Crystallizing the switch to hydrogen

Dec 02, 2011
Figure 1:  Heterometallic hydride clusters containing molybdenum atoms (purple spheres) and rare-earth yttrium metals (green spheres) are promising materials for on-board storage and release of hydrogen gas (light blue spheres). Credit: 2011 Zhaomin Hou

Hydrogen gas is an almost infinitely inexhaustible fuel source that emits only clean water during combustion. Switching from hydrocarbon-based transportation to systems powered by state-of-the-art fuel cells therefore seems a natural choice, but numerous obstacles have kept this technology confined to laboratories. A prime example is the problem of on-board hydrogen storage for vehicles: because ambient hydrogen gas is roughly 10,000 times less dense than gasoline, it would require impractically large tanks to obtain comparable mileage.

Compressing or liquefying it at -250 °C are two ways to increase its energy content by volume. However, chemists are developing a more attractive strategy using specially designed compounds, called metal hydride clusters, to produce high densities without extreme temperatures or pressures. The metal atoms within these molecules bind to large numbers of atoms, producing a solid that can reversibly add or remove hydrogen using mild heating or cooling.

Now, Zhaomin Hou from the RIKEN Advanced Science Institute in Wako and an international team of colleagues have isolated a new class of ‘heterometallic’ hydride clusters (Fig. 1) that may spur development of lighter and longer-lived devices. By incorporating multinuclear rare-earth metals into their compounds, the team has produced the first high-density storage molecules that have hydrogen addition properties that can be monitored directly using x-ray diffraction—a technique that provides clear insights into cluster structure and functionality.

Rare combinations

For the past 25 years, chemists have paired so-called ‘d-block transition metals’, such as tungsten (W) and molybdenum (Mo), with lightweight rare-earth metals, such as yttrium (Y), to increase the storage capacity of hydride clusters. Because the nuclei of rare earths are shielded by many electrons, these metals can pack high numbers of hydrogen atoms into small crystal volumes without suffering electronic repulsions. Unfortunately, once hydrogen gas binds to a rare-earth metal, it tends to stay there. Mixing in d-block metals alters the rare-earth reactivity so that on-demand hydrogen storage and release can occur.

Until now, most of these combined metal hydrides were constructed using mononuclear rare-earth building blocks, such as YH, with a mononuclear d-block metal. Using a different strategy, Hou and his colleagues recently devised innovative protocols to isolate polynuclear rare-earth hydrides using large molecular ligands to trap these typically unstable compounds in place. Polynuclear hydrides feature dense, interconnected networks of ‘bridging’ hydrogen atoms connected to two or more metals—characteristics that led the researchers to explore their potential for hydrogen storage applications. 

Figure 2: Monitoring the reversible addition and release of a hydrogen gas molecule to a molybdenum-yttrium cluster in real time with x-ray crystallography has revealed the first atom-resolved insights into hydrogen storage by organometallic crystals. Credit: 2011 Zhaomin Hou

“It is not difficult to imagine that hydrogen atoms could bond to multiple metal atoms in a polynuclear polyhydride complex, and the [mode of] bonding could be different with different metal combinations,” says Hou. “However, it is not easy to prepare quality polyhydride samples for high-precision structure determinations. Hydride complexes containing both rare-earth and d-block transition metals are even more difficult to prepare because of their air- and moisture-sensitivity.”

A five-way first

Performing their experiments inside nitrogen-filled and humidity-free enclosures, the team mixed one of their carefully prepared polynuclear complexes—four yttrium metals and eight hydrogen atoms held together by bulky organic ligands—with either a Mo or W pentahydride. After precipitating crystals out of the reaction, they used x-ray and neutron diffraction experiments to view their product’s atomic structure. These measurements showed that the two metallic components fused together, yielding a Y4MH11 (M = Mo, W) hydride with double-, triple-, and quadruple-bridged .

Zapping the penta-metallic polyhydride with ultraviolet light enabled the team to remove a protective phosphorus ligand and increase the hydrogen bridging density within the cluster. This produced the first hydride cluster where hydrogen is bonded to five metals in a distinctive symmetry known as trigonal bipyramidal. “The confirmation of a penta-coordinated hydrogen atom in this geometry is unprecedented,” says Hou.

Step-by-step scrutiny

Hou and colleagues’ experiments then demonstrated that their heterometallic clusters possessed critical hydrogen storage and release capabilities. Heating H2 and Y4WH11 to 80 °C caused an oxidative addition of the gas molecule to the cluster, which they could reverse through ultraviolet-light treatment. Despite the Y4MoH11 molecule not responding to the same chemical tricks, the researchers discovered that applying a vacuum could suck H2 from the cluster, giving a new Y4MoH9 complex. Exposing this compound to hydrogen gas at room temperature spontaneously regenerated the original molecule (Fig. 2).

According to Hou, the most striking aspect of this chemistry is that the hydrogen addition to the Y4MoH9 cluster can be followed from single crystal to single crystal—meaning that the starting material, the reaction intermediates, and the product all retain the same rigid morphology. “No metal hydrides have previously shown such excellent crystallinity,” he notes.

