A Better Platinum Catalyst for Fuel Cells
A new type of catalyst could lead to fuel cells that use a fifth of the platinum they use now. The new material, developed by researchers at the University of Houston, Technical University of Berlin in Germany, and the Department of Energy’s SLAC National Accelerator Laboratory in Menlo Park, CA, consists of nanoparticles with cores made of a copper-platinum alloy and an outer shell that is mostly platinum. The material is up to five times as efficient as regular platinum.
Platinum and platinum alloys are the most efficient catalysts for speeding up chemical reactions in hydrogen fuel cells. Platinum is the only metal that can withstand the acidic conditions inside such a cell, but it is expensive, and this has limited the broad, large-scale applications of fuel cells. Furthermore, about 90 percent of the world’s platinum supply comes from just two countries–South Africa and Russia.
The new material already meets the U.S. Department of Energy’s 2015 target for platinum catalysts: producing at least 0.44 amperes of electric current per milligram of platinum. It produces up to 0.49 amps per milligram of platinum, and the researchers believe it should be possible to increase the material’s catalytic activity even more. “If we could get another factor of two [improvement in catalytic activity], we think that the cost of platinum in these fuel cells would make the technology more practical,” says SLAC physicist Anders Nilsson.
“This is excellent work that should enable us to use less platinum in fuel cells,” says Jean-Pol Dodelet, a professor of energy, materials, and telecommunications at the Institut National de la Recherche Scientifique (INRS) in Quebec.
At the anode of a conventional proton exchange membrane (PEM) fuel cell, the catalyst splits hydrogen into hydrogen ions and electrons, with the latter flowing out of the cell to create current. At the cathode, oxygen molecules combine with electrons and hydrogen ions to form water. This reaction is sluggish and speeding it up requires 10 times as much platinum as is used at the anode. “If you’re trying to replace platinum, it is more important to replace the platinum at the cathode,” says Dodelet.
Peter Strasser, a chemical engineering professor at both the University of Houston and the Technical University of Berlin, started working on a new type of catalyst in 2005, depositing nanoparticles of a copper-platinum alloy onto carbon supports. When a cyclic alternating current is applied to the material, the copper separates from the surface region, giving the nanoparticles a platinum-rich outer layer.
In a recent Nature Chemistry paper, the researchers reveal the mechanism that makes this catalyst more active than regular platinum. By studying how x-ray beams are scattered by the new catalyst, they discovered that the distance between the platinum atoms that are left on the surface of the nanoparticles is less than the distance in pure platinum nanoparticles. A good catalyst should be able to split up oxygen molecules into atoms but should not bind too strongly with the free atoms; the shorter distance between platinum atoms in the new material makes it a more effective catalyst because it binds even more weakly with the oxygen atoms.
There are alternatives to using platinum as a catalyst. Dodelet and his group have worked with General Motors to develop a promising iron-based catalyst that they are now working to commercialize. Meanwhile, low-cost carbon nanotube catalysts and nickel catalysts are in the works for alkaline fuel-cell chemistries.
Platinum-free catalysts have advantages other than their low cost, says Liming Dai, a materials engineering professor at the University of Dayton, in Ohio, who is working on carbon nanotube catalysts. Platinum nanoparticles tend to lose their catalytic efficiency by aggregating into larger particles over time or when carbon monoxide sticks to their surface. Carbon nanotubes are more robust in the long-term, Dai says.
“This is interesting work and an important advance because the mechanism could be applied to other catalysts,” Dai says of the new platinum catalyst. “It would be interesting to check out the long-term stability and carbon monoxide [surface] poisoning effect for this kind of core-shell catalyst.”
Strasser agrees that the new catalyst will need further testing. However, the larger size of the core-shell particles makes them intrinsically more stable than pure platinum, he says. The choice of this metal also makes a difference. “We are confident that alternative non-platinum metals in the core, like cobalt or nickel, will solve the stability problem while maintaining the activity advantage of the core-shell structure,” Strasser says.
The new material has also been tested in working fuel cells, which could be a crucial market advantage. “Most of these other catalysts were measured in electrochemical measurements,” he says. “They have potential for use in the future, but this [new catalyst] is something we have that you can put in real fuel cells today.”
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