One of the biggest roadblocks to adoption of environmentally friendly hydrogen fuel cells for automotive propulsion is the high cost of the platinum-based catalysts that operate at the electrodes of the cell. At the cathode in particular, considerable quantities of the precious-metal coatings are needed to promote the relatively slow electrochemical reduction of atmospheric oxygen to produce water.
But with a market value greater than the price of gold—more than $1800 an ounce—platinum is a non-starter.
Not surprisingly, chemists and materials engineers have been working for decades to replace prohibitively expensive platinum catalysts in proton exchange membrane (PEM) fuel cells. Recently, scientists at Los Alamos National Laboratory (LANL) have done just that.
In a paper published in the journal Science, LANL researchers Gang Wu, Christina Johnston, and Piotr Zelenay, as well as Karren More of Oak Ridge National Laboratory, described the use of novel, platinum-free catalysts in a hydrogen fuel cell.
Despite the lack of platinum the new carbon-iron-cobalt catalysts yielded high power output, good efficiency, and promising longevity, said Zelenay, the team leader. “We reported several hundred hours of operation,” he noted.
The researchers found in addition that fuel cells containing the catalysts, which were synthesized by Wu, not only generated power levels that are comparable to the output of precious-metal-catalyst fuel cells but also held up favorably when cycled on and off, which can damage inferior catalysts relatively rapidly, Zelenay explained.
Removing the cost
“By avoiding platinum, we have in essence removed the costs from the catalyst,” he said. Moreover, the carbon-iron-cobalt catalysts create “only tiny amounts of hydrogen peroxide, an undesirable reaction product that is detrimental both to the power output of the cell and the durability of the polymer membrane,” he explained.
The researchers' main accomplishment thus far is identifying a catalyst with good durability and cycle lifetime relative to platinum-based catalysts. Zelenay noted, however, that in a pure oxygen atmosphere the new formulations still display insufficient chemical stability for practical use.
The group developed the non-precious-metal catalysts using inexpensive iron and cobalt salts together with carbon as a substrate (which was partially derived from polyaniline) in a high-temperature (900ºC) process.
Most of the funding for the Los Alamos research came from the U.S. Department of Energy's Energy Efficiency and Renewable Energy (EERE) Office. The researchers have filed a patent for the new catalysts.
Mysteries yet to solve
The next step in the team’s research will focus on achieving a greater understanding of what exactly is happening at the catalysis sites. Such insight “will help us gain better control of the material so we can increase activity and durability," the LANL chemist noted. "Right now, we’re not sure what active sites are. The transition metals iron and cobalt is the most likely interpretation, but the community is divided.”
Zelenay said that further analysis is not simple because no straightforward surface technique exists for identifying the active sites as the carbon absorbs most incoming light, which limits the number of useful techniques.
His group is collaborating with researchers at Argonne National Laboratory, where the scientists will try to address the problem by using synchrotron radiation from Argonne’s Advanced Photon Source on “oxygen analogues as a reference” to help determine the identity of the constituents of the active sites.