Georgia Tech researchers have developed a new ceramic material that could help expand the applications for solid oxide fuel cells – devices that generate electricity directly from a wide range of liquid or gaseous fuels without the need to separate hydrogen.
Though the long-term durability of the mixed ion conductor material still needs to be proven, its development could address two problems facing the solid oxide fuel cells: tolerance of sulfur in fuels, and resistance to carbon build-up known as coking. The new material could also allow solid oxide fuel cells – which convert fuel to electricity more efficiently than other fuel cells – 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,” said Meilin Liu, a Regent’s professor in the School of Materials Science and Engineering. “This new material would potentially allow the fuel cells to run with dirty hydrocarbon fuels without the need to clean them and supply water.”
Like all fuel cells, solid oxide fuel cells (SOFCs) use an electrochemical process to produce electricity by oxidizing a fuel. SOFCs use a ceramic electrolyte known as 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 electrolyte – which is also YSZ.
The anode has three drawbacks: even small amounts of sulfur in fuel “poison” the anode to reduce efficiency, 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 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, reducing energy efficiency. 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 - BZCYYb (Barium-Zirconium-Cerium-Yttrium-Ytterbium Oxide) - addresses all of these anode issues. BZCYYb 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 degrees Celsius.
The material could be used 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.
BZCYYb has provided steady performance for up to 1,000 hours of operation in a small laboratory-scale SOFC. To be commercially viable, the material will have to be proven in operation for up to five years, which is the expected lifespan of a commercial SOFC.
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.
“Solid oxide fuel cells offer high energy efficiency, the potential for direct utilization of all types of fuels including renewable biofuels, and the possibility of lower costs since they do not use any precious metals,” said Liu. “We are working to reduce the cost of solid oxide fuel cells to make them viable in many new applications, and this new material brings us much closer to doing that.”