Gadolinia-doped ceria (GDC) is one of the most promising electrolyte for intermediate temperature solid oxide fuel cells (SOFCs). In particular, the production of GDC as nanopowders leads to an higher reactivity that allows better performances, milder sintering conditions and lower production costs. However, nanopowders can be produced only by carefully tailoring their production process. The choice and optimization of the synthesis process is therefore a key step for the production of powders suitable for efficient SOFC components. In this work nanocrystalline GDC (Ce0.8Gd0.2O2-delta ) particles were successfully obtained by one-step microwave-assisted synthesis from a diethylene glycol solution of metal nitrates under mild conditions (170?C, 1 atm). The as-prepared powder showed good crystallinity with specific surface area of 50 m2/g. The sintering and electrochemical properties were compared with a nanometric commercial powder. The MW-produced powder showed an improved sintering behaviour and a uniform sub-micronic microstructure. Electrochemical tests for the MW-produced GDC showed at 600?C twice the conductivity of the corresponding commercial sampl

Albonetti, Stefania

Boldrini, Stefano

Gondolini, Angela

Mercadelli, Elisa

Sanson, Alessandra

PUblication MAnagement

P072 
 1 
 
MW-Assisted Polyol Mediated Synthesis of Gadolinium-
Doped Ceria Nanopowders 
  
 
Elisa Mercadelli, Angela Gondolini, Alessandra Sanson, Stefania Albonetti§, Stefano 
Boldrini* 
CNR – ISTEC 
Via Granarolo, 64  
48018 Faenza (RA) - ITALY 
Tel.: +39-0546-699743  
Fax: +39-0546-46381 
elisa.mercadelli@istec.cnr.it 
§
 Dip. Chimica industriale e dei Materiali, INSTM, Research Unit of Bologna 
Viale Risorgimento, 4  
40136 Bologna - ITALY 
*CNR – IENI 
Corso Stati Uniti, 4  
35127 Padova - ITALY 
 
 
Abstract 
 
Gadolinia-doped ceria (GDC) is one of the most promising electrolyte for intermediate 
temperature solid oxide fuel cells (SOFCs). In particular, the production of GDC as 
nanopowders leads to an higher reactivity that allows better performances, milder sintering 
conditions and lower production costs. However, nanopowders can be produced only by 
carefully tailoring their production process. The choice and optimization of the synthesis 
process is therefore a key step for the production of powders suitable for efficient SOFC 
components. 
In this work nanocrystalline GDC (Ce0.8Gd0.2O2- ) particles were successfully obtained by 
one-step microwave-assisted synthesis from a diethylene glycol solution of metal nitrates 
under mild conditions (170°C, 1 atm). The as-prepared powder showed good crystallinity 
with specific surface area of 50 m2/g. The sintering and electrochemical properties were 
compared with a nanometric commercial powder. The MW-produced powder showed an 
improved sintering behaviour and a uniform sub-micronic microstructure. Electrochemical 
tests for the MW-produced GDC showed at 600°C twice the conductivity of the 
corresponding commercial sample.  
 
