Strategy to improve phase compatibility between proton conductive BaZr0.8Y0.2O3-δ and nickel oxide

Article abstract of DOI:10.1039/c5ra26947d

BaZr_0.8Y_0.2O_3-δ (BZY20) is a promising candidate as an electrolyte in protonic ceramic fuel cells (PCFCs), and nickel (Ni) is known to show good electrode properties for the anode reaction. However, their compatibility seems to be questionable, since during the co-sintering process for cell fabrication, a second phase of BaY₂NiO₅ formed due to a reaction between BZY20 and NiO. The results in this work revealed that BaY₂NiO₅ was unstable against high temperature (1500 and 1600 °C), and could also be reduced in a hydrogen atmosphere at 600 °C. The products of these reactions may affect fuel cell performance. A systematic work was then performed to provide fundamental insight into the reactivity between BZY20 and NiO, which was found to be impacted significantly by the compositional homogeneity of the BZY20 powder used for cell fabrication, and also the BaO activity during the co-sintering process. It is concluded that improving the compositional homogeneity of BZY20, by elevating the final heating temperature for BZY20 from 1300 to 1600 °C in this work, and choosing a proper sintering strategy may improve effectively the phase purity of the cell.

Full text of DOI:10.1039/c5ra26947d

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To: RSC Advances
Strategy to Improve Phase Compatibility between Proton Conductive BaZr0.8Y0.2O3ꢀδ and
Nickel Oxide
Donglin Han a, *, Yuki Otani a, Yohei Noda a,b, Takayuki Onishi a, Masatoshi Majima b
and Tetsuya Uda a, *
a Department of Materials Science and Engineering, Kyoto University,
Yoshida Honmachi, Sakyoꢀku, Kyoto 606ꢀ8501, Japan
b Sumitomo Electric Industries, Ltd.,
1ꢀ1ꢀ1, Koyakita, Itamiꢀshi, Hyogo 664ꢀ0016, Japan
* Corresponding authors: Donglin Han (han.donglin.8n@kyotoꢀu.ac.jp)
and Tetsuya Uda (uda_lab@aqua.mtl.kyotoꢀu.ac.jp)
TEL: +81ꢀ75ꢀ753ꢀ5445, FAX: +81ꢀ75ꢀ753ꢀ5284
1

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Abstract
BaZr0.8Y0.2O3ꢀδ (BZY20) is a promising candidate as an electrolyte in protonic ceramic fuel cells
(PCFCs), and nickel (Ni) is known to show good electrode property for the anode reaction.
However, their compatibility seems to be questionable, since during the coꢀsintering process for cell
fabrication, a second phase of BaY2NiO5 formed due to reaction between BZY20 and NiO. The
results in this work revealed that BaY2NiO5 was unstable against high temperature (1500 and 1600
o
oC), and also reduced in hydrogen atmosphere at 600 C. The products of these reactions may
effect fuel cell performance. A systematic work was then performed to provide fundamental insight
into the reactivity between BZY20 and NiO, which was found to be impacted significantly by the
compositional homogeneity of BZY20 powder used for cell fabrication, and also the BaO activity
during the coꢀsintering process. It is concluded that improving the compositional homogeneity of
o
BZY20, by elevating the final heating temperature for BZY20 from 1300 to 1600 C in this work,
and choosing a proper sintering strategy may improve effectively the phase purity of the cell.
2

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1. Introduction
Yꢀdoped BaZrO3 (BZY) is an attractive material due to its high protonic conductivity in humid
atmosphere [1ꢀ3]. Incorporation of BZY into fuel cells as an electrolyte therefore seems to be quite
promising, since the operation temperature can thereby be decreased to an intermediate temperature
range (450 – 700 oC), lower than that of conventional solid oxide fuel cells (SOFCs) using oxide ion
o
conductive electrolytes (around 750 – 1000 C) [4, 5]. Great efforts have been devoted to the
development of BZY electrolyteꢀbased fuel cells. [6ꢀ13] However, a lot of challenges still remain.
Referring to the anode, it is regarded to be a good choice to use a composite one (generally, a
mixture of nickel oxide (NiO) and the corresponding electrolyte material) which has already been
widely applied in the SOFC community. [5] The same strategy has also been introduced into the
BZY electrolyteꢀbased fuel cells with prospective applicability. [8ꢀ14]
However, during the coꢀsintering process for fuel cell fabrication, a second phase of BaY2NiO5
formed due to the reaction between BZY and NiO. [14ꢀ16] Although a positive role of BaY2NiO5
in improving sinterability of BZY was suggested by Tong et al. [17], a recent work by Fang et al.
o
reported that such second phase decomposed in reducing or humid environment at 900 C [15].
