nitrogen oxides gas sensor based on al3+ ion conducting solid electrolyte
TRANSCRIPT
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Available online at www.sciencedirect.com
Sensors and Actuators B 130 (2008) 46–51
Nitrogen oxides gas sensor based on Al3+ ion conducting solid electrolyte
Shinji Tamura, Isao Hasegawa, Nobuhito Imanaka ∗Department of Applied Chemistry, Faculty of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan
Available online 25 July 2007
bstract
A solid electrolyte type nitrogen oxides (NOx) gas sensor was fabricated by applying an Al3+ cation conducting solid electrolyte with the KNO3-
oped (Gd0.4Nd0.6)2O3 solid solution and Al metal as the auxiliary sensing electrode and the reference electrode, respectively. A rapid, reproduciblend continuous sensor response was obtained with obeying the theoretical Nernst relationship for NO and NO2 gases at the temperature as low as50 ◦C, one of typical temperature of emitted gases.2007 Elsevier B.V. All rights reserved.
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eywords: Nitrogen oxides; Solid electrolyte; Rare earth oxide; Aluminum ion
. Introduction
Nitrogen oxides (NOx) are one of the typical air pollutantas species and the suppression of the gas emission into thetmosphere has become a global urgent issue. Among the variousOx gas sensing methods proposed using semiconductors [1–8],
olid electrolytes [9–13], and organic compounds [14–18], theolid electrolyte type gas sensor has an advanced merit of highelective and quantitative gas sensing performance with rapidesponse.
In our previous communication [13], we proposed aolid electrolyte type NOx gas sensor combined with twoinds of solid electrolytes of the Al3+ ion conductingAl0.2Zr0.8)20/19Nb(PO4)3 [19] and the O2− ion conductingttria stabilized zirconia (YSZ) with the K+ ion conducting.54Gd2O3–0.46KNO3 solid solution [20] as the auxiliary sens-ng electrode. The sensor exhibited such a superior NOx sensingerformance with obeying the Nernstian theoretical response at50 ◦C. However, there is a great demand to realize the NOx
as sensing at an intermediate temperature region down to sev-ral hundred degrees centigrade in order to prevent the thermaleterioration of the sensor components and to reduce the power
onsumption for sensor operation. Although it is the essentialatter for overcoming these requests to lower the operation tem-erature, our previous sensor described above could not operate
∗ Corresponding author. Tel.: +81 6 6879 7352; fax: +81 6 6879 7354.E-mail address: [email protected] (N. Imanaka).
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925-4005/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.snb.2007.07.077
minum metal
elow 450 ◦C because of the increase of sensor cell resistance,esulting in generating a substantial electrical noise.
In this paper, since we considered that the predominant factoro generate such high sensor cell resistance is the yttria stabilizedirconia (YSZ) showing the lowest conductivity among the sen-or components at the target temperature region below 450 ◦C,he sensor element was redesigned by changing the YSZ solidlectrolyte to Al metal film. Furthermore, in order to obtainsmooth electrochemical reaction at the surface of auxiliary
ensing electrode, we modified the auxiliary sensing electrodef Gd2O3–KNO3 solid. Here, KNO3-doped (Gd0.4Nd0.6)2O3olid solution was selected as the NOx sensing auxiliary elec-rode because the K+ ion conductivity in (Gd0.4Nd0.6)2O3 solidolution was ca. 5 times higher than that of the KNO3-dopedd2O3 solid [20], and the NOx sensing properties of the sensorith the (Gd0.4Nd0.6)2O3–KNO3 solid solution and Al metal
s the auxiliary sensing electrode and the reference electrode,espectively, were investigated at temperature as low as 250 ◦C.
