The characteristics of nano-sized Gd-doped CeO 2 particles prepared by spray pyrolysis

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1 Journal of Alloys and Compounds 398 (2005) The characteristics of nano-sized Gd-doped CeO 2 particles prepared by spray pyrolysis Hee Sang Kang a, Jong Rak Sohn b, Yun Chan Kang c,, Kyeong Youl Jung b, Seung Bin Park a a Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology, Guseong-dong, Yuseong-gu, Daejeon , Republic of Korea b Advanced Materials Division, Korea Research Institute of Chemical Technology, P.O. Box 107, Yuseong-gu, Daejeon , Republic of Korea c Department of Chemical Engineering, Konkuk University, 1 Hwayang-dong, Gwangjin-gu, Seoul , Republic of Korea Received 11 January 2005; received in revised form 7 February 2005; accepted 7 February 2005 Available online 11 March 2005 Abstract Gd-doped ceria particles with nanometer size were prepared by spray pyrolysis from spray solution containing ethylene glycol. The effect of ethylene glycol and gadolinium dopant on the morphology of the ceria particles prepared by spray pyrolysis was investigated. The Gddoped ceria particles prepared from pure aqueous solution had submicron size, spherical shape and dense structure after post-treatment. The Gd-doped ceria particles prepared from solution containing ethylene glycol had loosely aggregated structure of the primary particles with nanometer size after post-treatment at 900 and 1050 C. The Gd-doped ceria particles had more loosely aggregated and regular morphology than the pure ceria particles at the post-treatment temperature of 900 C. The gadolinium component used as dopant suppressed grain growth in ceria particles prepared by spray pyrolysis. Nanometer size Gd-doped ceria particles were prepared by a ball milling process of the particles obtained by spray pyrolysis. The mean size of the Gd-doped ceria particles was 46 nm after a post-treatment at 1050 C. The ionic conductivity of the Gd-doped ceria particles prepared by spray pyrolysis was compared with that of the commercial yttria-stabilized zirconia (YSZ) Elsevier B.V. All rights reserved. Keywords: Ceria; Spray pyrolysis; Fuel cell; Electrolyte; Nano-particle 1. Introduction Low operating temperature (below 800 C) solid oxide fuel cells (SOFCs) have recently attracted much attention because the cost of materials and fabrication will be dramatically reduced. A lower operating temperature also implies several advantages such as less thermal mismatch between cell components, wider choice of sealing materials, and longer operational life. However, decreasing the operating temperature generally results in increased losses of power density at a constant efficiency. One of the causes of this major problem is the ohmic drop through the electrolyte. The ionic Corresponding author. Tel.: ; fax: address: yckang@konkuk.ac.kr (Y.C. Kang). conductivity of Y 2 O 3 -stabilized ZrO 2 (YSZ), the most commonly used electrolyte material in SOFC, is not sufficient at reduced operating temperatures. Therefore, using other types of solid electrolytes with high ionic conductivity is one possibility to increase the performance of single cells at low operating temperatures. The trivalent rare-earth-doped ceria has been extensively studied as electrolytes in low operating temperature SOFCs because of high ionic conductivity at low temperatures [1 4]. The ionic conductivities of ceriarare earth oxide systems depended on the ionic radii of the added cation. Gadolinium- and samarium-doped ceria have the highest electrical conductivity because of the close ionic radii of Gd 3+ and Sm 3+ compared to the radius of Ce 4+ [4 7]. Another method to decrease voltage losses at low operating temperature of SOFCs is a reduction of the electrolyte /$ see front matter 2005 Elsevier B.V. All rights reserved. doi: /j.jallcom

