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
Nano Letters.
If hydrogen is ever to be used to deliver energy for wide
commercial applications, one of the requirements is finding a fast, inexpensive way to produce hydrogen. One of the most common
hydrogen production
techniques is splitting water into hydrogen and oxygen. There are
several ways to split water, such as with an electric current (
electrolysis), heat, sunlight, or a substance that chemically reacts with water. Such substances include aluminum, zinc, and silicon.
As the scientists explained, silicon-
water oxidation reactions have so far been slow and uncompetitive with other
water splitting
techniques. However, silicon does have some theoretical benefits, such
as being abundant, being easy to transport, and having a high
energy density.
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
silicon nanoparticles, which have not previously been studied for hydrogen generation. Because silicon nanoparticles have a larger
surface area
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."
Enlarge
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 (Al
2O
3)
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 m
2/g, about 6 times greater than the surface area of 100-nm silicon, which is 32 m
2/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
silicon nanoparticles may serve to both temper their reactivity and increase the
hydrogen generation capacity of the system by replacing the added base (e.g., KOH in the published paper) with a material that also generates
hydrogen."
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
Copyright 2013 Phys.org
All rights reserved. This material may not be published, broadcast,
rewritten or redistributed in whole or part without the express written
permission of Phys.org.
