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Sensing characteristics of hydrogen peroxide sensor using carbon-based electrode loaded with perovskite-type oxide
ELSEVIER
B Sensors and Actuators B 34 (1996) 493-498
CHEMICAL
Sensing characteristics of hydrogen peroxide sensor using carbon-based electrode loaded with perovskite-type oxide Youichi Shimizu~,*, Hiroki Komatsu a, Satoko Michishita a, Norio Miura b, Noboru Yamazo" aDepartment of Chemistry, Faculty of Engineering, Kyushu Institute of Technology, S::asui-cho 1-1. Tobata, gita~ushu 804, Japan bDepartment of Materials Science and Technology, Graduate School qf Engineering Sciences, Kyushu University, gasugakaen 6-1, Kasuga, Fukuoka 816, Japan Accepted ! 8 January 1996
Abstract Carbon-based electrodes loaded with large surface area perovskite-type oxides showed good properties of potentiometric as well as amperometric sensing to H202. Electromotive force (EMF) of the electrode loaded with Lao.6Cao.4Ni0.TFe0.303 was found to vary logarithmically with H202 concentration between 10-5 and 10-3 M. The 90% response time was ca. 5 min at 25°C. Interestingly, EMF of the sensor element was hardly affected by the concentration of dissolved oxygen in the solution. The electrodes loaded with large surface area perovskite-ty!~e oxide gave high activity to the electrochemical oxidation of H20 ~, a limiting current was observed at the potential range between +0.6 and +1.0 V (versus SCE). The sensing current at +0.75 V of the carbon-based electrode loaded with Lao.6Cao,4MnO3 was found to be approximately proportional to H202 concentration up to 0.5 mM. The response time was as short as ca: 60 s at 25°C.
Keywords: H202 sensor; Carbon-based electrode; Perovskite-typeoxide; Limiting current-type; Potentiometric-type
I. Introduction
Detection of the exact amount of hydrogen peroxide, the main product from most enzyme reactions, is important for fabricating high performance enzyme-based biosensors [ 1-3]. The importance of the H202 sensor is also increasing in the fields of environmental and chemical industries. For the practical monitoring of H202 concentration, analytical instruments such as based on UV absorption or chemiluminescence are commonly used. These physical H202 sensor systems are rather good on accuracy and stability, however they have some problems with respect to price, portability, and the application to biosensing devices. So far, many kinds of compact H202 sensors have been investigated, most of which are based on electrochemical [1,2] or optical [3] devices using noble metal electrodes or optical fibers with luminescent materials, respectively. For the electrochemical
* Correspondingauthor.
0925-4005/96/$15.00 © 1996 Elsevier Science S.A. All fights reserved PII S0925-4005(96)01856-4
monitoring of H202 concentration, a Pt electrode is most widely used for biosensors at present. Although the platinum-based H202 sensors show high sensitivity and stability to H202, they are rather expensive for a disposable sensor. The cross sensitivity to dissolved oxygen of this kind sensor is also one of the drawbacks to fabricate a high performance enzyme sensor. It is well known that some perovskite-type oxides exhibit remarkable catalytic activities for electrochemical oxygen reduction and/or decomposition of H202 comparable to a platinum catalyst, and are expected to be promising materials for oxygen electrode [4-7]. In this study, we have investigated about H202 sensing properties of carbon-based electrodes loaded with La-based perovskitetype oxides. It was turned out that the carbon-based electrodes loaded with large surface area perovskite-type oxides prepared by an amorphous malate precursor method showed rather good properties of potentiometric as well as amperometric sensing to H202, We report here that the sensing properties as well as the sensing mechanism of this new electrode system.
Y. Shimizu et al. t Sensors and Actuators B34 (1996) 493--498
494 [ Metal nitrates I
I [Drying at 80"cl
lAmo~Vho.sprecursor I Evaporation I at 350~C
~t
Calcination I at 650~,2h
80 mesh Cu screen to form an electrode, which has 1 cm 2 area and ca. 0.5 mm thick. The electrode thus obtained was dried at ca. 100°C for 6 h and finally sintered at 300°C for 30 min in air. Preparation of Pt-loaded (0.6 wt%) electrode was the same as already reported [10]. The electrode thus obtained was finally fixed on the end of a glass tube (7 mm in diameter) with an epoxy resin to form a sensor element.
2. 3. Sensor response characteristics
] Perovskite.typeoxide J Fig. I, Preparation process of large surface area perovskite-type oxides by an amorphous malate precursor (AMP) method.
2. Experimental
2.1. Preparation of perovskite-rype oxides Perovskite-type oxides with large surface area were prepared by an amorphous malate precursor (AMP) method as shown in Fig. 1 [8]. The precursors prepared from malic acid and the nitrates of constituent metal were heated at 6500C for 2 h in an ambient atmosphere. Preparation by a conventional acetate decomposition (AD) method [9] also carried out for comparison. An aqueous solution dissolving constituent acetates or nitrates was evaporated to dryness and calcined at 8500C for 10 h in air. X-Ray diffraction (XRD) analysis revealed that the oxides prepared by both methods showed wellcrystallized and almost single-phase perovskite-type oxides. Specific surface areas of the oxides prepared by the AMP method were as large as 8-30 m2/g, which was about 5-10 times larger than those prepared by the AD method.