. Experimental
The starting materials of Al(OH)3, ZrO(NO3)2·2H2O,b2O5 and (NH4)2HPO4 were mixed in an agate mortar in aolar ratio of 8:32:19:114 and calcined at 1000 ◦C for 12 h
nd then pelletized and heated again at 1200 ◦C for 12 h andhen at 1300 ◦C for 12 h in air. The resulting Al3+ ion con-ucting (Al0.2Zr0.8)20/19Nb(PO4)3 pellet was pulverized andhen made into pellet after mixing boron oxide (6 wt.%) as
d Actuators B 130 (2008) 46–51 47
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Fig. 1. Illustration of (a) our previously proposed nitrogen oxides gas sensorcell by combining Al3+ ion conducting (Al0.2Zr0.8)20/19Nb(PO4)3 and O2− ionconducting YSZ solid electrolytes with the 0.54Gd2O3–0.46KNO3 solid as theauxiliary sensing electrode, and (b) the presently redesigned sensor cell by com-b(a
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S. Tamura et al. / Sensors an
sintering additive [21] and sintered at 1300 ◦C for 12 h inir. The (Gd0.4Nd0.6)2O3 solid solution was prepared by a co-recipitation method using oxalic acid. A mixed solution ofmol L−1 Gd(NO3)3 and 1 mol L−1 Nd(NO3)3 with a stoichio-etric ratio was dropped into 0.5 mol L−1 oxalic acid aqueous
olution. After stirring for 24 h, the oxalate precipitates were fil-ered off and dried at 80 ◦C for 24 h and the dried powder waseated at 900 ◦C in air for 12 h. The (Gd0.4Nd0.6)2O3 and KNO2owders were mixed by ball milling apparatus (Fritsch GmbH,ulverisette 7). The mixture was pelletized and heated at 500 ◦Cor 12 h and 600 ◦C for 12 h in air, and then sintered at 600 ◦Cor 12 h in air. The samples were characterized by X-ray powderiffraction using Cu K� radiation (Rigaku, Multiflex 2 kW), flu-rescence X-ray spectrometer (Rigaku, ZSX100e) and the laseraman spectrometer (JASCO, NRS-3100).
Reference electrode of aluminum metal thin film was loadedn the sintered (Al0.2Zr0.8)20/19Nb(PO4)3 pellet by the vacuumeposition apparatus (JEOL, JEE-420), and then, it was coveredith sputtered Pt thin film by the ion coater (Eiko Engineer-
ng, IB-3). The auxiliary sensing electrode of KNO3-dopedd2O3–Nd2O3 solid solution was attached on the opposite
urface of the sintered (Al0.2Zr0.8)20/19Nb(PO4)3 pellet as illus-rated in Fig. 1(b) and heated up to the operating temperature of50 ◦C. The NOx gas concentration was regulated between 200nd 2000 ppm by mixing the 1% NO or NO2 (N2 valance) gasith O2 and N2 (Oxygen partial pressure was keep constant at.1 × 104 Pa). The total gas flow rate was fixed at 100 mL min−1
nd the sensor output was monitored by an electrometer (Advan-est, R8240).
. Results and discussion
Fig. 2 presents the XRD patterns of the KNO3-dopedGd0.4Nd0.6)2O3 solid. Among the samples prepared, the sam-les with the KNO3 content (x) smaller than 0.45 were foundo hold the single phase of cubic rare earth oxide with the peakhift toward lower angle with increasing the x value. The lat-ice volume of cubic phase which is estimated from the XRDeak angles was linearly increased with the KNO3 content forhe single phase samples (x ≤ 0.44) and any meaningful fur-her lattice expansion was not observed for the samples with≥ 0.45. In addition, since the existence of both K+ and NO3
−ons was confirmed by the X-ray fluorescence analysis and theaman spectroscopic analysis, it was clear that the KNO3 and
Gd0.4Nd0.6)2O3 solid successfully form the solid solution forhe samples with x up to 0.44.
