Yttrium doped BaCeO3 thin films by spray pyrolysis technique for application in solid oxide fuel cell

Journal of Alloys and Compounds 587 (2014) 664–669

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Yttrium doped BaCeO3 thin films by spray pyrolysis technique for application in solid oxide fuel cell S.U. Dubal a, A.P. Jamale a, S.T. Jadhav a, S.P. Patil a, C.H. Bhosale a, L.D. Jadhav b,⇑ a b

Department of Physics, Shivaji University, Kolhapur 416 004, India Department of Physics, Rajaram College, Kolhapur 416 004, India

a r t i c l e

i n f o

Article history: Received 16 July 2013 Received in revised form 9 October 2013 Accepted 13 October 2013 Available online 24 October 2013 Keywords: Yttrium doped barium cerate Spray pyrolysis technique Solid oxide fuel cell Thin film X-ray diffraction Scanning electron microscope

a b s t r a c t Yttrium doped barium cerate (BCY) a solid state ion conductor which exhibits proton conductivity under proper atmospheric conditions, is used as an electrolyte in a solid oxide fuel cell (SOFCs). In present work, nanocrystalline BaCe0.8Y0.2O2.9 (BCY20) thin films were successfully deposited onto alumina substrates by simple and economical spray pyrolysis technique (SPT) at 250 °C. The effect of solution concentration and annealing on physico-chemical properties of BCY20 thin film has been studied. The X-ray diffraction (XRD) studies of spray pyrolysed BCY20 films revealed polycrystalline (crystallite size 35 nm) orthorhombic structure with lattice parameters a = 8.77 Å, b = 6.234 Å and c = 6.223 Å. The scanning electron micrographs showed dense morphology which is very useful for electrolyte. The stoichiometry was confirmed by elemental analysis and the estimated atomic ratio was in good agreement with that of the precursor solution ratio. The most intense band at 353.26 cm1 observed in room temperature Raman spectrum of BCY20 film was due to vibrational mode of barium cerate. The FTIR spectra with heat treatment shows no carbon based vibration bonds, revealing absence of carbon based surface impurities in the sample. The dc conductivities measured in air and argon atmospheres at 600 °C were 1.7  103 and 4.25  103 S cm1, respectively. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction An electrolyte, key component in the development of solid oxide fuel cell (SOFC), should have properties like high ionic conductivity, low electrical conductivity, thermal and chemical stability. The three basic materials BaZrO3, SrZrO3 and BaCeO3 are used as electrolyte in protonic SOFCs [1]. Out of these, BaZrO3 and SrZrO3 materials have less proton conductivity and also poor sinterability, while BaCeO3 exhibits best conduction properties [2]. Further its conductivity and chemical stability increased if doped with 20% yttrium at ceria site. So 20% yttrium doped barium cerate (BCY20) has been studied intensively [3–8]. BCY20 has perovskite structure in which oxygen vacancies are formed by replacing Ce4+ by Y3+ ions. It shows proton conductivity either in hydrogen or moist atmosphere. The incorporation of hydrogen proceeds in this case by the reaction;

H2 O þ V o þ Oxo $ 2ðOHÞo

ð1Þ

BCY20 has been prepared by several methods such as solid state reaction [9], combustion [10], freeze-dried precursor method [11] and co-precipitation [12]. All these methods need high sintering temperature for prolonged time and also require hazardous ⇑ Corresponding author. Mobile: +91 9890694409; fax: +91 231 2537840. E-mail address: [email protected] (L.D. Jadhav). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.10.093

