detection of offensive odorant in air with a planar-type potentiometric gas sensor based on ysz with...
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Sensors and Actuators B 191 (2014) 351– 355
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical
journa l h om epage: www.elsev ier .com/ locat e/snb
hort communication
etection of offensive odorant in air with a planar-typeotentiometric gas sensor based on YSZ with Au and Pt electrodes
asami Moria, Yoshiteru Itagakia,∗, Yoshihiko Sadaokab, Shin-ichi Nakagawac,asahito Kidac, Takio Kojimac
Department of Materials Science and Biotechnology, Graduate School of Science and Engineering, Ehime University, Matsuyama, Ehime 890-8577, JapanThe Cooperative Center of Scientific and Industrial Research, Ehime University, Matsuyama, Ehime 890-8577, JapanFuture Products Project Team, New Business Advancement Group, NGK Spark Plug Co., Ltd., Komaki, Aichi 485-8510, Japan
r t i c l e i n f o
rticle history:eceived 13 June 2013eceived in revised form 20 August 2013ccepted 1 October 2013vailable online 10 October 2013
a b s t r a c t
A potentiometric gas sensor with a planar-type structure was fabricated and used to detect some offensiveodorants in harsh environments such as ammonia, trimethyl-amine, methyl mercaptan and hydro-gen sulfide. The electrode potential of the planar type sensor with a (Au/YSZ)|YSZ|(Pt/YSZ) structureresponded to such compounds at sub-ppm levels. The sensitivity at 450 ◦C was in the order of methylmercaptan > hydrogen sulfide > ammonia > trimethylamine.
© 2013 Elsevier B.V. All rights reserved.
eywords:ffensive odormmoniarimethylamineydrogen sulfide ethyl mercaptanlectrode potential
. Introduction
Since air quality deterioration by offensive odors has become serious problem for health in several areas of human life, theesponsible odorants in the air need to be strictly regulated to anxtremely low concentration level. Therefore, continuous moni-oring of offensive odors is now strongly demanded, especially inrban spaces. Among the various sensing techniques used to detectuch harmful gases, solid-state sensors are of practical interest dueo their small size and ease of use. So far, zirconia-based electro-hemical sensors have been widely investigated for the detectionf NOx, CO and hydrocarbons [1–20]. Furthermore, a zirconia-ased oxygen sensor is an attractive technique to electrochemicallyetect some offensive odorants/smells in air such as nitrides andulfides.
In a potentiometric oxygen sensor based on an oxide-ion con-uctor with Pt electrodes, P1(O2), Pt(SE)/oxide conductor/Pt(REF),2(O2), the output voltage of the sensing electrode can be predicted
∗ Corresponding author. Tel.: +81 89 927 9755.E-mail addresses: [email protected], [email protected]
Y. Itagaki).
925-4005/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.snb.2013.10.005
by the Nernst equation assuming a reversible transfer of oxygenfrom cathode to anode:
E =(
RT4F
)ln
[P1(O2)P2(O2)
](1)
However, under the condition of P1(O2)/P2(O2) /= [O2ad(1)]/[O2ad(2)], a non-Nernstian relationship should be experimentallyobserved. This phenomenon occurs when the contamination ofother molecules induces a decrease in the concentration of theadsorbed oxygen on the electrode surface by the replacementand/or electrochemical reactions. Such an oxygen exclusion effectwould depend on the chemical and physical structure of theelectrode.
In this study, planar-type potentiometric oxygen sensors basedon zirconia with porous Pt and Au electrodes were fabricated andevaluated their feasibility to detect sub-ppm levels of offensiveodorants contaminating the air. In the planar-type sensor structure,the output signal is given by the difference between the half-cellpotentials of the two electrodes. Therefore, the half-cell potentialsof the electrodes were also investigated. As a result, the half-cellpotential of the Pt electrode well responded not only to the oxygenconcentration, but also to the contamination of air by several gases.
2. Experimental
The structures of the planar-type sensor are illustrated in Fig. 1.A 5.4 mol% Y2O3/ZrO2 (YSZ) was used as the electrolyte and coated
352 M. Mori et al. / Sensors and Actuat
Fs
oA1
Ft
ig. 1. Schematic illustrations of the sensor structure (top) and pictures of the usedensor element (bottom).
n an alumina substrate. The composite electrodes, Pt/YSZ andu/YSZ, were then attached to the YSZ surface and sintered at400 and 1000 ◦C, respectively. The electrodes were connected to
Fig. 2. SEM photographs of the Pt/YSZ and Au/YSZ electrodes formed on the YS
ig. 3. O2 concentration dependence of the half-cell potentials of the Pt/YSZ and Au/YSZhe italic form are the estimated n-values. Temperature in ◦C is also indicated in figure. Th
ors B 191 (2014) 351– 355
Au-wire using Au paste (TR-1530; Tanaka-Matthey). Finally, the Pt-heater was formed on the opposite side of the alumina substrate.SEM photographs of the electrode surfaces are shown in Fig. 2. Boththe electrodes exhibited porous structures even after the sinteringprocess. Metal electrodes such as Pt and Au tend to sinter at hightemperature, and this may cause instability of a sensor in a longperiod. Adding the secondary phase of YSZ into the metals maystabilize the morphology during the sensor operation. The sensorwas set in a 20 cm3 measuring chamber and the total flow rate was200 cm3/min. The test gases were prepared by mixing of the syn-thetic air of G2 grade and the vapor. The air-balanced gases wereprepared by mixing the vapors through diffusion tubes at regulatedtemperatures with cylinder air at the prescribed flow rate ratios(Gastech, G1). The electrode potential of the sensor was measuredusing a digital electrometer (Advantest, TR8652). The sensing tem-perature was controlled by applying a dc voltage to the Pt-heater.Before the measurements, the sensor elements were pre-heated at1000 ◦C.
