exhibits an internal resistance to either electronic or ionic current flow, often ... tubular SOFC stac designed by Siemens/Westinghouse. On the other hand, ...
SPECIAL SECTION: ENERGY
Solid Oxide Fuel Cells and Membranes Fuel cells and membranes are similar in that they employ materials that transport ions and electrons. Both can play important roles in making clean energy a reality.
Kyle Brinkman Clemson Univ. Kevin Huang Univ. of South Carolina
A
sustainable future calls for the development of clean,
-
-
-
-
Solid oxide fuel cells Hydrocarbon Fuel Load
2–
2–, from the cathode to the anode 2
e–
H2 + O2– = H2O + 2e– CO + O2– = CO2 + 2e–
O2–
I 2– e– 2 2
–
Anode (Fuel Electrode)
0.5O2 + 2e– = O2–
Solid Electrolyte
V
+
Cathode (Air Electrode)
Air
Figure 1. In a solid oxide fuel cell, the cathode reduces oxygen (from air) to O2–, which the electrolyte transports from the cathode to the anode, where it reacts with a fuel to produce H2O, CO2, and electrons.
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SOFC stack designs
help reduce the power losses on current connections, which ceramic interconnect, or porous inactive insulator can serve
Advantages of SOFCs 2–
2
2
-
and function as interconnects and current collectors simulta-
Figure 2. The SOFC substrate supporting the thin electrolyte film can be made into a tubular (a) or planar (b) shape. The segment-in-series design (c) is a special type of planar construction made by depositing multiple cells in series on an electrochemically inactive and electrically insulating substrate. Source: Adapted from (1).
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SPECIAL SECTION: ENERGY
-
for several reasons:
2
and
-
Applications of SOFCs -
SOFCs: Challenges and opportunities -
-
Figure 3. (a) Siemens/Westinghouse has demonstrated a 220-kWe SOFC/micro-turbine hybrid generator that achieved a net electrical efficiency of 53%. (b) A different unit, a 100-kWe SOFC generator, has more than 35,000 hours of operation, making it the longest-running SOFC ever demonstrated. Source: Photos courtesy of Siemens/Westinghouse.
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2
2
• supports both ionic and electronic conduction, or • forms a two-phase composite of an ionic conductor and
2
2
Membranes
-
2–
2
–
2 2–
(4). 2–
2
2–
and two electrons in opposite
and 2e– at the other surface
2–
2
–
in chemical potential between the feed and permeate sides of side of the membrane to the low-partial-pressure side, while
-
membranes can be viewed as a short-circuit fuel cell, with
Table 1. Solid-oxide fuel cell systems can be used for power generation at different scales. Size
Small-Scale
Class Rating
Efficiency (Net Alternating Current, LHV)
48% >60%
Pipeline natural gas
Larger industrial units
Coal gas
Small communities
Pipeline natural gas
Baseload power generation
Coal gas
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SPECIAL SECTION: ENERGY
-
(11), methanol (12),
(13). -
environment (14). membrane performance (6). 2
2
o
(15).
2
-
2
2 separation (7), while carbonate ion conductors 2 separation (8).
-
2
Applications of membranes e.g., renew-
2
-
2
2
2
-
separation membranes are composites
or 2 Li2 salt as the carbonate ion conductor within a porous metal support such as stainless steel or silver, which serves as the electronic conductor (9).
(16).
(10), 2
capture (17). 2
Figure 4. A mixed ionic-electronic conductor (MIEC) membrane can be thought of as a short-circuit fuel cell, with electron transport occurring within the material rather than through an external circuit. Source: Adapted from (5).
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2
transport; -
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Another application for membranes in the production of
2
(18). (16). 2
2
Membranes: Challenges and opportunities Porous Support
High-Pressure Syngas (External)
Thin Active Membrane Low-Pressure Air (Internal)
Internal Flow Channels
High-Pressure Syngas (External)
Figure 5. An oxygen separation membrane can be used for syngas production. Source: Adapted from (16).
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Literature Cited 1.
Huang, K., and J. B. Goodenough,
10. Wang, H., et al.,
2.
Huang, K.,
cations, 7 11. Wilhelm, D. J., et al.,
3.
Huang, K., and S. C. Singhal,
4.
Journal of Power Sources, 237, Sunarso, J., et al.,
5.
Membrane Science, 320 Reifsnider, K. L., et al.
6.
Society, 160 Lane, J. A., and J. A. Kilner,
Catalysis Communi-
Fuel Processing Technology, 71 12. Taylor, S. H., et al.,
Catalysis Today, 42
Journal of
13. Lunsford, J. H., Catalysis Today, 63 14. Jacobson, M. Z., et al.,
Journal of The Electrochemical
Science, 308 15. Twigg, M. V., “
Solid State 7. 8.
Ionics, 136–137, Phair, J. W., and S. P. S. Badwal, Ionics, 12 Chuang, S. J., et al.,
16. Miller, C. F., et al., Catalysis Today, 228 17. Christie, M., et al. Industrial and Engi-
9.
neering Chemistry Research, 44 Xu, N. S., et al., Journal of Membrane Science, 401–402,
2
18. Zeng, Y., et al., 2
from Journal of Membrane Science, 150
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