expensive chemicals and different atmospheric conditions. The powder obtained by these methods is pressed into disc shaped pellets which, however, offer very high ohmic resistance. Since last few years research in SOFCs has been focused on thin films. Especially, electrolyte materials having lower thickness reduce the ohmic losses and thereby improve the cell performance. In present work, BCY20 thin films have been prepared by spray pyrolysis deposition technique (SPT). It is simple, low cost and environmentally benign thin film deposition technique wherein physical, electrical and optical properties depends on various parameters such as precursor solution concentration, distance between nozzle to substrate, substrate temperature, air pressure, spray rate, deposition rate and cooling rate after deposition [13–15]. Further electrode/electrolyte interfaces would be formed at low temperatures [16,17]. The deposition of BCY20 was carried out on alumina substrates using aqueous solution. The films were characterized by different physio-chemical techniques and the effect of concentration and annealing on their structural and morphological properties was also investigated. The films were also studied by Raman and FTIR and their electrical conductivities were measured. 2. Experimental Prior to deposition, the precursor solution of cerium nitrate (Ce(NO3)36H2O, 99.9%; ALFA AESAR), barium nitrate (Ba(NO3)2, 99.9%, ALFA AESAR) and yttrium nitrate (Y(NO3)3:6H2O, 99.9%; ALFA AESAR) in double distilled water was prepared

S.U. Dubal et al. / Journal of Alloys and Compounds 587 (2014) 664–669 according to desired BaCe0.8Y0.2O2.9 phase. The concentration of solution was varied from 0.05 to 0.2 M. The solution was sprayed using glass nozzle with spray rate 1 ml/min and air as carrier gas on preheated alumina substrate. The crystallization of material was achieved by subsequent annealing at 700–1000 °C. Thermogravimetric and differential thermal analysis (TG–DTA) was carried out using a Perkin Elmer TGA–DTA–DSC instrument (model SDT-2960) to determine the decomposition temperature of precursor and phase stability of spray deposited material. The prepared films were then characterized for their structural, elemental, morphological and functional properties. The structural properties of the prepared film were determined using X-ray diffractometer (PHILIPS PW-3710) with Cu Ka radiation source. Elemental and morphological properties were analyzed using scanning electron microscope (SEM, JEOL-JSM-6360) with attached EDAX unit. The infrared spectrum was recorded by using Perkin Elmer infrared spectrophotometer with scanning range 400–4000 cm1. Functional analysis of BCY20 was done using Jobine Yon HR 800 UV Raman spectrometer system with excitation wavelengths of 325 and 554 nm. DC-conductivity of prepared BCY20 film was measured by two probe method using lab made resistivity set up in air and argon atmosphere.

3. Results and discussion

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powder scratched from as spray deposited film from 0.1 M solution. The as prepared powder is less crystalline and carries nitrates as evidenced from XRD (not shown here). In order to predetermine the appropriate phase formation temperature of BCY20, the TG– DTA has been carried out. TGA shows total weight loss of 19.71% in the temperature range of room temperature to 650 °C. A first sluggish weight loss is observed in the temperature range of room temperature to 565.40 °C. The weight loss up to 100 °C is ascribed to moisture removal. The gradual weight loss continues up to 565.40 °C along with a broad exothermic peak in DTA at 260.18 °C, which may be due to oxidative decomposition of residual precursor [20]. A sharp fall in the weight after 565.40 °C along with endothermic peaks at 565.40 °C and 641.31 °C is due to removal of nitrate based entities. TGA shows no further weight loss after 650 °C, which confirms formation of stable phase of BCY20. Accordingly, as prepared thin films were annealed at 700– 1000 °C for 6 h in ambient condition at a heating rate of 1 °C min1.

3.1. Film formation Fig. 1 shows schematic representation of spray pyrolysis technique. It involves many processes occurring either simultaneously or sequentially. This includes aerosol generation and transport, solvent evaporation, droplet impact with consecutive spreading, and precursor decomposition. Except aerosol generation, all processes are temperature dependent. Consequently, the substrate surface temperature is the main parameter that determines the film morphology and properties. With an increase in temperature the film morphology can change from a cracked to a porous microstructure but at optimized substrate temperature uniform pinhole-free and well adherent films would be possible [18]. Another spray parameter, which is equally important and affects the morphology of the film, is the solution concentration. It needs to be also optimized. A porous to dense morphology is reported for high to low solution concentration. So solution concentration was varied from, 0.05 M to 0.2 M keeping the substrate temperature fixed at 250 °C. The possible chemical reaction of this thermal decomposition is 900 C

