Solid State Ionics xxx (2013) xxx–xxx

SOSI-13071; No of Pages 5

Contents lists available at ScienceDirect

Solid State Ionics

j ourna l homepage: www.e lsev ie r .com/ locate /ss i

CO2 and SO2 tolerant Fe-doped metal oxides for solid state gas sensors

Suresh Mulmi, Ramaiyan Kannan, Venkataraman Thangadurai ⁎University of Calgary, Department of Chemistry, 2500 University Drive NW, Calgary, AB T2N 1N 4, Canada

⁎ Corresponding author. Tel.:+1403 210 8649; fax:+1 4E-mail address: vthangad@ucalgary.ca (V. Thangadura

0167-2738/$ – see front matter © 2013 Elsevier B.V. All rihttp://dx.doi.org/10.1016/j.ssi.2013.09.050

Please cite this article as: S. Mulmi, et al., Sol

a b s t r a c t

a r t i c l e i n f o

Article history:Received 17 May 2013Received in revised form 13 September 2013Accepted 25 September 2013Available online xxxx

Keywords:Resistive CO2 sensorsSO2 sensorsPerovskitesMixed conductorsBa2Ca0.66Nb0.68Fe0.66O6 − δ (BCNF)BaMg0.33Nb0.34Fe0.33O3 − δ (BMNF)

Perovskite-type BaMg0.33Nb0.34Fe0.33O3 − δ (BMNF) and double perovskite-type Ba2Ca0.66Nb0.68Fe0.66O6 − δ

(BCNF) were prepared by conventional solid-state reaction at 1400 °C in air. Ex-situ powder X-ray diffraction(PXRD) and high temperature PXRD (30–700 °C) of BCNF and BMNF under CO2 and SO2 exhibited excellentchemical stability. Chemical stability under SO2 was further confirmed by scanning electron microscopy (SEM)combined with energy dispersive X-ray analysis (EDX), where no sulfur-containing compounds were detectedwithin the detection limit of the equipment. BCNF and BMNF were tested for their sensing properties in CO2

and SO2 at 500–700 °C by applying constant DC voltage (0.1 V). Ag and Pt current collectors were employed aselectrodes for BCNF and showed similar response at 600 °C (t90= 7min) for ppm level of CO2 in dry syntheticair. BCNF sensor was found highly stable and very selective toward CO2 in presence of SO2 (0– 20 ppm)

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

Recently, detecting greenhouse gases has become essential to con-trol the damage they cause to the environment and quality of life[1–3]. The demand for convenient and low-cost sensor for continuouslymonitoring gaseous emissions from various processes is steeply grow-ing. Monitoring or detecting the concentration of gases in the environ-ment is generally based on optical, chromatography, and spectroscopymethods. These techniques are expensive to use and real-time on-linemonitoring at high temperature is not practical since gaseous speciesneed to be cooled down and require dust-free environments to operate.Hence, there is a need to develop alternative gas sensing technologieswhich can provide reliable and reproducible sensing results.

A practical sensor should be compact, easy to fabricate and minia-turize, inexpensive, and should provide rapid response relative tothe targeted gas in the presence of other gases [4–7]. Solid-state ionicconductor-based electrochemical sensors fall in this category, whichgive electrical signals that can be directly related to the gas present inthe system. The best-known example is a widely used commercial O2

sensor based on oxide ion conducting yttria-stabilized zirconia (YSZ).Fast alkali ion conducting Na3Zr2Si2PO12 has highly been considered fordeveloping CO2 and SO2 gas sensors using carbonate and sulfateauxiliary electrodes, respectively [7–10]. Yamazoe et al. [11,12] usedbinary carbonate and sulfate auxiliary phases such as Na2CO3–BaCO3

and Na2SO4–BaSO4 as sensing electrode to avoid humidity effect andchemical degradation [13,14]; however, the reliability, reproducibilityand long-term stability have remained as major challenges. Ishiharaet al. [15] reported solid state thin film CO2 (ppm level to 10%) sensors

03 210 9364,+1403 289 9488.i).

ghts reserved.

id State Ionics (2013), http://

based on semiconducting BaTiO3–CuO, where high resistivity, re-producibility and durability issues still persist. Recently, we haveemployed mixed conducting perovskite-type BaMg0.33Nb0.67− xFexO3− δ

and double perovskite-type Ba2Ca0.66Nb1.34 − xFexO6 − δ for detectionof CO2 (ppm level) in dry synthetic air using Au current collectors[16–18]. Here, we report the role of current collectors on sensing CO2,and investigate sensing SO2 in dry synthetic air at elevated temperatureusing BaMg0.33Nb0.34Fe0.33O3 − δ (BMNF) and Ba2Ca0.66Nb0.68Fe0.66O6 − δ

(BCNF).

