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Understanding crystallization processes of NiO/Ce0.9Gd0.1O2�d sol–gelprocessed thin films for the design of efficient electrodes: an in situ thermalellipsometry analysis†

Guillaume M€uller,ab C�edric Boissi�ere,a David Grosso,a Armelle Ringued�e,b Christel Laberty-Robert*a

and Cl�ement Sancheza

Received 12th December 2011, Accepted 7th March 2012

DOI: 10.1039/c2jm16550c

We describe a simple, non-destructive method, in situ thermal ellipsometry analysis (TEA), for

understanding the different processes (decomposition of organics, crystallization, and sintering)

occurring upon heating hybrid organic–inorganic films. According to these studies, a thermal treatment

was tailored in order to obtain robust, nanocrystalline inorganic mesoporous 100–150 nm thick films

with efficiently connected porosity surrounded by a crystalline inorganic network. Polymodal porous,

nanocrystalline NiO/Gd-doped Ceria composites or Ni/Gd-doped Ceria films, interconnected network

of open pores ranging from macro- to micro-pores, have been synthesized. The inorganic network is

built from connected crystalline nanoparticles with mean diameters of 12 � 3 nm, whose small size is

still preserved even at 800 �C. We also show that the thermal ellipsometry analysis is readily extendable

to MO/Gd-doped Ceria with M ¼ Cu, Ni, Co, etc., therefore demonstrating the interest of this

technique in understanding thermal phenomena in complex ceramic and composite systems. This is

trivial for designing electrodes with efficient microstructure.

Introduction

Mesoporous inorganic films open up a land of opportunity for the

design of new materials1,2 for photonics,3,4 catalysis,5

membranes,6,7 sensing8 and energy conversion9,10 owing to their

high specific surface area, their nanoparticles and their well-con-

nected inorganic and porous networks. Gadolinium doped Ceria

(Gd–CeO2, GDC) is a strategic material used as a cathodic

interface in Solid Oxide Fuel Cells (designated as SOFCs), while

Ni/Gd–CeO2 cermets are used as anodes for low-temperature

SOFCs.11,12 Other nanocomposite materials such as Au–CeO2

have been extensively studied for water gas-shift reactions.13–16

Although these materials are well-established for these types of

applications, the development of materials with a controlled

nanoarchitecture continues with an emphasis on achieving

enhanced stability, important ionic and electronic conductivities,

and a continuous porous network for gas diffusion.

aLaboratoire de Chimie de La Mati�ere Condens�ee de Paris, UMRUPMC-CNRS-Coll�ege de France 7574, Universit�e Pierre et MarieCurie, Paris 6, Coll�ege de France, 11 Place Marcellin Berthelot, 75231Paris, France. E-mail: christel.laberty@upmc.fr; Fax: +33 1 44 27 1504; Tel: +33 1 44 27 15 28bLaboratoire d’Electrochimie, Chimie des Interfaces et Mod�elisation pourl’Energie, UMR ENSCP-CNRS 7575, Chimie Paristech, 11 Rue Pierreet Marie Curie, 75231 Paris, France. E-mail: armelle-ringuede@chimie-paristech.fr; Fax: +33 1 44 27 67 50; Tel: +33 1 55 42 12 35

† Electronic supplementary information (ESI) available. See DOI:10.1039/c2jm16550c

9368 | J. Mater. Chem., 2012, 22, 9368–9373

Ceria-based thin films dominate the landscape of active layers

for micro-SOFCs17 due to their high ionic conductivities at low

temperature18,19 and to their both chemical and thermal stabili-

ties at temperatures below 500 �C.20–23 Various methods such as

pulsed laser deposition,24,25 electrostatic spray deposition,26 spin

and dip-coating27,28 and electrophoretic infiltration29 have been

explored to synthesize composite NiO/Gd–CeO2 films. However,

none of these methods report the optimization of the porosity of

these films. However, this physical characteristic is very impor-

tant as it influences gas diffusion and electro-catalytic active sites

distribution. For example, efficient gas diffusion is important as

it will allow homogeneous reduction of NiO to Ni. Finally,

a percolated-Ni network could be achieved, limiting the voltage

losses of the fuel cell at high and low current density. The elec-

trochemical performance of these layers is not yet competitive,

but its mixed conductivities improve markedly when synthesized

in high-surface area, nanoscale forms.26 The electrochemical

properties of Ceria-based thin films may be further enhanced by

controlling the pore-solid network, as we recently described for

a pure ionic conductor Gd0.1Ce0.9O2.10

In the present paper, we propose to use ‘‘the sol–gel process’’

combined with dip-coating to tune the pore solid network to

fabricate, for the first time via this approach, a composite elec-

trode, NiO/Gd–CeO2, with controlled microstructure. Hereby,

the pore size and connectivity are mainly due to the decompo-

sition of an amphiphilic block-copolymer within the hybrid

organic–inorganic layer.

