New Sulfur- and Coking-Resistant Ceramic Material Could Expand Applications for Solid Oxide Fuel Cells

2 October 2009

Researchers at Georgia Tech have developed a new ceramic material for use in a solid oxide fuel cell that resists deactivation by carbon buildup (coking) from hydrocarbon fuels or by sulfur contamination (poisoning)—two of the most vexing problems facing SOFCs. The material also exhibits high ionic conductivity at relatively low temperatures of 500-700 °C. A paper on their work appears in the 2 Oct issue of the journal Science.

If the long-term durability of the new mixed ion conductor material is proven, it could expand the applications for SOFCs—devices that generate electricity directly from a wide range of liquid or gaseous fuels without the need to separate hydrogen.

The development of this material suggests that we could have a much less expensive solid oxide fuel cell, and that it could be more compact, which would increase the range of potential applications. This new material would potentially allow the fuel cells to run with dirty hydrocarbon fuels without the need to clean them and supply water.

—Meilin Liu, a Regent’s professor in the School of Materials Science and Engineering at the Georgia Institute of Technology

The conventional anode of a fuel cell uses a composite of yttria-stabilized zirconia (YSZ) and the metal nickel. This anode provides excellent catalytic activity for fuel oxidation, good conductivity for collecting current generated, and compatibility with the cell’s ceramic electrolyte, which is also YSZ.

However, the YSZ material has three significant drawbacks:

• Even small amounts of sulfur in fuel “poison” the anode to dramatically reduce efficiency (e.g., just 2.5 ppm of H₂S in reformed natural gas fed to a SOFC operating at 800°C results in an observed electrochemical performance loss of 12.5%);

• The use of hydrocarbon fuels creates carbon build-up which clogs the anode; and

• Because YSZ has limited conductivity at low temperatures, SOFCs must operate at high temperatures.

As a result, fuels used in SOFCs, such as natural gas or propane, must be purified to remove sulfur, which increases their cost. Water in the form of steam must also be supplied to a reformer that converts hydrocarbons to hydrogen and carbon monoxide before being fed to the fuel cells, adding complexity to the overall system and reducing energy efficiency. And the high-temperature operation means the cells must be fabricated from costly exotic materials, which keeps SOFCs too expensive for many applications.

The new material developed at Georgia Tech addresses all three of those anode issues. BaZr_0.1Ce_0.7Y_0.2–χY_bχO_3–γ, referred to as BZCYYb Barium-Zirconium-Cerium-Yttrium-Ytterbium Oxide) tolerates hydrogen sulfide in concentrations as high as 50 parts-per-million, does not accumulate carbon, and can operate efficiently at temperatures as low as 500 ° Celsius.

The BZCYYb material could be used in a variety of ways: as a coating on the traditional Ni-YSZ anode, as a replacement for the YSZ in the anode and as a replacement for the entire YSZ electrolyte system. Liu believes the first two options are more viable.

So far, the new material has provided steady performance for up to 1,000 hours of operation in a small laboratory-scale SOFC. To be commercially viable, however, the material will have to be proven in operation for up to five years—the expected lifespan of a commercial SOFC.

The researchers don’t yet understand how their new material resists deactivation by sulfur and carbon. However, they theorize that its ability to resist deactivation by sulfur and coking is linked to the mixed conductor’s enhanced catalytic activity for sulfur oxidation and hydrocarbon cracking and reforming, as well as enhanced water adsorption capability.

In addition to its tolerance of sulfur and resistance to coking, the BZCYYb material’s conductivity at lower temperature could also provide a significant advantage for SOFCs.

If we could reduce operating temperatures to 500 or 600 degrees Celsius, that would allow us to use less expensive metals as interconnects. Getting the temperature down to 300 to 400 degrees could allow use of much less expensive materials in the packaging, which would dramatically reduce the cost of these systems.

—Meilin Liu

Beyond its use in fuel cells, the material developed by Liu and his team—which also included Lei Yang, Shizhong Wang, Kevin Blinn, Mingfei Liu, Ze Liu and Zhe Cheng——could also be used for fuel reforming to feed other types of fuel cells.

In a Perspective in the same issue of Science, Dr. J. R. Selman of the Illinois Institute of Technology commented that the design approach taken by Liu and his team—using the BZCYYb material that exhibits mixed conduction but exists as a single phase—could simplify anode structure considerably.

...it has the promise of a “3-in-1 material” that resists sulfur poisoning and inhibits coking of the anode while potentially simplifying anode structure, even if nickel or copper might have to be used as an electronic and electrocatalytic backbone...Implicit in the work of Yang et al. is that the mixed rare-earth doped BaZr cerate is not only a solid electrolyte but functions as a catalyst for the anodic oxidation (somewhat like the apparent function of ceria in ceria-Ni cermet). If corroborated, this would open interesting possibilities—for SOFC technology as well as hybrid high-temperature cells that could use new design strategies.

—Selman, 2009

The research was supported by the US Department of Energy’s Basic Energy Science Catalysis Science Program under grant DE-FG02-06ER15837.

Resources

• Lei Yang, Shizhong Wang, Kevin Blinn, Mingfei Liu, Ze Liu, Zhe Cheng, Meilin Liu (2009) Enhanced Sulfur and Coking Tolerance of a Mixed Ion Conductor for SOFCs: BaZr_0.1Ce_0.7Y_0.2–χY_bχO_3–γ. Science Vol. 326. no. 5949, pp. 126 - 129 doi: 10.1126/science.1174811

• J. R. Selman (2009) Poison-Tolerant Fuel Cells. Science Vol. 326. no. 5949, pp. 52 - 53 doi: 10.1126/science.1180820