Since H2 will be fed to the anode, existence of BaY2NiO5 is rather unfavorable, whose
decomposition introduces electrochemical insulator phase (Y2O3), and also potentially results in
cracks or delamination in the anode [15]. However, although BaY2NiO5 formed after heating BZY
o
o
– NiO mixture at 1400 C, its existence was not confirmed by elevating the temperature to 1600 C.
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[18] Such information suggests the possibility to suppress the formation of BaY2NiO5. But first
of all, it is necessary to perform a detailed investigation on the behavior of BaY2NiO5 at high
temperature, and also the reaction between BZY and NiO. We therefore conducted this work.
2. Experimental
2.1 Material preparation
Samples of BaZr0.8Y0.2O3ꢀδ (BZY20) and BaY2NiO5 were prepared by a conventional solid state
reaction method. Starting materials (BaCO3, ZrO2 and Y2O3 for BaZr0.8Y0.2O3ꢀδ, and BaCO3, Y2O3
and NiO for BaY2NiO5) were mixed at the desired ratios, and ballꢀmilled for 24 h. Mixtures were
then pressed into pellets under 9.8 MPa and heatꢀtreated at 1000 oC in ambient atmosphere for 10 h.
After ballꢀmilling for 10 h, the samples were pressed into pellets under 9.8 MPa again, and kept at
o
1300 C in ambient atmosphere for 10 h for synthesizing. The asꢀsynthesized samples were
ballꢀmilled for 100 h and 24 h for BZY20 and BaY2NiO5, respectively. The BaY2NiO5 powder was
pressed at 392 MPa to prepare pelletꢀlike samples. For all the heatꢀtreatments in this work, the
o
o
heating rates from room temperature to 1000 C, and 1000 C to higher temperature (1300, 1400,
1500 and 1600 oC) were 4.17 and 3.33 oCminꢀ1, respectively.
The asꢀsynthesized BZY20 powder was pressed into pellets at 392 MPa. After being embedded in
sacrificial powder with the composition of the asꢀsynthesized BZY20, these BZY20 pellets were
o
heated at 1600 C for 24 h in oxygen atmosphere for sintering. The asꢀsintered BZY20 was
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pulverized by ballꢀmilling for 50 h. For the sake of clarity, BZY20 with the final heating
o
o
o
temperature of 1300 and 1600 C are named as BZY20 (1300 C) and BZY20 (1600 C),
respectively, in this work. Then, both BZY20 (1300 oC) and BZY20 (1600 oC) were mixed with 70
wt% NiO, and ballꢀmilled for 10 h for mixing. The mixture was then pressed at 392 MPa to form
pelletꢀlike samples.
2.2 Sintering Strategy
With the purpose to create environment containing different BaO activity during the sintering
process, as shown in Fig. 1, different strategies were attempted for sample setting. To evaluate the
o
stability of BaY2NiO5 at 1500 and 1600 C, a method as shown in Fig. 1(a) with the name of
openꢀsintering was used. Magnesia (MgO) containers were inserted between the BaY2NiO5
pelletꢀlike samples and alumina (Al2O3) plateꢀlike crucibles to prevent their direct touch and
unfavorable reaction.
In addition to the openꢀsintering, two other methods named as
coverꢀsintering (Fig. 1(b)) and embedꢀsintering (Fig. 1(c)) were also applied to evaluate the
reactivity between BZY20 and NiO by heatꢀtreating the BZY20 – 70 wt% NiO pelletꢀlike samples.
In the coverꢀsintering method, the BZY20 – 70 wt% NiO pelletꢀlike samples were covered with
Al2O3 caps. Sacrificial powder (BZY20 (1300 oC) – 10 wt% BaCO3) was added to prevent a direct
touch of the samples from the Al2O3 plateꢀlike crucibles, and also seal the space between the Al2O3
caps and the plateꢀlike crucibles in certain degree. In the embedꢀsintering method, the samples
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were embedded in sacrificial powder (only BZY20) in the MgO containers. After heatꢀtreatment,
the BaY2NiO5 and BZY20 – 70 wt% NiO pelletꢀlike samples were all quenched in the ambient
atmosphere.
2.3 Characterization
Xꢀray diffraction (XRD) measurements were performed using Cu Kα radiation with X’PertꢀProMPD
(PANalytical, Almelo, Netherland). Rietveld refinement was carried out utilizing a commercial
software X’Pert HighScore Plus to simulate the XRD patterns. Microstructures were observed by
scanning electron microscopy (SEM) with VEꢀ7800 (Keyence Co., Osaka, Japan) and scanning
transmission electron microscopy (STEM) with JEMꢀ2100F (JEOL, Tokyo, Japan). Energy
dispersion Xꢀray spectroscopy with GenesisꢀXM2 (SEMꢀEDS, EDAX, Mahwah, NJ) and also
JEDꢀ2300 (STEMꢀEDS, JEOL, Tokyo, Japan) were used for composition measurement.
