Chemical reactivity between Ce0.7RE0.2Mo0.1O2 (RE=Y, Sm) and 8YSZ, and conductivity studies of their solid solutions

Solid State Ionics xxx (2014) xxx–xxx

SOSI-13345; No of Pages 5

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Solid State Ionics

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Chemical reactivity between Ce0.7RE0.2Mo0.1O2 (RE = Y, Sm) and 8YSZ,and conductivity studies of their solid solutions

Kalpana Singh 1, Venkataraman Thangadurai ⁎Department of Chemistry, University of Calgary, 2500 University Drive, NW, Calgary, AB, T2N 1N4, Canada

⁎ Corresponding author. Tel.: +1 403 210 8649; fax: +E-mail address: [email protected] (V. Thangadura

1 Tel.: +1 403 210 8649; fax: +1 403 289 9488.

http://dx.doi.org/10.1016/j.ssi.2014.03.0300167-2738/© 2014 Elsevier B.V. All rights reserved.

Please cite this article as: K. Singh, V. Thangadies of ..., Solid State Ionics (2014), http://dx.d

a b s t r a c t
a r t i c l e i n f o
Article history:Received 11 May 2013Received in revised form 22 March 2014Accepted 29 March 2014Available online xxxx

Keywords:Mixed conductorsCe0.7RE0.2Mo0.1O2 (RE = Y, Sm)Chemical reactivitySOFC anodesElectronic conductivity

In this paper, we report the chemical reactivity between Ce0.7RE0.2Mo0.1O2 (RE = Y, Sm) (CRMO) and 8 mol%yttria-stabilized zirconia (8YSZ), at an elevated temperature in air and investigate the crystal structure, morphol-ogy, and electrical conductivity of their reaction product. Solid state reaction between CRMO and 8YSZ starts tooccur on heating at 1000 °C and yields single phase Mo-doped fluorite solid solution at 1200 °C, as confirmedby powder X-ray diffraction (PXRD). Thermo-gravimetric analysis in 5% H2/N2 showed that weight loss in thecase of solid solution is less compared to pure CRMO. In order to study the ability of CRMO to be co-fired withYSZ, compatibility tests between CRMO and 8YSZ in the pellet formwere also performed. It was found that elec-trical conductivity data of CRMO-YSZ solid solution (CRZMO) in the air and wet H2was lower than CRMO, due tothe formation of defect associates and decrease in concentration of charge carriers.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

Ceria–zirconia solid solutions find their applications in automotivethree way catalysts (TWC), catalyst for methane oxidation and watergas shift reactions [1–4], as solid oxide fuel cells (SOFCs) anodes andelectrolytes [5–7]. The wide use of ceria in these applications is mainlydue to unique redox properties of ceria, where it can release or gainoxygen by changing its oxidation state reversibly from Ce4+ to Ce3+

and vice versa. The most famous and commercial application of ceriais in TWC as a promoter for oxygen storage capacity (OSC). However,when un-doped ceria is used, it loses OSC, due to increase in particlesize during operation, and addition of small size Zr increases the ther-mal stability of ceria and the OSC by decreasing activation energy (Ea)for oxide ions mobility in the lattice. Ceria–zirconia solid solutionbased TWC catalysts work for years under automotive exhaust.

Nowadays SOFCs are gaining tremendous attention due to their abil-ity to convert chemical reaction energy into electricity with high efficien-cy and the ability to run on various fuels, including hydrogen,hydrocarbons and carbon monoxide due to its high temperature opera-tion. Typical SOFC anodes should show redox properties, OSC, electronicand ionic conductivity, chemical compatibility with electrolytes, electro-chemical oxidation of fuel, and resistance towards coke and sulfur poi-soning. Due to poor tolerance of current Ni-YSZ (YSZ = Yttria-stabilized zirconia) anodes towards coke and sulfur poisoning, there is

1 403 289 9488.i).

urai, Chemical reactivity betwoi.org/10.1016/j.ssi.2014.03.0

constant search for alternate anode materials for SOFCs. One of the lead-ing ceramicmaterials for SOFC anodes is ceria, due to itsmixed-ionic elec-tronic conducting (MIEC) nature and good activity towards fueloxidations. Since, the redox and thermal stability of ceria is poor, it isdopedwith trivalent ions such as Sm3+ and Y3+. Also, the doping of tri-valent cations can increase the ionic conductivity of ceria due to forma-tion of oxide ion vacancies [8].