After gingerly sealing a Y4MoH9 crystal into a thin, hydrogen-filled capillary tube, the researchers monitored the spontaneous addition reaction over 60 hours. As the cluster gradually took in hydrogen and changed color from black to red, they watched—at precision greater than one-millionth of a meter—yttrium and molybdenum atoms separate and shift within the crystal unit cell. By providing the first-ever atom-resolved views of active sites and bonding modes for hydrogen addition to an organometallic crystal, these findings should aid design of more sophisticated alloys in the future.

Theoretical calculations performed by the researchers indicated that combining two metals with starkly different electronic properties played a big role in giving the clusters their unique reactivity. With wide swaths of the periodic table available for exploring using this technique, breakthroughs in heterometallic hydride materials may have only just begun.

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More information: Shima, T., et al. Molecular heterometallic hydride clusters composed of rare-earth and d-transition metals. Nature Chemistry 3, 814–820 (2011). doi:10.1038/nchem.1147 
 
Nishiura, M. & Hou, Z. Novel polymerization catalysts and hydride clusters from rare-earth metal dialkyls. Nature Chemistry 2, 257–268 (2010). doi:10.1038/nchem.595

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CapitalismPrevails
3 / 5 (10) Dec 02, 2011
"Hydrogen gas is an almost infinitely inexhaustible fuel source that emits only clean water during combustion."

OMFG, i can't stand it every time i hear this. Hydrogen is NOT an energy source! It's a storage medium...
rawa1
1 / 5 (4) Dec 02, 2011
In context cold fusion technology, it's quite effective fuel source. Unfortunately for scientists involved, just the cold fusion will render their research practically useless.
PinkElephant
5 / 5 (2) Dec 02, 2011
"Hydrogen gas is an almost infinitely inexhaustible fuel source that emits only clean water during combustion."

OMFG, i can't stand it every time i hear this. Hydrogen is NOT an energy source! It's a storage medium...
They did say "fuel source", not "energy source". But reading comprehension's overrated...
topkill
1 / 5 (4) Dec 02, 2011
@PinkElephant,

Are you just having fun being a SMARTA$$ or do you really think they didn't "mean" to infer that hydrogen is an energy source?

And let's forget that little stupidity and let's say they really did mean a "fuel source" in the strict context...exactly WHERE do they think hydrogen exist as an endless source of ANY type? What? Are they going to import it from the Sun? ROFLMAO

They make it from steam reformed natural gas and it gives off just as much pollution to do that as burning the natural gas anyway.

So...OMFG...everyone STOP acting like hydrogen is magic unicorn farts and it's somehow "clean"!!!!!! It's just natural gas converted to hydrogen..at a cost and WITH pollution.
Newbeak
not rated yet Dec 02, 2011
They did say "fuel source", not "energy source". But reading comprehension's overrated...

The wording is disingenuous.OF COURSE hydrogen is inexhaustible,but the trick is obtaining it in a cost effective and ecologically sound manner.
Shakescene21
5 / 5 (1) Dec 03, 2011
"WHERE do they think hydrogen exist as an endless source of ANY type? What? Are they going to import it from the Sun? ROFLMAO"

@Topkill -- Yes, the energy will come from the sun. The green source of hydrogen will be from electrolysis of water, using electricity generated from photovoltaics or geothermal electric power. Obtaining hydrogen by steam reforming NG will be the fading technology.
Newbeak
not rated yet Dec 03, 2011
"WHERE do they think hydrogen exist as an endless source of ANY type? What? Are they going to import it from the Sun? ROFLMAO"


I suppose they mean the most common element in the universe is hydrogen,but of course,it is almost always chemically bound to other elements.
PinkElephant
not rated yet Dec 05, 2011
exactly WHERE do they think hydrogen exist as an endless source of ANY type?
Compared to Lithium, or most other rare-earth metals used in batteries, I'd say it's practically endless. Like Shakescene21 said, electrolysis of water on the fuel-production side; reconstitution of water on the fuel-combustion side. A complete cycle, with energy being gathered from renewable (solar or geothermal or tidal) sources (solar includes hydro, wind, PV, concentrators, biomass, wave.) That makes a sustainable energy storage and distribution infrastructure.

Provided you can get the hydrogen packed densely enough to be economical, and provided you don't leak it in substantial quantities (whereupon it will rise to the stratosphere and react with ozone, thereby wiping out the ozone layer...), and provided you can find cheap and plentiful catalyst materials for the fuel cells (to replace Platinum...)

Yes, promising perhaps, but this tech pathway does have *a few* hurdles yet to overcome...
unknownorgin
not rated yet Dec 06, 2011
Hydrogen a fuel source? When did a fusion reactor go on line?
Newbeak
not rated yet Dec 07, 2011
Hydrogen a fuel source? When did a fusion reactor go on line?

If fusion reactors ever become cost effective,they would be a great source of helium for dirigibles,and other lighter than air vehicles,as current supplies are getting scarcer.
Callippo
not rated yet Dec 07, 2011
The cold fusion produces no helium and other reactors cannot compete with it in cost effectiveness.
Newbeak
not rated yet Dec 07, 2011
The cold fusion produces no helium and other reactors cannot compete with it in cost effectiveness.

I was talking about hot fusion,not pie in the sky..

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