One factor that hinders the widespread use of environmentally friendly hydrogen fuel cells in automobiles, trucks, and other vehicles is the cost of platinum catalysts.
One way to use less expensive platinum is to combine it with other cheaper metals, but these alloy catalysts tend to degrade rapidly under fuel cell conditions.
Now, researchers at Brown University have developed a new type of alloy catalyst that can both reduce the use of platinum and maintain good performance in fuel cell testing.
According to "Joule" magazine, this catalyst is made of platinum alloy and cobalt in nanoparticles, and exceeds the US Department of Energy's (DOE) 2020 goals in terms of reactivity and durability.
"The durability of alloy catalysts is a big issue in this field," said Junrui Li, a graduate student in chemistry at Brown University.
"Research shows that the initial performance of the alloy is better than pure platinum, but in fuel cells, the non-noble metal part of the catalyst will be quickly oxidized and filtered out."
To solve this leaching problem, Li and his colleagues developed a special structure of alloy nanoparticles.
These particles have a pure platinum shell, surrounded by a core of alternating layers of platinum and cobalt atoms.
Shouheng Sun, professor of chemistry at Brown University and senior author of the study, said that this layered core structure is the key to catalyst reactivity and durability.
"The layered arrangement of atoms in the core helps smooth and tighten the platinum lattice in the shell," Sun said.
"This increases the reactivity of platinum and prevents cobalt atoms from being eaten during the reaction. This is why these particles perform better than alloy particles when the metal atoms are randomly arranged."
Details on how ordered structures enhance catalyst activity are briefly described in Joule papers, but more specifically, in another computer modeling paper published in the Journal of Chemical Physics.
The modeling work was led by Andrew Peterson, an associate professor at the Brown School of Engineering and a co-author of the Joule paper.
In order to carry out the experimental work, the researchers tested the ability of the catalyst to perform the oxygen reduction reaction, which is critical to fuel cell performance and durability.
On the side of a proton exchange membrane (PEM) fuel cell, the electrons stripped from the hydrogen fuel will generate a current that drives the motor. At the other end of the battery, oxygen atoms absorb these electrons to complete a cycle.
This is done through an oxygen reduction reaction.
Preliminary tests have shown that the catalyst performs well in a laboratory environment and is superior to more traditional platinum alloy catalysts.
The new catalyst remained active after 30,000 voltage cycles, while the performance of the traditional catalyst decreased significantly.
However, although laboratory tests are important for evaluating the performance of catalysts, the researchers say they do not necessarily show the performance of catalysts in actual fuel cells.
Compared with the laboratory test environment, the temperature of the fuel cell environment is higher and the acidity is also different, which will accelerate the degradation of the catalyst.
To find out how long the catalyst will last in this environment, the researchers sent the catalyst to Los Alamos National Laboratory for testing in an actual fuel cell.
Tests have shown that the catalyst is superior to the goals set by the US Department of Energy (DOE) in terms of initial activity and long-term durability.
The U.S. Department of Energy requires researchers to develop a catalyst that will have an initial activity of 0.44 amperes per milligram of platinum by 2020 and an activity of at least 0.26 per milligram of platinum after 30,000 voltage cycles (roughly equivalent to 5 years of fuel cell vehicle use) ampere.
Tests on the new catalyst showed that its initial activity was 0.56 amperes per milligram, and after 30,000 cycles the activity was 0.45 amperes per milligram.
"Even after 30,000 cycles, our catalyst still exceeded the Department of Energy's original activity goal," Sun said.
"In a real fuel cell environment, this performance is really promising."
The researchers have applied for a temporary patent for the catalyst, and they hope to continue to develop and improve it.
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