For years people have talked of fuel cells as as a salvation from our
current oil-based economy to a new, enlightened hydrogen-based economy
where pollution from energy will be a thing of the past.
While this, like many visions of the future, is a bit
grandiose, it is based in fact. Fuel cells are operate on the
same principle as all modern engines: they seek to turn potential
chemical energy stored in a fuel into useful mechanical energy.
The big difference from the engines of today is that the main
source of fuel for a fuel cell is hydrogen, and its by-product is
merely fresh water. One promising class of fuel cells are
polymer electrolyte fuel cells, an example of a proton
exchange fuel cell. While these are not a new
technology—they were first used on the Apollo missions in the 1960s
and 1970s—they have one prohibitive barrier: cost.
Current PEFC consist of a polymer membrane sandwiched between an anode
and a cathode, which are then connected to form an electrical current.
The basic chemistry of a fuel cell is that molecular hydrogen
(H2) is present on the anode side where it is
oxidized into two protons
(H+) and two electrons (e-).
The electrons flow to the
cathode creating an electrical current that can be used to power
devices. The free protons migrate across the polyelectrolyte
membrane to participate in an oxygen reduction reaction at the cathode.
Here on the cathode side, pure oxygen (O2) or
present and it combines with the electrons that have flowed across the
circuit, and along with the protons that have migrated across the
membrane react to form pure water (H2O).
While this is very
simple in theory, performing the oxidation and reduction reactions is
not easy, therefore catalysts are used to facilitate these reactions.
Currently precious metal catalysts are used to aid these
reactions. These catalysts are primarily platinum (Pt) and to a
lesser extent palladium
(Pd) and ruthenium (Ru). Use of these metals presents a few
mentioned above they are expensive (Pt is currently selling for
$45/gm), these metals are rare (Pt occurs at only 37 parts per billion
in the Earth's crust), and their catalytic activity can significantly
degrade over time.
work by scientists at Los Alamos National Laboratory may alleviate some of these
issues. The researchers have found a class of non-precious
metal composites that can replace the platinum on the cathode side of a
PEFC. While it would be ideal to be able to replace the platinum on both electrodes on the cell, replacing it on the
cathode only will result in a large decrease in the amount of catalyst
needed. This is due to the fact that the slower oxygen reduction reaction requires much more
catalyst than the hydrogen oxidation reaction that takes place at the
anode. The researchers found that a composite consisting
of cobalt (Co, a non-precious metal) covalently trapped
in-between polypyrolle (PPY, [C4NH3]n)
backbones on an amorphous carbon
support could give both decent fuel cell performance, and excellent
stability, all at a low cost. Fuel cells built with this
catalyst were tested in a pure H2 and O2
environment and were reported
to be capable of generating ~0.2 A/cm2 at 0.5V
While this is less that what may be obtained with a Pt-based
catalyst system (~ 1 A/cm2, from early reported
numbers), it was shown
to be able to maintain this output level for the entirety of a 100 hour
test. While it may not produce the same amount of energy per unit time,
this technology has shown that it can produce energy at a fraction of
the cost—cobalt costs ~$0.20/gm. It can also sustain
output for long periods of time, longer than the traditional precious
metal catalyst are capable. This work represents a large step
towards economically viable hydrogen fuel cells, something many the
world have been waiting for.