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Solid State Ionics 179 (2

New type of sulfur dioxide gas sensor based on trivalent Al3+ ionconducting solid electrolyte

Yuichi Inaba, Shinji Tamura, Nobuhito Imanaka ⁎

Department of Applied Chemistry, Faculty of Engineering, Osaka University 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan

Received 20 June 2007; received in revised form 29 November 2007; accepted 29 November 2007

Abstract

A new solid electrolyte type sulfur dioxide gas sensor was fabricated by the combination of the trivalent aluminum cation conducting(Al0.2Zr0.8)20/19Nb(PO4)3 solid and the divalent oxide anion conducting yttria stabilized zirconia (YSZ) with the NaAl(SO4)2 solid as a detectingauxiliary electrode. Since the present sensor showed such a superior sensing performance of theoretical response, which obeys the Nernstrelationship between the sensor EMF output and the logarithm of the SO2 gas concentration, along with the stable, continuous, and reproducibleresponse, it is greatly expected to be applied to a practical SO2 gas sensing device.© 2007 Elsevier B.V. All rights reserved.

Keywords: Gas sensor; Sulfur dioxide; Solid electrolyte

1. Introduction

Sulfur oxides (SOx) exhausted from electric power plants,boilers, industrial plants by the combustion of fossil fuel, arerepresentative air pollutant gas species causing acid precipitants.Recently, the suppression of the SOx gas emission into theatmosphere is an urgent issue for the environmental conserva-tion. In order to effectively reduce the SOx gas emission, it isessential to develop the SOx gas sensing tool which can detectSOx gas rapidly and exactly at every emitting site. While someinstrumental apparatuses using IR absorption [1] and chemicalluminescence [2] have been widely utilized for the measurementof SOx gas concentration, they are generally too expensive andtoo large in size to install at every emitting site. Furthermore,since some pre-treatments for the sample gas are always requiredfor the accurate sensing, they are not suitable in situ gas sensingtool. To overcome these disadvantages, various types of compactSOx gas sensors have been proposed such as semiconductor type[3,4] and solid electrolyte type [5,6]. Although the semiconduc-tor type sensor can detect very low concentration gas (ppb level),the selectivity is potentially low because the sensing mechanismof the semiconductor type sensor is based on the electrical

⁎ Corresponding author.E-mail address: imanaka@chem.eng.osaka-u.ac.jp (N. Imanaka).

0167-2738/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.ssi.2007.11.029

resistance change of the semiconductor by the gas adsorption onits surface. Therefore, it is difficult to detect only the target gas inthe atmosphere where various kinds of gas species exist. On theother hand, solid electrolyte type sensor has an advancedmerit ofhigh selectivity because its gas sensing mechanism is based on aunique characteristic of solid electrolyte that only single ionspecies can migrate in solids. Therefore, the solid electrolytetype sensor is expected to be a practical candidate for theaccurate SOx gas sensing tool. For the SOx gas sensor with solidelectrolyte, Y. Yan et al. [7] proposed the YSZ-based SOx sensorusing the different metal sulfates, such as Li2SO4, Na2SO4,Li2SO4–CaSO4. Although the sensors exhibit good sensingproperties for 10–200 ppm SO2, the operating temperature ofthis type of sensor is high (873–1073 K), and, therefore, thelowering of the operation temperature is necessary for thepractical application use.

Recently, we have proposed a solid electrolyte type SO2 gassensor [8] which was combined with two kinds of solidelectrolyte of Al3+ ion conducting (Al0.2Zr0.8)20/19Nb(PO4)3 [9]and O2− ion conducting yttria stabilized zirconia (YSZ) havingthe Li2SO4 doped La2O2SO4 solid as the detecting auxiliaryelectrode. Although this sensor could detect the SO2 gas at773 K, there was a problem that the sensor response graduallydeteriorated. In this sensor, the sensor output is obtained as theEMF according to the Nernstian relationship, which is derived

Fig. 2. The representative sensor response curves observed when SO2 gasconcentration was varied from 200 to 2000 ppm and vice versa at 803 K.

1626 Y. Inaba et al. / Solid State Ionics 179 (2008) 1625–1627

from the equilibrium reactions at each solid component of thesensor element and interfaces between solids. In our previouspaper [8], we assumed that the ion exchange reaction of Li+ andAl3+ was occurred at the interface between detecting auxiliaryelectrode and (Al0.2Zr0.8)20/19Nb(PO4)3 solid electrolyte. There-fore, a plausible reason for the deterioration of the sensingresponse is thought to be caused by the resistance increase at theinterface caused by such ion exchange.