2. Experimental

2.1. Preparation and characterization

Oxides of the nominal chemical formula of Ba2Ca0.66Nb0.68Fe0.66O6− δ

(BCNF) and BaMg0.33Nb0.34Fe0.33O3−δ (BMNF)were preparedusing high-temperature solid-state (ceramic) synthesis method. Stoichiometricamounts of Ba(NO3)2 (99+ %, Alfa Aesar), CaCO3 (99% Fisher ScientificCompany), MgCO3 (J. T. Baker®), Nb2O5 (99.5%, Alfa Aesar) and Fe2O3

(99+%, Alfa Aesar) were mixed together with 2-propanol in a zirconiabowl and ball milled (Pulverisette Fritsch, Germany) for 6 h at200 rpm, followed by firing at 1000 °C in air for 12 h. The sampleswere then ground using agate mortar and pestle until fine powder ofthe compoundwas obtained andpressed isostatically into dense pellets.BCNF pellet was sintered at 1400 °C, while BaMg0.33Nb0.34Fe0.33O3 − δ

(BMNF) was sintered at 1300 °C. BMNF samples were ball milledagain by adding 2-propanol followed by isostatic pellet pressing andre-sintered at 1200 °C in air.

The sampleswere characterized by powder X-ray diffraction (PXRD)(Bruker D8 powder X-ray diffractometer) (Cu Kα) (2θ = 10°–80°;scan rate of 0.02o and a counting time of 6 s per scan) at room

dx.doi.org/10.1016/j.ssi.2013.09.050

Fig. 2. In-situ high temperature PXRD of (a) BCNF [17] and (b) BMNF at 30 °C, 500 °C and700 °C under the flow of 1% CO2 balanced in dry synthetic air.

2 S. Mulmi et al. / Solid State Ionics xxx (2013) xxx–xxx

temperature. The high temperature PXRD (HT-PXRD) patterns werecollected at 30 °C, 500 °C and 700 °C (at 6 °C/min) in the presence of1% CO2 balanced in dry synthetic air (21% O2 in N2) using PXRDequipped with variable temperature and atmosphere stage (AntonPar HTK2000). The gas flow rate was maintained at 100 sccm. Scan-ning electron microscopy (SEM, Philips XL30 SEM) coupled with anenergy-dispersive X-ray analysis (EDX) was used for the determina-tion of chemical composition and microstructure characterization. Sam-ples were crushed into fine powders and treated in 30 ppm SO2 gas indry synthetic air at 700 °C for 72 h and resultant product was examinedusing PXRD.

2.2. Sensor fabrication and electrical measurements

Sintered pellets of BCNF and BMNF having average diameter of 1cmwere cut into ~1mm thickness using diamond saw (Buehler, Isomet®5000). The circular pellets were used for two-electrode electrochemicalsensor fabrication. Au, Ag, and Pt pastes (Heraeus Inc., LP A88-11 SGermany) were painted on both sides of the investigated metal oxides.Au and Ag pasted perovskites (BCNF and BMNF) were fired at 600 °Cand Pt pasted electrolyte at 800 °C for 2 h in air to remove the organicbinders. The pellets were kept inside a gas-tight quartz cell and heatedusing a Barnstead tubular furnace (model no. 21100) until it reachedthe desired temperature (Fig. 1). The required ratio of CO2 gas was pre-pared by diluting 3000 ppm CO2 with dry synthetic air (or N2 + O2)(Praxair, Inc., Canada). The total flow rate of sample gas or the base N2

gas was adjusted at 100 sccm by means of mass flow controllers(MCS-100 SCCM-D/5M, 5IN). A constant dc voltage of 0.1V was applied(Solartron, SI 1287) to obtain the response current detecting CO2

concentration.

