Preparation of a porous cermet SOFC anode substrate by gelcasting of NiO-YSZ powders

Article abstract of DOI:10.1111/j.1551-2916.2006.01385.x

A porous NiO-YSZ substrate for anode-supported solid oxide fuel cells has been prepared by geleasting of NiO-YSZ powders using urea-formaldehyde monomers, followed by humidity-controlled drying, binder removal, and sintering of the gelled bodies. The gelled bodies had sufficient strength to remove even 2-mm-thick samples from the mold immediately after gelation. A gel-cast NiO-YSZ sample sintered at 1450°C for 2 h showed an open porosity of ～53 vol%, and the porosity increased to ～58% upon reduction with hydrogen. Pore sizes measured on the scanning electron microscopy photomicrograph of NiO-YSZ and Ni-YSZ cermet substrates are in the range of 2-5 μm. Urea-formaldehyde polymer, present in a high amount (～13 wt%) in the gelcast body, acts as a template for pores.

Full text of DOI:10.1111/j.1551-2916.2006.01385.x

J. Am. Ceram. Soc., 90 [2] 622–625 (2007)
DOI: 10.1111/j.1551-2916.2006.01385.x
r 2006 The American Ceramic Society
ournal
J
Preparation of a Porous Cermet SOFC Anode Substrate by Gelcasting of
NiO–YSZ Powders
Kuttan Prabhakaran,w Anand Melkeri, Mohd Oveas Beigh, Nitin Madhusudan Gokhale, and
Suresh Chandra Sharma
Naval Materials Research Laboratory, Addl. Ambernath, Thane 421 506, India
A porous NiO–YSZ substrate for anode-supported solid oxide
fuel cells has been prepared by gelcasting of NiO–YSZ powders
using urea–formaldehyde monomers, followed by humidity-con-
trolled drying, binder removal, and sintering of the gelled bodies.
The gelled bodies had sufficient strength to remove even 2-mm-
thick samples from the mold immediately after gelation. A gel-
cast NiO–YSZ sample sintered at 14501C for 2 h showed an
open porosity of B53 vol%, and the porosity increased to
B58% upon reduction with hydrogen. Pore sizes measured on
the scanning electron microscopy photomicrograph of NiO–
YSZ and Ni–YSZ cermet substrates are in the range of 2–5
lm. Urea–formaldehyde polymer, present in a high amount
(B13 wt%) in the gelcast body, acts as a template for pores.
main advantage of the urea–formaldehyde system is that oxy-
gen will not interfere with the polymerization reaction so that
the casting process could be carried out in air atmosphere.
Unlike acrylamide, the urea–formaldehyde forms strong gels
even at low ceramic powder loading. Also, urea–formaldehyde,
being a thermosetting polymer, would decompose without de-
formation of the gelcast body even if it is present in a large
amount. In addition, the urea–formaldehyde present in a high
amount would act as a template for pores. The objective of the
present work is to use a large amount of urea–formaldehyde for
gelcasting of NiO–YSZ powders for the preparation of a porous
NiO–YSZ substrate and its subsequent reduction to Ni–YSZ
cermet for SOFC anode application.
I. Introduction
II. Experimental Procedure
OLID oxide fuel cells (SOFC) have been identified as clean,
The 8 mole% yttria-stabilized zirconia (8YSZ) and nickel oxide
powders used in the present work were prepared in the laboratory
by adopting the sucrose combustion method reported else-
where.25–27 Accordingly, the 8YSZ and NiO powders were pre-
pared separately by heating aqueous solutions containing the
corresponding metal nitrates and sucrose in a mole ratio of 1:3 on
a hot plate to form a resin, followed by drying and subsequent
combustion. The combustion products were calcined at 6001C for
removal of the organics and formation of the crystalline phase.
The calcined products were deagglomerated by planetary ball
milling for 24 h in iso-propanol medium using zirconia grinding
media. A charge to ball ratio of 1:4 was used for ball milling. The
ball-milled powders were sieved through a mesh [Mesh 140
(ASTM)] and then calcined at 12001C for 1 h. The 8YSZ and
NiO powders were characterized by X-ray diffraction (XRD)
(Philips analytical PW 1710, the Netherlands), particle size anal-
ysis (Malvern Master Size Analyzer 2000, Malvern Instruments
Ltd., Worcestershire, U.K.), surface area measurement (Sorpto-
matic 1990, Thermo Finnigan, Italy), and microstructure analysis
(scanning electron microscopy (SEM), LEO 1455, U.K.).
Urea (CDH, New Delhi, India) and formaldehyde solution
(37 w/v%, S.D. Fine Chemicals, Mumbai, India) used for gel-
casting were of analytical reagent grade. Deionized water was
used for the preparation of solutions and slurries. Ammonia
solution (Merck, Mumbai, India) and HNO3 (Merck) used for
pH adjustment were of analytical reagent grade. A flowchart for
the gelcasting process is shown in Fig. 1. Urea and formalde-
hyde solution were mixed in the molar ratio 1:4, the pH was
adjusted to 8.5 using ammonia solution, and then the solution
was aged for 24 h to form a methylol urea monomer. A 25 vol%
slurry was prepared by dispersing the NiO and YSZ powders in
the methylol urea solution. The amount of NiO and YSZ used
for the slurry preparation was equivalent to 45 vol% Ni–YSZ
cermet composition. The slurry was ball milled for 6 h in a
polyethylene container using zirconia grinding media. The slurry
was then supplemented with additional urea to make the mole
ratio of urea and formaldehyde 1:2 after adjusting its pH to
S
efficient, and high-capacity futuristic energy devices.1–4 An-
ode-supported SOFC technology seems to be cost effective as it
uses less of costly lanthanum strontium manganite compared
with the cathode-supported design and reduces the operation
temperature due to less ohmic resistance offered by the thin
(10–25 mm) layer of electrolyte compared with the electrolyte-
supported type.5–14 Of the anode-supported SOFC designs,
although the tubular design has more mechanical integrity and
does not require sealant material, it has a lower power density
compared with the planar design.1 Tape casting and Coat-Mixs
process are the two methods used for the preparation of anode
substrates for planar SOFC.15–19 In the tape-casting process,
NiO–YSZ slurry containing a dispersing agent, a polymer bind-
er, and a plasticizer is used to prepare thin flexible green sheets
using a doctor blade. The green sheets are subsequently heat
treated for binder removal and sintering to form a rigid and
porous NiO–YSZ substrate. In the coat mix process, a polymer
binder is coated on NiO–YSZ powder particles and the binder-
coated powder is warm pressed in a metallic dye. The green
body thus obtained is further heat treated for binder removal
and sintering.
Gelcasting is a process for preparation of near-net-shape
bodies from concentrated ceramic powder suspensions by set-
ting them in a mold by in situ polymerization of organic mono-
mers.20–22 Nearly 2–5 wt% of the organic monomer has been
used for gelcasting of ceramic powder suspensions for prepar-
ation of dense ceramic components. Organic monomers such as
acrylamide and urea–formaldehyde have widely been used for
gelcasting of aqueous ceramic powder suspensions.20–24 The
P. Holtappels—contributing editor
Manuscript No. 21951. Received June 28, 2006; approved August 31, 2006.
wAuthor to whom correspondence should be addressed. e-mail: kp2952002@
yahoo.co.uk
622

