Iron 'veins' are secret of promising new hydrogen storage material

Aug 31, 2011 By Chad Boutin
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. Credit: NIST

(PhysOrg.com) -- With a nod to biology, scientists at the National Institute of Standards and Technology 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 , 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 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 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 seconds," says Bendersky, "and they can do so at only 150 degrees Celsius 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 percent 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 percent hydrogen by weight.

Bendersky adds that the measurement technique could be valuable more generally, as it can reveal details of how a material absorbs more effectively than the more commonly employed technique of X-ray diffraction—a method that is limited to analyzing a material's averaged properties.

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More information: Z. Tan, et al. Thermodynamics, kinetics and microstructural evolution during hydrogenation of iron-doped magnesium this films. International Journal of Hydrogen Energy, 36 (2011), pp. 9702-9713, DOI: 10.1016/j.ijhydene.2011.04.196

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SteveL
5 / 5 (2) Aug 31, 2011
The energy density of gasoline is roughly 2-1/2 that of hydrogen in either gas or liquid form. If this methodology can hold up to 7% hydrogen by weight then just storing the equivalant energy would require at least 2.8 times the weight of the present fuel tank. (Liquid hydrogen weighs 0.07g/cc and gasoline weighs 0.75g/cc - http://www-formal...n.html.)

Hauling around that extra weight will reduce effeciency and require an even heavier/stronger vehicle to support it. Hydrogen is still not the answer yet - especially for vehicles. Actually, because of the added weight even more hydrogen would need to be stored to achieve the ranges we are used to, increasing weight and lowering effeciency even more.
BLP
not rated yet Aug 31, 2011
SteveL. You have to factor in the relative inefficiencies of an internal combustion engine versus a fuel-cell. That's why hydrogen storage of equal or greater than 6% by weight is considered acceptable.
Eikka
not rated yet Aug 31, 2011
Hydrogen at 6% weight delivers 1998 Wh/kg which reduces to roughly 1000 Wh/kg after the fuel cell, and about 800-900 Wh/kg as mechanical work at the motor shaft.

A reasonably good engine will deliver roughly 3500 Wh/kg from gasoline. 6% is not really great - it's just somewhat tolerable. 40 kilograms of this material would drive the car roughly 150 miles, whereas 40 kilograms of gasoline can do 500-600 miles.

The next question is, what is required to get the hydrogen out? 150 degrees of heat? How much energy is wasted on that?
antialias_physorg
not rated yet Aug 31, 2011
what is required to get the hydrogen out? 150 degrees of heat?

Not much. You only need to get a very small part up to that temp (not the entire tank) - after that you use the heat which comes off the fuel cell.

This seems to be an interesting concept as it allows for easy refueling, safe storage and does not require any exotic materials.

Whether you need a larger tank is not the only issue:
If the fuel cell plus electric motor is smaller than an equivalent gasoline motor plus transmission then you can afford to have a larger tank.
yoatmon
not rated yet Aug 31, 2011
It's interesting and partly amusing to note that the "wheel" is constantly being reinvented.
"Metallacarboranes thus combine boron, carbon and metal atoms in a cage-like structure. The team investigated the hydrogen storage capacity of metallacarborane-based MOF. Their study shows that metal in metallacarboranes can bind multiple hydrogen molecules, while carbon can link the clusters to form three-dimensional frameworks. They found that replacing carboranes in MOF by metallacarboranes enhances the wt % due to adsorption of additional H2 on metal atoms via Kubas interaction. This leads to storage of up to 8.8 wt % in metallacarboranes."
The quotation is cited from:
http://www.greenc...tml#more
Eikka
not rated yet Aug 31, 2011
Not much. You only need to get a very small part up to that temp (not the entire tank) - after that you use the heat which comes off the fuel cell.


It would be technically more sensible to have a small accumulator for the hydrogen to spend before the tank is fully hot.