One of the effective ways to control the ion exchange reactionat the interface between detecting auxiliary electrode and the(Al0.2Zr0.8)20/19Nb(PO4)3 electrolyte is to select Al3+ ion as acation species in the detecting auxiliary electrode. In this study,Al2(SO4)3 was selected as a component of detecting auxiliaryelectrode. However, the electrical resistance of Al2(SO4)3 is sohigh that the electrochemical reaction with ambient SO2 gas cannot be accelerated. Therefore, we mixed, here, the high Na+ ionconductive Na2SO4 with Al2(SO4)3 to form NaAl(SO4)2, andinvestigated the SO2 gas sensing properties of the sensor based onthe two solid electrolytes of (Al0.2Zr0.8)20/19Nb(PO4)3 and YSZwith the NaAl(SO4)2 solid as a detecting auxiliary electrode.

2. Experimental

(Al0.2Zr0.8)20/19Nb(PO4)3 was prepared by mixing the startingmaterials of Al(OH)3, ZrO2, Nb2O5, and (NH4)2H(PO4)3 in amolar ratio of 8:32:19:114. The mixed powder was heated at1273 K for 12 h, 1473 K for 12 h, and then, 1573 K for 12 h in air.The (Al0.2Zr0.8)20/19Nb(PO4)3 obtained was pelletized andsintered at 1573 K for 12 h in air. Yttria-stabilized zirconia(YSZ), (ZrO2)0.92(Y2O3)0.08, was synthesized bymixingZrO2 andY2O3 in a molar ratio of 92:8, and themixed powder was heated at1873 K for 6 h. The YSZ powder was pelletized and sintered at1873 K for 12 h. NaAl(SO4)2 was prepared by mixing Al2(SO4)3and Na2SO4 (molar ratio=6:4) in an agent pot at a rotation speedof 300 rpm for 4 h with a planetary ball mill apparatus (FRITSCHGmbH, P-7). The mixed powder was pelletized and heated at873 K for 12 h in air. The samples obtained were identified by X-ray powder diffraction (XRD) with Cu-Kα radiation (Rigaku,Multiflex 2 kW). Thermal analysis of the NaAl(SO4)2 was

Fig. 1. Schematic illustration of the SO2 gas sensor element by the combinationof Al3+ ion conducting solid electrolyte and YSZ with the NaAl(SO4)2 detectingauxiliary electrode.

performed by using a thermal gravimetric and differential thermalanalysis (TG-DTA) apparatus (Shimadzu, DTG-50H).

Two solid electrolyte pellets of (Al0.2Zr0.8)20/19Nb(PO4)3 andYSZ were tightly fixed by inorganic adhesive agent (Asahi,Sumiceram 17-D), and the NaAl(SO4)2 detecting auxiliaryelectrode was put on the (Al0.2Zr0.8)20/19Nb(PO4)3 surface(Fig. 1), and then, the sensor element was heated up to theoperating temperature of 803 K. The SO2 gas concentration wasregulated by mixing pure SO2 with air. The total gas flow ratewas kept constant at 100 ml/min, and the sensor output wasmonitored by an electrometer (Advantest, R8240).

3. Results and discussion

From the XRD measurement, 0.6Al2(SO4)3–0.4Na2SO4 wasidentified to be NaAl(SO4)2 (with a very small amount of Al2(SO4)3). Furthermore, the sample showed no weight change upto 873 K, indicating that the NaAl(SO4)2 solid holds an enoughthermal stability for using as the detecting auxiliary electrode.

Fig. 2 shows the representative sensor response curve ob-served when the SO2 gas consentration was varied from 200 to2000 ppm and vice versa. The response time to obtain a 90%total response was within seven minutes and a continuous andreproducible response was observed.

The plausible reactions occurred at the NaAl(SO4)2 detectingauxiliary electrode, the interface between the detecting auxiliary

Fig. 3. The relationship between the sensor EMF output and the logarithm of theSO2 gas concentration at 803 K. Solid line shows the theoretical slope calculatedfrom the Nernst Eq. (7).

1627Y. Inaba et al. / Solid State Ionics 179 (2008) 1625–1627

electrode and the Al3+ ion conducting electrolyte, the interfacebetween the two solid electrolytes, and the reference electrodeare as follows.

