Pt electrode, i.e., the supply of oxygen and the removal of H,O, was assisted by permeation through the porous alumina substrate. Based on these findings, the following sensing mechanism was estimated for the present sensor. The inner Pt electrode, which is separated from the gas phase by the porous alumina, is less accessible to H, molecules than the outer Pt electrode. It follows that the anodic reaction (1) takes place at a higher rate on the outer electrode than on the inner one. Under the short-circuit condition, the protons produced more abundantly on the outer electrode would migrate to the inner electrode through the proton-conductor film, giving rise to the stationary electric-current flow. Since the anodic reaction ( 1) is a diffusion-limited process [ 11, the rate of proton production and therefore that of proton migration (short-circuit current) would be proportional to the Hz concentration in the gas phase, as was observed.
129
4. coIdlsions The results of the present work are summarized as follows: (1) The use of a proton-conductor thick film is highly promising for developing cordless, micro gas sensors capable of operation at room temperature. (2) The thick film is applicable to both the potentiometric and amperometric elements. (3) Both elements show excellent responses to Hz (and CO), with a 90% response time of about 10 s at 30 “C. (4) The spin-coating method gives a better quality thick film than the screen-printing method. Acknowledgements We gratefully acknowledge the use of the sputtering instrument at the Center of Advanced Instrumental Analysis, Kyushu University. This work was partially supported by grants from the Ministry of Education, Science and Culture of Japan and Casio Science Promotion Foundation. References 1 N. Miura, H. Kato, N. Yamazoe and T. Seiyama, Proton conductor sensors for H, and CO operative at room temperature, Proc. Znt. Meet. Chemical Sensors, Fukuoka,
Japan, Sept. 19-22, 1983, Kodansha, Tokyo/Elsevier, Amsterdam, pp. 233-238. 2 N. Miura, H. Kato, N. Yamazoe and T. Seiyama, An improved type of proton conductor sensor sensitive to Ha and CO at room temperature, Chem. Lat., (1983) 15731576. 3 N. Miura, H. Kato, Y. Omwa, N. Yamazoe and T. Seiyama, Amperometric gas sensor using solid state proton conductor sensitive to hydrogen in air at room temperature, Chem. Lett., (1984) 1905- 1908. 4 N. Miura, H. Kato, N. Yamazoe and T. Seiyama, Amperometric proton-conductor sensor for detecting hydrogen and carbon monoxide at room temperature, Fkaken~als and Applications of Chemical Sensors, American Chemical Society, Washington, DC, 1986, pp. 203-214. N. Miura, H. Kaneko and N. Yamazoe, A four-probe type gas sensor using a solid-state proton conductor sensitive to hydrogen at room temperature, J. Electrochem. Sot., 134 (1987) 1875- 1876. N. Miura and W. L. Worrell, Sensing characteristics of a solid-state ammonia sensor at ambient temperatures, Solid State Zonics, 27 ( 1988) 175 - 179. N. Miura and N. Yamazoe, Development of a solid-state gas sensor using proton conductor operative at room temperature. Chemical Sensor Technology, Vol. 1, Kodansha, Tokyo/Elsevier, Amsterdam, 1988, pp. 123 - 139. N. Miura, T. Harada and N. Yamazoe, Sensing characteristics and working mechanism of four-probe type solidstate hydrogen sensor using proton conductor, J. Electroc~em.Soc., 136 (1989) i21j- 1219. G. Velasco. J. Ph. Schnell and M. Croset. Thin solid state electrochemical gas sensors, Sensors and Actuators, 2 (1982) 371- 384. A. J. Polak, A. J. Beuhler and S. Petly-Weeks, Hydrogen sensors based on proton conducting polymers, Proc. kd Znt. Conf. Solid-State Sensors and Actuators (Transducers ‘85). Philadelphia, PA, U.S.A., June 11-14, 1985, pp. 85-88.
