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Pt potentiometric oxygen sensor with reference air
Sensors and Actuators B 143 (2009) 56–61
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Detection of sub-ppm level of VOCs based on a Pt/YSZ/Pt potentiometric oxygen sensor with reference air Masami Mori a , Hiroyuki Nishimura a , Yoshiteru Itagaki a , Yoshihiko Sadaoka a,∗ , Enrico Traversa b a b
Department of Materials Science and Biotechnology, Graduate School of Science and Engineering, Ehime University, 3 Bunkyo-cho, Matsuyama, Ehime 890-8577, Japan International Research Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan
a r t i c l e
i n f o
Article history: Received 12 February 2009 Received in revised form 29 July 2009 Accepted 1 September 2009 Available online 6 September 2009 Keywords: YSZ VOC sensor Oxygen sensor EMF Sub-ppm
a b s t r a c t Potentiometric oxygen sensors with Pt|YSZ|Pt structure were applied for detecting several volatile organic compounds (VOCs; acetic acid, methylethylketone, ethanol, benzene, toluene, o- and p-xylene) at sub-ppm levels in the temperature range of 400–500 ◦ C. The electromotive force (EMF) output from the sensors linearly changed with logarithm of VOC concentration, and sufficiently large output was confirmed at sub-ppm levels. Among the examined VOCs, ethanol exhibited the highest sensitivity (−54.1 mV/decade) at 400 ◦ C. However, the sensitivity of ethanol drastically decreased at 500 ◦ C. Thus the sensitivity of all the VOCs examined in this study was tunable with temperature. An increase in temperature decreased the sensitivity but effectively modified the response and recovery times. Analysis of multi-dimensional plots combining the sensor outputs at the different operating temperatures (400, 450 and 500 ◦ C) suggested the possibility to discriminate the different VOCs. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Since air quality contamination by volatile organic compounds (VOCs), especially indoor, has become a serious problem for health in several scenes of human life [1], their concentration in air needs to be strictly regulated to an extremely low level. Therefore, continuous monitoring of VOCs is now strongly demanded, especially in urban spaces. Among of the various sensing techniques used to detect such harmful gases [2,3], solid-state sensors are of practical interest due to their small size and ease of use [4]. Zirconia-based electrochemical sensors tailored for NOx , CO, and hydrocarbon detection have been widely investigated [4–24]. An interesting approach to electrochemically detect VOC gases is exploiting zirconia-based oxygen sensors due to their low cost and easy use. In a potentiometric method, the sensor output voltage can be evaluated from the Nernst equation assuming a reversible transfer of oxygen from cathode to anode. For a sensor having a simple pa (O2 ), Pt|YSZ|Pt, pc (O2 ) geometry, where pa (O2 ) and pc (O2 ) are the different oxygen partial pressures at the anode and the cathode, respectively, the Nernst equation can be written as follows: EMF =
p (O ) c 2
RT ln 4F pa (O2 )
∗ Corresponding author. E-mail address: [email protected] (Y. Sadaoka). 0925-4005/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2009.09.001
(1)
where R is the gas constant, T the absolute temperature and F the Faraday’s constant. The observed output voltage value can deviate from the theoretical one, estimated from Eq. (1), when reducing gases, such as VOCs, coexist with oxygen. In 1977, Fleming first interpreted this deviation from a viewpoint of non-Nernstian behavior, resulting from the simultaneous occurrence of two electrochemical reactions at the working electrode [4]. Since then, many research groups have exploited the non-Nernstian behavior to develop several electrochemical sensors. In this work, potentiometric oxygen sensors with a Pt|YSZ|Pt structure were fabricated and used to check the feasibility to detect sub-ppm levels of contaminating VOCs in air at an intermediate temperature region of 400–500 ◦ C. The results were promising and showed that a three-dimensional plot of the data can allow discrimination between the presence of different VOCs. The sensing mechanism was also discussed. 2. Experimental Illustration of the sensor device is shown in Fig. 1. 8YSZ sheets (8 mol% Y2 O3 /ZrO2 , Tosoh Co.) of 200 m in thickness were used as electrolytes. Pt electrodes were deposited on both 8YSZ sheet surfaces by magnetron sputtering. Au current collectors were attached onto a corner of the Pt sputtered electrodes using a Pt paste. The whole sensing elements were then heated to 600 ◦ C for 1 h. For measurements, each sensing element was fixed on the top of an alumina tube with an inorganic adhesive to separate the atmospheres surrounding the two Pt electrodes. One electrode side was
M. Mori et al. / Sensors and Actuators B 143 (2009) 56–61
Fig. 1. Illustration of the VOC sensor with Pt|YSZ|Pt structure.
exposed to synthetic air from a gas-cylinder and the other side was exposed to synthetic air contaminated by the different VOC gases, i.e., methylethylketone (MEK), acetic acid, ethanol, benzene, toluene, o- and p-xylene, at concentrations lower than 10 ppm. The air-balanced VOC gases were prepared with the permeator (Gastech G1) by mixing the VOC vapors through diffusion tubes at regulated temperatures with a cylinder air at prescribed flow rate ratios. The sensor element was placed in a temperature-controlled tubular furnace. The EMF change of the element was measured using a digital electrometer (Advantest R8340). 3. Results and discussion 3.1. VOC sensing response The surface morphology of the two sides of the supplied 8YSZ sheets was clearly different from each other, as easily detected by eyes. The surface of one side was more gloss/smooth than the surface of the other side and the difference of the surface morphology
57
could be confirmed by SEM observation as shown in Fig. 2. In this study, a more gloss surface was defined as SS and other surface as RS. A detailed study on the 8YSZ sheet surfaces and their effect in gas sensing properties has been reported in a previous paper [24]. It has been found that the different morphology has a strong influence on the sensing properties. When the Pt electrode deposited on the RS side was used as a sensing electrode, a faster and more stable response to oxygen concentration changes was observed. Moreover, the electron number for the oxygen sensing reaction evaluated at 400 ◦ C in the 0.1–100% oxygen concentration range was 4 ± 0.2, close to the theoretical value. Therefore, in this work the sensor elements were prepared using the Pt electrode sputtered on the RS side as a sensing electrode for oxygen and VOCs, whereas the Pt electrode sputtered on the SS side was used as a reference electrode. When electrodes were prepared on the same surface and exposed simultaneously to the same atmosphere, negligible EMFs were measured. Fig. 3 shows the EMF response at 400 and 450 ◦ C of the Pt|YSZ|Pt sensor to different ethanol concentrations. In these experiments, the sensor exposed to uncontaminated air and then a contaminated air with ethanol. After repeated two times, the concentration of ethanol was changed from 0.5 to 10 ppm in steps. After the measurements for a series of VOCs, the temperature changed. Good repeatable curves for ethanol were observed even after the measurements of a series of VOCs at the constant temperature (after about 60 h of the first measurement). When both side of the sensor were exposed to the air, a negative EMF value of −25 mV was detected at 400 ◦ C, deviating from the expected zero value. The different potential values can be explained in terms of the different surface structure and morphology of the two sides of 8YSZ sheets, which might affect the redox reaction kinetics. The negative EMF output, despite both sides of the sensing element were exposed to the same atmosphere, indicates that the sensing electrode (RS side) was more cathodic with respect to the reference electrode. This is possibly due to the morphology-induced slightly larger oxygen ion equilibrium activity at the RS surface than that at the SS side [24]. The EMF offset in air decreased from (−) 25 to (−) 6 mV with increasing the temperature up to 500 ◦ C. This change may be due to
Fig. 2. SEM photographs of the rough surfaces (RS) and smooth (SS) of the 8YSZ sheet.
