Nanosilicon rapidly splits water without light, heat, or electricity

Jan 24, 2013 by Lisa Zyga feature
Illustration of the multidisciplinary approach for producing hydrogen through silicon oxidation. The approach involves synthesizing silicon nanoparticles, the silicon-water reaction which generates hydrogen on demand, and using the hydrogen in a fuel cell for portable power. Credit: Folarin Erogbogbo, et al. ©2013 American Chemical Society

(Phys.org)—Although scientists know that when silicon mixes with water, hydrogen is produced through oxidation, no one expected how quickly silicon nanoparticles might perform this task. As a new study has revealed, 10-nm silicon nanoparticles can generate hydrogen 150 times faster than 100-nm silicon nanoparticles, and 1,000 times faster than bulk silicon. The discovery could pave the way toward rapid "just add water" hydrogen generation technologies for portable devices without the need for light, heat, or electricity.

The researchers, Folarin Erogbogbo at the University of Buffalo and coauthors, have published their paper on using nanosilicon to generate hydrogen in a recent issue of .

If hydrogen is ever to be used to deliver energy for wide , one of the requirements is finding a fast, inexpensive way to produce hydrogen. One of the most common techniques is splitting water into hydrogen and oxygen. There are several ways to split water, such as with an electric current (), heat, sunlight, or a substance that chemically reacts with water. Such substances include aluminum, zinc, and silicon.

As the scientists explained, silicon- reactions have so far been slow and uncompetitive with other techniques. However, silicon does have some theoretical benefits, such as being abundant, being easy to transport, and having a high . Further, upon oxidation with water, silicon can theoretically release two moles of hydrogen per mole of silicon, or 14% of its own mass in hydrogen.

For these reasons, the scientists decided to take a closer look at silicon, specifically , which have not previously been studied for hydrogen generation. Because silicon nanoparticles have a larger than larger particles or bulk silicon, it would be expected that the nanoparticles can generate hydrogen more rapidly than the larger pieces of silicon.

But the improvements the scientists discovered with silicon nanoparticles far exceeded their expectations. The reaction of 10-nm silicon particles with water produced a total of 2.58 mol of hydrogen per mol of silicon (even exceeding theoretical expectations), taking 5 seconds to produce 1 mmol of hydrogen. In comparison, the reaction with 100-nm silicon particles produced a total of 1.25 mol of hydrogen per mole of silicon, taking 811 seconds to produce each mmol of hydrogen. For bulk silicon, total production was only 1.03 mol of hydrogen per mol of silicon, taking a full 12.5 hours to produce each mmol of hydrogen. For a rate comparison, the 10-nm silicon generated hydrogen 150 times faster than 100-nm silicon and 1,000 times faster than bulk silicon.

"I believe the greatest significance of this work is the demonstration that silicon can react with water rapidly enough to be of practical use for on-demand hydrogen generation," coauthor Mark Swihart, Professor of Chemical and Biological Engineering at the University of Buffalo, told Phys.org. "This result was both unexpected and of potential practical importance. While I do not believe that oxidation of silicon nanoparticles will become a feasible method for large-scale hydrogen generation any time soon, this process could be quite interesting for small-scale portable applications where water is available."

A comparison of hydrogen generation rates for different forms of silicon. Maximum rates are in the left column with images of the samples on them. Average rates are in the right column. The red line indicates the maximum reported rate for hydrogen generated from aluminum. Credit: Folarin Erogbogbo, et al. ©2013 American Chemical Society

In addition to producing hydrogen faster than larger silicon pieces, the 10-nm silicon also produces hydrogen significantly faster than aluminum and zinc nanoparticles. As Swihart explained, the explanation for this inequality differs for the two materials.

"Compared to aluminum, silicon reacts faster because aluminum forms a denser and more robust oxide (Al2O3) on its surface, which limits the reaction," he said. "In the presence of a base like KOH [potassium hydroxide], silicon mostly produces soluble silicic acid (Si(OH)4). Compared to zinc, silicon is simply more reactive, especially at room temperature."

Although the larger surface area of the 10-nm silicon compared with larger silicon pieces contributes to its fast hydrogen production rate, surface area alone cannot account for the huge rate increase that the scientists observed. The surface area of 10-nm silicon is 204 m2/g, about 6 times greater than the surface area of 100-nm silicon, which is 32 m2/g.

