Solid oxide fuel cell operable over wide temperature range

Details Solid oxide fuel cell operable over wide temperature range Solid oxide fuel cell operable over wide temperature range

1999-06-29

2001-03-27

Kalafut, Stephen (Department: 1745)

Chemistry: electrical current producing apparatus, product, and

With pressure equalizing means for liquid immersion operation

C429S006000, C429S047000, C427S115000

Reexamination Certificate

active

06207311

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to solid electrolyte electrochemical cells, and more particularly relates to solid oxide fuel cells which are operable over a wide temperature range.
2. Background Information
High temperature solid oxide fuel cells (SOFC) have demonstrated the potential for high efficiency and low pollution in power generation. However, some problems remain associated with the high temperature processing and operation of such conventional cells. For example, any interaction between the lanthanum oxide-based perovskite air electrode (AE) and the zirconia-based electrolyte (EL) to form a low conducting compound at the AE/EL interface increases both cell resistance and the air electrode polarization, which can seriously affect the cell performance and result in loss of SOFC power. One technique to avoid such interaction is to apply an interfacial material at the AE/EL interface which does not increase cell resistance or increase air electrode polarization. As disclosed in U.S. Pat. No. 5,106,706 to Singh et al., which is incorporated herein by reference, the use of ceria as an AE/EL interface material has proven effective in improving high temperature SOFC performance. However, the thermal expansion match with the electrolyte and the electrical conduction characteristics of the interface modifier need further improvement.
Successful operation of SOFCs for power generation has been limited to temperatures of around 1000° C. due to insufficient electrical conduction of the electrolyte and high air electrode polarization loss at lower temperatures. U.S. Pat. Nos. 4,547,437 and 4,692,274 to Isenberg et al., which are incorporated herein by reference, disclose solid oxide fuel cells operable at relatively high temperatures. Increased versatility in generator design and operation would result if the SOFCs could operate over a wider temperature range and under a wider temperature gradient. In addition to large-scale power generation, SOFCs operable at lower temperatures would be useful in additional applications such as in powering light-duty vehicles.
Conventional solid oxide fuel cells which incorporate yttria stabilized zirconia (YSZ) electrolytes have shown good performance at high temperatures of around 1000° C. owing to the chemical stability and adequate ionic conduction of YSZ. However, with a decrease in operating temperature, the power losses increase, resulting from the substantial increase in the YSZ bulk ionic resistance, the electrolyte/electrode interfacial resistance and the electrode reaction polarization. The high temperature operation also sets limitations on generator materials, which can increase costs.
In order to allow SOFCs to operate at lower temperatures, the use of ultrathin film YSZ and non-zirconia based electrolytes, such as those based on ceria, have been explored. While the application of ultrathin film YSZ electrolytes of a few microns or thinner does reduce the bulk electrolyte resistance to some extent, such ultrathin films reduce the cell long-term reliability and the resistance to gas and electron leakage through the electrolyte. The use of ceria-based electrolytes requires major modifications of the other SOFC components, such as the air electrode and interconnection, to achieve a thermal expansion match. Moreover, ceria-based electrolytes demonstrate significant electronic conduction in fuel atmospheres, resulting in substantial non-power generating fuel consumption.
SUMMARY OF THE INVENTION
In accordance with one embodiment of the present invention, the above-noted problems are solved by applying an interfacial material at the AE/EL interface which does not adversely increase cell resistance or air electrode polarization. The interfacial material provides a barrier which controls the AE/EL interaction. It also acts as an interfacial modifier to reduce the polarization loss through the reduction of the AE/EL interfacial electrical resistance. The interfacial material is substantially chemically inert to both the electrolyte and the air electrode materials such that deleterious interactions are prevented. The interfacial material also is a good electronic and oxide ionic mixed conductor.
The interfacial material preferably comprises a zirconia-based mixed conductor of the formula Zr
1-x-y
Y
x
Tb
y
O
z
, wherein x preferably ranges from about 0.12 to about 0.2, y preferably ranges from about 0.15 to about 0.5, and z is less than 2. The value of z depends on the oxygen partial pressure to which it is exposed. Such compounds may be referred to as “YZTb” or terbia/yttria stabilized zirconia. Such compounds exhibit oxide ionic and p-type electronic mixed conduction due to the particular electronic structure of terbium, which results in a Tb
4
O
7
or TbO
1.75
composition.
In accordance with another embodiment of the present invention, an improved electrolyte in the form of SC
2
O
3
-stabilized zirconia (ScSZ) having higher electrical conductivity than conventional yttria-stabilized zirconia (YSZ) is provided. The use of a ScSZ electrolyte combined with an improved interface layer in a solid oxide fuel cell in accordance with the present invention has been found to substantially increase the operable temperature range of the SOFC, by substantially improving the operation at temperatures below 1000° C. The ScSZ electrolyte may be provided as a very thin layer in order to reduce resistance. The ScSZ electrolyte is preferably used in combination with a YZTb interface layer between the air electrode and electrolyte which reduces interfacial resistance and air electrode polarization in the SOFC.


REFERENCES:
patent: 4547437 (1985-10-01), Isenberg et al.
patent: 4692274 (1987-09-01), Isenberg et al.
patent: 5106706 (1992-04-01), Singh et al.
patent: 5518830 (1996-05-01), Worrell et al.
patent: 5589285 (1996-12-01), Cable et al.
patent: 5993989 (1999-11-01), Baozhen et al.
patent: 6-96791 (1994-04-01), None
Soral, P., et al. “Comparison of Power Densities and Chemical-Potential Variation in SOFC's with Multi-Layer and Single-Layer Oxide Electrolytes,” Proceedings of the Electrochemical Society, vol. 97-40, pp. 264-273, May 1997.*
P. Han et al., “Mixed-Conducting; Oxides in Solid Oxide Fuel Cells”, EPRI/GRI Fuel Cell Workshop on Fuel Cell Technology Research and Development, Atlanta, Georgia (1994). (No Month).

Affiliated with

Also associated with

Rating

Solid oxide fuel cell operable over wide temperature range does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Solid oxide fuel cell operable over wide temperature range, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Solid oxide fuel cell operable over wide temperature range will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2435744