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Researchers demonstrate water splitting to generate hydrogen using ultra-small Si nanoparticles

18 January 2013

Erogbogbo
Schematic showing CO2 laser pyrolysis synthesis of silicon nanoparticles transferred to a custom stainless steel prototype cartridge used to generate hydrogen for fuel cell applications. Credit: ACS, Erogbogbo et al. Click to enlarge.

A team of researchers from the University at Buffalo (SUNY) have demonstrated that hydrogen generation from ultra-small silicon nanoparticles (10 nm diameter) proceeds much more rapidly than expected based upon extrapolation of rates obtained using larger particles. The ultra-small particles react with water to generate hydrogen 1,000 times faster than bulk silicon, 100 times faster than previously reported Si structures, and 6 times faster than competing metal formulations.

In a paper published in the ACS journal Nano Letters, they report that the hydrogen production rate using 10 nm Si is 150 times that obtained using 100 nm particles—significantly exceeding the expected effect of increased surface to volume ratio. These results imply that nanosilicon could provide a practical approach for on-demand hydrogen production without the addition of heat, light, or electrical energy, they suggested.

Silicon nanoparticles may be of practical use for on-demand hydrogen generation, based upon on their enhanced activity relative to other air-stable hydrogen-generating materials such as aluminum and zinc. Integration of nanosilicon with appropriate cartridge technologies could provide a “just add water” hydrogen-on-demand technology that would promote adoption of hydrogen fuel cells in portable power applications.

However, scalable and energy-efficient processes for nanoparticle production must be implemented to expand the potential use of silicon-based H2 generation beyond niche applications. Laser pyrolysis has been demonstrated at kg/h scales, and thus it may be one such process.

—Erogbogbo et al.

Conventional means of splitting water to produce hydrogen include electrolysis, thermolysis, photocatalysis, and combinations of those. Water can also be split chemically using a substance that can be oxidized by water, such as aluminum or silicon, the authors notes. The silicon-water reaction is slow and can be self-limiting, via oxide formation. However, they add, silicon can theoretically release two moles of hydrogen per mole of silicon, is abundant and safe, has high energy density, and releases no carbon dioxide.

Upon oxidation with water, silicon can generate 14% of its own mass in hydrogen, via overall reactions such as:

Si(cr) + 4H2O(l) → Si(OH)4(aq) + 2H2(g)
Si(cr) + 2H2O(l) → SiO2(s) + 2H2(g)

Because of their high surface area per volume, similar to that of other nanoparticles for hydrogen generation, silicon nanoparticles are naturally expected to generate hydrogen more rapidly than bulk silicon. Moreover, many properties of silicon nanocrystals are known to differ from those of bulk silicon in uniquely advantageous ways. Nonetheless, the advantages of silicon for rapid generation of hydrogen have not previously been investigated.

—Erogbogbo et al.

They used silicon particles with diameters of about 10 nm synthesized in their lab; particles under 100 nm (Sigma Aldrich); and 325 mesh (<40 μm, Sigma Aldrich) using various aqueous bases and reaction conditions.

Ergbogbo2
Conceptual illustration of a shrinking core model that describes H2 generation from 10 nm Si NP. Brown spheres represent NPs, and the dotted circle indicates shrinkage from the original sphere size. The gray and blue areas represent hydrogen contributed by the silicon and by water, respectively. Credit: ACS, Erogbogbo et al. Click to enlarge.

One of the clear differences they observed was that the 10 nm Si shows instantaneous generation of hydrogen that plateaus only after the silicon has been depleted, while larger particles exhibit an induction period. In addition, the hydrogen generated from the 10 nm silicon exceeds the expected 2 mols of H2 per mole of silicon. They attributed this to to the presence of hydrogen on the surface of the particles remaining from their production in a hydrogen rich environment.

To understand the much greater hydrogen production rate by the 10 nm silicon than can be accounted for by the difference in the specific surface area, they conducted experiments in which they stopped the etching process before the silicon had been fully consumed. (Etching is the removal of material by a liquid or vapor etchant.)

They attributed the greater production rate to a change in the etching dynamics at the nanoscale from anisotropic etching (uniformity in the vertical direction) of larger silicon to effectively isotropic etching (uniformity in all directions) of 10 nm silicon.

