Combustion Synthesis of Ce 0.9 Gd 0.1 O 1.95 for Use as an Electrolyte for SOFCs

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1 Journal of Metals, Materials and Minerals, Vol.18 No.2 pp , 2008 Combustion Synthesis of Ce 0.9 Gd 0.1 O 1.95 for Use as an Electrolyte for SOFCs Waraporn NUALPANG 1, Navadol LAOSIRIPOJANA 1, Suttichai ASSABUMRUNGRAT 2, Uthaiwan INJAREAN 3, Pipat PICHESTAPONG 3, Sumittra CHAROJROCHKUL 4 1 The Joint Graduate School of Energy and Environment, King Mongkut s University of Technology, Thonburi, Bangkok, Thailand 2 Department of Chemical Engineering, Chulalongkorn University, Bangkok, Thailand 3 Thailand Institute of Nuclear Technology, Bangkok, Thailand 4 National Metal and Materials Technology Center, Pathumthani Abstract Received Feb. 18, 2008 Accepted Feb. 19, 2009 Doped ceria has been used as an electrolyte for intermediate temperature solid oxide fuel cells. However, the price is more expensive than the conventional materials due to the raw material cost and preparation cost. A combustion synthesis technique has been used to manufacture particles of doped ceria in a single step. It is a very cost effective technique that the overall reaction is complete in much shorter time than other conventional powder processing techniques. Therefore, the aim of this study is to prepare Ce 0.9 Gd 0.1 O 1.95 using a low cost combustion synthesis technique. In addition, chemicals prepared from Thailand Institute of Nuclear Technology (TINT) have been used as initial reactants. Ce 0.9 Gd 0.1 O 1.95 prepared from TINT chemicals have been compared with those synthesized from commercial chemical reactants. The effect of ratio between fuel and oxidant for the combustion synthesis technique is studied. The synthesized powder has been characterized using XRD for phase identification, SEM for microstructural analysis and Langmuir for surface area measurement. The particle size of both sample groups analyzed using a mastersizer is around 300 nm. The grain boundary is significant for both synthesized powder group. Ce 0.9 Gd 0.1 O 1.95 powders prepared from commercial chemical shown higher total conductivity than these prepared from local chemicals. Key words : Combustion synthesis; Gadolinia-doped ceria; IT-SOFC Introduction An intermediate temperature solid oxide fuel cells (IT-SOFC) operated at reduced temperature (<800 C) is currently interesting due to advantages in choices of low cost materials, increasing a system reliability, operation life-time, and reducing the cost of cell fabrication. (1, 2) Ceria-based materials have been a candidate material for use as an electrolyte in reduced temperature SOFCs due to its high ionic conductivity at low temperature (Huang, et al. 2007, Chourashiga, et al and Leng, et al. 2004) in comparison to that of the traditional electrolyte yttria-stabilized zirconia (YSZ). (5) Oxygen ion vacancies are responsible for the ionic conductivity observed in doped ceria. (6) The 10 mol % Gd 2 O 3 -doped CeO 2 shows the greatest ionic conductivity and the lowest (3, 7) activation activity among gadolinium-doped ceria. While the study of Peng and Zhang, (2007) showed that the highest conductivity was found for 20% of Gd doped ceria. Different synthesis methods have been adopted to prepare cerium oxide based materials. (9-12) A combustion synthesis has been interesting since this method is a simple, cost-effective method and requires a short reaction time. (13) It was observed that the physical properties of synthesized powder prepared by combustion synthesis depend on both (14, 15) the fuel-to-oxidant ratio and the type of fuel. In the present work, Ce 0.9 Gd 0.1 O 1.95 has been prepared via a combustion synthesis technique. The physical and electrical properties of as prepared powder are compared when the source of initial chemical reactants are different. The effect of fuel to oxidant ratio on those properties were Phone , Fax Sumittrc@mtec.or.th