BaðNO3 Þ2 þ 0:8CeðNO3 Þ3 6H2 O þ 0:2YðNO3 Þ3 6H2 O ƒƒƒƒ! BaCe0:8 Y0:2 O2:9 1 þ 12H2 O þ 2:5ðNO2 Þ þ O2 ð2Þ 2

3.2. Thermogravimetric analysis Thermo-gravimetric and differential temperature analysis (TG– DTA) is important in studying the transformation of a solid sample in a thermal process [15,19]. Fig. 2 shows TG–DTA of BCY20

3.3. XRD studies XRD patterns of the BCY20 films deposited from various solution concentrations and annealed at 900 °C are shown in Fig. 3.1 while Fig. 3.2 shows XRD patterns of BCY20 films from 0.1 M concentrations and annealed at different temperatures. The films are polycrystalline with orthorhombic crystal structure (JCPDS card No. 82-2372) [20]. XRD patterns indicate strong orientation along (0 0 2), (0 2 2), (2 1 3) and (0 4 0) planes, also some weak reflections (0 2 1), (3 1 1), (1 1 3), (3 1 3) and (2 4 1) are present. Reflections from alumina substrate are observed in all patterns. As shown in Fig. 3.1 with an increase in solution concentration intensity of reflection peaks of BCY20 increase while those of alumina substrate decrease and is very weak for 0.2 M indicating formation of thick film. Besides, XRD for 0.2 M shows additional reflections (5 1 0) and (5 0 2) from BCY20 and remaining reflections are due to cerium–yttrium oxide (Ce0.503Y0.497O1.751) (JCPDS card No. 083-0326) [2]. Intensity of (0 0 2) reflection is more for 0.1 M concentration and the crystallite size was calculated from Scherrer formula.

D¼

0:9  k b  cos h

ð3Þ

where D is the crystallite size, k is the wavelength of X-ray (15.406 nm), b is the full-width at half-maximum (FWHM) in radian, and h is the angle of diffraction. The effect of concentration variation on lattice parameter and crystallite size is shown in Table 1. At concentration 0.05 M, less material was deposited on

Fig. 1. Schematic representation of spray pyrolysis technique.

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Fig. 2. TG–DTA of BCY20 powder scratched from as spray deposited film from 0.1 M solution.

the substrate indicating low intensity reflections. As the concentration was increased to 0.1 M, uniform and compact thin film was deposited on the substrate, also the planer density enhanced representing formation of highly crystalline material. But for further increase in concentration from 0.15 to 0.2 M due to weaker bond strength between substrate and material, instead of adherent film powder was formed on the substrate. For this high solution concentration, powder piled up over the substrate with incomplete nucleation and hence resulting into two phases. Hence optimized solution concentration is 0.1 M. Fig. 3.2 reveals the effect of sintering temperature on structural properties of BCY20 films prepared from 0.1 M solution. As revealed in TG–DTA the phase has not been formed during spray and need annealing which also helps to improve crystallization. The films were annealed at 700, 800, 900 and 1000 °C. The crystallanity is observed to be improve and the crystallite size grows from 15.68 (700 °C) to 35.11 nm (900 °C). However, a small reflection from CeO2 is observed at 1000 °C and intensity of (0 0 2) reflection of BCY20 is decreased [21]. Fig. 3.2 also shows XRD pattern of BCY20 powder collected from spray deposited film heat treated at 900 °C. It reveals phase pure BCY20 with lattice parameters a = 8.8735 Å, b = 6.246 Å and c = 6.216 Å, which are in good agreement with [20]. 3.4. Scanning electron micrograph

Fig. 3.1. XRD patterns of BCY20 films deposited from various solution concentration and annealed at 900 °C.