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M. Mori et al. / Sensors and Actuators B 143 (2009) 56–61
Fig. 3. Response to the application of cycles of air and ethanol/air at 400 ◦ C (a) and 450 ◦ C (b).
Fig. 4. Response to 0.5 ppm VOCs at 400 ◦ C. The test gas changed from air to 0.5 ppm VOC/air and then returned.
the increases in ionic conductivity of the 8YSZ sheet with the temperature. The absolute value of EMF became close to zero and this might be explained by the fact that the kinetic difference, due to the difference in surface morphologies of the two sides, might diminish because of thermal activation of the oxygen exchange reaction and/or of an increase in 8YSZ conductivity [25]. Concentration-dependent EMF changes were measured by exposing the sensor to VOCs. Fig. 4 shows the sensor response to 0.5 ppm VOCs at 400 ◦ C. The EMF response level at 0.5 ppm is large enough for reliable measurements. Table 1 summarizes the 90% response (air → VOC) and recovery (VOC → air) times in ethanol, measured at 400 and 500 ◦ C. Both the times tend to increase with the increase in the ethanol concentration. The 90% recovery times are 2.0–2.4 times longer than the response time. This trend suggests that VOC or oxidized species strongly bound on the surface of the Pt electrode and it slows the replacement by an oxygen molecule. Increasing the temperature up to 500 ◦ C resulted in considerably shortened response and recovery times. Recovery times became almost half compared to those at 400 ◦ C. On the other hand response drastically shortened to be 2.5–25% of those at 400 ◦ C. This
result supports that large energy is required for VOC (or oxidized species) desorption. Similar response and recovery behaviors were also observed for all the tested VOCs. To eliminate the effects of the offset measured in air, the difference between the EMF measured upon exposure to VOCs and in air, EMF, was defined as in Eq. (2).
Table 1 90% response and recovery times to ethanol. Ethanol concentration (ppm)
0.5 1 3 7 10
90% response time (s)
90% recovery time (s)
400 ◦ C
500 ◦ C
400 ◦ C
500 ◦ C
155 185 185 180 204
4 10 15 45 30
355 364 415 435 480
240 190 245 205 245
EMF = EMFVOC − EMFair
(2)
Fig. 5 shows the concentration dependence of the EMF of the Pt|8YSZ|Pt sensor upon exposure to several VOCs. The EMF changed linearly with the logarithm of the VOC concentration, EMF = a + b log C, where a is the EMF at 1 ppm of VOC, b the sensitivity in mV/decade and C the concentration of VOC. Table 2 Table 2 Sensing characteristics of the sensor to VOCs.
Sensitivity (mV/decade) Acetic acid MEK Ethanol Benzene Toluene o-Xylene p-Xylene EMF/mV at 1 ppm VOCs Acetic acid MEK Ethanol Benzene Toluene o-Xylene p-Xylene
400 ◦ C
450 ◦ C
500 ◦ C
−38.2 −38.5 −54.1 −30.9 −38.7 −41.5 −37.1
−35.9 −19.9 −37 −27.7 −34.3 −37 −37
−22.1 −3.1 −10 −14.1 −37.6 −28.9 −36.5
18.7 22.1 39.2 16.1 21.8 28.3 24.8
22.9 6.2 14.7 4.6 7.7 8.7 22.5
4.4 0.6 4.9 3.4 7.3 7.7 7.8
M. Mori et al. / Sensors and Actuators B 143 (2009) 56–61
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Fig. 6. Three-dimensional plot of the EMF values measured at 400, 450 and 500 ◦ C.
reports the sensitivity, defined as the slope (mV/decade) of the linear fits of the EMF data reported in Fig. 5. The sensitivity resulted in the order: ethanol o-xylene > toluene, MEK, acetic acid, p-xylene > benzene at 400 ◦ C. Among those VOCs, a distinctly large sensitivity was observed for ethanol (−54.1 mV/decade). It is predicted that ethanol is the most catalytically activated species among the VOCs for the oxidation reaction. At 500 ◦ C, the sensitivity to all the VOCs decreased, and the sensitivity order changed to: toluene, p-xylene > o-xylene > acetic acid > benzene, ethanol, MEK (Table 2). The reduction of sensitivity by elevating temperature is probably caused by the enhancement of desorption of adsorbed oxygen on the surface of Pt electrode. In particular, the sensitivity to ethanol remarkably decreased, being less than one-fifth of the value measured at 400 ◦ C. This finding may suggest that, at higher temperature, ethanol mainly reacts outside of the three-phase boundary, such as in gas phase, resulting in a minor contribution of ethanol to the sensing reactions. The observed large temperature dependence of sensitivity, distinct for each of the VOCs, suggested us the possibility to discriminate the various VOCs using a combination of EMF data measured at different temperatures. Fig. 6 shows a three-dimensional plot that reports in the x, y, and z axes the EMF measured at 400, 450 and 500 ◦ C, respectively. The values in the graph were determined by taking for each point the EMF values measured at the three temperatures for the same concentration of a given VOC, and thus each point of the graph represents a different VOC concentration. Each VOC trace can be well separated and the three-dimensional analysis enhances discrimination of each VOC especially at large EMF values, which represents VOC concentrations above 3 ppm.
