Wednesday, 7 August 2013

Synthetic polymers enable cheap, efficient, durable alkaline fuel cells

Synthetic polymers enable cheap,
efficient, durable alkaline fuel
cells
This is an example membrane -- a
cost-efficient and durable alternative
for fuel cells consisting of two
electrodes with a clear membrane
sandwiched between them. Credit:
Patrick Mansell
A new cost-effective polymer
membrane can decrease the cost of
alkaline batteries and fuel cells by
allowing the replacement of
expensive platinum catalysts without
sacrificing important aspects of
performance, according to Penn State
researchers.
"We have tried to break this
paradigm of tradeoffs in materials
(by improving) both the stability and
the conductivity of this membrane at
the same time, and that is what we
were able to do with this unique
polymeric materials design," said
Michael Hickner, associate professor
of materials science and engineering .
In solid-state alkaline fuel cells,
anion exchange membranes conduct
negative charges between the
device's cathode and anode—the
negative and positive connections of
the cell—to create useable electric
power. Most fuel cells currently use
membranes that require platinum-
based catalysts that are effective but
expensive.
Hickner's new polymer is a unique
anion exchange membrane—a new
type of fuel cell and battery
membrane—that allows the use of
much more cost-efficient non-
precious metal catalysts and does not
compromise either durability or
efficiency like previous anion
exchange membranes.
"What we're really doing here is
providing alternatives, possible
choices, new technology so that
people who want to commercialize
fuel cells can now choose between
the old paradigm and new
possibilities with anion exchange
membranes," Hickner said.
Creating this alternative took some
intuition and good fortune . In work
spearheaded by Nanwen Li, a
postdoctoral researcher in materials
science and engineering, Hickner's
team created several variations of the
membrane, each with slightly
different chemical compositions. They
then ran each variation under
simulated conditions to predict which
would be optimal in an actual fuel
cell. The researchers published their
results in a recent issue of the
Journal of the American Chemical
Society .
A membrane electrode assembly is
shown being inserted into a fuel cell
testing stand. By creating several
variations of membranes and
studying them under similar
conditions, the research team can
predict the most optimal structure in
an active and stable fuel cell. Credit:
Patrick Mansell
Based on these initial tests, the
group predicted that the membranes
with long 16-carbon structures in
their chemical makeup would provide
the best efficiency and durability, as
measured respectively by conductivity
and long-term stability.
Chao-Yang Wang, William E.
Diefenderfer Chair of Mechanical
Engineering, and his team then
tested each possibility in an
operating fuel cell device. Yongjun
Leng, a research associate in
mechanical and nuclear engineering,
measured the fuel cell's output and
lifetime for each material variation.
Despite predictions, the membranes
containing shorter 6-carbon
structures proved to be much more
durable and efficient after 60 hours
of continuous operation.
"We were somewhat surprised…that
what we thought was the best
material in our lab testing wasn't
necessarily the best material in the
cell when it was evaluated over
time," said Hickner, who added that
researchers are still trying to
understand why the 6-carbon
variation has better long-term
durability than the 16-carbon sample
in the fuel cell by studying the
operating conditions of the cell in
detail.
Because the successful membrane
was so much more effective than the
initial lab studies predicted,
researchers are now interested in
accounting for the interactions that
the membranes experienced while
inside the cell.
"We have the fuel cell output—so we
have the fuel cell efficiency, the fuel
cell life time—but we don't have the
molecular scale information in the
fuel cell," Hickner said. "That's the
next step, trying to figure out how
these polymers are working in the
fuel cell on a detailed level."
Provided by Pennsylvania State
University
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