Fig. 1. Powder X-ray diffraction (PXRD) patterns of (i) Ce0.43Zr0.4Y0.12Mo0.05O2; (ii) Ce0.35Zr0.35Y0.2Mo0.1O2 and (iii) Ce0.35Zr0.35Sm0.1Y0.1Mo0.1O2 powders synthesized in airthrough mixed oxide route at 1450 °C. Selected area PXRD clearly shows the effect ofchemical doping.

een Ce0.7RE0.2Mo0.1O2 (RE = Y, Sm) and 8YSZ, and conductivity stud-30

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Table 1Data for lattice constant, bulk conductivity and Arrhenius parameters of CRMO [10] and CRZMO. Lattice constant for 8YSZ is also included for comparison [13].

Composition Lattice constant (nm) σbulk (S/cm); atmosphere; T (°C) Ea (eV)

Ce0.7Y0.2Mo0.1O2 [10] 0.53908 (13) 4.22 × 10−2; wet H2; 450 °C 0.46 (250–700 °C)2.42 × 10−3; air; 600 °C 0.89 (300–800 °C)

Ce0.7Sm0.2Mo0.1O2 [10] 0.54180 (14) 7.82 × 10−2; wet H2; 450 °C 0.28 (250–700 °C)3.90 × 10−3; air; 600 °C 0.44 (600–800 °C)

Ce0.43Zr0.4Y0.12Mo0.05O2 0.522 (3) 3.73 × 10−3; wet H2; 450 °C 0.37 (450–600 °C)2.35 × 10−4; air; 600 °C 1.09 (350–800 °C)

Ce0.35Zr0.35Y0.2Mo0.1O2 0.524 (3) 1.74 × 10−3; wet H2; 450 °C 0.28 (450–600 °C)5.3 × 10−5; air; 600 °C 1.09 (350–800 °C)

Ce0.35Zr0.35Sm0.1Y0.1Mo0.1O2 0.525 (3) 1.87 × 10−3; wet H2; 450 °C 0.60 (450–600 °C)1.22 × 10−4; air; 600 °C 1.15 (350–800 °C)

8YSZ [13] 0.5417 –

2 K. Singh, V. Thangadurai / Solid State Ionics xxx (2014) xxx–xxx

Learning from the success of ceria–zirconia solid solutions in automo-tive applications, it is encouraging to look into the use of ceria–zirconiasolid solutions in SOFC anodes. Ahn et al. [5] have shown that anodeperformance of Cu–Ce0.6Zr0.4O2 was less affected by high temperaturetreatment when compared to un-doped Cu–CeO2 anode. Song et al.[6] also demonstrated improved performance on symmetrical electro-chemical cells, where nanoparticulate Ce1 − xZrxO2 (x = 0.1; 0.25)anode showed lower polarization resistances in 5% humidified CH4

against the ceria under same conditions. Also, coke deposition on con-ventional Ni-YSZ anode under hydrocarbons was mitigated by usingCe0.75Zr0.25O2 layer [9].

In our previous work, it was shown that the total (electronic andionic) conductivity of ceria can be increased when co-doped withRE (Y and Sm) and Mo. At 450 °C under wet H2, Ce0.7RE0.2Mo0.1O2

(RE = Y and Sm) (CRMO) showed the conductivity in the order of10−2 S/cm with an activation energy of 0.28–0.46 eV [10]. With itsMIEC nature, together with excellent electrochemical oxidation proper-ties of ceria, CRMO looks promising as potential anode material forSOFCs. Using powder X-ray diffraction, we have shown that at above1200 °C, CRMO and 10YSZ form single-phase cubic fluorite solid solution[11]. In this study the electrical properties of resultant solid solution

Fig. 2. SEM showing images of (a) Ce0.7Y0.2Mo0.1O2 (CYMO) surface; (b) 8YSZ surface1400 °C for 12 h in the air.

