Powering Fuel Cells: Oxide Materials May Facilitate Small-scale Hydrogen Production
Scientists have long known that oxides of the rare earth elements cerium (Ce), terbium (Tb), and praseodymium (Pr) can produce hydrogen from water vapor and methane in continuous "inhale and exhale" cycles. By doping iron atoms into the oxides, researchers at the Georgia Institute of Technology have lowered the temperatures at which these "oxygen pump" materials produce hydrogen, potentially allowing the process to be powered by solar energy.
This promising early-stage research was reported in the journal Advanced Materials 15 (2003) pp 521-526.
"This is a new approach for producing hydrogen that has several advantages compared to conventional production technology," said Zhong L. Wang, a professor in Georgia Tech's School of Materials Science and Engineering and director of the Center for Nanoscience and Nanotechnology. "For some applications, particularly those in the home, this could provide an alternative way to supply hydrogen for small-scale fuel cells."
Traditional reforming processes use metallic catalysts and temperatures in excess of 800 degrees Celsius to produce hydrogen from hydrocarbons such as methane. While efficient in industrial-scale production, the traditional reforming process may not be ideal for the small-scale hydrogen production needed to power fuel cells in homes or vehicles.
By operating at lower temperatures, the oxide system being developed at Georgia Tech could provide a lower-cost alternative that uses less energy and less water to operate.
The system would take advantage of the oxides' unique crystalline structure, which allows as much as 20 percent of the oxygen atoms to leave the lattice without structural damage. That would permit cycling oxygen atoms out of and back into the structure through a sequence of oxidation and reduction processes that both produce hydrogen, first from methane and then from water vapor. By providing an oxygen supply, the oxide system could reduce the amount of water required for hydrogen production.
First, temperatures of 700 degree Celsius drive oxygen out of the material, where it oxidizes carbon in the methane to form carbon oxides and free hydrogen. Temperatures as low as 375 degrees Celsius are then used to reduce water vapor, pulling oxygen from water to replenish the crystalline structure -- producing more hydrogen.
"By cycling the temperature back and forth in the presence of methane or water, you can continuously produce hydrogen," Wang said.