To understand what causes the much larger increase in the hydrogen production rate, the researchers conducted experiments during the silicon etching process. They found that, for the 10-nm particles, etching involves the removal of an equal number of lattice planes in each direction (isotropic etching). In contrast, for 100-nm particles and microparticles, unequal numbers of lattice planes are removed in each direction (anisotropic etching).

The researchers attribute this etching difference to the different geometries of different-sized crystals. As a result of this difference, the larger particles adopt non-spherical shapes that expose less reactive surfaces compared to the smaller particles, which remain nearly spherical, exposing all crystal facets for reaction. Larger particles also develop thicker layers of oxidized silicon byproducts through which water must diffuse. Both of these factors limit the rate of the reaction on larger particles.

To confirm that that the 10-nm silicon-water reaction generates hydrogen with no byproducts that could interfere with applications, the researchers used the silicon-generated hydrogen to operate a fuel cell. The fuel cell performed very well, producing more current and voltage than the theoretical amount of pure hydrogen, which is due to the fact that the 10-nm particles generated more hydrogen than the theoretical 14 wt %.

The researchers hope that this surprising ability of silicon nanoparticles to rapidly split water and generate hydrogen could lead to the development of a hydrogen-on-demand technology that could enable fuel cells to be used in portable devices. This technology would require a large-scale, energy-efficient method of silicon nanoparticle production, but could have some advantages compared to other hydrogen generation techniques.

"The key advantage of silicon oxidation for hydrogen generation is its simplicity," Swihart said. "With this approach, hydrogen is produced rapidly, at room temperature, and without the need for any external energy source. The energy needed for hydrogen generation is effectively stored in the silicon. All of the energy input required for producing the silicon can be provided at a central location, and the silicon can then be used in portable applications.

"The key disadvantage of silicon oxidation is its relative inefficiency. The energy input required to create the silicon nanoparticles is much greater than the energy available from the hydrogen that is finally produced. For large scale applications, this would be a problem. For portable applications, it is not. For example, the cost of electricity supplied by an ordinary household battery can easily be 10 to 100 times higher than the cost of electricity from a utility, but batteries still play an important role in our lives."

In the future, the researchers plan to further increase the hydrogen generation capacity of silicon oxidation by experimenting with different mixtures.

"One direction that we are presently pursuing is the use of mixtures of silicon nanoparticles with metal hydrides, which also react with water to produce hydrogen," Swihart said. "Compounds like lithium hydride and sodium hydride react with water to produce the base (LiOH or NaOH) that is needed to catalyze the silicon oxidation. However, they can react too fast with water (explosively) and are not stable in air. Mixing them with silicon nanoparticles or coating them with nanoparticles may serve to both temper their reactivity and increase the capacity of the system by replacing the added base (e.g., KOH in the published paper) with a material that also generates ."

Explore further: Graphene sensor tracks down cancer biomarkers

More information: Folarin Erogbogbo, et al. "On-Demand Hydrogen Generation using Nanosilicon: Splitting Water without Light, Heat, or Electricity." Nano Letters. DOI: 10.1021/nl304680w

Journal reference: Nano Letters search and more info website

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tpot
4.1 / 5 (8) Jan 24, 2013
Perhaps a little sili powder and a thimble of water could replace using precious helium in party balloons. They would make fun little explosions over the birthday cake candles.
master chief049
3.4 / 5 (8) Jan 24, 2013
This would be great for military applications! a vehicle that would run by just adding water no need to transport fuel to the war zone
Shabs42
4.5 / 5 (8) Jan 24, 2013
Another repeat article from a couple of days ago.

http://phys.org/n...and.html

Between the ridiculous comments, repeat articles, and inconsistent writing this site is getting tougher to read.
hemitite
4.7 / 5 (3) Jan 24, 2013
"Anything worth doing, is worth doing poorly"
- G.K.Chesterton
omatwankr
2.5 / 5 (6) Jan 24, 2013
"precious helium in party balloons"

"But John Lee, chairman of the UK's Balloon Association, insists that the helium its members put into balloons is not depriving the medical profession of the gas. 'The helium we use is not pure,' says Lee. 'It's recycled from the gas which is used in the medical industry, and mixed with air. We call it balloon gas rather than helium for that reason.'""
http://science.sl...balloons
El_Nose
1.4 / 5 (5) Jan 24, 2013
~10 minutes per gram H

scaleble and cheap -- as long as you don't pour the silicon into a major water way like an ocean, it shouldn't become an uncontrolled loss of water.
antialias_physorg
4.4 / 5 (8) Jan 24, 2013
as long as you don't pour the silicon into a major water way like an ocean, it shouldn't become an uncontrolled loss of water.

