An example of a platinum island of optimized thickness and shape, containing a high number of very active sites (shown in yellow). The graph shows how the predicted catalytic activity (the black dots) decreases as such islands increase in width.

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, recently joined forces to investigate a
catalyst that promotes energy-releasing reactions in fuel cells.

They
discovered that the catalyst, made of ultrathin platinum layers grown
on a single crystal of rhodium, would work better and last longer if its
structure—as well as its composition—was carefully engineered.

Stable,
highly active catalysts are particularly needed in fuel cells, which
could power electric vehicles without the range limitations of current
batteries. But the catalysts are often made of scarce and expensive
materials like platinum. Attempts to engineer such catalysts have
failed, however, because the most active catalysts are not stable
enough, and the most stable catalysts aren’t active enough.

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 a theoretical tool called 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.

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.

Source: Stanford University