Platinum is widely used as a catalyst in hydrogen vehicles, but the element’s high cost is a factor in holding back adoption of the technology. Researchers have long sought to combine the precious metal with other, more abundant elements to form alloys. However, these alloy catalysts tend to degrade quickly in fuel cell conditions, with the non-platinum component getting oxidised and leached away.
To overcome this conundrum, the Brown team fashioned platinum-cobalt nanoparticles with a specialised structure designed to withstand the conditions inside a fuel cell. The particles have a pure platinum outer shell surrounding a core made from alternating layers of platinum and cobalt atoms. According to the researchers, that layered core structure is key to the catalyst's reactivity and durability.
"The layered arrangement of atoms in the core helps to smooth and tighten the platinum lattice in the outer shell," said Shouheng Sun, Professor of chemistry at Brown and senior author of the research, which appears in Joule.
"That increases the reactivity of the platinum and at the same time protects the cobalt atoms from being eaten away during a reaction. That's why these particles perform so much better than alloy particles with random arrangements of metal atoms."
Initial lab testing found the new catalyst maintained its activity after 30,000 voltage cycles, whereas the performance of a traditional pure-platinum catalyst dropped off significantly. The team then sent the catalyst to the Los Alamos National Lab to be tested in an actual fuel cell, beating targets set by the Department of Energy (DOE) for both initial activity and longer-term durability. The DOE has challenged 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 per milligram after 30,000 voltage cycles (approximate to five years of use in a fuel cell vehicle). Testing of the layered platinum catalyst showed that it had an initial activity of 0.56 amps per milligram and an activity after 30,000 cycles of 0.45 amps.
"Even after 30,000 cycles, our catalyst still exceeded the DOE target for initial activity," said Sun. "That kind of performance in a real-world fuel cell environment is really promising."
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