Sensors and Actuators, Bl (1990) 125-129
Cordless Solid-state Hydrogen Sensor Using Proton-conductor Thick Film NORIO MIURA,
TATSURO
HARADA,
YOUICHI
SHIMIZU
and NOBORU
YAMAZOE
Department of Materials Science and Technology, Graduate School of Engineering Sciences, Kyushu University, Kasuga-shi, Fukuoka-ken 816 (Japan)
Abstract
Potentiometric as well as amperometric solidstate gas sensors using a thick film ( zz 10 pm) of proton-conductor have been developed for detecting small amounts of hydrogen in air at room temperature. The sensor elements are composed of the following electrochemical cell: reference (or counter) electrode (Au or Pt) 1 proton-conductor thick film 1sensing electrode (Pt). The thick film is formed on an alumina substrate by applying a paste of antimonic acid-polyvinyl alcohol mixture, mainly by means of a spin-coating method. The electromotive force of the potentiometric sensor varies logarithmically with H2 concentration, while the short-circuit current of the amperometric sensor is found to be proportional to H, concentration. Both give a 90% response time of about 10 s at 30 “C.
(4) The choice of materials for sensor fabrication can be less limited compared with hightemperature sensors. From these viewpoints, we have investigated new types of solid-state gas sensors that can work at room temperature. We have already reported that sensors using proton-conductors such as antimanic acid can detect small amounts of H,(CO, NH3) in air even at room temperature [l-8]. However, these investigations were mainly aimed at elucidating the basic properties of proton-conductor sensors and the sensor elements used were of rather primitive structure, based on a disc or sheet of proton-conductor. In this paper we report that proton-conductor (antimonic acid) thick films can be successfully introduced into sensor elements, which not only have an advanced structure suitable for microfabrication, but also show even better sensing performance than those elements using a disc or a sheet of proton-conductor.
1. Introduction
There are many excellent solid-state gas sensors already put into practical use, of which inflammable gas sensors using oxide semiconductors and oxygen sensors using stabilized zirconia are typical examples. Most of these gas sensors are designed to operate at elevated temperatures in order to achieve sufficient sensitivities and response rates to objective gases. These sensors must be equipped with a heater connected to external electric power sources. Compared with such sensors, the solid-state gas sensor, which can operate at room temperature, has the following advantages: (1) Room-temperature sensing seems to create interesting new application fields for gas sensors. (2) With no heater needed for the sensor element, the sensor may be operated with a small power source such as a battery. This may lead to a portable (so-called ‘cordless’) sensor. (3) The elimination of a heater simplifies the structure of the element, which is convenient for the fabrication of microsensors or integrated sensors.
2. Expesimental The proton-conductor thick film was obtained as follows. A powder mixture containing 80 wt.% of antimonic acid (Sbz05*2H,0, donated by Toagosei Chemical Ind.) and 20 wt.% of polyvinyl alcohol (PVA, degree of polymerization: 500) was mixed with water, followed by partial evaporation of water at about 80 “C. The resulting paste was applied on a porous alumina substrate (10 x 10 x 3 mm, apparent porosity: 43%) either by a screen-printing method or by a spin-coating method (rotating rate of 3500 rpm). The protonconductor fihn about 10 pm thick thus obtained was then dried at room temperature for about one day. Two types of sensor elements were fabricated, both being composed of the following electrochemical cell: reference (or counter) electrode 1 proton-conductor thick film 1 sensing electrode. A planar type had both sensing and reference electrodes on top of the proton-conductor film (see 0 Elsevier Sequoia/Printed
in The Netherlands
126
Fig. 3). The electrodes (Pt or Au) were thinly deposited by RF sputtering (Nichiden Anelva, SPF-2 IOHRF) or vacuum evaporation (Hitachi, HUS-SGB). A laminated-type element had the two electrodes sandwiching the proton-conductor film (see Fig. 6). For preparing this type of element, a reference Pt electrode was first fitted to the substrate, and after the application of the proton-conductor film, the Pt electrode was fitted on it. Gas-sensing experiments were carried out in a flow ( 100 cm3 mm-‘) of a mixture of Hz (or other reducing gases) and air. To prevent the proton-conductor film from drying, the gas was humidified by passing through water. For potentiometric sensing, the difference in potential between the sensing and reference electrodes (electromotive force) was measured by an electrometer (Advantest, TR8652) at 30 “C. For amperometric sensing, the short-circuit current flowing through the element was measured by an ammeter (Hokuto Denko, Zero Shunt Ammeter HM-101) at 30 “C.