3. Results and discussion
For the planar-type sensor with the (Au/YSZ)|YSZ|(Pt/YSZ) struc-
ture, the output voltage is observed even when the electrodes areexposed to the same ambient oxygen concentration. The resultis shown in Fig. 3. Over- and undershooting are observed whenthe concentration is switched between 21% and 10.5%. It mightZ sheet. The electrodes were sintered at 1400 and 1000 ◦C, respectively.
electrodes, determined versus Pt (ref. 21%O2) reference. The numbers indicated ine output EMF transient response curve is inserted in figure.
M. Mori et al. / Sensors and Actuators B 191 (2014) 351– 355 353
Fa
b(FtdwehlPilswiawembstl
VilTtwoototstmttaltifi�t
ig. 4. The transient sensor responses to air-contaminations by hydrogen sulfidend ammonia at 450 ◦C. The numbers indicated in figure are their concentrations.
e possible that the introducing and withdrawing a dilution gasN2) may momentarily vary sensor temperature. The result inig. 3 indicates that the oxygen activity is different between thewo electrodes. Therefore, we measured the oxygen concentrationependence of the half-cell potentials at different temperatures inhich the half-cell potential was evaluated relative to the Pt ref-
rence electrode in 21% oxygen. The result is shown in Fig. 3. Thealf-cell potentials at both electrodes linearly increased with the
ogarithm of the oxygen concentrations. The half-potential of thet/YSZ electrode is higher than that of the Au/YSZ electrode, andt did not significantly change with temperature. Meanwhile, theevel of the half-cell potential on the Au/YSZ electrode side moreensitively changed with the oxygen concentration and decreasedith an increase in the operating temperature. Based on the exper-
mental curves, the electron transport number, n, was determinedssuming the Nernst equation. The n value at the Pt/YSZ electrodeas n = 4, equal to the theoretical number, while those at the Au/YSZ
lectrode side were n = 1.32–1.40 and much lower than 4, whicheans the half-cell potential cannot be expressed by a Nernst
ehavior. In other words, the electrode potential on the Au/YSZide is more sensitive to the oxygen concentration changes thanhat on the Pt/YSZ side, or the oxide ion activity of the former isower than that of the latter.
It is known that a separate-type oxygen sensor responds to theOC contaminations in air [1,2]. The planar type sensor evaluated
n this study also well responded to contaminations of ppm orower levels of hydrogen sulfide or ammonia as shown in Fig. 4.he contamination with such odorants induced a shift in the poten-ial in the negative direction, suggesting that the Au/YSZ electrodeas more anodic than the Pt/YSZ electrode. That is, the activity
f the oxide ion on the Au/YSZ electrode side is lower than thatf the oxide ion on the Pt/YSZ electrode side. The defined poten-ial, �E = EVOC – Eair, is shown in Fig. 5. All the electrode potentialsf the planar type sensor responded to four kinds of odorants inhe following order of magnitude: methyl mercaptan > hydrogenulfide > ammonia ≈ trimethylamine. The output of �E was propor-ional to the odorant concentration below 3 ppm. The sensitivity to
ethyl mercaptan is distinctly higher than the other odorants. Theemperature dependences of �EMF are shown in Fig. 6. Even inhe uncontaminated air, �EMF was not zero and increased withn increase in the temperature. The non-zero output is due to theower oxide ion activity of the Au/YSZ than that of the Pt/YSZ, buthe increasing temperature reduces this difference. A monotonous
ncrease in the electrode potential was observed in hydrogen sul-de and methyl mercaptan. For ammonia and trimethylamine, theEMF initially decreased with the increasing temperature untilhey reached a minimum and then began to increase in �EMF by
Fig. 5. �EMF responses to the VOCs vs. log Cvoc (a) and Cvoc (b) when the appliedheater voltage was 7.0 V. The numbers in parentheses in (b) are the slopes of thecalibration curves in V/ppm.