Detecting auxiliary electrode:

1=2Naþ þ 1=2Al3þ þ SO2 þ O2 þ 2e�⇆1=2NaAlðSO4Þ2 ð1ÞInterface between the detecting auxiliary electrode and the

(Al0.2Zr0.8)20/19Nb(PO4)3 electrolyte:

2=3Al3þ þ 19=24ðNa0:6Zr0:8Þ20=19NbðPO4Þ3⇄1=2Naþ

þ 1=2Al3þ þ 19=24ðAl0:2Zr0:8Þ20=19NbðPO4Þ3 ð2Þ

Interface between the two solid electrolytes:

1=3Al2O3⇄2=3Al3þ þ O2� ð3ÞReference electrode:

O2−⇄1=2OII2 þ 2e� ð4Þ

From Eqs. (1)–(4), overall reaction of the present sensor isdescribed as follows.

1=3Al2O3 þ 19=24ðNa0:6Zr0:8Þ20=19NbðPO4Þ3 þ SO2

þ OI2⇄1=2NaAlðSO4Þ2

þ 19=24ðAl0:2Zr0:8Þ20=19NbðPO4Þ3 þ 1=2OII2 ð5Þ

Therefore, the following Nernst equation is derived.

E ¼ E0 þ ðR=nFÞT lnfðaAl2O3Þ1=3ðaðNa0:6Zr0:8Þ20=19NbðPO4Þ3Þ19=24

�ðPSO2ÞðPOI2ÞðaNaAlðSO4Þ2Þ−1=2

�ðaðAl0:2Zr0:8Þ20=19NbðPO4Þ3Þ−19=24ðPOII2 Þ−1=2g ð6Þ

E0 is constant, n=2.00 in this case, and R, T and F are the gasconstant, absolute temperature and the Faraday's constant,respectively. Because Al2O3, (Na0.6Zr0.8)20/19Nb(PO4)3, NaAl(SO4)2, and (Al0.2Zr0.8)20/19Nb(PO4)3 are all in a solid form,activities (a) of these solids are constant at a fixed temperature.In addition, as the whole sensor cell is set in the same at-mosphere, O2

I is exactly equal to O2II and oxygen partial pressure

was kept constant at 0.21 atm. Therefore, the Eq. (6) is sim-plified as follows.

E ¼ CðconstantÞ þ RT=nF lnðPSO2Þ ðn ¼ 2:00Þ ð7ÞThe sensor EMF outputs observed in Fig. 2 are plotted in Fig. 3

as open squares and closed circles for increasing and decreasing theSO2 gas concentration, respectively. The sensor EMF outputsmonotonously decreased with decreasing the SO2 gas concentra-

tion and a linear 1:1 relationship was clearly observed. Thetheoretical slope (n=2.00) calculated from the Nernstian relation-ship Eq. (7) is also depicted as a solid line in Fig. 3. Since the nvalues (n=2.03 and 2.08 for increasing and decreasing the SO2 gasconcentration, respectively) estimated from the slope for theobserved EMFs well coincided with the theoretical one (n=2.00),it was found that the present SO2 gas sensor exhibited thetheoretical response for SO2 gas sensing. In addition, the presentsensor was found to maintain the theoretical response for SO2 gassensing over a month, while the previously proposed sensor withLa2O2SO4–Li2SO4 as a detecting auxiliary electrode deterioratedafter ca.15 days from the sensor operation. The reason why thepresent sensor shows such a long term high stability for SO2 gassensing is thought to be as follows. Since the NaAl(SO4)2 solidcontains Al3+, the ion exchange reaction between Na+ in detectingauxiliary electrode and Al3+ in (Al0.2Zr0.8)20/19Nb(PO4)3 solidelectrolyte was effectively controlled, preventing the formation ofthe high resistive phase. (We have confirmed that no chemicalreaction occured at the interface between NaAl(SO4)2 and(Al0.2Zr0.8)20/19Nb(PO4)3 by SEM observation and XRDmeasurement.)

4. Conclusions

A new type of sulfur dioxide gas sensor operating at 803 Kwas fabricated by the combination of the Al3+ ion conductingand O2−ion conducting solid electrolytes with NaAl(SO4)2 asthe detecting auxiliary electrode. Since the present sensor withNaAl(SO4)2 detecting auxiliary electrode, which contains bothNa+ and Al3+ ions, showed a continuous, reproducible, andtheoretical response over a month with obeying the theoreticalNernst relationship, it is greatly expected to be applied for the insitu SO2 gas sensing tool at every emitting site.

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