3.2. Sensing mechanisms
Fig. 5. Concentration dependence of the EMF at 400 ◦ C (a and b) and 450 ◦ C (c and d).
Considering the VOC sensing mechanisms, the following reactions can be considered. If a reaction between the VOC and oxygen molecules adsorbed on the Pt electrode surface is involved in the sensing mechanism, the reaction probably proceeds according to the Langmuir–Hinshelwood (L–H) mechanism and/or to the Eley–Rideal (E–R) mechanism [26,27]. The former proceeds by a two molecular reaction between the adsorbed VOC and oxygen molecules, whereas the latter proceeds by a VOC in gaseous phase attaching to the primarily adsorbed oxygen molecules (or vice versa). In both cases, the VOC exposure causes a decrease in the concentration of oxygen adsorbed on the Pt electrode surface, simultaneously resulting in a given EMF change. Furthermore, a contribution of the YSZ lattice oxygen should be also considered.
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Fig. 7. Surface model for adsorption of VOC molecules and reactions with oxygen.
Consequently, the following processes are expected to occur. O2 (g) → O2 (ad)
(adsorption of oxygen)
O2 (g) + VOC(g) → oxidized products O2 (ad) + 4e− → 2O2− (lattice)
(combustion in gas phase)
(electrochemical reduction)
O2 (ad) + VOC(g) → O2 (g) + VOC(ad) VOC(ad) + O2 (ad) → oxidized products
(oxygen replacement) Fig. 8. Correlation between the EMF values observed at the different temperatures.
(L–H mechanism)
VOC(g) + O2 (ad) → oxidized products (E–R mechanism) VOC(ad, g) + nO2− (lattice) → oxidized products + 2ne− (electrochemical oxidation) Fig. 7 shows a scheme of the surface model for the adsorption of VOC molecules and the reactions with oxygen species, summarizing all the mentioned processes. Three types of oxygen species, i.e., gas [O2 (g)], adsorbed oxygen [O2 (ad)], and dissociated oxygen [O2− ] present in the interlayer between Pt electrode and 8YSZ are taken into account. Dissociated oxygen directly contributes to the oxygen activity at the three-phase boundary (TPB). The oxygen at TPB should be in equilibrium with the oxygen on the Pt electrode surface, O2 (ad), and the oxygen in the gas phase, O2 (g). Therefore, the Nernst equation in Eq. (1) can be rewritten as the following:
EMF =
ah (O2− ) RT ln 2F al (O2− )
(3)
where ah and al are the higher and lower oxygen activities at the electrode TPB, respectively. Therefore, an EMF response upon exposure to VOCs can be generated only if a certain degree of change in oxygen activity occurs. This can be achieved according to two possible processes: (1) the reduction of the number of oxygen adsorption sites (decrease in oxygen activity at the electrode) due to the adsorption of VOCs, and (2) a mixed potential at the TPB generated by the coexistence of electrochemical reduction of oxygen and electrochemical oxidation of VOCs. As a consequence of both processes, oxygen activity at the TPB, i.e., at the electrolyte/electrode/gas interface, strongly depends on physical and chemical surface properties, like the number of adsorption sites or catalytic activity, and on the reaction kinetics at the electrode surface. A direct oxidation (combustion) reaction between oxygen and VOC taking place outside the TPB, i.e., in the gas phase, is also possible. However, this is not likely to be the rate determining
process for the sensing reaction, because the VOCs concentration is just at ppm levels and is vanishingly small compared to the oxygen concentration. For the same reason, the reactions associated with VOC(g) can be also excluded. Nonetheless, a significant EMF value of several tenths of mV was observed at 400 ◦ C in ambient containing 1 ppm of VOCs. This suggests that the EMF response can be observed only when due to: (1) a reaction between VOC(ad) and O2 (ad) on the electrode surface (L–H mechanism), (2) a replacement of the adsorbed oxygen by a VOC molecule, or (3) a reaction between VOC(ad) and O2− (lattice). As shown in Fig. 8a and b, the absolute value of EMF, |EMF|, decreased with increasing the temperature from 400 to 500 ◦ C. At higher temperatures, the adsorption/desorption equilibrium of VOCs shifts towards desorption, resulting in a reduced contribution of VOCs to the sensing reaction, especially under very low VOC concentrations, and then |EMF| is reduced. In this case, a large effect on the |EMF| reduction is also due to O2 (ad) desorption, hence the reduction of molecular oxygen coverage. On the other hand, the electrochemical oxygen reduction reaction, O2 (ad) + 4e− → 2O2− (lattice), will be promoted with increasing the temperature, due to the fact that the O2− (lattice) species are thermally activated and more reactive with VOCs at high temperatures. Therefore, reaction (3) would increase |EMF| with a temperature increase. Thus, we conclude that the reaction between VOC and O2− (lattice) is a minor process in the temperature range investigated, 400–500 ◦ C. This suggests that a mixed potential is a minor component in the output of the potentiometric sensor. The degree of the EMF deviation from the Nernst value with changing the temperature was strongly dependent on the various VOCs. From the above discussion, this difference can be attributed to the difference in the adsorption/desorption dynamics of the tested VOCs. Therefore, the sensors showed a sharp decrease in |EMF| with increasing the temperature upon exposure to ethanol and MEK, which are better ready to decrease their coverage on the Pt surface at higher temperatures.
