Palladium/Iron-Platinum Core/Shell Nanoparticle Catalysts Promising for Practical Fuel Cell Applications

26 May 2010

Chemists at Brown University have synthesized novel core and shell palladium/iron-platinum (Pd/FePt) nanoparticles that use far less platinum yet perform more efficiently and last longer than commercially available pure-platinum catalysts for the oxygen reduction reaction (ORR) in hydrogen fuel cells.

In testing, the current density generated from a 5 nm core/1 nm shell Pd/FePt nanoparticle (NP) was around 12 times higher than that from a commercial platinum catalyst. The 5 nm/1 nm Pd/FePt NPs had no noticeable change in morphology after the ORR test and 10,000 potential cycles and the core/shell structure was maintained. These nanoparticles, the researchers concluded, are promising new catalysts for practical fuel cell applications.

The oxygen reduction reaction takes place at the fuel cell’s cathode, creating water as its only waste. The cathode is also where up to 40% of a fuel cell’s efficiency is lost, so “this is a crucial step in making fuel cells a more competitive technology with internal combustion engines and batteries,” said Shouheng Sun, professor of chemistry at Brown and co-author of the paper published online 24 May in the Journal of the American Chemical Society.

The research team, which includes Brown graduate student and co-author Vismadeb Mazumder and researchers from Oak Ridge National Laboratory in Tennessee, created a five-nanometer palladium (Pd) core and encircled it with a shell consisting of iron and platinum (FePt). The trick, Mazumder said, was in molding a shell that would retain its shape and require the smallest amount of platinum to pull off an efficient reaction.

The team created the iron-platinum shell by decomposing iron pentacarbonyl [Fe(CO)₅] and reducing platinum acetylacetonate [Pt(acac)₂], a technique Sun first reported in a paper published in 2000 in the journal Science. The result was a shell that uses only 30% platinum, although the researchers say they expect they will be able to make thinner shells and use even less platinum.

The researchers demonstrated for the first time that they could consistently produce the unique core-shell structures. In laboratory tests, the palladium/iron-platinum nanoparticles generated 12 times more current than commercially available pure-platinum catalysts at the same catalyst weight. The output also remained consistent over 10,000 cycles, at least ten times longer than commercially available platinum models that begin to deteriorate after 1,000 cycles.

The team created iron-platinum shells that varied in width from one to three nanometers. In lab tests, the group found the one-nanometer shells performed best.

The Pd/FePt NPs showed FePt shell-dependent catalytic properties, and those having a 1 nm coating exhibited drastic increases in durability and activity (15 times more active with a 140 mV gain in onset potential in comparison with those having a 3 nm coating).

The multimetallic core/shell NPs reported here have several distinct advantages over any of the previously reported NP systems: (1) the catalytically active shell is deposited uniformly on the core surface and is readily activated; (2) the shell thickness is controlled, and core/shell interactions can be tuned to optimize catalytic performance; and (3) the synthesis is versatile and allows different metals to be incorporated into either the core or the shell structure, offering a rich variety of core/shell NPs for catalytic applications.

—Mazumder et al.

Mazumder and Sun are studying why the palladium core increases the catalytic abilities of iron platinum, although they think it has something to do with the transfer of electrons between the core and shell metals. To that end, they are trying to use a chemically more active metal than palladium as the core to confirm the transfer of electrons in the core-shell arrangement and its importance to the catalyst’s function.

Miaofang Chi and Karren More at the Oak Ridge Laboratory also contributed to the paper. The US Department of Energy’s Office of Energy Efficiency and Renewable Energy funded the research as part of its Fuel Cell Technologies Program.

Resources

• Vismadeb Mazumder, Miaofang Chi, Karren L. More and Shouheng Sun (2010) Core/Shell Pd/FePt Nanoparticles as an Active and Durable Catalyst for the Oxygen Reduction Reaction. J. Am. Chem. Soc., Article ASAP doi: 10.1021/ja1024436