Innovative Deposition Method May Spur Development Of Solid Oxide Fuel Cells

January 14, 2000

University Park, Pa. -- A faster, cheaper method of creating a gas-tight coating may help manufacturers commercialize a pollution-free way to use hydrocarbon fuels without burning them, according to Penn State researchers.

Solid oxide fuel cells convert gaseous hydrogen-rich fuels like natural gas, biogas, alcohols or coal-derived gas, directly into electrical energy.

They do this in a reaction that eventually breaks the fuel down into water and carbon dioxide. The tubular solid oxide fuel cells consist of bundles of tubes with oxygen flowing inside and gaseous hydrocarbon flowing over the tubes and operates at about 1800 degrees Fahrenheit.

"There must be a gas-tight layer between the hydrogen-rich fuel and the oxygen, otherwise, when they meet, there will be an explosion," says Dr. Rajendra N. Basu, postdoctoral associate in materials science and engineering.

Working with Dr. Merrilea J. Mayo and Dr. Clive A. Randall, both, associate professors of materials science and engineering, Basu developed a method to apply a gas-tight layer of zirconia on the tubes.

"Putting down the gas-tight layer can be accomplished using electrochemical vapor deposition, but it is a very expensive, time consuming and complicated process," says Basu, who is a scientist at Central Glass and Ceramic Research Institute, a national laboratory in Calcutta, India. "Other researchers have tried using electrophoretic deposition for this application, but with limited success."

The Penn State researchers are using electrophoretic deposition with a difference that appears to work. In electrophoretic deposition, a suspension of yttrium-doped zirconium oxide powder is made in very high concentration acetic acid. The application of an electrical potential allows the charged powder to move towards and deposit on the electrode with the opposite charge. The object, with the powder coating, is then fired at very high temperature so that the coating forms into a continuous film on the underlying material.

However, depositing zirconium oxide on the bare porous ceramic cathode tube surfaces of the tubular solid oxide fuel cells leads to an inhomogeneous coating that was not gas tight. The Penn State researchers deduced that the pores of the cathode tube were the source of the problem and decided to try a fugitive layer of carbon between the tube and the coating. This graphite layer serves as a uniform cathode and the zirconium oxide deposits evenly on the surface. During firing, the graphite sublimes and the coating deposits evenly on the tube's surface.

"We achieved a very even, homogeneous layer without any porosity," says Basu. "We can also easily adapt this process to coating long tubes."

Solid oxide fuel cells of the future might be used for stand-alone or back-up power and produce anywhere from 500 watts to 10 megawatts. These bundles of tubes would be about the size of a refrigerator with tubes as long as 70 inches. Zirconia coatings on those tubes must be continuous and gas-tight for the length of the tube.

Basu, Mayo and Randall have applied for a patent, "Fabrication of Zirconia Electrolyte Films by Electrophoretic Deposition." This work was funded by the Gas Research Institute with support from Siemens Westinghouse Power Corporation.

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Contacts:

A'ndrea Elyse Messer (814) 865-9481 (o)/ (814) 867-1774 (h)

Vicki Fong (814) 865-9481 (o)/ (814) 238-1221(h)

EDITORS: Dr. Basu is at (814) 865-6883 or Dr. Mayo is at (814) 863-7330 or Dr. Randall is at (814) 863-1328 or by email.