3. Results
3.1 Stability of BaY2NiO5 in H2 and O2 Atmospheres
As shown in Fig. 2(a), the asꢀsynthesized BaY2NiO5 sample was a single phase after the
heatꢀtreatment at 1300 oC. The single phase was also confirmed after keeping the sample powder at
o
600 C in dry O2 atmosphere for 72 h (Fig. 2(b)). However, when the atmosphere was changed to
dry H2, as shown in Fig. 2(c), BaY2NiO5 decomposed completely (eqn (1)). Since the sample was
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lately exposed to the ambient atmosphere containing H2O and CO2 for the XRD measurements,
Ba(OH)2·H2O, BaCO3 were identified. Such result agrees with that reported by Fang et al. by
heating BaY2NiO5 at 900 oC in dry H2. [15]
o
600
C
(1)
BaY2NiO5 + H2 (g) → Ba(OH)2 + Y2O3 + Ni
3.2 Stability of BaY2NiO5 at 1500 and 1600 oC
o
In order to evaluate the behaviour at 1500 and 1600 C, BaY2NiO5 pelletꢀlike samples were
heatꢀtreated at these temperatures in ambient atmosphere for desired time, and finally quenched. It
is worth noting here that at first, we placed the samples directly on Al2O3 plateꢀlike crucibles, but
o
found that BaY2NiO5 reacted with Al2O3 to form BaAl2O4 and Ba3Y2AlO7.5 at 1500 and 1600 C
(detailed information is given in Fig. S1 to S3). In order to prevent such reaction, we then used
MgO containers to accommodate the BaY2NiO5 samples (a schematic as shown in Fig. 1(a)). As
o
shown in Fig. 3, when heating at 1500 C, the samples kept the round pelletꢀshape, and no obvious
o
change in their appearance was confirmed. However, when heating at 1600 C, although the shape
of the pellets can still be identified, it is obvious that black liquidꢀlike product formed, which further
spread over the entire surface of the MgO containers with the heating time increased to 2 h. After
the sample was heated for 24 h, such black liquidꢀlike product disappeared, and the MgO crucibles
showed green appearance possibly due to the diffusion of Ni inward.
These residues of BaY2NiO5 pelletꢀlike samples were pulverized and analysed by powder XRD. As
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shown in Fig. 4, when heating at 1500 oC for 0 h (quenched immediately after heating up to 1500 oC)
and 1 h, only the diffraction peaks belonging to the BaY2NiO5 single phase were observed. When
o
the sample was kept at 1500 C for 2 h, diffraction peaks of Y2O3 appeared. And the intensity of
Y2O3 peaks further increased with the heating time extended to 5 and 24 h. These results suggest
that at 1500 oC, BaY2NiO5 is still a solid phase. However, theoretical calculation based on reported
thermodynamic data [19ꢀ21] reflects the fact that Ba and Ni oxides and hydroxides have relatively
high partial pressure at high temperature, as shown in Fig. 5. For example, the partial pressure of
o
Ba(OH)2 was calculated to be as high as 7.32 × 10ꢀ5 atm at 1500 C, if the reaction among BaO(s),
H2O (g) (partial pressure assumed as 0.01 atm), and Ba(OH)2 (g) reached equilibrium. Such
volatile property of Ba and Ni oxides and hydroxides results in a gradual decomposition of
BaY2NiO5, with Y2O3 remained (eqn (2)).
o
1500
C
(2)
BaY2NiO5 (s) → Y2O3 (s) + BaO (g) + NiO (g)
The case for heating at 1600 oC is different. As shown in Fig. 6(a), even the sample was just heated
up to 1600 oC, in addition to BaY2NiO5, diffraction peaks belonging to Y2O3 and BaNiO2 appeared.
When the sample was heated for 24 h, the BaNiO2 peaks disappeared. Such results suggest a totally
different phase relationship at 1600 oC, compared with the case at 1500 oC. SEMꢀEDS analysis was
o
then performed on the sample heated at 1600 C for 0 h (quenched immediately after heating up to
o
1600 C). As shown in Fig. 7(a), the area marked with number 1 and 2 has the composition close
to BaY2NiO5 (detailed SEMꢀEDS point analysis results are given in Table S1), whereas some
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adjacent areas (Point 6) is compositionally Yꢀrich, indicating possible existence of Y2O3. The areas
with the composition close to BaNiO2 (Points 4 and 5) showed a clear liquidꢀlike appearance with
some small precipitates embedded (a SEM image with large magnification is given in Fig. 7(b)).
