A chemist at Washington University in St. Louis hopes to find the
right stuff to put the element hydrogen in a sticky situation.
Lev Gelb, Ph.D., Washington University assistant professor of
chemistry, prepares theoretical models of molecules that may be used
to store and transport hydrogen gas.

Image: Storing hydrogen is problematic. A WUSTL chemist and his
colleagues are exploring different approaches to help make hydrogen
fuel more practical.

Gaseous at room temperature, hydrogen is even lighter and less dense
than natural gas and thus harder to store. So, while hydrogen has a
high energy-per-weight, it has a low energy-per-volume.

"If you had a kilogram of hydrogen at atmospheric pressure, you'd have
to store it in about 100 big balloons, if you can picture that," said
Gelb. "A kilogram of gasoline, on the other hand — that would be a
small container."

Gelb works on one possible solution to this storage problem, a process
called gas physical adsorption.

"The idea here is to create materials composed of molecules hydrogen
likes to stick to," said Gelb. "If hydrogen stuck to these particles
you could carry around the substance, along with the hydrogen."

Such a substance would have to be relatively light-weight and very
porous, having a high surface area, in order to adsorb as much
hydrogen as possible. Then it is hoped that the hydrogen can be
removed at the site of combustion by applying some low-energy force
such as a vacuum.

"The problem is that as far as we know, nothing is sticky enough
without being too heavy," said Gelb.

But this doesn't stop him: his theoretical chemistry work aims at
calculating what the properties of such a material would be — what the
material should be made of, what it should look like. Currently, Gelb
and some of his post-doctoral researchers are looking at a class of
materials called coordination polymers, recently synthesized, highly
porous materials that have shown some promise in hydrogen gas
adsorption.

Building molecular models

By focusing on building molecular models of such materials, Gelb can
screen potentially promising molecules. This way he can have a good
idea whether a certain material might be a good candidate before
someone else devotes the time and energy involved in synthesizing it.

"Hydrogen gas has a lot of promise," said Gelb. "It has two basic
advantages: it is an efficient fuel and produces no pollutant
by-product."

Hydrogen, or H2, burns in the same way as natural gas. It is a
promising alternative energy, however, because its chemical energy can
be directly and efficiently converted to electricity in special fuel
cells that are easily miniaturized. In burning natural gas, on the
other hand, chemical energy first must be converted to mechanical
energy in order to create electricity, an extra step that reduces
efficiency.

Hydrogen also has a very high energy-to-weight ratio, higher than that
of natural gas and gasoline. Most appealing, perhaps, is that hydrogen
is clean burning — its combustion yields only water. Natural gas,
along with all fossil fuels, burns to produce water and carbon
dioxide, the most abundant greenhouse gas.

Unfortunately, there are many problems that have prevented and
continue to prevent hydrogen from being used on a large scale, of
which storage and transport is only one.

There are several other possible solutions to the storage/transport
problem, but each has significant downsides.

Pressing matter

The most likely option in the near future, said Gelb, is to simply
compress the gas at very high pressure. Hydrogen-powered car
prototypes made by General Motors, for example, use this storage
option. There are several drawbacks, however; storage tanks are
expensive and inherently dangerous, especially since hydrogen is
combustible. Additionally, it is energetically costly to compress the
hydrogen, making a net efficient usage of energy difficult to achieve.

Another potential storage solution involves cooling the gas to
extremely low temperatures until the gas becomes a liquid. This
option, however, would also be energetically costly and presents the
problem of evaporation.

A third idea involves chemically incorporating the hydrogen in a solid
material, for instance in a class of materials called metal hydrides.
Hydrogen can be stored in these materials at such high densities as to
surpass the density of liquid hydrogen. Unfortunately, it is very
difficult to get the hydrogen out of the material, requiring more
energy. Also, these hydrides are often very reactive, dangerous
materials — many react violently with both air and water and cease
working.

But the biggest problem with hydrogen, according to Gelb, is producing
it.

For one thing, considering hydrogen gas to be an energy 'source' is a
misnomer — it does not naturally occur on the earth; it must be
derived from something else. While hydrogen is the most abundant
element in the universe, on our planet all of it is bound with other
elements.

Water, for example, is two parts hydrogen, one part oxygen, and it is
also bound up in hydrocarbons and a milieu of other compounds. Thus,
hydrogen production is the larger problem that stands in the way of
ever achieving a 'hydrogen economy.'

Currently, the vast majority of hydrogen gas is produced from natural
gas in a process called steam reforming. Besides using up natural gas,
this process also creates carbon dioxide — the byproduct absent in
hydrogen combustion, which contributes to much of its promise as a
'green' fuel.

While there has been some progress in sequestering this carbon dioxide
in places where it cannot seep into the atmosphere, such as deep
underground, producing hydrogen via steam reforming has only limited
promise for reducing greenhouse emissions, and is not a renewable
strategy.

"The case has been made persuasively that you'd be better off just
burning the natural gas, rather than going to the trouble of producing
hydrogen from natural gas and going through all the problems
associated with its storage and transport," said Gelb.

But that doesn't stop him from trying to solve these problems.

Gelb, in fact, is working in collaboration with several other
Washington University researchers in energy-related science.

This work is supported by the recently established Washington
University Center for Materials Innovation. His Washington University
colleagues in this endeavor are: Pratim Biwas, William Buhro, Dewey
Holten, Ramki Kalyanaraman, Kenneth Kelton, Richard Loomis, Thomas
Vaid, and Amy Walker.

Source: Washington University in St. Louis (By Doug Main)

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