Effect of solution concentration and annealing temperature on morphology of the BCY20 films are demonstrated respectively in Figs. 4.1 and 4.2. Micrographs show agglomerated dense morphology. A slightly porous morphology is observed for 0.05 M while more compact and denser morphology is observed for 0.1 M concentration and the later is very useful for electrolyte in order to avoid mixing of gases during the working of SOFCs. For still higher concentration, porosity increases. Fig. 4.2 shows effect of sintering temperature on morphology of optimized concentration of 0.1 M. At 700 °C and 800 °C, the film is porous while at 900 °C dense and compact morphology is observed. At still high temperature, evaporation of barium leads to porous structure. In spray pyrolysis, generally droplets approaching substrate wets the substrate followed by subsequent nucleation and growth process. The whole process has been reported to be function of substrate temperature [22,23] and also solution concentration. For low solution concentration, droplets hit the substrate before complete evaporation causing more wetting of the substrate. Further, for low solution concentration, the deposited material traps the solvent which while evaporation during annealing leaves behind some porosity or develops cracks over the surface. The optimum solution concentration results into dense structure. However, for still higher concentration, droplets result into vapor formation before wetting the substrate and lump of powders get collected over the substrate resulting into highly porous and less adherent structure. However, this is not a general rule; it strongly depends on the type of solution, solvent, substrate and the experimental set up. 3.5. Elemental analysis

Fig. 3.2. XRD patterns of BCY20 films from 0.1 M concentrations and annealed at different temperatures.

The thickness of films was measured with the surface profiler and it is 1.4 lm. Fig. 5 shows elemental analysis of BCY20 thin film obtained from 0.1 M solution concentration and annealed at 900 °C. It shows uniform elemental distribution of Ba, Ce and Y. The precursor for spray deposition was formed to produce atomic ratio of 1:0.8 of Ba over Ce. EDAX mapping gives atomic ratio value of 1:0.77, relatively close to that in precursor. It has been often observed that during high temperature processing of BCY20, barium evaporates leading to barium deficiency and forming secondary

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Table 1 Effect of concentration variation on different parameters. Solution concentration

a (Å)

b (Å)

c (Å)

Crystallite size (nm)

Intensity of (0 0 2) peak in a.u.

0.05 M 0.1 M 0.15 M 0.2 M

8.710 8.725 8.700 8.818

6.240 6.240 6.237 6.236

6.324 6.269 6.306 6.282

26.19 35.11 34.55 24.14

1145.35 2926.77 1383.60 1357.92

Fig. 4.1. SEM of BCY20 films from (a) 0.05 M, (b) 0.1 M, (c) 0.15 M and (d) 0.2 M concentration and annealed at 900 °C.

Fig. 4.2. SEM of BCY20 films from 0.1 M concentration and annealed at (a) 700 °C, (b) 800 °C, (c) 900 °C and (d) 1000 °C.

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Fig. 5. Elemental analysis of BCY20 thin film obtained from 0.1 M concentration and annealed at 900 °C.

powder shows vibration bands in the range 500–1000; 1350– 1780 and around 3400 cm1, which can readily be assigned to M–O vibrations, vibrations arising from nitrate based impurities and presence of moisture, respectively [25,26]. The intensity of these bands decreased after applying heat treatment due to crystallization in the film. The FT-IR spectra of annealed film shows weak broad band centered around 3500 cm1 of O–H stretching vibration indicating presence of residual moisture absorbed by the sample during the measurements. The vibration maxima at 861.61, 509.77 cm1 indicates the formation of metal oxide. The bands in the range 1350–1780 cm1 are almost absent expect the one at 1438 cm1, which is due to H–O–H bending modes [27]. The FT-IR spectrum shows no carbon based vibration bonds revealing absence of carbon based surface impurities in the sample. This is the uniqueness of spray pyrolysed films over the other methods. 3.7. DC-conductivity

phases [20]. However, in the present case, though the film has been heated at 900 °C, no barium loss was detected and XRD pattern also showed absence of any secondary phases.

