Gadolinium-doped ceria nanopowders synthesized by urea-based homogeneous co-precipitation (UBHP)

Accepted Manuscript Gadolinium-doped ceria nanopowders synthesized by Urea-Based Homogeneous co-Precipitation (UBHP)

G. Accardo, L. Spiridigliozzi, R. Cioffi, C. Ferone, E. Di Bartolomeo, Sung Pil Yoon, G. Dell’Agli PII:

S0254-0584(16)30893-8

DOI:

10.1016/j.matchemphys.2016.11.060

Reference:

MAC 19326

To appear in:

Materials Chemistry and Physics

Received Date:

25 July 2016

Revised Date:

18 October 2016

Accepted Date:

29 November 2016

Please cite this article as: G. Accardo, L. Spiridigliozzi, R. Cioffi, C. Ferone, E. Di Bartolomeo, Sung Pil Yoon, G. Dell’Agli, Gadolinium-doped ceria nanopowders synthesized by Urea-Based Homogeneous co-Precipitation (UBHP), Materials Chemistry and Physics (2016), doi: 10.1016/j. matchemphys.2016.11.060

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ACCEPTED MANUSCRIPT     

Urea-based homogeneous co-precipitation is applied to synthesize nanocrystalline GDC. Dense GDC samples at different sintering temperatures were characterized. SEM and TEM revealed a well define microstructure and controlled composition. Correlation between electrochemical properties by EIS and microstructure was discussed. UBHP method can be used to prepare high performance GDC electrolytes.

ACCEPTED MANUSCRIPT

Gadolinium-doped ceria nanopowders synthesized by Urea-Based Homogeneous co-Precipitation (UBHP). G. Accardo1*, L. Spiridigliozzi2, R. Cioffi3, C. Ferone3, E. Di Bartolomeo4, Sung Pil Yoon1 , G. Dell’Agli2 1

Fuel Cell Research Center, Korea Institute of Science and Technology, Hwarangno 14-gil, Seongbuk-gu, Seoul 136-791, South

Korea 2

Department of Civil and Mechanical Engineering and INSTM Research Unit, University of Cassino and Southern Lazio, Via G. Di

Biasio 43, 03043 Cassino (FR) Italy 3

Department of Engineering and INSTM Research Unit, University Parthenope of Naples, Centro Direzionale, Is. C4, 80143 Napoli,

Italy 4

Department of Chemical Science and Technology, University of Rome “Tor Vergata” Viale della Ricerca Scientifica 00133, Rome,

Italy

*Corresponding author: Grazia Accardo, Fuel Cell Research Center, Korea Institute of Science and Technology, phone: +82-2-958-5885: fax: +82-2-958-5199; E-mail: [email protected]

Abstract: Gadolinium (10%)-doped ceria was successfully synthesized by using an urea-based coprecipitation method (UBHP). A single fluorite phase was obtained after a low temperature (400 °C) calcination treatment. The resulting powders showed grains of nanometric size with some agglomerations and an overall good sinterability. Pellets were sintered at 1300 and 1500 °C for 3 h. The ionic conductivity was measured by electrochemical impedance spectroscopy measurements and a correlation between electrical properties and microstructure was revealed. The promising conductivity values showed that the synthesized powders are suitable for intermediate temperature solid oxide fuel cells (IT-SOFCs) applications. Keywords:Urea-Based Homogeneous co-Precipitation, nanopodwers, gadolinium doped ceria, electrolytes, solid oxide fuel cell

