Trends in Transportation - Hydrogen and Fuel Cells: Advances in Transportation and Power - page 171

contaminate the membrane.
Researchers at 3M have been able to increase catalytic activity with
nanotextured membrane surfaces that employ tiny columns to increase the
catalyst area. Other materials include nonprecious metal catalysts such as
cobalt and chromium along with particles embedded in porous composite
structures.
A fuel cell vehicle must have enough hydrogen to provide a reasonable
driving range. For a range of 400 miles, 5-7 kilograms of hydrogen may be
required, most fuel cell prototypes hold a little more than about half this
amount.
Hydrogen can be stored in high pressure tanks as a compressed gas
at ambient temperature and there has been much work on increasing the
pressure capacity of composite pressure tanks. Liquid hydrogen systems
that store the fuel at temperatures below -253°C have been successful.
But, almost one third of the energy available from the fuel is needed to
maintain the temperature and keep the hydrogen in a liquid state. Even
with thick insulation, evaporation and losses through seals results in a
loss every day of about 5% of the total stored hydrogen.
Alternative storage technologies include metal hydride systems
where metals and alloys are used to hold hydrogen on their surfaces until
heat releases it for use. They act like a sponge for hydrogen. ECD Ovonic,
a part of Texaco Ovonic Hydrogen Systems, has been active in this area.
The hydrogen gas in the high pressure storage tank chemically bonds to
the crystal lattice of the metal or alloy in a reaction that absorbs heat. The
resulting compound is a metal hydride.
Waste heat from the fuel cell is used to reverse the reaction and
release the fuel. In 2005 GM and Sandia National Laboratories launched a
$10-million program to develop metal hydride storage systems based on
sodium aluminum hydride.
Metal hydride storage systems can be heavy and weigh about
300 kilograms. Researchers at the Delft University of Technology in the
Netherlands have found a way to store hydrogen in water ice, a hydrogen
hydrate, where the hydrogen is trapped in molecule sized cavities in ice.
This approach is much lighter than metal alloys.
In the past, hydrogen hydrates have been difficult to produce, since
they require low temperatures and pressures in the range of 36,000 psi.
Working with the Colorado School of Mines, the Delft group used a
promoter chemical (tetrahydrofuran) to stabilize the hydrates at 1,450 psi.
This approach would allow about 120 liters (120 kilograms) of water to