However, we did not succeed in determining the composition of these precipitates by SEMꢀEDS,
because they are too small. Since the shape of the round pellets can still be identified even after
o
heating for 24 h (Fig. 3(f) – (j)), it is reasonable to believe that at 1600 C, BaY2NiO5 do not melt,
but peritecticly decomposed to Y2O3 and a liquid phase, as given in eqn (3). But during the
quenching from 1600 oC, the liquid phase decomposed to BaNiO2 and the small precipitates (eqn (4)).
Based on previous reports on relevant systems [22ꢀ25], we here propose a schematic pseudoternary
o
phase diagram of BaO – YO1.5 – NiO at 1600 C (Fig. 8), in which a qualitative indication of the
phase relationship was given. Anyhow, quantitative determination of the phase boundary might be
an interesting topic in the future.
o
1600
C
(3)
(4)
BaY2NiO5 → Y2O3 (s) + liquid phase
o
cooling from 1600
C
liquid phase → BaNiO2 (s) + small precipitates (s)
3.3 Dependence of BZY20ꢀNiO Reactivity on Final Heating Temperature of BZY20
It is thereby an interesting and also important topic that under what kind of conditions would
BaY2NiO5 form in the anode. Pelletꢀlike samples with the composition of BZY20 : NiO = 30 : 70
wt% were heated in ambient atmosphere at the temperature range of 800 to 1600 oC for 10 h with the
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openꢀsintering method (Fig. 1(a)). Both the BZY20 powders finally heatꢀtreated at 1300 and 1600
o
oC were used. As shown in Fig. 9(a), for the BZY20 (1300 C) – 70 wt% NiO mixture, peaks
o
belonging to Y2O3 appeared even after heating at 800 and 900 C, possibly due to the improvement
of crystallinity of the Y2O3 residue. When the temperature was elevated to 1000 oC, the Y2O3 peaks
disappeared, whereas those of BaY2NiO5 appeared. The existence of BaY2NiO5 was confirmed
o
o
from 1000 to 1400 C. In addition, at 1400 C, the peaks of Y2O3 rose again to coꢀexist with those
o
of BaY2NiO5. When the temperature was further increased to 1500 and 1600 C, BaY2NiO5
disappeared. Only the Y2O3 peaks were observed.
However, the mixture added with the BZY20 powder finally heated at 1600 oC behaved in a different
way. As shown in Fig. 9(b), no second phase was confirmed when the sample was heatꢀtreated
o
from 800 to 1100 C. The peaks belonging to BaY2NiO5 appeared only with the temperature
o
o
elevated to 1200 and 1300 C. Further heating at 1400 ꢀ 1600 C results in the formation of Y2O3,
but BaY2NiO5 disappeared.
The XRD patterns were simulated by Rietveld refinement to estimate weight amounts of the second
o
phases. As shown in Fig. 10, when BZY20 (1300 C) was added, the weight amount of BaY2NiO5
o
was larger than 3 wt% after heating at 1000 – 1200 C, and decreased with the increasing
o
temperature higher than 1200 oC. However, using BZY20 (1600 C) powder did not only lead to a
o
relatively narrow temperature range for BaY2NiO5 confirmation (1200 ꢀ 1300 C), and also reduced
the BaY2NiO5 amount (1.1 and 1.7 wt% at 1200 and 1300 oC, respectively). In addition, in both the
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two cases, Y2O3 appeared at 1400 oC, with its amount increased with the elevating temperature.
Variation of lattice constants of the perovskite phase (BZY20) in the BZY20 – 70 wt% NiO mixture
is shown in Fig. 11. It is clear that without adding NiO, the lattice constant of BZY20 finally
o
o
heatꢀtreated at 1300 C is smaller than that heatꢀtreated at 1600 C. The lattice constant of the
o
perovskite phase in BZY20 (1600 C) – 70 wt% NiO decreased with the increasing heating
temperature, due to diffusion of Ni cations into the BZY20 lattice, accompanied by an intraꢀgrain
o
Baꢀloss [3]. But the lattice constant of the perovskite phase in BZY20 (1300 C) – 70 wt% NiO
increased slightly with the increasing temperature, which is regarded to be a combined effect from
the Ni diffusion (shrinking the lattice) and improvement in compositional homogeneity (expanding
the lattice). The status of the perovskite phase in these two samples seems to get approached after
o
heating at 1500 C for 10 h, since the lattice constants was very close. However, after heating at
1600 oC, the lattice constant of the perovskite phase in BZY20 (1600 oC) – 70 wt% NiO is relatively
small, compared with that that in BZY20 (1300 oC) – 70 wt% NiO. It is attributed to a more severe
o
Baꢀloss in the openꢀsintering mode at such high temperature (1600 C), and also a more significant
segregation of Y2O3 (as shown in Fig. 10).