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ACCEPTED MANUSCRIPT 1. Introduction The choice of metal oxides for intermediate temperature solid oxide fuel cells (IT-SOFC) application primarily depends on the ionic conductivity that is strongly related to their microstructure, and thus to their fabrication methods [1-2]. Moreover, powders for electrolytes have to be easily sinterable into dense membranes with an enough mechanical strength, reduced thickness and large area to minimize the overall cell resistance. Thus, a fully dense microstructure is necessary to maximize the conductivity and minimize the reactants cross-over. The required ionic conductivity value is as large as ~10-2 S/cm with an activation energy lower possibly than 1 eV and a negligibly electronic conductivity [3-5]. Finally, the electrolytes should be low costly and mechanically and chemically compatible with electrodes to avoid the formation of blocking insulating phases at the interface. Currently, fluorite structure materials such as zirconia-based and ceria-based oxides, and perovskite oxides such as LaGaO3-based materials have shown great potential as electrolytes for SOFC applications. Because of their large ionic conductivity at reduced operating temperatures, ceriabased electrolytes, typically with Ce+4 substituted by Gd+3 (GDC), offer many advantages over traditional zirconia-based YSZ electrolytes for applications in intermediate temperature range [6]. Furthermore, doped ceria shows good chemical compatibility with highly performing cathode materials [7] and can be also incorporated into anodes improving the fuel oxidation electro-catalytic activity and providing a viable path toward internal reforming and/or direct utilization of hydrocarbon fuels [8]. Usually, the processing of gadolinium doped ceria (GDC) pellets consists of three steps: powder synthesis, shaping and sintering [9-11]. Among those steps, the synthetic route plays a critical role on the final particle size and it largely affects the microstructure of the sintered pellets as well as their ionic conduction. The sintering profile (atmosphere, heating/cooling rate, highest sintering 2

ACCEPTED MANUSCRIPT temperature and dwell time) can also obviously affect the microstructure and the shaping, i.e. pellets prepared by isostatic pressing are denser than that produced by extrusion. To improve GDC sinterability, the use of a chemical synthetic method is a key point to get a controlled, homogeneous and nanometric resulting powders. Several chemical methods have been used for GDC production such as: homogenous precipitation [12], sol gel process [13-14], hydrothermal synthesis [15], glycine-nitrate process [16]. Among the cited several methods, the urea-based homogeneous precipitation (UBHP) is particularly promising to produce ceria nano-powders with regular microstructure [17-18]. UBHP, method was used for a variety of inorganic ceramic powders with uniform size distribution and well defined stoichiometry and morphology by taking advantages from the slow decomposition of urea at elevated temperatures (>83 °C), which serves as a reservoir for precipitating anions [19-20]. Silver et al. [21] have shown that if a preliminary careful check of starting precursors is carried out, the UBHP method ensures a precise and predictable type of co-precipitate. Wu et al. [22] have found that, regardless of the starting environment (basic, acid or neutral), the stable pH achieved during the decomposition of urea in aqueous solution is an additional benefit for an accurate control of the composition of ultrafine powders. Moreover, it also been reported [23-25] that a narrow particle distribution and an high specific surface area of powders can be achieved in comparison to powders synthesized by using conventional co-precipitation methods even if a proper chemical and physical condition can be necessary [26]. Thus, the UBHP technique represents an alternative method to prepare GDC electrolyte. The existing background about the employment of UBHP method in the production of electrolytes is concentrated mainly on the synthesis process and morphology of the powders. To the best of our knowledge, only few data were reported regarding the electrical properties of sintered pellets. Therefore, the aim of this work is to investigate the microstructure and conductivity properties of nano-ceria doped with 10mol% Gd (GDC) synthesized by UBHP method. 3

ACCEPTED MANUSCRIPT The thermal behavior of as-synthesized and calcined powders was evaluated both by thermal gravimetric analysis (TG-DTA) and in-situ high temperature X-ray diffraction, the crystallites size was determined by X-ray diffraction analysis (XRD). No sintering aids, dispersant or protective agent were used during sintering cycle at 1300 °C and 1500 °C and the relative densities were evaluated by Archimedes’ methods. The microstructure was investigated by electron scanning microscopy (SEM) and transmission electron microscopy (TEM), the ionic conductivity was measured by electrochemical impedance spectroscopy (EIS).

