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.”