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Previous work on mesoporous GDC10 highlights that

a controlled thermal treatment is necessary for both the crystal-

lization of the inorganic network and the formation of the porous

network. This step is crucial as it tunes the microstructure of the

electrode and therefore their electrochemical performances. In

general, competition between decomposition of the organic

network and crystallization of the inorganic network occurs and

in some cases, it is difficult to preserve the porous network.30,31

This could be achieved through a perfect tuning of the heat

treatment. Understanding thermal events occurring during the

heating of the films allows the determination of the adequate heat

treatment. This step is important in order to design the efficient

electrode microstructure i.e. (i) connected porous network for gas

diffusion, (ii) well-connected GDC network through optimized

grain boundaries for good ionic transport, (iii) developed

NiO/Gd–CeO2 interface for achieving good triple phase bound-

aries, and (iv) well-connected NiO network for achieving

a percolated-Ni network upon reduction treatment. The sintering

temperature can be determinedwhich indicates the temperature at

which the nanoparticles start to grow. Then, understanding the

thermal behaviour of the films allows us to predict the collapse of

themicrostructure and then its stability under various conditions.

The present work aimed to study via a non-destructive tech-

nique the different thermal events occurring during the heating of

these mesoporous thin films. Since these films exhibit optical

quality, an in situ thermal ellipsometry (TE) analysis was used.32

This technique previously developed in the laboratory allows us

to follow the change in refractive index, absorption coefficient

and thickness upon heating a mesostructured film under air to

understand the different thermal events.32 Previous works on

mesoporous TiO2 films and dense, nanocomposite TiO2/SiO2

films have shown the interest of the technique. Louis et al. have

demonstrated how appropriate it can be to understand pores

formation, crystallization and sintering phenomena for designing

materials with adequate properties.32,33 In this study, our interest

will be less to relate to the porous structure than the under-

standing of thermal-treatment phenomena. We further demon-

strate that the developed technique is relevant to more complex

systems that are relevant to electrodes for micro-SOFCs.

Experimental section

Chemical

Nickel(II) chloride hexahydrate (Aldrich, 99.999%), cerium(III)

chloride heptahydrate (Aldrich, 99.999%), gadolinium(III) chlo-

ride hexahydrate (Aldrich, 99.999%), copper(II) nitrate (Aldrich,

99.999%), ethylalcohol(PS-b-PEO), polystyrene (40 000)-b-

polyethylene (49 000) oxide (Polymer Sources Inc.) and tetra-

hydrofuran were used, as received.

0.413 mmol of CeCl3$7H2O, 0.05 mmol of GdCl3$6H2O and

1.540 mmol of NiCl2$6H2O were dissolved in a mixture of

ethanol–water (0.087 mol/0.042 mol) and stirred for 1 h (solution

A). A solution (B) containing 100 mg of PS-b-PEO dissolved in

�2.3 mL (0.027 mol) of THF is stirred for 1 hour in order to form

and stabilize the micelles in solution. The solution B was added

to the solution A and stirred for �24 h at room temperature

under air. Because of the contrast between hydrophilic and

hydrophobic groups and the different interaction existing

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between the solvent and the polymer chains, spherical micelles

averaging �30 nm in diameter (estimated by Dynamic Light

Scattering analysis, DLS) were formed. In the final solution,

these micelles are surrounded by the hydrated Ce(III), Gd(III), and

Ni(II) cations.

A solution containing Cu was also prepared from

a Cu(NO3)2$2.5H2O precursor. Pluronic� F127 was deliberately

used as a template. We have chosen these templates to generalize

the analysis to low cost block-copolymer templates, even if it

decomposes at a lower temperature than PS-b-PEO. 120 mg of

Pluronic� F127 was added to 8 mL of solution A containing

copper and stirred for 24 h.

Film synthesis

The NiO/GDC and CuO/GDC films were deposited by dip

coating on Si substrates with a withdrawal speed of 3–6 mm s�1.