Please cite this article as: K. Singh, V. Thangadurai, Chemical reactivity betwies of ..., Solid State Ionics (2014), http://dx.doi.org/10.1016/j.ssi.2014.03.0

obtained using 10YSZ and 8YSZ and CRMO will be investigated with theaim of understanding reduction behavior of ceria zirconia solid solutionin the presence of Mo. In the case of Mo-doped ceria, neutron diffractionstudies and density functional theory analyses have shown the presenceof oxygen interstitials and cation deficiency [12].

2. Experimental methods

Ce0.7RE0.2Mo0.1O2 (RE = Y and Sm) (CRMO) powders were synthe-sized by conventionalmixed oxide solid state reaction as reported in theliterature [10]. High purity CeO2 (N99.9%), Sm2O3 (N99.9%), Y2O3

(N99.9%), MoO3 (N99.5%) and ZrO2 (N99.9%) oxides from Alfa Aesarwere used for synthesis. For chemical compatibility tests betweenCRMO in the pellet form and 8YSZ, commercial 8YSZ powder samplefrom Fuel Cell Materials was used. A CRMO pellet (20 mm diameter)was prepared and a 8YSZ slurry was painted on it for chemical reac-tivity test.

Zr-doped CRZMO with nominal chemical compositions of Ce0.43Zr0.40Y0.12Mo0.05O2, Ce0.35Zr0.35Y0.2Mo0.1O2 and Ce0.35Zr0.35Y0.1Sm0.1

Mo0.1O2, were prepared by solid-state reaction using respective ox-ides in the air. The powders were ball-milled (Pulverisette, Fritsch,

; (c) Ce0.7Sm0.2Mo0.1O2 (CSMO) surface and (d) 8YSZ surface after treatment at



Fig. 3.Characteristic AC impedance plots (a) at 400 °C in air; and (b) at 450 °C inwetH2 for(i) Ce0.43Zr0.4Y0.12Mo0.05O2; (ii) Ce0.35Zr0.35Y0.2Mo0.1O2 and (iii) Ce0.35Zr0.35Sm0.1Y0.1Mo0.1O2. In (a) the open symbols represent the experimental data and solid line passingthrough the data points correspond to fitted data, using the equivalent circuit shown inthe inset.

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Germany) in isopropanol for 6 h at 200 rpm, by using zirconia balls.After drying, powders were heated at 1000 °C in the air for 24 hand ball milled again for 6 h. After drying, the powders were pressedinto ~1 cm diameter and ~2 cm length pellets using an isostatic pres-sure of 200 kN. These pressed pellets were then sintered at 1450 °Cfor 24 h, two times in air with ball milling step after the firstsintering. Phase analysis of CRZMO powders were done at room tem-perature by using a Bruker D8 powder X-ray diffractometer (Cu–Kα,40 kV, 40 mA).

Solartron electrochemical impedance spectroscopy (model: SI 1260;AC amplitude: 100 mV; 0.1 Hz–10 MHz) was used to investigate theelectrical conductivity of CRZMO in air and wet H2 in the range of300 °C to 800 °C. Sintered CRZMO pellets (~1 cm in diameter and~0.15 cm in thickness) were coated by Pt paste (LP A88-11S, HeraeusInc., Germany) by using a paint brush and cured at 800 °C for 1 h inair. During the electrical measurements, both the surfaces of pelletswere attached to Pt wires through spring loaded-contact. Barnstead tu-bular furnace (model 21100) was used to keep the cell and getting thedesired temperature range. Before making conductivity measurementsat desired temperature, the sampleswere allowed to attain thermal sta-bility by keeping the temperature constant for at least an hour to over-night. The microstructure of pellets was studied by employing scanningelectron microscopy (SEM) and energydispersive X-ray spectroscopy(EDX) analysis (Philips XL30). Thermo-gravimetric analysis was donein 5%H2/N2 by using TGA/DSC fromMettler Toledo. The lattice constantswere determined from the powder X-ray diffraction (PXRD) data.