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Communications of the American Ceramic Society
623
The polymer content in the dried sample was estimated by
thermo-gravimetric analysis (Hi Res TGA 2950, TA Instru-
ments, New Castle, DE). Polymer removal from the dried gel-
cast samples was carried out by heating at a rate of 51C/h up to
3001C and then 601C/h up to 6001C. After binder removal, the
substrate samples were sintered at a temperature of 14501C for 2
h. The heating rate used was 3001C/h up to the sintering tem-
perature. The sintered NiO–YSZ substrate samples were re-
duced in a hydrogen atmosphere at 8501C. The porosity of the
sintered NiO–YSZ samples and the Ni–YSZ cermet was meas-
ured by the water absorption method. In this method, previ-
ously weighed sintered NiO–YSZ and Ni–YSZ cermet samples
were kept in boiling water for 1 h. The samples were wiped with
a wet cloth and then weighed. The weight increase was taken as
the volume of open porosity. Microstructures of the sintered
NiO–YSZ and the cermet samples were observed on a fractured
surface using a SEM (LEO 1455).
Urea + Formaldehyde (solution)
Mole ratio of urea and formaldehyde = 1:4
pH = 8.5, Aging for 24 h
Dil.HNO
NiO & YSZ Powder
3
Slurry (pH=2)
Milling
Urea
Casting
Gelation
Mould removal & Drying
Binder burnout
Sintering
III. Results and Discussion
Porous NiO-YSZ substrate
The 8YSZ and NiO powders prepared by the sucrose combus-
tion method showed all characteristic peaks in the XRD spec-
trum. The 8YSZ powder contains particles in the size range of
0.42–8.71 mm with a D50 value of 1.62 mm. The particle size
range in the NiO powder is from 0.48 mm to 8.8 mm. However,
the D50 value for the NiO powder is 2.70 mm. Figure 2 shows the
particle size distribution of the YSZ and NiO powders. The
surface area of the 8YSZ and NiO powders was 6.3 and 4.8 m2/
g, respectively. Figure 3 shows an SEM photomicrograph of the
YSZ and NiO powders.
Reduction with H
2
Ni-YSZ Cermet anode
Fig. 1. Flow chart of the gelcasting process for preparation of NiO–
YSZ substrate.
It has been reported that urea and formaldehyde in the
mole ratio 1:4 under the basic condition (pH 5 8.5) form a
stable solution of methylol urea.23 This solution, after adjusting
105
95
Fig. 2. Particle size distribution of the 8YSZ and NiO powders.
85
2 using dilute nitric acid. The slurry was then cast in a cylindrical
glass mold of inner diameter 30 mm. After gelation, the slurry
was removed from the mold and dried at room temperature
at a relative humidity of 75%. The viscosity of the NiO–YSZ
gelcasting slurry was measured using a cone and plate vis-
cometer (Model NCR T20; Haake Rotoviscometer, Haake
GmbH, Germany).
75
0
100
200
300
400
500
600
–1
Shear rate (s
)
Fig. 4. Viscosity versus shear rate plot of the NiO–YSZ gelcasting
slurry.
Fig. 3. Scanning electron microscopy photomicrograph of the (A) 8YSZ and (B) NiO powders.

