Particles of pure magnesium (left) can only collect a limited amount of hydrogen on their outer surfaces, and the process is slow. But when the magnesium is doped with iron (right), far more hydrogen is delivered through the iron layers, which also results in much faster charging. Image: NIST |
With a nod to biology, scientists at NIST have a new
approach to the problem of safely storing hydrogen in future fuel-cell-powered
cars. Their idea: molecular scale “veins” of iron permeating grains
of magnesium like a network of capillaries. The iron veins may transform
magnesium from a promising candidate for hydrogen storage into a real-world
winner.
Hydrogen has been touted as a clean and efficient
alternative to gasoline, but it has one big drawback: the lack of a safe, fast
way to store it onboard a vehicle. According to NIST materials scientist Leo
Bendersky, iron-veined magnesium could overcome this hurdle. The combination of
lightweight magnesium laced with iron could rapidly absorb—and just as
importantly, rapidly release—sufficient quantities of hydrogen so that grains
made from the two metals could form the fuel tank for hydrogen-powered vehicles.
“Powder grains made of iron-doped magnesium can get
saturated with hydrogen within 60 sec,” says Bendersky, “and they can
do so at only 150 C and fairly low pressure, which are key factors for safety
in commercial vehicles.”
Grains of pure magnesium are reasonably effective at
absorbing hydrogen gas, but only at unacceptably high temperatures and
pressures can they store enough hydrogen to power a car for a few hundred
kilometers—the minimum distance needed between fill-ups. A practical material
would need to hold at least 6% of its own weight in hydrogen gas and be able to
be charged safely with hydrogen in the same amount of time as required to fill
a car with gasoline today.
The NIST team used a new measurement technique they devised
that uses infrared light to explore what would happen if the magnesium were
evaporated and mixed together with small quantities of other metals to form
fine-scale mixtures. The team found that iron formed capillary-like channels
within the grains, creating passageways for hydrogen transport within the metal
grains that allow hydrogen to be drawn inside extremely fast. According to
Bendersky, the magnesium-iron grains could hold up to 7% hydrogen by weight.
Bendersky adds that the measurement technique could be
valuable more generally, as it can reveal details of how a material absorbs
hydrogen more effectively than the more commonly employed technique of X-ray
diffraction—a method that is limited to analyzing a material’s averaged
properties.