The silicon substrateswerewashedwith ethanol and acetone prior

to deposition. The compositions of the films studied resulted in

aNi/GDCvolumetric ratio of 20/80, 30/70, 50/50, 80/20, and100/0

(puremetal), which correspond to aNiO/GDCvolumetric ratio of

29.7/70.3, 42.0/58.0, 62.8/37.2, 87.1/12.9 and 100/0, respectively.

For convenience 50/50 vol% corresponds to 62.8/37.2 vol% of

NiO/GDC and 64/36 vol% of CuO/GDC and most of the char-

acterization has been done on these compositions.

Characterizations

The thickness (h), the refractive index (n), and the absorption

index (k) of optical films were measured by spectroscopic

ellipsometry. Measurements were performed within the

UV-Visible range (240–1000 nm) at an incident angle of 70� usinga (VASA) M-2000U from Woollam. In situ Thermal Ellipsom-

etry Analyses (TEAs) were performed, using a covered heating

unit connected and monitored by a programmable temperature

regulator. The thermocouple regulator was directly in contact

with the sample. After the deposition, the as-prepared samples

were heated at 80 �C for 10 minutes inside the covered heating

unit, just before analysis measurements. Data analysis was per-

formed with Wvase32 software, where ellipsometric J and D

angle plots were fitted using a single Lorentz layer model

composed of one oscillator having its maximal energy fixed

at 0 eV, and another having the energy modulated in the

UV-Visible range.

The microstructure of the films obtained after different heat

treatment was observed by Field Emission Gun Scanning Elec-

tronMicroscopy (FE-SEM, Zeiss Ultra 55) and High Resolution

Transmission Electronic Microscopy (HR-TEM; JEOL JEM

2011). For HR-TEM analyses, mesoporous films are scratched

from the substrates and the obtained powders are deposited on

coated carbon–copper grids.

The structures of the NiO/GDC and CuO/GDC films were

analyzed using a Bruker D8 Advance X-ray diffractometer with

monochromatic Cu Ka radiation (40 kV, 40 mA) and Ni filter. A

2q survey scan was performed from 20� to 60� at 0.3� min�1 with

a sample interval of 0.01�. After fitting peaks by a Gauss model

with Igor Pro software and correcting the machine broadening

and the Cu Ka2 peak, the mean particle size was evaluated using

the Debye–Scherrer equation.34

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Optical spectroscopies (UV-Visible and FT-IR) were used on

both NiO/GDC films and NiO/GDC solutions. UVIKON XL

160 from Secoman has been used for UV-Visible analysis and

a Spectrum 400 from PerkinElmer for the FT-IR analysis.

Results and discussion

While different methods have been reported for the synthesis of

NiO (50 vol%)/GDC (50 vol%) (noted as NiO/GDC) composite

thin films,24–26,35 the evaporation induced self-assembly approach

to produce well-defined mesoporous NiO/GDC nanocomposite

films has not been reported yet to the best of our knowledge. This

method allows for the preparation of inorganic films with both

a mesoporous morphology and a nanocrystalline frame-

work.1,36,37Briefly, an isotropic THF-ethanolic PS-b-PEOdiblock

copolymer solution containing the corresponding hydrated metal

salt is dip-coated onto a polar substrate. Upon evaporation of the

volatile constituents, the various non-volatile species co-assemble

to form a mesostructured inorganic–organic composite layer.

Thermal treatment is used to initiate hydrolysis and condensation

reactions, to decompose the PS-b-PEO template and to induce the

crystallization of both GDC and NiO oxides. The PS-b-PEO

template has been chosen because of its high amphiphilic contrast

and the thermal stability of the PS block. Generally, it is suitable

for the preparation of ordered mesoporous thin films with both

highly crystalline walls and large pores. This template produces

walls close to 15 nm thick, which usually enables the formation of

relatively large but stable crystalliteswithout significant distortion

of the pore network symmetry during crystallization. The as-

synthesized films exhibit optical quality after stabilization at 70 �Cfor 24 hours, as already observed for GDC films.10 After heat

treatment at 500 �C for 1 hour under air, the calcined films are

transparent, and exhibit the crystalline, pearl-necklace structure

characteristic of single GDC films (Note: 500 �CNiO/GDC films)

(Fig. 1a). The 500-calcined-GDC films retain the pore-solid

network.

