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.

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