Water and rust – those are the key ingredients that are putting a cheap new twist on what has been an expensive technology for turning sunlight into hydrogen.

It’s only in the experimental stages right now, but if it pans out, this advance holds the potential of providing an economically viable way to store solar energy by converting it to hydrogen, researchers say.

Photoelectrochemical tandem solar cell with iron oxide semiconductor
Photoelectrochemical tandem solar cell with iron oxide semiconductor. (image via EPFL)

Here are the nuts and bolts: Scientists from the École Polytechnique Fédérale de Lausanne report they have combined an oxide semiconductor with a dye-sensitized cell in a self-contained device that uses the power from light “to break up water molecules and reform the pieces into water and hydrogen.”

The kicker here is that the semiconductor – where the oxygen evolution takes place – is made with iron oxide. That’s right, rust.

The choice came about because the EPFL team took an approach seemingly opposite of that taken by other researchers. Instead of trying to find a material that would drive up the efficiency of the process, no matter how exotic and expensive the material might be, they were bent on using something cheap and abundant.

“A U.S. team managed to attain an impressive efficiency of 12.4 percent,” says EPFL researcher 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.”

Iron oxide is a lot cheaper than that, although the researchers admit that their iron oxide is a bit more complicated than the stuff developing on that shovel you left out in the backyard.

“Nanostructured, enhanced with silicon oxide, covered with a nanometer-thin layer of aluminum oxide and cobalt oxide – these treatments optimize the electrochemical properties of the material, but are nonetheless simple to apply,” EPFL reports.

rust water solar cell
Solar cell producing bubbles of gas (image via EPFL)

Now, there still is the matter of efficiency. The EPFL cell has tested at between 1.4 and 3.6 percent, depending on the precise prototype used.

“With our less expensive concept based on iron oxide, we hope to be able to attain efficiencies of 10% in a few years, for less than $80 per square meter. At that price, we’ll be competitive with traditional methods of hydrogen production,” Sivula says.

The counterpart to the semiconductor, the dye-sensitized solar cell – which lets electrons transferred by the iron oxide gain the energy to pull hydrogen from water – is made up of standard ingredients, a dye and titanium dioxide.

The EPFL research is published in the journal Nature Photonics.

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