3.4 STEMꢀEDS Analysis on BZY20 Finally HeatꢀTreated at 1300 and 1600 oC
In our previous works, we supposed that the status of BZY20 is different after finally heating at 1300
o
and 1600 C due to an obvious difference in lattice constant [18, 26]. Moreover, it is a very
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common method to fabricate the anode by mixing NiO with asꢀsynthesized BZY20 [8ꢀ14]. And
o
1300 C is a typical synthesizing temperature for using solid state reaction method to synthesize
o
BZY20 [26ꢀ31]. So, a detailed analysis of the asꢀsynthesized BZY20 after heating at 1300 C is
highly necessary.
Fang et al. [33] suggested that by mechanical mixing (such as ballꢀmill in this work), it was difficult
to achieve a homogeneous mixing of the raw materials of BaCO3, ZrO2 and Y2O3. It seems to be
o
true, since we confirmed the residue of these raw materials in BZY20 heatꢀtreated at 1300 C by
STEM observation, as shown in Fig. 12 (although these residues were not observed from XRD
patterns (Fig. S4)). However, such raw material residue was not observed in the sample
o
o
heatꢀtreated at 1600 C. Elevating the final heating temperature from 1300 to 1600 C decreased
o
effectively the residue of raw materials, raising a great difference between BZY20 (1300 C) and
BZY20 (1600 oC).
Fig. 13(a) shows a bright field STEM (BFꢀSTEM) image of the perovskite phase area in BZY20
(1300 oC), which exhibits a different morphology from that containing residue of raw materials (Fig.
12). Then, STEMꢀEDS point analysis of several different areas was performed to determine the
local composition of such perovskite phase (examples for the STEMꢀEDS analysis are given in the
electronic supplemental information.). As shown in Fig. 13(c) (BaCO3, Y2O3 and ZrO2 residues
were excluded), a very obvious compositional scattering, especially in the Y content, can be seen.
A quite significant amount of the analyzing points locate in the area where the Y content is lower
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than the nominal value, and existence of highly Yꢀrich grains was also detected. Such result clearly
indicates that the cation distribution in the asꢀsynthesized BZY20 is not so homogeneous, possibly
due to the insufficient diffusion of cations to achieve a uniform distribution under the current
o
o
synthesizing condition (1300 C for 10 h) or different phase relationship at 1300 C. And we
consider that such compositional imhomogeneity should be the reason why the peak shape of BZY20
o
o
(1300 C) is broad and asymmetric, and its lattice constant is smaller than that of BZY20 (1600 C)
o
[18, 23]. In contrary, for BZY20 finally heatꢀtreated at 1600 C (a BFꢀSTEM image is shown in
Fig. 13(b) for the powder sample after ballꢀmilling), significantly improved homogeneity in
composition was confirmed by STEMꢀEDS. As shown in Fig. 13(d), most of the grains analyzed
were determined to have the composition close to the nominal one. Although some grains are
detected to be compositionally deviated from the nominal value, but the amount is very small. Such
improvement in compositional homogeneity agrees well with our previous work [27]. The
difference in compositional homogeneity is a quite interesting and important factor.
o
It is thereby definite that the final heating temperature of BZY20 (1300 or 1600 C), which greatly
influence the status of BZY20 (compositional homogeneity, residue of the starting materials, etc.),
makes BZY20 to show different reactivity with NiO.
3.5 Dependence of BZY20ꢀNiO Reactivity on BaO Activity
With the aim to control the BaO activity during sintering, different sintering strategies, namely the
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openꢀsintering (Fig. 1(a)), coverꢀsintering (Fig. 1(b)), and embedꢀsintering (Fig. 1(c)), were
attempted. The openꢀsintering and the embedꢀsintering methods are considered to maintain the
lowest and highest BaO activity, respectively. The BZY20 – 70 wt% NiO pelletꢀlike samples were
o
heated at 1500 C for desired time (0, 2, 5, and 10 h) with a subsequent quench in the ambient
atmosphere.
