PORTLAND, Ore. Brookhaven National Laboratory says it has found a way to keep the platinum used as a catalyst in fuel cells from dissolving and degrading performance. The U.S. Department of Energy thinks the breakthrough could help put the federally mandated Advanced Energy Initiative back on track toward realizing all-electric vehicles by 2020.
The DOE, which has patented the Brookhaven discovery and hopes to license it to fuel cell makers worldwide, started getting reports about 18 months ago that stop-and-go driving was seriously degrading the lifetime of next-generation fuel cell-powered vehicles--as much as 45 percent over five days of accelerated testing. At this rate, hybridswhich use an electric motor to supplement a gasoline-powered enginewould never graduate to all-electric vehicles, as called for by the Advanced Energy Initiative.
The culprit? Wide voltage swings caused by the discharging-then-charging cycles of stop-and-go driving were dissolving the thin layer of platinum from the fuel cell's carbon electrodes. "When the platinum on the electrode dissolves, it seriously degrades fuel cell performance," eventually causing premature failure, said Radoslav Adzic, a researcher at the DOE's Brookhaven National Laboratory (Upton, N.Y.).
Adzic's team solved the problem by impregnating clusters of 10 to 15 gold atoms into the surface of palladium nanoparticles, which were then used in a thin coating on a fuel cell's electrodes. The gold mitigates the voltage swings associated with charging and discharging during stop-and-go driving, and as a consequence, keeps the platinum from dissolving. Carbon coated with palladium is a cheaper electrode, "since it only has a monolayer of platinum," said Adzic.
The Brookhaven National Laboratory experiment cycled a fuel cell through more than 30,000 simulated stop-and-go driving events and the resultant complementary oxidation and reduction cycles they caused at the fuel cell's electrodes. At the lab's National Synchrotron Light Source, the team used X-ray probes in a scanning transmission microscope (STM) to observe the electrochemical process and verify that the use of gold clusters was reducing the oxidation of platinum. "Basically, what we discovered is that if you coat 3- to 5-nanometer platinum nanoparticles with even smaller gold clusters of about 10 or 15 atoms each, then the voltage fluctuation is less, from 0.75 V to 1.1 V," compared with the typical 1.2 V, Adzic said. "This is enough of a change to keep the platinum from dissolving."
Gold is twice as expensive as palladium but half the price of platinum. Consequently, the new electrode composition should not increase the price of fuel cells, but it probably won't decrease it either.
The Brookhaven team is still investigating the exact mechanism that makes its method work, but estimates that the resulting optimal particle architecture is about 10 percent gold and 90 percent platinum.
For the future, the Brookhaven teamwhich included fellow chemistry department researchers Junliang Zhang and Kotaro Sasaki, along with Eli Sutter from Brookhaven's Center for Functional Nanomaterialsplans to test its discovery at Los Alamos National Laboratory's Institute for Hydrogen and Fuel Cell Research.
"Next, we are going to synthesize a few grams of our gold-protected platinum catalyst and take it out to Los Alamos National Laboratory, where they can validate that it works in real production fuel cells in the field," said Adzic. "Although our results have been very promising, so far we've only tested them under laboratory conditions."