Erm...it's not a catalytic reaction. The Silicon is used up. Once all silicon is oxidized the reaction stops.
It's a 2 component fuel. Either component is inert, Mix 'em, et voila: hydrogen (and waste)

But at a 10 to 1 (or 100 to 1) energy cost for producing the silicon nanoparticles vs. energy gotten via the released H2 don't expect this to be the solution for mass energy storage or hydrogen cars.
Whydening Gyre
2.3 / 5 (6) Jan 24, 2013
Don't know about anyone else, but I would find it helpful to also know the actual times involved in producing a reaction. That's how we could determine maximum efficiency, isn't it?
PressingTheIssue
not rated yet Jan 24, 2013
This is a great discovery. I can see a portable backpack power station type device. I wonder how long a backpack sized power generator could be effective and what it could power?
k_m
1 / 5 (4) Jan 24, 2013
Perhaps a little sili powder and a thimble of water could replace using precious helium in party balloons. They would make fun little explosions over the birthday cake candles.

And cute 'Chinese Lanterns' too.

This would be great for military applications! a vehicle that would run by just adding water no need to transport fuel to the war zone

And when the vehicle reaches the destination, let it simmer and stew, then detonate any left-over hydrogen.
DonaldJLucas
5 / 5 (4) Jan 25, 2013
Perhaps a little sili powder and a thimble of water could replace using precious helium in party balloons. They would make fun little explosions over the birthday cake candles.


I will never forget the demonstration in Chemistry 101 my freshman year in college of just how much energy is contained in one party balloon full of hydrogen! The professor inflated what I recall as just a standard sized party balloon with hydrogen from a portable tank. He tied a string to the balloon and let it rise about 10 feet above the lectern. He then tied a small birthday cake candle to the end of a bamboo pole, lit the candle, and then raised it up to the balloon. The resulting explosion hurt our ears (approximately 300 students in a very large lecture hall) and produced a flame ball about 2-3 feet in diameter.

I really don't think hydrogen in kid's party balloons is a very good idea at all ;-(
gwrede
1 / 5 (2) Jan 25, 2013
I wouldn't rub my hair against one.
antialias_physorg
3.7 / 5 (3) Jan 25, 2013
The resulting explosion hurt our ears

You sure that was hydrogen? I remember a very similar experiment in school which wasn't impressive at all. There was a bang, but not particularly intense.
Eikka
1 / 5 (1) Jan 25, 2013
'It's recycled from the gas which is used in the medical industry, and mixed with air. We call it balloon gas rather than helium for that reason.'


That's a pretty poor argument, considering that naturally occurring helium isn't pure either. It's mixed with a whole bunch of petroleum products and has to be separated to be used in the medical industry.

That same process could take the spent helium and recycle it back, instead of wasting it into party balloons.

Once helium becomes actually rare, it will also become really expensive, and people won't be able to afford to waste it in party balloons. Nor will they afford to run MRI machines either.
antialias_physorg
3.7 / 5 (3) Jan 25, 2013
Since it's currently obviously cheaper for hospitals to buy helium than to recycle it - what's the fuss?

If it's discarded by the medical industry then I see no reason to fault those who pick it up. If anything you should be calling the medical industry on poor recycling practices.
DonaldJLucas
5 / 5 (2) Jan 25, 2013
The resulting explosion hurt our ears

You sure that was hydrogen? I remember a very similar experiment in school which wasn't impressive at all. There was a bang, but not particularly intense.

Its possible that they mixed oxygen in with the hydrogen in the balloon in an ideal ratio, but that is how I remember it (close to 40 years ago...) I was sitting very close to the front of the class room, near the experiment, so it may have been louder for me than for others.
Whydening Gyre
1.6 / 5 (5) Jan 25, 2013
The resulting explosion hurt our ears

You sure that was hydrogen? I remember a very similar experiment in school which wasn't impressive at all. There was a bang, but not particularly intense.

We had a neighbor who would float oxy-acetyline "balloons" over bonfires. THAT was loud. Simple fact is - if something burns - and burns fast - it will create an audible shockwave...
People will forever be driven to investigate that which surprises them. However, if that investigation will cause irreperable harm - DON'T.
Eikka
3 / 5 (2) Jan 25, 2013
Since it's currently obviously cheaper for hospitals to buy helium than to recycle it - what's the fuss?


