Friday, 9 August 2013

Five times less platinum: Fuel cells could become economically more attractive thanks to novel aerogel catalyst

Five times less platinum: Fuel cells
could become economically more
attractive thanks to novel aerogel
catalyst
Thomas J. Schmidt. Head of the
Electrochemistry Laboratory and
Rüdiger Kötz, Head of the research
group Electrocatalysis and interfaces,
were in charge of the
characterisation of the novel aerogel
catalyst at PSI. Credit: Markus
Fischer/Paul Scherrer Institute.
Fuel cells that convert hydrogen into
power and only produce pure water
as a by-product have the potential to
lead individual mobility into an
environmentally friendly future. The
Paul Scherrer Institute (PSI) has been
researching and developing such low-
temperature polymer electrolyte fuel
cells for more than 10 years and
initial field tests have already
demonstrated the successful use of
these fuel cells in cars and buses.
However, further research is still
required to improve the durability
and economic viability of the
technology. An international team of
researchers involving the PSI has now
manufactured and characterised a
novel nanomaterial that could vastly
increase the efficiency and shelf-life
of these fuel cells – as well as reduce
material costs.
In a hydrogen fuel cell , hydrogen is
converted into power and water
through electrochemical reactions . A
key step in these reactions is the
reduction of oxygen at the cell's
positive electrode, where oxygen
molecules fed into the cell are
converted into water. As this reaction
takes place very slowly under normal
conditions, catalysts are needed to
speed up the conversion process. In
conventional cells, precious metals
such as platinum fullfill this catalytic
function . The thin nanoparticles used
for this purpose are supported by a
substrate typically made of high
surface area carbon. However, the
carbon substrate can easily become
corroded during the common start/
stop operation in city traffic or during
idling; thereby compromising the
function of the catalyst, which in turn
shortens the service life of the entire
fuel cell . Consequently, researchers
have long been looking for catalysts
for oxygen reduction that do not
need a support and still display a
high specific surface area with a large
number of catalytic centres as well as
good long-term stability.
An international team of researchers
involving the PSI has now made
considerable progress in this
direction. Using a three-dimensional
aerogel made of a platinum
palladium alloy, they were able to
increase the catalytic activity for
oxygen reduction at the positive
electrode of a hydrogen fuel cell
fivefold compared to normal catalysts
made of platinum on carbon
supports. This means that the same
amount of oxygen can now be
converted with only a fifth of the
amount of precious metals. If this
reduction of the necessary platinum
load could be transferred onto an
industrial scale, it would slash the
production costs for these fuel cells.
The aerogel, which is a kind of
nanostructured foam, has also
passed long-term tests in the lab,
where the typical operating
conditions in a vehicle were
simulated.
Schematic representation of the
oxygen reduction reaction at the
positive electrode of a low-
temperature polymer electrolyte fuel
cell. Each oxygen atom from the
oxygen molecules fed into the cell
captures two electrons, which is
followed by the reaction with
hydrogen nuclei to form water.
Credit: Wiley-VCH Verlag GmbH & Co.
KGaA
Light-weight mesh of nanowires
The aerogel now synthesised and
characterised by researchers at
Dresden University of Technology and
at PSI forms a three-dimensional
network of nanowires and it features
a high porosity and large inner
surface. The latter properties
facilitate the adsorption of many
oxygen molecules onto the
catalytically active platinum atoms –
a prerequisite for the efficient
conversion of oxygen. While catalysts
used in commercial fuel cells also
exhibit a high degree of porosity and
large surfaces, they achieve that only
when they consist of platin
nanoparticles on a carbon substrate.
The key advantage of the new
aerogel is that it combines these
assets with an extensive three-
dimensional structure, which means
there is no need for a support
whatsoever.
Bimetal alloy aerogel synthesised
for the first time
Because of its excellent properties
for many applications in
electrochemistry and sensing
applications, aerogels have attracted
a lot of attention in recent years.
Numerous teams of researchers all
over the world have been chancing
their arm at producing new aerogels,
mostly safe in the knowledge that
useful applications will follow. Until
now, however, their success has
been limited to a small group of
chemical substances: most aerogels
are made of oxides or single metals.
That said, theoretical considerations
had suggested that catalysts made of
particular metal alloys would display
greater catalytic activity and stability
and this had sparked attempts to
implement those features in an
aerogel catalyst. Synthesising a
bimetal aerogel, however, has proven
to be easier said than done. "This is
the first time that an aerogel made
of a metal alloy has ever been
synthesised" says Thomas Justus
Schmidt, head of the
Electrochemistry Laboratory at the
PSI and co-author of the study.
The new results confirm the high
hopes for these materials. The key to
improving the activity of the new
aerogel, for instance, is that the alloy
with palladium optimises the bond
strength between the platinum atoms
and the oxygen-containing species. In
other words, the bond is so strong
that the oxygen molecules remain
adsorbed just long enough for the
conversion into water but not too
strong as to induce the formation of
oxides on the catalyst's surface. The
fact that the conversion to water is
more favourable than the formation
of oxides optimises at each point in
time the number of available
catalytic centres which in turn leads
to oxygen molecules being adsorbed
and converted at a considerably high
rate.
Electron microscope image of the
platin/palladium aerogel (alloy ratio:
50% platine, 50% palladium). Credit:
Wiley-VCH Verlag GmbH & Co. KGaA
Some questions still unanswered
The researchers are yet to
understand another advantage of the
alloy, namely the greater stability of
the bimetal alloy aerogel compared
to monometal aerogels made of pure
platinum. "Obviously, the presence of
palladium in the aerogel plays a key
role here, too, but we don't know yet
exactly what impact this has on the
stability of the catalyst," explains T.J.
Schmidt. The scientists would now
like to spend the next three years
focusing on this and other questions
regarding the new nanomaterial in a
follow-up project. "We have just
finished the draft for a funding
application together with Dresden
University of Technology to give the
project we have been funding
internally up to now a broader
financial footing."
More information: Liu, W. et al.
Bimetall-Aerogele: hoch effiziente
Elektrokatalysatoren für die
Sauerstoffreduktion, Angewandte
Chemie . DOI: 10.1002/
ange.201303109
Provided by Paul Scherrer Institute

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