3. Results and discussion

3.1. Structure and chemical stability

Fig. 2 shows the in situ HT-PXRD structural patterns of Ba2Ca0.66Nb0.68Fe0.66O6 − δ (BCNF) and BaMg0.33Nb0.34Fe0.33O3 − δ (BMNF) at30 °C, 500 °C and 700 °C with the continuous flow of 1% CO2 balancedin dry synthetic air (21% O2 in N2). The PXRD patterns for BCNF andBMNF are attributed to their parent compound Ba2Ca0.66Nb1.34O6

(BCN) and BaMg0.33Nb0.67O3 (BMN) structure [16,17]. Ca and Nb or-dered at the B-site shows double perovskite structure with spacegroup Fm-3m (No. 221), while Mg exhibits disordered perovskite withspace group Pm-3m (No. 221). Although, both compounds vary in thestructure (B-site order/disorder), they provide excellent stabilityunder 1% CO2 at elevated temperature.

SEM images for pellet samples of as-prepared and after 30ppm SO2

in synthetic air (700°C for 72h) exposed Fe-doped BCNF and BMNF areshown in Fig. 3(a). No traces of sulfur containing compound were de-tected using EDX combined with SEM for BCNF and BMNF samplesafter SO2 treatment. Furthermore, ex-situ PXRD patterns before andafter exposure of SO2 show the similar diffraction patterns (Figs. 2a,

Fig. 1. Schematic diagram illustrating an experimental set up for Ba2Ca0.66Nb0.68Fe0.66O6-δ

(BCNF) and BaMg0.33Nb0.34Fe0.33O3-δ (BMNF) based CO2 sensor.

Please cite this article as: S. Mulmi, et al., Solid State Ionics (2013), http://

2b and 3b). The absence of secondary phase in BCNF and BMNF afterSO2 treatment comparing to their parent phase (Fig. 2) indicates thatboth samples are chemically stable under SO2. PXRD data was alsoused to calculate the theoretical density (ρt) as:

ρt ¼M·ZV·N

ð1Þ

where, M, Z, V and N are molecular weight, number of formula unitsper unit cell, volume of the unit cell and Avogadro's constant respec-tively. ρA, Archimedes density, was calculated by:

ρA ¼ Mdry

Msat·Msusð2Þ

where, Mdry is dry weight of pellet in air after drying by gently wip-ing out water from pellet's surface. Msat is the weight of pellet inwater and Msus in air before weighing Mdry. For determining ρA,the pellets were immersed in boiled water for few hours prior tomeasuring the weights (Msat, Msus, Mdry) for letting the water topenetrate the finest pores. The theoretical densities (ρt) are foundto be 5.53 g/cm3 and 6.09 g/cm3 and the Archimedes densities (ρA)5.12 g/cm3 and 5.10 g/cm3 for BCNF and BMNF, respectively.

dx.doi.org/10.1016/j.ssi.2013.09.050

Fig. 3. (a) Typical SEM images with EDX results of as-prepared and SO2 treated BCNF andBMNF samples, (b) ex-situ PXRD of SO2 treated BCNF and BMNF samples at 700 °C.

Fig. 4. (a) Response and recovery transients of BCNF at 600 °C using Ag (solid line) and Pt(broken line) electrodes. (b) The steady-state current density vs. log CO2 using Au [17], Ptand Ag electrodes at 600 °C (Applied voltage= 0.1 V, total gas flow=100 sccm).

3S. Mulmi et al. / Solid State Ionics xxx (2013) xxx–xxx

3.2. CO2 and SO2 sensing properties

Fig. 4(a) shows the response to CO2 for Ba2Ca0.66Nb0.68Fe0.66O6 − δ

(BCNF) using Ag and Pt current collectors/electrodes at 600 °C. For thisstudy, the maximum temperature was kept at 600°C to avoid potentialdiffusion of Ag in solids at temperatures ≥650 °C [19]. The time toacquire 90% steady state current (t90) was approximately 7 min at600 °C for both Ag and Pt electrodes using BCNF. However, the currentdensity for Pt (broken line) electrode was higher than Ag (solid line)electrode. Au electrode, in our previous study, exhibited t90 of 5min at600 °C [17]. The CO2 sensing properties of investigated perovskites at600°C using Au, Pt and Ag are shown in Fig. 4b, where the response be-havior is linear against the log CO2 in ppm level.