It would remain relatively cheap for longer if it wasn't simply wasted.

When it finally becomes rare enough that you have to recycle it, it will also be so expensive that a lot of the applications become economically infeasible.
antialias_physorg
3.3 / 5 (3) Jan 25, 2013
It would remain relatively cheap for longer if it wasn't simply wasted.

I agree. But why are you ripping into the people who aren't responsible for that shortcoming? They're just using stuff that would be (currently) wasted.
SteveL
5 / 5 (1) Jan 27, 2013
Since it's currently obviously cheaper for hospitals to buy helium than to recycle it - what's the fuss?


It would remain relatively cheap for longer if it wasn't simply wasted.

When it finally becomes rare enough that you have to recycle it, it will also be so expensive that a lot of the applications become economically infeasible.
Helium is also a very important component in TiG and MiG welding as a shield gas. It's also used in the electronics industry in silicon wafer manufacturing. Considering how much electronics and welding goes into making the variety of items we use, and the heavy equipment used to produce the items we use, helium isn't just important for cryogenics and the medical field. As for its use as a purge gas, nitrogen is beginning to replace helium in many applications.
gwrede
1 / 5 (2) Jan 29, 2013
The resulting explosion hurt our ears
You sure that was hydrogen? I remember a very similar experiment in school which wasn't impressive at all. There was a bang, but not particularly intense.
Its possible that they mixed oxygen in with the hydrogen in the balloon in an ideal ratio, but that is how I remember it (close to 40 years ago...) I was sitting very close to the front of the class room, near the experiment, so it may have been louder for me than for others.
Usually they insert electrodes into a jar of water and let a voltage break water into hydrogen and helium. The combined gas automatically contains the right ratio. Detonating this gives a very loud explosion, whereas if you only collect the hydrogen, you get a much quieter one. (The hydrogen has to be paired up with oxygen from initially outside the bubble, which takes time and energy.)

Many teachers only collect the hydrogen, because the mixture is a bit too potent for regular size classroom safety.
Eikka
1 / 5 (1) Jan 29, 2013
I agree. But why are you ripping into the people who aren't responsible for that shortcoming? They're just using stuff that would be (currently) wasted.


Because they are still wasting it, and they can't excuse themselves by saying they're simply wasting someone else's waste, because that first waste shouldn't even be happening.

It's like saying, "Oh, this aluminium I'm throwing in the river? It's no harm, because I got it from a dude who was throwing it away into a landfill. He didn't want to put it in recycling, so it's okay that I'm wasting it."

What if instead of party balloons, they'd take the helium and sell it back to reprocessing? Oh right: party balloons make more profit.
Eikka
1 / 5 (1) Jan 29, 2013
Or even worse still; the hospitals might have considered recycling the helium, but the party balloon guys simply paid more for it.
antialias_physorg
3 / 5 (2) Jan 29, 2013
"Oh, this aluminium I'm throwing in the river? It's no harm, because I got it from a dude who was throwing it away into a landfill. He didn't want to put it in recycling, so it's okay that I'm wasting it."

well, yeah - because it's still better than having the aluminum float downriver. The problem isn't with the guy fishing it out - it's with the guy throwing it in the river in the first place.

What if instead of party balloons, they'd take the helium and sell it back to reprocessing?

Obviously no one wants to. Because otherwise the hospital would sell it for reprocessing and not give it to people who want to make balloons.
LeoVuyk
1 / 5 (2) Jan 30, 2013
Is there reason to suggest that this process has so called "OVERUNITY" CHARACTERISTICS?
antialias_physorg
3 / 5 (2) Jan 30, 2013
Is there reason to suggest that this process has so called "OVERUNITY" CHARACTERISTICS?

No there is not. It takes 10 to 100 times more energy to make the silicon nanoparticles than you get out interms of hydrogen when using them up. It's not a catalytic reaction but a simple redox reaction. It's a 2-component fuel (water and silicon nanoparticles get dumped together and react with one another to make silicic acid an hydrogen.). Both get used up.
SteveL
not rated yet Jan 30, 2013
@ qwrede:
Usually they insert electrodes into a jar of water and let a voltage break water into hydrogen and helium.
Hydrogen and Oxygen.

It's a 2-component fuel (water and silicon nanoparticles get dumped together and react with one another to make silicic acid an hydrogen.). Both get used up.
And creates a new waste stream for an acid. Effectively taking pure water (safe) and silicon to convert one form of energy to another that when used creates an acid. Hm.... No issues with this?