(4
3. Results and Discussion 3.1. SEiU Observation of Films Figure 1 represents SEM (Hitachi, S-510) photographs of the proton-conductor films prepared by a screen-printing method (a) as well as by a spin-coating method (b). Although the same paste was used in both methods, the resulting films were quite different in external appearance from each other, i.e., the screen-printed film had a rather rough surface with small through holes running vertically in places, while the spin-coated one had a fine uniform microstructure. Figure 2 shows a cross-sectional view of a spin-coated film formed on an alumina substrate. It is seen that the film is about 10 pm thick and stuck tight on the substrate. 3.2. Potentiometric Sensor The potentiometric sensing properties of the thick-film sensor were examined with the planartype element shown in Fig. 3. Previously we reported that Ag could be used as a reference electrode because of its inertness to dilute Hz [2]. Subsequently we found that Au was not only almost inert to H2 but also much more stable for long-term sensor operation than Ag. Thus an Au reference electrode was adopted in the present study. The 0.2 pm thick Pt sensing electrode and the 0.3 pm thick Au reference electrode had a 1 mm space between them. The responses of the element to several sample gases are shown in Fig. 4. The difference in elec-
0) Fig. 1. External appearance of the antimonic-acid thick til (SEM). (a) Screen-printed film; (b) spin-coated film.
trode potential, AEm_,++ responded quite well to H,, giving a change as large as about 200 mV to 1000 ppm Hz. The 90% response time was as short as about 10 s at 30 “C, which was ev‘en shorter than that of the previous disc-type eleme:nt
)
antimonic acid
alumina i Fig. 2. Cross-sectional view of the antimonic acid flm deposited on an alumina substrate (SEM).
127 sensing Pt electrode
(O.Zpm)
antimonic acid film /
(ca. IOvm)
alumina substrate
(-150mVldecade)
1000
Fig. 3. Structure of the potentiometric thick-film sensor (planar type).
[ 1,2]. The sensor also responded to CO (1000 ppm) in air, but it was insensitive to methane ( 15 000 ppm), propane (7000 ppm) and isobutane (1000 ppm). Figure 5 shows the variation of AE~R_Aujwith the concentration of H, and CO in air. AEcPt_Auj was found to be linear to the logarithm of H, concentration in the range 200 ppm to l%, with a slope of about - 150 mV/decade. A similar linear relationship with a slope of about -80 mV/ decade was also observed for CO in the concentration range 20- 1000 ppm. The less steep slope for CO indicates that the thick-film sensor is less sensitive to CO than to H,, as was also the case in the disc-type sensor element [7J. The gas-sensing mechanism of this sensor is described briefly below. There have been reports of thick-film H, sensors using proton-conductors such as hydrogen uranyl phosphate [9] and polyvinyl alcohol impregnated with phosphoric acid [lo], which aimed at detecting Hz, especially in inert gas. These sensors were designed to operate as an H, concentration cell and the presence of oxygen reportedly interfered severely with them. In our sensor, however, the abundant presence of oxygen is essential for steady operation. As already shown previously [ 11, the electric potential of the sensing Pt electrode on exposure to a mixture of H, + air is determined by the electro-
Time
Fig. 4. Responses of the potentiometric sensor to various sample gases at 30 “C (Hz: 1000 ppm, CO: lOOOppm, CH,: 15 000 ppm, C,Hs: 7000 ppm, i-C.,H,,: 1000 ppm).