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Vol. 90, No. 2
Fig. 5. Thermo-gravimetric analysis–Differential thermal analysis plot
of the NiO–YSZ gelcast sample.
Fig. 7. Scanning electron microscopy photomicrograph of the NiO–
YSZ substrate prepared by the gelcasting process.
the pH to an acidic range (pHo3) when supplemented with
additional amount of urea to make the mole ratio of urea
and formaldehyde 1:2, undergoes gelation due to conden-
sation polymerization of the methylol urea and urea.23,28
Ramanathan et al.15 reported by x potential and viscosity
measurements that NiO–YSZ powder mixtures disperse well
in water medium both in acidic and basic pH ranges. As
in water, the NiO–YSZ powders disperse well in the solution
of methylol urea at pH 2 as evidenced from viscosity measure-
ment of the slurry. Figure 4 shows the viscosity of the 25 vol%
NiO–YSZ slurry prepared in the methylol urea solution
measured at various shear rates. The slurry showed low viscos-
ity with shear thinning flow behavior. The viscosity of the slurry
measured at a shear rate in the range of 66–754 sÀ1 is in the
range of 104–62 mPa.s. The slurry with this viscosity range is
suitable for gelcasting.
The slurry undergoes gelation nearly 10 min after casting. The
gelled slurry has sufficient handling strength and can be
removed from the mold immediately after gelation. The gelcast
bodies did not show any crack or deformation during drying in
a humidity chamber at 75% relative humidity at ambient
temperature (B301C). The cylindrical gelcast bodies showed a
diametrical shrinkage of 1.6% during drying.
decomposes through exothermic reactions in the temperature
range of 2001–3001C, yielding more thermally stable structures.
Weight loss observed in the temperature range of 4001–5501C
is due to exothermic decomposition of these stable structures.
The TGA-DSC plot of the dried gelcast sample is shown in
Fig. 5. As nearly 84% of the polymer is decomposing below
3001C, a slow heating rate of 51C/h was used for the polymer
removal up to 3001C.
The binder-removed NiO–YSZ gelcast samples showed a
diametrical shrinkage of 13.4% during sintering at 14501C for
2 h. Circular and rectangular samples of 2 mm thickness
could be prepared. Figure 6 shows a photograph of a sintered
NiO–YSZ substrate. The sintered NiO–YSZ samples, when
aged in boiling water for 1 h, absorbed 53.3 vol% of water,
indicating the presence of 53.3 vol% of open porosity. Figure 7
shows an SEM photomicrograph of a sintered NiO–YSZ
gelcast sample. Most of the pores observed in the microstruc-
ture have sizes in the range of 2 to 5 mm. The porosity of the
NiO–YSZ sample increased to 58.2 vol% during reduction in
hydrogen atmosphere. Porosity increase during reduction is
due to shrinkage caused when NiO is reduced to Ni. Figure 8
shows an SEM photomicrograph of the Ni–YSZ cermet sample.
The Ni–YSZ cermet sample obtained by reducing the sintered
NiO–YSZ gelcast sample in hydrogen atmosphere showed only
YSZ and Ni peaks in the XRD spectrum, indicating complete
conversion of NiO to Ni. Figure 9 shows an XRD pattern of the
Ni-YSZ cermet sample. A qualification of the substrate in real
cells is still pending.
The thermal decomposition of the urea–formaldehyde poly-
mer in the NiO–YSZ gelcast sample is similar to that reported
by Camino et al.29 for neat urea–formaldehyde polymer. Weight
loss observed up to 2001C, corresponding to 13% of the total
weight loss, is due to the urea–formaldehyde cross-linking reac-
tions. A corresponding endothermic peak is observed in the
differential scanning calorimetry (DSC) plot. Nearly 71% of
the urea–formaldehyde polymer present in the gelcast sample
Fig. 6. Photograph of NiO–YSZ substrates fabricated by the gelcasting
process.
Fig. 8. Scanning electron microscopy photomicrograph of the Ni–YSZ
cermet prepared by reducing the gelcast NiO–YSZ substrate.

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