2 H.S. Kang et al. / Journal of Alloys and Compounds 398 (2005) thickness by using an electrode supported thin film electrolyte. The reduction of electrolyte thickness is effective in low operating temperature SOFCs because decreasing the electrolyte thickness has low voltage losses at low temperatures [8]. The electrolyte also has to be gas tight as well as to have a high ionic conductivity. Therefore, nano-sized electroceramic particles should be obtained near full density at low temperature and as thin electrolyte membrane. In this work, nano-sized 10 mol% Gd-doped ceria particles were synthesized by a large-scale spray pyrolysis process. The as-prepared particles with hollow and porous morphology obtained by spray pyrolysis were post-treated at high temperature. In spray pyrolysis, hollow particles can be formed when a solute concentration gradient is created during evaporation of the droplet. The solute precipitates first at the more highly supersaturated surface if sufficient time is not available for solute diffusion in the droplet. Thus, the morphology of particles is strongly influenced by the preparation condition in spray pyrolysis. In this work, the Gd-doped ceria particles were prepared under severe preparation conditions such as high solution concentration, short residence time and high gas flow rate to obtain a more hollow and porous morphology. Moreover, the puff-type particles were obtained by adding ethylene glycol into the spray solution to synthesize the Gd-doped ceria particles with nanometer size and loosely aggregated structure. The ionic conductivity of the prepared pellets was determined via electrochemical impedance spectroscopy at temperatures from 600 to 900 C in air. The crystal structures of the as-prepared and post-treated ceria particles were investigated by using X-ray diffractometry (XRD, Rigaku DMAX-33). The mean crystallite size of the ceria particle was estimated from Scherrer s equation. The morphological characteristics of the particles were investigated in using scanning electron microscopy (SEM, Philips XL 30S FEG and Jeol JSM-6700F) and transmission electron microscope (TEM, Carl Zeiss EM912). 3. Results and discussion The morphology of the Gd-doped ceria particles prepared by spray pyrolysis was strongly affected by the types of spray solution. Figs. 1 and 2 show the SEM photographs of the Gddoped ceria particles prepared from pure aqueous and ethylene glycol solution, respectively. The concentration of ethylene glycol used as organic additive was 60 mol% of cerium nitrate, in which the Gd-doped ceria particles had good morphology. The as-prepared particles using spray pyrolysis at 1000 C were post-treated at 900 and 1050 C for 3 h to improve the crystallinity of the Gd-doped ceria particles. The Gd-doped ceria particles prepared from pure aqueous solution had submicron size, spherical shape and dense structure 2. Experimental The spray pyrolysis system consists of droplet generator, quartz reactor, and particle collector. A 1.7 MHz ultrasonic spray generator having six vibrators was used to generate a large amount of droplets, which are carried into the hightemperature tubular reactor by a carrier gas. Droplets and particles evaporated, decomposed, and/or crystallized in the quartz reactor. The length and diameter of the quartz reactor are 1200 and 50 mm, respectively. The cerium and gadolinium precursor used in this work was cerium and gadolinium nitrate. Ethylene glycol was used as additive to the spray solution to change the morphology of the as-prepared particles obtained by spray pyrolysis. The concentration of cerium and gadolinium nitrate was fixed at 0.5 M. The doping concentration of gadolinium was 10 mol% of the cerium content. The concentration of ethylene glycol was adjusted from 0 to 100 mol% of the cerium nitrate. The flow rate of air used as carrier gas was 40 l/min. The ceria particles as-prepared at 1000 C by spray pyrolysis were post-treated in the box furnace at 900 and 1050 C in air atmosphere to change the crystallite size and morphology of the particles. A ball milling process using zirconia balls (2 mm in diameter) was then applied to form nano-sized ceria particles. For the measurement of the ionic conductivity, pellets with a thickness of 2 mm were prepared using Gd-doped ceria particles with a mean size of 46 nm and commercial yttria-stabilized zirconia particles. The sintering temperature of the pellets was 1300 C. Fig. 1. SEM photographs of Gd-doped ceria particles prepared from aqueous solutions at different post-treatment temperature.

