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Solid Oxide Fuel Cells

Technology History Applications

The essay below outlines the technology and history of solid oxide fuel cells. If you have artifacts, photos, documents, or other materials that would help to improve our understanding of these devices be sure to respond to the questionnaire:
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Solid Oxide Fuel Cell Technology

A solid oxide fuel cell (SOFC) uses a hard ceramic electrolyte instead of a liquid and operates at temperatures up to 1,000 degrees C (about 1,800 degrees F). A mixture of zirconium oxide and calcium oxide form a crystal lattice, though other oxide combinations have also been used as electrolytes. The solid electrolyte is coated on both sides with specialized porous electrode materials.

diagram of tubular solid oxide fuel cell component, 2000
Fuel flows over this tubular solid oxide fuel cell; air flows through the center.
At these high operating temperature, oxygen ions (with a negative charge) migrate through the crystal lattice. When a fuel gas containing hydrogen is passed over the anode, a flow of negatively charged oxygen ions moves across the electrolyte to oxidize the fuel. The oxygen is supplied, usually from air, at the cathode. Electrons generated at the anode travel through an external load to the cathode, completing the circuit and supplying electric power along the way. Generating efficiencies can range up to about 60 percent.

In one configuration, the SOFC consists of an array of tubes (see image below). Another variation includes a more conventional stack of disks. Since SOFCs operate at such high temperatures, a reformer is not required to extract hydrogen from the fuel. Some demonstration units have capacities up to 100 kilowatts.



Solid Oxide Fuel Cell History
photo of solid oxide fuel cell bundle, 2000
Tubular solid oxide fuel cells arranged in bundles.

Both solid oxide and molten carbonate fuel cells are high temperature devices. The technical history of both cells seems to be rooted in similar lines of research until the late 1950s.

Swiss scientist Emil Baur and his colleague H. Preis experimented with solid oxide electrolytes in the late 1930s, using such materials as zirconium, yttrium, cerium, lanthanum, and tungsten. Their designs were not as electrically conductive as hoped and reportedly experienced unwanted chemical reactions between the electrolytes and various gases, including carbon monoxide.

In the 1940s, O. K. Davtyan of Russia added monazite sand to a mix of sodium carbonate, tungsten trioxide, and soda glass "in order to increase the conductivity and mechanical strength." Davtyan's designs, however, also experienced unwanted chemical reactions and short life ratings.

By the late 1950s, research into solid oxide technology began to accelerate at the Central Technical Institute in The Hague, Netherlands, Consolidation Coal Company, in Pennsylvania, and General Electric, in Schenectady, New York. A 1959 discussion of fuel cells noted that problems with solid electrolytes included relatively high internal electrical resistance, melting, and short-circuiting due to semiconductivity. It seems that many researchers began to believe that molten carbonate fuel cells showed more short-term promise.

Not all gave up on solid oxide, however. The promise of a high-temperature cell that would be tolerant of carbon monoxide and use a stable solid electrolyte continued to draw modest attention. Researchers at Westinghouse, for example, experimented with a cell using zirconium oxide and calcium oxide in 1962. More recently, climbing energy prices and advances in materials technology have reinvigorated work on SOFCs, and a recent report noted about 40 companies working on these fuel cells.



Solid Oxide Fuel Cell Applications

Like molten carbonate fuel cells, solid oxide cells require high operating temperatures, and their most common application is in large, stationary power plants. The high temperatures open the opportunity for "cogeneration"–using waste heat to generate steam for space heating, industrial processing, or in a steam turbine to make more electricity.

Solid oxide fuel cells, like most other types, produce little pollution. Although they require inverters to change their direct current to alternating current, they can be manufactured in relatively small, modular units. The compact size and cleanliness of SOFCs make them especially attractive for urban settings like Tokyo, where 25 kw units are already on line.

Siemens Westinghouse tubular solid oxide fuel 
Siemens Westinghouse tubular
solid oxide fuel cell.
In April 2000, the U.S. Department of Energy announced that a SOFC-microturbine cogeneration unit will be evaluated by the National Fuel Cell Research Center and Southern California Edison. The fuel cell was built by Siemens Westinghouse and the microturbine by Northern Research and Engineering Corporation. According to Siemens Westinghouse, the 220 kw SOFC operated for nearly 3400 hours, and achieved an electrical efficiency of about 53%.

Other companies working on SOFC technology include Fuel Cell Energy (which acquired Global Thermoelectric's Fuel Cell Division in late 2003). Cermatec is continuing work on units for mobile power generation.


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