Both the mixtures containing BZY20 finally heating at 1300 and 1600 oC were examined here, but in
general, these two samples behaved in a similar way, because the heat treatment temperature is 1500
oC. With the openꢀsintering method, as shown in Fig. 14(a) and 14(b), BaY2NiO5 existed even the
samples were just heated up to 1500 oC (0 h), but the peak intensity of BaY2NiO5 decreased with the
increasing heating time. Meanwhile, the peaks belonging to Y2O3 appeared when the sample was
o
kept at 1500 C for 2 h with its intensity increased with the time. Such phenomenon indicates that
BaY2NiO5 formed during the elevating of temperature, but gradually decomposed during the static
o
heating at 1500 C. A quite similar result was obtained by using the coverꢀsintering method, as
shown in Fig. 14(c) and 14(d). But it is worth noting here that a delayed rising of Y2O3 peak
occurred with the coverꢀsintering method (2 h for the openꢀsintering method, but 5 h for the
coverꢀsintering method).
When the embedꢀsintering method was applied, as shown in Fig. 14(e) and 14(f), peaks belonging to
o
BaY2NiO5 were observed after just heating up to 1500 C, but became weakened and finally
o
disappeared after keeping at 1500 C for 2 and 5 h, respectively (see a peak around 32.5o). It is
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especially interesting to see that Y2O3 was not observed, regardless of the heating time. No second
o
phase appeared when the sample was kept at 1500 C for 5 and 10 h. The results imply that
relatively high BaO vapor assists incorporation of Y2O3 into the perovskite phase. These results
indicate that controlling BaO activity is important.
4. Discussion
o
Tong et al. suggested that the melting point of BaY2NiO5 was between 1450 – 1500 C [17].
o
However, in this work, we confirmed that BaY2NiO5 to be a solid phase at 1500 C. But it is
unstable due to the evaporation of oxides and hydroxides of barium and nickel at such high
o
temperature. Furthermore, at 1600 C, BaY2NiO5 peritecticly decomposed to Y2O3 and a liquid
phase. Anyhow, remaining of BaY2NiO5 in the fuel cells seems to be rather problematic, since it
decomposes in the hydrogen atmosphere at 600 oC, which is an expected temperature for the BZY20
electrolyteꢀbased fuel cell to operate.
In the anode, BaY2NiO5 formed due to the reaction between NiO and BZY20, which is revealed in
this work to be a rather sophisticated process depending on a couple of parameters. Especially, the
status of BZY20 after synthesizing is a very important factor. BZY20 which is relatively poor in
homogeneity, and contains residue of raw materials (BaCO3, Y2O3) exhibited high reactivity with
NiO to maintain a wide temperature range for BaY2NiO5 to exist. However, a positive effect on
suppressing the BaY2NiO5 formation was achieved by improving the compositional homogeneity of
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BZY20. Such improvement can be simply realized by just elevating the final heating temperature
of BZY20 (namely, from 1300 oC to 1600 oC, in this study).
The sintering strategy, or the BaO activity during sintering in another word, also impacts greatly the
phase appearance in the BZY20 – NiO mixture. With the openꢀsintering (Fig. 1(a)) and
coverꢀsintering (Fig. 1(b)) methods, which have relatively low BaO activity, Y2O3 was identified to
o
be the only second phase after keeping at 1500 C for 10 h. Such Y2O3 is considered to be
generated from the decomposition of BaY2NiO5 residue formed at the low temperature range during
the heating up process, because as shown in Fig. 14, BaY2NiO5 already existed in the sample just
o
heated up to 1500 C. We then increased the BaO activity during sintering by using the
embedꢀsintering method (Fig. 1(c)), and was excited to see that the segregation of Y2O3 did not
occur. Furthermore, there is even no second phase identified from XRD measurements after
heating at 1500 oC for 5 and 10 h. It seems that Y, and possibly also Ni, return to the crystal lattice
of barium zirconate, if a properly sufficient BaO activity can be supplied during the sintering.
5. Conclusions
The results in this work revealed that BaY2NiO5 was unstable at high temperature (1500 and 1600 oC)
o
in a different way, and also in a reducing atmosphere at 600 C. Remaining of BaY2NiO5 as a
second phase in BZY20ꢀbased fuel cells seems quite problematic. A systematic work was therefore
performed to provide fundamental insight into the reactivity between BZY20 and NiO. It was
16

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found that improving the compositional homogeneity of BZY20 powder, which was used in
composing the electrolyte and anode layers, reduced or even suppressed effectively the formation of
BaY2NiO5. It is rather interesting that such improvement in the compositional homogeneity can be
yielded simply by just elevating the final heating temperature for BZY20, such as from 1300 to 1600
oC. Furthermore, a proper BaO activity in the environment during the sintering is another key
factor, which can be adjusted by choosing appropriate sintering method. Improving the
compositional homogeneity of BZY20, and controlling precisely the BaO activity, therefore provide
a potential strategy to prepare a BZY20 electrolyteꢀbased fuel cell without second phases.