3 242 H.S. Kang et al. / Journal of Alloys and Compounds 398 (2005) Fig. 2. SEM photographs of Gd-doped ceria particles prepared from ethylene glycol solution at different post-treatment temperature. after post-treatment. The submicron size particles prepared from pure aqueous solution had a hard-aggregated structure of primary particles with nanometer size. The degree of aggregation and grain growth of the primary particles was increased at high post-treatment temperature. On the other hand, the Gd-doped ceria particles prepared from ethylene glycol solution had hollow and porous morphology. The submicron size particles prepared from ethylene glycol solution had loosely aggregated structure of the primary particles with nanometer size after post-treatment at 900 and 1050 C. The Gd-doped ceria particles post-treated at 1050 C had larger size primary particles than that of the particles post-treated at 900 C. In the preparation of the Gd-doped ceria particles by spray pyrolysis, the post-treatment temperature was efficient to the control of the primary particle size. The Gddoped ceria particles prepared from ethylene glycol solution had finer grain size than that of the particles prepared from pure aqueous solution at the same post-treatment temperature. The morphology change of the post-treated Gd-doped ceria particles resulted from the different morphology of the as-prepared particles obtained from pure aqueous and ethylene glycol solutions by spray pyrolysis. The as-prepared particles obtained from pure aqueous solution by spray pyrolysis had a more dense morphology than those obtained from ethylene glycol solution. The spherical shape and dense morphology of the as-prepared particles obtained from pure aqueous solution was maintained after post-treatment at 900 and 1050 C. On the other hand, the as-prepared particles obtained from ethylene glycol solution had hollow and porous morphology with thin wall structure. The ceria particles had hollow and porous morphology due to the evolving gas resulting from decomposition of the ethylene glycol used as organic additive during post-treatment process. Thus, the asprepared particles obtained from the ethylene glycol solution turned into loosely aggregated particles with nanometer size primary particles after post-treatment at 900 and 1050 C. Pure ceria particles were also prepared from the ethylene glycol solution to show the effect of the gadolinium dopant on the morphology of the ceria particles. Fig. 3 shows the SEM photograph of the pure ceria particles. The asprepared particles prepared from ethylene glycol solution were post-treated at 900 C for 3 h. The pure ceria particles had a hardly aggregated morphology with fine primary particle size. In Figs. 2 and 3, the Gd-doped ceria particles had a more loosely aggregated and regular morphology than the pure ceria particles after the post-treatment temperature of 900 C. The gadolinium component used as dopant suppressed grain growth of the ceria particles prepared by spray pyrolysis. Dopants such as Gd 3+,Y 3+ were reported to be effective in suppressing grain growth in ceria particles [9 11]. The Gd-doped ceria particles have smaller grain size in comparison with undoped ceria particles [11]. In the spray pyrolysis, phase separation between gadolinium and cerium components were minimized. Therefore, the Gd-doped ceria particles prepared by spray pyrolysis from ethylene glycol solution consisted of primary particles with fine size and narrow size distribution. Fig. 4 shows the TEM photograph of Gd-doped ceria particles prepared from an ethylene glycol solution by spray pyrolysis. The as-prepared particles were post-treated at 1050 C. Slightly ball milled particles were used as the sample for the TEM photograph. The mean size of the primary particles measured from the TEM photograph was 46 nm. Fig. 5 shows the XRD spectra of the as-prepared and posttreated Gd-doped ceria particles prepared from pure aqueous and ethylene glycol solutions. Both the as-prepared ceria particles obtained from pure aqueous and ethylene glycol solu- Fig. 3. SEM photograph of the pure ceria particles prepared from ethylene glycol solution.