M. Mori et al. / Sensors and Actuators B 143 (2009) 56–61
4. Conclusions The effects of the contamination of several VOCs in air at ppm levels on the response of Pt|YSZ|Pt potentiometric oxygen sensors were investigated in the intermediate temperature region of 400–500 ◦ C. For all the examined VOCs, sufficiently large responses were obtained even at sub-ppm level. In particular ethanol exhibited the highest sensitivity at 400 ◦ C, but drastically decreased in its sensitivity at 500 ◦ C. For all the VOCs, the response and recovery times were shortened with increasing the temperature, whereas the EMF levels decreased. From the findings, it was deduced that the main processes for the sensing reaction are reaction (oxidation) and/or replacement of the surface adsorbed oxygen with VOC molecules. For all the examined VOCs, the EMF linearly increased with increasing the logarithm of VOC concentration. The combination of the EMF observed at 400, 450, and 500 ◦ C in a three-dimensional plot allows a preferential discrimination for each VOC, in particular for concentrations above 3 ppm. Therefore, the potentiometric oxygen sensor with Pt/YSZ/Pt structure is promising to detect several VOCs in contaminated air at ppm or even sub-ppm levels, at an operating temperature of around 450 ◦ C. Acknowledgements This study was subsidy supported by JST (Japan Science and Technology Agency). References [1] A.P. Jones, Indoor air quality and health, Atmos. Environ. 33 (1999) 4535–4564. [2] Y. Itagaki, M. Mori, Y. Hosoya, H. Aono, Y. Sadaoka, O3 and NO2 sensing properties of SmFe1−x CoxO3 perovskite oxide, Sens. Actuators B 122 (2007) 315–320. [3] T. Sasahara, H. Kato, A. Saito, M. Nishimura, M. Egashira, Development of a ppblevel sensor based on catalytic combustion for total volatile organic compounds in indoor air, Sens. Actuators B 126 (2) (2007) 536–543. [4] W.J. Fleming, Physical principles governing nonideal behavior of the zirconia oxygen sensor, J. Electrochem. Soc. 124 (1977) 21–28. [5] A. Vogel, G. Baier, V. Schüle, Non-Nernstian potentiometric zirconia sensors: screening of potential working electrode materials, Sens. Actuators B 15–16 (1993) 147–150. [6] G. Lu, N. Miura, N. Yamazoe, High-temperature hydrogen sensor based on stabilized zirconia and a metal oxide electrode, Sens. Actuators B 35–36 (1996) 130–135. [7] R. Sorita, T. Kawano, A highly selective CO sensor using LaMnO3 electrodeattached zirconia galvanic cell, Sens. Actuators B 40 (1997) 29–32. [8] N. Miura, T. Raisen, G. Lu, N. Yamazoe, Highly selective CO sensor using stabilized zirconia and a couple of oxide electrodes, Sens. Actuators B 47 (1998) 84–91. [9] T. Hibino, S. Kakimoto, M. Sano, Non-Nernstian behavior at modified Au electrodes for hydrocarbon gas sensing, J. Electrochem. Soc. 146 (1999) 3361–3366. [10] E.L. Brosha, R. Mukundan, D.R. Brown, F.H. Garzon, J.H. Visser, M. Zanini, Z. Zhou, E.M. Logothetis, CO/HC sensors based on thin films of LaCoO3 and La0.8 Sr0.2 CoO3−ı metal oxides, Sens. Actuators B 69 (2000) 171–182. [11] T. Hibino, A. Hashimoto, S. Kakimoto, M. Sano, Zirconia-based potentiometric sensors using metal oxide electrodes for detection of hydrocarbons, J. Electrochem. Soc. 148 (2001) H1–H5. [12] J.W. Yoon, M.L. Grilli, E. Di Bartolomeo, R. Polini, E. Traversa, The NO2 response of solid electrolyte sensors made using nano-sized LaFeO3 electrodes, Sens. Actuators B 76 (2001) 483–488. [13] M.L. Grilli, E. Di Bartolomeo, E. Traversa, Electrochemical NOx sensors based on interfacing nano-sized LaFeO3 perovskite-type oxide and ionic conductors, J. Electrochem. Soc. 148 (2001) H98–H102.
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[14] E.L. Brosha, R. Mukundan, D.R. Brown, F.H. Garzon, Mixed potential sensors using lanthanum manganate and terbium yttrium zirconium oxide electrodes, Sens. Actuators B 87 (2002) 47–57. [15] A. Dutta, N. Kaabbuathong, M.L. Grilli, E. Di Bartolomeo, E. Traversa, Study of YSZ-based electrochemical sensors with WO3 electrodes in NO2 and CO environments, J. Electrochem. Soc. 150 (2003) H33–H37. [16] E. Di Bartolomeo, M.L. Grilli, E. Traversa, Sensing mechanism of potentiometric gas sensors based on stabilized zirconia with oxide electrodes: is it always mixed potential? J. Electrochem. Soc. 151 (2004) H133–H139. [17] E. Di Bartolomeo, N. Kaabbuathong, A. D’Epifanio, M.L. Grilli, E. Traversa, H. Aono, Y. Sadaoka, Nano-structured perovskite oxide electrodes for planar electrochemical sensors using tape cast YSZ layers, J. Eur. Ceram. Soc. 24 (2004) 1187–1190. [18] E. Di Bartolomeo, N. Kaabbuathong, M.L. Grilli, E. Traversa, Planar electrochemical sensors based on tape-cast YSZ layers and oxide electrodes, Solid State Ionics 171 (2004) 183–190. [19] N.F. Szabo, P.K. Dutta, Correlation of sensing behavior of mixed potential sensors with chemical and electrochemical properties of electrodes, Solid State Ionics 171 (2004) 173–181. [20] N. Miura, M. Nakatou, S. Zhuiykov, Development of NOx sensing devices based on YSZ and oxide electrode aiming for monitoring car exhausts, Ceram. Int. 30 (2004) 1135–1139. [21] B. White, E. Traversa, E.D. Wachsman, Investigation of La2 CuO4 /YSZ/Pt potentiometric NOx sensors with electrochemical impedance spectroscopy, J. Electrochem. Soc. 155 (2008) J11–J16. [22] L. Chevallier, E. Di Bartolomeo, M.L. Grilli, M. Mainas, B. White, E.D. Wachsman, E. Traversa, Non-Nernstian planar sensors based on YSZ with a Nb2 O5 electrode, Sens. Actuators B 129 (2008) 591–598. [23] L. Chevallier, E. Di Bartolomeo, M.L. Grilli, E. Traversa, High temperature detection of CO/HCs gases with non-Nernstian planar sensors using Nb2 O5 electrodes, Sens. Actuators B 130 (2008) 514–519. [24] M. Mori, H. Nishimura, H. Yahiro, Y. Sadaoka, Potentiometric VOCs detection using 8YSZ based oxygen sensors, J. Ceram. Soc. Jpn. 116 (2008) 777–780. [25] D.S. Tannhauser, J.A. Kilner, B.C.H. Steele, The determination of the oxygen self-diffusion and gas–solid exchange coefficients for stabilized zirconia, Nucl. Instrum. Methods Phys. Res. 218 (1–3) (1983) 504–508. [26] N. Guillet, R. Lalauze, C. Pijolat, Oxygen and carbon monoxide role on the electrical response of a non-Nernstian potentiometric gas sensor; proposition of a model, Sens. Actuators B 98 (2004) 130–139. [27] E. McCarthy, J. Zahradnlk, G.C. Kuczynskl, J.J. Carberry, Some unique aspects of CO oxidation on supported Pt, J. Catal. 39 (1) (1975) 29–35.
