March 25-29, 2007
NSLS Users Present Energy Research at ACS Meeting
Two NSLS users – Brookhaven chemists Radoslav Adzic and Jose Rodriguez – presented their research on fuel cells and energy production at the 233rd National Meeting of the American Chemical Society (ACS). The details of their talks are highlighted below:
Giving Platinum Catalysts a Golden Boost for Fuel Cells
Platinum might outweigh gold in the jewelry market, but as part of an ongoing effort to produce efficient and affordable fuel cells, scientists at the U.S. Department of Energy’s (DOE) Brookhaven National Laboratory are studying how gold atoms might enhance the value of the pricier metal. Specifically, they’re looking for ways to use gold to prevent the destruction of platinum in the chemical reactions that take place in fuel cells. Brookhaven chemist Radoslav Adzic described this research at the ACS Meeting on March 27, 2007, at McCormick Place South, Chicago, Illinois.
 | |
|
Platinum is the most efficient electrocatalyst for accelerating chemical reactions in fuel cells. However, in reactions during the stop-and-go driving of a fuel-cell-powered electric car, the platinum dissolves. In accelerated tests, as much as 45 percent of the catalyst can be lost during five days. “Platinum is by far the best single component catalyst for the oxygen reduction reaction, and we have to find a way to protect it,” Adzic said. Under lab conditions that imitate the environment of a fuel cell, Adzic and a team of Brookhaven researchers, including Junliang Zhang, Kotaro Sasaki, and Eli Sutter, added gold clusters to a platinum electrocatalyst, which kept it intact during an accelerated stability test that simulates stop-and-go driving in an electric car.
The details: A fuel cell converts hydrogen and oxygen into water and, as part of the process, produces electricity. Hydrogen is oxidized at the device’s anode (the terminal where current flows in) when electrons are released and hydrogen ions are formed; the released electrons supply current for an electric motor. These electrons flow to the cathode (the terminal where current flows out) to reduce oxygen, and in a reaction with hydrogen ions, water, the only byproduct of a fuel cell reaction, is produced. Platinum electrocatalysts are used to speed up the oxidation and reduction reactions involved in this process, but as a result, they, too, are oxidized (lose electrons) and dissolve.
In the unique method used at Brookhaven, researchers place gold on carbon-supported platinum nanoparticles by displacing a single layer of copper and subject it to several sweeps of voltage. The copper is needed to reduce the charged gold particles to neutral atoms; it then conveniently forms a monolayer of platinum by an adsorption process, the binding of molecules or particles to a surface. Using x-rays as probes at Brookhaven’s National Synchrotron Light Source, a scanning transmission microscope at Brookhaven’s Center for Functional Nanomaterials, and electrochemical techniques in the laboratory, the scientists can show that less platinum is oxidized with this method. As predicted, during laboratory testing, the platinum electrocatalyst remains stable when under conditions mimicking stop-and-go driving conditions. Next, researchers will test the catalyst in real fuel cells at the DOE’s Los Alamos National Laboratory in New Mexico.
“The very promising properties of fuel cells have been known for many decades,” Adzic said. “But it’s only now that we can look at the activities and qualities of the catalysts and find something stable enough to be used in cars or residential applications.”
This research is funded through the U.S. Department of Energy’s Hydrogen Program, which implements the President’s Hydrogen Fuel Initiative, a five-year program that began in 2003 to sponsor research, development, and demonstration of hydrogen and fuel cell technologies. Specifically, the funding derived from DOE’s Office of Basic Energy Sciences and its Office of Energy Efficiency and Renewable Energy.
Gold, Copper Nanoparticles Take Center Stage in the Search for Hydrogen Production Catalysts
X-ray studies at Brookhaven National Laboratory are pointing the way to less costly and more efficient catalysts for improving the performance of fuel cells. The studies, which were presented by Brookhaven chemist Jose Rodriguez, show that copper can be substituted for gold in reactions that keep fuel cells functioning longer while eliminating unwanted byproducts.
 |
Jose Rodriguez |
With the goal of efficient fuel cell operation in mind, researchers first need to turn their attention to hydrogen, which is one of the leading energy sources being investigated by scientists sponsored by the DOE as part of its mission to ensure the nation’s future energy needs. A major problem facing today’s most promising fuel-cell technologies is that the same hydrogen-rich materials feeding the reaction often contain high levels of carbon monoxide (CO), which is formed during hydrogen production. Within a fuel cell, CO “poisons” the expensive platinum catalysts that convert hydrogen into electricity, deteriorating their efficiency over time and requiring their replacement.
Rodriguez discussed how the use of gold and copper nanoparticles might provide a solution to this problem on March 28, 2007, at McCormick Place South, Chicago, Illinois. “We’re trying to find a catalyst that achieves two things: produces hydrogen while removing a large amount of CO,” Rodriguez said.
One way to eliminate the CO byproduct is to combine it with water to produce hydrogen gas and carbon dioxide in a process known as the “water-gas shift” reaction. With the assistance of proper catalysts, the water-shift reaction can convert nearly 100 percent of the CO into carbon dioxide. Using catalyst characterization techniques at Brookhaven’s National Synchrotron Light Source (NSLS), Rodriguez and coworkers Jonathan Hanson and Jan Hrbek found that nanoparticles of either gold or copper, supported on a metal, can perform this catalytic role. In particular, they found that the greatest catalytic activity is achieved with extremely small nanoparticles – less than 4 nanometers (4 billionths of a meter) – supported on the metal cerium oxide, or ceria.
“Metal nanoparticles alone are not able to do the catalysis,” Rodriguez said. “But when you put them on the ceria, you see tremendous catalytic activity.”
At the nanoscale, gold has long been known to exhibit chemical reactivity that makes it a potent catalyst. The problem, however, comes with its hefty price tag. “We wanted a material that was less expensive,” Rodriguez said. “We wanted to see if we could replace the gold with copper.” Using x-ray diffraction, absorption, and spectroscopy studies at the NSLS, Rodriguez’s group showed that the substitution is indeed possible. Although gold nanoparticles continue to show the greatest catalytic activity, copper is almost as reactive and its cost is much lower.
This research was funded by the Office of Basic Energy Sciences within the DOE’s Office of Science.
ARTICLE BY: Kendra Snyder