Supercheap hydrogen recipe: Sunlight + water + rust
Water and rust can team up to produce hydrogen from sunlight supereconomically, a group at École Polytechnique Fédérale de Lausanne (EPFL) in Switzerland has found.
Researchers have been pursuing the conversion of solar energy into neutral-carbon-footprint hydrogen for more than four decades. EPFL joined the hunt in the 1990s through the work of Michael Grätzel, the inventor of the dye-sensitized solar cell. Working with a colleague at the University of Geneva, he invented the photoelectrochemical tandem solar cell, a technique for producing hydrogen directly from water.
“A US team managed to attain an impressive efficiency of 12.4 percent,” said EPFL’s Kevin Sivula. “The system is very interesting from a theoretical perspective, but with their method, it would cost $10,000 to produce a 10-square-centimeter surface.”
Sivula and his colleagues purposefully limited themselves to inexpensive materials and easily scalable production processes to enable an economically viable solar hydrogen production method.
“The most expensive material in our device is the glass plate,” Sivula said.
The self-contained device produces electrons used to break up water molecules and re-form the pieces into oxygen and hydrogen. In the same liquid, two distinct layers in the device have the job of generating electrons when stimulated by light: an oxide semiconductor, which performs the oxygen evolution reaction, and a dye-sensitized cell, which liberates the hydrogen.
The semiconductor, which performs the oxygen evolution reaction, is just iron oxide, or rust.
“It’s a stable and abundant material. There’s no way it will rust any further. But it’s one of the worst semiconductors available,” Sivula admitted.
That’s why their iron oxide is more sophisticated than what you would find on an old nail. Nanostructured, enhanced with silicon oxide and covered with a nanometer-thin layer of aluminum oxide and cobalt oxide – these treatments optimize the electrochemical properties of the material, but are still simple to apply. “We needed to develop easy preparation methods, like ones in which you could just dip or paint the material,” Sivula said.
The second part of the device is composed of a dye and titanium dioxide, the basic ingredients of a dye-sensitized solar cell. This second layer lets the electrons transferred by the iron oxide gain enough energy to extract hydrogen from water.
“Right now, the efficiency of our device is still quite modest. We’re only able to convert about 1.2 percent of the solar light into hydrogen,” Sivula said in an EPFL video on the project. “But the fact that we’re using very inexpensive materials like rust, which can potentially convert up to 16 percent of solar energy into hydrogen, gives us hope that soon we’ll be able to find a very economical way to create chemical energy from sunlight.”
The team hopes to be able to attain efficiencies of 10 percent in a few years, for less than $80 per square meter, Sivula said. “At that price, we’ll be competitive with traditional methods of hydrogen production.”
The experimental device is described in Nature Photonics (doi: 10.1038/nphoton.2012.265).
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