Biographies Masami Mori received her B.Sci. degree in analytical chemistry from Ehime University in 2003. She obtained Dr.Eng. degree from Ehime University in 2008. She is research associate at department of materials science and engineering since 2003. Her main interest is nano-particle applications for chemical sensor and catalyst. Hiroyuki Nishimura received his M.Eng. degree in materials science and engineering from Ehime University in 2009. Yoshiteru Itagaki received his M.Eng. degree in industrial chemistry from Hiroshima University in 1995. He obtained Dr.Eng. degree from Hiroshima University in 1998. He is assistant professor at department of materials science and engineering since 2003. His main interests are inorganic and organic composites for chemical sensor and fuel cell. Yoshihiko Sadaoka received his M.Eng. degree in industrial chemistry from Ehime University in 1971. He has been on the faculty of engineering at Ehime University since 1971. He obtained Dr.Eng. degree from Kyushu University in 1979. He is a professor at department of materials science and engineering since 1996. His main interests are inorganic and organic functional materials for chemical sensor and green materials. Enrico Traversa received his “Laurea” (Italian doctoral degree) in chemical engineering from the University of Rome La Sapienza in 1986. In 1988, he joined the University of Rome Tor Vergata where he is currently professor of materials science and technology and director of the doctorate course in materials for environmental and energy applications. His research is focused on studies and development of solid oxide fuel cells, sensors, gas separation membranes and more recently materials for tissue engineering.
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Detection of sub-ppm level of VOCs based on a Pt/YSZ/Pt potentiometric oxygen sensor with reference air Masami Mori a , Hiroyuki Nishimura a , Yoshiteru Itagaki a , Yoshihiko Sadaoka a,∗ , Enrico Traversa b a b
Department of Materials Science and Biotechnology, Graduate School of Science and Engineering, Ehime University, 3 Bunkyo-cho, Matsuyama, Ehime 890-8577, Japan International Research Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan
a r t i c l e
i n f o
Article history: Received 12 February 2009 Received in revised form 29 July 2009 Accepted 1 September 2009 Available online 6 September 2009 Keywords: YSZ VOC sensor Oxygen sensor EMF Sub-ppm
a b s t r a c t Potentiometric oxygen sensors with Pt|YSZ|Pt structure were applied for detecting several volatile organic compounds (VOCs; acetic acid, methylethylketone, ethanol, benzene, toluene, o- and p-xylene) at sub-ppm levels in the temperature range of 400–500 ◦ C. The electromotive force (EMF) output from the sensors linearly changed with logarithm of VOC concentration, and sufficiently large output was confirmed at sub-ppm levels. Among the examined VOCs, ethanol exhibited the highest sensitivity (−54.1 mV/decade) at 400 ◦ C. However, the sensitivity of ethanol drastically decreased at 500 ◦ C. Thus the sensitivity of all the VOCs examined in this study was tunable with temperature. An increase in temperature decreased the sensitivity but effectively modified the response and recovery times. Analysis of multi-dimensional plots combining the sensor outputs at the different operating temperatures (400, 450 and 500 ◦ C) suggested the possibility to discriminate the different VOCs. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Since air quality contamination by volatile organic compounds (VOCs), especially indoor, has become a serious problem for health in several scenes of human life [1], their concentration in air needs to be strictly regulated to an extremely low level. Therefore, continuous monitoring of VOCs is now strongly demanded, especially in urban spaces. Among of the various sensing techniques used to detect such harmful gases [2,3], solid-state sensors are of practical interest due to their small size and ease of use [4]. Zirconia-based electrochemical sensors tailored for NOx , CO, and hydrocarbon detection have been widely investigated [4–24]. An interesting approach to electrochemically detect VOC gases is exploiting zirconia-based oxygen sensors due to their low cost and easy use. In a potentiometric method, the sensor output voltage can be evaluated from the Nernst equation assuming a reversible transfer of oxygen from cathode to anode. For a sensor having a simple pa (O2 ), Pt|YSZ|Pt, pc (O2 ) geometry, where pa (O2 ) and pc (O2 ) are the different oxygen partial pressures at the anode and the cathode, respectively, the Nernst equation can be written as follows: EMF =
p (O ) c 2
RT ln 4F pa (O2 )
∗ Corresponding author. E-mail address: [email protected] (Y. Sadaoka). 0925-4005/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2009.09.001
(1)
where R is the gas constant, T the absolute temperature and F the Faraday’s constant. The observed output voltage value can deviate from the theoretical one, estimated from Eq. (1), when reducing gases, such as VOCs, coexist with oxygen. In 1977, Fleming first interpreted this deviation from a viewpoint of non-Nernstian behavior, resulting from the simultaneous occurrence of two electrochemical reactions at the working electrode [4]. Since then, many research groups have exploited the non-Nernstian behavior to develop several electrochemical sensors. In this work, potentiometric oxygen sensors with a Pt|YSZ|Pt structure were fabricated and used to check the feasibility to detect sub-ppm levels of contaminating VOCs in air at an intermediate temperature region of 400–500 ◦ C. The results were promising and showed that a three-dimensional plot of the data can allow discrimination between the presence of different VOCs. The sensing mechanism was also discussed. 2. Experimental Illustration of the sensor device is shown in Fig. 1. 8YSZ sheets (8 mol% Y2 O3 /ZrO2 , Tosoh Co.) of 200 m in thickness were used as electrolytes. Pt electrodes were deposited on both 8YSZ sheet surfaces by magnetron sputtering. Au current collectors were attached onto a corner of the Pt sputtered electrodes using a Pt paste. The whole sensing elements were then heated to 600 ◦ C for 1 h. For measurements, each sensing element was fixed on the top of an alumina tube with an inorganic adhesive to separate the atmospheres surrounding the two Pt electrodes. One electrode side was
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Fig. 1. Illustration of the VOC sensor with Pt|YSZ|Pt structure.
