Although the long-term durability of the new mixed ion-conductor material needs to be proven, its development could address two of the most vexing problems facing SOFCs: tolerance of sulfur in fuels, and resistance to carbon build-up (‘coking’). The new ceramic – described in the journal Science – could also allow SOFCs to operate at lower temperatures, potentially reducing material and fabrication costs.
‘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,’ says Professor Meilin Liu, in the School of Materials Science & Engineering at the Georgia Institute of Technology. ‘This new material would potentially allow the fuel cells to run with dirty hydrocarbon fuels, without the need to clean them and supply water.’
SOFCs use a ceramic electrolyte, generally yttria-stabilized zirconia (YSZ). The fuel cell’s anode uses a composite consisting of YSZ and nickel. This anode provides excellent catalytic activity for fuel oxidation, good conductivity for collecting current generated, and compatibility with the cell’s YSZ electrolyte.
However, the material has three significant drawbacks. Even small amounts of sulfur in fuel poison the anode to dramatically reduce efficiency; the use of hydrocarbon fuels creates carbon build-up which clogs the anode; and because YSZ has limited conductivity at low temperatures, and 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 CO before being fed to the fuel cells – making the overall system more complex, 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 these anode issues. The BZCYYb (barium-zirconium-cerium-yttrium-ytterbium oxide) material tolerates hydrogen sulfide in concentrations as high as 50 ppm, does not accumulate carbon, and can operate efficiently at temperatures as low as 500°C.
The BZCYYb material could be used in a variety of ways in SOFCs: 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.
In addition to its sulfur tolerance and coking resistance, 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°C, that would allow us to use less expensive metals as interconnects,’ notes Liu. ‘Getting the temperature down to 300–400°C could allow use of much less expensive materials in the packaging, which would dramatically reduce the cost of these systems.’