 
Introduction 
 
In the recent years, gadolinia-doped ceria (GDC) has attracted a lot of interesting as 
material for intermediate temperature SOFC [1]. Unfortunately, CeO2-based ceramics 
reach a suitable density at temperatures well above 1300°C [2], involving therefore high 
production costs. Moreover, the high sintering temperature induces the reduction of CeIV to 
CeIII with the consequent creation of an electronic leakage that decreases the system 
effectiveness and leads to poor mechanical stability [3].  
To overcome these problems the research is recently considering nano-scale row 
materials. Nano-powders in fact due to their high reactivity allow milder sintering conditions 
Presented at 9th European Solid Oxide Fuell Cell Forum, Lucerna, 29 june-2 july 2010
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and as a consequence, better performances and lower production costs. Different 
chemical techniques have been proposed for the synthesis of GDC including combustion 
[4], cation complexation [5], nano-blast [6], carbonate co-precipitation [7], self-propagating 
room temperature [8], polymeric precursor [9], hydrothermal [10] synthesis, homogeneous 
precipitation [11], spray pyrolysis [12], co-precipitation [13], gel casting [14]. In most cases 
however the procedure needs long dwelling time and/or the application of a pressure. 
Moreover the powders obtained require an additional calcination step to transform the 
initial amorphous precursors into a crystalline phase. This process can cause severe 
aggregation and consequently the loss of nanometric nature of the powders. 
Conversely, the “polyol route” allows the direct precipitation of oxides in a high-boiling 
alcohol [15] that hinders the formation of hydroxide as intermediate phase, preventing at 
the same time particle agglomeration as a consequence of its surface capping action [16]. 
Due to its high dipole moment, the polyol can also serve as an excellent solvent for 
microwave heating. Compared to conventional procedure, microwave dielectric heating 
has some unique advantages in nanostructured materials synthesis, such as rapid 
volumetric heating (that can lead to fast homogeneous nucleation) and the possibility of 
forming novel nanostructures. 
In this study, a novel method for the preparation of nanocrystalline GDC is reported. 
Ce0.8Gd0.2O2-  powders were successfully prepared by microwave irradiation (MW) under 
mild conditions. The ceramics obtained showed enhanced electrochemical properties for 
SOFC applications. 
 
 
Experimental 
 
GDC powders were prepared following the procedure reported in Figure 1. Gd(NO3)3 
∙7H2O (99.9%, Alfa Aesar) and Ce(NO3)3∙7H2O (99.5%, Alfa Aesar) were used as starting 
materials. The stoichiometric mixtures of reagents were dispersed in diethylene glycol 
(DEG) to have a final metal concentration of 0.1M. The solution was heated under reflux in 
a commercial microwave oven designed for chemical synthesis (MicroSYNTH Plus, 
Milestone, USA) at 170°C for 2h. To improve the salts hydrolysis, a water excess (1:10 = 
[M]:[H2O]) was added to the DEG solution at 140°C. The precipitated powders were 
separated by centrifugation, washed in ethanol and characterized by X-ray diffraction. 
XRD measurements were carried out at room temperature with a Bragg/Brentano 
diffractometer (X'pertPro Panalytical) equipped with a fast X’Celerator detector, using Cu 
anode as X-ray source (Kα = 1.5418 Å). The chemical composition of the as-synthesized 
powders was determined by Inductively Coupled Plasma-Atomic Emission Spectrometry 
(ICP-AES) (Liberty 200, Varian, Clayton South, Australia). The powder morphology was 
investigated by scanning electron microscopy (SEM, Leica Cambridge Stereoscan 360). 
The specific surface area of the powders was measured by BET method (Sorpty 1750, 
Carlo Erba, Italy). The sintering behaviour was compared to commercial GDC powder by 
dilatometer analysis (Netzsch Dilatometer 402E, Germany). The GDC powders were 
uniaxially (700 MPa) pressed in bars and sintered at 1400°C for 6h. Conductivity 
measurements were made in O2 atmosphere in the 250-900 °C temperature range (50 °C 
step) by a.c. impedance spectroscopy (IS). Impedance spectra were collected using a 
frequency response analyser Autolab PGSTAT100, applying 100mV amplitude signal in 
the frequency range 1MHz to 1Hz (10 points per decade). Measurements were carried out 
on pellets using a two-electrode cell configuration and under symmetrical cell conditions 
with Pt deposition on 2 opposite faces (about 200 nm of Pt via DC magnetron sputtering), 
mounting them on a ProboStat (NorECs) test-rig.  
Impedance spectra consisted of a higher frequency arc related to phenomena occurring in 
the electrolyte bulk and of a lower frequency arc representing the grain boundary 
Presented at 9th European Solid Oxide Fuell Cell Forum, Lucerna, 29 june-2 july 2010
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response, aside from the contribution at low frequency ascribed to electrode/electrolyte 
interface phenomena. Total ionic conductivities were determined with the lower frequency 
intercept of grain boundary arc for each temperature.  
 
 
 
 
Figure 1 Scheme of the synthesis of the GDC powders. 
 
