ABSTRACT RESULTS Solid-oxide fuel cells (SOFCs) are ... XRD analysis of pure and ...
Electrolyte material for Solid Oxide Fuel Cells
Viktoriya Yugay and Dr. Rick Ubic
Department of Materials Science and Engineering, Boise State University
ABSTRACT
Solid-oxide fuel cells (SOFCs) are high-efficiency devices that can convert a wide variety of fuels
( e.g.
, hydrogen, methane, butane, etc .) into electrical energy. The main components of the fuel cell include the anode, electrolyte, cathode, and interconnect. This project involves the synthesis and characterization of ceramic compositions for potential application as electrolyte materials.
INTRODUCTION
The historical use of fluorite-structured Y
2
O
3
stabilized zirconia (YSZ) as electrolytes in SOFCs is based on the fast O 2 conduction of YSZ at elevated temperatures. It is a key element in Westinghouse
SOFCs, and NASA is studying it for their planar SOFC design. The doping of ZrO
2
with 8mol% Y
2
O
3
not only stabilizes the cubic phase but simultaneously creates a large concentration of chargecompensating oxygen vacancies which are mobile at high temperatures and provide sufficiently high conductivity (0.18 S/cm at 1000 ° C). The ionic conductivity of the electrolyte determines, to a large extent, the available power and operating temperature of an SOFC.
Pyrochlores (Fig. 1) are ordered forms of fluorite which contain two different cations and in which the unit cell is doubled. There is some evidence that ordered pyrochlore phases can be better ionic conductors than disordered fluorite phases of similar compositions.
GOAL
The objective of this project is to explore whether anion disorder can exist independently of cation disorder in pyrochlores and what implication this possibility may have for the development of better ceramic electrolytes. Pyrochlore stability generally exists for cation radius ratios ~1.4 ≤ r
A
/ r
B
≤ 1.8
(Fig. 2). The goal is to maximize the number of oxygen vacancies by appropriate doping while still retaining the pyrochlore structure and avoiding low-mobility vacancy clusters. Analysis will help determine independently the contributions of both the r
A
/ r
B
ratio and oxygen-vacancy concentration on structure and conductivity.
RESEARCH POSTER PRESENTATION DESIGN © 2012 www.PosterPresentations.com
Figure 1. A generic pyrochlore unit cell.
EXPERIMENT
Pure Y
2
Zr
2
O
7
, with r
A
/ r
B
= 1.4153 and no stoichiometric anion vacancies, was prepared via conventional mixed-oxide synthesis as a benchmark. In addition, two doped compositions were prepared. The first was a singly-doped composition in the (Y
2x
Ca x
)Zr
2
O
7x /2
system ( x =
0.1505) with r
A
/ r
B
= 1.4258 and 7.525% oxygen vacancies on the 8b site. Next, a co-doped composition in the
(Y
2-200 x /99
Ca
x
Li
101 x /99
)Zr
2
O
7-301 x /198 system ( x = 0.0495) was synthesized to achieve the same vacancy concentration but with a lower r
A
/ r
B ratio, equal to that for pure Y
2
Zr
2
O
7
.
Figure 2. <110> selected-area electron diffraction pattern of Tb
2
Hf
2
O
7
, a pyrochlore on the verge of fluorite stability ( r
A
/ r
B
= 1.46). The faint arcs of diffuse intensity correspond to the breakdown of pyrochloric ordering and correspond to antiphase domains in the microstructure.
RESULTS
XRD analysis of pure and doped powders calcined at 1450 ° C for 6 hours showed the presence of unreacted Y
2
O
3
(Fig. 3). The Y
2
Zr
2
O
7 calcined at 1500 ° C for 6 hours was coarse but fully reacted (Fig. 3).
powder
Figure 3. X-ray diffraction data for
Y
2
Zr
2
O
7
, (Y
1.8495
Ca
0.1505
)Zr
2
O
6.9248
,
(Y
1.9
Ca
0.0495
Li
0.0505
)Zr
2
O
7
all calcined at 1450°C for 6 hours and
Y
2
Zr
2
O
7
calcined at 1500°C for 6 hours.
The density measurements of sintered Y
2
Zr
2
O
7 pellets (Fig. 4) showed that lower pressing pressures yielded better results. The best densities for Y
2
Zr
2
O
7
(~95% r th
) were achieved by calcining at
1450 ° C for 6 hours and sintering at 1750 ° C for 4 hours (Fig. 5).
Density vs Pressure
5.50
5.40
5.30
5.20
5.10
5.00
4.90
4.80
4.70
5.009
5.083
5.148
5.159
5.172
5.458
78 73 61 49 37 24
1700°C/4hours 1750°C/4hours
Pressure, MPa
Figure 4. Density measurements of
Y
2
Zr
2
O
7
pellets pressed under different load and sintered at 1700 ° C (78, 73, 61 MPa loads) and 1750 ° C (49, 37, 24
MPa loads) for 4 hours.
5
4.75
4.5
4.25
4
3.75
3.5
3.84
Density of Y
2
Zr
2
O
7
4.5203
4.8362
5.083
5.143
5.186
4.5771
Sintering parameters
Figure 5. Density measurements of
Y
2
Zr
2
O
7
pellets sintered at different temperatures. Average pressing pressure for the pellets is 55MPa.
The sample measured as
95% dense was still visibly porous (Fig. 5), meaning that the actual density is < 95%.
Figure 5. SEM image of Y
2
Zr
2
O
7
The EDS analysis showed that the Y
2
Zr
2
O
7 pellets sintered at 1750 ° C for 4 hours were single-phase, with no leftover Y
2
O
3
found (Table 1).
Element Weight% Atomic%
Y 50.24
Zr 49.76
Total 100.00
50.88
49.12
100.00
Table 1. Proportion of elements calculated from EDS spectra of Y
2
Zr
2
O
7 pellets sintered at 1750
°
C for 6 hours.
These data suggest that, although a single-phase pellet is produced upon sintering, the calcination temperature of 1450 ° C is insufficient to achieve a single-phase powder, which may hinder the subsequent densification.
CONCLUSIONS
Single-phase Y
2
Zr
2
O
7
was achieved by calcining at
1500 ° C for 6 hours. The calculated density for
Y
2
Zr
2
O
7
calcined at 1450 ° C for 6 hours and sintered at 1750 ° C for 4 hours was 95%, but the observed density was lower.
FUTURE WORK
More doped compositions are required to establish trends, and increased sintering times are probably warranted to increase densities. Transmission electron microscopy (TEM) and neutron diffraction are required to establish the crystal structures.
Finally, developing an apparatus to conduct ionic conductivity tests at high temperatures will help relate the structural effects to conductivity.