2. Experimental Ceria doped with 10 mol% Gd (GDC), named as GDC10, was synthesized using commercial cerium (III) nitrate hexahydrate(Ce(NO3)3∙6H2O, 99+% purity), Gadolinium nitrate hexahydrate (Gd(NO3)3∙6H2O, 99.9+% purity) and Urea (CO(NH2)2 99+% purity) as starting precursors. Basically, several steps are involved in the UBHP method: dissolution of precursors, decomposition of urea and formation of metal oxide particles with binary composition as the case of GDC. Urea and metal nitrates were dissolved in the same solution to promote the formation of an homogeneous co-precipitate and molar concentrations were chosen of 2 M and 0.1 M for urea and total metallic cations, respectively. In situ decomposition of urea, for temperature > 83 °C, is in synchrony with the active release, homogeneously into the solution, of OH− and CO32− ions that leads to the precipitation of oxide particles avoiding the localized distribution of the reactants. The nucleation and growth of the precipitate with controlled particle morphology is possible thanks to these phenomena [19, 27]. The formed co-precipitate is aged in boiling conditions under vigorous stirring for 1 h. During the last two steps a total reflux apparatus is used to avoid the escape of the evaporated solution so keeping constant conditions. The co-precipitate was filtered and washed repeatedly with deionized water in 4

ACCEPTED MANUSCRIPT order to remove the undesired ions, after drying at 60 °C overnight and then a calcinations step was carried out in air at 400 °C for 1 h. The powders were finally grinded and compacted in cylindrical pellets by cold isostatic pressing at 160 MPa. Sintering was performed in air using different rates for heating and cooling, 3 °C min-1 and 10 °C min-1 respectively. Two sintering temperature were investigated, 1300 and 1500 °C with a dwell time of 3 hours. The relative density of sintered pellets was determined by hydrostatic balance. The crystalline structure of both as-prepared and calcined powders was evaluated by X-ray diffraction (XRD) using a Panalytical X'PERT MPD diffractometer. The XRD analysis was also carried out at high temperature (up to 1300°C) in air by an Anton Paar HTK 16 high temperature stage. The heating rate was 5°C/min while a dwell time of 10 min was set up at each temperature before starting the X-ray analysis. The primary particles size was calculated by the Scherrer equation (using X’Pert High Score Panalytical software for the peak profile with a pseudo-Voigt function as peak modelling): d=K λ/(B cosθ)

(1)

where K is the shape factor (assumed equal to 0.89),  the X-ray wavelength (0.1541 nm for Cu K),  the Bragg’s angle of the most intense diffraction peak and B the full width at half maximum of the same peak, corrected for the instrumental broadening, given by: B=Bsample - Binstr

(2)

where Binstr was determined using standard polycrystalline silicon. The thermal decomposition behavior of GDC10 powders was evaluated by simultaneous differential scanning calorimetry and thermogravimetric analysis (DSC and TGA, Thermoanalyzer STA 409, Netzsch) in air at a heating rate of 10 °C/min up to 1200 °C and using Al2O3 as a reference.

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ACCEPTED MANUSCRIPT The specific surface area of synthesized samples was measured by the BET method using a Micromeritics Gemini apparatus and utilizing nitrogen as an adsorbate, after drying the powders under vacuum at 100 °C. Morphology, particle size and distribution of the powders were analyzed by Transmission Electron Microscope (TEM, FEI Tecnai G2 Spirit Twin) while the microstructure of the sintered pellets was observed by SEM, FEI Quanta 200 FEG and Philips XL30 respectively, while the EDX analysis was performed by using an Energy Dispersion Spectrometer Oxford Inca Energy System 250 equipped with INCAx-act LN2-free detector, working at 20 kV. Electrochemical impedance spectroscopy (EIS) measurements were performed on sintered pellets using symmetric electrodes. Gold conductive paste was deposited on both sides of the pellets and thermal treated at 600°C for 1h to ensure good adhesion and proper electric contact. EIS measurements were carried out in the 300-800 °C temperature range using a frequency response analyzer (FRA, Solartron 1260), coupled with a dielectric interface (Solartron 1296) in a frequency range between 0.1 Hz and 1 MHz with an AC voltage amplitude of 100 mV. The data analysis of impedance plots was carried out by ZPlot and Zview software.