Fig. 5(a) shows the long-term stability of BCNF using Au elec-trodes; however, displays no difference in steady state current den-sity at different ppm levels of SO2. This result has led to further test

Please cite this article as: S. Mulmi, et al., Solid State Ionics (2013), http://

BCNF's sensing properties using Pt and Ag electrodes in similarconcentration ranges of SO2 in dry synthetic air. The obtained re-sponse and recovery behavior at 600 °C are shown in Fig. 5(c).The response for Pt and Ag was slower (t90=8min) (Fig. 5d) com-pared to Au electrode at 600 °C (t90 = 3 min) (Fig. 5b). However,the issue of monitoring lower concentration of SO2 (b30 ppm)still persists with Au, Pt and Ag electrodes. Hori et al. [20] have re-ported CO2 reduction where the effect of Au seemed to be morepronounced comparing to that of Ag and Pt. They have shownthat the CO2 can be reduced at constant –2.8 V vs. Ag/AgClin non-aqueous electrolyte. All three electrodes predominantlyyielded CO from CO2, where Au showed the highest Faradic effi-ciency than Ag and Pt.

The potentialmechanism for CO2 has been described in our previousCO2 sensor studies [16–18]. Briefly, theO2 produced fromCO2 reductiondue to catalytic activity of Fe in BCNF and BMNF as well as O2 present indry synthetic air tend to increase the current density by creating mixedconduction. Wetchkakun et al. have further provided additional evi-dences for CO2 and SO2 sensing mechanisms [21].

Furthermore, an interesting CO2 sensitivity result in the presence ofSO2 at similar conditions (700 °C and 0.1V) has been obtained (Fig. 6).The ppm level of CO2 in dry synthetic air was varied in the range of100–1500ppm(solid line). In similarmanner, the CO2 sensingmeasure-ments were carried out in dry synthetic air with 10ppm (dash line) and20 ppm SO2 (dotted line) at 700 °C. All three measurements exhibitedthe same trend for response and recovery transients without any rise

dx.doi.org/10.1016/j.ssi.2013.09.050

Fig. 5. Long-term sensing properties of BCNF at low concentrations (1–20ppm) of SO2 in dry synthetic air using (a) Au electrode at 700°C, (b)magnified portion of (a) in 20–40min range,(c) Pt and Ag electrodes at 600 °C and (d) magnified portion of first 60min in (c).

4 S. Mulmi et al. / Solid State Ionics xxx (2013) xxx–xxx

or fall in the current densities. Although, the stable and reproducibleresponse current density by BCNF for CO2 with ppm level of SO2 in drysynthetic air atmosphere is promising, further tests with higher ppmlevel of SO2 and other different gases at various temperatures are re-quired tounderstand the sensingproperties. It is very crucial tomeasureCO2 and SO2 in ppm range, in particular, in solid oxide fuel cells (SOFCs),carbon capture and storage (CCS), and other industrial applications dueto catalyst poisoning and safety issues.

Fig. 6. Response and recovery transients of Ba2Ca0.66Nb0.68Fe0.66O6-δ (BCNF) at different con-centrations of CO2 (100–1500ppm) balanced in dry synthetic air using Au as current collec-tors. Three different sensing measurements were carried out by adding no SO2 (solid line),10 ppm SO2 in N2, (dashed line) and 20 ppm SO2 in N2 (dotted line). (Flow rate: 100 sccm;applied voltage=0.1 V.)