The evolution of the crystallinity of the as-synthesized

NiO/GDC films is studied via ex situ X-ray diffraction analyses

(Fig. 2a). The 300 �Ccalcined films exhibit the peaks characteristic

Fig. 1 HR-TEM images of NiO/GDCmesoporous thin films calcined at

500 �C (a) and 300 �C (b) during the ‘‘in situ’’ thermo-ellipsometric

analysis, in air (heating rate: 10 �C min�1) and the average particle size

distributions measured on a large zone for the two samples.

9370 | J. Mater. Chem., 2012, 22, 9368–9373

of GDC and NiO oxides while the 200 �C calcined films are

amorphous. The calcined films crystallize to both GDC and NiO

single phase, as evidenced by X-ray diffraction analyses; the

d-spacings match those of the fluorine GDC and NiO structure.

The crystallization temperature is quite low compared to the solid

state method,38–40 thus preventing detrimental particle growth,

which is advantageous for the projected applications of these

materials. Using the Scherrer equation, the particle size of GDC

andNiO is similar and the value is�2 nm for 300 �C calcined films

and �5 to 7 nm for 500 �C calcined films. These results are

confirmed by HR-TEM analyses (Fig. 1a and b). The calcined

nanoarchitectures exhibit well-defined particles with edges and

steps; 300 �C calcined films have 2 nm of GDC and NiO crystal-

lites that grow to �5 nm after calcination at 500 �C under air.

These crystallites are relatively small compared to single GDC

films, which is expected because the solid composite network of

the mesostructured films inhibits particle growth.41

Transformation of the amorphous nanoarchitecture into

nanocrystalline NiO/GDC films is accompanied by the decom-

position of the template, the nucleation and the crystallization of

the nanoparticles. These different thermal events can interfere

Fig. 2 (a) XRD diffractogram of mesoporous NiO/GDC thin films

calcined at various temperatures for 1 h in air (heating rate: 1 �C min�1).

The extra-peaks correspond to the Si-wafer. (b) Variation of the thickness

h (black) as a function of the temperature (heating rate: 10 �C min�1) for

synthesized NiO/GDC mesoporous thin films stabilized at 80 �C and

derivative of the thickness curve for NiO/GDC (red), GDC (blue) and

NiO (green) as a function of the temperature.

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Fig. 4 Crystallization temperatures of GDC (blue) andNiO (green) with

different percentages of the Ni(O) phase in the NiO/GDC mesoporous

thin films extract to in situ thermal ellipsometric analysis.

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and the pore collapses could be observed, especially if the crys-

tallization of NiO and/or GDC occurs after the decomposition of

the different blocks of the template, i.e. PEO and PS.

We studied the thermal behaviour of the mesostructured

NiO/GDC films through Thermal Ellipsometry (TE) analyses.

TE analyses were performed on stabilized hybrid organic–inor-

ganic NiO/GDC films in the wavelength range of 400 to 900 nm.

The ellipsometric data, tan(J) and cos(D), were fitted with

Lorentz Ocillator model and the thickness (h) (Fig. 2b), and the

real (n) and imaginary (k) parts of the complex refractive index

(Fig. 5a and b) of the films were evaluated. The variation of the

film thickness as a function of the temperature is reported in

Fig. 2b. The derivative of the NiO/GDC thin film thickness curve

exhibits four peaks, related to various phenomena. The first peak

at low temperature corresponds to small variations of the

thickness. This step can be attributed to the evaporation of

residual solvents such as H2O, THF, and EtOH. Between 190 �Cand 220 �C, a variation of both the thickness and refractive index

is observed, and corresponds to the decomposition of the PEO

block to CO and CO2 gases. This is confirmed by IR analyses

where the C–H aliphatic stretch at �2920 cm�1, the scissor at

950 cm�1 and the C–O stretch at �1070 cm�1 disappear between

150 �C and 300 �C, indicating the decomposition of the PEO

blocks (Fig. 3). Between 220 �C and 260 �C, a higher decrease of

the thickness (�13.6%) with the temperature appears and is due

to the crystallization of Ce0.9Gd0.1O2�d nanocrystallites (nucle-

ation growth). Prior XRD investigations of NiO/GDC films have

shown the presence of GDC peaks on X-ray patterns for 300 �Ccalcined films (Fig. 2a). Additionally, TE analyses of meso-

structured GDC films show that GDC crystallizes at 300 �C, asa well-defined peak is present in the derivative dh/dT plots

(Fig. 2b).10 In the same way, the crystallization temperature of

the GDC phase in these nanocomposite films has been deduced

from the peak position in the derivative dh/dT plots (T¼ 250 �C)(Fig. 2b). Between 260 �C and 320 �C, both a large decrease in

thickness (�24%) and a change in the real refractive index of

7.2% are observed and are attributed to both the crystallization

Fig. 3 Ex situ IR analysis of NiO(50)–GDC(50) mesoporous thin films

calcined at different temperatures.

