Hydrogen fuel for thought: Researchers find metallacarboranes may meet DOE storage goals

September 30, 2010

(PhysOrg.com) -- New research by Rice University scientists suggests that a class of material known as metallacarborane could store hydrogen at or better than benchmarks set by the United States Department of Energy (DOE) Hydrogen Program for 2015.

The work could receive wide attention as hydrogen comes into play as a fuel of the future for cars, in fuel cells and by industry.

The new study by Rice theoretical physicist Boris Yakobson and his colleagues, which appears in the online , taps the power of scandium and titanium to hold a load of hydrogen molecules -- but not so tightly that they can't be extracted.

A matrix made of metallacarboranes would theoretically hold up to 8.8 percent of its weight in , which would at least meet and perhaps surpass DOE milestones issued a year ago for cars that would run on .

Yakobson, a professor in mechanical engineering and materials science and of chemistry at Rice, said inspiration for the new study came from the development of metallacarboranes, now well-known molecules that combine boron, carbon and metal atoms in a cage-like structure.

"A single metal atom can bind multiple ," Yakobson said, "but metals also tend to aggregate. Without something to hold them, they clump into a blob and are useless."

Abhishek Singh, lead author of the study, a former postdoctoral researcher for Yakobson and now an assistant professor at the Indian Institute of Science in Bangalore, India, calculated that boron clusters would grip the titanium and scandium, which would in turn bind hydrogen. "The metals fit like a gem in a setting, so they don't aggregate," Yakobson said. Carbon would link the clusters to form a matrix called a metal organic framework (MOF), which would act like a sponge for hydrogen.

Investigation of various transition metals showed scandium and titanium to have the highest rate of adsorption (the adhesion of transient molecules -- like hydrogen -- to a surface). Both demonstrate an affinity for "Kubas" interaction, a trading of electrons that can bind atoms to one another in certain circumstances. "Kubas is a special interaction that you often see mentioned in hydrogen research, because it gives exactly the right binding strength," Yakobson said.

"If you remember basic chemistry, you know that covalent bonds are very strong. You can bind hydrogen, but you cannot take it out," he said. "And on the other extreme is weak physisorption. The molecules don't form chemical bonds. They're just exhibiting a weak attraction through the van der Waals force.

"Kubas interaction is in the middle and gives the right kind of binding so hydrogen can be stored and, if you change conditions -- heat it up a little or reduce pressure -- it can be taken out. You want the framework to be like a fuel tank."

Kubas allows for reversible storage of hydrogen in ambient conditions -- ranging from well above to well below room temperature -- and that would make metallacarborane materials highly attractive for everyday use, Yakobson said. Physisorption of hydrogen by the carbon matrix, already demonstrated, would also occur at a much lower percentage, which would be a bit of a bonus, he said.

Other studies have demonstrated how to make carborane-based MOFs. "That means they can already make three-dimensional frameworks of material that are still accessible to gas. This is very encouraging to us," Yakobson said. "There are many papers where people analyze a cluster and say, 'Oh, this will also absorb a ,' but that's not useful. One cluster is nothing.

"But if chemists can synthesize this particular framework with metallacarborane as an element, this may become a reality."

Explore further: Brookhaven Scientists Working Toward Practical Hydrogen-Storage Materials

More information: pubs.acs.org/doi/abs/10.1021/ja104544s

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5 / 5 (2) Sep 30, 2010
So, 8.8% hydrogen, which means the bare material would have an energy density of 10.56 MJ/kg.

Taking into account the differences in use, one gets roughly 5 MJ/kg out of the hydrogen and 15 MJ/kg out of gasoline. For the same weight in "fuel", the gasoline powered car still goes three times further.

Compared to today's lithium batteries, the energy density would be about tenfold, so instead of 550 kg of batteries, you'd have a 55 kg tank of MOF.

The downside is that the hydrogen system would use 3-4 times as much energy as the battery electric vehicle, and only slightly less than the gasoline powered vehicle, and consume vast amounts of clean water in the hydrogen production stage.
3 / 5 (2) Sep 30, 2010
If using hydrogen, we have to consider that we're already using up fresh groundwater at a faster rate than it is replenished.