further increasing the temperature. The minimum points observedin Fig. 6 may be related to the diversity of the reaction/oxidationof the N-contained compounds. It should be noted that the �EMFresponse tends to decrease at higher temperature for all the odor-ants, but for ammonia the decrease in the response is rathersluggish. It might be possible that ammonia is partially convertedinto nitrogen oxides at high temperature to further decrease theoxygen activity at the TPB-site. In fact, Schönauer et al. [19] reportedthat NH3 was converted into NO and N2O (not into NO2) on theAu-based electrode at 550 ◦C. At the electrode, the following reac-tions are expected and some other species as products are alsoconsidered.(
12
)O2 + 2e− + V∗∗
O ⇔ O2−(at TPB) (2)
(23
)NH3 + O2− → H2O +
(13
)N2 + 2e− + V∗∗
O (3)
(2
25
)(CH3)3N + O2− →
(6
25
)CO2 +
(9
25
)H2O +
(2
25
)NO2 + 2e−
+ V∗∗O (4)
(13
)H2S + O2− →
(13
)H2O +
(13
)SO2 + 2e− + V∗∗
O (5)
(16
)CH3SH+O2− →
(16
)CO2+
(26
)H2O +
(16
)SO2 + 2e− + V∗∗
O (6)
For example, from the electrochemical reactions (2) and (3), amixed potential, Emix, can be estimated using Eq. (7).
Emix ∝ (kT/2e)[(
23
)ln CNH3 −
(12
)ln CO2 − ln CH2O
](7)
where k is the Boltzmann constant, T is the absolute temperature inK and e is the electron charge. For the monolithic type sensor, this
354 M. Mori et al. / Sensors and Actuators B 191 (2014) 351– 355
. Conc
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4
coolushsaifsof
R
[
[
[
[
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[
[
[
[
[
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Fig. 6. Temperature dependence of �EMF responses
elationship (7) should be applied to both the Au/YSZ and/or Pt/YSZlectrodes. The mixed potential model suggests a semi-logarithmicelationship between the electrode potential and the concentrationf species even for the monolithic-type sensor. The mathematicallytted equations suggested a linear relationship (�EMF = �CVOC)ather than a semi-logarithmic relationship. We need to furthernvestigate the sensing mechanism. The estimated sensitivity, �, isndicated in Fig. 5. In addition, the concentration giving 10 mV of
E was 0.038, 0.099, 0.25, and 0.264 ppm for the methyl mercap-an, hydrogen sulfide, ammonia, and trimethylamine, respectively.iven the Guideline/Regulation in Japan for offensive odors in
he atmosphere of generally 0.002, 0.02, 1.0, and 0.005 ppm,espectively, further enhancement of the sensitivity should beequired.
. Conclusion
An interesting sensor concept for exhaust gas detection appli-ations is a concentration cell and/or the mixed potential cell basedn YSZ as a solid electrolyte. Initial investigations using this typef sensor for sensing offensive odors such as ammonia, trimethy-amine, hydrogen sulfide and methyl mercaptan were conductedsing the monolithic-type sensor with the (Au/YSZ)|YSZ|(Pt/YSZ)tructure. The corresponding concentration for 10 mV of �E wasigher than that established Japanese Guideline value for offen-ive odors. More detailed investigations for the electrode materialsnd design of the sensor structure will provide the possibility tomprove the sensing characteristics. Although we in this reportocused on initial properties, we are now examining the long-termtability and reproducibility of the current sensor with changingdorant concentration and temperature, and will report in the nearuture.
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Biographies
Masami Mori received her B.Sc. degree in analytical chemistry from Ehime Uni-versity in 2003. She obtained a Dr. Eng. degree from Ehime University in 2008.She is a research associate in the Department of Material Science and Engineering
Actuat
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YUsptig
M. Mori et al. / Sensors and
ince 2003. Her main interest is fine-particle applications for chemical sensors andatalysts.
oshiteru Itagaki received his M.E. in 1995 in Industrial Chemistry from Hiroshimaniversity. He obtained a Dr. Eng. from Hiroshima University in 1998. He is a seniorssistant professor at Ehime University since 2011. His main interests are the designnd fabrication of functional ceramic membranes for chemical sensors and fuel cells.
oshihiko Sadaoka received his M.E. degree in industrial chemistry from Ehimeniversity in 1971. He has been on the Faculty of Engineering at Ehime University
ince 1971. He obtained a Dr. Eng. degree from Kyushu University in 1979. He is arofessor in the Department of Material Science and Engineering since 1996 and inhe Cooperative Center of Scientific and Industrial Research since 2012. His mainnterests are inorganic and organic functional materials for chemical sensors andreen materials.
ors B 191 (2014) 351– 355 355
Shin-ichi Nakagawa received his M.E. degree in material engineering from KyushuUniversity in 1994. He is a supervisor in the Future Products Project Team, NewBusiness Advancement Group, NGK Spark Plug Co. Ltd. since 2001. His main interestis MEMS sensor development.
Masahito Kida received his M.E. degree in applied chemistry from Nagoya Instituteof Technology in 1991. He is an assistant manager in the Future Products ProjectTeam, New Business Advancement Group, NGK Spark Plug Co. Ltd., since 1996. Hismain interest is MEMS sensor development.
Takio Kojima received his B.E. degree in machine engineering fromNagoya University in 1984. He is a general manager in the Future Prod-ucts Project Team, New Business Advancement Group, NGK Spark Plug Co.Ltd., since 1984. His main interests are automotive parts and MEMS sensordevelopment.