2.2. Preparation of the H20,. sensor elements H202 sensor elements were based on poly(tetrafluoroethylene)(PTFE)-bonded carbon electrodes with or without a catalyst, as shown in Fig. 2. Carbon black (30 wt%; AKZO Ltd., Ketjen black 600DJ), perovskite-type oxide (50 wt%), and FFFE dispersion (P'I~'E content 20 wt%; Daikin Kogyo Co., Ltd.; Polyflon D-2) were kneaded into a paste with water. The paste obtained was applied on an
Potentiometric as well as amperometric sensing properties to H202 were carried out in a conventional electrochemical half-cell system with an SCE reference electrode and a Pt-wire counter electrode at 25°C. Phosphate buffer solution adjusted to pH 7.1 was used as an electrolyte. The electromotive force (EMF) responses of the sensor element, i.e., the difference in potential between the sensor element and the SCE electrode was measured in the solution with various H202 concentration by using an electrometer (Toho Technical Research; PS-14). I-V characteristics were determined by scanning the potential of the sensor element against the reference electrode at a rate of 5 mVIs, by using a potentiostat (Toho Technical Research; PS-14) and a function synthesizer (NF Electronic Instruments, 1732). The amperometric responses to H202 were evaluated by the electric current following between the sensor element and the Pt counter electrode under applying a fixed external voltage against the SCE electrode by using the potentiostat.
3. Results and discussion
3.1. Potentiometric sensing characteristics First, potentiometric sensing properties of carbonbased electrodes w¢~e investigated. It was found that the carbon-based electrodes showed EMF responses to H202, the performance of which was largely depending on the oxide used. As shown in Fig. 3, EMF response of the only carbon electrode without oxide catalyst was linear to the
100 E
PelxJw~klt~tylt~ o~lde+Carbon.l, PTFE
~
Cu.raesh Epo~ r¢sin
.i00 L . . . . . . . . . . . . . . . . . . . . .
i0 --~
|0-4
10 "3
H 2 0 2 cone. / M
~
cu.,,ir*
Fig. 2. H202 sensor clement using carbon-based electrode.
Fig. 3. Dependence of EMF response of the only carbon electrode on H202 concentration at 250C.
495
Y. Shimizu et aLI Sensors and Actuators B34 (1996) 493--498
350
104
I
25 mV
N ~ ~
_3~I
250
04
-32 mV/decade (n=l.8)
m
IO rain
~
(
(b)
Time Fig. 4. Response transient of the potentiometric carbon-based electrode loaded with Lao.6Cao.4Nio.7Feo.303 to H202 at 25°C.
logarithm of H202 concentration in the range between 3 x 105 and I x 10-~ M, with a Nernst's slope of +118 mV/decade at 25°C. The slope shows the electrode reaction is a 0.5-electron reduction per a H202 molecule at 25°C. In this case, the electrode reaction should be written as Eq. (1). H202 + ( i / 2 ) H + + ( l / 2 ) e - = H 2 0 + ( I / 2 ) H O 2
(l)
The 90% response time of the only carbon electrode for the change from 3 x 10-s to 1 x 10-4 M H202 was as long as ca. 30 min. It was further found that the carbon-based electrode loaded with 50 wt% Lao.6Cao.4Nio.TFeo.303 showed rather good potentiometric response characteristics to H202. As shown in Fig. 4, the 90% response time for the change from 3 x 10-4 to 1 x 10-3 M H202 was ca. 5 min at 25°C. EMF of the sensor element decreased with increasing H202 concentration, and varied logarithmically with H202 concentration between 10-s and 10-3 M at 25°C as shown in Fig. 5a. The observed Nernst's slope of -32 mV/decade is completely different from the only carbon electrode, and is consistent with an electrochemical 2-electron oxidation per a H202 molecule. It should be expressed as Eq. (2). H 2 0 2 = 2H + + 02 + 2e-
(2)
EMF response of the carbon-based electrode using a Pt-catalyst was also examined for comparison. Little change in EMF of the element was observed in the H202 concentration range between 10-5 and 10-3 M as shown in Fig. 5b. This indicates that a potentiometric H202 sensor is hardly constructed with a Pt-catalyzed carbon-based electrode. Effects of the concentration of dissolved oxygen (DO) in the solution on the EMF response were further investigated. Fig. 6 shows response transients of the elements for the carbon-based electrodes using Lao.6Cao.4Nio.TFe0.303 and Pt at the change in the concentration of DO in the solution. The concentration of DO was changed by bubbling through the solution with nitrogen, synthetic air, and pure oxygen. Interestingly, EMF of the sensor element using Lao.6Cao.4Nio.7Feo.303 was found
ISO
. . . . . . . . . . . . . . . . .
10.5
10-4 H 2 0 2 cone.
10.3
/M
Fig. 5. EMF responses of the carbon-based electrodes loaded wilh (a) Lao.6Ca0,4Nio,7Feo.303 or (b) Pt to 1-1202 at 25°C.
to be hardly affected by the concentration of DO in the solution, while that using Pt showed large drift with increasing the concentration of DO. Table 1 summarizes the potentiometric sensing properties of the various carbon-based electrodes to H202. Most caxbon-based electrodes using perovskite-type oxides could detect H202 potentiometricaily, however the 90% response time of the elements was as long as 2040 min except for that using Lao.6Cao.4Ni0.TFeo303. It was further found that the PTFE-bonded electrode using the only perovskite-type oxides (without carbon) gave poor EMF responses to H202. These results led to the conclusion that the potentiometric H202 sensor with highest performance could be fabricated by the carbon-based electrode loaded with Lao.6Cao.aNio.TFeo.303 in the elements examined. The Nernst's slope for the carbon-based electrodes with Lao.6Cao.4BO3 (B =Cr, Mn, Co) or Lao.eCao.4FeO3 indicate a one- or a two-electron reduction per H202 molecule, respectively. In the cases, reactions (3) or (4) should be occurred on the electrodes, respectively.
