A nanoparticle of indium tin oxide (green and red) braces platinum nanoparticles (blue) on the surface of graphene (black honeycomb) to make a hardier, more chemically active fuel cell material. Credit: Pacific Northwest National Laboratory. |
A new combination of nanoparticles and graphene results
in a more durable catalytic material for fuel cells, according to work
published online at the Journal of the
American Chemical Society. The catalytic material is not only
hardier but more chemically active as well. The researchers are confident the
results will help improve fuel cell design.
“Fuel cells are an important area of energy
technology, but cost and durability are big challenges,” said chemist Jun
Liu. “The unique structure of this material provides much needed
stability, good electrical conductivity, and other desired properties.”
Liu and his colleagues at the Department of Energy’s
Pacific Northwest National Laboratory, Princeton Univ. in Princeton, N.J., and
Washington State Univ. in Pullman, Wash., combined graphene with metal oxide
nanoparticles to stabilize a fuel cell catalyst and make it better available to
do its job.
“This material has great potential to make fuel
cells cheaper and last longer,” said catalytic chemist Yong Wang, who has
a joint appointment with PNNL and WSU. “The work may also provide lessons
for improving the performance of other carbon-based catalysts for a broad range
of industrial applications.”
Muscle metal oxide
Fuel cells work by chemically breaking down oxygen and hydrogen gases to create
an electrical current, producing water and heat in the process. The centerpiece
of the fuel cell is the chemical catalyst—usually a metal such as platinum—sitting
on a support that is often made of carbon. A good supporting material spreads
the platinum evenly over its surface to maximize the surface area with which it
can attack gas molecules. It is also electrically conductive.
Fuel cell developers most commonly use black carbon, but
platinum atoms tend to clump on such carbon. In addition, water can degrade the
carbon away. Another support option is metal oxides, but what metal oxides make
up for in stability and catalyst dispersion, they lose in conductivity and ease
of synthesis. Other researchers have begun to explore metal oxides in
conjunction with carbon materials to get the best of both worlds.
As a carbon support, Liu and his colleagues thought
graphene intriguing. The honeycomb lattice of graphene is porous, electrically
conductive and affords a lot of room for platinum atoms to work. First, the
team crystallized nanoparticles of the metal oxide known as indium tin oxide—or
ITO—directly onto specially treated graphene. Then they added platinum
nanoparticles to the graphene-ITO and tested the materials.
Platinumweight
The team viewed the materials under high-resolution microscopes at EMSL, DOE’s
Environmental Molecular Sciences Laboratory on the PNNL campus. The images
showed that without ITO, platinum atoms clumped up on the graphene surface. But
with ITO, the platinum spread out nicely. Those images also showed catalytic
platinum wedged between the nanoparticles and the graphene surface, with the
nanoparticles partially sitting on the platinum like a paperweight.
To see how stable this arrangement was, the team
performed theoretical calculations of molecular interactions between the
graphene, platinum and ITO. This number-crunching on EMSL’s Chinook
supercomputer showed that the threesome was more stable than the metal oxide
alone on graphene or the catalyst alone on graphene.
But stability makes no difference if the catalyst doesn’t
work. In tests for how well the materials break down oxygen as they would in a
fuel cell, the triple-threat packed about 40% more of a wallop than the
catalyst alone on graphene or the catalyst alone on other carbon-based supports
such as activated carbon.
Last, the team tested how well the new material stands up
to repeated usage by artificially aging it. After aging, the tripartite
material proved to be three times as durable as the lone catalyst on graphene
and twice as durable as on commonly used activated carbon. Corrosion tests
revealed that the triple threat was more resistant than the other materials
tested as well.
The team is now incorporating the platinum-ITO-graphene
material into experimental fuel cells to determine how well it works under real
world conditions and how long it lasts.