If there is not enough clean water, hydrogen must be produced from desalinated water and that would add immensely to the energy costs of hydrogen production, making it practically pointless.
1.5 / 5 (2) Sep 30, 2010
If there is not enough clean water, hydrogen must be produced from desalinated water and that would add immensely to the energy costs of hydrogen production, making it practically pointless.

Been saying this for some time.

You need massive solar power to produce the hydrogen from sea water. In a hydrogen fuel cell the hydrogen is more of a form of "energy storage" rather than a "fuel".

I have devised better schemes of clean energy transportation using external power source for pure electric vehicles, rather than on-board batteries or fuel cells.

If you have ever ridden bumper cars at a carnival you might understand what I would propose.

Effectively, you would have an entire infrastructure based on "bumper cars" which are powered by contacting an eletrical surface.
4.5 / 5 (2) Sep 30, 2010
Hydrogen is doomed.

Bio-diesel is a far superior carrier of energy and we already have a massive infrastructure for its transportation (of hydrocarbons). Unlike Hydrogen, it is non-toxic, bio-degradable, non-volatile, leaks do not deplete the ozone layer, does not require expensive and exotic metals for storage and transportation, and is more efficient to produce.

Bio-diesel can be just as clean to burn and is carbon-neutral, exhaust containing CO2 and water. The CO2 released representing CO2 that was absorbed from the atmosphere for its production.

Bio-diesel will effectively displace current usage in heavy industry and transportation and offset fossil fuel CO2 production. Hydrogen would require a massive, expensive, and energy intensive new infrastructure to be built.

Bio-diesel keeps many thousands of people employed using their skills with hydrocarbon machines, and the public can still repair their cars themselves.
2 / 5 (1) Sep 30, 2010

My proposal is carbon-negative in the long term...

I have also considered the possibility of magnetic levitation automobiles that use an infrastructure that could be overlayed on top of existing roadways. This approach would also be carbon-negative in the long term.
1 / 5 (2) Oct 01, 2010
The downside is that the hydrogen system would use 3-4 times as much energy as the battery electric vehicle, and only slightly less than the gasoline powered vehicle, and consume vast amounts of clean water in the hydrogen production stage.

Your "downside" doesn't seem all that bad to me. People love vehicles that have at least 300 miles range. Also, H-O fuel cells don't consume water; they use hydrogen (and sometimes oxygen) as energy storage particles that are returned to the environment upon re-combining. In fact, if you really want clean, fresh water, try the chemically pure H2O from a fuel cell's exhaust. It's delicious.
2 / 5 (1) Oct 01, 2010
In fact, if you really want clean, fresh water, try the chemically pure H2O from a fuel cell's exhaust. It's delicious.

That would actually be harmful to you, in the same way that drinking distilled water is. If you put living cells in absolutely pure water, the osmotic pressure fills them up like balloons until they burst.

Hydrogen production does use a lot of water, and it is infeasible to capture said water back at the cars, because one kilogram of hydrogen generates 9 times its weight in H2O, so in addition to the size of the actual fuel tank, you'd need an extra tank for the expelled water and more space for the cooling system to turn the water into liquid as it exits the fuel cell as steam and vapor. (a fuel cell is only 50% efficient)

Essentially, the weight of the fuel system would double as you run through a tank of hydrogen, and you end up with 20 gallons of water.
not rated yet Oct 04, 2010
Whether or not a hydrogen economy ever happens, it is still exciting that new materials that can store hydrogen are cropping up. Hopefully the substance can stand water, and oxygen and is not a fire or environmental hazard.

Of course research should advance on all fronts, so that the graphene flywheel that can spin at 60000000 RPM to store energy has potential too, or the superstrong materials that could work as a winding spring to make a clockwork operated car. For energy density it is going to be hard to beat a lithium / air battery with discharge stations on the roadside to deal with the solid waste and allow for recycling.

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