4 H.S. Kang et al. / Journal of Alloys and Compounds 398 (2005) Fig. 6. Ionic conductivity of post-treated 10 mol% Gd-doped ceria particles prepared from ethylene glycol solution. Fig. 4. TEM photograph of post-treated Gd-doped ceria particles prepared from ethylene glycol solution. tions had poor crystallinity because of the short residence time of the particles inside the tubular reactor maintained at 900 C. The crystallinity of the particles increased with increasing post-treatment temperature. The particles prepared from ethylene glycol solution had lower crystallinity than those prepared from the pure aqueous solution after a posttreatment at 900 and 1050 C. The crystallite size of the Gddoped ceria particles prepared from pure aqueous solution was 48 and 78 nm after post-treatment at 900 and 1050 C, respectively. On the other hand, the crystallite size of the Gddoped ceria particles prepared from ethylene glycol solution was 38 and 59 nm after post-treatment at 900 and 1050 C, respectively. As shown in Figs. 2 and 3, the gadolinium component used as dopant suppressed grain growth in ceria particles prepared by spray pyrolysis. The ionic conductivity of the Gd-doped ceria particles was compared with that of commercial yttria-stabilized zirconia (YSZ) particles. Gd-doped ceria particles prepared from ethylene glycol solution by spray pyrolysis were used as the sam- ple for the measurement of the ionic conductivity after the ball milling process. Pellets with a thickness of 2 mm were prepared using the Gd-doped ceria particles with a mean size of 46 nm and the commercial yttria-stabilized zirconia particles. The sintering temperature of the pellets was 1300 C. The ionic conductivity of the prepared pellets was measured by electrochemical impedance spectroscopy at temperatures from 600 to 900 C in air. Fig. 6 shows Arrhenius plots of the ionic conductivities for the Gd-doped ceria and the commercial YSZ pellets with thickness of 2 mm. The ionic conductivity of the Gd-doped ceria had similar values as the commercial YSZ. In this work, characteristics of the Gd-doped ceria particles such as the crystallinity and the morphology were not optimized to achieve high ionic conductivity of particles. Additionally, the optimum doping concentration of the gadolinium in the ceria particles prepared by spray pyrolysis was not varied in order to optimize the ionic conductivity of the Gd-doped ceria particles. Therefore, the ionic conductivity of the nano-sized Gd-doped ceria particles prepared by spray pyrolysis could be improved by optimization of the preparation and the post-treatment processes. 4. Conclusion Fig. 5. XRD spectra of gadolinium-doped ceria particles prepared from aqueous and ethylene glycol solutions. A modified spray pyrolysis process was applied to the preparation of nano-sized Gd-doped ceria particles. The hollow and porous morphology of the as-prepared particles obtained from spray solution containing ethylene glycol produced nano-sized Gd-doped ceria particles after posttreatment at high temperature. The evolving gas resulting from the decomposition of the ethylene glycol led to the ceria particles with hollow and porous structure. The Gd-doped ceria particles had a more loosely aggregated and regular morphology than the pure ceria particles after a post-treatment at 900 C. The gadolinium component used as dopant suppressed grain growth in the ceria particles prepared by spray pyrolysis. The ionic conductivity of the Gd-doped ceria par-

5 244 H.S. Kang et al. / Journal of Alloys and Compounds 398 (2005) ticles was compared with that of the commercial yttriastabilized zirconia particles. The ionic conductivity of the Gd-doped ceria had similar values as the commercial yttriastabilized zirconia in the range of C. The nanosized Gd-doped ceria particles prepared by spray pyrolysis might be applied as electrolyte in low operating temperature solid oxide fuel cells (SOFCs) after improving the ionic conductivity of the particles by optimization of the preparation and the post-treatment processes. Acknowledgements This work was supported by the Korea Science and Engineering Foundation (KOSEF, R ). References [1] B.C.H. Steele, Solid State Ionics 129 (2000) 95. [2] C. Xia, M. Liu, Solid State Ionics (2002) 423. [3] J.R. Jurado, J. Mater. Sci. 36 (2001) [4] J. Van herle, T. Horita, T. Kawada, N. Sakai, H. Yokokawa, M. Dokiya, Solid State Ionics (1996) [5] H. Yahiro, K. Eguchi, H. Arai, Solid State Ionics 36 (1989) 71. [6] K. Eguchi, T. Setoguchi, T. Inoue, H. Arai, Solid State Ionics 52 (1992) 165. [7] D.Y. Chung, E.H. Lee, J. Alloys Compd. 374 (2004) 69. [8] E.I. Tiffee, A. Weber, D. Herbstritt, J. Eur. Ceram. Soc. 21 (2001) [9] P.L. Chen, I.W. Chen, J. Am. Ceram. Soc. 77 (1994) [10] P.L. Chen, I.W. Chen, J. Am. Ceram. Soc. 79 (1996) [11] M. Mogensen, N.M. Sammens, G.A. Tompsett, Solid State Ionics 129 (2000) 63.