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"The existing limitations facing PEM fuel cell
technology applications in the transportation sector could be
eliminated with the development of stable cathode catalysts with
several orders of magnitude increase in activity over today's
state-of-the-art catalysts, and that is what our discovery has the
potential to provide," said Vojislav Stamenkovic, a scientist with
dual appointments in the Materials Sciences Division of both Berkeley
Lab and Argonne.
Stamenkovic and Argonne senior scientist Nenad
Markovic are the corresponding authors of a study whose results are
now available online from the journal Science. The paper, entitled
Improved Oxygen Reduction Activity on Pt3Ni(111) via Increased Surface
Site Availability, reports a platinum-nickel alloy that increased the
catalytic activity of a fuel cell cathode by an astonishing 90-fold
over the platinum-carbon cathode catalysts used today.
"This surface sets a new bar for catalytic activity
in PEM fuel cells and makes it feasible to meet U.S. Department of
Energy (DOE) targets for platinum-specific power densities without a
loss in cell voltage," Stamenkovic said.
Other authors of the Science paper in addition to
Stamenkovic and Markovic were Philip Ross and Bongjin Mun of Berkeley
Lab, Ben Fowler and Christopher Lucas of England's University of
Liverpool, and Guofeng Wang, of the University of South Carolina.
By converting chemical energy into electrical
energy without combustion, fuel cells represent perhaps the most
efficient and clean technology for generating electricity. This is
especially true for fuel cells designed to directly run off hydrogen,
which produce only water as a byproduct. The hydrogen-powered fuel
cells most talked about for use in vehicles are PEM fuel cells (also
known as "proton exchange membrane fuel cells") because they can
deliver high power in a relative small, light-weight device. Unlike
batteries, PEM fuel cells do not require recharging, but rely on a
supply of hydrogen and access to oxygen from the atmosphere.
PEM fuel cells have admirably served NASA's space
program, but they remain far too expensive for use in cars or most
other Earth-bound applications. The biggest cost factor is their
dependency on platinum, which is used as the cathode catalyst. A PEM
fuel cell consists of a cathode and an anode (the negatively charged
electrode) that are positioned on either side of a polymer electrolyte
membrane, which is a specially treated substance that conducts
positively charged protons and blocks negatively charged electrons.
Like other types of fuel cells, PEM fuel cells
carry out two reactions, an oxidation reaction at the anode and an
oxygen reduction reaction (ORR) at the cathode. For PEMs, this means
that hydrogen molecules are split into pairs of protons and electrons
at the anode. While the protons pass through the membrane, the blocked
electrons are conducted via a wire (the electrical current), through a
load and eventually onto the cathode. At the cathode, the electrons
combine with the protons that passed through the membrane plus atoms
of oxygen to produce water. The oxygen (O) comes from molecules in the
air (O2) that are split into pairs of O atoms by the cathode catalyst.
"Massive application of PEM fuel cells as the basis
for a renewable hydrogen-based energy economy is a leading concept for
meeting global energy needs," said Stamenkovic.
"Since the only byproduct of PEM fuel cell
exploitation is water vapor, their widespread use should have a
tremendously beneficial impact on greenhouse gas emissions and global
warming."
A challenge has been the platinum. While pure
platinum is an exceptionally active catalyst, it is quite expensive
and its performance can quickly degrade through the creation of
unwanted by-products, such as hydroxide ions. Hydroxides have an
affinity for binding with platinum atoms and when they do this they
take those platinum atoms out of the catalytic game. As this
platinum-binding continues, the catalytic ability of the cathode
erodes. Consequently, researchers have been investigating the use of
platinum alloys in combination with a surface enrichment technique.
Under this scenario, the surface of the cathode is covered with a "skin"
of platinum atoms, and beneath are layers of atoms made from a
combination of platinum and a non-precious metal, such as nickel or
cobalt. The subsurface alloy interacts with the skin in a way that
enhances the overall performance of the cathode.
For this latest study, Stamenkovic and Markovic and
their colleagues created pure single crystals of platinum-nickel
alloys across a range of atomic lattice structures in an ultra-high
vacuum (UHV) chamber. They then used a combination of
surface-sensitive probes and electrochemical techniques to measure the
respective abilities of these crystals to perform ORR catalysis. The
ORR activity of each sample was then compared to that of platinum
single crystals and platinum-carbon catalysts.
The researchers identified the platinum-nickel
alloy configuration Pt3Ni(111) as displaying the highest ORR activity
that has ever been detected on a cathode catalyst – 10 times better
than a single crystal surface of pure platinum(111), and 90 times
better than platinum-carbon. In this (111) configuration, the surface
skin is a layer of tightly packed platinum atoms that sits on top of a
layer made up of equal numbers of platinum and nickel atoms. All of
the layers underneath those top two layers consist of three atoms of
platinum for every atom of nickel.
According to Stamenkovic, the Pt3Ni(111)
configuration acts as a buffer against hydroxide and other
platinum-binding molecules, blunting their interactions with the
cathode surface and allowing for far more ORR activity. The reduced
platinum-binding also cuts down on the degradation of the cathode
surface.
"We have identified a cathode surface that is
capable of achieving and even exceeding the target for catalytic
activity, with improved stability for the cathodic reaction in fuel
cells," said Stamenkovic. "Although the platinum-nickel alloy itself
is well-known, we were able to control and tune key parameters which
enabled us to make this discovery. Our study demonstrates the
potential of new analytical tools for characterizing nanoscale
surfaces in order to fine-tune their properties in a desired direction."
The next step, Stamenkovic said, will be to
engineer nanoparticle catalysts with electronic and morphological
properties that mimic the surfaces of pure single crystals of
Pt3Ni(111). |