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of the NiO phase and the decomposition of the PS block of the

copolymer. From the dh/dT curves, the crystallization tempera-

ture of the NiO phase is estimated to be around 290 �C. PreviousTE analyses and ex situ X-ray diffraction analyses on NiO films

indicated that NiO crystallizes in air at temperatures around

290 �C.We changed the vol% of NiO into the GDC mesostructured

thin films in order to synthesize films with different compositions

and microstructure, achieving probably different functionalities.

TE-analyses were performed and the results are reported in the

ESI†. The TEA curves present the same behaviour. The different

crystallization temperatures of both NiO and GDC oxides were

estimated and their variations as a function of the NiO content in

the nanocomposite films are plotted in Fig. 4. We do not observe

any change in the crystallization temperature for both GDC and

NiO oxides. This is due to the fact that the particle growth is

driven by the curvature of the first nuclei which is independent of

the amount of nickel in the composite.41

Obtaining a single phase of both GDC and NiO using sol–gel

chemistry rather than mixed oxides is important for the use of

this class of nanocomposites in SOFC anodes. Because we were

able to create two different networks, we posit that both Ni–O–

Ni and Ce–O–Gd oligomers form, which upon heating create

separated NiO and GDC oxide networks. UV-Vis analyses

combined with the variation of k with the temperature measured

during the TE analyses do also support the formation of two

different networks at a different stage of the fabrication and heat

treatment (Fig. 5).

The absorption peak observed for 500 �C calcined films below

310 nm is attributed to the metal–ligand transition between Ceria

and oxygen (Fig. 5c) while no peak is observed in the wavelength

range of 405 nm and 760 nm, corresponding to the d–d transition

of Ni(II). The variation of the absorption profile k was studied

from in situ TE analyses between 400 and 900 nm. In this studied

range, the change of the Ni-ligand can be followed. We do

observe the same absorption profile between 400 and 900 nm for

the sample stabilized at 80 �C, which is characteristic of Ni(II) in

solution as seen in Fig. 5c, red curve. This observation confirms

that NiO oxide crystallizes separately, and at a temperature

J. Mater. Chem., 2012, 22, 9368–9373 | 9371

Fig. 6 (a) Variation of the thickness h (black) and the refractive index n

(blue) as a function of the temperature (heating rate: 10 �C min�1) for

synthesized CuO/GDC mesoporous thin films stabilized at 80 �C. Thederivative of the thickness curve (red) as a function of the temperature.

(b) XRD pattern of mesoporous CuO 1 and CuO/GDC 2 thin films

calcined at 400 �C in air (heating rate: 10 �C min�1). The extra peaks

correspond to the Si-wafer.

Fig. 5 Variation of the refraction index n as a function of the temper-

ature (a) and variation of the absorption index k as a function of the

temperature (b) (heating rate: 10 �C min�1) for synthesized NiO/GDC

mesoporous thin films pre-stabilized at 80 �C. (c) UV-Visible spectra of

the precursors solution (red line) and NiO/GDC mesoporous thin films

calcined at 500 �C for 1 h, 10 �Cmin�1 in air. Note that the absorbance of

the glass substrate is reported as a black line.

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higher thanGDC. Because the absorption peak of Ni2+ at 750 nm

moves to UV domains as the temperature increases, a change of

the Ni-ligand from the aqua form to hydroxyl-, oxy-hydroxyl- to

the final oxide form is observed.42,43 Alder and Feinleib44 observe

the same feature; stoichiometric NiO exhibits absorption peaks

below 4 eV due to intra-ionic d8–d8* transition (1–2 eV) at high

temperature (227–327 �C) (Fig. 5b).Finally, based on both the TE results and the other results

from ex situ characterization techniques (i.e. XRD, IR, UV-Vis,

and FE-SEM), we propose a four-stage mechanism leading to

the formation of separate NiO and GDC oxides: (i) decompo-

sition of PEO block, (ii) thermally promoted hydrolysis and

condensation reactions to form GDC, (iii) decomposition of PS

block, and (iv) thermally promoted hydrolysis and condensation

reactions to form NiO.