Concentration
1Ocxxl
/ ppm
Fig. 5. Dependence of the potentiometric response on gas concentration at 30 “C.
chemical hydrogen oxidation (1) and the electrochemical oxygen reduction (2) : H2 +2H+
+ 2e-
(1)
( 1/2)02 + 2H+ + 2e- +H,O
(2) These reactions proceed simultaneously to form a local cell and the electrode potential eventually reaches a steady value, a mixed potential (EM), when the corresponding anodic and cathodic currents become equal in magnitude. In other words, the mixed potential is given by the intersection of the anodic- and cathodic-polarization curves. Under the particular condition that the concentration of Hz (C,,) is far smaller than that of 02, the anodic polarization curve will be dominated by a limiting current region where the anodic current is proportional to C,, , while the cathodic one will follow a Tafel-like equation. The intersecting potential, EM, will be given in the form EM = a + b log C,,
(a, b are constants)
(3) This equation verifies the linear dependence of AECPt_Aujon the logarithm of H2 concentration shown in Fig. 5. The same explanation also holds for CO sensing when eqn. (1) is replaced by the reaction CO+HzO+C0,+2H+
+2e-
(4)
3.3. Amperometric Sensor Although a potentiometric sensor covers a broad range of gas concentrations, its accuracy of gas detection is usually inferior to that of an amperometric sensor, which gives a signal output in direct proportion to the sample gas concentrations. We have already reported that the disc-type sensor elements fabricated from a mixture of antimanic acid and a Teflon binder can operate amperometrically under the short-circuit condition [3,4]. It was found in this study that an amperometric sensor element could be fabricated successfully by using a proton-conductor thick film. The fabricated sensor element had a laminatedtype structure, as shown in Fig. 6, in order to
128 antimonlc acid film (ca.~o~~) sensing Pt electrode
\
\
r
Ag reference
electrode
(0.2pm) antimonic acid film
T alumina /
(4
cdunter Pt electrode (0.2 pm 1
alumina
substrate
(3mm)
5cQl
Fig. 6. Structureof the amperometricthick-filmsensor(laminated type). minimize the electric resistance of ionic conduction between the electrodes. Only the spin-coated proton-conductor film, having no through holes, was applicable to this type of structure. The H, sensing performance of the laminated element is shown in Fig. 7. The short-circuit current (output signal) was found to increase linearly with the H, concentration in air up to 6000 ppm. The 90% response time to 1000 ppm Hzwas again as short as about 10 s at 30 “C. It is thus clear that, with such an excellent performance, the laminated element is quite promising as an amperometric H2 sensor element. Although the output signal to a given H2 concentration is still small compared with that of the previous disc-type element, it could be intensified through optimization of the sensor design and fabrication techniques. To clarify the sensing mechanism of the above element, one should know the behaviour of each Pt electrode in the sample gas. For this purpose, the element was fitted with an additional Ag electrode on the outer surface of the proton-conductor film, as shown in Fig. 8(a). Under the open-circuit condition, the electric potential of each Pt electrode referring to the Ag electrode was measured in varying concentrations of Hz in air. As shown in Fig. 8(b), each electrode potential had a linear dependence on the logarithm of H2 concentration, while the outer electrode was slightly more dependent than the inner one. The two electrodes
0
2ooo
Loo0
H, concentration
6M I ppm
Fig. 7. (a) Dependenceof the amperometricresponseon H, concentrationin air and (b) a response transient of the shortcircuit current at 30 “C.
substrate
inner electrode
t ‘“k d
outerelectrode
01 100
500 1000
Hz concentration
5000
I
ppm
@I
Fig. 8. (a) Laminated-type element with an additional Ag electrode; (b) behaviour of electric potential of Pt electrodes with reference to Ag in varying concentrations of H, in air.
