Solar Powered Hydrogen Production from Rust
Source: MIT Technology
Review
Researchers in Switzerland have demonstrated more-efficient water-splitting
solar cells based on a cheap, abundant, and long-lasting material: rust.
The advance could lead to a cheap and energy-efficient way to generate
hydrogen for fuel-cell vehicles using solar energy.
Water-splitting solar panels would have important advantages over existing
technologies in terms of hydrogen production. Right now, the primary way
to make hydrogen is to separate it from natural gas, a process that generates
carbon dioxide and undercuts the main motivation for moving to hydrogen
fuel-cell vehicles: ending dependence on fossil fuels. The current alternative
is electrolysis, which uses electricity to break water into hydrogen and
oxygen, with the two gases forming at opposite electrodes. Although electrolysis
is costly, it can be cleaner if the source of the electricity is wind,
sun, or some other carbon-free source.
But if the source of the electricity is the sun, it would be much more
efficient to use solar energy to produce hydrogen by a photochemical process
inside the cell itself. By improving the efficiency of such solar panels,
Michael Grätzel, chemistry professor at the Ecole Polytechnique Fédérale
de Lausanne, in Switzerland, and his colleagues have taken an important
step toward this goal.
The researchers have shown that by including small amounts of silicon
and cobalt, they can grow nanostructured thin films of iron oxide that
convert sunlight into the electrons needed to form hydrogen from water.
And the iron oxide films do this more efficiently than ever before with
this material.
Iron oxide has long been an appealing material for such solar panels,
in part because it holds up well in contact with water. But although it
can absorb sunlight, the resulting charge carriers could not easily escape
the material, so they recombined, canceling each other out before they
could split any water. By doping the rust with silicon, the researchers
coaxed the material to form cauliflower-like structures with extremely
high surface area, ensuring that a large part of the atoms in the material
were in contact with the water, or very close to it. That way, holes could
easily escape into the water, where they prompt the generation of oxygen
gas. The silicon also improves electron conductivity in the material,
which is important for generating hydrogen gas at an opposite electrode.
The researchers further improved the process by adding cobalt, which acts
as a catalyst for the reactions.
Grätzel's new iron-oxide films can convert an impressive and, according
to the researchers, "unprecedented" 42 percent of ultraviolet
photons in sunlight into electrons and holes. But the system's overall
efficiency is only about 4 percent, in part because iron oxide doesn't
absorb all the parts of the solar spectrum.
The main achievement of Grätzel's new research, which appears in
the current issue of the Journal of the American Chemical Society, is
that it examines the interactions at work in the system in great detail,
says Brian Holcroft, CEO of Hydrogen Solar, a company based in Guildford,
UK, that is developing ways to mass-produce panels inspired by Grätzel's
materials. The findings suggest several strategies that could help the
iron-oxide-based panel reach the 10 percent efficiency level that would
make the technology competitive with current ways of creating hydrogen,
Holcroft says. (Iron oxide could theoretically be as much as 20 percent
efficient.) These include adjusting the amount and arrangement of silicon
and cobalt, and improving the structure of the films.
If this level of efficiency can be met, hydrogen-generating solar energy
could mitigate some of the challenges that threaten to make hydrogen fuel-cell
vehicles impractical, says George Sverdrup, hydrogen technology manager
at the National Renewable Energy Laboratory (NREL), in Golden, CO. For
example, if consumers and businesses used these panels to make hydrogen,
rather than getting hydrogen from a large facility, it would cut out the
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