SLAC-Stanford study suggests that tailored 3D nanostructures can enhance activity and stability of fuel cell catalysts
18 August 2012
A study by researchers from two SLAC-Stanford joint institutes—the Stanford Institute for Materials and Energy Sciences (SIMES) and the SUNCAT Center for Interface Science and Catalysis—has found that engineering fuel cell catalysts with tailored 3D nanostructures—with increased occurrence of the most active sites—could strongly enhance catalyst stability and activity.
The results argue for adding three-dimensional nanostructuring as an additional catalyst design criterion and so provide insight into novel approaches to catalyst design that could result in less expensive catalysts for fuel cells. A paper on their work was recently published in the Journal of the American Chemical Society.
Stable, highly active catalysts are needed in fuel cells; however, the catalysts are scarce and expensive materials such as platinum. Attempts to engineer other catalysts have fallen short because the most active catalysts are not stable enough, and the most stable catalysts aren’t active enough.
Bimetallic catalyst materials are of great interest due to their wide variability of the electronic structure that allows for “tuning” of the catalyst affinity to various reaction intermediates. Such a tuning is needed in particular for the oxygen reduction reaction (ORR) in fuel cells, where a significant reduction of Pt loading is essential for economic viability.
In comparison...with experimentally determined ORR activities for a number of bimetallic systems, including Pt monolayer catalysts, we note two important characteristics: first, with the exception of the more recent development of Pt3Sc and Pt3Y catalysts, the materials near the top of the volcano are alloys of Pt with late 3d transition metals, which can be unstable under fuel cell operating conditions due to dissolution of the non-noble component. Second, it is remarkable that some model catalysts, i.e., Pt/Au(111), Pt/ Ir(111), Pt/Rh(111), and Pt/Ru(0001), appear much more active than predicted theoretically. This raises the question whether an additional catalyst design criterion besides ligand and strain effects can alter the oxygen adsorption energy, enhancing the ORR activity for these systems.
Here, we show that such an enhancement can be due to three-dimensional nanostructuring of the Pt monolayer, which has not been accounted for in the previous studies, thus validating the theoretical predictions.—Friebel et al.
The SIMES team, led by Associate Staff Scientist Daniel Friebel, studied two very different platinum-rhodium nanostructures: one with platinum deposited on the rhodium crystal in a single, atom-thick layer, and the other with the same amount of platinum forming thicker islands with bare rhodium in between. They used the high-resolution X-ray spectrometer at the Stanford Synchrotron Radiation Lightsource’s Beam Line 6-2 to examine the surface chemistry that determines the catalytic activity of both structures.
The two samples showed markedly different behavior: The platinum islands were much better at grabbing and holding oxygen atoms than the platinum monolayer, which captured almost no oxygen on its surface. In fuel cells, atomic oxygen is the key intermediary between bond-breaking and bond-making chemical reactions on a fuel cell’s cathode, where oxygen molecules, current-carrying electrons and protons that have been generated from hydrogen at the fuel cell’s anode are transformed into water.
How strongly oxygen atoms are held by the catalyst is a vital consideration when determining its efficacy, said Friebel. “If the oxygen is too weakly attached to the catalyst, the initial bond-breaking never gets going. If, however, oxygen gets stuck, it will throttle the bond-making that is needed to complete the reaction.”
Venkat Viswanathan, a graduate student at SUNCAT, was able to explain the SIMES results using density functional theory. For each platinum surface atom he found a simple description of its ability to grab oxygen, which depends on how many platinum and rhodium atoms are in its immediate neighborhood. Generally, platinum atoms with fewer neighboring metal atoms can bind oxygen more strongly, but where a neighbor atom exists, another platinum atom is preferred. A rhodium neighbor spoils platinum’s appetite for oxygen more than a platinum neighbor.
This interaction between platinum and rhodium is why the thicker platinum islands bind oxygen more strongly than the atom-thick platinum layer, but still more weakly than pure platinum.
Researchers were also able to identify the most active sites on the platinum islands and predict that an optimized platinum-rhodium nanostructure could be up to five times more active than pure platinum. Moreover, such a structure is expected to resist degradation much better than platinum-nickel or platinum-cobalt catalysts with comparable activity, thus fulfilling requirements for both high activity and stability.
The high amount of edges and corners in the 3D island nanostructure adds further enhancements in O affinity in a very similar manner to stepped Pt(111) surfaces. This can overcompensate ligand and strain effects, thus opening a new field of catalyst design through nanostructuring, where catalytic activity becomes much less dependent from the choice of constituent elements. Hence, the often-encountered incompatibility between catalyst activity and stability could be overcome.—Friebel et al.
Although rhodium, like platinum, is too expensive to use as a catalyst, the knowledge gained from these studies enables novel approaches to inexpensive catalyst design.
Daniel Friebel, Venkatasubramanian Viswanathan, Daniel J. Miller, Toyli Anniyev, Hirohito Ogasawara, Ask H. Larsen, Christopher P. O’Grady, Jens K. Nørskov, and Anders Nilsson (2012) Balance of Nanostructure and Bimetallic Interactions in Pt Model Fuel Cell Catalysts: In Situ XAS and DFT Study. Journal of the American Chemical Society 134 (23), 9664-9671 doi: 10.1021/ja3003765
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