(a)
(b)
bubhlillg ga~,: N2
ail
I):~
I
"l'inac Fig. 6. Response transients of the carbon-based electrodes loaded with (a) Lao.6Cao.4Ni0.TFe0.303 or (b) Pt to the change of concentration of dissolved oxygen at 25°C.
Y. Shiraizu et aLI Sensors and Actuators B34 (1996) 493-498
496 Table I
Effects of the electrode materials on the potentiometric sensing characteristics of the H202 sensors. Sensor material
H202 responsea
Nemst slope (mV/decade)
nb
Carbo? Pt + carbon Lao.6Cao.4CtO 3 + carbon Lao.6Ca0.4MnO 3 + carbon l.,ao.6Cao.4FeO 3 + carbon l.,ao.6Cao.4Co03 + carbon Lao.6Cao.4NiO ~ + carbon L~o.6Cao.4Ni0,?Fe0.303 + carbon La0.6Ca0,4Ni0.?Fe0.303
A x A A A A × 0 x
+118
0.5
36
-
-
0.7 1.3 1.5 1.0 1.8 -
20 25 40 35 5 -
t~.6Cao,4Co03
x
- t 50 to -25
-
-
-8 +80 +45 +40 +60 +10 -32 -85 to - 10
90% response time (min)c
"O, excellent, A. good, x. poor. bn is the number of electron(s) involved in the electrode reaction per H202 molecule. el x 10"4-~ 3 x 10-4 M H202 .
H20~ = H+ + 02 + 2e- = OH + H20
(3)
H202 = 2H* + 02 + 2e- = 2H20
(4)
As the only carbon electrode and the only oxide electrode showed cathodic and anodic reactions, respectively, EMF should be generated from the mixed potential of both reactions in some cases, although further studies are necessary to verify these reactions.
3.2. Amper~metric sensing characteristics Second, amperometric sensing properties of carbonbased electrodes were investigated. It was turned out that the carbon-based electrodes using large surface area perovskite-type oxides showed high activity to the electrochemical oxidation of H~O2. Fig. 7 shows I-V characteristics of the carbon-based electrodes loaded with AMP and AD-Lao,6Cao,~FeO3 at various H202 concentrations at 25~C, For the electrode using AMP-Lao,6C%.4FeO3 (specific surface area (SA) 30 m2/g), the current increased with increasing the electrode potential above ca. 0.2 V
versus SCE, although that for AD-Lao6Cao.4FeO 3 (SA 3.8 m2/g) showed small current density even at the higher potential of 1.0 V, A limiting current was observed for the former electrode at the potential between 0.7 and 1.0 V, and the current increased with increasing H202 concentration. It suggests that the carbon-based electrode itself functions as a diffusion limiting layer for H202. The I-V characteristics indicate that the amperometric sensing of H202 is possible by the use of the carbon-based electrodes using large surface area perovskite-type oxides. Fig. 8 shows the dependence of sensing current on H202 concentration for the sensor element with Lao.6Cao.4FeO 3 (AMP) at various applied electrode potential. Although no great dependence on HzO~_concentration was found at the applied electrode potential of +0.5 V, the current was found to be increased with increasing H202 concentration at the applied electrode potential of +0.75 V. This agrees with the fact that a limiting current appeared at the electrode potential above +0.7 V in the I-V characteristic curves. Amperometric sensing characteristics of the carbon-based electrodes loaded with various perovskite-type
3,o H202 cone. (M) --
K' 1(|4
Eat +0.75 V (vs. St'El
I
]
e,i
E u
|,(l
y
(bl __
0 0
•
I 0.5 E vs. SCE / V
_
¢104 ,~10 ~' I" 0 I,O
Fig, "L I-V characteristics of the carbon-based electrodes loaded with two kinds of LaO.6Cao,4FeO3 (AMP or AD) at various H202 concentration, (5 mWs, 25°C)
0
0
0.2
0.4 0.6 0.8 H202 conc. !mM
1.0
Fig. 8. Dependence of sensing current of the carbon electrode loaded with Lao.6Cao.4FeO 3 (AMP) on H202 concentration at various electrode potentials.