Resources

  • Folarin Erogbogbo, Tao Lin, Phillip M. Tucciarone, Krystal M. LaJoie, Larry Lai, Gauri D. Patki, Paras N. Prasad, and Mark T. Swihart (2013) On-Demand Hydrogen Generation using Nanosilicon: Splitting Water without Light, Heat, or Electricity. Nano Letters doi: 10.1021/nl304680w

January 18, 2013 in Hydrogen Production, Nanotech | Permalink | Comments (12) | TrackBack (0)

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Comments

When hydrogen will be produced cheaply from water with or without e-power, lower cost FCs may become a worthwhile alternative to generate on-board electricity for EVs and/or for fixed applications.

So where does one get ultra-small nanoparticles of silicon? What are the costs of producing it? Is the expense of creating nanoparticlized silicon greater than the market value of the hydrogen produced?

Look at the guy with the night vision scope in the graphic.
This suggests a military market, and high costs.
I suppose it is like a very powerful, non-rechargeable battery.

I wonder does it say anything about catalysts for electrical splitting of H2O ?
It Would be useful for energy storage if it ended up being better than current techniques.

I had a look for the technology, and found several papers on making the silicon nanoparticles by laser pyrolysis.
They were all well above my head, and also are really early stage demos, and so it is premature to talk about the efficiency in terms of either cost or energy.

It is certainly interesting stuff though, and the uses they illustrated were not just for military goggles but also for a laptop, so maybe at least the hope is that costs will be reasonable.

I hope they start using it for almost free fuel thereafter. Gasoline price will collapse right after it. Some fuelcell cars will be needed so it take hydrogen cars and some hydrogen infrastructure to start at the same time. Also conventionnal ice car can be modified to get an auxillary hydrogen tank to add range and lower the fuel cost so it will cost less to be able to use hydrogen as a fuel.

Isn't silica a finite resource? Doesn't mining it require massive amounts energy - most of it being fossil fuels put into the mining machinery? If silica was 100% renewable, this would be awesome.

@ejj:
We aren't going to run out of sand, aka silica dioxide.

Harvey,

This is a possible EV solution even accounting of the water that must be added as per the two possible reactions between water and the nano silicon. The process generates either 4kg of hydrogen per 100 kg or 64 kg of water and silicon, which implies 4% to 6.25% or about 5% hydrogen by weight of “fuel”.

For example, 98.8 kWh FC energy output at 60% efficiency would require 4.35 kg of hydrogen produced by 30.45 kg of silicon. The same output from a 25% efficient ICE requires 10.9 gallons of gasoline costing about $40. So, for a FC to compete economically with an ICE, a hermetically sealed nano silicon refill needs to be in the $1/kg range and the FC itself must be in an ICE price range. Even a $100/ton carbon tax will only jack up the price of 10.9 gallons by less than $11.

Looks like you can get 50g of 5-15 nm of Si for $79.10 from this company:
http://www.sigmaaldrich.com/materials-science/material-science-products.html?TablePage=18010474

...so there is a long way to go to get to $1 per kg.

The important part is how much energy it will take to regenerate the silicon particles. It may prove to be a low cost way of transporting hydrogen, no cryogenics or high pressures needed.

there is no pure Silica to be mined on earth (except maybe very deep below the crust.
So we use a very expensive method to reduce SiO2 to Si, which we can use as a fuel. We transform sand into fuel, which returns to sands after "burning".
For reducing the SiO2 to Si, we obviously need external energy and expensive technology.
Once we are at the point that we make fuel from SiO2, why nog simply make fuel from CO2. This is not expensive technology, and most probably much more efficient than through laser blasting of metal. (synthetic CH4 is already being made inexpensively from wind-electricity in germany)
The waste from the Si is nanoparticles of SiO2. That's exactly what you make to induce silicose, a severe pulmonary disease. The waste product from synthetic CH4 is CO2 : completely harmless and easy to dispose off (if it is recycled CO2 to start with).

Except maybe for exotic military applications, this will never be of any use for transportation.
If we have the renewable electricity to make Si-nanoparticles, we certainly have the renewable electricity to make syntetic carbon-based fuels, which are much less dangerous and much cheaper.

This is still under study. Look what is ALREADY WORKING and get the same thing cheaper, greener and above all it works:

http://www.reflectia.eu/alhymotion/index.php?lang=eng

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