3. Results and Discussion The co-precipitate GCD10 powders synthesized from UBHP method showed a white color. Generally, the white color of the co-precipitates denotes that gadolinium-doped ceria with fluorite structure is still not formed. This feature was confirmed by the XRD analysis (data not shown). The diffraction pattern showed that a crystalline oxycarbonate phase was formed corresponding to cerium oxide carbonate hydrate, Ce2O(CO3)2.H2O, reference pattern ICDD card n. 43-602, according to previously reported literature data, indicating that lanthanides generally form basiccarbonate in the presence of urea decomposition [20]. Very likely in this case a mixed ceriumgadolinium oxide carbonate hydrate is actually formed as confirmed by an evident shift of the 2 6

ACCEPTED MANUSCRIPT peaks positions due to the formation of the solid solution. The thermal behavior of the co-precipitate GDC10 powder was evaluated by DSC-TG as reported in Figure 1. A pronounced and sharp endothermic peak ascribable to the thermal decomposition of the carbonate-based phase was revealed at 316 °C. The thermal decomposition of the oxycarbonate phase is associated to a remarkable weight loss of 20% which does agree to the theoretical value of 20.7 % related to the simultaneous evolution of carbon dioxide, water and formation of fluorite phase, according to the following global reaction (3): (Ce0.9Gd0.1)2O(CO3)2.H2O + ½ O2→ 2Ce0.9Gd0.1O1.95+ 2CO2 + H2O

(3)

Figure 2 shows the XRD patterns of co-precipitate GDC10 at different temperatures from 200 to 1300 °C. The fluorite-based structure of Gadolinium-doped ceria (ICCD card n. 01-075-0161 used as reference pattern) was formed above 250 °C and this transformation was completed at 300 °C. At 200 °C (Figure 2a) only the oxycarbonate phase was visible as a consequence of urea coprecipitation synthesis, whereas at 250 °C (Figure 2b) the XRD peaks of fluorite structure come out, even though very broad, together with the oxy-carbonate phase. At 300 °C (Figure 2c) only the fluorite phase was detectable, probably with a residual amorphous phase. Finally, heating up to 1300 °C no other structural transformations occurred and an evident peak sharpening due to irreversible grain growth was observed. It is also visible an increasing peaks shift towards smaller 2 angles with increasing the temperature of the thermal treatment. Very probably this is related to the thermal expansion of the material with the temperature which induces an increase of the cell parameter and in turns a decrease of the Bragg angle of the XRD peaks. The calculated average crystallite size for the fluorite phase using the Scherrer formula at the main diffraction plane (111) is reported in Table 1.

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ACCEPTED MANUSCRIPT Temperature (°C)

Crystal size (nm)

300 400 600 800 1000 1300

10.6 10.8 12.0 20.4 47.9 71.2

Table 1 Crystal size of the fluorite phase deriving from the calcination of the coprecipitate at various temperatures up to 1300 °C

The obtained values showed the nano-crystalline nature of powders at all measured temperatures being the crystallite size always smaller than 100 nm. Clearly, increasing the temperature, the broadening of reflection peaks decreases and the crystallinity increases. The crystal growth started from 300 °C and increased exponentially with temperature. According to the endothermic peak temperature further confirmed by the XRD analysis, a calcination step at 400 °C for 1 hour was chosen to get powders with pure fluorite structure minimizing the effect of the crystal growth. The GDC10 fluorite single phase was readily obtained as reported in the diffraction pattern of Figure 3. The diffraction peaks correspond to crystallographic planes 111, 200, 220, 311, 222, 400, 331, 420, 422 and 333 (ICDD Card n. 01-075-0161). No peaks shift was observed denoting the formation of single-phase cubic GDC with fluorite-type structure. In addition, the absence of gadolinium oxide diffraction peaks indicated that Gd+3 cations mostly replaced Ce+3 and Ce+4 cations in the lattice. A quantitative crystallographic study of the calcined powders was performed using Rietveld refinement and MAUD suite [28]. For the cubic phase the Fm3m space group (n. 225) was assumed, with Gd+3 and Ce+4cations and O-2 anions in 4a and 8c positions, respectively. The results of Rietveld refinements are reported in Table 2. The lattice parameter deriving from the refinement 0.542 nm was in very good agreement with the value reported in the reference pattern 01-075-0161 (0.5418 nm). The average crystallite size of the calcined powders was about 14 nm. The difference in the calculated crystallite size at 400 °C by Scherrer formula, reported in Table 1, can be ascribed to the shorter exposition to 400 °C during in-situ XRD analysis. 8