497
F. Shimizu et al. I Sensors and Actuators B34 (1996) 493-498
oxides (AMP: Lao.6Cao.4BO3; B = Cr, Mn, Fe, Co, Ni, Nio.TFe03) were further investigated and shown in Fig. 9. The magnitude of the sensing current at the potential of +0.75V was in the order of ( B = ) Mn>Fe> Ni > Nio.TFeo.3 = Co > Cr. Although sensing current for the most sensor elements was saturated at higher H202 concentration range, the carbon-based electrode with La0.6Cao.4MnO3 was found to show rather good sensing properties among the elements tested. The current response was approximately proportional to the H202 concentration up to 0.5 raM. Fig. 10 shows the response transient of the element using Lao.6CaoAMnO 3 (AMP) to the change in H202 concentration up to 1 0 0 g M at 25°C. The 90% response time was as short as ca. 60 s even for 1 0 g M H202 at 25°C. As previously described, the potentiometric sensing o f H202 had a response time more than ca. 5 rain, thus the amperometric sensing has the great advance in the response rate as well as the detection of small amount of H2Ov The amperometric sensing mechanism of the carbonbased electrodes should be considered as follows. As the potential of the sensor element is applied positive, the electrochemical oxidation of H202 should take place at the electrode as expressed by Eq. (5), to yield oxygen and protons.
(5)
H202 = 2H + + O2 + 2e-
The difference in magnitude of the sensing current seems to be come from the electrochemical activities of the loaded perovskite-type oxides to thz reaction (5). The perovskite-type oxides like LaBO3 (B = Co, Ni) and related systems exhibited high catalytic activities to the decomposition of H20215-71, which expressed as Eq. (6).
(6)
H:zO2 = H20 + 0/2)O2
The consumption of H202 by Eq. (6) should bring about the decrease in the current response for the oxides of
t.q.
E I
||202 cone. : I00 ~M r""
(
10.l mA $0 ~M
l
JO~MI 5 mitt
Time Fig. 10. Response transient of the carbon-basedelectrode loaded with Lao.6Cao.4MnO3 to H202 al 25°C. (electrode potential +0.75 V vs. SCE). B = Co and/or Ni. Whereas, further investigation should be necessary to clarify the mechanism of the amperometric H202 sensing on the electrodes. References
[I] S. Yamauchi, Y. Ikariyama and M. Yaoita, Enzyme embodied electrode - a new amperometric biosensing device, Chemical Sensor Technology, 2 (1989) 205-223. [2] M Shichiri, R. Kawamori, Y. Yamasaki and N. Ueda, Medical applications of the glucose sensor, Chemical Sensor Technology, 1 (1988) 209-220. [3] M. Aizawa, Y. lkariyama, H. Shinohara and M Tanaka, Optical immunosensors, Chemical Sensor Technology, 2 (1989) 225-236. [4l D.B. Medowcrofi, Low-cost oxygen electrode material, Nature, 226 (1970) 847-848. [5l Y. Shimizu, K. Uemura, H. Matsuda, N. Miura and N. Yamazoc, Bi-functional oxygen electrode using large surface area LaI _ xCa~CoO3 for reehargcable metal-air battery, .k Eleclro. chent Sot., [ 37 (1990) 3430-3433. [6l Y. Shlmizu, H. Matsuda, N. Miura and N. Yamazoe, Bi-functional oxygen electrode using large surface area perovskite-typeoxide catalyst lot reehargeable metal-air batteries, Chem. Lett., 0992) 1033-1036. 171 H. Falcon and R.E. Catbonio, Study of the heterogeneous decomposition of hydrogen p,.roxide: its application to the development of catalysts for carbon, based oxygen cathodes, Z Electroanal. Chem., 339 (1992) 69-83. [8] Y. Teraoka, H..Kakebayashi, I, Monguehi and S, Kagawa, Hydroxy acid-added synthesis of perovskite-type oxides of cobalt and manganese, Chem. Lett., (1991)673-676. [9] H.M. Zhang, Y. Shimizu, Y. Teraoka, N. Miura and N. Yamazoe, Oxygen sorption and catalytic properties of LaI _xSrxCOl _y FevO3 perovskite-typeoxides, J. Catal., 121 (1990) 432--440. [lO] S.Motoo, M. Watanabe and N. Furoya, Gas diffusion electrode of high performance,J. Electroanal. Chem., 160 (1984) 351-357.
Ni Biographies
°10
0.2 0.4 H202 cone. /mM
0.6
Fig. 9. Dependence of the sensing current of the carbon-b~ed electrodes loaded with Lao.6Cao.4BO3 (B = Cr, Mn, F¢, Co, Ni, Nio.7Feo.3) on H202 concentration at 25°C (electrode potential +0.75 V vs. SCE).
Youichi Shimizu has been an Associate Professor at Kyushu Institute of Technology since 1993. He received the B. Eng, degree in applied chemistry in 1983 and the Dr. Eng. degree in 1992 from Kyushu University, His current research interests include the solid-state gas sensors, and ion sensors.
498
E Shimizu et al. I Sensors and Actuators B34 (1996) 493.-.498
Hiroki Komatsu received his B. Eng. degree in applied chemistry in 1994 from Kyushu Institute of Technology. He is currently working at Koransha Co., Ltd. Satoko Michishita received her B. Edu. degree in chemistry in 1991 from Fukuoka University of Education. She has been a research assistant at Kyushu Institute of Technology since 1991, and is currently studying ion sensors.
Norio Miura has been an Associate Professor at Kyushu University since 1982. He received his B. Eng. degree in applied chemistry in 1973 from Hiroshima University
and his Dr. Eng. degree in 1980 from Kyushu University. His current research concentrates on chemical sensors based on solid electrolyte, piezoelectric crystals, and semiconductive oxides. Noboru Yamazoe has been a Professor at Kyushu University since 1981. He received the B. Eng. degree in applied chemistry in 1963 and the Dr. Eng. degree in 1969 from Kyushu University. His current research interests include the development and application of functional inorganic materials.