Fig. 3 shows the compositional dependencies of the+ ion conductivity at 250 ◦C for the single phase of
1 − x)(Gd0.4Nd0.6)2O3–xKNO3 solid solution (x ≤ 0.44). Theonductivity at 250 ◦C of the 0.54Gd2O3–0.46KNO3 solidhich is the auxiliary sensing electrode of our previous sen-
or is also depicted as a star symbol. Among the single phaseamples (x ≤ 0.44), the 0.56(Gd0.4Nd0.6)2O3–0.44KNO3 solid
ossesses the highest conductivity which is ca. five times asigh as that of the 0.54Gd2O3–0.46KNO3 solid. Since the higheronductivity, i.e., the lower resistivity would realize a smoothlectrochemical reaction with NOx gas at the surface of auxil-tcin
ining Al3+ ion conducting (Al0.2Zr0.8)20/19Nb(PO4)3 with the KNO3-dopedGd0.4Nd0.6)2O3 solid solution and Al metal as the auxiliary sensing electrodend the reference electrode, respectively.
ary sensing electrode, we applied the high K+ ion conductive.56(Gd0.4Nd0.6)2O3–0.44KNO3 solid as a new auxiliary sens-ng electrode.
Fig. 4 depicts the comparison of the sensor responseime, which was defined as the time to obtain a 90% ofotal response when the NO gas concentration was variedrom 400 to 1000 ppm at 450 ◦C, for the sensors whichas combined with Al3+ ion conducting (Al0.2Zr0.8)20/19b(PO4)3 and O2− ion conducting YSZ solid elec-
rolytes with the 0.56(Gd0.4Nd0.6)2O3–0.44KNO3 solid or the.54Gd2O3–0.46KNO3 solid (previous sensor [13]) as the aux-liary sensing electrode (Fig. 1(a)). The response time for theensor with 0.56(Gd0.4Nd0.6)2O3–0.44KNO3 solid as the auxil-ary sensing electrode was ca. 5 min shorter than that for theensor with the 0.54Gd2O3–0.46KNO3 solid. This fact indi-ates that the fast electrochemical reaction at the surface ofuxiliary sensing electrode was successfully realized and sup-orts the above idea that the 0.56(Gd0.4Nd0.6)2O3–0.44KNO3olid is more appropriate auxiliary sensing electrode. In addi-
ion, although it was found that the sensor with the high K+ iononducting 0.56(Gd0.4Nd0.6)2O3–0.44KNO3 solid as the auxil-ary sensing electrode could operate even at 400 ◦C (The data areot shown here.), the sensor with YSZ (Fig. 1(a)) could not oper-
48 S. Tamura et al. / Sensors and Actuators B 130 (2008) 46–51
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Fig. 4. Comparison of the sensor response time at 450 ◦C for the sensors whichwas combined with Al3+ ion conducting (Al0.2Zr0.8)20/19Nb(PO4)3 and O2− ionconducting YSZ solid electrolytes with the 0.56(Gd0.4Nd0.6)2O3–0.44KNO3
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ig. 2. XRD patterns of the KNO3-doped (Gd0.4Nd0.6)2O3 solid,1 − x)(Gd0.4Nd0.6)2O3–xKNO3. Here, x is the analytical potassiumontent in the sample.
te at the target intermediate temperature region around 250 ◦C.herefore, we redesigned the sensor construction and fabricatednew type sensor cell as illustrated in Fig. 1(b). In this sensor,ecause a metallic aluminum showing a high electrical conduc-ivity was applied instead of the low ionic conductive YSZ, it isxpected the considerable reduction of the sensor cell resistance,ealizing the lower temperature sensor operation.
The plausible electrochemical reactions for NO detectiont the 0.56(Gd Nd ) O –0.44KNO solid auxiliary sensing
0.4 0.6 2 3 3lectrode, interface between the auxiliary sensing electrode andhe Al3+ ion conducting solid electrolyte, and the reference elec-rode of Al metal, are speculated as follows.ig. 3. The relationship between the KNO3 concentration and the conductivityf the (1 − x)(Gd0.4Nd0.6)2O3–xKNO3 solid solution (x ≤ 0.44) at 250 ◦C. Theorresponding datum of the 0.54Gd2O3–0.46KNO3 solid is also depicted as atar symbol.
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olid (solid line) or the 0.54Gd2O3–0.46KNO3 solid (broken line). The NO gasoncentration was varied from 400 to 1000 ppm.
