Normally, fuel cells need pure hydrogen to function correctly, and most of them are not immune to carbon and sulfur. Solid oxide fuel cells (SOFC), however, don’t necessarily need hydrogen to work, since they can dissociate methane 0r propane to produce electricity. Still, those gases have to be sulfur-free, otherwise they’ll “poison” the fuel cell’s anode and reduce its efficiency dramatically. Removing sulfur and other impurities from fuels rises their price.
Researchers from Georgia Institute of Technology have invented a new type of ceramic material that could widen the uses of SOFCs, so they could also be used with a wide range of liquid or gaseous fuels, without the need of separating hydrogen or cleaning them. Also, the new material allows fuel cells to work at much lower temperatures, making the need of expensive precious metals to disappear.
Normally, SOFCs use an electrochemical process to produce electricity by oxidizing the fuel. They use a ceramic electrolyte, a material known as yytria-stabilized zirconia (YSZ). aterial known as yttria-stabilized zirconia (YSZ). The new fuel cell ceramic material discovered at Georgia Tech is named “BZCYYb”, and it tolerates hydrogen sulfide in concentrations as high as 50 parts-per-million (ppm), does not accumulate carbon deposits, like YSZ cells do, and works fine at temperatures of below 500°C.
The BZCYYb (Barium-Zirconium-Cerium-Yttrium-Ytterbium Oxide) 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. Professor Meilin 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.
“We don’t see any problems ahead for fabrication or other issues that might prevent scale-up,” said Liu. “The material is produced using standard solid-state reactions and is straightforward.”
The researchers don’t yet understand how their new material resists deactivation by sulfur and carbon, but theorize that it may provide enhanced catalytic activity for oxidizing sulfur and both cracking and reforming hydrocarbons.
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,” Liu noted. “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.”
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.
Though the technology for solid oxide fuel cells is currently less mature than that for other types of fuel cells, Liu believes SOFCs will ultimately win out because they don’t require precious metals such as platinum and their efficiency can be higher – as much as 80 percent with co-generation use of waste heat.