B Sensors and Actuators B 34 (1996) 493-498
CHEMICAL
Sensing characteristics of hydrogen peroxide sensor using carbon-based electrode loaded with perovskite-type oxide Youichi Shimizu~,*, Hiroki Komatsu a, Satoko Michishita a, Norio Miura b, Noboru Yamazo" aDepartment of Chemistry, Faculty of Engineering, Kyushu Institute of Technology, S::asui-cho 1-1. Tobata, gita~ushu 804, Japan bDepartment of Materials Science and Technology, Graduate School qf Engineering Sciences, Kyushu University, gasugakaen 6-1, Kasuga, Fukuoka 816, Japan Accepted ! 8 January 1996
Abstract Carbon-based electrodes loaded with large surface area perovskite-type oxides showed good properties of potentiometric as well as amperometric sensing to H202. Electromotive force (EMF) of the electrode loaded with Lao.6Cao.4Ni0.TFe0.303 was found to vary logarithmically with H202 concentration between 10-5 and 10-3 M. The 90% response time was ca. 5 min at 25°C. Interestingly, EMF of the sensor element was hardly affected by the concentration of dissolved oxygen in the solution. The electrodes loaded with large surface area perovskite-ty!~e oxide gave high activity to the electrochemical oxidation of H20 ~, a limiting current was observed at the potential range between +0.6 and +1.0 V (versus SCE). The sensing current at +0.75 V of the carbon-based electrode loaded with Lao.6Cao,4MnO3 was found to be approximately proportional to H202 concentration up to 0.5 mM. The response time was as short as ca: 60 s at 25°C.
Keywords: H202 sensor; Carbon-based electrode; Perovskite-typeoxide; Limiting current-type; Potentiometric-type
I. Introduction
Detection of the exact amount of hydrogen peroxide, the main product from most enzyme reactions, is important for fabricating high performance enzyme-based biosensors [ 1-3]. The importance of the H202 sensor is also increasing in the fields of environmental and chemical industries. For the practical monitoring of H202 concentration, analytical instruments such as based on UV absorption or chemiluminescence are commonly used. These physical H202 sensor systems are rather good on accuracy and stability, however they have some problems with respect to price, portability, and the application to biosensing devices. So far, many kinds of compact H202 sensors have been investigated, most of which are based on electrochemical [1,2] or optical [3] devices using noble metal electrodes or optical fibers with luminescent materials, respectively. For the electrochemical
* Correspondingauthor.
0925-4005/96/$15.00 © 1996 Elsevier Science S.A. All fights reserved PII S0925-4005(96)01856-4
monitoring of H202 concentration, a Pt electrode is most widely used for biosensors at present. Although the platinum-based H202 sensors show high sensitivity and stability to H202, they are rather expensive for a disposable sensor. The cross sensitivity to dissolved oxygen of this kind sensor is also one of the drawbacks to fabricate a high performance enzyme sensor. It is well known that some perovskite-type oxides exhibit remarkable catalytic activities for electrochemical oxygen reduction and/or decomposition of H202 comparable to a platinum catalyst, and are expected to be promising materials for oxygen electrode [4-7]. In this study, we have investigated about H202 sensing properties of carbon-based electrodes loaded with La-based perovskitetype oxides. It was turned out that the carbon-based electrodes loaded with large surface area perovskite-type oxides prepared by an amorphous malate precursor method showed rather good properties of potentiometric as well as amperometric sensing to H202, We report here that the sensing properties as well as the sensing mechanism of this new electrode system.
Y. Shimizu et al. t Sensors and Actuators B34 (1996) 493--498
494 [ Metal nitrates I
I [Drying at 80"cl
lAmo~Vho.sprecursor I Evaporation I at 350~C
~t
Calcination I at 650~,2h
80 mesh Cu screen to form an electrode, which has 1 cm 2 area and ca. 0.5 mm thick. The electrode thus obtained was dried at ca. 100°C for 6 h and finally sintered at 300°C for 30 min in air. Preparation of Pt-loaded (0.6 wt%) electrode was the same as already reported [10]. The electrode thus obtained was finally fixed on the end of a glass tube (7 mm in diameter) with an epoxy resin to form a sensor element.
2. 3. Sensor response characteristics
] Perovskite.typeoxide J Fig. I, Preparation process of large surface area perovskite-type oxides by an amorphous malate precursor (AMP) method.
2. Experimental
2.1. Preparation of perovskite-rype oxides Perovskite-type oxides with large surface area were prepared by an amorphous malate precursor (AMP) method as shown in Fig. 1 [8]. The precursors prepared from malic acid and the nitrates of constituent metal were heated at 6500C for 2 h in an ambient atmosphere. Preparation by a conventional acetate decomposition (AD) method [9] also carried out for comparison. An aqueous solution dissolving constituent acetates or nitrates was evaporated to dryness and calcined at 8500C for 10 h in air. X-Ray diffraction (XRD) analysis revealed that the oxides prepared by both methods showed wellcrystallized and almost single-phase perovskite-type oxides. Specific surface areas of the oxides prepared by the AMP method were as large as 8-30 m2/g, which was about 5-10 times larger than those prepared by the AD method.