3. Results and discussion

Fig. 1 shows the PXRD pattern for as-prepared Ce0.43Zr0.4Y0.12Mo0.05O2, Ce0.35Zr0.35Y0.2Mo0.1O2 and Ce0.35Zr0.35Sm0.1Y0.1Mo0.1O2 (CRZMO)powders through a mixed oxide route. The lattice constant values andPXRD pattern confirm the dissolution of Ce into the YSZ structureand formation of ceria–zirconia solid solution with cubic fluorite struc-ture (Table 1) [10,13]. The decrease in lattice constant upon Zr dopingis in agreement with the small ionic radius of Zr as compared to Ce(Ce4+ = 0.097 nm, Zr4+ = 0.084 nm) [14]. The doping of Zr, Y, andMo in ceria could be described by the following doping reactions:

CexCe sð Þ→ZrO2 ZrxCe sð Þ; ð1Þ

CexCe sð Þ þ Oxo→

Y‐dopingY=Ce sð Þ þ 0:5V••

o sð Þ þ 0:5O2 gð Þ; ð2Þ

CexCe sð Þ→MoO3 Mo••Ce sð Þ þ O==

i sð Þ; ð3Þ

CexCe sð Þ þ Oxo→

Mo‐dopingMo••Ce sð Þ þ 2Ce=Ce sð Þ; ð4Þ

3CexCe sð Þ→Mo‐doping

2Mo••Ce sð Þ þ V====Ce ; ð5Þ

where, CeCe(s)x , ZrCe(s)x Oox, YCe(s)

/ , Vo(s)•• , MoCe(s)•• , Oi(s)

// , CeCe(s)/ , and VCe//// rep-

resent tetravalent cerium at the tetravalent cerium site, tetravalent zir-conium at the tetravalent cerium site, lattice oxygen, trivalent Y at thetetravalent cerium site with effective negative charge, oxide ion vacan-cy with effective two positive charges, hexavalentMo at the tetravalentcerium sitewith effective two positive charges, oxygen at the interstitialsite with effective two negative charges, trivalent Ce at the regulartetravalent ceriumsitewith effective four negative charges, respectivelyfour negative charges., respectively. In the present case of Zr, Y andMo-doped ceria, onewould also anticipate that Mowould give rise to inter-esting defect chemistry with oxygen interstitials, cation vacancies,along with an increase in electronic conductivity due to its ease in re-ducibility. Further diffraction and computational studies are required


to confirm the proposed doping mechanism, especially, the cation va-cancy and interstitial anion in the investigated system.

For the chemical stability test, the white layer of 8YSZ turned intofragile yellow colored layer after heating at 1400 °C. In the ceria-YSZ in-terface, there is selective diffusion of Ce ions towards the YSZ phase [15].Fig. 2 shows the SEM images for compatibility tests between CRMO and8YSZ (Fig. 2(a)–(d)) after heat treatment at 1400 °C for 12 h in the air.Fig. 2(a) shows the SEM image of Ce0.7Y0.2Mo0.1O2 (CYMO) surfaceand Fig. 2(b) shows the SEM image of YSZ surface after the reaction.EDX analysis of YSZ surface in Fig. 2(b) confirmed the formation ofsolid solution with Mo also as the dopant. Fig. 2(c–d) show the SEMimages for compatibility tests between Ce0.7Sm0.2Mo0.1O2 (CSMO) and8YSZ after heat treatment at 1400 °C for 12 h in the air.

Fig. 3(a) and (b) shows the characteristic AC impedance plot forCe0.43Zr0.4Y0.12Mo0.05O2, Ce0.35Zr0.35Y0.2Mo0.1O2 and Ce0.35Zr0.35Sm0.1

Y0.1Mo0.1O2 (CRZMO) in ambient air and wet H2, at the temperature of400 °C and 450 °C, respectively. For all the investigatedCRZMOsamples,the impedance plots were highly reproducible during heating andcooling cycles between difference batches of samples. In all composi-tions, the contribution due to bulk (high frequency range), grain-boundary and electrode effect was observed at 400 °C in the air, asshown in Fig. 3(a). In wet H2 (Fig. 3(b)), CRZMO shows the frequencyindependent resistance behavior. Inductance effect due to experimentalsetup is seen at high frequency regime.