exposed to synthetic air from a gas-cylinder and the other side was exposed to synthetic air contaminated by the different VOC gases, i.e., methylethylketone (MEK), acetic acid, ethanol, benzene, toluene, o- and p-xylene, at concentrations lower than 10 ppm. The air-balanced VOC gases were prepared with the permeator (Gastech G1) by mixing the VOC vapors through diffusion tubes at regulated temperatures with a cylinder air at prescribed flow rate ratios. The sensor element was placed in a temperature-controlled tubular furnace. The EMF change of the element was measured using a digital electrometer (Advantest R8340). 3. Results and discussion 3.1. VOC sensing response The surface morphology of the two sides of the supplied 8YSZ sheets was clearly different from each other, as easily detected by eyes. The surface of one side was more gloss/smooth than the surface of the other side and the difference of the surface morphology
57
could be confirmed by SEM observation as shown in Fig. 2. In this study, a more gloss surface was defined as SS and other surface as RS. A detailed study on the 8YSZ sheet surfaces and their effect in gas sensing properties has been reported in a previous paper [24]. It has been found that the different morphology has a strong influence on the sensing properties. When the Pt electrode deposited on the RS side was used as a sensing electrode, a faster and more stable response to oxygen concentration changes was observed. Moreover, the electron number for the oxygen sensing reaction evaluated at 400 ◦ C in the 0.1–100% oxygen concentration range was 4 ± 0.2, close to the theoretical value. Therefore, in this work the sensor elements were prepared using the Pt electrode sputtered on the RS side as a sensing electrode for oxygen and VOCs, whereas the Pt electrode sputtered on the SS side was used as a reference electrode. When electrodes were prepared on the same surface and exposed simultaneously to the same atmosphere, negligible EMFs were measured. Fig. 3 shows the EMF response at 400 and 450 ◦ C of the Pt|YSZ|Pt sensor to different ethanol concentrations. In these experiments, the sensor exposed to uncontaminated air and then a contaminated air with ethanol. After repeated two times, the concentration of ethanol was changed from 0.5 to 10 ppm in steps. After the measurements for a series of VOCs, the temperature changed. Good repeatable curves for ethanol were observed even after the measurements of a series of VOCs at the constant temperature (after about 60 h of the first measurement). When both side of the sensor were exposed to the air, a negative EMF value of −25 mV was detected at 400 ◦ C, deviating from the expected zero value. The different potential values can be explained in terms of the different surface structure and morphology of the two sides of 8YSZ sheets, which might affect the redox reaction kinetics. The negative EMF output, despite both sides of the sensing element were exposed to the same atmosphere, indicates that the sensing electrode (RS side) was more cathodic with respect to the reference electrode. This is possibly due to the morphology-induced slightly larger oxygen ion equilibrium activity at the RS surface than that at the SS side [24]. The EMF offset in air decreased from (−) 25 to (−) 6 mV with increasing the temperature up to 500 ◦ C. This change may be due to
Fig. 2. SEM photographs of the rough surfaces (RS) and smooth (SS) of the 8YSZ sheet.
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M. Mori et al. / Sensors and Actuators B 143 (2009) 56–61
Fig. 3. Response to the application of cycles of air and ethanol/air at 400 ◦ C (a) and 450 ◦ C (b).
Fig. 4. Response to 0.5 ppm VOCs at 400 ◦ C. The test gas changed from air to 0.5 ppm VOC/air and then returned.
the increases in ionic conductivity of the 8YSZ sheet with the temperature. The absolute value of EMF became close to zero and this might be explained by the fact that the kinetic difference, due to the difference in surface morphologies of the two sides, might diminish because of thermal activation of the oxygen exchange reaction and/or of an increase in 8YSZ conductivity [25]. Concentration-dependent EMF changes were measured by exposing the sensor to VOCs. Fig. 4 shows the sensor response to 0.5 ppm VOCs at 400 ◦ C. The EMF response level at 0.5 ppm is large enough for reliable measurements. Table 1 summarizes the 90% response (air → VOC) and recovery (VOC → air) times in ethanol, measured at 400 and 500 ◦ C. Both the times tend to increase with the increase in the ethanol concentration. The 90% recovery times are 2.0–2.4 times longer than the response time. This trend suggests that VOC or oxidized species strongly bound on the surface of the Pt electrode and it slows the replacement by an oxygen molecule. Increasing the temperature up to 500 ◦ C resulted in considerably shortened response and recovery times. Recovery times became almost half compared to those at 400 ◦ C. On the other hand response drastically shortened to be 2.5–25% of those at 400 ◦ C. This
result supports that large energy is required for VOC (or oxidized species) desorption. Similar response and recovery behaviors were also observed for all the tested VOCs. To eliminate the effects of the offset measured in air, the difference between the EMF measured upon exposure to VOCs and in air, EMF, was defined as in Eq. (2).
Table 1 90% response and recovery times to ethanol. Ethanol concentration (ppm)
0.5 1 3 7 10
90% response time (s)
90% recovery time (s)
400 ◦ C
500 ◦ C
400 ◦ C
500 ◦ C
155 185 185 180 204
4 10 15 45 30
355 364 415 435 480
240 190 245 205 245
EMF = EMFVOC − EMFair
(2)
Fig. 5 shows the concentration dependence of the EMF of the Pt|8YSZ|Pt sensor upon exposure to several VOCs. The EMF changed linearly with the logarithm of the VOC concentration, EMF = a + b log C, where a is the EMF at 1 ppm of VOC, b the sensitivity in mV/decade and C the concentration of VOC. Table 2 Table 2 Sensing characteristics of the sensor to VOCs.