2.2. Preparation of the H20,. sensor elements H202 sensor elements were based on poly(tetrafluoroethylene)(PTFE)-bonded carbon electrodes with or without a catalyst, as shown in Fig. 2. Carbon black (30 wt%; AKZO Ltd., Ketjen black 600DJ), perovskite-type oxide (50 wt%), and FFFE dispersion (P'I~'E content 20 wt%; Daikin Kogyo Co., Ltd.; Polyflon D-2) were kneaded into a paste with water. The paste obtained was applied on an
Potentiometric as well as amperometric sensing properties to H202 were carried out in a conventional electrochemical half-cell system with an SCE reference electrode and a Pt-wire counter electrode at 25°C. Phosphate buffer solution adjusted to pH 7.1 was used as an electrolyte. The electromotive force (EMF) responses of the sensor element, i.e., the difference in potential between the sensor element and the SCE electrode was measured in the solution with various H202 concentration by using an electrometer (Toho Technical Research; PS-14). I-V characteristics were determined by scanning the potential of the sensor element against the reference electrode at a rate of 5 mVIs, by using a potentiostat (Toho Technical Research; PS-14) and a function synthesizer (NF Electronic Instruments, 1732). The amperometric responses to H202 were evaluated by the electric current following between the sensor element and the Pt counter electrode under applying a fixed external voltage against the SCE electrode by using the potentiostat.
3. Results and discussion
3.1. Potentiometric sensing characteristics First, potentiometric sensing properties of carbonbased electrodes w¢~e investigated. It was found that the carbon-based electrodes showed EMF responses to H202, the performance of which was largely depending on the oxide used. As shown in Fig. 3, EMF response of the only carbon electrode without oxide catalyst was linear to the
100 E
PelxJw~klt~tylt~ o~lde+Carbon.l, PTFE
~
Cu.raesh Epo~ r¢sin
.i00 L . . . . . . . . . . . . . . . . . . . . .
i0 --~
|0-4
10 "3
H 2 0 2 cone. / M
~
cu.,,ir*
Fig. 2. H202 sensor clement using carbon-based electrode.
Fig. 3. Dependence of EMF response of the only carbon electrode on H202 concentration at 250C.
495
Y. Shimizu et aLI Sensors and Actuators B34 (1996) 493--498
350
104
I
25 mV
N ~ ~
_3~I
250
04
-32 mV/decade (n=l.8)
m
IO rain
~
(
(b)
Time Fig. 4. Response transient of the potentiometric carbon-based electrode loaded with Lao.6Cao.4Nio.7Feo.303 to H202 at 25°C.
logarithm of H202 concentration in the range between 3 x 105 and I x 10-~ M, with a Nernst's slope of +118 mV/decade at 25°C. The slope shows the electrode reaction is a 0.5-electron reduction per a H202 molecule at 25°C. In this case, the electrode reaction should be written as Eq. (1). H202 + ( i / 2 ) H + + ( l / 2 ) e - = H 2 0 + ( I / 2 ) H O 2
(l)
The 90% response time of the only carbon electrode for the change from 3 x 10-s to 1 x 10-4 M H202 was as long as ca. 30 min. It was further found that the carbon-based electrode loaded with 50 wt% Lao.6Cao.4Nio.TFeo.303 showed rather good potentiometric response characteristics to H202. As shown in Fig. 4, the 90% response time for the change from 3 x 10-4 to 1 x 10-3 M H202 was ca. 5 min at 25°C. EMF of the sensor element decreased with increasing H202 concentration, and varied logarithmically with H202 concentration between 10-s and 10-3 M at 25°C as shown in Fig. 5a. The observed Nernst's slope of -32 mV/decade is completely different from the only carbon electrode, and is consistent with an electrochemical 2-electron oxidation per a H202 molecule. It should be expressed as Eq. (2). H 2 0 2 = 2H + + 02 + 2e-
(2)
EMF response of the carbon-based electrode using a Pt-catalyst was also examined for comparison. Little change in EMF of the element was observed in the H202 concentration range between 10-5 and 10-3 M as shown in Fig. 5b. This indicates that a potentiometric H202 sensor is hardly constructed with a Pt-catalyzed carbon-based electrode. Effects of the concentration of dissolved oxygen (DO) in the solution on the EMF response were further investigated. Fig. 6 shows response transients of the elements for the carbon-based electrodes using Lao.6Cao.4Nio.TFe0.303 and Pt at the change in the concentration of DO in the solution. The concentration of DO was changed by bubbling through the solution with nitrogen, synthetic air, and pure oxygen. Interestingly, EMF of the sensor element using Lao.6Cao.4Nio.7Feo.303 was found
ISO
. . . . . . . . . . . . . . . . .
10.5
10-4 H 2 0 2 cone.
10.3
/M
Fig. 5. EMF responses of the carbon-based electrodes loaded wilh (a) Lao.6Ca0,4Nio,7Feo.303 or (b) Pt to 1-1202 at 25°C.
to be hardly affected by the concentration of DO in the solution, while that using Pt showed large drift with increasing the concentration of DO. Table 1 summarizes the potentiometric sensing properties of the various carbon-based electrodes to H202. Most caxbon-based electrodes using perovskite-type oxides could detect H202 potentiometricaily, however the 90% response time of the elements was as long as 2040 min except for that using Lao.6Cao.4Ni0.TFeo303. It was further found that the PTFE-bonded electrode using the only perovskite-type oxides (without carbon) gave poor EMF responses to H202. These results led to the conclusion that the potentiometric H202 sensor with highest performance could be fabricated by the carbon-based electrode loaded with Lao.6Cao.aNio.TFeo.303 in the elements examined. The Nernst's slope for the carbon-based electrodes with Lao.6Cao.4BO3 (B =Cr, Mn, Co) or Lao.eCao.4FeO3 indicate a one- or a two-electron reduction per H202 molecule, respectively. In the cases, reactions (3) or (4) should be occurred on the electrodes, respectively.