Acknowledgement
The authors want to thank Mr. Kenji Kazumi for STEMꢀEDS analysis.
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Chemical Reference Data, Monograph 9, American Chemical Society, American Institute of
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1995.
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Thermodynamic Properties of Nickel and Its Inorganic Compounds, U.S. Bureau of Mines,
Washington, 1976.
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(a) Open-sintering
(b) Cover-sintering
(c) Embed-sintering
Sacrificial powder (BZY20-10wt% BaCO3)
Sacrificial powder
(BZY20)
Sample
MgO container
Al2O3 plate-like
crucible
Sample
Sample
Al2O3 cap
MgO container
Al2O3 plate-like
crucible
Al2O3 plate-like
crucible
Evaluation of
Stability of BaY2NiO5
Evaluation of
Reactivity between BZY20 and NiO
Fig. 1 Schematics of methods for sample setting in this work. For evaluating the stability of
BaY2NiO5 and reactivity between BZY20 and NiO, three different methods, namely (a)
openꢀsintering, (b) coverꢀsintering and (c) embedꢀsintering, were adopted. In order to prevent the
reaction between Al2O3 and BaY2NiO5, MgO containers were used in the openꢀsintering and embed
sintering methods to avoid direct touch between the samples and the Al2O3 plateꢀlike crucible.
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(a)
(b)
(c)
as-synthesized
1300 oC, air, 10 h
BaY2NiO5 (00-047-0090)
600 oC, O2, 72 h
600 oC, H2, 72 h
•
Ba(OH)2 H2O (00-026-0154)
BaCO3 (00-045-1471)
Ni (00-051-0370)
Y2O3 (00-041-1105)
20
30
40
50
60
70
80
90
2θ / degree
o
Fig. 2 Powder XRD patterns of (a) asꢀsynthesized BaY2NiO5 after heatꢀtreatment at 1300 C in air
o
for 10 h, then heatꢀtreated at 600 C in (b) dry O2, or (c) dry H2 atmosphere for 10 h. BaY2NiO5
powder was used in the heatꢀtreatment at 600 oC in dry O2 and H2. XRD patterns were collected in
ambient atmosphere after each heatꢀtreatment. The reported reference patterns of BaY2NiO5
(JCPDS No. 00ꢀ047ꢀ0090), Ba(OH)2·H2O (JCPDS No. 00ꢀ026ꢀ0154), BaCO3 (JCPDS No.
00ꢀ045ꢀ1471), Ni (JCDPS No. 00ꢀ051ꢀ0370), and Y2O3 (JCPDS No. 00ꢀ041ꢀ1105) are also plotted.
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Fig. 3 Optical images of BaY2NiO5 pellets heatꢀtreated at 1500 and 1600 oC in ambient atmosphere
for various time. The BaY2NiO5 pellets were placed on MgO containers, which were placed on
Al2O3 plateꢀlike crucibles, the same as shown in Fig. 1(a). Blue residue on the Al2O3 plateꢀlike
o
crucibles was due to past experiments. The heating rates from room temperature to 1000 C, and
o
o
o
1000 C to 1500 or 1600 C were 4.17 and 3.33 Cminꢀ1, respectively. All the pellets were finally
quenched in ambient atmosphere.
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Quenched after Heating at 1500 oC, using MgO container
(a) Just reached 1500 oC (0 h)
BaY2NiO5 (01-080-0979)
Y2O3 (00-041-1105)
(b) Keep at 1500 oC for 1 h
(c) Keep at 1500 oC for 2 h
(d) Keep at 1500 oC for 5 h
(e) Keep at 1500 oC for 24 h
20
30
40
50
60
70
80
90
2θ / degree
Fig. 4 Powder XRD patterns of the residues of BaY2NiO5 pelletꢀlike samples heatꢀtreated at 1500
oC for various time. The pellets were placed on MgO containers, which were placed on Al2O3
o
o
plateꢀlike crucibles (Fig. 1(a)). The heating rates from room temperature to 1000 C, and 1000 C
o
o
to 1500 C were 4.17 and 3.33 Cminꢀ1, respectively. All the pellets were finally quenched in
ambient atmosphere.