In yttrium doped barium cerate, each yttrium dopant is compensated by half oxygen vacancy, which plays imperative role in

3.6. Fourier transform raman and infrared spectroscopy The barium cerate is a perovskite structure with either cubic, or tetragonal or orthorhombic structure, which is strongly dependent on synthesis route. In the present case, it is crystallized into orthorhombic structure which has space group D(Pbnm) and four formula groups per unit cell (Z = 4) [24]. Raman spectroscopy is a very powerful technique to study soft modes and structural phase transitions [21]. Fig. 6 shows Raman spectrum of BCY20 material. It shows pronounced bands at 147.84, 231, 256.59, 353.26 and 454.19 cm1. These pecks have been identified as arising from BCY20. The most intense band at frequency 353.26 cm1 indicates vibrational mode of barium cerate. Weak bands at 550.5 and 624.4 cm1 are due to dopant-oxygen ion symmetric stretching vibrations [24]. Fourier Transform Infrared (FT-IR) spectroscopy is one of the most common spectroscopic techniques used by organic and inorganic chemists. The main goal of FT-IR spectroscopic analysis is to determine the chemical functional groups in the sample. Different functional groups absorb characteristic frequencies of FT-IR radiation. Fig. 7 shows FT-IR spectra in KBr mixture of sample scratched from as spray deposited and annealed film. As spray deposited

Fig. 7. Fourier transform infrared (FTIR) spectra of as spray deposited and annealed BCY20 thin films.

Fig. 6. Raman scattering spectrum of BCY20 thin film obtained from 0.1 M solution concentration and annealed at 900 °C.

Fig. 8. DC-conductivity of BCY20 thin film in (a) air, (b) argon atmosphere.

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conduction. In the moist or hydrogen atmosphere, protons hop by Eq. (1) to give proton conductivity. In the oxygen atmosphere, oxygen ion diffuses into the lattice through vacancies giving rise to oxide ion conductivity. Fig. 8 shows dc conductivity of BaCe0.8Y0.2O3d in air and argon atmosphere in the temperature range 400–600 °C. The conductivity is slightly more in argon atmosphere than in air. It is 1.7  103 S cm1 in air and 4.25  103 S cm1 in argon atmosphere at 600 °C. Suksamai and Metcalfe [6] have reported that in air and argon atmosphere BCY exhibits oxide ion conductivity at low temperature while p-type conductivity at high temperature associated with different activation energies. However, in air no significant change in conduction phenomena is observed and the activation energy is 0.97 eV. In argon atmosphere, however, conduction phenomena is observed to change at 442 °C indicative of two processes one with activation energy of 1.5 eV and 0.83 eV respectively in low and high temperature regions. This is because, in argon, p-type conduction is increased with the temperature. 4. Conclusions Paper reports low temperature synthesis of single phase orthorhombic 20% yttrium doped barium cerate (BCY20) thin films by the novel spray pyrolysis technique. The solution concentration and annealing temperatures were optimized to get phase pure BCY20 films. The films obtained from 0.1 M solution and annealed at 900 °C were crystalline and dense. The thickness of the films is 1.4 lm, which can be monitored to suit it for SOFC application. The Raman and FTIR spectra confirmed absence of any surface impurities and assured phase pure BCY20 formed at optimized solution concentration and temperature. The dc conductivity measurement in argon atmosphere showed two types of conduction: oxide ion conduction and p-type conduction in low and high temperature regions, respectively. Acknowledgement Miss S.U. Dubal is highly grateful to University grant commission, New Delhi for its support through Rajiv Gandhi Junior Research Fellowship.