(a)
(b)
bubhlillg ga~,: N2
ail
I):~
I
"l'inac Fig. 6. Response transients of the carbon-based electrodes loaded with (a) Lao.6Cao.4Ni0.TFe0.303 or (b) Pt to the change of concentration of dissolved oxygen at 25°C.
Y. Shiraizu et aLI Sensors and Actuators B34 (1996) 493-498
496 Table I
Effects of the electrode materials on the potentiometric sensing characteristics of the H202 sensors. Sensor material
H202 responsea
Nemst slope (mV/decade)
nb
Carbo? Pt + carbon Lao.6Cao.4CtO 3 + carbon Lao.6Ca0.4MnO 3 + carbon l.,ao.6Cao.4FeO 3 + carbon l.,ao.6Cao.4Co03 + carbon Lao.6Cao.4NiO ~ + carbon L~o.6Cao.4Ni0,?Fe0.303 + carbon La0.6Ca0,4Ni0.?Fe0.303
A x A A A A × 0 x
+118
0.5
36
-
-
0.7 1.3 1.5 1.0 1.8 -
20 25 40 35 5 -
t~.6Cao,4Co03
x
- t 50 to -25
-
-
-8 +80 +45 +40 +60 +10 -32 -85 to - 10
90% response time (min)c
"O, excellent, A. good, x. poor. bn is the number of electron(s) involved in the electrode reaction per H202 molecule. el x 10"4-~ 3 x 10-4 M H202 .
H20~ = H+ + 02 + 2e- = OH + H20
(3)
H202 = 2H* + 02 + 2e- = 2H20
(4)
As the only carbon electrode and the only oxide electrode showed cathodic and anodic reactions, respectively, EMF should be generated from the mixed potential of both reactions in some cases, although further studies are necessary to verify these reactions.
3.2. Amper~metric sensing characteristics Second, amperometric sensing properties of carbonbased electrodes were investigated. It was turned out that the carbon-based electrodes using large surface area perovskite-type oxides showed high activity to the electrochemical oxidation of H~O2. Fig. 7 shows I-V characteristics of the carbon-based electrodes loaded with AMP and AD-Lao,6Cao,~FeO3 at various H202 concentrations at 25~C, For the electrode using AMP-Lao,6C%.4FeO3 (specific surface area (SA) 30 m2/g), the current increased with increasing the electrode potential above ca. 0.2 V
versus SCE, although that for AD-Lao6Cao.4FeO 3 (SA 3.8 m2/g) showed small current density even at the higher potential of 1.0 V, A limiting current was observed for the former electrode at the potential between 0.7 and 1.0 V, and the current increased with increasing H202 concentration. It suggests that the carbon-based electrode itself functions as a diffusion limiting layer for H202. The I-V characteristics indicate that the amperometric sensing of H202 is possible by the use of the carbon-based electrodes using large surface area perovskite-type oxides. Fig. 8 shows the dependence of sensing current on H202 concentration for the sensor element with Lao.6Cao.4FeO 3 (AMP) at various applied electrode potential. Although no great dependence on HzO~_concentration was found at the applied electrode potential of +0.5 V, the current was found to be increased with increasing H202 concentration at the applied electrode potential of +0.75 V. This agrees with the fact that a limiting current appeared at the electrode potential above +0.7 V in the I-V characteristic curves. Amperometric sensing characteristics of the carbon-based electrodes loaded with various perovskite-type
3,o H202 cone. (M) --
K' 1(|4
Eat +0.75 V (vs. St'El
I
]
e,i
E u
|,(l
y
(bl __
0 0
•
I 0.5 E vs. SCE / V
_
¢104 ,~10 ~' I" 0 I,O
Fig, "L I-V characteristics of the carbon-based electrodes loaded with two kinds of LaO.6Cao,4FeO3 (AMP or AD) at various H202 concentration, (5 mWs, 25°C)
0
0
0.2
0.4 0.6 0.8 H202 conc. !mM
1.0
Fig. 8. Dependence of sensing current of the carbon electrode loaded with Lao.6Cao.4FeO 3 (AMP) on H202 concentration at various electrode potentials.
497
F. Shimizu et al. I Sensors and Actuators B34 (1996) 493-498
oxides (AMP: Lao.6Cao.4BO3; B = Cr, Mn, Fe, Co, Ni, Nio.TFe03) were further investigated and shown in Fig. 9. The magnitude of the sensing current at the potential of +0.75V was in the order of ( B = ) Mn>Fe> Ni > Nio.TFeo.3 = Co > Cr. Although sensing current for the most sensor elements was saturated at higher H202 concentration range, the carbon-based electrode with La0.6Cao.4MnO3 was found to show rather good sensing properties among the elements tested. The current response was approximately proportional to the H202 concentration up to 0.5 raM. Fig. 10 shows the response transient of the element using Lao.6CaoAMnO 3 (AMP) to the change in H202 concentration up to 1 0 0 g M at 25°C. The 90% response time was as short as ca. 60 s even for 1 0 g M H202 at 25°C. As previously described, the potentiometric sensing o f H202 had a response time more than ca. 5 rain, thus the amperometric sensing has the great advance in the response rate as well as the detection of small amount of H2Ov The amperometric sensing mechanism of the carbonbased electrodes should be considered as follows. As the potential of the sensor element is applied positive, the electrochemical oxidation of H202 should take place at the electrode as expressed by Eq. (5), to yield oxygen and protons.