2 224 NUALPANG, W. et al. also studied and discussed. The electrical property of this study was evaluated using an AC impedance measurement. Materials and Experimental Procedures Powder Preparation A commercial chemical reactant of Cerium(III) nitrate hexahydrate (Ce(NO 3 ) 3 6H 2 O, 99.5%) and gadolinium(iii) nitrate hexahydrate (Gd(NO 3 ) 3 6H 2 O, 99.9%) were supplied from Alfa Aesar while local initial reactants were produced from Thailand Institute of Nuclear Technology using a by-product from other chemical process. Cerium(III) nitrate hexahydrate and gadolinium(iii) nitrate hexahydrate from Alfar Aesar were taken to the required ratio and mixed with urea (NH 2 CONH 2, 98%, Alfa Aesar) with a few drops of deionized water. The molar ratios of the fuel to oxidant were varied as 2:1, 2.5:1 (stoichiometry) and 3:1. The mixed solution was then heated with an excess temperature until an autoignition ocurred. The as-synthesized powders were pulverized for 10 mins in a mortar. Characterization An X-ray diffractometer (JEOL, JDX-3530) using Cu-Kα was carried out on the synthesized powder for a phase identification. Particle sizes distributions of pulverized powder were determined using a laser diffraction particle size analyzer (Malvern, Mastersizer-S). The specific surface area (Langmuir) is determined using a nitrogen adsorption isotherm (Quantachrome, Autosorb-1). Particle morphology and microstructure were examined using a scanning electron microscope (JEOL, JSM6301F). The powders were further pressed into pellets at 150 psi. The pellets were sintered at 1400 C for 4 h. The pellets were painted with Au paste to form electrodes and then fired at 800 C for 1 h. The Au-coated pellets were analyzed using an AC impedance analyzer (SI1260, Impedance/Gainphase analyzer, Solartron). The measurements were conducted in the temperature range of C with an interval of 25 C. The impedance spectra of the cells were recorded in the frequency range from 0.05 Hz to 10 MHz. The impedance spectra data were fitted to distinguish the bulk resistance and grain-boundary resistance. Results and Discussion Physical Properties In our work, Ce 0.9 Gd 0.1 O 1.95 prepared from both local and commercial chemical reactants are compared as well as varying the fuel to oxidant ratio to investigate the effect in the physical and electrical properties of these samples for the further use as an electrolyte for SOFCs. The powders of Ce 0.9 Gd 0.1 O 1.95 prepared from both local and commercial initial reactant were hereafter referred to as -L and, respectively, as shown in Table 1. Table 1. Effect of fuel to oxidant ratio on the measured temperature, particle size (D mastersizer ) and Langmuir specific surface area (S) of Ce 0.9 Gd powder. Fuel to oxidant ratio Sample code The maximum me asured temperature ( C) D mastersizer (nm) S (m 2 g -1 ) 2.0 -L1 204 ± L2 264 ± L3 274 ± ± ± ± The reaction temperature was in the range of 200 to 350 C. The temperature of reaction increased with the fuel to oxidant ratio. A flameless reaction was observed for a preparation of 1 with a fuel to oxidant ratio of 2:1 while the preparation of from local reactant were autoignited for all the fuel to oxidant ratios. The colors of group are light yellow while these of -L group are orange-yellow color. The particles shown in SEM images (Figure. 1) are around 0.2 μm. However, the average particle sizes of the pulverized powder determined by a laser diffraction particle size analyzer were ranging from 270 to 290 nm. It tends to agglomerate to each other. The specific surface area of pulverized powder synthesized with the fuel to oxidant ratio of 2:1 and 2.5:1 are slightly close to each other but the one with the fuel to oxidant ratio of 3:1 is the highest for -L3 and the lowest for 3.

3 Combustion Synthesis of Ce 0.9 Gd 0.1 O 1.95 for Use as an Electrolyte for SOFCs 225 (a) (c) (b) (d) of these materials that separate out the bulk and grain boundary contribution to the total resistance. The total resistance of an electrolyte is given by: R t = R b + R gb (1) where R b is a bulk resistance (Ω), R gb is a grain boundary resistance (Ω) and R t is a total resistance (Ω). The conductivity is defined as: (e) (f) t = 1 R (2) s where t is the sample thickness (cm) and S is the sample area (cm 2 ). The conductivity data were plotted using an Arrhenius equation: E = 0 a T exp (3) kt Figure 1. SEM micrographs of pulverized Ce 0.9 Gd 0.1 O 1.95 powders (a) -L1, (b) -L2, (c) - L3, (d) 1, (e) 2 and (f) 3 Figure 2. Shows the X-ray diffraction patterns of the synthesized powders. The cubic fluorite structure has been obtained from powders of all conditions. No second phase is observed in any ratios. where is the activation energy of electrical conduction (ev), k is the Boltzman s constant (ev/k), T is the absolute temperature (K) and 0 is a pre-exponential factor being a constant in a certain temperature range. The electrical conductivity were extrapolated and the activation energy were calculated from the slope of Arrhenius plots as listed in Table Z" (ohm cm) L1 -L2 -L Z' (ohm cm) Figure 2. XRD patterns of the synthesized powder (a) -L1, (b) -L2, (c) -L3, (d) 1, (e) 2 (f) 3 (g) standard Gd 0.1 Ce 0.9 O 1.95 JCPDS AC-impedance Spectroscopy The AC impedance technique has been used for the measurement of the electrical conductivities Figure 3. Typical semicircles observed for all samples at 300 C The two arcs for all samples observed in Figure 3 shows Nyquist plot that corresponding to bulk at the left side and grain boundary conductivities of sample at the right side at 300 C, respectively. The grain boundary resistivity is greater than the bulk resistivity. This result agrees well with the