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References [1] H. Iwahara, T. Yajima, T. Hibino, K. Ozaki, H. Suzuki, Solid State Ionics 61 (1993) 65–69. [2] X. Fu, J. Luo, A.R. Sanger, N. Luo, K.T. Chuang, J. Power Sources 195 (2010) 2659–2663. [3] A.N. Virkar, H.S. Maiti, J. Power Sources 14 (1985) 295–303. [4] T. Hibino, A. Hashimoto, M. Suzuki, Electrochem. J. Soc. 149 (2002) A1503– A1508. [5] A. Longo, F. Giannici, A. Balerna, C. Ingrao, F. Deganello, A. Martorana, Chem. Mater. 18 (2006) 5782–5788. [6] W. Suksamai, I.S. Metcalfe, Solid State Ionics 178 (2007) 627–634. [7] N. Maffei, L. Pelletier, A.M. Farlan, J. Power Sources 175 (2008) 221–225. [8] E. Gorbova, V. Maragou, D. Medvedev, A. Demin, P. Tsiakaras, J. Power Sources 181 (2008) 207–213. [9] K. Takeuchi, C.K. Loong, J.W. Richardson Jr., J. Guan, S.E. Dorris, U. Balachandran, Solid State Ionics 138 (2000) 63–77. [10] M. Jacquin, Y. Jing, A. Essoumhi, G. Taillades, D.J. Jones, J. Rozière, J. New Mater. Electrochem. Syst. 10 (2007) 243–248. [11] M. Amsifa, D. Marrero-Lopez, A. Magraso, J.P. na-Martinez, J.C. Ruiz-Morales, P.N. nez, J. Eur. Ceram. Soc. 29 (2009) 131–138. [12] R. Koferstein, L. Jager, S.G. Ebbinghaus, J. Mater. Sci. 45 (2010) 6521–6527. [13] A.P. Jamale, S.U. Dubal, S.P. Patil, C.H. Bhosale, L.D. Jadhav, Appl. Surf. Sci. 286 (2013) 78–82. [14] L.D. Jadhav, A.P. Jamale, S.R. Bharadwaj, S. Varma, C.H. Bhosale, Appl. Surf. Sci. 258 (2012) 9501–9504. [15] M.G. Chourashiya, S.H. Pawar, L.D. Jadhav, Appl. Surf. Sci. 254 (2008) 3431– 3435. [16] M.G. Chourashiyaa, L.D. Jadhav, Int. J. Hydrogen Energy 36 (2011) 14984– 14995. [17] M.G. Chourashiya, S.R. Bhardwaj, L.D. Jadhav, J. Solid State Electrochem. 14 (2010) 1869–1875. [18] A.A. Rizkalla, A.H. Lefebvre, J. Eng. Power 97 (1975) 173–177. [19] A.D. Sheikh, V.L. Mathe, J. Mater. Sci. 43 (2008) 2018–2025. [20] M. Amsifa, D. Marrero-López, A. Magrasó, J. Pe~ na-Martínez, J.C. Ruiz-Morales, P. Núnez, J. Eur. Ceram. Soc. 29 (2009) 131–138. [21] S.K. Knight, N. Bonanos, J. Mater. Chem. 4 (1994) 899. [22] T. Scherban, R. Villeneuve, L. Abello, G. Lucazeau, Solid State lonics 61 (1993) 93–98. [23] G. Chiodelli, L. Malavasi, C. Tealdi, S. Barison, M. Battagliarin, L. Doubova, Fabrizio, C. Mortalò, R. Gerbasi, J. Alloys Comp. 470 (2009) 477–485. [24] M.G. Chourashiya, S.H. Pawar, L.D. Jadhav, J. Appl. Surf. Sci. 254 (2008) 3431– 3435. [25] L.D. Jadhav, M.G. Chourashiya, K.M. Subhedar, A.K. Tyagi, J.Y. Patil, J. Alloys Comp. 470 (2009) 383–386. [26] L.D. Jadhav, M.G. Chourashiya, A.P. Jamale, A.U. Chavan, S.P. Patil, J. Alloys Comp. 506 (2010) 739–744. [27] J.F. Scott, J.P. Mathieu (Eds.), Advances in Raman Spectroscopy 1 (1972) 115.