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The temperature dependence of conductivity for CRZMO in air andwet H2 is shown in Fig. 4(a) and (b), respectively. For comparison, con-ductivity of CRMO data and 8YSZ are also plotted in the Fig. 4 [10,16]. Itcan be clearly seen that electrical conductivity, decreased due to the ad-dition of Zr in the CRMO lattice. Table 1 lists activation energy and bulkelectrical conductivity at 600 °C in air and at 450 °C in wet H2. The acti-vation energy (Ea) in air is increased in the case of CRZMO compared toCRMO (Table 1) suggesting that, the effective free oxygen vacancies forionic conduction are lowered due to formation of defect associates dueto Zr substitution in CRMO. For fluorite type solid solution, it is generallyaccepted that high concentration of oxygen vacancies leads to loweringin ionic conductivity due to formation of defect associates [17–19].

In the case ofwetH2, the decrease in conductivity ismore pronouncedfor CRZMO compared to CRMO. Also, each composition exhibits two acti-vation energies, in the range from 300–450 °C and 450–600 °C. Usingcomputer simulation, Balducci [20] have shown that the Ce4+/Ce3+ re-duction energy is reduced on addition of Zr for Ce in CeO2, also there isincrease in oxygen mobility on addition of Zr substitution compared tothat of undoped ceria. Under the reducing environment, Ce4+ reducesto Ce3+, and to maintain electroneutrality in the lattice, oxide ion vacan-cies are formed, which can be described using the defect equilibrium

Fig. 4. Arrhenius plots for electrical conductivity of CRZMO in (a) the air; and (b) wet H2.For comparison, conductivity data for CRMO in the air and wet H2 [10] and 8YSZ [16] arealso shown.


reaction:

Oxo sð Þ þ CexCe sð Þ→0:5V ••

o sð Þ þ Ce=Ce sð Þ þ 0:5O2 gð Þ ð6Þ

In the case of pure ceria, electronic conductivity arises due to hop-ping of electrons between the cerium sites. In the present case, due tothe presence of Zr in the equal concentrationwith Ce ions, which cannotchange its oxidation state, the Ce concentration is low to form conduc-tion band and thus exhibit lower electronic conductivity compared toceria. The low electronic conductivity in reducing environments of theinvestigated CRZMO is due to the effect of decreased concentration inCe3+ ions (hopping carriers) [21,22]. The trend in reduction of electronicconductivity is also supported by TGA in 5% H2/N2, where theweight lossin the case of CRZMO is lower than that of CRMO (Fig. 5 (a)). FromFig. 5(b), it can also be seen that electronic conductivity in CRZMO islower than CRMO [10] and Ce0.9Gd0.1O2 [23]. This shows that the effectof Zr addition in lowering the electronic conductivity ismore pronouncedthan the presence of easily reducible Mo ion in the CRZMO.

4. Conclusions

Through chemical compatibility tests in pellet form, we have shownthe extent of chemical reaction between Ce0.7RE0.2Mo0.1O2 (RE = Y andSm) (CRMO) and commercial solid oxide ion electrolyte YSZ. The

Fig. 5. (a) Thermogravimetric curves of CRZMO and CRMO obtained under 5% H2/N2 from50 °C to 900 °C and (b) Arrhenius plot comparing the electrical conductivity data ofCRZMO, CRMO [10] and Ce0.9Gd0.1O2 [23] in wet H2.