Auxiliary sensing electrode:
KNO3(in Gd2O3–Nd2O3) � K+ + NO + O2 + e− (1)
Interface between the auxiliary sensing electrode and the(Al0.2Zr0.8)20/19Nb(PO4)3 electrolyte:
K+ + (19/12)(Al0.2Zr0.8)20/19Nb(PO4)3
� (1/3)Al3+ + (19/12)(K0.6Zr0.8)20/19Nb(PO4)3 (2)
Reference Al metal electrode:
(1/3)Al3+ + e− � Al (3)
rom Eqs. (1)–(3), total reaction can be expressed as follows:
NO3 + (19/12)(Al0.2Zr0.8)20/19Nb(PO4)3
� Al + (19/12)(K0.6Zr0.8)20/19Nb(PO4)3 + NO + O2
(4)
his leads to the following Nernst equation:
= E0 − R
nFT
× ln
{(aAl)(a(K0.6Zr0.8)20/19Nb(PO4)3 )19/12(pNO)(pO2 )
(aKNO3 )(a(Al0.2Zr0.8)20/19Nb(PO4)3 )19/12
}
(E0 = constant, n = 1.00) (5)
he activity of solids is constant if the temperature is fixed andO2 is constant at 2.1 × 104 Pa, the above Nernst Eq. (5) can benally simplified into the following relation:
= E1 − R
nFT ln(pNO) (E1 = constant, n = 1.00) (6)
n the case for the NO2 gas detection, the plausible reaction (1)s rewritten to
NO3(in Gd2O3–Nd2O3) � K+ + NO2 + (1/2)O2 + e− (7)
d Actuators B 130 (2008) 46–51 49
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Fig. 6. The sensor output EMFs observed in Fig. 5 for increasing and decreasingthe NO gas concentration as open squares and filled circles, respectively. Thes
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S. Tamura et al. / Sensors an
s a result, the final Nernst equation is expressed as
= E2 − R
nFT ln(pNO2 ) (E2 = constant, n = 1.00) (8)
ig. 5 depicts one of the representative sensor response curvesf the sensor with the 0.56(Gd0.4Nd0.6)2O3–0.44KNO3 solidnd Al metal as the auxiliary sensing electrode and the refer-nce electrode, respectively, by varying the NO concentrationf a typical gas emission from ca. 200 to ca. 2000 ppm and viceersa. The response time was within 5 min, demonstrating thatrapid, continuous, and reproducible response was successfullybtained even at 250 ◦C which is 200 ◦C lower than the opera-ion limit temperature of the previous sensor [13]. In addition,he response time of the present sensor at 250 ◦C is as fast as thatf the sensor with YSZ solid electrolyte at 450 ◦C (see Fig. 4).or this type of sensor, the factors defining the response time are
he reaction rates of the electrochemical reaction at sensing elec-rode (Eq. (1)) and subsequent reaction at the interface betweenensing electrode and the Al3+ ion conducting solid electrolyteEq. (2)), which are influenced by the temperature, and the reac-ion rates become slow with lowering the temperature. In thease for the present sensor, we have successfully obtained theast response even at 250 ◦C due to using the high K+ ion con-ucting 0.56(Gd0.4Nd0.6)2O3–0.44KNO3 solid and aluminumetal as the auxiliary sensing electrode and reference electrode,
espectively.Sensor output EMFs observed in Fig. 5 for increasing and
ecreasing the NO gas concentration are plotted in Fig. 6 as openquares and filled circles, respectively. The EMFs monotonouslyecreased with increasing the NO gas content and an exact 1:1inear relationship was observed in the relation of EMF ver-us log(pNO). The slope of R/nF (n = 1.00) calculated from Eq.6) is also plotted as a solid line in Fig. 6. The n values esti-ated from the slope for increasing and decreasing the NO gas
ontent are 1.07 and 1.03, respectively, which are well consis-ent with the theoretical value of n = 1.00, indicating that theresent sensor shows a theoretical Nernst response for NO gasetection.
