One factor that hinders widespread use of environmentally friendly hydrogen fuel cells in automobiles, trucks, and other vehicles is the cost of the platinum catalyst that operates the battery. One way to use less precious metals is to combine them with other less expensive metals, but these alloy catalysts tend to degrade rapidly under fuel cell conditions.

Now, researchers at Brown University have developed a new alloy catalyst that both reduces the use of platinum and maintains good conditions in fuel cell testing. According to the test results described in the Joule journal, the catalysts made by alloying platinum and cobalt alloys in nanoparticles outweighed the US Department of Energy (DOE) 2020 goal in terms of reactivity and durability.

"The durability of alloy catalysts is a big problem in this area," said Junrui Li, Brown's graduate student in chemistry, the lead author of the study. “It turns out that the initial performance of the alloy is better than pure platinum, but in this case, inside the fuel cell, the non-precious metal part of the catalyst is oxidized and leached quickly.”

To solve this leaching problem, Li and his colleagues developed alloy nanoparticles with a special structure. The particles have a pure platinum outer shell that surrounds the core made of alternating layers of platinum and cobalt atoms. Shouheng Sun, a professor of brown chemistry 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 to smooth and tighten the platinum lattice in the outer shell,” Sun said. “This increases the reactivity of platinum, while protecting cobalt atoms from being eaten during the reaction. This is why these particles perform better than alloy particles of randomly arranged metal atoms.”

Details on how the ordered structure enhances catalyst activity are briefly described in the Joule paper, but more specifically in a separate computer modeling paper published in the Journal of Chemical Physics . The modeling work was led by Andrew Peterson, an associate professor at Brown Engineering, who is also a co-author of the Joule paper.

For experimental work, the researchers tested the ability of the catalyst to perform an oxygen reduction reaction , which is critical to the performance and durability of the fuel cell. On one side of the proton exchange membrane (PEM) fuel cell, electrons stripped from the hydrogen fuel produce a current that drives the motor. On the other side of the cell, oxygen atoms absorb these electrons to complete the circuit. This is done by 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 maintains its activity at 30,000 voltage cycles, while the performance of conventional catalysts is significant.

But the researchers say that while laboratory testing is important to assess the properties of the catalyst, they do not necessarily show the performance of the catalyst in an actual fuel cell. The fuel cell environment is hotter and has a different acidity than the laboratory test environment, which accelerates catalyst degradation. To find out the stability of the catalyst in this environment, the researchers sent the catalyst to the Los Alamos National Laboratory for actual fuel cell testing.

Tests have shown that the catalyst beats the DOE's goal of initial activity and long-term durability. The US Department of Energy has asked researchers to develop catalysts with an initial activity of 0.44 amps per milligram of platinum by 2020 and an activity of at least 0.26 amps/mg after 30,000 voltage cycles (approximately five years for fuel cell vehicles). Testing of the new catalyst showed that it had an initial activity of 0.56 amps/mg and an activity of 0.45 amps after 0.45 cycles.

“Even after 30,000 cycles, our catalysts still exceed the DOE's initial activity goals,” Sun said. "In the real world fuel cell environment, this performance is really promising."

The researchers applied for a temporary patent on the catalyst and they hope to continue to develop and improve the catalyst.

Further exploration: Efficient monoatomic catalysts can help the automotive industry

For more information: Joule (2018). DOI: 10.1016 / j.joule.2018.09.0 16