Proton-exchange membrane fuel cells (PEMFCs) have a high energy-conversion efficiency, which could make them the best type of fuel cell to eventually replace the gasoline and diesel internal combustion engine.

By introducing a “magic solvent” (in this case, benzyl ether) when synthesising platinum-nickel nanocrystals for PEMFCs, the team, led by Younan Xia, succeeded in significantly reducing the amount of surfactants on the catalytic active surface while keeping the much desired octahedron shape of the crystals. As measured by a team led by Minhua Shao at UTC Power, the clean (111) surface of these octahedra has a “Pt mass activity” of 3.3 A/mg at 0.9 V, a record-breaking value that is 17 times higher than that of conventional Pt/C electrocatalysts.

Gram quantities

Xia and colleagues prepared Pt-Ni octahedral crystals just 9 nm in size in benzyl ether using oleylamine and oleic acid as surfactants and carbon monoxide released from W(CO)6 as a capping agent. The oleylamine and oleic acid helped narrow the size distribution of the Pt-Ni octahedra, explains Xia, and the W(CO)6 helped promote the {111} facets in the presence of Ni. “What is more, we were able to easily prepare the catalysts in gram quantities,” he said. This is 15 times more than in previously reported protocols and will be enough for a typical fuel cell testing.”

For pure Pt, the {111} facet is normally as active as {110} and much more active than {100} for the ORR in acidic media, he continued. However, when alloyed with Ni, the ORR activity on the Pt3Ni(111) single-crystal surface appears to be drastically enhanced when compared to Pt3Ni(110) and Pt3Ni(100).

Large-scale commercialization of PEMFCs?

“Our new catalysts could potentially allow for the large-scale commercialisation of proton-exchange membrane fuel cells, something that is currently being held back by the high cost associated with Pt-based ORR catalysts,” Xia told nanotechweb.org. “According to a US Department of Energy (DOE) report, the activity of ORR catalysts needs to be increased by about 3-4 fold, so that the load of Pt can be reduced accordingly. Our catalyst shows a 17-fold increase in ORR activity – much higher than stipulated by the DOE.”

Although octahedral Pt-Ni has the highest catalytic activity for the ORR among all the Pt-transition metal bimetallic nanocrystals, this point needs to be reaffirmed in further lab experiments, says Xia. The GeorgiaTech researchers are therefore now looking at how to make Pt-transition metal octahedra with controlled sizes and compositions, and measuring their catalytic properties for the ORR. “At the same time, we are trying to improve the stability of the highly active Pt-Ni octahedra,” revealed Xia.

The research is detailed in Nano Letters DOI: 10.1021/nl401881z.

Further reading

CNTs tuned to provide electrocatalyst support (Jan 2009)
Nitride nanocrystals for fuel cell catalysts (Dec 2007)