The deviations of the sensor EMF for 200, 400, 1000, and
000 ppm NO are plotted in Fig. 7. Although the sensor EMFsere slightly changed due to the deviation of the base EMF,he n values were kept constant (1.0 ≤ n ≤ 1.1), indicating thatresent sensor exhibits a theoretical response for NO over 2
ig. 5. A representative sensor response curve obtained by varying the NOoncentration of a typical gas emission from ca. 200 to ca. 2000 ppm and viceersa at 250 ◦C.
c(
F(
lope (n = 1.00) calculated from Eq. (6) is also plotted as a solid line.
onths. This long-term stability is thought to be caused by theensor operation at low temperature.
In the case for the NO2 gas detection, similar sensing prop-rties to the case for the NO detection were observed even at50 ◦C. The sensor EMFs at low concentration NO2 gas werelittle deviated in increasing and decreasing NO2 gas concen-
ration as shown in Fig. 8, and the n values were 1.01 and 1.24or increasing and decreasing NO2 gas content, respectively.owever, for the NO2 concentration higher than 400 ppm, the nalues for increasing and decreasing NO2 gas content were 0.92nd 1.10, respectively, which are coincided with the theoreticalalue (n = 1.00) calculated from Eq. (8). This result indicateshat the present sensor shows the theoretical response for NO2hose concentration is higher than 400 ppm. The reason why
he sensor EMF at 200 ppm NO2 is a little deviated from thextrapolated value which can be estimated from the EMFs at00–2000 ppm NO2 is thought to be due to the poor electro-hemical reaction at the surface of auxiliary sensing electrode
Eq. (7)).ig. 7. The sensor EMF deviations for 200 (�), 400 (�), 1000 (�), and 2000 ppm�) NO at 250 ◦C.
50 S. Tamura et al. / Sensors and Act
Fig. 8. (a) A representative sensor response curve for the NO2 concentrationfrom 200 to 2000 ppm and vice versa at 250 ◦C, and (b) the sensor output EMFsffip
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or increasing and decreasing the NO2 gas concentration as open squares andlled circles, respectively. The slope (n = 1.00) calculated from Eq. (8) is alsolotted as a solid line.
. Conclusions
A new type of nitrogen oxides (NOx) gas sensor applica-le at a moderate temperature of 250 ◦C, was fabricated by theombination of trivalent aluminum cation conducting solid elec-rolyte with the KNO3-doped (Gd0.4Nd0.6)2O3 solid solutionnd aluminum metal as the auxiliary sensing electrode and theeference electrode, respectively. The present sensor shows theigh satisfactory sensing characteristics of rapid, continuous andeproducible response for NO and NO2 gases with obeying theernst theoretical relationship. Since the present sensor also
xhibits the long-term stability over 2 months, the sensor woulde a suitable in situ sensing tool.
cknowledgements
This work was partially supported by the Industrial Technol-gy Research Grant Program in ’05 (Project ID: 05A18011d)rom the New Energy and Industrial Technology Developmentrganization (NEDO) of Japan. This work was also supported
[
uators B 130 (2008) 46–51
y Grant-in-Aid for Science Research (Nos. 15550172 and7750196) from the Ministry of Education, Science, Sports andulture, and by Hyogo Science and Technology Association,he Iwatani Naoji Foundation and Yazaki Memorial Foundation
or Science & Technology.
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iographies
hinji Tamura was born in Osaka, Japan in 1972. He received his B.E. (1997)egree in Applied Chemistry from Osaka University. He then obtained his.E. (1999) Ph.D. degree (2001) in Materials Chemistry from Osaka Uni-
ersity. He has joined the faculty at Osaka University since 2004 and he is
Bvhme
uators B 130 (2008) 46–51 51
ssistant Professor. His main research fields are solid electrolytes and chemicalensors.
sao Hasegawa was born in Tochigi Pref., Japan in 1982. He obtained his.E (2004) degree in Applied Chemistry and M.E. (2006) degree in Materialshemistry from Osaka University.
obuhito Imanaka was born in Kawanishi, Hyogo, in 1958. He earned his
.E.(1981) and M.E.(1983) degrees in Applied Chemistry from Osaka Uni-ersity. He then obtained a Ph.D. degree from Osaka University in 1986. Heas joined the faculty at Osaka University since 1988 and he is Professor. Hisain research fields include rare earths and functional materials such as solidlectrolytes and chemical sensors.