A new process increases the energy output of methanol fuel cells by 50 percent.
- From: Jack Linthicum <jacklinthicum@xxxxxxxxxxxxx>
- Date: Tue, 28 Oct 2008 02:39:27 -0700 (PDT)
The one-day delay imposed by Google Groups makes it difficult to find
a thread to attach this to, despite a recent bit on fuel cells. I
wonder if anyone has ideas on why the household fuel cell has never
developed sufficiently to be a commercial product. Large utilities and
their campaign donattions? Just read a bit in the Anchorage News about
the attempts to wean the isolated native villages and towns from
diesel dependence.
http://www.adn.com/anchorage/story/567169.html
November/December 2008
Fuel-Cell Power-Up
A new process increases the energy output of methanol fuel cells by 50
percent.
By Kristina Grifantini
In her lab at MIT, chemical-engineering professor Paula Hammond
pinches a sliver of what looks like thick Saran wrap between tweezers.
Though it appears unremarkable, this polymer membrane can
significantly increase the power output of a methanol fuel cell, which
could make that technology suitable as a lighter, longer-lasting, and
more environmentally friendly alternative to batteries in consumer
electronics such as cell phones and laptops.
Methanol is a promising energy source for fuel cells because it is a
liquid at room temperature, so it's easier to manage than hydrogen.
But so far, its commercial applications have been limited. One reason
has to do with the properties of the proton-conducting membranes at
the heart of fuel-cell technology.
On one side of a methanol fuel cell, a catalyst causes methanol and
water to react, yielding carbon dioxide, protons, and free electrons.
The protons pass through a membrane to a separate compartment, where
they combine with oxygen from air to form water. The electrons, which
can't cross the membrane, are forced into wires, generating a current
that can be used to power electronic devices.
The more protons cross the membrane, the more power is generated. But
the polymers that conduct protons well also tend to let the methanol
solution into the other compartment. The resulting loss of fuel lower
s the cells' power output. To limit such "methanol crossover,"
researchers have to either use polymers that don't conduct protons as
well or make thicker membranes. But both of those options decrease
efficiency, too.
In work published last spring in Advanced Materials, Hammond used an
elegant, inexpensive process to reduce methanol crossover in a
commercial fuel-cell membrane, increasing the efficiency of a methanol
fuel cell by more than 50 percent. "What we've done is generate a very
thin film that actually prevents the permeation of methanol but at the
same time allows a rapid rate of proton transport," says Hammond.
Encouraged by this success, her team is now working to build such
membranes from scratch, which could make them less expensive.
A Modified Process
A layer-by-layer assembly technique is the key to Hammond's membranes.
In earlier work, her team altered a membrane made of Nafion, a polymer
manufactured by DuPont that is commonly used in fuel cells. It
conducts protons well but also permits some methanol leakage, and it's
relatively expensive to make.
To begin the new process, Avni Argun, a postdoc in the lab and lead
author on the Advanced Materials paper, mounts a specially treated
silicon disc in a lab hood and starts the disc slowly rotating. Facing
the membrane are four sprayer nozzles. Each nozzle is connected to a
separate container. One contains a positively charged polymer solution
and one a negatively charged polymer solution; two hold water.
Argun starts the sprayer system, which mists the disc with the
positive solution for a few seconds, then with a water rinse, then
with the negatively charged polymer, and finally with water again. A
two-layer film forms within about 50 seconds. The thickness of this
"bilayer" depends on the polymers and can range from 3 to 50
nanometers. In about six hours, the sprayer can apply between 400 and
600 bilayers, creating a membrane about 20 micrometers thick. The
membrane described in Advanced Materials was made up of three bilayers
on top of a Nafion membrane, adding only 260 nanometers to its
thickness. By using a combination of positive and negative polymers,
the researchers maintained Nafion's high conductivity while reducing
its methanol crossover.
Other researchers have tried to reduce membrane permeability by using
new polymers or blending two different polymers. Blending often
doesn't work well, though, because polymers with different structures
tend to separate, making the membrane less stable. With the layer-by-
layer assembly process--common in other areas of materials
science--"we combine two different materials, but on a nanometer-
length scale so they're really intermingled," Hammond says.
Testing Grounds
After the membrane dries, Argun carefully peels it off the disc and
tests its permeability and electrical resistance, which allows him to
calculate its conductivity. With a large clip, he fastens the membrane
between a plastic chip and a base that holds platinum wires that will
measure resistance. After putting the assembly in a sealed plastic box
that allows him to control temperature and humidity, he manipulates
the membrane using a pair of gloves that reach through the box and
into the chamber. Most membranes perform better under high
temperature and humidity, so both conditions must be noted. Argun
connects the assembly to an external analyzer to test the membrane's
resistance. Measuring its permeability is more straightforward; he
simply notes the amount of methanol that diffuses through it over a
specific amount of time.
If a membrane fares well in these initial tests, Argun couples it to a
positive and a negative electrode (where the electricity-producing
reactions take place) to see how it would perform in an actual fuel
cell. He places the electrodes--two black, circular carbon cloths
studded with particles of platinum and a metal alloy--on either side
of the membrane. Then he sandwiches the whole apparatus inside an
insulating gasket that looks like thin cardboard. Finally, he seals
the unit using a hot press.
Graduate student Nathan Ashcraft takes over from here. Ashcraft puts
the membrane-electrode assembly into an active fuel cell, into which
air and methanol are carefully pumped. Two square slabs of steel,
about the size of slices of bread, make up the outside of the cell;
they contain heaters that allow Ashcraft to precisely control the
temperature of the reaction. Between the steel slabs, two gold-plated
electrodes sandwich graphite blocks with small channels etched into
them. Ashcraft places the membrane-electrode assembly between the
blocks and secures it with screws. He then pumps methanol and air
through the channels to either side of the assembly. He measures and
records the resulting current, along with the system's temperature.
Hammond's team has not yet devised a completely new membrane that
conducts as well as Nafion. However, "we feel like we're very close,"
she says. The team is also experimenting with membrane thickness; if a
membrane is too thin, it will tear in the fuel cell, but thicker
membranes don't conduct protons as well. The membrane that the lab
ends up with will probably be about 50 micrometers thick, Ashcraft
says. Hammond also plans to try building membranes that incorporate
additional polymers.
.
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