Though it still takes a lot of energy to heat something like a hundred kilograms of stuff from ambient temperatures to 150 C before each drive. Presumably you have to cool it down to stop the hydrogen from building up pressure inside.

Let's suppose it has the specific heat capacity of iron. 450 J/kg/K. That means you need 5.9 MJ to heat 100 kg up from 20 to 150 C. That's 1.625 kWh of energy, which would technically be enough to drive the car for three miles.

So it's not a vehicle for short commutes.
Ricochet
not rated yet Aug 31, 2011
The idea with hydrogen storage is to use it in a combustion engine, correct? So the fuel cell could, essentially be heated by the engine after a few minutes. The only challenge then would be extraction and delivery until the fuel cell is warmed... The accumulator would be a good idea. It could collect the hydrogen that's extracted after the car is stopped, as it will take the fuel cell a bit to cool down again, and may be actively releasing hydrogen until then.
antialias_physorg
not rated yet Sep 01, 2011
Though it still takes a lot of energy to heat something like a hundred kilograms of stuff from ambient temperatures to 150 C before each drive.

As noted: you don't need to heat all of it all the time. heating small parts is not very energy intensive (Your car battery heats your catalytic converter in your car to 250 degrees Celsius within 10-15 seconds because your engine doesn't get hot fast enough to be effective in the early stages of a ride. Later the motor causes the converter to go to 400-900 degrees).

The idea with hydrogen storage is to use it in a combustion engine, correct?

No. Using it in a fuel cell and an electric motor is much more efficient.
Eikka
not rated yet Sep 01, 2011

As noted: you don't need to heat all of it all the time. heating small parts is not very energy intensive


I get the point, but it's not technically very robust.

If you segment the fuel tank into small compartments, you have to figure out a way to pipe the waste heat from the fuel cell between these compartments, which involves a whole bunch of pipes and valves. That not only adds weight, but increases the mechanical complexity of the system and gives it a lot of failure points as compared to a single pipe running through the bigger canister.

antialias_physorg
not rated yet Sep 01, 2011
I get the point, but it's not technically very robust.

So build the fuel cell right next to the hydrogen tank. Where's the problem? You'll need piping anyhow (for air conditioning/heating). Basically do it the same way it's been done in cars for the past 120 years.
Eikka
not rated yet Sep 01, 2011
I get the point, but it's not technically very robust.

So build the fuel cell right next to the hydrogen tank. Where's the problem? You'll need piping anyhow (for air conditioning/heating). Basically do it the same way it's been done in cars for the past 120 years.


That's not the issue at all. It's the number of valves and segments you have. A car's cooling system has one mechanically operated valve that shunts the water past the radiator if the engine is not hot enough.

You probably fail to appreciate how much 1.6 kWh is. Split the tank five ways, and you still have 320 Wh to heat up in each segment. At 3 kW heat output from the fuel cell, corresponding to driving at 20-30 mph, it still takes 6½ minutes to heat it up.

That means that the fuel tank takes the first 2-3 miles of your trip to heat up before it gives off enough hydrogen to go faster. That is, unless you want to speed it up by wasting energy from your battery, or by burning the hydrogen in the tanks.
Eikka
5 / 5 (1) Sep 01, 2011
And another very interesting question: how do you measure how full the tank is?

antialias_physorg
not rated yet Sep 01, 2011
A car's cooling system has one mechanically operated valve that shunts the water past the radiator if the engine is not hot enough.

You'll need exactly one valve to funnel the correct amount past the tank (and I'm not at all sure that you need to regulate it to such exact limits). I think we have come to be able to handle that kind of 'complexity', dont you?

If that's still too complicated then just use some of the electricity from the fuel cell to heat parts of the tank electrically. That would reduce operational efficiency a tiny bit (if you thermally isolate the tank enough - which you'll have to do anyways and which should be easy to incorporate into any structural safety features. With good isolation you'
ll not need to continualy heat the tank - just for the first few minutes)

If you really want to split it I wouldn't split it equally. A small part for use in the first few minutes and then a split for the rest should do it.