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resultant solid solution phases, Ce0.43Zr0.4Y0.12Mo0.05O2, Ce0.35Zr0.35-Y0.2Mo0.1O2 and Ce0.35Zr0.35Sm0.1Y0.1Mo0.1O2 (CRZMO), show totalelectrical conductivity in the order of 10−3S/cm at 450 °C in wetH2. The bulk conductivities for current CRZMO compositions in wetH2 at 450 °C were found to be an order of magnitude lower than thatof parent CRMO; however, further optimisation of the CRZMO canlead to a highly conducting anode for SOFCs. In future studies, studyingthe conductivities of CRZMO over awide range of partial pressure of ox-ygen, will give the insight into defect chemistry, which will help to as-certain the correct defect model for observed trend in ionic andelectronic conductivity. In conclusion, through chemical compatibilitytests between CRMO and YSZ, we have found one potential mixedionic electronic conductor based on ceria–zirconia solid solution.

Acknowledgments

We thank the Natural Science and Engineering Research Council(NSERC), Solid Oxide Fuel Cell Canada (SOFCC) Strategic Research Net-work and other sponsors listed at www.sofccanada.com for their finan-cial support.

References

[1] H. Sobukawa, R&D Rev. Toyota CRDL 37 (2002) 1–5.


[2] S.Y. Choung, M. Ferrandon, T. Krause, Catal. Today 99 (2005) 257–262.[3] S. Larrondo, M.A. Vidal, B. Irigoyen, A.F. Craievich, D.G. Lamas, I.O. Fabregas, G.E.

Lascalea, N.E.W. de Reca, N. Amadeo, Catal. Today 107 (2005) 53–59.[4] S. Ricote, G. Jacobs, M. Milling, Y. Ji, P.M. Patterson, B.H. Davis, Appl. Catal. A Gen. 303

(2006) 35–47.[5] K. Ahn, H. He, J.M. Vohs, R.J. Gorte, Electrochem. Solid-State Lett. 8 (2005)

A414–A417.[6] S. Song, R.O. Fuentes, R.T. Baker, J. Mater. Chem. 20 (2010) 9760–9769.[7] Y. Mishima, H. Mitsuyasu, M. Ohtaki, K. Eguchi, J. Electrochem. Soc. 145 (1998)

1004–1007.[8] G.B. Balazs, R.S. Glass, Solid State Ionics 76 (1995) 155–162.[9] H. He, J.M. Hill, Appl. Catal. A Gen. 317 (2007) 284–292.

[10] Q. Li, V. Thangadurai, J. Mater. Chem. 20 (2010) 7970–7983.[11] H.T. Handal, K. Singh, Q. Li, V. Thangadurai, ECS Trans. 45 (2012) 453–463.[12] F. Ramezanipour, M. Shishkin, K. Singh, J.P. Hodges, T. Ziegler, V. Thangadurai, J.

Mater. Chem. A. 1 (2013) 8344–8347.[13] N.M. Sarnrnes, Zhihong Cai, Solid State Ionics 100 (1997) 39–44.[14] R.D. Shannon, Acta Crystallogr. A3225 (1976) 751–767.[15] H. Mitsuyasu, Y. Nonaka, K. Eguchi, H. Arai, J. Solid State Chem. 129 (1997) 74–81.[16] S. Hui, J. Roller, S. Yick, X. Zhang, C.D. Petit, 'Y. Xie, R. Maric, D. Ghosh, J. Power

Sources 172 (2007) 493–502.[17] C.H. Lee, G.M. Choi, Solid State Ionics 135 (2000) 653–661.[18] B. Cale's, J.F. Baumard, J. Electrochem. Soc. 131 (1984) 2407–2413.[19] C. Leach, N. Khan, B.C.H. Steele, J. Mater. Sci. 27 (1992) 3812–3816.[20] G. Balducci, J. Kaspar, P. Fornasiero, M. Graziani, J. Phys. Chem. B 101 (1997)

1750–1753.[21] R.E.W. Casselton, Phys. Status Solidi A 1 (1970) 787–794.[22] R.F. Reidy, G. Simkovich, Solid State Ionics 62 (1993) 85–97.[23] K.Q. Huang, M. Feng, J.B. Goodenough, J. Am. Ceram. Soc. 81 (1998) 357–362


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Chemical reactivity between Ce0.7RE0.2Mo0.1O2 (RE=Y, Sm) and 8YSZ, and conductivity studies of their solid solutions