4 226 NUALPANG, W. et al. conductivities reported by D. Perez-Coll et.al. (16) Moreover, they had reported that the bulk resistance increase with an increase in Gd 3+ -content (Gd 3+ -content = 5, 10, 20 and 30%), whereas the grain boundary resistance shows the opposite trend. Table 2. Bulk ( b) and grain boundary ( gb) conductivities at 700 C and activation energy ( ) -L1 -L2 -L Bulk conductivity b Grain boundary conductiv ty gb Total conductivit y t It can be seen from Table 2 that -L3 has the highest conductivity among the local reactants while 1 shows the highest conductivity among the commercial reactant. The bulk conductivities of both groups are not much different but the contributions from grain boundary are significant. Conclusions Ce 0.9 Gd 0.1 O 1.95 powder has been prepared by a urea-nitrate combustion synthesis technique with different fuel to oxidant ratios from local and commercial chemical sources. All samples show a single phase of cubic Ce 0.9 Gd 0.1 O The powder with the particle size distribution around 300 nm can be obtained after grinding for 10 mins. For the Ce 0.9 Gd 0.1 O 1.95 prepared from local chemicals, the fuel to oxidant ratio of 3:1 showed the greatest total conductivity, whereas the fuel to oxidant ratio of 2:1 demonstrated the greatest conductivity of powder prepared from commercial chemicals. The total conductivity of Ce 0.9 Gd 0.1 O 1.95 powders prepared from commercial chemical are greater than these prepared from local chemical. Acknowledgments The authors would like to acknowledge the research funding from National Research Council of Thailand (NRCT), the Joint Graduate School of Energy and Environment (JGSEE), King Mongkut s University of Technology Thonburi and National Metal and Materials Technology Center (MTEC). References 1. Huang, B., Wang, S.R., Liu, R.Z., Ye, X.F., Nie, H.W., Sun, X.F. and Wen T.L Performance of Ni/ScSZ cermet anode modified by coating with Gd 0.2 Ce 0.8 O 2 for a SOFC. Mater. Res. Bull. 42(9) : Sarat, S., Sammes, N. and Smirnova, A Bismuth oxide doped scandia-stabilized zirconia electrolyte for the intermediate temperature solid oxide fuel cells. J. Power Sources. 160(2) : Chourashiya, M.G., Patil, J.Y., Pawar, S.H. and Jadhav, L.D Studies on structural, morphological and electrical properties of Ce 1-x Gd x O 2-(x/2). Mater. Chem. Phys. 109(1) : Leng, Y.J., Chan, S.H., Jiang, S.P. and Khor, K.A Low-temperature SOFC with thin film electrolyte prepared in situ by solid-state reaction. Solid State Ionics. 170(1-2) : Sha, X., Lu, Z., Huang, X., Miao, J., Liu, Z., Xin, X., Zhang, Y. and Sua, W Influence of the sintering temperature on electrical property of the Ce 0.8 Sm 0.1 Y 0.1 O 1.9 electrolyte. J. Alloys Comp. 433(1-2) : Skinner, S.J. and Kilner, J.A Oxygen ion conductors. Mater. Today. 6(3) : Stelzer, N., Nolting, J. and Riess, I Phase Diagram of Nonstoichiometric 10 mol% Gd 2 O 3 -Doped Cerium Oxide Determined from Specific Heat Measurements. J. Solid State Chem. 117(2) : Peng, C. and Zhang, Z Nitrate-citrate combustion synthesis of Ce 1-x Gd x O 2-x/2 powder and its characterization. Ceram. Inter. 33(6) :

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