(5)
H202 = 2H + + O2 + 2e-
The difference in magnitude of the sensing current seems to be come from the electrochemical activities of the loaded perovskite-type oxides to thz reaction (5). The perovskite-type oxides like LaBO3 (B = Co, Ni) and related systems exhibited high catalytic activities to the decomposition of H20215-71, which expressed as Eq. (6).
(6)
H:zO2 = H20 + 0/2)O2
The consumption of H202 by Eq. (6) should bring about the decrease in the current response for the oxides of
t.q.
E I
||202 cone. : I00 ~M r""
(
10.l mA $0 ~M
l
JO~MI 5 mitt
Time Fig. 10. Response transient of the carbon-basedelectrode loaded with Lao.6Cao.4MnO3 to H202 al 25°C. (electrode potential +0.75 V vs. SCE). B = Co and/or Ni. Whereas, further investigation should be necessary to clarify the mechanism of the amperometric H202 sensing on the electrodes. References
[I] S. Yamauchi, Y. Ikariyama and M. Yaoita, Enzyme embodied electrode - a new amperometric biosensing device, Chemical Sensor Technology, 2 (1989) 205-223. [2] M Shichiri, R. Kawamori, Y. Yamasaki and N. Ueda, Medical applications of the glucose sensor, Chemical Sensor Technology, 1 (1988) 209-220. [3] M. Aizawa, Y. lkariyama, H. Shinohara and M Tanaka, Optical immunosensors, Chemical Sensor Technology, 2 (1989) 225-236. [4l D.B. Medowcrofi, Low-cost oxygen electrode material, Nature, 226 (1970) 847-848. [5l Y. Shimizu, K. Uemura, H. Matsuda, N. Miura and N. Yamazoc, Bi-functional oxygen electrode using large surface area LaI _ xCa~CoO3 for reehargcable metal-air battery, .k Eleclro. chent Sot., [ 37 (1990) 3430-3433. [6l Y. Shlmizu, H. Matsuda, N. Miura and N. Yamazoe, Bi-functional oxygen electrode using large surface area perovskite-typeoxide catalyst lot reehargeable metal-air batteries, Chem. Lett., 0992) 1033-1036. 171 H. Falcon and R.E. Catbonio, Study of the heterogeneous decomposition of hydrogen p,.roxide: its application to the development of catalysts for carbon, based oxygen cathodes, Z Electroanal. Chem., 339 (1992) 69-83. [8] Y. Teraoka, H..Kakebayashi, I, Monguehi and S, Kagawa, Hydroxy acid-added synthesis of perovskite-type oxides of cobalt and manganese, Chem. Lett., (1991)673-676. [9] H.M. Zhang, Y. Shimizu, Y. Teraoka, N. Miura and N. Yamazoe, Oxygen sorption and catalytic properties of LaI _xSrxCOl _y FevO3 perovskite-typeoxides, J. Catal., 121 (1990) 432--440. [lO] S.Motoo, M. Watanabe and N. Furoya, Gas diffusion electrode of high performance,J. Electroanal. Chem., 160 (1984) 351-357.
Ni Biographies
°10
0.2 0.4 H202 cone. /mM
0.6
Fig. 9. Dependence of the sensing current of the carbon-b~ed electrodes loaded with Lao.6Cao.4BO3 (B = Cr, Mn, F¢, Co, Ni, Nio.7Feo.3) on H202 concentration at 25°C (electrode potential +0.75 V vs. SCE).
Youichi Shimizu has been an Associate Professor at Kyushu Institute of Technology since 1993. He received the B. Eng, degree in applied chemistry in 1983 and the Dr. Eng. degree in 1992 from Kyushu University, His current research interests include the solid-state gas sensors, and ion sensors.
498
E Shimizu et al. I Sensors and Actuators B34 (1996) 493.-.498
Hiroki Komatsu received his B. Eng. degree in applied chemistry in 1994 from Kyushu Institute of Technology. He is currently working at Koransha Co., Ltd. Satoko Michishita received her B. Edu. degree in chemistry in 1991 from Fukuoka University of Education. She has been a research assistant at Kyushu Institute of Technology since 1991, and is currently studying ion sensors.
Norio Miura has been an Associate Professor at Kyushu University since 1982. He received his B. Eng. degree in applied chemistry in 1973 from Hiroshima University
and his Dr. Eng. degree in 1980 from Kyushu University. His current research concentrates on chemical sensors based on solid electrolyte, piezoelectric crystals, and semiconductive oxides. Noboru Yamazoe has been a Professor at Kyushu University since 1981. He received the B. Eng. degree in applied chemistry in 1963 and the Dr. Eng. degree in 1969 from Kyushu University. His current research interests include the development and application of functional inorganic materials.