differed by 1OO- 150 mV in electric potential in the examined H, concentration range, and such a large potential difference would be a driving force for the current flow under the short-circuit condition. It was found that the output current of the laminated element decreased drastically to almost insignificant values when the outer Pt electrode was covered with another spin-coated film of antimonic acid. This’ implies that the spin-coated film was sufficiently tight to gas permeation. The same drastic decrease in output current also resulted when part of the alumina substrate of the element was covered with epoxy resin. This indicates that the gas transport necessary for the inner Pt electrode, i.e., the supply of oxygen and the removal of H,O, was assisted by permeation through the porous alumina substrate. Based on these findings, the following sensing mechanism was estimated for the present sensor. The inner Pt electrode, which is separated from the gas phase by the porous alumina, is less accessible to H, molecules than the outer Pt electrode. It follows that the anodic reaction (1) takes place at a higher rate on the outer electrode than on the inner one. Under the short-circuit condition, the protons produced more abundantly on the outer electrode would migrate to the inner electrode through the proton-conductor film, giving rise to the stationary electric-current flow. Since the anodic reaction ( 1) is a diffusion-limited process [ 11, the rate of proton production and therefore that of proton migration (short-circuit current) would be proportional to the Hz concentration in the gas phase, as was observed.
129
4. coIdlsions The results of the present work are summarized as follows: (1) The use of a proton-conductor thick film is highly promising for developing cordless, micro gas sensors capable of operation at room temperature. (2) The thick film is applicable to both the potentiometric and amperometric elements. (3) Both elements show excellent responses to Hz (and CO), with a 90% response time of about 10 s at 30 “C. (4) The spin-coating method gives a better quality thick film than the screen-printing method. Acknowledgements We gratefully acknowledge the use of the sputtering instrument at the Center of Advanced Instrumental Analysis, Kyushu University. This work was partially supported by grants from the Ministry of Education, Science and Culture of Japan and Casio Science Promotion Foundation. References 1 N. Miura, H. Kato, N. Yamazoe and T. Seiyama, Proton conductor sensors for H, and CO operative at room temperature, Proc. Znt. Meet. Chemical Sensors, Fukuoka,
Japan, Sept. 19-22, 1983, Kodansha, Tokyo/Elsevier, Amsterdam, pp. 233-238. 2 N. Miura, H. Kato, N. Yamazoe and T. Seiyama, An improved type of proton conductor sensor sensitive to Ha and CO at room temperature, Chem. Lat., (1983) 15731576. 3 N. Miura, H. Kato, Y. Omwa, N. Yamazoe and T. Seiyama, Amperometric gas sensor using solid state proton conductor sensitive to hydrogen in air at room temperature, Chem. Lett., (1984) 1905- 1908. 4 N. Miura, H. Kato, N. Yamazoe and T. Seiyama, Amperometric proton-conductor sensor for detecting hydrogen and carbon monoxide at room temperature, Fkaken~als and Applications of Chemical Sensors, American Chemical Society, Washington, DC, 1986, pp. 203-214. N. Miura, H. Kaneko and N. Yamazoe, A four-probe type gas sensor using a solid-state proton conductor sensitive to hydrogen at room temperature, J. Electrochem. Sot., 134 (1987) 1875- 1876. N. Miura and W. L. Worrell, Sensing characteristics of a solid-state ammonia sensor at ambient temperatures, Solid State Zonics, 27 ( 1988) 175 - 179. N. Miura and N. Yamazoe, Development of a solid-state gas sensor using proton conductor operative at room temperature. Chemical Sensor Technology, Vol. 1, Kodansha, Tokyo/Elsevier, Amsterdam, 1988, pp. 123 - 139. N. Miura, T. Harada and N. Yamazoe, Sensing characteristics and working mechanism of four-probe type solidstate hydrogen sensor using proton conductor, J. Electroc~em.Soc., 136 (1989) i21j- 1219. G. Velasco. J. Ph. Schnell and M. Croset. Thin solid state electrochemical gas sensors, Sensors and Actuators, 2 (1982) 371- 384. A. J. Polak, A. J. Beuhler and S. Petly-Weeks, Hydrogen sensors based on proton conducting polymers, Proc. kd Znt. Conf. Solid-State Sensors and Actuators (Transducers ‘85). Philadelphia, PA, U.S.A., June 11-14, 1985, pp. 85-88.