Sensitivity (mV/decade) Acetic acid MEK Ethanol Benzene Toluene o-Xylene p-Xylene EMF/mV at 1 ppm VOCs Acetic acid MEK Ethanol Benzene Toluene o-Xylene p-Xylene
400 ◦ C
450 ◦ C
500 ◦ C
−38.2 −38.5 −54.1 −30.9 −38.7 −41.5 −37.1
−35.9 −19.9 −37 −27.7 −34.3 −37 −37
−22.1 −3.1 −10 −14.1 −37.6 −28.9 −36.5
18.7 22.1 39.2 16.1 21.8 28.3 24.8
22.9 6.2 14.7 4.6 7.7 8.7 22.5
4.4 0.6 4.9 3.4 7.3 7.7 7.8
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Fig. 6. Three-dimensional plot of the EMF values measured at 400, 450 and 500 ◦ C.
reports the sensitivity, defined as the slope (mV/decade) of the linear fits of the EMF data reported in Fig. 5. The sensitivity resulted in the order: ethanol o-xylene > toluene, MEK, acetic acid, p-xylene > benzene at 400 ◦ C. Among those VOCs, a distinctly large sensitivity was observed for ethanol (−54.1 mV/decade). It is predicted that ethanol is the most catalytically activated species among the VOCs for the oxidation reaction. At 500 ◦ C, the sensitivity to all the VOCs decreased, and the sensitivity order changed to: toluene, p-xylene > o-xylene > acetic acid > benzene, ethanol, MEK (Table 2). The reduction of sensitivity by elevating temperature is probably caused by the enhancement of desorption of adsorbed oxygen on the surface of Pt electrode. In particular, the sensitivity to ethanol remarkably decreased, being less than one-fifth of the value measured at 400 ◦ C. This finding may suggest that, at higher temperature, ethanol mainly reacts outside of the three-phase boundary, such as in gas phase, resulting in a minor contribution of ethanol to the sensing reactions. The observed large temperature dependence of sensitivity, distinct for each of the VOCs, suggested us the possibility to discriminate the various VOCs using a combination of EMF data measured at different temperatures. Fig. 6 shows a three-dimensional plot that reports in the x, y, and z axes the EMF measured at 400, 450 and 500 ◦ C, respectively. The values in the graph were determined by taking for each point the EMF values measured at the three temperatures for the same concentration of a given VOC, and thus each point of the graph represents a different VOC concentration. Each VOC trace can be well separated and the three-dimensional analysis enhances discrimination of each VOC especially at large EMF values, which represents VOC concentrations above 3 ppm.
3.2. Sensing mechanisms
Fig. 5. Concentration dependence of the EMF at 400 ◦ C (a and b) and 450 ◦ C (c and d).
Considering the VOC sensing mechanisms, the following reactions can be considered. If a reaction between the VOC and oxygen molecules adsorbed on the Pt electrode surface is involved in the sensing mechanism, the reaction probably proceeds according to the Langmuir–Hinshelwood (L–H) mechanism and/or to the Eley–Rideal (E–R) mechanism [26,27]. The former proceeds by a two molecular reaction between the adsorbed VOC and oxygen molecules, whereas the latter proceeds by a VOC in gaseous phase attaching to the primarily adsorbed oxygen molecules (or vice versa). In both cases, the VOC exposure causes a decrease in the concentration of oxygen adsorbed on the Pt electrode surface, simultaneously resulting in a given EMF change. Furthermore, a contribution of the YSZ lattice oxygen should be also considered.
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Fig. 7. Surface model for adsorption of VOC molecules and reactions with oxygen.
Consequently, the following processes are expected to occur. O2 (g) → O2 (ad)
(adsorption of oxygen)
O2 (g) + VOC(g) → oxidized products O2 (ad) + 4e− → 2O2− (lattice)
(combustion in gas phase)
(electrochemical reduction)
O2 (ad) + VOC(g) → O2 (g) + VOC(ad) VOC(ad) + O2 (ad) → oxidized products
(oxygen replacement) Fig. 8. Correlation between the EMF values observed at the different temperatures.
(L–H mechanism)
VOC(g) + O2 (ad) → oxidized products (E–R mechanism) VOC(ad, g) + nO2− (lattice) → oxidized products + 2ne− (electrochemical oxidation) Fig. 7 shows a scheme of the surface model for the adsorption of VOC molecules and the reactions with oxygen species, summarizing all the mentioned processes. Three types of oxygen species, i.e., gas [O2 (g)], adsorbed oxygen [O2 (ad)], and dissociated oxygen [O2− ] present in the interlayer between Pt electrode and 8YSZ are taken into account. Dissociated oxygen directly contributes to the oxygen activity at the three-phase boundary (TPB). The oxygen at TPB should be in equilibrium with the oxygen on the Pt electrode surface, O2 (ad), and the oxygen in the gas phase, O2 (g). Therefore, the Nernst equation in Eq. (1) can be rewritten as the following:
EMF =
ah (O2− ) RT ln 2F al (O2− )
(3)
where ah and al are the higher and lower oxygen activities at the electrode TPB, respectively. Therefore, an EMF response upon exposure to VOCs can be generated only if a certain degree of change in oxygen activity occurs. This can be achieved according to two possible processes: (1) the reduction of the number of oxygen adsorption sites (decrease in oxygen activity at the electrode) due to the adsorption of VOCs, and (2) a mixed potential at the TPB generated by the coexistence of electrochemical reduction of oxygen and electrochemical oxidation of VOCs. As a consequence of both processes, oxygen activity at the TPB, i.e., at the electrolyte/electrode/gas interface, strongly depends on physical and chemical surface properties, like the number of adsorption sites or catalytic activity, and on the reaction kinetics at the electrode surface. A direct oxidation (combustion) reaction between oxygen and VOC taking place outside the TPB, i.e., in the gas phase, is also possible. However, this is not likely to be the rate determining
process for the sensing reaction, because the VOCs concentration is just at ppm levels and is vanishingly small compared to the oxygen concentration. For the same reason, the reactions associated with VOC(g) can be also excluded. Nonetheless, a significant EMF value of several tenths of mV was observed at 400 ◦ C in ambient containing 1 ppm of VOCs. This suggests that the EMF response can be observed only when due to: (1) a reaction between VOC(ad) and O2 (ad) on the electrode surface (L–H mechanism), (2) a replacement of the adsorbed oxygen by a VOC molecule, or (3) a reaction between VOC(ad) and O2− (lattice). As shown in Fig. 8a and b, the absolute value of EMF, |EMF|, decreased with increasing the temperature from 400 to 500 ◦ C. At higher temperatures, the adsorption/desorption equilibrium of VOCs shifts towards desorption, resulting in a reduced contribution of VOCs to the sensing reaction, especially under very low VOC concentrations, and then |EMF| is reduced. In this case, a large effect on the |EMF| reduction is also due to O2 (ad) desorption, hence the reduction of molecular oxygen coverage. On the other hand, the electrochemical oxygen reduction reaction, O2 (ad) + 4e− → 2O2− (lattice), will be promoted with increasing the temperature, due to the fact that the O2− (lattice) species are thermally activated and more reactive with VOCs at high temperatures. Therefore, reaction (3) would increase |EMF| with a temperature increase. Thus, we conclude that the reaction between VOC and O2− (lattice) is a minor process in the temperature range investigated, 400–500 ◦ C. This suggests that a mixed potential is a minor component in the output of the potentiometric sensor. The degree of the EMF deviation from the Nernst value with changing the temperature was strongly dependent on the various VOCs. From the above discussion, this difference can be attributed to the difference in the adsorption/desorption dynamics of the tested VOCs. Therefore, the sensors showed a sharp decrease in |EMF| with increasing the temperature upon exposure to ethanol and MEK, which are better ready to decrease their coverage on the Pt surface at higher temperatures.