Our sol–gel protocol can be readily adapted to synthesize

nanocomposite materials with various compositions (MO/GDC

with M ¼ Cu, Ni, Co,.). We, also, synthesize CuO–GDC mes-

ostructured films. After reducing CuO to Cu, a mesoporous,

nanocomposite Cu–GDCfilm is beneficial as an anodematerial in

direct oxidation of hydrocarbons. Indeed, copper avoids carbon

cooking.45However, the lowmelting point of Cu (1080 �C) allowsthe sintering of CuO–GDC films at low temperature.

In order to define the adequate heat-treatment in terms of

keeping inter-connected pores and crystalline nanoparticles in

the inorganic wall, TE analyses were performed and the results

are reported in Fig. 6. To confirm the decomposition tempera-

ture of the previous template, those experiments were performed

with another template (Pluronic� F127) that is less stable.

Between 100 and 140 �C, a decrease of the films thickness is

observed with an increase of the refractive index. This step

corresponds, as we previously discussed, to the contraction of the

film accompanied by the evaporation–desorption of physisorbed

9372 | J. Mater. Chem., 2012, 22, 9368–9373

solvent. Between 145 and 180 �C, the film thickness continues to

decrease and this step corresponds to the crystallization (nucle-

ation-growth) of CuO, as evidenced by ex situ XRD analysis. An

increase of the refractive index indicates that condensation of the

inorganic matter occurs without the decomposition of the

copolymer block. Between 180 and 210 �C, the variation of both

the thickness and the refractive index is mainly due to the

decomposition of the Pluronic� F127. As observed previously,

the next step (220–245 �C) corresponds to the nucleation-growth

and crystallization of the GDC nanoparticles. Those experiments

confirm that (i) TE analyses are an universal technique that can

be used to understand thermal events of films containing single

oxide, composite, as soon as they exhibit optical quality, (ii) TE

analyses can be used as an aid to define the heat treatment, where

the inter-connected pores network surrounded by an inorganic

network is conserved, and (iii) TE analyses do also indicate the

temperature at which the sintering of nanoparticles occurs,

accompanied by a collapse of the porous network. By analogy,

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we expect that a similar nanocomposite structure composed of

a complex oxide such as La0.6Sr0.4Co0.2Fe0.8O3 and GDC

nanoparticles will contain optimized three-phase boundary

catalytic sites for use as micro-SOFC cathodes.

We also prepared mesostructured Ni/GDC films, by heating

the NiO/GDC mesoporous films at 550 �C under 10% H2/N2.

During this step, NiO is reduced to Ni, as it is evidenced by ex

situ XRD analyses (S.I.). We, then, measured the polarization

resistance of the films at 500 �C and found that the film exhibits

mixed conductivity as the film resistance is much lower than the

one observed for GDC mesostructured films at the same

temperature (13.9 U cm2 for GDC and 0.71 U cm2 at 500 �C for

Ni(50)/GDC(50)). This indicates that NiO is reduced to Ni and

the Ni network is percolated.

Conclusions

In summary, we describe a low-temperature, facile route to

synthesize GDC-based nanocomposites as mesoporous thin films

for micro-SOFC anodes. The thermal behaviour of these

mesoporous GDC-based nanocomposites was studied from

TE-analyses in order to characterize the decomposition of the

template, the crystallization and the sintering of these films. This

technique combined with other characterization techniques

(XRD, FE-SEM, IR,.) allows optimization of thermal

processes that permit conservation of interconnected porous

networks and crystalline inorganic pore walls. Such a micro-

structure is suitable for the development of efficient SOFC

compounds because it yields mixed conductivity. In conclusion,

we demonstrate that the sol–gel method containing a surfactant

and combined with the dip-coating approach is readily adapted

to multi-component GDC-based nanocomposites containing

simple (NiO and CuO) or more complex oxides (La0.6Sr0.4-

Co0.2Fe0.8O3) of relevance to electrocatalytic reactions.

Acknowledgements

The authors thank the EADS Foundation for supporting

G. M€uller PhD grant. They also thank D. Montero for doing

FE-SEM analyses.

Notes and references

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