ACCEPTED MANUSCRIPT GDC10 a (nm) Crystallite size (nm) Chi squared 2

0.542212±4.7x10-5 13.952±0.081 1.274

Table 2 Rietveld refinement results of the calcined powder at 400 °C for 1 hour

The morphology of the calcined powders was revealed by TEM analysis reported in Figure 4. Very small spherical particles of few tens of nanometers were visible together with few larger particles of irregular shape likely aggregates of the smaller particles. This morphology is derived directly from the morphology of the as-synthesized powders as it was reported by Li for the (Y1-xGdx)2O3 compounds [20]. The specific surface area of the calcined powders measured by BET method was 87.95 m2/g. The average diameter of the primary particle was calculated, considering all particles as spherical and using the following relationship [28]: dp= 6000 / (ρ S)

(4)

where dp is the equivalent spherical diameter (in nm),  is the density (7.22 g/cm3 according to ICDD card n. 01-075-0161) and S is the specific surface area (in m2/g). The obtained value, close to 10 nm is in good agreement with the value obtained by XRD analysis. Pellets of calcined nanopowders were sintered in air at 1300 °C and 1500 °C and the relative densities were 90% and 95%, respectively. The densification kinetics of these types of electrolytes are sensible to the sintering atmosphere. In fact, as reported in [30], when a reducing atmosphere for calcination and sintering is preferred to air, an enhanced densification can be obtained and the reduction of some Ce4+ to Ce3+ leads to the formation of more oxygen vacancies. The SEM micrographs reported in Figure 5 show the microstructure of the pellets sintered at different temperatures. Chemical analysis was performed by EDX on the samples in four different positions. The atomic ratio between cerium and gadolinium resulted 0.895:0.105, 0.885:0.115, 0.884:0.116 and 0.902:0.098 namely in very good agreement with the nominal composition 9

ACCEPTED MANUSCRIPT confirming that during the UBHP synthesis a well stoichiometric composition can be achieved even in the presence of binary systems. Figure 5a shows the surface of the pellet sintered at 1300 °C. Pores of micrometric dimension, little fractures and grains of different shapes were observed. Apparently, the submicrometric grain size was too low to induce a significant grain growth at 1300 °C. The grain size observable from Figure 5a is about 500 nm, while the crystallite size reported in Table 1 after XRD at 1300 °C is 71.2 nm. This difference is due to the grain growth conditions because, in the XRD analysis, the noncompacted powders are exposed at high temperature only for a short period of time, thus limiting the growing process. The sintering process, i.e. long time exposition at high temperature of compacted powders, converts the powders in a dense ceramic electrolyte. Different morphological features were observed for the pellet sintered at 1500 °C as shown in Figure 5b. The micrograph reveals high densification, low porosity, no interspaces or cracks. The sample surface showed a texture formed by grains of few microns. Impedance measurements were performed on GDC10 pellets sintered at 1300 and 1500 °C in the 300-800 °C temperature range. The measured data reported as Nyquist plots were fitted using an equivalent circuit to estimate the total resistivity values. From 800 to 550 °C the Nyquist plot showed a single semicircle due to bulk contribution, while at lower temperatures there is a double semicircle due to bulk and grain boundary contributions, as reported in Figure 6 at 500 °C. This behavior is in agreement with literature [31]. Figure 7 shows the Arrhenius plots of the total conductivity of GDC10 samples sintered at 1300 °C and 1500 °C. The sample sintered at 1500 °C showed the largest total ionic conductivity values in the whole temperature range with a maximum value of 4.1∙10-2 S cm-1 at 800 °C. This feature is in agreement with the microstructural analysis reported in Figure 5 being the pellets sintered at higher temperature denser and less porous and thus more conductive than the pellets sintered at 1300 °C. In both samples a change in the linear slope was observed around 550 °C the same temperature value at which the grain boundary contribution 10