Please cite this article as: S. Mulmi, et al., Solid State Ionics (2013), http://

4. Conclusion

In summary, PXRD results revealed the high chemical stability ofas-prepared BCNF and BMNF against 1% CO2 in dry synthetic air and30ppmSO2 indry synthetic air aswell. BCNF at 500 and 600°C exhibitedsimilar sensing trends for CO2 usingAg andPt. The t90 valueswere foundto be ~7min at 600°C for both Pt andAg. Sensing tests on lower range ofSO2 concentration (0–20 ppm) employing BCNF showed no differencein current density despite of using different (Au, Pt and Ag) electrodes;however, it demonstrated excellent long-term stabilities. In addition,BCNF displayed its good selectivity properties towards CO2 in the pres-ence of SO2 gas (ppm). Thus, further research is required to (i) improvethe selectivity properties and develop new materials to detect lowerppm level of SO2 in SOFC operating conditions; and (ii) understandthe sensing linearity relationship between logPCO2 and current densitytogether with their chemical stability of investigated perovskitesunder ppm level of SO2 and CO2.

Acknowledgments

This work was supported by the Institute for Sustainable Energy, En-vironment and Economy (ISEEE) of the University of Calgary, Natural Re-sources Canada and CarbonManagement Canada (CMC), and a CanadianNetwork Center of Excellence (NCE). The authors would like to expresstheir sincere appreciation to Prof. Viola I. Birss (University of Calgary)and Prof. Abdelhamid Sayari (University of Ottawa) for their enthusiasticparticipation in discussions and helpful advice during our research. Oneof us, SureshMulmi, is thankful to Alberta Innovates Technology Futures(AITF) for graduate scholarship.

dx.doi.org/10.1016/j.ssi.2013.09.050

5S. Mulmi et al. / Solid State Ionics xxx (2013) xxx–xxx

References

[1] N. Yamazoe, N. Miura, Sens. Actuators B 20 (1994) 95–102.[2] http://www.epa.gov/air/criteria.html(accessed on Sept. 12, 2013).[3] F. Sonni, F. Chinnici, N. Natali, C. Riponi, Food Chem. 129 (2011) 1193–1200.[4] H.H. Mobius, J. Solid State Electrochem. 8 (2004) 94–109.[5] C.O. Park, J.W. Fergus, N. Miura, J. Park, A. Choi, Ionics 15 (2009) 261–284.[6] J. Liu, W. Weppner, Appl. Phys. A55 (1992) 250–257.[7] T. Maruyama, S. Sasaki, Y. Saito, Solid State Ionics 23 (1987) 107–112.[8] M. Zhou, A. Ahmad, Sens. Actuators B 122 (2007) 419–426.[9] S. Yao, Y. Shimizu, N. Miura, N. Yamazoe, Chem. Lett. (1990) 2033–2036.

[10] N. Imanaka, T. Kawasato, G.-Y. Adachi, Chem. Lett. (1990) 497–500.[11] N. Miura, S. Yao, Y. Shimizu, N. Yamazoe, J. Electrochem. Soc. 139 (1992) 1384.

Please cite this article as: S. Mulmi, et al., Solid State Ionics (2013), http://

[12] N. Miura, S. Yao, Y. Shimizu, N. Yamazoe, Jpn. J. Appl. Phys. 31 (1992) L197.[13] J. Yoon, G. Hunter, S. Akbar, P.K. Dutta, Sens. Actuators B 182 (2013) 95–103.[14] E.D. Bartolomeo, E. Traversa, J. Sol-Gel Sci. Technol. 19 (2000) 181–185.[15] T. Ishihara, K. Kometani, Y. Nishi, Y. Takita, Sens. Actuators B 28 (1995) 49–54.[16] R. Kannan, S. Mulmi, V. Thangadurai, J. Mater. Chem. A 1 (2013) 6874–6879.[17] S. Mulmi, A. Hassan, P.P. Almao, V. Thangadurai, Sensors Actuators B 178 (2013)

598–605.[18] S. Mulmi, V. Thangadurai, J. Electrochem. Soc. 160 (2013) B1–B7.[19] Y. Gong, C. Qin, K. Huang, ECS Electrochem. Lett. Soc. 2 (2013) F4–F7.[20] Y. Hori, Mod. Asp. Electrochem. 42 (2008) 89–187.[21] K. Wetchakun, T. Samerjai, N. Tamaekong, C. Liewhiran, C. Siriwong, V. Kruefu,

A. Wisitsoraat, A. Tuantranont, S. Phanichphant, Sens. Actuators B 160 (2011)580–591.

dx.doi.org/10.1016/j.ssi.2013.09.050