Turning organic waste into hydrogen now works without expensive platinum.
- From: Jack Linthicum <jacklinthicum@xxxxxxxxxxxxx>
- Date: Mon, 23 Feb 2009 04:18:57 -0800 (PST)
In the future you will process your day's garbage and then use the
resulting hydrogen to power the family car. Ships are an even bigger
target.
Monday, February 23, 2009
Cheap Hydrogen from Scraps
Turning organic waste into hydrogen now works without expensive
platinum.
By Nora Schultz
It sounds almost too good to be true: add a few bugs to food scraps
and waste water to generate clean hydrogen fuel. But over the past few
years, researchers have been gradually working toward this promising
scheme for producing hydrogen.
Now, with the help of an unassuming stainless-steel brush, microbial
electrolysis cells (MECs) have taken another step forward. The steel
brush can be used to replace the expensive platinum normally employed
as the electrolysis cell's cathode, slashing costs by more than 80
percent.
Hydrogen is an appealing, environmentally friendly fuel because
burning it creates only water as a waste product. MECs harness the
electrons produced by certain bacteria as those bacteria feed on
biodegradable material. The bacteria sit on an electrode--the anode--
as they metabolize organic matter in an oxygen-devoid chamber. Not
being able to react with oxygen, the electrons travel from the anode
to the counter-electrode--the cathode--where they combine with protons
to form hydrogen.
In late 2007, a team led by Bruce Logan, Kappe professor of
environmental engineering at Pennsylvania State University, showed
that they could improve the efficiency of this process: by adding a
small jolt of electricity (0.25 volts) at the cathode. Until now,
however, the researchers have relied on a platinum catalyst on the
cathode to make the process fast enough.
"The need to use a precious metal catalyst had been holding back
further development of the technique, but now we have found a way to
do it without platinum," says Logan.
Compared with platinum, which acts as an effective catalyst when
applied in a thin layer to a flat piece of carbon cloth, a simple
piece of stainless steel is two-thirds less effective. But when
Logan's team increased the surface area of the stainless-steel cathode
by arranging the material in the form of a high-density bristle brush,
hydrogen production rates increased to values that matched or even
exceeded those of the platinum cathode. While the platinum cathode
costs around 15 cents, the stainless-steel brush only set the
researchers back 3 cents.
Logan hopes that further modifying the chemistry of the brush will
improve the results even more. "We now already know more about which
types of stainless steel work best," he says. "And we will also want
to minimize hydrogen bubbles being trapped between the bristles
because this can make recovery of the gas less efficient."
He also emphasizes that high surface area is not everything. A brush
made from carbon with an even higher surface area did 14 times worse
than the naked steel-brush core, and when the researchers cut the
steel brush in half to allow closer spacing of the two electrodes,
they got even better results than with the full brush, even though
they lost half of the surface area.
Lars Angenent, an associate professor of biological and environmental
engineering at Cornell University, says that big challenges remain,
and he argues that the effect of electrode spacing is going to be one
of the biggest limitations of MEC technology. "I think this work is
great, but the next question is, can you scale it up so it's
economical?" he says. "In a larger system, moving ions through liquid
between cathode and anode is more difficult, so you will produce less
hydrogen per unit volume."
Patrick Hallenbeck, a professor of bacteriology at the University of
Montreal, in Canada, agrees with Angenent that scaling will be a
challenge. However, he is optimistic that with the platinum limitation
gone, the outlook for MECs is good: "By showing that platinum can be
effectively replaced by stainless steel, Logan's group have removed a
critical barrier. These devices were first described only four years
ago, and there has been tremendous progress since then. Further
developments may very well move MEC devices into the realm of
practical application."
Copyright Technology Review 2009.
.
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