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4. Conclusions The effects of the contamination of several VOCs in air at ppm levels on the response of Pt|YSZ|Pt potentiometric oxygen sensors were investigated in the intermediate temperature region of 400–500 ◦ C. For all the examined VOCs, sufficiently large responses were obtained even at sub-ppm level. In particular ethanol exhibited the highest sensitivity at 400 ◦ C, but drastically decreased in its sensitivity at 500 ◦ C. For all the VOCs, the response and recovery times were shortened with increasing the temperature, whereas the EMF levels decreased. From the findings, it was deduced that the main processes for the sensing reaction are reaction (oxidation) and/or replacement of the surface adsorbed oxygen with VOC molecules. For all the examined VOCs, the EMF linearly increased with increasing the logarithm of VOC concentration. The combination of the EMF observed at 400, 450, and 500 ◦ C in a three-dimensional plot allows a preferential discrimination for each VOC, in particular for concentrations above 3 ppm. Therefore, the potentiometric oxygen sensor with Pt/YSZ/Pt structure is promising to detect several VOCs in contaminated air at ppm or even sub-ppm levels, at an operating temperature of around 450 ◦ C. Acknowledgements This study was subsidy supported by JST (Japan Science and Technology Agency). References [1] A.P. Jones, Indoor air quality and health, Atmos. Environ. 33 (1999) 4535–4564. [2] Y. Itagaki, M. Mori, Y. Hosoya, H. Aono, Y. Sadaoka, O3 and NO2 sensing properties of SmFe1−x CoxO3 perovskite oxide, Sens. Actuators B 122 (2007) 315–320. [3] T. Sasahara, H. Kato, A. Saito, M. Nishimura, M. Egashira, Development of a ppblevel sensor based on catalytic combustion for total volatile organic compounds in indoor air, Sens. Actuators B 126 (2) (2007) 536–543. [4] W.J. Fleming, Physical principles governing nonideal behavior of the zirconia oxygen sensor, J. Electrochem. Soc. 124 (1977) 21–28. [5] A. Vogel, G. Baier, V. Schüle, Non-Nernstian potentiometric zirconia sensors: screening of potential working electrode materials, Sens. Actuators B 15–16 (1993) 147–150. [6] G. Lu, N. Miura, N. Yamazoe, High-temperature hydrogen sensor based on stabilized zirconia and a metal oxide electrode, Sens. Actuators B 35–36 (1996) 130–135. [7] R. Sorita, T. Kawano, A highly selective CO sensor using LaMnO3 electrodeattached zirconia galvanic cell, Sens. Actuators B 40 (1997) 29–32. [8] N. Miura, T. Raisen, G. Lu, N. Yamazoe, Highly selective CO sensor using stabilized zirconia and a couple of oxide electrodes, Sens. Actuators B 47 (1998) 84–91. [9] T. Hibino, S. Kakimoto, M. Sano, Non-Nernstian behavior at modified Au electrodes for hydrocarbon gas sensing, J. Electrochem. Soc. 146 (1999) 3361–3366. [10] E.L. Brosha, R. Mukundan, D.R. Brown, F.H. Garzon, J.H. Visser, M. Zanini, Z. Zhou, E.M. Logothetis, CO/HC sensors based on thin films of LaCoO3 and La0.8 Sr0.2 CoO3−ı metal oxides, Sens. Actuators B 69 (2000) 171–182. [11] T. Hibino, A. Hashimoto, S. Kakimoto, M. Sano, Zirconia-based potentiometric sensors using metal oxide electrodes for detection of hydrocarbons, J. Electrochem. Soc. 148 (2001) H1–H5. [12] J.W. Yoon, M.L. Grilli, E. Di Bartolomeo, R. Polini, E. Traversa, The NO2 response of solid electrolyte sensors made using nano-sized LaFeO3 electrodes, Sens. Actuators B 76 (2001) 483–488. [13] M.L. Grilli, E. Di Bartolomeo, E. Traversa, Electrochemical NOx sensors based on interfacing nano-sized LaFeO3 perovskite-type oxide and ionic conductors, J. Electrochem. Soc. 148 (2001) H98–H102.
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Biographies Masami Mori received her B.Sci. degree in analytical chemistry from Ehime University in 2003. She obtained Dr.Eng. degree from Ehime University in 2008. She is research associate at department of materials science and engineering since 2003. Her main interest is nano-particle applications for chemical sensor and catalyst. Hiroyuki Nishimura received his M.Eng. degree in materials science and engineering from Ehime University in 2009. Yoshiteru Itagaki received his M.Eng. degree in industrial chemistry from Hiroshima University in 1995. He obtained Dr.Eng. degree from Hiroshima University in 1998. He is assistant professor at department of materials science and engineering since 2003. His main interests are inorganic and organic composites for chemical sensor and fuel cell. Yoshihiko Sadaoka received his M.Eng. degree in industrial chemistry from Ehime University in 1971. He has been on the faculty of engineering at Ehime University since 1971. He obtained Dr.Eng. degree from Kyushu University in 1979. He is a professor at department of materials science and engineering since 1996. His main interests are inorganic and organic functional materials for chemical sensor and green materials. Enrico Traversa received his “Laurea” (Italian doctoral degree) in chemical engineering from the University of Rome La Sapienza in 1986. In 1988, he joined the University of Rome Tor Vergata where he is currently professor of materials science and technology and director of the doctorate course in materials for environmental and energy applications. His research is focused on studies and development of solid oxide fuel cells, sensors, gas separation membranes and more recently materials for tissue engineering.