 
Results and Discussion 
 
Nanocrystalline ceria doped particles were produced by one-step microwave-assisted 
synthesis from a diethylene glycol solution of cerium nitrate and gadolinium nitrate by 
adjusting the synthesis conditions. 170°C for 2h were the reaction conditions able to give 
crystalline nanometric powders with sufficient reaction yield. Under these mild conditions 
the powders show a well-defined fluoritic structure (Figure 2) with the stoichiometric 
composition of Ce0.795±0.005Gd0.205±0.002O1.95 determined by ICP analysis. The as-
synthesized powders show an average crystallite size of about 5 nm, estimated by the 
XRD pattern using the Debye-Scherrer equation.  
The morphology of the powders synthesized is reported in Figure 3. Sub-micrometric, 
round shape, soft-agglomerated uniform powders with a specific surface area of 50 m2/g 
were obtained. 
The sintering behaviour of Gd-doped ceria powders was studied by dilatometry in the 
range between 25 and 1500°C and compared with the one of a commercial GDC 
nanopowders (Fuel Cell Materials, USA) (Figure 4). The GDC obtained via the polyol 
method show a sintering behaviour that is very similar to the one of the commercial 
powders. However, the maximum shrinkage for the MW-polyol powders occurs in the 
range 300 - 900°C. This temperatures range is decisively lower than the one needed to 
densify commercial powders (600-1200°C). The improvement of the sintering behaviour is 
related to the higher specific surface area of the MW-polyol-GDC powders respect to the 
commercial ones that promotes the densification at lower temperatures. 
The as-synthesized powders and commercial nanometric GDC were uniaxially pressed 
and sintered at 1400°C for 6 hours. Archimede’s densities of both sintered samples are 
over 95%, suggesting a good densification.  
Presented at 9th European Solid Oxide Fuell Cell Forum, Lucerna, 29 june-2 july 2010
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Figure 2  XRD diffractogram of as-obtained powders at 170°C, 2h. 
 
 
 
 
 
Figure 3  SEM micrographs of the as-prepared GDC powders. 
 
 
The microstructures of the sintered ceramics are shown in Figure 5. The micrographs 
confirm that an uniform sub-micronic microstructure is obtained at 1400°C starting from 
either commercial and MW-polyol GDC powders. The mean grain size decreases from 1.3 
to 0.8 µm starting from the commercial or MW-polyol GDC respectively, as a consequence 
of the lower particles size of the latter. 
Finally the GDC obtained through the optimization of the MW-polyol synthesis process 
exhibited values of ionic conductivity systematically higher than those showed by 
commercial produced nano-powders of the same composition by a solid state synthesis 
(Figure 6). In particular at 600°C a conductivity of 0.01 S·cm-1 was recorded, twice the 
values of the corresponding commercial sample.  
Presented at 9th European Solid Oxide Fuell Cell Forum, Lucerna, 29 june-2 july 2010
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Figure 4  Sintering curves for the commercial and MW-polyol GDC powders. 
 
 
 
 
 
Figure 5  SEM micrographs of polished and etched surfaces of the sintered pellets at 1400 
for 6 h: a) MW-polyol GDC and b) Commercial GDC. 
 
Presented at 9th European Solid Oxide Fuell Cell Forum, Lucerna, 29 june-2 july 2010
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Figure 6  Arrhenius plots of total conductivity for MW-polyol and commercial GDC 
ceramics. 
 
 
Conclusions 
 
Applying the polyol process combined with microwave heating, nanocrystalline with a 
uniform size distribution Ce0.8Gd0.2O2-  (GDC) particles were successfully produced under 
very mild conditions (170°C, 1 atm). This one-step method allows the direct formation of 
the fluoritic phase without the intermediate decomposition and/or calcination steps 
required by the other chemical synthesis routes. The powders obtained show an improved 
sintering behavior and better ionic conductivity than the other commercial nanometric 
powders. 
This is a very promising method to easily produce ultrafine GDC powders for SOFC 
applications with enhanced electrochemical properties.  
 
 
References 
 
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[13] A. Tsoga, A. Naoumidis, W, Jungen, D. Stover, J. Eur. Ceram. Soc., 19 (1999) 907-
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MW-Assisted polyol mediated synthesis of gadolinium-doped ceria nanopowders

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MW-Assisted polyol mediated synthesis of gadolinium-doped ceria nanopowders

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