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W. Guo et al. / Journal of Alloys and Compounds 504 (2010) L21–L24
conducting phase was formed at the bilayer interface [13]. Chan et
al. simulated yttria-doped bismuth oxide (YDB) as the substrate
electrolyte and a thin layer of yttria-stabilized zirconia (YSZ) as
a coating on the anode side. It was found that depositing a very
thin layer of YSZ onto the YDB in a thickness ratio of 1–10,000 (e.g.
1
0 nm-thick YDB layer and 100 m-thick YSZ layer) significantly
increased the interfacial oxygen partial pressure in the bilayer elec-
trolyte and reduced the penetration of the electronic current [14].
They reported that the fabrication of an ultra thin but dense layer
of YSZ onto a substrate electrolyte is technically very difficult and
should be overcome. Leng and Chan adopted a cost-effective wet
ceramic process/painting/screen-printing method to successfully
fabricate a YDB film (of thickness ∼16 m) on gadolinia-doped
ceria (GDC) substrates and found that such a YDB film was effec-
tive in blocking electronic penetration through the GDC electrolyte
[
15]. Therefore the investigation of a new YDB/LSGM/LDC trilayer
is very interesting, because YDB layer can block the electron pene-
tration of LSGM induced by diffusion of Ni during high temperature
co-firing process and it has been reported an increase of the open-
circuit voltages using doped bismuth/doped ceria bilayers such as
ESB/GDC, YDB/YSZ and YDB/GDC.
As mentioned above, bilayer electrolytes have been commonly
introduced to prevent the decomposition of doped bismuth oxide
in low partial pressures of oxygen. In order to apply YDB electrolyte
to an LSGM electrolyte layer as a buffer layer for IT-SOFCs, we have
designed trilayer electrolyte composed of a YDB layer on the oxi-
dizing side and an LSGM/LDC composite layer on the reducing side.
The advantage of this design is that LSGM, LDC, and YDB are all
sufficiently conductive to serve as IT-SOFC electrolytes. In addi-
tion, compared with the previous reports summarized above, YDB
is fabricated onto the LSGM electrolyte as a buffer layer, which is
also effective in improving the electrochemical performance of the
resulting trilayer electrolyte SOFCs [7,8,16,17].
Fig. 1. XRD patterns of the as-prepared powders: (a) LSGM and (b) YDB.
In the present work, a YDB buffer layer has been employed in
the fabrication of anode-supported SOFCs with an LSGM electrolyte
layer. To the best of our knowledge, there has not hitherto been any
report on the application of a YDB/LSGM/LDC composite electrolyte
in an anode-supported SOFC in the literature.
◦
vehicle [22]. The mixture was applied to the electrolyte and then fired at 600 C for
2
2
h in air. The cathode area was 0.20 cm .
A single cell was assembled by attaching a cell pellet to one end of an alumina
tube using silver paste as a sealant and joint material. The cells were tested with
an electrochemical instrument (Autolab PGSTAT30, Holland). Hydrogen was passed
over the anode at a flow rate of 75 mL min , while the cathode was exposed to
ambient air. After reducing the NiO-containing anode at 500 C in H2 for several
−
1
◦
◦
2.
Experimental
hours, the performance of the cell was tested from 500 to 700 C. The morphology
and microstructure of the single cells were characterized with a Philips XL-30FEG
scanning electron microscope (SEM). AC impedance spectroscopy was typically car-
La0.9Sr0.1Ga0.8Mg0.2O3−ı (LSGM) and La0.4Ce0.6O1.8 (LDC) powders were synthe-
sized by conventional solid-state reactions [7]. Bi0.75Y0.25O1.5 (YDB) was prepared by
3+
3+
ried out in the frequency range from 0.01 Hz to 100 kHz with a signal amplitude of
reverse-titration chemical co-precipitation from a Bi - and Y -containing aque-
ous solution [18]. For this, analytically pure Bi(NO3)3·5H2O and Y(NO3)3·6H2O, in
a molar ratio of 3:1, were dissolved in dilute nitric acid to prepare a nitrate solu-
◦
2
0 mV under open-circuit conditions at temperatures between 500 and 700 C.
3+
3+
−1
tion containing a total metal ion (Bi , Y ) concentration of 0.1 mol L . This nitrate
solution was added drop wise to aqueous ammonia solution in a reaction vessel at
room temperature. The solution was continuously stirred using a magnetic needle
and was maintained at around pH 12. During the chemical titration, a 1 wt% solu-
3. Results and discussion
XRD analysis showed the XRD pattern of the as-prepared LSGM
powder (Fig. 1a) to be in good agreement with the diffraction data
for a perovskite phase, but a few small impurity peaks of LaSrGaO4
and LaSrGa3O7 can be seen in XRD pattern. The XRD pattern of YDB
tion of PEG4000 (polyethylene glycol of average molecular weight 4000) in aqueous
◦
ammonia was added as a dispersing agent. The precipitate was oven-dried at 80
C
◦
and calcined at 750 C for 2 h to form YDB powder. Sm0.2Ce0.8O1.9 (SDC) powder was
prepared using a citric–nitrate process and was calcined at 800 C for 2 h [19]. Nickel
◦
oxide (NiO) powder was obtained by the precipitation method, using Ni(NO3)2·6H2O
powder (Fig. 1b) shows good agreement with the expected peaks
as the raw material and ammonia as the precipitant. The precipitate was calcined
for a ␦-Bi O3 phase, with no indication of any second phase.
2
◦
at 400 C for 2 h. All of the powders were characterized by X-ray powder diffraction
Fig. 2 shows SEM images of cross-sectional views of anode-
supported SOFCs with the YDB/LSGM/LDC composite electrolyte
◦
(
XRD) on a Shimadzu XD-3A diffractometer with a scanning step of 0.02 .
The as-prepared NiO and SDC powders were mixed in a weight ratio of 6:4.
(Fig. 2). The total thickness of the YDB/LSGM/LDC composite elec-
After supplementing the mixture with 15 wt% starch as a pore former, the anode
powder was pressed into pellets of diameter 13 mm and thickness 0.6 mm. The green
trolyte film was 50 m (18 m YDB + 19 m LSGM + 13 m LDC). As
can be seen from the SEM images, the YDB, LSGM and LDC trilayers
were good intimate contact and surrounded electrodes, showing
that the materials sintered well together. The LSGM and LDC lay-
ers were relatively dense, and the location of YDB layer closed to
LSGM layer was tight while the location away from LSGM layer
was actually porous. A further increase in the sintering tempera-
ture of the YDB layer would have led to a change in its composition
due to the evaporation of bismuth [15]. The obtained YDB layer
with a porous microstructure still exhibited good compatibility
◦
pellets were pre-sintered at 1000 C for 4 h to obtain the NiO–SDC anode substrates.
LSGM/LDC bilayer electrolytes were fabricated by a centrifugal casting technique.
In this way, layers of first LDC and then LSGM were sequentially deposited on the
NiO–SDC anode substrates [7,20]. The thicknesses of the LSGM and LDC layers were
◦
controlled by the amount of slurry. The deposited pellets were co-fired at 1400
C
for 4 h. A YDB layer was also fabricated on the LSGM/LDC by a centrifugal casting
technique. The anode substrates with the YDB/LSGM/LDC trilayer were then sintered
◦
at 900 C for 2 h. Before application of the cathodes, the gas tightness of as-fired
samples was characterized with a vacuum measurement apparatus as previously
described by Dollen and Barnett [21]. The porous cathode was prepared using a
mixture of Ag and YDB in a weight ratio of 6:4 with an ethylcellulose–terpineol
Guo, Weimin
