Fuel cell aimed at handhelds
By Ted Smalley Bowen,
Technology Research News
Fuel cells have
been tagged as promising energy sources for everything from buildings and
cars to cell phones and yet-to-emerge micro-devices.
With an eye toward the small end of the scale, a researcher at Lawrence
Livermore National Laboratory has made a fuel cell that could eventually
power consumer electronics.
The thin-film fuel cell consumes methanol, and is smaller and would be cheaper
to make than rechargeable batteries, according to Jeff Morse, a researcher
in the lab’s center for micro-technology engineering.
Morse made prototype fuel cells using materials and processes borrowed from
computer chip manufacturing, the emerging field of microelectromechanical
systems (MEMS), and microfluidics.
A fuel cell works by converting chemical energy into electrical energy and
heat. Like a battery, it uses a cycle of chemical reactions between positive
and negative electrodes to produce an electric current. Unlike batteries,
fuel cells are supplied by fuel -- commonly some form of hydrogen -- and
need to be refueled rather than recharged.
In a typical hydrogen fuel cell, the hydrogen fuel is flowed into the cathode,
or negative electrode, and air flowed into the anode, or positive electrode.
The porous metal electrodes act as catalysts to speed the reactions of hydrogen
gas from the fuel and oxygen from the air with the electrolyte, a pool of
chemicals bathing the electrodes. At the anode, the oxygen reacts with water
in the electrolyte to form hydroxide ions. At the cathode, the hydrogen
fuel reacts with the hydroxide ions to form water, releasing two electrons
per hydrogen molecule.
The released electrons flow through an external circuit. This electrical
current can be tapped to produce work. The chemical cycle also produces
The Livermore fuel cell is designed to use replaceable fuel cartridges filled
with methyl alcohol, or methanol. Each cartridge fuels the power pack for
twice as long as a lithium ion battery lasts on a single charge, and could
eventually last close to three times as long, according to Morse.
Fully loaded, the fuel cell weighs 8 grams and produces 1 to 2 Watts of
power, or about 3,000 work hours per kilogram, according to Morse. The cells
could be made as small as 5 by 3 by 3 centimeters, he said.
The small fuel cell is made from a thin polymer film, with thin-film electrodes
of platinum and ruthenium. Key to shrinking the fuel cell was combining
microelectronics with microfluidic plumbing. A micro-machined flow field
of channels etched into silicon circulates methanol over the anode and air
over the cathode. The channels range from a millimeter to one-tenth of a
The fuel cell also contains electrical elements that heat the components,
which speeds the ions bound from the anode to the cathode, enabling the
cell to produce more power.
The researchers' design must become more efficient before it can challenge
existing fuel cell designs, according to Morse. “Our fuel conversion efficiency
is slightly less than conventional fuel cells. Our estimates for present
designs are thirty-five percent,” he said.
Conventional large fuel cells extract about 60 percent of the energy available
in the fuel. Batteries, meanwhile, can convert about 75 percent of the chemical
energy they contain into electrical current. Fuel cells are more often considered
as an alternative to internal combustion engines, which burn fuel rather
than chemically extracting electricity from it. The combustion engines in
today's cars produce much more waste heat and are only about 15 to 20 percent
To date, diminutive fuel cells have not measured up well against conventional
batteries, said Lutgard C. De Jonghe, professor of ceramics in the department
of materials science and engineering at the University of California at
Berkeley and senior scientist in the materials sciences division of Lawrence
Berkeley National Laboratory. “In general, comparisons of small fuel cells
with same size batteries is quite unfavorable for the fuel cell, both in
power and energy density."
One inherent problem with thin-film fuel cells is seepage of fuel before
it can react with the anode. "Important issues such as methanol crossover
have been a problem for polymer membrane fuel cells,” said De Jonghe. This
methanol crossover can sap around 20 percent of a fuel cell’s fuel and slightly
lower its voltage, dropping its overall efficiency.
The Livermore prototype’s fuel reforming devices address this by converting
the methanol water-fuel mixture to hydrogen at the fuel cell anode, said
Morris. This may produce another problem, however, he added. “Catalytic
reforming converts the methanol to hydrogen and carbon dioxide first. Our
concern is the impact of any carbon monoxide generated.”
The MEMS fuel cell could find uses in portable electronics, small sensors,
and military electronics, according to Morse. It could be ready for practical
use within three years, he said.
Morris presented the work at the Lawrence Livermore National Laboratory
NanoSIG conference in Livermore, California on Februrary 14, 2002. The work
was funded by the lab.
Timeline: <3 years
TRN Categories: Energy; Materials Science and Engineering
Story Type: News
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