ACCEPTED MANUSCRIPT comes out and can be deconvoluted as a separate arc. As a consequence, two distinct values of the activation energies can be retrieved from data fitting: lower values at T>550 °C, higher values at T<550 °C as resumed in Table 3. Below 550 °C the total conductivity significantly decreased due to the grain boundary contribution thus the sample sintered at 1300 °C is mainly affected. However, the measured conductivity values are still in the range required for applications in IT-SOFC [3233].

Ea 550°C (eV) Ea <550°C (eV) tot800 °C (Scm-1) tot650 °C (Scm-1)

GDC sintered 1300 °C

GDC sintered 1500 °C

0.82 0.88 1.9˙10-2 0.5˙10-2

0.67 0.70 4.1˙10-2 1.55˙10-2

Table 3 Activation Energy and ionic conductivity values for GDC samples sintered at 1300 and 1500°C

Finally, the UBHP method was successful employed to produce nanometric GDC10 powders able to be sintered at reduced sintering temperature with conductivity values suitable for application in IT-SOFCs. It is worth noting that this synthesis procedure is generally applied for catalysts with different purpose from electrolytes, but the good results obtained can encourage the further development and application of the UBHP method to solid oxides.

4. Conclusions The urea-based homogeneous co-precipitation method was applied to synthesize nanocrystalline powders for gadolinium doped ceria. To decompose the oxycarbonate phase present in the assynthesized powder and obtain a single-phase fluorite structure, a calcination step at 400 °C was carried out. The low temperature thermal treatment was able to preserve the nanometric size of the powders and to hinder undesirable grain growth and agglomeration phenomena. Dense GDC samples with a relative density of 95% and a uniform microstructure with grains of few microns were achieved after a sintering at 1500 °C for 3 hours. This sample showed conductivity values of 11

ACCEPTED MANUSCRIPT 1.1∙10-2 and 4.1∙10-2 S/cm in the temperature range of 650-800 °C remarkable for application as electrolyte in SOFCs. With this work, we can confirm a renewed interest towards the urea co-precipitation method, highlighted by the numerous papers published in the last years, as a method to synthesized mixed oxide with very interesting properties for applications in SOFCs. Future work will be devoted to the optimization of the chemical and physical conditions of the synthesis to further improve the morphology of the powders.

Acknowledgments This work was supported by KRF - Korean Research Fellowship Program through the National Research Foundation of Korea, funded by the Ministry of Science, ICT and Future Planning of /Republic of Korea (Grant Number:2016H1D3A1908428 ).

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ACCEPTED MANUSCRIPT Figure Captions Figure 1 Thermograph of the coprecipitate GDC10. Figure 2 XRD patterns of the co-precipitate GDC10 at different temperatures: a) 200 °C; b) 250 °C; c) 300 °C; d) 400 °C; e) 600 °C; f) 800 °C; g) 1300 °C. Figure 3 XRD patterns of co-precipitate GDC10 after calcination at 400 °C for 1 hour. Figure 4 TEM micrographs of GDC10 powders calcined at 400 °C Figure 5 SEM micrographs of GDC10 pellet sintered at: a) 1300 °C and b) 1500 °C. Figure 6 Nyquist plot recorded at 500°C for GDC10 pellet sintered at 1300 °C and 1500 °C. Figure 7 Arrhenius plot of total ionic conductivity of pellets sintered at 1300 and 1500 °C.

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