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Temperature (oC)
800 900 1000 1100 1200 1300 1400 1500 1600 1700
-2
-4
-6
-8
-10
-12
-14
-16
-18
-20
-22
(3) BaO (s) = BaO (g)
(4) BaZrO3 (s) + H2O (g, 0.01 atm)
= Ba(OH)2 (g) + ZrO2 (s)
1000 1100 1200 1300 1400 1500 1600 1700 1800 1900 2000
Temperature (K)
Fig. 5 Theoretical calculation of partial pressure of BaO, NiO, Ba(OH)2, Ni(OH)2 against
temperature based on the reported thermodynamic data [19ꢀ21]. The partial pressure of water vapor
was assumed to be 0.01 atm.
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Quenched after Heating at 1600 oC, using MgO container
(a) Just reached 1600 oC (0 h)
BaY2NiO5 (01-080-0979)
Y2O3 (00-041-1105)
BaNiO2 (00-072-0403)
?
(b) Keep at 1600 oC for 1 h
?
(c) Keep at 1600 oC for 2 h
?
(d) Keep at 1600 oC for 5 h
?
(e) Keep at 1600 oC for 24 h
?
20
30
40
50
60
70
80
90
2θ / degree
Fig. 6 Powder XRD patterns of the residues of BaY2NiO5 pelletꢀlike samples heatꢀtreated in
o
ambient atmosphere at 1600 C for various time. The pellets were placed on MgO containers,
which were placed on Al2O3 plateꢀlike crucibles (Fig. 1(a)). The heating rates from room
o
o
o
o
temperature to 1000 C, and 1000 C to 1600 C were 4.17 and 3.33 Cminꢀ1, respectively. All the
pellets were finally quenched in ambient atmosphere. Some small peaks unable to be identified are
marked with question marks.
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Fig. 7 (a) A SEM image of the surface of the BaY2NiO5 pellet heatꢀtreated in ambient atmosphere
at 1600 oC for 0 h (the sample was quenched in ambient atmosphere just after heating up to 1600 oC).
Remaining of BaY2NiO5 (Points 1 and 2), and existence of BaNiO2 (Points 3, 4 and 5), and Y2O3
(Point 6) were confirmed by SEMꢀEDS point analysis. The detailed composition of each point was
listed in Table S1. The morphology of the areas with the composition close to BaNiO2 (Points 4
and 5) was shown in (b) with large magnification.
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BaO
Schematic
Diagram
0.0
1.0
0.1
0.9
BaO + Liquid
BaO
+ Ba3Y4O9
+ Liquid
0.2
0.8
0.7
0.6
0.5
0.4
5
.
1
O
0.3
0.4
0.5
Y
X
X
Ba3Y4O9
+ Liquid
B
a
O
Ba3Y4O9
0.6
Y2O3
+ Liquid
Liquid
Y2O3
0.7
0.8
0.9
1.0
0.3
0.2
0.1
+ Ba3Y4O9
+ Liquid
BaY2NiO5
NiO + Liquid
Y2O3 + NiO + Liquid
0.0
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
YO1.5
NiO
XNiO
o
Fig. 8 A schematic pseudoternary phase diagram of BaO – YO1.5 – NiO system at 1600 C based
on previous reports on the relevant systems [22ꢀ25]. The phase relationship is established based on
the experimental results of the present work, but a more detailed investigation is expected in future to
determine the precise phase boundary.
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BZY20 (1300 oC) - 70 wt% NiO
BZY20 (1600 oC) - 70 wt% NiO
(a)
(b)
BaY2NiO5 (01-080-0979)
Y2O3 (00-041-1105)
BaY2NiO5 (01-080-0979)
Y2O3 (00-041-1105)
Before heating
Before heating
BZY20 (011)
BZY20 (011)
800 oC
10 h
800 oC
10 h
900 oC
10 h
900 oC
10 h
1000 oC
10 h
1000 oC
10 h
1100 oC
10 h
1100 oC
10 h
1200 oC
1200 oC
10 h
1300 oC
10 h
1300 oC
10 h
1400 oC
10 h
1400 oC
10 h
1500 oC
10 h
1500 oC
10 h
1600 oC
10 h
1600 oC
10 h
27
28
29
30
31
32
33
27
28
29
30
31
32
33
2θ / degree
2θ / degree
Fig. 9 Powder XRD patterns between 27 – 33 degree of the BZY20 – 70 wt% NiO mixtures. The
BZY20 powder added was finally heatꢀtreated at (a) 1300 oC in ambient atmosphere for 10 h, and (b)
1600 oC in O2 for 24 h, respectively. These samples were kept in ambient atmosphere at the desired
o
temperature between 800 – 1600 C for 10 h using the openꢀsintering method (Fig. 1(a)). The
heating rates in the temperature ranges of room temperature to 1000 oC, and 1000 oC to 1600 oC were
4.17 and 3.33 oCminꢀ1, respectively. All the samples were finally quenched in ambient atmosphere.
9