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Arvind Varma, from left, Purdue Univ.'s R. Games Slayter Distinguished Professor of Chemical Engineering, postdoctoral researcher Hyun Tae Hwang and doctoral student Ahmad Al-Kukhun review data from a new process for storing and generating hydrogen to run fuel cells in cars and portable consumer electronics. The process, called hydrothermolysis, uses a powdered material called ammonia borane, which contains one of the highest hydrogen contents of all solid materials. (Purdue Univ. photo/Andrew Hancock)
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A new process for storing and generating hydrogen to run
fuel cells in cars has been invented by chemical engineers at Purdue Univ.
The process, given the name hydrothermolysis, uses a
powdered chemical called ammonia borane, which has one of the highest hydrogen
contents of all solid materials, said Arvind Varma, R. Games Slayter Distinguished
Professor of Chemical Engineering and head of the School of Chemical
Engineering.
"This is the first process to provide exceptionally
high hydrogen yield values at near the fuel-cell operating temperatures without
using a catalyst, making it promising for hydrogen-powered vehicles," he
said. "We have a proof of concept."
The new process combines hydrolysis and thermolysis, two
hydrogen-generating processes that are not practical by themselves for vehicle
applications.
Research findings were presented June 15 during the
International Symposium on Chemical Reaction Engineering in Philadelphia. The research also is detailed
in a paper appearing online in the AIChE
Journal, published by the American Institute of Chemical Engineers, and
will be published in an upcoming issue of the journal.
Ammonia borane contains 19.6% hydrogen, a high weight
percentage that means a relatively small quantity and volume of the material
are needed to store large amounts of hydrogen, Varma said.
"The key is how to efficiently release the hydrogen
from this compound, and that is what we have discovered," he said.
The paper was written by former Purdue doctoral student
Moiz Diwan, now a senior research engineer at Abbott Laboratories in Chicago; Purdue
postdoctoral researcher Hyun Tae Hwang; doctoral student Ahmad Al-Kukhun; and
Varma. Purdue has filed a patent application on the technology.
In hydrolysis, water is combined with ammonia borane and
the process requires a catalyst to generate hydrogen, while in thermolysis the
material must be heated to more than 170 degrees Celsius, or more than 330
degrees Fahrenheit, to release sufficient quantities of hydrogen.
However, fuel cells that will be used in cars operate at
about 85 degrees Celsius (185 degrees Fahrenheit). Hydrogen fuel cells generate
electricity to run an electric motor.
The new process also promises to harness waste heat from
fuel cells to operate the hydrogen generation reactor, Varma said.
The researchers conducted experiments using a reactor
vessel operating at the same temperature as fuel cells. The process requires
maintaining the reactor at a pressure of less than 200 pounds per square inch,
far lower than the 5,000 psi required for current hydrogen-powered test
vehicles that use compressed hydrogen gas stored in tanks.
In some experiments, the researchers used water containing
a form of hydrogen called deuterium. Using water containing deuterium instead
of hydrogen enabled the researchers to trace how much hydrogen is generated
from the hydrolysis reaction and how much from the thermolysis reaction,
details critical to understanding the process.
At the optimum conditions, hydrogen from the
hydrothermolysis approach amounted to about 14% of the total weight of the
ammonia borane and water used in the process. This is significantly higher than
the hydrogen yields from other experimental systems reported in the scientific
literature, Varma said.
"This is important because the U.S. Department of
Energy has set a 2015 target of 5.5 weight percent hydrogen for hydrogen
storage systems, meaning available hydrogen should be at least 5.5% of a
system's total weight," he said. "If you're only yielding, say, 7%
hydrogen from the material, you're not going to make this 5.5% requirement once
you consider the combined weight of the entire system, which includes the
reactor, tubing, the ammonia borane, water, valves and other required
equipment."
The researchers determined that a concentration of 77%
ammonia borane is ideal for maximum hydrogen yield using the new process.
The research has been funded by the U.S. Department of
Energy by a grant through the Energy Center in Purdue's Discovery Park.
Future work on hydrothermolysis will explore scaling up
the reactor to the size required for a vehicle to drive 350 miles before
refueling. Additional research also is needed to develop recycling technologies
for turning waste residues produced in the process back into ammonia borane.
The technology may also be used to produce hydrogen for
fuel cells to recharge batteries in portable electronics, such as notebook
computers, cell phones, personal digital assistants, digital cameras, handheld
medical diagnostic devices and defibrillators.
"The recycling isn't important for small-scale
applications, such as portable electronics, but is needed before the process
becomes practical for cars," Varma said.
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